Evolution of microflares associated with bright points in coronal holes and in quiet regions

We aim to find similarities and differences between microflares at coronal bright points found in quiet regions and coronal holes, and to study their relationship with large scale flares. Coronal bright points in quiet regions and in coronal holes were observed with Hinode/EIS using the same sequence. Microflares associated with bright points are identified from the X-ray lightcurve. The temporal variation of physical properties was traced in the course of microflares. The lightcurves of microflares indicated an impulsive peak at hot emission followed by an enhancement at cool emission, which is compatible with the cooling model of flare loops. The density was found to increase at the rise of the impulsive peak, supporting chromospheric evaporation models. A notable difference is found in the surroundings of microflares; diffuse coronal jets are produced above microflares in coronal holes while coronal dimmings are formed in quiet regions. The microflares associated with bright points share common characteristics to active region flares. The difference in the surroundings of microflares are caused by open and closed configurations of the pre-existing magnetic field.Comment: 9 pages, 11 figures, accepted for publication in A&

Spatially Dependent Heating and Ionization in an ICME Observed by Both ACE and Ulysses

The 2005 January 21 interplanetary coronal mass ejection (ICME) observed by multiple spacecraft at L1 was also observed from January 21-February 4 at Ulysses (5.3 AU). Previous studies of this ICME have found evidence suggesting that the flanks of a magnetic cloud like structure associated with this ICME were observed at L1 while a more central cut through the associated magnetic cloud was observed at Ulysses . This event allows us to study spatial variation across the ICME and relate it to the eruption at the Sun. In order to examine the spatial dependence of the heating in this ICME, we present an analysis and comparison of the heavy ion composition observed during the passage of the ICME at L1 and at Ulysses . Using SWICS, we compare the heavy ion composition across the two different observation cuts through the ICME and compare it with predictions for heating during the eruption based on models of the time-dependent ionization balance throughout the event.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98582/1/0004-637X_760_2_105.pd

Solar hard X-ray microflares : atmospheric response and implications for coronal heating models

Die Sonne besitzt eine ausgedehnte Atmosphäre, deren äußerste Schicht die sog. Korona ist. Die Korona besteht aus Plasma mit Temperaturen zwischen 1-3 MK. Dies sind um zwei Größenordnungen höhere Temperaturen als auf der Sonnenoberfläche. Ein vielversprechendes theoretisches Modell entwickelt die Idee, dass sog. "Micro- und Nanoflares" (Sonneneruptionen oder Flares mit einem Energieinhalt von 10 -6 bis 10 -9 mal dem Energieinhalt der größten je beobachteten Flares) die Korona heizen können. Der Energiefreisetzungsmechanismus in Microflares ist derselbe wie in grossen Flares, nämlich magnetische Rekonnexion.Magnetische Rekonnexion ist ein resistiver Plasmaprozess, in dem sich Magnetfeldlinien öffnen und danach in einer neuen Konfiguration verbinden. Es ist jedoch wenig erforscht, ob Microflares tatsächlich auf dieselbe Art ausgelöst werden wie große Flares (magnetische Rekonnexion) und dass in ihnen auch dieselben Prozesse ablaufen (Teilchenbeschleunigung, Plasmaheizung, großräumige Plasmaflüsse). Das Ziel der Dissertation ist daher die Bestimmung der allgemeinen Eigenschaften von Microflares im Vergleich zu regulären Flares. Die Ergebnisse zeigen auf, ob und wie sich Microflares hinsichtlich ihres Entstehungsmechanismus und ihrer physikalischen Abläufe mit regulären Flares vergleichen lassen. Mittels einer aufwändigne Datenanalyse wird eine große Zahl einzelner Microflares untersucht, um deren Eigenschaften wie Signaturen im Röntgenbereich, im extremen UV, im optische Bereich und Radio herauszufinden. Weiters werden spektrale Eigenschaften (Röntgenbereich, EUV, Radio) untersucht. Microflares werden identifiziert mit Beobachtungen des Röntgensatelliten RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Image). Weitere Daten kommen vom Sonnenobservatorium Kanzelhöhe, Dutch Open Telescope, Transition Region and Coronal Explorer, dem Radiospektrographen des Astrophysikalischen Institutes Potsdam und vom Solar and Helospheric Observatory.The Sun has an extensive atmosphere, the outermost layer of which is called the corona. The corona consists of very tenuous and hot plasma with temperatures of 1-3 MK. These temperatures are by two orders of magnitude higher than on the solar surface. One model among modern theories proposes that a large number of 'micro-and nanoflares', i.e.flares downsized in their energy content by a ratio of 10 -6 to 10 -9 compared to the largest observed flare energies are able to heat the corona. The energy release mechanism in micro- and nanoflares is supposed to be the same process as in large flares, i.e. magnetic reconnection. Magnetic reconnection is a resistive plasma process in which magnetic field lines break up and connect again into a configuration containing less free magnetic energy. However, it is not well ascertained that microflares involve the same creation mechanism (magnetic reconnection) and the same processes as regular flares (particle acceleration, plasma heating, plasma flows into the corona, etc). The aim of this thesis project is the study of X-ray microflares with respect to their basic properties by means of an extensive data analysis. Emphasis is given to the study of selected single events in order to obtain information on their basic characteristics as compared to regular flares (appearance in X-rays, extreme ultraviolet, the optical range and radio plus spectral and magnetic field properties, associated mass flows). Information on these issues is obtained by means of an extensive data analysis. Microflares are searched for and identified with observations of the X-ray satellite RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager). Further data are provided by the Kanzelhöhe Solar Observatory, the Dutch Open Telescope, the Transition Region and Coronal Explorer, the radio spectrograph of the Astrophysical Institute Potsdam and the Solar and Heliospheric Observatory.Sigrid Berkebile-StoiserAbweichender Titel laut Übersetzung der Verfasserin/des VerfassersGraz, Univ., Diss., 2009OeBB(VLID)20123

