P. Hoyng

High time resolution analysis of solar hard X-ray flares observed on board the ESRO TD-1A satellite

AbstractThe Utrecht solar hard X-ray spectrometer S-100 on board the ESRO TD-1A satellite covers the energy range above 25 keV with 12 logarithmically spaced channels. Continuous sun-pointing is combined with high time resolution: 1.2 s for the four low energy channels (25–90 keV) and 4.8 s for the others. It is emphasized that the instrument design and calibration yield data virtually free of pile-up and other instrumental defects.A complete set of observations is presented for all well-observed flares during the period March 12, 1972 to October 1, 1973, including four from the highly active period August 1–8, 1972. Photon spectra are computed every 1.2 s for each event by deconvolution through the instrument response, rather than by fitting techniques. Using these actual photon spectra, the index γ for the best fitting single power law and the minimum (thick target) injection rate of electrons above 25 keV, F25, are calculated.Results for γ and F25 at 1.2 s intervals are presented for each event. Examination of all these results tentatively suggests a real distinction between events of a purely impulsive nature and prolonged events.Techniques of time series analysis are applied to the burst time profiles. Specifically:(1)The fluctuations present in the series are shown to be compatible with Poisson noise in the count rate.(2)It is emphasized that, without spatial resolution, the X-ray source must be characterized by the e-folding time scale τ of the total count rate; examination of individual τs through the event shows very few statistically real τs as short as 1.2 s, confirming (1).(3)For all events, the series are Fourier analysed; no small events showed statistically significant periodicities, but the large event of August 4, 1972 exhibited real periods of 30, 60 and 120 s in both the flux and the spectral index.(4)Statistically real, small timing differences (∼0.2 s) are shown to exist between spike peaks at different photon energies. A search is made for correlations between instantaneous values of inferred parameters (e.g. F25, γ and the time scales). Most results are negative, but in the August 4 and 7, 1972 events a very well defined path was followed through the (F25, γ)-plane, giving insight into the electron acceleration process.Finally some general conclusions are drawn concerning the implications of our analysis for the physics of particle acceleration, including the possibility of two classes of event. Specifically, the severe problems posed by the large electron fluxes (equivalent current ∼1017 A) demanded by the data are discussed in relation to flare theories. Some possibilities for getting around these problems, such as by reacceleration in a confinement region, are briefly considered.

The Hard X-ray Imaging Spectrometer (HXIS)

The HXIS, a joint instrument of the Space Research Laboratory at Utrecht, The Netherlands, and the Department of Space Research of the University of Birmingham, U.K., images the Sun in hard X-rays: Six energy bands in energy range 3.5–30 keV, spatial resolution 8″ over Ø 2′40″ and 32″ over Ø 6′24″ field of view, and time resolution of 0.5–7 s depending on the mode of operation. By means of a ‘flare flag’ it alerts all the other SMM instruments when a flare sets in and informs them about the location of the X-ray emission. The experiment should yield information about the position, extension and spectrum of the hard X-ray bursts in flares, their relation to the magnetic field structure and to the quasi-thermal soft X-rays, and about the characteristics and development of ‘type IV’ electron clouds above flare regions.

X-ray imaging of three flares during the impulsive phase

AbstractThe impulsive phases of three flares that occurred on April 10, May 21, and November 5, 1980 are discussed. Observations were obtained with the Hard X-ray Imaging Spectrometer (HXIS) and other instruments aboard SMM, and have been supplemented with Hα data and magnetograms. The flares show hard X-ray brightenings (16–30 keV) at widely separated locations that spatially coincide with bright Hα patches. The bulk of the soft X-ray emission (3.5–5.5 keV) originates from in between the hard X-ray brightenings. The latter are located at different sides of the neutral line and start to brighten simultaneously to within the time resolution of HXIS. Concluded is that:(1)The bright hard X-ray patches coincide with the footpoints of loops.(2)The hard X-ray emission from the footpoints is most likely thick target emission from fast electrons moving downward into the dense chromosphere.(3)The density of the loops along which the beam electrons propagate to the footpoints is restricted to a narrow range (109 < n < 2 × 1010 cm-3), determined by the instability threshold of the return current and the condition that the mean free path of the fast electrons should be larger than the length of the loop.(4)For the November 5 flare it seems likely that the acceleration source is located at the merging point of two loops near one of the footpoints. It is found that the total flare energy is always larger than the total energy residing in the beam electrons. However, it is also estimated that at the time of the peak of the impulsive hard X-ray emission a large fraction (at least 20%) of the dissipated flare power has to go into electron acceleration. The explanation of such a high acceleration efficiency remains a major theoretical problem.

Observations of a post-flare radio burst in X-rays

More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 109 cm−3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 106 K. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H∥ = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15–30 min after its onset.

