Solar X-ray Events Research Articles

Characteristics of flight delays during solar flares

Solar flares are one of the severest solar activities that have important effects on near-Earth space. Previous studies have shown that flight arrival delays increase as a result of solar flares, but the intrinsic mechanism behind this relationship is still unknown. In this study, we conducted a comprehensive analysis of flight departure delays during 57 solar X-ray events by using a huge amount of flight data (~ 5 × 106 records) gathered over a 5-year period. It is found that the average flight departure delay time during solar X-ray events increased by 20.68% (7.67 min) compared to quiet periods. Our analysis also revealed apparent time and latitude dependencies, with flight delays being more serious on the dayside than on the nightside and longer (shorter) delays tending to occur in lower (higher) latitude airports during solar X-ray events. Furthermore, our results suggest that the intensity of solar flares (soft X-ray flux) and the Solar Zenith Angle directly modulate flight departure delay time and delay rate. These results indicate that communication interferences caused by solar flares directly affect flight departure delays. This work expands our conventional understanding of the impacts of solar flares on human society and provides new insights for preventing or coping with flight delays.

Global ionospheric flare detection system (GIFDS)

The Global Ionospheric Flare Detection System (GIFDS) is currently under development at the German Aerospace Center as a ground based detector for continuous monitoring of the solar flare activity in order to provide real time warnings on solar X-ray events. GIFDS is using Very Low Frequency (VLF) radio transmissions in the northern hemisphere which respond to enhanced ionization in the bottomside ionosphere caused by X-ray flares. Since solar flares can only be detected during daytime, VLF receivers have to be installed around the globe to guarantee continuous records at the dayside sector. GIFDS consists of a network of Perseus SDR (Software Defined Radio) receivers equipped with a MiniWhip antenna each. Reliable detection of solar flares is ensured by recording multiple frequency channels ranging from 0 to 500kHz. The applicability of the system is demonstrated in a first analysis by comparing VLF measurements with GOES's (Geostationary Operational Environmental Satellite) X-ray flux data. The high potential of GIFDS for a permanent monitoring of solar flares in near real time is discussed.

Many-fold coincidence pileup in silicon detectors: Solar X-ray response of charged particle detector systems for space

A technique for computing counting rates of silicon detectors in case of multiple absorption events has been developed. It enables quantitative analysis of the response of solid-state detectors flown in space to solar X-ray events of different intensities and temperatures. The technique is applied to computing the responses of generic types of satellite detectors to solar X-ray emission spectra in the 5 to 40 MK temperature range. Comparison with the data available for the Energetic Particle Experiment (EPE) flown on the IMP 8 satellite confirms that many-fold coincidence of photon absorption pulses can account for the response of solid-state detectors to solar X-rays. With an example of a two-threshold detector, it is shown that silicon detectors can be used to measure both the temperature and the emission measure of solar X-ray flares.

The frequency of long-duration solar X-ray events

It is shown that the long-duration X-ray events tend to ignore the sunspot cycle. This is particularly true for events with durations of 6 hr or more.

Expressions to determine temperatures and emission measures for solar X-ray events from GOES measurements

We have developed expressions which give the effective color temperatures and corresponding emission measures for solar X-ray events observed with instruments onboard any of the GOES satellites. Since 1976, these satellites have been used to monitor continuously the full-Sun X-ray emission in two broadband wavelength intervals (approximately 0.5–4 Å and 1–8 Å) with a time resolution of 3 s. To simulate the solar X-ray input at a variety of plasma temperatures, we used theoretical spectra provided by D. L. McKenzie. These spectra were folded through the wavelength dependent transfer functions for the two GOES detectors as given by Donnelly et al. (1977). The resulting detector responses and their ratio as a function of plasma temperature were then fit with simple analytic curves. Over the entire range between 5 and 30 million degrees, these fits reproduce the calculated color temperatures within 2% and the calculated emission measures within 5%. With the theoretical spectra provided by McKenzie, we can determine similar expressions for any pair of broadband X-ray detectors whose sensitivities are limited to wavelengths between 0.2 and 100 Å.

