1,908 research outputs found
Supernova Reverse Shocks and SiC Growth
We present new mechanisms by which the isotopic compositions of X-type grains
of presolar SiC are altered by reverse shocks in Type II supernovae. We address
three epochs of reverse shocks: pressure wave from the H envelope near t =
10s; reverse shock from the presupernova wind near 10s; reverse
shock from the ISM near 10s. Using 1-D hydrodynamics we show that the
first creates a dense shell of Si and C atoms near 10s in which the SiC
surely condenses. The second reverse shock causes precondensed grains to move
rapidly forward through decelerated gas of different isotopic composition,
during which implantation, sputtering and further condensation occur
simultaneously. The third reverse shock causes only further ion implantation
and sputtering, which may affect trace element isotopic compositions. Using a
25M supernova model we propose solutions to the following unsolved
questions: where does SiC condense?; why does SiC condense in preference to
graphite?; why is condensed SiC Si-rich?; why is O richness no obstacle
to SiC condensation?; how many atoms of each isotope are impacted by a grain
that condenses at time t at radial coordinate r? These many
considerations are put forward as a road map for interpreting SiC X grains
found in meteorites and their meaning for supernova physics.Comment: 28 pages, 14 figures, animation for Figure 3 and machine-readable
Table 3 can be found at
http://antares.steelangel.com/~edeneau/supernova/DHC_2003, Submitted to Ap
Flux Rope Formation Preceding Coronal Mass Ejection Onset
We analyse the evolution of a sigmoidal (S shaped) active region toward
eruption, which includes a coronal mass ejection (CME) but leaves part of the
filament in place. The X-ray sigmoid is found to trace out three different
magnetic topologies in succession: a highly sheared arcade of coronal loops in
its long-lived phase, a bald-patch separatrix surface (BPSS) in the hours
before the CME, and the first flare loops in its major transient intensity
enhancement. The coronal evolution is driven by photospheric changes which
involve the convergence and cancellation of flux elements under the sigmoid and
filament. The data yield unambiguous evidence for the existence of a BPSS, and
hence a flux rope, in the corona prior to the onset of the CME.Comment: ApJ Letters, in pres
Free Magnetic Energy in Solar Active Regions above the Minimum-Energy Relaxed State
To understand the physics of solar flares, including the local reorganization of the magnetic field and the acceleration of energetic particles, we have first to estimate the free magnetic energy available for such phenomena, which can be converted into kinetic and thermal energy. The free magnetic energy is the excess energy of a magnetic configuration compared to the minimum-energy state, which is a linear force-free field if the magnetic helicity of the configuration is conserved. We investigate the values of the free magnetic energy estimated from either the excess energy in extrapolated fields or the magnetic virial theorem. For four different active regions, we have reconstructed the nonlinear force-free field and the linear force-free field corresponding to the minimum-energy state. The free magnetic energies are then computed. From the energy budget and the observed magnetic activity in the active region, we conclude that the free energy above the minimum-energy state gives a better estimate and more insights into the flare process than the free energy above the potential field state
Evolution of magnetic fields and energetics of flares in active region 8210
To better understand eruptive events in the solar corona, we combine sequences of multi-wavelength observations and modelling of the coronal magnetic field of NOAA AR 8210, a highly flare-productive active region. From the photosphere to the corona, the observations give us information about the motion of magnetic elements (photospheric magnetograms), the location of flares (e.g., H, EUV or soft X-ray brightenings), and the type of events (H blueshift events). Assuming that the evolution of the coronal magnetic field above an active region can be described by successive equilibria, we follow in time the magnetic changes of the 3D nonlinear force-free (nlff) fields reconstructed from a time series of photospheric vector magnetograms. We apply this method to AR 8210 observed on May 1, 1998 between 17:00 UT and 21:40 UT. We identify two types of horizontal photospheric motions that can drive an eruption: a clockwise rotation of the sunspot, and a fast motion of an emerging polarity. The reconstructed nlff coronal fields give us a scenario of the confined flares observed in AR 8210: the slow sunspot rotation enables the occurence of flare by a reconnection process close to a separatrix surface whereas the fast motion is associated with small-scale reconnections but no detectable flaring activity. We also study the injection rates of magnetic energy, Poynting flux and relative magnetic helicity through the photosphere and into the corona. The injection of magnetic energy by transverse photospheric motions is found to be correlated with the storage of energy in the corona and then the release by flaring activity. The magnetic helicity derived from the magnetic field and the vector potential of the nlff configuration is computed in the coronal volume. The magnetic helicity evolution shows that AR 8210 is dominated by the mutual helicity between the closed and potential fields and not by the self helicity of the closed field which characterizes the twist of confined flux bundles. We conclude that for AR 8210 the complex topology is a more important factor than the twist in the eruption process
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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