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

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.

Figures (7)

• Figure 1: Overview of the CME/ICME measurements. a) Early CME measurements as observed on the limb in SDO/AIA wavelength 193 Å and later in LASCO and STEREO-A coronagraphs. b) Spacecraft configuration at the time of CME detection, as well as the direction and extent of the CME as derived from the GCS reconstruction. c) Mesh of the GCS reconstructed 3D CME geometry as viewed in the Earth plane-of-sky, along with the position of pulsar at the time of pulsar measurement (red dot). d) In situ measurements of the ICME detected by SolO in local spacecraft Radial-Tangential-Normal (RTN) coordinates, where the ICME sheath and flux rope are shaded green and red, respectively.
• Figure 2: Top panel: DM time-series for PSR J1022+1001 during the 2021 solar conjunction. Bottom panel: DM and RM variations measured from the LOFAR-core observation closest to the estimated time at which the CME arrived at the pulsar LoS. The 40 minute observation was divided into 4 minute sub-integrations to obtain each point in the lower plot.
• Figure 3: CMEchaser plot showing the position of J1022+1001 with a white crosshair symbol at 13:40 UT on August 20, 2021. The background image shows both the LASCO C3 and C2 images of the event.
• Figure 4: The LOFAR rotation measure (RM) spectra as a function of time for the 4-minute sub-integrations. Dashed curves show the normalized RM spectra, vertically offset for clarity, and solid curves show the corresponding Gaussian fits. The colour bar indicates the time in minutes from the start of the observation. The dotted line track the peaks of the Gaussian fits over time.
• Figure 5: 3DCORE fitting results: a) Magnetic field measurements from SolO (solid lines) along with the 3DCORE ensemble fit. The shaded region represents the ensemble's 2$\sigma$ spread, while the dashed colored lines depict the mean of the ensemble. Vertical black dashed lines indicate the start and end of the time window used for the 3DCORE reconstruction. The grey dashed lines mark the fitting points where the model output is compared to the in situ data. The magnetic field data is shown in the local spacecraft Radial-Tangential-Normal (RTN) coordinate system. b) 3D reconstruction of the ensemble mean, together with three example magnetic field lines within the toroidal structure, shown 24 hours after the CME reached 21.5 $r_\odot$. Also shown are the positions of Earth (green dot) and SolO (orange square), along with its past and future trajectory (dashed and solid lines, respectively). The straight magenta line extending from Earth indicates the LoS to the pulsar, with synthetic spacecraft placed along it (back dots).