Simultaneous relativistic electron and auroral particle access to the polar caps during interplanetary magnetic field B_z northward: A scenario for an open field line source of auroral particles

Abstract

The large magnetic storm of February 1986 provides a unique opportunity to assess whether existing open‐closed theories of the magnetosphere can consistently account for precipitating particle signatures measured at 840 km under a wide variety of solar wind and solar particle conditions. Here we report the simultaneous observation of relativistic electrons associated with a solar proton event and of auroral ions and electrons associated with a polar cap arc event, made across the central polar cap using detectors on board the Defense Meteorological Satellite Program (DMSP) satellites. On February 7 while a solar proton event was still in progress, polar cap arc activity (interplanetary magnetic field (IMF) Bz large and positive, Kp = 3+) occurred prior to a prolonged period of oval arc activity (IMF Bz large and negative, Kp = 6−). Throughout these periods the following are identified: the equatorward edge of the relativistic electron precipitation; the outer zone electron poleward and equatorward boundaries; the equatorward auroral electron and ion boundaries; and the auroral electron transition boundary. The history of the boundaries is presented as the transition from intense polar cap activity to intense auroral oval activity takes place. The data clearly show that the polar cap region of relativistic electron precipitation does not change significantly as the sign of Bz changes. On the other hand, the boundary layer population, which contains auroral arcs and 1‐ to 10‐keV ion precipitation, expands to fill most of the polar cap for Bz northward and greatly contracts as Bz turns southward. The expansion is not symmetric between poles. The fact that in a broad region of the central polar cap, auroral arcs and relativistic electrons are found on the same field lines presents some difficulty for jointly maintaining theories that propose the formation of polar cap arcs strictly on closed field lines and theories that explain relativistic electron entry by means of direct access along open field lines. We argue that the data support polar cap arc formation on open field lines and present a scenario in which the tail lobe boundary plasma is the source of auroral particles at high latitudes.

Multi-instrumentation observations of a transpolar arc in the northern hemisphere

“…Transpolar arcs are thought to be the optical manifestation of particle precipitation on closed field lines (Frank et al, 1982Zhu et al, 1997). However, other observations, which demonstrate the occurrence of polar rain electrons and relativistic electrons adjacent to and outside of polar cap arc, suggest the arcs are on open field lines, (Hardy et al, 1982;Hardy, 1984;Gussenhoven and Mullen, 1989). A recent study (Eriksson et al, 2005) suggests that the apparent continuous band of Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Convection and Electrodynamic Signatures in the Vicinity of a Sun-Aligned Arc: Results from the Polar Acceleration Regions and Convection Study (Polar Arcs)

“…These observations may be accounted for by sun-aligned arc formation on closed field lines threading an expanded (antisunward convecting) LLBL, but are also consistent with a model in which they formed on open field lines on the dusk side of a bifurcated tail lobe (Figure 7 (b)). A model in which the polar cap is entirely open (Figure 7 (a)) appears to be ruled out, since it is inconsistent with the strong ion precipitation observed over the center of the polar cap (unless you invoke the Gussenhoven and Mullen [1989] scenario of an earthwardstreaming lobe ion component). The antisunward convection signature appears least consistent with a model in which the polar cap arc field lines map to an expanded plasma sheet (Figure 7 (c)), since one would expect sunward convection in this region.…”

“…These models are here called the 'open' [Hardy et al, 1982;Burke et al, 1982;Chiu, 1989;Gussenhoven and Mullen, 1989], 'bifurcated tail' [Frank et al, 1982;Kan and Burke, 1985;Frank and Craven, 1988;Toffoletto and Hill, 1990], 'expanded plasma sheet' [Meng, 1981;1988;Murphree et al, 1982;Makita et al, 1991], and 'expanded low-latitude boundary layer (LLBL)' [Lundin and Evans, 1985;Lundin et al, 1990] models. A fifth model, which relies on the rotation of the tail x-line, is also introduced and discussed briefly [Reiff et al 1992].…”

“…Polar cap arcs have been argued to occur on open field lines on the basis of two types of observations: the measurement of electron spectra within polar cap arcs with the appearance of an accelerated polar rain (magnetosheath) distribution [Hardy et al, 1982;Burke et al, 1982] and, secondly, observations of sun-aligned arcs imbedded in regions containing simultaneous solar flare (> 100 keV) and polar rain electron precipitation [Gussenhoven and Mullen, 1989]. If the Polar ARCS data are interpreted in terms of an open model, shown in Figure 7 (a), the weak, sun-aligned arc would map along open field lines to a position in the tail lobe denoted by the black dot.…”

“…On the dayside, the electrons carrying the majority of this upward current have either been interpreted as precipitating on closed field lines threading the LLBL [Lassen and Danielsen, 1989;Lundin et al, 1990] or on open field lines threading the plasma mantle [Newell et al, 1991]. At other local times, it has been suggested that weak, polar cap arcs result from the precipitation of accelerated polar rain electrons on open field lines [Hardy et al, 1982;Burke et al, 1982;Gussenhoven and Mullen, 1989], or from plasma sheet or low latitude boundary layer electrons on closed field lines [Meng, 1981;Murphree et al, 1982;Lundin and Evans, 1985;Lundin et al, 1990].…”