Ionospheric signatures of internal reconnection for northward interplanetary magnetic field: Observation of “reciprocal cells” and magnetosheath ion precipitation

Abstract

[1] Magnetic reconnection that involves overdraped lobe field lines is called internal reconnection since it occurs inside the magnetopause. When the interplanetary magnetic field (IMF) is due northward and the Earth's dipole is tilted significantly, internal reconnection occurs in the winter hemisphere, not only between a summer lobe field line and a winter lobe field line but also between a summer lobe field line and a closed field line. The latter internal reconnection drives ''reciprocal cells'' in the winter ionosphere that circulate exclusively in the closed field line region. The reciprocal cells are intimately related to the lobe cells in the summer ionosphere in that in the steady state, the reconnection voltage associated with merging of IMF and open field lines is equal to the sum of the lobe cell potential and the reciprocal cell potential. In this paper we present observations of convection patterns consistent with those expected for reciprocal cells, using ionospheric radar and low-altitude satellite data. We also show the concurrence of lobe cells and reciprocal cells. The observations of reciprocal cells provide support for the internal reconnection between a summer lobe field line and a closed field line. In addition, we show that equatorward of the polar cap boundary, magnetosheath-like ions are drifting from noon toward the flankside in both hemispheres. We suggest that these ions are of magnetosheath origin and that they entered the closed region of the magnetosphere through the rotational discontinuity associated with internal reconnection. These magnetosheath-like ion observations strongly support the occurrence of internal reconnection.

Interhemispheric observations of dayside convection under northward IMF

“…Therefore, the pass is considered to be through closed field line regions. The F13 pass is more symmetric about local noon than in Figure 7, and shows the typical northward IMF summer particle precipitation characteristics observed by Watanabe et al [2006]. Between 13:57 and 13:59, signatures of energetic ions centered around approximately 1 keV are seen.…”

“…[8] The present study expands upon the work done by Watanabe et al [2006] and Wilder et al [2010] by investigating, in detail, the simultaneous development of reverse convection in both hemispheres for a case of strongly northward IMF near the northern winter solstice. SuperDARN radar measurements are used to determine time series of the reverse convection velocity magnitude, and DMSP particle precipitation data is used to demonstrate how open-closed boundaries correspond to dayside convection in each hemisphere.…”

“…It is interesting to note that at the highest latitude, the distribution includes ions with energy greater than 10 keV, which are not typical for ions of magnetosheath origin. Aside from this anomaly, which is at present unexplained, this DMSP pass shows the characteristics of open field lines mapping to the merging site in the magnetosheath, which has been observed in the past [Watanabe et al, 2006]. We identify the open field line region to be between 13:56 UT and 14:00 UT.…”

“…By observing where the energetic particle flux returns to values seen equatorward of the auroral zone, one can determine open-closed boundaries. This will be done in a manner consistent with the methodology used by Watanabe et al [2006] to determine the open-closed boundary features within the two different forms of NBZ cells.…”

“…[6] Using data from the Super Dual Auroral Radar Network (SuperDARN) and energetic particle spectrograms from the Defense Meteorological Satellite Program (DMSP), Watanabe et al [2006] demonstrated these differences in ionospheric convection and magnetic field line topology under northward IMF by investigating one event in winter and one event in summer. They did not, however, provide a quantitative simultaneous comparison of the effectiveness of each form of magnetic merging due to lack of radar measurements from the southern hemisphere.…”