The morphological and statistical properties of solar X-ray events with long decay times

Photographic data from the S-054 X-ray telescope on Skylab has been used to study the morphological and statistical properties of a class of X-ray events with long decay times. The events were selected for analysis based on their Solrad 1-8 A flux signatures. These events are associated statistically with long-lived active regions, white-light coronal transients, and H alpha prominence activations. These X-ray events usually consist of an arcade of loops with a bright linear structure running along the arcade. The heights of the loops, their expansion velocities, their lifetimes, and morphological appearances all suggest that at least some of these events are the X-ray analog of the loop prominence systems observed in H alpha by Bruzek.

Polarization results of solar X-rays from OSO-7

Polarization measurements of solar X-ray events that were obtained with an instrument on OSO-7 are presented. The results appear to be consistent with the results of Tindo et al. on the existence and magnitudes of polarization. A comparison with polarization predictions when X-rays are produced by radial beams of electrons gives two examples of deviations from such a model.

Electron loss processes in the lower D-region during the decay of solar X-ray flare events

Data concerning the recovery of the lower D-region during the decay of solar X-ray events, are examined. A simple discussion of such results in terms of a two positive-ion model, has shown possible ranges for some significant parameters of the D-region, in agreement with previous experimental determinations.

On anisotropy of solar hard X-ray emission

A number of solar X-ray events above 10 keV and 20 keV were compiled in order to test for evidence of anisotropic emission. The results are not definite, although the two samples show apparently different behaviours.

D-region recombination coefficients and the short wavelength X-ray flux during a solar flare

VLF phase and amplitude measurements were made on five different frequencies at São Paulo, Brazil during a solar flare which occurred on 22nd January 1972. The phase and amplitude measurements during the decay phase of the flare were combined with the full wave solutions of Wait and Spies (1964) to calculate the recombination coefficient in the lower ionosphere. The values thus obtained are lower than those reported by Reid (1970), but are compatible with those reported by Montbriand et al. (1972) during Solar X-ray events. The effective loss rates have been utilized to calculate the ion-production at the maximum of the flare, which in turn has been utilized to calculate the incident X-ray flux as a function of wavelength at the maximum of the flare. Extensions to the calculations are discussed.

The Hard Solar X-Ray Spectrum Observed from the Third Orbiting Solar Observatory

The hard solar X-ray scintillation-counter telescope on the OSO-Ill satellite covers the energy range 7.7-210 keV with 15-sec time resolution, and six logarithmically spaced energy channels. Approximately 55 per cent time coverage has been obtained for the period following the date of launching, March 8, 1967, until failure of the on-board tape recorder June 28, 1968 This paper is based mainly on solar X-ray events observed during the first two weeks of data accumulation. Approximately ten bursts per day were de- tected during the interval March 9-March 23, 1967, above the threshold sensitivity of 10-8 erg (cm2 sec)-1 for 7.7 «= liv «= 12.5 keY. About once per day a burst of peak energy flux greater than 1.6 X 1O~ erg (cm2 sec)' was observed. Although many variations were observed, the typical event had an e-folding rise time of 86 sec and a decay time of 458 sec. The bursts occurred in correlation with almost all listed flares and subflares (88 per cent), microwave bursts (92 per cent), and SID's (100 per cent). Numerous bursts were also detected without these accompanying phenomena. The correlation with type III radio bursts, although still positive, is not as good (31 per cent), a fact which suggests that coronal disturbances are not an inevitable consequence of the process which produces X-ray emission. The X-ray spectrum is appreciably non-thermal in the initial phase of the burst and thermal in the decay phase, with an effective temperature often exceeding 50 X 106 °K. The average peak temperature of subfiares exceeds 10 X 106° K, while that of importance 1 or greater exceeds 14 X 1060 K. The emission measure flefliV has a constant value of about 1.4 X 1O~ cm3 both during an X-ray burst and from burst to burst. The solar X-ray bursts therefore differ mainly in the maximum temperature attained. A less detailed examination of later data shows that the phenomena we describe are not peculiar characteristics of a single active re- gion