Solar wind and ionospheric conductivity studied for role in inducing Venus or Mars magnetosphere

Johnson, F. S.

Midgley, J. E.

English

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INDUCED MAGNETOSPHERE OF VENUS 
Francis S. Johnson and James E. Midgley 
Southwest Center f o r  Advanced Studies 
Dallas,  Texas, USA 
Prepared f o r  Presentat ion a t  
The COSPAR 11th Plenary Meeting 
Tokyo, Japan 
May 9-21, 1968 
f. 16 
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INDUCED MAGNETOSPHERE OF VENUS 
Francis S. Johnson and James E. Midgley 
Southwest Center fo r  Advanced Studies 
Dal las ,  Texas, USA 
Abstract  
It is  suggested t h a t  the conduct ivi ty  of t he  ionosphere of Venus 
(or  Mars) gives rise t o  an induced magnetosphere formed by p i l i n g  up 
magnetic f i e l d  from the s o l a r  wind; i t  i s  assumed t h a t  the p lane t  has no 
i n t r i n s i c  f i e l d .  Dessler has noted t h a t  the ionospheric conductivity 
i s  such a s  t o  ser ious ly  impede the convection of in te rp lane tary  f i e l d ,  
causing f i e l d  t o  bui ld  up t o  a s tagnat ion value s u f f i c i e n t  t o  d i v e r t  
the s o l a r  wind and cause a shock wave t o  develop. It i s  suggested here 
t h a t  c o l l i s i o n s  of s o l a r  wind p a r t i c l e s  with atmospheric p a r t i c l e s  w i l l  
lead t o  fu r the r  accumulation of f i e l d ,  thickening the region of s tagnat ion 
f i e l d  s t rength  t o  the poin t  where the s o l a r  wind i s  diver ted a t  high 
enough a l t i t u d e  s o  t h a t  c o l l i s i o n s  with neut ra l s  no longer s ign i f icant ly  
impede the flow, The outer  boundary of the s tagnat ion f i e ld  region 
cons t i t u t e s  a magnetopause above which any ions formed from atmospheric 
p a r t i c l e s  a r e  quickly swept away by the e l e c t r i c  f i e l d  associated with 
the s o l a r  wind; this  l eve l  i s  iden t i f i ed  with t h a t  near 500 km a l t i t u d e  
where the daytime ioniza t ion  drops of f  t o  an undetectable value according 
t o  the observations of the  Stanford Group i n  Mariner 5. 
- 1- 
The flow of the s o l a r  wind i s  ser ious ly  dis turbed when a planetary 
s ized conductor i s  placed i n  it .  The moon i s  an example of a poor con- 
ductor t h a t  does not se r ious ly  d i s t u r b  the  flow o ther  than by producing 
a shadow zone i n  it; the  s o l a r  wind impinges d i r e c t l y  on the lunar  
surface and i s  neutral ized the re ,  and no shock wave i s  produced i n  the 
flow ahead of the moon. This w a s  shown by measurements made i n  Explorer 
35 (Ness e t  a l . ,  1967; Colburn e t  a l . ,  1967) ,  and analyses of the problem 
have indicated the nature of the in t e rac t ion  t o  be expected i f  the moon 
were a b e t t e r  conductor (Sonnett and Colburn, 1967; Johnson and Midgley, 
1968). 
When a planet  with an atmosphere i s  exposed t o  the so l a r  wind, the 
The atmosphere presence of the atmosphere complicates the  in t e rac t ion .  
normally provides an in su la t ing  layer  t h a t  e l iminates  the conductivity of 
the p lane t  i t s e l f  from the problem f o r  a l l  except t r ans i to ry  changes i n  
the in te rp lane tary  magnetic f i e l d ,  which can induce eddy cur ren ts  i n  the 
planet.  However, i f  there  i s  an ionosphere within the  atmosphere, i t s  
conductivity should play an important ro l e  i n  the in te rac t ion .  
(1968) has pointed out t h a t  the ionospheric conductivity resists the rapid 
passage of in te rp lane tary  magnetic f i e l d  through i t ,  and t h a t  a s  a r e s u l t  
the  magnetic f i e l d  should build up s t rength  ahead of the ionosphere u n t i l  
