**5. Instrumentation and modulations detection capabilities**

We analyze the VLF (< 0.01 Hz) modulations of solar microwave bursts recorded in Metsähovi Radio Observatory (Finland) with the 14-m and 1.8 m radio telescope antenna at 37 GHz and 11.7 GHz, respectively. The key selection criterion for the analyzed microwave data was their synchronism with the oscillating loops observed in extreme ultraviolet (EUV) by TRACE (Aschwanden et al., 2002). The width of the antenna beam pattern of the Metsähovi radio telescope at 37 GHz is 2.4', the sensitivity of the receiver is about 0.1 sfu (10−<sup>23</sup> W m−<sup>2</sup> Hz−1), and time resolution, 0.05 <sup>÷</sup> 0.1 s. Therefore, at 37 GHz the spatial resolution of the radio telescope is sufficient for identification of an active region that contains a radiating source. This enables to analyze microwave radiation emitted directly from the region, imaged in EUV (e.g., observed by TRACE), and to perform the comparison of the radiation features

c

d

**0 4 6 8 10 12 14 16** Frequency, mHz

Burst on 23.03.2000 Spectrum averaged on T=90-240

7.8 mHz 8.4 mHz

12.0 mHz

15.6 mHz

at 37 Ghz **<sup>30</sup>**

6.0 mHz

Time (UT)

Relative spectral density

Fig. 3. (a) SOHO/MDI Magnetogram of the Sun on 2000-Mar-23, white arrow points at the active region AR8910; (b) The Sun image in 304 Å on 2000-Mar-23 from SOHO/EIT, white arrow points at the active region AR8910; (c) Intensity profile and corresponding VLF modulation dynamic spectrum of the microwave radiation, recorded from the active region AR8910 on 2000-Mar-23, at 11:30-12:00; Color codes the dynamic spectral relative intensity

interval 1.7 ± 0.3 mHz of the modulation feature revealed by the analysis of the microwave

(arbitrary units), more dark features correspond to stronger (better pronounced)

modulations; (d) averaged spectral density of the VLF modulation.

**25**

a b

SOHO/EIT 304 Å,

2000-Mar-23

<sup>155</sup> Analysis of Long-Periodic Fluctuations of Solar

Microwave Radiation, as a Way for Diagnostics of Coronal Magnetic Loops Dynamics

**20**

**15**

**10**

**5**

**0**

**2**

1.7 mHz

3.4 mHz

**11:20 11:40 12:00**

Solar burst on 23.03.2000 at 37 GHz

Frequency,

**0.01**

**0.008 0.006 0.004 0.002 0 1E+4**

SOHO/MDI

Magnetogram,

2000-Mar-23

Dynamic spectrum burst on 2000-Mar-23

Intensity, a.u

radiation.

**8E+3**

**4E+3**

 Hz

and dynamics of coronal loops. At 11.7 GHz the radiation is collected from the whole solar disk and the position of the radiating source cannot be resolved. However, even in this case, by comparison with observations in other wavelengths and timing of the events, it is usually possible to identify the microwave radiation features related to the energy release and dynamic phenomena in particular active regions.

Variations of the background magnetic field in a radiating source may cause not only the amplitude modulation of microwave radio emission from solar active regions (according (1)), but also could result in a certain frequency modulation (due to the dependence of electron gyro-frequency *ν<sup>B</sup>* on the magnetic field). The estimated width of the gyro-frequency variation interval due to this effect is from several tens to several hundreds MHz. At the same time, the bandwidth of the receiver at Metsähovi is much larger than this interval and a possible frequency modulation of the microwave radio emission cannot be resolved. Thus, one can detect only the effects of varying magnetic field, manifested in the intensity modulation of the microwave signal, due to (1).
