Coronal Polarimetry: Determining the Magnetic Origins of Coronal Mass Ejections

,


Synopsis:
The mechanism for the release of stored magnetic energy in solar eruptions remains a major unsolved problem of Heliophysics. Choosing between triggers requires knowledge of pre-eruptive magnetic fields ( B ) . Although linear polarization in visible/infrared (VIR) coronal emission lines reveals intriguing clues about coronal mass ejection (CME) precursor topology, small telescope apertures limit current capability for measuring vector field. Current coronal observations are insufficient to diagnose 3D coronal magnetic fields in CME precursors.
Coronal cavities are the ideal candidates for CME precursor studies. B LOS in cavities is a direct measure of stored magnetic free energy, and the presence and location of topological X-points (reconnection locations) and O-points (circulation of B about axis) distinguish predictions of the flux rope-torus-instability and sheared-arcade-breakout CME models.
• Finding: Coronal cavities are ubiquitous throughout the solar cycle, erupt as CMEs, and have coronal polarimetric signatures that distinguish between model predictions. • Recommendation: Coronal cavities should be targeted in a comprehensive multiwavelength study of CME precursors & associated eruption-driving mechanisms.
Large ground-based VIR telescopes can measure both coronal magnetic field topology and strength through a combination of the saturated Hanle and B LOS -sensitive Zeeman effects.
• Finding: The 4m Daniel K. Inouye Solar Telescope (DKIST) and proposed 1.5m Coronal Solar Magnetism Observatory Large Coronagraph (COSMO-LC) make a major leap forward in VIR coronal sensitivity, enabling measurement of vector B. • Recommendation: A dedicated coronal synoptic telescope with a large field-of-view (i.e., COSMO-LC ) is needed to identify the dominant CME driving mechanisms, by measuring coronal magnetic fields from precursor state into eruption.
In the UV, a new opportunity has arisen to make use of the strong H I Lyman-α coronal line, which obtains a measurement of B that is independent of the Zeeman effect.
• Finding: Small-telescope spectropolarimetric capability in the unsaturated Hanle regime provides an independent and complementary coronal magnetic diagnostic to large ground-based telescope measurements. • Recommendation: The proposed 12-cm Coronal Lyman-α Resonance Observatory (CLARO) spectropolarimetric coronagraph demonstrates a path forward for space-based observations of the coronal magnetic field that should be incorporated into future missions away from the Sun-Earth line (e.g., COMPLETE ). Current density magnitude vs time, illustrating the reconnection-driven breakout eruption scenario (Lynch et al. 2016;high-res ). (Bottom) Ideal torus instability triggering eruption in a magnetic flux rope (Fan & Gibson 2007;high-res. ) Coronal mass ejections (CMEs) are solar eruptions associated with potentially devastating space weather. CMEs are thought to be driven by the free energy stored in twisted or sheared magnetic fields, but the mechanism underlying the release of this energy remains controversial. The key issue of solving the controversy and determining the CME mechanism has boiled down to identifying the coronal magnetic configuration prior to the eruption (Patsourakos et al. 2020). One scenario depends on magnetic reconnection above a multipolar magnetic structure (breakout reconnection, e.g., Antiochos et al. 1999;Fig. 1, top). Here, a sheared magnetic field pushes up against a critical topological point (oppositely directed magnetic fields, i.e., an X-point ), forming a current sheet. Reconnection at this sheet removes the overlying field, enabling the energized, sheared field to erupt outward after forming a flux rope. Another type of model involves a pre-existing magnetic flux rope (where magnetic field circulates about an axis, i.e., an O-point ) which undergoes the ideal torus instability as the flux rope axis rises past a critical point in the gradient of the overlying magnetic field (Kliem & Török 2006;Fan & Gibson 2007;Fig. 1, bottom). Choosing between models is essential for space weather prediction but requires knowledge of CME precursor topology, i.e., the existence and location of X-and O-points 1 .
Determining the coronal magnetic field from solar surface measurements is difficult for several reasons. If simplifying assumptions such as the current-free or potential limit to the magnetic field in the corona are made, a unique solution can be determined that yields a good first-order characterization of the global coronal magnetic field but ignores the energy-carrying currents that drive the CME. Vector magnetic information at the photospheric boundary can be used to solve for a non-linear force-free field (NLFFF), but since the photospheric field is not force-free (i.e., not magnetically dominated), these models are prone to inconsistencies and non-uniqueness (de Rosa et al. 2009). The photospheric boundary alone is insufficient to constrain the coronal magnetic field, motivating polarimetric observations in the corona.
A range of multiwavelength (radio, visible/infrared (VIR), ultraviolet (UV)) measurements yield direct information on the coronal magnetic field through sensitivities to a variety of physical mechanisms ; see Table 1). However, the only daily synoptic coronal magnetic diagnostics obtained to date have been measurements of linear polarization by the Coronal Multichannel Polarimeter (CoMP) (Tomczyk et al. 2008) and its replacement, the Upgraded CoMP (UCOMP). Since the VIR lines accessible to the ground are from forbidden transitions that lie in the saturated regime of the Hanle effect (Casini & Judge 1999), these measurements determine magnetic field plane-of-sky (POS) direction but not the magnetic strength. Even so, measurements of linear polarization consistently demonstrate a "lagomorphic" (rabbit-head-shaped) structure as predicted for forward-modeled magnetic flux ropes (  Multipolar magnetic structures have their own distinctive signatures in FeXIII linear polarization, as the van Vleck effect creates lobes converging at the X-point Fig. 3b-c). A sheared arcade-quadrupolar simulation leading to magnetic breakout (Dahlin et al. 2021) predicts a structure of this nature, in which the magnetic X-point is shifted asymmetrically in response to different degrees of shear in the magnetic lobes (Fig. 3d). Such structures are observed by CoMP in association with coronal pseudostreamers (e.g., Gibson et al. 2017;Fig. 3a,e), although the precise location of the X-point can be difficult to ascertain (Fig.  3e). Linear polarization in VIR coronal emission lines can reveal X-points, but current observations are limited by telescope aperture in resolving these features.
Recently and for the first time, the POS component of coronal magnetic fields was mapped using VIR coronal seismology (Yang et al. 2020a(Yang et al. , 2020b. Analytical (Plowman 2014, Dima & Schad 2020) and forward-model (Dalmasse et al. 2019; Paraschiv & Judge 2022) capabilities for inverting full vector magnetic fields are currently maturing but depend on measuring the circular polarization , sensitive to B LOS . However, circularly polarized light is extremely faint in the VIR corona, making its detection with small-to-medium aperture telescopes extremely rare (Lin et al. 2000(Lin et al. , 2004. In radio, the Expanded Owens Valley Solar Array (EOVSA) is enabling unprecedented measurements of coronal magnetism during solar flares (Chen et al. 2020; see white paper by Chen et al. 2022 for discussion of future directions in radio). At UV wavelengths, measurements may be made in the unsaturated Hanle regime of permitted transitions and utilized to diagnose magnetic field direction and strength (Bommier & Sahal-Brechot 1982;Fineschi et al. 1991;1993;Casini et al. 2017;Trujillo Bueno et al. 2017), but to date these have only been obtained in the upper chromosphere and transition region (Woodgate et al. 1980;West et al. 2006;Ishikawa et al. 2021;Kano et al. 2017). The only UV coronal spectropolarimetric observation was a very special case in which the solar and heliospheric observatory (SOHO) satellite rotated, serendipitously turning the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument into an effective (if somewhat inefficient) spectropolarimeter (Raouafi et al. 2002). Coronal observations to date have not been sufficient to diagnose the 3D coronal magnetic field in CME precursors.

