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 (so-called double) auroral oval, which map to very different regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diffuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux 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 diffuse 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 diffuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near-Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward-stretched magnetic field lines.
Highlights
Being visible in global scale, the auroras provide the images of global dynamics of the magnetospheric processes
A remarkable feature and important manifestation of the magnetospheric substorm is the auroral dynamics: it starts from the brightening of the most equatorial discrete auroral arc and continues as the development of the auroral bulge (Akasofu, 1968), which forms by continuous poleward motion of the bright discrete arc and/or by progressive poleward formation of new arcs (Sergeev and Yahnin, 1979)
We considered the observations made in the northern hemisphere during the steady magnetospheric convection (SMC) event on 24 November 1981
Summary
Being visible in global scale, the auroras provide the images of global dynamics of the magnetospheric processes. The overlapping of poleward bright part of the double oval with the velocity-dispersed ion structures (VDIS) (Kovrazhkin et al, 1987; Zelenyi et al, 1990), as well as the mapping in the specially prepared magnetospheric model for two SMC events (Sergeev et al, 1996), both con®rmed that these high-latitude discrete auroras come from the far tail or PSBL Another scheme, proposed by Feldstein and Galperin (1985) and Galperin and Feldstein (1991), placed the earthward edge of the discrete-arc source region deep into the inner magnetosphere. Those NOAA/TIROS orbits were selected which crossed the auroral zone inside (or very close to) the ®eld of view of all-sky cameras operated during the time of crossing Both satellite trajectories and detected auroral forms were mapped to the common altitude 110 km and put on the map in the invariant latitude ± MLT coordinates. We will describe the events briey, presenting ®rst the observations made in August 1979 and those made in November 1981
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