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Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate? (2) How do solar transients drive heliospheric variability? (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere? (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans (SOOPs), resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime., Comment: 20 pages, 1 figure, accepted by Astronomy & Astrophysics

Context. The remote observations of solar flare ion acceleration are rather limited. There are theoretical predictions for signatures of ion acceleration in EUV line profiles. Previous tests involve observations of flares with no evidence for energetic ions. Aims. We aim to examine a source flare of impulsive (or 3He-rich) solar energetic particle events with EUV line spectroscopy. Methods. We inspect all (90+) reported 3He-rich flares of previous solar cycle 23 and find only four (recurrent) jets in the field of view of SOHO CDS. The jet with the most suitable spatial and temporal coverage is analyzed in detail. Results. Two enhanced (non-thermal) line broadenings are observed in the cooler chromospheric / transition-region lines and they are localized near the site where the closed magnetic loops reconnect with the open magnetic field lines. Both enhanced broadenings are found in the sites with redshifts in the lines, surrounded by the region with blueshifts. One enhanced line broadening is associated with a small flare without energetic particle signatures while another occurs just after the particle acceleration signatures of the main flare terminated. Conclusions. The observed excess broadening appears to be not directly related to the energetic ion production and motions. Further investigations where the critical impulsive phase of the flare is covered are required, ideally with high-resolution spectrometers intentionally pointed to the 3He-rich solar energetic particle source., Comment: accepted for publication in Astronomy & Astrophysics

A class X1.5 flare started on the solar limb at 00:43 UT on 21 April 2002, which was associated with a CME observed at 01:27 UT by LASCO C2. The coordinated analyses of this flare include TRACE 195 {\AA} images and SUMER spectra in lines of Fe XXI, Fe XII, and C II. We find that: 1) The flare began with a jet seen by TRACE, which was detected by SUMER in the C II line as a strong brightening with blue shifts up to 170 km s$^{-1}$. At that time only weak emission was detected in Fe XII and Fe XXI. 2) Subsequently, a weak looplike brightening started south of the jet, moving outwards with an average speed of about 150 km s$^{-1}$. The SUMER spectra responded this moving loop as separatingly brightenings, visible only in the Fe XXI line. The southwards moving component contains red- and blue-shifted emission features and has an apparent speed of $\sim$120 km s$^{-1}$. The absence of signatures in Fe XII and C II lines indicates that the moving weak loop seen by TRACE corresponds to the emission from very hot plasma, in a blend line in the 195 {\AA} bandpass due to Fe XXIV formed at T > 10 MK. 3) The trigger mechanism of the flare and associated CME can be interpreted in the same way as that proposed by Wang et al. (2002) for an event with similar initial features., Comment: 4 pages, 3 figures, 2 animations; published in ESA SP-505 in 2002

Context: Photospheric flows create a network of often mixed-polarity magnetic field in the quiet Sun, where small-scale eruptions and network flares are commonly seen. Aims: The aim of this paper is (1) to describe the characteristics of the flows that lead to these energy releases, (2) to quantify the energy build up due to photospheric flows acting on the magnetic field, and (3) to assess its contribution to the energy of small-scale, short-lived X-ray flares in the quiet Sun. Methods: We used photospheric and X-ray data from the SoHO and Hinode spacecraft combined with tracking algorithms to analyse the evolution of five network flares. The energy of the X-ray emitting thermal plasma is compared with an estimate of the energy built up due to converging and sheared flux. Results: Quiet-Sun network flares occur above sites of converging opposite-polarity magnetic flux that are often found on the outskirts of network cell junctions, sometimes with observable vortex-like motion. In all studied flares the thermal energy was more than an order of magnitude higher than the magnetic free energy of the converging flux model. The energy in the sheared field was always higher than in the converging flux but still lower than the thermal energy. Conclusions: X-ray network flares occur at sites of magnetic energy dissipation. The energy is probably built up by supergranular flows causing systematic shearing of the magnetic field. This process appears more efficient near the junction of the network lanes. Since this work relies on 11 case studies, our results call for a follow-up statistical analysis to test our hypothesis throughout the quiet Sun.

