Coronal Transient Waves and Coronal Shock Waves

Details Coronal Transient Waves and Coronal Shock Waves Coronal Transient Waves and Coronal Shock Waves

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.446..477m&link_type=abstract

8th SOHO Workshop: Plasma Dynamics and Diagnostics in the Solar Transition Region and Corona. Proceedings of the Conference hel

Computer Science

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52

Scientific paper

Coronal transient (or EIT) waves have been discovered by the EIT instrument aboard the SOHO spacecraft as a global wave phenomenon in the low corona. Most of them are associated with solar type II radio bursts appearing predominantly in the radio frequency range 40-100 MHz. Such type II radio bursts are signatures of shock waves travelling outwards in the upper corona. The mean EIT wave velocity of 290 km/s is well above the sound speed in the corona. Therefore, these waves are considered as fast magnetosonic waves propagating nearly perpendicular to the ambient magnetic field in the low corona. On the other hand, the type II burst related shock waves have mean velocities of 970 km/s, which must be well above the local Alfven speed. Considering both phenomena, i.e., coronal transient waves and type II burst related shock waves, to be caused by the same initial energy release (flare), these waves can be used as diagnostic tools for the magnetic field in the solar corona. Thus, a magnetic field strength of about 5 G is deduced from the EIT wave speeds at 0.08 solar radii above the photosphere. Such values are well expected above nonactive regions in the low corona. In the upper corona, i.e., at 0.5 solar radii above the photosphere, typical magnetic field strengths of about 2.5 G are deduced from the measurements. This value corresponds to a typical Alfven speed of 600-1000 km/s, which is well below the type II related shock speeds as expected.

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