Simulated observations of the electron coronal density irregularity\overline {n^2 } /(\bar n)^2

A technique to derive the coronal density irregularity factor , wheren is the electron density, has been proposed by Fineschi and Romoli (1993). This technique will exploit the unique UVCS capability of cotemporal and cospatial measurements of both UV line radiation and K-coronal polarized brightness,pB.The ratio of the measured H I Lyman (Ly-) line intensity to the resonant-scattering dominated H I Lyman (Ly-) intensity can be used to extract the collisional component of the Ly-. This component yields an estimate of . The quantity is then obtained from the UVCS white-light K-coronal measurements.We present simulated observations of the UVCS for coronal atmosphere models with different filling factors and electron density profiles, and for different coronal structures (e.g., coronal holes, streamers). These simulations will show how the proposed technique may be used to probe inhomogeneities of the solar corona.     

Gamma-ray line production in astronomical objects

This review focuses on the conditions for -ray line production in the most interesting astronomical objects, in light of the planned experiments: Gamma-1, GRO, Sigma, GRASP, and others. Among these objects are the Sun, the galactic center region, molecular and dust clouds, accreting and exploding stars.     

Probing the solar wind acceleration region using spectroscopic techniques

Measurements of the intensities and profiles of UV and EUV spectral lines can provide a powerful tool for probing the physical conditions in the solar corona out to 8 R and beyond. We discuss here how measurements of spectral line radiation in conjunction with measurements of the white light K-corona can provide information on electron, proton and ion temperatures and velocity distribution functions; densities; chemical abundances and mass flow velocities. Because of the fundamental importance of such information, we provide a comprehensive review of the formation of coronal resonance line radiation, with particular emphasis on the H i L line, and discuss observational considerations such as requirements for rejection of stray light and effects of emission from the geocorona and interplanetary dust. Finally, we summarize some results of coronal H i L and white light observations acquired on sounding rocket flights.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

On the derivation of empirical limits on the helium abundance in coronal holes below 1.5R s

We present a simple technique describing how limits on the helium abundance, , the ratio of helium to proton number density, can be inferred from measurements of the electron density, temperature and their gradients below 1.5R _s. As an illustration, we apply this technique to emission line intensities in the extreme ultraviolet, measured in polar coronal holes. The example indicates that can be significantly large in the inner corona. This technique could be applicable to the more extensive data to be obtained from coordinated ground and space-based observations during the Ulysses south polar passage and the Spartan flight, and subsequently during the SOHO mission. Limits on the helium abundance in the solar wind can thus be derived from its source region and compared to interplanetary values.     

Spartan 201 coronal spectroscopy during the polar passes of ulysses

Spartan 201 is a shuttle deployed spacecraft that is scheduled to perform ultraviolet spectroscopy and white light polarimetry of the extended solar corona during two 40 hour missions to occur in September 1994 and August 1995. The spectroscopy is done with an ultraviolet coronal spectrometer which measures the intensity and spectral line profile of HI Ly up to heliocentric heights of 3.5 solar radii. It also measures the intensities of the OVI doublet at 1032 and 1037 Å and of Fe XII at 1242 Å. The HI Ly line profile measurements are used to determine the random velocity distribution of coronal protons along the line-of-sight. The absolute HI Ly intensities can be used together with electron densities from the white light coronagraph to estimate electron temperatures from hydrogen ionization balance calculations, and bulk outflow velocities from models of Doppler dimmed resonant scattering. Intensities of minor ion lines are used to determine coronal abundances and outflow velocities of O⁵⁺. Ultraviolet spectroscopy of extended coronal regions from the 11 April 1993 mission of Spartan 201 are discussed.     

Acceleration of the solar wind

In this review, we discuss critically recent research on the acceleration of the solar wind, giving emphasis to high-speed solar wind streams emanating from solar coronal holes. We first explain why thermally driven wind models constrained by solar and interplanetary observations encounter substantial difficulties in explaining high speed streams. Then, through a general discussion of energy addition to the solar wind above the coronal base, we indicate a possible resolution of these difficulties. Finally, we consider the question of what role MHD waves might play in transporting energy through the solar atmosphere and depositing it in the solar wind, and we conclude by examining, in a simple way, the specific mechanism of solar wind acceleration by Alfvén waves and the related problem of accelerating massive stellar winds with Alfvén waves.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.On leave from the Auroral Observatory, Institute of Mathematical and Physical Sciences, University of Tromsø, N-9001 Tromsø, Norway.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

