SOlar Heliospheric Observatory (SOHO) Solar Wind ANisotropies (SWAN) Instrument Overview A full discussion of the Solar Wind ANisotropies instrument and main observations of the interplanetary medium are found in the paper by Bertaux et al. (1995). Aspects related to comet observations are found in the paper by Combi et al. (2000) from which excerpts are provided here. SWAN Overview ------------- The SOHO spacecraft is stationed at the L1 Lagrange point which lies about 1.5 million km in front of the Earth, enabling continuous temporal coverage of solar and solar wind events. One of the on-board instruments, the Solar Wind ANisotropies (SWAN) camera, makes all-sky hydrogen Lyman-alpha (H Ly-alpha) images of the hydrogen distribution in the interplanetary medium (IPM), providing a global picture both of the solar Ly-alpha flux, which is responsible for illuminating the hydrogen, and of the solar wind, which is responsible for shaping the hydrogen distribution through charge impact ionization. The typical brightness of Ly-alpha emission from the IPM seen from the vicinity of Earth is in the range of 500 to 1000 Rayleighs. On top of the IPM emission background, brightnesses of comets in the field of view can range from a few tens to tens of thousands of Rayleighs. SWAN has two identical sensor units, SU+Z and SU-Z, for observing north and south of the ecliptic, respectively. SWAN's initial absolute calibration was such that 1 Rayleigh of intensity gives 0.84 counts per second per pixel in SU+Z and a factor of 2.6 less in SU-Z. A new calibration was performed using Hubble Space Telescope Goddard High Resolution Spectrometer observations of the IPM, and new calibration factors that varied in time over the first several years of SWAN operation were found. Details are provided in the paper by Combi et al. (2011). Each sensor has an instantaneous field of view (IFOV) of 5 degrees by 5 degrees with a multianode detector of 25 1 degree by 1 degree pixels. Full coma images are made by mosaicking the IFOV across the sky. Some comets in the early years were specifically targeted with special observations that oversampled the 1 degree pixels by a factor, but most comet observations were derived from the standard daily full-sky images. The uncertainties in the tabulated data reflect the mode in which the observations were made. References ---------- 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., and Holzer, T. 1995. SWAN: A Study of Solar Wind Anisotropies on SOHO with Lyman Alpha Sky Mapping. Solar Physics 162, 403-439. Combi, M.R., Lee, Y., Patel, T.S., Maekinen, J.T.T., Bertaux, J.-L., and Quemerais, E. 2011. SOHO/SWAN Observations of Short-Period Spacecraft Target Comets. Astron. J. 141, 128. Combi, M.R., Reinard, A.A., Bertaux, J.L., Quemerais, E., and Maekinen, T. 2000. SOHO/SWAN Observations of the Structure and Evolution of the Hydrogen Lyman-alpha Coma of Comet Hale-Bopp (1995 O1). Icarus 144, 191-202.