714,541 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
Signatures of the non-Maxwellian -distributions in optically thin line spectra. II. Synthetic Fe XVII--XVIII X-ray coronal spectra and predictions for the Marshall Grazing-Incidence X-ray Spectrometer (MaGIXS)
We investigated the possibility of diagnosing the degree of departure from
the Maxwellian distribution using the Fe XVII - Fe XVIII spectra originating in
plasmas in collisional ionization equilibrium, such as in the cores of solar
active regions or microflares. The original collision strengths for excitation
are integrated over the non-Maxwellian electron -distributions
characterized by a high-energy tail. Synthetic X-ray emission line spectra were
calculated for a range of temperatures and . We focus on the 6-24 A
spectral range to be observed by the upcoming Marshall Grazing-Incidence X-ray
Spectrometer MaGIXS. We find that many line intensity ratios are sensitive to
both and . Best diagnostic options are provided if a ratio
involving both Fe XVII and Fe XVIII is combined with another ratio involving
lines formed within a single ion. The sensitivity of such diagnostics to
is typically a few tens of per cent. Much larger sensitivity, of about
a factor of two to three, can be obtained if the Fe XVIII 93.93 A line observed
by SDO/AIA is used in conjuction with the X-ray lines. We conclude that the
MaGIXS instrument is well-suited for detection of departures from the
Maxwellian distribution, especially in active region cores.Comment: Astronomy & Astrophysics, accepte
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
A solar spectroscopic absolute abundance of argon from RESIK
Observations of He-like and H-like Ar (Ar XVII and Ar XVIII) lines at 3.949
Angstroms and 3.733 Angstroms respectively with the RESIK X-ray spectrometer on
the CORONAS-F spacecraft, together with temperatures and emission measures from
the two channels of GOES, have been analyzed to obtain the abundance of Ar in
flare plasmas in the solar corona. The line fluxes per unit emission measure
show a temperature dependence like that predicted from theory, and lead to
spectroscopically determined values for the absolute Ar abundance, A(Ar) = 6.44
pm 0.07 (Ar XVII) and 6.49 pm 0.16 (Ar XVIII) which are in agreement to within
uncertainties. The weighted mean is 6.45 pm 0.06, which is between two recent
compilations of the solar Ar abundance and suggest that the photospheric and
coronal abundances of Ar are very similar.Comment: 4 figure
Studies of electroweak boson production in the forward region with LHCb
Studies of electroweak boson production at LHCb are detailed and discussed.
Proposed signal selection schemes and background suppression strategies are
described and the projected performance is estimated using Monte Carlo data.
Due to the unique pseudorapidity coverage and triggering capabilities of LHCb,
these studies will probe an unexplored region of x, Q^2 space.Comment: Presented at "XVIII International Workshop on Deep-Inelastic
Scattering and Related Subjects", (DIS2010) Firenze, 19-23rd April 201
Investigating the tetraquark structure of the new mesons
Using the QCD sum rule approach we investigate the possible four-quark
structure of the recently observed mesons , firstly observed
by BaBaR, X(3872), firstly observed by BELLE and observed by
BELLE. We use diquark-antidiquark currents and work in full QCD, without
relying on expansion. Our results indicate that a four-quark structure
is acceptable for these mesons.Comment: 4 pages 1 eps figure, proceedings of the XVIII Workshop on Hadronic
Interactions (RETINHA-18) Sao Paulo-S
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