MAGNETOSPHERIC SOURCE REGION OF DISCRETE AURORAS INFERRED FROM THEIR RELATIONSHIP WITH ISOTROPY BOUNDARIES OF ENERGETIC PARTICLES

According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border o f the wide (so-called double) auroral oval, which map to very differen t regions of the magnetotail. To find common physical conditions for t he auroral-arc generation in these magnetotail regions, we study the s patial relationship between the diffuse and discrete auroras and the i sotropic boundaries (IBs) of the precipitating energetic particles whi ch can be used to characterise locally the equatorial magnetic field i n the tail. From comparison of ground observation of auroral forms wit h meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we f ound that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of > 30-keV electrons, whereas (2) the IB o f the > 30-keV protons is often seen inside the diffuse aurora. These relationships hold true for both quiet and active (substorm) condition s in the premidnight-nightside (18-01-h) MLT sector considered. In som e events the auroral arcs occupy a wide latitudinal range. The most eq uatorial of these arcs was found at the poleward edge of the diffuse a uroras (but anyway in the vicinity of the electron IB), the most polew ard arcs were simultaneously observed on the closed field lines near t he polar-cap boundary. These observations disagree with the notion tha t 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 held lines.