Doppler characteristics of radar echoes reflected by solar coron

Harrington, J. V.

English

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CENTER FOR SPACE RESEARCH c-
MASSACHUSETTS INSTITUTE OF TICHNOLOGY 
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ff 653 July 65 
https://ntrs.nasa.gov/search.jsp?R=19660010248 2020-03-16T22:45:06+00:00Z
NASA Grant NCR-22-009-064 
El Campo Solar  Radar System 
Second Semiannual Report 
July 1 to December 31, 1965 
DSR Project 4539 
CSR PR-4539-2 
MASSACHUSETTS INSTITUTE OF TECHNOLOGY 
CENTER FOR SPACE RESEARCH 
SECOND SEfilIANNUAL REPORT 
For the Period 
July 1 to December 31, 1965 
on 
NASA Grant NGR-22-009-064 
EL CAMPO SOLAR RADAR SYSTEM 
DSR Project 4539 
To The 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
Office of Grants and Research Contracts 
Subnitted by: J, V, Harrington 
Principal Investigator 
CSR PR-4539-a/ 
. 
I 
During the per iod  covered by t h i s  report the p r i n c i p a l  
1 
e f f o r t  has been t h e  performince of daily solar r adz r  experi-  
ments. The problems associated w i t h  chanqes i n  experimental  
procedure and w i t h  improvements i n  the t r a n s m i t t e r ,  antenna, 
receiver, and data processor  are con t inua l  ones. /  For example, 
dur ing  t h e  sp r ing  of 1965 the rece iv ing  bandpass was increased  
t o  2OOKcps so t h a t  t h e  wide Doppler spectra of the s o l a r  echoes 
could be more f a i t h f u l l y  s t u d i d .  This increased  bandpass 
demanded more of the magnetic tape rccoi-der i n  o rde r  for the  
spectra t o  be f a i t h f u l l y  r e p r o d ~ c e d .  Many a t tempts  were made 
t o  improve the q u a l i t y  of t h e  t a p e  recording and the  character- 
ist ics of new t a p e  machines w x e  i nves t iga t ed ,  with t h e  con- 
c l u s i o n  that analog recording aE a weak wideband s i g n a l  bur ied  
i r ?  20 decibels of no i se  w i l l  involve an apprec iab le  loss  of 
s ignal- to-noise  ra t io  even for t h e  most advanced tape machines. 
[The t e n t a t i v e  conclusion is t h a t  the best way t o  study t h e  
echo s p e c t r a  is t o  perform the s i g n a l  i n t e g r a t i o n  for t he  vari- 
ous segments of the spectra in real  t i m e  as t he  s i g n a l  is 
received.  I 
The solar echo a t  t i m e s  shows weak scatterers t w o  t o  three 
or nore solar radi i  i n  f r o n t  of t h e  sun's center .  The bulk of 
t h e  echo energy is re turned  from a coronal  he igh t  of about 1 .3  
t o  1 . 5  s o l a r  r a d i i ,  and these main echo components have a much 
broader Doppler spreading and larger Doppler s h i f t s  than  the 
weak camponents from t w o  t o  t h r e e  solar radi i  h e i g h t s .  These 
high-corona echoes are very i n t e r e s t i n g ,  and a s tudy  of them 
- 2 -  
- .. 
_ -  
is considered very important beccuse of their unusual Dop2ler 
characteristics and because they imply i r r e g u l a r i t i e s  having 
i o n i z a t i o n  d e n s i t i e s  s e v e r a l  hundred t i m e s  the  mean coronal  
dens i ty .  /For these resons considerable e f fo r t  w a s  expended 
during t h i s  r e p o r t  period i n  s tud l ing  t h e s e  high-corona echoes. 
A s e p a r a t e  bank of i n t e g r a t o r s  was used t o  s tudy  these echoes 
i n  real t i m e  because they are weak an6, as mentioned, there i s  
a loss  of s e n s i t i v i t y  through tape recording.[ Various s o l a r  
models designed to  explain there echoes have a l s o  been inves- 
t i g a t e d .  ( It has been found that these echoes are p resen t  a t  
many range i n t e r v a l s  simultaneously,  t h a t  i s  they are n o t  
p r e s e n t  a t  one range one day and a t  some other range another  
day. Furthermore, when they appear a t  earlier range i n t e r v a l s  
than  t h e  main echo, they a l s o  appear a t  l a te r  range i n t e r v a l s .  