We introduce analogue black holes (BHs) based on ideal magnetohydrodynamic equations. Similar to acoustic BHs, which trap phonons and emit Hawking radiation (HR) at the sonic horizon where the flow speed changes from super- to sub-sonic, in the horizon of magnetoacoustic and Alfv\'{e}nic BHs, the magnetoacoustic and Alfve'n waves will be trapped and emit HR made of quantized vibrations similar to phonons which we call magnephonons and Alphonons. We proposed that magnetoacoustic and Alfve'nic BHs may be created in the laboratory using a tube with variable cross section embedded in a uniform magnetic field, and a super-magnetoacoustic or a super-Alfve'nic flow. We show that the Hawking temperature for both BHs is a function of the background magnetic field, number density of fluid, and radius of the tube. For a typical setup, the temperature is estimated to be about 0.0266 K., Comment: 9 pages, 3 Figures

It is well-known that extreme ultraviolet emission emitted at the solar surface is absorbed by overlying cool plasma. Especially in active regions dark lanes in EUV images suggest that much of the surface activity is obscured. Simultaneous observations from IRIS, consisting of UV spectra and slit-jaw images give vital information with sub-arcsecond spatial resolution on the dynamics of jets not seen in EUV images. We studied a series of small jets from recently formed bipole pairs beside the trailing spot of active region 11991, which occurred on 2014 March 5 from 15:02:21 UT to 17:04:07 UT. There were collimated outflows with bright roots in the SJI 1400 {\AA} (transition region) and 2796 {\AA} (upper chromosphere) that were mostly not seen in AIA 304 {\AA} (transition region) and AIA 171 \AA\ (lower corona) images. The Si IV spectra show strong blue-wing but no red-wing enhancements in the line profiles of the ejecta for all recurrent jets indicating outward flows without twists. We see two types of Mg II line profiles produced by the jets spires: reversed and non-reversed. Mg II lines remain optically thick but turn into optically thin in the highly Doppler shifted wings.The energy flux contained in each recurrent jet is estimated using a velocity differential emission measure technique which measures the emitting power of the plasma as a function of line-of-sight velocity. We found that all the recurrent jets release similar energy (10$^8$ erg cm$^{-2}$ s$^{-1}$ ) toward the corona and the downward component is less than 3\%., Comment: Accepted for publication in ApJ, 6 fiugres

In addition to their anomalous abundances, 3He-rich solar energetic particles (SEPs) show puzzling energy spectral shapes varying from rounded forms to power laws where the later are characteristics of shock acceleration. Solar sources of these particles have been often associated with jets and narrow CMEs, which are the signatures of magnetic reconnection involving open field. Recent reports on new associations with large-scale EUV waves bring new insights on acceleration and transport of 3He-rich SEPs in the corona. We examined energy spectra for 32 3He-rich SEP events observed by ACE at L1 near solar minimum in 2007-2010 and compared the spectral shapes with solar flare signatures obtained from STEREO EUV images. We found the events with jets or brightenings tend to be associated with rounded spectra and the events with coronal waves with power laws. This suggests that coronal waves may be related to the unknown second stage mechanism commonly used to interpret spectral forms of 3He-rich SEPs., Comment: Presented at 15th Annual International Astrophysics Conference "The Science of Ed Stone". Accepted for publication in Journal of Physics: Conference Series

This paper presents multiwavelength observations of a flare-generated type II radio burst. The kinematics of the shock derived from the type II closely match a fast EUV wave seen propagating through coronal arcade loops. The EUV wave was closely associated with an impulsive M1.0 flare without a related coronal mass ejection, and was triggered at one of the footpoints of the arcade loops in active region NOAA 12035. It was initially observed in the AIA 335 \AA\ images with a speed of ~800 km/s and accelerated to ~1490 km/s after passing through the arcade loops. A fan-spine magnetic topology was revealed at the flare site. A small, confined filament eruption (~340 km/s) was also observed moving in the opposite direction to the EUV wave. We suggest that breakout reconnection in the fan-spine topology triggered the flare and associated EUV wave that propagated as a fast shock through the arcade loops., Comment: ApJ, 12 Pages, 8 Figures

