C H A PT E R
I
1 . 1
N O IS E
1.1.1
STO R M
longer
i n
the
O
radio
noise
by
constitute
many
techniques
such
sources
physics
ment
of
and
solar
to
the
subsequent
noise
storm
Hey
in
19^2
at
was
so
intense
that
were
noise
lasts
extends
advent
of
and
from
of
noise
(Wild
et
been
at
decay
of
al.,
O
I
radar
jammed.
from
storms
storms
decameter
of
the
plasma
have
been
was
telescopes,
from
their
activity.
this
It
established
radio
type
is
also
have
in
its
receivers,
the
III
in
confirmed.
i n
the
to
the
develop­
by
radiation
the
that
wavelengths.
appeared
1965;
and
bursts
of
Historically,
operating
’d a y s '
of
inter*
accidentally
Sometimes
now
obser*
time
birth,
discovered
of
Models
receivers
is
extensively
radio
understanding
their
processes
variety
constructed
and
most
using
polarimeters.
decameter
Kundu,
one
complex
sources;
comprising
1963;
the
'hours’ to
to
meter,
perhaps
wavelength.
wavelength
noise
at
better
of
satellite-borne
kilometer
reviews
and
storm
decimeter
storms
I
simple
radiation
meter
wavelengths
storm
have
arrive
noise
of
as
noise
order
I
investigators
spectrometers
i n
C
Noise
ferometers,
storm
U
storms
atmosphere.
investigated
time
D
manifestations
solar
vational
H
BACKGROUND
wavelengths
spectacular
T
PH EN O M EN A
H IS T O R IC A L
Solar
N
I
meter
the
spectrum
With
the
existence
hectometer
Excellent
literature.
Zheleznyakov,
1970;
-
2
-
Wild and. Sxaerd', 1972; Fainberg and Stone,, 197^
Elgar^y,
19775.
A metric and decametric noise storm consists of a
smooth background radio emission, superimposed on it are
narrow band short-lived type I bursts at metric band and
type III bursts with or without fine structure at decameter
wavelengths.
Brightness temperature of the continuum and
the burst may exceed 10^ °K.
Obviously thermal processes
cannot account for such high temperatures and so gyrosynchrotron and plasma radiation processes are suggested.
Both the continuum and siperimposed bursts are circularly
polarized in the same sense or opposite sense.
High degree
of circular polarization suggests the presence of strong
magnetic fields up in the corona.
Noise storm centers are
found to be associated with visible optical regions on the
sun’s disk.
In general, noise storm activity and flare
activity are two aspects of a center of activity on the
sun though they may occur independently of each other.
Even though the noise storms were discovered as
early as 19^2 by Hey, consistent source size measurements
were available only after the installation in late 1950’s
of Nancay interferometer in France at meter wavelength
(operating frequency 169 MHz).
Source sizes in the deca­
meter range were estimated earlier by Wild and Sheridan
(1958) and later by Gergely and Kundu (1975).
A significant
innovation took place when the Culgoora radioheliograph at
Culgoora in Australia, operating at 80, 160 and H-3,25 MHz
could obtain radio pictures of the Sun in two dimensions
and two senses of circular polarization at intervals of
1 sec (Wild, 1968; Sheridan et al., 1973)*
It has signifi­
cantly assisted in the better understanding of noise storm
phenomenon and in developing the widely accepted model by
Kai and Sheridan (197H-) using multi-frequency radiohelio­
graph data of Culgoora.
The decametric type III burst is found to start at
the frequency where the metric type I burst stops.
The
type I to type III transition region is most interesting
which is around two solar radii above the photosphere
(Ho MHz plasma level) since in this region, co-rotation of
the corona with the sun stops and the solar wind enters
into the interplanetary space.
1.1.2
NOISE STORMS AT METER WAVELENGTH
At meter wavelengths noise storm is a common pheno­
menon.
Most of the noise storms at meter wavelengths occur
in the frequency range of 350 to 100 MHz.
Dynamic spectra
of meter wavelength noise storm bursts were made in 1950
(Wild and McCready, 1950) in the frequency range 10 to
30 MHz and later in the range 5 to 2H0 MHz.
-
If -
W hether th e background and. s h o r t - l i v e d b u r s ts a re
two d i s t i n c t components o r have a common o r i g i n i s an
i n t e r e s t i n g q u e s tio n and i s w e ll d is c u s s e d i n th e l i t e r a t u r e (McCready, Pawsey and P a y n e -S c o tt, 19^7; T akakura,
1956).
According t o Fokker ( I 9 6 0 ) ,
o b s e r v a tio n a l ev id en ce
such as th e o ccu rren ce o f storm b u r s ts i n c l u s t e r s and a ls o
s h o r t - l iv e d b u r s t s i n th e absence o f f l u c t u a t i o n s i n th e
background r a d i a t i o n o ver a d u ra tio n o f few m inutes
fav o u rs th e h y p o th e s is t h a t th e background and b irrst
components a re two d i s t i n c t phenomena.
B o t h
s h o r t - l i v e d
( W i l d ,
t h e
b a c k g r o u n d
b u r s t s
a r e
s t r o n g l y
1 9 5 1 )
a n d
b o t h
( P a y n e - S c o t t
a n d
L i t t l e ,
s t u d i e s
o f
n o i s e
a n d
s e n s e s
s t o r m s
o f
195 1 ) *
a r e
m o s t
o f
t h e
c i r c u l a r l y
s u p e r i m p o s e d
p o l a r i z e d
p o l a r i z a t i o n s
D e t a i l s
d i s c u s s e d
i n
o f
a r e
o b s e r v e d
p o l a r i z a t i o n
s e c t i o n
1.2.^,
E a rly a tte m p ts were made to a s s o c ia te n o ise storm
phenomenon w ith th e su n sp o t and o th e r o p t i c a l m a n if e s ta tio n
o f th e s o l a r a c t i v i t y by McCready, Pawsey and P a y n e -S c o tt
(1 9 ^ 7 ), L i t t l e and P a y n e -S co tt (1951), Maltby (1958) and
Fokker (1 9 6 0 ).
C onclusive evidence f o r a s s o c ia tio n o f
th e n o ise storm c e n tre w ith 'o p t i c a l a c t i v i t y 1 was p o ss­
i b l e o n ly when th e m u lti-e le m e n t in te r f e r o m e te r te c h n iq u e
was used by B oisehot (1958) a t 169 MHz a t Nancay i n
F ran ce,
-
5-
N o i s e s t o r m s are a s s o c i a t e d w i t h v i s i b l e
spots
and two dimensional measurement of p o s i t i o n have shown
t h a t n o i s e s t o r m s d o not lie v e r t i c a l l y abo v e t he o p t i c ­
ally active centres
(Malinge,
1963).
Detailed discussion
o n a s s o c i a t i o n of o p t i c a l a c t i v i t y of t he
sun with the
no i s e s t o r m i s g i v e n l a t e r i n se c t i o n 1.1.5.
I t has
also b e e n o b s e r v e d t h a t t h e p r o b a b i l i t y o f o b s e r v i n g
n o i s e s t o r m i n c r e a s e s t o w a r d s t h e CMP.
storm centers
Moreover the
a p p e a r t o be m o r e i n t e n s e and n u m e r o u s o n
the w e s t e r n h a l f of the
s u n a nd t h i s a s y m m e t r y is o p p o s i t e
to t h a t o f t y p e III b u r s t as o b s e r v e d b y W i l d
( 1959 )? F o k k e r (i96 0 )
and S u z u k i
(1961).
et al.
This
asymmetry
o f s t o r m c e n t e r s h a v e b e e n a c c o u n t e d as p r o p a g a t i o n s !
effect
(Malinge,
1963).
M e a s u r e m e n t s of a l t i t u d e of t h e n o i s e s t o r m
sources f r o m t h e p h o t o s p h e r e
are m o r e a c c u r a t e f o r o p t i c ­
a l l y a s s o c i a t e d l i m b sources.
sources
at
169
a v e rage b e i n g
M H z lies b e t w e e n
0 .5
R r> .
The altitude of the limb
0 .3
a nd
7
R
q
, the
T h e d i a m e t e r o f th e s t o r m s o u r c e
decreases w i t h increasing frequency e . g . , V
at 200 MHz?
1.7
! t o 9 ! arc at
in the East-West directions
150
MHz? 8'
(Malinge,
to 7 r arc
arc at 169 M H z
1963).
Moreover,
t h e s o u r c e does not m a i n t a i n t h e same size d u r i n g i ts
l i f e t i m e a n d t h e r e is no r e l a t i o n b e t w e e n t he flux
«w»
density
the
and
diameter
storm burst
of the
is m u c h
(3 m
source*
less
The
tha n that
angular
of
noise
size
of
storm
source.
1. 1 .3
NOISES S T O R M S
Decametric
region of type
Decametric
of type
III
of
III
Smerd,
dents
bursts
1972|
de
Groot,
swept
of
(1)
(see
are
in
USA,
and K u n d u
namely
They
"On-fringe"
position
of these
bursts
generally
found two
type
of
and
III
not
).
We
have
(decametric).
a large
succession,
Excellent
number
occasion­
and time
reviews
literature
on
(Wild
Utretch,
and
Correspon­
197^).
studied the
storm with the
at
help
kinds
"off-fringe"
bursts
of
as
65-25
of type
III
follows:
They
Starting
are
associated with flares
frequency
generally
and
occur
MHz
Observa­
coincide in position
source.
MHz.
character­
Clark Lake Badio
different
continuum
is b e l o w 50
asso­
transition
in frequency
in the
(1975)
noise
"On-fringe"
as
Astronomy Group,
Smith,
a decametric
the
burst
quick
1.2.2),
available
197*H
with the
and
sec.
of
III
always
al., 1970
et
looked upon
frequency interferometer
tory in
burst
type
structures
Solar Badio
Gergely
istics
to
occurring
fine
almost
significance
(metric)
bursts
encountered
type
the
are
(Boischot
continuum can be
a l l y -varieties
are
storms
storms
out
I
DECAMETER WAVELENGTH
noise
ciated with metric
already pointed
AT
w^eak
over
a
-
7
-
narrow freq u e n c y range 30-20 MHz.
(2 )
" O f f - f r in g e ” ty p e I I I H u rsts a re d is p la c e d from
th e p o s i t i o n o f th e continuum source by 0.1
t o 0 .5 R^, •
Most o f them a re i n te n s e , a s s o c ia te d w ith f l a r e s and
appear a l l o v e r th e fre q u e n c y range 65-25 MHz,
As e a r l y as 1958, a tte m p ts were made to d eterm in e
th e source s iz e o f d e c a m e tric n o ise storm s c e n te r s .
