CHROMOSPHERIC
SATELLITE
EXPLOSIONS
SUNSPOTS*
D A V I D M.
(Mount
Wilson
and Palomar
AND
RUST
Observatories,
Pasadena,
Calif.,
U.S.A.)
ABSTRACT
Observations of the longitudinal c o m p o n e n t of the photospheric magnetic fields near sunspots
imply that surges and Ellerman b o m b s occur at neutral points in the magnetic fields (i.e. | £ | = 0 ) in
the chromosphere. The neutral points appear above satellite sunspots, which are defined as polarity
reversals (in B\\) near the edges o f large-spot penumbrae. A series o f magnetograph observations
s h o w s a point o f satellite polarity vanishing during a period o f almost c o n t i n u o u s surge activity. The
lost magnetic energy is comparable to the energy release evidenced by the surge.
1. Introduction
Several researchers h a v e used solar m a g n e t o g r a p h s t o s t u d y t h e
magnetic-field
structure of large s u n s p o t s in active regions. T h e y have found t h a t very frequently
these spots were s u r r o u n d e d by a n u m b e r of 'satellite' m a g n e t i c features. T h e polarity
o f t h e s a t e l l i t e s w a s o p p o s i t e t o t h a t of t h e c e n t r a l s p o t ( B u m b a , 1960, 1 9 6 2 ; H o w a r d ,
1959). I n m o s t c a s e s t h e s e s a t e l l i t e s w e r e n o t visible o n w h i t e - l i g h t p h o t o g r a p h s .
However,
G o p a s y u k et al. (1963) o b t a i n e d m a n y o b s e r v a t i o n s o f
flare-producing
r e g i o n s w h i c h i n c l u d e d l a r g e s u n s p o t s a c c o m p a n i e d b y s m a l l , visible s p o t s i m b e d d e d
in p e n u m b r a e . T h e s m a l l e r s p o t s ('satellites') u s u a l l y h a d p o l a r i t y o p p o s i t e t o t h a t
of t h e m a i n s p o t . I u s e d t h e m a g n e t o g r a p h a t t h e M o u n t W i l s o n O b s e r v a t o r y t o m a p
t h e l o n g i t u d i n a l c o m p o n e n t ( a s i n d i c a t e d b y Z e e m a n s p l i t t i n g in t h e p h o t o s p h e r i c
F e i line a t A5250) o f t h e m a g n e t i c field o f t h e a c t i v e r e g i o n of J u n e 2 - 3 , 1967. T h e
r e g i o n p a s s e d c e n t r a l m e r i d i a n in t h e N o r t h e r n h e m i s p h e r e o n J u n e 3. T h e o b s e r ­
v a t i o n s s h o w m a n y satellite s u n s p o t s (invisible in i n t e g r a t e d l i g h t ) n e a r t h e p e n u m ­
b r a o f t h e l a r g e s t s p o t i n t h e r e g i o n . T h e spacial r e s o l u t i o n of t h e s c a n s w a s e i t h e r 5 o r
10 sec o f a r c a n d t h e n o i s e level w a s a b o u t 2 g a u s s . T h e o b s e r v e d size of t h e satellite
m a g n e t i c f e a t u r e s is a t t h e r e s o l u t i o n limit.
2. Observations
A s the magnetic m e a s u r e m e n t s were being m a d e , personnel of the L o c k h e e d Solar
O b s e r v a t o r y m a d e l a r g e - s c a l e H a m o t i o n p i c t u r e s of t h e r e g i o n . F i g u r e 1 is a n H a
p h o t o g r a p h o f t h e r e g i o n a t 1947 U T o n J u n e 2. A s u r g e is o c c u r r i n g a t t h e p e n u m b r a
* Presented by R . H o w a r d .
Kiepenheuer
(ed.),
Structure
and Development
of Solar Active
Regions,
7 7 - 8 4 . C I. A. if.
