Barbara J. I. Bromage

The Coronal Diagnostic Spectrometer for the Solar and Heliospheric Observatory

The Coronal Diagnostic Spectrometer is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet wavelength range 150–800 A. By observing the intensities of selected lines and line profiles we may derive temperature, density, flow and abundance information for the plasmas in the solar atmosphere. Spatial and temporal resolutions of down to a few arcseconds and seconds, respectively, allow such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of large wavelength bands provides the capability for simultaneously observing the properties of plasmas across the wide temperature ranges of the solar atmosphere.

Solar EUV spectroscopic observations with SOHO/CDS I. An in-flight calibration study

An in-flight calibration study of the Coronal Diagnostic Spectrometer (CDS) instrument on board SOHO (Solar and Heliospheric Observatory) is presented. The relative intensity calibration of CDS is a funda- mental requirement for deriving important physical parameters of the solar transition region and corona from the observations. This comprehensive study provides the rst complete in-flight relative calibration of all nine CDS channels, rst and second order. This has been achieved with the use of a spectroscopic calibration method, mainly based on the comparison between observed line ratios and theoretical predictions provided by the CHIANTI atomic database. The calibration method has been applied to a large number of observations (on-disc, o-limb, quiet sun, active region), to enable the use of a wide range of spectral lines from low to high temperatures of formation. The results are compared to the pre-launch calibration and other post-launch studies. Signicant dierences with the ground calibration results are found, while there is good agreement with the post-launch studies, based on rocket flights. It is also shown that the relative calibration has not signicantly changed over a long period of time, thus conrming the excellent stability of the CDS instrument.

SOHO-Ulysses Spring 2000 Quadrature: Coronal Diagnostic Spectrometer and SUMER Results

We present results from SOHO/CDS and SOHO/SUMER coordinated observations of coronal streamers made during the spring 2000 quadrature of SOHO, the Sun, and Ulysses. These observations form part of the JOP 112, which is primarily aimed at investigating the composition of the regions of the corona and the solar wind observed by both SOHO and Ulysses. SUMER and CDS observed the low corona from the limb up to about 1.3 R☉ from the Sun center. The physical parameters of the plasma, such as electron density, temperature, emission measure, and composition, are measured along the radial direction, toward Ulysses. The variation in these parameters over the 6 day series of observations was examined, and no significant change was seen. The dependence of these parameters on the solar latitude was also investigated. The results indicate that the observed streamers are homogeneous, of coronal composition, and nearly in hydrostatic equilibrium. The structures remained almost stable during the whole period of the observations.

Temperature structure of active regions deduced from the helium-like sulphur lines

Solar active-region temperatures have been determined from the full-Sun spectra of helium-like sulphur (Sxv) observed by the Bragg Crystal Spectrometer on board theYohkoh satellite. The average temperature deduced from Sxv is demonstrated to vary with the solar activity level: A temperature of 2.5 × 106 K is derived from the spectra taken during low solar activity, similar to the general corona, while 4 × 106 K is obtained during a higher activity phase. For the latter, the high-temperature tail of the differential emission measure of active regions is found most likely due to the superposition of numerous flare-like events (micro/nano-flares).

The laboratory calibration of the SOHO Coronal Diagnostic Spectrometer

The laboratory end-to-end testing of the Coronal Diagnostic Spectrometer (CDS) experiment on the ESA/NASA SOHO mission is reported. A brief overview of CDS, which operates in the extreme-ultraviolet wavelength range, is given. Pertinent details of the calibration source are presented, followed by an account of the source beam characterization. A section is devoted to the determination of the instrument apertures and this includes an outline of the measurements, the results from both the grazing incidence and normal incidence aperture scans and their interpretation to yield estimated aperture areas. Next the measurement of spectrometer bandwidths and their comparison with expected values are described. Then the pre-launch wavelength calibrations are obtained. The section on sensitivity starts with an evaluation of the effects of polarization on the measurements. The expected sensitivities are then derived. The measurements, their analysis and a comparison of measured and expected sensitivities are presented for both the normal incidence and grazing incidence spectrometers. The application of the laboratory calibration to in-flight solar data is discussed.

