I N S T I T U T D ' A E R O N O M I E S P A T I A L E D E B E L G I Q U E 3 - A v e n u e C i r c u l a i r e
B - 1 1 8 0 B R U X E L L E S
A E R O N O M I C A A C T A
A - N° 2 6 4 - 1 9 8 3
The interpretation of ATS 6 protons and electrons observations within the frame of the E 3 H electric
field model
by
J. LEMAIRE
B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R O N O M I E
3 - Ringlaan
FOREWORD
This paper describes the results obtained by the author when he was visiting the Physics Department of the University of California, San Diego, at La Jolla. It will be published in the Bulletin de l'Académie Royale de Belgique, Classe des Sciences.
AVANT-PROPOS
Ce travail présente les résultats obtenus par l'auteur lors de son séjour au Physics Department of the University of California, San Diego, La Jolla. Les résultats seront publiés dans le Bulletin de l'Académie Royale de Belgique, Classe des Sciences.
VOORWOORD
Dit werk omvat de resultaten vastgesteld door de auteur bij het bezoeken van de Physics Department of the University of California, San Diego, La Jolla. Deze resultaten zullen gepubliceerd worden in het Bulletin de l'Académie Royale de Belgique, Classe des Sciences.
VORWORT
Diese Arbeit beschreibt die Resultaten des Autors wenn er
das Physics Department of the University of California, San Diego, La
Jolla, besuchte. Die Resultaten werden in dem Bulletin de l'Académie
Royale de Belgique, Classe des Sciences, herausgegeben werden.
T H E I N T E R P R E T A T I O N OF A T S 6 P R O T O N S A N D E L E C T R O N S O B S E R V A T I O N S W I T H I N T H E F R A M E O F T H E E3H E L E C T R I C
F I E L D M O D E L
b y
J. L E M A I R E
A b s t r a c t
T h e " E 3 H " electric field model has been deduced by Mcllwain from dynamical proton and electron energy spectrograms measured along the geostationary orbit of A T S 5 satellite. It is shown that this empirical model, which has often been regarded with some reticence by a number of theoreticians, fits well with the measurements, also made in the equatorial region of the magnetosphere, onboard another geo- stationary satellite ( A T S 6) d u r i n g an extended period of time when the geomagnetic activity index K
pranged between 1 and 2. T h i s confirmation validates the E3H model, at least for such extended periods of time when K = 1 and 2+
P
From the dynamical spectrograms of A T S 6 observed on 19
July 1974, some evidence for patchy or localized injection of a magneto-
sheath plasma cloud across the late afternoon local time sector of
magnetopause has been gained.
Résumé
Le modèle de champ électrique "E3H" a été déduit par Mclwain des spectres d'énergie des protons et des électrons mesurés par le satellite géostationnaire A T S 5. Ce modèle empirique, que les théoriciens ont considéré très souvent avec une certaine réserve, est aussi en bon accord avec les mesures faites dans la région équatoriale de la magnéto- sphère, à bord d'un autre satellite géostationnaire ( A T S 6), d u r a n t une période prolongée pendant laquelle l'index d'activité géomagnétique K
pétait compris entre 1 et 2. De cette façon, le modèle E3H est confirmé, au moins pour des périodes de temps prolongées lorsque K^ est compris entre 1 et 2.
A partir des spectres dynamiques observés par A T S 6 le 19 juillet 1974, on a pu mettre en évidence la présence d'éléments de plasma de la magnétogaine injectés de manière irrégulière et localisée au travers de la magnétopause dans le secteur de temps local entre 12.00h et I8.00h.
- 2 -
Samenvatting
Het model van e l e k t r i s c h veld "E3H" w e r d door Mcllwain afgeleid uit de energiespectrogrammen v a n protonen en e l e k t r o n e n , gemeten door de geostationaire satelliet A T S 5. Dit empirisch model, door de theoretici v a a k in beschouwing genomen met een z e k e r e t e r u g - houdendheid, stemt eveneens goed overeen met de metingen die v e r r i c h t werden in het equatoriaal gebied van de magnetosfeer, aan boord v a n een andere geostationaire satelliet ( A T S 6 ) , g e d u r e n d e een v e r l e n g d e periode tijdens dewelke de geomagnetische a c t i v i t e i t s i n d e x K^ tussen 1 en 2 begrepen was, tenminste voor de v e r l e n g d e periodes waar K
ptussen 1 en 2 begrepen is.
