Photometry of Solar Flares.

Abstract

view Abstract Citations (19) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photometry of Solar Flares. Dodson, Helen W. ; Hedeman, E. Ruth ; McMath, Robert R. Abstract On the basis of photometric light-curves for 194 flares and subflares, the following data have been derived. 1.Ha intensity often varies differently in different parts of the same flare. 2.The change of Ha intensity with time in a flare can be well represented by the series of curves given by the equation iog =(iog Ii ax) t a(t-trnax), where t is measured in minutes and a is positive for the ascending branch, negative for the descending branch, and takes values between 0.40 and 0.00313. For the descending branch of the light-curve, a is always smaller than for the ascending branch. The most usual value of a for the ascending branch is 0.10; for the descending branch a is -<0.025 for 87 per cent of the cases. 3.Ha flare intensities have been measured in units of both the local and the centra] continuum at x 6590. In the former, average Ha intensity at flare maximum ranged from 0.66 for subflares and 0.88 for flares of importance 1 to intensity 1.17 and 1.28 for flares of importance 2 and 3, respectively. In units of the central continuum the average values are about 0.1 lower. The brightest measured maximum was 1.9 times the local continuum for a flare at CMD 72 . 4.No convincing relationship has been found between rate of increase of intensity and maximum intensity attained by the flare. However, the slowest rate of increase apparently diminishes the chances of attaining a maximum as bright as the contmuous spectrum. 5.There is evidence for center-to4imb variation in numerous characteristics of the Ha flares in this study. The slowest rates of rise and decline were observed only for flares ithin 55 of the central meridian, and the fastest rates of decline only for flares within 40 of the solar limb. Average intensity in units of the local continuum increased with increasing central meridian distance of the flares. In units of the central continuum, the average intensity diminished from center to limb. "Limb-darkening" equations for the flares and subflares are as follows: Flares: lc=O.63+O.34COS 0, Subilares: lc=O.36+O.26CC)S 0. For flares of the same importance, average duration diminished with increasing distance from the center of the disk. Near the center of the disk, the Ha flare usua]ly lasted as long as, or longer than, the associated ionospheric disturbance, but for flares near the limb the reverse was true. 6.For the flares in this study, area and intensity increase together, but the correlation coefficient of these two measures is only 0.58 + 0.03 p.e. 7.Flare duration was associated more closely with flare area than with maximum intensity. The duration of flares as measured by photometric light-curves is much longer than the average duration of flares reported in the Quarterly Bulletin of Solar Activity. 8.The reported starting times of SID's usually fall during the period of increasing intensity in the flare. Except in unusual cases, SID's were not reported unless the flare intensity was at least 0.8 of the local continuum and the area was of the order of 200 millionths of the solar disk. Near the limb the SID often lasts longer than the Ha flare. 9.There is apparently less diminution from center to limb in the ionizing radiation from flares than in Ha. 10. The ionospheric disturbances were unusually small for certain large and relatively bright flares associated with the onsets of 200-Mc/s noise storms. 11. Examination of ionospheric records for the times of the flares and subflares in this study has provided evidence for a new type of flare-associated ionospheric effect, a gradual ionospheric disturbance, in contradistinction to the sudden SID or Dellinger effect. The 45 reported SID's have been augmented by 19 additional, well-confirmed ionospheric disturbances and by 23 less certain events. 12. There is no evidence to indicate that subflares are not just smaller, fainter examples of the flare phenomenon. Flares and subflares in this study have similar disk distributions and rates of increase and decrease of intensity. Ionospheric disturbances accompanied at least four of the brightenings here classified as subflares. Publication: The Astrophysical Journal Supplement Series Pub Date: February 1956 DOI: 10.1086/190027 Bibcode: 1956ApJS....2..241D full text sources ADS |