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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.

Magnetotail earthward‐propagating fast plasma flows provide important pathways for magnetosphere‐ionosphere coupling. This study reexamines a flow‐related red‐line diffuse‐like aurora event previously reported by Liang et al. (2011, https://doi.org/10.1029/2010ja015867), utilizing THEMIS and ground‐based auroral observations from Poker Flat. We find that time domain structures (TDSs) within the flow bursts efficiently drive electron precipitation below a few keV, aligning with predominantly red‐line auroral intensifications in this non‐substorm event. The diffuse‐like auroras sometimes coexisted with or potentially evolved from discrete forms. We forward model red‐line diffuse auroras due to TDS‐driven precipitation, employing the time‐dependent TREx‐ATM auroral transport code. The good correlation (∼0.77) between our modeled and observed red line emissions underscores that TDSs are a primary driver of the red‐line diffuse‐like auroras, though whistler‐mode wave contributions are needed to fully explain the most intense red‐line emissions. Plain Language Summary Fast plasma flows in the magnetotail, traveling earthward at several hundred kilometers per second, transport energetic particles and magnetic flux into the inner magnetosphere. Upon braking near Earth's high magnetic flux regions, they trigger plasma instabilities and waves, leading to increased electric currents and particle precipitation in the polar regions. This precipitation, depending on its driver, results in either diffuse auroras from electron pitch‐angle scattering, or discrete auroras from field‐aligned electron acceleration and currents. Our case study highlights the important role of time‐domain structures in diffuse‐like aurora generation during flow braking. This reveals a new aspect of magnetosphere‐ionosphere coupling: the generation of diffuse auroras through electron scattering by time‐domain structures in braking flow bursts. Key Points Predominantly red‐line auroras are linked to flow bursts, TDSs, and <1 keV electron precipitation For the first time, red‐line diffuse‐like auroras have been forward‐modeled using TREx‐ATM with time domain structure (TDS) inputs A good correlation between forward‐modeled and observed red‐line emissions suggests that TDSs are a major driver

The process that generates Earth’s most intense aurora is found to occur at Jupiter, but is of only secondary importance in generating Jupiter’s much more powerful aurora. Jupiter's awesome aurora The most intense aurora on Earth are generated by a 'discrete' process whereby electrons are accelerated coherently. Weaker aurora arise from wave scattering of magnetically trapped electrons. As Jupiter's aurora is orders of magnitude more powerful than Earth's, it was naturally assumed that the former process was responsible, yet early in situ observations by the Juno spacecraft found no evidence of the discrete process. Barry Mauk and collaborators report discrete downward accelerations of electrons on some auroral crossings, but the energy flux is much less than that caused by broadband processes, with broadband characteristics that are very different from those at Earth. The most intense auroral emissions from Earth’s polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth’s space environment to its polar regions 1 , 2 . In contrast, Earth’s less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvénic aurora) 3 , 4 . Jupiter’s relatively steady main aurora has a power density that is so much larger than Earth’s that it has been taken for granted that it must be generated primarily by the discrete auroral process 5 , 6 , 7 . However, preliminary in situ measurements of Jupiter’s auroral regions yielded no evidence of such a process 8 , 9 , 10 . Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter’s auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth 11 . Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.

Diffuse aurora is an important phenomenon that is responsible for significant magnetospheric energy input into the ionosphere. Pulsating aurora is one distinct type of diffuse aurora, caused by quasi‐periodic electron precipitation with energies ranging from a few keV to tens of keV. Recent studies have suggested that pulsating aurora can also be accompanied by relativistic electron precipitation, which can penetrate into the E/D‐layer of the ionosphere and lead to a significant ionospheric response. However, the energy extent and occurrence rate of the relativistic, quasi‐periodic precipitation are not well understood. In this study, we perform a statistical analysis of quasi‐periodic electron precipitation events observed by the low‐altitude, polar‐orbiting ELFIN CubeSats. Our results show that these precipitation events predominantly occur in the dawn sector, and approximately 15% of them extend to relativistic energies (>500${ >} 500$keV). Possible mechanisms for the quasi‐periodic relativistic precipitation are also discussed.