Van de dynamische spectrogrammen, door A T S 6 waargenomen
op 19 juli 1974, heeft men de aanwezigheid kunnen vaststellen van
plasma-elementen die op onregelmatige en gelocaliseerde wijze door de
magnetopauze heen geïnjecteerd werden in de lokale tijdssector tussen
12.00h en 1 8 . 0 0 h .
Zusammenfassung
Mcllwain hat das Modell van elektrisches Feld " E 3 H " deduziert aus Energiespektrogrammen von Protonen und Elektronen, gemessen von dem geostationären Satellit A T S 5. Dieses empirische Modell, das der Theoretiker oft betracht mit einer Reserve, stimmt auch g u t überein mit M e s s u n g e n getan ins Äquatorialgebiet von der Magnetosphäre, an B o r d eines anderen geostationären Satellites ( A T S 6), während einer a u s - gebreiteten Periode worin der geomagnetische Aktivitätsindex K
pliegt zwischen 1 und. 2, wenigstens in den augebreiteten Perioden worin K
pliegt zwischen 1 und 2.
Von den dynamische Spektrogrammen, die A T S 6 beobacht hat, könnte man Plasma-Elementen sehen, die h i n d u r c h die Magneto- pause auf unregelmässiger und lokalisierte Weise eingespritzt wurden in dem lokalen Zeitsektor zwischen 12.00h und 18.00h.
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I N T R O D U C T I O N
T h e E3H electric field d i s t r i b u t i o n has been deduced b y Mcllwain ( 1 9 7 2 , 1974) from electron and proton e n e r g y spectra o b s e r v e d along t h e geostationary o r b i t of A T S 5 ( D e f o r e s t and M c l l w a i n , 1 9 7 1 ) .
Fig. 1 shows equatorial equipotential lines (<|> = c t ) corresponding to Mcllwain's electrostatic f i e l d . T h i s electric field d i s t r i b u t i o n has many features in common with t h e f i r s t magnetospheric convection electric field published in 1961 b y A x f o r d and Hines ( 1 9 6 1 ) . Indeed in both models the g r a d i e n t of the potential, <|>, ( o r t h e electric field Ê ) , has a large radial component in the p o s t - m i d n i g h t local time sector. Note also t h e significant d a w n - d u s k asymmetry in t h e e q u i - potential contours. In both the E3H model and in A x f o r d and Hines
1electrostatic field d i s t r i b u t i o n , t h e last closed equipotential extends up to t h e magnetopause near 1800 L T ; t h e last closed equipotential for t h e E3H model coïncides with the outer edge of the shaded zone of f i g . 1.
F u r t h e r m o r e , t h e r e is no singular point where t h e electric field vanishes and consequently where the E x B / B d r i f t velocity tends to zero ( i . e . a stagnation p o i n t ) .
A f t e r the discovery of t h e plasmapause b o u n d a r y b y C a r p e n t e r (1963, 1966) a n d , a f t e r it had been found t h a t this surface of discontinuity has a d a w n - d u s k asymmetry shown b y t h e innermost shaded area in f i g . 1, a number of d i f f e r e n t mathematical models f o r the magnetospheric electric field have been proposed b y many theoreticans ( B r i c e , 1967; Kavanagh et a l , 1968; Volland, 1973; S t e r n , 1977 and o t h e r s ) . Fig. 2 shows the equatorial cross section of equipotential surfaces for one of these mathematical models.
A common f e a t u r e of all these theoretical models is t h a t the
last closed equipotential does not extend up to t h e magnetopause, as it
does in t h e E3H model. T h e last equipotential in all these theoretical
models is always chosen to coïncide with the observed positions of t h e
F i g . 1.- A contour plot of model E3H electric equipotential lines in the equatorial plane of the magneto- sphere. The dashed lines are also streamlines
(drift paths) of zero-energy protons and electrons. The last closed equipotential coincides with the outer edge of the shaded areas. This*
limit intersects the geostationary orbit (R = 6.6;
shown by a dotted circle) at 2230 LT and 1044 L T . The position of the plasmapause as observed by Carpenter (1966) is shown by the solid line with a bulge at 1800 LT.
