U. of Iowa 67-19

MICROBURST P H E " A

2. A u r o r a l Zone F,lectrons*

1 M. N. Oliven Department of Physics and Astronomy

Universi ty of Iowa, Iowa City

D. Venkatesan Department of Physics

Universi ty of Calgary, Calgary, Alberta

and

2 K. G. McCracken Southwest Center f o r Advanced Studies

Dallas, Texas

September 1967

* Work a t t h e Universi ty of Iowa by Off ice of Naval Research Contract 1509(06), a t t h e University of Calgary by National Research Council Contract NRC A-3865, and a t t h e Southwest Center f o r Advanced Studies by NASA Contract NAS-r-198.

NASA Graduate Trainee I

Current Address: Physics Department, Universi ty of Adelaide, Adelaide, South Aus t r a l i a

https://ntrs.nasa.gov/search.jsp?R=19670030587 2018-07-16T09:14:03+00:00Z

2

ABSTRACT

Observations made during a high-time reso lu t ion mode

of operation of the I n j u n 3 s a t e l l i t e have i d e n t i f i e d aurora l

zone e lec t ron microbursts, the parent phenomenon of the of ten

observed bremsstrahlung x-ray microbursts.

microbursts, detected i n the

de tec tor and l e s s f requent ly i n the E

detector , have time p r o f i l e s similar t o those of both t h e sym-

These e lec t ron

Ee 2 40 keV (180") (p rec ip i t a t ing )

2 40 keV ( 9 0 " ) (trapped) e

met r ic and asymmetric daughter x-rajr microbursts and exhibi t

5 t y p i c a l peak f luxes of Jo (Ee 2 40 keV) - 3 x 10

em see sterad-' i n t he 180" detector. S t a t i s t i c a l s tud ies

e lectrons

-2 -1

of these sa te l l i t e -observed microbursts revea l t h e i r occurrence

predominantly i n t h e regions of 6 5 L I 8.5 and 4:30 s magnetic

l o c a l time S l2:3O. Detailed s t a t i s t i c a l d i s t r ibu t ions of

microburst occurrence as functions of l o c a l time and L a r e

presented.

3

INTRODUCTION

I n order t o augment the study of aurora l zone x-ray micro-

burs t s , an inves t iga t ion was car r ied out t o determine whether

s a t e l l i t e de tec tors observe any phenomena which m i g h t be connected

with the microbursts observed from balloons.

microbursts have previously been discussed i n grea t d e t a i l (Anderson

and Milton [ 19643 ; Anderson [ 19651 ; Oliven and Gurnett [ 1967aI ; and

Venkatesan e t al. [ 19671), and high-time reso lu t ion measurements of

t hese x-ray microbursts have been c a r r i e d out f o r severa l years.

The Injun 3 s a t e l l i t e provided a good opportunity t o determine

whether t he causat ive p rec ip i t a t ing p a r t i c l e s which must r e s u l t i n

t h e production of x-ray bremsstrahlung microbursts could be observed

above t h e x-ray production l aye r . Consequently, da t a from charged

p a r t i c l e de tec tors aboard t h e Injun 3 s a t e l l i t e were examined

i n d e t a i l fo r t he period, January-October 1963. Although t h i s

s a t e l l i t e was not i n operation as l a t e as August 1965, t h e time

a t which the balloon observations of microbursts were made

(Venkatesan e t al. [ 19671 ), it nevertheless provides use fu l in for -

Auroral zone x-ray

mation i n the inves t iga t ion of f a s t temporal changes i n p a r t i c l e

p r e c i p i t a t i o n phenomena because of i t s incorporation of high-time

4

reso lu t ion detectors . The Injun 4 s a t e l l i t e , magnetically

or ien ted as Injun 3 and containing s imi l a r detectors , and i n

o r b i t during t h e per iod of t h e aforementioned balloon observa-

t ions , was incapable of resolving times of less than a second,

and hence could not be u t i l i z e d f o r t he study of the fast micro-

b u r s t s t o be discussed herein.

