Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi‐stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi‐potential structure itself may be part of an Alfvén wave.
Abstract. Hot ion shell distributions could possibly contain enough free energy for waves that could power electron energisation above auroral inverted-V regions. Using both linear theory (WHAMP) and two-dimensional electrostatic simulations, we show that ion shell distributions can cause unstable ion Bernstein mode emissions with high temporal growth rates, as well as perpendicular and parallel e-folding distances, that are in accordance with the tranverse dimensions of auroral arcs and the parallel size of the energisation region, respectively. The phase velocities of the waves are in the proper range to give parallel energisation to electrons with a Landau resonance. The simulation shows that about 90% of the energy goes into electrons and 10% goes into cold ion perpendicular heating. An electron heating rate of ∼ 80 eV/s is obtained.
Abstract. We derive the current-voltage relationship in the auroral region taking into account magnetospheric electrons for the bi-Maxwellian and kappa source plasma distribution functions. The current-voltage formulas have in principle been well known for a long time, but the kappa energy¯ux formulas have not appeared in the literature before. We give a uni®ed treatment of the bi-Maxwellian and kappa distributions, correcting some errors in previous work. We give both exact results and two kinds of approximate formulas for the current density and the energy¯ux. The ®rst approximation is almost generally valid and is practical to compute. The ®rst approximation formulas are therefore suitable for use in simulations. In the second approximation we assume in addition that the thermal energy is small compared to the potential drop. This yields even simpler linear formulas which are suitable for many types of event studies and which have a more transparent physical interpretation than the ®rst approximation formulas. We also show how it is possible to derive the ®rst approximation formulas even for those distributions for which the exact results can not be computed analytically. The kappa ®eld-aligned conductance value turns out always to be smaller than the corresponding Maxwellian conductance. We also verify that the obtained kappa current density and energy¯ux formulas go to Maxwellian results when j 3 I.
We have used data from the European Incoherent Scatter (EISCAT) facility to determine the characteristics of particle precipitation before and within auroral breakups and westward traveling surges. The EISCAT data of electron density versus altitude were transformed into energy spectra of the precipitating electrons [Kirkwood and Osepian, 1995]. We have used other ground‐based data such as auroral images and magnetic recordings to put the EISCAT observations into the context of the substorm development. In particular, we tried to determine the exact location of EISCAT with respect to the initial auroral forms of an onset. Characteristic differences are found in the deduced energy spectra of the precipitating electrons for the onset and the WTS events. Both the WTS and onset spectra showed a tendency of superposed spectral peaks at 10 keV and 20–30 keV respectively, which could be related to a near Earth acceleration mechanism. The conductance ratio in the onset events was twice as high as in the WTS events and also the measured particle fluxes were higher. We will discuss the implementations of our results for our understanding of particle acceleration mechanisms during the very onset of the substorm.
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