Coronal mass ejections (CMEs) are large-scale plasma and magnetic discipline expulsions from the photo voltaic corona into the heliosphere. The magnetic discipline entrained within the CME plasma is essential to understanding their propagation, evolution, and geo-effectiveness. Among the many totally different observables at radio wavelengths, spectral modeling of faint gyrosynchrotron (GS) emission from CME plasma has been considered one of the vital promising distant observing strategies for estimating spatially resolved CME magnetic fields. Imaging the very low flux density CME GS emission close to the Solar, which has orders of magnitude increased flux density, has confirmed relatively difficult. After the primary detection and modeling of spatially resolved faint GS emission from CME plasma by Batian et al. 2001, there are solely a handful of research (Maia et al. 2007, Tun & Vourlidas 2013, Bain et al. 2014, Mondal et al. 2020) which have managed to detect this emission.
This instrumental problem has solely lately been met utilizing the excessive dynamic vary imaging functionality of the Murchison Widefield Array (MWA). With excessive dynamic-range spectropolarimetric meter wavelength photo voltaic photos supplied by the MWA, faint GS emissions from a CME are detected out to ∼8.3 photo voltaic radii, the biggest heliocentric distance reported up to now. For the primary time within the literature, round polarization (Stokes V) detection has collectively been used with whole depth (Stokes I) spectra to constrain GS fashions. We anticipated that the inclusion of polarimetric measurement would supply tighter constraints on the GS mannequin parameters. As a substitute, we discovered that homogeneous GS fashions, which have been utilized in all prior works, are unable to mannequin each the full depth and round polarized emission concurrently. This strongly suggests the necessity for utilizing inhomogeneous GS fashions to robustly estimate the CME magnetic discipline and plasma parameters.
A quick overview of observations
The CME studied on this work had a sky-plane velocity of round 490 km/s, however was very faint in LASCO coronagraph photos. This CME was reported as a partial-halo occasion within the LASCO CDAW catalog. This CME erupted from the southwestern energetic area (AR 12047), which was noticed by the intense ultraviolet photos of the Atmospheric Imaging Meeting (AIA) onboard Photo voltaic Dynamics Observatory (SDO). This CME propagated in direction of the southwest. Radio observations have been accessible protecting frequencies between 80 to 240 MHz utilizing the MWA. Excessive dynamic vary spectropolarimetric photos supplied by the state-of-the-art calibration and imaging algorithm, P-AIRCARS (Kansabanik et al, 2022, Kansabanik et al., 2023a) enable us to detect faint GS radio emission at a number of frequencies. Apparently the radio emission is detected behind the forefront of the CME, which has been hypothesied as a result of native reconnections in disturbed preexisting streamer and the CME. Complete depth (Stokes I) emission at 80 MHz is proven by contours overlaid on LASCO C2 and C3 base distinction coronagraph photos, and the CME forefront is marked by the yellow curve. The CME studied on this work is marked by the cyan field in Determine 1. The noise achieved on this picture is near the thermal noise restrict of the remark (~1.3 occasions). This suggests that there’s little additional scope for enhancing the sensitivity by enhancing calibration or imaging. Regardless of the top quality of calibration and imaging achieved right here, Stokes V emission is detected solely at a single spectral chunk at 98 MHz over a small patch of the CME (left panel of Determine 2). Nonetheless, these observations place delicate higher limits on the Stokes V flux densities for different areas and spectral chunks. The inclusion of the Stokes V detection and stringent Stokes V higher limits together with the Stokes I spectrum enable us to review the validity of widespread assumptions within the fashions used for modeling CME GS emissions.
Determine 1: Stokes I emissions at 80 MHz are proven by the white contours overlaid on the LASCO C2 and C3 operating distinction picture of the CME. Contour ranges are at 0.5%, 1%, 2%, 4%, 6%, 8%, 20%, 40%, 60%, and 80% of the height flux density. Radio emission marked by the inexperienced field is related to CME and is the main target of this work.