An optical outburst from the periodic recurrent X-ray transient A0538-66

We present optical spectroscopy and photometry and IUE spectroscopy of the counterpart of the LMC recurrent X-ray transient A0538-66 during an outburst at the end of December 1980 which was consistent with the 16.6 day X-ray period (Skinner, 1980). The optical spectra show steadily increasing Balmer and HeI emission (indicative of a shell phase) superposed on a B2 IV spectrum with a substantial brightness increase of 2^m and the sharp turn-on of HeII 686 at the peak. Significant radial velocity changes have been detected but they show no correlation with the 16.6 day period. IUE spectra during a subsequent outburst show very strong and broad (5000 km s^–1) emission from C IV 1550 and HeII 1640. This behaviour is compared with other galactic transients and shell/Be stars.     

A solar extreme ultraviolet telescope and spectrograph for Shuttle/Spacelab

An instrument for advanced studies of the solar corona is described. Its optical system provides nearly stigmatic imaging of selected portions of the Sun over the spectral range from 22.5 to 44.0 nm. Both spectroheliograms and emission line profiles of coronal features will be obtained over a wide range of coronal temperatures.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.This paper was presented at the conference by U. Feldman.     

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.     

Concerning the interpretation of the ultraviolet radiation from B stars

Measurements of the shape of the ultraviolet spectrum from B stars are compared with the theoretical spectra predicted from a homogeneous series of eight model atmospheres which are known to be close to a state of radiative equilibrium and to give a good representation of the ordinarily observed spectral region. The broad-band photometer measurements of Byram, Chubb, and Friedman in the region 1314 indicate that the stars become brighter in the ultraviolet as their temperature increases. The theoretical spectra reproduce this trend. However, the theoretical spectra are about three times as bright at 1314 relative to their brightness at 5560 as is observed.The spectral observations at 50Å resolution of Stecher and Milligan of six absorption-line stars are compared in detail with theoretical spectra. The observed shape of the spectrum is reproduced well by the models from 2600 to longer wavelengths. At wavelengths shorter than 2600 Å, the observed fluxes from B stars are less than the predicted fluxes. At 2000 the deficiency is between a factor two and a factor four. The spectrum of Canis Majoris is observed to have a different shape from that found for four other early-type stars. In the case of Canis Majoris the deficiency at 2000 is about a factor 13.The proper manner in which to compare theory and observation is discussed and some astrophysical terminology is explained. Theoretical fluxes, , are given in Table 1 for eight early B type model atmospheres at wavelengths between the Lyman limit and 6251. These fluxes have been computed without consideration of the opacity due to line blanketing. It is shown that line blanketing can probably account for the differences noted between predicted and observed ultra-violet spectra of B stars. It is not necessary at present to invoke unusual sources of opacity in the stellar atmosphere or in the space between the star and the earth in order to explain the observations. Spectra of B stars in the 2000 region at sufficient resolution to show the line spectrum would clarify the problem.     

Coronal transient phenomena

Solar coronal transients, particularly those caused by flares and eruptive prominences, play a major role in the fields of solar-terrestrial physics and astrophysics. In the former field, coronal transients and their associated interplanetary disturbances are responsible for solar and galactic cosmic ray modulations, as well as planetary magnetospheric and ionospheric disturbances. In the latter field, supernovae remnants are scaled-up manifestations of such disturbances; that is they are stellar, rather than solar, coronal transients. Study of the more accessible solar transients is proving invaluable in both fields and is, therefore, selected for attention in this paper.A series of coronal transient observations is discussed in the spirit of a representative overview following some introductory remarks on the background solar wind. One of these observations is chosen because its interplanetary signature-the shock wave-was detected by two spacecraft at different heliocentric radii. Other cases are chosen because of the extended observations of embedded eruptive prominences. Progress is also being made in the interdisciplinary areas of optical imagery complemented with radio astronomical techniques.Finally, several recent theoretical models and MHD computer simulation studies are summarized. It is suggested that further comparison of specific events with such models promises a rich harvest of physical understanding of the origin, structure and interplanetary progeny of coronal transients.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Magnetic fields,plasmas, and coronal holes: The inner solar system

