Statistical analysis of orientation, shape, and size of solar wind switchbacks

Abstract

Context. One of the main discoveries from the first two orbits of Parker Solar Probe (PSP) was the presence of magnetic switchbacks, whose deflections dominated the magnetic field measurements. Determining their shape and size could provide evidence of their origin, which is still unclear. Previous work with a single solar wind stream has indicated that these are long, thin structures although the direction of their major axis could not be determined Aims. We investigate if this long, thin nature extends to other solar wind streams, while determining the direction along which the switchbacks within a stream were aligned. We try to understand how the size and orientation of the switchbacks, along with the flow velocity and spacecraft trajectory, combine to produce the observed structure durations for past and future orbits. Methods. The direction at which the spacecraft cuts through each switchback depended on the relative velocity of the plasma to the spacecraft and the alignment direction for that stream. We searched for the alignment direction that produced a combination of a spacecraft cutting direction and switchback duration that was most consistent with long, thin structures. The expected form of a long, thin structure was fitted to the results of the best alignment direction, which determined the width and aspect ratio of the switchbacks for that stream. Results. We find that switchbacks consistently demonstrate a non-radial alignment in the same sense as the Parker spiral field, but not the background flow direction within each stream. This alignment direction varied between streams. The switchbacks had a mean width of 50, 000 km, with an aspect ratio of the order of 10. Conclusions. We conclude that switchbacks are not aligned along the background flow direction, but instead aligned along the local Parker spiral, perhaps suggesting that they propagate along the magnetic field. Since the observed switchback duration depends on how the spacecraft cuts through the structure, the duration alone cannot be used to determine the size or influence of an individual event. For future PSP orbits, a larger spacecraft transverse component combined with more radially aligned switchbacks will lead to long duration switchbacks becoming less common.

Multiscale Solar Wind Turbulence Properties inside and near Switchbacks measured by Parker Solar Probe

“…The origin of this discrepancy between PSP and previous measurements is not easy to understand due to PSP 's unique trajectories compared to previous missions, nearly co-rotating with the Sun's surface during significant fractions of its first encounters (Bale et al 2019;Badman et al 2020a;Rouillard et al 2020). This specific trajectory is likely causing the observed magnetic field deflections to have shorter duration in measurements for larger radial distance, as the spacecraft could be slicing through an elongated structure, possibly aligned along the Parker spiral (Laker et al 2020). Apart from these unexpected characteristics, many similarities between PSP and previous SB observations remain.…”

Nonlinear dynamics of small-scale Alfvén waves

“…This may explain why AW are ubiquitous in the solar wind 5 , especially close to the sun and in the corona, currently being explored by NASA's Parker Solar Probe (PSP) 6 . PSP observations in particular show patches of extremely large amplitude waves (δB/B ∼ 1) that can even reverse the direction of the (mainly radial) background magnetic field: for this reason, these structures have been dubbed "switchbacks", and their morphology, formation, and evolution are current topics of active observational [6][7][8][9][10][11][12][13] and theoretical [14][15][16][17][18][19][20][21][22][23] interest.…”

Turbulent generation of magnetic switchbacks in the Alfvénic solar wind

“…3. Switchbacks appear to be field-aligned, elongated structures with a typical aspect ratio of 10 near the first perihelion (Horbury et al 2020;Laker et al 2020).…”