Recent Observations of Low-Energy Charged Particles in the Earth's Magnetosphere [chapter]

L. A. Frank
1968 Astrophysics and Space Science Library  
his research was sup Space Administration -of the Magnetosphere, Distribution of this document is unlimited. 2 Ai3STFWT Several recent observations of low-energy proton and e l e c t r o n i n t e n s i t i e s within t h e energy range -100 eV to 50 keV i n t h e e a r t h ' s r a d i a t i o n zones with a s e n s i t i v e array of e l e c t r o s t a t i c analyzers borne on t h e e a r t h -s a t e l l i t e OGO 3 during mid-1966 a r e summarized. Measurements of charged p a r t i c l e s
more » ... p a r t i c l e s of t h e e x t r a t e r r e s t r i a l r i n g current during a moderate geomagnetic storm, of t h e low-energy proton and e l e c t r o n d i s t r i b u t i o n s i n t h e v i c i n i t y of t h e midnight 'trapping boundary' near t h e magnetic equatorial. plane, and of upper limits for proton and i o n (100 eV < -< 50 keV) energy f l u x e s deep within t h e inner r a d i a t i o n zone a r e presented together with several introductory comments concerning t h e morphology of t h e omnidirectional i n t e n s i t i e s of energetic e l e c t r o n s (E > 40 keV, > 230 keV and > 1.6 MeV) a t t h e magnetic equator i n t h e outer r a d i a t i o n zone. E -Q ----3 INTRODUCTION A l a r g e c o l l e c t i o n of d e t a i l e d observations of energetic charged p a r t i c l e s within t h e v a s t region e n c i r c l i n g t h e e a r t h which i s dominated by t h e geomagnetic f i e l d has been acquired since t h e i n i t i a l discovery of intense fluxes of energetic protons and e l e c t r o n s i n t h e s e regions, t h e inner and outer r a d i a t i o n zones, with earths a t e l l i t e measurements i n 1958. observations of electrons E 2 10 keV and of protons E 2 100 key with instrumentation borne on earth-orbiting s a t e l l i t e s a t low and high a l t i t u d e s ,several hundred kilometers t o 30 have been published over t h e p a s t several years [ Van AJ-len, 1963; O'Brien, 1964a; Shabansky, 1965; Hess, 1965; Frank, 1965al. d i r e c t e d toward d e t a i l e d s t u d i e s of e l e c t r o n and proton i n t e n s i t i e s i n t h e above energy ranges within t h e e a r t h ' s r a d i a t i o n zones, lower energy charged p a r t i c l e s with energies E < 10 keV dominate t h e s e regions i n terms of energy d e n s i t i e s and number d e n s i t i e s . For example, t h e r a t i o s of t h e number d e n s i t i e s and energy d e n s i t i e s of t h e oftenobserved e l e c t r o n (E > 1 MeV) i n t e n s i t i e s i n t h e h e m t of t h e outer r a d i a t i o n zone near t h e magnetic equatorial plane a t 4.5 t o t h e corresponding number and energy d e n s i t i e s of e l e c t r o n s (E 5 10 keV) axe frequently -3 X 10 and 2 X 10-, respectively. Although t h e s e energetic Current reviews concerning t h e (%, e a r t h r a d i u s ) , Although t h e major f r a c t i o n of t h e l i t e r a t u r e has been Tu Tu -6 3 4 charged p a r t i c l e d i s t r i b u t i o n s have been and a r e p r e s e n t l y being thoroughly surveyed v i a i n s i t u measurements, t h e observations of lower energy charged p a r t i c l e s a r e l i m i t e d not through oversight of t h e importance of t h e i r d i s t r i b u t i o n s throughout t h e magnetosphere but due to t h e d i f f i c u l t i e s of designing instruments with s u f f i c i e n t s e n s i t i v i t y , energy thresholds and immunity from background responses a t t r i b u t a b l e to penetrating, energetic charged p a r t i c l e s and t o u l t r a v i o l e t r a d i a t i o n s . Recently observations of low-energy proton and e l e c t r o n i n t e n s i t i e s over t h e energy range extending from -1 0 0 eV -t o 50 keV have been obtained i n t h e e a r t h ' s r a d i a t i o n zones near t h e magnetic e q u a t o r i a l plane with a s e n s i t i v e a r r a y of e l e c t r o s t a t i c analyzers borne on t h e s a t e l l i t e OGO 3 [Frank 1967a, b , e; Frank and Swisher , 19671. r e s u l t s concerning t h e e x t r a t e r r e s t r i a l r i n g current, t h e possible existence of l a r g e energy f l u x e s of low-energy p o s i t i v e ions deep within t h e e a r t h ' s inner r a d i a t i o n zone, and t h e s t r u c t u r e of t h e "trapping" boundary a t -8 t h e midnight meridional plane together with s e v e r a l introductory comments concerning t h e d i s t r i b u t i o n s of energetic e l e c t r o n s (E > 40 keV) i n t h e outer r a d i a t i o n zone a r e presented herein as a swnmary of i n i t i a l , unique observations of t h e low-energy charged p a r t i c l e d i s t r i b u t i o n s within t h e e a r t h ' s r a d i a t i o n zones. These measurements and s e v e r a l unpublished, preliminary geocentric radial distance i n -5 SE3TFXA. L COMNENTS ON THE DISTRIBUTIONS --OF m R G E T I C ELECTRON INTENSITIES I N TKE OUTER RADLCITION ZOrJE ----A s w a s mentioned e a r l i e r t h e energetic eharged p a r t i c l e populations of t h e inner and outer r a d i a t i o n zones have been under i n t e n s i v e examination during t h e p a s t s e v e r a l years. Comprehensive surveys with Geiger-Mueller tubes, s c i n t i l l a t o r s , and s o l i d -s t a t e devices a r e available for regions near t h e magnetic e q u a t o r i a l plane throughout t h e r a d i a t i o n zones, at low a l t i t u d e s (-1000 km) from equator to poles, and for t h e e n t i r e inner r a d i a t i o n zone within t h e energy ranges E >lo0 keV for protons and E > 50 keV for e l e c t r o n s [cf McIlwain, 1963; Frank, 1965a, Frank, Van Allen and H i l l s , 1964; Davis, 1965; Freden, Blake and Paulikas , 1565; Gabbe and Brown, 1966; Vette , 19661. organization of a massive abundance of measurements i n t o a coherent , concise d e s c r i p t i o n of t h e complex speciesand energy-dependent morphology of t h e e a r t h ' s r a d i a t i o n zones. An example of t h e morphology of energetic e l e c t r o n i n t e n s i t i e s i n t h e outer r a d i a t i o n zone at t h e magnetic equator i s shown i n Figures 1 and 2 [Owens and Frank, 19671 which summarize four months of observations of e l e c t r o n (E > 40 keV, -> 230 keV and -> 1.6 MeV) i n t e n s i t i e s with an a r r a y OS Geiger-Mueller tubes borne on Explorer 1 4 during t h e period beginning a t launch on 2 October 1962 through January 1963 ( r e f e r to Frank, ' u * One of t h e p r i n c i p a l observational t a s k s i s t h e 6 V a n Allen and H i l l s [1964] f o r d e t a i l s concerning instrumentation and o r b i t a l parameters). O f immediate i n t e r e s t a r e t h e o r b i t a l i n c l i n a t i o n 3 3 O , i n i t i a l apogee geocentric r a d i a l distance 16.4 RE, i n i t i a l perigee a l t i t u d e 281 kilometers and period 36.4 hours. swnmarize contours of constant omnidirectional i n t e n s i t i e s of e l e c t r o n s with energies exceeding t h e above detector thresholds as functions of L and time at t h e magnetic equator. The d a i l y sum K included a t t h e bottom of each of t h e s e grayhs i n order to i n d i c a t e periods of r e l a t i v e magnetic a c t i v i t y . Several f e a t u r e s of t h e morphology of t h e s e energetic e l e c t r o n i n t e n s i t i e s are immediately evident upon perusal of t h e s e graphic summaries: (1) t h e apparent complexity of t h e temporal v a r i a t i o n s and t h e dependence of t h e gross temporal behavior upon e l e c t r o n energy, (2) t h e increases of e l e c t r o n i n t e n s i t i e s following