Investigating the polar ionosphere during the development of neutral density enhancements on 24–25 September 2000

We focus on the well‐known northern daytime neutral density spikes detected by CHAMP on 25 September 2000 and related coupled magnetospheric‐ionospheric‐thermospheric processes. We investigate the underlying magnetic events and resultant thermospheric variations plus the state of the ionospheric polar region by employing multi‐instrument CHAMP and DMSP data. Results show the unfolding of a weak (SYM‐HMin ≈ −27 nT; ~0345 UT) magnetic storm during which these northern density spikes occurred. Some smaller southern daytime density spikes were also detected prior to this storm on the previous day. All these density spikes were detected in or near polar convection flow channels (FCs). Each FC was characterized by strong antisunward zonal ion drifts that excited the zonal and meridional neutral winds leaving the signature of FC in the CHAMP neutral wind measurements and thus providing direct observational evidence of FC underlying the density spike. Additional to the small‐scale field‐aligned current (SS‐FAC) filaments, the sudden intensifications of ionospheric closure current in the FC fueled the thermosphere and contributed to the development of upwelling and density spike. Some smaller density increases occurred due to the weak intensification of ionospheric closure currents. Equatorward (poleward) directed meridional neutral winds strengthened (weakened) the density spike by moving the neutral density up and along (down and against) the upwelling fueled by the ionospheric closure current and SS‐FAC filaments.

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