These later range i n t e r v a l s  undoubtedly r ep resen t  r e f l e c t i o n s  
from the  limbs and from coronal regions a t  greater d i s t a n c e s  
th=m t h e  center of the sun. This  sugqests  t h a t  the dis turbances  
r e spons ib l e  for these echoes are present  simultaneously a l l  over 
t h e  sun. Could they be some type of sound waves t r a v e l i n g  
around the  per iphery of the corona? D o  they r ep resen t  the  solar 
wind v e l o c i t i e s  a t  t w o  t o  three solar rad i i?  
s e n t  solar wind velocit ies,  then  the s o l a r  wind v e l o c i t y  a t  
t h a t  l o c a t i o n  is less than predicted by Parker. A s t a t i s t i c a l  
s tudy  of the  occurrence of these echoes has shown t h a t  they 
have a s t rong  tendency t o  appear on days when t h e  main echo is  
larger than average. 
If they do repre- 
- 3 -  
During t h i s  r e p o r t  per iod most of t h e  solar echo d a t a  
obtained s i n c e  1 9 6 1  was punched on IEM cards. Various analyses  
I .  
of data w e r e  t hen  made using t h e  computer f a c i l i t i e s  a t  t he  
. -_ 
nearby Wharton County Jtlnior College. For example, one program 
computed and normalized t h e  cross s e c t i o n ,  range depth, and 
mean range of a l l  runs. The cross s e c t i o n s  were then averaged 
fo r  three-month periods and plotted along wi th  a three-month 
running average of the  sun s p o t  number. I n  a gene ra l  sense t h e  
cross s e c t i o n  inc reases  w i t h  sun snot number, b u t  there is  n o t  
a s t rong  c o r r e l a t i o n  w i t h  t h e  s h o r t  period v a r i a t i o n s .  The 
average cross s e c t i o n  for this r e p o r t  per iod w a s  about 1.2nRo2 
f o r  energy received i n  a wide band, where Ro is  t h e  photo- 
s p h e r i c  rad ius .  This  cross s e c t i o n  is n o t  so l a r g e  as expected 
on t h e  basis of the  inc rease  i n  sun spot number s i n c e  1964.  
The average cross section i n  1961  was about 20nR02. 
The average range depth and mean range of t h e  echoes have 
also been found t o  vary w i t h  sun m o t  number. The three-month 
averages of range depth a r e  f a i r l y  well c o r r e l a t e d  w i t h  t h e  
three-month averages of cross sec t ion .  The range depth i s  
def ined  as t w i c e  the  second moment about t h e  mean of the  d is -  
t r i b u t i o n  of echo energy w i t h  radar range. The average range 
depth for t h i s  r e p o r t  period was about 0.55 solar  rad i i ,  and 
the average range depth i n  1961 was about 1 . 4  solar  rad i i .  
The mean range w a s  computed as t h e  c e n t r o i d  of t h e  d i s t r i -  
T h i s  cen t ro id  was then bu t ion  of echo energy w i t h  radar range. 
related t o  t h e  c e n t e r  of the sun, assuming t h a t  the a d d i t i o n a l  
. 
- 4 -  
radar group delay t i m e  was 1 .6  seconds, the computed value for 
the  c e n t r a l  ray  using the  Baumbach-Allen model. Actual ly  t h i s  
group delay is n o t  cons t an t  but  v a r i e s  w i t h  coronal dens i ty .  
The mean range for  this r e p o r t  period w a s  about 1.07 solar  r a d i i  
from the sun's c e n t e r  toward the earth. The m a n  range i n  1 9 6 1  
was about 1.22 solar  radi i .  In gene ra l  the mean range v a r i e s  
wi th  sun s p o t  number, bu t  the  c o r r e l a t i o n  i s  n o t  so high as i s  
t h e  c o r r e l a t i o n  of range depth and cross s e c t i o n  w i t h  sun s p o t  
number. The c o r r e l a t i o n  would probably be h igher ,  and the  
change i n  mean range w i t h  t i m e  would be greater, i f  a more 
accura t e  account could be taken of the  a d d i t i o n a l  group delay.  
On the  basis of a given model, it would be possible t o  deter- 
mine the  v a r i a t i o n  of coronal  i o n i z a t i o n  dens i ty  from radar data. 
