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Using the statistical wave power spectral profiles obtained from CRRES wave data within the 0000–0600 MLT sector under different levels of geomagnetic activity and a modeled latitudinal variation of wave normal angle distribution, we examine quantitatively the effects of lower band and upper band chorus on resonant diffusion of plasma sheet electrons for diffuse auroral precipitation in the inner magnetosphere. Whistler mode chorus‐induced resonant scattering of plasma sheet electrons is geomagnetic activity dependent, varying from above the strong diffusion limit (timescale of an hour) during active times (AE* > 300 nT) with peak wave amplitudes of >50 pT to weak scattering (timescale of a day) during quiet conditions (AE* < 100 nT) with typical wave amplitudes of ≤10 pT. Chorus waves present at different magnetic latitudes make distinct contributions to the net diffusion rates of plasma sheet electrons, largely depending on the latitudinal variation of wave power. Upper band chorus is the controlling scattering process for electrons from ∼100 eV to ∼2 keV, and lower band chorus is most effective for precipitating the higher energy (>∼2 keV) plasma sheet electrons in the inner magnetosphere. Efficient scattering by the combination of active time lower band and upper band chorus can cover a wide energy range from ∼100 eV to >100 keV and a broad interval of equatorial pitch angle, thereby accounting for the formation of observed electron pancake distribution. Decreased chorus scattering during less disturbed times can also modify the magnetic local time distribution of plasma sheet electrons. Compared to the effects of chorus waves, electron cyclotron harmonic wave‐induced resonant diffusion coefficients are at least 1 order of magnitude smaller and are negligible under any geomagnetic condition, indicating that chorus waves act as the major contributor dominantly responsible for diffuse auroral precipitation in the inner magnetosphere. Chorus‐driven momentum diffusion and mixed diffusion are also important. Lower band and upper band chorus can cause strong momentum diffusion of plasma sheet electrons in the energy ranges of ∼500 eV to ∼2 keV and ∼2 keV to ∼3 keV, respectively, which can significantly result in energization of the electrons and attenuation of the waves. Key Points Chorus can cause both efficient pitch angle scattering and momentum diffusion Chorus dominates over ECH waves to account for diffuse auroral precipitation Chorus scattering can also explain the formation of electron pancake distribution

Using statistical wave power spectral profiles obtained from CRRES and the latitudinal distributions of wave propagation modeled by the HOTRAY code, a quantitative analysis has been performed on the scattering of plasma sheet electrons into the diffuse auroral zone by multiband electrostatic electron cyclotron harmonic (ECH) emissions near L = 6 within the 0000–0600 MLT sector. The results show that ECH wave scattering of plasma sheet electrons varies from near the strong diffusion rate (timescale of an hour or less) during active times with peak wave amplitudes of an order of 1 mV/m to very weak scattering (on the timescale of >1 day) during quiet conditions with typical wave amplitudes of tenths of mV/m. However, for the low‐energy (∼100 eV to below 2 keV) electron population mainly associated with the diffuse auroral emission, ECH waves are only responsible for rapid pitch angle diffusion (occasionally near the limit of strong diffusion) for a small portion of the electron population with pitch angles αeq < 20°, dependent on electron energy and geomagnetic activity level. ECH scattering alone cannot account for the rapid loss of plasma sheet electrons during transport from the nightside to the dayside, nor can it explain the formation of the pancake electron distributions strongly peaked at αeq > 70°. Computations of the bounce‐averaged coefficients of momentum diffusion and (pitch angle, momentum) mixed diffusion indicate that both mixed diffusion and energy diffusion of plasma sheet electrons due to ECH waves are very small compared to pitch angle diffusion and that ECH waves have little effect on local electron acceleration. Consequently, the multiple harmonic ECH emissions cannot play a dominant role in the occurrence of diffuse auroral precipitation near L = 6, and other wave‐particle interaction mechanisms, such as whistler mode chorus‐driven resonant scattering, are required to explain the global distribution of diffuse auroral precipitation and the formation of the pancake distribution in the inner magnetosphere. Key Points ECH scattering varies from strong diffusion limit to weak diffusion ECH scattering is confined to a small electron population with low pitch angles ECH scattering alone cannot account for the diffuse auroral precipitation

The separation and classification of ionospheric troughs in the winter evening and morning ionospheres of the southern hemisphere were performed using CHAMP satellite data for high solar activity (2000–2002). In the high-latitude ionosphere, the main ionospheric trough (MIT) was separated from the high-latitude trough (HLT). The separation was carried out using a thorough analysis of all the characteristic structures of the ionosphere in the framework of the auroral diffuse particle precipitation model. Two types of high-latitude troughs were identified: (1) a wide trough associated with zone II of diffuse precipitation on the poleward edge of the auroral oval and (2) a narrow trough of ionization, which is presumably associated with an electric field action. The poleward wall of MIT is as ever formed by diffuse precipitation in zone I on the equatorward edge of the auroral oval. The HLT and MIT separation is most difficult at the longitudes of the eastern hemisphere, where all structures are located at the highest latitudes and partially overlap. In the mid-latitude ionosphere, all the characteristic structures of the ionosphere were also identified and considered. MIT was separated from the ring ionospheric trough (RIT), which is formed by the decay processes of the magnetospheric ring current. The separation of MIT and RIT was performed based on an analysis of the prehistory of all geomagnetic disturbances during the period under study. In addition to the RIT, a decrease in the electron density equatorward of the MIT was found to be often formed at the America–Atlantic longitudes, which masks the MIT minimum. For completeness, all cases of a clearly defined polar cavity are also presented.

