Small-scale inhomogeneities in the solar corona: Evidence from meter-λ radio bursts

A number of inconsistencies between simple theory and observations of solar radio bursts indicate that mode-mode coupling in the solar corona is much stronger than predicted. The inconsistencies include the absence of predicted reversal of the sense of polarization in a type 1 storm at CMP, and the anomalously weak polarization of type II and type III emission. The strong mode coupling could be explained in terms of small scale inhomogeneities (L _N? 100 km) throughout the relevant regions of the corona. The relevant regions are those with open magnetic field lines overlying active regions. It is suggested that the coronal plasma is confined to magnetically self-pinched sheets, and it is pointed out that another inconsistency, namely the anomalously small amount of Faraday variation in type III bursts, could be explained if the value of n _e B in the inter-sheet region were two orders of magnitude less than in the sheets.     

Numerical simulations of the K-corona and of the λ 5303 emission line corona

More and more observations tend to prove that the lower corona is very heterogeneous and that the active regions are quite exclusively arch-structured. So, we have attempted to see what would be the result of simulations of a corona structured only with arches. In a previous work we had made the computations for both the K-corona and the 5303 emission line corona, for which we have much observational data. The complexity of computations has led us to make the comparisons with observations for the vertical intensity gradients only. A priori, it seemed impossible to obtain a simulation close to reality with a corona structured only with arches, at least as we have defined them in this paper, the important fact being a conspicuous lack of matter beyond a certain height. We have made new simulations with a different electron density distribution and for a different region. These latter calculations show us that the material can be confined in the feet of very high arches or in open structures as has already been suggested.     

Does the ion cyclotron exist in the inner corona?

The present work is about the interpretation of the linear polarization of the O VI D₂ (λ1032) coronal line observed by SUMER/SoHO. We take into account the effect of the Doppler redistribution due to the scattering ions motion. We consider the cases of isotropic and anisotropic velocity field distributions. The latter can be interpreted by the ioncyclotron effect that affects heavy ions in the solar corona. The comparison of the numerical results with the observations yields constraints on the solar wind outflow speed and on the velocity field distribution of the O⁵⁺ ions at low coronal altitudes in the polar holes.     

Physical properties of the quiet solar chromosphere–corona transition region

The physical properties of the quiet solar chromosphere–corona transition region are studied. Here the structure of the solar atmosphere is governed by the interaction of magnetic fields above the photosphere. Magnetic fields are concentrated into thin tubes inside which the field strength is great. We have studied how the plasma temperature, density, and velocity distributions change along a magnetic tube with one end in the chromosphere and the other one in the corona, depend on the plasma velocity at the chromospheric boundary of the transition region. Two limiting cases are considered: horizontally and vertically oriented magnetic tubes. For various plasma densities we have determined the ranges of plasma velocities at the chromospheric boundary of the transition region for which no shock waves arise in the transition region. The downward plasma flows at the base of the transition region are shown to be most favorable for the excitation of shock waves in it. For all the considered variants of the transition region we show that the thermal energy transfer along magnetic tubes can be well described in the approximation of classical collisional electron heat conduction up to very high velocities at its base. The calculated extreme ultraviolet (EUV) emission agrees well with the present-day space observations of the Sun.     

Rotational characteristics of the green solar corona: 1964–1989

Fe XIV 5303 coronal emission line observations have been used for the estimation of the rotation behaviour of the green solar corona. A homogeneous data set, created from measurements carried out within the framework of the world-wide coronagraphic network, has been examined with a correlation analysis to reveal the averaged synodic rotation period as a function of latitude and time over the epoch from 1964 to 1989.The values of the synodic rotation period obtained for the epoch 1964–1989 for the whole range of latitudes and for a latitude band ±30° are 28.18±0.12 days and 27.65±0.13 days, respectively. The differential rotation of the green solar corona was confirmed, together with local maxima of the rotation period at latitudes 45° and -60° and a minimum at the equator, but no clear cyclic variation of the rotation has been found for the epoch examined.     

