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 0.1	2.13 0.2	2.21 0.13
Pre burst phase	11:58	31.5	0.66 0.04	2.32 0.08	2.15 0.28
Main phase	12:29	6.0			2.24 0.13
Post burst phase	12:35	21.0	0.45 0.08	1.90 0.15	1.91 0.08
March 23, 1991 (Fig. 3 (click here))	12:58	47.5	0.66 0.06	2.31 0.12	2.29 0.14
March 23, 1991	14:00	61.1	0.57 0.04	2.13 0.08	2.21 0.14
March 24, 1991 (Fig. 2 (click here))	10:00	63.1	0.58 0.13	2.15 0.26	1.99 0.11
Pre burst phase	10:00	5.1	0.44 0.05	1.87 0.1	1.96 0.11
Main phase	10:05	14.0			2.37 0.11
Post burst phase	10:19	44.0	0.41 0.05	1.82 0.1	1.90 0.36
March 24, 1991	11:09	58.3	0.49 0.25	1.92 0.5	2.04 0.15
Pre burst phase	11:09	7.0	0.39 0.03	1.77 0.05	1.77 0.56
Main phase	11:16	2.5			2.71 0.16
Post burst phase	11:18	48.8	0.25 0.04	1.49 0.08	1.55 0.59
March 24, 1991	13:41	91.0	0.52 0.15	2.04 0.3	2.14 0.62
Pre burst phase	13:41	28.0	0.49 0.04	1.98 0.08	1.79 0.61
Main phase	14:09	20.0			2.15 0.63
Post burst phase	14:29	43.0	0.47 0.04	1.93 0.08	2.14 0.31
August 2, 93: Quiet Sun	9:58	17.8	0.22 0.02	1.44 0.04	1.37 0.59
August 2, 93: Sky	10:42	21.8	0.14 0.02	1.27 0.04	1.03 0.96

Table 1: Scaling exponents calculated by Eq. (2 (click here)) and spectral indices of the analyzed mm-wave bursts. Spectral indices

are calculated by Eq. (6 (click here)), and

are determined from the scalegrams (Figs. 11 (click here)-13 (click here), 15 (click here)-17 (click here)). We omitted the values of

and

for the main burst phases because their scatter becomes exceedingly large in such short intervals

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).

2. The microwave data