Coronal mass ejections (CMEs) are large-scale expulsion of plasma and electromagnetic fields from the solar corona into the heliosphere. Electromagnetic field entrained in the CME plasma is important to comprehend their advancement, proliferation, and geo-effectiveness. Among the various observables at radio wavelengths, spectral modeling of faint gyrosynchrotron (GS) emission from CME plasma has been related to as among the most promising remote observing methods for approximating spatially fixed CME magnetic fields. Imaging the really low flux density CME GS emission in close distance to the Sun with orders of magnitude higher flux density has, nevertheless, shown to be rather tough. After the very first detection and modeling of spatially fixed faint GS emission from CME plasma by Batian et al. 2001, there are only a handful of studies (Maia et al. 2007, Tun & & Vourlidas 2013, Bain et al. 2014, Mondal et al. 2020) which have handled to identify this emission.
This critical difficulty has actually only just recently been met using the high dynamic range imaging capability of the Murchison Widefield Array (MWA). Regular detection of GS is now within reach, the challenge has shifted to constraining the big number of free criteria in GS models, a few of which are degenerate, utilizing the restricted number of spectral points at which the observations are generally readily available. In this research study, we show that high-fidelity and high dynamic variety spectropolarimetric imaging with the MWA can offer robust constraints on the circular polarization of faint GS emission. Using this information together with the overall intensity spectrum can considerably improve the constraints on plasma specifications of the CME.
A quick introduction of observations
The CME studied in this work is a sluggish CME, having three-dimensional speed around 450 km/s. High vibrant variety spectropolarimetric images offered by the cutting edge calibration and imaging algorithm, P-AIRCARS (Kansabanik et al, 2022, Kansabanik et al., 2023a) allows us to detect faint GS radio emission at multiple frequencies and covering almost the whole white-light CME. Addition of these stringent Stokes V upper limitations along with Stokes I spectrum limits the stage space offered for GS model criteria and for this reason improves their effectiveness.
Figure 2: Regions where spectra have been extracted. The red areas are those where spectrum fitting is done and spectrum fitting is not done for green regions. Spectrum fitting is likewise done for area 7, which is marked by cyan, while keeping some specifications fixed. Region 9, which is marked by magenta, just has a single spectral point.
Bastian, T. S., Pick, M., Kerdraon, A., Maia, D., & & Vourlidas, A. 2001, ApJL, 558, L65 Bain, H. M., Krucker, S., Saint-Hilaire, & P., & Raftery, C. L. 2014, ApJ, 782, 43 Maia, D. J. F., Gama, R., Mercier, C., et al. 2007, ApJ, 660, 874 Mondal, S., & Oberoi, D., & Vourlidas, A. 2020, & ApJ, 893, 28 Tun, S. D., & Vourlidas, A. 2013, ApJ &, 766, 130 Kansabanik, D., Oberoi, D., & & Mondal, S. 2022c, ApJ, 932, 110 Kansabanik, D., Bera, A., Oberoi, D., & Mondal, S. 2023, ApJS, 264, 47
Amongst the different observables at radio wavelengths, spectral modeling of faint gyrosynchrotron (GS) emission from CME plasma has been concerned as one of the most promising remote observing strategies for approximating spatially solved CME magnetic fields. Regular detection of GS is now within reach, the obstacle has actually shifted to constraining the large number of totally free parameters in GS designs, a few of which are degenerate, utilizing the limited number of spectral points at which the observations are typically readily available. It likewise reveals GS spectra corresponding to 1,000 random realizations from the posterior distribution of GS model criteria. The black lines represent the Stokes I and V GS spectra corresponding to the average values of the posterior distributions of the GS model specifications. Light-yellow lines show the GS spectra for 1000 realizations chosen arbitrarily from the posterior distributions of the GS design criteria.
The Stokes V spectra are revealed in the 2nd row. The black lines represent the Stokes I and V GS spectra corresponding to the average worths of the posterior circulations of the GS design parameters. Light-yellow lines show the GS spectra for 1000 awareness picked randomly from the posterior circulations of the GS model specifications.