The Solution, Part One: Coronal Cavities as CME precursors
Coronal prominences (a.k.a. filaments when observed on the solar disk) often erupt in CMEs, but these mass-loaded structures trace only a small portion of the 3D fields that encompass them. Filament channels are known to concentrate free magnetic energy along underlying neutral lines and represent the fundamental magnetic structure of CME precursors (Mackay et al. 2010). At the solar limb, filament channels oriented along the line of sight extend up into the corona and manifest as dark cavities in emission. B LOS in coronal cavities is a measure of stored magnetic free energy (see, e.g., Corchado-Albelo et al. 2021).
Coronal cavities have been associated with eruptions involving both bipolar and quadrupolar configurations (Fig. 4;Yurchyshyn 2002;Vršnak et al. 2004;Gibson et al. 2006;Maričić et al. 2009;Gibson 2015;Karna et al. 2021). They occur frequently throughout the solar cycle ( Fig. 5) even with an observational bias toward cavities aligned with the observer's line of sight; this useful selection effect favors ~axisymmetric geometries, facilitating identification of X-points (Fig. 3) and O-points (Fig. 6). Ruminska et al. (2022) analyzed >1000 coronal cavities and found >80% of 570 unique cavities had CoMP lagomorphic signatures during their lifetime. Coronal cavities are ideal candidates for the study of pre-eruption magnetic structures.   Finding: Coronal cavities are ubiquitous throughout the solar cycle, erupt as CMEs, and have coronal polarimetric signatures that distinguish between model predictions. Recommendation: Coronal cavities should be targeted in a comprehensive multiwavelength study of CME precursors and associated eruption-driving mechanisms.

