Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections

Abstract

A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images.

Our key conclusion is that the differentiation of pre-eruptive configurations in terms of SMAs and MFRs seems artificial. Both observations and modeling can be made consistent if the pre-eruptive configuration exists in a hybrid state that is continuously evolving from an SMA to an MFR. Thus, the ‘dominant’ nature of a given configuration will largely depend on its evolutionary stage (SMA-like early-on, MFR-like near the eruption).

Appendix: A Suggested Approach Towards Deciphering the Pre-Eruption Configuration

We suggest that vector magnetic field measurements at multiple heights spanning from the photosphere to the lower corona and multi-viewpoint hot plasma observations in the corona could advance our understanding of pre-eruptive configurations and may eventually lead to predicting their eruptions. To demonstrate the potential of such observations, we create and analyze synthetic data from an MHD flux-emergence simulation, as follows.

First, we extract data-cubes of the vector magnetic field in the lower solar atmosphere from the MHD flux-emergence simulation of Syntelis et al. (2017) (SAT17, hereafter). Note that the SAT17 simulation had a different objective. It was not meant to simulate a large-scale CME but it has all the necessary characteristics to demonstrate our approach. We select representative pre-eruptive configurations from this simulation at two consecutive times (Fig. 19). The configuration at the earlier time (top panels) consists predominantly of sheared, high-arching field lines of similar orientation, resembling a ‘2.5D-like’ SMA that has a much simpler structure than the flat 3D SMAs described in Sect. 3.1. At the later time (bottom panels), the configuration contains also twisted field lines, i.e., it has transitioned into an MFR. The central and right panels show the horizontal magnetic field, $B_{h}$, at two different heights (corresponding roughly to the lower chromosphere and middle transition region). The orientation of $B_{h}$ with respect to the PIL is significantly different for the two magnetic configurations. For the SMA, $B_{h}$ points roughly along the same direction in both atmospheric layers. For the MFR, B_h is much more sheared and flips orientation across the PIL with increasing height, which suggests a transition across the center of an MFR. This suggests that mapping the orientation of B_h as a function of height in the low solar atmosphere may be a powerful approach for assessing the evolutionary stage (SMA-to-MFR transition) of pre-eruptive configurations. The height of this transition will depend on the physical parameters of a given region on the Sun. For this reason, multiple height coverage extending to coronal heights will be needed to properly capture the evolution of the system across the range of ARs and phases of solar activity. As a side benefit, the use of constraints from multiple atmospheric layers will greatly improve the quality of NLFFF (or non-force free) extrapolations and should lead to a more accurate specification of the magnetic field in the corona.

Magnetic field observations above the photosphere should also capture field changes induced by eruptions (e.g., Fleishman et al. 2020). Such changes have been notoriously difficult to detect in photospheric magnetograms (Sudol and Harvey 2005), leading to uncertainties of the amount of magnetic flux removed by CMEs. We demonstrate this by exploiting the recurrent eruption characteristic of the SAT17 simulation in Fig. 20. We see that while the photospheric magnetic flux exhibits a smooth behavior characterized by a sharp increase followed by a plateau (black line), the magnetic flux higher up undergoes several dips, each associated with an eruption (colored lines). It is, therefore, conceivable, that we could measure the flux (and by extension, the magnetic energy) that participates in an eruption by following the magnetic flux evolution above the photosphere. Constraining the energetics of eruptions with such direct measurements would greatly advance our understanding of explosive energy release and help establish the geo-effective potential of CMEs at a very early stage of the eruption process (Vourlidas et al. 2019).

Fig. 20

Temporal evolution of magnetic flux at various heights for the Syntelis et al. (2017) simulation. The snapshots employed in Figs. 19 and 21 are taken before t=50 min. The vertical dashed lines show the kinetic energy maxima inside the numerical domain indicating the four eruptions in the simulation

Finally, we calculate synthetic images resulting from LOS-integration of the squared electric current density, $J^{2}$, (Fig. 21). These images can be considered as proxies for high-temperature plasma emissions in the EUV or SXR. We use the same simulation snapshots as in Fig. 19. The top view of the SMA (Fig. 21, top left) is reminiscent of a sigmoidal structure. This is, however, easily dismissed by inspecting the side view (top right). On the other hand, both views of the MFR snapshot display a clear sigmoidal structure. We emphasize that this is just a proof-of-concept attempt. It should be expanded with more rigorous calculations by, for instance, calculating synthetic Stokes profiles from the MHD simulations and inverting them as done with real data.

Fig. 21

LOS-integrations of ${J}^{2}$ for the two simulation snapshots of Fig. 19. The left panels correspond to top views of the SMA and MFR in Fig. 19. The right panels correspond to side views of the SMA and MFR. A reverse color-table is used

These diagnostics can be exploited once vector magnetic field observations at several layers above the photosphere become available. Photospheric and chromospheric vector magnetic field observations (albeit over a limited FOV) will be available from DKIST, beginning in late 2020. In addition, the Cryogenic Near Infra-Red Spectro-Polarimeter (Cryo-NIRSP) of DKIST will obtain off-limb observations in HeI 10830 Å and FeXII 10747 Å for estimates of magnetic field magnitudes and orientation in prominences and coronal loops. Even though these are not vector magnetograms, the polarization information along with forward modeling can be used to decipher the nature of pre-eruptive structures within coronal cavities as shown by Rachmeler et al. (2013). In late 2021, the remote sensing instruments on Solar Orbiter (Müller et al. 2020; Zouganelis et al. 2020) will start science operations, including photospheric vector magnetic field measurements by the Polarimetric and Helioseismic Imager (PHI) from non-Earth viewpoints, thus providing additional constraints on the photospheric boundary conditions for magnetic-field extrapolations and possibly allowing concurrent coronal and photospheric magnetic field estimates when combined with Cryo-NIRSP.

Multi-viewpoint observations of hot plasmas are less certain in the near future, although some research can be undertaken either with past (2007-2014) EUVI 284 Å observations from STEREO-A and -B or now with comparisons between STEREO/EUVI 284 Å (currently at L5 and approaching Earth) and SUVI 284 Å observations. In addition, the off-ecliptic viewpoints from Solar Orbiter will supply complementary high-resolution views of prominences and hot plasmas from the EUI and SPICE instruments, which could be compared with Earth-based observations (e.g., SDO/AIA and SUVI).

Further in the future, we are looking forward to exciting mission and instrument concepts. The proposed COSMO observatory (Tomczyk et al. 2016) will derive some coronal magnetic field quantities (e.g. strength, azimuth) and plasma properties further from the limb than DKIST. The SOLAR-C mission, recently approved by JAXA, will carry a next-generation upper atmospheric imaging spectrograph that will greatly enhance our diagnostics of the thermal structure and the dynamics of pre-eruptive configurations. Based on our earlier discussion, an EUV channel similar to the 131 Å channel of SDO/AIA, at an off Sun-Earth viewpoint, along with a vector magnetograph would make great addition to a scientific payload for an L5 mission (Vourlidas 2015). An observatory at that location could play a major role in understanding CME initiation because it would increase the observational coverage of the solar surface magnetic field and the identification and tracking of magnetic regions would be possible for longer times (e.g., Mackay et al. 2016). All these observations combined with a well-crafted modeling program will allow us to finally fully characterize the nature and evolution of pre-eruptive configurations towards eruption.