During t h i s  report period work was done on three papers 
which are soon to  be presented for pub l i ca t ion  i n  the  A s t r o -  
phys i ca l  and Astronomical Journals .  
sent t i e  s o l a r  radar r e s u l t s  obtained at E 1  Campo since 1961; 
The first paper w i l l  pre- -
the  second w i l l  p r e sen t  the 1964 Venus saclar exneriment per- 
formed a t  E l  Campo; and t h e  t h i r d  w i l l  d i s cuss  t h e  effective 
s i z e  of the sun i n  occu l t ing  radio energy from t h e  galaxy. 
t h i r d  paper w i l l  be based on o c c u l t a t i o n  experiments performed 
a t  El Campo during December of each year  when t h e  sun was on 
t h e  galactic equator ,  and w i l l  also be based upon t h e  r e s u l t s  
Of Taurus:.occultation observat ions made a t  El Campo i n  June. 
For p o i n t  sources  the occu l t ing  zone of t he  sun i s  about three 
The 
or f o u r  degrees i n  diameter, b u t  for  a large d i s t r i b u t e d  source 
. .  
- 5 -  
t he  e f f e c t i v e  occu l t ing  zone is n o t  t h i s  large because of t h e  
energy mirrored i n  t h e  corona. For a cons t an t  g a l a c t i c  back- 
ground the effective occul t ing  zone is only about one solar 
diameter for a wide range of  f requencies .  
i ng  zone is based on n e t  loss of galactic energy due t o  the 
presence of t h e  sun. 
The effective occul t -  
During J u l y  and August 1965, t h e  two l i nea r ly -po la r i zed  
El Campo a r r a y s  were connected t o  produce the t w o  c i r c u l a r l y  
po la r i zed  components of echo energy and solar noise .  The 
r e s u l t s  of these experiments are t h a t  t h e  echo energy is approx- 
imately evenly divided between t h e  two c i r c u l a r  components. 
Perhaps w i t h  a narrow antenna beam t h a t  could r e so lve  a s m a l l  
region of the sun one component might predominate when there 
were s t rong  magnetic f i e l d s .  
O t h e r  accomplishments during this r enor t ing  period include: 
(1) A monitoring of J u p i t e r  at 38Mcps (no b u r s t s  were 
observed) ; 
Various t h e o r e t i c a l  i n v e s t i g a t i o n s  pe r t a in ing  t o  t h e  
solar r e f l e c t i o n  process; and 
( 2 )  
(3) A drawing of a sky map of contour l i n e s  of cons t an t  
galactic temperature a t  38Mcps from cosmic noise 
data received a t  E l  Campo. 
The p r e s e n t  opinion as t o  t h e  type of reflectors respon- 
sible for the  solar  echo i s  t h a t  the  i r r e g u l a r i t i e s  a r e  caused 
by some type  of hydromagnetic shocks which are propagating a t  
v a r i o u s  d i r e c t i o n s  i n  t h e  corona, b u t  predominately r a d i a l l y  
I 
- 6 -  
outward. These shock f r o n t  v e l o c i t i e s  are from 100 t o  2OOkm/s, 
and a r e  probably respons ib le  for  coronal  heat ing.  The shocks 
vary i n  number and i n  i n t e n s i t y  w i t h  t i m e  b u t  are always pres- 
e n t .  The coronal  s h e l l  throughcut which m o s t  r e f l e c t i o n s  t a k e  
p l ace  is probably less than one-tenth s o l a r  r a d i i  th ick .  The 
cross s e c t i o n  inc reases  when the nunber dens i ty  and i o n i z a t i o n  
density of the shock fronts increase.  
Addit ional  f a c i l i t i e s  recomended for  f u t u r o  solar radar 
experiments are: 
(1) An antenna t h a t  would allow more than one experiment 
per day to  be performed; 
(2 )  A n  increase i n  system s e n s i t i v i t y  by a t  least  t e n  
decibels; 
(3) -An antenna w i t h  an angular  r e so lu t ion  of a t  least  
one-fourth degree: and 
( 4 )  A bank of real time i n t e g r a t o r s  for  spectral s t u d i e s .  
r .  
I .  


El Campo solar radar system Second semiannual report, Jul. 1 - Dec. 31, 1965

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