Probing the magnetic field of a coronal mass ejection with PSR J1022+1001

TL;DR

This study demonstrates that low-frequency pulsar observations can estimate the magnetic field along the line of sight during a CME, and compares these estimates with 3D CME models constrained by Solar Orbiter data.

Contribution

It introduces a method to derive LoS magnetic fields from pulsar DM and RM measurements during a CME and validates these against 3D CME reconstructions.

Findings

Pulsar RM and DM can estimate LoS magnetic fields during CMEs.

Only a specific flux rope configuration matches the observed magnetic field evolution.

Modeled magnetic field amplitudes are systematically larger than observational estimates.

Abstract

We investigate whether low-frequency pulsar observations can provide LoS magnetic field estimates and whether these are consistent with synthetic LoS signatures extracted from a three-dimensional CME reconstruction constrained by Solar Orbiter data. We analyze a CME occultation of the LoS to PSR J1022+1001 on 20 August 2021, observed simultaneously with LOFAR and NenuFAR. From LOFAR, we derive time-resolved dispersion measure (DM) and rotation measure (RM) and isolate the CME contributions using background estimates for interstellar, solar wind and ionospheric components. We then infer the density-weighted LoS-averaged magnetic field component <B||>_PSR from the ratio delta-RM/delta-DM. In parallel, we reconstruct the CME using a semi-empirical 3DCORE model fitted to Solar Orbiter in-situ magnetic field observations at 0.65 au. We sample the modeled magnetic field along the pulsar LoS using fixed spatial sampling points and compute synthetic LoS-averaged signatures <B||>_3D for different flux rope configurations. The derived <B||>_PSR increases from approximately -9 nT to a peak near 63 nT during the observed interval. Comparison with synthetic signatures shows that the polarity and temporal evolution of the LoS signal are strongly dependent on the flux rope configuration and only a South-West-North (SWN) configuration (confirmed by Solar Orbiter in-situ data) reproduces the observed sign and overall evolution, whereas alternative configurations are incompatible. The modeled amplitudes, however, are systematically larger than the pulsar-derived values by roughly a factor of five. We show that simultaneous low-frequency pulsar DM and RM measurements can provide LoS magnetic field estimates for a CME and can be used to test CME magnetic structure against data-constrained three-dimensional reconstructions.