Images of post-flare coronal structures in X-rays

This is an extended abstract of several papers mentioned in the references describing extensive coronal structures related to radio continua and imaged in > 3.5 keV X-rays.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Thermal structures associated with post-flare coronal arches

Shortly after the dynamic flare of 14 44 UT on 6 November, 1980, which initiated the second revival in the sequence of post-flare coronal arches of 6–7 November, a moving thermal disturbance was observed in the fine field of view of HXIS. From 15 40 UT until about 18 UT, when it left the field of view, the disturbance rose into the corona, as indicated by a projected velocity of 7.4 km s^-1 in the south-east direction. The feature was located above the reconnection region of the dynamic flare and was apparently related to the revived coronal arch. Observations in the coarse field of view after 18 UT revealed a temperature maximum in the revived arch, rising with a velocity of 7.0 km s^-1 directly in continuation of the thermal disturbance. The rise velocity of the disturbance was initially (at least until 17 20 UT) very similar to the rise velocities observed for the post-flare loop tops of the parent flare. This suggests that the rise of the reconnection point, in the Kopp and Pneuman (1976) mechanism responsible for the rise of the loop tops, also dictates the rise of the disturbance. From energy requirements it follows that in this phase the disturbed region is still a separate magnetic island, thermally isolated from the old arch structure and the post-flare loops. After 18 UT the rise of the post-flare loop tops slowed down to 2 km s^-1, which is significantly slower than the rise of the brightness and temperature maxima of the revived arch in the coarse field of view. Thus in this phase the Kopp and Pneuman mechanism is no longer directly responsible for the rise of the thermal structure and the rise possibly reflects the merging of the old and the new arch structures.A similar thermal disturbance was observed after the dynamic flare of 07: 53 UT on 4 June, 1980. On the other hand, the confined flare of 17 25 UT on 6 November, 1980, did not show this phenomenon. Apparently this type of disturbance occurs after dynamic flares only, in particular when the flare is associated with an arch revival.     

SUMER spectral observations of post-flare supra-arcade inflows

On 21 April 2002 a large eruptive flare on the west limb of the Sun developed a bright, very dynamic, post-flare arcade. In TRACE 195 Å images, a series of dark, sunward moving flows were seen against the bright extreme ultraviolet (EUV) arcade. SUMER obtained a series of spectra of the dark EUV flows in the lines Cii, Fexii, and Fexxi at a fixed position above the limb. These spectra give spatially resolved line-of-sight velocities and emission measures for the arcade plasma over a temperature range 2×10⁴ to 10⁷ K. The flows are dark in all SUMER lines. The UV continuum longward (1350 Å) and shortward (675 Å) of the hydrogen Lyman limit is used to determine whether the dark 195 Å inflows are due to regions of low plasma density (plasma voids) or cold absorbing material. There is some evidence of absorption near the front of one of the inflows; however, along most of the dark channels there is no change in continuum ratio and we therefore conclude, as originally suggested by McKenzie and Hudson (1999), that they are plasma voids.     

Slow-shock heating in post-flare arches

The heating of a coronal arch, following the occurrence of a dynamic (two-ribbon) flare, is discussed. We investigate whether slow-shock heating, occurring during the reconnection process in the dynamic flare and responsible for the heating of the post-flare loops, is also a workable proposition for the heating of a coronal arch. Contrary to the flare loops, the shock structure in the arch is generally not modified greatly by thermal conduction effects. As a result slow-shock heating may be investigated in terms of the familiar MHD shock jump relations. The observed enhanced arch density with respect to the surrounding corona is explained as a direct consequence of the reconnection process. For a combination of high arch temperatures and low values of coronal magnetic field and density thermal conduction may become important and will lead to an extra density enhancement in the arch. Our interpretation of the arch of 21–22 May, 1980 suggests that the formation of the arch took approximately one hour, and that observed temperature, density and maximum energy content can be consistently explained by the slow-shock heating mechanism.Currently: Center for Astrophysics and Space Sciences, C-011, University of California at San Diego, La Jolla, CA 92093, U.S.A.     

Large-scale electric fields in post-flare loops

As the electrical conductivity along the magnetic field in solar atmosphere is large, parallel electric fields have been neglected in most investigations. We will first demonstrate their importance for post-flare loops, and then introduce a model for them which takes into account the effect of parallel electric fields. The electric field calculated from the model is consistent with the electric field observed by Foukal et al. (1983).     

