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Intense sunward (westward) plasma flows, named Subauroral Polarization Stream (SAPS), have been known to occur equatorward of the electron auroras for decades, yet their effect on the upper thermosphere has not been well understood. On the one hand, the large velocity of SAPS results in large momentum exchange upon each ion‐neutral collision. On the other hand, the low plasma density associated with SAPS implies a low ion‐neutral collision frequency. We investigate the SAPS effect during non‐storm time by utilizing a Scanning Doppler Imager (SDI) for monitoring the upper thermosphere, SuperDARN radars for SAPS, all‐sky imagers and DMSP Spectrographic Imager for the auroral oval, and GPS receivers for the total electron content. Our observations suggest that SAPS at times drives substantial (>50 m/s) westward winds at subauroral latitudes in the dusk‐midnight sector, but not always. The occurrence of the westward winds varies withAEindex, plasma content in the trough, and local time. The latitudinally averaged wind speed varies from 60 to 160 m/s, and is statistically 21% of the plasma. These westward winds also shift to lower latitude with increasingAEand increasing MLT. We do not observe SAPS driving poleward wind surges, neutral temperature enhancements, or acoustic‐gravity waves, likely due to the somewhat weak forcing of SAPS during the non‐storm time.

We examined the source region of dayside large‐scale traveling ionospheric disturbances (LSTIDs) and their relation to cusp energy input. Aurora and total electron content (TEC) observations show that LSTIDs propagate equatorward away from the cusp and demonstrate the cusp region as the source region. Enhanced energy input to the cusp initiated by interplanetary magnetic field (IMF) southward turning triggers the LSTIDs, and each LSTID oscillation is correlated with a TEC enhancement in the dayside oval with tens of minutes periodicity. Equatorward‐propagating LSTIDs are likely gravity waves caused by repetitive heating in the cusp. The cusp source can explain the high LSTID occurrence on the dayside during geomagnetically active times. Poleward‐propagating ΔTEC patterns in the polar cap propagate nearly at the convection speed. While they have similar ΔTEC signatures to gravity wave‐driven LSTIDs, they are suggested to be weak polar cap patches quasiperiodically drifting from the cusp into the polar cap via dayside reconnection.

The extreme substorm event on 5 April 2010 (THEMIS AL = −2,700 nT, called supersubstorm) was investigated to examine its driving processes, the aurora current system responsible for the supersubstorm, and the magnetosphere‐ionosphere‐thermosphere (M‐I‐T) responses. An interplanetary shock created shock aurora, but the shock was not a direct driver of the supersubstorm onset. Instead, the shock with a large southward IMF strengthened the growth phase with substantially larger ionosphere currents, more rapid equatorward motion of the auroral oval, larger ionosphere conductance, and more elevated magnetotail pressure than those for the growth phase of classical substorms. The auroral brightening at the supersubstorm onset was small, but the expansion phase had multistep enhancements of unusually large auroral brightenings and electrojets. The largest activity was an extremely large poleward boundary intensification (PBI) and subsequent auroral streamer, which started ~20 min after the substorm auroral onset during a steady southward IMFB_zand elevated dynamic pressure. Those were associated with a substorm current wedge (SCW), plasma sheet flow, relativistic particle injection and precipitation down to the D‐region, total electron content (TEC), conductance, and neutral wind in the thermosphere, all of which were unusually large compared to classical substorms. The SCW did not extend over the entire nightside auroral activity but was localized azimuthally to a few 100 km in the ionosphere around the PBI and streamer. These results reveal the importance of localized magnetotail reconnection for releasing large energy accumulation that can affect geosynchronous satellites and produce the extreme M‐I‐T responses.

Recent studies of Pc5‐band (150–600 s) ultralow frequency waves found that foreshock disturbances can be a driver of dayside compressional waves and field line resonance, which are two typical Pc5 wave modes in the dayside magnetosphere. However, it is difficult to find spatial structure of dayside Pc5 waves using a small number of satellites or ground magnetometers. This study determines 2‐D structure of dayside Pc5 waves and their driver by utilizing coordinated observations by the THEMIS satellites and the all‐sky imager at South Pole during two series of Pc5 waves on 29 June 2008. These Pc5 waves were found to be field line resonances (FLRs) and driven by foreshock disturbances. The ground‐based all‐sky imager at South Pole shows that periodic poleward moving arcs occurred simultaneously with the FLRs near the satellite footprints over ~3°latitude and had the same frequencies as FLRs. This indicates that they are the auroral signature of the FLRs. The azimuthal distribution of the FLRs in the magnetosphere and their north‐south width in the ionosphere were further determined in the 2‐D images. In the first case, the FLRs distribute symmetrically in the prenoon and postnoon regions with out‐of‐phase oscillation as the odd toroidal mode in the equatorial plane. In the second case, the azimuthal wavelengths of the 350–500 s and 300–450 s period waves were ~8.0 and ~5.2 Re in the equatorial plane. It also shows a fine azimuthal structure embedded in the large‐scale arcs, indicating that a high azimuthal wave number (m~ 140) mode wave coupled with the low‐wave number FLRs.