Measuring the Magnetic Origins of Solar Flares, Coronal Mass Ejections, and Space Weather

Abstract

We take a broad look at the problem of identifying the magnetic solar causes of space weather. With the lackluster performance of extrapolations based upon magnetic field measurements in the photosphere, we identify a region in the near-UV (NUV) part of the spectrum as optimal for studying the development of magnetic free energy over active regions. Using data from SORCE, the Hubble Space Telescope, and SKYLAB, along with 1D computations of the NUV spectrum and numerical experiments based on the MURaM radiation–magnetohydrodynamic and HanleRT radiative transfer codes, we address multiple challenges. These challenges are best met through a combination of NUV lines of bright Mg ii, and lines of Fe ii and Fe i (mostly within the 4s–4p transition array) which form in the chromosphere up to 2 × 104 K. Both Hanle and Zeeman effects can in principle be used to derive vector magnetic fields. However, for any given spectral line the τ = 1 surfaces are generally geometrically corrugated owing to fine structure such as fibrils and spicules. By using multiple spectral lines spanning different optical depths, magnetic fields across nearly horizontal surfaces can be inferred in regions of low plasma β, from which free energies, magnetic topology, and other quantities can be derived. Based upon the recently reported successful sub-orbital space measurements of magnetic fields with the CLASP2 instrument, we argue that a modest space-borne telescope will be able to make significant advances in the attempts to predict solar eruptions. Difficulties associated with blended lines are shown to be minor in an Appendix.

Optimal Spectral Lines for Measuring Chromospheric Magnetic Fields

“…Further, lines of Fe + exist with diverse opacities similar to and less than those of Ca + , but at wavelengths relatively close to Mg + . While Judge et al (2021) argued for the combination of Mg + and Fe + lines at wavelengths between 2560 and 2810 Å as a critical wavelength range to perform chromospheric polarimetry, here we confirm and extend this result to show that it is also optimal, no matter the wavelength region selected for observations.…”

The characteristics of flare- and CME-productive solar active regions

“…New properties now incorporate more sophisticated information from image processing, photospheric flow fields, spectroscopy and data-driven modelling, the last two of which are, at the moment, our only way to probe higher atmospheric layers. On the one hand, spectra and observables from transition region and the corona provide information more relevant to the atmospheric layers where the initiation of flares and CMEs takes place, with promising future implications [Judge et al, 2021]. On the other hand, data-driven modelling, although it cannot fully compensate for our lack of coronal magnetic field measurements, is our only way to probe the threedimensional vector magnetic field in the solar atmosphere and assess the competing roles of the core and ambient magnetic field in CME initiation [Amari et al, 2018].…”

Improved AI-generated Solar Farside Magnetograms by STEREO and SDO Data Sets and Their Release

“…Magnetic fields play a fundamental role in producing solar extreme events, i.e., solar flares and coronal mass ejections (Wiegelmann et al 2014;Judge et al 2021). A series of ground-based and space-borne magnetographs have provided solar magnetic field data to study the field's origin and evolution over the last few decades (Pietarila et al 2013).…”