Auroral, polar cap, and sudden cosmic noise absorption estimates for 136 mc/s satellite radio signa

Basler, R. P.

Leinbach, H.

Owren, L.

English

NASA Technical Reports Server

Microfiche (MF) 
f 653 Jury 65 
. ESTIVATED ARSORPTIOM OF 1% MC/S SATELLITE RADIO S I G N A L S  
In te r im Technical Report No, 1 
prepared for 
NATIONAL AERONAUTICS AND S P A C E  A D M I N I S T R A T I O N  
GODDARD S P A C E  F L I G H T  CENTER 
under 
Contract No, MAS 5-1413 
February 1962 
t 
* 
Report prepared by: 
Roy P, Bas le r  
Haro ld  Leinbach 
Leif Owren 
, 
Pr inc ipa l  Inves t iga to r :  
Lei f '  Wren 
https://ntrs.nasa.gov/search.jsp?R=19660018653 2020-03-16T19:34:32+00:00Z
total 
by 
where 
by iicy Basier 
According t o  t h e  Appleton-Hartree magnero-ionic i.ne~i’;, L ~ L  
non-deviative absorp t ion  of r a d i o  field s t r eng th  is giver ;  
[NV ds e. 59 2?r e 
2 mc / Absorption (db) = 
(a +, 
N = e l e c t r o n  d e n s i t y  
v = c o l l i s i o n  frequency 
cu = 2rf = sbserv ing  frequency (angular)  
u+, = l ong i tud ina l  component of gyromagnetic frequcncy 
d s  = element of p a t h  lengch 
For f requencies  i n  t h e  high H F  and the  VHF range, LCI caln k k  
neglected,  i n  which case 
L, 
Nv db Absorption = 1 .2  x 10-9 --
f 2  k m  
3 -1 where f i s  i n  mc/s, N i n  electrons/cm , and v i n  sec . 
Absorption of 27.6 mc/s ctJsmic noise  monitored ;;ith il krclaz 
tl;=arr! antenns (,J Elenlent yagi) has 3et-n records2 f o r  s evz ra l  ;.;,-a:-s 
a t  Collsge, A.L ;~LK~.  Ar t h i s  frequency cht- iiigsiic-st :>t served 
values of au,-oral abso rp t io r :  are  i n  t h e  range f ron  10-2@ d?. 
These high qialues ,771 auro ra l ly  a s s z c i a t e d  a b s o q t i o ; i  occut’ ms7;ly 
i i l  tkt fall and winrc;l. .ionths and a x  a1:no::t absent  i n  tric 
suimer. The anoLinT; uf‘ Q ; I : J O I > Z ~ ~  abso rp t ion  ar 3 . 6  mc/s as a 
I 
. 
func t ion  of frequency of occurrence f o r  t h e  month of February 
1958 i s  shown i n  t h e  following graph. 
every 15 minutes, s o  some of the peak absorp t ion  valuss  may not  
be included if they  occurred during the  15 min i n t e r v a l  between 
sca l ings ;  however, t h e  graph r ep resen t s  a f a i r  sampling of t he  
data. Febniary 1958 was a n  extremely d i s tu rbed  month, so t h i s  
The records  were sca led  
graph should be regarded no t  as a t y p i c a l  expec ta t ion  curve but 
ra ther  as an  i n d i c a t i o n  of what might be expected during pe r iods  
of maximum a u r o r a l  a c t i v i t y .  
Auroral absorp t ion  i s  characterist ically irregular,  and it 
can change by as much as 2-3 db/min a t  27.6 mc/s.  
fas t  as  4-5 db/min have been observed, but are rare. 
Changes as 
Taking a maximum of 20 db a t  27.6 mc/s (which i s  extreme), 
u s ing  the inve r se  frequency squared r e l a t i o n ,  and neglec t ing  f o r  
the moment the fact  that cosmic noise  is a diffuse source and the 
s a t e l l i t e  i s  a po in t  source, the m a x i m  absorp t ion  which would 
be expected a t  136 mc/s  i s  
2 
Abs (max) = [s] 27*6 x 20 = 0.8 db. 