This paper reviews the recent results concerning streams and magnetic fields in the inner solar system. Specifically, it discusses in situ magnetic field and plasma observations within 1 AU which describe MHD stream flows and Alfvénic fluctuations, and it discusses the latest theories of those phenomena. Observationally, there have been significant advances in our understanding of streams and fluctuations as the result of acquiring nearly complete sets of high resolution plasma and magnetic data simultaneously at two or more points by IMPs 6, 7, and 8, Mariner-Venus-Mercury, HELIOS-1, and HELIOS-2. HELIOS and IMP observations and coronal hole observations demonstrated that streams can have very thin boundaries in latitude and longitude near the Sun. This has necessitated a revision of earlier views of stream dynamics, for it is now clear that magnetic pressure is a major factor in the dynamics of stream in the inner solar system and that nonlinear phenomena are significant much closer to the Sun than previously believed. Simultaneous IMP 6, 7, and 8 observations of Alfvénic fluctuations have shown that they are probably not simply transverse Alfvén waves; they suggest that Alfvénic fluctuations are better described as nonplanar, large-amplitude, general Alfvén waves moving through an inhomogeneous and discontinuous medium, and coupled to a compressive mode.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Fourier parameters of heliospheric current sheet and their significance

If the path of the neutral line on the coronal source surface is expressible as a singlevalued function (colatitude vs longitude ), then Fourier analysis of ctn with respect to leads to a simple algorithm for realistically mapping the neutral line outward to model the heliospheric current sheet (HCS) at distancesr1 AU. To be compatible with MHD, the source surface used for this mapping should be prolate (aligned with dipole axis) rather than spherical. Orientation of the Sun's magnetic-dipole moment is indicated by them=1 Fourier amplitude (a ₁ sin +b ₁ cos ) of ctn on the source surface. Physical features (including the neutral line) on a prolate source surface intrinsically map to lower dipole latitudes atr1 AU in the heliosphere, and Ulysses observations of a unipolar field at latitudes beyond 30°S (when the neutral line on the source surface still reached 39°S) confirm the expected geometry.     

Gamma Rays From Molecular Clouds

High energy -rays from individual giant molecular clouds contain unique information about the hidden sites of acceleration of galactic cosmic rays, and provide a feasible method for study of propagation of cosmic rays in the galactic disk on scales 100 pc. I discuss the spectral features of ⁰-decay -radiation from clouds/targets located in proximity of relatively young proton accelerators, and speculate that such `accelerator+target systems in our Galaxy can be responsible for a subset of unidentified EGRET sources. Also, I argue that the recent observations of high energy -rays from the Orion complex contain evidence that the level of the `sea of galactic cosmic rays may differ significantly from the flux and the spectrum of local (directly detected) particles.     

NGC 4151 and MCG 8-11-11: Two X-ray seyfert galaxies with strong soft γ-ray emission

During a balloon flight of the MISO telescope on the 30th September 1979, the Seyfert galaxies NGC 4151 and MGC 8-11-11 were studied in the hard X-ray range (E_X > 20 keV) and low-energy -ray range up to 19 MeV. An emission at the 4.5 level above 20 keV (4 above 260 keV) was detected in the direction of NGC 4151. -ray emission at the 3.9 level above 90 keV was also observed from the direction of MCG 8-11-11. The emission photon spectrum shows a high-energy cutoff at about 3 MeV. A large amount of the observed low-energy -ray diffuse background could be produced by a few percent of the X-ray emitting Seyfert galaxies having a -ray luminosity comparable to that observed from the regions of NGC 4151 or MCG 8-11-11.     

