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

Abstract

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 Earth's plasma sheet, where thermalized hot plasma with ion temperatures of about 5 x 10(7) K has been observed. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earth's 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 Earth's 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.

Electric field measurements on Cluster: comparing the double-probe and electron drift techniques

“…By using the ISEE-1 relaxation sounder, Etcheto and Saint-Marc (1985) found a plasma component below 30 eV sometimes dominating the plasma sheet boundary layer, reaching densities of 5 cm −3 . More recently, Seki et al (2003) found a similar plasma component in Geotail data from the plasma sheet itself, where we clearly had spurious fields in our example (Sect. 2.2.3).…”

Survey of cold ionospheric outflows in the magnetotail

“…With Geotail in eclipse, rendering a negative spacecraft potential, Seki et al (2003) were able to detect cold ions coexisting with the hot plasma sheet population. Olsen (1982) made similar measurements of cold ions when two different spacecraft (Applied Technology Satellite 6 and SCATHA) were in eclipse.…”

“…In addition, even if the cold ions are exempted from heating, the hot plasma sheet population will prevent the creation of a wake inhibiting measurement of cold ions. Detection of cold ions coexisting with the hot plasma sheet population, as revealed by Olsen (1982) and Seki et al (2003) is therefore not possible with our method.…”

Energetic O+and H+ions in the plasma sheet: Implications for the transport of ionospheric ions

“…First, during active times, the ion energy spectrum is generally more complex and may not be represented by a function or by any single function [Christon et al, 1991]. Second, the present approach disregards any coexisting cold ion component even though its density can be comparable to that of the hot component [e.g., Seki et al, 2003]. Therefore, we should keep in mind that those deduced parameters are concerned exclusively with the hot component.…”