The effect of magnetopause motion on fast mode resonance

Abstract:

The Earth's magnetosphere supports several types of Ultra Low Frequency (ULF) waves; these include trapped fast mode waves often referred to as cavity modes, waveguide modes, and tunneling modes/virtual resonance. All trapped fast mode waves require a stable outer boundary to sustain wave activity. The magnetopause, usually treated as the outer boundary for cavity/waveguide modes in the dayside magnetosphere, is often not stable, particularly during geomagnetic storms. We examine how magnetopause motion affects the magnetosphere's ability to sustain trapped fast mode waves on the dayside using idealized simulations obtained from the BATSRUS global magnetohydrodynamic (MHD) code. We present the first observations of cavity modes in BATSRUS, replicating results from other global MHD codes. We further show that for most solar wind conditions magnetopause motion negligibly affects fast mode resonance; other mechanisms are responsible for the observed low cavity mode occurrence rates in the outer magnetosphere. However, extreme solar wind conditions with large magnetopause displacements, such as CIR events, may affect cavity mode occurrence. Our simulation results agree with observations from the WIND and GOES spacecraft suggesting that cavity modes are not significantly affected by magnetopause boundary motion.