Photometric observations of pulsating aurora

Miller, R. E.

Zeitz, W. M.

English

NASA Technical Reports Server

XAVIER 
UNIVERSITY 
DEPARTMENT OF 
PHYSICS 
January 1 9 7 1  
CASE F I L E  
RESEARCH REPORT 
OBSERVATIONS ON 
PULSATING AURORAS 
by 
R. E. M i l l e r  and 
Wm. M. Z e i t z  
Ciwritrrrati. Ohio 45207 
https://ntrs.nasa.gov/search.jsp?R=19710009640 2020-03-12T00:12:07+00:00Z
RESEARCH REPORT 
.I. 
R, E ,  M i l l e r  and Wm. M. ~ e i t z -  
Xavier Un ive r s i ty ,  C inc inna t i ,  Ohio 
Accepted f o r  Pub l i ca t ion  
Journa l  o f  P lane ta ry  and Space Science 
9 This  r e sea rch  supported by t h e  Nat iona l  Aeronaut ics  and 
Space Adminis t ra t ion ,  
* 
Freder ick  A, Hauck Undergraduate Research Scholar .  
ABSTRACT 
-I- O p t i c a l  a u r o r a l  p u l s a t i o n s  i n  A3914 N and 1 5 5 7 7  01 have  2 
been measured i n  t h e  f requency  r a n g e  0 - 0 1  t o  1.0 H e r t z  by  
a doub le  photometer  sys tem d u r i n g  t h e  NASA A i r b o r n e  A u r o r a l  
E x p e d i t i o n .  The s p e c t r a l  d e n s i t y  c a l c u l a t i o n s  i n v a r i a b l y  
show a pronounced peak i n  t h e  0.05 t o  0.2 f requency  r a n g e ,  
The p u l s a t i n g  forms were c o n f i n e d  t o  t h e  r e g i o n  between 
62O and 69O North  l a t i t u d e .  No p u l s a t i o n s  were d e t e c t a b l e  
i n  t h e  A4861 HB emiss ion .  
INTRODUCTION 
This  paper i s  concerned w i t h  t h e  measurement o f  
p u l s a t i o n s  i n  t h e  a u r o r a l  luminos i ty  du r ing  t h e  NASA Airborne 
Expedi t ion 19  January t o  6  February,  1968. This  exped i t i on ,  
based a t  F o r t  C h u r c h i l l ,  Manitoba, c o n s i s t e d  o f  t h e  
coord ina ted  experiments o f  groups from n i n e  u n i v e r s i t i e s  and 
r e s e a r c h  i n s  ti t u t i o n s  aboard t h e  NASA Convair 990 j e t  a i r c r a f t .  
Aurora l  p u l s a t i o n s  have been observed by Campbell and 
Rees (1961) and by Paulson and Shepard (1966) u s i n g  photo- 
e l e c t r i c  photometers,  and more r e c e n t l y  by Cresswel l  and Davis 
(1966) u s ing  an image o r t h i c o n  t e l e v i s i o n  system. The 
p u l s a t i o n s  a r e  quas i -pe r iod ic  and u s u a l l y  appear  on ly  a s  a  
s l i g h t  modulation of  t h e  t o t a l  luminosi ty .  The p u l s a t i o n s  
a r e  o f t e n  connected w i t h  q u i e t  forms and r e p r e s e n t  a  f a i r l y  
s t a t i o n a r y  phenomenon i n  c o n t r a s t  t o  r a p i d  and complex motions 
a s s o c i a t e d  w i t h  t he  break-up phase of  an a u r o r a l  d i s p l a y .  The i r  
cause  i s  unknown, b u t  i t  seems l i k e l y  t h a t  they  r e f l e c t  
o s c i l l a t i o n s  e i t h e r  i n  t h e  p a r t i c l e  source  o r  i n  t h e  
m a g n e t ~ s p h e r i c  c o n d i t i o n s  a long  the  pa th  s f  t h e  p a r t i c l e s ,  
Thus a knowledge of t h e  f r e q u e n c i e s  and i n t e n s i t i e s  ef these 
f l u c t u a t i o n s  cou ld  be s f  impordance in f o r m u l a t i n g  a complete 
auroral  theory ,  
INSTRUMENTATION 
The a u r o r a l  luminos i ty  w a s  observed through an o p t i c a l  
window i n  t h e  roof  of t h e  a i r c r a f t  u s ing  two i d e n t i c a l  
photometers which were p r e c i s e l y  a l i gned  s o  a s  t o  view 
i d e n t i c a l  r e g i o n s o  Each photometer c o n s i s t e d  of  an f/5 
