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A balloon observation of auroral X-rays has been carried out by using two types of X-ray imagers. The first is a new type of imager consisting of a two-dimensional 5×5 matrix array of Si(Li) semiconductor detectors whereas the other is a one-dimensional array of 8 NaI(Tl) scintillation detectors. Both are pinhole camera type imagers. The balloon carrying these imagers was launched from Abelver, Norway, on July 5, 1985, and flew westward over Iceland. Near the location of L=5.2 at the geomagnetic midnight during a geomagnetic substorm, temporal variations of auroral X-ray images were observed with 3-s time intervals by the Si(Li) imager. These images were confirmed simultaneously by the NaI(Tl) imager. A time sequence of the auroral X-ray images did not show a stable arc structure, but rather a localized small-scale structure with rapid time variation. The present observation suggests that localized intermittent precipitations of energetic electrons with energies of several tens keV occurred in the post-break-up phase of an auroral substorm. It is fully possible that localized energetic electron precipitations are due to the localized field-aligned electric field.

The Japanese Akebono (EXOS-D) satellite was successfully put into a semipolar, elliptical orbit in order to study particle acceleration processes in the auroral magnetosphere. The onboard LEP (low energy particle) instrument makes three distinct types of charged particle observations: 1) energy and pitch-angle distributions of auroral electrons and ions, 2) mass per charge analysis of positive ions, and 3) onboard detection of particle flux modulations in the HF and VLF ranges. In this report we describe the LEP instrumentation, and present some preliminary results from item 1 above. Due to the initial orbital configuration, emphasis is placed on dayside phenomena, including the polar cusp. Unique examples of high particle activity in the polar cap and the hot plasma distribution along the transpolar arc are also presented. The observations in the cusp region reveal intense but patchy electron fluxes accompanied by keV-range ions with a characteristic energy versus time/space dispersion. The data are interpreted as signatures of longitudinal and/or temporal fluctuations of the magnetosheath plasma injection into the polar cusp. The dayside is characterized by the frequent appearance of low-energy ion conics and beams accompanied by intense low-energy electron precipitation. Finally it is noted that low-energy accelerated ions upgoing along the magnetic field line are observed in a very limited region around the dayside edge of the transpolar arc.

Plasma waves and sounder experiments (PWS) onboard the Akebono (EXOS-D) satellite have been successfully made after deployment of antenna and turn-on of the instruments on March 4, 1989. In natural plasma wave observations, the dynamic spectra of the plasma waves in the whole range of the orbits are observed in the PWS frequency range from 20kHz to 5MHz. The results are summarized as follows.i) The upper hybrid emissions that cover a frequency range from the electron cyclotron frequency fC to the upper hybrid frequency fUHR for the case where fC is higher than the local plasma frequency fP, or cover from fP to fUHR for the case of fP>fC, are always observed in the altitude range from 1000km to apogee of the orbit of the satellite. From the global distribution of the upper hybrid frequency emissions, the structures of the plasmasphere are clarified.ii) Auroral kilometric radiations are observed in the region associated with the auroral particle acceleration regions. Polarization and Poynting vectors of AKR are also precisely measured. The data show the possibility of studies on the structure of the acceleration regions in addition to the generation mechanism of AKR. The AKR data that have been obtained in the acceleration region of the auroral particles are especially important, suggesting the possible co-existence of the electrostatic plasma waves with the Z-mode waves.iii) In the equatorial region of the plasmasphere the enhancement of the upper hybrid mode waves is discovered; the enhancement shows two possible plasma turbulences, the first is the enhancement of the microscopic instabilities that are identified only as the enhancements of the intensity of the upper hybrid emissions and called “equatorial enhancement of the wave turbulence (EPWAT)”; and the second is the enhancement of the irregular change of the upper hybrid frequency suggesting the turbulent variation of the plasma density that is called “equatorial enhancement of the density turbulence (EPDET)”. EPWAT's encircle the magnetic equator forming a disc within the altitude range of 1000km to 9000km, while EPDET's encircle the geographic equator within the altitude range of 1000km to 9000km, forming another disc.iv) Sounder experiments of the PWS have also been carried out; the results show the principal resonances such as plasma resonances at fP, upper hybrid resonances at fUHR, and harmonics of the electron cyclotron frequency at nfC (n=1, 2, 3, ...). The sequence of diffuse plasma resonances fDn with the static plasma wave resonance fQn are also detected even in a high altitude range higher than 3000km. Very long ducted echoes of sounder transmitting pulses have also been detected even at the altitude of 6000km. All these results of the experiments reveal that the PWS/EXOS-D data provide a very promising key for studies of the plasma and plasma waves throughout the entire global ionosphere and plasmasphere, including the regions of auroral particles acceleration.

The obtained sheath capacity data of the spherical impedance probe with a radius of 6cm, installed on the TAIYO satellite, indicate periodic oscillations corresponding to wavy patterns, for the electron density distribution in the region of the ionosphere. The wave patterns can be identified with the substorm type geomagnetic disturbance within a possible delay time of 2 hours 30min to 6 hours from the onset time of the substorms. There were spread F echoes indicated in ISIS-2 data in the region of the South Atlantic anomaly coinciding with the time of the occurrence of the wavy pattern of the electron density profile detected by TAIYO in which the solar X-ray detector is also installed; the detector of the solar X-ray can be operated for the monitor of the energetic particle precipitation during night time conditions when no solar X-ray is injected to the detector. The occurrence of the spread F echoes is associated with the enhancement of energetic particle precipitations. The most likely process that can provide a consistent interpretation of these observed evidences is the TID effect that is triggered as additional effects of the enhanced particle precipitation of the high energetic particles from the radiation belt caused by the arrival of the TID in the region of the F-2 layer. The possible TID is inferred to be caused by the auroral break up in a form of the acoustic gravity wave which propagates through the F layer with group velocity ranging from 250m/sec to 760m/sec, period of about 103sec and the wavelength of about 500km.

Les analyses preleminaires des donnees de fusees-sondes transportant des detecteurs de modulation de flux d'electrons auroraux indiquent que, sous des conditions relativement calmes les electrons auroraux sont presque uniformes dans les etendues de hautes et moyennes frequences tandis que sous des conditions aurorales actives les resultats montrent des modulations BF du flux d'electrons associees avec des regions de precipitations d'electrons intenses

Energy spectra and pitch angle distributions of auroral electrons were obtained by means of two sounding rockets launched respectively into stable auroral arcs in geomagnetically quiet condition, and into active auroral arcs during the expansion phase of a substorm. These rockets were launched from Syowa Station, Antarctica in the austral winter in 1985.It was found that the observed energy spectra of downward electrons show a plateau extending from about 1keV to several keV in the center of an auroral arc, while they have a mono-energetic peak at the edge, or just outside an auroral arc. This feature suggests that strong wave-particle interaction processes are occurring on auroral field lines.The pitch angle distribution of auroral electrons at the mono-energetic peak was isotropic in the range from 0° to 90° (0° is upward along the magnetic field line), and was almost isotropic in the energy below the peak. The former feature suggests acceleration of auroral electrons at high altitude, and the latter feature is attributable to secondary electrons produced by auroral electrons.Just outside an auroral arc, the peak energy decreased with the distance from the arc. This feature can be considered to be an inverted-V structure of precipitating electrons observed at the rocket altitude.