The Fe xii 195.1 Å/1242 Å emission line ratio in the solar corona

Recent R-matrix calculations of electron impact excitation rates in Fe xii are used to derive the theoretical emission line ratio R ₁ = I(195.1 Å)/I(1242 Å), which is potentially a useful electron density diagnostic for the solar inner corona (r 1.05 61-01). These results are found to be significantly different from the earlier estimates of Withbroe and Raymond (1984), but are in good agreement with the observed values of R ₁, for the quiet Sun and an active region. Adoption of the R-matrix atomic data for the 1242 Å line in the coronal iron abundance determination removes an existing discrepancy between results derived from the EUV transition and other iron lines in the solar XUV spectrum. The R-matrix calculations confirm the prediction of Withbroe and Raymond that the earlier discrepancies in R ₁ and the iron abundance were due to the 1242 Å line excitation rates being underestimated by a factor of ~2. Withbroe and Raymond's paper is, therefore, an excellent example of how astronomical observations can be used to accurately predict atomic physics data.     

Electron density diagnostics for various plasma structures of the solar corona based on Fe XI-FeXIII lines in the range 176–207 Å measured in the SPIRIT/CORONAS-F experiment

The relative intensities of FeXI-Fe XIII lines in the range 176–207 Å have been measured for various plasma structures of the solar corona using data from the XUV spectroheliograph of the SPIRIT instrumentation onboard the CORONAS-F satellite with an improved spectral sensitivity calibration. Electron density diagnostics of a plasma with temperatures 0.8–2.5 MK has been carried out in active regions, quiet-Sun and off-limb areas, and, for the first time, in extremely intense solar flares. The density range is (1.6–8) × 10⁹ cm^?3 for flares, (0.6–1.6) × 10⁹ cm^?3 for active regions, and ～5 × 10⁸ cm^?3 for quiet-Sun areas. The calibration accuracy of the spectral sensitivity for the spectroheliograph has been analyzed based on spectral lines with density-independent intensity ratios.     

Smooth models of overshooting at the base of the solar convective zone

A set of smoothed temperature gradient profiles around overshooting layers at the solar convective zone bottom is considered. In classical local theories of convection the one point defined according to the Schwarzschild criterion is enough to describe a convective boundary. To get a sophisticated picture of the overshooting we use four points to compute the transition overshooting functions. Analyzing the transition gradient profiles we found that the overshooting convective flux may be either positive or negative. A negative overshooting flux appears in nonlocal convective theories and causes a steep temperature gradient profile. But we propose an evenly smoothed gradient which corresponds to a convective flux positive everywhere. To outline the effect of the temperature gradient on the solar oscillations the squared Brunt–Väisälä frequency N ² is calculated. In local convective theories the N ² profile shows the discontinuity of the first derivative at the convective boundary, while all smoothed profiles eliminate the break.     

Electron density diagnostics for solar ultraviolet lines of O V

We determine the electron densities for a range of solar features using new calculations for the Ov line ratio, R=I(761.1)/I(760.4), in conjunction with observational data obtained with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory (SOHO). The densities obtained from this diagnostic are in good agreement with earlier measured values. This provides support for the theoretical diagnostics presented in this paper, and hence the atomic data used in their derivation. We conclude from these results that this particular Ov ratio is a useful diagnostic for many types of solar features.     

On the fine structure of the Fe I λ532.4185-nm line profile in the spectrum of the solar disk center

Based on the digital spectra taken with double monochromators of high spectral resolution, we have constructed the profile of the Fe I λ532.4185-nm line in the spectrum of the solar disk center. Basic spectrophotometric characteristics of the line profile have been determined with a high accuracy. The fine structure of the line profile is studied in detail. The profile asymmetry parameters have been determined.     

The existence of long-lived rays of the coronal streamer belt – Radial density and velocity distributions of the solar wind flowing in them

A technique is proposed for separating the rays of the streamer belt with quasi-stationary and non-stationary solar wind (SW) flows. It is shown that the lifetime of rays with a quasi-stationary SW can exceed 20 days. A new method has been developed for measuring the relative density distribution of a quasi-stationary slow SW flowing along the streamer belt's ray of increased brightness, based on the LASCO/SOHO data. It is shown that the density n for such SW flows varies with the radius R according to the relationship nR ^–, where =₁3.3–3.9 within 4 R ₀ R 6 R ₀ (here R ₀ is the solar radius), and decreases gradually further away. It is also shown that the V(R)-profiles in some rays of the streamer belt differ little from each other, although the value of the mass flow density, j _E, at the Earth's orbit in them can vary more than by a factor of 4. This distinguishes in a crucial respect a slow SW in the streamer belt's rays from a fast SW originating in coronal holes, for which j _Econstant and the dependences V(R) in different fast flows can differ greatly.     

