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Session 1 - Chromosphere, Corona, Flares.
Display session, Friday, June 27
Ballroom B, Chair: Charles Kankelborg
We have used SUMER to obtain deep exposures of the 1300--1400 Å\ spectrum, at the extreme limb and off-limb. Previous ``translimb'' studies in the thermal infrared had revealed remarkable extensions of cold material (T\sim3000 K)---traced by carbon monoxide emission lines---into the heart of the hot chromosphere. A main objective of our program was to search for corresponding far-UV signatures of the ``thermally-bifurcated'' low chromosphere; for example, radiatively fluoresced emissions of the CO A--X 4th-positive system (collisional excitation would be negligible in cold gas). We conducted two separate observing programs with SUMER. Both made use of the 1^\prime\prime-diameter circular aperture, translated across the limb in the minimum motor step increments of 0.\!^\prime\prime375, along the central meridian in the Northern polar coronal hole. The first program executed for nine hours beginning 19\,UT 25 Oct 1996. The full wavelength range was 1340--1400 ÅIt was recorded in two overlapping segments, placing key regions of the spectrum alternately on the KBr and bare parts of the detector, to help isolate 2nd-order features. Each segment was integrated for 500 s, and 32 pairs were obtained to span a 12^\prime\prime swath centered on the optical limb. The second program was conducted 00--09\,UT 01 Dec 1996. It consisted of a single wavelength setting (1300--1340 Åwith exposure time 500 s, but twice the spatial coverage of the earlier series: 64 steps, for a total displacement of 24^\prime\prime. The strong chromospheric resonance lines of atomic oxygen (1302--1305 Åand ionized carbon (1334--1335 Åwere observed on the bare part of the MCP camera.
We report our progress in cataloging the rich, diverse translimb emission spectrum; and our efforts to deduce fundamental properties of the thermally heterogeneous chromosphere. [-3mm]
SOHO\/ is a project of international cooperation; the participation of TRA was supported by NASA grant NAG5-3226.
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