A
sw ep t-freq u en cy in te r f e r o m e te r was used i n th e freq u en cy
range o f 65 t o
MHz.
Mean source s iz e s determ in ed a t
65 MHz and *+5 MHz were 6* and 10' a rc r e s p e c t iv e ly (W ild
and S h e rid a n , 1958).
Also so u rc e s iz e s d e c re a se w ith
in c r e a s in g freq u e n cy and th e d e c re a se i s f a s t e r compared
to t h a t a t m eter w avelength,
G ergely and ICundu (197 5) j
ta k in g i n t o account th e e f f e c t o f r e f r a c t i o n and s c a t t e r ­
ing (F o k k er, 1965; and L eb lan c, 1973)j d eterm in ed th e
h e ig h t and source s iz e s i n th e range o f 65 to 25 MHz.
Mean so u rce s iz e tu rn e d out to be 0.51 R^, and 1.3 R<t> a t
65 MHz and 30 MHz r e s p e c t iv e ly .
The h e ig h ts were d e te r ­
mined by a p p ly in g l e a s t sq u are a n a ly s is m ethod, and
assuming c o n s ta n t r o t a t i o n a l r a t e o f th e so u rc e,w h ic h a t
U0 MHz and 30 MHz tu rn e d o u t t o be (1 .9 + 0.5)R^-, and
(2 .3 + 0.25)R ,.
r e s p e c t iv e ly .
The most I n p o r ta n t
c h a r a c t e r i s t i c o f d eca m e tric storm so u rc es i s t h a t th e
s iz e o f th e so u rce i s no t c o n s ta n t b u t changes over a
s h o rt p e rio d o f tim e .
M oreover, changes i n s iz e a t
8
n e a r b y f r e q u e n c i e s are u s u a l l y not correlated.
The author
fe e l s t h a t t h e v a r i e t i e s o f fine s t r u c t u r e o b s e r v e d i n
time and frequency plane
for the
(see sec,
1.2.2) m a y b e r e s p o n s i b l e
s o u r c e size v a r i a t i o n as s e e n b y G e r g e l y and K u n d u
(197 5 )*
L i k e t h e m e t r i c n o i s e storm,
d e c a m e t r i c noi s e
s t o r m c o n t i n u u m is f o u n d t o h a v e more d i r e c t i v i t y at the
c e n t r e o f t h e disk.
T h e e a s t - w e s t a s y m m e t r y as o b s e r v e d
i n t h e m e t r i c b a n d is also o b s e r v e d i n t he d e c a m e t r i c b a n d
by Gergely and Kundu
of Newkirk
A c o m p a r i s o n w it h t he m o d e l
(1 9 6 7 ) and o t h e r models o b t a i n e d f r o m ra d i o
observations
Erickson,
(1 9 7 5 ).
(Wild et a l . ,
1966)
1 9 5 9 ; Weis s , 1963; M a l i t s o n and
shows t h a t t h e storms o r i g i n a t e i n m o d e r a t e l y
l o w t o h i g h d e n s i t y regions.
D i r e c t i v i t y and
systematic
p o l a r i z a t i o n studies of t h e d e c a m e t r i c noi s e storms are,
however, still lacking.
1 . 1,k
N O I S E S T O R M S IT H E C T O M E T E R AN D K I L O M E T E R W A V E L E N G T H S
R a d i o A s t r o n o m y E x p l o r e r (RAB)
l a u n c h e d i n 1968,
Satellite
1,
a n d d e v o t e d to rad i o o b s e r v a t i o n s at l o w
f r e q u e n c i e s , p r o v i d e d f o r t h e first t i m e a n o p p o r t u n i t y to
o b s e r v e n o i s e sto r m s c o m p r i s i n g c o n t i n u u m r a d i a t i o n and
t h o u s a n d s o f d r i f t i n g b u r s t s i n h e c t o m e t e r and k i l o m e t e r
w a v e l e n g t h r a n g e s (Fairiberg and,Stone,
and W e b e r et a l . ,
1971 ).
1970 a,b;
1971a,b;
- 9 -
IMP- 6 spacecraft observations show that the source
position of continuum component and that of type III burst
were the same (Fainberg and Stone, 197^).
During a storm
a large number of type III bursts, 1 every 10 seconds, were
observed
near 2,8 MHz, They occur in large number when the
source moves away from CMP,
This effect can be either due
to change in the source activity or due to refractive
focusing due to corona of electromagnetic radiation in the
direction of steepest density gradient.
Simultaneous studies in the range of 20 to 60 MHz
(decameter)- and in the range of 5 MHz to 300 kHz (hectokilometer)- showed that, while continuum is observed in
the decametric range, simultaneous hectometer emission is
composed of large fast drifting bursts.
These observations
have been interpreted by Stone et al. (1969) as the local­
ized region of decametric continuum acting as the storage
of particles responsible for hectometric drifting bursts.
Noise storm provides an opportunity to estimate
exciter speed independent of coronal electron density
models.
The August 1968 noise storm was observed over a
period of half solar rotation.
Mean frequency drift rate
of burst was found to be varying with longitude, being
maximum at CMP,
This variation in the drift rate was
estimated quantitatively since at large distances from
the sun, assuming the path of the outward moving sources
10
f r o m s i m p l e g e o m e t r i c a l c o n s i d e r a t i o n a nd f i n i t e v e l o c i t y
(c) o f p r o p a g a t i o n of r a d i o emission.
ting technique,
determined
T h e n b y curve f it­
a b s o l u t e v a l u e o f t h e s o u r c e speed w as
as o , 3 7 c t o 0 , 1!- c (Fainberg a nd Stone,
Further,
1970b),
a s s u m i n g t h e e m i s s i o n t o b e at fun d a m e n t a l ,
d e n s i t i e s w e r e d e r i v e d but t h e s e d e n s i t i e s were a n o r d e r
o f m a g n i t u d e h i g h e r t h a n t h a t r e q u i r e d f or solar wind.
The densities
e s t i m a t e d o n t h e bas i s o f s e c o n d h a r m o n i c
e m i ssion, h o w e v e r ,
are a f a c t o r o f U- l o w e r
and are i n
b e t t e r a g r e e m e n t w i t h N e w k i r k ' s s u m m a r y of t h e d e n s i t y
models
(1967).
T h i s f a c t o r m a y be s t i l l f u r t h e r r e d u c e d
i f r e f r a c t i o n a n d s c a t t e r i n g ef f e c t s a re t a k e n into
account.
T h i s gives an i n d i r e c t s upport f o r e m i s s i o n at
l o w e r f r e q u e n c i e s to be
p l a s m a f r equency,
Steinberg
at t h e 2 nd h a r m o n i c o f t he l o c a l
From theoretical consideration,
(1973) sta t e s that h e c t o - k i l o m e t r i c b u r s t seems
to originate
at s e c o n d h armonic,
W e b e r et al.
and H A S - 2 satellite,
(1977)
w i t h the h e l p o f H e l i o s - 1 a nd 2
f o r t he fir s t t i m e h a v e d e t e r m i n e d
two d i m e n s i o n a l p o s i t i o n s w i t h o u t
s i t y model.
a s s u m p t i o n of any d e n ­
O n l y a s s u m p t i o n t h e y h a v e m a d e is that t h e
e m i s s i o n is at 2nd h a r m o n ic.
The positions obtained
f r o m d i r e c t i o n finding f r o m th e spin m o d u l a t i o n b y
H e l i o s - 1 and 2 agree w e l l w i t h th e a r r i v a l t i m e d e t e r ­
m i n ations,
This
shows t h a t
c o r r e c t i o n s due t o p r o p a g a t i o n s !
11
e f f e c t s s u c h as
-
r e f r a c t i o n an d scat t e r i n g
are not l a r g e
e v e n i f s i m p l e g e o m e t r i c a l p a t h l e n g t h is assumed.
Polarization observations i n the l o w frequency
r a n g e w i l l b e v e r y m u c h rewarding.
E x c e l l e n t r e v i e w s of
l o w f r e q u e n c y o b s e r v a t i o n a n d stereo r a d i o
been made b y Fainberg and Stone
astronomy have
(197*+) and S t e i n b e r g
(1977).
1 . 1 .5
A S S O Cliff I O N O F N O I S E S T ORKS W I T H O P T I C A L
ACTIVITY OF THE SUN
Noise
storms a l l o v e r t h e o b s e r v a b l e f r e q u e n c y
r a n g e h a v e b e e n f o u n d t o b e a s s o c i a t e d w i t h sunspots.
But t h e 2 7 - d a y cy c l e w h i c h i s o b s e r v e d i n t h e case of
act i v e r e g i o n s
(or sunspots) i s not n e c e s s a r i l y o b s e r v e d
i n t h e c a s e o f no i s e storms.
be r e l a t e d t o c e r t a i n p h a s e s
regions.
Thus,
n o i s e s t o r m s s e e m to
of development of active
T h e s t o r m a c t i v i t y appears t o h a v e
some co r r e ­
l a t i o n w i t h the n u m b e r of m a j o r suns p o t s o n t h e d i s k bub
it is f o u n d t h a t l a r g e r sunspots are not always a s s o c i a t e d
w i t h n o i s e s t o r m activity,
F o k k e r (1965),
whi l e stud y i n g
t h e a s s o c i a t i o n of m e t e r w a v e l e n g t h no i s e s t o r m s w i t h
sunspots, f o u n d t h a t t h e noi s e s t o r m a c t i v i t y i n c r e a s e s
with the
a r e a as w e l l as t he m a g n e t i c f i e l d s t r e n g t h o f
t h e sunspot.
by
S i m i l a r r e s u l t s were also o b t a i n e d p r e v i o u s l y
Payne-Scott
ever,
and L i t t l e
s t o r m cent r e s
(195D
a nd M a l i n g e
(1963).
How­
are not r a d i a l l y s i t u a t e d above the
12
o p t i c a l c e n te r o f a c t i v i t y b u t th e r e i s enough o bserva­
t i o n a l evidence to show t h a t d e ea m e trie n o ise storm s o r i ­
g in a te i n n o n - r a d ia lly o r ie n te d c o ro n a l s t r u c tu r e s
( H a lin g e , 1963; S tew art and Labrum, 1972 ).
R e c e n tly , th e dependence o f some o f th e storm
c h a r a c t e r i s t i c s i n m eter w avelength re g io n such as mean
i n t e n s i t y , number o f storm s p e r y e a r e t c . , on th e 1 1 -y ear
s o l a r c y c le has been e s ta b lis h e d (K o ro lev , 197^).