Context: Flare kernels brighten simultaneously in all SDO/AIA channels making it difficult to determine their temperature structure. IRIS is able to spectrally resolve Fe xxi emission from cold chromospheric brightenings, so can be used to infer the amount of Fe xxi emission in 131 channel. Aims: We use observations of two small solar flares seen by IRIS and SDO to compare the EMs deduced from the IRIS Fe xxi line and the AIA 131 channel to determine the fraction of Fe xxi emission in flare kernels in the 131 channel of AIA. Methods: Cotemporal and cospatial pseudo-raster AIA images are compared with the IRIS results.We use multi-Gaussian line fitting to separate the blending chromospheric emission so as to derive Fe xxi intensities and Doppler shifts in IRIS spectra. Results: We define loop and kernel regions based on the brightness of the 131 and 1600 {\AA} intensities. In the loop regions the Fe xxi EMs are typically 80% of the 131 ones, and range from 67% to 92%. Much of the scatter is due to small misalignments but the largest site with low Fe xxi contributions was probably affected by a recent injection of cool plasma into the loop. In flare kernels the contribution of Fe xxi increases from less than 10% at the low intensity 131 sites to 40-80% in the brighter kernels. Here the Fe xxi is superimposed on bright chromospheric emission and the Fe xxi line shows blue shifts, sometimes extending up to the edge of the spectral window, 200 km/s. Conclusions: The AIA 131 emission in flare loops is due to Fe xxi emission with a 10-20% contribution from continuum, Fe xxiii, and cooler background plasma emission. In bright flare kernels up to 52% of the 131 is from cooler plasma. The wide range seen in the kernels is caused by significant structure in the kernels which is seen as sharp gradients in Fe xxi EM at sites of molecular and transition region emission., Comment: 10 pages, 19 figures and 1 table, accepted by A&A

Small 3He-rich solar energetic particle (SEP) events with their anomalous abundances, markedly different from solar system, provide evidence for a unique acceleration mechanism that operates routinely near solar active regions. Although the events are sometimes accompanied by coronal mass ejections (CMEs) it is believed that mass and isotopic fractionation is produced directly in the flare sites on the Sun. We report on a large-scale extreme ultraviolet (EUV) coronal wave observed in association with 3He-rich SEP events. In the two examples discussed, the observed waves were triggered by minor flares and appeared concurrently with EUV jets and type III radio bursts but without CMEs. The energy spectra from one event are consistent with so-called class-1 (characterized by power laws) while the other with class-2 (characterized by rounded 3He and Fe spectra) 3He-rich SEP events, suggesting different acceleration mechanisms in the two. The observation of EUV waves suggests that large-scale disturbances, in addition to more commonly associated jets, may be responsible for the production of 3He-rich SEP events., Comment: accepted for publication in The Astrophysical Journal

Coronal loops on the east limb of the Sun were observed by SUMER on SOHO for several days. Small flare-like brightenings are detected very frequently in the hot flare line Fe~{\small XIX}. We find that the relatively intense events are in good coincidence with the transient brightenings seen by Yohkoh/SXT. A statistical analysis shows that these brightenings have durations of 5-84 min and extensions along the slit of 2-67 Mm. The integrated energy observed in Fe~{\small XIX} for each event is in the range of $3\times10^{18}-5\times10^{23}$ ergs, and the estimated thermal energy ranges from $10^{26}-10^{29}$ ergs. Application of the statistical method proposed by Parnell \& Jupp (2000) yields a value of 1.5 to 1.8 for the index of a power law relation between the frequency of the events and the radiated energy in Fe~{\small XIX}, and a value of 1.7 to 1.8 for the index of the frequency distribution of the thermal energy in the energy range $>10^{27}$ ergs. We examine the possibility that these small brightenings give a big contribution to heating of the active region corona., Comment: 5 pages, 7 figures, Proc. of 11th European Solar Physics Meeting, 2005

Discovered more than 40 years ago, impulsive solar energetic particle (SEP) events are still poorly understood. The enormous abundance enhancement of the rare 3He isotope is the most striking feature of these events, though large enhancements in heavy and ultra-heavy nuclei are also observed. Recurrent 3He-rich SEPs in impulsive events have only been observed for limited time periods, up to a few days which is typically the time that a single stationary spacecraft is magnetically connected to the source active regions on the Sun. With the launch of the two STEREO spacecraft we now have the possibility of longer connection time to solar active regions. We examined the evolution of source regions showing repeated 3He-rich SEP emissions for relatively long time periods. We found that recurrent 3He-rich SEPs in these long-lived sources occur after the emergence of magnetic flux., Comment: accepted for publication in Journal of Physics: Conference Series

