Instrumental oscillations in RHESSI count rates during solar flares

Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV energy range during the decay phase of the flares of 2004 November 4 and 6. Methods: The various motions of the RHESSI spacecraft which may contribute to the manifestation of oscillations are studied. The response of each detector in turn is also investigated. Results: We find that on 2004 November 6 the observed oscillations correspond to the nutation period of the RHESSI instrument. These oscillations are also of greatest amplitude for detector 5, while in the lightcurves of many other detectors the oscillations are small or undetectable. We also find that the variation in detector pointing is much larger during this flare than the counterexample of 2004 November 4. Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead to clearly observable oscillations in count rates, posing a significant hazard for data analysis. This issue is particularly problematic for detector 5 due to its design characteristics. Dynamic correction of the RHESSI counts, accounting for the livetime, data gaps, and the transmission of the bi-grid collimator of each detector, is required to overcome this issue. These corrections should be applied to all future oscillation studies.Comment: 8 pages, 10 figure

“Drifting tadpoles” in wavelet spectra of decimetric radio emission of fiber bursts

Aims. The solar decimetric radio emission of fiber bursts was investigated searching for the “drifting tadpole” structures proposed by theoretical studies. Methods. Characteristic periods with the tadpole pattern were searched for in the radio flux time series by wavelet analysis methods. Results. For the first time, we have found drifting tadpoles in the wavelet spectra of the decimetric radio emission associated with the fiber bursts observed in July 11, 2005. These tadpoles were detected at all radio frequencies in the 1602-1780 MHz frequency range. The characteristic period of the wavelet tadpole patterns was found to be 81.4 s and the frequency drift of the tadpole heads is -6.8 MHz s-1. These tadpoles are interpreted as a signature of the magnetoacoustic wave train moving along a dense flare waveguide and their frequency drift as a motion of the wave train modulating the radio emission produced by the plasma emission mechanism. Using the Aschwanden density model of the solar atmosphere, only low values of the Alfvén speed and the magnetic field strength in the loop guiding this wave train were derived which indicates a neutral current sheet as the guiding structure. The present analysis supports the model of fiber bursts based on whistler waves

Global statistics of 0.8-2.0 GHz radio bursts and fine structures observed during 1992-2000 by the Ondřejov radiospectrograph

681 solar radio events observed by the Ondřejov 0.8-2.0 GHz radiospectrograph during 1992-2000 are analyzed and corresponding bursts and fine structures classified into ten different classes. A new rare type of fine structure with rapid frequency variation we called lace pattern was included. Drifting pulsation structures, observed usually at the beginning of the impulsive flare phase, were recognized among pulsations. Furthermore, a new sub-class of zebra patterns with many zebra lines (~30) superimposed on fibers was identified. For all defined types of burst and fine structures basic characteristics of their parameters are presented. Distributions of various types of burst and fine structures in the years 1992-2000 in dependence on the changes of solar activity during the cycles 22 and 23, occurrences of studied types of burst in association with GOES class flares as well as their relationship to GOES flare maxima are shown. Finally, the association of the analyzed bursts with the metric type III bursts observed at Potsdam-Tremsdorf Observatory was studied

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

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

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

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, crosscorrelations, 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

Drifting radio bursts and fine structures in the 0.8-7.6 GHz frequency range observed in the NOAA 9077 AR (July 10-14, 2000) solar flares

The 0.8-7.6 GHz global and detailed radio spectra of the four most intense flares observed in the NOAA 9077 active region (July 10-14, 2000) are presented. The radio bursts of these flares and their sequence reveal features indicative of topological similarities among the flares under study. The drifting pulsation structures were found to be the typical signatures of these flares. Furthermore, many other fine structures such as narrowband drifting lines, drifting harmonic structure with zebra patterns, drifting branches of narrowband dm-spikes, and structures with fast positively and negatively drifting bursts are shown in the context of the whole radio flares. Some of them were observed for the first time. The relationships among them and the resulting interpretations are summarized. The characteristic periods of the drifting pulsation structures and the magnetic field in the zebra radio source are determined

Radio bursts with rapid frequency variations -Lace bursts

The Ondřejov radiospectrograph operating in the 0.8-2.0 GHz frequency range recorded in recent years (1998-2000), three (August 10, 1998; August 17, 1999; June 27, 2000) unique bursts with rapid frequency variations (lace bursts) lasting for several minutes. On August 17, 1999, the same burst was recorded simultaneously by the Brazilian Solar Spectroscope in the 1.0-2.5 GHz frequency range. The frequency variations of these bursts in four time intervals were analyzed by the Fourier method and power-law spectra with power-law indices close to -2 were found. The Fourier spectra show the presence of frequency variations in the 0.01-3.0 Hz interval which indicate fast changes of plasma parameters in the radio source. Due to the similarities in the line features of these bursts with zebra pattern lines, a model similar to that of the zebra pattern was suggested. The model radio spectra, computed using this model with a turbulent state of the solar flare atmosphere, are similar to those observed by the radiospectrographs