CLUSTER observations of electron outflowing beams carrying downward currents above the polar cap by northward IMF

Abstract

Abstract. Above the polar cap, at about 5-9 Earth radii (R E ) altitude, the PEACE experiment onboard CLUSTER detected, for the first time, electron beams outflowing from the ionosphere with large and variable energy fluxes, well collimated along the magnetic field lines. All these events occurred during periods of northward or weak interplanetary magnetic field (IMF).These outflowing beams were generally detected below 100 eV and typically between 40 and 70 eV, just above the photoelectron level. Their energy gain can be explained by the presence of a field-aligned potential drop below the spacecraft, as in the auroral zone. The careful analysis of the beams distribution function indicates that they were not only accelerated but also heated. The parallel heating is estimated to about 2 to 20 eV and it globally tends to increase with the acceleration energy. Moreover, WHISPER observed broadband electrostatic emissions around a few kHz correlated with the outflowing electron beams, which suggests beam-plasma interactions capable of triggering plasma instabilities.In presence of simultaneous very weak ion fluxes, the outflowing electron beams are the main carriers of downward field-aligned currents estimated to about 10 nA/m 2 . These electron beams are actually not isolated but surrounded by wider structures of ion outflows. All along its polar cap crossings, Cluster observed successive electron and ion outflows. This implies that the polar ionosphere represents a significant source of cold plasma for the magnetosphere during northward or weak IMF conditions. The successive ion and electron outflows finally result in a filamented current system of opposite polarities which connects the polar ionosphere to distant regions of the magnetosphere.

Study of nonlinear ion- and electron-acoustic waves in multi-component space plasmas

“…The multi-component plasmas containing cold and hot electrons and ions, formed by the mixing of two plasmas of different temperatures could exist on a time scale shorter than the thermalization time, which in the collisionless space plasmas could be very large, of the order of several hours to days in the magnetosphere. Several spacecrafts have indeed observed multispecies plasmas in the magnetosphere (Tokar and Gary, 1984;Lin et al, 1984;Koskinen et al, 1990;Onsager et al, 1993;Matsumoto et al, GRL, 1994;Pottelette et al, 1999;Vogiatzis et al, 2006;Teste et al, 2007;Backrud-Ivgren et al, 2005;Pickett et al, 2005).…”

Polar cap ion beams during periods of northward IMF: Cluster statistical results

“…Above the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static fieldaligned electric field that can extend to altitudes higher than 7 R E (Maggiolo et al, 2006;Teste et al, 2007).…”

“…Above the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static fieldaligned electric field that can extend to altitudes higher than 7 R E (Maggiolo et al, 2006;Teste et al, 2007).Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas, we have been able to extract about 200 events of accelerated upgoing ion beams above the polar caps from the Cluster database. Most of these observations are taken at altitudes lower than 7 R E and in the Northern Hemisphere.…”

Distribution of Field‐Aligned Electron Events in the High‐Altitude Polar Region: Cluster Observations

“…This suggests an alternative path of entering the low-altitude polar region. Some authors considered that solar wind or magnetosheath electrons could be the main source for downward electrons in the cusp region (Lavraud et al, 2004;Shi et al, 2014;Smith & Lockwood, 1996), and the magnetotail electrons should be the main source for the downward electrons or precipitation in the nightside auroral oval (Barth et al, 2004;Berko & Hoffman, 1974;Crooker, 1992;Teste et al, 2007).…”