th ree- inch  diameter  coa ted  acromat,  f i e l d  l e n s ,  in te rchange-  
a b l e  two-inch diameter  i n t e r f e r e n c e  f i l t e r ,  v a r i a b l e  
diaphragm, and a r e f l e c t i n g  cone l i g h t  p i p e  which coupled 
t h e  i n t e r f e r e n c e  f i l t e r  t o  t h e  one-inch diameter  cathode of  
an EMR p h o t o m u l t i p l i e r ,  This  l i g h t  p i p e  i n c r e a s e d  t h e  
s e n s i t i v i t y  by a lmost  a  f a c t o r  of four  w i thou t  t h e  i n c r e a s e  
i n  dark  c u r r e n t  whish would have r e s u l t e d  from us ing  a  l a r g e r  
diameter  cathode,  With t h i s  des ign  and t h e  u se  of  h igh  
q u a l i t y  e l e c t r o n i c s ,  t h e  s e n s i t i v i t y  f o r  r eco rd ing  p u l s a t i o n s  
was Limited on ly  by t h e  s t a t i s t i c a l  photon n o i s e ,  The 
narrow-band i n t e r f e r e n c e  f i l t e r s  were chosen t o  measure A4861 HOP 
-I- 
X5577 01, o r  A3914 N2 a u r o r a l  emiss ions .  
A l l  d a t a  were recorded simuLtaneously on s t r i p  c h a r t  and 
magnetic t ape  r e c o r d e r s ,  
Regular photometr ic  c a l i b r a t i o n s  were c a r r i e d  o u t  u s i n g  
It44 
a s t anda rd  tiang-sten l a m p  and a @ raclioackl~se l i g h t  source 
( c o u r t e s y  o f  R, M, Eather) , 
A I L  measurements were obtal  ned during quiet magnet~c 
- 
periods w i t h  K values i n  the range of 0 - 3 , The a u r o r a l  
P 
abso rp t ion  a t  2 0  and 30  MHz was simulkaneously measured 
aboard t h e  a i r c r a f t  by Bradbury and J o k i  (1969) and found t o  
be  l e s s  than 1 db du r ing  a l l  t h e  recorded p e r i o d s  of  p u l s a t i o n ,  
DATA ANALYSIS 
Records of t h e  a i r c r a f t ' s  s t a b i l i t y  show t h a t  t h e  r o l l  
motion about  t h e  l o n g i t u d i n a l  a x i s  o c c a s i o n a l l y  had an 
ampli tude a s  l a r g e  as one degree  peak-to-peak, This  4 - 6  s e c  
p e r i o d i c  o s c i l l a t i o n  was d i s c e r n i b l e  i n  only  two o f  t h e  p u l s a t i o n  
r eco rds ,  
A l l  s u i t a b l e  r eco rds  were d i g i t i z e d  u s i n g  t h e  Nyquist  o r  
Shannon c r i t e r i o n  of sampling s o  as t o  avoid any p o s s i b l e  
a l i a s i n g  e r r o r ,  Assuming tha-t  t h e  f l ue tu -a t ions  i n  time of  
a u r o r a l  b r i g h t n e s s  c o n s t i t u t e  a p h y s i c a l  r e a l i z a t i o n  of  a 
second-order s t a t i o n a r y  p roces s ,  t h e  power s p e c t r a l  d e n s i t i e s  
were computed us ing  the d i s c r e t e  Four ie r  t rans form a lgor i thm of 
Cooley and 'Eukey (1965) followed by a Hanning smoothing procedure  
(Blackman and Tukey, 1958 )  .,
Figure  1 shows a p o r t i o n  sf the X 5 5 7 7  UP i n t e n s i t y  v e r s u s  
time record for a v e r y  vrvad and somewhat o v a l  shaped pakeh o f  
s t r o n g l y  pulsating aurora that was recorded while flying a t  an 
G 
a l t i t u d e  of 33,0013 f e ~ k  a t  geomagnetic coord ina tes  62 $1- 
0 Lat i tude  and 38 01' W longx tude  fLOd, 4 hrs, 5 1  min, U T j ,  
Th i s  even l a s t e d  f o r  s l i g h t l y  over  fou r  minutes and w a s  
unusual  i n  t h a t  t h e  f l u c t u a t i o n s  were o f t e n  L O O  p e r c e n t  of  t h e  
t o t a l  i n t e n s i t y ,  A v i s u a l  i n s p e c t i o n  of  t h e  record  i n d i c a t e d  
t h a t  i t  was d iv ided  i n t o  fou r  s e c t i o n s ,  The f i r s t  t h r e e  
