Multiwavelength Observations of an Eruptive Flare: Evidence for Blast Waves and Break-out

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 figure

Effects of magnetic fields on radiatively overstable shock waves

We discuss high-resolution simulations of one-dimensional, plane-parallel shock waves with mean speeds between 150 and 240 km/s propagating into gas with Alfven velocities up to 40 km/s and outline the conditions under which these radiative shocks experience an oscillatory instability in the cooling length, shock velocity, and position of the shock front. We investigate two forms of postshock cooling: a truncated single power law and a more realistic piecewise power law. The degree of nonlinearity of the instability depends strongly on the cooling power law and the Alfven Mach number: for power-law indices \alpha < 0 typical magnetic field strengths may be insufficient either to stabilize the fundamental oscillatory mode or to prevent the oscillations from reaching nonlinear amplitudes.Comment: 11 text pages, LaTeX/AASTeX (aaspp4); 5 figures; accepted by Ap

Simulations of small-scale explosive events on the Sun

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 Physic

Undercover EUV Solar Jets Observed by the Interface Region Imaging Spectrograph

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 fiugre

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

STEREO quadrature observations of coronal dimming at the onset of mini-CMEs

Context: Using unique quadrature observations with the two STEREO spacecraft, we investigate coronal dimmings at the onset of small-scale eruptions. In CMEs they are believed to indicate the opening up of the coronal magnetic fields at the start of the eruption. Aims: It is to determine whether coronal dimming seen in small-scale eruptions starts before or after chromospheric plasma ejection. Methods: One STEREO spacecraft obtained high cadence, 75 s, images in the He II 304A channel, and the other simultaneous images in the Fe IX/FeX 171A channel. We concentrate on two well-positioned chromospheric eruptions that occurred at disk center in the 171A images, and on the limb in 304A. One was in the quiet Sun and the other was in an equatorial coronal hole. We compare the timing of chromospheric eruption seen in the 304A limb images with the brightenings and dimmings seen on disk in the 171A images. Further we use off-limb images of the low frequency 171A power to infer the coronal structure near the eruptions. Results: In both the quiet Sun and the coronal hole eruption, on disk 171A dimming was seen before the chromospheric eruption, and in both cases it extends beyond the site of the chromospheric eruption. The quiet Sun eruption occurred on the outer edge of the enclosing magnetic field of a prominence and may be related to a small disruption of the prominence just before the 171A dimming. Conclusions: These small-scale chromospheric eruptions started with a dimming in coronal emission just like their larger counterparts. We therefore suggest that a fundamental step in triggering them was the removal of overlying coronal field.Comment: 4 pages, 8 figures. To appear A&A Letters. Movies accompanying this Letter are at http://www.mps.mpg.de/data/outgoing/innes/dims