Takumi ABE

Numerical simulations of Boltzmann's equation have been carried out in order to investigate the anisotropy of the electron energy distribution in the topside F region ionosphere. The equation contains both electron-electron collisions and electron-ion collisions. The simulations were compared with the observational data obtained with the electron temperature probe on board the Ohzora satellite. The results show that the anisotropy of the electron energy distribution can be generated in the topside F region ionosphere and that the electron energy distribution can differ from the Maxwell distribution, when an electric field or temperature gradient (heat flux) exists in the plasma. The simulations also indicate the dependence of electron density on the anisotropy of the ionospheric electron energy distribution.

We investigate the heliographic latitude-longitude structure of solar wind speed between Sept. 1985 and Jan. 1987 through the analysis of the solar wind data obtained by Sakigake, Suisei, and IMP-8. Until Jan. 1986,both polar regians had a high-speed stream extended across the equatorial region. After Feb. 1986,high-speed streams were rarely seen on the equator and low-speed streams were ranged along the equator, although there remained a slight wavy pattern in the latitude-longitude structure. This wavy pattern has been stable till the end of 1986.

27-day recurrence-associated phenomena of the solar wind (SOW), the plasma wave (PWP), and the interplanetary magnetic field (IMF) were detected by the Sakigake spacecraft. The heliospheric structures, having a neutral (current) sheet of ∿2×10^4km thickness and a "plasmasheet" of ≳7×10^5km thickness, during April, 1985-August, 1986 and September, 1986-July, 1987,were in the excursion and aligned phases, respectively. In the neutral sheet the IMF direction usually reverses in a minute. Using high time-resolution data, the average ion density and temperature in the heliospheric plasma sheet are also found to be ≳60/cc and ∿20×10^4K, respectively.

Periodic components were picked up by applying maximum entropy method to the visual brightness of comet Halley which was measured from the ground. 2.22 days' period appears between November 1985 and February 1986. This period is the same as obtained by using Planet-A and Vega imaging experiments. This rotational period shifts to 2.33 days after April 1986. 7.4 days' period is detected from March to April 1986. Millis and Schleicher also indicate that the rotational period of the comet is near 7.4 days. Between March and May 1986,3.4 days' period appears, which can be explained by assuming that 2.33 days' period is modulated by the period of 7.4 days.

Solar wind parameters measured by SAKIGAKE around the time of encounter with Halley comet are discussed Sakigake was in or near the neutral sheet of Heliomagnetosphere and therefore the solar wind velocily was comparatively slow. Interplanetary space was in a small disturbance period. Although a firm evidence does not exist that solar wind velocity was decelerated by Halley's comet, frequency spectrum analysis of the solar wind parameters shows the exisistence of water group ions.

Electron temperature was measured with planar plobes boarding on ISAS 10th scientific satellite "OHZORA". The simultaneous measurments of electrons moving along geomagnetic line of force with those moving perpendicularly revealed anisotropy of the electron temperature. In the region of geomagnetic latitude above 20 degrees, electron temperature measured with the probe which surface is perpendicular to the geomagnetic line of force, T_⫽ is, in most cases, higher than that measured with the one parallel to geomagnetic field, T_⊥. Below 20 degree, T_⊥ was higher than T_⫽ with some exceptions. A celear local time dependence can be seen; in night time the events that T_⫽ was higher than T_⊥ were more frequently observed than those that T_⫽ was lower than T_⊥. No correlation between temperature anisotropy and Kp index was observed. The anisotropy of electron temperature will interpret some discripancies of electron temperature measured so far.