Ultra Low Frequency Waves at the Ground Driven by the Kelvin-Helmholtz
Instability Associated with Reconnection: A Case Study
Abstract
The Kelvin-Helmholtz instability (KHI) and its effects relating to the
transfer of energy and mass from the solar wind into the magnetosphere
remain an important focus of magnetospheric physics. One such effect is
the generation of Pc4-Pc5 ultra low frequency (ULF) waves (periods of
45-600 s). On 3 July 2007 at $\sim$ 0500 magnetic local
time (MLT) the Cluster space mission encountered Pc4 frequency
Kelvin-Helmholtz waves (KHWs) at the magnetopause with signatures of
persistent vortices. Such signatures included bipolar fluctuations of
the magnetic field normal component associated with a total pressure
increase and rapid change in density at the vortex edges, oscillations
of magnetosheath and magnetospheric plasma populations, wave frequencies
within the expected range of the fastest growing KH mode, and
magnetopause conditions favorable to the onset of the KHI. The event
occurred during a period of southward polarity of the interplanetary
magnetic field. Most of the KHI vortices were associated with
reconnection indicated by the Walén relation, the presence of
deHoffman-Teller frames and field-aligned ion beams. Global
magnetohydrodynamic (MHD) simulation of the event also resulted in KHWs
at the magnetopause. The observed KHWs associated with reconnection
coincided with recorded ULF waves at the ground whose properties suggest
that they were driven by the KHWs. Such properties were the location of
Cluster’s magnetic foot point, the Pc4 frequency, and the solar wind
conditions.