652 research outputs found
Shock waves in thermal lensing
We review experimental investigation on spatial shock waves formed by the
self-defocusing action of a laser beam propagation in a disordered thermal
nonlinear media.Comment: 9 pages, 12 figure
Bernoulli type polynomials on Umbral Algebra
The aim of this paper is to investigate generating functions for modification
of the Milne-Thomson's polynomials, which are related to the Bernoulli
polynomials and the Hermite polynomials. By applying the Umbral algebra to
these generating functions, we provide to deriving identities for these
polynomials
Coronal hole boundaries at small scales: III. EIS and SUMER views
We report on the plasma properties of small-scale transient events identified
in the quiet Sun, coronal holes and their boundaries.
We use spectroscopic co-observations from SUMER/SoHO and EIS/Hinode combined
with high cadence imaging data from XRT/Hinode. We measure Doppler shifts using
single and multiple Gauss fits of transition region and coronal lines as well
as electron densities and temperatures. We combine co-temporal imaging and
spectroscopy to separate brightening expansions from plasma flows. The
transient brightening events in coronal holes and their boundaries were found
to be very dynamical producing high density outflows at large speeds. Most of
these events represent X-ray jets from pre-existing or newly emerging coronal
bright points at X-ray temperatures. The average electron density of the jets
is logNe ~ 8.76 cm^-3 while in the flaring site it is logNe ~ 9.51 cm^-3. The
jet temperatures reach a maximum of 2.5 MK but in the majority of the cases the
temperatures do not exceed 1.6 MK. The footpoints of jets have temperatures of
a maximum of 2.5 MK though in a single event scanned a minute after the flaring
the measured temperature was 12 MK. The jets are produced by multiple
microflaring in the transition region and corona. Chromospheric emission was
only detected in their footpoints and was only associated with downflows. The
Doppler shift measurements in the quiet Sun transient brightenings confirmed
that these events do not produce jet-like phenomena. The plasma flows in these
phenomena remain trapped in closed loops.Comment: 16 pages, accepted for publication in A&
Evolution of microflares associated with bright points in coronal holes and in quiet regions
We aim to find similarities and differences between microflares at coronal
bright points found in quiet regions and coronal holes, and to study their
relationship with large scale flares. Coronal bright points in quiet regions
and in coronal holes were observed with Hinode/EIS using the same sequence.
Microflares associated with bright points are identified from the X-ray
lightcurve. The temporal variation of physical properties was traced in the
course of microflares. The lightcurves of microflares indicated an impulsive
peak at hot emission followed by an enhancement at cool emission, which is
compatible with the cooling model of flare loops. The density was found to
increase at the rise of the impulsive peak, supporting chromospheric
evaporation models. A notable difference is found in the surroundings of
microflares; diffuse coronal jets are produced above microflares in coronal
holes while coronal dimmings are formed in quiet regions. The microflares
associated with bright points share common characteristics to active region
flares. The difference in the surroundings of microflares are caused by open
and closed configurations of the pre-existing magnetic field.Comment: 9 pages, 11 figures, accepted for publication in A&
Coronal Temperature Diagnostic Capability of the Hinode/X-Ray Telescope Based on Self-Consistent Calibration
The X-Ray Telescope (XRT) onboard the Hinode satellite is an X-ray imager
that observes the solar corona with unprecedentedly high angular resolution
(consistent with its 1" pixel size). XRT has nine X-ray analysis filters with
different temperature responses. One of the most significant scientific
features of this telescope is its capability of diagnosing coronal temperatures
from less than 1 MK to more than 10 MK, which has never been accomplished
before. To make full use of this capability, accurate calibration of the
coronal temperature response of XRT is indispensable and is presented in this
article. The effect of on-orbit contamination is also taken into account in the
calibration. On the basis of our calibration results, we review the
coronal-temperature-diagnostic capability of XRT
On the ultraviolet signatures of small scale heating in coronal loops
Studying the statistical properties of solar ultraviolet emission lines could
provide information about the nature of small scale coronal heating. We expand
on previous work to investigate these properties. We study whether the
predicted statistical distribution of ion emission line intensities produced by
a specified heating function is affected by the isoelectronic sequence to which
the ion belongs, as well as the characteristic temperature at which it was
formed. Particular emphasis is placed on the strong resonance lines belonging
to the lithium isoelectronic sequence. Predictions for emission lines observed
by existing space-based UV spectrometers are given. The effects on the
statistics of a line when observed with a wide-band imaging instrument rather
