Observations of fluxes of charged particles with energies E < 30 keV at low and equatorial latitudes of the inner magnetosphere

N. M. Shutte¹, M. V. Tel'tsov², B. V. Mar'in², V. I. Lazarev², Z. Nemechek³, Ya. Shafrankova³, Ya. Shmilauer⁴, and I. Shimunek⁴

¹ Institute of Space Research, Moscow, Russia ² Nuclear Physics Research Institute, Moscow State University, Moscow, Russia ³ Charles University, Prague, Czech Republic ⁴ Geophysical Institute, Prague, Czech Republic

Contents

Abstract

Introduction

Rocket and satellite experiments have shown [Heikkila, 1971; Prange, 1978; Shutte, 1983, 1985; Shutte and Izhovkina, 1985, 1987, 1991; Shutte et al., 1979] that intense fluxes of charged particles with energies from hundreds of electronvolts to tens of kiloelectronvolts can be observed at low and near-equatorial latitudes under quiet geophysical conditions [Shutte, 1983, 1985; Shutte et al., 1979]. In daytime, the intensity of these fluxes and the energy distributions are comparable to those usually detected in the auroral regions.

Analysis of the experimental data obtained by the Cosmos 900 satellite, which had a polar (inclination of ~83^o) quasi-circular orbit with an altitude of ~500 km, made it possible to ascertain the interrelation of parameters of the low-latitude particle fluxes (the probability of occurrence, intensity, and dynamics of the energetic distribution function) with characteristics of the geomagnetic field and ring current [Heikkila, 1971; Shutte, 1985; Shutte and Izhovkina, 1985, 1987, 1991]. However, the data obtained so far are not adequate to provide a comprehensive explanation for the origin of the observed intense fluxes of charged particles in the low-latitude upper ionosphere and their spatial and temporal variations.

This paper presents some preliminary results of observations of charged particle fluxes obtained by the APEX satellite using the PEAS energy-angle spectrometer at low and equatorial latitudes.

Equipment

The APEX satellite (together with a smaller subsatellite) was launched in a polar near-Earth orbit on December 13, 1991 (apogee of 3080 km, perigee of 437 km, and inclination of 82.5^o), and had a three-axis orientation, in which one of the satellite axes (longitudinal) is always directed tangentially to the Earth's surface. The satellite velocity vector is stabilized in the plane perpendicular to the longitudinal axis. In addition to the plasma wave and magnetometer instruments, the satellite was equipped with an electron gun and a plasma injector.

In this paper, we discuss the results of measurements of charged particles obtained by using the PEAS equipment only in the so-called "passive" mode of operation of the satellite, with its electron gun and plasma injector switched off.

It should be noted that owing to the orbit precession, it was possible to obtain information over a 3-month period covering the altitude range ~450-3000 km at both low and high latitudes and all local times.

The PEAS equipment included two identical energy-angle spectrometers AP-1 and AP-2 arranged on board the satellite in mutually orthogonal planes. Each spectrometer is substantially a torus electrostatic analyzer consisting of three concentric axisymmetric electrodes divided into 12 sectors. The analyzer is capable of measuring independently and simultaneously, in the energy range from 50 eV to 30 keV, energy spectra of electrons and ions from 12 directions at an angular resolution of ~ 20 5^o in the angle range 360 5^o. Analyzing voltage was varied stepwise, the number of steps being 64, 32, 16, and 8, depending on the mode of operation of the equipment. The energetic resolution of the analyzing tracts was 5%. The AP-1 spectrometer analyzers are located in the plane of the satellite longitudinal axis oriented toward the Earth. The AP-2 spectrometer analyzers are located in the plane perpendicular to the longitudinal axis, that is, in the plane of the satellite velocity vector. Thus the measurement of the total angular distribution of the particles (from 360^o) in the mutually perpendicular planes was provided.

Measurement Results

Although the measurements at low and equatorial latitudes covered all the orbital altitudes, the signal measured at altitudes over ~1300 km, when the satellite was in the inner radiation zone, was completely masked by the background signal generated by the intense fluxes of high-energy protons. Because of this, in analyzing the information of the PEAS energy spectrometers at low and near-equatorial latitudes, we only considered the crossings of the perigee sections of the orbit with altitudes H<1000 km, where the background signal is 2-3 orders of magnitude lower and can be unambiguously estimated (except in the South Atlantic Magnetic Anomaly region, where, because of the lower values of the geomagnetic field, the effect of the radiation belts may take place at lower altitudes, too).

