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.     

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.     

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.     

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.     

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.     

The solar chromosphere and its transition to the corona

Our present knowledge on the average physical properties of the chromosphere and of the transition region between chromosphere and corona is reviewed. It is recalled that shock wave dissipation is responsible for the high temperatures observed in the chromosphere and corona and that, due to the non-linear character of the dissipation mechanism, no satisfactory explanation of the structure of the outer solar layers has yet been given. In this paper, the main emphasis is on the observations and their interpretation.Evidence for the non-spherically symmetric structure of the atmosphere is given; the validity of interpreting the observations with the help of a fictitious spherically symmetric atmosphere is discussed.The chromosphere and the transition region are studied separately: for each region, the energy balance is considered and recent homogeneous models derived from ultra-violet, infrared and radio observations are discussed.It is stressed that although in the chromosphere, a study of the radiative losses may lead to the determination, as function of height, of the amount of mechanical energy dissipated as function of height, a more detailed analysis of the velocity field is necessary to find the periods and the wavelengths of the waves responsible for the heating. The methods used for wave detection and some results are presented.Observational and theoretical evidence is given for the non-validity of the assumption of hydrostatic equilibrium which is commonly used in modeling the transition region.We conclude that a better understanding of the heating mechanism will come through a higher spatial resolution (less than 0.2) and more accurate absolute measurements, rather than from sophisticated hydrodynamical calculations.     

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.     

Ultraviolet spectroscopy of the extended solar corona during the Spartan 201 mission

The instruments on the Spartan 201 spacecraft are an Ultraviolet Coronal Spectrometer and a White Light Coronagraph. Spartan 201 was deployed by the Space Shuttle on 11 April 1993 and observed the extended solar corona for about 40 hours. The Ultraviolet Coronal Spectrometer measured the intensity and spectral line profile of HI Ly and the intensities of OVI 103.2 and 103.7 nm. Observations were made at heliocentric heights between 1.39 and 3.5 R. Four coronal targets were observed, a helmet streamer at heliographic position angle 135°, the north and south polar coronal holes, and an active region above the west limb. Measurements of the HI Ly geocorona and the solar irradiance were also made. The instrument performed as expected. Straylight suppression, spectral focus, radiometric sensitivity and background levels all appear to be satisfactory. The uv observations are aimed at determining proton temperatures and outflow velocities of hydrogen, protons and oxygen ions. Preliminary results from the north polar coronal hole observations are discussed.     

Radio evidence of long lasting release of nonthermal energy in the solar corona

This paper presents a short summary of observations of microwaves giving evidence of the presence of sources of release of nonthermal energy in most active regions. The analysis is based on spectral-polarization observations carried out on the radio telescope RATAN-600 and describes three kinds of the nonthermal components of different physical nature.     

Interplanetary scintillation observations of the high-latitude solar wind

Until the ULYSSES spacecraft reached the polar regions of the solar wind, the only high-latitude measurements available were from indirect techniques. The most productive observations in regions of the solar wind between 5R and 200R have been the family of radio scattering techniques loosely referred to as Interplanetary Scintillation (IPS) (Coles, 1978). Useful observations can be obtained using a variety of radio sources, for example spacecraft beacons, planetary radar echoes and compact cosmic sources (quasars, active galactic nuclei, pulsars, galactic masers, etc.). However for measurement of the high-latitude solar wind cosmic sources provide the widest coverage and this review will be confined to such observations. IPS observations played a very important role in establishing that polar coronal holes (first observed in soft x-ray emission) were sources of fast solar wind streams which occasionally extend down to the equatorial region and are observed by spacecraft. Here I will review the IPS technique and show the variation of both the velocity and the turbulence level with latitude over the last solar cycle. I will also outline recent work and discuss comparisons that we hope to make between IPS and ULYSSES observations.     

Possible evidence for a rigidity-dependent release of relativistic protons from the solar corona

A model of the time evolving relativistic solar proton spectra for the 7 May 1978 ground level solar cosmic ray event is presented. This event, with associated cosmic ray neutron monitor increases of over 100% and containing relativistic particles with energies greater than 10 GeV/nucleon was characterized by an extreme anisotropy and a rapidly evolving spectrum, particularly during the initial phase. The observational data from cosmic ray neutron monitors viewing in the anti-Sun direction (180° away from the initial solar particle direction) indicates that a back scatter pulse of 4% of the primary pulse was observed at the Earth 20 min after the event onset. Previous attempts to model the solar particle spectrum found consistent and systematic differences between the theoretically calculated cosmic ray increase and the actual increase as observed by neutron monitors. In order to reconcile these differences, we have concluded that the observational data give evidence for a rigidity dependent release of relativistic solar protons from the solar corona during the very early stages of this event.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Ground-based solar radio observations of the August 1972 events

