H. Hayakawa

The SEEK (Sporadic‐E Experiment over Kyushu) Campaign

The SEEK (Sporadic-E Experiment over Kyushu) campaign was conducted in late August 1996 from the southern region of Kyushu, Japan, to investigate the mechanism for the generation of quasi-periodic (QP) radar backscatter from field-aligned irregularities imbedded in nighttime sporadic-E (Es) layers. SEEK was designed to determine in-situ small-scale electrodynamical properties using two sounding rockets and large-scale dynamics and electrodynamics using ground-based sensors, which included a transportable radar and other radio and optical instruments deployed in the vicinity of the rocket range. It was observed by this campaign that Es layers existed in a convergent wind shear region, where large electric fields were induced and when active atmospheric gravity waves existed in the mesosphere. However, there was little evidence which positively supported the hypothesis that Es layers were deeply modulated in altitude.

Cross polar cap diameter and voltage as a function of PC index and interplanetary quantities

Measurements of precipitating particles and the electric field on board EXOS D spacecraft for the period January–June 1990 have been used for estimation of the diameter of the polar cap along dawn-dusk meridian and the polar cap voltage. Identification of the polar cap boundaries has been made on the basis of specific features of precipitating ions. The data on the polar cap boundary location obtained for different geophysical conditions have been used to derive the statistical relationship between the polar cap diameter and PC index. The analysis has shown an approximately linear relationship between the polar cap diameter and the PC index for values PC < 3, the diameter tending to be asymptote when the PC index reaches large positive values. Cross polar cap voltage derived from EXOS D data is in good correlation with interplanetary quantities including the interplanetary magnetic field (IMF) southward component. The best correlation is obtained for the merging electric field υBT sin2 θ/2, with a coefficient of correlation higher than 0.82. Almost the same correlation is observed between polar cap voltage and PC index. The effect of “saturation” is not traced in the voltage dependencies on the PC index and interplanetary quantities up to values BT ≤ 10 nT.

A statistical study of variations in the near and middistant magnetotail associated with substorm onsets: GEOTAIL observations

We have studied the three-dimensional structure of substorm-associated variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 RE (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 RE, corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn-dusk electric field, first starts to be enhanced around (X, Y) ∼ (−20,7) RE about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around (X, Y) ∼ (−18,7) RE about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn-dusk electric fields both calculated with E = −V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 RE and X ∼ −28 RE immediately after onset, while those in the plasma sheet around.(X, Y) ∼ (−20,5) RE do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy release, where the near-Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around (X, Y) ∼ (−19,6) RE. These features are consistent with a thin-current reconnection model.

Cold ions in the hot plasma sheet of Earth's magnetotail

Most visible matter in the Universe exists as plasma. How this plasma is heated, and especially how the initial non-equilibrium plasma distributions relax to thermal equilibrium (as predicted by Maxwell–Boltzman statistics), is a fundamental question in studies of astrophysical and laboratory plasmas. Astrophysical plasmas are often so tenuous that binary collisions can be ignored, and it is not clear how thermal equilibrium develops for these ‘collisionless’ plasmas. One example of a collisionless plasma is the Earths plasma sheet, where thermalized hot plasma with ion temperatures of about 5 × 107 K has been observed. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earths plasma sheet in coexistence with thermalized hot ions. This cold component cannot be detected by plasma sensors on satellites that are positively charged in sunlight, but our observations in the Earths shadow show that the density of the cold ions is comparable to that of hot ions. This high density is difficult to explain within existing theories, as it requires a mechanism that permits half of the source plasma to remain cold upon entry into the hot turbulent plasma sheet.

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.

Low-energy charged particle measurement by MAP-PACE onboard SELENE

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the SELENE (SELenological and ENgineering Explorer) satellite. PACE consists of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measure the distribution function of low-energy electrons below 15 keV, while IMA and IEA measure the distribution function of low energy ions below 28 keV/q. Each sensor has a hemispherical field of view. Since SELENE is a three-axis stabilized spacecraft, a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA) are necessary for obtaining a three-dimensional distribution function of electrons and ions. The scientific objectives of PACE are (1) to measure the ions sputtered from the lunar surface and the lunar atmosphere, (2) to measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer onboard SELENE simultaneously as an electron reflectometer, (3) to resolve the Moon-solar wind interaction, (4) to resolve the Moon-Earth’s magnetosphere interaction, and (5) to observe the Earth’s magnetotail.

Statistical nature of impulsive electric fields associated with fast ion flow in the near-Earth plasma sheet

Statistical characteristics of impulsive electric field (IEFD) associated with fast earthward ion flows in the inner central plasma sheet were studied by using electric field data obtained from the double-probe instrument on board the Geotail satellite. It is shown that the strongest electric field is produced in the region between X AGSM = -15 R E and -9 R E where the fast earthward flow is decelerated. The IEFDs are short-lived with a timescale of ∼2 min on the average, compatible with the timescale of other signatures of substorm fine structures. This strong cross-tail electric field, coincident with the braking of the fast flow and correlated with the magnetic field dipolarization, has important implications for the particle acceleration in the near-Earth plasma sheet.

