Monday, June 18, 2007

GEM Workshop, Monday breakout sessions

I attended two breakout sessions devoted to the focus group on Magnetosphere-Ionosphere Coupling Electrodynamics and Transport. Bill Lotko of Dartmouth and Josh Semeter of Boston University led the discussion. I spoke in the morning session, the second-to-last talk before lunch. Other speakers in the morning session were Yi-Jiun Su (UT Arlington) on the electron cyclotron maser instability in the Alfvénic acceleration region, R. P. Sharma (India Institute of Technology) on nonlinear kinetic Alfvén waves, Peter Damiano (St. Andrews) on Alfvén wave induced electron precipitation, Joo Hwang (Colorado) on test particle simulations of the effect of moving double layers on ion outflow, and Alex Glocer (Michigan) on ion outflow modeling in the "gap" region. Speakers in the afternoon session were Matt Zettergren on a kinetic-fluid coupled model for computing ion outflow, Bill Lotko on the importance of 0.1-1 Hz Alfvén waves in ion outflow, Andrew Wright (St. Andrews) on the effects of field-aligned currents on the ionosphere, Josh Semeter on an observational perspective of M-I coupling in the auroral region, Jo Baker (APL) on midlatitude ionospheric convection as observed with the SuperDARN Wallops radar, and Jerry Goldstein on "SAPpy musings" about M-I coupling.

Some highlights of the sessions:

Damiano showed that at least 40% of Alfvén wave energy in a field line resonance is lost every half period due to induced electron beam precipitation (the loss is higher with hotter electrons). He can derive a Knight-like I-V relation for warm electrons in a field line resonance; as Lotko pointed out, this implies that the wave period is much longer than the electron transit time. The initial current perturbation must broaden so that the electric field can replenish the loss cone.

Hwang showed significantly enhanced (by an order of magnitude), but episodic, ion outflow in the presence of moving double layers as compared with the extended static parallel electric field structure examined by Gorney et al. (1985). She pointed out that the outflowing ions need not pass through a moving double layer.

Lotko noted that if ion parallel dynamics are properly included, the ion density depletion in the lower F region can be ~80%, rather than ~1% if the ion dynamics are not included. The extra depletion is due to a nonlinear coupling of Alfvén waves to ion sound waves. The ions are pushed upward.

Wright noted that a downward parallel current can deplete the E region on time scales of ~30 seconds. The parallel current region must broaden to compensate. The reflection coefficient, and therefore the ionospheric Alfvén resonator, are significantly modified. He derived a parameter which determines whether depletion is (nearly) complete and a time scale for the depletion. He raised the question of whether the depletion of E region plasma implies current closure in the F region.

Semeter showed that the apparent motion of auroral arcs may be due to dispersive propagation of Alfvén wave packets.

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