Hard X-ray imaging of two flares in active region 2372

We discuss hard X-ray images of two flares observed by the Hard X-Ray Imaging Spectrometer (HXIS) aboard SMM on 1980 April 7 and 10. A comparison with H..cap alpha.. images and the photospheric magnetic field maps shows that the emission originated in (arcades of) loops which differ greatly in the hardness of the X-ray spectra. On April 7 the hardest X-ray emission coincided with the brightest H..cap alpha.. patch. On April 10 the most intense X-ray emission appeared to be concentrated in a looplike structure with a softer spectrum at the top and a harder spectrum in the legs. Temperature estimates from flux ratios in different energy bands tend to confirm that small, hot components are embedded in more extensive, cooler flaring regions. Temperatures in excess of 8 x 10/sup 7/ K have been found in the impulsive phase, but alternately a power-law spectrum with ..gamma..roughly-equal5.4 might fit the observed counts better.

Diagnostics of solar flare hard X-ray sources

The dynamics of hard X-ray producing electron beams in solar flares can be strongly affected by the occurrence of a reverse current. The parameter diagram for a beam can be divided into three regimes, one of which is the usual thick target case, the two others being due to two different possible consequences of the reverse current. The use of this parameter diagram as a possible diagnostic tool for solar flare hard X-ray sources is discussed, together with the necessary observations and their interpretation.The forthcoming Solar Maximum Mission, complemented with concurrent ground-based efforts provide the next possibility to obtain these observations, given a good coordination of observing programs. We stress the importance of microwave (GHz) ratio observations with good temporal (≲few sec) and spatial resolution (≲1″) in one dimension, and of reliable spectroscopic methods to determine the density in solar flare hard X-ray sources.

The limb flare of 1980 April 30 as seen by the hard X-ray imaging spectrometer

X-ray imaging of the limb event of 1980 April 30 shows that the flaring involved two distinct components: a pointlike component, which was the source of the initial hard X-ray burst and an extensive tongue reaching some 30,000 km above the limb. The tongue had a higher temperature than the other parts of the structure and seemed to be enhanced by energetic electrons that derived their energy from the initial source.

The structure and evolution of a solar flare as observed in 3.5–30 keV X-rays

On July 5, 1980 the Hard X-Ray Imaging Spectrometer on board the Solar Maximum Mission observed a complex flare event starting at 22 : 32 UT from AR 2559 (Hale 16955), then at N 28 W 29, which developed finally into a 2-ribbon flare. In this paper we compare the X-ray images with Hα photographs taken at the Big Bear Solar Observatory and identify the site of the most energetic flare phenomena. During the early phases of the event the hard X-rays (>16 keV) came from a compact source located near one of the two bright Hα kernels; we believe the latter are at the footpoints of a compact magnetic loop. The kernel identified with the X-ray source is immediately adjacent to one of the principal sunspots and in fact appears to ‘rotate’ around the sunspot over 90° in the early phase of the flare. Two intense X-ray bursts occur at the site of the rotating kernel, and following each burst the loop fills with hot, X-ray emitting plasma. If the first burst is interpreted as bremsstrahlung from a beam of electrons impinging on a collisionally dominated medium, the energy in such electrons, >16 keV, is ∼ 5 × 1030 erg. The altitude of the looptop is 7–10 × 103 km. The temperature structure of the flare is extremely non-homogeneous, and the highest temperatures are found in the top of the loop.A few minutes after the hard X-ray bursts the configuration of the region changes; some of the flare energy is transferred along a system of larger loops that now become the defining structure for a 2-ribbon flare, which is how the flare develops as seen in Hα. In the late, cooling phase of the flare 15 min after maximum, we find a significant component of the plasma at temperatures between 25 and 30 × 106 K.

On the determination of the photospheric velocity distribution from profiles of weak Fraunhofer lines

We derive the conditions under which the profile of a weak Fraunhofer line can be described as the convolution of the separate profiles of damping, thermal and non-thermal motions at the average depth of formation of the line. The average velocity distribution along the line of sight, rather than its customary chosen macro- and micro-turbulent components, is then found from the deconvolution of the observed profile with the known other contributions. Reversely, the observed profiles can be compared to predicted profiles on the basis of De Jagers (1974) theoretical turbulence broadening curves.These two methods are tested on four suitable lines. The results indicate a non-gaussian velocity distribution with a rms velocity of about 3 km s−1 and show that the photospheric turbulence is not incompatible with Kolmogoroffs law.

Radiation from a source in a cold magnetoactive plasma, re‐examined. Application to cyclotron and multipole radiation

An analysis is presented of the radiation emitted by an arbitrary source embedded in a cold, magnetoactive plasma and physically distinct from the latter. The plasma is supposed to be infinite and homogeneous; its dielectric properties are described by a dielectric tensor e. Expressions for the radiation fields are derived using the technique of Fourier decomposition. An expression for the vector potential is constructed and elaborated as far as possible for an arbitrary current source. The approach differs from that in previous work on technical points, the main one being the sequence in which the various integrations are carried out. The radiation flux is defined on the basis of Poynting’s vector S; a distinction is made between current sources behaving as a given function of time and randomly fluctuating sources. In the latter case an ensemble average is preferred over a time average. A comparison is made with existing treatments in the literature, and a variety of defects is pointed out. The general r...