Propagation of Moreton Waves

Abstract

With the Flare-Monitoring Telescope (FMT) and Solar Magnetic Activity Research Telescope (SMART) at Hida observatory of Kyoto University, 13 events of Moreton waves were captured at H$\alpha$ center, H$\alpha$$\pm$0.5 Å, and H$\alpha$$\pm$0.8 Å wavebands since 1997. With such samples, we have studied the statistical properties of the propagation of Moreton waves. Moreton waves were all restricted in sectorial zones with a mean value of 92$^\circ$. However, their accompanying EIT waves, observed simultaneously with SOHO/EIT at extreme-ultraviolet wavelength, were very isotropic with a quite extended scope of 193$^\circ$. The average propagation speeds of the Moreton waves and the corresponding EIT waves were 664 km s$^{-1}$ and 205 km s$^{-1}$, respectively. Moreton waves propagated either under large-scale close magnetic flux loops, or firstly in the sectorial region where two sets of magnetic loops separated from each other and diverged, and then stopped before the open magnetic flux region. The location swept by Moreton waves had a relatively weak magnetic field as compared to the magnetic fields at their sidewalls. The ratio of the magnetic flux density between the sidewall and the path falls in the range of 1.4 to 3.7 at a height of 0.01 solar radii. Additionally, we roughly estimated the distribution of the fast magnetosonic speed between the propagating path and sidewalls in an event on 1997 November 3, and found a relatively low-fast magnetosonic speed in the path. We also found that the propagating direction of Moreton waves coincided with the direction of filament eruption in a few well-observed events. This favors an interpretation of the “Piston” model, although further studies are necessary for any definitive conclusion.

Moreton and EUV Waves Associated with an X1.0 Flare and CME Ejection

“…Thus, the "field-line valley" to the north of AR 12017 having low Alfvén speed implies a region of high compression ratio, consequently a shock wave traveling through it can produce a strong compression over the chromosphere and eventually a Moreton signature. This is in agreement with the results found by Zhang et al (2011), who analyzed the magnetic field configuration present in 13 Moreton events and concluded that the waves mostly propagate either in regions of large-scale closed magnetic loops or along valleys delimited by two sets of separated magnetic loops. Furthermore, given that close to the solar surface we expect a lower fast magneto-acoustic speed, the wavefronts will tend to curve downward, favoring compression over the chromosphere (Zhang et al, 2011).…”

“…A region of open magnetic field lines can be seen in correspondence with sectors 35 -43, which sets favorable conditions for the propagation of a coronal MHD shock able to generate Moreton disturbances (Zhang et al, 2011). Furthermore, the shock speed is fundamentally determined by the coronal magnetic field value, but the compression ratio decreases with increasing magnetic field (Krause et al, 2015).…”

Large-scale Globally Propagating Coronal Waves

“…In these cases, the associated flares tend to be strong (M and X-class), but shock events were even found for C-class flares (Warmuth, 2010;Zhang et al, 2011). These flares are comparatively impulsive.…”

“…A more recent study (considering the interval from 1997 March to 2001 August) gives rates of 3 and 4 waves per year for the Kanzelhöhe and Big Bear solar observatories, respectively (Warmuth et al, 2004a). Warmuth (2010) found 27 waves from 1997 to 2006 (from several observatories), which gives a rate of 4 waves per year, while Zhang et al (2011) reported 13 Moreton waves from Hida Observatory in the same time range, yielding a yearly rate of 7 waves. This means that Moreton waves occur at a rate of only ≈5% the rate of than coronal EUV waves, which implies that for the formation of a Moreton wave, more stringent conditions have to be met as compared to coronal EUV waves.…”

“…Actual observed shapes are thus dependent on both the 3D structure of the feature and the viewing angle (see Section 4.1.3). The propagation of Moreton waves is almost always confined to a certain angular extent, on average ≈ 90° (Smith and Harvey, 1971;Warmuth et al, 2004a;Zhang et al, 2011, see right of Figure 17 for an example). Only three events have been reported where Moreton fronts were observed to propagate in a full (or broken) circle like the "textbook" EUV waves (Pick et al, 2005;Liu et al, 2006;Muhr et al, 2010;Balasubramaniam et al, 2010).…”