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DAWN
SUM 12
DUSK
Fig. 2.- A contour plot of the uniform dawn-dusk electric
field model by Kavanagh et al. (1968). The last
closed equipotential passing through the singular
point at 1800 LT, is choosen to fit the equatorial
position of the plasmapause. The dotted curves are
streamlines of electrons and ions confined within
a region which has been identified with the plasma-
sphere, according to the MHD-theory for the plasma-
pause formation.
plasmapause surface. Indeed, according to an early MHD theory by Brice (1967), the plasmapause should coïncide with the last closed equi- potential surface of the magnetospheric electric field distribution! Since the actual plasmpause does not extend up to the magnetopause, these latter modelers had to assume the existence of a singular point in their semi-empirical E-field models. Indeed, this mathematical singularity determines uniquely the position of the last closed equipotential and, according to the MHD theory, the position of the plasmapause.
When Mcllwain (1972, 1974) deduced the E3H electric field from A T S 5 particle observations along geostationary orbit, his empirical model was criticized for not having a singular point, i.e. a point where the electric field becomes equal to zero, and, across which the last closed equipotential would pass.
It is mainly for this reason that the E3H electric field model has been widely disregarded, even though it is not simply a theoretical model but is based on electron and proton dynamical spectrograms directly observed in the magnetosphere, like that of f i g . 3a.
Meanwhile, an alternative theory was proposed for the
formation of the plasmapause (Lemaire, 1975, 1976; Lemaire and
Kowalkowski, 1981). According to this new theory the plasmapause does
not coïncide with the last closed equipotential of the magnetospheric
electric field. Indeed, when the physical process of plasma interchange
motion is included in the theory, a well defined plasmapause is
continuously forming at finite radial distances in the magnetosphere,
even for electric field distributions (like the E3H model) which have no
mathematical singular point. Consequently, the main reason for d i s -
regarding empirical models, like those of Axford and Hines
1(1961) or
Mcllwain (1972, 1974), is no longer valid. Nevertheless, the E3H model
is still being treated with some reserve by most of the theoreticians.
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F i g . 3 a . - The s p e c t r o g r a m of data from ATS 6 a c q u i s e d on 19 J u l y 1974. The
e l e c t r o n and p r o t o n e n e r g y s p e c t r a h a v e been m e a s u r e d w i t h the plasma
s p e c t r o m e t e r s w h i c h are d i r e c t e d n e a r l y p e r p e n d i c u l a r to the magnetic-
field d i r e c t i o n . The flux i n t e n s i t y is d e t e r m i n e d by the b r i g h t n e s s of
the p h o t o g r a p h i c d e n s i t y . Local times and U n i v e r s a l times of the
s a t e l l i t e ATS 6 are given r e s p e c t i v e l y by the upper and lower s c a l e s .
T h e top p a n e l s c o n t a i n the i n t e n s i t y (BJ the B c o m p o n e n t and the
I n c l i n a t i o n angle of the local m a g n e t i c field m e a s u r e d at the p o s i t i o n
of A T S 6 .
T h e aim of this article is to compare t h e model E3H with observations from another geostationary satellite ( A T S 6 ) , and to check w h e t h e r or not it gives a realistic approximation to t h e actual d i s t r i b u - tion of electric fields in the equatorial region of t h e magnetosphere.
S P E C T R O G R A M D E S C R I P T I O N
Fig. 3a shows the e n e r g y spectra of 90° equatorial pitch angle electrons and protons between 5eV to 5 0 k e V . T h e evolution of these spectra along the A T S 6 geostationary o r b i t is g i v e n as a function of Universal Time for 19 July 1974. Local Times of the satellite are defined by : LT = U T - 0620.
T h e top panel shows the magnetic field i n t e n s i t y ( B , in n T ) , t h e B^ component and the inclination angle as measured by a magneto- meter along A T S 6 o r b i t . T h e corresponding equatorial values of B for Mcllwain's M2 magnetic field model are shown for comparison by the dashed line in f i g . 3b ( M c l l w a i n , 1972). It can be seen from t h e top panel of f i g . 3a t h a t the observed d i u r n a l variation of B corresponds to u n d i s t u r b e d conditions. T h e K index did not exceed 3 - on the day
P
before this observation. I t remained low (K = 1 & 2+) from 7 / 1 8 / 7 4 at 0300 U T till 7 / 2 0 / 7 4 at 0600 U T , including the period of time corresponding to f i g . 3a. T h e photographic density in the two lower panels is a logarithmic function of t h e particle f l u x measured with t h e East/West detectors which are spinning sound an axis almost perpendicular to the magnetic field d i r e c t i o n .
