J. Leonard Culhane

Coronal Plasma Motions near Footpoints of Active Region Loops Revealed from Spectroscopic Observations with Hinode EIS

The solar active region 10938 has been observed from the disk center to the west limb with the Hinode EUV Imaging Spectrometer. In the disk-center observation, subsonic upflow motions of tens of km s -->−1 and enhanced nonthermal velocities have been found near the footpoints of the active region loops assuming a single Gaussian approximation for the emission-line profiles. When the same part of the active region is observed near the limb, both upflows and enhanced nonthermal velocities essentially decrease. There is a strong correlation between Doppler velocity and nonthermal velocity. Significant deviations from a single Gaussian profile are found in the blue wing of the line profiles for the upflows. These suggest that there are unresolved high-speed upflows. We discuss the implications for coronal heating mechanisms.

Multispectral observations of chromospheric evaporation in the 1991 November 15 X-class solar flare

We analyze simultaneous H(alpha) images and spectra (from Mees Solar Observatory), and soft and hard X-ray images and spectra (from YOHKOH) during the early phase of an X1.5/3B flare. We investigate the morphological relationship between chromospheric downflows, coronal upflows, and particle precipitation sites, and the energetic relationship between conductive heating, nonthermal particle heating, and the chromospheric response. We find that the observations consistently fit the chromospheric evaporation model. In particular, we demonstrate that the observed upflowing coronal and downflowing chromospheric plasma components originate in the same locations, and we show that our unique set of optical and X-ray observations can clearly distinguish between conductively driven and electron beam driven evaporation.

Yohkoh Soft X-Ray Determination of Plasma Parameters in a Polar Coronal Hole

The Yohkoh Soft X-Ray Telescope (SXT) has been used to study the emission from a coronal hole surrounding the north pole of the Sun. Stronger emission from closed coronal structures in the line of sight can interfere with attempts to measure properties of coronal hole plasma. SXT observations indicate that the north polar region was free of such contamination on 1992 October 3. Measured X-ray intensities, corrected for background and scattered X-rays, are compared with a theoretical coronal hole model. They are found to be broadly consistent with the model predictions for variation of intensity with height and for limb brightening, although the electron density is lower than would be appropriate for model predictions based on solar maximum densities. Electron temperatures estimated by the filter ratio method are also consistent with the model and with an in situ estimate of the maximum electron temperature in the solar wind by the Ulysses ionic charge composition experiment.

Eruption of a Flux Rope on the Disk of the Sun: Evidence for the Coronal Mass Ejection Trigger?

The first evidence of acceleration of a flux rope from the disk of the Sun using the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) is presented. A distinct blueshifted emission component (-480 km s-1) was observed by the EUV spectrometer on SOHO at the start of the impulsive phase of an X2.3 flare. There is a halo coronal mass ejection associated with this event. Based on a sequence of velocity measurements, we determine the acceleration of the erupting material. These results are supported by simultaneous EUV imaging data from the Transition Region and Coronal Explorer spacecraft, which shows the projected motion of the flux rope. The CDS spectra reveal an initial rapid acceleration phase (3.5 km s-2), followed by a transition to a more gradual acceleration (0.68 km s-2). This may indicate energy input via explosive reconnection.

Extreme-ultraviolet imaging spectrometer designed for the Japanese Solar-B satellite

The Extreme-ultraviolet Imaging Spectrometer combines, for the first time, high spectral, spatial and temporal resolution in a satellite based, solar extreme ultraviolet instrument. The instrument optical design consists of a multilayer-coated off- axis paraboloid mirror telescope followed by a toroidal grating spectrometer. The instrument includes thin film aluminum filters to reject longer wavelength solar radiation and employs CCD detectors at the focal plane. The telescope mirror is articulated to allow sampling of a large fraction of the solar surface from a single spacecraft pointing position. Monochromatic images are obtained either by rastering the solar image across the narrow entrance slit or by using a wide slit or slot in place of the slit. Monochromatic images of the region centered on the slot are obtained in a single exposure. Half of each optic is coated to maximize reflectance at 195 angstrom; the other half is coated to maximize reflectance at 270 angstrom. The two EUV wavelength bands were selected to optimize spectroscopic plasma diagnostic capabilities. Particular care was taken to choose wavelength ranges with relatively bright emission lines to obtain precision line of sight and turbulent bulk plasma velocity measurements from observed line profiles. The EIS spectral range contains emission lines formed over a temperature range from approximately 105 - 107 K. The wavelength coverage also includes several density sensitive emission line pairs. These line pairs provide spatial resolution independent density diagnostics at nominal coronal temperatures and densities. Each wavelength band is imaged onto a separate CCD detector. The main EIS instrument characteristics are: wavelength bands -- 180 - 204 angstrom and 250 - 290 angstrom; spectral resolution -- 0.0223 angstrom/pixel (23 - 34 km/second-pixel); slit dimensions -- 4 slits: 1 X 1024 arc- seconds and 50 X 1024 arc-seconds with two positions unspecified as of this writing; fine raster range -- >6 arc-minutes on the sun; coarse raster range -- > 1600 arc- seconds on the sun; largest spatial field of view in a single exposure -- 50 X 1024 arc-seconds; nominal time resolution for active region velocity studies -- 3.4s. The Solar-B satellite is scheduled for launch in August 2005 into a nominal 600 km sun-synchronous orbit.

The magnetic topology of a sigmoid

Abstract Recent surveys of solar features have linked the “sigmoid-to-arcade” scenario observed in the soft X-ray corona to coronal mass ejection (CME) onset (Geophys. Res. Lett. 26 (1999) 627, Geophys. Res. Lett. 14 (1998) 2481). Further to these observations, incorporation of extreme-ultraviolet, white light and H-alpha data into such a survey (Geophys. Res. Lett. 27 (2000) 2161) has illustrated the need for a quantitative definition of the term “sigmoid” and further understanding of such features if they are to be used as a means by which to predict CME onset. We analyse two sample active regions in detail, each appearing both sigmoidal and eruptive in Yohkoh soft X-ray telescope (SXT) full-disk data. Both regions were observed during October 1997 and each produced a flare displaying eruptive characteristics. In each case, formation of a flare-arcade was observed by both SXT and the extreme ultraviolet imaging telescope (EIT) following the event. EUV dimming and coronal EIT waves were also observed in each case. We have studied each active region both before and after eruption using soft X-ray, EUV and H-alpha data. A linear force-free field extrapolation has also been applied as a means by which to determine the active region field deviation from potential in each case. Each active region was observed to erupt by means of a different mechanism and while both events show signatures of eruption and consequently, mass ejection, only one produced a CME large enough to be observed by the SoHO large angle spectroscopic coronagraph. The implications of these observations in terms of CME prediction are discussed.

Coronal Diagnostic Spectrometer: an extreme-ultraviolet 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 15.0 - 80.0 nm. By observing the intensities of selected lines and line profiles, it is possible to derive temperature, density, flow, and abundance information for the plasmas in the solar atmosphere. Spatial resolution down to a few arcseconds and temporal resolution of seconds, allows such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of a large wavelength band provides the capability for simultaneously observing the properties of plasma across the wide temperature ranges of the solar atmosphere. The CDS design makes use of a Wolter-Schwarzschild II telescope which simultaneously illuminates two spectrometer systems, one operating in normal incidence the other in grazing incidence. In this paper we describe the salient features of the design of the CDS instrument and discuss the performance characteristics of CDS as established through pre-delivery test and calibration activities.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.