SWAN is dedicated to the determination of the large scale solar wind distribution in the heliosphere by remote sensing in the Lyman-α (Ly-α) UV light of interplanetary (IP) H atoms. Thus, the word "anisotropies" in SWAN's name refers mainly to the variation with latitude of the solar wind. Therefore SWAN can be qualified of "solar wind mapper", though from an instrumental point of view, it could better be described as a full-sky Ly-α imager.
Solar Wind distribution and solar Ly-α are also modified with solar cycle changes. From solar cycle minimum to solar cycle maximum, solar Ly-α can have up to 60% modulation, the radiation field can have up to a factor of three variation and the solar wind also varies. All these changes result in a variation of ionization rate up to a factor of two, in the ecliptic plane. These variations are reflected on IP neutrals' distribution and thus on IP Ly-α emission.
SWAN's mapping of IP Ly-α also traces the IS neutral H flow inside the Interplanetary Medium, which is modified by coupling with protons via charge-exchange at the heliospheric interface. Thus, SWAN observations contribute to the understanding and tracing of the heliopause signature.
The presence of active regions (sunspots) is another indice of solar activity. Both the intensity and the line profile of the solar Ly-α line changes from quiet regions to active regions. Active regions on Sun's surface illuminate IP neutrals which are in front of them. IP neutrals re-emit the Ly-α radiation received from the active regions. The whole phenomenon looks a lot like a wall screen reflecting the light of a torch pointing at it. SWAN beeing able to observe the gas illuminated both from the visible or from the hidden hemisphere of the Sun, it can easily track the 'reflection' of active regions moving along with solar rotation, even when active regions lie on the hidden face of the Sun. |
The Ly-α emission is the most intense UV emission in comets ("Dirty Snowballs"): Water-ice from the comet's nucleus is evaporated as the comet approaches the Sun. Solar UV radiation splits up water molecules and the liberated H atoms glow in UV light. |
Last updated on Mon, Feb 7, 2005 | DK |