J. Michael Ruohoniemi

Identification of the temperature gradient instability as the source of decameter-scale ionospheric irregularities on plasmapause field lines

[1]xa0Recent observations with the new mid-latitude SuperDARN HF radar located at Wallops Island, Virginia have identified a class of ionospheric irregularities that is prevalent in the nightside sub-auroral ionosphere under low-to-moderate Kp conditions. These irregularities can be observed for many hours and generally exhibit very low Doppler velocities. A recent collaborative experiment using the Wallops radar and the Millstone Hill incoherent scatter radar has determined that these irregularities are located at the ionospheric footprint of the plasmapause and in a region of opposed electron density and electron temperature gradients. We conclude that the irregularities are produced by the temperature gradient instability (TGI) or by turbulent cascade from primary irregularity structures produced from this instability. This is the first experimental confirmation that the TGI is effective in producing decameter-scale ionospheric irregularities.

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

[1]xa0Magnetic 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 Earths 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.

First radar measurements of ionospheric electric fields at sub-second temporal resolution

[1]xa0A new multipulse sounding technique currently being used at the Wallops Island and Goose Bay SuperDARN radars has produced significant improvements in the temporal resolution of Doppler velocity measurements from which plasma velocities and electric fields are determined. The new technique allows Doppler velocities to be determined from every 200 ms multipulse sequence transmitted by the radar (equivalent to a 5 Hz measurement rate). To our knowledge, this is the highest Doppler measurement rate that has ever been attained with ionospheric radars. Tests of the new technique with the Wallops radar and Ottawa magnetometer revealed bursts of subauroral electric and magnetic field pulsations with periods of 13–20 s during a substorm expansion phase. These results indicate that SuperDARN measurements can be used to study highly dynamic processes in the coupled magnetosphere-ionosphere system, including storm and substorm electrodynamics, short-period pulsations and short-term variability in Joule heating.