6,430 research outputs found
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
erg cm s ) 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 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
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, , can be defined so that modes with are unstable, while modes with
are stable. The perturbative analysis is complemented by several numerical
simulations to follow the time-dependent evolution of the system for different
values of . 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
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