Mean
i n t e n s i t y as w e ll as th e number o f n o is e storm s go on
d e c re a sin g tow ards th e s o la r minimum.
I t has a ls o been
e s ta b lis h e d t h a t th e r e q u ir e d spot a re a s a re sm a ll i n
s o l a r a c t i v i t y minimum compared t o th e re q u ire d a re a s o f
th e sp o t i n th e s o la r a c t i v i t y maximum f o r storm s to o c c u r.
Most o f th e storm s a re p reced ed by a f l a r e w ith in
an hour (M alinge, 1963) and by im p o rta n t f l a r e s w ith in
30 m in u tes.
S im ila r s tu d ie s c a r r i e d o u t i n sunspot minimum
p e rio d r u l e o u t th e a s s o c ia tio n o f f l a r e s w ith n o ise storm s
by ch ance.
W hile stu d y in g th e a s s o c ia tio n o f d eea m e trie
n o ise storm s w ith f l a r e s
G ergely and B ric k so h (1975) found
t h a t i n t e n s i f i c a t i o n o r o n se t o f d e eam etrie storm does not
d i r e c t l y r e l a t e w ith f l a r e s .
During th e sto rm , s e v e r a l
a c tiv e re g io n s \rere p re s e n t on th e d is k and a number o f
s u b f la re s were observed i n th e s e a c tiv e r e g io n s .
These
a c tiv e re g io n s appear to form a complex a t th e chromo­
sp h e ric l e v e l and a l l d e ea m e trie storm c e n te rs a re
associated
each
with
other.
rence
of
This
show
regions
(Vaiana
concluded
closed
A
for
when
the
number
the
metric
with
storm
netic
source
of
is
on
with
an
occur­
other
during
SKYLAB
several
(1973)
al.
features
have
observed
showed
sources
The
frequently
by
Gergely
a
flare
in
basis
the
that
were
of
OMP
of
date
position
coincided
to
of
flare
turned
random
and
out
the
active
frequency,
to
be
twice
potential
positions
higher
of
deca-
associated
or
lower
decametric
the
(1975)
occurrences.
structures
be
with
Kundu
distant
calculated
magnetic
found
in
observed
progress,
the
and
mag­
continuum
filaments
or
chains
filaments.
the
ing.
and
et
as
some
among
coronal
of
studies
around
Studies
of
Vaiana
the
in
obtained
simultaneously
storm
arches.
of
association
expected
source
evidenced
subflare
pictures
developed
that
fields
he
interact
configuration,
Simultaneous
coronal
a
1973)*
majority
noise
can
which
inter-connections
al.
technique
showed
the
et
loop
studying
region
X-ray
complex
that
regions
following
Excellent
mission
active
interaction
sub-flares
region.
were
these
sun
with
However,
type
region
I
of
hecto-and
Sakurai
noise
McMath
association
storm
9597
on
kilometer
(1971)
were
the
with
has
the
noise
shown
associated
disk
in
optical
the
storms
that
with
activity
are
lack­
hectometer
the
August
active
1968
storm.
~
Th u s j
1V-
i n vi e w of the l a c k of definite association
of t h e s t o r m s w i t h t h e l a r g e m a g n e t i c suns p o t s
c o r r e l a t i o n b e t w e e n the l a r g e s t sunspot o f
i n t e n s i t y o f t h e noise
st o r m radi a t i o n ,
and a p o o r
a group and
c o n d i t i o n f o r t he
g e n e r a t i o n o f n o i s e s t o r m (other t h a n o n l y the p r e s e n c e
of h i g h m a g n e t i c fields)
electrons
1.2
s u c h as t he p r e s e n c e of e n e r g e t i c
seems t o he necessary.
H I G H R E S O L U T I O N S T U D I E S O F MQ I S E S T O R M S O U R C E S
It h a s b e e n n o t e d e a r l i e r that t h e noi s e st o r m
c o n s i s t s o f a c o n t i n u u m r a d i a t i o n and an e n o r m o u s n u mber
o f b u r s t s h a v i n g n a r r o w band-widths and short duration.
U n d e r s t a n d i n g o f t h e s e b u r s t s an d t h e i r r e l ation, i f any,
to the continuum radiation is facilitated through h i g h
s e n s i t i v i t y and h i g h t i m e - f r e q u e n c y - a n d s p a t i a l - r e s o l u t ­
io n observations.
B y o b s e r v i n g t e m p o r a l a nd s p e c t r a l
v a r i a t i o n of solar r a d i o e m i s s i o n w i t h h i g h r e s o l u t i o n
techniques,
one c a n d i s t i n g u i s h b e t w e e n p h e n o m e n a i n t r i n ­
sic t o t h e sou r c e and s u b s e q u e n t p r o p a g a t i o n ef f e c t s i n
the overlying m edium
(Sawant et a l . , 1976 and C h a p t e r III
o f t h i s t h e sis).
T h e m o s t i m p o r t a n t r e g i o n of o b s e r v a t i o n is f r o m
20 t o 80 MHz.
H i g h spatial resolution observations i n
decameter range are lacking because t h e y require large
antennas.
Multi-frequency two-dimensional observations
i n t h e r a n g e o f 20 t o
110 M Hz b e i n g p l a n n e d b y M a r y l a n d
-
15
-
U n i v e r s i t y w i l l d e f i n i t e l y a s s i s t i n o u r u n d e r s ta n d in g
o f some o f t h e p ro b le m s m e n tio n e d i n t h e f o llo w in g s e c ­
tio n s .
1 ,2 .1
HIGH RESOLUTION STUDIES OF NOISE STORM
SOURCES AT METER WAVELENGTHS
For a b e t t e r u n d e rsta n d in g o f ty p e I b u r s ts and
t h e i r r e l a t i o n to continuum , h ig h r e s o lu tio n s tu d ie s i n
fre q u e n c y and tim e were s t a r t e d by E lg a ro y (1959) around
200 MHz.
S im ila r e f f o r t s were c o n tin u e d by o th e r w orkers
(de G root, 1959? E lg a ro y , 19655 de l a Noe and B o isc h o t,
1972; Ifc ise r and F a in b e rg , 197*+; de Groot e t a l . , 1976).
R e c e n tly , h ig h s e n s i t i v i t y h ig h r e s o lu tio n sp e c tro sc o p e i n
t h e ran g e o f9 3-186 MHz has gone i n to o p e ra tio n w ith a f r e ­
quency r e s o l u ti o n 100 kHz and tim e r e s o l u ti o n 50 msec
(K o ro lev , 1975).
But not much e f f o r t has gone to im prove
th e s p a t i a l r e s o l u t i o n b e t t e r th a n t h a t achieved i n th e
Nancay in te r f e r o m e te r a t 169 MHz and i n C ulgoora h e lio ­
g rap h o p e ra tin g a t 160, 80 and *4-3,25 MHz.
Type I , t y p e I I I and d r i f t p a i r s a r e t h e m a jo r
s p e c t r a l f e a t u r e s o b s e r v e d i n m e t r i c s to rm s o v e r and
ab o v e t h e c o n tin u u m .
Type I e m is s io n i s c h a r a c t e r i z e d
b y s h o r t d u r a t i o n 0 .1 5 t o 0 .5 s e c and n a rro w b a n d w id th s
3 t o 6 MHz.
Some b u r s t s show f r e q u e n c y d r i f t s
d e g re e o f c i r c u l a r p o la r iz a t io n .
and h ig h
T h e se b u r s t s a r e
o b s e r v e d w i t h i n t h e ra n g e o f 50 t o *4-00 MHz and h av e
- 16
b r ig h tn e s s te m p e ra tu re o f th e o rd e r o f 1010 t o 1 0 °K
(E lg a ro y , 1965).
These b u r s ts o f te n appear s in g le i n
th e tim e -fre q u e n c y p la n e .
Also t h e r e i s a ten d e n cy t o
c l u s t e r to g e th e r i n te n s e o r hundreds t o form what a re
c a lle d narrow band nc h a in s n o f ty p e I b u r s t (W ild, 1957;
E lg a ro y , 1961; H anasz, 1966; de G root, 1976).
W hether
ty p e I b u r s t appears s in g ly o r i n c h a in s , t h e r e i s no
d i s t i n c t i o n i n t h e i r p r o p e r tie s (H anasz, 1966).
c h a in s slo w ly d r i f t i n freq u e n cy .
The
Most ch ain s a re found
t o appear below 310 MHz (E lg aro y and Ugland, 1970) where
t h e h ig h freq u en cy c u t o f f f o r n o ise storm s l i e (Kundu,
1965 ).
O b serv a tio n s show t h a t weak ty p e I I I b u r s ts grow
o u t o f " c h a in s n which appear alm ost p a r a l l e l t o th e tim e
a x is on th e dynamic s p e c tr a i n d ic a t i n g th e band s p l i t t i n g
phenomenon (H anasz, 1966) as shown i n F ig . 1,1 ( a ) .
How­
e v e r, de Groot e t a l . (1976) have no t observed such pheno­
menon w ith in th e freq u e n cy range (160 to 320 MHz).
T his
may be due to d i f f e r e n t c o n d itio n s p r e v a ilin g e i t h e r a t
th e so u rc e o r i n th e p ro p a g a tio n medium a t th e tim e o f
o b s e r v a tio n s , such as d i f f e r e n t lo n g itu d e o f s o u rc e , mag­
n e t i c f i e l d c o n fig u r a tio n , e tc .
Prom th e h ig h r e s o l u ti o n
(tim e and p o l a r i z a t io n ) o b s e rv a tio n s o f n o ise storm s
Chernov e t a l . (1972) concluded t h a t ty p e I b u r s t and
c h a in s o f ty p e I b u r s t a t m eter w avelengths p o ss e s s
ZHW A o uan b sJJ
(b)
Fig. 1.1 (a)
Simultaneous type XIX a to ra and type X a to m i n
decameter range July 23, 1970 (Ifellei^y«d«rs«ut 19'A)
Mask type I I I b u rst "growing'* oat o f type I chri.ua.
6S MSI
February 26, 1962
17
-
18
-
d i f f e r e n t s p e c t r a l end p o l a r i z a t i o n c h a r a c t e r i s t i c s which
i n a d d itio n showed day to day v a r i a b i l i t y .
At low f r e q u e n c ie s , th e ty p e I b u rs t d is a p p e a rs
g iv in g way to d e c a m e tric ty p e I I I b u r s t .
T his t r a n s i t i o n
u s u a l ly occurs i n th e range o f 60-^0 MHz (M a lv ille , 1962;
H anasz, 1963; S te w a rt, 1972; G ergely and Kundu, 1975) as
shown i n F ig . 1.1 ( b ) .
