Electron trapping in evolving coronal structures during a large gradual hard X-ray/radio burst

Gradual hard X-ray/radio bursts are characterized by their long duration, smooth time profile, time delays between peaks at different hard X-ray energies and microwaves, and radiation from extended sources in the low and middle corona. Their characteristic properties have been ascribed to the dynamic evolution of the accelerated electrons in coronal magnetic traps or to the separate acceleration of high-energy electrons in a second step process. The information available so far was drawn from qualitative considerations of time profiles or even only from the common occurrence of emissions in different spectral ranges. This paper presents model computations of the temporal evolution of hard X-ray and microwave spectra, together with a qualitative discussion of radio lightcurves over a wide spectral range, and metric imaging observations. The basic hypothesis investigated is that the peculiar gradual features can be related to the dynamical evolution of electrons injected over an extended time interval in a coronal trap, with electrons up to relativistic energies being injected simultaneously. The analyzed event (26 April, 1981) is particularly challenging to this hypothesis because of the long time delays between peaks at different X-ray energies and microwave frequencies. The observations are shown to be consistent with the hypothesis, provided that the electrons lose their energy by Coulomb collisions and possibly betatron deceleration. The access of the electrons to different coronal structures varies in the course of the event. The evolution and likely destabilisation of part of the coronal plasma-magnetic field configuration is of crucial influence in determining the access to these structures and possibly the dynamical evolution of the trapped electrons through betatron deceleration in the late phase of the event.     

On the formation of coronal cavities

We present a theoretical study of the formation of a coronal cavity and its relation to a quiescent prominence. We argue that the formation of a coronal cavity is initiated by the condensation of plasma which is trapped by the coronal magnetic field in a closed streamer and which then flows down to the chromosphere along the field lines due to lack of stable magnetic support against gravity. The existence of a coronal cavity depends on the coronal magnetic field strength; with low strength, the plasma density is not high enough for condensation to occur. Furthermore, we suggest that prominence and cavity material is supplied from the chromospheric level. Whether a coronal cavity and a prominence coexist depends on the magnetic field configuration; a prominence requires stable magnetic support.We initiate the study by considering the stability of condensation modes of a plasma in the coronal streamer model obtained by Steinolfson et al. (1982) using a 2-D, time dependent, ideal MHD computer simulation; they calculated the dynamic interaction between outward flowing solar wind plasma and a global coronal magnetic field. In the final steady state, they found a density enhancement in the closed field region with the enhancement increasing with increasing strength of the magnetic field. Our stability calculation shows that if the density enhancement is higher than a critical value, the plasma is unstable to condensation modes. We describe how, depending on the magnetic field configuration, the condensation may produce a coronal cavity and/or initiate the formation of a prominence.NRC Research Associate.     

Evolution of X-ray cavities

A wide range of recent observations have shown that active galactic nuclei (AGN) driven cavities may provide the energy source that balances the cooling observed in the centres of 'cool-core' galaxy clusters. One tool for better understanding the physics of these cavities is their observed morphological evolution, which is dependent on such poorly understood properties as the turbulent density field and the impact of magnetic fields. Here, we combine numerical simulations that include subgrid turbulence and software that produces synthetic X-ray observations to examine the evolution of X-ray cavities in the absence of magnetic fields. Our results reveal an anisotropic size evolution of the cavities that is dramatically different from simplified, analytical predictions. These differences highlight some of the key issues that must be accurately quantified when studying AGN-driven cavities, and help to explain why the inferred pV energy in these regions appears to be correlated with their distance from the cluster centre. Interpreting that X-ray observations will require detailed modelling of effects, including mass entrainment, distortion by drag forces and projection. Current limitations do not allow a discrimination between purely hydrodynamic and magnetically dominated models for X-ray cavities.     

Flow-tube dynamics and coronal holes

We reexamine the well-known polytropic flow-tube model of the expanding solar corona, and find that as the divergence of the flow tube increases the expansion speed increases throughout the flow, over a stated parameter range. Corresponding to a specified flow-tube geometry the terminal speed of the fluid may be far in excess of the value corresponding to purely spherically symmetric flow. The implications of the results for the modelling of high-speed streams emanating from coronal holes are discussed.     