We report on the first direct observation of a fast-mode wave propagating along and perpendicular to cool (171 {\AA}) arcade loops observed by the SDO/AIA. The wave was associated with an impulsive/compact flare, near the edge of a sunspot. The EUV wavefront expanded radially outward from the flare center and decelerated in the corona from 1060-760 km/s within ~3-4 minute. Part of the EUV wave propagated along a large-scale arcade of cool loops and was partially reflected back to the flare site. The phase speed of the wave was about 1450 km/s, which is interpreted as a fast-mode wave. A second overlying loop arcade, orientated perpendicular to the cool arcade, is heated and becomes visible in the AIA hot channels. These hot loops sway in time with the EUV wave, as it propagated to and fro along the lower loop arcade. We suggest that an impulsive energy release at one of the footpoints of the arcade loops causes the onset of an EUV shock wave that propagates along and perpendicular to the magnetic field., Comment: ApJ letters (in press), 8 pages, 5 figures

Context: One aspect of understanding the dynamics of the quiet Sun is to quantify the evolution of the flux within small-scale magnetic features. These features are routinely observed in the quiet photosphere and were given various names, such as pores, knots, magnetic patches. Aims: This work presents a new algorithm for tracking the evolution of the broad variety of small-scale magnetic features in the photosphere, with a precision equal to the instrumental resolution. Methods: We have developed a new technique to track the evolution of the individual magnetic features from magnetograms, called "magnetic balltracking". It quantifies the flux of the tracked features, and it can track the footpoints of magnetic field lines inferred from magnetic field extrapolation. The algorithm can detect and quantify flux emergence, as well as flux cancellation. Results: The capabilities of magnetic balltracking are demonstrated with the detection and the tracking of two cases of magnetic flux emergence that lead to the brightening of X-ray loops. The maximum emerged flux ranges from 10^{18} Mx to 10^{19} Mx (unsigned flux) when the X-ray loops are observed.

The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by IRIS (Interface Region Imaging Spectrograph) reveal that it is difficult to determine what is up and down even in the cool 6000-K photosphere just above the solar surface: this region hosts pockets of hot plasma transiently heated to almost 100,000 K. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection., Comment: published in Science 346, 1255726 (2014), 30 pages, 13 figures; for associated movie, see http://www2.mps.mpg.de/data/outgoing/peter/papers/2014-iris-eb/fig1-movie.mov

We report high resolution observations from the SDO/AIA of intensity oscillations in a hot, T~8-10 MK, loop. The AIA images show a large coronal loop that was rapidly heated following plasma ejection from one of the loop's footpoints. A wave-like intensity enhancement, seen very clearly in the 131 and 94 \AA\ channel images, propagated ahead of the ejecta along the loop, and was reflected at the opposite footpoint. The wave reflected four times before fading. It was only seen in the hot, 131 and 94 \AA\ channels. The characteristic period and the decay time of the oscillation was $\sim$630 and $\sim$440 s, respectively. The phase speed was about 460-510 km/s which roughly matches the sound speed of the loop (430-480 km/s). The observed properties of the oscillation are consistent with the observations of Doppler shift oscillations discovered by the SOHO/SUMER and with their interpretation as slow magnetoacoustic waves. We suggest that the impulsive injection of plasma, following reconnection at one of the loop footpoints, led to rapid heating and the propagation of a longitudinal compressive wave along the loop. The wave bounces back and forth a couple of times before fading., Comment: 8 pages, 5 figures

We study the origin of 3He-rich solar energetic particles (<1 MeV/nucleon) that are observed consecutively on STEREO-B, ACE, and STEREO-A spacecraft when they are separated in heliolongitude by more than 90{\deg}. The 3He-rich period on STEREO-B and STEREO-A commences on 2011 July 1 and 2011 July 16, respectively. The ACE 3He-rich period consists of two sub-events starting on 2011 July 7 and 2011 July 9. We associate the STEREO-B July 1 and ACE July 7 3He-rich events with the same sizeable active region producing X-ray flares accompanied by prompt electron events, when it was near the west solar limb as seen from the respective spacecraft. The ACE July 9 and STEREO-A July 16 events were dispersionless with enormous 3He enrichment, lacking solar energetic electrons and occurring in corotating interaction regions. We associate these events with a small, recently emerged active region near the border of a low-latitude coronal hole that produced numerous jet-like emissions temporally correlated with type III radio bursts. For the first time we present observations of 1) solar regions with long-lasting conditions for 3He acceleration and 2) solar energetic 3He that is temporary confined/re-accelerated in interplanetary space., Comment: accepted for publication in The Astrophysical Journal