Note t h a t injected protons and electrons of almost zero e n e r g y are detected along the geostationary o r b i t at t h e same local time as t h e y would be if t h e y were convected from a common source region towards the position of t h e satellite. T h e i r d r i f t path between t h e source and t h e detectors is determined only by the d i s t r i b u t i o n of V
£= ? x E^/B , t h e electric d r i f t velocity in t h e magnetosphere. T h i s electric d r i f t velocity is parallel to the equipotential lines shown in f i g . 1. Protons
- 1 0 -
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3b. A plot showing the type of particle trajectories encountered by the
ATS 6 geostationary satellite according to the E3H and M2 electric and
magnetic field models. The scales are the same as in Fig. 3a.
with n o n - z e r o energies experience an additional westward g r a d i e n t - B d r i f t .
U N I F O R M I N J E C T I O N ALONG T H E MAGNETOPAUSE
Let us f i r s t assume t h a t t h e magnetopause surface or t h e Plasma B o u n d a r y L a y e r , is a steady and uniform source of protons and electrons with energies r a n g i n g from 0 to 50 k e V . In this case t h e f i r s t z e r o - e n e r g y particles d r i f t i n g inwards from t h e magnetopause will be detected at 2230 LT where t h e last closed equipotential intersects t h e geostationary o r b i t . T h i s last closed equipotential is t h e outer b o u n d a r y of t h e shaded area in f i g . 1. T h e geostationary o r b i t is shown by t h e dashed line at R = 6 . 6 R
£.
All z e r o - e n e r g y particles detected a f t e r 2230 LT at R = 6 . 6 have either been emitted along t h e duskside f l a n k of t h e Plasma B o u n d a r y Layer ( P B L ) a f t e r 1710 L T , or t h e y are charged particles d r i f t i n g down t h e tail toward t h e E a r t h . These zero-magnetic moment particles can be detected until 1044 LT where t h e last closed e q u i - potential of t h e E3H field intersects again t h e geostationary o r b i t . In the dynamical spectrogram shown in f i g . 3b all z e r o - e n e r g y particles d r i f t i n g inwards from t h e PBL or from the magnetotail should t h e r e f o r e be observed between points A
1(2230 L T , 0450 U T ) and A
2(1044 L T , 1704 U T ) .
A f t e r 1044 L T b u t before 2230 LT t r a p p e d z e r o - e n e r g y electrons or protons can come n e i t h e r d i r e c t l y from Plasma B o u n d a r y
Layer nor from t h e magnetotail, but must be of ionospheric o r i g i n .
Electrons of higher e n e r g y experience an eastward g r a d i e n t - B in addition to t h e electric d r i f t velocity V
E >T h e g r a d i e n t - B d r i f t is proportional to t h e perpendicular e n e r g y of t h e p a r t i c l e . As a consequence t h e trajectories of electrons with n o n - z e r o magnetic moments d i f f e r from t h e streamlines shown in f i g . 1. T h e last closed
- 1 2 -
streamline for - 0.1 k e V / n T electrons intersects the geostationary orbit at 2231 LT and 1031 L T . Particles of this magnetic moment, that have been observed between these two local time limits, must have been emitted at the P B L or have drifted earthwards down from the magneto- tail. These local time limits depend on the s i g n of the electric c h a r g e of the particles since the g r a d i e n t - B drift velocity is eastward for negatively charged particles and westward for positive ions. The upper solid line ( a ) in f i g . 3b g i v e s the local time limits between injected and trapped electrons as a function of e n e r g y . It can be seen that for electron energies above a certain limit 30 k e V ) , all trajectories intersecting the geostationary orbit are closed. A n y electrons observed along geostationary orbit with an energy greater than 30 keV (in the direction perpendicular to E^) must be particles trapped inside the magnetosphere.
There is also an upper energy limit beyond which all protons detected by A T S 6 at a given LT are on trapped orbits and can never penetrate into either the P B L or in the magnetotail. T h i s local time- dependent energy limit is shown by the lower solid line ( b ) in f i g . 3b.