5

SATELLITE DETECTORS

In jun 3, a magnetically or iented s a t e l l i t e , launched

on December 13, 1963, had an o r b i t of apogee a l t i t u d e 2785 km,

per igee a l t i t u d e 237 km, o r b i t a l i nc l ina t ion 7O.4", and a per-

iod of 116 minutes. The descr ipt ion of the s a t e l l i t e , designed

and b u i l t at the Universi ty of Iowa, and t h e on-board de tec tors ,

i s given i n a paper by O 'Er i en e t al. [1964].

p a r t i c u l a r i n t e r e s t a r e t h e 213 Anton Geiger counters viewing

e l ec t rons with energies Ee 2 40 keV, or ien ted a t 90" ( t rapped

p a r t i c l e s ) and 180" (p rec ip i t a t ing p a r t i c l e s ) with respec t t o

t h e magnetic f i e l d , i n t h e northern hemisphere, and having f i e l d s

of view and geometric f ac to r s of 26" diameter and (0.6 x

cm s t e r , and 86" diameter and ( 5 x cm s t e r , respec t ive ly

(O'Brien e t al. [19641).

The de tec tors of

2 2

O f t h e severa l modes of operation ava i l ab le i n Injun 3

(only one such mode may operate f o r any epoch of observat ions)

only two were capable of resolving times of l e s s than one second,

which i s a necessary r e q u i s i t e i n t he i d e n t i f i c a t i o n of microbursts.

These were modes 1 and 4, which provide de tec tor readings f o r both

t h e 90" and 180" de tec tors , every 1/4 second and 1/16 second,

6

respect ively.

were obtained i n mode 1, with only nine passes (each pass of

t y p i c a l duration

t i on .

Most of t h e da ta f o r t h e l i f e t i m e of t h e s a t e l l i t e

10 minutes) ava i lab le employing mode 4 opera-

7

THE IDENTIFICATION OF ELECTRON MICROBURSTS

A l l n ine passes ava i lab le i n t h e mode 4 operation of t h e

s a t e l l i t e were examined i n d e t a i l . It w a s found t h a t only two

passes, occurring respec t ive ly on January 17 and January 24, 1963

contained extended per iods of fast f luc tua t ions i n both t h e 90"

and 180" Ee 2 40 keV detectors .

f l uc tua t ions with time sca les of the order of severa l seconds,

examples of t h i s type of var ia t ions having been given i n Figure 6

o f O'Brien 's paper [1964]. In addition, f a s t e r va r i a t ions (< 1

second) were a l so seen i n fewer number. The pass of January 17

revealed a l a r g e number of examples of va r i a t ions of <1 second,

and severa l t y p i c a l cases of t h i s type of f luc tua t ion a r e presented

i n Figure 1. It can be seen tha t t he re a r e i s o l a t e d bu r s t s , as

we l l as many occurring i n t r a i n s ( o r i n t h e terminology of

Anderson and Milton, [1964] combs) present i n these data. Visu-

a l l y , t hese events have many of t he same c h a r a c t e r i s t i c s as t h e

balloon-observed bremsstrahlung x-ray microbursts (both symmetric

and asymmetric) as discussed by Anderson and Milton [1964],

Edwards e t al. [1966], Venkatesan e t al. [1967], Oliven and Gurnett

[1967al, and severa l o ther groups.

The pass on January 24 contained

Peak f luxes of t h e order of

8

5 -1 -1 J (E 2 40 keV) - 3 x 10 p a r t i c l e s sec s t e r above t h e

background levels f o r t h e 180" detector were commonly encountered

f o r t h i s type of event.

e

The fast time f luc tua t ions were seen e i t h e r by t h e 180"

detec tor , alone, o r by both t h e 180" and t h e 90" detectors . On

no occasion was a fast f luc tua t ion observed i n t h e 90" detector ,

alone. The absolute change i n p a r t i c l e f l u x observed by t h e 90"

de tec to r w a s t y p i c a l l y comparable, or somewhat l e s s (by a f a c t o r

of - 2) than t h a t observed by t h e 180" de tec to r . The background

f l u x upon which t h e f a s t changes were superposed was t y p i c a l l y

considerably g rea t e r i n the case o f t h e 90" (trapped) de t ec to r

than i n t h e case of t h e 180" ( p r e c i p i t a t i n g ) de tec tor , an order

of magnitude d i f f e rence being commonly observed (Parthasarathy,

e t al. [ 19661 ). Consequently, the s t a t i s t i c a l f l uc tua t ions

i n t h e background rate of t h e 90" de tec to r were w 3 times

those i n t h e 180" r a t e , and thus, on many occasions t h e

microbursts i n t h e 90" detector would b e masked by t h e l a r g e

s t a t i s t i c a l f l u c t u a t i o n s i n the counting r a t e .

9

The obvious question a r i s e s as t o whether t h e f a s t f luc tua-

t i o n s observed by Injun 3 were due t o temporal, or s p a t i a l

c h a r a c t e r i s t i c s of t h e e lec t ron p rec ip i t a t ion phenomenon.