Faint Hα emission in the solar corona: Morphological,situational and hydromagnetic analysis

Very faint H emission in the solar corona registered on over-exposed photographs made by a coronagraph and an H filter is studied. The over-exposed filtergrams have been processed by a Joyce Loebl automated microdensitometer and the two-dimensional scans have been analysed by the residual image method. A classification of the faint H emission objects, revealed on the isodensity maps, is proposed and the latitudinal distribution, the morphology, and the location with respect to the other active phenomena are analysed. Taking into account some possible plasma effects that could be caused by coronal magnetic field changes, a hydromagnetic interpretation of the faint H emission is proposed.     

Dynamics of the solar granulation – On the Time Variation of the Granular Flow

The emergence and evolution of large granules shows thegranular dynamics particularly well. We therefore investigate the time dependence of the convective flows within a regular and an exploding granule. The observational material for this study was taken at the center of the solar disk with the German VTT in Izaña (Tenerife, Spain) during an observing campaign in the year 1994. It consists of series of spectrograms of high spatial resolution, which were digitized and processed with wavelet techniques. Among other features, our data show the dynamical portrait of a regular and an exploding granule. We can follow their temporal evolution over more than 12 min. Using absorption lines of different strength we are able to see the dynamical change of both granules at several heights within the first 200 km above ₅₀₀₀=1. The observations reveal significant changes of the convective flow of both granules over time as well as over height, which are discussed in detail.     

Stochastic models of hot planetary and satellite coronas: Atomic oxygen in Europa’s corona

This paper analyzes the formation, kinetics, and transport of hot oxygen atoms in the atmosphere of the Jovian satellite Europa. Atmospheric sources of suprathermal oxygen atoms are assumed to be represented by the processes of dissociation of molecular oxygen, which is the main component of the atmosphere, by solar UV radiation and electron fluxes from the inner magnetosphere of Jupiter, as well as by the reaction of dissociative recombination of the main ionospheric ion O ₂ ⁺ which thermal electrons. It is shown that dissociation in Europa’s near-surface atmosphere is balanced by the processes of the loss of atomic oxygen due to the effective escape of suprathermal oxygen atoms into the inner magnetosphere of Jupiter along the orbit of Europa and due to ionization by magnetospheric electrons and catalytic recombination of oxygen atoms on the icy surface of the satellite. It thus follows that atomic oxygen is only a small admixture to the main atmospheric component—molecular oxygen—in the near-surface part of the atmosphere. However, the outer exospheric layers of Europa’s atmosphere are populated mostly by suprathermal oxygen atoms. The near-surface molecular envelope of Europa is therefore surrounded by a tenuous extended corona of hot atomic oxygen.     

Comparisons between stellar models and reliability of the theoretical models

The high quality of the asteroseismic data provided by space missions such as CoRoT (Michel et al. in The CoRoT Mission, ESA Spec. Publ. vol. 1306, p. 39, 2006) or expected from new operating missions such as Kepler (Christensen-Dalsgaard et al. in Commun. Asteroseismol. 150:350, 2007) requires the capacity of stellar evolution codes to provide accurate models whose numerical precision is better than the expected observational errors (i.e. below 0.1 μHz on the frequencies in the case of CoRoT). We present a review of some thorough comparisons of stellar models produced by different evolution codes, involved in the CoRoT/ESTA activities (Monteiro in Evolution and Seismic Tools for Stellar Astrophysics, 2009). We examine the numerical aspects of the computations as well as the effects of different implementations of the same physics on the global quantities, physical structure and oscillations properties of the stellar models. We also discuss a few aspects of the input physics.     

