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

Abstract

Abstract. According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (socalled double) auroral oval, which map to very di erent regions of the magnetotail. To ®nd common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the di use and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic ®eld in the tail. From comparison of ground observation of auroral forms with meridional pro®les of particle¯ux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of >30-keV electrons, whereas (2) the IB of the >30-keV protons is often seen inside the di use aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18± 01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the di use auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed ®eld lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the nearEarth portion of plasma sheet or exclusively on the PSBL ®eld lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailwardstretched magnetic ®eld lines.

Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

“…Such consistency is unlikely to be fortuitous only. As a matter of fact, the above result is fully consistent with a number of existing observations and theoretical proposals that an equatorward-most arc likely maps to somewhere in the earthward portion of a stretched thin current sheet, on the verge of a transition into the quasidipolar inner magnetosphere (Lui and Burrows, 1978;Galperin et al, 1992;Yahnin et al, 1997;Galperin and Bosqued, 1999;Sergeev et al, 2012). In particular, using an event-adaptive magnetic field model (Kubyshkina et al, 2011) tuned by THEMIS, GOES, and NOAA observations, Sergeev et al (2012) mapped the prebreakup arc to the CPS, and found that the AR was situated at the outer edge of a "magnetic wall" region with strong B z and R c gradient.…”

“…Compared to the above-inferred location of the earthward boundary of AR at ∼ 05:11 UTC; X ∼ −6.7; R ∼ 9.8 R E , even taking into account a slow migration of the NTR during the two measurement epochs, we are able to deduce the following key inference on the relative location of the AR with respect to the NTR: the AR itself is embedded in a highly stretched current sheet region with a relatively stable B z , but its earthward edge is on the verge of a transition into a quasi-dipolar magnetosphere with much steepened B z gradients. The above inference of the AR location is fully consistent with a number of existing observations and theoretical proposals that an equatorward-most arc likely maps to the earthward portion of a stretched current sheet region, at the periphery of the quasi-dipolar magnetosphere (Lui and Burrows, 1978;Galperin et al, 1992;Yahnin et al, 1997;Galperin and Bosqued, 1999;Sergeev et al, 2012).…”

Occurrence characteristics and lowest speed limit of subauroral polarization stream (SAPS) observed by the SuperDARN Hokkaido East radar

“…The region where precipitation flux is over 10 0 mW/m 2 is identified as auroral oval, whereas that where the flux is below the value is identified as subauroral region (Yahnin et al 1997). In this manner, we selected only subauroral region flows.…”

Properties of current sheet thinning at x ∼− 10 to −12 RE

“…Early in situ observations of CS thinning around x =−10 to −12 R E in the near-Earth magnetosphere [Mitchell et al, 1990;Pulkkinen et al, 1991;Sergeev et al, 1993;Sanny et al, 1994] and estimates of equatorial magnetic field gradients from precipitation models [Sergeev et al, 1990;Yahnin et al, 1997] suggested that CS thickness can decrease to 500-1000 km (see, also, review by Ganushkina et al [2015]). This scale is comparable to or even smaller than the local ion gyroradius.…”