Electric field fluctuations and charged particle precipitation in the cusp

In the cusp region, irregular fluctuations of the electric field are often observed by EXOS D at altitudes of several thousands of kilometers. Amplitudes of the fluctuations sometimes reach 100 mV/m and their spectra are broad. The electric to magnetic field ratios in the frequency range of 0.5-3 Hz agree well with the Alfven velocity at observation point. Hence the electric fluctuations are considered to be Alfven waves. The flux of precipitating ions at 500 eV to 10 keV and that of the electrons at 70-500 eV are enhanced in the cusp and they are well correlated to each other. On the other hand, correlation coefficients between the power spectral density (PSD) of the electric field at 1 Hz and the precipitating particle flux vary from case to case. When the latitude of the cusp is low and IMF is expected to be southward the coefficient is high. This suggests that the waves are generated in association with the injection of particles into the magnetosphere when reconnection occurs. On the other hand, when the latitude of the cusp is high and IMF is expected to be northward, the coefficient is low and the PSD of the electric field is smaller for the same flux of particles than when IMF is southward. In these cases the intensity of the electric fluctuations in the region of the particle injection is possibly not so great as that when reconnection occurs.

Two types of ion energy dispersions observed in the nightside auroral regions during geomagnetically disturbed periods

The Akebono satellite has observed two types of energy dispersion signatures of discrete ion precipitation event in the nightside auroral regions during active geomagnetic conditions. The charged particle experiments and electric and magnetic field detectors on board Akebono provide us with essential clues to characterize the source regions and acceleration and/or injection processes associated with these two types of ion signatures. The magnetic field data obtained simultaneously by the geosynchronous GOES 6 and 7 satellites and the ground magnetograms are useful to examine their relationships with geomagnetic activity. Mass composition data and pitch angle distributions show that different sources and processes should be attributed to two types (Types I and II) of energy dispersion phenomena. Type I consists of multiple bouncing ion clusters constituted by H+. These H+ clusters tend to be detected at the expansion phase of substorms and have characteristic multiple energy-dispersed signatures. Type II consists of O+ energy dispersion(s), which is often observed at the recovery phase. It is reasonable to consider that the H+ clusters of Type I are accelerated by dipolarization at the equator, are injected in the field-aligned direction, and bounce on closed field lines after the substorm onset. We interpret these multiple energy dispersion events as mainly due to the time-of-flight (TOF) effect, although the convection may influence the energy-dispersed traces. Based on the TOF model, we estimate the source distance to be 20–30 RE along the field lines. On the other hand, the O+ energy dispersion of Type II is a consequence of reprecipitation of terrestrial ions ejected as an upward flowing ion (UFI) beam from the upper ionosphere by a parallel electrostatic potential difference. The O+ energy dispersion is induced by the E × B drift during the field-aligned transport from the source region to the observation point.

On the sources of energization of molecular ions at ionospheric altitudes

During geomagnetically active times, the suprathermal mass spectrometer on the Akebono satellite frequently observes upflowing molecular ions (NO+, N2+, O2+) in the 2-3 Earth radii geocentric distance regions in the auroral zone. Molecular ions originating at ionospheric altitudes must acquire an energy of the order of 10 eV in order to overcome gravitation and reach altitudes greater than 2 RE. This energy must be acquired in a time short compared with the local dissociative recombination lifetime of the ions; the latter is of the order of minutes in the F region ionosphere (300-500 km altitude). Upflowing molecular ions thus provide a test particle probe into the mechanisms responsible for heavy ion escape from the ionosphere. In this paper we analyze the extensive complement of plasma, field, and wave data obtained on the Akebono satellite in a number of upflowing molecular ion events observed at high altitudes (5000 - 10,000 km). We use these data to investigate the source of energization of the molecular ions at ionospheric altitudes. We show that Joule heating and ion resonance heating do not transfer enough energy or do not transfer it fast enough to account for the observed fluxes of upflowing molecular ions. We found that the observed field-aligned currents were too weak to support large-scale field-aligned current instabilities at ionospheric altitudes. The data suggest but in the absence of high-resolution wave measurements in the 300 to 500 km altitude range cannot ascertain the possibility that a significant fraction of escape energy is transferred to molecular ions in localized regions from intense plasma waves near the lower hybrid frequency. We also compared the energization of molecular ions to that of the geophysically important O+ ions in the 300 to 500 km altitude range, where the energy transfer to O+ is believed to occur via small-scale plasma instabilities, ion resonance, and ion-neutral frictional heating. Direct observation of energy input to the ionosphere from all of these sources in combination with in situ measurements of the density and temperature of neutral and ionized oxygen in the 300 to 500 km range are required to determine the relative importance of these energy sources in providing O+ with sufficient energy to escape the ionosphere.