Robert Henry Esser

The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory

The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined by in situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii.

EUV Spectral Line Profiles in Polar Coronal Holes from 1.3 to 3.0 R

Spectral line profiles have been measured for H I λ1216, O VI λλ1032, 1037, and Mg X λ625 in a polar coronal hole observed during 1997 September 15-29, at projected heliographic heights ρ between 1.34 and 2.0 R☉. Observations of H I λ1216 and the O VI doublet from 1997 January for ρ=1.5-3.0 R☉ are provided for comparison. The O VI lines are well fit to a narrow and broad component which appear to be associated with regions of higher and lower spectral radiance, respectively. The narrow components dominate at low heights and become a small fraction of the lines at higher heights. Mg X λ625 is observed to have a narrow component at ρ=1.34 R☉ which accounts for only a small fraction of the observed spectral radiance. In the case of the broad components, the values of v1/e for O VI are only slightly larger than those for H I at ρ=1.34 R☉ but are significantly larger at ρ=1.5 R☉ and much larger for ρ>1.75 R☉. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least ρ=2.0 R☉ but never reach the values of O VI.

How reliable are coronal hole temperatures deduced from observations

Given the importance of the temperature at the base of the corona for the modeling of the solar wind, we investigate the range of temperatures which have been deduced from remote measurements in coronal holes, within a heliocentric distance of 1.6 solar radii, and the accuracy to which these temperatures have been inferred. Results are presented from an analysis of EUV observations made simultaneously in three spectral lines at the limb in a polar coronal hole, with little contamination from quiet region emission. A temperature range of 7.8-9.3 x 10 exp 5 K is obtained, between 1.02 to 1.07 solar radii for the coronal hole, with a very different temperature range of 9.4 x 10 exp 5 - 1.2 x 10 exp 6 K for the quiet regions bordering it. Inhomogeneities within the coronal hole contribute to a 14 percent variation in inferred temperature. The elemental abundance, which is one of the parameters that influence the temperature inference, can in turn be significantly constrained when intensity ratios from three spectral lines are used.

Ultraviolet spectroscopy of solar energetic particle source regions

A problem of fundamental importance for future space travel to the Moon and Mars is the determination and prediction of the radiation environment generated by the Sun. The sources of solar energetic particles (SEP) and the physical processes associated with their acceleration and propagation are not well understood. Ultraviolet coronagraphic spectroscopy uniquely has the capabilities for determining the detailed plasma properties of the likely source regions of such particles. This information can be used to develop empirical models of the source regions for specific events, and it can provide the key information needed to identify and understand the physical processes that produce SEP hazards. UVCS/SOHO observations have provided the first detailed diagnostics of the plasma parameters of coronal mass ejections (CMEs) in the extended corona. These observations have provided new insights into the roles of shock waves, reconnection and magnetic helicity in CME eruptions. Next generation ultraviolet coronagraph spectrometers could provide additional diagnostic capabilities. This paper summarizes past observations, and discusses the diagnostic potential of advanced ultraviolet coronagraphic spectroscopy for characterizing two possible sites of SEP production: CME shocks and reconnection current sheets.

Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory: instrument description and calibration overview

The SOHO ultraviolet coronagraph spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliographic height of 12 R. In this paper, the salient features of the design of the UVCS instrument are described. An overview of the UVCS test and calibration activities is presented. The results from the calibration activity have demonstrated that the UVCS can achieve all its primary scientific observational goals.