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 July 3, 2007 at ∼ 0500 magnetic local time the Cluster space mission encountered Pc4 frequency Kelvin-Helmholtz waves (KHWs) at the high latitude 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 vortex edges; oscillations of magnetosheath and magnetospheric plasma populations; existence of fast-moving, low-density, mixed plasma; quasi-periodic oscillations of the boundary normal and an anti-phase relation between the normal and parallel components of the boundary velocity. The event occurred during a period of southward polarity of the interplanetary magnetic field according to the OMNI data and THEMIS observations at the subsolar point. Several of the KHI vortices were associated with reconnection indicated by the Walén relation, the presence of deHoffman-Teller frames, field-aligned ion beams observed together with bipolar fluctuations in the normal magnetic field component, and crescent ion distributions. Global magnetohydrodynamic 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 those waves. Such properties were the location of Cluster’s magnetic foot point, the Pc4 frequency, and the solar wind conditions.
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Feedback Interactions Between the Ionosphere and Magnetosphere at Middle Latitude
Observations show that magnetic pulsations with frequencies around 1 mHz are frequently detected simultaneously at different latitudes on the ground, in the inner magnetosphere, and in the solar wind. The coupling between oscillations in the dynamic pressure or magnetic field carried by the solar wind and the ultra‐low frequency (ULF) waves detected on the ground at high latitudes has been suggested in several studies. We present results from a numerical study of ultra‐low‐frequency waves detected by the ground magnetometers at middle latitudes during substorm. We investigate the hypothesis that these waves are generated by the ionospheric feedback instability driven by the large‐scale electric field in the ionosphere. This field is associated with the surface waves propagating along the ambient magnetic field on a strong perpendicular gradient in the plasma density occurring in the equatorial magnetosphere. The gradient in the plasma density is associated with the plasmapause. The plasmapause moves to the middle latitude when the plasmasphere erodes during substorm. The energy from the external driver can be coupled to the large‐scale surface Alfvén waves traveling along the field lines into the ionosphere and generating small‐scale intense ULF waves and field‐aligned currents at middle latitudes. The simulations of the two‐fluid magnetohydrodynamics model confirm this scenario, and the numerical results show a good quantitative agreement with the observations.
more » « less- Award ID(s):
- 1803702
- PAR ID:
- 10375863
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 127
- Issue:
- 2
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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