Analysis of the intensities and profiles of the spectral line Mg xii 8.42 Å in the solar X-ray spectrum

High resolution profiles of the Mg xii 8.42 Å line in the solar X-ray spectrum were recorded from the Intercosmos 7 satellite. The Mg xii line intensity provides a sensitive indicator of the hot plasma content (T ? 3 × 10⁶ K) in coronal condensations and X-ray flare volumes. The ratio of the line intensity to the intensity of the adjacent continuum has been used to compute approximate thermal models of the emitting regions. For all the investigated coronal condensations the temperature distribution of plasma has been found to be a function monotonically decreasing with temperature. But for some X-ray bursts there occurred a distinct excess of the hot plasma of temperature between 6–10 × 10⁶K. FWHM values of the Mg xii line profiles have been used to estimate ion temperature in the emitting regions.     

Investigation of Quasi-periodic Variations in Hard X-rays of Solar Flares

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for the investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by the good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap, particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong pulses. The advantage of our model of oscillating magnetic traps is that it can explain both the pulses of electron acceleration and quasi-periodicity of their distribution in time.     

Investigation of Quasi-periodic Variations in Hard X-Rays of Solar Flares. II. Further Investigation of Oscillating Magnetic Traps

In our recent paper (Jakimiec and Tomczak, Solar Physics 261, 233, 2010) we investigated quasi-periodic oscillations of hard X-rays during the impulsive phase of solar flares. We have come to the conclusion that they are caused by magnetosonic oscillations of magnetic traps within the volume of hard-X-ray (HXR) loop-top sources. In the present paper we investigate four flares that show clear quasi-periodic sequences of the HXR pulses. We also describe our phenomenological model of oscillating magnetic traps to show that it can explain the observed properties of the HXR oscillations. The main results are the following: i) Low-amplitude quasi-periodic oscillations occur before the impulsive phase of some flares. ii) The quasi-periodicity of the oscillations can change in some flares. We interpret this as being due to changes of the length of oscillating magnetic traps. iii) During the impulsive phase a significant part of the energy of accelerated (non-thermal) electrons is deposited within a HXR loop-top source. iv) The quick development of the impulsive phase is due to feedback between the pressure pulses by accelerated electrons and the amplitude of the magnetic-trap oscillation. v) The electron number density and magnetic field strength values obtained for the HXR loop-top sources in several flares fall within the limits of N≈(2 – 15)×10¹⁰ cm⁻³, B≈(45 – 130) gauss. These results show that the HXR quasi-periodic oscillations contain important information about the energy release in solar flares.     

Cosmic-ray intensity versus solar soft X-ray background in cross-correlation analysis

The relationship between the galactic cosmic ray modulation (CR) and the non-flare coronal level, as given by the solar soft X-ray background (XBG), is investigated from 1 July 1968 to 30 June 1980 on a daily basis. The stationarity problem of a multivariate time series, as well as the role of the short- and medium-term corona variability are faced. From them it is found that the CR/XBG relation is variable during the considered heliomagnetic semicycle, while CR and XBG are highly anticorrelated on a long-time scale (12-month averages). The CR/XBG relationship during the declining phase of solar activity shows a moderately strong anticorrelation, on short- and medium-term time scales (coefficient up to –0.77 for 27-day running averages), went towards insignificant values in the minimum phase and is only partially reconstructed during the rise of the following solar cycle. During the solar activity maximum of cycle 20 the cosmic-ray modulation is only related to the short-term coronal fluctuations (no other time scales are significant, supporting the reliability of the so-called `Gnevyshev gap' in solar parameters).     

Comparison of the microwave and soft X-ray emission above a sunspot

The Westerbork Synthesis Radio Telescope (WSRT) 6 cm radio observations of the active region HL 16864 large spot (Strong, Alissandrakis, and Kundu, 1984) are compared with X-ray data obtained from the Flat Crystal Spectrometer (FCS) onboard the Solar Maximum Mission satellite on May 25, 1980. The X-ray data confirm the presence of a temperature depression above the spot umbra in agreement with suggestions obtained from radio data analysis. Significant differences in the spatial distribution of both kinds of emission observed in the corona above this spot are attributed mainly to the strong resonant character of the cyclotron radio radiation. Some differences are also caused by both the relatively low efficiency and the low spatial resolution of the FCS. Deconvolution of X-ray images allows to see the new structures and enhances the mutual correlation between X-ray and radio pictures.     

Determination of Electron Density and Filling Factor for Soft X-Ray Flare Kernels

In the standard method of electron density determination for soft X-ray (SXR) flare kernels, it is necessary to assume what the extent is of a kernel along the line of sight. This is a source of significant uncertainty of the obtained densities.     

Investigation of the Mg XII 8.42 Å doublet in solar flare spectra

The intensity ratio of the components of the Mg xii 8.42 Å (1s ² S _1/2 – 2p ² P _{1/2, 3/2}) doublet in solar flare spectra has been investigated using observations recorded from the Intercosmos 7 satellite. The observed values of the ratio fall within the interval 0.38–0.66 and have been compared with recent theoretical predictions based on an optically thin collisional-radiative model. It has been found that for the flare plasma the low values of the ratio cannot be explained since they fall below the smallest theoretical value. The highest values on the other hand require that an unacceptably high electron density be postulated. It is suggested that both high and low values may be caused by the resonance line scattering of the Mg xii quanta in the flare volume, provided that the volume is elongated and not spherical.The intensity of the nearby satellite lines is also investigated. Good agreement between the theoretical and observed intensities is found.     

Investigation of X-Ray Flares with Long Rising Phases

We have used Yohkoh and GOES X-ray observations to investigate flares with a long rising phase. We have found that a characteristic feature of such flares is a long time interval, Δ t ≥ 20 min, between the temperature maximum and the maximum of the emission measure. We have carried out detailed analysis for 10 limb flares of this type. Time variation of the heating function, E_H(t), has been determined for their loop-top X-ray kernels. The time variation of E_H(t), together with the temperature–density diagnostic diagrams, have been used to explain the large value of the time interval, Δ t. The main point is that for these flares the heating function E_H(t) decreases so slowly after the temperature maximum, that for the long time, Δ t, the energy flux reaching flare foot points is sufficient to maintain significant chromospheric evaporation. Investigation of the flare evolution in the temperature–density diagnostic diagrams allowed us to work out a new method of determination of the density for flare kernels. This method can be applied to all the kernels for which their altitudes can be estimated. The advantage of this method is that for the density determination it is not necessary to assume what is the extension of the emitting plasma along the line of sight.     

Quasi-periodic Variations in the Hard X-ray Emission of a Large Arcade Flare

We investigated the quasi-periodic oscillations of the hard X-ray (HXR) emission of the large flare of 2 November 1991 using HXR light curves and soft X-ray and HXR images recorded with the Yohkoh X-ray telescopes. We analysed these observations and report five main results: i) The observations confirm that electrons are accelerated in oscillating magnetic traps that are contained within the cusp magnetic structure. ii) The chromospheric upflow increases the density within the magnetic traps, which in turn together with the higher amplitude of the trap oscillations increases the amplitude of the HXR pulses. iii) This increase stops when the density inside the traps increases progressively and inhibits the acceleration of electrons. iv) The model of oscillating magnetic traps is able to explain the time variation of the electron precipitation, the strong asymmetry in the precipitation of the accelerated electrons, and the systematic differences in the precipitation of 15 and 25 keV electrons. v) We have obtained direct observational evidence that strong HXR pulses are the result of the inflow into the accelerated volume of dense plasma from chromospheric evaporation.