A t  136 mc/s the  r a d i o  t ransmiss ion  i s  along t h e  l i ne -o f - s igh t  
pa th  from t h e  s a t e l l i t e  t o  the  rece iv ing  antenna. By recording 
the r a d i o  s i g n a l  strength as the s a t e l l i t e  sweeps a c r o s s  the sky 
it i s  p o s s i b l e  t o  d e t e c t  the presence of any absorbing regions,  
even the very s m a l l  ones. I n  order  t o  sense the  abso rp t ion  
s t r u c t u r e  as e f f e c t i v e l y  as  can be done w i t h  sa te l l i tes ,  cosmic 
noise would have t o  be monitored with a p e n c i l  beam a n t e m a ,  
The s i z e s  of the  absorbing i r r e g u l a r i t i e s  i n  the a u r o r a l  zone are 
2 
I 
I not known s ince  almost a l l  of t h e  s t u d i e s  of au ro ra l  abso rp t in s  
~ 
have been made with broad beam antennas, but  it i s  not  unreason- 
able t o  expect some f i n e  s t r u c t u r e  w i t h  angular dimensions of one 
o r  t w o  degrees  as seen from the ground. It i s  thus d i f f i c u l t  t o  
Correct  f o r  the e f f e c t  of antenna beam width f o r  t h e  commonly 
l 
~ 
I 
I 
occurr ing  auroral absorp t ion  events,  but  the m a x i m u m  absorpt ion 
t o  b e  expected a t  136 mc/s can s t i l l  be estimated a t  l e s s  than  
one db since the f i g w e  of 20 db =sed i n  the above c a l c u l a t i o n  
probably r e f e r s  t o  an occasion when t h e  e n t i r e  antenna beam was 
very near ly  f i l l e d  with bright aurora, t hus  removing the  e f f e c t  
of t h e  antenna pa t t e rn .  
A s  an a l t e r n a t i v e  approach t o  t he  est imate  of the maximum 
absorpt ion,  it i s  possible t o  use t h e  spectroscopic  information 
that  t h e  max imum e l e c t r o n  dens i ty  i n  a br ight  a u r o r a l  d i sp l ay  i s  
of t h e  o rde r  of 10 7 el./crn 3 
of the  order  of 10 5 sec- l  ( t h e  approximate value a t  100 km), t h e  
Assuming the t o t a l  absorp t ion  t o  
occur i n  a l a y e r  10 km t h i c k  with an average c o l l i s i o n  frequency 
absorp t ion  a t  136 mc/s i s  given by  
- -  Absorption = 1 . 2  x 10 -9 Nv db x 10 k m  
f 2  k m  
-9 10‘ x 10’ 
(136) 
= 1.2 x 10 x 10 db 
Thus the maximum expected a u r o r a l l y  a s soc ia t ed  absorp t ion  a t  
v e r t i c a l  incidence i s  less than one dec ibe l ,  For  obl ique 
incidence t h i s  f i g u r e  must be mul t ip l ied  by t h e  secant  of t h e  
angle  of incidence.  
3 
The above estimate i s  based on the Appleton-Hartree magneto- 
i o n i c  theory,  which i s  not  now considered s t r i c t l y  applicable*. 
However, i t  i s  easily accu ra t e  enough t o  use  i n  such o rde r  of 
magnitude c a l c u l a t i o n s  as have been c a r r i e d  out above. 
Absorption measurements have been made simultaneously over a 
north-south d i s t a n c e  of over 1000 km, spanning the a u r o r a l  zone, 
i n  both  Alaska and Norway, These s t u d i e s  have shown that pe r iods  
of d l s t u ~ b a n c e  ge~erall.;  ccinzide o v e r  a I?;agnetiC la+4 I,I L,UUC ---,.. I aLi&e 
of 4 t o  8 degrees, centered on the a u r o r a l  zone. 
d e t a i l e d  s t r u c t u r e  of absorpt ion events  may d i f f e r  s i g n i f i c a n t l y  
over d i s t a n c e s  of l e s s  than 200 km, i n d i c a t i n g  tha t  small scale 
s t r u c t u r e  of t he  absorp t ion  does e x i s t .  
c o n s i s t e n t  with t h e  observat ion tha t  absorp t ion  i s  c l o s e l y  
c o r r e l a t e d  t o  the  presence of v i sua l  au ro ra in  the beam of the r i o -  
meter antenna. 
However, t h e  
T h i s  r e s u l t  i s  i t s e l f  
The Norwegian s t u d i e s  (Norwegian Defence Research Es tab l i sh -  
ment Report No, 35, par t  1-111, Apri l  1961) show that t h e  d e t a i l e d  
c o r r e l a t i o n  between s t a t i o n s  separated by 200-300 k m  tencis t o  be 
b e t t e r  along a l i n e  of  constant magnetic l a t i t u d e  than  along a 
magnetic meridian, 
Alaska suggest that  a c l o s e  degree of c o r r e l a t i o n  exis ts  o v e r  
magnetic east-west base l ine  as long as TOO km, 
Some recent  and very pre l iminary  s t u d i e s  i n  
a 
*IJe cannot use  t h e  a c t u a l  mean absorp t ion  frequency i n  the 
Appleton-Hartree formula. 
e f f e c t i v e  c o l l i s i o n  frequency, the  c a l c u l a t i o n s  should be good 
first order  estimates. 
