Long-lived hot coronal structures observed with CORONAS-F/SPIRIT in the Mg XII line

Large-scale hot features were detected and observed several times high in the solar corona in the high-temperature Mg XII line (T = 5–20 MK, T _max = 10 MK) with the soft X-ray telescope of the SPIRIT instrumentation complex onboard the CORONAS-F spacecraft. These features look like a spider up to 300000 km in size and live up to a few days. Their bright cores observed at heights were from 0.1 to 0.3 solar radii are connected with active regions by darker legs, giant loops. These features are disposed above arcades, which are simultaneously observed in cooler emission lines sensitive to temperatures of 1 to 2 MK. For the core of such a feature observed December 28–29, 2001, Zhitnik et al. (2003a) estimated an electron temperature of 10 MK and a number density of n _e ≈ 10¹⁰ cm^?3. A high activity and an association with eruptive phenomena were found for such features in continuous (up to 20-day) observations with a cadence of 0.6–1.7 min. In the present paper, we discuss the relation of such features to coronal structures, which are known from previous studies. We identify such off-limb features observed with SPIRIT on October 22, November 12, and December 28–29, 2001, with hot upper parts of post-eruptive arcades. The results of multifrequency analysis of these features based on the data obtained in various spectral ranges by different instruments (Yohkoh/SXT, SOHO/EIT, SOHO/LASCO, Nobeyama and SSRT radioheliographs) are briefly discussed. We address the physical conditions of the long-term existence of giant hot coronal structures. It is demonstrated that the post-eruptive energy release must be prolonged and the condition β ? 1 is not satisfied in these structures. It is argued that the so-called “standard flare model” should be better considered as a “standard post-eruptive energy release model.”     

Reconciling Observational Challenges to the Impulsive-Piston Shock-Excitation Scenario. II. Shock Waves Produced in CME-Less Events with a Null-Point Topology

Solar Physics - The problem of bias, meaning over- or under-estimation, of the component perpendicular to the line-of-sight [ $B_{\perp }$ ] in vector magnetic-field maps is discussed. Previous...     

Radio-heliographic diagnostics of the potential flare productivity of active regions

As deduced from the data with high spatial resolution obtained at the radio heliographs of the Siberian Solar Radio Telescope (SSRT, 5.7 GHz) and the Nobeyama radio heliograph (NoRH, 17 GHz), radio brightness centers in the distribution of the Stokes parameter I are shifted relative to the distribution of the parameter V 1–2 days before an intense flare. It has been shown that this phenomenon can be related to the behavior of quasi-stationary sources over the inversion line of the radial component of the magnetic field (neutral-line associated sources (NLSs)). These sources have a brightness temperature up to 106 K and a circular polarization up to 90%. The origination of NLSs is associated with the outflow of a new magnetic flux into the atmosphere of an active region that is a classical factor of the flare activity. Therefore, an NLS is a precursor of power solar flares and can be used as a forecast factor. Owing to the high resolution of the SSRT, the deviation of the observed polarization distribution of microwave radiation of the active region from the normal one within the solar disk zone containing the active region can be used as a precursor of the preflare state of the active region. As a result, the single-frequency Tanaka-Enome criterion is modified. The use of the data from two radio heliographs (SSRT and NoRH) allows us to propose a two-frequency criterion of normal longitudinal zones that is more efficient for short-term forecasting of solar flares. Preflare features associated with the displacement of brightness centers in I and V, which is manifested as the transformation of NLSs into spot sources, are fine attributes added to forecast according to the two-frequency criterion. This is illustrated by an example of active region 10930, which produced power proton flares on December 6 and 13, 2006.     

Moreton wave, “EIT wave”, and type II radio burst as manifestations of a single wave front

We show that a Moreton wave, an “EIT wave,” and a type II radio burst observed during a solar flare of July 13, 2004, might have been a manifestation of a single front of a decelerating shock wave, which appeared in an active region (AR) during a filament eruption. We propose describing a quasi-spheroidal wave propagating upward and along the solar surface by using relations known from a theory of a point-like explosion in a gas whose density changes along the radius according to a power law. By applying this law to fit the drop in density of the coronal plasma enveloping the solar active region, we first managed to bring the measured positions and velocities of surface Moreton wave and “EIT wave” into correspondence with the observed frequency drift rate of the meter type II radio burst. The exponent of the vertical coronal density falloff is selected by fitting the power law to the Newkirk and Saito empirical distributions in the height range of interest. Formal use of such a dependence in the horizontal direction with a different exponent appears to be reasonable up to distances of less than 200 Mm around the eruption center. It is possible to assume that the near-surface shock wave weakens when leaving this radius and finally the active region, entering the region of the quiet Sun where the coronal plasma density and the fast-mode speed are almost constant along the horizontal.     