Protons observed at geostationary orbit which none higher energies have not been convected down from the tail, but are westward drifting trapped ions which have been injected at an earlier substorm and form the Ring C u r r e n t . Note that the position of the solid line ( b ) is independent of the mass of the ions which are measured.
The numbers given along the line ( a ) are the local times
where the particles have been emitted at the inner edge of the Plasma
B o u n d a r y Layer. It can be seen that these local times range from
1200 LT to 1745 LT (i.e. the afternoon local time sector); the protons
and electrons observed by A T S 6 at 0500 UT with energies below 1 keV
are all injected along the magnetopause in a rather narrow range of
local times in the later afternoon sector between 1647 LT and 1734 L T .
The last Plasma B o u n d a r y Layer electrons and protons observed by
A T S 6 at 1700 UT with energies below 1 . keV have penetrated into the
magnetosphere between 1651 LT and 1725 LT (see f i g . 3b).
Note the good coïncidence of t h e line ( a ) with t h e s h a r p S - s h a p e d spectral f e a t u r e , seen in f i g . 3a, between 0400 U T and 0500 U T the proton and electron spectrograms below 1 k e V . For these low e n e r g y particles t h e electric field d i s t r i b u t i o n in t h e magnetosphere plays the dominant role, since t h e g r a d i e n t - B d r i f t is small f o r energies smaller than 1 k e V . T h e good agreement between these experimental results and t h e model calculation argues f o r the relevance of t h e E3H electric field model. "
If t h e whole Plasma B o u n d a r y Layer and magnetotail were a steady and uniform source of. electrons and protons of all e n e r g i e s , a uniform d i s t r i b u t e d by f l u x would be observed almost e v e r y w h e r e between t h e solid lines ( a ) and ( b ) in t h e spectrogram of f i g . 3 b . Since this is not t h e case in f i g . 3a, one can i n f e r t h a t t h e source region has a f i n i t e e x t e n t in local time along the magnetopause f l a n k , i . e . t h a t particles are convected to the geostationary o r b i t from a r a t h e r limited range of local times along the inner edge of the Plasma B o u n d a r y L a y e r .
T h i s is not the only possible i n t e r p r e t a t i o n of t h e f e a t u r e seen in t h e spectrogran of f i g . 3a between 0400 U T and 0500 U T . To check this w o r k i n g hypothesis one would need two or more magneto- spheric satellites simultaneously in o r b i t to measure ,the proton and electron f l u x e s between 0 and 50 k e V . Until such coordinated and multisatellite observations become available one cannot, h o w e v e r , rule out t h e i n t e r p r e t a t i o n given above.
P A T C H Y I N J E C T I O N REGION A T T H E MAGNETOPAUSE
It can be seen from the spectrograms of f i g . 3a t h a t t h e f l u x of low e n e r g y electrons observed at R = 6 . 6 R
£is limited w i t h i n a narrow local time s t r i p . T h i s seems to indicate t h a t the source of t h e low e n e r g y protons and electrons is limited in local time as indicated above. T h e dashed line ( c ) in f i g . 3b gives t h e Universal Time at
- 1 4 -
which A T S 6 can detect particles which have penetrated across the magnetopause at 1900 L T . These particles have penetrated into the magnetosphere at different Universal Times indicated along the line ( c ) , e . g . a zero-energy particle encountered b y A T S 6 at 0525 U T has penetrated into the magnetosphere at 0253 U T in the 1900 Local Time sector along the magnetopause.lt can be seen that the particles observed by A T S 6 between 0400 U T and 0600 UT could have been injected into the magnetosphere over a time period extending from before 0225 U T to after 0310 U T .
Between the injection region at the magnetopause and the location of the satellite, the particles have gained kinetic e n e r g y because of the change of the electric potential along their drift paths.
A s a consequence of the conservation of the f i r s t adiabatic invariant the perpendicular e n e r g y of these particules is proportional to the magnetic field intensity. The magnetic field intensity at their injection point at the magnetopause is 7 n T , while the magnetic field at geostationary orbit is 90 n T . Therefore the energy of a particle detected b y A T S 6 with an e n e r g y of 1 keV was originally 13 times smaller i.e. 77 eV. In other words, the Betatron acceleration mechanism in the E3H + M2 electric and magnetic field distributions can account for how magneto- sheath protons or electrons of 70 eV are able to acquire an e n e r g y of 1 keV at geostationary orbit.
