4 Results

1.

Almost all type II bursts - 19 from 21 or 90% - are associated with EIT waves. The remaining events either do not have an EIT observation (24 July 1997) or the event starts far behind the limb (7 October 1997);

2.

Some EIT waves seem to start earlier than the type II bursts. However, they ever start in the time window around associated type III bursts (see Fig. 2). The cadence of EIT images and uncertainties in timing are too large for a definite conclusion;

Figure 11: Top-event on 28 May 1997, bottom event on 17 September 1997

Figure 12: Event on 24 September 1997

3.

As shown in Fig. 3, the type II velocity and the EIT wave speed are uncorrelated in our sample. On average, the type II speed is about three times larger than the EIT wave speed (see Fig. 4);

4.

The "best pronounced'' EIT waves are observed if the type II bursts are accompanied by strong type IV emission (e.g. the cases 7 April, 12 May and 6 November 1997).

Figure 13: Top-event on 25 September 1999, bottom event on 9 October 1997

Figure 14: Event on 28 September 1997

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 $R_\odot$ 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 ([1997]) the temperature of coronal plasma is 1.6$\$10⁶K in active regions but 1.25$\$10⁶K in quiet regions. We assume 1.4$\$10⁶K as a representative temperature value. Thus, the sound speed is $c_{\rm s}$=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.

Figure 15: Event I on 3 November 1997

The wave speed V of such waves is $V=(v^2_{\rm A} + c^2_{\rm s})^{1/2}$ (Priest [1982]) with $v_{\rm A}$ the Alfvén speed. Then, $v_{\rm A} = 203$ km s^-1 follows as a typical value in EIT line emission regions. Outside active regions, a barometric density law (Koutchmy [1994]) and a particle number density of 8.78$\$10⁸ cm^-3 (Newkirk [1961]) can be assumed at the base of the corona. Therefore we find at 0.08 $R_\odot$ from $v_{\rm A} = 203$ km s^-1 a particle number density of 4.22$\$10⁸ 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 [1982]). 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 $R_\odot$, 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.

Figure 16: Event II on 3 November 1997

Figure 17: Event on 6 November 1997

Figure 18: Event on 27 November 1997