Figure 1 shows the energetic spectra of ions ( I ) and electrons ( E ) in two mutually perpendicular angular sectors for two satellite passages on April 30, 1992 (2050.19-2102.38 UT), and May 1, 1992 (1058.11-1110.28 UT), at the unlit parts of the orbit under relatively quiet geomagnetic conditions ( Kp = 1-, SKp = 18- and Kp = 1+, SKp = 21, respectively). Four spectra of ions and electrons were measured within each of the time intervals indicated. The entrance apertures of the analyzers were oriented (as is shown by arrows in Figure 1) toward the Earth, from the Earth, along the satellite velocity vector, and against it. The logarithms of the particle energy are shown at the abscissas. At the right-hand side of Figure 1 the logarithmic scale of the particle differential fluxes log P (per cm ² s sr keV) is shown. Temporal sequence of the spectrum measurements goes from the bottom to the top. The altitude values in kilometers and the L parameter during the spectrum measurement are shown at the left-hand side.

The arrangement of the AP-1 and AP-2 detectors at the fixedly oriented APEX satellite is such that it can be assumed that, in the near-equatorial latitude zone, one of the two mutually orthogonal planes of recording of charged particle fluxes (AP-2 detector) was close to the plane of the field lines B_|.

The energetic distributions shown in Figure 1 demonstrate, for the given interval of variation of parameter L and altitude H, a pronounced altitudinal dependence both in the transverse direction B (1-4) and in the longitudinal direction along the magnetic field (5-8). Particle fluxes of maximum intensity were observed at altitudes below 600 km. First, with decreasing altitude, soft electron fluxes appear, the intensity of which increases from 10⁶ to 10⁸ cm ^-2 sr ^-1 s ^-1 keV ^-1. Ion fluxes with an intensity of ~ 10⁷ cm ^-2 sr ^-1 s ^-1 keV ^-1 appear somewhat later, near perigee altitudes.

Characteristic of the aforementioned energetic distributions is their fluctuating, non-Maxwellian character, with considerable energy-dependent variations in intensity. This was most clearly evident in the field-aligned plane (5-8).

Despite the low recording rate and, accordingly, rather poor spatial resolution, it is possible to trace the variation in form of the energetic distributions for particles of both signs in these sections of the orbit upon a simultaneous change in parameter L and altitude H of the satellite. Comparison of energetic distributions in two sections of the orbit, differing in that, in one case, the decrease in altitude was accompanied by increasing parameter L (April 30, 1992) and, in the other case (May 1, 1992), by its decrease, shows that at lower altitudes, the share of electrons with energies below 1 keV (flux of ~ 5 10⁷ cm ^-2 sr ^-1 s ^-1 keV ^-1 ) increases, and so does the flux of kiloelectronvolt ions. At equal altitudes the intensity of softer electrons was higher under lower values of parameter L.

In the transverse direction, spatial anisotropy of predominantly earthward ion fluxes with a maximum energetic distribution of ~1 keV is more clearly pronounced. In the longitudinal direction, at altitudes H>600 km, anisotropy of ion fluxes with energies of ~1 keV is also observed.

Figure 2 shows the angular distributions of the electrons (top panels 1a and 2a) and ions (bottom panels 2a and 2b) measured on April 30, 1992 (2050.19-2102.38 UT), for three fixed particle energies (0.1 keV, 1.0 keV, and 10 keV) in the plane of the satellite velocity vector (2a and 2b) and in the longitudinal plane (1a and 1b). The angles from which the measurements were conducted are shown at the abscissas. In the velocity vector plane (2a and 2b) the zero angle is the angle where the analyzer aperture is directed along the velocity vector. In the longitudinal plane (1a and 1b) the zero angle is the angle where the analyzer aperture coincides with the longitudinal axis and is oriented from the Earth. The logarithms of the particle count rate in the corresponding channels are shown at the ordinates. The logarithmic scale of the particle counting is shown at the right-hand side of the figure (  log C in counts per second). The time sequence of the spectrum measurements goes from the bottom to the top. The values of the altitude in kilometers and L parameter corresponding to the time of the measurements are shown at the left-hand side.