But as  long as we adopt a s u i t a b l e  
2. NON-AURORAL ABSORPTION AND OTHER CONSIDERATIONS 
by Earold Leinbach ~ 
~ 
I 
(i)a P O l S Y  Cap Abser@lm 
The above c a l c u l a t i o n s  have treated au ro ra l ly -a s soc ia t ed  
absorp t ion ,  A second c l a s s  of events ,  important a t  high l a t -  
i t u d e s ,  i s  the p o l a r  cap absorp t ion  (PCA), 
t i o n  results from bombardment of the polar cap ionosphere by 
s o l a r  cosmic r a y s  e j e c t e d  by c e r t a i n  large solar f lareso These 
T h i s  type of absorp- 
events  a r e  r e l a t i v e l y  rare compared w i t h  a u r o r a l  absorption; 
some 10 PCA events  may be recorded during a year of high sunspot 
a c t i v i t y ,  During the minimum y e a r s  of the sunspot cycle ,  the  
frequency of PCA events  may drop t o  zero per year. 
PCA events  are important, however, because of the l a r g e  
magnitude o f  absorp t ion  which may be a t t a i n e d ,  and because the 
high values of a b s o q t i o n  nay pers i s t  f o r  s e v e r a l  successive 
dap. P o l a r  c q  zbsorpt ion occurs i n  a m i f o r m  afid t h i c k  h o r i -  
zon ta l  l aye r ,  as has been demonstrated both experimental ly  and 
t h e o r e t i c a l l y ,  
served a t  say 27.6 mc/s t o  t h e  absorp t ion  exFected a t  h igher  
f requencies  can be =de with f a i r  c e r t a i n t y .  
that  the inve r se  frequency dependence holds  f o r  the absorp t ion  
a t  f requencies  above rf mc/8; t h i s  conten t ion  has been exper- 
imental  v e r i f i e d  i n  s e v e r a l  events, A s m a l l  co r r ec t ion  should 
be made t o  t h e  absorp t ion  measured w i t h  a wide v e r t i c a l  beam i n  
o rde r  t o  es t imate  t he  absorpt ion p e r  u n i t  v e r t i c a l  column, T h i s  
c o r r e c t i o n  does not change the  order of magnitude es t imate  below. 
Therefore ex t r apo la t ion  of the absorp t ion  ob- 
The assumption i s  
5 
I .  , -  
‘tie assume that the  maximum probable value of a b s o q t i o n  a t  
27.6 mc/s, using a wide  v e r t i c a l  beam, i s  30 db. Then t h e  ex- 
pLb,,Gu n a n f  n.4 absorp t ion  p e p  v e r t ~ z a l  c o ~ m n  a t  1% m / s  i s  approx- 
imate ly  (27.6/136j2 x 30 db = 1.2 db. 
)(, the expected absorp t ion  i s  then  1.2 x see x db. 
A t  moderate z e n i t h  angles ,  
I n  a given year, a t  least  50% of  the  PC4 events  should have 
maximum absorp t ion  values  far  less  t h a n  the  extreme of 30 db used 
above. 
(ii) Sudden Ionospheric Disturbances (SIC:  s and S C N A ’ s )  
Sudden ionospheric  dis turbances ( S I D 1 s ) ,  and the  equiva len t  
sudden cosmic noise  absorpt ion (SCNA) events  r e s u l t  from i on iza -  
t i o n  of‘ t he  D-region by electromagnetic r a d i a t i o n  from solar 
flares.  Thus S I D ’ S  and SCNA:s are r e s t r i c t e d  t o  t h e  s u n l i t  
hemisphere. A t  t h e  l a t i t u d e  of College, SCITA1s are seldom ob- 
served with i! v e r t i c a l  27.6 mcis riome’cer; events  witn magnitudes 
g r e a t e r  than  1 o r  2 db are ve ry  ra re .  One of the  l a r g e s t  3CNA’s 
ever  observcl  occurred on July 29, 1958, when a maximw value of 
absorp t ion  of abour; 6 d.5 was recorded a t  27.6 mcl’s, 
wGuld be equivalent  t o  about 0,251 db &t 136 mc/s ( v e r t i c a l  
inc idence) ,  assuming a uniform absorbing l aye r ,  and an  inve r se  
frequency dependence o f  the  z b s o q t i o n ,  
T h i s  value 
We conclude t h a t  SCNA’S w i l l  not be important f a c t o r s  i n  
t h e  recept ion  of 136 mc/s  s i g n a l s  f r o m  s a t e l l i t e s .  