Polar crown prominences are made of chromospheric plasma partially circling the Suns poles between 60 and 70 degree latitude. We aim to diagnose the 3D dynamics of a polar crown prominence using high cadence EUV images from the Solar Dynamics Observatory (SDO)/AIA at 304 and 171A and the Ahead spacecraft of the Solar Terrestrial Relations Observatory (STEREO-A)/EUVI at 195A. Using time series across specific structures we compare flows across the disk in 195A with the prominence dynamics seen on the limb. The densest prominence material forms vertical columns which are separated by many tens of Mm and connected by dynamic bridges of plasma that are clearly visible in 304/171A two-color images. We also observe intermittent but repetitious flows with velocity 15 km/s in the prominence that appear to be associated with EUV bright points on the solar disk. The boundary between the prominence and the overlying cavity appears as a sharp edge. We discuss the structure of the coronal cavity seen both above and around the prominence. SDO/HMI and GONG magnetograms are used to infer the underlying magnetic topology. The evolution and structure of the prominence with respect to the magnetic field seems to agree with the filament linkage model., Comment: 24 pages, 14 figures, Accepted for publication in Solar Physics Journal, Movies can be found at http://www2.mps.mpg.de/data/outgoing/panesar/

The sites of chromospheric excitation during solar flares are marked by extended extreme ultraviolet ribbons and hard X-ray footpoints. The standard interpretation is that these are the result of heating and bremsstrahlung emission from non-thermal electrons precipitating from the corona. We examine this picture using multi-wavelength observations of the early phase of an M-class flare SOL2010-08-07T18:24. We aim to determine the properties of the heated plasma in the flare ribbons, and to understand the partition of the power input into radiative and conductive losses. Using GOES, SDO/EVE, SDO/AIA and RHESSI we measure the temperature, emission measure and differential emission measure of the flare ribbons, and deduce approximate density values. The non-thermal emission measure, and the collisional thick target energy input to the ribbons are obtained from RHESSI using standard methods. We deduce the existence of a substantial amount of plasma at 10 MK in the flare ribbons, during the pre-impulsive and early-impulsive phase of the flare. The average column emission measure of this hot component is a few times 10^28/cm^5, and we can calculate that its predicted conductive losses dominate its measured radiative losses. If the power input to the hot ribbon plasma is due to collisional energy deposition by an electron beam from the corona then a low-energy cutoff of around 5 keV is necessary to balance the conductive losses, implying a very large electron energy content. Independent of the standard collisional thick-target electron beam interpretation, the observed non-thermal X-rays can be provided if one electron in 10^3 - 10^4 in the 10 MK (1 keV) ribbon plasma has an energy above 10 keV. We speculate that this could arise if a non-thermal tail is generated in the ribbon plasma which is being heated by other means, for example by waves or turbulence., Comment: 14 Pages, 13 Figures

Using the SIT instrument aboard STEREO we have examined the abundance of the 3He during the ascending phase of solar cycle 24 from January 2010 through December 2012. We report on several cases when 3He-rich solar energetic particle events were successively observed on ACE and STEREO-A with delays consistent with the Carrington rotation rate. In the investigated period ACE and STEREO-A were significantly separated in the heliolongitude corresponding to solar rotation times of 5 to 10 days. We inspect STEREO-A EUV images and use the potential-field source-surface extrapolations together with in-situ magnetic field data to identify responsible solar sources. We find the 3He/4He ratio highly variable in these events and correlated between the spacecraft for the cases with the same connection region on the Sun., Comment: 4 pages, 3 figures, 33rd ICRC, Rio de Janeiro, Brazil, 2-9 July 2013

Images of an east-limb flare on 3 November 2010 taken in the 131 \AA\ channel of the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory provide a convincing example of a long current sheet below an erupting plasmoid, as predicted by the standard magnetic reconnection model of eruptive flares. However, the 171 \AA\ and 193 \AA\ channel images hint at an alternative scenario. These images reveal that large-scale waves with velocity greater than 1000 km/s propagated alongside and ahead of the erupting plasmoid. Just south of the plasmoid, the waves coincided with type-II radio emission, and to the north, where the waves propagated along plume-like structures, there was increased decimetric emission. Initially the cavity around the hot plasmoid expanded. Later, when the erupting plasmoid reached the height of an overlying arcade system, the plasmoid structure changed, and the lower parts of the cavity collapsed inwards. Hot loops appeared alongside and below the erupting plasmoid. We consider a scenario in which the fast waves and the type-II emission were a consequence of a flare blast wave, and the cavity collapse and the hot loops resulted from the break-out of the flux rope through an overlying coronal arcade., Comment: Solar Physics (published), 15 pages, 8 figures