The solar flare plasma: Observation and interpretation

In the past several years, X-ray observations of the Sun made from rockets and satellites have demonstrated the existence of high temperature (20 × 10⁶ – 100 × 10⁶ K), low density plasmas associated with solar flare phenomena. In the hard X-ray range ( < 1 ), spectra of the flaring plasma have been obtained using proportional and scintillation counter detectors. It is possible from these data to determine the evolution of the hard X-ray flare spectrum as the burst progresses; and by assuming either a non-thermal or thermal (Maxwellian) electron distribution function, characteristic plasma parameters such as emission measure and temperature (for a thermal interpretation) can be determined. Thermal interpretations of hard X-ray data require temperatures of 100 × 10⁶ K.In contrast, the soft X-ray flare spectrum (1 <<30 ) exhibits line emission from hydrogen-like and helium-like ions, e.g. Ne, Mg, Al, Si,... Fe, that indicates electron energies more characteristic of temperatures of 20 × 10⁶ K. Furthermore, line intensity ratios obtained during the course of an event show that the flare plasma can only be described satisfactorily by assuming a source composed of several different temperature regions; and that the emission measures and temperatures of these regions appear to change as the flare evolves. Temperatures are determined from line ratios of hydrogen-like to helium-like ions for a number of different elements, e.g., S, Si, and Mg, and from the slope of the X-ray continuum which is assumed to be due to free-free and free-bound emission. There is no obvious indication in soft X-ray flare spectra of non-thermal processes, although accurate continuum measurements are difficult with the data obtained to date because of higher order diffraction effects due to the use of crystal spectrometers.Soft X-ray flare spectra also show satellite lines of the hydrogen-like and helium-like ions, notably the 1s ²2s ² S-1s2s2p ² P transition of the lithium-like ion, and support the contention that in low density plasmas these lines are formed by dielectronic recombination to the helium-like ion. Also, series of allowed transitions of hydrogen-like and helium-like ions are strong, e.g., the Lyman series of S up to Lyman-, and ratios of the higher member lines to the Lyman- line can be compared with theoretical calculations of the relative line strengths obtained by assuming various processes of line formation.This review will discuss the X-ray spectrum of solar flares from 250 keV to 0.4 keV, but will be primarily concerned with the soft X-ray spectrum and the interpretation of emission lines and continuum features that lie in this spectral range.     

Three-dimensional energetic ion observations and the tangential magnetopause electric field

Three-dimensional distributions for 24.0–44.5 keV protons (ions) are presented from the ISEE-1 medium energy particles instrument during a magnetopause traversal at 01:10 UT on 20 November 1977. Local time of the traversal was 1030. Ion fluxes were observed coming generally from the subsolar region, but over a wide range of latitudes. Enhanced fluxes were observed at the magnetopause crossing with strong components from the subsolar region and from the +Z _SE direction. These observations are compared with the simultaneous electric field observations presented by Mozer et al. (1978). Ion streaming in a direction consistent with the Y-component of the drift velocity was observed whereas streaming along the X and Z-components is not seen. Based on energy arguments we conclude that in this case, 24 keV ions are not the major energy carrier of the locally measured · dissipation.     

Magnetic fields in the ionosphere of Venus

This review surveys the observations of the ionospheric magnetic fields of Venus as observed on the Pioneer Venus Orbiter and the models that have been developed to describe them over the last decade. The models for the large-scale ionospheric field have developed to the advanced stage of one-dimensional, self-consistent, multi-fluid MHD models which provide a detailed picture of the field in the subsolar region for specific upper boundary conditions. In contrast, the models for the small-scale fields and the nightside fields have only reached a rudimentary stage. Much challenging work remains to be done on the origin of the ionospheric flux ropes and nightside ionospheric hole fields. On the whole, the subject of the ionospheric fields would greatly benefit from 3-dimensional global MHD models with self-consistent treatments of the ionosphere.     

Quiet sun

We underline the diagnostic strength of recent observations of the oscillating quiet Sun. While high quality (k, ) power spectra permit a better knowledge of the convection zone, long and continuous survey of oscillations of the integrated Sun provides an efficient sounding of the inner solar body.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Plasma waves at the dayside magnetopause

Experimental investigations of plasma waves at the magnetopause, including recent results from the AMPTE/IRM satellite, show that both E and B fluctuations typically have a featureless spectrum which monotonically decreases with frequency; integrated rms amplitudes are typically a few mV m^-1 for E and 10 nT for B, though in particular E can be as much as an order of magnitude larger in exceptional cases. Surveys show a lack of correlation between wave parameters and the magnetopause parameters. Under the assumption that crossing the diffusion region would give a pronounced signature in the waves, the survey data allow an upper limit to be placed on the latitudinal extent of the diffusion region, which is about 1000 km — implying that it is not surprising that the wave data surveys have so far failed to detect it. The observed wave turbulence levels have been used to estimate diffusion coefficients under different assumptions for the wave mode, but the resulting diffusion coefficient is always too small to explain either reconnection or boundary layer formation. Recent work of Galeev et al. (1986) indicates that the dominant diffusion process may be magnetic field migration, which is a macroscopic process involving the interaction of tearing mode islands. Assuming this mode to be present at the observed level of B, a particle diffusion coefficient of nearly 10⁹ m² s^-1 is obtained. Another macroscopic diffusive process which could occur at the magnetopause is stochastic E × B scattering, which also implies a diffusion coefficient the order of 10⁹ m² s^-1 if the observed E spectrum is assumed to be a turbulent cascade consisting of convective cells.