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D A V I D M. R U S T
78
'0
F I G . 1. The active region of June 2,1967. North is at the top and East is on the left. This photograph
was made in the center of Ha at 1947 UT. During the period of observation (1635 UT, June 2, to 0116
UT on June 3) nearly continual surge activity took place on the Northwest
side of the large leading
spot. A small surge is shown in this picture. Many flare brightenings occurred in the center and left-ofcenter plages in the region.
of t h e l a r g e , l e a d i n g s u n s p o t , a n d s o m e t r a c e s o f a flare b r i g h t e n i n g a r e e v i d e n t in t h e
m i d d l e o f t h e p i c t u r e . T h e r e w e r e s e v e r a l flares i n t h e r e g i o n o n J u n e 2 a n d 3 . T h e
m o t i o n p i c t u r e s s h o w c o n t i n u a l s u r g e a c t i v i t y in t h e r e g i o n . T h e p o s i t i o n s o f all t h e
o b s e r v e d s u r g e s a r e s h o w n in F i g u r e 2, w h i c h is a m a g n e t o g r a m of t h e r e g i o n a s it
a p p e a r e d o n J u n e 3. D a s h e d lines i n c l o s e n e g a t i v e fields; solid lines inclose p o s i t i v e
fields. Solid t r i a n g l e s s h o w a p p r o x i m a t e s u r g e size a n d d i r e c t i o n . S o l i d d o t s a r e Ellerm a n b o m b s a n d t h e flare a r e a s a r e h a t c h e d . Every bomb, every surge, and many
brightenings
originate
at the satellite
sunspots.
flare
B e c a u s e of t h e relatively l o w r e s o l u t i o n
of t h e m a g n e t i c m e a s u r e m e n t s , it is i m p o s s i b l e t o k n o w t h e real size a n d t h e p e a k field
i n t e n s i t y o f t h e satellites. E v e n if t h e i r c h a r a c t e r i s t i c fields s h o u l d b e several h u n d r e d
g a u s s a n d t h e i r c r o s s - s e c t i o n s o n l y 2 sec o f a r c , t h e m a g n e t o g r a p h w o u l d t r a n s f o r m
t h e m i n t o b r o a d 5 o r 10 g a u s s f e a t u r e s o n t h e m a g n e t i c m a p s . All flaring r e g i o n s o f
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1
areas of negative
by solid triangles.
intensity are dis­
map reveals that
arc.
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https://doi.org/10.1017/S0074180900021355
F I G . 2. Contour map of the longitudinal magnetic fields in the region at 1600 UT on June 3, 1967. Dashed lines inclose
field, and solid lines inclose areas of positive field. Contour levels are 5,10,20,40,80,
and 160 gauss. Dark surges are indicated
Solid dots denote Ellerman bombs (moustaches)
and areas of flare brightening are indicated by hatching. Valleys' in the field
tinguished from 'hills' by small converging tick marks. A comparison of the visual sunspot polarities and some valleys on this
the valleys are really sites of polarity reversals not completely detected with the magnetograph.
The resolution is 5 sec of
S
N
80
DAVID
M.
RUST
J u n e 2 h a v e b e e n i n c l u d e d in t h e figure. P e r h a p s i n c l u s i o n of o n l y t h e p o i n t s w h i c h
s h o w e d a flash p h a s e w o u l d give a h i g h e r c o r r e l a t i o n b e t w e e n satellite s p o t a n d flareb r i g h t e n i n g p o s i t i o n s . M o r e r e s e a r c h is n e e d e d o n t h i s p o i n t .
3 . The Lines of Force
W h a t d o t h e fieldlines a b o v e satellite s u n s p o t s l o o k like? T o find t h i s o u t , I u s e d t h e
m e t h o d o f c o m p u t i n g t h e m a g n e t i c - f i e l d lines d e v e l o p e d b y S c h m i d t ( 1 9 6 4 ) . H i s
m e t h o d is t o a s s u m e t h a t t h e s p a c e a b o v e t h e p h o t o s p h e r e is c u r r e n t - f r e e . P o t e n t i a l
t h e o r y m a y t h e n b e u s e d t o c a l c u l a t e t h e field f r o m t h e o b s e r v e d flux d i s t r i b u t i o n in
t h e p h o t o s p h e r e . F i g u r e 3 s h o w s t h e lines of force n e a r t h e l a r g e s t s u r g e s . The
follow
the lines of force*
O n this
figure
surges
I h a v e d r a w n t h e p r i n c i p a l n e u t r a l lines
s
F I G . 3. Magnetic lines of force (heavy lines with arrows) near the region of most intense surge
activity. A comparison with Figure 2 shows that the surges follow the lines of force. The fieldlines were
constructed according to the potential theory by considering the chromosphere current-free. The sources
of the field are fixed in the photosphere,
which is assumed plane-parallel.
