Forward-modeling of Polarization Signals: Gaining A New Perspective on the Solar Corona

by Rachel Hauser

Solar, climate, weather and similar models are designed to replicate natural phenomena. To check model accuracy, it is typical to compare real-world observations with modeled. When it comes to considering solar dynamics, modeling by its nature is more nuanced and complex than Earth-bound models not least because of our relatively remote link to the Sun – distance alone reduces immediacy of understanding. Assisting with this effort are polarimeters, instruments that measure the Sun’s magnetic activity. Polarimeters estimate magnetic field by measuring several components of visible radiation; the brightness of the Sun’s polarized light is proportional to the strength of the magnetic field along the line of sight. Developed in NCAR’s High Altitude Observatory (HAO), the coronal multichannel polarimeter (CoMP) provides, for the first time, ongoing views of magnetism in regions of the solar atmosphere where coronal energy is stored. With CoMP,  solar physicists at HAO and around the world are pushing understanding of coronal dynamics forward.

Forward modeling a coronal prominence cavity (Gibson et al., 2010). (a) STEREO EUVI-A image of the cavity with (b) intensity contours overlaid. (c) Forward modeled (line-of-sight integrated) EUV emission using model density and temperature with (d) intensity contours overlaid. Contours are in units of “Data Number”(DN) per second.

CoMP provides a full view on the solar corona and its magnetic fields, tracking magnetic activity around the edge of the Sun. With sensitive, low-noise infrared sensors, CoMP can measure the intensity, velocity, and polarization of the solar corona’s magnetic field, with readings available every 15 minutes, if needed. Today, CoMP data are providing new insights on solar events such as coronal mass ejections (CMEs), among other stellar dynamics. For instance, CoMP’s detailed observations can be compared to “forward models” that incorporate theoretical descriptions of the evolution of magnetic fields and other properties – such as variations in plasma density and temperature – to reproduce coronal structures and dynamics.

HAO’s Sarah Gibson has led a concerted forward-modeling effort during the past two years. With support from the International Space Science Institute (ISSI), an international team of researchers is comparing CoMP and other data to models in order to better understand and predict coronal prominence cavity morphology and dynamics. Understanding prominence cavities will feed CME understanding that, in turn, influence space weather and the storms that can disrupt satellite communications and air transportation (particularly at high latitudes), as well as impact human space exploration. Prominence cavities are regions where magnetic energy is stored, the more energy stored, the stronger the CME and more serious the resulting weather and storm effects on regions of space surrounding the Earth. Coronal prominences consist of dense, relatively cool mass that collects along coronal magnetic energy lines. Such a prominence is often surrounded by a dark, circular-cross-section cavity that extends as a tunnel along its length. These structures can also frequently be seen erupting within coronal mass ejections.

Modeling coronal white light (e.g., as observed by HAO’s Mk4 Coronameter) is simplest, explains Gibson, because white light generation depends on a single solar characteristic – plasma density. Other coronal observables that aid understanding of solar activity, like extreme ultraviolet (EUV) and soft X-ray (SXR), also depend on coronal temperature.  

“But the concept is the same. A 3-dimensional distribution of density and temperature from a theoretical model is used to predict what the corona would look like in EUV or SXR,” says Gibson.

Seen in the accompanying movie, forward modeling was used to predict morphology of a coronal prominence cavity. The animation shows EUV observations of a cavity from NASA’s STEREO spacecraft on the top/left, as well as the forward-modeled predicted EUV for the coronal cavity on the bottom/right. As time goes by in the movie, it’s possible to see the 3-dimensional, tunnel-like cavity rotate past.
Because its observations include information on magnetic fields as well as density and temperature profiles, CoMP data are slightly more complex than that of STEREO, however, forward modeling can still be run. Jim Dove, an astrophysicist at Colorado’s Metro State College and an HAO visitor in 2010, has worked with Gibson and other HAO scientists, including Laurel Rachmeler, Steve Tomczyk, and Phil Judge, to compare theoretical models of coronal magnetic fields and plasma density and temperature to CoMP coronal observables. In doing so, the scientists were able to establish that CoMP observations of a coronal cavity were well reproduced by a coronal magnetohydrodynamic model of twisted, slinky-like magnetic fields.

With the initial forward-modeling effort completed, the next step will be to compare CoMP observables side by side for a range of magnetic field models, including HAO models, and those from the community, says Laurel Rachmeler, a post-doctoral student working with Gibson in HAO.  

“This work relies on CoMP observations and will reveal new coronal magnetic signatures that allow us to choose between theoretical models, assessing which one provides the most accurate representation of coronal activity,” Rachemeler says. “It’s exciting work because the observations provided by CoMP are unprecedented, and can greatly help advance current models.”  

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