This collaborative research team will study plasma physics processes occuring in the solar corona, and the investigators are motivated by the fact that the corona provides physical regimes which are inaccessible to laboratory experiments. The researchers will concentrate on making remote sensing measurements of the corona and inner solar wind using the Very Large Array (VLA) radiotelescope, as well as its upgrades (collectively called the Expanded Very Large Array, or EVLA), and interpreting those observations in terms of coronal plasma physics. The team's primary observational technique will be the measurement of Faraday Rotation of background radio sources that are occulted by the solar corona.
The team will make Faraday Rotation measurements using the EVLA that will be deeper in the corona than previously obtained, at altitudes of 2 to 5 solar radii. This approach will yield information on the properties of the coronal magnetic field and magnetohydrodynamic (MHD) turbulence in this inner region. The team will then use MHD models of the corona to create synthetic Faraday Rotation measures for the same line-of-sight paths measured with the VLA and EVLA, in order to compare results and test of the accuracy of their models against actual observations.
The lead Principal Investigator (PI) in Iowa will mentor University of Iowa undergraduate and graduate students participating in this project, and the Co-PI in Boston will offer research experiences for these students at the Harvard-Smithsonian Center for Astrophysics. The lead PI will also engage in public outreach through the Eastern Iowa Observatory and Learning Center (EIOLC) and collaborations with amateur astronomy groups, particularly the Cedar Amateur Astronomers of Cedar Rapids and the Astronomy Club of the Quad Cities, both in Iowa.
Summary of project The goal of this research project was to evaluate a new technique for testing how well theoretical models of our Sun predict the density and magnetic field in the extended solar atmosphere. Our Sun is a surrounded by a million degree atmosphere called the solar corona. Models of the density of the corona can be tested by examining photographs of the corona obtained on Earth during solar eclipses or from space with a satellite, but there are few direct measurements of the magnetic field within the corona. One way to detect the coronal magnetic field is to monitor the polarization of radio emission from galaxies as they move behind the Sun. In a process called Faraday rotation, the angle of polarized emission is rotated in the presence of a dense plasma with a magnetic field. By measuring this rotation, we can place a constraint on the density and magnetic field in the corona along the line from Earth to the galaxy. Previously, several galaxies have been monitored by the Very Large Array (VLA) over a period of several weeks as they first approached and then moved away from the Sun. During these periods Faraday rotation of the emissions from the galaxies was observed, and the magnitude of the variation was consistent with simple models of conditions in the corona. In this project we compared the existing observations with magnetohydrodynamic simulations of the global solar corona. These simulations are much more sophisticated than earlier simpler models, and make use of direct measurements of the magnetic field on the surface of the Sun as an input. By comparing our Faraday rotation observations with the numerical simulations, we were able to accomplish two things: estimate how reliable the coronal Faraday rotation observations are, and determine if the Faraday rotation observations are useful for testing the ability of the numerical simulations to predict the coronal fields. Two intervals, one in May 1997 and the other in March 2005 were selected for study, allowing us to compare periods with high and low levels of solar activity. Intellectual merit of the project This project successfully advanced our knowledge of the magnetic field in the solar corona by showing for the first time that numerical simulations of the corona driven by observations of the photospheric magnetic field produce predictions of the coronal field that are generally consistent with Faraday rotation observations out to about eight solar radii. The project furthermore demonstrated that the observations with the largest disagreement between the observations and the simulations occur near the boundaries between streams of different types of coronal material and solar wind, suggesting that these interfaces must be modeled in more detail in order to successfully describe the coronal field. We recommend that Faraday rotation observations be used in the future for validation and testing of large scale coronal models. Our main findings were as follows: (1) even in solar minimum, when the surface of the Sun and the corona seem very steady, one can only get good agreement between the simulation and the observations if the most recent photospheric magnetic field measurements were used; (2) previously published variability in the level of Faraday rotation over the course of a four hour observation is generally consistent with the variation produced as the model corona rotates past the observer; (3) Most of the time the predicted level of Faraday rotation agreed with the observations, except when the line of sight to the galaxy passed close to an interface between different types of solar wind. Broader impact of this project Knowing the magnetic field in the corona is critical to understanding how the Sun heats the corona to extreme temperatures, and for forecasting space weather, including solar flares and eruptions that can impact communications and navigation at Earth. A particular challenge for effective forecasting of space weather is our limited ability to predict the magnitude and direction of the magnetic field in the solar wind near Earth. If we can improve our knowledge of the coronal magnetic field, then we can also improve our ability to forecast conditions in interplanetary space. This project demonstrated that Faraday rotation observations can be used to produce a unique new measurement of the coronal magnetic field.
