The Structure and Dynamics of Poynting Flux in the Geomagnetic Tail and its Role
in Generating Aurora

J.R. Wygant


Recent observations from the Polar and Cluster spacecraft have provided evidence
that a variety of processes observed in different locations in the geomagnetic
tail during intervals of strong magnetic activity generate intense Alfvenic and
steady state Poynting flux directed along magnetic field lines towards the
Earth. This Poynting flux is often capable of providing the power necessary for
the acceleration of auroral electrons and creating intense auroral displays.
Strong electric and magnetic fields associated with MHD waves carrying Poynting
flux directed towards the Earth are observed in the high beta plasma sheet near
9 Re during repetitive intervals of magnetic field dipolarization during major
geomagnetic storms. Intense Poynting flux associated with Alfvenic surface waves
have been also been observed in the low beta Plasma sheet boundary layer (PSBL)
at altitudes of 4-7 Re immediately above the auroral acceleration region near
the poleward boundary of the auroral zone. During the main phase and peak of
major geomagnetic storms, measurements of electric and magnetic fields from
Polar in the near Earth equatorial plasma sheet show evidence for spatially
extensive (1-3 Re) regions of intense Poynting flux (both steady state and
Alfvenic) towards the Earth. This Poynting flux maps to low latitudes over
widths of 2-5 degrees. Finally, observations from the Cluster spacecraft studies
demonstrate that strong Poynting flux is radiated from the reconnection region
near ~20 Re in association with magnetic field energy conversion in the tail.
The in-situ values of Poynting flux from these regions are 0.1 to 1.0 mW/m2 and
may be related to the observations in the PSBL The value of Poynting flux mapped
to altitudes of 100 km are on the order 10-100 mW/m2 and are sufficient to power
some of the most intense aurora observed in the Polar spacecraft UV Image and
the IMAGE spacecraft WIC data sets. The properties of the structure and dynamics
of the MHD waves associated with the Poynting flux will be summarized and
compared to the temporal and spatial structure of the auroral forms
simultaneously observed by the IMAGE and Polar Space based imagers.

_______________ 
Presentation, Fall Meeting, American Geophysical Union, San Francisco, USA, 
8-12 December 2003