Studies on post-flare loop prominence of 1981 April 27

By use of the H observations of the Astrophysical Observatory in Catania, Italy and the Purple Mountain Observatory in Nanking, China as well as hard X-ray and gamma-ray burst data from the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS), a major eruptive loop prominence was studied during the limb solar flare event of 1981 April 27.Our preliminary analysis shows that there seems to exist a second abrupt energy release for this event, almost 20 min after the end of the impulsive phase of the flare. This energy release is probably associated with the rapidity in upward motion or activation of the loop prominence.A possible candidate for such a process could be the reconnection of the old magnetic field with a newly emerging magnetic field.A theoretical gross estimate for the energy release and particle acceleration has also been made in this work. It appears that the proposed model for charged particle acceleration is very efficient.     

Study of the post-flare loops on 29 July 1973

Bright and dark curvilinear structures observed between the two major chromospheric ribbons during the flare of 29 July 1973 on films from the Big Bear Solar Observatory are interpreted as a typical system of coronal loops joining the inner boundaries of the separating flare ribbons. These observations, made through a 0.25 Å H filter, only show small segments of the loops having Doppler shifts within approximately ± 22 km s^–1 relative to the filter passband centered at H, H -0.5 Å or H +0.5 Å. However, from our knowledge of the typical behavior of such loop systems observed at the limb in H and at 5303 Å, it has been possible to reconstruct an appoximate model of the probable development of the loops of the 29 July flare as they would have been viewed at the limb relative to the position of a prominence which began to erupt a few minutes before the start of the flare. It is seen that the loops ascended through the space previously occupied by the filament. On the assumption that H fine structures parallel the magnetic field, we can conclude that a dramatic reorientation of the direction of the magnetic field in the corona occurred early in the flare, subsequent to the start of the eruption of the filament and prior to the time that the H loops ascended through the space previously occupied by the filament.     

Decimeter continuum radio emission from a post-flare loop

Radio observations offer an important means for providing estimates of magnetic fields in post-flare loops.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

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.     

Study of the post-flare loops on 29 July 1973

We present revised values of temperature and density for the flare loops of 29 July 1973 and compare the revised parameters with those obtained aboard the SMM for the two-ribbon flare of 21 May 1980. The 21 May flare occurred in a developed sunspot group; the 29 July event was a spotless two-ribbon flare. We find that the loops in the spotless flare extended higher (by a factor of 1.4–2.2), were less dense (by a factor of 5 or more in the first hour of development), were generally hotter, and the whole loop system decayed much slower than in the spotted flare (i.e. staying at higher temperature for a longer time). We also align the hot X-ray loops of the 29 July flare with the bright H ribbons and show that the H emission is brightest at the places where the spatial density of the hot elementary loops is enhanced.presently at Big Bear Solar Observatory, Caltech, California, U.S.A.presently at The Marshall Space Flight Center, Huntsville, Alabama, U.S.A.     

Pre- and post-flare X-ray variations in active regions

Extremely low background noise of the HXIS experiment aboard the SMM made it possible to detect > 3.5 keV X-ray emissions from non-flaring active regions which are 10³–10⁴ times weaker than the X-ray flux from flares. Short-lived X-ray bursts and long-lived X-ray enhancements of various intensities seem to characterize active regions in different phases of their development. After major two-ribbon flares, giant X-ray arches are seen in the corona, slowly decaying for many hours after the flare end. Associated with these arches appear to be quasi-periodic flare-like variations of purely coronal nature.     

Dynamics of post-flare ejections and magnetic loop geometry

Flare-associated mass ejections have been observed at the solar limb on June 29, 1980 in the H line, with the Multichannel Subtractive Double Pass spectrograph of the Meudon solar tower. Radial velocities were measured as a function of time in a two dimensional field, and kinematics investigated in one selected fine structure. A simple model of locally dipole-type magnetic field increasing with time can be fitted to the data. It can be checked from extrapolation that the model is consistent with an ejection starting roughly from the same point at the same time. Height of the loops ( 135 000 km) is consistent with other determinations. Magnetic field is found to be increasing locally by a factor 1.14 within 10 min.