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.     

Radio and X-ray imaging observations of solar flares and coronal transients

We present a review of selected studies based upon simultaneous radio and X-ray observations of solar flares and coronal transients. We use primarily the observations made with large radio imaging instruments (VLA, BIMA, Nobeyama, and Nançay) along with Yohkoh/SXT and HXT and CGRO experiments. We review the recent work on millimeter imaging of solar flares, microwave and hard X-ray observations of footpoint emission from flaring loops, metric type IV continuum bursts, and coronal X-ray structures. We discuss the recent studies on thermal and nonthermal processes in coronal transients such as XBP flares, coronal X-ray jets, and active region transient brightenings.Dedicated to Cornelis de Jager     

X-ray bright points,coronal heating and the solar cycle

Recent Skylab observations about the bright points in the solar X-ray images seem to confirm an essential prediction of a model proposed by this author for the appearance and the disappearance of the photospheric fields during a solar cycle.The segments of the individually rising strands of the fundamental flux-loops proposed in the model may lead to the X-ray bright points with the observed properties.The emergence of such strands may substantially contribute to the coronal heating at different heights.     

Multifrequency radio observations of coronal holes,filaments and cavities on RATAN–600

On the basis of multifrequency solar radio observations made on RATAN–600 radiotelescope with high spatial resolution at nine wavelengths in the 2–32–wavelength range is shown that filaments and cavities are well detected on the solar scans at short centimeter wavelengths as the regions of low radio brightness with angular dimensions of 25′–80′ in E—W direction. The tendency of decreasing radio sizes for cavities and filaments from 2.0 to 8.0 cm is observed. The coronal hole (CH) is more contrast in the range of 8–32 cm. The radio size of CH in E—N direction increases from 2′ (at 8.2) to 5′.0 (at 31.6 cm). The spectra of the brightness temperature of CH and the quiet Sun are obtained. The brightness temperature of CH is twice lower than that of the quiet Sun at wavelength of 31.6 cm.     

Soft X-ray observation of a large-scale coronal wave and its exciter

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfvén speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022904125479 deceased     

X-ray resonance scattering in a spherically symmetric coronal model

In the solar corona the opacities of some of the prominent X-ray emission lines are on the order of 1 over typical coronal path lengths. We present and discuss a particular solution of the radiative transfer problem involving an extended, spherically symmetric coronal shell radiating isotropic, homogeneous emission in which single-scattering also takes place. Within the context of this simplified model we find that scattered radiation is an important contribution to the total emergent resonance line flux and that for the He-like family of resonance (r), intercombination (i), and forbidden (f) lines, the ratio G=(f + i)/r would decrease as a function of optical depth for disk-center emission in an extended spherically symmetric corona.     

Observational evidence of continual heating in X-ray emitting coronal loops

A 90 s time resolution study of the soft X-ray emission from three active region loops shows the emission to be constant to about two percent over the half hour period of observation. Soft X-ray observations in two wavebands are used to deduce the temperature and density of these loops. The data unambiguously demonstrate that energy is supplied to each loop during the observations. If heating is due to discrete events, the time interval between events is shown to be less than 10 min, which is short relative to the radiative cooling time of the loops.Skylab Solar Workshop Post-Doctoral Appointee, 1975–1977. The Skylab Solar Workshops are sponsored by NASA and NSF and managed by the High Altitude Observatory, National Center for Atmospheric Research.     

The dynamics of driven magnetic reconnection in coronal arcades

The dynamics of interacting coronal loops and arcades have recently been highlighted by observations from theYohkoh satellite and may represent a viable mechanism for heating the solar corona. Here such an interaction is studied using two-dimensional resistive magnetohydrodynamic (MHD) simulations. Initial potential field structures evolve in response to imposed photospheric flows. In addition to the anticipated current sheet about theX-point separating the colliding flux systems, significant current layers are found to lie all the way along the separatrices that intersect at theX-point and divide the coronal magnetic field into topologically distinct regions. Shear flows across the separatrices are also observed. Both of these features are shown to be compatible with recent analytical studies of two-dimensional linear steady-state magnetic reconnection, even though the driven system that has been simulated is not strictly ‘open’ in the sense implied by steady-state calculations. The implications for future steady-state models are also discussed. The presence of the neutral point also brings into question any constant-density approximations that have previously been used for such quasi-steady coronal evolution models. This results from the intimate coupling between the neutral point and its separatrices communicated via the gas pressure. In terms of the detailed energetics during the arcade evolution, preliminary results reveal that on the order of 3% of the energy injected by the footpoint motions is lost purely through ohmic dissipation. We would therefore anticipate a local hot spot between the interacting flux systems, and a brightening distributed along the length of any separatrix field lines. Furthermore, as the resistivityη is reduced, the flux annihilation rate and the ohmic dissipation rate are found to scale independently ofη.     

