EIT coronal transient waves are excited in active regions. The observations demonstrate that they can propagate over the whole disc. Clearly a certain height range contributes to the visibility of the wave phenomenon. In the sense of a working hypothesis we assume an effective propagation height level of 0.08 above the photosphere for the EIT waves. Outside active regions the background magnetic field is radially aligned. Therefore, the EIT waves propagate dominantly transversal to the magnetic field. According to Landi & Landini () the temperature of coronal plasma is 1.6106K in active regions but 1.25106K in quiet regions. We assume 1.4106K as a representative temperature value. Thus, the sound speed is =179kms-1. The mean EIT wave speed of 271kms-1 is significantly above the sound speed. From both facts together, we conclude that EIT waves can be regarded as fast magnetosonic ones.
The wave speed V of such waves is (Priest ) with the Alfvén speed. Then, km s-1 follows as a typical value in EIT line emission regions. Outside active regions, a barometric density law (Koutchmy ) and a particle number density of 8.78108 cm-3 (Newkirk ) can be assumed at the base of the corona. Therefore we find at 0.08 from km s-1 a particle number density of 4.22108 cm-3 and a magnetic field strength of 1.9 G. These values correspond with a magnetic field strength of 2.2 G in the photosphere due to magnetic flux conservation. Such low magnetic field values are actually expected in the photosphere outside of active regions (Priest ). The solar type II radio bursts occur predominantly in the frequency range 40-100 MHz. According to a barometric
density law the 100 and 40 MHz plasma levels are located at a height of 0.35 and 0.63 , respectively. The magnetic flux conservation provides field strengths of 1.4 and 0.8 G leading to an Alfvén speed of 255 and 365 kms-1 in the corresponding height levels. Because solar type II bursts are excited by shock waves the driver speed has to exceed the local Alfvén speed. Just this is in agreement with our observations: the solar type II bursts point to a mean speed of 739 kms-1 well above both values.
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