Recent Progress and Future Prospects in Solar Physics, and Their Relevance for Planet Earth

Understanding our star, the Sun, is of fundamental interest for life on Earth. In this paper, the status of knowledge in solar physics of roughly two decades ago is summarised, and the most recent developments in this very active field are shown, many of them achieved by means of space based missions. The Sun-Earth connection is described and, finally, an outlook on future solar research is give

A nanoflare model of quiet Sun EUV emission

Nanoflares have been proposed as the main source of heating of the solar corona. However, detecting them directly has so far proved elusive, and extrapolating to them from the properties of larger brightenings gives unreliable estimates of the power-law exponent $\alpha$ characterising their distribution. Here we take the approach of statistically modelling light curves representative of the quiet Sun as seen in EUV radiation. The basic assumption is that all quiet-Sun EUV emission is due to micro- and nanoflares, whose radiative energies display a power-law distribution. Radiance values in the quiet Sun follow a lognormal distribution. This is irrespective of whether the distribution is made over a spatial scan or over a time series. We show that these distributions can be reproduced by our simple model.Comment: 13 pages, 18 figures, accepted for publication by A&

Structural investigations on bredigite from the Hatrurim Complex

Bredigite, Сa7Mg(SiO4)4, is an indicator mineral of metasomatic rocks of the sanidinite facies formed at high temperatures (>800 °C) and low pressures (<1–2 kbar). Bredigite samples from ternesite-gazeevite-larnite pyrometamorphic rocks of the Hatrurim Complex (Negev Desert, Israel) have been studied by electron probe micro analysis and single-crystal diffraction using synchrotron radiation. They are characterized by a relatively uniform composition. The empirical formula calculated on the basis of 16 O atoms per formula unit is: (Ca7.006Na0.015Ba0.014)Σ7.035Mg0.938(Si4.000P0.014)Σ4.014O16. Basic crystallographic data of a sample studied by X-ray diffraction are as follows: orthorhombic symmetry, space group Pnnm, a = 18.38102(17) Å, b = 6.74936(7) Å, c = 10.90328(11) Å, V = 1352.66(2) Å3, Z = 4. Structure solution and subsequent least-squares refinements resulted in a residual of R(|F|) = 0.023 for 2584 independent observed reflections with I > 2σ(I) and 149 parameters. To the best of our knowledge this is the first detailed structural investigation on natural bredigite. In contrast to previous studies on samples retrieved from metallurgical slags there was no need to describe the structure in the acentric space group Pnn2. Furthermore, the problem of Ba incorporation into the bredigite structure is discussed. Data on the composition of Ba-bearing bredigites from pyrometamorphic rocks of the Hatrurim Complex from Jordan with simplified formula Ba0.7Ca13.3Mg2(SiO4)8 (based on 32 oxygen atoms) are provided for the first time, pointing out perspectives of finding new Ba-bearing minerals isostructural with bredigite in nature

All Coronal Loops are the Same: Evidence to the Contrary

The 1998 April 20 spectral line data from the Coronal Diagnostics Spectrometer (CDS) on the {\it Solar and Heliospheric Observatory} (\SOHO) shows a coronal loop on the solar limb. Our original analysis of these data showed that the plasma was multi-thermal, both along the length of the loop and along the line of sight. However, more recent results by other authors indicate that background subtraction might change these conclusions, so we consider the effect of background subtraction on our analysis. We show Emission Measure (EM) Loci plots of three representative pixels: loop apex, upper leg, and lower leg. Comparisons of the original and background-subtracted intensities show that the EM Loci are more tightly clustered after background subtraction, but that the plasma is still not well represented by an isothermal model. Our results taken together with those of other authors indicate that a variety of temperature structures may be present within loops.Comment: Accepted for publication in ApJ Letter

A nanoflare model for active region radiance: application of artificial neural networks

Context. Nanoflares are small impulsive bursts of energy that blend with and possibly make up much of the solar background emission. Determining their frequency and energy input is central to understanding the heating of the solar corona. One method is to extrapolate the energy frequency distribution of larger individually observed flares to lower energies. Only if the power law exponent is greater than 2, is it considered possible that nanoflares contribute significantly to the energy input. Aims. Time sequences of ultraviolet line radiances observed in the corona of an active region are modelled with the aim of determining the power law exponent of the nanoflare energy distribution. Methods. A simple nanoflare model based on three key parameters (the flare rate, the flare duration time, and the power law exponent of the flare energy frequency distribution) is used to simulate emission line radiances from the ions Fe XIX, Ca XIII, and Si iii, observed by SUMER in the corona of an active region as it rotates around the east limb of the Sun. Light curve pattern recognition by an Artificial Neural Network (ANN) scheme is used to determine the values. Results. The power law exponents, alpha 2.8, 2.8, and 2.6 for Fe XIX, Ca XIII, and Si iii respectively. Conclusions. The light curve simulations imply a power law exponent greater than the critical value of 2 for all ion species. This implies that if the energy of flare-like events is extrapolated to low energies, nanoflares could provide a significant contribution to the heating of active region coronae.Comment: 4 pages, 5 figure

High-resolution spectro-polarimetry of a flaring sunspot penumbra

We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904 during a weak flare eruption (GOES magnitude B7.8), obtained with the Swedish Solar Telescope (SST) in La Palma, Canary Islands. High-resolution \ion{Ca}{ii} $H$ images show a typical two-ribbon structure that has been hitherto only known for larger flares, and the flare appears in a confined region that is discernible by a bright border. The underlying photosphere shows a disturbed penumbral structure with intersecting branches of penumbral filaments. High-resolution Doppler- and vector-magnetograms exhibit oppositely directed Evershed flows and magnetic field vectors in the individual penumbral branches, resulting in several regions of magnetic azimuth discontinuity and several islands where the vertical magnetic field is reversed. The discontinuity regions are co-spatial with the locations of the onset of the flare ribbons. From the results, we conclude that the confined flare region is detached from the global magnetic field structure by a separatrix marked by the bright border visible in \ion{Ca}{ii} $H$. We further conclude that the islands of reversed vertical field appear because of flux emergence and that the strong magnetic shear appearing in the regions of magnetic azimuth discontinuity triggers the flare.Comment: 20 pages + 1 online Figure for A&