This work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019, which provides a comprehensive SAPS climatology using a significantly larger database of radar observations than was used in seminal earlier works. Key features of SAPS and associated Ne/Ti/Te are investigated using a superposed epoch analysis method. The characteristics of these parameters are investigated with respect to magnetic local time, season, geomagnetic activity, solar activity, and interplanetary magnetic field orientation, respectively. The main results are as follows: (1) Conditions for SAPS are more favorable for dusk than near midnight, for winter compared to summer, for active geomagnetic periods compared to quiet time, for solar minimum compared to solar maximum, and for IMF conditions with negative By and negative Bz. (2) SAPS is usually associated with a midlatitude trough of 15–20\% depletion in the background density. The SAPS-related trough is more pronounced in the postmidnight sector and near the equinoxes. (3) Subauroral ion and electron temperatures exhibit a 3–8\% (50–120 K) enhancement in SAPS regions, which tend to have higher percentage enhancement during geomagnetically active periods and at midnight. Ion temperature enhancements are more favored during low solar activity periods, while the electron temperature enhancement remains almost constant as a function of the solar cycle. (4) The electron thermal content, Te \times Ne, in the SAPS associated region is strongly dependent on 1/Ne, with Te exhibiting a negative correlation with respect to $Ne$.

Rapid radiation belt recovery following storm time depletion involves local acceleration of multi-MeV electrons in nonlinear interactions with VLF chorus waves. Previous studies of an apparent impenetrable barrier at L ~ 2.8 focused on precipitation loss mechanisms for an explanation of the sharp reduction of multi-MeV electron fluxes earthward of L ~3. Van Allen Probes observations for cases when the plasmasphere is contracted earthward of L ~ 3 indicate that strong coherent signals from VLF transmitters play significant roles in the suppression of nonlinear chorus wave growth earthward of L ~ 3. As a result, local acceleration of 100s keV electrons to MeV energies does not occur in this region. During the recovery of the outer radiation belt when the plasmasphere is significantly contracted, it is the suppression of chorus wave growth and local acceleration by the action of the transmitter waves at the outer edge of the VLF bubble that is responsible for the sharp inner edge of the new MeV electron population and the formation of the impenetrable barrier.

We describe mid-latitude plasma density striations (MDS) modulating the evening side of Storm Enhanced Density (SED) by magnetosphere-ionosphere coupling. The MDS are magnetically conjugate, and they consist of elongated density structures [enhancements (plumes) and depletions (troughs)] that extend from the equator to the main trough equatorward boundary. Each density perturbation is associated with a flow channel, and they develop progressively at all latitudes. We present a detailed analysis of the MDS during the 7-8 September 2017 storm, by virtue of remote and in-situ observations of the magnetosphere-ionosphere system. We find that the density plumes are a result of local plasma uplift, and poleward and westward plasma transport guided by the adjacent flow channels. While the MDS’s troughs bear some resemblance to the depletion patterns associated with equatorial plasma bubbles, it has been found to be quite distinct, both in terms of its observational manifestations and its formation mechanism. Namely, the trough is associated with enhanced flow channels peaking at the edges, with elevated electron and ion temperatures. Crucial spacecraft measurements of plasma parameters in the ionosphere and plasmasphere near the equatorial plane ($L\approx1.9$) unambiguously show conjugate nature of the MDS. In particular, the magnetospheric electric field intensifications lie just earthward of the injected $<$200~keV ions at the ion pressure gradient.