Kenneth J. H. Phillips

The soft X-ray polychromator for the Solar Maximum Mission

The 1.4–22.4 Å range of the soft X-ray spectrum includes a multitude of emission lines which are important for the diagnosis of plasmas in the 1.5–50 million degree temperature range. In particular, the hydrogen and helium-like ions of all abundant solar elements with Z > 7 have their primary transitions in this region and these are especially useful for solar flare and active region studies. The soft X-ray polychromator (XRP) is a high resolution experiment working in this spectral region. The XRP consists of two instruments with a common control, data handling and power system. The bent crystal spectrometer is designed for high time resolution studies in lines of Fe i-Fe xxvi and Ca xix. The flat crystal scanning spectrometer provides for 7 channel polychromatic mapping of flares and active regions in the resonance lines of O viii, Ne ix, Mg xi, Si xiii, S xv, Ca xix, and Fe xxv with 14″ spatial resolution. In its spectral scanning mode it covers essentially the entire 1.4–22.5 Å region.This paper summarizes the scientific objectives of the XRP experiment and describes the characteristics and capabilities of the two instruments. Sufficient technical information for experiment feasibility studies is included and the resources and procedures planned for the use of the XRP within the context of the Solar Maximum Mission is briefly discussed.

The Bragg Crystal Spectrometer for SOLAR-A

The Bragg Crystal Spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employs four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like Fexxvi and He-like Fexxv, Caxix, and Sxv in four narrow wavelength ranges with a resolving power (λ/Δλ) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarizes the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.

Solar flare X-ray spectra from the Solar Maximum Mission flat crystal spectrometer

High-resolution solar X-ray spectra obtained with the Flat Crystal Spectrometer aboard the Solar Maximum Mission from two solar flares and a nonflaring active region are analyzed. The 1--22 A region was observed during the flare on 1980 August 25, while smaller spectral regions were repeatedly covered during the 1980 November 5 flare. Voigt profiles were fitted to spectral lines to derive accurate wavelenths and to resolve blends. During the August 25 flare, 205 lines were found in the range 5.68--18.97 A, identifications being provided for all but 40 (mostly weak) lines. Upper limits to flare densities are derived from various line ratios, the hotter (Troughly-equal10/sup 7/ K) ions giving N/sub e/

Extreme-ultraviolet and X-ray spectroscopy of a solar flare loop observed at high time resolution: A case study in chromospheric evaporation

We present extreme-ultraviolet (EUV) and X-ray light curves and Doppler velocity measurements for a GOES class M2 solar flare observed in NOAA Active Region 9433 on 2001 April 24 at high time resolution with the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) satellite (9.83 s) and the Bragg Crystal Spectrometer (BCS) and Hard X-Ray Telescope (HXT) on board the Yohkoh satellite (9.00 s). Coordinated imagery with SOHOs Extreme-ultraviolet Imaging Telescope and the Transition Region and Coronal Explorer satellite reveal that the CDS slit was centered on the flare commencement site; coordinated magnetograms from SOHOs Michelson Doppler Imager are consistent with this site being the footpoint of a flare loop anchored in positive magnetic field near the outer edge of a sunspots penumbra. CDS observations include the preflare quiescent phase, two precursors, the flare impulsive and peak phases, and its slow decline. We find that (1) the average wavelengths of O III, O IV, O V, Ne VI, and He II lines measured during the preflare quiescent phase are equal (within the measurement uncertainties) to those measured during the late decline phase, indicating that they can be used as reference standards against which to measure Doppler velocities during the flare; (2) the EUV lines of O III, O IV, O V, and He II exhibit upflow velocities of ~40 km s-1 during both precursor events, suggestive of small-scale chromospheric evaporation; (3) the Fe XIX EUV intensity rises and stays above its preflare noise level during the second (later) precursor; (4) the maximum upflow velocities measured in Fe XIX with CDS (64 km s-1) and in Ca XIX (65 km s-1) and S XV (78 km s-1) with BCS occur during the flare impulsive phase and are simultaneous within the instrumental time resolutions; (5) the Fe XIX EUV intensity begins its impulsive rise nearly 90 s later than the rise in intensities of the cooler lines; (6) hard X-ray emission arises nearly 60 s after the cool EUV lines begin their impulsive intensity rise; and (7) the EUV lines of O III, O IV, O V, and He II exhibit downflow velocities of ~40 km s-1 during the flare impulsive phase, suggesting momentum balance between the hot upflowing material and the cool downflowing material. Our observations are consistent with energy transport by nonthermal particle beams in chromospheric evaporation theory.

