Marcos E. Machado

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.

The observed characteristics of flare energy release. I - Magnetic structure at the energy release site

It is shown that flaring activity as seen in X-rays usually encompasses two or more interacting magnetic bipoles within an active region. Soft and hard X-ray spatiotemporal evolution is considered as well as the time dependence of the thermal energy content in different magnetic bipoles participating in the flare, the hardness and impulsivity of the hard X-ray emission, and the relationship between the X-ray behavior and the strength and observable shear of the magnetic field. It is found that the basic structure of a flare usually consists of an initiating closed bipole plus one or more adjacent closed bipoles impacted against it. 119 references.

Radiative backwarming in white-light flares

We examine empirical atmospheric structures that are consistent with enhanced white-light continuum emission in solar flares. This continuum can be produced either by hydrogen bound-free emission in an enhanced region in the upper chromosphere, or by H- emission in an enhanced region around the temperature minimum. In the former case, weak Paschen jumps in the spectrum will be present, with the spectrum being dominated by a strong Balmer continuum, while in the latter case the spectrum exhibits a weaker, flat enhancement over the entire visible spectrum.We find that when proper account is taken of radiative backwarming processes, the two enhanced atmospheric regions above are not independent, in that irradiation by Balmer continuum photons from the upper chromosphere creates sufficient heating around the temperature minimum to account for the temperature enhancements there. Thus the problem of main phase white-light flare production reduces to one of creating temperature enhancements of order 104 K in the upper chromosphere; radiative backwarming then naturally accounts for the enhancements of order 100 K around the temperature minimum.Heating by electron and proton bombardment, and by XUV irradiation from above, are then considered as candidates for creating the necessary enhancements in the upper chromosphere. We find that electron bombardment can be ruled out, whereas bombardment by protons in the few-MeV energy range is a viable candidate, but one without strong observational support. The XUV irradiation hypothesis is examined by incorporating it self-consistently into the PANDORA radiative transfer algorithm used to construct the empirical model atmospheres; we find that the introduction of XUV radiation, with flux and spectrum appropriate to white-light flare events, does indeed produce sufficient radiative heating in the upper chromosphere to balance the radiative losses associated with the required temperature enhancements.In summary, we find that the radiative coupling of (i) the upper chromosphere and temperature minimum regions (through Balmer continuum photons) and (ii) the transition region and upper chromosphere (through XUV photons) can account for white-light emission in solar flares.

Analysis of the August 7, 1972 white light flare: Its spectrum and vertical structure

Spectral data on a white light wave occurring during the explosive phase of the August 7 flare were obtained simultaneously with three telescopes at the Sacramento Peak Observatory. Spectrograms in the region λλ3530 to 5895 and sequences of filtergrams (∼ 200 Å halfwidth) at 4950 Å and 5900 Å constitute the most complete record of white light flare emission obtained to date. Analysis of the iron line spectrum and of the CN and CH molecular lines shows that the maximum depth of the emission in the flare wave is about 200 km above the photosphere of the Harvard-Smithsonian Reference Atmosphere. Analysis of the Balmer lines gives an electron density of 3 × 1013 cm−3 where the continuum emission is present. From the Balmer line analysis, it is concluded that, in agreement with Canfield (1974) and Shmeleva and Syrovatskii (1973), the flare occurs in a thin layer and that the heating and ionization of the flaring layers are due to the injection of 100 keV electrons. There is no need to postulate filamentary structure in the flaring layer in order to explain the observations. Analysis of the continuum emission in the wave indicates that it is produced by free-bound transitions of hydrogen at a temperature of ∼ 8500 K. In the impulsive phase of the flare emission arose from short-lived bluish knots which could not be studied in detail. In the following phase, the one to which the conclusions in this paper refer, the continuum emission coincided with the Hα ribbon expansion (the explosive phase). We identify it with the ‘yellowish-white’ flares reported by Trouvelot (1891) and others.

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.

Spatial and temporal evolution of soft and hard X-ray emission in a solar flare

We study the spatial and temporal characteristics of the 3.5 to 30.0 keV emission in a solar flare on April 10, 1980. The data were obtained by the Hard X-ray Imaging Spectrometer aboard the Solar Maximum Mission Satellite. It is complemented in our analysis with data from other instruments on the same spacecraft, in particular that of the Hard X-ray Burst Spectrometer.Key results of our investigation are: (a) Continuous energy release is needed to substain the increase of the emission through the rising phase of the flare, before and after the impulsive phase in hard X-rays. The energy release is characterized by the production of hot (5 × 107 ≲ T ≲ 1.5 × 108 K) thermal regions within the flare loop structures. (b) The observational parameters characterizing the impulsive burst show that it is most likely associated with non-thermal processes (particle acceleration). (c) The continuous energy release is associated with strong chromospheric evaporation, as evidenced in the spectral line behavior determined from the Bent Crystal Spectrometer data. Both processes seem to stop just before flare maximum, and the subsequent evolution is most likely governed by the radiative cooling of the flare plasma.

