Tomoko Nakagawa

Net current density of photoelectrons emitted from the surface of the GEOTAIL spacecraft

The current density carried by photoelectrons emitted from the GEOTAIL spacecraft is estimated from the electric potential of the spacecraft measured in the single probe mode of GEOTAIL/EFD and plasma density and temperature obtained by GEOTAIL/LEP during the period from September 14, 1993 to October 31, 1998, by assuming balance of the currents carried by photoelectrons and ambient thermal electrons. Behaviour of the photoelectron current as a function of spacecraft potential is consistent with the current profile predicted by Grard (1973), and the emitted photoelectrons consist of several components with different temperatures. The saturation density of the low energy component of the photoelectron current is 85 ± 33 × 10−6 [Am−2]. Number density of the photoelectrons is estimated to be 2.9 ± 1.4 × 109 [m−3] at the surface of the spacecraft, and the average energy of the photoelectrons is 2.1 ±0.5 [eV]. These values are higher than the prediction by Grard but consistent with previous in-flight measurements from GEOS-1, ISEE-1 or Viking.

GEOTAIL observation of upstream ULF waves associated with lunar wake

Left-handed, circular polarized ULF waves with frequency of 0.3−1.1 Hz were detected by GEOTAIL at 27 lunar radii upstream of the moon when the spacecraft was magnetically connected with the lunar wake. The wave was detected twice at 16:45–17:00 and 18:55–19:02 on October 25, 1994, when the spacecraft and the moon were on the dawn side of the Earth’s magnetosphere. The ULF wave was propagating in a direction nearly parallel to the background magnetic field. The observed frequency and polarization are explained by reversal of polarization of right-handed, sunward-propagating electron whistler waves with frequencies above 1.4 Hz in the solar wind frame of reference, which were excited through the interaction with electron beams flowing in anti-sunward direction downstream of the lunar wake. The downstream flow of electron beam is explained by filtering effect of the potential drop at the boundary of the lunar wake. Low-energy components of electrons are reflected back by the potential drop, and the rest components, with energies higher than that of the electric potential penetrate through the wake. The velocity distribution of downstream electrons would be modified to have some bump or shoulder in energy range to form a beam, which is likely to excite whistler mode wave through cyclotron resonance. The lowest energy of the resonant electrons was calculated to be 0.96–2.5 (keV) from the lower boundary of the detected frequency. The variation in the lowest frequency suggests that there are some regions of the lunar wake where potential drop is reduced.

Electromagnetic full particle simulation of the electric field structure around the moon and the lunar wake

The electric field structure around the moon is studied using a 2-dimensional electromagnetic full particle simulation. By considering absorption of the plasma particles at the surface of the moon, we obtain an intense electric field at the terminator region where the electric field produced by the negatively charged lunar surface and the ambipolar electric field at the wake boundary are in the same direction. The intensity of the electric field is 2.2E0 (E0 = m0υeωp/q0) at the terminator, corresponding to 3.5 V m−1 in the solar wind. It has a large horizontal component due to the potential difference between the negatively charged, antisolarside surface of the moon and the electrically neutral, solar-side surface, even though the emission of photoelectrons are not taken into consideration in this study. The half width of the electric field structure is of the order of Debye shielding length. The electric field at the downstream wake boundary at x = 6.5RL is stillas large as 0.1E0 ∼ 0.16 V m−1, which is strong enough to cause the pitch angle diffusion of the solar-wind electron beam, as is expected in the generation mechanism of the wake-related whistler wave. The ion acceleration occurs in the close vicinity of the moon and can be explained by the acceleration by the electric field produced by the surface charging of the moon.

Solar source of the interplanetary planar magnetic structures

Planar magnetic structure (PMS) is an interplanetary magnetic structure in which magnetic field vectors are all parallel to a plane but highly variable in both magnitude and direction in that plane. This magnetic structure corresponds to re-entrant loops of magnetic field lines in the photosphere that emanate into interplanetary space. To find information on the generation site, occurrence properties of PMSs are investigated by using the interplanetary magnetic field data obtained by Sakigake and ISEE-3 spacecraft. No significant correlation is found between PMS occurrence and the solar wind velocity gradient which would suggest interplanetary formation of PMSs. No significant correlation is found between the PMS events and flares or filaments, either. Instead, a half of the PMSs were projected to the vicinity of the sector boundary in the source surface magnetic field, although there are exceptions when PMS appeared in the center of a sector. The PMS planes were not parallel to the current sheet at the sector boundary. Sometimes PMSs were observed recurrently at the same heliospheric longitude in successive rotations of the Sun, suggesting persistence of the source of PMS on the Sun. The orientation of the PMS planes were not conserved in the recurrent PMSs.

