Giovanni Peres

The Sun as an X-Ray Star. II. Using the Yohkoh/Soft X-Ray Telescope-derived Solar Emission Measure versus Temperature to Interpret Stellar X-Ray Observations

This paper is the second of a project dedicated to using solar Yohkoh/SXT data as a guide and a template to interpret data on stellar coronae. In the light of the large diUerences in scope and approach between solar and stellar studies, we have developed a method to translate Yohkoh/SXT data of the whole solar corona into stellar-like data, i.e., to put them in the same format and context as the stellar ones. First from the Yohkoh/SXT images we derive the whole-Sun X-ray emission measure versus tem- perature (EM(T )), in the range 105.5¨108 K, during the speci—c observation. Then, we synthesize the solar X-ray spectrum; —nally, we fold the spectrum through the instrumental response of nonsolar X-ray observatories, for instance, ROSAT /PSPC and ASCA/SIS. Finally, we analyze such solar coronal data in the same band and with the same methods used for stellar observations, allowing a direct and homoge- neous comparison with them. In this paper we present in detail our method and, as an example of results, we show and discuss EM(T ) and stellar-like spectra for three phases of the solar cycle: maximum, intermediate phase, and minimum. The total amount and the distribution of the emission measure change dramatically during the cycle, in particular at temperatures above 106 K. We also show the EM(T ) of the whole solar corona during a large —are. The ROSAT /PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star that we obtain are discussed in the context of stellar coronal physics. The Suns coronal total luminosity in the ROSAT /PSPC band ranges from B2.7 ) 1026 ergs s~1 (at minimum) to B4.7 ) 1027 ergs s~1 (at maximum). We discuss future developments and possible applica- tions of our method. Subject headings: Sun: coronaSun: X-rays, gamma rays

Coronal closed structures. IV - Hydrodynamical stability and response to heating perturbations

The response of magnetically confined atmospheres to perturbations in the temperature and density distribution, and the local heating rate by means of a one-dimensional time-dependent hydrodynamical code, which incorporates the full energy, momentum and mass conservation equations is studied. These studies extend the linear instability analysis of Habbal and Rosner (1979) into the finite-amplitude regime, and generalize the confined atmosphere models of Serio et al., to the time-dependent domain. The results show that closed coronal atmospheres are stable against finite-amplitude perturbations if the chromospheric response is taken into account; and observed correlated increases in coronal density and temperature can only be achieved under quiescent conditions by increasing the heat deposition rate relatively more in the chromosphere than in the corona.

Crushing of interstellar gas clouds in supernova remnants - I. The role of thermal conduction and radiative losses

We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M = 30 and 50, corresponding to a post-shock temperature T 1.7 x 10 6 K and 4.7 x 10 6 K, respectively), we explore two very different physical regimes: for M = 30. the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M = 50, the thermal conduction dominates the evolution of the shocked cloud. which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries.

Temperature and Density Structure of Hot and Cool Loops Derived from the Analysis of TRACE Data

We study the transversal structure (in particular the filamentation) and the longitudinal plasma stratifica- tion in two sets of solar coronal loops observed with TRACE in the 171 and 195 Apassbands. The density stratification and the thermal structuring of the plasma along the fibrils that form the loops are derived using two techniques simultaneously: (1) a filter ratio diagnostic (195/171 A ˚ ) and (2) modeling intensity profiles along the fibrils with hydrostatic models; in both techniques we remove the background flux. We find consistent values of temperature and density with both methods in the coronal structures selected. We find evidence of a very warm fibril (T � 5 � 10 6 K) and of rather cold ones (T � 2 � 10 5 K). The hot fibril appears to have a thermal structure with a maximum at the apex compatible with temperature and density stratification, dictated by energy balance, typical of a nonisothermal hydrostatic loop. The cold fibrils appear to be isothermal and are probably in dynamical conditions. Subject headings: Sun: corona — Sun: UV radiation On-line material: color figures

On the origin of asymmetries in bilateral supernova remnants

Aims. We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field. Methods. We perform 3D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons. Results. We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations. Conclusions. BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs.