( l e n g t h s  of 7 8 ,  6 8 ,  and 58 seconds,  respec tbve ly)  were 
s epa ra t ed  from each o t h e r  by 1 0  s e c ,  p e r i o d s  o f  c o n s t a n t  
i n t e n s i t y ,  a £  t e r  which t h e  p u l s a t ~ o n  f  requenc ies  app rec i ab ly  
i nc reased  (30 t o  6 0  p e r c e n t ) ,  The f o u r t h  s e c t i o n  was 
d i s t i n g u i s h e d  from t h e  t h l r d  on ly  by an ab rup t  i n c r e a s e  i n  
frequency t h a t  l a s t e d  f o r  only  18 seconds,  The prominent 
f r equenc ie s  of  t h e  p u l s a t i o n s  and t h e i r  r e l a t i v e  i n c r e a s e  w i t h  
time can be seen  i n  F ~ g u r e  2 where t h e  s p e c t r a l  d e n s i t i e s  f o r  
each of t h e  fou r  s e c t i o n s  a r e  given as a func t ion  of frequency,  
I t  is i m p o s s ~ b l e  t o  know from t h e s e  obse rva t ions  whether 
%he f r e q u e n c y  spec t rum,  such a s  shown i n  Figure  2 ,  i s  a c t u a l l y  
that.  of a s i n g l e  pu lsa txng  form o r  t h e  sum of s e v e r a l  t h a t  
l i e  i n  the f i e l d  of  v i e w ,  However, Cresswel l  and Davis ( 1 9 6 6 )  
have shown, using a n  image o r t h l c o n  t e l e v i s i o n  system, t h a t  
whereas the p u l s a t i o n  of ail i i-idsvidval farria i s  observed 50 be 
everphere  ~ r t .  phase ( w ~ t h ~ n  the ~ n s t r r l m e n t a %  faeld sf v i e w )  , 
a d j a c e n k  p o r t ~ o n s  ot t h e  sky are observed t o  p u l s a t e  andependen t ly .  
F i g u r e  3 shows a por t ion  of the o r i g i n a l  record of t h e  
4- A4861 HB and hS914 N2 i n t e n s i . t i e s  f o r  a  subv i sua l  aurora  
0 
ob ta ined  a t  geomagnetic coo rd ina t e s  65 2 7 '  N l a t i t u d e  and 
+ 37O 34'  W l ong i tude  (37d. 1 0  h r s .  38 min, UT). The 13914 Nz 
r a d i a t i o n  c o n s i s t e d  of  approximately a  25 p e r c e n t  modulation 
of  t h e  200 Rayleigh average i n t e n s i t y ,  The computed s p e c t r a l  
d e n s i t i e s  f o r  v a r i o u s  p o r t i o n s  of t h e  1 2  minutes d u r a t i o n  o f  
t h e  p u l s a t i o n s  showed t h a t  t h e  frequency composit ion changed 
on ly  s l i g h t l y  w i t h  t i m e  and t h a t  t h i s  a u r o r a  had more power 
i n  t h e  h ighe r  f r equenc ie s  ( g r e a t e r  than 0,3 Her tz )  than t h a t  
of  Figure  2, 
During t h i s  exped i t i on  t h e  X486l HA w a s  s imul taneous ly  
measured and recorded,  as shown i n  Figure  3,  a long  w i t h  e i t h e r  
-I- 
X5577 01 o r  A3914 N emiss ions .  The very  sma l l  v a r i a t i o n s  2  
i n  t h e  HR s i g n a l  a r e  completely random and a r e  t h e  r e s u l t  o f  
s t a t i s t i c a l  photon n o i s e ,  An a n a l y s i s  Q £  t h e  e n t i r e  c o l l e c t e d  
d a t a  i n d i c a t e s  a t o t a l  absence of HB f l u c t u a t i o n s  a t  l e a s t  t o  
t h e  l i m i t  of d e t e c t i o n  of about  0 , 2  Rayleigho This  i n d i c a t e s  
t h a t  t h e  p u l s a t i o n s  were due e n t l r e l y  t o  e l e c t r o n  e x c i t a t i o n ,  
I t  should be poin ted  o u t  t h a t  considerabbe c a r e  must be 
exe rc i sed  i n  eaLeuLat ing and interpreting power s p e c t r a l  
densities of a u r s r d l  r adxa lnon  s i n c e  a change in phase o f  the 
f l u c t u a t i o n s  can r e s u l t  i n  the appearance o f  s t r o n g  h igher  
harmonics t h a t ,  i n  f a c t ,  have no physical reality, Another  
common fault i s  the a t t empt  to give a physlcal i n t e r p r e t a t i o n  
