S. K. Sarkar

Some studies on the association of solar optical flares and microwave bursts with sudden ionospheric disturbances

Abstract The association of solar optical flares (numbering 8230) reported under the dualclassification and also of different types of microwave bursts (numbering 1670) with the sudden ionospheric disturbances (SIDs) during the peak phase of current solar cycle (cycle 20) have been examined. Important results obtained are: 1. (1) Percentage association of flares with SIDs of all types increases with the intensity of the flares irrespective of their areas; 2. (2) Out of 51 flares associated simultaneously with all the six different types of SIDs, 37 were of bright intensity (b), and only two were of large area (class 3); 3. (3) Microwave burst-SID association increases with the increase of peak flux. Further, percentage association is higher when the bursts are complex or contain ‘fluctuations’; 4. (4) Of the 47 microwave bursts associated simultaneously with all the six SIDs, 83 per cent show fluctuations; 5. (5) SFDs are found to have their peaks attained earlier than those of both the associated flares and microwave bursts. Results obtained from the present investigation lead to the conclusion that a bright flare with an associated microwave burst of either complex type or having fluctuations is a true indication of the simultaneous occurrences of all the different types of sudden ionospheric disturbances.

Spectrum of average flux of the S-component of solar radio emission

Spectrum of average flux of the S-component of solar radio emission observed during the peak phase of the present solar cycle has been determined statistically. Daily values of the mean solar flux at 606, 1415, 2695, 4995 and 8800 MHz observed at the Sagamore Hill Solar Radio Observatory have been examined. The superposed epoch method (Chree analysis) has been used for determining the true nature of the S-component at all these frequencies. Spectrum has been obtained after elimination of the basic components at the respective frequencies. The important results obtained from the present statistical investigation are: (1) the basic component increases with frequency, (2) the S-component shows a maximum at 4995 MHz (∼6 cm), (3) the spectrum is independent of the phase of the 27-day cycle and (4) S-components at all the frequencies have slopes which are both more uniform and higher in the ascending phase than those in the descending phase of the 27-day cycle. In the descending phase slopes increase with frequency.

A comparative study of Hα-flares of different visual features in relation to radio bursts and sunspots

Solar Hα-flares now reported with their distinctive visual features have been statistically examined for a period of about eight years in relation to their different characteristics, flare-burst and flare-sunspot association. Important results obtained are: (i) Integrated intensity changes from the highest to the lowest values in the order F, H, E, and D flare type, whereas, impulsiveness in the order H, F, E, and D type, (ii) Flare-burst association is frequency dependent and is highest and lowest for H and D types respectively in almost all the frequencies, (iii) Most of the flares of D, E, and F types are associated with sunspots of βp, αp, and δ configurations having field strength 1500–2500 G, while that of H type with βp and β configurations having field strength 1000–2000 G.

The role of photospheric magnetic field in the development of solar flares

A statistical investigation has been made about the flare-process in relation to the photospheric magnetic field and configuration. It is understood from the analysis that the flare energy bears a linear relationship with the rate of change of flux of the longitudinal component of photospheric magnetic field.

Wide-band average spectra of solar radio bursts

Peak flux spectra of solar radio bursts in a wide frequency band have been statistically determined for different morphological types of bursts, for various ranges of magnetic field of the burst-associated sunspots and also for the bursts occurring in the central and limb region of the solar disk. Important results obtained are: (i) The generalised spectra have two peaks, one near to meter-wave and the other in the centimeter-wave region, the former peak being more pronounced than the latter; (ii) identical spectral shape is observed for the great and impulsive types and also for GRF and PBI types of bursts; (iii) the radio emission intensity is relatively higher in the central part than that in the limb part of the solar disk for frequencies 1–10 GHz, while the reverse is true for frequencies 0.245–1 GHz and 10–35 GHz; (iv) the optical depth of the absorbing layer above the source of a burst is found to be the same for meter to centimeter-wavelength bursts, implying that the radio sources in this wide band have uniform characteristics with respect to optical thickness; (v) in case of simultaneous emission in the dekameter to X-ray band, most of the decimetric bursts are seen to be very prompt and coincident with the associated flares starting time. The interpretations of the obtained spectra give an insight into the possible generation mechanisms, pointing to the location of the source region in the solar atmosphere.

Some studies on Hα-flares and microwave bursts in relation to sunspot magnetic configurations

Sunspot associated Hα-flares and microwave bursts occurring during the period 1972 to 1974 have been examined in relation to the magnetic strength and configurations of the sunspots and sunspot groups (abbreviated as ‘spots’). Important results obtained are: (i) percentage occurrences of flares exceeds those of microwave bursts up to a magnetic field strength of 2000 G while the reverse is true for higher field strength of ‘spots’, (ii) flare productivity (average number of flares per spot) and also burst productivity are comparatively higher in the case of ‘βγ’ and ‘δ’ types of spots than in the case of other types of spots, (iii) the above productivities are predominantly high when magnetic configuration of ‘spots’ changes during their life time, and (iv) ‘impulsive’ type of microwave bursts are more associated with ‘spots’ having ‘changing type’ of magnetic configuration.

Some studies on the solar microwave bursts in relation to the slowly varying component

The occurrences of 5772 microwave bursts recorded by the Sagamore Hill and Manilla Solar Radio Observatories over the period January 1968 to July 1970, covering the maximum phase of the current solar cycle at frequencies 2695, 4995 and 8800 MHz and their energy excesses have been examined in relation to the S-component of solar radio emission. The average slowly varying component has been determined by the superposed epoch method commonly known as the Chree analysis. Similar treatment of the bursts, data, mentioned above has been made to examine any probable 27-day variation and the results obtained have been compared with that of the S-component. Further, spectra of the microwave bursts under the so-called spectral type - ‘inverted U’, particularly those having a peak at 4995 MHz, have also been examined and compared with the average spectrum of the S-component. Some of the important results obtained from the present analysis are: (1) the nature of variation of both the average number of occurrences and energy excesses of the microwave bursts follow in general the average 27-day variation of the S-component, (2) the number of occurrences and energy excesses of the microwave bursts are comparatively greater in the ascending phase of the 27-day cycle than those in the descending phase, (3) bursts at progressively higher frequencies originate at lower levels in the solar atmosphere than those of the associated S-component, and (4) the average spectrum of the microwave bursts of ‘inverted U’ spectral type having a peak at 4995 MHz is quite identical in nature to that of the S-component.