Understanding the Coronal Magnetic Structure of CMEs

If we could understand the magnetic structure that leaves the Sun in a coronal mass ejection (CME), how it propagates through and interacts with the solar wind, and how it impacts the Earth’s magnetosphere and couples with the upper atmosphere - we could predict many of the space weather effects of the CME.  Tackling such a broad problem requires examining the chain of phenomena from Sun to Earth and evaluating both our theoretical understanding of relevant physical processes, and the sufficiency of current observations to constraint and guide that understanding. In this manner we can identify the weakest links and prioritize our research.

What is the internal magnetic structure of the CME as it interacts with the Earth’s magnetosphere?  This is the central question of WG3, the “Bz Challenge”. It is largely unanswered, in part because of the “weak link” of a lack of capability in quantifying energized coronal magnetic fields.

Our strategy is to strive for improved coronal polarimetric observations, in particular in the form of the proposed Coronal Solar Magnetism Observatory (COSMO). In parallel we must also develop analysis tools so that when we obtain COSMO observations we are able to use them to quantify coronal magnetic fields.

For the past three summers, SOARS student Marcel Corchado-Albelo from the University of Puerto Rico, Mayaguez, has worked with HAO scientists Sarah Gibson, Kevin Dalmasse, Yuhong Fan, and Anny Malanushenko to develop a method of identifying “hot spots” of energized coronal magnetic fields. In a paper submitted to the Astrophysical Journal (Corchado-Albelo et al., submitted, 2019), he has defined a quantitative diagnostic of non-potentiality that could be calculated from a comparison of coronal polarimetric observations (such as those to be obtained by COSMO) and the corresponding polarization signal synthesized from a potential field extrapolated from photospheric magnetograms.  Using a simulation of a magnetic flux rope he has shown that this diagnostic correlates with free magnetic energy, i.e., the magnetic energy in excess over the ground-state (current-free) potential field that can be released in the CME (Figure).

Time evolution (time steps 30, 54, and 89, top to bottom) of the maps of non-potentiality diagnostic for the flux rope simulation of Fan (2017). Left column shows the difference between synthesized linear polarization fraction for the simulation vs. potential field. Middle column shows the same for circular polarization. Right column shows column density of non-potential magnetic (free) energy.

This work is significant because by using coronal and photospheric polarimetric observations we will be able to identify regions of non-potentiality. These then can guide and constrain models of the coronal magnetic field, providing improved measures of the fields at the heart of the CME. In addition, it provides a method of identifying highly-energized coronal magnetic fields, and thus a new tool for predicting CMEs before they erupt!