B. C. Monsignori Fossi

Spectroscopic diagnostics in the VUV for solar and stellar plasmas

SummaryThe VUV emission spectra from the solar atmosphere and stellar atmospheres have been intensively studied during the past 25 years with several major space programs. In this review we discuss the spectroscopic diagnostic techniques used to study astrophysical plasmas, the atomic processes involved, the recent observations and the plans for future space missions.

The Xray-Euv Spectrum of Optically Thin Plasmas

The new observations of the Extreme Ultraviolet Explorer Spectrometers on EUVE and the future high resolution observation by Coronal Diagnostic Spectrometer (CDS) and SUMER on SOHO have suggested the revision of the Xray-EUV spectral code of the authors (1990) for optically thin plasmas. More accurate atomic data computations are now available and several lines have been added. Work is in progress to update the Xray-EUV code following the suggestions of the reviewers of the workshop on “Atomic data assessment for SOHO” held in Abingdon (March 1992). Special care is given to the highly ionized iron lines in the EUV spectral region.

Models for inner corona parameters

Abstract The structure of the inner corona are the main sources of the XUV line radiation from the sun. The line intensities are determined by the temperature and density of the plasma via the differential emission measure (DEM) distribution. The DEM contains a lot of information concerning the physics of the coronal features; the methods to evaluate differential emission measure distribution from the observational data are reviewed and discussed.

The coronal structure of active regions

A four-parameter model which assumes a Gaussian dependence of both temperature and pressure on distance from center is used to fit the compact part of coronal active regions as observed in X-ray photographs from a rocket experiment. The four parameters are the maximum temperature TM, the maximum pressure PM= 2NMkTM, the width of the pressure distribution σP, and the width of the temperature distribution σT = α1/2σP. The maximum temperature TM ranges from 2.2 to 2.8 × 106K, and the maximum density NM from 2 to 9 × 109cm−3. The range of σP is from 2 to 4 × 109 cm and that of α from 2 to 7.

A simple formula for the total dielectronic recombination coefficient

Two simple relationships for the total dielectronic recombination coefficient are developed. The first is for isoelectronic sequences of H, He, Ne, K-Ni and the second for Li-F, Na-A and Cu-Kr. Comparison with the extended computations of Jordan and Elwert is made.

Abundance determination in solar active region

Abstract An improved method has been developed to evaluate element abundances from emission line intensities of thin plasmas, depending on the differential emission measure (DEM) of the source. Observations made by the SERTS Rocket EUV Spectrograph are used to perform a detailed analysis of the DEM distribution for temperatures larger than 10 5 K in a solar active region. Comparison of the DEM distributions obtained by means of lines from different elements allows the verification of relative abundances for the most common elements of the solar corona, and gives an abundance estimates for the minor components, such as Na, Al, Ar, Cr, Mn and Zn.

Thermal and non-thermal soft X-ray bursts

X-ray bursts observed for energies lower than 25 keV are usually interpreted as being produced by a thermal plasma with several million degrees of temperature.A small number of events recorded at Arcetri by real time telemetry of SOLRAD 9 satellite agrees with a thermal interpretation and gives temperatures ranging between 10 × 106 and 30 × 106K and emission measures, ∫Ne2dV, between 1047 and 1048 cm−3.An impulsive event recorded on January 7, 1969 shows an anomalous behaviour. In this case the emission has been attributed to bremsstrahlung radiation from electrons with a power law energy distribution dN = KE-γ dE. The values of the spectral index and of the emission measure are given.A tentative interpretation of the event is suggested and the way to produce non-relativistic electrons with a power law energy distribution is investigated.

Non-thermal ionization and recombination processes during solar flares

Ionization and recombination processes are studied for a plasma of which the electrons follow a power-law energy distribution.The rates for collisional ionization, radiative and dielectronic recombination and for autoionization are evaluated.Numerical computations are performed for H-like, He-like and Li-like ions from neon to nickel as a function of the spectral index of the electron distribution. The ionization equilibrium is evaluated as well as the ratios of fluxes emitted in two lines pertaining to two successive ionization stages of the same element. A comparison with a few experimental data is made and the possibility of a non-thermal interpretation of X-ray line emission during solar flares is discussed.

Soft x-ray emitting regions in the solar corona

The soft X-ray emission of the solar corona is investigated by comparison of the signals of several broad band photometers carried on the Solrad 9 satellite, and sensitive to the region 0.5–20 Å. Temperature from 1.5 × 106 to 25 × 106 K have been measured with ‘emission measure’ ∫ Ne2 dV ranging between 1050 cm−3 to 1047 cm−3.By means of the observational data and assuming magnetic confinement and hydrostatic equilibrium, the model of an active region is investigated. For temperatures larger than 107K the emission is due to flare activity and two sets of emission measure are observed which appear to be related to the evolution of flares.

X-Ray and EUV Emission from Optically Thin Plasma

The computation of the emission of an optically thin and low density plasma is performed at 104 < T < 108 K between 1 and 2000 A. The computation is applied to explain the far EUV observations of Feldman et al. (1981) of interstellar medium emission towards the North galactic pole and the solar quiet region observations of Vernazza and Reeves (1978) in the spectral region from 300 to 1350 A.