Assessment of the near-Sun axial magnetic field of the 10 March 2022 CME observed by Solar Orbiter from active region helicity budget

(Solar Orbiter Nugget #52 by S. Koya^1,2, S. Patsourakos¹, M. K Georgoulis^3,4, and A. Nindos¹)

1. Introduction

Coronal Mass Ejections (CMEs) are energetic eruptions of plasma and magnetic fields from the solar atmosphere [1,2,3]. Understanding the near-Sun magnetic field strength of CMEs is crucial as it plays a key role in shaping the early evolution of CME dynamics and their potential geoeffectiveness. However, there are no routine, continuous observations of near-Sun CME magnetic fields. In [4], a method based on the magnetic helicity of the CME was introduced, which can be indirectly used to estimate the near-Sun magnetic field of the CMEs. The overarching principle for this assessment is the conservation of magnetic helicity accumulated in an Active Region (AR) and transferred to CMEs upon launch. By following the methodology proposed by [4] with a more refined estimation of the helicity budget of the CME, we estimated the near-Sun magnetic field of a CME observed on 10 March 2022 by Solar Orbiter, STEREO, SOHO and SDO. This study provided us with an enhanced understanding of the event and its magnetic field evolution with heliocentric distance.

Figure 1. Spacecraft configuration of STEREO-B (blue circle), STEREO-A (red circle), SolO (magenta circle), Bepi Colombo (orange circle), Earth (green circle) and Sun (yellow) on 10 March 2022 at 00:00 UT in heliocentric Earth ecliptic (HEE) system. The dotted lines show the angular difference between  STEREO A and B from the Sun-Earth line. Units are in AU.
 

2. Methodology

We estimated a CME’s near-Sun axial magnetic field that erupted from AR 12962 on 10 March 2022. This event was observed from an advantageous spacecraft configuration with Solar Orbiter (SolO), located just 7.8 degrees east of the Sun-Earth line at a heliocentric distance of 0.43 AU, in good alignment with it and the L1 measurements by the WIND spacecraft at 0.99 AU (See Fig.1) . We followed the methodology of [4] to estimate the near-Sun axial magnetic field of the CME. First, we tracked the temporal evolution of the instantaneous relative magnetic helicity of the CME’s source AR, NOAA AR 12962, using a nonlinear force-free, magnetic connectivity-based (CB) method [5], that revealed a significant decrease in the helicity budget of the AR prior to the CME eruption. Next, we determined a set of CME geometrical parameters (e.g., radius R, length L) from forward-modelling geometrical fits of multi-view coronal observations, utilising data from the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment (SOHO/LASCO) and the Solar Terrestrial Relations Observatory/Sun-Earth Connection Coronal and Heliospheric Investigation (STEREO/SECCHI) coronagraphs. For this, we applied the Graduated Cylindrical Shell (GCS) model [6]. We then estimated the near-Sun axial magnetic field of the CME at 0.03 AU and its maximum height by employing the Lundquist flux rope model [7]. Assuming a power-law index variation for the magnetic field with heliocentric distance, we estimated a best-fit single power-law index by incorporating the estimated near-Sun magnetic field at 0.03 AU and magnetic-field measurements at 0.43 AU and 0.99 AU.

Figure 2. Top to bottom: a) Temporal evolution of magnetic properties of NOAA AR 12962. The blue curve  represents the instantaneous values of the relative magnetic helicity estimated from the CB method. The over-plotted orange curve and green error bars are the running averages of the estimated values over a 48-minute window, with  standard deviation, , respectively. b) Red dots represent the base difference intensity of the darkest region within the dimming. c) The blue curve represents GOES 1 – 8 Å X-ray flux 1-minute data. In all panels, vertically dashed black lines indicate the dimming start and peak time, and the orange dashed line represents the GOES flare peak time.

3. Results 

We tracked the temporal evolution of the instantaneous relative magnetic helicity of the source AR of the CME. The net helicity difference between the pre- and post-eruption phase in the AR was estimated as (−7.1 ± 1.2) × 1041 Mx2 (see Fig 2), which is assumed to be bodily transported to the CME. Assuming a Lundquist flux-rope model and geometrical parameters obtained through the GCS CME forward modelling (see Fig.3), we determined the CME axial magnetic field at a GCS-fitted height of 7.6 R⊙ (0.03 AU) as 2067 ± 405 nT.

Figure 3. GCS model fits of  the 10 March 2022 CME. Top row: Images of the CME  taken by LASCO/C2/SOHO (left) and SECCHI COR2/STEREO-A (right). Bottom row: The same images, with the GCS wireframe overlaid.

Assuming a power-law variation of the magnetic field with distance, we estimated a single power-law index of −1.23 ± 0.18 from 0.03 AU to L1 (see Fig.4). While there have been  studies in this direction, mainly focused on the inner heliospheric region from  0.3 AU to 1 AU, our study offers a distinct perspective by estimating a single power-law index of −1.23 ± 0.18, incorporating data points from near-Sun to 0.43 AU and up to 0.99 AU. Our findings indicate a less steep decline in the magnetic field strength with distance compared to previous studies. However, they align with studies including near-Sun in situ magnetic field measurements, such as from the Parker Solar Probe [8,9].

Figure 4. CME magnetic field variation with distance. Blue points with error bars depict the maximum magnetic field B0 and its uncertainties, respectively.The near- Sun field is estimated from the application of our methodology, while the fields at 0.43 AU (Solar Orbiter) and 0.99 AU (WIND) stem from  in-situ measurements. The 0.99 AU measurements have minimal error (on the order pT), and the red line represents the least-squares best fit of the CME magnetic field at the respective three locations.

4. Conclusions

In summary, this study not only estimates the near-Sun magnetic field of the CME and enhances understanding of the power-law variation of the CME magnetic field with heliocentric distance but also confirms that differences in pre- and post-eruptive helicity in source ARs can be explored to study the resulting CME. The availability of multiple viewpoints and co-aligned observations of CME events enables the development of a database to determine a maximum likelihood near-Sun CME magnetic field​ based on observations rather than models. Our methodology offers a foundation for routine calculations of magnetic helicity in the lower solar atmosphere, complementing existing geometric models of CMEs in the outer corona.

Creating a near-Sun CME axial magnetic filed​ database and our approach could contribute to a more systematic understanding of magnetic field evolution of the CME. Additionally, the maximum likelihood values of near-Sun CME axial magnetic filed​ and insights into its variation with heliocentric distance could serve as initial conditions for inner helospheric CME propagation such as the European Heliospheric FORecasting Information Asset (EUHFORIA) model [10].

For further reading, see full publication Koya et al. (2024).

Affiliations

(1) Section of Astrogeophysics, Department of Physics, University of Ioannina, 45110 Greece
(2) Institute of Physics, University of M. Curie-Skłodowska, Pl. M. Curie-Skłodowskiej 1, 20-031 Lublin, Poland
(3) Space Exploration Sector, Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, USA
(4) Research Center for Astronomy and Applied Mathematics, Academy of Athens, 11527 Athens, Greece

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Acknowledgements

This work is part of the SWATNet project funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 955620.