t o  t h e  approximately exponen t i a l l y  dec reas ing  s p e c t r a l  d e n s i t y  
w i t h  i n c r e a s i n g  f requency,  s i n c e  t h i s  can be  t h e  r e s u l t  of  
t a k i n g  a d i s c r e t e  Four i e r  t rans form of  a  f i n i t e ,  quas i -pe r iod ic  
f u n c t i o n  of t i m e  ., 
I n  o rde r  t o  o b t a i n  t h e  r e l a t i v e  occurrence of  t h e  v a r i o u s  
f r equenc ie s  of  p u l s a t i o n s ,  t h e  prominent f r equenc ie s  w e r e  
taken from t h e  s p e c t r a l  d e n s i t y  p l o t s  f o r  a l l  f l u c t u a t i n g  
a u r o r a s  recorded du r ing  t h e  e x p e d i t i o n -  The r e s u l t  i s  shown 
by t h e  his togram i n  Figure  4 where a  frequency i n t e r v a l  of  
0-05 Hertz  w a s  chosen s i n c e  t h i s  i s  t h e  approximate r e s o l u t i o n  
of t h e  s p e c t r a l  d e n s i t y  c a l c u l a t i o n s o  The p l o t  shows a  broad 
maximum i n  t h e  0,05 t o  0 , 2  Her tz  r eg ion  w i t h  a peak a t  t h e  
0-05 t o  O , L  Her tz  i n t e r v a l ,  
An a n a l y s i s  s f  all a u r o r a s  observed du r ing  t h i s  exped i t i on  
shows t h a t  p u l s a t i n g  forms occur red  between no r th  geomagnetic 
0 o l a t i t u d e s  of 62 and 69 and t h a t  t h e i r  maximum occurrence 
i s  i n  t h e  few hours  fol lowing l o c a l  geomagnetic midnight ,  
W e  are g ~ a t e f u l  to Ms, l J O u  Haughraey sf the Axrborne 
Sciences O f f i c e ,  NASA-Ames Resear eh Center, and to the pilots 
and cEew for the success of the I 9 6 8  Airbs~ne Auroral 
Expedi ti on, 
We would lake ts thank Prafessor  W ~ . I E i a r n  G o  Fasthe of 
The Johns Hopkins U n ~ v e ~ s i t y  for esnkinual support and many 
useful diseuss~sns, 
This research was supported under grant NGR-3 6-014-006 
from the Nakional Aeronautics and Space AdsninFstration 
B l a c k m a n ,  R, B, and Tukey, J, W, ( 1 9 5 8 )  , 
" 'The M e a s u r e m e n t  of P o w e r  Spectra", D o v e r .  
B r a - d b u r y ,  J, N. and ~ o k i ,  E ,  G .  ( 1 9 6 9 )  , 
T r a n s ,  A m e r ,  G e o p h y s ,  U n i o n  2 7 2 ,  
C a m p b e l l ,  w. H, and R e e s ,  M ,  B, ( 1 9 6 1 ) -  
J. G e o p h y s .  R e s .  - 66  41, 
C o o l e y ,  V, W, and T u k e y ,  J. W, ( 1 9 6 5 )  . 
Math. C o m p .  9 2 9 4 ,  
C r e s s w e l l ,  G ,  R,  and D ~ V ~ S ,  T  N, ( 1 9 6 6 ) ,  
J e  G e o p h y s ,  R e s  - 7 1 3 1 5 5 ,  
Paubson,  K ,  V ,  and Shepherd, G ,  G ,  ( 1 9 6 6 )  . 
C a n a d ,  J. Physacs - 44 8 3 7 ,  
F I G U M  CAPTIONS 
l, P a r t  of  t h e  o r i g i n a l  record  of t h e  A5577 01 r a d i a t i o n  
of a  ve ry  s t r o n g l y  p u l s a t i n g  a u r o r a ,  
2 ,  The power s p e c t r a l  d e n s i t y  of  fou r  consecut ive  a u r o r a l  
b r i g h t n e s s  f l u c t u a t i o n  r eco rds  of A5577 01 r a d i a t i o n  
0 
ob ta ined  20d, 3 h r ,  57m, UT and 6 2  41'  N. Geom. La t .  
and 38O 0 1 '  W Geom. Long. 
+ 
3 .  Simultaneous r eco rd ing  of  t h e  A4861 HA and A3914 N 2  
r a d i a t i o n s  from a  p u l s a t i n g  au ro ra ,  
4 ,  The r e l a t i v e  occurrence of t h e  v a r i o u s  f r equenc ie s  of  
p u l s a t i o n s  f o r  a l l  f l u c t u a t i n g  a u r o r a s  recorded du r ing  
t h e  exped i t i on ,  

,"I -2 .3 -44 
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Observations on pulsating auroras

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