Relativistic electron transport processes associated with magnetospheric substorms

Radiation Measurements, Vol. 26, No. 3, pp. 343-345, 1996

Pergamon P l l : S1350-4487(96)00009-1

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RELATIVISTIC ELECTRON TRANSPORT PROCESSES ASSOCIATED WITH MAGNETOSPHERIC SUBSTORMS A. P. KROPOTKIN Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia Abstract--A preliminary study is presented of the magnetospheric processes of transport and energization for high*energy electrons which are presumably important for atmospheric coupling and ozone depletions. These processes are basically associated with magnetospheric substorrn field variations. Both global "slow" (of the order of an hour) and localized "fast" (of the order of a minute) constituents of those variations might be important. Copyright © 1996 Elsevier Science Ltd

1. INTRODUCTION Among the particle populations in the near-Earth space, of particular interest are those which possibly may be a link in the coupling of solar variability to the Earth's lower atmosphere. The solar activity control of weather and climate is a very controversial topic. However, convincing evidence of such control has accumulated recently and its possible mechanisms are widely discussed (Reid, 1991; FriisChristensen and Lassen, 1991;Labitzke and van Loon, 1990; Pudovkin and Raspopov, 1992). Particle populations which affect the atmosphere in the most direct manner must be those which can penetrate deep into the atmosphere. Thus relativistic electrons (RE) appear to be among the most important candidates (Baker, 1992). Other agents of this type, also heavily dependent on solar activity, are solar energetic protons (of MeV range energies). We try to examine specific magnetospheric processes which might be involved in the forming of the RE fluxes precipitating into the middle and lower atmosphere. The highly variable character of those fluxes is due to the control of outer radiation belt populations by intense sporadic magnetospheric disturbances, i.e. magnetospheric substorms. The way in which this control may determine the behaviour of near-equatorial RE populations is the subject of this preliminary study.

solar cycle. These factors directly affect only the dilute layers of the upper and middle atmosphere, down to the stratosphere. However, these influences may be very important. In particular, they do result in the ozone abundance variations at stratospheric heights (through intermediate abundances of odd nitrogen oxides). Ozone variations are known to be important from two points: (1) they affect the UV flux penetrating to the Earth and strongly affecting living organisms; and (2) they have an influence upon the temperature variations at stratospheric heights. In turn, that affects propagation and damping of waves penetrating from the troposphere, i.e. Rossby waves and internal gravity waves, and these are intimately associated with tropospheric circulation structures. It must be pointed out that the relative role of RE fluxes in the stratosphere, in comparison with galactic cosmic rays and solar protons, remains a subject of considerable controversy. The most optimistic estimates (CaUis et al., 1991), based on RE measurements at the geosynchronous orbit, and their projection to the atmospheric heights with the use of a rather crude model, yields [03] variations of tens of percent, being substantially higher than the effects due to other agents. In any case both rather high RE fluxes in the magnetosphere, in the region of the outer radiation belt, and intense and fast variations of those fluxes, are well established observational facts; up to now their nature lacks a reliable theoretical explanation.

2. A T M O S P H E R I C I M P A C T Relativistic electrons are one of those factors affecting the atmosphere which are relatively faint in the sense of energetics, but on the other hand, vary greatly in association with solar activity (about an order of magnitude in the case of RE) contrary to the "solar constant" characterizing the total energy flux from the Sun which varies only by ~ 0.1% during a

3. MAGNETOSPHERIC SUBSTORM INFLUENCE ON RE Despite the possibility that RE fluxes are coming from the interplanetary space having, presumably, in part, the Jovian origin (Baker, 1992), most probable however, is local origin, i.e. in the highly variable 343

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plasma of the outer magnetosphere. Their acceleration and transport may be associated with electromagnetic disturbances in that region. In the outer belt region the main electromagnetic disturbances are the sharp day-night asymmetric substorm field variations. They consist of a "slow" global component and a "fast" component localized in a narrow near-midnight sector. A characteristic feature of the substorm magnetic field variation is the tailward stretching of the field lines at the nightside during the substorm growth phase, penetrating deep into the magnetosphere down to L ~ 5-7. This variation causes a high field line curvature near the equator which could be a reason for adiabaticity violation for the high-energy portion of trapped particles. Such violation has been invoked by Imhof et al. (1991) to explain the observed sharp bursts of energetic electron precipitation as a result of non-adiabatic pitch angle changes of previously near-equatorial electrons. This effect should be incorporated into the general pattern of electron transport and acceleration. Another aspect of that transport should be associated with "slow" substorm equatorial field intensity variations causing an asymmetric decrease of the field with predominance at the nightside, from the start of the growth phase up to the beginning of the recovery. This is due to the quasi-steady magnetosphere reconfiguration, with the tail diameter increasing, and both the dayside magnetopause and the plasma sheet earthward displacement. To examine this process in detail a "dynamical" magnetospheric model is needed, which has not been elaborated up until now. However, in a simple model, involving the change of the magnetopause curvature radius due to that process, it is easily shown that field magnitude B variations occur of the order of subsolar point field magnitude.