An a ttem p t t o e x p la in t h i s f e a tu r e
was made by Gordon (1971) i n term s o f n o n lin e a r plasm a
p ro c e sse s.
A ccording to B o isch o t e t a l . (1970) and B oischot
(197^)s th e ty p e I - ty p e I I I t r a n s i t i o n re g io n co rresp o n d s
t o t r a n s i t i o n from c lo s e d f i e l d l i n e to open f i e l d l i n e s
I n th e corona.
T h is su g g e stio n i s su p p o rte d by th e la r g e
a n g u la r s iz e o f th e s o u rc e s , which in c r e a s e s r a p i d l y w ith
d e c re a s in g freq u e n cy and th e r e f o r e w ith in c re a s in g h e ig h ts
from th e p h o to sp h e re .
S te reo experim ent c a r r i e d o u t by C aroubalous and
S te in b e rg (197b-) d eterm in ed th e r a d i a ti o n power p a t t e r n
o f ty p e I and ty p e I I I b u r s t s .
Same p o la r e v e n ts o bserved
a t 169 MHz sim u lta n e o u s ly from th e e a r t h and w ith a d i s t a n t
S o v ie t space p robe M ars-3 } have shown 3 db beam w id th
s m a lle r th a n 25° i n th e e c l i p t i c p la n e .
F u r th e r , th e s e
o b s e rv a tio n s have shown t h a t ( i ) so u rc e s o f ty p e 1 a re
s m a lle r th a n as seen from th e ground and a re s i t u a t e d i n
o v er dense f i b r e s o r arch es which ex ten d u pto 0 .8 R
q
above th e p h o to sp h e re , ( i i ) a n g le o f r a d i a ti o n w ith th e
-
m a g n e t i c f i e l d is l e s s t h a n
19
~
(iii)
the low e r c o r o n a
above t h e a c t i v e r e g i o n is f u l l o f f i b r e s a nd arches and
r a n d o m s c a t t e r i n g i n t h a t m e d i u m accounts f o r all t h e
o b s e r v e d p r o p e r t i e s of t y p e I burst, a n d (iv) type III
bursts
are less d i r e c t i v e c o m p a r e d t o t y p e I bu r s t a nd
t h e r e is e n o u g h e v i d e n c e f or t he p r e s e n c e o f large scale
d e n s e s t ructures.
Observations obtained with Culgoora Hadioheliog r a p h c o m b i n e d w i t h o p t i c a l d a t a suggest t h a t t y p e I
sources
the
are l o c a t e d o n m u l t i p l e strong f i e l d loops a nd
s o u r c e s o f s p o radic t y p e III bur s t o n w i d e l y d i v e r g ­
i n g f i e l d lines
(KaA a n d S h e r idan,
At p r e s e n t t h e r e i s
p l a i n t y p e I burs t s
197*+)»
no cons i s t e n t t h e o r y t o ex­
a l o n g w i t h t he f o r m a t i o n o f
" c h a i n s 1'
o n t h e h i g h e r f r e q u e n c y side o f s p e G t r a a nd w h i c h g r o w
i n t o t y p e I I I b u r s t a n d o n t h e l o w e r side o f spectra,
t r a n s i t i o n o f 'type I t o d e c a m e t r i o t y p e III.
Many theories have been put forward b y various
workers
s u c h as W i l d a n d T l a m i c h a (196**-), T r a k h t e n g e r t s
(1966),
G o r d o n (1971),
Z a i t s e v and
Fomlchev
(1973)
w h i c h e x p l a i n o n e o r t h e o t h e r f e a t u r e s l e a v i n g the
r e m a i n i n g o n e s u n e x p l a i ned.
O t h e r m a j o r s p e c t r a l f eature o b s e r v e d is the
d r i f t pair.
A drift p a i r contains t w o i d e n t i c a l ele­
ments, t h e s e c o n d b e i n g th e
r e p e t i t i o n of t h e first a n d
20
-
i s "observed a l t e r a d e la y o f 1 t o 2 seconds (R o b e rts , 1958).
Each elem ent has a t o t a l d u r a tio n o f 0 .5 t o U- seconds and
an in s ta n ta n e o u s band-width being from 0 ,3 t o if MHz,
T y p ic a l tim e d e la y s betw een th e two elem ents of a
d r i f t p a i r o f 1 t o 2 seconds were e x p la in e d by R oberts
(1958) as due to an echo phenomenon a t th e second harm onic
e m issio n .
I n f a c t , o b s e rv a tio n s of a s in g le t r a c e a t h a l f
th e fre q u e n c y (de l a Noe and Mo11e r-P e d e rse n , 1971)>
su p p o rt t h i s h y p o th e s is . However, to
e x p la in o th e r
o b serv ed s p e c t r a l f e a t u r e s , such as d u ra tio n a t fix e d
f r e q u e n c ie s , a s s o c ia tio n w ith ty p e I I I b u r s t , d r i f t r a t e s ,
narrow band e m issio n , de l a Noe
p roposes an e x p la n a tio n
f o r th e d r i f t p a i r s based on th e p o s s ib le e x is te n c e o f
so u rc e s f o r e le c tr o n a c c e le r a tio n i n th e upper co ro n a.
1 .2 .2
HIGH RESOLUTION STUDIES IN DBC.AMETBR WAVELENGTH
R e s o lu tio n s i n tim e and freq u e n cy com parable to t h a t
o b serv ed i n m e tric band was n o t r e a l i s e d i n th e decam eter
band t i l l l a t e 196o’s . E l l i s & McCulloch (1966) i n i t i a t e d
h ig h r e s o l u ti o n work i n tim e (20 ms) and fre q u e n c y (30 kHz)
i n th e range 28 to 36 MHz.
T able 2,1 i n C hapter I I summar­
i z e s th e h ig h r e s o l u ti o n sp e c tro sc o p e s and m u lti-c h a n n e l
r e c e iv e r s developed from tim e t o tim e i n th e decam eter
ra n g e . O b serv a tio n s i n th e d eca m e tric range were i n i ­
t i a t e d by de l a Noe and B oischot (1972) w ith h ig h s e n s i t i ­
v i t y and h ig h r e s o lu tio n in time
(1 0 ms) and freq u e n cy
21
-
(60 KHz) o n l y recently,
V a r i e t i e s o f s p e c t r a l features
storm bursts,
dr i f t p a i r s ,
stria bursts,
s u c h as fast dri f t
split p a i r s , tr i p l e t s , t y p e Illb,
V - s h a p e d b u r s t s , t y p e Illd, h o o k b u r s t s,etc.,
h a v e b e e n o b s e r v e d i n d e c a m e t r i c noi s e s t o r m o b s e r v a t i o n s
o b t a i n e d b y e m p l o y i n g h i g h t i m e - f r e q u e n c y r e s o l u t i o n and
h i g h sensitivity techniques.
Some o f t h e s e burs t s h a v e
b e e n s h o w n i n Fig.
c, d).
1,2 (a, b,
O b s e r v a t i o n s of
t h e s e s p e c t r a l f e a t u r e s r e n d e r v a l u a b l e i n f o r m a t i o n s u c h as:
a)
N a t u r e o f p l a s m a i n t he c o r o n a a r o u n d t wo solar
radii
( T a k a k u r a and Yousef,
1975;
Melrose,
1975;
S a w a n t et a l . , 1976).,
b)
L o n g i t u d i n a l d e p e n d e n c e of n o i s e s t o r m b u r s t s f or
s t u d y i n g the p r o p a g a t i o n ef f e c t s
c)
High sensitivity and h i g h time
(de l a Noe,
1975).
and f r e q u e n c y
r e s o l u t i o n d e c a m e t r i c o b s e r v a t i o n s of the quiet
s u n a n d b u r s t s for t h e u n d e r s t a n d i n g o f s c a t t e r i n g
effects i n the c o r o n a a nd n a t u r e o f ex c i t e r r e s p o n s ­
i b l e for bursts.
M a i n f e a t u r e s o f t h e c o m m o n l y o b s e r v e d burs t s h a v e
b e e n s u m m a r i z e d below:
-
22
l0-8-<6
10 - 8 - 7 6
3 5 -0
R
FREQUENCY
HIA1 )
3 4 -6
3 4 - 20 4 01 31 0 4 01 32
TIME U.T HOURS
iw
' '
03 53 0 0 03 53 01
TIME U-T. HOURS
13 - 1-77
31-3-76
34-3
(c )
050143
050146
TIME U T HOURS
3 4 -0
(a)
0 6 :46:58
TIME
50
1.2
0 6 :47:00
U T HOURS
V a rie tie s o f deeajaetric b u rs ts observed by H„R.S.
a t Ahmedabad: (a ) S t r i a b u rs t, (b) S p lit p a i r ,
(c ) Long d u ra tio n t r i p l e t and (d) type I l l d b u rs ty
i n R» and L - p o la r iz a tio n s .
SPLIT PAIR
E l l i s and McCulloch (1966) d isc o v e re d " s p l i t p a ir s "
as c o n s is tin g o f two
p a r a l l e l s h o r t l iv e d h u r s t s , sometimes
d r i f t i n g tow ards th e low fre q u e n c y .
These h u r s ts a re m ostly
o b serv ed below 60 MHz w ith th e average d u r a tio n o f 1-2 sec
as shown i n F ig . 1 .2 ( b ) .
Two elem ents of th e b u r s t do not u s u a lly b e g in
e x a c tly a t th e same tim e 5 th e lo ite r freq u e n cy elem ent
s t a r t s on an average e a r l i e r th a n th e h ig h freq u e n cy one.
I n g e n e ra l, th e h ig h freq u e n cy elem ent i n a p a i r i s more
in te n s e th a n th e low fre q u e n c y one and i n t r i p l e t s th e
m iddle i s more in te n s e th a n th e o th e r two.
The d r i f t r a t e o f each elem ent i s o f th e o rd e r o f
0 ,1 MHz/sec. and th e freq u e n c y s e p a r a tio n betw een two
elem ents in c r e a s e s w ith fre q u e n c y , th e average v a lu e being
0 .1 MHz a t 25 MHz t o 1 MHz a t 60 MHz.
The bandw idth o f
t h e h ig h fre q u e n c y elem ent i s about 0.05 MHz.
These
b u r s t s can occu r i n i s o l a t i o n , i n ch ain s and a s s o c ia te d
w ith ty p e I I I b u r s t s .
In about 10^ o f th e o bserved s p l i t
p a i r b u r s t s , t r i p l e s p l i t t i n g was o b se rv e d .