i t  i s  s t rong enough t o  divert  the  s o l a r  wind, thus cu t t ing  of f  the rapid 
i n f l u x  of addi t iona l  in te rp lane tary  f i e l d  i n t o  the ionospheric region. 
divers ion of t he  s o l a r  wind i n  addi t ion  causes the development of a shock 
f ront ,  a fea ture  which was observed near Venus by Mariner 5 (Bridge e t  a l . ,  
1967). 
Dessler 
The 
The ove ra l l  p i c tu re  then i s  one i n  which magnetic f i e ld  p i l e s  up above 
a cur ren t  system i n  the ionosphere t o  a value ab le  t o  withstand the  
-2- 
stagnat ion pressure of the  s o l a r  wind, i .e. ab le  t o  d e f l e c t  t he  s o l a r  
wind. 
ing  the  ionosphere t r ans fe r s  i t s  pressure t o  the magnetic f i e ld .  
magnetic f i e l d  i n  turn  t r ans fe r s  i t s  pressure t o  the neu t r a l  atmosphere 
through the  mechanism of the  ionospheric cur ren t  system. The region of 
b u i l t  up magnetic f i e l d  cons t i t u t e s  an induced magnetosphere. The thick- 
ness of the b u i l t  up f i e l d  region must be governed by the conditions t h a t  
control  the  en t ry  of f i e l d  a t  the  top and the e x i t  of f i e l d  from the 
bottom of the ionosphere. Since the  f i e l d  s t rength  cannot be b u i l t  up 
above i t s  s tagnat ion value,  the thickness of the region of s tagnat ion 
s t rength  f i e l d  must increase u n t i l  the r a t e  of en t ry  of f i e ld  i s  reduced 
t o  match the  losses .  
The s o l a r  wind passes through a shock f ron t ,  and then on approach- 
The 
We have calculated the r a t e  of leakage of magnetic f i e ld  through the 
ionosphere assuming the  presence of no f i e l d  below the ionosphere and a 
s tagnat ion f i e l d  i n  the  ionosphere above the l eve l  where ionospheric 
cur ren ts  can flow. The r a t e  of f i e l d  leakage, fo r  reasonable assumptions 
f o r  e lec t ron  and neu t r a l  p a r t i c l e  concentrations,  is  about 10 
f i e l d  convected up to  the  planet  by the s o l a r  wind. 
t h a t  a l l  but one p a r t  i n  ten  thousand of the magnetic f lux  convected up 
t o  the planet  by the  s o l a r  wind i s  def lected around the planet.  
f ac to r  probably appl ies  fo r  the proportion of the s o l a r  wind t h a t  flows 
around the  p lane t  r a the r  than impinging upon the atmosphere. 
-4 of the 
Therefore i t  appears 
The same 
A value fo r  the thickness of the region i n  which the magnetic f i e ld  
i s  b u i l t  up t o  a s tagnat ion value can be deduced from observation. 
t i on  measurements made with Mariner 5 (Mariner Stanford Group, 1967) indicated 
a plasmapause o r  sudden drop off  of ion concentration from a value near 
10 cm t o  a value indis t inguishable  from the in te rp lane tary  concentration 
Occulta- 
4 -3 
-3- 
about 400 km above the ionospheric maximum. 
t h i s  with the ou te r  boundary of the b u i l t  up magnetic f i e l d ,  o r  the  
magnetopause, assuming the boundary t o  be r e l a t i v e l y  sharp and the f i e l d  
s t r eng th  t o  be r a the r  uniform from near the ionospheric maximum out  t o  
the boundary. I n  this case,  any ions formed from the neu t r a l  atmosphere 
by photoionization wi th in  the magnetic f i e l d  region have r e l a t i v e l y  long 
l i f e t imes ,  while any formed beyond the induced magnetosphere a r e  rap id ly  
swept  away by the solar wind, the e l e c t r i c  f i e l d  i n  the s o l a r  wind pro- 
viding the  acce lera t ion  mechanism. 
It i s  tempting t o  i d e n t i f y  
The p r inc ipa l  problem associated with the p i c tu re  of an induced 
magnetosphere i s  the physics of the in t e rac t ion  between the shocked s o l a r  
