On the dependence of dayside Kelvin-Helmholtz activity on IMF orientation

With its similar to12 h-long period of strongly northward magnetic field, the coronal mass ejection which passed Earth on April 11, 1997 affords an excellent opportunity of studying Kelvin-Helmholtz (KH) activity at the dayside magnetopause as a function of the clock angle, theta, of the interplanetary field (IMF). A correlation between the wavelength of the KH waves and theta may be expected on the basis of our recent model, where activity is generated in strips which broaden as theta decreases. We identify two 2-hour long intervals of small, but different, clock angles, and make a preliminary test of model predictions, using records from two ground magnetometer chains, both on the dayside. Taking into account the local time of the magnetometer arrays and the geometry of the KH-active strips, we show that the resonant stations measured considerably more spectral power density in the 3-4 mHz range during the phase with smaller theta. Moreover, as theta increases, the spectral power spectrum shifted to higher frequencies that were almost absent for smaller theta. (C) 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.