Parker Solar Probe Enters the Magnetically Dominated Solar Corona

The high temperatures and strong magnetic fields of the solar corona form streams of solar wind that expand through the Solar System into interstellar space. At 09:33 UT on 28 April 2021 Parker Solar Probe entered the magnetized atmosphere of the Sun 13 million km above the photosphere, crossing below the Alfvén critical surface for five hours into plasma in casual contact with the Sun with an Alfvén Mach number of 0.79 and magnetic pressure dominating both ion and electron pressure. The spectrum of turbulence below the Alfvén critical surface is reported. Magnetic mapping suggests the region was a steady flow emerging on rapidly expanding coronal magnetic field lines lying above a pseudostreamer. The sub-Alfvénic nature of the flow may be due to suppressed magnetic reconnection at the base of the pseudostreamer, as evidenced by unusually low densities in this region and the magnetic mapping.

Figure 1

An overview of the eighth solar encounter (E8) by PSP. (a) Total amplitude ($B$, black) and radial component ($B_r$, blue) of magnetic field, enveloped by the scaled $B$ prediction (red). The green triangle indicates a perihelion at $15.9R_{\odot }$ on 29 April 2021 at 08:48 UT. (b) Electron density ($n_e$, black) and the expected proton density ($n_{p\mathrm{expt}}$ based on speed and distance, red). (c) Proton radial speed ($v_r$, black) and the Alfvén speed ($v_A$, blue). (d) Proton (${\beta }_p$, black) and electron (${\beta }_e$, blue) plasma beta together with the kinetic to magnetic energy density ratio ($E_k/E_B$, red). (e) Radial Alfvén Mach number ($M_A$). (f) Normalized electron pitch angle distribution at 316.4 eV. The date (month/day), distance $r$, and latitude $\lambda °$ relative to the solar equator are indicated at daily intervals. PSP was in sub-Alfvénic flow for three periods in this encounter as shown by the shaded regions.

Figure 2

Power spectrum (upper panel) and spectral index (lower panel) of magnetic ($E_b$), velocity ($E_v$), and total energy ($E_t$) fluctuations in a 5 h period within I1. The break between the low-frequency “$1/f$” range and the turbulent inertial range is marked with vertical dashed lines at $f_{\mathrm{sc}}\sim 2\times {10}^{-3}\mathrm{Hz}$ (black) and the ion cyclotron period (red). The horizontal dotted lines correspond to $-1$ for the $1/f$ range and $-3/2$ and $-5/3$ as model predictions for the turbulence inertial range [41].

Figure 3

Mapping of solar wind from PSP to the solar surface using a PFSS magnetic field model. Green and purple lines show connection from PSP to photosphere and color of lines shows Alfvén Mach number, with thick purple and white being the sub-Alfvénic periods. (a) 2D Carrington projection with the heliospheric current sheet (black line), model coronal holes (red or blue regions), and PSP’s trajectory projected down to $2.0R_{\odot }$ (blue or red line) with a square showing location at the start of 1 May 2021. (b) 3D PSP connected field lines in green or purple, and underlying pseudostreamer field lines in black. PSP traverses from left to right in this plot. PSP crossed about 7° in longitude in the first interval while the photospheric sources map to two small regions about 40° apart on the Sun. The modeled angular distance from the HCS was approximately 2°–4° at this time.

Figure 4

Comparison of PSP trajectory (red) and prediction of location of the Alfvén critical surface (blue background) using Wind observations at 1 A.U. (updated from [15] with same power law scalings for radial variations in density and magnetic field) and smoothed sunspot number (green). PSP crossing into sub-Alfvénic flow is consistent with 1 au observations from 2021 extrapolated inwards. Probability and duration of sub-Alfvénic encounters should increase as Alfvén surface expands outwards due to increasing solar activity and PSP perihelion is lowered through additional encounters with Venus.