Long period variations of dm-radio and X-ray fluxes in three X-class flares

¹ Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic e-mail: hana@asu.cas.cz
² Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovak Republic

Received: 2 June 2006
Accepted: 24 August 2006

Abstract

Aims.Long period (≥60 s) variations of the radio (0.8-4.5 GHz) and X-ray fluxes observed during the July 14, 2000, April 12, 2001, and April 15, 2001 flares by the Ondřejov radiospectrograph and Yohkoh spacecraft are studied by statistical methods.

Methods.In the flares under study, characteristic periods are searched for by the Fourier and wavelet methods. To understand the origin of the 0.8-4.5 GHz drifting burst with long period variations, observed at the beginning of the April 15, 2001 flare, cross-correlations, time shifts, coherence, and phase differences in its time series are computed.

Results.The global statistical study of these flares revealed characteristic periods in the interval of 60-513 s in the radio (0.8-4.5 GHz) and 60-330 s in the X-ray Yohkoh fluxes. Cross-correlations between the radio fluxes at different frequencies helped us to determine the bursts generated by plasma or gyro-synchrotron mechanisms. In the April 12, 2001 flare, soft X-ray fluxes of the sources located at the loop-top and footpoints of a flare loop vary with the period of 60-320 s, and they are highly correlated. But their relation to the radio (1.1 GHz - plasma emission and 4.0 GHz - gyro-synchrotron emission) is complex. At the beginning of the April 15, 2001 flare, in the 0.8-4.5 GHz range, a broadband drifting radio burst with the time variation of 61-320 s was observed at times of flare loop ejection. Its detailed statistical analysis shows that this burst consists of two parts, and, that first part is generated by the plasma emission mechanism and the second, probably, by the gyro-synchrotron one. The characteristic period of about 300 s found in three X-class flares in their dm-radio and X-ray emissions is discussed.