t h e onset of a period of r e l a t i v e magnetic disturbance (high M ) with peak values of omnidirectional i n t e n s i t i e s occurring with increasing time delay a f t e r onset of t h e disturbance with increasing e l e c t r o n energy ( r e f e r t o t h e l a t e December 1962 enhancements f o r a c l e a r example), (3) t h e ebbing of t h e i n t e n s i t i e s of electrons (E > 230 keV and -> 1.6 MeV) during a sustained period of r e l a t i v e magnetic quiescence beginning 23 December through 9 January, (4) t h e sudden decreases of e l e c t r o n ( i n p a r t i c u l a r E > 1.6 MeV) i n t e n s i t i e s concurrent with t h e onset of magnetic disturbance and (5) t h e p e r s i s t e n t occurrence of peak e l e c t r o n (E > 230 keV and -> 1.6 MeV) Figures 1 and 2 P' =P9 has been P ----7 omnidirectional i n t e n s i t i e s a t L -, , , 4.5 to 5.5 and of peak e l e c t r o n (E -> 40 keV) i n t e n s i t i e s over a r e l a t i v e l y broad L-value range L --4 to 7. ref Frank, Van Allen and H i l l s , 1964; Frank, 1965a, b , e] but t h i s presentation provides t h e most comprehensive panorama of t h e Explorer 1 4 measurements p r e s e n t l y a v a i l a b l e and o f f e r s r a p i d i n s i g h t i n t o t h e complex temporal behavior of outer zone e l e c t r o n i n t e n s i t i e s . several of t h e temporal v a r i a t i o n s i n i n t e n s i t i e s evident i n Figures 1 and 2 may be at l e a s t i n p a r t understood without knowledge of t h e dominant lower energy p a r t i c l e s (for example, inward r a d i a l d i f f u s i o n [Frank, 1965bl and adiabatic r e l o c a t i o n and a c c e l e r a t i o n due to an Many of t h e s e observations have been reported previously Although enhanced e x t r a t e r r e s t r i a l r i n g current during magnetic storms [McIlwain, 1966]), heavily upon a thorough a n a l y s i s of t h e low-energy p a r t i c l e populations throughout t h e s e regions. of t h e s e low-energy charged p a r t i c l e i n t e n s i t i e s a r e discussed i n t h e following summaries. a complete t h e o r e t i c a l i n v e s t i g a t i o n depends Several i n i t i a l observations 8 UPPER LIMITS FOR TKF: ENERGY FLUXES _. OF LOW-ENERGY --PROTONS AND IONS I N THE INNER RADIATION -ZONE -----Early low-altitude surveys of t h e energy fluxes of low-energy protons or ions within t h e energy range extending from 500 eV to 1 MeV with an a r r a y of CdS c r y s t a l s borne on t h e low-altitude s a t e l l i t e Injun 1 provided t h e remarkable r e s u l t t h a t l a r g e energy f l u x e s , -50 ergs (em -see-sr) zone at, 1.25 5 L 5 1.70 [Freeman, 19621. i n a B-L coordinate system i s included i n Figure 3 . These measurements implied a strong a c c e l e r a t i o n mechanism f o r protons or p o s i t i v e ions within a region which i s deeply imbedded within t h e e a r t h ' s magnetosphere, and hence presumably r e l a t i v e l y remote from t h e outer magnetosphere where t h e s o l a r wind i s an able energy source f o r charged p a r t i c l e acceleration. An order-of-magnitude estimate of t h e source strength necessary to maintain t h e s e energy fluxes may be easiLy obtained by assuming t h a t (1) t h e charged p a r t i c l e s a r e -1 keV protons, (2) t h e f l u x e s a r e confined to a volume bounded by t h e L-shells 1.25 and 1.70 and a spherical surface a t -1000 km a l t i t u d e above t h e e a r t h ' s 2 -1 , were present deep within t h e inner r a d i a t i o n A summary of t h e s e observations surface and ( 3 ) t h e p r i n c i p a l l o s s mechanism i s charge-exchange (although Coulomb s c a t t e r i n g i s a l o s s mechanism of s i m i l a r magnitude for protons i n t h i s energy range) i n t h e ambient t e r r e s t r i a l exosphere [Liemohn, 19611. This estimate of t h