I n th e
double c h a in o f b u r s t s , c h ain s were found t o be harm onic­
a lly re la te d .
A v a r i e n t o f s p l i t p a i r i s "V" shaped b u r s t
(B aselyan e t a l . , 197^ I ) and i s d is c u s s e d i n C hapter IV .
-
2 lf -
The h ig h s e n s i t i v i t y ( 1 0
—2 2
„ p
Wm
„ -i
Hz
) and h ig h
r e s o l u ti o n i n tim e and freq u e n cy o b s e rv a tio n s i n th e range
o f 20 - M3 MHz and ^0 - 80 MHz o f de l a Hoe and B oischot
(1972) along -with th e p o s i t i o n a l in fo rm a tio n o f th e storm
(de l a Noe, 1975) d u rin g two conse-cutive h a l f s o la r- r o t a ­
tio n s have le d t o th e fo llo w in g new r e s u l t s .
STRIA BURSTS
A l a r g e number o f s in g le b u r s ts i d e n t i c a l t o an
i n d iv id u a l elem ent o f s p l i t p a i r were observed and were
named as " s t r i a " b u r s ts (de l a Noe and B o isc h o t, 1972).
Of th e s e 25$ were grouped to form s p l i t p a i r s .
H e lio g ra p h ic l o n g it u d i n a l dependence o f th e spec­
t r a l p a ra m e te rs o f s t r i a , s p l i t p a i r , and t r i p l e t a l l
grouped i n to ty p e I l l b were in v e s tig a te d .
The fo llo w in g
r e s u l t s came out o f th e a n a ly s is :
1,
The d a il y average freq u e n cy o f o c cu rren c e i n case
o f s t r i a , s p l i t p a i r , t r i p l e t s and ty p e I l l b b u r s ts shoxfed
minimum when a c tiv e re g io n s were lo c a te d n e ar th e c e n t r a l
m erid ia n o f th e Sun,
2.
The average d u r a tio n of s t r i a , s p l i t p a i r , t r i p ­
l e t and ty p e I l l b in c r e a s e s from h ig h t o low freq u e n cy
and fo llo w s an e x p o n e n tia l law .
T his d u ra tio n i s l a r g e r
f o r so u rc es n e a r th e lim b th a n t h a t a t CMP.
- 25 3.
Average d r i f t r a t e s v a ry from day to day b u t th e
v a r i a t i o n i s random i n term s o f th e average lo n g itu d e . The
average d r i f t r a t e ran g es betw een 0-150 k H z/sec.
In s ta n ta n e o u s ban d \fid th and freq u e n c y s p l i t t i n g
i n a s p l i t p a i r and t r i p l e t a re independent o f lo n g itu d e .
I n s p l i t p a i r s and t r i p l e t s , elem ents have in s ta n ta n e o u s
bandw idth n arrow er th a n th o s e o f s in g le s t r i a e , th e aver­
age r a t i o b ein g 1 s1 .5 .
DIFFUSE STRIA BURST
E s s e n t i a l l y th e s e d i f f u s e b u r s ts a re o f lo n g e r
d u r a tio n and e x h ib it no fre q u e n c y d r i f t (B a sely a n e t a l . ,
197^ I , I I ) .
Average d u r a tio n a t 25 MHz i s c=-10 s e c . The
in s ta n ta n e o u s bandw idth o f d if f u s e s t r i a b u r s t i s 1 .5 to
2 tim e s l a r g e r th a n t h a t of th e o rd in a ry s t r i a b u r s ts
( 0 .1 2 MHz a t 25 MHz).
They may be a ls o grouped t o form
s p l i t p a i r , t r i p l e t and c h a in s .
Chains o f d if f u s e s t r i a
b u r s t s a re named as ty p e I l l d b u r s t and i n g e n e ra l
ty p e H i d b u r s t fo llo w s ty p e I I I b u r s t and a re r e l a t e d
by F-H r e l a ti o n s h i p (B asely an e t a l . , 197^ I I ) .
No p o lar--
i z a t i o n i s o b serv ed f o r th e s e b u r s ts (de l a Noe, 1975).
TYPE IH to BURSTS
Chains o f s t r i a , s p l i t p a i r s , and t r i p l e t s have
been named as ty p e I I l b b u r s ts by de l a Noe and B oischot
(1 9 7 2 ).
Type I l l b b u r s ts a re o bserved i n decam eter
26
-
wavelength regions as frequently as decametric type III
hursts and rarely in the meter wavelength (Ellis and
McCulloch, 1967? Boischot,
1973; Baselyan et al., 197*4- (I,
II); Takakura and Yousef, 1975).
Drift rate of type IXIh
hursts are approximately twice the drift rate of type III
hurst in the same range (Baselyan et al., 197*4-11).
The
main spectral properties of type Illh hursts are summarized
as follows;
Type Illh hursts are observed more frequently at
a low frequency around 35 MHz (de la Noe and Boischot,
1972).
The observations reported in this thesis have
been made around the same frequency range.
The number of elements in type Illh hurst is found
to have longitudinal dependence and is minimum at
(de la Noe, 1975).
CMP
This number can he as large as *4-0 in
80 to 25 MHz range (Boischot, 1973).
In general, the
number of elements in type Illh is small and in the limit?
stria hurst can he Illh hurst reduced to a single element.
Detailed analysis of the time delay and frequency
ratio between the two related type Illh and type III hurst
favours the F-H hypothesis (Ellis and McCulloch, 1 9 6 7 ;
Baselyan et al., 197*4- (I, II); Takakura and Yousef, 1975;
Meiser and Fainberg, 1975).
de la Noe and Boischot (1972)
feel that type Illh is likely to he a precursor of type
III hurst (de la Noe, 1973).
An excellent example of
-
U burst
27
s u p p o r t i n g F - H h y p o t h e s i s is g i v e n b y St e w a r t
(1975)*
S t r i a t i o n i n f r e q u e n c y is o b s e r v e d i n the f u n d a ­
m e n t a l w h i l e t h e h a r m o n i c is s m o o t h (Fig.
relationship
III b u r s t s
1.3 )• F - H
and p r e c u r s o r h y p o t h e s i s f o r t y p e I l l b - t y p e
are d i s c u s s e d i n d e t a i l i n C h a p t e r I V of t h i s
thesis,
T h e C u l g o o r a s p e c t r o g r a p h and r a d i o h e l i o g r a p h
observations
s h o w t h a t t y p e Illb p o s i t i o n m a y v a r y i n a
t y p e III group.
No s i g n i f i c a n t d i f f e r e n c e w as fou n d b e t ­
w e e n t h e p o s i t i o n o f t y p e I I l b p r e c u r s o r a nd the f o l l o w i n g
t y p e III burst.
T h e a n g u l a r size o f t y p e I l l b an d a s soci­
a t e d t y p e III b u r s t o c c u r r i n g
as a p a i r as
judged f r o m
0 . 3 5 t i m e s t h e m a x i m u m c o n t o u r tu r n s ou t to b e about
t o 2 0 ’ o f arc at the
1^'
s a me p o s i t i o n at 80 MHz.
A b r a n i n et a!,
(1975) h a v e d e t e r m i n e d t he a n g u l a r
s i z e o f s o u r c e s i n 25 to
12,5 M Hz range.
The
size o f
i n d i v i d u a l s t r i a c a n b e less t h a n 10* o f arc i n 26-2*f MHz
band.
A n g u l a r sizes o f d i f f u s e t races,
second h a r m o n i c ,
are 20'
a nd
which
ace p r o b a b l y
o f a rc at 25 and 12.5 MHz
respectively.
No s i g n i f i c a n t change i n t h e sizes o f t h e
sources o f t h e
s e c o n d h a r m o n i c was o b v i o u s
l i f e t i m e o f t y p e I l l b burst.
d u r i n g t he
T h e e n t i r e r e g i o n of emis­
s i o n i s u s u a l l y no l a r g e r t h a n the s o u r c e o f t he s e c o n d
harmonic
(type III)
a n d c o i n c i d e s w i t h it i n position.
2
£
28 -
FREQUENCY (MHz)
to
-
06h31m 10
Fig, 1.3
3lm 305
UT
In v e rte d -IJ b u r s t rec o rd ed by th e Culgoora
sp e c tro g ra p h . On th e o r i g i n a l spectrogram
th e b rig h te n in g a t th e b e g in n in g o f th e
down s tro k e o f th e fundam ental i s seen to
c o n s is t o f two c l e a r l y s e p a ra te d s t r i a e .
I n t e n s i t y v a r i a ti o n s o b serv ed i n th e range
o f 110 t o 200 MHz a re in s tr u m e n ta l o r i g in
(S te w a rt, 1975).
-
29
-
Apart from these commonly observed features, other
features such as fast drift storm burst, hook burst, drift
pair, etc,, are also observed.
Fast drift storm bursts are
observed during decametrie noise storm.
groups of 10 to 20 bursts.
They occur in
Drift rates of these bursts are
■within the range 1 to 2 MHz/sec and bandwidths ^
30 kHz.
HOOK BURSTS
Hook bursts are associated with drift pairs and
look like U burst.
Drift rate and bandwidths are similar
to that of a drift pair but the slope is suddenly reversed
producing a curved trace of increasing frequency.
On
dynamic spectra, it looks like VLF hooks produced by elec­
tron streams in the Earth's atmosphere (Ellis, 1969).
High resolution observations during decametrie
noise storms in the range (70 to 2 h ,^ MHz) for about 15
months (December 1972 to February 197*+) were analyzed by MollerPedersen (197^) to find the dependence of spectral features
on heliographic longitude.
Very interesting results came
out; type Illb bursts were observed when the storm was
located away from CMP while type III bursts were observed
when the storm was near CMP,
Drift pairs are observed
only over narrow range of longitude near CMP within the
small cone of emission.
Their studies show that character­
istics of decametrie noise storm does not depend on density
-
30
-
o r m agnetic f i e l d and a re c o n s is te n t -with c o n c lu sio n s drawn
by K orolev ( 197*+).
They have a ls o su g g e ste d t h a t a l l storm
b u r s ts a re e m itte d sim u lta n e o u s ly b u t a re e m itte d i n d i f f ­
e re n t d i r e c t i o n s .
T h is in fe re n c e sh o u ld be checked by
S o la r s te r e o ra d io astronom y experim ents w ith space c r a f t s .
We have seen t h a t a d ecam etric n o ise storm o f long
d u r a tio n g e n e r a lly ev o lv es as ty p e I l l b n e a r CMP becomes
as ty p e I I I and ends a s ty p e I l l b ,
Storms o f o n ly ty p e
I l l b , ty p e I I I and ty p e I l l - d r i f t p a i r s have a ls o been
o b serv ed .