The Square Kilometre Array (SKA) is the most ambitious radio telescope ever planned. With a collecting area of about a square kilometre, the SKA will be far superior in sensitivity and observing speed to all current radio facilities. The scientific capability promised by the SKA and its technological challenges provide an ideal base for interdisciplinary research, technology transfer, and collaboration between universities, research centres and industry. The SKA in the radio regime and the European Extreme Large Telescope (E-ELT) in the optical band are on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI) and have been recognised as the essential facilities for European research in astronomy. This "White Paper" outlines the German science and R&D interests in the SKA project and will provide the basis for future funding applications to secure German involvement in the Square Kilometre Array., Comment: Editors: H. R. Kl\"ockner, M. Kramer, H. Falcke, D.J. Schwarz, A. Eckart, G. Kauffmann, A. Zensus; 150 pages (low resolution- and colour-scale images), published in July 2012, language English (including a foreword and an executive summary in German), the original file is available via the MPIfR homepage

Solar tornados are dynamical, conspicuously helical magnetic structures mainly observed as a prominence activity. We investigate and propose a triggering mechanism for the solar tornado observed in a prominence cavity by SDO/AIA on September 25, 2011. High-cadence EUV images from the SDO/AIA and the Ahead spacecraft of STEREO/EUVI are used to correlate three flares in the neighbouring active-region (NOAA 11303), and their EUV waves, with the dynamical developments of the tornado. The timings of the flares and EUV waves observed on-disk in 195\AA\ are analyzed in relation to the tornado activities observed at the limb in 171\AA. Each of the three flares and its related EUV wave occurred within 10 hours of the onset of the tornado. They have an observed causal relationship with the commencement of activity in the prominence where the tornado develops. Tornado-like rotations along the side of the prominence start after the second flare. The prominence cavity expands with acceleration of tornado motion after the third flare. Flares in the neighbouring active region may have affected the cavity prominence system and triggered the solar tornado. A plausible mechanism is that the active-region coronal field contracted by the `Hudson effect' due to the loss of magnetic energy as flares. Subsequently the cavity expanded by its magnetic pressure to fill the surrounding low corona. We suggest that the tornado is the dynamical response of the helical prominence field to the cavity expansion., Comment: 7 pages, 8 Figures, accepted for publication in A&A, Movies can be found at http://www.mps.mpg.de/data/outgoing/panesar/tornado/

A prominence eruption on 7 June 2011 produced spectacular curtains of plasma falling through the lower corona. At the solar surface they created an incredible display of extreme ultraviolet brightenings. The aim is to identify and analyze some of the local instabilities which produce structure in the falling plasma. The structures were investigated using SDO/AIA 171A and 193A images in which the falling plasma appeared dark against the bright coronal emission. Several instances of the Rayleigh-Taylor instability were investigated. In two cases the Alfven velocity associated with the dense plasma could be estimated from the separation of the Rayleigh-Taylor fingers. A second type of feature, which has the appearance of self-similar branching horns, is also discussed., Comment: 6 pages, 7 figures, submitted A&A. Movies are at http://www.mps.mpg.de/data/outgoing/innes/arcs_movie.avi and http://www.mps.mpg.de/data/outgoing/innes/horns_movie.avi

Small-scale extreme ultraviolet (EUV) dimming often surrounds sites of energy release in the quiet Sun. This paper describes a method for the automatic detection of these small-scale EUV dimmings using a feature based classifier. The method is demonstrated using sequences of 171 A images taken by STEREO/EUVI on 13 June 2007 and by SDO/AIA on 27 August 2010. The feature identification relies on recognizing structure in sequences of space-time 171\AA\ images using the Zernike moments of the images. The Zernike moments space-time slices with events and non-events are distinctive enough to be separated using a Support Vector Machine (SVM) classifier. The SVM is trained using 150 event and 700 non-event space-time slices. We find a total of 1217 events in the EUVI images and 2064 events in the AIA images on the days studied. Most of the events are found between latitudes -35 degree and +35 degree. The sizes and expansion speeds of central dimming regions are extracted using a region grow algorithm. The histograms of the sizes in both EUVI and AIA follow a steep power law with slope about -5. The AIA slope extends to smaller sizes before turning over. The mean velocity of 1325 dimming regions seen by AIA is found to be about 14 km/s., Comment: 6 pages, 11 figures Accepted for ApJ

