From: Matson, Robert (
Date: Mon Jun 05 2000 - 11:54:37 PDT

  • Next message: Mark A. Hanning-Lee: "Obs 6/2, 6/4"

    > An Iridium flare of magnitude -8 was supposed to flare from Iridium #38 at
    > on Jun 2nd.I saw the flare but in an unusual way.The flare was right on
    time but it flared
    > to a -4 magnitude in a rust color before fading into the morning sky.Can
    anyone tell me
    > why it was a rust color and missed the -8 it was supposed to flare.
    This was a penumbral event, with the peak of the flare chopped off
    by a reverse sunrise at the satellite's location.  In other words,
    the geometry *would have been* correct for a -8 flare at your
    location, only the earth got in the way!
    Actually, from a purely geometry perspective, the sun was below
    the horizon at the time of your flare.  Atmospheric refraction bent
    the sunlight by several tenths of a degree.  Under extremely good
    atmospheric conditions, the sun is visible at lower tangent heights,
    allowing greater atmospheric bending.  For example, if you were
    able to see the sun all the way down to the "hard-earth" limb,
    then refraction would bend the sun's image more than 1 degree.
    IRIDFLAR, however, uses a 20-km tangent height cutoff for spring/
    summer predictions (10-km for fall/winter) since atmospheric
    extinction is usually quite high below this altitude.
    The reason your flare was red was due to the high atmospheric
    absorption and scattering of the shorter wavelengths (same reason
    the sun appears redder at sunset).  This was also why it was dimmer
    than the predicted -8.  Which brings up an interesting observation
    for people to try -- seeing the "green flash" from an Iridium
    satellite!  --Rob
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