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Yohkoh SXT: Introduction

The Yohkoh Science Nuggets

The Authors

Chronological Listing

Topical Listing

We're Fascinated

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Introduction: The Yohkoh Soft X-ray Telescope,
A Fruitful Collaboration

H. S. Hudson, L. W. Acton, and S. Tsuneta

The Yohkoh Soft X-ray Telescope (SXT) emerged from a very perceptive and constructive collaborative agreement between Japan's Institute of Space and Astronautical Science (ISAS) and the USA National Aeronautics and Space Administration (NASA) to cooperate on an ISAS mission to study high energy processes in the sun's atmosphere. The SXT itself was conceived and built by the National Observatory of Japan and the Lockheed Martin Solar and Astrophysics Laboratory. The scientific planning for SXT, and its operation, has involved scientific groups in Japan and the USA. A very strong SXT team priority lay with the early implementation of a comprehensive software system for data handling and analysis. This subsequently evolved into the familiar and powerful SolarSoft system now in use by many solar groups for a large variety of experiments.

The Yohkoh launch (August, 1991) gave us the first solar soft X-ray telescope equipped with a CCD camera: SXT. Although SXT's angular resolution is comparable to the Skylab telescopes, its performance is quite uniform over the entire sun, it has much lower scattered light, much more telemetry, and most importantly, the CCD itself. Such a detector is inherently linear and stable, and (much to our pleasure) robust; it is still going strong ten years later. Almost each day its images bring new thrills, especially since Yohkoh has survived into its second solar maximum. With time we've learned much better how to observe flares and CMEs with a soft X-ray telescope.

The SXT has recorded some well-known things much, much better than previous experiments, e.g., the canonical magnetic reconnection model now has a much firmer observational foundation (see [1], [2], [3] for relevant science nuggets) thanks to SXT images. Trans-equatorial loops ([1], [2], [3]) turn out to be hot and to be related to a class of CMEs. SXT discovered X-ray jets ([1], [2], [3]), and dimmings ([1], [2], [3]) and sigmoids ([1], [2], [3]) associated with coronal mass ejections. Along with the Yohkoh Hard X-ray Telescope, the SXT images have also met Yohkoh's primary objective and have greatly clarified the relationship between hard and soft X-ray sources in solar flares ([1], [2], [3]).

Yohkoh now embarks on its second decade, and from the point of view of orbital dynamics it appears that it could study an entire Hale cycle (22 years), producing an incalculably valuable data base on coronal activity in an era of concern about "space weather." Yohkoh data are freely available to the scientific community at data centers in Japan, the USA and the U.K. We are pleased to share on this CD-ROM some of the scientific "nuggets" from the first decade of Yohkoh observing.