Intermingled open and closed magnetic field lines near the radial origin of the heliospheric current sheet

TL;DR

This study analyzes the complex magnetic field structures near the Sun's heliospheric current sheet, revealing intermingled open and closed magnetic field lines and associated electrostatic waves, based on Parker Solar Probe data.

Contribution

It provides detailed observations of the intermingling of open and closed magnetic field lines and their wave phenomena near the Sun, advancing understanding of magnetic field geometry at the heliospheric current sheet.

Findings

Sign change in magnetic field component indicates crossing of current sheet.

Regions with open and closed magnetic field lines exhibit distinct plasma and wave characteristics.

Electrostatic waves are linked to ion acoustic waves, with harmonic frequencies related to proton plasma frequency.

Abstract

Aims. To investigate the magnetic field geometry and waves in the region near the Sun where the heliospheric current sheet is formed. Methods. One good example of apparent open and closed field lines was found and its fields and plasmas were analyzed. Results. The radial component of the magnetic field (the Z-component) measured on the Parker Solar Probe (PSP) changed sign between 12:00 and 13:00 UT on March 30, 2024, when the spacecraft was at 13 solar radii. This sign change may have occurred because the spacecraft crossed the heliospheric current sheet on long open magnetic field lines or it may have occurred because the spacecraft crossed from one side of the equator to the other on much shorter closed magnetic field lines. During this crossing, two distinct regions having different magnetic field geometries, strahl flows, plasma densities, and electric field spectra were observed and identified as regions with open and closed magnetic field lines, respectively. The two regions intermingled on time scales less than 100 milliseconds to create a complex magnetic field geometry. The waves observed in both regions were electrostatic and composed of wide band signatures (in the open field lines regions) and well-structured frequency harmonics (in both the open and closed field lines regions). These harmonic frequencies correlated with the proton plasma frequency, fpp, with the lowest frequency at ~0.1fpp. This result plus the field aligned electric field perturbations and plasma density fluctuations, require that the observed intense electrostatic mode and associated harmonics were ion acoustic waves. The absence of broadband electrostatic signals in closed field line regions is explained by the lower (than in open field line regions) hot and core electron density and higher ratio of the electron plasma frequency to the electron gyrofrequency, suppressing wave generation.