Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm42a0595k&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM42A-0595
Physics
2400 Ionosphere, 2407 Auroral Ionosphere (2704), 2409 Current Systems (2708), 2431 Ionosphere/Magnetosphere Interactions (2736), 2455 Particle Precipitation
Scientific paper
In a recent study of IMAGE FUV data, Frey et al. (2003) examined cases of high-latitude dayside aurora caused by precipitating electrons predominantly during northward IMF with a strongly positive IMF By component. Analysis of FAST data showed that the auroral spots coincide with regions of localized upward field-aligned currents (FACs). In order to determine the conditions that control these auroral emissions, we examined field-aligned currents observed by the Iridium constellation for 25~events of prolonged steady IMF orientation with clock angles between +60o and +90o. We find that localized FACs are observed under all solar wind conditions but that the occurrence of high latitude aurora are restricted to solar wind proton densities below 4 {cm-3} and peak current densities higher than 1.0 {μ A/m2}. These conditions were met for the 11~events with auroral emissions while emissions are not observedduring the remaining 14~events for which these conditions are not met. Our findings can be qualitatively explained with the Knight relation [Knight, 1973], which states that the field-aligned potential drop is directly proportional to the field-aligned current density and inversely proportional to the energy density of the precipitating population. Auroral emissions are generated, if the field-aligned potential drop, which accelerates electrons into the ionosphere, is sufficiently large whereas either low current density or high particle density leads to lower potential and correspondingly weaker aurora. The results also indicate that the particle density on field lines mapping to the upward current region is largely governed by the solar wind density.
Anderson Benjamin J.
Frey Harald U.
Immel Thomas J.
Korth Haje
Mende Stephen B.
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