Citizen scientists, along with satellite and ground-based sensors, have revealed a new arc boundary at subauroral latitudes.

This Letter reports a modeling study on the formation of streamer discharges from an isolated ionization column under subbreakdown condition. Numerical simulations show that positive streamers are able to form from the tip of an ionization column in a uniform applied electric field well below the breakdown threshold field. However, even when the applied field approaches the breakdown threshold field, negative streamers fail to originate from the other tip of the ionization column after the positive streamer has propagated a certain distance. The results reported explain some puzzling observations on streamer discharges in nature such as the predominant initiation of sprites by downward propagating positive streamers and help advance the initiation theories of sprites and lightning.

[1] This paper reports a modeling study on sprite streamer formation at subbreakdown conditions, i.e., the initiation of sprite streamers in a lightning field below the breakdown threshold field E k . Successful formation of streamers is observed for a variety of electric fields with the lowest field of 0.3E k when an appropriate ionization column is introduced. The simulation results also indicate that the initial ionization patch from which a streamer is initiated may become very bright following the streamer formation, and its brightness persists as the streamer continues its propagation. The overall luminous structure of the streamer and the ionization patch is very similar to the appearance of initiation of a single sprite streamer on high-speed images. The brightness of the patch depends on its initial density, and the patch with smaller density is brighter than the denser one. An analytical formula describing the temporal and spatial variation of the electric field in the streamer channel is derived to explain the brightening of the ionization column as well as the luminous streamer trail. Finally, comparisons between the streamers from the ionization patch and those forming in the vicinity of a conducting sphere in an electric field below E k show that the exponential growth rates associated with streamer characteristics are similar.Citation: Kosar, B. C., N. Liu, and H. K. Rassoul (2012), Luminosity and propagation characteristics of sprite streamers initiated from small ionospheric disturbances at subbreakdown conditions,

To understand magnetosphere‐ionosphere conditions that result in thermal emission velocity enhancement (STEVE) and subauroral ion drifts (SAID) during the substorm recovery phase, we present substorm aurora, particle injection, and current systems during two STEVE events. Those events are compared to substorm events with similar strength but without STEVE. We found that the substorm surge and intense upward currents for the events with STEVE reach the dusk, while those for the non‐STEVE substorms are localized around midnight. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations show that location of particle injection and fast plasma sheet flows for the STEVE events also shifts duskward. Electron injection is stronger and ion injection is weaker for the STEVE events compared to the non‐STEVE events. SAID are measured by Super Dual Auroral Radar Network during the STEVE events, but the non‐STEVE events only showed latitudinally wide subauroral polarization streams without SAID. To interpret the observations, Rice Convection Model (RCM) simulations with injection at premidnight and midnight have been conducted. The simulations successfully explain the stronger electron injection, weaker ion injection, and formation of SAID for injection at premidnight, because injected electrons reach the premidnight inner magnetosphere and form a narrower separation between the ion and electron inner boundaries. We suggest that substorms and particle injections extending far duskward away from midnight offer a condition for creating STEVE and SAID due to stronger electron injection to premidnight. The THEMIS all‐sky imager network identified the east‐west length of the STEVE arc to be ~1900 km (~2.5 h magnetic local time) and the duration to be 1–1.5 h.