4. SLOW FIELD VARIATIONS AND RE ORBITS These variations are "slow" in comparison with energetic electron drift periods, being of the order of minutes. Thus corresponding drift trajectory changes should conserve the third invariant. For near-equatorial particles the drift trajectory coincides with the B---const contour, due to magnetic moment ~ / B conservation. Therefore the change in particle position is determined by quasi-steady transitions between such contours, with conservation of the magnetic flux ¢ through the contour at any time moment. For a given dynamical field model this could be straightforwardly calculated. Again the scale magnitude of particle displacement may be evaluated by means of a simple model. Let the field magnitude variation at the equator be equal to b ( t ) = - h(t)(R/Ro)cosdp

b=0

n>tp>nt2;

- x/2 < q~ < n/2, -~/2>qb>

-n,

(1)

R being the radial distance, 4~ being the local time angle with tk = 0 at midnight. Initially the particle drifts at a distance R0, B(R0) = B0. At any moment t at dayside the drift path remains circular at R = R~ with I~/R~ = B~, R~ = Ro + r~. At nightside B, = U/R'(q~) - h R ( 4 , ) c o s 4 , / ~

;

R(O) = ~ + r0(4,).

(2)

Hence

r I --

r 0 '~

COS

~

(3)

With the use of the • = const condition we arrive at R0 h R0 h (l/n-cos~b) r j = ~ if0 ; r 0 - 3 B0

(4)

We see now that for h being a considerable fraction of B0 (and that is the case for the outer belt region as follows from the earlier estimates) the inward displacement r0 is large enough (it may be of the order of several RE) especially near midnight. This displacement is evidently reversible for adiabatic "slow" field variations. Note, however, that just at nightside those variations also include field line stretching. Thus an initially equatorial electron can be drawn a considerable way inside and then pitch-angle scattered. Both these effects should be studied in their interrelation more thoroughly; their cooperative action presumably can be a mechanism for fast radial transport times on the scale of tens of minutes to hours, those to be compared to much longer scale times of traditional radial diffusion which moreover dramatically increase with diminishing radial distance.

5. SUBSTORM ELECTRON INJECTION AND ACCELERATION As was mentioned earlier the region where RE are mostly observed at the nightside partly coincides and is generally slightly earthward from that region where most intense magnetic field variations occur during substorms, i.e. field line stretchings and dipolarizations. This domain has a relatively small radial extension, so that the field z-component intensively decreases radially at a short distance (Lui, 1992). This implies that a small radial adiabatic (/z = const) displacement of a near-equatorial particle here should be followed by its considerable energization. Such transfer and energization do occur in this region during "microsubstorms', i.e. substorm intensifications which result in this domain in sharp field dipolarization events with intense transient electric fields (Lopez et al., 1989; Kropotkin, 1990). For the dominant plasma population with energies

RELATIVISTIC ELECTRON TRANSPORT up to tens of keV this implies the process of substorm "injection" (Mcllwain and Whipple, 1986). Drift periods of those particles are of the order of hours and besides they are strongly affected by convective transport so that they have little chance to return to the nightside acceleration region during that same substorm. To the contrary, the most energetic populations with drift periods of the order of minutes have a good chance to return to that region many times during the same substorm and thus to be energized there several times in successive dipolarization events. To estimate possible effects in particle fluxes, note that a typical electric field during dipolarization is estimated as E ~ 10 mV/m; the local time interval of the process is Ark --~ 3-5h LT, the radial distance is R ~ 5-8 RE. The drift period is r ~ 22/Le ~ 1-4 rain for electrons with energies e ~ 1-2 MeV. Then the time of E field action on the electron is Az ~ z(Aq~/ 2n) ~ 10-40 s; the energy gain is Ae ~ 200--500 keV; the magnetic field increase on the electron orbit is A B / B ~ 10-50% (remember e/B = const). Typically the electron flux at higher energies in the outer belt can be presented in the exponential form J e , , ~ e x p ( - e / e o ) with e 0 ~ 0 . 3 MeV. With these figures in mind we obtain the flux increase at every acceleration act to be AJ~ ~ 100-300%.

6. S U M M A R Y The main points of our argument in this study are as follows: 1. In the "slow" phase of a substorm disturbance energetic electrons can penetrate very deep into the magnetosphere at the nightside. 2. Substorm field variations can sporadically cause pitch-angle scattering of energetic electrons due to adiabatic motion violation in a sharply curved magnetic field. 3. Short-lived intense electric fields, associated with localized "dipolarization" events which are

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short-term constituents of a substorm at nightside, can substantially energize 100 keV to MeV electrons. This is a one-sided process so that successive events provide more and more acceleration. The process is in a sense regular rather than stochastic.

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