A solar flare observed with the SMM and Einstein satellites

We present X-ray observations of the 21 July, 1980 flare which was observed both with the Einstein Observatory Imaging Proportional Counter (IPC) and the X-Ray Polychromator (XRP) and Gamma-Ray Spectrometer onboard the SMM satellite. The Einstein observations were obtained in scattered X-ray light, i.e., in X-rays scattered off the Earth's atmosphere. In this way it is possible to obtain spatially unresolved X-ray data of a solar flare with the same instrument that observed many X-ray flares on other stars. This paper juxtaposes the results and implications of the stellar interpretation to those obtained from the far more detailed SMM observations. The result of this calibration observation is that the basic properties of the flaring plasma can be reliably determined from the stellar data, however, the basic physics issues can only be studied through models.     

Kinematical analysis of flare spray ejecta observed in the corona

The mass ejection event on 17 January 1974 was a classsic spray associated with a flare from an over the limb region. The structure of the accompanying coronal transient was typical of well-observed mass ejections, with coronal loops and a forerunner racing ahead of the rising prominence. Observations in H, soft X-ray, white light and radio wavelengths allowed us to track both cool (T _e10⁴ K) and hot (T _e>10⁶ K) material from limb de-occultation to 6R . We determined the kinematics and thermodynamics of the internal material, and the overall mass and energy budget of the event. The majority of the mass and energy was linked with coronal material, but at least 20% of the ejected mass originated as near-surface prominence material. We conclude that the upper part of the prominence was being continuously heated to coronal temperatures as it rose through the corona. Above 2R nearly all of the material was completely ionized. The primary acceleration of the prominence occurred below 3.5 × 10⁴ km with all of the material exhibiting constant velocity above 1.5R . We found evidence that a moving type IV burst, indicative of strong magnetic fields, was associated with the upper part of the prominence. Our observations suggest that both the cool and hot material were acted upon by a similar, continuous force(s) to great heights and over a long time interval. We find that the observations are most consistent with magnetic propulsion models of coronal transients.     

Analysis of the emission line spectra of a solar flare observed from Skylab

The EUV emission spectra in the wavelength range 110–1900 Å of the 5 September 1973 flare observed with the NRL slit spectrograph on Skylab are studied. The results are: (1) The chromospheric and transition-zone lines are greatly enhanced during the flare. In particular, the allowed lines are enhanced more than the intersystem lines. The Ni ii and P ii lines show the greatest enhancement with a factor of 800 increase in intensity. Other lines such as O i, C i, Si iii, S iii, S iv, O iv, O v, and N v show increases in intensity 10–100 times during the flare. (2) The chromospheric lines, although greatly enhanced during the flare, maintain their sharp and gaussian profiles and are not appreciably broadened. The transition zone lines, on the other hand, show a red-shifted component during the initial phase of the flare. The deduced downward velocity in the transition zone is 50 km s^–1. In addition, there are large turbulent mass motions. The downward mass motion is probably caused by the pressure imbalance between the flare hot plasma at 13 × 10⁶ K and the cooler plasma at 10⁵ K. (3) The density of the 10⁵ K flare plasma, as deduced from density-sensitive lines, is greater than 10¹² cm^-3. The depth of the 10⁵ K plasma in the flare transition zone is only of the order of 0.1 km, giving a steep temperature gradient. Consideration of the energy balance between the conductive flux and the radiative energy losses shows that, indeed, the high density in the transition zone requires that its thickness be very small. This is a consequence of the maximum radiative efficiency at the temperature around 10⁵ K in the solar plasma.Ball Brothers Research Corporation.     

Multiwavelength observations of the impulsive flare of November 19, 1990

We present the observation and interpretation of a solar radio burst whose evolution of the source position at 48 GHz has been correlated with microwave spectral observations from 3.1 to 19.6 GHz and H imaging spectrograms. The event of November 19, 1990 showed 4 impulsive peaks in microwaves and 2 H kernels. There exists strong evidence that the impulsive emission has originated from nonthermal electrons including an electron beam during the rising phase of the third microwave peak. The complex evolution of the source position at 48 GHz is attributed to two inhomogeneous and spatially separated sources with changing relative brightness.     

