32 research outputs found
A Unified Picture of the FIP and Inverse FIP Effects
We discuss models for coronal abundance anomalies observed in the coronae of
the sun and other late-type stars following a scenario first introduced by
Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with
the partially neutral gas. Instead of considering wave heating of ions in this
location, we explore the effects on the upper chromospheric plasma of the wave
ponderomotive forces. These can arise as upward propagating waves from the
chromosphere transmit or reflect upon reaching the chromosphere-corona
boundary, and are in large part determined by the properties of the coronal
loop above. Our scenario has the advantage that for realistic wave energy
densities, both positive and negative changes in the abundance of ionized
species compared to neutrals can result, allowing both FIP and Inverse FIP
effects to come out of the model. We discuss how variations in model parameters
can account for essentially all of the abundance anomalies observed in solar
spectra. Expected variations with stellar spectral type are also qualitatively
consistent with observations of the FIP effect in stellar coronae.Comment: 25 pages, 4 figures, submitted to Ap
Radiative Hydrodynamic Models of the Optical and Ultraviolet Emission from Solar Flares
We report on radiative hydrodynamic simulations of moderate and strong solar
flares. The flares were simulated by calculating the atmospheric response to a
beam of non-thermal electrons injected at the apex of a one-dimensional closed
coronal loop, and include heating from thermal soft X-ray, extreme ultraviolet
and ultraviolet (XEUV) emission. The equations of radiative transfer and
statistical equilibrium were treated in non-LTE and solved for numerous
transitions of hydrogen, helium, and Ca II allowing the calculation of detailed
line profiles and continuum emission. This work improves upon previous
simulations by incorporating more realistic non-thermal electron beam models
and includes a more rigorous model of thermal XEUV heating. We find XEUV
backwarming contributes less than 10% of the heating, even in strong flares.
The simulations show elevated coronal and transition region densities resulting
in dramatic increases in line and continuum emission in both the UV and optical
regions. The optical continuum reaches a peak increase of several percent which
is consistent with enhancements observed in solar white light flares. For a
moderate flare (~M-class), the dynamics are characterized by a long gentle
phase of near balance between flare heating and radiative cooling, followed by
an explosive phase with beam heating dominating over cooling and characterized
by strong hydrodynamic waves. For a strong flare (~X-class), the gentle phase
is much shorter, and we speculate that for even stronger flares the gentle
phase may be essentially non-existent. During the explosive phase, synthetic
profiles for lines formed in the upper chromosphere and transition region show
blue shifts corresponding to a plasma velocity of ~120 km/s, and lines formed
in the lower chromosphere show red shifts of ~40 km/s.Comment: 21 pages, 15 figures. Will appear in 2005 September 1 issue of the
Ap
Local re-acceleration and a modified thick target model of solar flare electrons
The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts
has become an almost 'Standard Model' of flare impulsive phase energy transport
and radiation. However, it faces various problems in the light of recent data,
particularly the high electron beam density and anisotropy it involves.} {We
consider how photon yield per electron can be increased, and hence fast
electron beam intensity requirements reduced, by local re-acceleration of fast
electrons throughout the HXR source itself, after injection.} {We show
parametrically that, if net re-acceleration rates due to e.g. waves or local
current sheet electric () fields are a significant fraction of
collisional loss rates, electron lifetimes, and hence the net radiative HXR
output per electron can be substantially increased over the CTTM values. In
this local re-acceleration thick target model (LRTTM) fast electron number
requirements and anisotropy are thus reduced. One specific possible scenario
involving such re-acceleration is discussed, viz, a current sheet cascade (CSC)
in a randomly stressed magnetic loop.} {Combined MHD and test particle
simulations show that local fields in CSCs can efficiently
accelerate electrons in the corona and and re-accelerate them after injection
into the chromosphere. In this HXR source scenario, rapid synchronisation and
variability of impulsive footpoint emissions can still occur since primary
electron acceleration is in the high Alfv\'{e}n speed corona with fast
re-acceleration in chromospheric CSCs. It is also consistent with the
energy-dependent time-of-flight delays in HXR features.Comment: 8 pages, 2 figure
