University of Minnesota
School of Physics & Astronomy

Space Physics Seminar

Tuesday, October 30th 2018
1:25 pm:
Space Physics Seminar in Tate 301-20
Speaker: John Wygant, University of Minnesota
Subject: The role of Poynting flux in powering ion and electron energy flux of the cusp wind

In this talk, we present measurements of steady state and wave Poynting flux, as well as ion and electron energy flux flowing parallel to the magnetic field in the cusp region of the Earth's magnetosphere in order to assess the in flow and out flow of energy along these magnetic flux tubes. These measurements are obtained by the Polar spacecraft at 3-5 Re geocentric distances and by the FAST spacecraft at 1.6 Re and below. We find that during "active times" at Polar altitudes, there is a very substantial and steady net flow of ion energy flux away from the Earth which ranges between 10 and >100 mW/ m2 (normalized to flux tube area at 100 km). Average values of the net ion energy flux at 3-5 Re during less active periods are on the order of 1 mW/m2. The energy flux of ions and electrons at the lower altitudes observed by FAST are generally downward. The observations of large net energy flux out of the ionosphere indicate that the process of ion energization and outflow in the cusp is one of the most energy intensive processes in the Earth's magnetosphere rivaling the energy invested in the collisionless acceleration of electron beams associated with auroral arcs. During extreme events, it involves the formation of a low beta (0.1) subsonic wind of plasma flowing away from the Earth on cusp field lines at velocities of 50 km/s to several hundred km/s. A related question is what powers this system. The Polar spacecraft measurements provides evidence for large values of "steady state" and "wave" Poynting flux flowing earthward along magnetic field lines over a band-pass from 0.1 mHz to 1 Hz (spacecraft frame) and there is strong evidence that this is the only mode of energy transfer parallel to the magnetic field capable of powering this energetic wind. A comparison of incident Poynting flux to outflowing ion energy flux shows that the energy coupling/acceleration mechanism is likely to be quite efficient. In addition, measurements show the Poynting flux is concentrated in the large-scales, not in the smaller scale waves. These observations motivate interest in a number of unresolved issues related to if and how the large scale Poynting flux is transferred to small scales waves that can efficiently heat the plasma. This energy flow and its partitioning is an interesting contrast to that observed in conjunction with auroral electron acceleration on the nightside. This wind is also analogous in some important ways to the solar wind, which is also believed to be driven by Poynting flux. In principle, similar winds could also exist at other planetary magnetospheres.

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