wind and the  s tagnat ion magnetic f i e l d .  There a r e  two aspects  t o  t h i s  
problem: the flow of cur ren t  between the plasma and the  ionosphere, and 
the e f f e c t s  of c o l l i s i o n s  between p a r t i c l e s  i n  the shocked s o l a r  wind and 
the neu t r a l  atmosphere. With magnetic f i e l d  s l ipp ing  through the iono- 
sphere a t  a slow r a t e  on the  lower s ide  of the s tagnat ion f i e l d  region, 
there  must be an equal introduct ion of magnetic f i e l d  from the upper s ide  
t o  maintain cont inui ty;  the  de ta i led  means by which t h i s  occurs has not 
been recognized, bu t  i t  i s  probably regulated by c o l l i s i o n s  between s o l a r  
wind and atmospheric p a r t i c l e s .  A re la ted  question is why there  should 
be a sudden t r a n s i t i o n  a t  the outer  boundary of the  s tagnat ion f i e l d  region 
r a the r  than a gradual decrease i n  magnetic f i e l d  and a gradual t r a n s i t i o n  
i n t o  the  solar wind plasma. However, the  concept t h a t  the magnetic f i e l d  
bui lds  up t o  the poin t  where it i s  ab le  t o  d i v e r t  the s o l a r  wind implies 
t h a t  there should be a magnetospheric type boundary i n  which the magnetic 
f i e l d  pressure in s ide  the boundary is  balanced by the plasma pressure out- 
s i d e  the  boundary. 
-4- 
The ro l e  of the  b u i l t  up magnetic f i e l d  i s  t o  d i v e r t  the  s o l a r  wind. 
However, i t  must not a c t  i n  such a way a s  t o  break the e l e c t r i c a l  con- 
nection between the  s o l a r  wind plasma and the  ionosphere, a s  t h a t  would 
cu t  of f  the cur ren t  source t o  the  ionosphere and el iminate  the  magnetic 
f i e ld  build up i t s e l f .  
the  magnetic f i e l d  build up s tops j u s t  sho r t  of cu t t i ng  o f f  the e l e c t r i c a l  
connection. The minimum thickness of stagnation-value f i e l d  tha t  might 
conceivably do t h i s  i s  the thickness of a Ferraro sheath (a few k i lo-  
meters) o r ,  i f  the  sheath i s  neut ra l ized ,  perhaps a proton gyro radius  i n  
the s tagnat ion f i e l d .  
layer.  The conducting l aye r  provided by the ionoshere i s  th in  and of 
broad geographic ex ten t ,  thus cons t i t u t ing  a la rge  t h i n  conducting p la te .  
The current  path from the s o l a r  wind t o  the  ionosphere i s  broad and shor t  i n  
length compared t o  the ionospheric conductor. The geometry therefore  sug- 
ges t s  t h a t  the  Pederson conductivity i n  the  b u i l t  up magnetic f i e l d  region 
may be three  o r  even four orders of magnitude lower than t h a t  near the 
ionospheric maximum and s t i l l  provide a good e l e c t r i c a l  path between the 
shocked s o l a r  wind and the ionospheric current  system. The e lec t ron  con- 
cen t r a t ion  required t o  produce t h i s  conductivity i n  a stagnation magnetic f i e ld  
4 -3 is  of the order of magnitude of 10 c m  
near t he  magnetopause (Mariner Stanford Group, 1967). We therefore  suggest 
t h a t  the  outer  boundary of the induced magnetosphere occurs a t  a t  t h a t  
a l t i t u d e  where the ambient atmosphere can no longer support a s u f f i c i e n t l y  
dense upper ionosphere t o  provide an adequate e l e c t r i c a l  connection be- 
tween the  s o l a r  wind and the  lower ionosphere. 
A se l f - regula t ing  process might develop i n  which 
However, the geometry of the problem suggests a th icker  
, which i s  about what was observed 
This research was supported by NASA Grant NsG-269. 


Induced magnetosphere of Venus

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