Posterior distributions of GS design specifications are displayed in Figure 4 for two cases– utilizing just Stokes I spectrum (blue contours) and using both Stokes I spectrum and Stokes V upper limitations (maroon shapes). When both Stokes I and V measurements are utilized, it is evident GS specifications are better constrained.
Figure 4: Comparison between Stokes I only and Stokes I, V joint modeling. Two-dimensional plots show the joint probability circulation of any 2 specifications. The contours are at 0.5, 1, 2, and 3σ. Blue contours represent the posterior distributions of specifications utilizing just Stokes I spectrum. Maroon shapes represent the posterior distributions utilizing the joint Stokes I and V spectra. Electromagnetic field (B) and line-of-sight with electromagnetic field (θ) are better constrained by Stokes I and V joint fitting.
Discussion
Addition of Stokes V upper limits in addition to Stokes I spectrum considerably lowers the spread in the circulation function of the model specifications and breaks a few of the degeneracies in the GS design, when compared to the scenario when just Stokes I spectrum is utilized. Making use of even strict Stokes V upper limitations jointly with Stokes I spectra enables us to omit the part of the specification area of GS models, which follows the Stokes I spectra but not with the Stokes V upper limitations. Analyzing the varieties of posterior distributions reveals that the unpredictabilities in the estimates of line-of-sight angle with magnetic field have actually reduced by ~ 44% and that in magnetic field strength by ~ 30% on using joint Stokes I and V modeling.
This work marks the next action beyond the earlier attempts of estimating CME plasma criteria utilizing GS emission. When used properly in combination with other available restraints, it demonstrates the usefulness of upper limits. Based on the results from present-day instruments like the MWA, there is no doubt that the much more delicate and larger bandwidth spectropolarimetric imaging from the upcoming instruments, like the Square Kilometre Array Observatory, the Next Generation Very Large Array, and the Frequency Agile Solar Radiotelescope; helped by the multi-vantage point coronagraph observations will enable GS modeling to be utilized as a regular and robust remote noticing method for approximating CME plasma specifications covering a large series of coronal heights.
Extra information
Based on a recent paper by Kansabanik, D., Mondal, S. and Oberoi, D. 2023, 2023 ApJ 950 164, https://doi.org/10.3847/1538-4357/acc385
Full list of authors: Devojyoti Kansabanik 1, Surajit Mondal 2, Divya Oberoi 1
1 National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune, India
2 Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, USA
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Figure 1: Radio emission from CME-1 at 80 MHz. Stokes I emissions at 80 MHz are revealed by the contours overlaid on a base distinction coronagraph image. The background shows the LASCO C2 and C3 coronagraph images from the nearest offered timestamps. The inner white-light image is from the C2 coronagraph and the outer image is from the C3 coronagraph. Radio image is at 01:24:55 UTC. Contours levels are at 0.5%, 1%, 2%, 4%, 6%, 8%, 20%, 40%, 60%, and 80% of the peak flux density. The radio emission marked by the cyan box is from CME-1, which is spotted on the sky aircraft out to 5.2 R ⊙ revealed by the red circle.
Spectro-polarimetric Modeling of the Faint GS Radio Emission
Faint GS radio emission from the CME is found from 80 MHz to 145 MHz. Spatially solved spectra are extracted at several independent adjacent point-spread-function (PSF) sized regions, as shown by the ellipses in Figure 2. Peaked Stokes I spectra (top panel of Figure 3) verify the emission system as GS emission.
The simplest GS emission design presumes an isotropic and uniform circulation of non-thermal electrons following a single power-law distribution. Even this simplistic GS design has ten free specifications and a few of them show degeneracy. Constraining all of them only utilizing Stokes I spectrum is not possible. Multi-vantage point observations utilizing white-light coronagraphs supply independent estimation of thermal electron density and maximum allowed source depth for the GS model. A lot of other parameters are constrained using a Monte-Carlo Markov-Chain (MCMC) base approach using both the Stokes I spectrum and the spectrum of Stokes V upper limitations. Observed and fitted Stokes I and V spectra for a few of the areas marked in red in Figure 2, are displayed in Figure 3. It also shows GS spectra corresponding to 1,000 random realizations from the posterior distribution of GS model parameters. As is obvious, the best fit designs are consistent with both observed Stokes I spectra and Stokes V upper limitations.