THE SOLAR FLARE 3.8-10 keV X-RAY SPECTRUM

The 3.8–10 keV solar flare spectrum includes lines of highly stripped Ca, Fe, and Ni ions, as well as a continuum steeply falling with energy. Groups of lines at � 7a nd� 8 keV, observed during flares by the broadband RHESSI spectrometer and called here the Fe line and Fe/Ni line features, are formed mostly of Fe lines but with Ni lines contributing to the � 8 keV feature. Possible temperature indicators of these line features are discussed: the peak or centroid energies of the Fe line feature, the line ratio of the Fe line to the Fe/Ni line features, and the equivalent width of the Fe line feature. The equivalent width is by far the most sensitive to temperature. However, results will be confused if, as is commonly believed, the abundance of Fe varies from flare to flare, even during the course of a single flare. With temperature determined from the thermal continuum, the Fe line feature becomes a diagnostic of the Fe abundance in flare plasmas. These results are of interest for other hot plasmas in coronal ionization equilibrium such as stellar flare plasmas, hot gas in galaxies, and older supernova remnants.

Observational Evidence for Mode Coupling in the Chromospheric Network

Oscillations in network bright points (NBPs) are studied at a variety of chromospheric heights. In particular, the three-dimensional variation of NBP oscillations is studied using image segmentation and cross-correlation analysis between images taken in light of Ca II K3, Hα core, Mg I b2, and Mg I b1 - 0.4 A. Wavelet analysis is used to isolate wave packets in time and to search for height-dependent time delays that result from upward- or downward-directed traveling waves. In each NBP studied, we find evidence for kink-mode waves (1.3, 1.9 mHz), traveling up through the chromosphere and coupling with sausage-mode waves (2.6, 3.8 mHz). This provides a means for depositing energy in the upper chromosphere. We also find evidence for other upward- and downward-propagating waves in the 1.3-4.6 mHz range. Some oscillations do not correspond to traveling waves, and we attribute these to waves generated in neighboring regions.

X-ray line widths and coronal heating

We present preliminary results of spectroscopy and imaging of a solar active region and flare plasma in soft X-ray emission lines. Observed X-ray line widths in a nonflaring active region are broader than the Doppler width corresponding to the local electron temperature. An analysis of 41 soft X-ray flares within a single active region reveals a preference for flares to occur at locations that already show enhanced X-ray emission and to favor magnetic complexity over high gradient. However, flares do not appear to be directly responsible for the heating and X-ray production of the active regions.

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 continua and two line features, at ~6.7 and ~8 keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at ~6.7 keV to derive the electron temperature Te, 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 Te. To minimize instrumental problems with high count rates and effects associated with multitemperature 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 Te = 12 MK; the observed N(Fe XXV)/N(Fe) values appear to be displaced from the most recent theoretical values by between 1 and 3 MK.

Solar Flare Abundances of Potassium, Argon, and Sulphur

The absolute abundance of potassium has been determined for the first time from X-ray solar flare line and continuum spectra. The absolute and relative abundances of Ar and S have also been determined. Assuming that the flare plasma is coronal, and since potassium has the lowest first ionization potential (FIP) of any common element on the Sun, this determination is of importance in the continuing debate concerning the nature of the coronal/photospheric element abundance ratios, which are widely considered to depend on the FIP. The measurements were made with the RESIK crystal spectrometer on the Coronas-F spacecraft. A differential emission measure DEM exp(-βTe) was found to be the most consistent with the data of three models considered. We find that the K/H abundance ratio is (3.7 ± 1.0) × 10-7, a factor of 3 times photospheric. Our measured values of the Ar/H ratio, (2.8 ± 0.2) × 10-6, and of the S/H ratio, (2.2 ± 0.4) × 10-5, are equal to previous coronal and photospheric determinations to within uncertainties. These measurements therefore fit a pattern in which low-FIP elements are enriched in the corona by a factor 3 and in which high-FIP elements (including S) have equal coronal and photospheric abundances.

X-ray spectra of solar flares obtained with a high-resolution bent crystal spectrometer

Preliminary results obtained for three solar flares with the Bent Crystal Spectrometer on the SMM are presented. Resonance and satellite lines of Ca XIX and XVIII and FeXXV and XXIV are observed together with the Fe XXVI Ly..cap alpha.. line. Plasma properties are deduced from line ratios and evidence is presented for changes of line widths coincident with the occurrence of a hard X-ray impulsive burst. Fe K..cap alpha.. spectra from a disk center and a limb flare agree with the predictions of a fluorescence excitation model. However, a transient Fe K..cap alpha.. burst observed in a third flare may be explained by the collisional ionization of cool iron by energetic electrons.