On the representation of inhomogeneous linear force-free fields

It is shown that there is a false assumption hidden in the description of a relaxed state with inhomogeneous boundary conditions as the vector sum of a potential field, satisfying the boundary conditions, and a sum of eigenfunctions of the associated eigenvalue problem expanded by certain coefficients. In particular, although the Jensen and Chu formula (1984) can provide the correct expansion coefficients, it contains an implicit paradox in its derivation according to a general vector theorem. The same paradox led Chu et al. (1999) to be concerned about a contradiction obtained by taking the curl of their magnetic field expansion which, if permitted, becomes inconsistent with a current normal to the surface. The assumption that the curl can be commuted across an infinite sum of terms is the mechanism leading to these, apparently paradoxical, conclusions. Two mechanisms for resolving this apparent paradox are possible, one of which will be described in some detail below and the other discussed further in a forthcoming, more theoretical paper (Laurence et al., 2000). The decomposition of the magnetic field above is valid with convergence in the mean squared sense, but a decomposition of the current needs to be reinterpreted in terms of negative Sobolev spaces. To avoid this, and remain in a more easily managable and familiar setting, we derive the expansion coefficients in a way that involves the commuting of the inverse curl (as opposed to the curl) and the series. The resulting series converges in a mean square sense. When this is done the calculation can conform to the general vector theorem and a new gauge-invariant expression for the coefficients is obtained. However the consequence of the non-commutability is nullified in the Jensen and Chu formula, in both simply and multiply connected domains, by the important extra requirement of a boundary condition on the vector potential eigenfunctions; this excludes magnetic field eigenfunctions that carry flux, but there remains a complete set for the expansion and all flux is carried by the potential field. The two formulas are then identical. On a different issue, it is shown that if the general expansion is taken over a half-space, by combining positive and negative eigenvalue terms, then the coefficients are anisotropic, that is they are tensors except when evaluated at the first eigenvalue. A specific example is presented to illustrate the situation and to validate the new method of deriving the coefficients.

EUV Observations Above Polar Coronal Holes

We derive electron temperature and density as a function of height up to 0.2 R ⊙ above the limb in polar coronal holes, using five EUV data sets recorded by the SOHO Coronal Diagnostic Spectrometer between July 1997 and February 1998. Radial T e and N e distributions, averaged in a 2° to 10° range of position angles, are the same above the North and South coronal holes. They do not show any time variability over a period of seven months. Polar plumes are found to have lower electron temperature and higher density than the interplume lanes. The electron density slope suggests that the proton temperatures are twice as high as the electron temperatures.

Preliminary results from coordinated SOHO-Ulysses observations

SOHO-Ulysses quadratures occur at times when the SOHO-Sun-Ulysses angle is 90° and offer a unique possibility to compare properties of plasma parcels observed in the low corona with properties of the same parcels measured, in due time, in situ. The June 2000 quadrature occurred at a time Ulysses was at 3.35 AU and at a latitude of 58.2 degrees in the south-east quadrant. Here we focus on the UVCS observations made on June 11, 12, 13, 16. UVCS data were acquired at heliocentric altitudes ranging from 1.6 to 2.2 solar radii, using different grating positions, in order to get a wide wavelength range. The radial direction to Ulysses, throughout the 4 days of observation, traversed a region where high latitude streamers were present. Analysis of the spectra taken by UVCS along this direction shows a variation of the element abundances in the streamers over our observing interval: however, because the radial to Ulysses crosses through different parts of streamers in different days, the variation could be ascrib...

The Solar Magnetic Field as a Coronal Hole Extension Forms: Effects of Magnetic Helicity and Boundary Conditions

An analytical solution is presented for linear force fields within a spherical shell, representing the solar corona. Allowing for a global magnetic helicity, we find magnetic fields over the entire corona with realistic inner boundary conditions obtained from transformation and extrapolation of photospheric magnetograms and considering alternative outer boundary conditions. Such fields are found for the well known coronal hole extension event of August 1996.

A Cosmic Ray Signature of Equatorial Coronal Holes

The evolution of open field regions on the Sun over the last cycle is illustrated by observations of coronal holes in SOHO EIT images. The development of a large equatorial coronal hole near solar minimum is discussed, indicating the processes which led to the appearance of open field regions at low latitude. The observed cosmic ray signature is presented and interpreted in terms of the passage of the Earth through the streamer belt, which at this time had become distorted by the coronal hole and associated active region. The times when such equatorial coronal holes might be expected to directly influence cosmic ray counts in this way are seen to be limited to the approach to solar minimum, around minimum and the approach to maximum.