The s a t e l l i t e w a s moving with a ve loc i ty of approximately 0.5 km

p e r 1/16 second, and consequently loca l i zed e lec t ron p r e c i p i t a t i o n

of a s p a t i a l s ca l e of approximately 4 km might masquerade i n t h e

da t a as "microbursts". The evidence aga ins t such an explanation

i s as follows:

1. To explain t h e f a s t r i s e , and slow f a l l times of many

of t he e lec t ron f l u x changes, the e lec t ron p r e c i p i t a t i o n would

have t o exhib i t an asymmetric d i s t r ibu t ion with respect t o s p a t i a l

co-ordinates; t h e d i s t r i b u t i o n almost c e r t a i n l y being under geo-

magnetic control . I n contradict ion t o t h i s hypothesis i s the

f a c t t h a t , while t h e s a t e l l i t e was moving from north t o south

during t h e January 24 p rec ip i t a t ion event, and south t o nor th

during t h a t of January 17, t h e electron f luc tua t ions observed on

both occasions exhibi ted a predominance of f a s t r i s e times, and

slow decay times.

2. A common c h a r a c t e r i s t i c of t h e x-ray microburst pheno-

menon i s t h e occurrence of "comb events' ' (Anderson and Milton [19643;

Venkatesan e t al. [1967] ) containing many indiv idua l microbursts.

10

Since a balloon detector i s s ta t ionary , and i n t e g r a t e s over a

l a r g e s p a t i a l extent (-- 100 km scale s i z e ) at the bremsstrahlung

production l e v e l , these observations imply the exis tence of

r e p e t i t i v e temporal f l uc tua t ions in e lec t ron p rec ip i t a t ion .

s p a t i a l extent of t hese e lec t ron p rec ip i t a t ion events has been

shown t o be 2 80 km (Parks, [1967]), consequently, a s a t e l l i t e such

a s Injun 3 would have t o observe "comb events" l a s t i n g of order 19

seconds before leaving the region of e l ec t ron p rec ip i t a t ion . Figure 2

p re sen t s ''comb events" seen by Injun 3.

balloon-borne de tec tors bear a s t r i k i n g s i m i l a r i t y of duration

and r e p e t i t i o n c h a r a c t e r i s t i c s (e. g., Anderson [ 19651 ) t o t hese

s a t e l l i t e observed combs.

In jun 3 t r a v e l l e d - 25 km, hence the simplest hypothesis, by

f a r , i s t h a t Injun 3 was observing temporal f l uc tua t ions as seen

commonly by balloon-borne equipment.

i n terms of a mult i -sheet s p a t i a l s t r u c t u r e could be advanced;

however, it would r equ i r e a number of d r a s t i c ad hoc assumptions

t o explain t h e s i m i l a r i t y t o t h e known temporal va r i a t ions observed

previously .

The

sombs observed by

During t h e per iod of time i n question,

An explanation of Figure 2

3. Paper 1 (Venkatesan e t al. [ 19671 ) has shown t h a t

-2 -1 peak photon f luxes of - 20 photons cm sec

are commonly observed by balloon-borne de tec tors .

f o r hv > 60 keV

An e lec t ron

11

5 -1 elec t rons cm-2 see p r e c i p i t a t i o n of order 3-5 X 10

required t o produce such photon f luxes. Typical shor t time-

sca l e counting r a t e f luc tua t ions (% 0.5 sec) observed by Injun

3 imply peak e lec t ron f luxes of 2 3 x 10 e lec t rons see-'

sterad-' i s

5

s t e r - l f o r E

6 Short t ime-scale f luc tua t ions having peak fluxes up t o - 4 x 10

e lec t rons cm sec s t e r (near de tec tor s a tu ra t ion ) are

observed, bu t i n fewer number.

2 40 keV i n t h e 180" detec tor (p rec ip i t a t ing ) . e

-2 -1 -1

On the b a s i s of t he foregoing points , we conclude t h a t

t h e temporal f l uc tua t ions of time sca l e 2 0.5 seconds commonly

observed by Injun 3 are due to temporal changes i n e lec t ron

p r e c i p i t a t i o n such as in fe r r ed previously from balloon-borne

observations. We s h a l l therefore refer t o these f luc tua t ions

as "electron microbur s t s " . A t y p i c a l asymmetric burs t observed by the s a t e l l i t e w a s

s tud ied i n an attempt t o compare it with t h e balloon observations.