Expansion of an X-ray coronal arch into the outer corona

An asymmetric, expanding arch, photographed in the inner corona with an X-ray telescope on 13 August, 1973, is identified as the source of the mass ejected in a white light transient in the outer corona. The morphology, angular position, estimated mass and apparent rate of upward acceleration of the lower coronal arch are similar to those of the arch seen passing through the outer corona. The mass of material removed from the lower corona is estimated at 2 × 10¹⁵ g, and the upward movement is consistent with a constant acceleration of 12.5 m s^–2 between 1.3 and 5 R.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

An analysis of Skylab X-ray pictures of a giant coronal arch

The limb event of 13/14 August, 1973, imaged by Skylab in soft X-rays, proved to be a giant arch, quite similar to those observed in 1980–1986 on SMM. High spatial resolution (by a factor of 4–5 better than in SMM data) made it possible to see the internal structure of the arch. Its brightest part consisted of loops very similar to, but higher than, post-flare loops, surrounded by a rich system of weak loop structures extending up to altitudes of 260 000 km. While the main brightest structure of the arch was newly formed, the weak very large loops had existed above the active region before and were only enhanced during the event.Skylab data support the model proposed by Kopp and Poletto that the giant arch is formed by reconnections high in the corona, different from the reconnection process in the underlying flare. However, contrary to Kopp and Poletto's suggestion, the data strongly indicate that the field lines that reconnect in the arch did not open before, as in the Kopp and Pneuman model: more likely, we encounter here an interaction of large-scale loops high in the corona. (The interaction of two of them is clearly seen.) Thus, while post-flare loops are formed by the Kopp and Pneuman mechanism, giant arches above eruptive flares may originate through interactive reconnections of large-scale magnetic field lines which form loops high in the corona. These loops are brought close to each other in consequence of changes in the coronal structure caused by the eruptive flare phenomenon. The arch-associated enhancement of the pre-existing large-scale active-region loops may be caused by electrons accelerated during the reconnection process and diffusing across field lines, as suggested by Achterberg and Kuipers (1984).     

Forward modeling of the coronal response to reconnection in an X-ray bright point

We use MDI magnetic field observations and the theory of reconnection through a separator to constrain a numerical simulation of an X-ray bright point observed in EUV by TRACE. A gasdynamic model is employed to describe the corona and transition region in the bright point loop. Nonlocal effects such as opacity and ambipolar diffusion are important to the transition region; these effects are approximated locally by modification of the radiative loss and thermal conduction. A straightforward comparison of measured light curves versus those generated by the simulation shows that the reconnection model is unable to account for the observations. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005205807984     

Analysis and interpretation of soft X-ray photographs of coronal active regions taken with Fresnel zone plates

Soft X-ray photographs of the Sun taken at O vii 21.6 Å and in a spectral band ranging from 13.2 to 22.1 Å have been analysed in order to establish spatially resolved maps of temperature and emission measure for several active regions in the corona. The photographs were taken on 11 March, 1971, and on 2 March, 1972, with Fresnel zone plate cameras which were flown on ESRO and NRL sounding rockets.In Part I of this paper we first deal with those aspects of the instrumentation which are important for setting up a suitable image analysis procedure. We discuss the characteristics of the wavelength dependent image formation by zone plates combined with absorption filters. Results of the calibration of the X-ray film are given. Then we describe a specific iterative data reduction procedure and finally present the resulting maps of temperature and emission measure for a selected active region. In Part II corresponding results for further active regions will be given and discussed.