Heavy dashed lines coincide
with the major lines of zero longitudinal field in the region and the polarities on either side are indicated
by symbols (+ and —). Thin arrows point out satellite sunspots detected by the routine Mount
Wilson
sunspot field measurements.
* I d o not intend to imply, however, that the magnetic field above the region was current-free.
The surges alone are g o o d evidence for currents. Nevertheless, it is apparent that the lines of force
do not deviate radically from those calculated under the current-free approximation.
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CHROMOSPHERIC
EXPLOSIONS AND
SATELLITE
SUNSPOTS
81
( h e a v y , d a s h e d lines) in t h e m e a s u r e d l o n g i t u d i n a l fields. T h e t h i n a r r o w s p o i n t t o t h e
few visible satellite s u n s p o t s in t h e r e g i o n . A flare o c c u r r e d n e a r t h e visible satellite in
t h e c e n t e r of t h e f i g u r e ; h o w e v e r , t h e o b s e r v e d s u r g e s a n d E l l e r m a n b o m b s all o c c u r r e d
n e a r invisible
satellites, i.e., n e a r p o i n t s of i s o l a t e d p o l a r i t y d e t e c t e d o n l y w i t h t h e
magnetograph.
4 . Neutral Points
T h e significance of t h e satellite s u n s p o t s for b o m b s a n d s u r g e s , a t least, is r e v e a l e d
in F i g u r e 4. T h e r e is m o s t likely a n e u t r a l p o i n t (i.e. \B\ = 0) in t h e m a g n e t i c field in
F I G . 4. Sketch of the fieldlines above a satellite sunspot. An X marks the neutral point where
explosive events probably originate. The satellite spots are small features easily overlooked in the large
scale (usually bipolar) pattern of the major regions, even though the satellites seem to be an integral
feature of most large spots in growing regions (Bumba,
1960).
t h e s o l a r a t m o s p h e r e a b o v e e v e r y satellite s u n s p o t . It a p p e a r s t h a t t h e e x p l o s i v e
events of J u n e 2 a n d 3 originated at the neutral points t h a t are implied by the geo­
m e t r y o f t h e u n d e r l y i n g fields. S e v e r n y (1965) a n d K o v a l (1965) a t t h e
Crimean
O b s e r v a t o r y h a v e c o m e t o s i m i l a r c o n c l u s i o n s . It is clear f r o m t h e figure t h a t t h e
l a r g e r t h e satellite s p o t r e l a t i v e t o its p a r e n t , t h e h i g h e r t h e n e u t r a l p o i n t will b e . F o r
e x a m p l e , b o m b s a r e very s m a l l a n d t h e y o c c u r in t h e l o w c h r o m o s p h e r e . L a r g e flares
o c c u r m u c h h i g h e r u p . A p p a r e n t l y , bigger, h i g h e r e v e n t s o c c u r a b o v e l a r g e r satellite
s p o t s , i.e., t h o s e s p o t s w h i c h h a v e t h e m o r e elevated n e u t r a l p o i n t s , b u t m o r e r e s e a r c h
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DAVID
82
M.
RUST
is n e e d e d t o e s t a b l i s h w h e t h e r t h i s p a t t e r n is g e n e r a l l y o b e y e d b y e x p l o s i v e e v e n t s .
N e u t r a l p o i n t s in t h e m a g n e t i c field h a v e p l a y e d a n i m p o r t a n t r o l e in a l m o s t all
theoretical discussions of explosive events. A t these points magnetic energy m a y be
converted to observable radiative and kinetic forms.
