Each SWAN sensor is equipped with a Hydrogen cell. The cell is placed in the optical path, and provides quantitative information on the spectral profile of the Ly-α line, with a resolving power of about 3 x105. The cell contains molecular hydrogen at few hundreds of Pascals pressure, dissociated by two heated tungsten filaments. When the cell is activated, hydrogen atoms, produced by the dissociation process, absorb in resonance the Ly-α line emitted by interplanetary (IP) hydrogen atoms.
The cell produces a maximum absorption when the Doppler shift between emitting atoms and absorbing atoms is zero, i.e. when the line-of-sight is perpendicular to the relative velocity vector between the interstellar gas and the satellite.
For most earth locations, this happens along a great circle on the sky , the so-called ZDSC or Zero Doppler Shift Circle. Actually, this is not exactly a circle, but close to.
| Last updated on Mon, Mar 3, 2005 | RL |