The Solution, Part Two: Finding Xs and Os in Coronal Cavities
In line-of-sight-aligned coronal cavities, the identification of magnetic O-points is a straightforward matter of looking for circular contours in B LOS . Simulations where a pre-eruption cavity is identified with a magnetic flux rope (Fig. 6, top) demonstrate that a ~10-25 Gauss field aligned with the cavity axis (B LOS ) is sufficient to drive a torus-instability CME with acceleration and final speed within the observed range (Bein et al. 2011;Fan 2018;Fan & Liu 2019). In contrast, sheared-arcade simulations with a magnetic breakout topology (quadrupole with X point above it; Fig. 6, bottom) store magnetic energy well below the cavity center with very little B LOS higher in the corona. Thus, although both models in Fig. 6 have one or more magnetic O-points, the existence and location of the O-and X-points relative to the cavity center distinguish between flux rope and sheared arcade models, and thus CME drivers. (Bottom) Dahlin et al. (2021) sheared arcade/quadrupolar breakout topology simulation. Here O-points form at the beginning of eruption (shortly after time step shown) and are associated with the lower-lying field concentrations of the filament channel and not the center of the density-depleted cavities. An X-point lies above the two magnetic lobes, as expected in a quadrupolar magnetic topology. High-res ( left , right ).
B LOS in coronal cavities has never been observed. In VIR, a ground-based telescope with significantly bigger aperture than the 20-cm CoMP is needed to measure the faint circular polarization (V/I) signal and allow direct inversion of B LOS . The 4m Daniel K. Inouye Solar Telescope (DKIST) provides an exciting new opportunity to measure the magnetic structure of small cavities for the first time, but is unlikely to capture an eruption because of its small field of view. The proposed 1.5m Coronal Solar Magnetism Observatory Large Coronagraph (COSMO-LC) has a global field of view dedicated to synoptic coronal observations and so is ideally suited to studies of CMEs and their precursors (see white paper by Tomczyk et al., 2022 ). Fig. 7 shows forward-modeled circular polarization with and without photon noise added for the COSMO telescope as in Fan et al. 2018 (note a weaker-field (10 G) flux rope was used in that study vs. Fan & Liu (2018;24 G)). Even for these relatively low intensity/magnetic field strength structures, a 1-min integration time resolves the magnitude of the axial field ( Fig. 7 b), and with a 5-min integration time (more than sufficient to capture evolution on time scales of cavity eruptions, see Fig. 4, left), the B LOS maximum at 1.1 Rs (O-point) is resolved. The proposed 1.5m COSMO-LC has the sensitivity, spatial and temporal resolution to measure magnetic field evolution on a ~1-min time scale, and to detect and follow the presence and height of a magnetic O-point during the slow rise phase of a magnetic flux rope erupting as a CME. Recommendation: A dedicated coronal synoptic telescope with a large field-of-view (i.e., COSMO-LC ) is needed to identify the dominant CME driving mechanisms, by observing coronal magnetic fields from precursor state into eruption.
In the UV, space-borne instruments can take advantage of the fact that the H I Lyman-α (Lyα) coronal line is the strongest UV emission from the Sun, extending far out from the solar surface. The sensitivity of Lyα to the unsaturated Hanle effect yields a measurement of B LOS that is independent of the Zeeman effect measured by VIR circular polarization. Instead, it is based on the rotation of the linear polarization direction (Azimuth) from the solar limb tangent (Raouafi et al. 2016;Zhao et al. 2019). The 12-cm Coronal Lyman-α Resonance Observatory (CLARO ; see white paper by Casini et al. 2022) utilizes an internally occulted Lyα coronagraph to be deployed to the International Space Station (ISS). Because of the brightness of the Lyα corona and its strong linear polarization by resonance scattering, despite its small aperture, CLARO is sufficiently sensitive to measure B LOS in coronal cavities and distinguish between the flux rope and sheared arcade models (Fig. 8). As the first solar coronal mission dedicated to the observation of linearly polarized light by resonance scattering in the UV, CLARO would demonstrate the diagnostic power of Lyα coronal spectropolarimetry. With its relatively small size, such an instrument could be deployed to vantage points off the Sun-Earth line (see COMPLETE white papers by Caspi et al. 2022 ) and potentially even over the Sun's poles, where B LOS would provide a unique view on the important geoeffective quantity B Z . CLARO's coronal Lyα spectropolarimetry provides a coronal magnetic diagnostic that complements those from large ground-based VIR telescopes, small enough to be deployed throughout the heliosphere, building a 4π view of the coronal magnetic field.

Fig. 8. Forward modeling demonstrates that CLARO can distinguish between models of CME precursors.
Forward-modeled Lyα linear polarization Azimuth (vector direction; scales with B LOS ) for sheared-arcade (left; high-res ) and flux-rope (right; high-res ) simulations as shown in Fig. 6, with photon noise added and determined for a 12 cm telescope, 1.5 hour integration, 3.5" pixels, and 0.0074 flux throughput. Contours of simulation density in the plane of sky are overplotted to identify spatial location of cavity relative to Lyα Azimuth signal.

Finding:
The proposed 12-cm Coronal Lyman-α Resonance Observatory (CLARO) spectropolarimetric coronagraph demonstrates a path forward for space-based observations of the coronal magnetic field, which may be incorporated into future missions away from the Sun-Earth line (e.g., COMPLETE ). Recommendation: In order to provide an independent and complementary coronal magnetic diagnostic to large ground-based telescopes measurements, small-telescope spectropolarimetric capability in the unsaturated Hanle regime should be explored.