Role of solar wind turbulence in the coupling of the solar wind to the Earth's magnetosphere

Abstract

[1] The correlation between the amplitude of the MHD turbulence in the upstream solar wind and the amplitude of the Earth's geomagnetic activity indices AE, AU, AL, Kp, ap, Dst, and PCI is explored. The amplitude of the MHD turbulence is determined by the fluctuation amplitude of the solar wind magnetic field. This ''turbulence effect'' in solar wind/magnetosphere coupling is more easily discerned when the interplanetary magnetic field (IMF) is northward, but the effect is also present when the IMF is southward. The magnitude of the effect is the same for northward and southward IMF, accounting for about 150 nT of the variability of the AE index. Tests are performed that conclude (1) that the turbulence effect is not caused by the turbulence amplitude acting as a proxy for jBj in the solar wind and (2) that reversals of the IMF from northward to southward in the turbulent fluctuations is not the cause of the correlations. An expression is derived for the total viscous-shear force on the surface of the magnetosphere; improved solar wind/ magnetosphere correlations result when this expression is used. With insight from fluidflow experiments, the turbulence effect is interpreted as an enhanced viscous coupling of the solar wind flow to the Earth's magnetosphere caused by an eddy viscosity that is controlled by the amplitude of MHD turbulence in the upstream solar wind: more upstream turbulence means more momentum transfer from the magnetosheath into the magnetosphere, resulting in more stirring of the magnetosphere, which produces enhanced geomagnetic activity indices.

Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions

“…In addition, Borovsky and Funsten (2002) demonstrated that the enhanced upstream IMF turbulence increases the momentum transfer from the magnetosheath into the magnetosphere, resulting in more stirring of the magnetosphere and higher geomagnetic activity.…”

Coronal mass ejections and their sheath regions in interplanetary space

“…18 and 19, sheaths have many properties that enhance solar wind-magnetosphere coupling efficiency (see the detailed discussion of this topic in Kilpua et al 2017); high dynamic pressure (e.g., Palmroth et al 2007;Myllys et al 2016), enhanced ULF wave power causing enhanced viscous interactions at the magnetopause (e.g., Borovsky and Funsten 2003) and high Alfvén Mach numbers causing stronger compression of the plasma and field at the bow shock and the cross polar cap potential saturating at larger driving electric fields than during low Alfvén Mach number conditions (e.g., Borovsky and Birn 2014;Myllys et al 2016). Largescale fluctuations in the IMF north-south component could also build the ring current up more efficiently than a smoother driver of similar magnitude due to periodic trapping of the particles in the ring current region (e.g., Kamide et al 1997 .…”

Global geomagnetic responses to the IMF <i>B</i><sub>z</sub> fluctuations during the September/October 2003 high-speed stream intervals

“…The 2003 HSS events have been studied by other workers (Korth et al, 2006;Lee et al, 2006;Lyons et al, 2009;Tsurutani et al, 2011;Bolzan et al, 2012). However, a global solar wind-magnetosphere interaction study during 2003 HSS intervals is still outstanding.…”