High s e n s i t i v i t y and h ig h tim e r e s o l u ti o n te c h n iq u e
has f o r th e f i r s t tim e p e rm itte d th e o b s e rv a tio n o f th e
q u ie t sun i n th e decam eter ra n g e .
O bserved low tem pera­
t u r e s can be e x p la in e d as due to s c a t t e r i n g o f th e r a d i ­
a tio n i n an inhomogeneous co ro n a.
i s o f th e o rd e r o f 0,5 x 10^ °K,
e n t (w hich
E stim a te d te n p e r a tu r e
A slo w ly v a ry in g coirpon-
i s w e ll o b serv ed i n th e m eter and d e cim e ter
w av elengths) needs to be m easured i n th e decam eter ra n g e .
D uring n o ise sto rm s, ty p e I I I b u r s t s oust above
th e q u ie t sun background a t 60, 36.9 and 2 ^ ,3 MHz were
observed (A ubier and B o is c h o t, 1972),
From th e observed
tim e p r o f i l e s th e y o b ta in e d average v a lu e s o f e x c i te r
f u n c tio n d u r a tio n as 3 .9 j 6 .2 and 7 .5 sec r e s p e c tiv e ly
and average decay tim e c o n s ta n t 2.0 se c a t a l l th e t h r e e
f r e q u e n c ie s .
A r e l a t i o n was e s ta b lis h e d betw een th e
-
duration
shown
time
of
and
that
the
coronal
constant
the
decay
an i n p r o v e d method,
results
were
(Barrow and
1*2.3
time.
derived
assumption
were
hut
obtained
Achong,
Using this
temperatures
w i t h the
plasma waves
31 -
lower
than
with less
in the
19755
of
it
from the
decay
expected.
By using
sensitivity,
and
HIGH R E S O L U T I O N O B S E R V A T I O N S
AMD KILOMETER WAVELENGTHS
was
collisions! damping
interval
Achong
relation
of
15
Barrow,
AT
to
similar
36
MHz
1975).
BECTO
At p r e s e n t h ig h r e s o l u ti o n o b s e rv a tio n s i n h e c to and K ilom eter range a re sim ply la c k in g ,
T his may be due
to c o n s tr a in t on space borne experim ents re g a rd in g s iz e
and w eig h t.
T ech nical co n sid er a tio n puts c o n str a in t on th e
s iz e o f antennas which in tu rn puts lim it on s e n s i t i v i t y
i
and a n g u la r r e s o l u ti o n .
S e n s i t i v i t y and dynamic ran g e o f
th e r e c e iv e r s a re a ls o hampered by space c r a f t 's su b -sy stem
r a d io freq u e n c y i n te r f e r e n c e (KFI) w hich most o f te n o c cu rs
i n low fre q u e n c y band and can
b o ard .
s a tu r a te th e r e c e iv e r s on
Time r e s o lu tio n and freq u en cy range o f th e r e c e iv e r s
a re lim ite d by th e te le m e tr y d a ta r a t e system s though th e
p r e s e n t te c h n o lo g y p e rm its th e use o f h ig h te le m e try
ra te s .
- 32 Radio astronom y s a t e l l i t e s and in g en eo u s a n a l­
y s i s methods have y ie ld e d c o n s id e ra b le knowledge o f th e
i n t e r p l a n e t a r y space and h e c to -k ilo m e te r n o is e storm s.
RAE-1 s a t e l l i t e launched i n J u ly 1969 was th e f i r s t s a te ­
l l i t e devoted e n t i r e l y to ra d io o b s e rv a tio n s a t low f r e ­
quencies (0 .2 t o 9 MHz) (Weber e t a l . , 1971).
Thousands
o f ty p e I I I b u r s t s and storm s have been d e te c te d
(F ain b erg and S to n e , 197*0.
The IMP-6 s a t e l l i t e was t h e
f i r s t t o p ro v id e th e o b s e rv a tio n s w ith h ig h s e n s i t i v i t y
and a c c u ra te d i r e c ti o n s o f ra d io so u rc es u sin g s p in modu­
l a t i o n te c h n iq u e su g g ested by S lysh (1 9 6 7 ).
A ccurate
a sp e c t in fo rm a tio n ^ 1 ° was o b ta in e d from o p t i c a l se n so rs .
I t a ls o p ro v id e d an o p p o rtu n ity f o r th e sim u ltan eo u s
o b s e rv a tio n s o f e n e rg e tic p a r t i c l e s (p ro to n s and e le c ­
tr o n s ) and ra d io em issio n .
F o r th e f i r s t tim e , a sweep freq u e n cy r e c e iv e r
i n th e ran g e if—2 M s w ith tim e r e s o l u ti o n o f 2 seconds
and freq u e n cy r e s o lu tio n o f 20 kHz was flow n on 0G0-3 i n
1966.
O b se rv a tio n s o f ty p e I b u r s t were in c o n c lu s iv e
(Haddock and G raodel, 1970),
I t w i l l be rew arding to
lo o k in to d a ta o b ta in e d f o r ty p e I l l b - l i k e r a d i a t i o n .
Time r e s o l u ti o n o f th e o rd e r o f 0 .5 sec and
e x is tin g freq u e n c y r e s o l u ti o n ( cm 10 kHz) along w ith
c i r c u l a r p o l a r i z a t i o n o b s e rv a tio n a re b a d ly needed a t
p r e s e n t f o r b e t t e r u n d e rsta n d in g o f th e I l l b b u r s ts
-
33
-
a n d t h e i r r e l a t i o n w i t h t y p e III.
S i m u l t a n e o u s ground-
b a s e d o b s e r v a t i o n s w i l l b e o f gre a t value.
We hope that w i t h the Voyager mission already
l a u n c h e d i n August
1977 a n d s i m u l t a n e o u s g r o u n d - b a s e d
o b s e r v a t i o n s w i l l t h r o w light o n some o f the cu r r e n t
p r o b l e m s i n t h e h e c t o - a n d k i l o m e t e r w a v e l e n g t h range.
P O L A R I Z A T I O N S T U D I E S O F N O I S E STOR M S
C o m p r e h e n s i v e studies o f t h e p o l a r i z a t i o n b e h a ­
v i o u r o f no i s e s t o r m s
made by Fokker
(i9 6 0 )
at m e t r i c w a v e l e n g t h s h a v e b e e n
a nd S u z u k i
(1961)
a r o u n d 200 MHz.
No c o m p r e h e n s i v e p o l a r i z a t i o n studies o n d e c a m e t e r n o i s e
storms
are c u r r e n t l y available.
h e c to-and kilometer ranges
Nevertheless,
P o l a r i z a t i o n st u d i e s i n
are s i m p l y not existing.
s c a n t y p o l a r i z a t i o n d a t a at d e c a m e t e r w a v e ­
lengths
are a v a i l a b l e f r o m W a r w i c k a n d D u l k ( 1 9 6 9 ), E l l i s
(1969))
de l a Noe a n d B ois e h o t
(1972)
a n d S a s t r y (1 9 7 2 ).
B o t h t h e c o n t i n u u m e m i s s i o n a n d the rad i o b u rsts
at m e t e r w a v e l e n g t h s
are s t r o n g l y c i r c u l a r l y p o l a r i z e d .
O p p o s i t e se n s e o f p o l a r i z a t i o n i s
also o b s e r v e d o f t he
c o n t i n u u m a n d t h a t o f the s u p e r i m p o s e d bursts.
O f t e n the
c o n t i n u u m and b u r s t s h a v e th e same s e n s e o f p o l a r i z a t i o n ,
W e a k l y p o l a r i z e d o r u n p o l a r i z e d s t o r m s h a v e also b e e n
o b s e r v e d but t h e i r p o s i t i o n s
t h o s e o f t h e p o l a r i z e d storm.
are f o u n d to b e h i g h e r t h a n
.
-
31)--
Malinge studied the sense of polai-ization of noise
storms as a function of their position at 1 69
I960 ; 1963).
About
MHz (Malinge,
90$ of right handed polarized storms
are situated in the northern hemisphere and about
90% of
the left handed polarized storms are situated in the south­
ern hemisphere.
The storms with variable polarization are
found near the equator.
The polarization of noise storms is very strong
near the CMP and it tends to decrease and become unpolar­
ized near the limb.
This fact can be explained in terms
of the assumed variation of height of ;the storm center.
The sunspot magnetic field at greater heights near the
limb may be weak or disordered resulting in the polariza­
tion to be weak or random.
However, the fact that strongly
polarized storms are occasionally observed near the limb
and that an unpolarized storm near the limb may develop
into a strongly polarized storm in the course of a day
indicates that the polarization behaviour may be related
to the magneto-ionic conditions in the vicinity of the
source in the corona.
The sense of polarization of the noise storm
radiation is related to the sense of magnetic field of
the associated
center of activity and is related to the
leading spot of the largest in size in the group.
The
spot polarity is opposite in the two hemispheres and
-
35
-
reverses at each sunspot cycle (Thiessen, 1952; Babcock,
1959).
Two noise storms were studied (May 17-23, and
June 7-13, 1969) by Chernov et al,, 1972 in meter wave­
length with simultaneous high resolution observation of
spectrum, flux density and polarization.
Other research­
ers such as Bhonsle and McNarry (196 k), Rao (1965) and
Harvey and McNarry (1970) had observations of only one of
the above types and hence, could not study the microstruc­
ture of storm phenomena.
Consistent with other observers,
they found that same sense of polarization prevails for
type I burst, chains and continuum.
They also observed
that the degree of polarization of type I burst and chain
increases at the initial stage of storm development.
Once it attains 100$ then it remains same till the end of
storm.
The two storms mentioned above differed in polar­
ization so far as the isolated bursts are considered.
In
the case of May 1969 storm, the degree of polarization cf
the burst was weak but changed within the life time of
burst and reached maximum when intensity of the burst was
maximum.
On the contrary, in the June storm, the degree of
polarization remained the same during the life time of
burst, even when it was relatively weak.
This difference
in polarization and changes in other properties of burst
may be due to their different longitudinal positions,
storm centers and their evolutions.
-
36
-
In t h e decam eter ra n g e , h ig h tim e r e s o lu tio n spec­
t r a l measurement along w ith p o l a r i z a t io n was i n i t i a l l y
s t a r t e d hy Warwick and Bulk (1969) and E l l i s (1 9 6 9 ).