An emission measure analysis of two sunspots observed by the UVSP instrument on the SMM spacecraft

The EUV observations from the SMM satellite of two sunspots are presented here. These observations show the sunspots (a) to be regions of lower intensity than the surrounding plage, contrary to that found by previous authors, and (b) to have line intensities which vary little over a period of several hours. An upper limit to mass flows of 2km s^-1 is derived, indicating a relatively simple energy balance for the chromosphere-corona transition zone with thermal conduction being balanced by radiative losses. Electron densities derived from Niv to Civ line ratios imply electron pressures (log N _eT_e) of 15.0 to 15.3.     

Post-flare coronal arches observed with the SMM/XRP Flat Crystal Spectrometer

The phenomenon of post-flare coronal arches, initially discovered with the Hard X-Ray Imaging Spectrometer (HXIS), was investigated using observations made with the SMM Flat Crystal Spectrometer (FCS) on 20 through 23 January, 1985. Since these observations were made with a different type of instrument from HXIS, they provide independent information on the physical characteristics of the arch phenomenon and extend our knowledge to lower coronal temperatures.Conspicuous arch activity was observed after three flares and after a disturbance which could not be identified. (1) A dynamic flare starting on 20 January at 20: 39 UT was responsible for the formation of the primary arch structure. (2) An arch revival, showing characteristics very similar to those of the arch revivals observed with HXIS, took place after the dynamic flare starting on 21 January at 23: 50 UT. The most conspicious difference relates to the moving thermal disturbance observed very shortly after the onset of the parent flare, in particular to its propagation velocity. This difference in the arch revival is probably related to the different range of plasma temperatures covered by the FCS observations (3 × 10⁶ K through 6 × 10⁶ K) and the HXIS observations (>10⁷ K) and the consequently more important effects of radiative cooling in the FCS arch revival. (3) More arch activity was observed after a (possibly dynamic) flare starting at 03: 40 UT on 21 January and (4) after an unidentified event with estimated time of occurrence near 23: 00 UT on 22 January. Similar to the arch revival, this activity was primarily characterized by the energization of (i.e., input of energy to) a pre-existing arch structure. The activity after the unidentified event suggests the existence of a mode of arch activation which is different from the typical flare-associated revival and is characterized by the absence of significant activity at chromospheric levels.     

Electron-heated flare models - alive and well

The idea that deka-keV electrons, accelerated during the primary energy release, are responsible for the bulk of the impulsive phase energy transport in solar flare atmospheres has been around for quite some time now. With the advent of (i) detailed numerical calculations of the hydrodynamic response of the atmosphere to the energy input from such electrons, and (ii) the observations made during the last solar maximum of the spatial, spectral, and temporal behavior of flare-associated emissions in a variety of wavelength ranges, we have now reached a point where quantitative tests of the thick-target electron-heated model (Brown, 1973) are possible. Here I describe some recent results of such quantitative tests.     

Electron-heated flare models - alive and well

The idea that deka-keV electrons, accelerated during the primary energy release, are responsible for the bulk of the impulsive phase energy transport in solar flare atmospheres has been around for quite some time now. With the advent of (i) detailed numerical calculations of the hydrodynamic response of the atmosphere to the energy input from such electrons, and (ii) the observations made during the last solar maximum of the spatial, spectral, and temporal behavior of flare-associated emissions in a variety of wavelength ranges, we have now reached a point where quantitative tests of the thick-target electron-heated model (Brown, 1973) are possible. Here I describe some recent results of such quantitative tests.

