M. Hirahara

Slow-mode shocks in the magnetotail

We have identified slow-mode shocks between the plasma sheet and lobe in the midtail to distant-tail regions by using three-dimensional magnetic field data and three-dimensional plasma data including density, velocity, temperature, and heat flux of both ions and electrons observed by the GEOTAIL satellite. Analyzing the data obtained between September 14, 1993, and February 16, 1994, we have found 303 plasma sheet-lobe boundary crossings at distances between -XGSE ∼ −30RE and XGSE ∼ −210RE. Thirty-two out of these 303 boundaries are identified as slow-mode shocks. We have found back streaming ions on the upstream side of the slow-mode shocks, which may be important in understanding the dissipation mechanism of the slow shocks in collisionless plasma. We have also found acceleration of cold ions between the upstream and the downstream of the slow-mode shocks. These cold ions are often observed in the lobe, and they are usually flowing tailward. Upon entering the plasma sheet, they are accelerated and rotate around the magnetic field and at times show ring-shaped velocity distributions. These ions may reflect the kinetic structure of slow-mode shocks. Slow shocks are at times observed also on the front side of plasmoids. These slow shocks on the front side of plasmoids have a different orientation from that of the ordinary slow shocks observed at the plasma sheet-lobe boundaries, which suggests an existence of “heart” -shaped plasmoids predicted by a numerical simulation.

Statistical properties and possible supply mechanisms of tailward cold O+ beams in the lobe/mantle regions

We investigate statistical properties of cold O+ beams (COBs) streaming tailward at the velocity nearly equal to the major H+ component, which were observed by Geotail/low-energy particle (LEP) instrument in the tail lobe/mantle regions at geocentric distance between 8 and 210 RE (Earth radii) during the solar-minimum period (October 1993 to March 1995). The average O+ density is ∼1.3×10−3 cm−3, which corresponds to ∼1.2 % of the proton component. Properties of the flow velocity show that it is not the weakening of the magnetospheric convection but the large parallel velocity which enables the O+ ions to remain still in the distant lobe/mantle regions. The occurrence frequency of COBs suggests that O+ ions tend to exist in the mantle-like region rather than tenuous “pure lobe” and that their existence has a clear correlation with the geomagnetic activity. On the basis of the EMF By dependence of the double-peaked COB distribution along dawn-dusk direction, it is shown that COBs exist mostly on loaded quadrants in the north–south and dawn-dusk asymmetry of sheath plasma entry caused by the IMF By effect on the dayside reconnection process. The concentration on the loaded quadrants can be seen even in geomagnetic storms. It suggests that frequent COB occurrence at active times is mainly due to the southward orientation of IMF rather than the increase of dynamic pressure itself during the geomagnetic storms. That is, the statistics show that COBs are abundant at geomagnetically active times on loaded quadrants resulting from the dayside reconnection process, where the ions of solar wind origin bear the major component, and their field-aligned velocity is larger than usual. These COBs should originate in the dayside magnetosphere and/or the polar cap regions. From the COB energy of several keV, which is rather higher than that of cusp/cleft ion outflows, the necessity of extra energization(s) to elevate parallel velocity is suggested. Clear IMF By dependence, on one hand, provides other possibilities of the COBs source such as the energetic UFI beams and the equatorially trapped ions. The requirements for each candidate so as to be a main contributor to COBs are also discussed.

Geotail observation of cold ion streams in the medium distance magnetotail lobe in the course of a substorm

We observed cold ion beams in the magneto-tail lobe at XGSM ∼ −42 Re in the initial operations of the Low Energy Particle (LEP) experiment onboard the GEOTAIL satellite on August 22, 1992, when multiple onsets of substorms took place. These ion beams generally consisted of protons and singly-charged oxygen ions (O+), flowing tailward with nearly the same velocities ∼ 100–200 km/s. The H+ number density was generally of order 10−2 cm−3, while the O+ density was of order 10−3 cm−3 but at times increased sporadically by an order of magnitude. These ions presumably would be transported along magnetic field lines through the polar mantle from the dayside polar ionosphere and convected to the mid-magnetotail where the observation was made. That different ion species had equal velocities is consistent with the velocity filter effect due to E × B convection. Three-dimensional determinations of their velocity distributions have revealed off-ecliptic angles of the flow directions as large as several tens of degrees. We discuss this feature in terms of the enhanced convection velocity that is associated with the plasma-sheet thinning confined to a limited region in the Y direction. Characteristic changes in the flow velocity are also found in association with a substorm plasmoid passage.

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.