Since the highest particle fluxes near 0525 UT are confined with in a narrow vertical strip (i.e. only d u r i n g one hour U T ) one can also s u g g e s t that the injection region did not extend beyond 1900 LT along the magnetopause or Plasma B o u n d a r y Layer.
High energy injected electrons are missing above 5 keV in
f i g . 3a. The maximum observed energies are smaller than those
corresponding to the solid line ( a ) . T h i s discrepancy can be partially
resolved by further limiting the local time extent of the injection region
at the magnetopause. Indeed if, this injection region does not extend to
local times earlier than 1600 L T , the upper energy limit of injected electrons is lowered from line (a) to line (d) in f i g . 3b.
From the comparison of figs. 3a and 3b, it can be seen that the observed upper limit is still higher than the calculated line ( d ) . Therefore some other or an additional effect needs to be found to resolve the discrepancy. One possible explanation could be that the cloud of injected particles (source region), which has penetrated impulsively into the duskside flanks of the magnetotail did not contain many electrons with initial energies greater than 210 eV which would then have been measured at geostationary orbit with a final energy greater than 5 keV, as a result of the betatron acceleration process.
T h i s explanation for the low fluxes of electrons above 5 keV is supported by plasma observations in the magnetosheath which show that the solar wind electron energy spectra are rather depleted above 200 e V .
We have shown that the narrow band of enhanced electron flux observed between 0400 UT and 0525 UT along the A T S 6 trajectory can be accounted for by an injection region initially located along the magnetopause at local times later than 1600 LT and before 1900 L T . T h i s result can be considered as additional evidence for patchy or localized injection of magnetosheath plasma into the magnetosphere instead of steady-state diffusion or uniformly distributed penetration across the magnetopause.
T h e satisfactory agreement obtained between the position of this observed S-shaped spectral feature and the low energy portion of the line (a) in fig. 3b, indicates that the E3H electric field does indeed fit A T S 6 observations of electrons and protons between 0400 UT and 0600 U T .
T h e fading of the spectral intensity after 0600 UT within the bright strip feature in the electron spectrogram of fig. 3a, may be the
- 1 6 -
consequence of scattering and pitch angle diffusion as t h e beam of electron proceeds deeper into t h e magnetosphere. B u t it may also be evidence f o r t h e slow extinction of t h e source of injected p a r t i c l e s , i . e . f o r a depletion of t h e particle r e s e r v o i r as time proceeds.
Simultaneous observations at d i f f e r e n t locations would be r e q u i r e d to determine how much f a d i n g is due to the time v a r i a t i o n of the source, and how much is a consequence of continous s c a t t e r i n g of particles p e n e t r a t i n g into the deeper region of t h e magnetosphere.
T H E DEEP M I N i M U M IN T H E PROTON S P E C T R U M
T h e r e are some protons t h a t are injected at t h e magnetopause and p e n e t r a t e r a t h e r deep into the magnetosphere before t h e y arc detected at geostationary o r b i t . Such westward d r i f t i n g protons spend t h e i r longest time in t h e denser cold plasma region of t h e inner magneto- sphere where t h e i r d r i f t velocity is significantly reduced because of t h e large magnetic field intensity B. Indeed both t h e electric and g r a d i e n t - B d r i f t s are inversely proportional to the magnetic field i n t e n s i t y , B.
T h e r e f o r e these protons could be lost before t h e y t r a v e r s e t h e geo- stationary o r b i t for t h e second time. A deep minimum is expected in t h e proton spectrum f o r all energies corresponding to the o r b i t s p e n e t r a t i n g f a r inside the geomagnetic f i e l d . Mcllwain ( 1 9 7 2 ) has shown t h a t t h e E3 field model predicts t h e observed position of this minimum in t h e A T S 5 e n e r g y s p e c t r a . T h e dashed line ( e ) in f i g . 3b gives t h e e n e r g y f o r which the penetration in the E3H field is deepest : i . e . t h e t r a j e c t o r y with t h e minimum minimorum radial distance. T h e agreement between t h e line ( e ) and t h e observed minimum in t h e proton spectrogram of f i g . 2a, is v e r y satisfactory and is an additional confirmation of t h e relevance of the E3H electric field model.
