R. Pallavicini

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.

Thermal models of flaring region based on observations by the SOLRAD 10 satellite

A number of solar X-ray bursts, observed with the SOLRAD 10 satellite on 1971 September 14, 17, November 28 and 1972 August 1, have been analysed. From the differential emission measure distribution deduced from the observations the temporal and spatial distribution in a flaring region has been evaluated and compared with models of instantaneous or continuous energy injection and conductive redistribution. Assuming a power law for the differential emission measure in dependence of electron temperature the ratios of the counting rates observed in different spectral bands have been compared with theoretical predictions so as to test the validity of this particular class of models.

The Differential Emission Measure of λ And

Simultaneous X-ray and UV observations of λ And from EXOSAT and IUE are analyzed and the differential emission measure as a function of temperature is derived.