Coexistence of Earth-origin O+ and solar wind-origin H+/He++ in the distant magnetotail

In the lobe/mantle region at ∼159 RE away from the Earth during a geomagnetically disturbed period, we have found the coexistence of three ion species, H+, He++, and O+, streaming tailward with nearly the same flow velocity ∼200–500 km/s. Both H+ and O+ ions are detected almost continuously from near plasma sheet to near magnetopause region. From a positive correlation between the proton density and their velocity component parallel to the magnetic field VH+║, we conclude that most of protons have come from the solar wind. The existence of He++ further supports this conclusion, which implies the importance of solar wind contribution to the magnetotail. The existence of O+, on the other hand, suggests that the ions of ionospheric origin have mixed with those of solar wind origin. The lack of positive correlation between O+ density and VO+║ is consistent with the idea that O+; ions have some source mechanism different from that of protons. Simultaneously, curious velocity differences are also observed: VO+¶ appears to be often faster than VH+║; by ΔV¶ = 20–30 km/s. This observation may provide a key for further discussion.

Cold dense ion flows with multiple components observed in the distant tail lobe by Geotail

The Geotail spacecraft frequently observes cold dense ion flows (CDIFs) streaming tailward in the magnetotail lobe region (XGSM∼−10 to −210 RE). The density is often quite high (∼1/cm3), comparable to or larger than that in the plasma sheet, and the tailward speed is from 50 to 500 km/s. The CDIFs sometimes contain multiple (two or three) energy-per-charge components. The lowest- and highest-energy components are identified as H+ and O+, respectively. The intermediate-energy component is identified as He+. Therefore a part of the CDIFs in the distant lobes is believed to be of ionospheric origin. A possible source candidate is the upward flowing ions (UFIs) outgoing from the dayside auroral region or polar wind escaping from the polar cap and accelerated along the magnetic field direction. The high proton to O+ density ratio in the CDIFs suggests that the major component of the CDIFs originates from the solar wind and that the cold ion beams of ionospheric origin merge with the solar wind component penetrating into the magnetosphere at the mantle or the flank of the tail lobe. In some observations, the H+ ions continuously stream tailward both in the tail lobe and in the magnetosheath, and they couple with the solar wind ions through the magnetopause. Although the O+ and He+ ion flows cannot be recognized so clearly in the magnetosheath, weak O+ fluxes are sometimes present in the magnetosheath near the magnetopause. This implies that plasma of ionospheric origin may leak from the tail to the magnetosheath. The multicomponent CDIF events make it possible to discuss the sources and transport processes of the tail lobe plasma, and their behavior in or near the magnetopause provides important clues to the interactions between the lobe and the magnetosheath.

Auroral particle instrument onboard the index satellite

Abstract The INDEX satellite is a microsatellite which will be inserted into a low-altitude (680 km) polar orbit. A low-energy plasma particle instrument, which consists of two sensor heads (electron/ions sensors;,ESA/ISA) and a multi-spectral auroral camera (MAC) will be installed in the INDEX satellite in order to investigate formation mechanisms of fine-scale structures of optical auroral arc emissions. One of advantages of the INDEX mission is that the satellite is capable to observe fine-scale auroral arc emissions and corresponding auroral particles simultaneously by controlling an attitude of the satellite. Since the satellite velocity is relatively fast, a high time-resolution is necessary for the plasma measurement. The time resolution of the plasma instruments onboard the INDEX satellite is 20 ms, which corresponds to a spatial scale of ∼150m.

GEOTAIL low energy particle and magnetic field observations of a plasmoid at XGSM = −142 RE

Data from the GEOTAIL Low Energy Particle (LEP) instrument and magnetometer (MGF) for a plasmoid event on October 8, 1993 when the spacecraft was located at XGMS ∼−142 RE were analyzed. The event started 16 minutes after a substorm onset with a tailward flow of electrons whose characteristic energy was several keV with an apparent energy dispersion. This was followed by the arrival of a beam of high energy ions, which also had a similar energy dispersion, and an enhancement of the magnetic field intensity. There was appreciably high flux of cold ions in the lobe region, and it was further enhanced at the front of the plasmoid. These cold ions were energized at the boundary of the plasmoid where a sharp decrease in the total magnetic field intensity was observed. It was found that the main part of the plasmoid had a significant dusk-to-dawn magnetic field, indicating the plasmoid had a flux rope, or a distorted closed-loop magnetic field structure. After passing the main part of the plasmoid, the spacecraft entered a region characterized by high energy tailward flowing ions and coexisting cold ions. The level of low frequency magnetic field fluctuations was relatively high, and there were some time variations in both high-energy and cold ion fluxes. Synthesizing these features, we conclude that our observations are consistent with the creation of a plasmoid and surrounding energetic particle layers following a substorm as predicted by the near-Earth neutral line model of a magnetospheric substorm.

Terrestrial plasmaspheric imaging by an Extreme Ultraviolet Scanner on planet‐B

An extreme ultraviolet (XUV) scanner on board the Mars orbiter, Planet-B, observed the terrestrial plasmasphere while it was in a parking orbit around the earth. The image was available only for the dusk side of the plasmasphere. Nonetheless, this was the first image from the outside of the plasmasphere. Future missions, such as the NASA IMAGE mission in 2000 as well as the Japanese plasmaspheric imaging telescope on SELENE in 2003 will provide further information on this region.

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.