Counting r a t e s f o r average samples of both asymmetric and sym-

met r ic x-ray microbursts were in tegra ted over 60 mil l isecond

per iods. I n t h e case of t h e asymmetric bu r s t t h i s w a s performed

us ing t h e approximate representat ion fo r t he asymmetric bu r s t of

12

dt , with t i n mill iseconds. These -t/30) ,-t/200

0

i n t eg ra t ions , equivalent t o the 1/16 second accumulation time

of t h e s a t e l l i t e detectors , were begun at t = 0, -10, -20, . .. and -50 milliseconds i n order t o explore t h e consequences of

0

non-synchronous sampling of t h e electron pu l se by t h e s a t e l l i t e

de tec tor . The most advantageous f i t of t h e s e s ix s e t s of both

symmetric and asymmetric x-ray pulses t o an asymmetric e l ec t ron

pulse observed by Injun 3 i s presented i n Figure 3.

x-ray microburst measurement i s seen t o be i n good agreement with

t h e asymmetric e l ec t ron burst viewed by t h e s a t e l l i t e , and t h e r e f o r e

shows t h a t asymmetric e l ec t ron microbursts seen by t h e s a t e l l i t e

correspond, i n form, t o the asymmetric bursts seen by Venkatesan,

e t al. [1967] and cannot be accounted for by t h e sampling of a

symmetric b u r s t . A similar analysis of symmetric b u r s t s viewed

by both s a t e l l i t e and balloons l ikewise exh ib i t exce l l en t agree-

ment. We, therefore , conclude t h a t both t h e symmetric microburst

(as seen by u s and Anderson and Milton, [1964]), and t h e asymmetric

microburst (Edwards, e t al. [1966], and Venkatesan, e t al. [19671)

were observed by t h e Injun 3 sa t e l l i t e , and represent v a r i a n t s

of t h e one microburst phenomenon.

The asymmetric

STATISTICAL STUDY OF OCCURRENCE

Ac t iv i ty similar t o t h a t present i n mode 4 can be found

i n mode 1, i n which de tec tors sample f o r 1/4 second and de tec tor

readings a r e given a t 1/4 se- Land in t e rva l s . Most of t h e da t a

from the e n t i r e l i f e t i m e of t h e s a t e l l i t e were accumulated i n

t h i s mode of operation.

of t h e e lec t ron p r e c i p i t a t i o n event, and t h e sampling per iods of

t he s a t e l l i t e , t h e microbursts w i l l appear i n mode 1 e i t h e r as

an observed m a x i m u m value and one o r two po in t s marking t h e

decay, o r as a v a r i e t y of forms i n which one po in t i nd ica t e s

t h e lead ing edge, one po in t t h e m a x i m u m , and one o r two po in t s

t h e decay phase. I n all cases an ind iv idua l b u r s t r i s i n g out

of t h e background contains no more than t h r e e t o four def ining

p o i n t s (1/4 second p o i n t s ) .

Figures 4 and 5.

Depending upon t h e r e l a t i v e phasing

Several such examples a r e shown i n

A s t a t i s t i c a l study was undertaken t o inves t iga t e the

frequency of occurrence and any poss ib le reg iona l r e s t r i c t i o n s

upon t h e appearance of t hese microburst events. A t f i r s t , f i f t y

random samples of da t a from s a t e l l i t e passes of var ious L values

and l o c a l times were invest igated. It became apparent t h a t most

14

of t h e e lec t ron microbursts and groups of b u r s t s were occurring

during t h e l o c a l hours from 04:30 - 1 2 ~ 3 0 magnetic l o c a l time and L

values from 6.0 t o 8.5, the same regions i n which balloon-observed

x-ray microbursts most commonly appear.

Subsequently, de tec tor da ta from t h e e n t i r e l i f e t i m e of

the s a t e l l i t e were u t i l i z e d t o inves t iga te t h e occurrence of e lec t ron

microbursts. Passes covering a l l 24 hours i n magnetic l o c a l time and

L values from 2-11 were invest igated. All samples were chosen t o

be 8 seconds i n length. Thus, fo r example, a pass which remained

within a given l o c a l time and L range f o r 16 seconds was con-

s idered t o be two samples each 8 seconds i n length.

L-magnetic l o c a l time combination ( the re being a t o t a l of 10 x 24,

or 240 combinations) at l e a s t 10 such 8-second samples were

inves t iga t ed f o r t h e presence of microbursts. Whenever possible ,

10 d i f f e r e n t passes were used f o r each L value - magnetic l o c a l

t ime combination.

t h i s requirement of 10 samples not met.

