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Magnetic Switchbacks Are MHD Floaters in the Solar Wind

Presentation #107.11 in the session Solar Wind Posters.

Published onSep 18, 2023
Magnetic Switchbacks Are MHD Floaters in the Solar Wind

Switchbacks are a major discovery of the Parker Solar Probe (PSP). A switchback is a localized reversal in the magnetic field direction of the solar wind that the PSP frequently travels through and measures. Switchbacks were not expected prior to PSP and have not been explained yet to my knowledge.In my presentation I will describe a selfconsistent MHD model for switchbacks that I call the “floater” model because of the analogy with floaters bobbing in a stream. The solar wind close to the Sun, where PSP operates, can be described by MHD because the Debye length is small compared with typical length scales of the wind. But the plasma is not in thermal equilibrium — as the PSP data clearly bear out — because the collision frequency is relatively low.Considering that the magnetic field is divergenceless and applying Stokes’ theorem we find that there has to be a current around the switchback which we assume to be a ring current, since there is no preferred direction perpendicular to the magnetic field. The ring current generates a dipole field that reverses direction inside the ring and that can overwhelm the wind magnetic field there. The result is a switchback. I will demonstrate that the magnetic morphology of the ring current/switchback field resembles that of a bubble, or a “floater” within the wind that will stream along with wind.Ring currents in the wind are a natural consequence of magnetic reconnection near the base of the wind — that several authors already have pointed out as the likely origin of switchbacks - and also of cut-off reconnection further out that has been observed by LASCO. The cut-off of currents is very likely and these currents can only close in themselves since currents are divergenceless as well in MHD. These currents no longer have a driver, so they will gradually dissipate and the floater will disappear farther out in the wind. However, resistivity is rather low in the solar wind plasma so the timescale for dissipation of the floaters is long enough for them to exist near the Sun. I will present estimates as to how far out “floaters” can go.

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