Active Region UV Atlas

An ultraviolet spectral Atlas of a sunspot with high spectral and
spatial resolution in the wavelength region 1190 - 1730 A is
presented. The sunspot was observed with the High Resolution Telescope
and Spectrograph (HRTS). The HRTS instrument was built at the U.S.
Naval Research Laboratory (NRL), Washington, D.C. (Bartoe and
Brueckner, 1975). The instrument combines high spatial, spectral, and
time resolution with an extensive wavelength and angular
coverage. This makes HRTS particularly well suited for studies of fine
structure and mass flows in the upper solar atmosphere. HRTS has
flown six times on rockets between 1975 and 1989 and as a part of
Spacelab 2 in 1985.

The spectrograms used for the Atlas are from the second HRTS rocket
flight, known as HRTS II, flown on 13 February 1978 aboard a Black
Brant VC rocket (NASA Flight 21.042) at White Sands, New Mexico.
During the rocket flight the slit was oriented radially from the solar
disc center through the active region McMath 15139, including a
sunspot, and across the solar limb. The Solar Pointing Aerobee Rocket
Control System (SPARCS) kept the spectrograph slit positioned on the
solar surface during the observing time of 4.2 minutes. The spatial
resolution on this flight was 2 arcsec with a time resolution from 0.2
- 20.2 sec.

The HRTS spectra were recorded on Eastman Kodak 101-01 photographic
film. Microphotometry of the spectrograms has been carried out at the
Institute of Theoretical Astrophysics in Oslo. The data reduction
includes correcting the spectral images for geometrical distortion,
Fourier filtering to remove high frequency noise, transformation to
absolute calibrated solar intensity and calibration of the wavelength
scale.

The absolute intensity calibration was obtained by comparing relative
intensity scans of a quiet solar region with absolute intensities from
the Skylab S082B calibration rocket, CALROC The resulting absolute
intensities are accurate to within 30% (rms).

The wavelength scale was established using solar lines from neutral
and singly ionized atoms as reference lines. From this wavelength
scale velocities accurate to 2 km/s can be measured over the entire
wavelength range. The measured velocities are, however, relative to
the average velocity in the chromosphere where the reference lines are
formed.

The Atlas contains spectra of three different areas in the sunspot and
also of an active region and a quiet region. The selected areas are
averaged over several arcsec, ranging from 3.5 arcsec in the sunspot
to 18 arcsec in the quiet region. The transition region lines in the
Atlas show the most extreme example known of downflowing gas above a
sunspot, a phenomenon which seems to be commonly occurring in sunspots.

One of the selected areas in the sunspot is a light bridge crossing
the spot. This is the most interesting sunspot region where the
continuum radiation is enhanced and measurable throughout the HRTS
spectral range. A number of lines appear which do not occur in the
regular sunspot spectrum.

The Atlas is available in a machine readable form together with an IDL
program to interactively measure linewidths, total intensities and
solar wavelengths. See: http://zeus.nascom.nasa.gov/~pbrekke/HRTS/