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Study of auroral potentials using multi-satellite data from ESA’s Cluster mission
Aurora is created by beams of energetic electrons, spiraling towards Earth along the magnetic field, and stopped by collisions with the upper atmosphere, causing auroral emissions at altitudes between 100 km and 300 km. Adjacent to the aurora, there is a simultaneous outflow of electrons, carrying the auroral return current. High above the aurora the space plasma is very thin and limited in terms of current carriers. This leads to the formation of electric potential structures at altitudes between 5000 and 13000 km. When passing through such potential drops, the Earthward moving electrons will increase in energy, resulting in a deeper penetration down into the atmosphere and an intensification of the aurora. In the return current region, at altitudes between 1000 and 4000 km, potential structures of the opposite polarity are formed, accelerating electrons away from Earth (electron acceleration and intense aurora can also result from the interaction between electrons and various kinds of waves). An auroral event typically starts by the occurrence of one or more east-west oriented, stable and homogeneous auroral arcs. After this follows a more active period with the appearance of more dynamic auroral curtains and spirals, varying in form and intensity.
2. Cluster auroral studies
This study, described below, forms part of a general survey of auroral potential structures and electrodynamics using multi-point data from the Cluster satellites, carried out by the research group at space and plasma physics, KTH. The four identical ESA Cluster satellites travel in formation in highly elliptical, polar orbits. The auroral data is selected from the Cluster orbit segments crossing auroral field lines at altitudes between 4 and 7 Earth radii. The main parameters to be used in this study are electric fields, magnetic fields, and time-energy spectra of electrons and ions. The multi-point data allows spatial and temporal variations to be separated, and monitoring of the evolution of structures in space and time.
3. Main goal of the study
The main goal of this study is to determine how well high-altitude potential structures extend down to the ionosphere, or expressed differently, the degree of ionospheric contribution to the high-altitude auroral potentials. For the aurora, this can be done by comparing the characteristic energy of upward ion beams, which provides an estimate of the parallel potential drop, with the perpendicular (to B) potential across the structure. This ratio, k, which normally is less than or equal to one, reveals whether the potential extends completely (k=0), is isolated from (k=1), or, which is most common case, partly extends (0 < k < 1) to the ionosphere. For the return current region, a similar comparison can be made, between the characteristic energy of upward electron beams and the perpendicular (to B) potential. Related to this issue is the Poynting flux which should be determined for the selected events.
In a similar study by Hwang et al (2006 a, b), focusing on the return current only and using single-satellite data, potentials associated with “curved” structures, such as spirals and folds, are shown to be relatively isolated from the ionosphere (0.5 < k < 1) whereas potentials associated with sheet-like structures, such as stable arcs, are shown to partly extend down to the ionosphere (0 < k < 0.5). They also found that the Poynting flux was typically directed downward for the curved structures and upward for the sheet-like structures.
The present study based on Cluster data benefits not only from the use of multi-point data, but also from the fact that the acceleration regions are located well below the Cluster orbit. Thus, since the magnetic field-aligned component of the electric field can be assumed to be zero, the two perpendicular (to B) components of the electric field are available from the electric field measurements.
This study requires good computer skills inc
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