than a spectrometer are also investigated. We use a hydrodynamic model to
simulate the UV emission of a loop system heated by nanoflares on small,
spatially unresolved scales. We select lines emitted at similar temperatures
but belonging to different isoelectronic groups: Fe IX and Ne VIII, Fe XII and
Mg X, Fe XVII, Fe XIX and Fe XXIV. Our simulations confirm previous results
that almost all lines have an intensity distribution that follows a power-law,
in a similar way to the heating function. However, only the high temperature
lines best preserve the heating function's power law index (Fe XIX being the
best ion in the case presented here). The Li isoelectronic lines have different
statistical properties with respect to the lines from other sequences, due to
the extended high temperature tail of their contribution functions. However,
this is not the case for Fe XXIV which may be used as a diagnostic of the
coronal heating function. We also show that the power-law index of the heating
function is effectively preserved when a line is observed by a wide-band
imaging instrument rather than a spectromenter
Chromospheric Magnetic Reconnection caused by Photospheric Flux Emergence: Implications for Jet-like Events Formation
Magnetic reconnection in the low atmosphere, e.g. chromosphere, is
investigated in various physical environments. Its implications for the
origination of explosive events (small--scale jets) are discussed. A
2.5-dimensional resistive magnetohydrodynamic (MHD) model in Cartesian
coordinates is used. It is found that the temperature and velocity of the
outflow jets as a result of magnetic reconnection are strongly dependent on the
physical environments, e.g. the magnitude of the magnetic field strength and
the plasma density. If the magnetic field strength is weak and the density is
high, the temperature of the jets is very low (~10,000 K) as well as its
velocity (~40 km/s). However, if environments with stronger magnetic field
strength (20 G) and smaller density (electron density Ne=2x10^{10} cm^{-3}) are
considered, the outflow jets reach higher temperatures of up to 600,000 K and a
line-of-sight velocity of up to 130 km/s which is comparable with the
observational values of jet-like events.Comment: 9 pages, 8 figures, 1 table, submitted to A&
Signatures of Coronal Heating Mechanisms
Alfven waves created by sub-photospheric motions or by magnetic reconnection
in the low solar atmosphere seem good candidates for coronal heating. However,
the corona is also likely to be heated more directly by magnetic reconnection,
with dissipation taking place in current sheets. Distinguishing observationally
between these two heating mechanisms is an extremely difficult task. We perform
1.5-dimensional MHD simulations of a coronal loop subject to each type of
heating and derive observational quantities that may allow these to be
differentiated.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Explosive events - swirling transition region jets
In this paper, we extend our earlier work to provide additional evidence for
an alternative scenario to explain the nature of so-called `explosive events'.
The bi-directed, fast Doppler motion of explosive events observed
spectroscopically in the transition region emission is classically interpreted
as a pair of bidirectional jets moving upward and downward from a reconnection
site. We discuss the problems of such a model. In our previous work, we focused
basically on the discrepancy of fast Doppler motion without detectable motion
in the image plane. We now suggest an alternative scenario for the explosive
events, based on our observations of spectral line tilts and bifurcated
structure in some events. Both features are indicative of rotational motion in
narrow structures. We explain the bifurcation as the result of rotation of
hollow cylindrical structures and demonstrate that such a sheath model can also
be applied to explain the nature of the puzzling `explosive events'. We find
that the spectral tilt, the lack of apparent motion, the bifurcation, and a
rapidly growing number of direct observations support an alternative scenario
of linear, spicular-sized jets with a strong spinning motion.Comment: 9 pages, 3 figures, accepted for publication in Solar Physic
Modeling CHANDRA Low Energy Transmission Grating Spectrometer Observations of Classical Novae with PHOENIX. I. V4743 Sagittarii
We use the PHOENIX code package to model the X-ray spectrum of Nova V4743
Sagittarii observed with the LETGS onboard the Chandra satellite on March 2003.
Our atmosphere models are 1D spherical, expanding, line blanketed, and in full
NLTE. To analyze nova atmospheres and related systems with an underlying
nuclear burning envelope at X-ray wavelengths, it was necessary to update the
code with new microphysics, as discussed in this paper. We demonstrate that the
X-ray emission is dominated by thermal bremsstrahlung and that the hard X-rays
are dominated by Fe and N absorption. The best fit to the observation is
provided at a temperature of T_eff = 5.8 x 10^5 K, with L_bol = 50 000 L_sun.
The models are calculated for solar abundances. It is shown that the models can
be used to determine abundances in the nova ejecta.Comment: 8 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
- …