Re: BSTAR value

From: Thomas Fly (
Date: Tue Apr 20 2004 - 03:11:23 EDT

  • Next message: Ted Molczan: "TJM obs of 2004 Apr 20 UTC"

    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).'
    '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:
    '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
    So the other drag terms apparently would change as the orbital altitude changes:
    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.
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