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SWAN: A Study of Solar Wind Anisotropies on SOHO with Lyman Alpha Sky Mapping

  • Authors: Bertaux, J. L.; Kyrola, E.; Quemerais, E.; Pellinen, R.; Lallement, R.; Schmidt, W.; Berthe, M.; Dimarellis, E.; Goutail, J. P.; Taulemesse, C.; Bernard, C.; Leppelmeier, G.; Summanen, T.; Hannula, H.; Huomo, H.; Kehla, V.; Korpela, S.; Leppala, K.; Strommer, E.; Torsti, J.; Viherkanto, K.; Hochedez, J. F.; Chretiennot, G.; Peyroux, R.; Holzer, T.
  • Publication: Solar Physics, v. 162, p. 403-439, 1995
  • Instrumentation
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  • Abstract: On board the SOHO spacecraft poised at L1 Lagrange point, the SWAN instrument is mainly devoted to the measurements of large scale structures of the solar wind, and in particular the distribution with heliographic latitude of the solar wind mass flux. This is obtained from an intensity map of the sky Lyman α-emission, which reflects the shape of the ionisation cavity carved in the flow of interstellar H atoms by the solar wind. The methodology, inversion procedure and related complications are described. The subject of the latitude variation of the solar wind is shortly reviewed: earlier Lyma α results from Prognox 1976 are confirmed by Ulysses. The importance of the actual value of the solar wind mass flux forthe equation of dynamics in a polar coronal hole is stressed. The instrument is composed of one electronic unit commanding two identical Sensor Units, each of them allowing to map a full hemisphere with a resolution of 1°, thanks to a two-mirrors periscope system. The desing is described in some details, and the rationale for choice between several varians are discussed. A hydrogen absoption cell is used to measure the shape of the interplanetary Lyman α line and other Lyman α emissions. Other types of observations are also discussed: the geocorona, comets (old and new), the solar corona, and a possible signature of the heliopause. The connexion with some other SOHO instruments, in particular LASCO, UVCS, SUMER, is briefly discussed.

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