1,254 research outputs found
3D Reconstruction of a Rotating Erupting Prominence
A bright prominence associated with a coronal mass ejection (CME) was seen
erupting from the Sun on 9 April 2008. This prominence was tracked by both the
Solar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, and
was seen to rotate about the line of sight as it erupted; therefore, the event
has been nicknamed the "Cartwheel CME." The threads of the prominence in the
core of the CME quite clearly indicate the structure of a weakly to moderately
twisted flux rope throughout the field of view, up to heliocentric heights of 4
solar radii. Although the STEREO separation was 48 degrees, it was possible to
match some sharp features in the later part of the eruption as seen in the 304
{\AA} line in EUVI and in the H\alpha-sensitive bandpass of COR1 by both STEREO
Ahead and Behind. These features could then be traced out in three-dimensional
space, and reprojected into a view in which the eruption is directed towards
the observer. The reconstructed view shows that the alignment of the prominence
to the vertical axis rotates as it rises up to a leading-edge height of \approx
2.5 solar radii, and then remains approximately constant. The alignment at 2.5
solar radii differs by about 115 degrees from the original filament orientation
inferred from H{\alpha} and EUV data, and the height profile of the rotation,
obtained here for the first time, shows that two thirds of the total rotation
is reached within \approx 0.5 solar radii above the photosphere. These features
are well reproduced by numerical simulations of an unstable moderately twisted
flux rope embedded in external flux with a relatively strong shear field
component.Comment: published in Solar Physics (Online First
Reconnection of a kinking flux rope triggering the ejection of a microwave and hard X-ray source. II. Numerical Modeling
Numerical simulations of the helical () kink instability of an
arched, line-tied flux rope demonstrate that the helical deformation enforces
reconnection between the legs of the rope if modes with two helical turns are
dominant as a result of high initial twist in the range . Such
reconnection is complex, involving also the ambient field. In addition to
breaking up the original rope, it can form a new, low-lying, less twisted flux
rope. The new flux rope is pushed downward by the reconnection outflow, which
typically forces it to break as well by reconnecting with the ambient field.
The top part of the original rope, largely rooted in the sources of the ambient
flux after the break-up, can fully erupt or be halted at low heights, producing
a "failed eruption." The helical current sheet associated with the instability
is squeezed between the approaching legs, temporarily forming a double current
sheet. The leg-leg reconnection proceeds at a high rate, producing sufficiently
strong electric fields that it would be able to accelerate particles. It may
also form plasmoids, or plasmoid-like structures, which trap energetic
particles and propagate out of the reconnection region up to the top of the
erupting flux rope along the helical current sheet. The kinking of a highly
twisted flux rope involving leg-leg reconnection can explain key features of an
eruptive but partially occulted solar flare on 18 April 2001, which ejected a
relatively compact hard X-ray and microwave source and was associated with a
fast coronal mass ejection.Comment: Solar Physics, in pres
Catastrophe versus instability for the eruption of a toroidal solar magnetic flux rope
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of force
balance governing the underlying equilibria. Using a toroidal flux rope in an
external bipolar or quadrupolar field as a model for the current-carrying flux,
we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for
several representative evolutionary sequences in the stable domain of parameter
space. We verify that this catastrophe and the torus instability occur at the
same point; they are thus equivalent descriptions for the onset condition of
solar eruptions.Comment: V2: update to conform to the published article; new choice for
internal inductance of torus; updated Fig. 2; new Figs. 3, 5, and
Shear band formation in granular media as a variational problem
Strain in sheared dense granular material is often localized in a narrow
region called shear band. Recent experiments in a modified Couette cell
provided localized shear flow in the bulk away from the confining walls. The
non-trivial shape of the shear band was measured as the function of the cell
geometry. First we present a geometric argument for narrow shear bands which
connects the function of their surface position with the shape in the bulk.
Assuming a simple dissipation mechanism we show that the principle of minimum
dissipation of energy provides a good description of the shape function.
Furthermore, we discuss the possibility and behavior of shear bands which are
detached from the free surface and are entirely covered in the bulk.Comment: 4 pages, 5 figures; minor changes, typos and journal-ref adde
Observational properties of a kink unstable coronal loop
Aims. Previous work on the dynamics of the kink instability has concentrated on the evolution of the magnetic field and associated current sheets. Here we aim to determine the observational consequences of the kink instability in short coronal loops, particularly what images TRACE would record of such an instability. This paper concentrates on the internal m = 1 mode where the kink structure
of the instability may not be apparent from the global field shape. This is most relevant to the observation of active region brightenings and coronal bright points.
Methods. An existing fluid code was modified to include the TRACE temperature response function in order to calculate temporally and spatially averaged, line of sight images in the 171, 195 and 284 Å band passes for straight, kink unstable flux tubes.
Results. Two new fluid effects of the kink instability are discovered: the circular enhancement of the density at the foot points and the appearance of a low density band running across the flux tube. The second of these effects is shown to be imagable by TRACE and hence would be a good candidate observational signature for an internal m = 1 kink unstable loop
The origin of net electric currents in solar active regions
There is a recurring question in solar physics about whether or not electric
currents are neutralized in active regions (ARs). This question was recently
revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical
simulations of magnetic flux emergence into the solar atmosphere. Such
simulations showed that flux emergence can generate a substantial net current
in ARs. Another source of AR currents are photospheric horizontal flows. Our
aim is to determine the conditions for the occurrence of net vs. neutralized
currents with this second mechanism. Using 3D MHD simulations, we
systematically impose line-tied, quasi-static, photospheric twisting and
shearing motions to a bipolar potential magnetic field. We find that such
flows: (1) produce both {\it direct} and {\it return} currents, (2) induce very
weak compression currents - not observed in 2.5D - in the ambient field present
in the close vicinity of the current-carrying field, and (3) can generate
force-free magnetic fields with a net current. We demonstrate that neutralized
currents are in general produced only in the absence of magnetic shear at the
photospheric polarity inversion line - a special condition rarely observed. We
conclude that, as magnetic flux emergence, photospheric flows can build up net
currents in the solar atmosphere, in agreement with recent observations. These
results thus provide support for eruption models based on pre-eruption magnetic
fields possessing a net coronal current.Comment: 14 pages and 11 figures (Accepted in The Astrophysical Journal
- …