     

Evidence for beamed electrons in a limb X-ray flare observed by HXIS

X-ray images taken by the Hard X-Ray Imaging Spectrometer (HXIS) aboard SMM during the 1980, November 18 limb flare are analysed. The temporal and spatial evolutions of the X-radiation are described. They differ significantly for hard and soft X-rays. During the elementary flare bursts energetic photons are predominantly emitted from a region close to the solar limb. In contrast, the soft X-ray sources are situated higher in the solar atmosphere. The observed X-ray spectra, in particular those emitted from small source regions at various altitudes, were fitted to power laws. Analysis of the spatial variation of the spectral index shows that there is a systematic tendency of the spectra to get harder with decreasing source altitude, especially during the elementary flare bursts. This fact is in agreement with the existence of nonthermal electron beams precipitating from the corona towards the denser layers of the solar atmosphere.     

Spatial structure and temporal development of a solar X-ray flare observed from Skylab on June 15, 1973

A solar flare on June 15, 1973 has been observed with high spatial and temporal resolution by the S-054 grazing-incidence X-ray telescope on Skylab. Both morphological and quantitative analyses are presented. Some of the main results are: (a) the overall configuration of the flare is that of a compact region with a characteristic size of the order of 30 at the intensity peak, (b) this region appears highly structured inside with complex systems of loops which change during the event, (c) a brightening over an extended portion of the active region precedes the flare onset, (d) the impulsive phase indicated by the non-thermal radio emission is a period during which a rapid brightening occurs in loop structures, (e) the X-ray emission is centered over the neutral line of longitudinal magnetic field, and the brightest structures at the flare onset bridge the neutral line, (f) loop systems at successively increasing heights form during the decay phase, finally leading to the large loops observed in the postflare phase, (g) different parts of the flare show distinctly different light curves, and the temporal development given by full disk detectors is the result of integrating the different intensity vs time profiles.The implications of these observations for mechanisms of solar flares are discussed. In particular, the flux profiles of different regions of the flare give strong evidence for continued heating during the decay phase, and a multiplicity of flare volumes appears to be present, in all cases consisting of loops of varying lengths.On leave from Arcetri Astrophysical Observatory, Florence, Italy.     

The impulsive and gradual phases of a solar limb flare as observed from the solar maximum mission satellite

Simultaneous observations of a solar limb flare in the X-ray and ultraviolet regions of the spectrum are presented. Temporal and spectral X-ray observations were obtained for the 25–300 keV range while temporal, spectral, and spatial X-ray observations were obtained for the 30–0.3 keV range. The ultraviolet observations were images with a 10 spatial resolution in the lines of O v (T _e 2.5 × 10⁵ K) and Fe xxi (T _e 1.1 × 10⁷ K). The hard X-ray and O v data indicate that the impulsive phase began in the photosphere or chromosphere and continued for several minutes as material was ejected into the corona. Impulsive excitation was observed up to 30 000 km above the solar surface at specific points in the flare loop. The Fe xxi observations indicate a preheating before the impulsive phase and showed the formation of hot post-flare loops. This later formation was confirmed by soft X-ray observations. These observations provide limitations for current flare models and will provide the data needed for initial conditions in modeling the concurrent coronal transient.     

Recombination edges observed in solar soft X-ray flare spectra

Edges in the solar soft X-ray flare continuum have been observed with the NRL Bragg crystal spectrometer aboard OSO-4. The edges near 2.06 Å, 2.8 Å, and 4.46 Å are interpreted to be due to an innershell dielectronic recombination process, details of which are presented. Two other edges, 3.59 Å and 3.31 Å, are interpreted to be due to recombination of the bare sulfur ion and innershell transitions of calcium.     

