             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.