The particle fluxes oriented along the field are seen to be more intense, especially at altitudes below ~700 km. Noteworthy is the presence of a significant angular anisotropy of the fluxes of particles of both signs. Spatial distributions have clearly pronounced, comparatively narrow (~30^o-60^o) maxima for both ions and electrons with energies E>0.1 keV. For all altitudes, these intensity maxima were observed in the angular sectors arranged along the field (~ 0^o and ~180^o) and in the perpendicular direction. With decreasing altitude, the particle fluxes become not only more intense, but also more isotropic.

In the transverse plane ( B ), noticeable intensities were only observed near perigee altitudes. Electron fluxes were nearly isotropic, their energy practically not exceeding ~100 eV. Ions with energies of ~100 and ~1000 eV featured a predominantly earthward spatial anisotropy.

Thus maximum intensities (up to ~ 10⁸ cm ^-2 sr ^-1 s ^-1 keV ^-1 ) and energies (up to ~10 keV) of nighttime near-equatorial fluxes evidently occur at the lowest values of orbital altitudes and parameter L. It is also evident that in the interval of variation of the latter from 1.1 to 1.9, the factor of decreasing altitude becomes more essential, especially for ions. Specific to ion fluxes is spatial anisotropy both along the field and in transverse directions. Spatial anisotropy of electron fluxes mainly occurred in the field-aligned plane.

Discussion

Measurements of fluxes of charged particles with energies of 50 eV to 30 keV at the APEX satellite in the region of low and equatorial latitudes show that in unlit sections of the orbit, electron and ion fluxes with an intensity of 10⁵-10⁸ cm ^-2 sr ^-1 s ^-1 keV ^-1 are regularly observed. The intensity of these fluxes increases with decreasing altitude, reaching maxima at altitudes below 600 km. The increase in intensity with altitude is accompanied by an increase in the contribution of the softer component.

Energetic distributions of near-equatorial fluxes have a "spiking," non-Maxwellian character, especially for the ion component in the longitudinal direction, along the field. The non-Maxwellian character of the near-equatorial charged particle fluxes had been indicated earlier by authors of experiments at the Cosmos 900 satellite [Shutte, 1983, 1985; Shutte et al., 1979] and ISIS 1 [Heikkila, 1971], who, independently, noted not only the fluctuating character of the energetic distributions, but also the antiphase position of maxima and minima of these distributions in the region of energies E<300 eV.

Low-energy electron fluxes ( E 100 eV) are practically isotropic both in the longitudinal plane and in the transverse direction. Spatial anisotropy was basically seen along the field for kiloelectronvolt ions and electrons.

The data obtained suggest that the intense fluxes of electrons and ions observed at ionospheric altitudes of the near-equatorial latitudes may be the result of the effects of collective interactions of ionospheric plasma and waves. For example, energetic protons of ring current can generate ion cyclotron waves, which, entering the ionosphere, interact resonancewise with the thermal ionospheric plasma (mainly with the O ⁺ ions), transferring to it part of the energy. In this context, in the case of a uniform (or constant) magnetic field (this condition is met in the magnetic equator zone), there are practically no restrictions for energy transfer from a wave to an ion and vice versa. Ungstrup et al. [1979], for example, show the possibility of an intense heating of ions of the upper ionosphere by ion cyclotron waves in a direction perpendicular to the geomagnetic field.

Besides, the electrostatic waves, observed at the ISEE 1 satellite [Bell and Ngo, 1988], stimulated by the coherent VLF electromagnetic whistler wave modes propagating along the field lines near and within the inner radiation belt may also be one reason for the resonance acceleration of the ionospheric plasma. Since the frequency band of these waves is, in the first approximation, determined by the magnitude of the local geomagnetic field [Bell and Ngo, 1988], the parameters of near-equatorial fluxes turn out to be associated with both the strength of the magnetic field [Shutte, 1985] and the characteristics of the ionospheric plasma (its composition, density, and, accordingly, plasma frequency). The fluctuating non-Maxwellian character of the near-equatorial energetic distributions observed in the above mentioned experiments [Heikkila, 1976; Prange, 1978; Shutte, 1983, 1985; Shutte et al., 1979] may be a signature of the effects of collective interactions between the particles and waves.