Images from the Solar Dynamics Observatory (SDO) at 211A are used to identify the solar source of the type III radio bursts seen in WIND/WAVES dynamic spectra. We analyse a 2.5 hour period during which six strong bursts are seen. The radio bursts correlate very well with the EUV jets coming from the western side of a sunspot in AR11092. The EUV jet emission also correlates well with brightening at what looks like their footpoint at the edge of the umbra. For 10-15 min after strong EUV jets are ejected, the footpoint brightens at roughly 3 min intervals. In both the EUV images and the extracted light curves, it looks as though the brightening is related to the 3-min sunspot oscillations, although the correlation coefficient is rather low. The only open field near the jets is rooted in the sunspot. We conclude that active region EUV/X-ray jets and interplanetary electron streams originate on the edge of the sunspot umbra. They form along a current sheet between the sunspot open field and closed field connecting to underlying satellite flux. Sunspot running penumbral waves cause roughly 3-min jet footpoint brightening. The relationship between the waves and jets is less clear., Comment: 4 pages, 7 figures, Accepted by A&A Letters. For associated gif movie, see http://www.mps.mpg.de/data/outgoing/innes/jets/losb_304_211_rd.gif

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&A

Twisting motions of different nature are observed in several layers of the solar atmosphere. Chromospheric sunspot whorls and rotation of sunspots or even higher up in the lower corona sigmoids are examples of the large scale twisted topology of many solar features. Nevertheless, their occurrence at large scale in the quiet photosphere has not been investigated. The present study reveals the existence of vortex flows located at the supergranular junctions of the quiet Sun. We use a 1-hour and a 5-hour time series of the granulation in Blue continuum and G-band images from FG/SOT to derive the photospheric flows. A feature tracking technique called Balltracking is performed to track the granules and reveal the underlying flow fields. In both time series we identify long-lasting vortex flow located at supergranular junctions. The first vortex flow lasts at least 1 hour and is ~20-arcsec-wide (~15.5 Mm). The second vortex flow lasts more than 2 hours and is ~27-arcsec-wide (~21 Mm)., Comment: 4 pages, 10 figures

Context: The atmosphere of the quiet Sun is controlled by photospheric flows sweeping up concentrations of mixed polarity magnetic field. Along supergranule boundaries and junctions, there is a strong correlation between magnetic flux and bright chromospheric and transition region emission. Aim: The aim is to investigate the relationship between photospheric flows and small flare-like brightenings seen in Extreme Ultraviolet images. Method: We describe observations of small eruptions seen in quiet Sun images taken with the Extreme UltraViolet Imager (EUVI) on STEREO. The photospheric flows during the eruption build-up phase are investigated by tracking granules in high resolution MDI continuum images. Results: Eruptions with characteristics of small coronal mass ejections (CMEs) occur at the junctions of supergranular cells. The eruptions produce brightening at the onset site, dark cloud or small filament ejections, and faint waves moving with plane-of-sky speeds up to 150 km/s. In the two examples studied, they appear to be activated by converging and rotating supergranular flows, twisting small concentrations of opposite polarity magnetic field. An estimate of the occurrence rate is about 1400 events per day over the whole Sun. One third of these events seem to be associated with waves. Typically, the waves last for about 30 min and travel a distance of 100 arcsec, so at any one time they cover 1/30th of the lower corona., Comment: 6 pages, 5 figures, to be published in A&A

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 figures

The Extreme ultraviolet Imaging Spectrometer (EIS) on board Hinode is the first solar telescope to obtain wide slit spectral images that can be used for detecting Doppler flows in transition region and coronal lines on the Sun and to relate them to their surrounding small scale dynamics. We select EIS lines covering the temperature range 6x10^4 K to 2x10^6 K that give spectrally pure images of the Sun with the 40 arcsec slit. In these images Doppler shifts are seen as horizontal brightenings. Inside the image it is difficult to distinguish shifts from horizontal structures but emission beyond the image edge can be unambiguously identified as a line shift in several lines separated from others on their blue or red side by more than the width of the spectrometer slit (40 pixels). In the blue wing of He II, we find a large number of events with properties (size and lifetime) similar to the well-studied explosive events seen in the ultraviolet spectral range. Comparison with X-Ray Telescope (XRT) images shows many Doppler shift events at the footpoints of small X-ray loops. The most spectacular event observed showed a strong blue shift in transition region and lower corona lines from a small X-ray spot that lasted less than 7 min. The emission appears to be near a cool coronal loop connecting an X-ray bright point to an adjacent region of quiet Sun. The width of the emission implies a line-of-sight velocity of 220 km/s. In addition, we show an example of an Fe XV shift with a velocity about 120 km/s, coming from what looks like a narrow loop leg connecting a small X-ray brightening to a larger region of X-ray emission., Comment: 12 pages, 8 figures, to be published in Solar Physics