e power required to maintain t h e s e energy Y fluxes i s Sol8 ergs(sec)-' which i s comparable to t h e average source strength necessary f o r e l e c t r o n p r e c i p i t a t i o n i n t o t h e auroral regions [cf Frank and Van Allen, 1964; O'Brien, 1964bl. Recently we have searched f o r t h e s e energy f l u x e s of low-energy protons or ions with t h e array of s e n s i t i v e e l e c t r o s t a t i c analyzers which was included on t h e OGO 3 spacecraft (launch date, 7 June 1966; i n i t i a l apogee and perigee 128,500 km and 6,700 km geocentric r a d i a l distances; i n c l i n a t i o n , 31") and were unable to discern any signature of l a r g e p o s i t i v e or negative ion i n t e n s i t i e s within t h e energy range 100 eV < -< 50 keV i n t h e region of t h e inner r a d i a t i o n zone surveyed with Injun 1 [Frank and Swisher, 19671. I n f a c t t h e responses of t h e e l e c t r o s t a t i c analyzers were m o d u l a t e d by t h e v a r i a t i o n s i n curvedp l a t e p o t e n t i a l and a r e background counting r a t e s a t t r i b u t a b l e to penetrating, energetic protons and electrons. Hence only upper l i m i t s f o r t h e energy fluxes of protons or p o s i t i v e ions (100 eV 4 E 4 50 keV) were generously determined by assuming t h a t t h e s e background responses were wholly due to low-energy ion fluxes. The corresponding upper l i m i t s f o r t h e s e energy f l u x e s a r e summarized i n Figure 3 i n order to E -& --Qf a c i l i t a t e comparison of t h e two r e s u l t s . i n t h e present discussion were obtained at l o c a l p i t c h angles a = 90". A cursory examination of Figure 3 shows t h a t t h e O W 3 upper l i m i t s f o r energy fluxes of protons or p o s i t i v e ions (100 eV < -< 50 keV) a r e l e s s than t h e Injun 1 measurements by f a c t o r s -10 to 100. E limits f o r negative-ion (100 eV < -< 50 keV) energy f l u x e s as derived -Q -A l l measurements reported E -Q -Upper 10 from t h e responses of companion e l e c t r o n e l e c t r o s t a t i c analyzers a r e within a f a c t o r -2 equal to t h e simultaneously measured upper l i m i t s f o r positive-ion energy fluxes reported here. It i s possible t h a t t h e energy f l u x e s observed with Injun 1 a r e predominantly shared by protons with energies exceeding t h e energy range of t h e OGO 3 e l e c t r o s t a t i c analyzers, t h e energy range 50 keV < E < 1 MeV. I n order to i n v e s t i g a t e t h i s p o s s i b i l i t y we have examined Injun 4 (launch, 21 November 1964; ' y i n i t i a l apogee and perigee a l t i t u d e s , 2502 km and 527 km; i n c l i n a t i o n , 81") observations of energy f l u x e s of protons within t h e energy range extending from -30 keV to 4.2 MeV and have summarized t h e r e s u l t s i n Table I. -50 ergs (em -see-sr) Again t h e r e i s no evidence of a l a r g e energy flux, 2 -1 , of protons i n t h i s energy range. It i s possible t h a t (1) t h e l a r g e energy f l u x e s of p o s i t i v e ions a r e present only near l o c a l midnight where a l l Injun 1 measurements were obtained [Freeman, 19621 and hence t h a t t h e OGO 3 observations during l o c a l day simply imply t h a t t h e s e low-energy p o s i t i v e ions a r e confined t o a r e s t r i c t e d local-time r a g e centered near l o c a l midnight ( t h e Injun 4 measurements were a l s o acquired near l o c a l midnight) or (2) t h e s e low-energy p o s i t i v e ions have disappeared via an occasional l o s s mechanism over t h e period extending from m i d -1961 (Injun 1) to mid-1966 (OGO 3 ) . findings of f i r m upper limits for t h e s e energy f l u x e s which a r e l e s s However, on t h e basis of t h e above 11 L, e a r t h r a d i i 1.35
doi:10.1007/978-94-010-3467-8_7 fatcat:lpa5i2skuvh37e3u5kkynodq6y