Spectral Modeling Utilizing a Homogeneous Supply Mannequin
Faint GS radio emission from the CME is detected from 80 MHz to 180 MHz. Spatially resolved spectra are extracted at a number of impartial adjoining point-spread-function (PSF) sized areas, marked by the ellipses in the fitting panel of Determine 2. Peaked Stokes I spectra (high panel of Determine 3) affirm the emission mechanism as GS emission. The only GS emission mannequin assumes a homogeneous and isotropic distribution of non-thermal electrons following a single power-law distribution. Thus far, all modeling of the GS radio emissions from CMEs has been accomplished primarily based on this mannequin (e.g., Batian et al. 2001, Tun & Vourlidas 2013, Bain et al. 2014, Mondal et al. 2020, Kansabanik et al., 2023b). In our method, GS mannequin parameters are constrained utilizing a Monte-Carlo Markov-Chain (MCMC) primarily based method utilizing each the Stokes I spectra, Stokes V detections, and stringent higher limits.
Noticed and fitted Stokes I and V spectra for among the areas are proven in Determine 3. It additionally reveals GS spectra from 1,000 random realizations from the posterior distribution of GS mannequin parameters. As is obvious, the best-fit fashions are in keeping with noticed Stokes I spectra and Stokes V higher limits however inconsistent with the Stokes V detection. In a scenario like the current, the place a superb mannequin match may be discovered for much less constraining information (solely Stokes V higher limits with Stokes I spectra), however because the constraints turn into tighter (inclusion of Stokes V detection), it’s now not potential to discover a good mannequin match, strongly suggests the necessity to critically look at the potential of a number of of the assumptions made by the mannequin being violated. We examined the next key assumptions — limiting the electron power distribution to a single energy regulation, ignoring any anisotropy within the nonthermal electron pitch-angle distribution, and the belief of homogeneity within the plasma current within the quantity being modeled by the GS mannequin. To look at these prospects and try and determine the precise assumption being violated, we systematically examined these assumptions.
Determine 2: Left panel: round polarization picture at 98 MHz. The background shade map reveals the share of round polarization and the contours symbolize the Stokes I emission. Proper panel: areas of the CME the place spectra have been extracted. Purple areas are these the place spectrum becoming is finished. Spectrum modeling shouldn’t be accomplished for inexperienced areas. Spectrum becoming can be accomplished for yellow areas, which even have Stokes V detection at 98 MHz.
Conclusion
Artificial spectrum evaluation confirmed that among the many a number of assumptions made within the usually used GS mannequin, the homogeneous distribution of plasma and magnetic fields alongside the LoS has the strongest impact on the Stokes V GS spectrum. Nonetheless, with the restricted constraints accessible from the very best observations accessible in the present day, it’s tough to constrain an inhomogeneous GS mannequin. Based mostly on the outcomes from present-day devices just like the MWA, there isn’t a doubt that the much more delicate and wider bandwidth spectropolarimetric imaging from the upcoming devices, just like the Sq. Kilometre Array Observatory anticipated to turn into accessible in direction of the top of this decade, and the Subsequent Era Very Giant Array, and the Frequency Agile Photo voltaic Radiotelescope; aided by the multi-vantage level coronagraph observations will allow GS modeling for use as a routine and sturdy distant sensing method for estimating CME plasma parameters spanning a wide variety of coronal heights.
Determine 3. Noticed and fitted spectra of among the areas marked by purple ellipses in Determine 2 utilizing a homogeneous GS mannequin. The Stokes I spectra are proven within the first row. The Stokes V spectra are proven within the second row.
Additional information
Based mostly on a latest paper by Kansabanik, D., Mondal, S. and Oberoi, D. 2024 ApJ 968 55, https://doi.org/10.3847/1538-4357/ad43e9
Full checklist of authors: Devojyoti Kansabanik 1,2,3, Surajit Mondal4 , Divya Oberoi3
1 Cooperative Packages for the Development of Earth System Science, College Company for Atmospheric Analysis, Boulder, CO, USA
2 The Johns Hopkins College Utilized Physics Laboratory, 11101 Johns Hopkins Street, Laurel, MD 20723, USA
3 Nationwide Centre for Radio Astrophysics, Tata Institute of Elementary Analysis, Pune, India
4 Middle for Photo voltaic-Terrestrial Analysis, New Jersey Institute of Expertise, Newark, NJ, USA
References
Bastian, T. S., Choose, 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
Kansabanik, D., Oberoi, D., & Mondal, S. 2022c, ApJ, 932, 110
Kansabanik, D., Bera, A., Oberoi, D., & Mondal, S. 2023a, ApJS, 264, 47
Kansabanik, D., Mondal, S., & Oberoi, D. 2023b, ApJ, 950, 164
Maia, D. J. F., Gama, R., Mercier, C., et al. 2007, ApJ, 660, 874
