11,751 research outputs found
Non-equilibrium of Ionization and the Detection of Hot Plasma in Nanoflare-heated Coronal Loops
Impulsive nanoflares are expected to transiently heat the plasma confined in
coronal loops to temperatures of the order of 10 MK. Such hot plasma is hardly
detected in quiet and active regions, outside flares. During rapid and short
heat pulses in rarified loops the plasma can be highly out of equilibrium of
ionization. Here we investigate the effects of the non-equilibrium of
ionization (NEI) on the detection of hot plasma in coronal loops.
Time-dependent loop hydrodynamic simulations are specifically devoted to this
task, including saturated thermal conduction, and coupled to the detailed
solution of the equations of ionization rate for several abundant elements. In
our simulations, initially cool and rarified magnetic flux tubes are heated to
10 MK by nanoflares deposited either at the footpoints or at the loop apex. We
test for different pulse durations, and find that, due to NEI effects, the loop
plasma may never be detected at temperatures above ~5 MK for heat pulses
shorter than about 1 min. We discuss some implications in the framework of
multi-stranded nanoflare-heated coronal loops.Comment: 22 pages, 7 figures, accepted for publicatio
Spatial distribution of X-ray emitting ejecta in Tycho's SNR: indications of shocked Titanium
Young supernova remnants show a characteristic ejecta-dominated X-ray
emission that allows us to probe the products of the explosive nucleosynthesis
processes and to ascertain important information about the physics of the
supernova explosions. Hard X-ray observations have recently revealed the
radioactive decay lines of 44Ti at ~67.9 keV and ~78.4 keV in the Tycho's SNR.
We here analyze the set of XMM-Newton archive observations of the Tycho's SNR.
We produce equivalent width maps of the Fe K and Ca XIX emission lines and find
indications for a stratification of the abundances of these elements and
significant anisotropies. We then perform a spatially resolved spectral
analysis by identifying five different regions characterized by high/low values
of the Fe K equivalent width. We find that the spatial distribution of the Fe K
emission is correlated with that of the Cr XXII. We also detect the Ti K-line
complex in the spectra extracted from the two regions with the highest values
of the Fe and Cr equivalent widths. The Ti line emissions remains undetected in
regions where the Fe and Cr equivalent widths are low. Our results indicate
that the post-shock Ti is spatially co-located with other iron-peak nuclei in
Tycho's SNR, in agreement with the predictions of multi-D models of Type Ia
supernovae.Comment: Accepted for publication in Ap
Magnetic shuffling of coronal downdrafts
Channelled fragmented downflows are ubiquitous in magnetized atmospheres, and
have been recently addressed from an observation after a solar eruption. We
study the possible back-effect of the magnetic field on the propagation of
confined flows. We compare two 3D MHD simulations of dense supersonic plasma
blobs downfalling along a coronal magnetic flux tube. In one, the blobs move
strictly along the field lines; in the other, the initial velocity of the blobs
is not perfectly aligned to the magnetic field and the field is weaker. The
aligned blobs remain compact while flowing along the tube, with the generated
shocks. The misaligned blobs are disrupted and merged by the chaotic shuffling
of the field lines, and structured into thinner filaments; Alfven wave fronts
are generated together with shocks ahead of the dense moving front. Downflowing
plasma fragments can be chaotically and efficiently mixed if their motion is
misaligned to field lines, with broad implications, e.g., disk accretion in
protostars, coronal eruptions and rain.Comment: 9 pages, 4 figures, proposed for acceptance, movies available upon
request to the first autho
The X-ray cycle in the solar-type star HD 81809
(abridged) Our long-term XMM-Newton program of long-term monitoring of a
solar-like star with a well-studied chromospheric cycle, HD 81809 aims to study
whether an X-ray cycle is present, along with studying its characteristics and
its relation to the chromospheric cycle. Regular observations of HD 81809 were
performed with XMM-Newton, spaced by 6 months from 2001 to 2007. We studied the
variations in the resulting coronal luminosity and temperature, and compared
them with the chromospheric CaII variations. We also modeled the observations
in terms of a mixture of active regions, using a methodology originally
developed to study the solar corona. Our observations show a well-defined cycle
with an amplitude exceeding 1 dex and an average luminosity approximately one
order of magnitude higher than in the Sun. The behavior of the corona of HD
81809 can be modeled well in terms of varying coverage of solar-like active
regions, with a larger coverage than for the Sun, showing it to be compatible
with a simple extension of the solar case.Comment: In press, Astronomy & Astrophysic
Post-flare UV light curves explained with thermal instability of loop plasma
In the present work we study the C8 flare occurred on September 26, 2000 at
19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the
impulsive phase to well beyond the disappearance in the X-rays. The emission
first decayed progressively through equilibrium states until the plasma reached
2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si
iii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium
conditions), are emitted at the same time and all evolve in a similar way. Here
we show that the simultaneous emission of lines with such a different formation
temperature is due to thermal instability occurring in the flaring plasma as
soon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative
start time of the light curves of each line in the correct order with a simple
(and standard) model of a single flaring loop. The agreement with the observed
light curves is greatly improved, and a slower evolution of the line emission
is predicted, if we assume that the model loop consists of an ensemble of
subloops or strands heated at slightly different times. Our analysis can be
useful for flare observations with SDO/EVE.Comment: 24 pages, 7 figures, accepted for publicatio
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