Effect of hot electrons on the polar wind

Abstract

A semikinetic model was used to study the effect that hot electron populations have on the polar wind. The model was used to describe the steady state plasma flow along diverging geomagnetic field lines in the collisionless regime at high altitudes. The plasma contained O+ and H+ ions and both hot and cold electron populations. Several hot electron populations were considered, including the polar rain, polar showers, and polar squall. Estimates of hot electron parameters based on characteristic energy and flux measurements indicate that the hot/cold electron temperature ratio varies from 10 to 104 and that the percentage of hot electrons varies from 0.1% to 10% at 4500 km. For ratios at the lower ends of these ranges the polar wind solutions with hot electrons are similar to those obtained previously for supersonic H+ outflow without hot electrons. For higher hot electron temperatures and a greater percentage of hot electrons, there is a discontinuity in the kinetic solution, which indicates the presence of a sharp transition. This transition corresponds to a contact surface between the hot and cold electrons. Along this surface, a double‐layer potential barrier exists which reflects the cold ionospheric electrons and prevents their escape. The presence of the hot electrons acts to increase the supersonic H+ outflow velocity and H+ energy but does not affect the already saturated H+ escape flux. The H+ energy gain may be as large as 1 to 2 keV. With regard to O+, the hot electrons act to reduce the potential barrier, thereby allowing more O+ ions to escape. A significant enhancement in the O+ escape flux can occur depending on the hot electron density and temperature.

Simulating the cleft ion fountain at polar perigee altitudes

“…Even if the effects of the escaping photoelectrons were important, it is still expected that the day-night density asymmetry is in large part controlled by the cleft ion fountain, as demonstrated in the present simulations. Effects of polar rain were not incorporated in the simulations either, since previous studies suggested that their accelerating effects are important at altitudes higher than 3-4 R E (Barakat and Schunk, 1984;Ho et al, 1992).…”

transport across the polar cap

“…Classical polar wind models are not able to predict O þ escape. Additional mechanisms that can explain how O þ ions are accelerated out of the ionosphere are: wave-particle heating by broadband emissions (Barghouthi, 1997), field-aligned potential drops set up by field-aligned currents (Mitchell et al, 1990;Ho et al, 1992;Barakat and Schunk, 1984; Barakat et al, 1998), hot and cold (thermal and photo) electron populations interacting with each other (Moore et al, 1999a;Su et al, 1998a,b;Schunk, 2000), centrifugal acceleration owing to convection, and flux tube compression and dilation (Horwitz, 1987;Cladis et al, 2000;Swift, 1990;Horwitz et al, 1994). With increasing height ion inertia, dynamic effects, and convection become increasingly important (Banks and Kokarts, 1973;Swift, 1990).…”

Heating of the sunlit polar cap ionosphere by reflected photoelectrons

“…Wilson et al [1997] argued that this mechanism was the most reasonable, and in kinetic simulations Wilson et al [1997] and Su et al [1998] were able to find zero current solutions without cold electron inflow or heavy ion outflow by inserting double layers with potential drops of tens of eV. One possible formation mechanism for these double layers is a contact discontinuity which forms between the cold ionospheric electron gas and the hot magnetosheath electron gas [Barakat and Schunk, 1984;Barakat et al, 1998b]. The simulations of Barakat et al [1998b] show that these double layers form between 2 and 6 R E in altitude, and their locations are highly dynamic as the field lines convect through different regions.…”