November 17, 2009
EIT Waves And Coronal Magnetic Field Diagnosis
Solar coronal seismology based on magnetic field-line stretching model of "EIT waves" is proposed, which is demonstrated to be potentially able to probe the mysterious magnetic field in the solar corona.
The study, which was led by Dr. Chen, is reported in Issue 52 of Science in China (G) because of its significant research value.Many explosive phenomena on the Sun, such as solar flares, involve the energy conversion from the magnetic energy to thermal and kinetic energies in the corona, which is the outer atmosphere of the Sun. Therefore, the coronal magnetic field is extremely crucial in the understanding of these eruptive phenomena. However, at present, only the magnetic field along the solar surface can be measured directly, whereas the magnetic field in the solar corona can hardly be measured. Despite some efforts of measuring through infrared spectral lines and of the inversion through radio emissions, the coronal magnetic field is generally approximated by extrapolating the magnetic field from the solar surface, which is however an ill-posed problem. Therefore, it would be great to have an alternative approach to diagnose the coronal magnetic field.
In 1997, the EUV Imaging Telescope (EIT in short) on board the European"“US satellite, Solar and Heliospheric Observatory (SOHO), discovered an unexpected wavelike phenomenon propagating in the solar corona, which was later named "EIT waves" after the telescope. "EIT waves" were explained successfully to be apparently propagating density enhancements compressed by the successive stretching of magnetic field lines during coronal mass ejections (CMEs), the largest-scale eruptive phenomenon on the Sun. According to this model, the "EIT waves" propagation velocity is intimately determined by the 3-dimensional distribution of the coronal magnetic field. Based on such an interesting property, Dr. Chen proposed recently that the profile of the "EIT wave" propagation velocity can be utilized to probe the coronal magnetic field.
Dr. Chen told the reporter: "You know, we can already diagnose the deep structure of the Earth by analyzing seismic waves. Similarly, we now can diagnose the magnetic field in the solar corona by analyzing EIT waves, which in some sense can be analogized as helioseismic waves." He commented that, in this sense, "EIT wave" observations open a new window for solar physicists to look into the mysterious magnetic field in the solar corona, and would help uncover the explosive nature of many explosive phenomena, including solar flares. As also commented by a reviewer, "This is an interesting paper describing the observations and modeling of EIT waves, and illustrating how they can be applied to probe the global magnetic field in the corona".
"EIT waves" were originally explained as the magnetoacoustic waves, i.e., sound waves coupled with the magnetic field. Such a model was also used to estimate the magnetic field in the low corona. However, the magnetoacoustic wave model cannot account for various characteristics of "EIT waves". To reconcile the discrepancies, Dr. Chen and his collaborators from China, USA, and Japan put forward the magnetic field-line stretching model since 2002, which has been widely recognized in the solar physics community. In this newly published paper, Dr. Chen demonstrated that it is feasible to diagnose the magnetic field in the solar corona using the observations of "EIT wave" velocity profiles.
With the application of the "EIT wave" diagnostics, the 3-dimensional distribution of the solar coronal magnetic field is expected to be revealed, which would finally help unveil the nature of solar flares and CMEs, the two major driving sources of hazardous space disturbances to human high-tech activities, including navigations, telecommunications, manned missions, etc.
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