For every

Jn only a few o f t h e higher L regions w a s

P a r t i c u l a r care was taken t o assure t h a t t hese va r i a t ions

were indeed phys ica l phenomena r a the r than noise da t a or b i t

e r r o r s i n t h e te lemetry transmission. Some of t h e procedures

followed have been previously described (Appendix 3, 0 'Brien [ 19641 ).

In addition, all changes i n the flux which were represented by

only one i s o l a t e d 1/4 second sample above t h e background l e v e l

were not considered t o be su f f i c i en t t o ind ica t e t h e presence

of microbursts. A l l counting r a t e increments of I; 2 JN of t h e

background counting l e v e l were automatically re jec ted . I n most

cases , samples which contained i d e n t i f i a b l e microbursts had

many such "subliminal" events. Additionally, f l u x changes which

were classed as microbursts seldom occurred alone within an

8-second sample. Only about 11% of t h e t o t a l 8-second samples

i d e n t i f i e d as containing any microbursts contained 5 3 s ing le

i d e n t i f i a b l e events. Many of these sec t ions of data were simul-

taneously received by two separated t racking s t a t ions , again

confirming t h a t t hese microbursts were indeed t rue occurrences.

It should be noted t h a t by imposing these s t r i c t se lec t ion

c r i t e r i a upon the i d e n t i f i c a t i o n of e lec t ron microbursts, i t

i s q u i t e poss ib le t h a t some genuine events have been re jec ted .

Thus, t h e s t a t i s t i c s which follow only represent events which

c l e a r l y qua l i fy as microbursts. The percentages of microbursts

occurrence would undoubtably be higher than those s t a t e d i f one

were t o employ l e s s r e s t r i c t i v e se lec t ion c r i t e r i a .

More than 2400 samples were inves t iga ted t o determine t h e

poss ib l e presence of microbursts. This study revealed t h a t e lec-

16

6 5 L <, 8.5 (corresponding t o 66" 5 invar ian t l a t i t u d e <, 70")

and magnetic l o c a l times of 4:30 <,magnetic l o c a l t i m e

with f a r fewer cases occurring i n surrounding regions. These

results a re summarized i n Table 1 and Figure 6. It can be seen

t h a t t he re a r e severa l notable exceptions t o t h i s ru l e , namely,

t he rare occurrence of microbursts i n the l o c a l night hours

within t h e 6 - 8.5 region of L.

represent t he exception r a t h e r than t h e rule.

l2:3O,

These are r a r e events and

The region of l a r g e s t occurrence frequency of e lec t ron

microbursts was fu r the r invest igated t o determine whether any

s ign i f i can t p a t t e r n of p rec ip i t a t ion could be found within t h i s

region.

of one hour (magnetic l o c a l time) by one-half L.

inves t iga ted were all taken t o be 8 seconds i n length, and each

pass w a s permit ted t o contr ibute only one such sample t o each

magnetic l o c a l time - L combination. Data from t h e e n t i r e l i f e -

time of t h e s a t e l l i t e were employed. These results, as seen i n

Figure 7, d i sp lay no d i s t i n c t i v e p a t t e r n of a c t i v i t y within t h e

region of i n t e r e s t .

Sample blocks were decreased i n s i z e t o encompass a region

The samples

An inves t iga t ion i n t o t h e poss ib le dependence of e lec t ron

microburst occurrence upon magnetic a c t i v i t y (as represented i n

l a r g e K values) revealed no c lear cor re la t ion . Within the region

of maximum occurrence as s t a t e d above, t he re appears t o be no

dependence of percentage occurrence upon t h e K value. Only a

very s l i g h t dependence upon K outside t h i s region i n t h e sur-

rounding times and L values i s found. The tendency f o r higher

percentage occurrence i n these regions during per iods of high

K

these regions and hence t h e s m a l l t o t a l number of such micro-

burs t s , the s t a t i s t i c s a r e inadequate t o e s t ab l i sh such a

dependence. I n addition, a study revealed t h a t t he re was no

s i g n i f i c a n t dependence of e lectron microburst occurrence upon

s a t e l l i t e a l t i t u d e .

P

P

P

(> 3) i s not iceable , bu t because of t he s m a l l occurrences i n P

18

SUMMARY AND CONCLUSIONS

The i d e n t i f i c a t i o n of electron microbursts, t h e parent

e lec t ron p r e c i p i t a t i o n phenomenon responsible f o r the production

of bremsstrahlung x-ray microbursts, has been herein establ ished.