5. T h e Field Changes
F i g u r e 5 s h o w s t h e a p p e a r a n c e o f a satellite s p o t u n d e r t h e s u r g e s b e f o r e a n d after
t h e m o s t v i o l e n t a c t i v i t y . T h e s m a l l s p o t o f p o s i t i v e p o l a r i t y is a s t a b l e f e a t u r e o f t h e
first t h r e e m a p s , t a k e n o n l y 8 m i n a p a r t . A f t e r a d e l a y o f 1 h o u r a n d 50 m i n , d u r i n g
which n o observations were obtained, the region was scanned three m o r e times. The
s a t e l l i t e - s p o t field h a d d i s a p p e a r e d . T h e d i s a p p e a r a n c e w a s n o t p e r m a n e n t , h o w e v e r ,
since t h e satellite p o l a r i t y a p p e a r e d o n m a p s m a d e 2\ h o u r s after t h e final s c a n s h o w n
in t h e figure. A l s o , t h e s p o t w a s d e t e c t e d o n a s c a n m a d e t h e f o l l o w i n g d a y . T h e satellite
s p o t w a s j u s t a b o v e t h e l i m i t o f r e s o l u t i o n for t h e s e m a p s (10 sec of a r c ) , a n d it is n o t
i m p o s s i b l e t h a t o b s e r v i n g c o n d i t i o n s h a d d e t e r i o r a t e d e n o u g h b y t h e s e c o n d series of
s c a n s t o m a k e t h e field u n d e t e c t a b l e . H o w e v e r , t h e fact t h a t it w a s d e t e c t e d several
"7 «
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JUNE 2,1967
10" APERTURE
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•
F I G . 5. A sequence of magnetic maps of the most active surge region. Contour levels are 5, 10, 20,
40, 80, and 160 gauss. The top three maps were made before the largest surges occurred. They all show
a small positive feature at the Northwest
(upper right) edge of the penumbra of the large negative
sunspot. The lower maps all lack the small feature which was at the location of the surges. Note that
other features of comparable size are unchanged during these
measurements.
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CHROMOSPHERIC
EXPLOSIONS
AND
SATELLITE SUNSPOTS
83
h o u r s l a t e r u n d e r e v e n w o r s e seeing c o n d i t i o n s , a n d t h e fact t h a t similar f e a t u r e s
e v e r y w h e r e else o n t h e m a g n e t o g r a m s d i d n o t u n d e r g o a s i m i l a r d i s a p p e a r a n c e t e n d
t o s u p p o r t t h e c o n c l u s i o n : t h e field o f t h e invisible satellite s p o t j u s t u n d e r t h e l a r g e s t
s u r g e s d e c r e a s e d significantly d u r i n g t h e m o s t p r o n o u n c e d s u r g e activity. T h e l a t e r
r e c o v e r y o f t h e field is c o n s i s t e n t w i t h o t h e r o b s e r v a t i o n s o f p h o t o s p h e r i c
field
c h a n g e s d u r i n g m i n o r e v e n t s in a g r o w i n g a c t i v e r e g i o n ( M i c h a r d et al., 1961).
