Secondary Rayleigh-Taylor type Instabilities in the Reconnection Exhaust Jet as a Mechanism for Supra-Arcade Downflows

Supra-arcade downflows (hereafter referred to as SADs) are low-emission, elongated, finger-like features usually observed in active-region coronae above post-eruption flare arcades. Observations exhibit downward moving SADs intertwined with bright upward moving spikes. Whereas SADs are dark voids, spikes are brighter, denser structures. Although SADs have been observed for decades, the mechanism of formation of SADs remains an open issue. In our three-dimensional resistive magnetohydrodynamic simulations, we demonstrate that secondary Rayleigh-Taylor type instabilities develop in the downstream region of a reconnecting current sheet. The instability results in the formation of low-density coherent structures that resemble SADs, and high-density structures that appear to be spike-like. Comparison between the simulation results and observations suggests that secondary Rayleigh-Taylor type instabilities in the exhaust of reconnecting current sheets provide a plausible mechanism for observed SADs and spikes

Jets or high velocity flows revealed in high-cadence spectrometer and imager co-observations?

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

The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution

The stability properties of one-dimensional radiative shocks with a power-law cooling function of the form $\Lambda \propto \rho^2T^\alpha$ are the main subject of this work. The linear analysis originally presented by Chevalier & Imamura, is thoroughfully reviewed for several values of the cooling index $\alpha$ and higher overtone modes. Consistently with previous results, it is shown that the spectrum of the linear operator consists in a series of modes with increasing oscillation frequency. For each mode a critical value of the cooling index, $\alpha_\textrm{c}$, can be defined so that modes with $\alpha < \alpha_\textrm{c}$ are unstable, while modes with $\alpha > \alpha_\textrm{c}$ are stable. The perturbative analysis is complemented by several numerical simulations to follow the time-dependent evolution of the system for different values of $\alpha$. Particular attention is given to the comparison between numerical and analytical results (during the early phases of the evolution) and to the role played by different boundary conditions. It is shown that an appropriate treatment of the lower boundary yields results that closely follow the predicted linear behavior. During the nonlinear regime, the shock oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle. The modes of oscillations during this phase do not necessarily coincide with those predicted by linear theory, but may be accounted for by mode-mode coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical Journa

Comparison between open-site and below-canopy climatic conditions in Switzerland for different types of forests over 10years (1998−2007)

We compared below-canopy and open-site climatic conditions for 14 different forest sites in Switzerland and analysed the forest influence on local summer and winter climate according to the forest type (coniferous, mixed, deciduous), soil type, slope orientation, basal area and tree height. We compared below-canopy and open-field data for minimum, maximum and daily mean temperature, relative humidity, maximum and daily mean photosynthetically active radiation (PAR) and wind speed from 1998 to 2007. We found clear differences between below-canopy and open-field temperatures, humidity, wind speed and PAR and could relate them to the specific site conditions and forest type. The forest influence on PAR and maximum temperature is clearly determined by the forest type, whereas the influence on minimum temperature is affected by both forest type and slope orientation and impact on humidity depends on the soil type. The wind speed is most impacted by topography and slope orientatio

EIS/Hinode observations of Doppler flow seen through the 40 arcsec wide slit

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 Physic

Planar Josephson Tunnel Junctions in a Transverse Magnetic Field

Traditionally, since the discovery of the Josephson effect in 1962, the magnetic diffraction pattern of planar Josephson tunnel junctions has been recorded with the field applied in the plane of the junction. Here we discuss the static junction properties in a transverse magnetic field where demagnetization effects imposed by the junction geometry and configuration of the electrodes are important. Measurements of the critical current versus magnetic field in planar Nb-based high-quality junctions with different geometry, size and critical current density show that it is advantageous to use a transverse magnetic field rather than an in-plane field to suppress the Josephson tunnel current and Fiske resonances in practical applications.Comment: 5 pages, 2 figures, submitted to Journal of Applied Physic

Small-scale flows in SUMER and TRACE high-cadence co-observations

We report on the physical properties of small-scale transient flows observed simultaneously at high cadence with the SUMER spectrometer and the TRACE imager in the plage area of an active region. Our major objective is to provide a better understanding of the nature of transient phenomena in the solar atmosphere by using high-cadence imager and spectrometer co-observations at similar spatial and temporal resolution. A sequence of TRACE Fe IX/X 171 A and high-resolution MDI images were analysed together with simultaneously obtained SUMER observations in spectral lines covering a temperature range from 10 000 K to 1 MK. We reveal the existence of numerous transient flows in small-scale loops (up to 30 Mm) observed in the plage area of an active region. These flows have temperatures from 10 000 K (the low temperature limit of our observations) to 250 000 K. The coronal response of these features is uncertain due to a blending of the observed coronal line Mg X 624.85 A. The duration of the events ranges from 60 s to 19 min depending on the loop size. Some of the flows reach supersonic velocities. The Doppler shifts often associated with explosive events or bi-directional jets can actually be identified with flows (some of them reaching supersonic velocities) in small-scale loops. Additionally, we demonstrate how a line-of-sight effect can give misleading information on the nature of the observed phenomena if only either an imager or a spectrometer is used.Comment: 7 pages, 6 figures, accepted by A&

Chapter 23: Climate change and Great Barrier Reef: industries and communities

Climate change is driving shifts in environmental conditions that, together with other human pressures, are impacting the Great Barrier Reef (GBR). Individuals, communities, and industries in the GBR catchment depend directly or indirectly on the GBR for ecosystem goods and services. These take the form of direct economic benefits (including commercial activities such as tourism and fishing), social services (including recreational activities and cultural linkages) and environmental services (including shoreline protection from barrier reefs and mangrove stands).This is Chapter 23 of Climate change and the Great Barrier Reef: a vulnerability assessment. The entire book can be found at http://hdl.handle.net/11017/13

Solar sail capture trajectories at Mercury

Mercury is an ideal environment for future planetary exploration by solar sail since it has proved difficult to reach with conventional propulsion and hence remains largely unexplored. In addition, its proximity to the Sun provides a solar sail acceleration of order ten times the sail characteristic acceleration at 1 AU. Conventional capture techniques are shown to be unsuitable for solar sails and a new method is presented. It is shown that capture is bound by upper and lower limits on the orbital elements of the approach orbit and that failure to be within limits results in a catastrophic collision with the planet. These limits are presented for a range of capture inclinations and sail characteristic accelerations. It is found that sail hyperbolic excess velocity is a critical parameter during capture at Mercury, with only a narrow allowed band in order to avoid collision with the planet. The new capture methodis demonstrated for a Mercury sample return mission