The real picture, of course, is not simple. Since the generated waves are nonmonochromatic, their propagation should be considered in conjunction with the effects of their attenuation in the ionosphere, and also with the condition of the surrounding plasma. Thus according to estimates of Ungstrup et al. [1979], the effects of transverse acceleration should take place at a very low density of the surrounding plasma (~300 cm ^-3 ).

Since meeting the conditions of driving oscillations requires the plasma density to exceed a critical value ( n₀>n_e ), this means that the region of electrostatic turbulence in space must be limited: basically, the upper ionosphere to altitudes of 1000 km in the daytime, when, because of the effect of photoionization, the range of critical values of density of the background plasma is widened. At night the upper altitude boundary should become lower, which is supported by the above presented data.

Observations of intense fluxes of charged particles by the APEX satellite at low and near-equatorial latitudes that agree well with earlier results [Heikkila, 1971; Prange, 1978; Shutte, 1983; Shutte et al., 1979], especially with those obtained at the Cosmos 900 satellite [Shutte et al., 1979; Shutte, 1983, 1985], in particular, the estimates of intensity of particle fluxes at night at altitudes of ~500 km, suggest that their existence in this region of the inner magnetosphere cannot be considered accidental.

The origin of the low-latitude and near-equatorial accelerated fluxes of charged particles in the upper ionosphere and inner magnetosphere, their relations with the geomagnetic field, and geomagnetic and solar activity levels seem to merit separate consideration. A theoretical model suitable for interpretation of the experimental data obtained is urgently needed.

Conclusion

1. Near-equatorial fluxes of charged particles with an intensity of 10⁵-10⁸ cm ^-2 sr ^-1 s ^-1 keV ^-1 are regularly observed at nightside at altitudes H<700 km.

2. Energetic distributions of the near-equatorial fluxes have a non-Maxwellian character, especially those along the field.

3. The observed low-latitude fluxes may be the result of the effects of collective resonance interactions of ionospheric plasma and waves. The condition of resonance transfer of the wave energy to the ionospheric plasma, under which the plasma density exceeds a critical value ( n₀>n_e ), must be met in the altitude interval ~300-1000 km. At night, the upper boundary corresponds to lower altitudes.

References

Bell, T. F., and H. D. Ngo, Electrostatic waves stimulated by coherent VLF signals propagating in and near the inner radiation belt, J. Geophys. Res., 93, 2599, 1988.

Heikkila, W. J., Soft particle fluxes near the equator, J. Geophys. Res., 76, 1076, 1971.

Prange, R., Energetic (keV) ions of ionospheric origin in the magnetosphere, Ann. Geophys., 34, 187, 1978.

Shutte, N. M., Trapped and precipitating charged particles with energies below 20 keV in low-latitude ionosphere, preprint of the Inst. of Space Res., no. Pr-818, 1983.

Shutte, N. M., Observations of electron and ion fluxes at low and equatorial latitudes of the inner magnetosphere, Kosm. Issled., 23 (4), 610, 1985.

Shutte, N. M., and N. I. Izhovkina, Charged particle fluxes in the inner magnetosphere in the region of the South-Atlantic magnetic anomaly and effects of a strong pitch-angle and radial plasma diffusion, Kosm. Issled., 23 (6), 921, 1985.

Shutte, N. M., and N. I. Izhovkina, On the role of ring current in the dynamics of fluctuating electron and ion fluxes in low-latitude magnetosphere, Kosm. Issled., 25 (3), 421, 1987.

Shutte, N. M., and N. I. Izhovkina, On properties of electrostatic turbulence and observations of non-Maxwellian distributions of charged particle fluxes in low-latitude magnetosphere, Kosm. Issled., 29 (3), 422, 1991.

Shutte, N. M., A. I. Puolokainen, and A. F. Kopylov, High and low latitude energy spectra of proton and electron precipitating into the ionosphere observed with the satellite Cosmos 900, Space Res., 19, 357, 1979.

Ungstrup, E., D. M. Klumpar, and W. J. Heikkila, Heating of ion to superthermal energies in the topside ionosphere by electrostatic ion cyclotron waves, J. Geophys. Res., 84, 4289, 1979.