THE OCCURRENCE AND POSSIBLE MEANING OF
THE'NIMBUS'
J. HOUTGAST
(Sterrewacht
' Sonnenborgh',
Utrecht,
The
Netherlands)
T h e r e a r e few o b s e r v a t i o n s of c a s e s in w h i c h a n i m b u s h a s d e v e l o p e d a r o u n d a
s o l a r flare of m a j o r i m p o r t a n c e . S p e c u l a t i o n s h a v e b e e n m a d e a b o u t t h e n a t u r e of
t h i s o b s c u r i n g c l o u d . W e shall give h e r e s o m e c o n s e q u e n c e s of t h e a s s u m p t i o n t h a t
we h a v e t o d o w i t h s c a t t e r i n g b y a c l o u d of e l e c t r o n s , ejected t o g e t h e r w i t h t h e p r o t o n s
during the formation of the
flare.
S i n c e t h e e l e c t r o n receives r a d i a t i o n f r o m a solid a n g l e 2n, h a l f t h e
c r o s s - s e c t i o n s h o u l d b e t a k e n , t h a t is ^ x 0 - 6 6 x 1 0 ~
cm
2 4
- 1
scattering
.
If w e s t a r t f r o m a d a r k e n i n g in a n i m b u s o f 3 % ( H o u t g a s t , 1962), w e g e t \ x 0-66 x
10"
2 4
N
= 0 0 3 , w h i c h yields for N . = 1 0 .
2 3
totm
t o t
F o r a h e i g h t of t h e c l o u d , a s s u m e d e q u a l t o its c r o s s r a d i u s , of a b o u t 1 0 0 0 0 0 k m =
1 0 c m , « = 1 0 per c m .
1 0
1 3
3
e
F o r c o m p a r i s o n : in a p r o m i n e n c e A? = 1 0 — 1 0
10
p h e r e t h e t o t a l n u m b e r o f p a r t i c l e s is 1 0
1 6
11
cm
- 3
, a t t h e b a s e of t h e c h r o m o s ­
c m ~ , for a h e i g h t of 1 0 0 0 k m it is 1 0
3
1 3
cm~ .
S i n c e t h e s c a t t e r i n g of e l e c t r o n s h o l d s for all w a v e l e n g t h s , t h e o b s e r v a t i o n
c o n t i n u o u s light is t o b e p r e f e r r e d for a v o i d i n g t h e influence o f
3
in
chromospheric
s t r u c t u r e a s seen in H a a n d o t h e r lines.
T h e g a s of p r o t o n s a n d e l e c t r o n s in a n i m b u s w o u l d c a u s e a series of a b s o r p t i o n s
d u e t o free-free a n d b o u n d - f r e e t r a n s i t i o n s . O f t h e s e t h e B a l m e r c o n t i n u u m is t h e
m o s t i m p o r t a n t . It c a n c o m p e t e w i t h e l e c t r o n s c a t t e r i n g for T of t h e o r d e r of 1 0 0 0 0 °
E
a n d /7 = 1 0
1 0
-10
n
c m " , i n c r e a s i n g w i t h n. S o it w o u l d b e i n t e r e s t i n g t o o b s e r v e a
3
n i m b u s i n a w a v e l e n g t h r e g i o n b e l o w t h e B a l m e r d i s c o n t i n u i t y , s a y a t k 3600 A .
O t h e r explanations of the n i m b u s p h e n o m e n o n are n o t excluded, b u t we wish to
stress t h e n e e d for o b s e r v a t i o n s w h i c h c o u l d give m o r e i n f o r m a t i o n . I s h o u l d very
m u c h like t o receive a v a i l a b l e , u n p u b l i s h e d , a n d f u t u r e o b s e r v a t i o n a l d a t a .
References
Houtgast, J. (1962)
(1963)
17, 56.
Bull. astr. Inst. Netherl,
in The Solar Corona,
Symposium
No. 16 of I.A.U.,
Ed. by John W. Evans, p. 231.
DISCUSSION
Kiepenheuer: The question of whether the nimbus effect could be explained by electron scattering
had been discussed already at the Cloudcroft Symposium (J. W. Evans, Proc. IAU Symposium No. 16:
Kiepenheuer
fed.),
Structure
and Development
of Solar Active
Regions,
483-484. t
I.A.U.
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 17 Jun 2017 at 18:45:03, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090002194X
484
J.
HOUTGAST
The Solar Corona, N e w York 1963, p. 233). The main difficulty involved seems to m e the very large
total number of electrons and consequently ions required to produce the observed effect, which is
of the order o f 1 0 electrons respectively ions. It is equivalent to about 1 0 times the electron re­
spectively ion number o f a well-developed prominence or of a flare surge. I can not quite see, h o w
such a large amount of material can be injected without observable effects.
Severny: The picture shows the nimbus for a flare just at the limb and it gives an idea about the
maximal height of the layer producing the 'nimbus'. The height of the layer responsible for nimbus
cannot be larger than 1 0 k m ( = 1 0 c m ) .
De Jager: Since the nimbus is a Hoc p h e n o m e n o n , its explanation should be looked for in the
Ha-source function. If it were due to electron scattering it should as well be visible in the continuous
radiation.
Beckers: If the nimbus were due to electron scattering I w o u l d expect it to be bright when seen
o n the disk in the centre o f Hoc. This because o f the large thermal m o t i o n s of the electrons which
cause D o p p l e r shifts of about 10 A r.m.s. for the incident radiation at a temperature o f 1 0 0 0 0 ° K .
S o the electrons see radiation c o m i n g from the continuum near the H a line causing a strong increase
in brightness (factor 5). Another factor to take into account is the solar limb darkening causing the
electron cloud to g o into emission near the limb for wavelengths where the limb darkening is large.
A t H a this may only be a small effect, however.
Svestka: A n electron density of 1 0 c m exists only in narrow filaments of the flare, the volume
of which is much smaller than y o u have considered. I also suspect very strongly that with the number
of free electrons y o u give, we s h o u l d have to observe continuous emission behind the Balmer limit,
which never has been the case. A n d as for the limb event s h o w n by Professor Severny, we should
observe s o m e emission hill o n the limb, since the light scattered by the electrons to all directions must
b e c o m e visible o n the limb. But we d o not observe anything like that.
Houtgast: The foregoing remarks are very m u c h to the point and are w e l c o m e reasonings regarding
the nimbus problem. Once m o r e they stress the need for a quantitative treatment, as well theoretically
as from the observational side.
4 3
5
4
9
1 3
- 3
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 17 Jun 2017 at 18:45:03, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090002194X