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Résumé Des éjections de matière associées à une éruption ont été observées au bord solaire le 29 Juin 1980, dans la raie H, avec le spectrographe à Double Passage Soustractif Multicanaux de la Tour Solaire de Meudon. Les vitesses radiales ont été mesurées au cours du temps sur un champ bidimensionnel, et la cinématique a été analysée pour une structure fine particulière. Un modèle simple de champ magnétique, localement dipolaire et croissant au cours du temps, peut être ajusté aux données. Par extrapolation, on peut vérifier que le modèle est compatible avec une éjection déclenchée sensiblement au même point et au même instant. La hauteur des boucles ( 135 000 km) est en accord avec d'autres déterminations. On trouve que le champ magnétique croît localement d'un facteur 1.14 en 10 min.

     

X-ray observations of limb flare loops and post-flare coronal arch

We present observations of another post-flare arch following an eruptive flare, detected in X-ray lines above the western solar limb on 2 May 1985.     

Spike emission at 21 cm wavelength in post-flare loops

Electron-cyclotron maser instability produced by accelerated energetic electrons is today's most favoured process for spike emission on ultra-short time scales at dm wavelengths. The process of electron acceleration occurs primarily in pre-impulsive and impulsive phases. We present observations of spike bursts at 21 cm in a limb event that occurred in AR 5629 on 1989 August 17. Optical observations of the event from Beijing Astronomical Observatory show coronal loops that interact just before the spike groups appeared. This implies that additional particle acceleration might be produced by the interaction between emerging loops and original large loops, and become the source of the energetic electrons responsible for the spike emission appearing in the descending phase. Some important parameters are deduced.Paper presented at the 4th CESRA workshop in Ouranopolis (Greece) 1991.     

Observation of multiple sausage oscillations in cool post-flare loop

Using simultaneous high spatial (1.3 arcsec) and temporal (5 and 10 s) resolution Hα observations from the 15 cm Solar Tower Telescope at Aryabhatta Research Institute of Observational Sciences (ARIES), we study the oscillations in the relative intensity to explore the possibility of sausage oscillations in the chromospheric cool post-flare loop. We use the standard wavelet tool, and find the oscillation period of ≈587 s near the loop apex, and ≈349 s near the footpoint. We suggest that the oscillations represent the fundamental and the first harmonics of the fast-sausage waves in the cool post-flare loop. Based on the period ratio   P ₁/ P ₂∼1.68  , we estimate the density scaleheight in the loop as ∼17 Mm. This value is much higher than the equilibrium scaleheight corresponding to Hα temperature, which probably indicates that the cool post-flare loop is not in hydrostatic equilibrium. Seismologically estimated Alfvén speed outside the loop is  ∼300–330  km s⁻¹  . The observation of multiple oscillations may play a crucial role in understanding the dynamics of lower solar atmosphere, complementing such oscillations already reported in the upper solar atmosphere (e.g. hot flaring loops).     

Real-time simulation of a potential magnetic field in a post-flare arch

We analyze the X-ray observations on 7–8 November of the last revival of a giant coronal arch first observed in the morning hours of 6 November, 1980. We compare these data with observations of earlier revivals of this arch, in order to determine whether the successive brightenings repeat without any apparent modification, or whether they show a varying behavior as they keep occurring. We also examine the magnetic structure of the revived arches in order to ascertain whether the subsequent phenomena involve the same magnetic configuration. The information about the magnetic structure of the last revival is the best we have, because the revival started close to the time of the Kitt Peak magnetogram which has been used for the magnetic modelling of the whole series of these giant arches.We conclude that while there seems to be some plasma depletion in the 7–8 November event, the real-time magnetic field modelling on this date seems to fit the observed footpoints better than the modelling on 6 November which had to use the same magnetogram for a situation occurring 24 to 36 hours earlier. The general topology does not show any significant change, apart perhaps in the field line inclination, but some flux tubes seem to have disappeared between 6 and 7 November. Nevertheless, both X-ray and H observations indicate that this magnetic structure basically kept its identity for more than 4 days.We also compare the two existing interpretations of the arch revivals: re-excitation of a permanent pre-existing structure (after Hick and vestka, 1987) and formation of each brightening through a separate reconnection (after Poletto and Kopp, 1988). Unfortunately, proximity to the limb does not allow one to decide between these two alternatives; it is suggested to look for other series of arch revivals utilizing our present knowledge about the behavior of the chromospheric footpoints of giant arches.     

