arXiv subjects

K. J. H. Phillips

Publications and source records attributed to K. J. H. Phillips.

31 records · Page 2Linked to original sources

arXiv2011

High time resolution observations of solar H-alpha flares - II. Search for signatures of electron beam heating

Aims. The H-alpha emission of solar flare kernels and associated hard X-ray (HXR) emission often show similar time variations but their light curves are shifted in time by energy transfer mechanisms. We searched for fast radiative response of the chromosphere in the H-alpha line as a signature of electron beam heating. Methods. We investigate the time differences with sub-second resolution between the H-alpha line emission observed with a Multi-channel Subtractive Double Pass (MSDP) spectrograph on the Large Coronagraph and Horizontal Telescope at Bialkow Observatory, Poland, and HXR emission recorded by the RHESSI spacecraft during several flares, greatly extending our earlier analysis (Paper I) to flares between 2003 and 2005. Results. For 16 H-alpha flaring kernels, observed in 12 solar flares, we made 72 measurements of time delays between local maxima of the RHESSI X-ray and H-alpha emissions. For most kernels, there is an excellent correlation between time variations in the H-alpha line emission (at line centre and in the line wings) and HXR (20-50 keV) flux, with the H-alpha emission following features in the HXR light curves generally by a short time lapse delta t = 1-2 s, sometimes significantly longer (10-18 s). We also found a strong spatial correlation. Conclusions. Owing to our larger number of time measurements than in previous studies, the distribution of delta t values shows a much clearer pattern, with many examples of short (1-2 s) delays of the H-alpha emission, but with some flares showing longer (10-18 s) delays. The former are consistent with energy transfer along the flaring loop legs by non-thermal electron beams, the latter to the passage of conduction fronts.

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arXiv2011

The Solar Flare Chlorine Abundance from RESIK X-ray Spectra

The abundance of chlorine is determined from X-ray spectra obtained with the RESIK instrument on {\em CORONAS-F} during solar flares between 2002 and 2003. Using weak lines of He-like Cl, \ion{Cl}{16}, between 4.44 and 4.50 Å, and with temperatures and emission measures from {\em GOES} on an isothermal assumption, we obtained $A({\rm Cl}) = 5.75 \pm 0.26$ on a scale $A({\rm H}) = 12$. The uncertainty reflects an approximately factor 2 scatter in measured line fluxes. Nevertheless our value represents what is probably the best solar determination yet obtained. It is higher by factors of 1.8 and 2.7 than Cl abundance estimates from an infrared sunspot spectrum and nearby \ion{H}{2} regions. The constancy of the RESIK abundance values over a large range of flares ({\em GOES} class from below C1 to X1) argues for any fractionation that may be present in the low solar atmosphere to be independent of the degree of solar activity.

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arXiv2010

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.

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arXiv2010

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.

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arXiv2010

Highly Ionized Sodium X-ray Line Emission from the Solar Corona and the Abundance of Sodium

{The \ion{Na}{x} X-ray lines between 10.9 and 11.2~Å have attracted little attention but are of interest since they enable an estimate of the coronal abundance of Na to be made. This is of great interest in the continuing debate on the nature of the FIP (first ionization potential) effect. } {Observations of the \ion{Na}{x} lines with the Solar Maximum Mission Flat Crystal Spectrometer and a rocket-borne X-ray spectrometer are used to measure the Na/Ne abundance ratio, i.e. the ratio of an element with very low FIP to one with high FIP.} {New atomic data are used to generate synthetic spectra which are compared with the observations, with temperature and the Na/Ne abundance ratio as free parameters.} {Temperature estimates from the observations indicate that the line emission is principally from non-flaring active regions, and that the Na/Ne abundance ratio is $0.07 \pm 50$\%.} {The Na/Ne abundance ratio is close to a coronal value for which the abundances of low-FIP elements (FIP $< 10$~eV) are enhanced by a factor of 3 to 4 over those found in the photosphere. For low-temperature ($T_e \leqslant 1.5$~MK) spectra, the presence of \ion{Fe}{xvii} lines requires that either a higher-temperature component is present or a revision of ionization or recombination rates is needed. }

