Parker Solar Probe Observations of Solar Wind Energetic Proton Beams Produced by Magnetic Reconnection in the Near-Sun Heliospheric Current Sheet

Abstract

We report observations of reconnection exhausts in the Heliospheric Current Sheet (HCS) during Parker Solar Probe Encounters 08 and 07, at 16 R _s and 20 R _s , respectively. Heliospheric current sheet (HCS) reconnection accelerated protons to almost twice the solar wind speed and increased the proton core energy by a factor of ∼3, due to the Alfvén speed being comparable to the solar wind flow speed at these near-Sun distances. Furthermore, protons were energized to super-thermal energies. During E08, energized protons were found to have leaked out of the exhaust along separatrix field lines, appearing as field-aligned energetic proton beams in a broad region outside the HCS. Concurrent dropouts of strahl electrons, indicating disconnection from the Sun, provide further evidence for the HCS being the source of the beams. Around the HCS in E07, there were also proton beams but without electron strahl dropouts, indicating that their origin was not the local HCS reconnection exhaust.

Keywords: heliospheric current sheet; magnetic reconnection; parker solar probe; particle acceleration; solar wind.

Figure 1

Parker Solar Probe (PSP) crossing of a reconnecting heliospheric current sheet (HCS) in E08. (a and b) proton and electron spectrograms in differential energy flux (eVs⁻¹cm⁻²ster⁻¹ eV⁻¹), (c) pitch angle distribution of 314 eV electrons, (d and e) differential energy fluxes of 314 eV electrons at 0°–30° and 150°–180° pitch angles, (f and g) magnetic field magnitude and components in RTN, (h and i) proton velocity and density, (j) proton temperature moment, (k) electron temperatures, and (l) schematic illustration of the standard reconnection exhaust and separatrix layers and the RTN coordinates. The vertical dashed lines mark the edges of the exhaust. The proton velocity is in the Sun's frame, while the spectrograms are in spacecraft frame. The PSP velocity in RTN at E08 perihelion was (−1,147, −4 km/s). The nearly indistinguishable black and green curves in panel (b) are twice the core T _e and the peak in differential energy fluxes.

Figure 2

Proton distributions in and around the E08 heliospheric current sheet (HCS). (a) Magnetic field, (b–d) proton radial velocity, temperature moment, and core temperatures, (e) ion spectrogram from ISOIS/EPI‐Lo, in counts/energy bin, and (f) proton spectrograms, (g–j) proton distributions summed and collapsed onto θ‐plane in SPAN‐ion instrument coordinates (Verniero et al., 2020): (g) near the outer edge of separatrix layer, (h) in the separatrix layer closer to the HCS, (i) in the exhaust, (j) in the weak |B| region of the exhaust, and (k) schematics showing the locations where the protons distributions g–j were sampled. The yellow arrow in panels g–j points along B, and its length represents the local V _A .

Figure 3

PSP crossing of a reconnecting heliospheric current sheet (HCS) in E07. The parameters are the same as in Figures 1 and 2. Velocity distribution functions (VDFs) in (m and n) were sampled outside the HCS, and VDF (o) was sampled inside the HCS near the left edge.

Figure 4

Zoom‐out of Figure 3. The black dashed lines mark the edges of the complete HCS crossing. The interval between the two green lines is a partial HCS crossing. The blue dashed lines mark the outer boundaries of the regions surrounding the HCS that showed persistent T _p|| > T _p⊥. The sketch in (m) shows a 2D exhaust without separatrix layers. It is unphysical because the black field lines end at the exhaust boundaries in a non‐physical way.