Hydrodynamic shock wave formation in the solar chromosphere and corona during flares

Basic mechanisms of the hydrodynamic shock wave formation in the solar atmosphere during flares are considered. Hydrodynamic plasma flows during flares arise due to fast energy release which is accumulated in the magnetic field of currents in the solar atmosphere. Shock waves arise as a result of rapid heating of the chromospheric upper layers from accelerated particles or heat fluxes. Powerful hydrodynamic phenomena can also arise due to explosive current sheet disruption in the region of strong magnetic field reconnection. Fundamental questions of shock wave formation and propagation in a non-homogeneous emitting solar atmosphere are discussed.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

The heating of the solar corona

In this paper a discussion is given of the present state of the theory of the heating of the solar corona by shock waves. Arguments are presented why the main contribution to the mechanical energy flux is of acoustic origin, while estimates for the amount of acoustic energy generated in the convection zone as well as the deviations from isotropy are given. During propagation through the atmosphere acoustic waves develop into shock waves after a distance of a few scale heights in the chromosphere. The heating of the outer layers by dissipation of shock waves is found to be sufficient to account for the observed radiative and corpuscular energy losses.Much emphasis is laid on the competitive role played by the four fundamental processes of energy transfer: mechanical heating, radiation, heat conduction and convection of energy in establishing the equilibrium structure of the corona. The atmosphere may be divided in several regions according to the predominance of one of the energy processes mentioned above.The physical properties of the chromosphere and the solar wind are discussed only where they are intimately connected with the problem of the heating of the corona.The most important aspects of the influence of a magnetic field on the structure and the heating of the corona in magnetically active regions are briefly mentioned. Special attention is paid to the strong channelling of heat flow along the field lines and its consequences for the structure and dynamics of the chromosphere-corona transition layer.     

The extension of explosive events from the transition region to the corona

We describe the properties of high velocity events in the corona and upper transition region and propose that they are the same phenomenon as the well studied explosive events seen in the lower transition region around T=10⁵ K. Furthermore, we discuss how the SOHO spectrometers, CDS and SUMER, may be used to check this conjecture. Magnetic reconnection has been considered a strong candidate for the physical mechanism causing explosive events. We present a phenomenological model showing how some of the observed properties of explosive events may be explained by reconnection occurring in small magnetic loops.     

X- and XUV-radiation of the solar corona

First a survey of the ionization states and emission lines of the ions existing in the corona is given. Then instruments for taking pictures of the Sun in the X- and the XUV-region as well as for measuring spectra emitted in interesting locations on the Sun are presented. Methods of plasma diagnostics, in particular for the determination of the mean temperature and the differential emission measure are described.In the following review of observations, which are related to the topic of the workshop, types of coronal structures especially coronal holes, active regions and large scale structures are described. Their relations to the photospheric magnetic fields are dealt with; methods to calculate coronal magnetic fields are briefly discussed. As for temporal variations results of the analysis of expanding X-ray arches and of structures becoming visible in the outer corona in white light are mentioned. Finally, plasma diagnostics by means of high-resolution spectra are dealt with, in particular methods for the determination of the particle density by lines of He-like ions and of the local temperature by Li-like satellites lines. Thus non-thermal random velocities and outward moving plasma can be inferred during flares.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

The transition region and corona associated with sunspots

To the present time, no structure has been identified immediately above the chromosphere in sunspots that is invariably present and that thus might be called the transition region and corona over the spot. But the magnetic flux tubes emerging from spots give rise to many of the plasma filled loops that characterize the active region corona. These emit strongly from ions characteristic of the transition region, or the corona, but seldom both simultaneously. This paper presents an overview of the morphology, evolution and theory of these structures.Invited review presented at the Joint Meeting of IAU Commissions 10, 12, and 44, The MHD of Sunspots, in Montreal, 20 August 1979.     

Optical obsdervations of the solar corona

The history of optical observations of the solar corona is traced through the development of lour topics: the observation of naturally occurring total solar eclipses, the discovery of forbidden emission lines in the solar corona, the development of narrow spectral bandpass observing techniques, and the invention of the coronagraph. These events occurred over a span of time from the middle of the last century until the present.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Electron beam propagation in the solar corona

The propagation of energetic electron flux in the solar corona is investigated with due regard for the influence of the neutralizing cold electron flux and the kinematic escape effect of the electrons with different velocities.     

Small scale structures in the solar corona

The observational characteristics of the small scale magnetic structures are summarized. The temperature structure and temporal variability of the emission from coronal bright points, that pervade the source region of the solar wind in coronal holes and the quiet sun, and from active regions are shown to be remarkably similar. Particular emphasis is given to observations, potentially feasible with SOHO, that could resolve some of the outstanding issues regarding the role of the small scale magnetic structures in the energy balance and properties of the solar wind.     