Auroral streamers are important meso‐scale processes that transport plasma and magnetic energy and drive dynamic magnetosphere‐ionosphere (MI) coupling and space weather. Although streamers are typically studied using imagers sensitive to energetic (>${ >} $ 1 keV) electron precipitation, such as all‐sky imagers, some are associated with low‐energy (<${< } $ 1 keV) precipitation better captured by red‐line auroral emissions. This paper reports such streamer‐like red‐line auroras observed poleward of a black aurora and an auroral torch, associated with a magnetospheric electron injection and braking ion flows. Using conjugate space‐ground observations, quasilinear theory, and auroral forward modeling, we establish the first direct linkage between streamer‐like red‐line auroras and plasma sheet electron pitch‐angle scattering by time‐domain structures. These results underscore the importance of wave‐driven diffuse auroral processes in generating low‐energy auroral streamers, distinct from the conventional quasi‐electrostatic coupling paradigm.

Dayside aurora is related to processes in the dayside magnetosphere and especially at the dayside magnetopause. A number of dayside aurora phenomena are driven by reconnection between the solar wind interplanetary magnetic field and the Earth’s internal magnetic field at the magnetopause. We summarize the properties and origin of aurora at the cusp foot point, High Latitude Dayside Aurora (HiLDA), Poleward Moving Auroral Forms (PMAFs), aurora related to traveling convection vortices (TCV), and throat aurora. Furthermore we discuss dayside diffuse aurora, morning side diffuse aurora spots, and shock aurora.

In this study, we identified 51 dayside diffuse auroral patches and examined their two‐dimensional evolutions by using the Time History of Events and Macroscale Interactions during Substorms probes and the ground‐based all‐sky imager at the South Pole. Two typical events show diffuse auroral patches associated with upstream dynamic pressure enhancements of the bow shock and magnetospheric compressions, followed by their east–west propagations. The statistical results suggest that most conjunction events were associated with foreshock activities, while the remaining events were associated with dynamic pressure enhancements in the pristine solar wind. These azimuthal motions can be either eastward or westward, with initial locations at ∼12–13 and ∼9–10 Magnetic Local Time, respectively, exhibiting a dawn‐dusk asymmetry. Additionally, poleward motions were found in all events. Larger dynamic pressure enhancements correspond to faster poleward motions and could push the initial diffuse auroral brightening toward lower latitudes. These characteristics of their poleward motions were consistent with the Tamao path.

We present the first global geospace simulation to reproduce auroral giant undulations (GUs). To identify their magnetospheric drivers, we employ the MAGE (Multiscale Atmosphere‐Geospace Environment) model in a case study of a geomagnetic storm for which there were spacecraft‐ and ground‐based observations of GUs. The model reproduces the spatial and temporal scales of the GUs as well as the presence of duskside subauroral polarization streams (SAPS) and plasmapause undulations. Based on our modeling, we are able to identify the magnetospheric drivers of GUs as mesoscale ring current injections which, after drifting westward, create inverted regions of flux‐tube entropy (FTE) and subsequent interchange instability. Outward‐protruding interchange fingers disrupt shielding of the inner magnetosphere, creating longitudinally localized ripples in magnetospheric convection equatorward of the magnetospheric instability, which structure the plasmapause and duskside diffuse precipitation. While not causal, SAPS and plasmapause undulations are a consequence of the unstable magnetospheric configuration. Plain Language Summary The visually dazzling display of the aurora during active periods is caused primarily by the precipitation of energetic electrons from the magnetosphere into the ionosphere. The auroral oval plays host to a variety of morphological features, or auroral forms, that are a reflection of magnetospheric processes and therefore a powerful tool for understanding the cross‐scale processes that bind together different geospace domains. Unlocking that power, however, requires an understanding of how magnetospheric processes are reflected in the aurora. Despite decades of study, that understanding has remained elusive, primarily due to limited in situ measurements and uncertainty in the magnetic mapping connecting them to the ionosphere. Only recently have new global geospace models emerged that can provide this understanding. In this letter we identify the magnetospheric driver of auroral giant undulations (GUs), wave‐like trains of undulations that form on the equatorward edge of the diffuse aurora with typical spatial scales of 100 km. We show that GUs are the consequence of a “buoyancy imbalance” formed during the buildup of the ring current and the subsequent disruption of the ionospheric current systems that typically shield the inner magnetosphere. Key Points We present the first global geospace simulation to reproduce auroral giant undulations (GUs) Model shows GUs result from localized under‐shielding as a consequence of interchange instability during the buildup of the ring current Interchange‐unstable regions drive ripples in magnetospheric convection, structuring the plasmapause and duskside diffuse precipitation