The Queens' flare: Its structure and development; precursors, pre-flare brightenings, and aftermaths

We continue previous research on the limb flare of 30 April, 1980, 20:20 UT, observed in X-rays by several instruments aboard the Solar Maximum Mission (SMM). It is shown quantitatively that the flare originated in an emerging magnetically confined kernel (diameter ∼ 20″) which existed for about ten to fifteen minutes, and from which energetic electrons streamed, in at least two injections, into a previously existing complicated magnetic loop system thus forming a less bright but extended and long-lived tongue. The tongue had a length of ∼ 35 000 km and lasted ∼ 90 min in X-rays (∼ 10 keV); at lower energies (∼ 0.7 keV) it was larger (∼ 80 000 km) and lasted longer. The total number of energetic electrons (≈ 1037) initially present in the kernel is of the same order as the number present in the tongue after the kernels decline. This gives evidence that the energetic electrons in the tongue originated mainly in the kernel. The electron number densities in the kernel and tongue at maximum brightness were ∼ 4.5 × 1011 and ∼ 1 × 1011 cm#X2212;3, respectively. During the first eight minutes of its existence the tongue was hotter than the kernel, but it cooled off gradually. Its decline in intensity and temperature was exponential; energy was lost by radiation and by conduction through the footpoints of the loop system. These footpoints have a cross-section of only ∼ 3 × 106 km2. This small value, as well as photographs in a Civ UV emission line, suggests a highly filamentary structure of the system; this is further supported by the finding that the tongue had a ‘filling factor’ of ∼ 10#X2212;2. Several faint X-ray brightenings (≲ 0.005 of the flares maximum intensity) were observed at various locations along the solar limb for several hours before and after the flare. At ∼ 30 min before the flares onset a faint (≲ 0.02) flare precursor occurred, coinciding in place and shape with the flare. First the kernel precursor was brightest but the tongue precursor increased continuously in brightness and was the brightest part of the precursor some 10–15 min after the first visibility of the kernel precursor, until the start of the main flare. This suggests (weak) continuous electron acceleration in the tongue during a period of at least 30 min. The main flare was caused by strong emergence of magnetic field followed by two consecutive field line reconnections and accelerations in a small loop system, causing footpoint heating. Subsequently plasma streamed (convectively) into a pre-existing system of larger loops, forming the tongue.

A solar burst with a spectral component observed only above 100 GHz during an M class flare

Context. Since the installation of submillimeter solar radio telescopes, a new spectral burst component was discovered at frequencies above 100 GHz, creating the THz burst category. In all the reported cases, the events were X-class flares and the THz component was increasing. Aims. We report for the first time an M class flare that shows a different submillimeter radio spectral component from the microwave classical burst. Two successive bursts of 2 min duration and separated by 2 min occurred in active region NOAA 10226, starting around 13:15 UT and having an M 6.8 maximum intensity in soft X-rays. Methods. Submillimeter flux density measured by the Solar Submillimeter Telescope (SST) is used, in addition to microwave total Sun patrol telescope observations. Images with Hα filters, from the Hα Solar Telescope for Argentina (HASTA), and extreme UV observations, from the Extreme-ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SoHO), are used to characterize the flaring region. An extensive analysis of the magnetic topology evolution is derived from the Michelson Doppler Imager (SoHO, MDI) magnetograms and used to constrain the solution space of the possible emission mechanisms. Results. The submillimeter component is only observed at 212 GHz. We have upper limits for the emission at 89.4 and 405 GHz, which are less than the observed flux density at 212 GHz. The analysis of the magnetic topology reveals a very compact and complex system of arches that reconnects at low heights, while from the soft X-ray observations we deduce that the flaring area is dense (n ∼ 10 12 cm −3 ). The reconnected arches are anchored in regions with magnetic field intensity differing by an order of magnitude. Accordingly, we conclude that the microwave emission comes from mildly relativistic electrons spiraling down along the reconnected loops. A very small portion of the accelerated electrons can reach the footpoint with the stronger magnetic field (2000 G) and produce synchrotron emission, which is observed at submillimeter frequencies. Conclusions. The finding of a submillimeter burst component in a medium-size flare indicates that the phenomenon is more universal than shown until now. The multiwavelength analysis reveals that neither positron synchrotron nor free-free emission could produce the submillimeter component, which is explained here by synchrotron of accelerated electrons in a rather complex and compact magnetic configuration.

The heating of the temperature minimum region in solar flares — A reassessment

As a sequel to the work by Machado et al. (1978), we discuss and evaluate the suggestions made by these authors on how to possibly reconcile the observed temperature enhancements at temperature-minimum levels in solar flares with some form of theoretical heating mechanism. After establishing the H− LTE assumption used by Machado et al., we then consider EUV irradiation, and joule heating by steady currents, as heating mechanisms. We find that, unless there are strong inhomogeneities associated with either mechanism, neither can reasonably be reconciled with observations. It is concluded that detailed, high resolution (both spatial and temporal) measurements are necessary to further our understanding of the flare process at temperature-minimum levels.

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.