Plasma Waves and Sounder (PWS) experiment onboard the Planet-B Mars orbiter

For the purpose of observing the Martian ionosphere under the condition of direct interaction with the solar wind by using a RF sounder together with a plasma density probe and high frequency plasma wave detectors, the Plasma Waves and Sounder (PWS) system onboard the Planet-B Mars orbiter has been developed through a proto-model (PM) which are followed by achievement of a flight-model (FM) for the Planet-B mission. The operation of the achieved system has been tested for function and stability in a simulated space environment. The PWS system is equipped with two sets of long deployable dipole antennas with a tip-to-tip length of 52 m. The high power (600 Watts) transmitter of the sounder experiment makes it possible to measure the electron density profiles of the Martian topside ionosphere in an altitude range from 300 to 3000 km in a density range from 102 to 106/cc with a time resolution of 25 µsec corresponding to a range resolution of 3.75 km. Observations of natural plasma waves and planetary radio waves in the frequency range from 20 kHz to 5 MHz are planned in the passive mode operation of the PWS system, identifying all the polarization characteristics together; observations of natural plasma waves caused by the direct interaction of the Martian ionosphere with the solar wind plasma are also a major purpose of the PWS experiment. For these purposes, a wide dynamic range and high sensitivity characteristics of the PWS receiver have been achieved. As the extended function of the PWS plasma sounder instrument, altitude measurement also will be carried out.

NOZOMI observation of the interplanetary magnetic field in 1998

The insertion of NOZOMI spacecraft into interplanetary space, where other spacecraft were operating, added a new array of multipoint observation that extended up to 1.6×106 km in y-direction of the GSE coordinate system. The coherency length of planar magnetic structures has been found to be less than 1.9×106 km in a direction tangential to the discontinuity surface. The region of low coherency seemed to coincide with region of high density plasma. An interplanetary magnetic flux rope which appeared on October 19, 1998, was identical over a distance of 1.9×106 km, while in a high-beta sheath preceding it, the magnetic field appeared differently at separate spacecraft.

Pitch angle diffusion of electrons at the boundary of the lunar wake

Velocity distribution of the solar wind electrons that penetrate through the lunar wake boundary is investigated by calculating orbits of the electrons injected into model structures of layers of electric fields. Only the electrons with sufficient energy to overcome the potential difference penetrate through the wake boundary. The electrons injected along the magnetic field lines which intersect the model structure undergo pitch angle scattering due to electric field component perpendicular to the magnetic field. After the passage through the electric field, the electrons have significant perpendicular component of velocity as well as the parallel component larger than a lower limit, which is dependent on the electric potential of the wake boundary. The velocity distribution can account for the cyclotron resonance with sunward-propagating whistler mode waves that were detected by GEOTAIL at 27 lunar radii upstream of the moon on October 25, 1994.

Plasma velocity in interplanetary planar magnetic structures

Abstract Non-Parker type interplanetary objects, planar magnetic structures, have been examined by using magnetic field and low-energy particle data obtained by GEOTAIL. Bi-directional electrons characteristic of closed field lines were not found in planar magnetic structures. Jumps in magnetic field in planar magnetic structures were tangential discontinuities across which there was no mass flux, with a few exception. It suggests that a planar magnetic structure is made of layers of plasmas bounded by several tangential discontinuities, and in a few cases, there is an interaction between the layers.

A reexamination of pitch angle diffusion of electrons at the boundary of the lunar wake

Velocity distribution of the solar wind electrons injected into the lunar wake boundary is re-examined by using a simple model structure of inward electric field. The electrons that were flowing along the magnetic field lines undergo pitch angle scattering due to the electric field component perpendicular to the magnetic field. The electrons obtain perpendicular speeds twice as much as the drift speed. On the basis of the GEOTAIL observations of the whistler mode waves and strahl electrons, the intensity of the electric field and the thickness of the wake structure are estimated to be 28-40 mVm-1 and less than 20 km, respectively.

Interplanetary planar magnetic structures associated with expanding active regions

Non-Parker type interplanetary objects, planar magnetic structures, have been examined in relationship to ‘active region expansions’ discovered in the Yohkoh soft X-ray observations. During the quiet period of the Sun from January 6 to November 11, 1993, solar sources of 5 typical planar magnetic structures, which were detected by Sakigake, were searched for in the Yohkoh soft X-ray telescope data. Loop structures were seen to expand outward above the corresponding active regions in the probable sources.