Solar coronal magnetic field measured with radio emission


(Guest post by Kazumasa Iwai)

In the solar corona, there exist various dynamical phenomena such as flares and coronal mass ejections (CMEs), which are thought to be caused by the interactions between the coronal magnetic field and plasma. Hence, coronal science usually requires a precise measurement of coronal plasma parameters, such as the magnetic field, which is usually difficult. The measurement of the magnetic fields of the Sun and stars are mainly based on remote-sensing, which is different from the terrestrial magnetic field directly measured by satellites or ground-based magnetometers. A noble technique to measure magnetic fields in the solar atmosphere using the radio wave has been demonstrated in the Frontier Letter by Iwai et al. (2014) published on Earth, Planets and Space.

Radio circular polarization

In magnetized plasma, radio thermal bremsstrahlung or so-called free–free emission has a circular-polarized component, which is converted to obtain the longitudinal component of the magnetic field. Iwai et al (2014) derived the coronal magnetic field from the radio polarization observation using the solar interferometer Nobeyama Radioheliograph (NoRH). They observed a post-flare loop on the west limb on 11 April 2013, and analyzed the radio circular polarization data accurately. The observed degree of polarization in the loop top region was 2.7% at 17 GHz, from which the longitudinal component of the magnetic field was calculated to be about 84 G.

Stereoscopy of the solar atmosphere

They also compare and discuss with the other simultaneous observations of the post-flare loops from the outer space by the extreme ultraviolet (EUV) imagers onboard the two spacecrafts launched by NASA; i.e. Solar Dynamics Observatory (SDO) which was located around the Earth in a geosynchronous orbit, and one of the two Solar Terrestrial Relations Observatory (STEREO) spacecrafts which was located approximately 133.7° from the Sun-Earth line (STEREO-A) in a heliocentric orbit ahead of the Earth. These multiple line-of-sight EUV observation enabled to derive the tilt angle of the loop structure, which was about 10° to 31°. Combining the tilt angle and line-of-sight magnetic field, they derived the magnetic field strength as 85~98 G at the loop top region. They also estimated the plasma density and temperature by combining radio wave and EUV observations, and further derived an important parameter ‘plasma beta’, a ratio between the magnetic pressure and the plasma pressure, at the loop top region, which was about 5.7 × 10−4 to 7.6 × 10−4. This is the first successful demonstration to delineate the three dimensional magnetic field and plasma beta from the radio wave observation. The new method will extend the research of various coronal magnetic phenomena such as flares in the future.

Summarized from ‘Coronal magnetic field and the plasma beta determined from radio and multiple satellite observations‘ by Iwai et al. (2014) published in Earth, Planets and Space

Updated June 14, 2016.

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