6. Energy for Surges
W h a t w a s t h e m a g n e t i c e n e r g y o f t h e v a n i s h e d s p o t ? T h e m a g n e t i c flux a s s o c i a t e d
with the spot was a b o u t 5 x 1 0
1 8
m a x w e l l s . T h e lines o f f o r c e s y m b o l i z i n g t h i s flux
m u s t p a s s t h r o u g h t h e p l a n e o f o b s e r v a t i o n t w i c e (see F i g u r e 4 ) . If t h e field d i d v a n i s h ,
we would expect b o t h positive and negative regions to weaken or disappear. However,
it is o b v i o u s l y m u c h e a s i e r t o d e t e c t a s m a l l c h a n g e i n t h e w e a k satellite s p o t t h a n in
t h e l a r g e p a r e n t s p o t w h i c h h a s e x t e n s i v e r e g i o n s o f s t r o n g fields. If I a s s u m e t h a t
t h e r e w a s a c h a n g e i n t h e n e g a t i v e fields t o a c c o m p a n y t h e c h a n g e in t h e s m a l l positive
s p o t a n d t h a t t h e a n n i h i l a t e d flux e x t e n d e d u p w a r d t o a h e i g h t o f 1000 k m , t h e n t h e r e
was about 2 x 1 0
2 6
e r g s a v a i l a b l e for t h e s u r g e s . If t h e r a d i a t i v e a n d k i n e t i c e n e r g y
r e l e a s e d in a s u r g e a r e c o m p a r a b l e , w e c a n find t h e t o t a l e n e r g y r e q u i r e d t o p r o d u c e a
surge from a n estimate of t h e kinetic energy of the o u t w a r d - m o v i n g material. T h e
velocity o f t h e s u r g e s is a b o u t 100 k m / s e c . If I a s s u m e a d i a m e t e r o f 1000 k m , a l e n g t h
of 5000 k m a n d a density of 1 0
energy of a b o u t 3 x 1 0
about 6 x 1 0
2 5
2 5
1 1
p r o t o n s / c m , t h e n I find t h a t t h e s u r g e h a d a k i n e t i c
3
ergs. T h u s , the kinetic a n d radiative energy of the surge was
e r g s . T h i s is c o m p a r a b l e t o t h e v a n i s h i n g m a g n e t i c e n e r g y . P e r h a p s
t h e n , we h a v e a c l e a r c a s e of a n n i h i l a t i o n of m a g n e t i c e n e r g y n e a r a n e u t r a l p o i n t w i t h
a t t e n d a n t b r i g h t e n i n g ( a t t h e b a s e of t h e s u r g e ) a n d a c c e l e r a t i o n of c h r o m o s p h e r i c
m a t t e r . Since v i r t u a l l y all e v e n t s w i t h p r o n o u n c e d flash p h a s e s o c c u r in o r n e a r s p o t
p e n u m b r a e , I t h i n k it is p r o b a b l e t h a t w e will find every s u c h e v e n t t a k i n g p l a c e
a b o v e t h e satellite s u n s p o t s f o u n d t h e r e .
Acknowledgements
I a m g r a t e f u l t o M r . T h o m a s C r a g g for s e c u r i n g t h e m a g n e t i c o b s e r v a t i o n s a n d to
D r . R o b e r t H o w a r d f o r critically r e a d i n g t h e m a n u s c r i p t . I t h a n k t h e p e r s o n n e l o f t h e
L o c k h e e d S o l a r O b s e r v a t o r y f o r t h e use o f t h e i r films a n d for s e v e r a l helpful dis­
cussions.
References
Bumba, V.
Bumba, V.
Gopasyuk,
Obs., 30,
(1960)
Publ. Crim. Astrophys. Obs., 2 3 , 212.
(1962)
Bull. Astr. Inst. Czech., 13, 48.
S.I., Ogir, M . B . , Severny, A . B . , Shaposhnikova, E . F . (1963)
15.
Publ. Crim.
Astrophys.
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available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900021355
84
DAVID
M.
RUST
H o w a r d , R. (1959)
Astrophys. J., 130, 193.
K o v a l , A . N . (1965)
Publ. Crim. Astrophys. Obs., 34, 278.
Michard, R., Mouradian, Z., Semel, M. (1961)
Ann. Astrophys., 24, 54.
Severny, A . B . (1965)
'Stellar and Solar Magnetic Fields', in l.A.U. Symposium No. 22, Ed. by
R. Lust, Interscience, N e w York, p. 255.
Schmidt, H . U . (1964)
A A S - N A S A Symposium o n the Physics of Solar Flares, NASA
Report
SP-50, p. 107.
DISCUSSION
Sturrock: The fact that surges occur a b o v e circular neutral lines (zero vertical magnetic field) fits
well with the fact that flares occur a b o v e neutral lines. However, if the surge energy is derived from
magnetic energy, the pre-surge state cannot be current-free and therefore not of the form depicted
by D r . Rust.
Howard: This is true.
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