On 6 February 2002 the NASA FAST satellite transited a substorm break up arc approximately 1 min after substorm onset as identified by the NASA IMAGE satellite far ultraviolet (FUV) instrument complement. These IMAGE data show that both the intense electron and proton precipitation features seen by FAST were not present prior to substorm onset. The most intense onset poleward surge was produced by superthermal electrons, and their energy spectrum and field‐aligned angular distribution were consistent with wave accelerated electrons. The low energy ion fluxes in the E × B direction and magnetometer measurements confirmed the presence of waves in this feature. Thus the leading edge of the auroral surge was not produced by quasi‐static field‐aligned currents and related “inverted V” electric fields. The onset arc was also the poleward boundary of intense energetic protons. The FUV proton auroral images indicate that substantial part of the ion energy was carried by protons of energy >25 keV. Equatorward of the surge, there was a broader region of electron precipitation with embedded quasi‐static “inverted V” electric fields. The FAST particle and magnetometer measurements are consistent with a field‐aligned current structure in which the bulk of the upward current was carried by the “inverted V” precipitation region. The region of the superthermal electrons carried very little net current. The high‐density upward field‐aligned current carried by the superthermal electrons was presumably balanced by oppositely directed downward currents carried by cold electrons. The ground onset location was separated from the closed/open field line boundary by an extended region of closed field lines. In this region there was some weak plasma sheet electron and proton precipitation, but there was no clear signature of any distant, prebreakup substorm onset activity.

The electromagnetic pulses (EMP) from tropospheric lightning produce transient luminous events (TLEs), known as elves, in the 80–90 km region above the lightning. The luminosity is evidence that the EMP carries sufficient electric field to excite optical emissions at these altitudes; however, it is still unknown whether the field is sufficient to ionize the atmosphere. The first multiwavelength, quantitative observatory on a free‐flying satellite dedicated to observing TLEs, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on FORMOSAT‐2, formerly called ROCSAT‐2, confirmed that a significant number of lightning events are accompanied by elves. The instrument consists of a low light level imager and a set of multichannel photometers. In a few cases where the lightning occurred beyond the solid Earth limb, pure spectral measurements of the elves were obtained. Here we analyze such an event and show that the elves contained significant 391.4 nm emission of the N₂⁺ ion. This is clear evidence that ionization takes place in elves. The ratio of cross sections for N₂ ionization and the production of the upper state of 391.4 nm emission is not a constant for low‐energy electrons found in TLEs. We attempted to find this ratio by comparing our photometric measurements of the TLE produced emissions to theoretically derived emission intensities. The electron energy distribution and the ratios of the modeled N₂ Lyman‐Birge‐Hopfield (LBH) and the N₂⁺ first negative to the second positive were computed as a function of the reduced electric field. From these ratios it was possible to obtain the reduced electric field from the ratios. We estimated that the reduced electric field, which characterizes the local electron energy distribution, was >200 Td. We also made comparisons of the theoretically derived intensities to our measurements of the N₂⁺ first positive and Lyman‐Birge‐Hopfield (LBH) band emission in the elves. On the basis of the ratio between the N₂⁺ first negative emission and the time‐integrated ionization production, we estimate that the elves produced an average electron density of 210 electrons cm⁻³ over a large (165 km diameter) circular region having an assumed 10 km altitude extent. These observations indicate that thunderstorms are a significant source of ionization in the low‐ to midlatitude nighttime D region.

We describe a mechanism to explain the subauroral emission feature called STEVE (Strong Thermal Emission Velocity Enhancement), with a focus on its continuum spectrum. Spacecraft observations show that emissions co‐occur with typically invisible plasma flows known as subauroral ion drifts. If these flows are fast enough, nitrogen molecules are vibrationally excited by collisions with ions, overcoming the activation energy of the N₂+O→NO+N reaction. The resulting NO combines with ambient O, producing NO₂ and spectrally broad light. Importantly, this mechanism also produces N, which reduces the lifetime of NO from hours to seconds and thus explains why the emission is confined to a discrete arc. The predicted emission altitude ( 130 km) and occurrence conditions ( 4‐km/s flows) match well with observations. We simulate this mechanism using a simple photochemical model to demonstrate its validity. This mechanism is initiated by fast ion flows and is thus distinct from auroral and airglow processes.

[1] Recently, a revised auroral substorm onset sequence has been proposed, in which onset is preceded by a poleward boundary intensification (PBI) and subsequent equatorward propagation of N‐S‐aligned auroral features to breakup arc latitudes. We reanalyzed 20 randomly selected samples of the Nishimura et al. (2010) 251‐event database and show in greater detail to what degree the observed features in this subset of events are consistent with the proposed scenario. To assess the sensitivity of space‐based imagers for seeing this scenario, we calibrated the absolute responsivity of the THEMIS ground‐based imagers. We also show that although not suitable for studies from apogee, IMAGE/FUV imagers can also observe a consistent scenario from a lower altitude. We conclude that in some cases PBIs and subsequent plasma flows can be effective in providing a trigger if the inner magnetosphere is ready to produce a substorm.