Using the nonlinear geometrical acoustics method in the problem of moreton and EUV wave propagation in the solar corona

Propagation of shock related Moreton and EUV waves in the solar atmosphere is simulated by the nonlinear geometrical acoustics method. This method is based on the ray approximation and takes account of nonlinear wave features: dependence of the wave velocity on its amplitude, nonlinear dissipation of wave energy in the shock front, and the increase in its duration with time. The paper describes ways of applying this method to solve the propagation problem of a blast magnetohydrodynamic shock wave. Results of analytical modeling of EUV and Moreton waves in the spherically symmetric and isothermal solar corona are also presented. The calculations demonstrate deceleration of these waves and an increase in their duration. The calculation results of the kinematics of the EUV wave observed on the Sun on January 17, 2010 are presented as an example.     

Fine structure superimposed on millimeter-wavelength (23–18 GHz) spectra of solar active regions

A millimeter-wavelength (23–18 GHz) variable frequency radiometer with frequency resolution of 1 GHz and time resolution of 0.1–16 s has been developed in conjunction with a 13.7 m-diameter antenna. In this paper we describe briefly this new instrument, and its use to observe active regions.Spectra of four active regions show that: (i) spectra of the quiet-Sun region and those of the active region were of the same nature and the spectral index of both varied between –0.4 and +0.3; (ii) for two cases the spectra of the active region remained almost flat during the observing period of about one hour; (iii) spectra of the two other active regions exhibited frequency fine structures similar to the trough and crest type with a width of the order of 2 GHz in frequency and lasting about 40 to 90 min.The spectra of the quiet-Sun region and those of the active regions are attributed to bremsstrahlung emission. Fine structures in frequency type crest and trough are attributed to the radio signatures of the temperature plateau in the chromosphere.     

Modelling the Propagation of a Weak Fast-Mode MHD Shock Wave near a 2D Magnetic Null Point Using Nonlinear Geometrical Acoustics

We present the results of analytical modelling of fast-mode magnetohydrodynamic wave propagation near a 2D magnetic null point. We consider both a linear wave and a weak shock and analyse their behaviour in cold and warm plasmas. We apply the nonlinear geometrical acoustics method based on the Wentzel–Kramers–Brillouin approximation. We calculate the wave amplitude, using the ray approximation and the laws of solitary shock wave damping. We find that a complex caustic is formed around the null point. Plasma heating is distributed in space and occurs at a caustic as well as near the null point due to substantial nonlinear damping of the shock wave. The shock wave passes through the null point even in a cold plasma. The complex shape of the wave front can be explained by the caustic pattern.     

Radio observations of compact solar sources located between sunspots

Data from SSRT form the basis for initiating a study of the properties of long-lived, compact microwave sources located outside sunspots. A step-like birth of such a source was detected and is described in this study. This sheds light on the relationship between the step-like phenomenon detected with SSRT and the peculiar source phenomenon observed at RATAN-600. Peculiar sources precede large flares and are projected onto the photospheric neutral line. It seems likely that the build-up of a large flare is also step-like in character. We also discuss a source overlying the neutral line which accompanied a large active region that did not produce any large flare.     

Weakly polarized microwave sources in active regions prior to large X-ray flares

The data set archive of the Siberian Solar Radio Telescope was used to synthesize two-dimensional images of more than 50 active solar regions where X-ray flares occurred. Reasonably bright microwave sources with a relatively low level of circular polarization were found to be present in 2/3 of cases prior (days, hours) to the flare. Such characteristics are normally inherent in sources located above the polarity inversion line of the photospheric magnetic field. This result, which has been obtained for the first time using such a representative data set, confirms the significance of these sources in the context of X-ray flare formation.     

Scintillation of Solar Radio Sources: Implication for Spikes

Possible scattering regimes of the emission from a solar radio source due to dielectric permitivity fluctuations of an extended coronal plasma co-rotating with the Sun are discussed. The exact and approximate expressions are given for the spectrum of temporal intensity fluctuations in the regime of weak scattering. The frequency, at which the spectrum shows a bend, is determined by the location of the effective scattering screen if the source size is not too large. In the regime of strong scattering of the emission from a broadbanded nonimpulsive radio source, the formation of random intensity spikes, namely millisecond, narrowbanded spike bursts is a possibility. Their apparent size can be quite significant. However, the sources with very small true sizes are required in order to produce strong spikes.