The cha rac t e r i s t i c times of these electron microbursts, as

seen i n the E

ha l f i n t e n s i t y points . Both symmetric and asymmetric ( f a s t r i s e

t i m e , slower decay time) time p r o f i l e s were observed. Many

e lec t ron microbursts appeared i n t r a ins , o r combs, as do the

daughter x-ray microbursts. No cha rac t e r i s t i c p e r i o d i c i t y has

been found, t o date, between these separate e lec t ron microbursts.

2 40 keV geiger tube were about 1/4 second at e

Typical peak f luxes above background of J (E 2 40 keV) e 5 -2 -1 - 3 x 10 p a r t i c l e s cm sec sterad-' for t he 180" detector

were commonly encountered f o r these microbursts.

microbursts observed at balloon a l t i t u d e have peak f luxes of

> 60 keV. The electron - 20 counts cm sec f o r Ex-ray

f l u x necessary t o produce t h i s flux of x-rays corresponds wel l

with the a fores ta ted s a t e l l i t e measurements.

Ty-picaL x-ray

-2 -1

The region of occurrence of e lec t ron microbursts i s found

t o be i n good agreement with t h e regions i n which x-ray microbursts

most commonly occur. This region has been es tab l i shed as

6 2 L I 8.5 and 4:30 I magnetic l oca l time I 12:30, from more

than 2400 cases s tudied i n 240 L, magnetic l o c a l time blocks.

S ign i f i can t microburst a c t i v i t y i s observed t o extend t o e a r l i e r

l o c a l times, v iz , 0200 l o c a l t ime.

The t h i r d paper, (Oliven and Gurnett [1967b]) w i l l explore

a connection found t o e x i s t between these e l ec t ron microbursts and

the V U phenomenon known a s chorus. It i s t h i s connection between

the au ro ra l zone e l ec t ron microbursts he re in reported, t h e i r r e s u l t a n t

au ro ra l zone x-ray bremsstrahlung microbursts, and the VLF' chorus

which promises t o give us the g rea t e s t i n s igh t ye t i n t o the plasma

i n s t a b i l i t i e s , acce le ra t ion mechanisms, and regions of occurrence

of these i n s t a b i l i t i e s i n the magnetosphere.

20

ACKNOWLEDGEMENTS

The authors wish t o express t h e i r thanks f o r t he advice

and support o f fe red i n t h i s pro jec t by Professor J. A. Van Allen

of t h e Universi ty of Iowa.

of Iowa by Off ice of Naval Research Contract 1509(06), at the

Universi ty of Calgary by National Research Council Contract NRC

A-3865, and a t t h e Southwest Center for Advanced Studies by

NASA Contract NAS-r-198.

This work w a s supported a t the Universi ty

21

Table 1

Occurrence i n L and Magnetic Local T ime of S a t e l l i t e Observed Microbursts Seen i n 180" Detector Only and i n Both 90"

and 180" Detectors of Injun 3

( i n percent of t h e t o t a l number of 8-second samples s tudied f o r t h e s p e c i f i c L and Magnetic Local T i m e ; unless

samples considered f o r each L-Magnetic Local Time combination)

otherwise ind ica t ed a t l e a s t t e n 8-second

Magnet i c Local T i m e

00 01 02 03 04 05 06 07

09 10 11 12 13 1 4 1 5 16 17 18 19 20 2 1 22 23

oa

2 3 4 5

0 0 0 10B 0 0 0 10B 0 0 0 1OA 0 0 0 0 0 0 0 0 0 0 0 20c 0 0 0 0 0 0 0 10A o o o 1 3 ~ 0 0 0 1gc o o o 31c 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

L Value

6

0 0

1OA 0

1 9 C 1 5 C 28c 43c 30c 29c 50c 38c

0 8A 0 0 0 0 0 0 0 0

10A 0

7

0 0

10A 25A 38c 26c 32c 50c 36c 28c 39c 18c 44c

8B 0 0 0 0 0

10A 0 0

10A 0

8

0 0 0

12A 2 2c 24C 39c 4 1 C 37c 36c 32c 50c 3OA

8A 0 0 0 0 0 0

10A 0

1oc 0

9

0 0 0

1OA 30A 0

20A 10A 106 20c 0

30c 0 0

1OA OCY 0 0 0 OCU OW 0 0 0

10

0 0 0 0 0

30c 10B

0 10A 20A 1OA 20A 0

10A 0 OCY OGJ 0 0 OCY OCY 0 0 0

11

0 0 10B

0 0

10B 0

10a 20Aa 0

20cCY 10a 0 0 OCY OCY OCY OW OCY OCY OCY 0 0 0

A = seen only i n 180" de tec tor . B = seen by both 90" and 180" de tec tor . C = both A and B occur wi th in the 8-second i n t e r v a l . CY = l ess than 10 samples ava i lab le .