(iii) . V a l i d i t y  of t h e  Inverse Frequency Squared and Secant Laws 
Two r iometers ,  one a t  27.6 mc/s and the  o t h e r  a t  50 mc/s, 
were operated a t  College for two years .  These r iometers  used 
6 
- s c a l e d  three-element,  ve r t i ca l ly -d i r ec t ed  Yagi antennas,  s o  that- 
t h e  abso rp t ion  could be d i r e c t l y  intercompared. Spot checks of 
t h e  r a t i o  o f  the  absorp t ion  at t h e  two f requencies  tvere made f o r  
numerous a u r o r a l  absorp t ion  events, wi th  t h e  conclusion t h a t  t h e  
r a t i o  d i d  not  depar t  s i g n i f i c a n t l y  from t h e  inve r se  frequency 
squared l a w .  More d e t a i l e d  checks of t h e  r a t i o  were made during 
the g r e a t  p o l a r  cap absorpt ion events  of July 1959, w i t h  t h e  
~ ~ n ~ l u s i ~ f i .  Iri the later case, t h e  only depai-tui-es from t h e  
i n v e r s e  frequency squared l a w  were towards h igher  than  p red ic t ed  
r a t i o s ,  that  is, towards smaller r e l a t i v e  absorp t ion  a t  t h e  h ighe r  
frequency than  expected. 
t h i s  r e s u l t .  
The secant  l a w  for t h e  absorp t ion  a t  oblique angles  has been 
No explanat ion has  ye t  been given f o r  
v e r i f i e d ,  by comparison of absorp t ion  data obtained w i t h  a 27.6 
mc/s r iometer  us ing  a ver t ical  three-element Yagi, and another  
27.6 mc/s r iometer  us ing  a north-pointing, ho r i zon ta l ly -d i r ec t ed  
four-element Yagi, wi th  a beam e l e v a t i o n  angle  o f  about 35 de- 
grees ,  Polar  cap absorp t ion  i s  always g r e a t e r  on t h e  oblique 
antenna, with a r a t i o  of absorpt ion to that  a t  v e r t i c a l  inc idence  
equal t o  that  given by t h e  secant l a w .  I n  most PCA events,  no 
inherent  l a t i t u d e  inc rease  of absorp t ion  occurs  over t h e  d i s t a n c e  
covered by the  two beams. 
The secant  l a w  has also been s u f f i c i e n t  t o  expla in  the r a t i o  
of absorp t ion  a t  v e r t i c a l  and obl ique incidence,  f o r  those  
a u m r a s  which were uniform over t h e  sky. Non-uniform a u r o r a s  may, 
of course,give h igher  absorpt ion i n  one beam than t h e  other .  
7 
. 
I .  
3,  S W R Y  AND CONCLUSIONS 
by Le i f  @wren 
The maximum ionospheric abscrptlon expected f o r  a 130 -. -r mc/s  
s a t e l l i t e  r a d i o  s i g n a l  observed from a s t a t i o n  i n  the a u r o r a l  
zone a t  the zen i th  angle  X is: 
Auroral absorp t ion  
Polar  cap absorp t ion  
0.8 sec X db 
L 2  622 x db 
Sudden cosmic noise absorp t ion  0.25 sec X db 
These maximum values apply t o  pe r iods  of very high s o l a r  
a c t i v i t y  and w i l l  be a t t a i n e d  only  on rare occasions,  
t y p i c a l  values  a t  136 mc/s might be: 
More 
Auroral abso rp t ion  0.3 see X db 
Polar  cap absorp t ion  0.5 sec  X db 
Sudden cosmic no i se  absorp t ion  Negl igible  
, 
For large z e n i t h  ang le s  the  secant  f a c t o r  may lead t o  an 
overest imate  of t h e  expected absorpt ion,  i n  p a r t i c u l a r  for a u r o r a l  
absorp t ion  when looking across  the a u r o r a l  zone. 
The a u r o r a l  abso rp t ion  zone t y p i c a l l y  extends over  7-8 de- 
grees of geomagnetic l a t i t u d e  during moderately d i s tu rbed  and 
d i s tu rbed  magnetic conditions.  
be centered approximately on the geomagnetic l a t i t u d e  of College, 
64,5"N. 
