Statistical Study of Magnetospheric Conditions for SAPS and SAID

Inner‐magnetospheric conditions for subauroral polarization streams (SAPS) and subauroral ion drifts (SAID) have been investigated statistically using Time History of Events and Macroscale Interactions during Substorms and RBSP observations. We found that plasma sheet electron fluxes at its earthward edge are larger for SAID than SAPS. The ring current ion flux for SAID formed a local maximum near SAID, but the ion flux for SAID was not necessarily larger than for SAPS. The median potential drop across SAID and SAPS is nearly the same, but the potential drop for intense SAID is substantially larger than that for SAPS. The plasmapause is sharper and electromagnetic waves were more intense for SAID. The SAID velocity peak does not strongly correlate with solar wind or geomagnetic indices. These results indicate that local plasma structures are more important for SAPS/SAID velocity characteristics as compared to global magnetospheric conditions. Plain Language Summary The mid‐latitude ionosphere and inner magnetosphere have fast plasma streams. Intense plasma streams have recently gained attention because they are associated with the STEVE phenomenon. However, it has not been understood what magnetospheric processes determine the strength and width of the fast plasma streams. We conducted a statistical study of plasma streams using >400 events from NASA's Time History of Events and Macroscale Interactions during Substorms and Van Allen Probes satellite observations to firmly address this question. We found that faster plasma streams are associated with higher electron fluxes than subauroral polarization streams. Ion fluxes for faster plasma streams are more localized but the magnitude is not enhanced. The peak velocity does not correlate with solar wind or geomagnetic conditions. We suggest that local plasma structures are more important for velocity characteristics of fast plasma streams compared to solar wind or global magnetospheric conditions. Key Points Subauroral ion drifts (SAID) are associated with higher plasma sheet electron fluxes and sharper flux gradients just outside the velocity peak Strong electron injection is the primary condition for creating SAID. SAID do not require strong ring current ions Local plasma structures are more important for subauroral polarization streams (SAPS)/SAID than IMF and global magnetospheric conditions