The stationary post-flare arch of May 21/22, 1980

On May 21/22, 1980 the Hard X-Ray Imaging Spectrometer aboard the SMM imaged an extensive coronal structure after the occurrence of a two-ribbon flare on May 21, 20:50 UT. The structure was observed from 22:20 UT on May 21 until its disappearence at 09:00 UT on May 22.At 22:20 UT the brightest pixel in the arch was located at a projected altitude of 95 000 km above the zero line of the longitudinal magnetic field. At 23:02 UT the maximum of brightness shifted to a neighbouring pixel with approximately the same projected altitude. This sudden shift indicates that the X-ray structure consisted of (at least) two separate arches at approximately the same altitude, one of which succeeded the other as the brightest arch in the structure at 23:02 UT.From 23:02 UT onwards the maximum of brightness did not change its position in the HXIS coarse field of view. With a spatial resolution of 32 this places an upper limit of 1.1 km s^-1 on the rise velocity of the arch. Thus, contrary to a similar arch observed on November 6/7, where rise velocities of the order of 10 km s^-1 were measured in the same phase of development, the May 22 arch was a stationary structure at an altitude of 145000 km.The following values were estimated for the physically relevant quantities of the May 21/22 arch at the time of its maximum brightness (23:00 UT): temperature T 6.3 × 10⁶ K, electron density n _e 1.1 × 10⁹ cm^-3, total emitting volume V 5 × 10²⁹ cm³, energy density 2.9 erg cm^–3, total energy contents E 1.4 × 10³⁰ erg, total mass M 9 × 10¹⁴ g.The top of the arch was observed at 145 000 km altitude within 1.5 hr after the flare occurrence. Since it seems unlikely that the structure already existed prior to the flare at 20:50 UT, the arch must have risen to its stationary position with an average velocity exceeding 17 km s^–1 (possibly much faster). We speculate that the arch was formed very fast at the flare onset, when (part of) the active region loop system was elevated within minutes to the observed altitude.     

Temporal variation of 2D multi-parameter fields of a post-flare loop system

The 2D H_β spectral data of the post-flare loop system (PFLs) of August 17, 1989 are obtained and analyzed quantitatively for three different times. Three physical quantities (i.e., the column number density of hydrogen atoms at the second level along the line-of-sight direction N₂, the excitation temperature T_ex and micro-turbulence velocity V_t) and their 2D fields are derived during the three times. The time variations of the 2D field are given for the three quantities more than 1 h after the maximum of the H_α flare. Our analyses show that the average values of N₂ and V_t decrease with time, while T_ex is nearly unchanged except for the top part of the PFLs where it is increasing slightly with time. A new evolution property is found in which the regions with the maxima of T_ex and N₂ move from the middle of the southern leg towards the top part of the PFLs, while the position of V_t maximum shifts from the top part towards the northern leg of the system. This scenario may be a result of successive formation of new loops at higher heights while under continuous cooling. The emission measure (EM), electron density n_e and pressure P_e in the PFLs are also estimated.     

Techniques For Measuring Radar Meteor Speeds

We compare the results from the application of four different methods to determine the speed of meteoroids from single station radar data. The methods used are the pre-t ₀ amplitude, post-t ₀ amplitude, pre-t ₀ phase and the Fresnel transform (FT) methods. Speeds from the first three methods are compared to the FT method since, requiring the use of the entire records of both the amplitude and phase data, this method is the most accurate of the four.     

Comparison Between Halo cme Expansion Speeds Observed on the Sun,the Related Shock Transit Speeds to Earth and Corresponding Ejecta Speeds at 1 au

We have compared characteristics of 38 halo coronal mass ejections observed on the Sun by the Large Angle and Spectrometric Coronagraph onboard SOHO with their corresponding counterparts observed near Earth by the magnetic field and plasma instruments onboard the ACE, WIND and SOHO satellites, in the period from January 1997 to April 2001. We only have selected events that have some associated interplanetary ejecta structure at 1 AU and we have compared the lateral expansion speeds of these halo CMEs and the corresponding ejecta speeds near Earth. We found that there is a high correlation between these two speeds. The results are very similar to the study done by Lindsay et al. (1999) using observations made by Solwind and SMM coronagraphs, and Helios-1 and PVO plasma and interplanetary field data from the period of 1979 to 1988. Also, we reviewed the relation between the CME-related shock transit speed to Earth and the ejecta speeds near Earth. This kind of relation is very important to estimate ejecta speeds of events for which no interplanetary observations are available.