A joint analysis of high-energy neutrons and neutron-decay protons from a flare

A joint analysis of neutron monitor and GOES data is performed to study the production of high-energy neutrons at the Sun. The main objects of the research are the spectrum of >50 MeV neutrons and a possible spectrum of primary (interacting) protons which produced those neutrons during the major 1990 May 24 solar flare. Different possible scenarios of the neutron production are presented. The high magnitude of the 1990 May 24 neutron event provided an opportunity to detect neutron decay protons of higher energies than ever before. We compare predictions of the proposed models of neutron production with the observations of protons on board GOES 6 and 7. It is shown that the precursor in high-energy GOES channels observed during 20:55–21:09 UT can be naturally explained as originating from decay of neutrons in the interplanetary medium. The ratio of counting rates observed in different GOES channels can ensure the selection of the model parameters.The set of experimental data can be explained in the framework of a scenario which assumes the existence of two components of interacting protons in the flare. A hard spectrum component (the first component) generates neutrons during a short time while the interaction of the second (soft spectrum) component lasts longer. Alternative scenarios are found to be of lesser likelihood. The intensity-time profile of neutron - decay protons as predicted in the framework of the two-component exponential model of neutron production (Kocharov et al., 1994a) is in an agreement with the proton profiles observed on board GOES. We compare the deduced characteristics of interacting high-energy protons with the characteristics of protons escaping into the interplanetary medium. It is shown that, in the 100–1000 MeV range, the spectrum of the second component of interacting protons was close to the spectrum of the prompt component of interplanetary protons. However, it is most likely that, at 300 MeV, the interacting proton spectrum was slightly softer than the spectrum of interplanetary protons. An analysis of gamma-ray emission is required to deduce the spectrum of interacting protons below 100 MeV and above 1 GeV.     

On the 24-day period observed in solar flare occurrence

Time series of daily numbers of solar Hα flares from 1955 to 1997 are studied by means of wavelet power spectra with regard to predominant periods in the range of ∼ 24 days (synodic). A 24-day period was first reported by Bai (1987) for the occurrence rate of hard X-ray flares during 1980–1985. Considering the northern and southern hemisphere separately, we find that the 24-day period is not an isolated phenomenon but occurs in each of the four solar cycles investigated (No. 19–22). The 24-day period can be established also in the occurrence rate of subflares but occurs more prominently in major flares (importance classes ≥ 1). A comparative analysis of magnetically classified active regions subdivided into magnetically complex (i.e., including a γ and/or δ configuration) and non-complex (α, β) reveals a significant relation between the appearance of the 24-day period in Hα flares and magnetically complex sunspot groups, whereas it cannot be established for non-complex groups. It is suggested that the 24-day period in solar flare occurrence is related to a periodic emergence of new magnetic flux rather than to the surface rotation of sunspots.     

Transition-zone observations of rapid flare events as observed by OSO-8

The rapid dissipation of flare energy has been observed in the transition-zone line of C iv at 1548.2 Å using the University of Colorado spectrometer aboard OSO-8. Impulsive brightenings have been resolved with characteristic risetimes as low as 3.5 s. One event is analyzed in detail, in which it is inferred that the electron density is greater than 2 × 10¹¹ cm^–3 at T = 60 000 K, and that the flare energy is deposited at a rate of 2 ergs cm^–3 s^–1 or greater. The temporal behavior of the intensity at the center of the C iv line is consistent with a non-equilibrium ionization of C iii through C v. If this event is a result of the multiple tearing mode instability as the primary energy release mechanism, then the observations indicate a pre-flare magnetic field of about 175 G.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Study of a solar flare on 2005 August 22 observed in hard X-rays and microwaves

We investigate the 2005 August 22 flare event(00:54 UT) exploiting hard X-ray(HXR) observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) and microwave(MW) observations from the Nobeyama Solar Radio Observatory. The HXR time profile exposes well-damped quasi-periodic pulsations with four sequential peaks, and the MW time profile follows the corresponding peaks.Based on this feature, we derive the time relationship of HXRs and MWs with multifrequency data from the Nobeyama Radio Polarimeter, and the spatially resolvable data from RHESSI and the Nobeyama Radioheliograph. We find that both frequency dependent delays in MWs and energy dependent delays in HXRs are significant.Furthermore, MW emissions from the south source are delayed with respect to those from the north source at both 17 GHz and 34 GHz, but no significant delays are found in HXR emissions from the different sources at the same energies. To better understand all these long time delays, we derive the electron fluxes of different energies by fitting the observed HXR spectra with a single power-law thick-target model, and speculate that these delays might be related to an extended acceleration process. We further compare the time profile of a MW spectral index derived from 17 and 34 GHz fluxes with the flux densities, and find that the spectral index shows a strong anticorrelation with the HXR fluxes.     

