Hi Rob & Tony, Your statements seem consistent: 'Actually, high solar activity doesn't "compress" the upper atmosphere -- it causes it to expand (to higher altitudes).' and... 'The major solar cycle variation in atmospheric density is to the increase of electromagnetic radiation heating the upper atmosphere. This is all in the Ultaviolet, and gets absorbed at heights above 100Km... During magnetic storms the increased flow of an an elecrtic current causes rapid changes in density because of the heating associated with the current flow.' However I still can't see how solar activity would explain the 20-day cycle that Moritz says he's noticed! According to: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html 'Also called the radiation pressure coefficient (or BSTAR), the parameter is another drag term in the SGP4 predictor.' So BSTAR itself shouldn't have anything to do with atmospheric density- though the other SGP4 drag constants apparently would: tle_def.html above, and http://www.hq.nasa.gov/osf/station/viewing/issvistech.html So the other drag terms apparently would change as the orbital altitude changes: http://www.hq.nasa.gov/osf/station/images/issalt.gif Both BSTAR and the other drag terms presumably would be affected according to whether or not the solar panels were turned toward the sun, or "trimmed" for minimum drag, but it doesn't seem likely to me that such a procedure would be carried out over a period as long as 20 days. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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