28,721 research outputs found
Temperature dependence of emission measure in solar X-ray plasmas. 1: Non-flaring active regions
X-ray and ultraviolet line emission from hot, optically thin material forming coronal active regions on the sun may be described in terms of an emission measure distribution function, Phi (T). A relationship is developed between line flux and Phi (T), a theory which assumes that the electron density is a single-valued function of temperature. The sources of error involved in deriving Phi (T) from a set of line fluxes are examined in some detail. These include errors in atomic data (collisional excitation rates, assessment of other mechanisms for populating excited states of transitions, element abundances, ion concentrations, oscillator strengths) and errors in observed line fluxes arising from poorly - known instrumental responses. Two previous analyses are discussed in which Phi (T) for a non-flaring active region is derived. A least squares method of Batstone uses X-ray data of low statistical significance, a fact which appears to influence the results considerably. Two methods for finding Phi (T) ab initio are developed. The coefficients are evaluated by least squares. These two methods should have application not only to active-region plasmas, but also to hot, flare-produced plasmas
Silicon abundance from RESIK solar flare observations
The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and
active region X-ray spectra in four channels covering the wavelength range 3.8
-- 6.1 \AA in its operational period between 2001 and 2003. Several highly
ionized silicon lines were observed within the range of the long-wavelength
channel (5.00 -- 6.05 \AA). The fluxes of the \sixiv Ly- line (5.217
\AA) and the \sixiii line (5.688 \AA) during 21 flares with
optimized pulse-height analyzer settings on RESIK have been analyzed to obtain
the silicon abundance relative to hydrogen in flare plasmas. As in previous
work, the emitting plasma for each spectrum is assumed to be characterized by a
single temperature and emission measure given by the ratio of emission in the
two channels of GOES. The silicon abundance is determined to be (\sixiv) and (\sixiii) on a logarithmic scale with
H = 12. These values, which vary by only very small amounts from flare to flare
and times within flares, are and times the
photospheric abundance, and are about a factor of three higher than RESIK
measurements during a period of very low activity. There is a suggestion that
the Si/S abundance ratio increases from active regions to flares.Comment: To be published, Solar Physic
The X-ray spectrum of Fe XVII revisited with a multi-ion model
The theoretical intensities of the soft X-ray Fe XVII lines arising from
2l-3l' transitions are reexamined using a three-ion collisional-radiative model
that includes the contributions to line formation of radiative recombination
(RR), dielectronic recombination (DR), resonant excitation (RE), and
inner-shell collisional ionization (CI), in addition to the usual contribution
of collisional excitation (CE). These additional processes enhance mostly the
2p-3s lines and not the 2p-3d lines. Under coronal equilibrium conditions, in
the electron temperature range of 400 to 600 eV where the Fe XVII line
emissivities peak, the combined effect of the additional processes is to
enhance the 2p-3s lines at 16.78, 17.05, and 17.10 A, by ~ 25%, 30%, and 55%,
respectively, compared with their traditional, single-ion CE values. The weak
2p-3d line at 15.45 A is also enhanced by up to 20%, while the other 2p-3d
lines are almost unaffected. The effects of DR and RE are found to be dominant
in this temperature range (400 - 600 eV), while that of CI is 3% at the most,
and the contribution of RR is less than 1%. At lower temperatures, where the Fe
XVII / Fe XVIII abundance ratio is high, the RE effect dominates. However, as
the temperature rises and the Fe XVIII abundance increases, the DR effect takes
over.
The newly calculated line powers can reproduce most of the often observed
high values of the (I17.05 + I17.10) / I15.01 intensity ratio. The importance
of ionization and recombination processes to the line strengths also helps to
explain why laboratory measurements in which CE is essentially the sole
mechanism agree well with single-ion calculations, but do not reproduce the
astrophysically observed ratios.Comment: Submitted to Ap
The Solar Photospheric-to-Coronal Fe abundance from X-ray Fluorescence Lines
The ratio of the Fe abundance in the photosphere to that in coronal flare
plasmas is determined by X-ray lines within the complex at 6.7~keV (1.9~\AA)
emitted during flares. The line complex includes the He-like Fe (\fexxv)
resonance line (6.70~keV) and Fe K lines (6.39, 6.40~keV), the
latter being primarily formed by the fluorescence of photospheric material by
X-rays from the hot flare plasma. The ratio of the Fe K lines to the
\fexxv\ depends on the ratio of the photospheric-to-flare Fe abundance,
heliocentric angle of the flare, and the temperature of the
flaring plasma. Using high-resolution spectra from X-ray spectrometers on the
{\em P78-1} and {\em Solar Maximum Mission} spacecraft, the Fe abundance in
flares is estimated to be and times the photospheric
Fe abundance, the {\em P78-1} value being preferred as it is more directly
determined. This enhancement is consistent with results from X-ray spectra from
the {\em RHESSI} spacecraft, but is significantly less than a factor 4 as in
previous work.Comment: Accepted for publication by MNRA
Search for Rapid Changes in the Visible-Light Corona during the 21 June 2001 Total Solar Eclipse
Some 8000 images obtained with the SECIS fast-frame CCD camera instrument
located at Lusaka, Zambia, during the total eclipse of 21 June 2001 have been
analyzed to search for short-period oscillations in intensity that could be a
signature of solar coronal heating mechanisms by MHD wave dissipation. Images
were taken in white- light and Fe XIV green-line (5303 A) channels over 205
seconds (frame rate 39 s-1), approximately the length of eclipse totality at
this location, with a pixel size of four arcseconds square. The data are of
considerably better quality than were obtained during the 11 August 1999 total
eclipse, observed by us (Rudawy et al.: Astron. Astrophys. 416, 1179, 2004), in
that the images are much better exposed and enhancements in the drive system of
the heliostat used gave a much improved image stability. Classical Fourier and
wavelet techniques have been used to analyze the emission at 29518 locations,
of which 10714 had emission at reasonably high levels, searching for periodic
fluctuations with periods in the range 0.1-17 seconds (frequencies 0.06-10 Hz).
While a number of possible periodicities were apparent in the wavelet analysis,
none of the spatially and time-limited periodicities in the local brightness
curves was found to be physically important. This implies that the pervasive
Alfven wave-like phenomena (Tomczyk et al.: Science 317, 1192, 2007) using
polarimetric observations with the CoMP instrument do not give rise to
significant oscillatory intensity fluctuations.Comment: Accepted by Solar Physics; 16 figure
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