Plasmapause surface waves (PSWs) near the plasmapause boundary are regarded to be the magnetospheric source of ionospheric auroral giant undulations (GUs) located at the equatorward boundary of diffuse aurora. However, the observational evidence of wave‐particle interaction connecting PSWs and GUs is absent. In this letter, we demonstrate GUs are driven by pitch‐angle scattering of time domain structures modulated by the PSWs, based on the conjugated ionospheric and magnetospheric observations. Specifically, ionospheric GUs are lighted by the pitch‐angle scattering of <1 keV thermal electron and ions and energetic ions with energy up to dozens of keV near the plasmapause. Further, the total fluxes during one PSW period and energy of scattered electron and ions determine the size and luminosity of GUs. Our research provides observational evidence that PSWs cause periodic electron precipitation via modulating the time domain structures rather than the previously predicted chorus or electron cyclotron harmonic waves. Plain Language Summary Boundary surface waves usually act as a kind of special oscillation along the boundary layer and are the widely existing physical phenomena in the universe. In our Earth, there are magnetopause surface wave and plasmapause surface wave. For the latter, the plasmapause surface wave has been confirmed to be a kind of sawtooth‐type auroral structures locating on the equatorial edge of aurora oval, named as giant undulations. But how can the plasmapause surface wave produce the auroral giant undulations is still unknown. Based on this question, we have provided the observational evidence of auroral giant undulations being driven by the periodic pitch‐angle scattering of time domain structures modulated by plasmapause surface waves. Our new results in this research would help us to better understand the energy conversion controlled by boundary dynamics and the crucial effect of boundary dynamics on the near‐surface space environment. Key Points Giant undulations (GUs) are lighted by the pitch‐angle scattering of <1 keV thermal electron and ions and energetic ions with energy up to dozens of keV Total fluxes during one plasmapause surface wave (PSW) period and energy of scattered electron and ions determine the size and luminosity of GUs PSWs can cause periodic electron precipitation by modulating time domain structures

The driver of energetic electron precipitation into Ganymede's atmosphere has been an outstanding open problem. During the Juno flyby of Ganymede on 7 June 2021, Juno observed significant downward‐going electron fluxes inside the bounce loss cone of Ganymede's polar magnetosphere. Concurrently, Juno detected intense whistler‐mode waves, both in the quasi‐parallel and highly oblique directions with respect to the magnetic field line. We use quasi‐linear model to quantify energetic electron precipitation driven by quasi‐parallel and very oblique whistler‐mode waves, respectively, in the vicinity of Ganymede. The data‐model comparison indicates that in Ganymede's lower‐latitude (higher‐latitude) polar region, quasi‐parallel whistler‐mode waves play a dominant role in precipitating higher‐energy electrons above ∼100s eV (∼1 keV), whereas highly oblique waves are important for precipitating lower‐energy electrons below 100s eV (∼1 keV). Our result provides new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphere. Plain Language Summary During the Juno flyby of Ganymede on 7 June 2021, the Juno spacecraft detected energetic electrons precipitating into Ganymede's atmosphere. Simultaneously, Juno detected intense electromagnetic whistler‐mode waves in the vicinity of Ganymede. We use a physics‐based model to quantify the role of the observed whistler‐mode waves in energetic electron precipitation. The comparison between the Juno observation and modeling results reveals that whistler‐mode waves potentially play a dominant role in precipitating energetic electrons into Ganymede's atmosphere over a broad energy range from tens of eV to several hundred keV. Our findings are potentially important for understanding the loss process of energetic electrons in Ganymede's magnetosphere, as well as the generation of Ganymede's diffuse aurora. Key Points We provide new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphere This finding is potentially important for the generation of aurora and the loss of energetic electrons in the vicinity of Ganymede Juno observations and quasi‐linear modeling are used to quantify energetic electron precipitation driven by whistler‐mode waves