Joseph B. Gurman

EIT: Extreme-UltraViolet Imaging Telescope for the SOHO Mission

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 Å), Fe XII (195 Å), Fe XV (284 Å), and He II (304 Å) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 106 K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

Deciphering solar magnetic activity. I. On the relationship between the sunspot cycle and the evolution of small magnetic features

Sunspots are a canonical marker of the Suns internal magnetic field which flips polarity every ~22 yr. The principal variation of sunspots, an ~11 yr variation, modulates the amount of the magnetic field that pierces the solar surface and drives significant variations in our stars radiative, particulate, and eruptive output over that period. This paper presents observations from the Solar and Heliospheric Observatory and Solar Dynamics Observatory indicating that the 11 yr sunspot variation is intrinsically tied to the spatio-temporal overlap of the activity bands belonging to the 22 yr magnetic activity cycle. Using a systematic analysis of ubiquitous coronal brightpoints and the magnetic scale on which they appear to form, we show that the landmarks of sunspot cycle 23 can be explained by considering the evolution and interaction of the overlapping activity bands of the longer-scale variability.

Comparison of the 1998 April 29 M6.8 and 1998 November 5 M8.4 Flares

We combined, and analyzed in detail, the Hα and magnetograph data from Big Bear Solar Observatory (BBSO), full-disk magnetograms from the Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO), coronagraph data from the Large Angle Spectrometric Coronagraph (LASCO) of SOHO, Fe XII 195 A data from the Extreme ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray telescope (SXT) data of the M6.8 flare of 1998 April 29 in National Oceanic and Atmospheric Administration (NOAA) region 8375 and the M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares have remarkable similarities: 1. Partial halo coronal mass ejections (CMEs) were observed for both events. For the 1998 April 29 event, even though the flare occurred in the southeast of the disk center, the ejected material moved predominantly across the equator, and the central part of the CME occurred in the northeast limb. The direction in which the cusp points in the postflare SXT images determines the dominant direction of the CMEs. 2. Coronal dimming was clearly observed in EIT Fe XII 195 A for both but was not observed in Yohkoh SXT for either event. Dimming started 2 hr before the onset of the flares, indicating large-scale coronal restructuring before both flares. 3. No global or local photospheric magnetic field change was detected from either event; in particular, no magnetic field change was found in the dimming areas. 4. Both events lasted several hours and, thus, could be classified as long duration events (LDEs). However, they are different in the following important aspects. For the 1998 April 29 event, the flare and the CME are associated with an erupting filament in which the two initial ribbons were well connected and then gradually separated. SXT preflare images show the classical S-shape sheared configuration (sigmoid structure). For the 1998 November 5 event, two initial ribbons were well separated, and the SXT preflare image shows the interaction of at least two loops. In addition, no filament eruption was observed. We conclude that even though these two events resulted in similar coronal consequences, they are due to two distinct physical processes: eruption of sheared loops and interaction of two loops.

Simultaneous measurements of sunspot umbral oscillations in the photosphere, chromosphere, and transition region

Measurements of umbral oscillations in a sunspot were made simultaneously from space (with the SMM/UVSP instrument) in the C IV transition-region line and from the ground (with the tower telescope at NSO/sunspot) in spectral lines formed in the photosphere and chromosphere. The power spectra of velocity and intensity variations show multiple peaks in the 3 min band (4.5-10 mHz). A strong oscillation at 5.5 mHz is coherent between the chromosphere and transition region. Another strong oscillation mode at 7.5 mHz is coherent between the photosphere and transition region and appears to have a node in the chromosphere. The rms velocity in the 3 min band is a little over 12 km/sec in both the chromosphere and transition region, but the kinetic energy density is lower in the transition region (by a factor of 10 or more) due to the lower mass density there. These measurements of amplitude and phase of the waves at different heights provided a new, independent method of testing or fitting models of the vertical temperature distribution in the umbral chromosphere and transition regions.

Observations of the longitudinal magnetic field in the transition region and photosphere of a sunspot

The Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission spacecraft has observed for the first time the longitudinal component of the magnetic field by means of the Zeeman effect in the transition region above a sunspot. The data presented here were obtained on three days in one sunspot, have spatial resolutions of 10 arc sec and 3 arc sec, and yield maximum field strengths greater than 1000 G above the umbrae in the spot. The method of analysis, including a line-width calibration feature used during some of the observations, is described in some detail in an appendix; the line width is required for the determination of the longitudinal magnetic field from the observed circular polarization.The transition region data for one day are compared with photospheric magnetograms from the Marshall Space Flight Center. Vertical gradients of the magnetic field are computed from the two sets of data; the maximum gradients of 0.41 to 0.62 G km−1 occur above the umbra and agree with or are smaller than values observed previously in the photosphere and low chromosphere.