The structure of the winter noon ionosphere of the southern hemisphere was studied. This structure includes the dayside cusp, associated high-latitude ionospheric trough (HLT), main ionospheric trough (MIT), electron density (Ne) peak at latitudes about 70°, mid-latitude ring ionospheric trough (RIT), and low-latitude quasi-trough. Data from the CHAMP satellite in the southern hemisphere for quiet geomagnetic conditions under high solar activity were selected for analysis. The DMSP satellite data and a model of auroral diffuse precipitation were also used. This model represents two zones of auroral diffuse precipitation on the equatorward and poleward edges of the auroral oval. It is shown that the situation in the winter noon ionosphere of the southern hemisphere depends cardinally on longitude. At sunlit longitudes, only the HLT is observed, and MIT is formed in the shadow region. At intermediate longitudes, both troughs can be observed and, therefore, there is a problem of their separation. The positions of all structures of the ionosphere depend on the longitude; in particular, the positions of the daytime MIT are changed by 6°−7°. At latitudes of the dayside cusp, both the peak and the minimum of Ne can be observed. A high and narrow peak of Ne is regularly recorded in the CHAMP data at latitudes of the equatorward zone of diffuse precipitation (68°−72°). In the shadow region, this peak forms the MIT poleward wall, and at sunlit longitudes a quasi-trough equatorward of this peak is sometimes observed. The RIT is rarely formed during the day, only at the American and Atlantic longitudes.

Using the statistical CRRES measurements of the electric field intensities of lower band chorus (LBC) and upper band chorus (UBC) around L = 6 under geomagnetically moderate conditions, we evaluate the variations in modeled magnetic field spectral intensity and the resultant changes in resonant scattering rates of plasma sheet electrons caused by different choices of the wave normal distribution. UBC scattering rates inferred from electric field measurements show a common trend of decreasing scattering with increasing peak wave normal angle, θm, for the plasma sheet electrons at all resonant pitch angles. This trend is mainly due to the lower power of magnetic field as derived from the electric field measurements for oblique waves. The LBC resonant diffusion inferred from electric field measurements shows a considerable increase in scattering rates with increasing θm for ∼1 keV electrons at all resonant pitch angles and for 3–30 keV electrons over certain ranges of pitch angles, which is contrary to the decrease in wave magnetic field amplitude and results mainly from the decrease in resonant energy and redistribution of the majority of wave power at large wave normal angles for increased peak wave normal angle. LBC‐induced scattering rates of 3–10 keV electrons decrease with increasing θm at low pitch angles, consistent with the decrease in wave magnetic field amplitude when θm increases. Our investigation demonstrates that the knowledge of the wave normal distribution of LBC and UBC is essential for an accurate quantification of the net resonant scattering rates and loss timescales of the plasma sheet electrons for an improved global simulation of diffuse auroral precipitation and the evolution of plasma sheet electron pitch angle distribution if only measurements of wave electric field intensity are available. In contrast, the diffuse auroral scattering rates calculated from magnetic field measurements are much less sensitive to the assumption on wave normal angle distribution. While UBC scattering with constant magnetic field power is roughly insensitive to the assumed wave normal distribution, LBC scattering with constant magnetic field power becomes more dependent on the assumed wave normal angle distribution, especially for ∼1 keV electrons. Key Points Importance of wave normal distribution to electric and magnetic field conversion Impact of wave normal distribution on chorus‐driven diffuse auroral scattering Relative roles of magnetic amplitude and wave power distribution over normal angle

We perform a comprehensive analysis of resonant scattering of diffuse auroral electrons by oblique nightside chorus emissions present along a field line with an equatorial crossing of 6 RE at 00:00 MLT, using various nondipolar Tsyganenko magnetic field models. Bounce‐averaged quasi‐linear diffusion coefficients are evaluated for both moderately and actively disturbed geomagnetic conditions using the T89, T96, and T01s models. The results indicate that inclusion of nondipolar magnetic field leads to significant changes in bounce‐averaged rates of both pitch angle and momentum diffusion for 200 eV to 10 keV plasma sheet electrons. Compared to the results using a dipole field, the rates of pitch angle diffusion obtained using the Tsyganenko models are enhanced at all resonant pitch angles for 200 eV electrons. In contrast, for 500 eV to 10 keV electrons the rates of pitch angle scattering are enhanced at intermediate and/or high pitch angles but tend to be considerably lower near the loss cone, thus reducing the precipitation loss compared to that in a dipole field. Upper band chorus acts as the dominant cause for scattering loss of 200 eV to 2 keV electrons, while lower band chorus scattering prevails for 5–10 keV electrons, consistent with the results using the dipole model. The first‐order cyclotron resonance and the Landau resonance are mainly responsible for the net scattering rates of plasma sheet electrons by oblique chorus waves and also primarily account for the differences in bounce‐averaged diffusion coefficients introduced by the use of Tsyganenko models. As the geomagnetic activity increases, the differences in scattering rates compared to the dipole results increase accordingly. Nonnegligible differences also occur particularly at high pitch angles for the diffusion rates between the Tsyganenko models, showing an increase with geomagnetic activity level and a dependence on the discrepancy between the Tsyganenko model fields. The strong dependence of bounce‐averaged quasi‐linear scattering rates on the adopted global magnetic field model and geomagnetic activity level demonstrates that realistic magnetic field models should be incorporated into future modeling efforts to accurately quantify the role of magnetospheric chorus in driving the diffuse auroral precipitation and the formation of electron pancake distributions. Key Points Chorus‐driven diffuse auroral scattering in nondipolar fields Main cause for the significant differences in diffusion rates Importance of using realistic magnetic fields to resonant scattering rates

This chapter contains sections titled: Introduction The Inner Magnetospheric Nightside Diffuse Auroral Precipitation The Outer Magnetospheric Nightside Diffuse Auroral Precipitation The Dayside Diffuse Auroral Precipitation Summary and Future Work

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.

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

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