Unusual spectral absorption observed in the 16 August 1989 limb flare

A relative complete set of He I 10830 Å profiles and their coincident slit-jaw Hα images of the large limb flare (2N/X20) of 16 August 1989 were observed by the solar spectrograph at Purple Mountain Observatory. In addition to the unusually broadened spectral profiles observed in the impulsive phase, more than half of the observed He I 10830 Å profiles are characterized by central reversals, which were detected not only in the impulsive phase but also in the late decaying phase. The central-reversed profiles may exist at different heights, ranging from the solar limb to (3–4) × 10⁴ km above. The absorption varies with time and position, with a typical lifetime and size of several minutes and 5–6 arc sec, respectively. Depths of the absorption profiles also change clearly. The absorptions are usually deeper at the loop footpoint near the solar limb and shallower at loop-top. However, the most unusual feature is that all the line-center wavelengths of them show no shift relative to that of the quiet chromosphere near the limb, implying the apparent velocities are zero while the associated emission profiles have different apparent velocities. Theoretical simulations demonstrate that the Doppler widths of the absorptions are in the range of (0.35–0.5)Å and increase with height, and the source functions are (0.11–0.3) times the disk center intensity. However, the absorptions have a relative large range of optical thickness (0.1–1.3) in the I ₃ component of the He I 10830 Å triplet. We have not observed such absorption in other limb flares, including the SB/X2.9 flare of 17 August 1989 that occurred in the same active region as the studied one (NOAA 5629). Our studies show that the absorption could not result from he scattering by the telluric atmosphere or from normal chromospheric absorption. This unique phenomenon may be related to extra intense X-ray flux and caused by diffuse and non uniform materials dissociated from the flare instead of self-absorption of the flare.     

Unusual spectral absorption observed in the 16 August 1989 limb flare

A relative complete set of He I 10830 Å profiles and their coincident slit-jaw Hα images of the large limb flare (2N/X20) of 16 August 1989 were observed by the solar spectrograph at Purple Mountain Observatory. In addition to the unusually broadened spectral profiles observed in the impulsive phase, more than half of the observed He I 10830 Å profiles are characterized by central reversals, which were detected not only in the impulsive phase but also in the late decaying phase. The central-reversed profiles may exist at different heights, ranging from the solar limb to (3–4) × 10⁴ km above. The absorption varies with time and position, with a typical lifetime and size of several minutes and 5–6 arc sec, respectively. Depths of the absorption profiles also change clearly. The absorptions are usually deeper at the loop footpoint near the solar limb and shallower at loop-top. However, the most unusual feature is that all the line-center wavelengths of them show no shift relative to that of the quiet chromosphere near the limb, implying the apparent velocities are zero while the associated emission profiles have different apparent velocities. Theoretical simulations demonstrate that the Doppler widths of the absorptions are in the range of (0.35–0.5)Å and increase with height, and the source functions are (0.11–0.3) times the disk center intensity. However, the absorptions have a relative large range of optical thickness (0.1–1.3) in the I ₃ component of the He I 10830 Å triplet. We have not observed such absorption in other limb flares, including the SB/X2.9 flare of 17 August 1989 that occurred in the same active region as the studied one (NOAA 5629). Our studies show that the absorption could not result from he scattering by the telluric atmosphere or from normal chromospheric absorption. This unique phenomenon may be related to extra intense X-ray flux and caused by diffuse and non uniform materials dissociated from the flare instead of self-absorption of the flare.