When shear is strong enough,switchback and isotropication occurs . In data obtained from PSp encounter-8 ,many many reversal in radial component of magnetic field
A switchback is defined not only in terms of the radial component of the magnetic field, B_R. The Sun has magnetic sectors, with magnetic field "away" from or "toward" the Sun, and such magnetic field orientation is carried out with the solar wind. From one of Maxwell's equations, a sudden change in magnetic field B implies a current sheet at the boundary. This is called the heliospheric current sheet (HCS). But a change in B_R due to a changing sector, or an HCS crossing, is *not* termed a switchback. In Parker Solar Probe's 8th encounter, there were several HCS crossings but actually very few switchbacks.
So how can you tell whether B_R is reversing as an HCS crossing or a switchback? Ideally you would use another type of data. Heat flux electrons, a.k.a. suprathermal or strahl electrons in the solar wind, are found to stream away from the Sun along the local B line. So if you see the pitch angle distribution of such electrons *not* changing, which B_R reverses, that is a switchback. Before/after an HCS crossing, the dominant pitch angle would reverse. Similarly, the cross-helicity of magnetic and velocity fluctuations is generally characteristic of outward-moving Alfven modes, so that behaves like the strahl electron pitch angle: it reverses before/after an HCS crossing but not during a switchback. Or a very simple but not physically rigorous way to identify a switchback is as a very temporary reversal of B_R, with the same sign before and after the switchback.
Finally, given that electron strahl and cross helicity are not reversing, many people define a switchback as B_R changing sign. However, a more general definition is rotation of the B vector beyond some angle, with the threshold angle as a variable parameter (see Dudok de Wit et al. 2020).
Hi D. Ruffolo,
Thank you for the thorough explanation. My question is, Is it possible to observe SB at any place in the heliosphere? Or Is it restricted close to the Sun?
They were observed over 20 years ago, but the word "switchback" was coined for Parker Solar Probe (PSP) analysis. They have been seen out to several AU. See Article Heliospheric magnetic field polarity inversions at high heli...
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