X-198* - NASA

. 8

U. of Iowa 67-18

MICROBURST P H E " A 1. Auroral Zone X-rays*

D. Venkatesan Department of Physics The U n i v e r s i t y of Calgary Calgary, Alberta, Canada

M. N. Oliven1 Department o f Physics and Astronomy

University of Iowa Iowa City, Iowa, USA

and

P. J. Edwards,2 K. G. McCracken2 and M. Steinbock 3

Southwest Center f o r Advanced Studies Dallas, Texas, USA

September 1967

* Work a t The U n i v e r s i t y of Calgary by National Research Council g r a n t A-3865, at t h e U n i v e r s i t y of Iowa by O f f i c e of Naval x-198* Research Contract 1509(06), and at t+Southwest Center f o r Advanced S t u d i e s by NASA Contract NAS

NASA Graduate Trainee

Current Address: Physics Department, University of Adelaide, Adelaide, South Australia

Address 1966-67: Physics Department, The U n i v e r s i t y of Calgary,

Calgary, Alberta, Canada

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ABSTRACT

High-time resolution x - r w equipment flown from Ft.

Churchill, Manitoba, Canada on August 11, 1965 provides e v i dence f o r species of a u r o r a l zone x-ray microbursts with an

asymmetric time p r o f i l e . These asymmetric microbursts are

- c h a r a c t e r i z e d by a r i s e of t h e form 1 e-t/TR where T~ i s about

30 milliseconds, and a decay of t h e form o f e-t/7n where T~ i s about 200 m i l l i s e c o n d s , and a t y p i c a l peak f l u x f o r t h e l a r g -

- e s t events of Jo (Ex-ray> 60 keV) 102 photons cm-* see-'at

. 10g/em2 An episode of t h e s e asymmetric b u r s t s was observed i n t h e

e a r l y morning hours ( a f t e r 4:30 l o c a l t h e ) and an episode of t h e more common symmetric microbursts began a f t e r 9 : 3 0 l o c a l time. The f a s t r i s e times, and t h e l a c k of d i s p e r s i o n ,> 10 milliseconds i n the x-ray bursts observed at different energies

i m p l i e s r e s t r i c t i o n s on t h e nature, and propagation of t h e parent electron microbursts.

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INTRODUCTION

Numerous r o c k e t and balloon studies i n t h e a u r o r a l zone have investigated the bremsstrahlung x-radiation produced by e l e c t r o n s impinging upon t h e upper atmosphere. With t h e a v a i l a b i l i t y of d a t a from charged p a r t i c l e d e t e c t o r s on p o l a r satell i t e s , measuring p r e c i p i t a t e d e l e c t r o n s , i t i s now p o s s i b l e t o i n v e s t i g a t e t h e behavior of both t h e parent electrons and t h e

. daughter b r em sst r ahlung x - r a y s

This study r e p o r t s on a p o r t i o n of t h e d a t a obtained during a series of seven b a l l o o n f l i g h t s made from F o r t C h u r c h i l l , Canada, of balloon-borne scintillator-photomultiplier x-ray detectors. The geographic c o o r d i n a t e s o f t h e b a l l o o n launch s i t e are 58.75"N

and 94.0g"W ( L = 8.66). The d a t a of i n t e r e s t h e r e i n , were o b t a i n e d

d u r i n g a b a l l o o n f l i g h t which commenced a t 0225 UT on August 11,

1965, during a p e r i o d of r e l a t i v e l y low geomagnetic a c t i v i t y .

The I n t e r n a t i o n a l P l a n e t a r y Index f o r geomagnetic a c t i v i t y , K P'

was 1 o r l e s s during t h e e n t i r e f l i g h t p e r i o d . 0 The b a l l o o n reached a c e i l i n g a l t i t u d e of 110,000 f e e t at

2315 l o c a l t i m e (0515 UT) and t h e r e a f t e r d r i f t e d i n a west-south-

w e s t e r l y d i r e c t i o n . It remained above 106,000 f e e t during t h e

p e r i o d of 1 5 hours, during which u s e f u l d a t a were acquired a t t h e

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telemetry receiving station. Table 1 gives the positions of the b a l l o o n during t h e p e r i o d of i n t e r e s t , namely 0700-1700 UT (0100-1100 LT) and t h e L coordinate corresponding t o t h e p o s i t i o n s . Also given, f o r comparison, are t h e L values f o r places from which other x-ray experiments have been flown before.

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INSTRUMENTATION

The d e t e c t i o n system, i l l u s t r a t e d i n Figure 1, c o n s i s t e d

of a 5 inch diameter, half inch thick NaI(T1) scintillation c r y s t a l , viewed by a Dumont 6364 p h o t o m u l t i p l i e r o f t h e same

diameter. The geometric f a c t o r of t h e d e t e c t o r f o r x-rays

i s o t r o p i c over t h e whole of t h e up2er hemisphere, was 476 cm2s t e r .

A l u c i t e l i g h t p i p e , of t h e same dimensions as t h e c r y s t a l , separated t h e c r y s t a l and t h e phototube by 2 cm, i n o r d e r t o minimize t h e e f f e c t s of photocathode non-uniformities upon t h e energy resolution of t h e system.

The p h o t o m u l t i p l i e r EKC was s u p p l i e d from a r e g u l a t e d DC c o n v e r t e r , which i n t u r n o b t a i n e d i t s power from a s e r i e s of mercury b a t t e r i e s . The whole h i g h v o l t a g e system w a s vacuum p o t t e d t o e l i m i n a t e t h e p o s s i b i l i t y of corona. A p-metal s h i e l d enclosed t h e photomultiplier dynode chain and t h e photocathode, and t h i s minimized the perturbation of t h e e l e c t r o n o r b i t s and hence t h e photomultiplier gain, by t h e e a r t h ' s magnetic f i e l d . The complete d e t e c t o r system was

observed t o y i e l d a r e s o l u t i o n of 50% FWHM f o r t h e 32 keV

x-ray from Cs137.

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