The near-Sun Heliospheric Current Sheet, fluid and kinetic properties

TL;DR

This work statistically characterizes the near-Sun HCS using Parker Solar Probe data, focusing on magnetic reconnection, the turbulence cascade, and mirror-mode–driven structures. Analyzing 39 HCS crossings below $50~R_ ext{⊙}$, the authors find reconnection jets in $82\%$ of cases, with outward jets attaining the local Alfvén speed $V_A$ while inward jets are sub-Alfvénic, indicating a distance-dependent asymmetry likely tied to field-line topology. Turbulence near the HCS shows enhanced power at ion kinetic scales and a dissipation-range slope near $-3$, consistent with reconnection-driven inverse cascades and sub-ion flux-rope production, while large-scale Alfvénic fluctuations are suppressed near the HCS. The study also reveals ubiquitous magnetic hole trains in high-$β$ HCS plasma, implicating mirror-mode instability in regulating ion temperature anisotropy during reconnection. Collectively, these results reveal a dynamically active, near-Sun HCS where reconnection, turbulence, and mirror-mode processes strongly couple to shape energy conversion and heliospheric structure, with implications for future multi-spacecraft and solar-terrestrial studies.

Abstract

The heliospheric current sheet (HCS) is an important large-scale structure of the heliosphere, and, for the first time, the Parker Solar Probe (PSP) mission enables us to study its properties statistically close to the Sun. We visually identify the 39 HCS crossings measured by PSP below 50~\Rs~during encounters 6 to 21, and investigate the occurrence and properties of magnetic reconnection, the behavior of the spectral properties of the turbulent energy cascade, and the occurrence of kinetic instabilities at the HCS. We find that 82\% of HCS crossings present signatures of reconnection jets, showing that the HCS is continuously reconnecting close to the Sun. The proportion of inward/outward jets depends on heliocentric distance, and the main HCS reconnection X-line has a higher probability of being located close to the Alfvén surface. We also observe a radial asymmetry in jet acceleration, where inward jets do not reach the local Alfvén speed, contrary to outward jets. We find that turbulence levels are enhanced in the ion kinetic range, consistent with the triggering of an inverse cascade by magnetic reconnection. Finally, we highlight the ubiquity of magnetic hole trains in the high $β$ environment of the HCS, showing that the mirror mode instability plays a key role in regulating the ion temperature anisotropy in HCS reconnection. Our findings shed new light on the properties of magnetic reconnection in the high $β$ plasma environment of the HCS, its interplay with the turbulent cascade and the role of the mirror mode instability.

Figures (13)

• Figure 1: Identification of HCS full and partial crossings during E16, while PSP was located below 50 $R_{\odot}.$ From top to bottom, panels show (a) the magnetic field's amplitude $B$ and radial component $B_R$, (b) the radial solar wind velocity $V_R$, (c) the QTN electron density $n_e$ and (d) the PAD for suprathermal electrons (300-800 eV) normalized to its maximum value for each timestamp. The time intervals of HCS crossings are indicated by red shading, while partial crossings are shaded in gray.
• Figure 2: Radial distance and Carrington longitude of HCS crossings measured by PSP from E6 to E21. We show PSP's orbits as grey lines and HCS crossings as full dots colored by encounter with a 1h time resolution. We also show long partial crossings lasting more than 1h as light grey dots. CMEs from E13 and E15 -- associated with a change of polarity in the magnetic field -- are indicated with stars.
• Figure 3: Width of HCS crossings, both in km and in ion inertial length ($d_i$) units. Black crosses represent events from Table \ref{['tab: app_HCS']} and red crosses are events from Phan_2021. Top and side panels show the width distribution in $d_i$ and km, with median values indicated as a dashed line.
• Figure 4: Examples of three representative HCS crossings. We show event #11 where no ion jet is identified (a), event #2 where a large-scale reconnection jet is observed (b), and event #19 where both a large-scale jet (c) and a small-scale jet (d) are present. For each, we display in black the $l$ component of the solar wind velocity. We over-plot in blue $V_l(t_0) \pm \delta V_A$, where $t_0$ is an arbitrary time of the crossing. Vertical black lines indicate a change in the correlation sign.
• Figure 5: Properties of HCS reconnection jets. In the top panel, we show for each HCS the median velocity variation (i.e., the second quartile Q$_2$, crosses) along with the first and third quartile range $\mathcal{Q}_1 - \mathcal{Q}_3$ (vertical lines) of $\Delta V_l/V_{A}$ as a function of radial distance. Events with no large-scale jets are in gray, events with visible reconnection outflows are in blue. When reconnection was observed, we show the maximum velocity variation (full dots) normalized by $V_{A}$. The small side jets of Table \ref{['tab: app_jets']} are also included in lighter blue. In the bottom panel, we show the proportion of inward (red) and outward (blue) reconnection outflows observed by radial distance bins of 2.5 $R_{\odot}.$