A microflare with hard X-ray-correlated gyroresonance line emission at 314 MHz

Context. Small energy release events in the solar corona can give insights into the flare process which are regularly hidden in the complex morphology of larger events. For one case we find a narrowband radio signal well correlated with the hard X-ray flare. We investigate wether these signals are probes for the flare current sheet. Aims. We aim to establish the relation between narrowband and short-duration features (<1% of the observing frequency in the spectral range 250–340 MHz, and some 5 s until 2  min, respectively) in dynamic radio spectral diagrams and simultaneously occuring HXR bursts. Methods. We use dynamic radio spectra from the Astrophysical Institute Potsdam, HXR images of RHESSI, TRACE coronal and chromospheric images, SOHO-MDI high resolution magnetogram data, and its potential field extrapolation for the analysis of one small flare event in AR10465 on September 26, 2003. We point to similar effects in e.g. the X-class flare on November 03, 2003 to demonstrate that we are not dealing with a singular phenomenon. Results. We confirm the solar origin of the extremely narrowband radio emission. From RHESSI images and the magnetic field data we identify the probable site of the radio source as well as the HXR footpoint and the SXR flare loop emission. The flare loop is included in an ongoing change of magnetic connectivity as confirmed by TRACE images of hot coronal loops. The flare energy is stored in the nonpotential magnetic field substructure around the microflare site which is relaxed to a potential one. Conclusions. We conclude that the correlated HXR footpoint/narrowband radio emission, and the transition to a second energy release in HXR without associated radio emission are direct probes of changing magnetic connectivity during the flare. We suppose that the narrowband radio emission is due to gyroresonance radiation at the second harmonic of the local electron cyclotron frequency. It follows an upper limit of the magnetic field in the radio source volume of less than 50% of the mean potential field in the same height range. This supports the idea that the narrowband radio source is situated in the immediate surroundings of the flare current sheet

Multi-wavelength fine structure and mass flows in solar microflares

Aims. We study the multi-wavelength characteristics at high spatial resolution, as well as chromospheric evaporation signatures of solar microflares. To this end, we analyze the fine structure and mass flow dynamics in the chromosphere, transition region and corona of three homologous microflares (GOES class <A9/0.7 with/without background), which occurred on July 4, 2006 in AR 10898. Methods. A multi-wavelength analysis using temporally and spatially highly resolved imaging data from the Dutch open telescope (Hα, Ca i