417,347 research outputs found
Spectroscopic Observations of Fe XVIII in Solar Active Regions
The large uncertainties associated with measuring the amount of high
temperature emission in solar active regions represents a significant
impediment to making progress on the coronal heating problem. Most current
observations at temperatures of 3 MK and above are taken with broad band soft
X-ray instruments. Such measurements have proven difficult to interpret
unambiguously. Here we present the first spectroscopic observations of the Fe
XVIII 974.86 AA emission line in an on-disk active region taken with then SUMER
instrument on SOHO. Fe XVIII has a peak formation temperature of 7.1 MK and
provides important constraints on the amount of impulsive heating in the
corona. Detailed evaluation of the spectra and comparison of the SUMER data
with soft X-ray images from the XRT on Hinode confirm that this line is
unblended. We also compare the spectroscopic data with observations from the
AIA 94 AA channel on SDO. The AIA 94 AA channel also contains Fe XVIII, but is
blended with emission formed at lower temperatures. We find that is possible to
remove the contaminating blends and form relatively pure Fe XVIII images that
are consistent with the spectroscopic observations from SUMER. The observed
spectra also contain the Ca XIV 943.63 AA line that, although a factor 2 to 6
weaker than the Fe XVIII 974.86 AA line, allows us to probe the plasma around
3.5 MK. The observed ratio between the two lines indicates (isothermal
approximation) that most of the plasma in the brighter Fe XVIII active region
loops is at temperatures between 3.5 and 4 MK.Comment: 12 pages, 5 figures. Submitted as letter to Ap
The Structure of Stellar Coronae in Active Binary Systems
A survey of 28 stars using EUV spectra has been conducted to establish the
structure of stellar coronae in active binary systems from the EMD, electron
densities, and scale sizes. Observations obtained by the EUVE during 9 years of
operation are included for the stars in the sample. EUVE data allow a
continuous EMD to be constructed in the range log T~5.6-7.4, using iron
emission lines. These data are complemented with IUE observations to model the
lower temperature range. Inspection of the EMD shows an outstanding narrow
enhancement, or ``bump'' peaking around log T~6.9 in 25 of the stars, defining
a fundamental coronal structure. The emission measure per unit stellar area
decreases with increasing orbital (or photometric) periods of the target stars;
stars in binaries generally have more material at coronal temperatures than
slowly rotating single stars. High electron densities (Ne>10^12 cm^-3) are
derived at ~10 MK for some targets, implying small emitting volumes. The
observations suggest the magnetic stellar coronae of these stars are consistent
with two basic classes of magnetic loops: solar-like loops with maximum
temperature around log T~6.3 and lower electron densities (Ne>10^9-10.5), and
hotter loops peaking around log T~6.9 with higher electron densities
(Ne>10^12). For the most active stars, material exists at much higher
temperatures (log T>6.9) as well. However, current ab initio stellar loop
models cannot reproduce such a configuration. Analysis of the light curves of
these systems reveals signatures of rotation of coronal material, as well as
apparent seasonal changes in the activity levels.Comment: 45 pages, 9 figures (with 20 eps files). Accepted for its publication
in ApJ
Solar microflares: a case study on temperatures and the Fe XVIII emission
In this paper, we discuss the temperature distribution and evolution of a
microflare, simultaneously observed by Hinode XRT, EIS, and SDO AIA. We find
using EIS lines that during peak emission the distribution is nearly isothermal
and peaked around 4.5 MK. This temperature is in good agreement with that
obtained from the XRT filter ratio, validating the use of XRT to study these
small events, invisible by full-Sun X-ray monitors such as GOES. The increase
in the estimated Fe XVIII emission in the AIA 94 {\AA} band can mostly be
explained with the small temperature increase from the background temperatures.
The presence of Fe XVIII emission does not guarantee that temperatures of 7 MK
are reached, as is often assumed. We also revisit with new atomic data the
temperatures measured by a SoHO SUMER observation of an active region which
produced microflares, also finding low temperatures (3 - 4 MK) from an Fe XVIII
/ Ca XIV ratio.Comment: 12-13 pages, 17 figures (22 eps-files), 4 tables, accepted by
Astronomy and Astrophysic
Abundance variations and first ionization potential trends during large stellar flares
The Solar First Ionization Potential (FIP) effect, where low-FIP elements are
enriched in the corona relative to the photosphere, while high-FIP abundances
remain unchanged, has been known for a long while. High resolution X-ray
spectroscopy has revealed that active stellar coronae show an opposite effect,
which was labeled the Inverse-FIP (IFIP) effect. The correlation found between
coronal activity and the FIP/IFIP bias suggested perhaps that flaring activity
is involved in switching from FIP to IFIP. This work aims at a more systematic
understanding of the FIP trends during stellar flares and complements an
earlier study based on Chandra alone. The eight brightest X-ray flares observed
with XMM-Newton are analyzed and compared with their respective quiescence
states. Together with six previous flares observed with Chandra, this
establishes the best currently available sample of flares. We look for
abundance variations during the flare and their correlation with FIP. For that
purpose, we define a new FIP bias measure. A trend is found where coronae that
are IFIP biased in quiescence, during flares show a FIP bias with respect to
their quiescence composition. This effect is reversed for coronae that are FIP
biased in quiescence. The observed trend is thus consistent with chromospheric
evaporation rather than with a FIP mechanism operating during flares. It also
suggests that the quiescent IFIP bias is real and that the large flares are not
the direct cause of the IFIP effect in stellar coronae.Comment: 12 pages, 6 figures, submitted to A&
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