22

REFERENCES

Anderson, K. A., "Balloon Measurements of X-Rays i n t h e Auroral Zone", Auroral Phenomena, ed. M. W a l t (Stanford Universi ty Press, Stanford, CaLifornia, 1965).

Anderson, K. A., and D. W. Milton, "Balloon Observations of X-Rays i n t he Auroral Zone. 3. High Time Resolution Studies ," J. Geophys. Res. - 69, 4457-4479 (1964).

Edwards, P. J., K. G. McCracken, M. Steinbock, M. Oliven, and D. Venkatesan, "Impulsive Prec ip i ta t ion of Electrons i n t h e Auroral Zone", Trans Am. Geophys. Union - 47, 139 (1966).

0 'Brien, B. J., "High Lat i tude Geophysical Studies with S a t e l l i t e Injun 3. 3. Prec ip i ta t ion of Electrons i n t o t h e Atmosphere", J. Geophys. Res. - 69, 13-43 (1964).

Oliven, M. N . , and D. A. Gurnett, " S t a t i s t i c a l Studies Concerning t h e Connection between 40 keV Electron Microbursts and VLF Chorus h i s s i o n s " , Trans. Am. Geophys. Union - 48, 74 (1967a).

Oliven, M. N., and D. A. Gurnett, "Microburst Phenomena 3. An Association Between Electron Microbursts and VLF Chorus," U. of Iowa 67-39 (submitted t o J. Geophys. Res. f o r publ icat ion) , (196711).

Microbursts", J. Geophys. Res. - 72, 215-226 (1967). Parks, G. K., "Spa t ia l Charac te r i s t ics of Auroral-Zone X-Ray

Parthasarathy, R. , F. T . Berkey, and D. Venkatesan, "Auroral Zone Electron Flux and I t s Relation t o Broadbeam Radiowave Absorption", Planet . Space Sei . - 14, 65-83 (1966).

Venkatesan, D. , M. N. Oliven, P. J. Edwards, K. G. McCracken, and M. Steinbock, "Microburst Phenomena. 1. Auroral Zone X-Rays", U. of Iowa 67-18 (submitted t o J. Geophys. Res. for publicat ion. )

23

FIGURF: CAPTIONS

Figure 1 Typical appearance of e lectron microbursts i n t h e

Injun 3 electron (Ee 2 40 keV) de tec to r s at s ixteen

samples pe r second.

Figure 2 Trains o r combs of e lectron microbursts seen by Injun

3 with similar f ea tu res as those seen i n balloon x-ray

de tec to r s of Venkatesan, e t al. [19671 and Anderson

[ 196 51 . Figure 3 Comparison of t h e most advantageous f i t of balloon

x-ray da ta of Venkatesan, e t al. [19671 with a sample of a s a t e l l i t e asymmetric microburst . The

asymmetric s a t e l l i t e bu r s t i s compared with an asym-

metr ic x-ray b u r s t and a symmetric x-ray bu r s t .

da t a have been accumulated f o r 60 mil l isecond i n t e r -

vals. The s a t e l l i t e microburst i s ind ica t ed i n

Figure 2 with an a s t e r i s k .

X-ray

Figure 4 Typical microburst appearances i n four samples (set)-' operation of the s a t e l l i t e de t ec to r s .

Figure 5 Typical microburst appearance i n t h e four samples -1 ( sec ) operation of the s a t e l l i t e de t ec to r . I s o l a t e d

microbursts as w e l l a s some appearing i n t r a i n s (combs)

can be seen.

24

Figure 6 The d i s t r i b u t i o n of microburst occurrence i n inva r i an t

l a t i t u d e and magnetic local time of t h e s a t e l l i t e as

observed by Injun 3. Each sample block contains a t

l eas t 10 samples.

Figure 7 The occurrence i n invariant l a t i t u d e and magnetic local

time of t h e electron microbursts detected by Injun 3.

25

G 66 - 524

t INJUN 3 JAN. 17,1963

0825 U.T., ALTITUDE - 660 KM

L = 4.81 L = 4.92 LOCAL TIME 4:31 LOCAL TIME 4133

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I I

5 IO 15 20 TIME IN SECONDS

FIGURE 1

26

2.0

I .5

I -0

0.5

0

3 .O

2 .o

I .o

0

667-776 ( R - 1 )

INJUN 3 JAN.17, 1963 L = 6.60 L O C A L TIME = 5:02

ELECTRONS E, 1 4 0 K e V

180" 213 DETECTOR

*

il 90" 213 DETECTOR

1 I I I 0 I 2 3 4

TIME IN SECONDS

FIGURE 2

\

I :LL :o ........... rl

..? \. ............. ..: _1 - w

rr) o_

t ..........