Spatial and Temporal Properties of Hot and Cool Coronal Loops

A suite of images from the XUV Doppler Telescope (XDT), the Yohkoh Soft X-ray Telescope (SXT), and the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) allow us to see the whole (T > 1 MK) temperature evolution of coronal loops. The detailed morphological comparison of an active region shows that hot loops seen in SXT (T > 3 MK) and cool loops seen in the the EIT 195 A band (T ~ 1.5 MK) are located in almost alternating manner. The anticoincidence of the hot and the cool loops is conserved for a duration much longer than the estimated cooling timescale. However, both hot and cool loops have counterparts in the intermediate-temperature images. The cross-correlation coefficients are higher for neighboring temperature pairs and lower for pairs with larger temperature differences. These results suggest that loops are not isothermal but rather have a differential emission measure distribution of modest but finite width that peaks at different temperatures for different loops.

Sunspot umbral oscillations in Mg II k

Time series observations of the profile of the Mgii k line λ2795.52 have been obtained in five sunspots with the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission. The three sunspots with umbrae larger than the 3″ × 3″ pixel size show significant oscillations in integrated line intensity and line centroid, with frequencies in the range 5.29–7.55 mHz (periods of 132–190 s).The frequencies of significant peaks in average umbral power spectra agree well with the frequencies of the three lowest-frequency transmission peaks predicted by a model of resonant transmission of acoustic waves. If radiative delays are unimportant, and the line centroid can be interpreted straightforwardly as a Doppler shift, the measured velocity-intensity phase differences indicate the superposition of upward- and downward-propagating waves in the umbral chromosphere; this is further evidence for the resonant transmission model.A single, quiet Sun time series of k core profiles yields power spectra and a phase difference consistent with the existence of a chromospheric p-mode.

The Virtual Solar Observatory: status and initial operational experience

The Virtual Solar Observatory (VSO) is a bottom-up grassroots approach to the development of a distributed data system for use by the solar physics community. The beta testing version of the VSO was released in December 2003. Since then it has been tested by approximately 50 solar physicists. In this paper we will present the status of the project, a summary of the communitys experience with the tool, and an overview of the lessons learned.

Measuring proton energies and fluxes using EIT (SOHO) CCD areas outside the solar disk images

An indirect proton flux measuring tool based on discrimination of the energy deposited by protons in 128 × 128 pixel EIT CCD areas outside the solar disk images is presented. Single pixel intensity events are converted into proton incident energy flux using modeled energy deposition curves for angles of incidence ±60° in four EIT spatial areas with different proton stopping power. The extracted proton flux is corrected for both the loss of one-pixel events in the range of angles of incidence as well as for the contribution to the single pixel events resulting from scattered middle-energy protons (low-energy or high-energy particles are stopped by the EIT components or pass through them, accordingly). A simple geometrical approach was found and applied to correct for a non-unique relation between the proton-associated CCD output signal and the incident proton energy. With this geometrical approximation four unique proton incident energy ranges were determined as 45–49, 145–154, 297–335, and 390–440 MeV. The indirect proton flux measuring tool has been tested by comparing Solar Energetic Particles (SEP) flux temporal profiles extracted from the EIT CCD frames and downloaded from the GOES database for the Bastille Day (BD) of 2000 July 14 and the more recent 2005 January 20 events. The SEP flux temporal profiles and proton spectra extracted from the EIT in the relatively narrow energy ranges between 45 and 440 MeV reported here are consistent with the related GOES profiles. The four additional EIT extracted ranges provide higher energy resolution of the SEP data. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The sunspot transition region - Where are the bright plumes and the downflows?

Measurements of umbral-to-quiet sun and umbral-to-plage contrast in five active regions have been obtained in the transition region emission lines Si IV 1402.77 A, C IV 1548.19 A, and O V 1371.29 A, using the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission. The umbral transition region in these lines appears generally indistinguishable from the quiet transition region. In addition, high-resolution profiles of the C IV lines 1548.19 A, 1550.77 A in the umbrae of eight individual sunspots in different active regions show only weak, mostly subsonic, redshifted components. This result differs sharply from the observations of multiple, strong, often supersonic downflows observed with the HRTS instrument (e.g., Brekke et al., 1987).