Exploration of the solar corona by high resolution solar decametric observations

In this review, current state of knowledge of high resolution observations at decameter wavelengths of the quiet Sun, the slowly varying component (SVC), type I to V bursts and noise storms is summarized. These observations have been interpreted to yield important physical parameters of the solar corona and the dynamical processes around 2R from the photosphere where transition from closed to open field lines takes places and the solar wind builds up. The decametric noise bursts have been classified into (i) BF type bursts which show variation of intensity with frequency and time and (ii) decametric type III bursts. The angular sizes of noise storm sources taking into account refraction and scattering effects are discussed. An attempt has been made to give phenomenology of all the known varieties of decametric bursts in this review. Available polarization information of decametric continuum and bursts has been summarized. Recent simultaneous satellite and ground-based observations of decametric solar bursts show that their intensities are deeply modulated by scintillations in the Earth's ionosphere. Salient features of various models and theories of the metric and decametric noise storms proposed so far are examined and a more satisfactory model is suggested which explains the BF type bursts as well as conventional noise storm bursts at decametric wavelengths invoking induced scattering process for 1 t conversion. Some suggestions for further solar decametric studies from the ground-based and satellite-borne experiments have been made.     

SOHO contribution to the understanding of mass supply and flows in the solar corona

We expect a variety of dynamic phenomena in the quiescent non-flaring corona. Plasma flows, such as siphon flows or convective flows of chromospheric material evaporating into the corona, are expected whenever a pressure differences is established either between the footpoints or between the coronal and chromospheric segments of a coronal loop. Such flows can induce phenomena of spatial and temporal brightness variability of the corona. In particular, evaporation induces a net mass input into the corona and consequently coronal density enhancements. Flows are also expected in the regions where energy is released during magnetic reconnection. From the observational point of view the dynamics of the solar atmosphere has been investigated in great detail mostly in the lower transition region with the HRTS, and during flares with theSolar Maximum Mission andYohkoh. The high spectral, temporal and spatial resolution of theSOHO ultraviolet spectrometers should enable us in the near future to fill the gap providing a continuous coverage from the chromosphere to the corona, in the 10⁴–10⁶ K domain, and therefore to best study the dynamics throughout the solar atmosphere.     

Structure and development of quiet loops in the solar corona

X-ray emission from solar coronal loops changes on two different timescales: a) flare loops and transient active region brightenings show a rapid variability, b) quiet region loops are quasi-steady and change only slowly with time. This different time behavior has been analyzed on the basis of Yohkoh SXT observations and we report here on the results from our analysis, mainly focussing on quiet loop variability.     

Jets and brightenings generated by energy deposition in the middle and upper solar chromosphere

Numerical simulations of energy depositions in the middle and upper solar chromosphere result in ejection of chromospheric material into the corona and heating of the chromospheric gas. These simulations may be capable of describing some of the features seen by the soft X-ray telescope on board theYohkoh satellite.     

Observational evidence for non-equilibrium ionization in the solar corona

We investigate whether temperature sensitive EUV line ratios can be used as observational signatures for the presence of non-equilibrium ionization in transition region plasma. We compute the total intensity of some EUV lines of carbon and oxygen expected from coronal loop models with a steady-state flow and which are known to have significant departures from ionization equilibrium, selecting lines whose intensity ratios are useful for deducing the electron temperature in the coronal plasma. We calculate the intensity ratios with and without the approximation of ionization equilibrium, in order to determine the effects of any deviations from equilibrium on the numerical values of the line ratios examined.     

Large scale structure of the solar corona in the declining phase of the solar cycle

Maps of the corona, obtained at meter wavelengths with the Nançay Radioheliograph (France), are used to study, on the disk, the radio counterpart of the coronal plasma sheet observed in K-corona on the limb. We study here the evolution of the coronal plasma sheet from the maximum of the activity cycle in 1980 to the minimum in 1986 and identify some of its large scale structures.     

Millimeter wave radiation sources visible in solar corona

Coronal millimeter wave sources (CMMS) have been observed at radio frequencies 22–90 GHz (at wavelengths 3–13 mm). The observed CMMS have been classified into three different categories according to their time scale and relation to other wave-length events. The CMMS indicate enhancement of electron density in the corona, as well as magnetic loop structures. The CMMS are evidence of dynamical processes taking place at the lower levels of the corona, propagating into the higher levels.     

Observed redshifts of transition region/corona lines

Solar UV observations reveal a redshifted emission at transition region temperatures, commonly interpreted as a net downflow of plasma. In earlier investigations the magnitude of the redshift has been found to increase with temperature, reaching a maximum at T=10⁵ K, and then to decrease towards higher temperatures. These observations, mostly from Skylab, suggested no significant shift of the O V line at 1218 Å formed at 2.4×10⁵ K. The variation of the downflow velocity with temperature is, however, uncertain since there are few reliable observations of lines formed at higher temperatures.Using spectrograms from the High Resolution Telescope and Spectrograph — HRTS we find an average net redshift of the O V lines at 1218 Å and 1371 Å at all locations extending from disk center to solar limb. A discrepancy between the observed flow velocity in the two lines is probably caused by uncertainty in the available laboratory wavelength of the intercombination line at 1218 Å (2s^{2 1}S₀-2s2p³P₁).The observed shift in O V is compared with corresponding measurements of lines formed at other temperatures (Si IV, C IV, N IV, O IV, and Fe XII). Large variations in the shift are found along the instrument slit. Thus, blueshifts are also observed with the sites of the largest upflow located in the sunspot umbrae and in a quiet region close to an active region.