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arXiv2010

The SECIS instrument on the Lomnicky Peak Observatory

Heating mechanisms of the solar corona will be investigated at the high-altitude solar observatory Lomnicky Peak of the Astronomical Institute of SAS (Slovakia) using its mid-size Lyot coronagraph and post-focal instrument SECIS provided by Astronomical Institute of the University of Wroclaw (Poland). The data will be studied with respect to the energy transport and release responsible for heating the solar corona to temperatures of mega-Kelvins. In particular investigations will be focused on detection of possible high-frequency MHD waves in the solar corona. The scientific background of the project, technical details of the SECIS system modified specially for the Lomnicky Peak coronagraph, and inspection of the test data are described in the paper.

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arXiv2010

Soft X-ray coronal spectra at low activity levels observed by RESIK

The quiet-Sun X-ray emission is important for deducing coronal heating mechanisms, but it has not been studied in detail since the Orbiting Solar Observatory (OSO) spacecraft era. Bragg crystal spectrometer X-ray observations have generally concentrated on flares and active regions. The high sensitivity of the RESIK (REntgenovsky Spectrometer s Izognutymi Kristalami) instrument on the CORONAS-F solar mission has enabled the X-ray emission from the quiet corona to be studied in a systematic way for the first time. Our aim is to deduce the physical conditions of the non-flaring corona from RESIK line intensities in several spectral ranges using both isothermal and multithermal assumptions. We selected and analyzed spectra in 312 quiet-Sun intervals in January and February 2003, sorting them into 5 groups according to activity level. For each group, the fluxes in selected spectral bands have been used to calculate values parameters for the best-fit that lead to a intensities characteristic of each group. We used both isothermal and multitemperature assumptions, the latter described by differential emission measure (DEM) distributions. RESIK spectra cover the wavelength range (3.3-6.1 A). This includes emission lines of highly ionized Si, S, Cl, Ar, and K, which are suitable for evaluating temperature and emission measure, were used. The RESIK spectra during these intervals of very low solar activity for the first time provide information on the temperature structure of the quiet corona. Although most of the emission seems to arise from plasma with a temperature between 2MK and 3MK, there is also evidence of a hotter plasma (T approx. 10MK) with an emission measure 3 orders smaller than the cooler component. Neither coronal nor photospheric element abundances appear to describe the observed spectra satisfactorily.

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arXiv2010

The solar X-ray continuum measured by RESIK

The solar X-ray continuum emission at five wavelengths between 3.495 Angstroms and 4.220 Angstroms for 19 flares in a seven-month period in 2002--2003 was observed by the RESIK crystal spectrometer on CORONAS-F. In this wavelength region, free--free and free--bound emissions have comparable fluxes. With a pulse-height analyzer having settings close to optimal, the fluorescence background was removed so that RESIK measured true solar continuum in these bands with an uncertainty in the absolute calibration of plus or minus 20 per cent. With an isothermal assumption, and temperature and emission measure derived from the ratio of the two GOES channels, the observed continuum emission normalized to an emission measure of 10^48 cm^-3 was compared with theoretical continua using the CHIANTI atomic code. The accuracy of the RESIK measurements allows photospheric and coronal abundance sets, important for the free--bound continuum, to be discriminated. It is found that there is agreement to about 25 per cent of the measured continua with those calculated from CHIANTI assuming coronal abundances in which Mg, Si, Fe abundances are four times photospheric.

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arXiv2010

Highly Ionized Potassium Lines in Solar X-ray Spectra and the Abundance of Potassium

The abundance of potassium is derived from X-ray lines observed during flares by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57 A. The lines include those emitted by He-like K and Li-like K dielectronic satellites, which have been synthesized using the CHIANTI atomic code and newly calculated atomic data. There is good agreement of observed and synthesized spectra, and the theoretical behavior of the spectra with varying temperature estimated from the ratio of the two GOES channels is correctly predicted. The observed fluxes of the He-like K resonance line per unit emission measure gives log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9. This is higher than photospheric abundance estimates by a factor 5.5, a slightly greater enhancement than for other elements with first ionization potential (FIP) less than about 10 eV. There is, then, the possibility that enrichment of low-FIP elements in coronal plasmas depends weakly on the value of the FIP which for K is extremely low (4.34 eV). Our work also suggests that fractionation of elements to form the FIP effect occurs in the low chromosphere rather than higher up, as in some models.

