Alfvén Waves from Interchange Reconnection at Separators as a Possible Source of Switchbacks

Abstract Parker Solar Probe has observed Alfvénic fluctuations of amplitude large enough to occasionally reverse the local magnetic field direction—a phenomenon known as a switchback. Some have attributed switchbacks to reconnection in the low corona between open and closed field lines, called interchange reconnection. This has not been investigated thoroughly enough to conclusively make the association. We make a small-scale magnetic model of a coronal hole to which PSP was briefly magnetically connected and show that no more than about 15% of the photosphere anchors magnetic field lines open to the heliosphere. The model’s field line mapping is characterized by a web of separatrices, an S-web, outlining ∼10 Mm cells reflecting the magnetic carpet. We find that PSP’s magnetic footpoint crossed S-web boundaries at a rate similar to encountering large magnetic field deflections. The model also reveals that interchange reconnection in the vicinity of an S-web separatrix generally occurs at a low-lying magnetic separator, and that such reconnection would launch a nonlinear Alfvén wave polarized within the separatrix surface. We use a thin flux tube model to infer that interchange reconnection at a low-lying separator can produce a wave with magnetic deflection as large as in a switchback. The average mass, momentum, and energy carried by a population of waves is estimated to be comparable to the solar wind itself. These findings appear to support the plausibility of interchange reconnection as a possible source of switchbacks.