Increase In X-ray Flux Research Articles

X-ray and radio emissions in the early stages of solar flares

Radio and X-ray observations are presented for three flares which show significant activity for several minutes prior to the main impulsive increase in the hard X-ray flux. The activity in this ‘pre-flash’ phase is investigated using 3.5 to 461 keV X-ray data from the Solar Maximum Mission, 100 to 1000 MHz radio data from Zurich, and 169 MHz radio-heliograph data from Nancay. The major results of this study are as follows: (1) Decimetric pulsations, interpreted as plasma emission at densities of 109–1010 cm−3, and soft X-rays are observed before any Hα or hard X-ray increase. (2) Some of the metric type III radio bursts appear close in time to hard X-ray peaks but delayed between 0.5 and 1.5 s, with the shorter delays for the bursts with the higher starting frequencies. (3) The starting frequencies of these type III bursts appear to correlate with the electron temperatures derived from isothermal fits to the hard X-ray spectra. Such a correlation is expected if the particles are released at a constant altitude with an evolving electron distribution. In addition to this effect we find evidence for a downward motion of the acceleration site at the onset of the flash phase. (4) In some cases the earlier type III bursts occurred at a different location, far from the main position during the flash phase. (5) The flash phase is characterized by higher hard X-ray temperatures, more rapid increase in X-ray flux, and higher starting frequency of the coincident type III bursts.

Slow X-ray bursts and flares with filament disruption

The data from OGO-5 and OSO-7 X-ray experiments have been compared with optical data from six chromospheric flares with filament disruption associated with slow thermal X-ray bursts. Filament activation accompanied by a slight X-ray enhancement precedes the first evidence of Hα flare by a few minutes. Rapid increase of the soft X-ray flux accompanies the phase of fastest expansion of the filament. Plateau or slow decay phases in the X-ray flux are associated with slowing and termination of filament expansion. The soft X-ray flux increases as F∼(A + Bh) h, where h is the height of the disrupted prominence at any given time and A and B are constants. We suggest that the soft X-ray emission originates from a growing shell of roughly constant thickness of high-temperature plasma due to the compression of the coronal gas by the expanding prominence.

X-ray Observations of Cyg X-3 Near the Time of Radio Outbursts

FOLLOWING the announcement of Gregory et al.1 and Hjellming and Balick2 that the radio source identified with Cyg X-3 had undergone a substantial increase in radio flux output some time between August 31, 1972, and September 2, 1972, we made a search through all data available for dates near this event from the Vela 5B spacecraft to determine if any associated increase in X-ray flux could be detected. Scans of the appropriate region of the sky were made on the four dates shown in Table 1. Cyg X-3 lies close enough on the sky to Cyg X-1 that the two sources were barely resolvable with the angular response of the X-ray detectors on the spacecraft; however, estimates of the strength of Cyg X-3 relative to Cyg X-1 were possible. In all cases, the flux observed from Cyg X-3 in the 3 to 12 keV energy range was consistent with the ratio of 1:3 (flux from Cyg X-3/flux from Cyg X-1) reported in previous observations of these sources (for example, Gursky et al.3).

Electron temperature and emission measure variations during solar X-ray flares

X-ray emission from seventeen X-ray flares was analyzed to obtain electron temperatures and emission measures associated with the source region in the solar corona. The source region was assumed to be isothermal with a Maxwellian electron velocity distribution. Flares which were characterized by a rapid initial X-ray flux increase were found to also have a rapid initial rise in electron temperature and emission measure. Flares which were characterized by a gradual initial X-ray energy flux increase were found to have a less rapid initial rise in electron temperature and emission measure. In all X-ray flares studied the peak temperature chronologically preceded the peak X-ray flux and the peak flux never came after the peak emission measure.

Six Minute Periodicity during X-ray Flares

THE satellite OSO-IV, launched on October 18, 1967, carried a group of detectors to observe the soft X-ray emission from the Sun1, Fig. 1 shows sample time profiles of the emission during flares as observed with a detector sensitive in the wavelength band 0.1 to 0.3 nm. In each case the time scale is the same but the intensity scale has been adjusted to show the full range of values during each event. During the visible class 1N flare represented in Fig. 1a, the X-radiation reached two peaks, the maxima being 6 min apart. Further evidence of such a time period is shown in Fig. 1b where the initial activity is considerably smaller than the second peak of the flare. Flares of this type have been noted by Teske2 and by Culhane and Phillips3; the initial event has been called the precursor to the main event. It is suggested here that the precursor is really a part of the main flare activity, but that unlike the increase in Fig 1a the initial development is relatively small compared with the second increase. Indeed, further evidence that this is the case seems to be provided by Fig. 1c. In this case there is a minor increase in X-ray flux about 6 min after the main increase. If the relative intensities of the two increases had been different, the first peak might have been called the precursor.