THE SUN AS AN X-RAY STAR. III. FLARES

In previous works we have developed a method to convert solar X-ray data, collected with the Yohkoh/SXT, into templates of stellar coronal observations. Here we apply the method to several solar flares, for comparison with stellar X-ray flares. Eight flares, from weak (GOES class C5.8) to very intense ones (X9) are selected as representative of the flaring Sun. The emission measure distribution versus temperature, EM(T), of the flaring regions is derived from Yohkoh/SXT observations in the rise, peak, and decay of the flares. The EM(T) is rather peaked and centered around T ≈ 107 K for most of the time. Typically, it grows during the rise phase of the flare, and then it decreases and shifts toward lower temperatures during the decay, more slowly if there is sustained heating. The most intense flare we studied shows emission measure even at very high temperatures (T ≈ 108 K). Time-resolved X-ray spectra both unfiltered and filtered through the instrumental responses of the nonsolar instruments ASCA/SIS and ROSAT/PSPC are then derived. Synthesized ASCA/SIS and ROSAT/PSPC spectra are generally well fitted with single thermal components at temperatures close to that of the EM(T) maximum, albeit two thermal components are needed to fit some flare decays. ROSAT/PSPC spectra show that solar flares are in a 2 orders of magnitude flux range (106-108 ergs cm-2 s-1) and a narrow PSPC hardness ratio range, however, higher than that of typical nonflaring solar-like stars.

1997 December 12 Helical Coronal Mass Ejection. II. Density, Energy Estimates, and Hydrodynamics

We use Ultraviolet Coronagraph and Spectrometer (UVCS) spectra to investigate the density range of the plasma ejected during the coronal mass ejection (CME) on 1997 December 12. Time-dependent ionization states for several phenomenological models, with the boundary conditions derived from the EUV Imaging Telescope (EIT) and UVCS observations, were computed and constraints on the density and temperature of the plasma at the early stage of the ejection are obtained. The role of physical mechanisms such as thermal conduction, radiation, and heating is also studied with a two-dimensional hydrodynamics simulation. The kinetic, thermal, and gravitational energies are estimated as well as the plasma heating. Whenever the ejected plasma has a density ≥109cm-3, a continuous supply of heat is required to meet the conditions observed at 1.7 R☉. Moreover heating mechanisms that release energy gradually during the outward motion of the plasma seem to be more appropriate than those that dump most of the energy when the plasma is lower in the corona. Our simulations also indicate that a three-dimensional self-similar expansion does not fit the UVCS observations. Comparisons with some CME models from the dynamical and energetics points of view are discussed.

Loop models of low coronal structures observed by the Normal Incidence X-Ray Telescope (NIXT)

The X-ray pictures obtained with the Normal Incidence X-Ray Telescope (NIXT), apart from the ubiquitous coronal loops well known from previous X-ray observations, show a new and peculiar morphology: in many active regions there are wide and apparently low-lying areas of intense emission which resemble H alpha plages. By means of hydrostatic models of coronal arches, we analyze the distribution of temperature, density, emission measure, and plasma emissivity in the spectral band to which NIXT is sensitive, and we show that the above morphology can be explained by the characteristics of high pressure loops having a thin region of high surface brightness at the base. We therefore propose that this finding might help to identify high-pressure X-ray emitting coronal regions in NIXT images, and it is in principle applicable to any imaging instrument which has high sensitivity to 10(exp 4) - 10(exp 6) K plasma within a narrow coronal-temperature passband. As a more general result of this study, we propose that the comparison of NIXT observations with models of stationary loops might provide a new diagnostic: the determination of the loop plasma pressure from measurements of brightness distribution along the loop.

Hydrodynamic modeling of an X-ray flare on Proxima Centauri observed by the Einstein telescope

Hydrodynamic numerical calculations of a flare which occurred on Proxima Centauri and was observed by the Einstein satellite on August 20, 1980 at 12:50 UT are presented. The highlights of the hydrodynamic code are reviewed, and the physical and geometrical parameters necessary for the calculations are derived and compared with observations. The results are consistent with the stellar flare being caused by the rapid dissipation of 5.9 x 10 to the 31st ergs, within a magnetic loop structure whose semilength is 7 x 10 to the 9th cm and cross-sectional radius is 7.3 x 10 to the 8th cm. The results provide evidence that flares on late-type stars can be described by a hydrodynamic model with a relatively simple geometry, similar to solar compact flares. 39 references.

Simulations of the Ca XIX spectral emission from a flaring solar coronal loop. I. Thermal case

Spectral profiles synthesized from the results of numerical simulations of solar coronal loop plasma are compared with the observed line profiles, and the sensitivity of this comparison to variations in the parameter values is tested. The comparison allows the procedures commonly adopted for obtaining temperatures and velocities directly from the bent crystal spectrometer observations, as well as the interpretation of parameters obtained from hydrodynamical calculations, to be independently validated. The characteristic spectral signatures of different models for the deposition of impulsive thermal energy in the loop are derived assuming two distinct spatial distributions of thermal heating: in one case, the heating function peaks near the apex of the loop, while in the second, energy is deposited directly in the lower atmosphere. 37 references.