We analyze several microwave bursts digitally observed at 36.8 GHz (8.2 mm wavelength) at the Metsähovi Radio Research Station of the University of Technology Helsinki/Finland (cf. Table 1 (click here)). The receiver is connected with a Cassegrain telescope of 14 m diameter resulting in a half-power beam width of 2.4 arcmin at this frequency. The quiet-Sun level at this frequency is 7800 K (Urpo et al. 1992). Typical time profiles are shown in Figs. 1 (click here)-3 (click here). Each data set has a duration of about one hour with a time resolution of 0.5 s.
Figure 1: Radio flux time profile of a burst at 36.8 GHz observed
on March 23, 1991 at the Metsähovi Radio Research Station of the
Helsinki University of Technology. A section of the event starting at
12:12 UT is shown
Figure 2: Radio flux time profile of a burst at 36.8 GHz observed
on March 24, 1991, 10:00 UT
Figure 3: Radio flux time profile of a burst at 36.8 GHz observed
on March 23, 1991, 12:58 UT. The flux time profiles of the
pre- and post-burst phases of the events presented in
Figs. 1 (click here)
and 2 (click here) look very similar to this one
Figure 4: Realization of fractional Brownian motion with a spectral index 2.25.
This signal resembles well the observed radio flux shown
in Fig. 3 (click here) and zoomed radio fluxes of the pre- and
post-burst phases of the events shown in
Figs. 1 (click here) and 2 (click here)
Date | Start | Duration | Scaling exponent | Spectral index (SF) | Spectral index (MRA) |
UT | (min) | ![]() |
![]() | ![]() | |
March 23, 1991 (Fig. 1 (click here)) | 11:58 | 58.5 | 0.57 ![]() |
2.13 ![]() | 2.21 ![]() |
Pre burst phase | 11:58 | 31.5 | 0.66 ![]() | 2.32 ![]() | 2.15 ![]() |
Main phase | 12:29 | 6.0 | 2.24 ![]() | ||
Post burst phase | 12:35 | 21.0 | 0.45 ![]() | 1.90 ![]() | 1.91 ![]() |
March 23, 1991 (Fig. 3 (click here)) | 12:58 | 47.5 | 0.66 ![]() | 2.31 ![]() | 2.29 ![]() |
March 23, 1991 | 14:00 | 61.1 | 0.57 ![]() | 2.13 ![]() | 2.21 ![]() |
March 24, 1991 (Fig. 2 (click here)) | 10:00 | 63.1 | 0.58 ![]() | 2.15 ![]() | 1.99 ![]() |
Pre burst phase | 10:00 | 5.1 | 0.44 ![]() | 1.87 ![]() | 1.96 ![]() |
Main phase | 10:05 | 14.0 | 2.37 ![]() | ||
Post burst phase | 10:19 | 44.0 | 0.41 ![]() | 1.82 ![]() | 1.90 ![]() |
March 24, 1991 | 11:09 | 58.3 | 0.49 ![]() | 1.92 ![]() | 2.04 ![]() |
Pre burst phase | 11:09 | 7.0 | 0.39 ![]() | 1.77 ![]() | 1.77 ![]() |
Main phase | 11:16 | 2.5 | 2.71 ![]() | ||
Post burst phase | 11:18 | 48.8 | 0.25 ![]() | 1.49 ![]() | 1.55 ![]() |
March 24, 1991 | 13:41 | 91.0 | 0.52 ![]() | 2.04 ![]() | 2.14 ![]() |
Pre burst phase | 13:41 | 28.0 | 0.49 ![]() | 1.98 ![]() | 1.79 ![]() |
Main phase | 14:09 | 20.0 | 2.15 ![]() | ||
Post burst phase | 14:29 | 43.0 | 0.47 ![]() | 1.93 ![]() | 2.14 ![]() |
August 2, 93: Quiet Sun | 9:58 | 17.8 | 0.22 ![]() | 1.44 ![]() | 1.37 ![]() |
August 2, 93: Sky | 10:42 | 21.8 | 0.14 ![]() | 1.27 ![]() | 1.03 ![]() |
The analyzed bursts originated in AR 6555 (Solar Geophysical Data). The observations are chosen because they show variations in a rather broad range of time scales. In particular the events of Figs. 1 (click here) and 2 (click here) exhibit short-period radio spikes which are known to appear frequently at dm-waves but are rare at solar bursts in the short cm- and mm-wave ranges (Slottje 1978; Benz 1986; Benz et al. 1992). The event shown in Fig. 3 (click here) appears to possess a different temporal dynamics than the other two bursts. Moreover, the temporal structure of the pre-burst flux profile in Fig. 1 (click here) appears to differ from that of the post-burst profile. This raises the question whether the structure of those diverse flux profiles can be quantitatively described in a uniform manner.
For later comparison, we add here in Fig. 4 (click here) the time profile of a synthetic realization of a fractional Brownian motion (fBm) process with a spectral index similar to that of the radio flux profile shown in Fig. 3 (click here).