Statistical studies of plasma waves and backstreaming electrons in the terrestrial electron foreshock observed by Geotail

Abstract

Abstract. We present statistical studies of the direction finding of 2fp radiation and the spatial distribution of plasma waves and energetic particles in the terrestrial electron foreshock observed by Geotail. First, we investigate the geometry of the electron foreshock which is assumed to be the "2fp radio source." The 2fp radio source is likely to be in the leading region of the electron foreshock where the most intense Langmuir waves are observed. The Langmuir wave activities and the population of energetic electrons gradually decrease in the region beyond 10 RE from the contact point. The decreasing rate of Langmuir wave activity is very small, about 10 -ø'øøs/RE. We also find that in the region around the contact point of the tangential interplanetary magnetic field (IMF) lines and the bow shock, the observed amplitude of the 2fp radiation seems to become weak. We think that it is due to the weak activities of Langmuir waves in the region close to the contact point and/or the directivity of 2fp radiation along the tangential IMF line. Next, we investigate the influence of the solar wind conditions on the activities in the electron foreshock. We confirm a positive correlation of the 2fp radio activity with the solar wind kinetic energy flux and a decrease of 2fp radio activity with decreasing IMF cone angle resulting in IMF lines tangent to the far flank of the bow shock. The 2fp radio activity is more affected by both parameters than the amplitude of Langmuir waves is affected. This suggests that the 2fp radio emissivity is very sensitive to the energy of Langmuir waves as expected from the generation process of 2fp radiation. Such high sensitivity also supports the concentration of the radio emissivity in the leading region of the electron foreshock and the limitation of the radio source extension along the magnetic field line. We also reinvestigate the comparison between the terrestrial and Venusian foreshocks. The differences between them suggest the nonsimilarity of shocks with different sizes.

Statistical behavior of foreshock Langmuir waves observed by the Cluster wideband data plasma wave receiver

“…3, are nearly identical. The nearsimultaneous observations of waves with similar amplitudes implies that the foreshock structure is fairly uniform on scales of 50 to 100 km, and supports earlier statistical studies showing that far away from the boundary significant changes in the average wave amplitudes in the foreshock occur on scales greater than ∼0.5-1.0 R E (Greenstadt et al, 1995;Kasaba et al, 2000).…”

“…Earlier studies of Langmuir waves in the Earth's foreshock conducted using data from ISEE (e.g. Greenstadt et al, 1995;, Geotail (Kasaba et al, 2000), and other spacecraft (e.g. Onsager et al, 1989) relied upon spectral density measurements, which can underestimate the wave amplitudes due to temporal and spectral averaging (see Robinson et al, 1993).…”

Low frequency wave sources in the outer magnetosphere, magnetosheath, and near Earth solar wind

“…The electron foreshock is indicated by the increase in the Langmuir wave activity and by the presence of waves with twice the electron plasma frequency, the so-called 2f p emissions (see Tsurutani and Rodriguez, 1981;Kasaba et al, 2000).…”

Plasma wave observation using waveform capture in the Lunar Radar Sounder on board the SELENE spacecraft

“…In the history of Japanese scientific spacecraft, the GEOTAIL mission is one of the most important projects investigating the phenomena in the earth's magnetosphere, by which so many new findings and paradigms were introduced by combining observation, computer simulations and theoretical studies. The WFC onboard SELENE is expected to clarify the detailed features of the wave phenomena observed by PWI instruments (Matsumoto et al, 1994a) on board the GEOTAIL spacecraft, such as Auroral Kilometric Radiation (AKR) (Murata et al, 1997;, nonthermal continuum and kilometric continuum (KC) (Hashimoto et al, 1999(Hashimoto et al, , 2005(Hashimoto et al, , 2006, continuum enhancement , low-frequency bursts (Anderson et al, 1997), electrostatic solitary wave (ESW) (Matsumoto et al, 1994b), narrowband electrostatic noise (NEN) , f p and 2 f p emissions (Kasaba et al, 2000), and solar radio emissions (Kasahara et al, 2001), among others. Emissions with frequencies higher than the maximum plasma frequency of the magnetosheath (about 30 kHz), such as AKR, KC, and continuum enhancement, and type II and III solar bursts are observed both inside and outside the magnetosphere.…”