Density gradients in the direction transverse to the mean magnetic field are though to increase the dissipation of Alfven waves in the corona and thereby promote coronal heating. We have studied Alfven waves propagating under conditions similar to coronal holes using the Large Plasma Device (LAPD) at the University of California, Los Angeles. LAPD produces a cylindrical plasma column in which we can launch and diagnose propagating Alfven waves. The transverse gradient was produced by using two plasma sources to generate a low density background with a high density core along the axis of LAPD. In such a plasma the Alfven speed varies radially due to the changing density. The Alfven wave fronts become distorted when propagating through such a plasma since the edges travel faster than the center. We have quantified this distortion using Fourier analysis and shown that the perpendicular wavenumber grows as the waves propagate away from the antenna and that the generation of high perpendicular wavenumbers is greater for stronger gradients. This trend is accompanied by greater energy reduction as the waves propagate. These results suggest that Alfven waves propagating through transverse gradients in the Sun, such as those in coronal magnetic flux tubes, may undergo significant damping.
Solar Heliospheric and INterplanetary Environment (SHINE 2019)
- Pub Date:
- May 2019