E l l i s (1969) found t h a t i n th e c ase o f a s p l i t p a i r "both
components were p o la r iz e d i n th e same sense v i z . , r i g h t
handed lo o k in g i n th e d i r e c t i o n o f p ro p a g a tio n correspond­
in g to th e o rd in a ry sense o f p o l a r i z a t i o n w ith re s p e c t t o
th e le a d in g sun sp o t o f th e group.
c ir c u la r p o la riz a tio n .
No b u r s t showed pure
S t r i a b u r s ts can be p u r e ly c i r c ­
u l a r l y p o la r iz e d .
O bviously, th e p o l a r i z a t i o n o f ty p e I l l b should be
th a t of s p lit p a ir,
A g iv e n ty p e I l l b b u r s t need not
n e c e s s a r ily be p o la r iz e d , but i f i t i s p o la r iz e d , th e n
t h e r e e x is ts a predom inent sen se o f p o l a r i z a t i o n (de l a
Noe and B o isc h o t, 1972).
1 .3
REFRACTION
and scattering effects i n the corona
Once e le c tro m a g n e tic waves a re g e n e ra te d a t th e
so u rc e th e y w i l l have to p ro p a g a te th ro u g h th e o v e rly in g
c o ro n a .
Near th e so u rc e , r e f r a c t i o n and s c a t t e r i n g e f f e c t s
w i l l dom inate and w i l l have to be ta k e n i n t o account f o r
th e d e te rm in a tio n o f source s iz e s and t h e i r p o s i t i o n s .
D uring subsequent p ro p a g a tio n , d i f f e r e n t i a l a b s o rp tio n ,
mode c o u p lin g , F araday r o t a t i o n , d is p e r s io n e f f e c t s w i l l
m odify th e s p e c t r a l and p o l a r i z a t i o n c h a r a c t e r i s t i c s o f
th e ra d ia tio n .
-
37
-
R e f ra c tio n i n th e co ro n a a r i s e s because o f th e
la r g e s c a le g ra d ie n ts i n th e e le c tr o n d e n s ity i n th e c o ro n a.
N eg lectin g th e e f f e c t o f m agnetic f i e l d and c o l l i s i o n s , th e
e f f e c t o f r e f r a c t i o n can be e s tim a te d .
I t can be shown
t h a t th e em ission which o r i g in a te s c lo s e t o th e lim b w ill
ap p ear t o be s h i f t e d tow ards th e c e n te r o f th e d is k .
On
t h e o th e r hand f o r so u rces c lo s e t o th e c e n te r o f th e d is k
t h e r e w i l l be n e g lig ib le s h i f t .
R e fra c tio n dom inates th e
em issio n n e a r th e plasm a l e v e l as compared to th e s c a t t e r ­
in g e f f e c t .
Harmonic r a d i a ti o n fo llo w s c lo s e ly th e p a th
o f u n s c a tte r e d r e f r a c t e d r a d i a t i o n (Fokker, 1965$ Fokker
and R u tte n , 1967$ S te in b e rg e t a l . , 1971$ and R id d le , 1972).
In a d d itio n t o smooth v a r i a ti o n s o f th e e le c tro n
d e n s ity i n th e c o ro n a, sm a ll s c a le d e n s ity f l u c t u a t i o n s
a re a lso p r e s e n t i n th e c o ro n a .
E f f e c ts o f s c a t t e r i n g have been e s tim a te d by using
n u m erical approach and assuming s p h e r ic a lly sym m etrical
c o ro n a w ith o u t c o n sid e rin g th e e f f e c t o f m agnetic f i e l d .
T his was done i n i t i a l l y by Fokker (1965) and Fokker and
R u tte n (1967) assuming i s o t r o p i c in h o m o g e n e itie s and
n e g le c tin g r e g u la r r e f r a c t i o n .
S te in b e rg e t a l . (1971)
in tro d u c e d r e g u la r r e f r a c t i o n . I t was found t h a t th e appa­
r e n t so u rce i n t e n s i t y i s a fu n c tio n o f h e lio g ra p h ic lo n g i­
tu d e .
R id d le (1972) extended t h i s tre a tm e n t to a n is o tro p ic
in h o m o g e n e itie s, r a d i a l l y e lo n g a te d , and coirputed th e e f f e c t
on ty p e I I I so u rc e a t 80 MHz w ith th e fo llo w in g c o n c lu sio n s:
-
i)
38
-
T h e d i r e c t i v i t y d e c r e a s e s for f u n d a m e n t a l and
h a r m o n i c r a d iation*
ii)
I n t h e t r a n s v e r s e p l a n e source size i n c r e a s e s h ut
i n the
r a d i a l d i r e c t i o n change is l i t t l e since i n
this plane
s y s t e m a t i c r e f r a c t i o n alwa y s re d u c e s the
a p p a r e n t source size.
iii)
T h e e f f e c t o n sizes o f h a r m o n i c r a d i a t i o n is
o b v i o u s l y small,
a nd i n fact source
m o n i c are s m a l l e r up t o 0 -
60°
sizes o f h a r ­
after t h e y b e c o m e
c o m p a r a b l e w i t h f u n d a m e n t a l and are lar g e for
s o u r c e s o n limb.
iv)
P o s i t i o n shift o f f u n d a m e n t a l and s e c o n d h a r m o n i c
is
shown to be away f r o m the centre and towards the
c e n t r e respect i v e ly,
d u e t o scattering.
i o n w h i c h is d o m i n a n t
t o w a r d the center.
But refrac-
at f u n d a m e n t a l w i l l shift
Thus,
while e s t i m a t i n g r e l a t i v e
p o s i t i o n o f f u n d a m e n t a l and s e c o n d harmonic, p a t h
e f f e c t s w i l l a d d and f or i s o t r o p i c a l l y emitting
sources, t r u e d i f f e r e n c e s o f p o s i t i o n i n funda~
m e n t a l and h a r m o n i c w i l l b e c a n c e l l e d out,
v)
L i f e t i m e of s o u r c e s w i l l i n c r e a s e since the
o b s e r v e d d u r a t i o n is t h e bu r s t d u r a t i o n p l u s
t h e s c a t t e r i n g d u ration.
-
39
-
S t u d i e s o f t y p e I I I "burst m a d e b y M c L e a n (1971)
Stewart
(1972)
Riddle (1972)
and
are c o n s i s t e n t -with t h e s e conclusions.
f u r t h e r m o d i f i e d this m o d e l w i t h o u t
s p h e r i c a l s y m m e t r y for s t u d y i n g the i n f l u e n c e o f a
s t r e a m e r w h e n t h e sour c e is l o c a t e d i n it.
R e c e n t l y S m i t h and R i d d l e
(1975) h a v e d e v e l o p e d a
m o d e l f o r t y p e III b u r s t c o n s i d e r i n g t h e effect of s catter­
ing of r a d i a t i o n i n the
s o urce itself.
T h e most i m p o r t a n t
result is t h a t t h e s c a t t e r i n g i n t h e sour c e does not alwa y s
h a m p e r t h e a m p l i f i c a t i o n o f t h e fundamental.
I n fact,
it
h a s b e e n s h o w n that t h e w e a k sca t t e r i n g u n d e r t he c i r c u m s t ­
ances h e l p s t h e p r o c e s s o f amplification.
I n general,
in
t y p e I l l b - t y p e III, t y p e I l l b is m o r e i n t e n s e and t y p e
III is m o r e diffuse.
h a r m o n i c r a d iation.
T h i s i s the si g n a t u r e o f f u n d a m e n t a l
The interpretation of these bursts
w i t h t h e i r i n t r i c a t e f i n e s t r u c t u r e i n t h e fir s t t r a c e is
simple i n t e r m s o f a m p l i f i c a t i o n p r o c e s s o n v e r y small
scale i n h o m o g e n e i t i e s .
This
can happen if the
source i s
extremely inhomogeneous
so t h a t s m a l l r e g i o n s exist w i t h
v e r y h i g h l e v e l o f the e n e r g y d e n s i t y i n p l a s m a waves
w h i c h a l l o w t h e f u n d a m e n t a l to b e a m p l i f i e d i n spite of
scattering, b u t c o n t r i b u t e l i t t l e to e m i s s i v i t y of t h e
harmonic.
-
Ho -
As e le c tro m a g n e tic waves p ro p a g a te o u t i n th e
c o ro n a , o th e r p ro p a g a tio n e f f e c t s w i l l a lso a f f e c t th e
ra d ia tio n c h a r a c te r is tic s .
Am plitude w i l l be damped
e i t h e r by c o l l i s i o n a l damping a n d /o r c o l l i s i o n l e s s damp­
in g , (Landau damping) A ubier and B o isch o t ( 1 9 7 2 ) , Harvey
and A ubier (1 9 7 3 ) and A ubier (1 9 7 H ).
I n v e s tig a tio n s of
t h i s had l e d t o th e u n d e rsta n d in g o f th e te m p e ra tu re o f
th e'co
ro n a a n d /o r th e n a tu re o f e x c i t e r fu n c tio n (S o la r
i
Radio Group, U tre c h t, 197H5 Barrow and Achong, 1975; and
Achong and Barrow, 1 9 7 5 ).
Because o f th e m agnetic f i e l d , e le c tro m a g n e tic
waves w ill be decomposed i n to two independent modes, and
th e y p ro p a g a te in d e p e n d e n tly w ith d i f f e r e n t p h ase v e lo c i­
tie s .
T hus, p h ase d if f e r e n c e w i l l be g e n e ra te d and a f t e r
coming o u t o f m a g n e to -a ctiv e medium th e y w i l l be combined.
Only th e p la n e o f p o l a r i z a t i o n w i l l be changed from th e
o rig in a l.
T his i s known as F arad ay r o t a t i o n and i s a func­
t i o n o f freq tien cy , e le c tr o n d e n s ity , and m agnetic f i e l d
along th e p a th .
S e v e ra l re s e a r c h e r s have
e s tim a te d t h i s
e ffe c t.
Two modes w i l l p ro p a g a te in d e p e n d e n tly i f th e
medium i s homogeneous b u t i n inhomogeneous magneto­
a c tiv e medium independence o f th e p ro p a g a tio n o f th e two
c h a ra c te ris tic
waves b re a k
down.
This w i l l cause two
modes to couple t i g h t l y and t h e r e w i l l not be F araday
ro ta tio n .
D e ta ils o f node c o u p lin g have "been d is c u s s e d by
Sudden (1 9 5 2 ), Cohen (1959)? Dodge (1972) and M elrose (1975).
Group d e la y s e f f e c t s a re d is c u s s e d i n s e c tio n s 3 .2 .3 & 3 .3 .