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arXiv2008

Highly ionized Fe X-ray lines at energies 7.7-8.6 keV

Fe XXV lines at 1.85 A (6.70 keV) and nearby Fe XXIV satellites have been widely used for determining the temperature of the hottest parts of solar flare and tokamak plasmas, though the spectral region is crowded and the lines are blended during flare impulsive stages. The aim of this work is to show that similarly excited Fe lines in the 7.7--8.6 keV (1.44--1.61 A) region have the same diagnostic capability with the advantage of not being so crowded. Spectra in the 7.7--8.6 keV range are synthesized using the CHIANTI spectral package for conditions (temperature, turbulent velocities) appropriate to solar flares. The calculated spectra show that the Fe lines in the 7.7--8.6 keV are well separated even when turbulent velocities are present, and Fe XXIV/Fe XXV line ratios should therefore provide valuable tools for diagnosing flares and tokamak plasmas. It is concluded that Fe lines in the 7.7--8.6 keV range are ideal for the measurement of flare temperature and for detecting the presence of low-energy nonthermal electrons present at flare impulsive stages. An indication of what type of instruments to observe this region is given.

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arXiv2008

RESIK observations of He-like Ar X-ray line emission in solar flares

The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s, p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal spectrometer aboard CORONAS-F between 2001 and 2003. The three line features include the Ar XVII w (resonance line), a blend of x and y (intercombination lines), and z (forbidden line), all of which are blended with Ar XVI dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies with electron temperature Te mostly because of unresolved dielectronic satellites. With temperatures estimated from GOES X-ray emission, the observed G ratios agree fairly well with those calculated from CHIANTI and other data. With a two-component emission measure, better agreement is achieved. Some S XV and S XVI lines blend with the Ar lines, the effect of which occurs at temperatures greater than 8MK, allowing the S/Ar abundance ratio to be determined. This is found to agree with coronal values. A nonthermal contribution is indicated for some spectra in the repeating-pulse flare of 2003 February 6.

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arXiv2006

RHESSI Observations of the Solar Flare Iron-line Feature at 6.7 keV

Analysis of RHESSI 3--10 keV spectra for 27 solar flares is reported. This energy range includes thermal free--free and free--bound continuum and two line features, at 6.7keV and 8keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at 6.7keV to derive the electron temperature T_e, the emission measure, and the Fe-line equivalent width as functions of time in each flare. The Fe/H abundance ratio in each flare is derived from the Fe-line equivalent width as a function of T_e. To minimize instrumental problems with high count rates and effects associated with multi-temperature and nonthermal spectral components, spectra are presented mostly during the flare decay phase, when the emission measure and temperature were smoothly varying. We found flare Fe/H abundance ratios that are consistent with the coronal abundance of Fe (i.e. 4 times the photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7 flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We find evidence that the Fe XXV ion fractions are less than the theoretically predicted values by up to 60% at T_e=25 MK appear to be displaced from the most recent theoretical values by between 1 and 3 MK.

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arXiv2001

Solar coronal observations at high frequencies

The Solar Eclipse Coronal Imaging System (SECIS) is a simple and extremely fast, high-resolution imaging instrument designed for studies of the solar corona. Light from the corona (during, for example, a total solar eclipse) is reflected off a heliostat and passes via a Schmidt-Cassegrain telescope and beam splitter to two CCD cameras capable of imaging at 60 frames a second. The cameras are attached via SCSI connections to a purpose-built PC that acts as the data acquisition and storage system. Each optical channel has a different filter allowing observations of the same events in both white light and in the green line (Fe XIV at 5303 A). Wavelet analysis of the stabilized images has revealed high frequency oscillations which may make a significant contribution on the coronal heating process. In this presentation we give an outline of the instrument and its future development.

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