Properties of Magnetic Switchbacks in the Near-Sun Solar Wind

Abstract

Magnetic switchbacks are fluctuations in the solar wind in which the interplanetary magnetic field sharply deflects away from its background direction so as to create folds in magnetic field lines while remaining of roughly constant magnitude. The magnetic field and velocity fluctuations are extremely well correlated in a way corresponding to Alfvénic fluctuations propagating away from the Sun. For a background field which is nearly radial this causes an outwardly propagating jet to form. Switchbacks and their characteristic velocity jets have recently been observed to be nearly ubiquitous by Parker Solar Probe with in situ measurements in the inner heliosphere within 0.3 AU. Their prevalence, substantial energy content, and potentially fundamental role in the dynamics of the outer corona and solar wind motivate the significant research efforts into their understanding. Here we review the in situ measurements of these structures (primarily by Parker Solar Probe). We discuss how they are identified and measured, and present an overview of the primary observational properties of these structures, both in terms of individual switchbacks and their collective arrangement into ``patches''. We identify both properties for which there is a strong consensus and those that have limited or qualified support and require further investigation. We identify and collate several open questions and recommendations for future studies.

Figures (20)

• Figure 1: Parker measurements of the young solar wind during the mission's first orbit, showing (a) the magnetic field amplitude and radial component as the spacecraft goes from 60 to 35 $R_{\odot}$ on November 6; (b) the radial solar wind speed together with a 20-minute average; (c) the electron density inferred using quasi-thermal noise spectroscopy (see Sect. \ref{['subsec: 1_instru']}) with a 20-minute average; (d) the pitch angle distribution of suprathermal electrons (314 eV).
• Figure 2: Observation from Mariner-II showcasing abrupt changes in the direction of the interplanetary magnetic field. Left panel: magnetic field and cosmic ray anisotropy; right panel: derived field line and (solar) cosmic ray flow. Reproduced from McCracken1966.
• Figure 3: Magnetic switchbacks observed by Parker at 28 R$_{\odot}$ during E5. The top panels display timeseries of (a) the magnetic field vector, (b) the radial solar wind velocity from SPAN, (c) the 1-minute average of the electron density from QTN, and (d) the PAD of suprathermal electrons (314 eV). The bottom panels show scatter plots of $B_R$ vs $B_T$ (e) and $B_R$ vs $B_N$ (f) colored by $B_R$ during the considered time interval.
• Figure 4: Impact of switchback definition. In panels $a$ to $c$, we show the magnetic field components in the RTN frame during E2, normalized by the closest approach radial distance of 35 R$_{\odot}$. We overplot the radial field (dotted lines), the Parker spiral field (full lines), and a 6h-mode field (dashed lines). In panels $d$ to $i$, we plot in white the 2D distribution contours of the normalized magnetic field components ($B_R$, $B_T$ in panels $d$, $f$, $h$, and $B_R$, $B_N$ in panels ($e$, $g$, $i$). Superimposed in color are the 2D histograms of the points that are located more than 60$^o$ away from the computed background fields, i.e., the radial direction ($d$, $e$), the Parker spiral ($f$, $g$), and the 6h-mode vector ($h$, $i$). We also add the normalized projected distributions on the side, as a black line for the full 2D distribution and color-shaded for the "more than 60$^o$ away" points. Reproduced from fargette_phd, copyright by the author
• Figure 5: Histogram showing the distribution of normalized deflection$z$, as defined by DudokdeWit2020, for individual switchback events during the E1--E13. Some common choices of thresholds in magnetic field deflections are indicated with a mention of the first study that made that choice to define a switchback.