A general association between discrete auroras and ion precipitation from the tail

Abstract

Using two‐dimensional observations from the spinning, polar‐orbiting S3‐3 satellite, we have compared the locations of regions that we identify as discrete auroral arcs with regions of isotropic ion precipitation. Regions of discrete auroral arcs are defined to be regions containing particle distributions consistent with field‐aligned potential drops ≳0.5 kV. Polar cap arcs and local times near noon (0900‐1500 MLT) are excluded from this study. Our most important result is that the inferred locations of discrete auroral arcs are almost exclusively confined to the region of isotropic ion precipitation at all local times studied. In the rare exceptions, the ion pitch angle distributions approach isotropy. This association is what is expected if magnetic field lines containing discrete auroral arcs thread the tail current sheet at distances where ion motion violates the guiding center approximations. We have also found that discrete arc regions are generally associated with spatial structure and boundaries in the precipitating ions. Additionally, we occasionally observe upward going, return‐current electrons, and these are also associated with the structured, isotropic ion precipitation.

Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles

“…They found bursts of the low-energetic (<20-keV) electrons, which they associated with discrete auroras, to be embedded into the region where both energetic electrons and energetic protons were isotropic. Lyons et al (1988) found the low-energy electron precipitation bursts to be located in the poleward part of the isotropic proton precipitation. Our results con®rm these ®ndings.…”

“…Our results con®rm these ®ndings. Lyons et al (1988) suggested that the proton isotropy is produced by the particle scattering in the current-sheet region, thus concluding that the discrete aurora originate from the farthest part of the magnetotail. This led them to the conclusion that discrete aurora are from the PSBL.…”

“…The ®rst of them emphasised that discrete auroras (hereafter we use this term in a more general sense as the name for very structured precipitation) are often seen in the poleward part of the auroral precipitation (referred to in the following as the auroral zone, to retain the original name``oval'' for the statistical pattern of the discrete forms). Lyons and Evans (1984) and Lyons et al (1988) suggested that they come from the outermost part of the magnetotail current sheet and associated this precipitation with the plasma-sheet boundary layer (PSBL) population (Eastman et al, 1984). This type of situation is quite typical for the recovery phase of a substorm where the so-called``double oval'' is formed (Elphinstone et al, 1995); it is also typical for long-duration events of the steady magnetospheric convection (SMC) (e.g.…”

Diurnal auroral occurrence statistics obtained via machine vision

“…Arcs are a pervasive morphology, occurring at essentially all local times (e.g. Stringer and Belon (1967); Gustafsson (1967)), from the poleward boundary of the oval equatorward to the proton and/or electron isotropic boundaries (Lyons et al, 1988;Yahnin et al, 1997). Statistical studies indicate that arc occurrence peaks on the night side (i.e.…”

The inner magnetosphere imager mission

“…It has been shown by Lyons and Evans [1984] and Lyons et al [1988] that discrete aurora, in general, occur in regions of nonadiabatic ion behavior on the nightside, rather than in regions of nonadiabatic electron behavior. In contrast, dayside nonadiabatic ion behavior may be associated with electron aurora [Lyons et al, 1987].…”