1.1+ BMERGE!ICS, EMISSION PROCESSES AND MDDELS OF
HOISE STORMS
At m eter w avelength f lu x d e n s i ti e s o f s in g le b u rs ts
were e s tim a te d from o b s e rv a tio n s (W ild, 1951, Hauge, 1956;
F o k k er, 1960; de G root, 1966; and S te in b e rg e t a l . , 197^).
O b se rv a tio n s show t h a t 5 x 10”
Wm
Hz”
can be ta k e n as
a t y p i c a l maximum i n t e n s i t y o f a storm b u r s t .
Then, f o r t y p i c a l so u rce s i z e (v-» 5 x 10^ Km),
energy d e n s ity o f th e
e le c tro m a g n e tic wave and o f th e
Langmuir wave (assum ing plasm a h y p o th e sis) and c o n v e rsio n
e f f i c ie n c y f o r l - - > t o f th e o rd e r o f 10” ^ (W ild e t a l . ,
1963) tu r n s ou t to be Eem = 10“ "^ erg cm” ^ and
E j = 10”
erg cm” 2.
For t y p i c a l plasm a p a ra m e te rs i n th e
co ro n a such as m agnetic f i e l d H = 3 .5 gauss (K ai, 1970),
E le c tr o n
density N
= 10° cm"J and te m p e ra tu re T = 10°
°K,
m agnetic (E ) and th e rm a l E^.^ (nKT) e n e rg ie s a re
Effl = 0 .5 erg cm” ° , E ^ = 0 . 1 erg cm“ ^.
Thus energy i n
th e b u r s t component i s q u ite sm a ll.
So a ls o th e energy i n th e background continuum
i s sm a ll as conpared w ith m agnetic and th e rm a l energy i n
th e so u rce re g io n (E lg a ro y , 1977).
-
Thus,
5+2
-
e v e n t h o u g h a n o i s e s t o r m is long d u r a t i o n
p h e n o m e n o n , o n l y a f r a c t i o n of e n e r g y s t o r e d i n large
c o r o n a l v o l u m e s i n the f o r m of m a g n e t i c and t h e r m a l e n e r g y
is r e l e a s e d a n d is v e r y m u c h s m a l l c o m p a r e d t o a n y o t h e r
e n e r g y r e s e r v i o r s i n t h e so l a r atmosphere.
e n e r g e t i c s o f n o i s e storms
O b viously,
alone w i l l not l e a d to a ry
t h e o r y b u t t h e o r i e s s h o u l d b e cons i s t e n t w i t h t he en e r g e t ­
i c s o f t h e p r o c e s s e s involved.
Theoretical interpretation of noise storms proce­
e d e d along t h e lines i n v o k i n g
p l a s m a r a diation processes,
of t h e o r d e r of 1
0
e i t h e r g y r o - s y n c h r o t r o n or
since brightness temperatures
to 10*^ ° K rules o ut t h e e m i s s i o n
process due to thermal mechanisms.
and K r use
have
Kiepenheuer
ot_. .al* (1956) a n d F u n g a nd Y i p
(19^6),
(1966)
p r o p o s e d g y r o - s y n c h r o t r o n r a d i a t i o n but t h e s e
t h e o r i e s h a v e s e r i o u s l i m i tations.
Denisse
(I960) fi r s t p r o p o s e d a p l a s m a m e c h a n i s m
to e x p l a i n the o r i g i n o f c o n t i n u u m radiation.
(1963)? Trakfcengertz (1966),
Takakura
a n d V e r e s h k o v (197^) h a v e
p r o p o s e d t h a t the A l f v e n waves
and MHD w a v e s p r o p a g a t i n g
along the c l o s e d m a g n e t i c f i e l d lines are r e s p o n s i b l e
f o r a c c e l e r a t i o n o f e l e c t r o n s w h i c h give r i s e t o type I
bursts.
Because of the observed similarities between
chains' o f t y p e I burs t s
and t y p e III b ursts,
Foraichev (1973) h a v e p r o p o s e d c o l l i s i o n l e s s
Zaitsev and
s h o c k wa v e s
■with a strong magnetic field ( (x)H
source of type I bursts and chains.
to be promising.
) as a possible
This suggestion seems
However, chains at 2nd harmonic have not
been observed so far, as one would expect as a consequence
of the above suggestion.
Various models have been proposed to explain
observational features of noise storms.
Common to all
models is magnetic loops or arches extending ip 1 - 2 R q
in the solar corona and joining the active regions of
opposite polarity. (Kai, 19705
Lantos and Jarry,
1970| McLean and Sheridan, 1972? Stewart and Labrum, 1972|
Kai and Sheridan, 197^5 Kai and Nakajina, 197*+).
Recently, Gergely and Kundu (197?) have given an
empirical model showing association of centimetre and
decimetre continuum and type I burst as shown in Fig. 1 A ,
They have shown possible position of decametric continuum
radiation above the filament on the top of closed lines.
"On-fringe1* bursts are generated some how from the same
place.
"Off-fringe" bursts are generated from the place
where particles generated in chromospheric flares have
direct
access to open field lines.
These aspects are
discussed in more detail in Chapter V.
-
Mf-
C o n tin u u m
F i g .
1.if
S a p i r i c a l
m o d e l
p o s i t i o n
o f
t y p e
b u r s t
i n
I I I
t h e
o t h e r
o f
s t o r m
+
n o i s e
t y p e
t o g e t h e r
r e g i o n s
s t o r m
r e g i o n
''on-fringe"
v d t h
( G e r g a l y
a n d
n o i s e
a n d
i n d i c a t i n g
”o f f - f r i n g e w
s t o r m
K u n d u ,
a c t i v i t y
1 9 7 5 ) *
- 1*5-
1 .5
MOTIVATION FOR THE PRESENT WORK
Main r e s e a rc h a c t i v i t i e s o f th e S o la r and
P la n e ta ry P h y sic s Area a t th e P h y s ic a l R esearch L ab o ra to ry
(PRL) f o r th e l a s t few y e a rs a re d ir e c te d t o stu d y th e
sun., i n te r p l a n e t a r y medium, m agnetosphere, io n o sp h e re ,
geomagnetism and cosmic r a y s .
For b e t t e r u n d e rsta n d in g
o f th e so ,la r t e r r e s t r i a l r e l a ti o n s h i p and i n o rd e r to keep
t r a c k ,o f th e s o l a r a c t i v i t y on d a y -to -d a y b a s i s , fo llo w in g
ex p erim ents a re conducted i n a d d itio n t o th e High R eso lu t­
io n S p ectro sco p e a t 35 MHz (C h ap ter I I ) .
1)
Wide band s o l a r ra d io sp e ctro sc o p e (h0-2l+0 MHz,
A f = 300 kHz,
& t = 0 .5 s) (B honsle and A lu rk a r,
1968) which i s m o d ified i n 1977 by th e a u th o r to
o p e ra te i n th e ran g e o f 25 - 75 MHz, w ith
L f = 60 kHz, t ,t
2)
s:
0 ,1
S .
S o la r microwave ra d io m e te r 2 .8 GHz w ith an accu racy
o f f l u x measurement +, 7>% (N arayanan and B honsle,
19715 Sawant e t al . , 197*+).
3)
Two elem ent phased sw itch ed in te r f e r o m e te r a t
60 MHz and a ra d io p o la rim e te r a t bo MHz cap ab le
o f re c o rd in g a l l S to k e s ' p a ra m eters- (o p e ra te d
as su p p o rtin g eq u ip m en ts).
I n a d d itio n , h ig h r e s o l u ti o n o p t i c a l o b s e rv a tio n s
a t V ed h sh ala’s Udaipur S o la r O b serv ato ry and th e ra d io
- If6 -
o b s e r v a t i o n s at P R L are b e i n g
c o o r d i n a t e d ( B h a t n a g a r et a l . }
1977).
W i t h t h e h e l p o f above inst r u m e n t s ,
studies i n r e s p e c t of
variety of
(a) p h y s i c s of s o l a r f l a r e s and s o l a r
terrestrial relationships
(Bhonsle et a l . , 1973)
(b) rad i o
b u r s t p h e n o m e n o n and t h e i r i n t e r p r e t a t i o n i n t e r m s of
plasma parameters
and s h o c k w a v e s i n t h e s o l a r a t m o s p h e r e
(Bhonsle et a l . , 1973* 1976;
& S a w a n t et a l . , 1975» 1976),
1 0 ^ radians) i n
(e)' m e a s u r e m e n t o f F a r a d a y r o t a t i o n
t h e m a g n e t o - i o n i c m e d i u m b e t w e e n th e s u n an d the e a r t h
( B h onsle a n d Mattoo,
197*+) h a v e b e e n c a r r i e d out.
T h e s o l a r radio
a s t r o n o m y o b s e r v a t o r y o f P E L is
s i t u a t e d at S p a c e A p p l i c a t i o n s Center,
Research Organization,
development,
A h medabad.
I n d i a n Sp a c e
Be s i d e s t h e design,
f a b r i c a t i o n and i n s t a l l a t i o n o f t h e H i g h
R e s o l u t i o n Sp e c t r o s c o p e ,
the author ha d overall respons­
i b i l i t y o f c o o r d i n a t i o n and d a y to d a y o p e r a t i o n o f t he
obser v a t o r y ,
Mattoo and Bhonsle
(197^)
f o u n d t h a t the time
p r o f i l e s o f c e r t a i n short d u r a t i o n b u rsts
observed o n two closely spaced frequencies
h
k H z n e a r 35 MHz
(3M97
(type I l l b ) ?
separated by
MH z and 3 ^ . 9 9 3 MHz)
were d is­
similar, i n l y i n g t h e e x i s t e n c e of fine s t r u c t u r e i n
frequency.
I n o r d e r to i n v e s t i g a t e t h i s p o i n t further,
as
it w a s n e c e s s a r y t o aeid.se a c o n t i n u o u s f r e q u e n c y s c a n ­
ning t y p e o f s p e c t r o s c o p e w i t h f r e q u e n c y and t i m e r e s o l u t ­
i o n o f 5 k H z and 10 m s r e s p e c t i v e l y .
This was acconplished
b y t h e a u t h o r i n t h e f o r m of H i g h R e s o l u t i o n S p e c t r o s c o p e
w h i c h y i e l d e d n e w i n f o r m a t i o n o n the m i c r o s t r u c t u r e s i n
decametric bursts
and n o i s e s t o r m p h e n o m e n a .
I
T h e a u t h o r ’s f i n d i n g s of th e i n t r i g u i n g p h e n o m e n a
of decametric bursts
and no i s e storms a n d t h e i r i n t e r p r e t ­
ations i n t e r m s o f p h y s i c a l p r o c e s s e s a re p r e s e n t e d i n
t h i s thesis.