This chapter reviews fundamental properties and recent advances of diffuse and pulsating aurora. Diffuse and pulsating aurora often occurs on closed field lines and involves energetic electron precipitation by wave-particle interaction. After summarizing the definition, large-scale morphology, types of pulsation, and driving processes, we review observation techniques, occurrence, duration, altitude, evolution, small-scale structures, fast modulation, relation to high-energy precipitation, the role of ECH waves, reflected and secondary electrons, ionosphere dynamics, and simulation of wave-particle interaction. Finally we discuss open questions of diffuse and pulsating aurora.

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On 26 March 2008, simultaneous measurements of a large substorm were made using the Poker Flat Incoherent Scatter Radar, Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft, and all sky cameras. After the onset, electron precipitation reached energies ≳100 keV leading to intense D region ionization. Identifying the source of energetic precipitation has been a challenge because of lack of quantitative and magnetically conjugate measurements of loss cone electrons. In this study, we use the maximum entropy inversion technique to invert altitude profiles of ionization measured by the radar to estimate the loss cone energy spectra of primary electrons. By comparing them with magnetically conjugate measurements from THEMIS‐D spacecraft in the nightside plasma sheet, we constrain the source location and acceleration mechanism of precipitating electrons of different energy ranges. Our analysis suggests that the observed electrons ≳100 keV are a result of pitch angle scattering of electrons originating from or tailward of the inner plasma sheet at ~9RE, possibly through interaction with electromagnetic ion cyclotron waves. The electrons of energy 10–100 keV are produced by pitch angle scattering due to a potential drop of ≲10 kV in the auroral acceleration region (AAR) as well as wave–particle interactions in and tailward of the AAR. This work demonstrates the utility of magnetically conjugate ground‐ and space‐based measurements in constraining the source of energetic electron precipitation. Unlike in situ spacecraft measurements, ground‐based incoherent scatter radars combined with an appropriate inversion technique can be used to provide remote and continuous‐time estimates of loss cone electrons in the plasma sheet.

The extreme substorm event on 5 April 2010 (THEMIS AL = −2,700 nT, called supersubstorm) was investigated to examine its driving processes, the aurora current system responsible for the supersubstorm, and the magnetosphere-ionosphere-thermosphere (M-I-T) responses. An interplanetary shock created shock aurora, but the shock was not a direct driver of the supersubstorm onset. Instead, the shock with a large southward IMF strengthened the growth phase with substantially larger ionosphere currents, more rapid equatorward motion of the auroral oval, larger ionosphere conductance, and more elevated magnetotail pressure than those for the growth phase of classical substorms. The auroral brightening at the supersubstorm onset was small, but the expansion phase had multistep enhancements of unusually large auroral brightenings and electrojets. The largest activity was an extremely large poleward boundary intensification (PBI) and subsequent auroral streamer, which started~20 min after the substorm auroral onset during a steady southward IMF B z and elevated dynamic pressure. Those were associated with a substorm current wedge (SCW), plasma sheet flow, relativistic particle injection and precipitation down to the D-region, total electron content (TEC), conductance, and neutral wind in the thermosphere, all of which were unusually large compared to classical substorms. The SCW did not extend over the entire nightside auroral activity but was localized azimuthally to a few 100 km in the ionosphere around the PBI and streamer. These results reveal the importance of localized magnetotail reconnection for releasing large energy accumulation that can affect geosynchronous satellites and produce the extreme M-I-T responses.Plain Language Summary Supersubstorms are extreme space weather events that involve unusually intense aurora. The goal of this study is to understand the driving processes and system responses during a supersubstorm event on 5 April 2010, when the Intelsat Galaxy-15 experienced an anomaly and stopped responding to ground commands. We found that the supersubstorm was associated with a particular type of aurora called poleward boundary intensification and a subsequent auroral streamer. This type of aurora can often occur, but the one in this event was unusually large (AL = −2,700 nT), in association with extremely intense currents and relativistic particle acceleration. The accelerated particles precipitated down to 60 km altitude, much lower than the typical height of aurora (>100 km). This event was caused by extremely intense magnetic reconnection and fast flows toward the Earth. This event also created a fast stream of neutral species in the upper atmosphere.

Relativistic electron precipitation (REP) is an important loss mechanism of the Earth's outer radiation belt electrons (Li & Hudson, 2019 and references therein), as well as a source of energy input into the Earth's atmosphere. It is widely accepted that electron precipitation is caused by wave-particle interactions that occur in the Earth's magnetosphere (e.g., Millan & Thorne, 2007;Thorne, 2010); however, sufficient stretching of magnetic field lines is another potential driver of electron and proton precipitation (e.g.,