T h e deep minimum in the proton spectra is most clearly
observed between 0000 L T and 1400 L T ; between 2200 LT and 2400 LT
t h e proton f l u x has a maximum i n t e n s i t y . T h e intensification d u r i n g this
latter local time i n t e r v a l is observed when t h e satellite p e n e t r a t e s into t h e p r e - m i d n i g h t local time sector where it detects t h e protons coming d i r e c t l y down t h e magnetotail, before t h e y pass t h r o u g h t h e i r deepest penetration point ( i . e . before t h e y have been s c a t t e r e d ) ; along t h e rest of geostationary o r b i t these 10-20 keV protons a r e measured a f t e r passing t h r o u g h lower L - s h e l l s .
Instead of using t h e minimum minimorum radial distance to determine t h e e n e r g y of t h e deep minimum in t h e proton s p e c t r a , Mc- llwain ( 1 9 7 2 ) has used two a l t e r n a t i v e c r i t e r i a : 1 ) t h e maximum maxi- morum magnetic field i n t e n s i t y and 2 ) t h e longest d r i f t time. All t h r e e c r i t e r i a lead to energies f o r t h e deep minimum which are close to each other and are sometimes h a r d to distinguish numerically.
T h e minimum in t h e proton spectra is a consistent f e a t u r e found in most A T S 5 and 6 spectrograms. A n y r e l e v a n t consistent magnetospheric electric and magnetic field models should t h e r e f o r e be able to p r e d i c t precisely t h e position of this deep minimum in t h e proton spectra. T h e E3H model was c a r e f u l l y tailored to do so ( M c l l w a i n , 1972, 1 9 7 4 ) .
C O N C L U S I O N S
Confirmations f o r the relevance of Mcllwain's electric field model E3H have been obtained using dynamic spectrograms of electrons and protons observed along t h e geostationary o r b i t of A T S 6 d u r i n g an extended period of time when the geomagnetic a c t i v i t y remained low and nearly constant ( K
p= 1 - 2 ) . Indeed t h e positions of t h e deep minimum observed in t h e proton e n e r g y spectra of f i g . 3a between 2 keV and 10 k e V , correspond to t h e e n e r g y for which these particles p e n e t r a t e deepest into t h e magnetosphere when this limit is computed with t h e E3H model. F u r t h e r m o r e , the S-shaped injection b o u n d a r y , in f i g . 3 b , separating the t r a p p e d electrons from those injected from t h e Plasma B o u n d a r y Layer or from the magnetotail, has been calculated using the
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E3H model; t h e theoretical r e s u l t fits v e r y precisely t h e experimental observations made onboard A T S 6 (see f i g . 3 a ) .
A l t h o u g h , a f a i r agreement has been found between t h e E3H model predictions and A T S 6 observations f o r a whole 24 hour period when Kp stayed between 1 and 2 , it should not be concluded t h a t all spectrograms observed d u r i n g more d i s t u r b e d days can be i n t e r p r e t e d in terms of t h i s empirical electric field model. Indeed u n d e r most of these circumstances, stationary electrostatic field d i s t r i b u t i o n s a r e useless; time d e p e n d e n t and i r r e g u l a r electric field models should t h e n be used instead of t h e E3H model, or of any other simpler approxima- tion of the equatorial electric field d i s t r i b u t i o n such as t h e uniform d a w n - d u s k model ( f i g . 2 ) or S t e r n - V o l l a n d ' s set of models.
I t has been suggested t h a t a localized injection of plasma from a narrow local time sector of t h e late afternoon Plasma B o u n d a r y Layer can explain t h e narrow band of l o w - e n e r g y electrons and protons (< 1 k e V ) observed between 0400 U T and 0600 U T in f i g . 3a. H o w e v e r , such an i n t e r p r e t a t i o n is t e n t a t i v e and would need additional experimental confirmation from f u t u r e coordinated and multisatellite observations.
ACKNOWLEDGEMENTS
I wish to t h a n k C . E . Mcllwain f o r making t h e A T S 6 data available to me as well as the computer facilities at t h e C A S S . I would like to t h a n k C . E . Mcllwain and E . C . Whipple also f o r t h e i r hospitality when I was v i s i t i n g at the U n i v e r s i t y of C a l i f o r n i a , San Diego.
T h e author wishes to t h a n k P r o f . M. Nicolet f o r his i n t e r e s t
and s u p p o r t d u r i n g t h e preparation of this a r t i c l e .
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