Ground-based observations of the variable solar radio emission ranging from few millimetres to decametres have been used here as a diagnostic tool to gain coherent phenomenological understanding of the great 2, 4 and 7 August, 1972 solar events in terms of dominant physical processes like generation and propagation of shock waves in the solar atmosphere, particle acceleration and trapping.The basic data used in this review have been collected by many workers throughout the world utilizing a variety of instruments such as fixed frequency radiometers, multi-element interferometers, dynamic spectrum analysers and polarimeters. Four major flares are selected for detailed analysis on the basis of their ability to produce energetic protons, shock waves, polar cap absorptions (PCA) and sudden commencement (SC) geomagnetic storms. A comparative study of their radio characteristics is made. Evidence is seen for the pulsations during microwave bursts by the mechanism similar to that proposed by McLean et al. (1971), to explain the pulsations in the metre wavelength continuum radiation. It is suggested that the multiple peaks observed in some microwave bursts may be attributable to individual flares occurring sequentially due to a single initiating flare. Attempts have been made to establish identification of Type II bursts with the interplanetary shock waves and SC geomagnetic storms. Furthermore, it is suggested that it is the mass behind the shock front which is the deciding factor for the detection of shock waves in the interplanetary space. It appears to us that more work is necessary in order to identify which of the three moving Type IV bursts (Wild and Smerd, 1972), namely, advancing shock front, expanding magnetic arch and ejected plasma blob serves as the piston-driver behind the interplanetary shocks. The existing criteria for proton flare prediction have been summarized and two new criteria have been proposed. Observational limitations of the current ground-based experimental techniques have been pointed out and a suggestion has been made to evolve appropriate observational facilities for solar work before the next Solar Maximum Year (SMY).     

High temporal resolution observations of electron heating at the bow shock

High temporal resolution measurements of solar wind electrons at the Earth's bow shock on the dawn side have been made with the LASL/MPI fast plasma experiments on ISEE-1 and 2. One dimensional, 1-d, temperatures, T _e , and densities, N _e , are obtained every 0.3 s and 2-d values are obtained every 3 s. Profiles of T _e and N _e at the shock usually are found to be similar to one another and also to the profile of the magnetic field magnitude. The time scale of electron thermalization varies from about 0.5 s to greater than 1 min, depending importantly on the shock motion and the orientation of the magnetic field. Typical thermalization times from 05:00–12:00 LT are 10 s, considerably shorter than proton thermalization times at the shock. This time scale corresponds to a distance of 100 km, comparable to but somewhat larger than the typical ion inertial length. The electron thermalization times are significantly longer than some of the values frequently cited in the past. At the end of the electron thermalization there typically is an overshoot in electron thermal pressure followed by an undershoot which give the pressure profile of the shock the appearance of a damped wave. Ion thermalization is essentially completed by the time the electron pressure wave is damped. The most probable value of the electron temperature ratio across the shock is 1.7, and this value is relatively independent of the Sun-Earth-satellite angle, _ss , for _ss between 25° and 100°.The Los Alamos Scientific Laboratory requests that the publisher identify this article as work performed under the auspices of the Department of Energy.By acceptance of this article, the publisher recognizes that the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes.     

Characteristics of shocks in the solar corona,as inferred from radio,optical, and theoretical investigations

Solar radio bursts of spectral type II provide one of the chief diagnostics for the propagation of shocks through the solar corona. Radio data on the shocks are compared with computer models for propagation of fast-mode MHD shocks through the solar corona. Data on coronal shocks and high-velocity ejecta from solar flares are then discussed in terms of a general model consisting of three main velocity regimes.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Satellite observations of type III solar radio bursts at low frequencies

Type III solar radio bursts have been observed from 10 MHz to 10 kHz by satellite experiments above the terrestrial plasmasphere. Solar radio emission in this frequency range results from excitation of the interplanetary plasma by energetic particles propagating outward along open field lines over distances from 5 R to at least 1 AU from the Sun. This review summarizes the morphology, characteristics and analysis of individual as well as storms of bursts. Substantial evidence is available to show that the radio emission is observed at the second harmonic instead of the fundamental of the plasma frequency. This brings the density scale derived by radio observations into better agreement with direct solar wind density measurements at 1 AU and relaxes the requirement for type III propagation along large density-enhanced regions. This density scale with the measured direction of arrival of the radio burst allows the trajectory of the exciter path to be determined from 10 R to 1 AU. Thus, for example, the dynamics and gross structure of the interplanetary magnetic field can be investigated by this method. Burst rise times are interpreted in terms of exciter length and dispersion while decay times refer to the radiation damping process. The combination of radio observations at the lower frequencies and in-situ measurements on non-relativistic electrons at 1 AU provide data on the energy range and efficiency of the wave-particle interactions responsible for the radio emission.     

Automatic stereo processing of high resolution radar imagery

Previously reported experiments with stereoscopic viewing of high resolution radar pictures demonstrated improvements in detection sensitivity up to 7 dB. This paper presents continued research into automatic algorithmic techniques for employing stereo information. A simple two-stage model for a human observer is proposed, and here we investigate the utility of an existing automatic stereo processor technique in application to radar imagery. It is shown that results obtained with the automatic processor very closely resemble those obtained through actual stereo viewing of radar pictures.     

Pioneer and Voyager observations of large-scale spatial and temporal variations in the solar wind

The Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were launched in 1972, 1974, and 1977, respectively. While these three spacecraft are all at compartively low heliographic latitudes compared with Ulysses, their observation span almost two solar cycles, a range of heliocentric distances from 1 to 57 AU, and provide a unique insight into the long-term variability of the global structure of the solar wind. We examine the spatial and temporal variation of average solar wind parameters and fluxes. Our obsevations suggest that the global structure of the outer heliosphere during the declining phase of the solar cycle at heliographic latitudes up to 17.5°N was charaterized by two competing phenomena: 1) a large-scale increase of solar wind density, temperature, mass flux, dynamic pressure, kinetic energy flux, and thermal enery flux with heliographic latitude, similar to the large-scale latitudinal gradient of velocity seen in IPS observations, 2) a small-scale decrease in velocity and temperature, and increase in density near the heliospheric current sheet, which is associated with a band of low speed, low temperature, and high density solar wind similar to that observed in the inner heliosphere.