Variation of the inner disk radius during the onset of the 2010 outburst of MAXI J1659–152

Low mass black hole binaries are generally transient sources and spend most of their time in the quiescent state. It is believed that the inner accretion flow in the quiescent state is in the form of advection dominated accretion flow and the cold outer accretion disk is truncated far away from the central black hole. During the onset of an outburst, the disk gradually extends towards the central black hole.However, the observational evidence for this general picture is indirect at best. Here we present the results of a study performed to understand the variation of the inner disk radius during the early phase of an outburst. We investigated the variation of the inner disk radius during the 2010 outburst of the black hole candidate MAXI J1659-152 using the method of simultaneous spectral fitting. We found that the inner edge of the disk is truncated at a large radius in the beginning of the outburst when the source was in the hard state. We found a systematic decrease in the inner disk radius as the outburst progressed. We also estimated an upper limit on the mass of the black hole to be 8.1 ± 2.9 M within the uncertainty of the distance and inclination angle.     

Structure of the region of solar wind—Interstellar medium interaction and its influence on H atoms penetrating the solar wind

The structure of the region of interaction between the solar wind and the interstellar medium in the two-shocks model (TSM), first suggested by Baranovet al. (1970), is numerically calculated.For this problem our model is true only for charged particles of the interstellar medium interacting with the solar wind, since the free paths of neutral particles are very long and any hydrodynamical approximation would be incorrect.The shapes of the outer and inner shocks, the shape of the contact surface and the distribution of the parameters inside the interaction region are calculated, and are universal and correct for other astrophysical applications such as interstellar bubbles (Weaveret al., 1977), the stellar wind flow around a globule (Dyson, 1975), the interaction of stellar winds in binaries (Prilutzky and Usov, 1976), and so on.The problem of the effect of the charge exchange of H atoms with interstellar gas protons decelerated by an outer shock on H atoms penetrating the solar system is considered using the calculated results (Wallis, 1975). This effect is shown to influence essentially the estimate of H-atom concentration in the interstellar medium based on theL -scattering data.     

Date of minimum of the “protracted” solar cycle 23

We offer an interpretation of the now widely discussed protracted onset of the epoch of solar activity minimum after cycle 23. The interpretation appeals to the Gnevyshev-Ohl rule, but in the context of a new statistical quantity-the product of the cycle amplitude by its duration. Considering this quantity, which has the same physical meaning as that of the integrated characteristic used by Gnevyshev and Ohl, yields a probable estimate for the onset of the minimum of the current cycle in the interval 2009.0–2012.4.     

Oscillations of the Hα emission in solar prominences

The time dependence of Doppler shift and line-center intensity is simultaneously observed for the H emission of three solar prominences, each one during about two hours. Doppler oscillations with periods near one hour and amplitudes between 1 and 2 km s^–1 are conspicuously visible in the recordings of all three prominences. Fourier analysis yields periods of 50, 60, and 64 min, as well as slight indications of short periods near 3 and 5 min. No oscillations are found in the line-center brightness.     

On ‘bimodality of the solar cycle’ and the duration of cycle 21

The period-growth dichotomy of the solar cycle predicts that cycle 21, the present solar cycle, will be of long duration (>133 mo), ending after July 1987. Bimodality of the solar cycle (i.e., cycles being distributed into two groups according to cycle length, based on a comparison to the mean cycle period) is clearly seen in a scatter diagram of descent versus ascent durations. Based on the well-observed cycles 8–20, a linear fit for long-period cycles (being a relatively strong inverse relationship that is significant at the 5% level and having a coefficient of determination r ² 0.66) suggests that cycle 21, having an ascent of 42 mo, will have a descent near 99 mo; thus, cycle duration of about 141 mo is expected. Like cycle 11, cycle 21 occurs on the downward envelope of the sunspot number curve, yet is associated with an upward first difference in amplitude. A comparison of individual cycle, smoothed sunspot number curves for cycles 21 and 11 reveals striking similarity, which suggests that if, indeed, cycle 21 is a long-period cycle, then it too may have an extended tail of sustained, low, smoothed sunspot number, with cycle 22 minimum occurring either in late 1987 or early 1988.