Coronal mass ejections (CMEs), as a large-scale eruptive phenomenon, often reveal some precursors in the initiation phase, e.g., X-ray brightening, filament darkening, etc, which are useful for CME modeling and space weather forecast. With the SOHO/SUMER spectroscopic observations of the 2000 September 26 event, we propose another precursor for CME eruptions, namely, long-time prominence oscillations. The prominence oscillation-and-eruption event was observed by ground-based H$\alpha$ telescopes and space-borne white-light, EUV imaging and spectroscopic instruments. In particular, the SUMER slit was observing the prominence in a sit-and-stare mode. The observations indicate that a siphon flow was moving from the proximity of the prominence to a site at a projected distance of 270$''$, which was followed by repetitive H$\alpha$ surges and continual prominence oscillations. The oscillation lasted 4 hours before the prominence erupted as a blob-like CME. The analysis of the multiwavelength data indicates that the whole series of processes fits well into the emerging flux trigger mechanism for CMEs. In this mechanism, emerging magnetic flux drives a siphon flow due to increased gas pressure where the background polarity emerges. It also drives H$\alpha$ surges through magnetic reconnection where the opposite polarity emerges. The magnetic reconnection triggers the prominence oscillations, as well as its loss of equilibrium, which finally leads to the eruption of the prominence. It is also found that the reconnection between the emerging flux and the pre-existing magnetic loop proceeds in an intermittent, probably quasi-periodic, way., Comment: 14 pages, 8 figures, submitted for publication in A&A

Concentrations of molecular hydrogen (H2) have been detected by SUMER in active region plage. The H2 is excited by O VI line emission at 1031.94 A which, although not observed, must be brightening along with the observed transition region line, Si III 1113.24 A. We have made co-ordinated observations of active region plage with the spectrometer SUMER/SoHO in lines of H2 1119.10 A and Si III 1113.24 A and with XRT/Hinode X-ray and SOT/Hinode Ca II filters. In six hours of observation, six of the seven H2 events seen occurred near a footpoint of a brightening X-ray loop. The seventh is associated with an unusually strong Si III plasma outflow. We conclude that microflare energy dissipation heats the chromosphere, reducing its opacity, so that O VI microflare emission is able to reach the lower layers of the chromosphere and excite the H2., Comment: 4 pages, 4 figures, accepted by A&A

We report on active region EUV dynamic events observed simultaneously at high-cadence with SUMER/SoHO and TRACE. Although the features appear in the TRACE Fe ix/x 171A images as jets seen in projection on the solar disk, the SUMER spectral line profiles suggest that the plasma has been driven along a curved large scale magnetic structure, a pre-existing loop. The SUMER observations were carried out in spectral lines covering a large temperature range from 10^4 K to 10^6 K. The spectral analysis revealed that a sudden heating from an energy deposition is followed by a high velocity plasma flow. The Doppler velocities were found to be in the range from 90 to 160 km/s. The heating process has a duration which is below the SUMER exposure time of 25 s while the lifetime of the events is from 5 to 15 min. The additional check on soft X-ray Yohkoh images shows that the features most probably reach 3 MK (X-ray) temperatures. The spectroscopic analysis showed no existence of cold material during the events.

Small-scale explosive events or microflares occur throughout the chromospheric network of the Sun. They are seen as sudden bursts of highly Doppler shifted spectral lines of ions formed at temperatures in the range 2x10^4 - 5x10^5 K. They tend to occur near regions of cancelling photospheric magnetic fields and are thought to be directly associated with magnetic field reconnection. Recent observations have revealed that they have a bi-directional jet structure reminiscent of Petschek reconnection. In this paper compressible MHD simulations of the evolution of a current sheet to a steady Petschek, jet-like configuration are computed using the Versatile Advection Code. We obtain velocity profiles that can be compared with recent ultraviolet line profile observations. By choosing initial conditions representative of magnetic loops in the solar corona and chromosphere, it is possible to explain the fact that jets flowing outward into the corona are more extended and appear before jets flowing towards the chromosphere. This model can reproduce the high Doppler shifted components of the line profiles but the brightening at low velocities, near the centre of the bi-directional jet, cannot be explained by this simple MHD model., Comment: 16 pages, 8 figures. To be published in Solar Physics