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

Abstract

We present statistical studies of the direction finding of 2ƒp 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 “2ƒp radio source.” The 2ƒp 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 10−0.008/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 2ƒp 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 2ƒp 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 2ƒp radio activity with the solar wind kinetic energy flux and a decrease of 2ƒp radio activity with decreasing IMF cone angle resulting in IMF lines tangent to the far flank of the bow shock. The 2ƒp radio activity is more affected by both parameters than the amplitude of Langmuir waves is affected. This suggests that the 2ƒp radio emissivity is very sensitive to the energy of Langmuir waves as expected from the generation process of 2ƒp 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.