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.

[1] Modeling results of sprite streamer formation from large ionospheric inhomogeneities or patches (several tens to hundreds of meter wide) indicate that positive streamers can be initiated at subbreakdown conditions from the inhomogeneities with a density comparable to sprite halo densities. For spherical patches with a given radius, the minimum density required for streamer initiation decreases with increasing ambient field. For a given density, the minimum size of the inhomogeneity decreases with increasing ambient field. The modeling results on the associated optical emissions show that a luminous spherical-like cap appears around the lower tip of the ionization patch before streamer initiation, and the streamer is initiated from the bottom of this cap, which appears to be similar to streamer initiation from luminous structures in the lower ionosphere recorded by recent high-speed videos. Our study suggests that if the sprite halo front is unstable, inhomogeneities developing from it can initiate sprite streamers at subbreakdown conditions. Citation: Kosar, B. C., N. Y. Liu, and H. K. Rassoul (2013), Formation of sprite streamers at subbreakdown conditions from ionospheric inhomogeneities resembling observed sprite halo structures, Geophys. Res. Lett., 40,[6282][6283][6284][6285][6286][6287]

As part of its International Capabilities Assessment effort, the Community Coordinated Modeling Center initiated several working teams, one of which is focused on the validation of models and methods for determining auroral electrodynamic parameters, including particle precipitation, conductivities, electric fields, neutral density and winds, currents, Joule heating, auroral boundaries, and ion outflow. Auroral electrodynamic properties are needed as input to space weather models, to test and validate the accuracy of physical models, and to provide needed information for space weather customers and researchers. The working team developed a process for validating auroral electrodynamic quantities that begins with the selection of a set of events, followed by construction of ground truth databases using all available data and assimilative data analysis techniques. Using optimized, predefined metrics, the ground truth data for selected events can be used to assess model performance and improvement over time. The availability of global observations and sophisticated data assimilation techniques provides the means to create accurate ground truth databases routinely and accurately.

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This technical report documents the details of Aurorasaurus citizen science data for the period spanning 2015 and 2016 as well as its routine data filtering protocols. Aurorasaurus citizen science data is a collection of auroral sightings submitted to the project via its website or apps and mined from social media. It is a robust data set and particularly abundant during strong geomagnetic storms when auroral precipitation models have the highest uncertainty. These data are offered to the scientific community for use through an open‐access database in its raw and scientific formats, each of which is described in detail in this technical report. Furthermore, by demonstrating its scientific utility, we aim to encourage its integration into auroral research.