I n  Alaska t h i s  zone appears  t o  
The a u r o r a l  absorp t ion  may change z t  a r a t e  of 0.1 db per 
minute a t  136 mc/s, The durat ion of a t y p i c a l  event is of the 
o r d e r  of 20-30 minutes but t h e  d i s tu rbed  pe r iod  may last f o r  a 
I 
8 
4 
c f e w  t o  s e v e r a l  hours. The frequency of occurrence may vary from 
n i n e t y  per cent of any given monthly per iod during very high solar 
a c t i v i t y  t o  a f e w  per cent  during low s o l a r  a c t i v i t y ,  
Polar  cap absorp t ion  extends i n  a f a i r l y  uniform l a y e r  o v e r  
t h e  a r c t i c  region down t o  about 6 2 O  geomagnetic l a t i t u d e  during 
the first 24 hours. 
storm it usually spreads t o  lower l a t i t u d e s  a l so .  
i n i t i a l  phase of a PCA event, t he  8hcnrp t ion  i n c r e a s e s  zt a ra te  
of up t o  0.1 db per hour a t  136 mc/s and reaches maximum af te r  
6-12 hours. 
1-2 days and the decay curve has the form of a damped o s c i l l a t i o n  
w i t h  daytime peaks and night t ime troughs.  
rence varies from about t e n  events  p e r  yea r  during maximum s o l a r  
a c t i v i t y  t o  perhaps one o r  no events  i n  a minimum year. 
With the  onset  of the a s soc ia t ed  magnetic 
During the 
The dura t ion  of a moderate event i s  of the o r d e r  of 
The frequency of occur- 
The estimated 136 mc/s absorp t ion  i s  based on e x t r a p o l a t i o n  
of' 27.6 mc/s cosmic noise  absorp t ion  measurements according t o  the 
i n v e r s e  frequency squared law. The v a l i d i t y  of t h i s  l a w  f o r  
a u r o r a l  and p o l a r  cap absorpt ion has been e s t ab l i shed  over t h e  
lower VHF range by simultaneous v e r t i c a l  beam observa t ions  of 
cosmic no i se  a t  27.6 and 50 mc/s. The secant  law f o r  ob l ique  
inc idence  has been tes ted by simultaneous v e r t i c a l  and obl ique 
beam observa t ions  on 27.6 mc/s. 
absorpt ion,  and may be used with some cau t ion  f o r  a u r o r a l  
absorpt ion.  
It i s  r e l i a b i e  f o r  p o l a r  cap 
The ex t r apo la t ion  of cosmic noise  absorp t ion  r e s u l t s  obtaiped. 
w i t h  wide antenna beams t o  s a t e l l i t e  r a d i a t i o n s  imp l i e s  t h e  
assumption tha t  t h e  absorbing medium i s  f a i r l y  homogeneous and 
9 
~ 
. -  
4 C C Z S  .IC T * i i Q i . T  c*a' -, l j  d * ; \ ~ u ~ , .  ~g ; A ' c ~ f + a  '1 .ll c = u ' Ab'' ,c*n 
i s  w e l l  j u s t i f i e d  f o r  polar cap and sudden c3smic noise  absorp- 
. L P ~ ;  i ; r ~ u  p - D b L b l y  a l s ?  k313.s fa?- a u r o r a l  a k s o q t i o n  s ince  The 
l a t t e r  g e n e r a l l y  i s  assoc ia ted  with d i f f u s e ,  pos t  -breakup a u r o r a l  
fo rms ,  
L .* 
During t h e  e a r l y  phase of  a u r o r a l  a c t i v i t y  when a u r o r a l  
forms are sharply del imi ted ,  t h i c k  layers of small e l e c t r o n  den- 
sity i r regulasi t ies  may fnm.  ~ h i c h  cause s igna l  at ter?catim by 
mul t ip l e  s c a t t e r i n g .  T h i s  a t t e n u a t i o n  could be more important 
t h a n  a poss ib l e ,  but  so fa r  experimeutally unver i f ied ,  t r u e  
absorp t ion  i n  small, underlying clouds. A d i s t i n c t i o n  between 
the  two types of a t t e n u a t i o n  would be d i f f i c u l t  for a sa t e l l i t e  
r a d i o  s i g n a l  observed w i t h  a s i n g l e  antenna. The a t t e n u a t i o n  of 
136 and 400 mc/ s  sa te l l i t e  radio s i g n a l s  by mul t ip l e  s c a t t e r i n g  
i n  t h e  a u r o r a l  ionosphere w i l l  be discussed and estimated i n  a 
separate report  
10 


Estimated absorption of 136 mc/s satellite radio signals Interim technical report no. 1

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