Spectro-Polarimetric Observation of an Emerging Flux Region: Triggering Mechanism of Ellerman Bombs
High spatial resolution observation of an emerging flux region (EFR) was done
using a vector magnetograph and a H-alpha Lyot filtergraph with Domeless Solar
Telescope at Hida Observatory on October 22, 2006. In H-alpha wing images, we
could see many Ellerman bombs (EBs) in the EFR. Two observation modes, slit
scan and slit fixed, were performed with the vector magnetograph, along with
H-alpha filtergraph. Using H-alpha wing images, we detected 12 EBs during slit
scan observation period and 9 EBs during slit fixed observation period. With
slit scan observation, we found that all the EBs were distributed in the area
where the spatial gradient of vertical field intensity was large, which
indicates the possibility of rapid topological change in magnetic field in the
area of EBs. With slit fixed observation, we found that EBs were distributed in
the areas of undulatory magnetic fields, both in vertical and horizontal
components. This paper is the first to report the undulatory pattern in
horizontal components of magnetic field, which is also evidence for emerging
magnetic flux by Parker instability. These results allow us to confirm the
association between EBs and emerging flux tubes. Three triggering mechanism of
EBs is discussed with respect to emerging flux tubes: 9 out of 21 EBs occurred
at the footpoints of emerging flux tubes, 8 out of 21 EBs occurred at the top
of emerging flux tubes, and 4 out of 21 EBs occurred at unipolar region. Each
case can be explained by magnetic reconnection in the low chromosphere.Comment: 30 pages, 17 figure
An XMM-Newton Study of the Coronae of Coronae Borealis
(Abridged) We present results of XMM-Newton observations of the RS CVn binary
Coronae Borealis. The RGS and EPIC MOS2 spectra were simultaneously
fitted with collisional ionization equilibrium plasma models to determine
coronal abundances of various elements. Contrary to the solar first ionization
potential (FIP) effect in which elements with a low FIP are overabundant in the
corona compared to the solar photosphere, and contrary to the ``inverse'' FIP
effect observed in several active RS CVn binaries, coronal abundance ratios in
CrB show a complex pattern as supported by similar findings in the
Chandra HETGS analysis of CrB with a different methodology (Osten et
al. 2003). Low-FIP elements ( eV) have their abundance ratios relative to
Fe consistent with the solar photospheric ratios, whereas high-FIP elements
have their abundance ratios increase with increasing FIP. We find that the
coronal Fe abundance is consistent with the stellar photospheric value,
indicating that there is no metal depletion in CrB. However, we
obtain a higher Fe absolute abundance than in Osten et al. (2003). Except for
Ar and S, our absolute abundances are about 1.5 times larger than those
reported by Osten et al. (2003). However, a comparison of their model with our
XMM-Newton data (and vice versa) shows that both models work adequately in
general. We find, therefore, no preference for one methodology over the other
to derive coronal abundances. Despite the systematic discrepancy in absolute
abundances, our abundance ratios are very close to those obtained by Osten et
al. (2003). Finally, we confirm the measurement of a low density in \ion{O}{7}
( cm), but could not confirm the higher densities
measured in spectral lines formed at higher temperatures.Comment: To appear in Astrophysical Journal (ApJ 10 September 2005, v630 2
issue
Long-term optical/infrared variability in the quiescent X-ray transient V404 Cyg
We present the results of optical and infrared photometry of the quiescent
X-ray transient V404 Cyg during the period 1992-2003. The ellipsoidal
modulations extracted from the most complete databases (years 1992, 1998 and
2001) show unequal maxima and minima with relative strength varying from year
to year although their peak to peak amplitudes remain roughly constant at
0.24+-0.01 magnitudes. Fast optical variations superimposed on the secondary
star's double-humped ellipsoidal modulation were detected every year with a
mean amplitude of ~0.07 mags. We have not found significant changes in the
activity during this decade which indicates that this variability is probably
not connected to the 1989 outburst. We have found periodicities in the 1998 and
2001 data near the 6 hr quasi-periodicity observed in 1992, although we
interpret it as consequence of the appearance of a flare event almost every
night. Significant variability is also present in the I and near infrared (J
and K_short) bands and this decreases slightly or remains approximately
constant at longer wavelengths. A cross correlation analysis shows that both
the R and I emission are simultaneous down to 40 s.Comment: 13 pages, 11 figures; accepted for publication in MNRA