Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm54b..03w&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM54B-03
Physics
[2704] Magnetospheric Physics / Auroral Phenomena
Scientific paper
The role that the structure of magnetic field-aligned Poynting flux and different forms of kinetic energy flow play in powering different auroral acceleration structures is still an outstanding issue in magnetospheric physics. In this talk we will present measurements of the contribution of the different forms of energy flux at quasi-static structures, Alfvenic acceleration structures, as well as at diffuse aurora and the cusp, at altitudes of ~ 1 Re and above. These measurements are compared to images from the UVI and VIS instruments on Polar and the WIC instrument on the IMAGE spacecraft. This altitude is above a significant fraction of the auroral acceleration region and above where most the energy has been dissipated in the collison-less acceleration process. The altitudes are low enough so that the mapping along magnetic field lines to auroral forms is accurate to a fraction of a degree. The data suggests that the shock-like electric field structures with amplitudes of 0.1- 1V/m responsible for the quasi-static acceleration of auroral electron beams and associated with quasi-stationary aurora are often associated with intense (4-40 ergs/cm2s) thin (~0.25 deg) sheets of Poynting flux flowing along the ambient magnetic fields towards the Earth. Measurements of electron kinetic energy flux from the Polar Hydra instrument indicates that the electrons provide a lesser contribution (20%) for aurora near the polar cap (67- 72 ILA). Thus, we provide evidence that Poynting flux is the dominant form of energy transfer from higher altitudes which power the quasi-static parallel acceleration of auroral electron beams and may in fact account for nearly all of the power delivered to and dissipated at these structures. The electric field structures are associated with upward field- aligned beam-like ions whose characteristic energies (300 eV- 5 keV) approximately track the line integral of the electric field along the spacecraft trajectory. This relation between the upgoing ions and the integral of the electric field has been documented in other studies (S3-3, FAST) and is used to argue the ions are accelerated by parallel electric field associated with closure of perpendicular electric field equipotential structures at lower altitudes. The E/B ratio varies from 5000 km/s to 100000 km/s, much larger than the ratio expected for current closure through the ionosphere. Downward Poynting flux strongly dominates over upward or reflected Poynting flux. The limitations and ambiquities of current single spacecraft measurements used to address energy flow problems will be discussed.
Cattell Cynthia A.
Dai Li
Fillingim Matthew O.
Frey Harald U.
Germany Glynn A.
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