$7: ....... i 4- r

I 1 I I

I

1 I I I I I 1 0 - 0 0 0 0 0

W In d- m N

SllNn AklVklll8klV

x n i d FIGURE 3

28

N N " W W W

5: In *

rc)

I Lo W

P

0

29

W

v) D 2 0 0 W

lnv)

I-

0

FIGURE 5

30 6 6 6 - 8 0 8

0 I

2 3 4 5 6 7 8 9

IO I I 12 13 14 15 16 17 18 19 20 21 22 23

1.5 2 .o I 1

1 3.0 4.0 5.0 8.0 I I I I I l l

INVARIANT LATITUDE

35O 40" 45O 50' 55O 60' 65' 70" 75O 80' I 1 1 1 1 1 I I ' I I ' 1 .... .. ......

.... . .. . . .. . ....... .... .. .. .. ..

. . . . . . . ... e.. . . . . . . . . . . . . . . 0% 0

o< ::I 10%

50% < I 60%

1- 1 1 i - 2 B s o - - 01

FIGURE 6

31 G 6 6 - 8 2 2

L = 6.0 6.5 7.0 7.5 8.0 8.5 INVARIANT ,= 6 6 O , 67O , 67.9' I 68.5O I 69.3O I 70"

L AT

W I I- -I

0 0 -I

- a

NUMBER INDICATES TOTAL NUMBER O F SAMPLES

0% 0

o< ::s 10%

10% < I I I I 20%

FIGURE 7

I 0 9 1 G l N A T I N G A C T I V I T Y fcorporate author)

University of Iowa Department of Physics and Astronomy

I 3 R E P O R T T I T L E

2 9 R E P O R T S E C U R I T Y C L A S S I F I C A T I O N

- UNCLASSIFIED Zh G R O U P

Microburst Phenomena 2. Auroral Zone Electrons

? a T O T A L NO. O F PAGES

31

t D E S C R I P T I V E NOTES (Type of report and inclustve da te s )

Progess August 1967 j AUTHORfS) (Las t name. f i rs t name. init ial)

76. NO. O F R E F S

9

Oliven, M. N., Venkatesan, D., McCracken, K. G.

C .

d .

R E P O R T D A T E

August 1967

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Nonr 1509 (06 ) b. P R O J E C T N O .

1 1 . S U P P L E M E N T A R Y NOTES 12. SPONSORING M I L I T A R Y A C T I V I T Y

Distribution of this document is unlimited.

Office of Naval Research

I 3 ABSTRACT

(See next page)

I D Z!% 1473 UNCLASSIFIED Security Classification

UNCLASSIFIED .I_---

I Security Classification 4.

K E Y WORDS -.

Microburst Phenomena

Microburst Phenomena 2. Auroral Zone Electrons

S a t e l l i t e Observed Auroral Zone P r e c i p i t a t i n g

Auroral Zone P rec ip i t a t ion

Electrons

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~

ABSTRACT

Observations made during a high-time r e so lu t ion mode

of operation of t h e Injun 3 sa te l l i t e have i d e n t i f i e d au ro ra l

zone e l ec t ron microbursts, t h e parent phenomenon of t h e of ten

observed bremsstrahlung x-ray microbursts. These electron

microbursts, detected i n the Ee 2 40 keV (180") ( p r e c i p i t a t i n g )

de t ec to r and l e s s f requent ly i n the Ee 2 40 keV (gO")(trapped)

de tec tor , have t i m e p r o f i l e s s imi la r t o those of both t h e sym-

me t r i c and asymmetric daughter x-ray microbursts and exh ib i t

t y p i c a l peak f luxes of Jo (Ee 2 40 keV) - 3 x 10

em sec sterad-' i n t h e 180" detector. S t a t i s t i c a l s tud ie s

of t h e s e sa t e l l i t e -obse rved microbursts r evea l t h e i r occurrence

predominantly i n t h e regions of 6 5 L s 8.5 and 4:3O 5 magnetic

l o c a l t i m e I l2:3O. Detai led s t a t i s t i c a l d i s t r i b u t i o n s of

microburst occurrence as functions of l o c a l time and L a r e

presented.

5 e l ec t rons

-2 -1