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DE-SC0016602: Fundamental Studies in Basic Plasma Science: Experimental Investigations of Alfven Wave Damping Processes Relevant to the Solar Corona

Award Status: Active
  • Institution: The Trustees of Columbia University in the City of New York, New York, NY
  • DUNS: 049179401
  • PM: Podder, Nirmol
  • Most Recent Award Date: 06/20/2018
  • Number of Support Periods: 3
  • PI: Savin, Daniel
  • Current Budget Period: 08/01/2018 - 07/31/2019
  • Current Project Period: 08/01/2016 - 07/31/2019
  • Supplement Budget Period: N/A
 

Public Abstract

A major unsolved problem in astrophysics is to determine what heats the solar corona.  One proposed solution is that magneto-hydrodynamic Alfven waves carry the energy from below the solar surface and deposit it in the corona.  Such low frequency small amplitude Alfven waves are predicted to damp only over length scales too long to to heat the corona.  Recent observations, though, show that these waves are actually damped in the low corona.  The plasma physics of the damping process has not been determined. There are several proposed mechanisms; all rely on inhomogeneities to drive flows and currents at small length scales where the energy can be more easily dissipated.  We will study Alfven wave damping in a laboratory plasma scaled to be similar to the solar corona.  Experiments will be performed using LAPD at the Basic Plasma Science Facility at UCLA.   

Experiments will focus on wave reflection from a gradient in the Alfven speed.  Reflection is a requirement for producing plasma turbulence, which is believed to be a fundamental process in many plasma environments.  However, predictions of Alven wave reflection from an Alfven-speed gradient have not been systematically tested.  Previously, Alfven wave reflection has been studied experimentally using conducting grids and other artificial configurations to reflect the waves.  Such setups may be relevant to the Earths magnetosphere, but not the solar corona.  Preliminary LAPD experiments have demonstrated wave reflection from a gradient.  We will measure the wave reflection and transmission for different frequencies and gradients.  The second task is to study plasmas with gradients transverse to the magnetic field. Cross- field variations in the wave phase speed cause waves on neighboring field lines to propagate out of phase, distorting the wave fronts and leading to dissipation by phase mixing.  By itself, this is likely too slow to explain the damping in the corona.  But some theories predict the damping rate to be enhanced by the generation of cross-field propagating waves, resonant absorption, and nonlinear processes.  A systematic study will be initiated using controlled radial density gradients.  Measurements will quantify phase mixing in the gradient layer, any new wave modes or flows that are generated, and heating.  The only previous comparable experiment was also done in LAPD and looked at waves at the edge of the plasma column.  But this was neither a systematic study nor relevant for the corona.

 



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