On the Dawn‐Dusk Asymmetry of the Kelvin‐Helmholtz Instability Between 2007 and 2013

Abstract

Using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS), a statistical study was performed to determine whether a dawn‐dusk asymmetry exists in the occurrence rates of the Kelvin‐Helmholtz instability during Parker‐Spiral (PS) and Ortho‐Parker‐Spiral (OPS) orientations of the interplanetary magnetic field (IMF). It is determined from the data that there is a strong preference toward the dawnside during PS orientation, and although a preference to the duskside during OPS is suggested, this requires further study for an unambiguous confirmation. The uncertainty in the OPS result is due to a low number of events, which satisfied our selection criteria. Because IMF is statistically in PS orientation, the Kelvin‐Helmholtz instability (KHI) preference for the dawn flank during this orientation may help explain the origin of the plasma sheet asymmetry of cold component ions because it has been shown that KHI can drive kinetic‐scale wave activity capable of ion heating.

Kelvin–Helmholtz Instability: Lessons Learned and Ways Forward

“…A revisit of a statistical survey of 7 years of THEMIS data (Kavosi and Raeder 2015), taking this aspect into account, tends to confirm this prediction observationally (Henry et al 2017). The normalized occurrence rates showed clear dawn flank preference during PS IMF, and dusk flank preference during northward IMF (Henry et al 2017).…”

The Variation of Resonating Magnetospheric Field Lines With Changing Geomagnetic and Solar Wind Conditions

“…The asymmetric plasma mass density distribution is also the cause of the lower eigenfrequencies in the afternoon sector. However, Henry et al () found that the KHI was statistically more common on the dawnside which could also account for this asymmetry. However, our discussion in the following sections establishes that both the KHI and pressure pulse mechanism could be drivers of the fundamental eigenmode.…”

Kinematic models of the interplanetary magnetic field

“…It is found that the wind becomes collimated toward the rotation axis of the Sun (or the star) by the magnetic pinching of the spiral or twisted field. In fact, any stationary, axisymmetric magnetized wind collimates toward the rotation axis at large distances (Heyvaerts and Norman, 1989). It is useful to introduce the poloidal-toroidal expression of the magnetic field in the two-dimensional MHD treatment:…”

“…Of course, irregular or transient phenomena (such as coronal mass ejections or co-rotating interaction regions) cause local, large-amplitude deviations from the mean field. Recent study by Henry et al (2017) indicates that the IMF (at the Earth orbit) can be regarded as the Parker spiral type when the IMF is sufficiently inclined to the Earth orbital plane, either (1) B x > 0.4B and B y < −0.4B or (2) B x < −0.4B and B y > 0.4B, where B x is the sunward component of the magnetic field (GSE-X direction), B y is the dawn-to-dusk component of the field (GSE-Y direction), and B is the magnetic field magnitude. The IMF can be more radial and of the ortho-Parker spiral type (valid under |B x | > 0.4B t , where B t denotes the transverse component of the magnetic field to the radial direction from the Sun, B t = B 2 y + B 2 z ) or oriented more northward or southward |B z | > 0.5B t .…”