These
are images of the
moon's sodium tail (spot) on two consecutive nights in November 1998.
The
spot was brighter on 19 November, closer to the time of New Moon and
then
dimmed
on
the following night. The
20 November image shows the spot shining
though
a pattern of
faint
ripples. These are waves passing through a layer of sodium gas that
occurs
in the
Earth's
upper atmosphere near 90 km altitude.
The lunar sodium tail is an extremely tenous stream of neutral sodium gas that originates from the moon's surface by the imact vaporization of micrometorites. The sodium atoms with velocities comparable to and greater than the lunar escape velocity are lost by the moon and are pushed anti-sunward by solar radiation pressure. In the discovery images above, the spot is located near the anti-lunar and -solar points in the sky. The above images were taken around the time of new Moon and ~2 days after the peak of the 1998 Leonid meteor shower. At that time, the shower produced an enhancement in the amount of impact-vaporized meteoritic material leading to a bright and easily detectable signature in the all-sky images. The existence of tail has been independently verified and the tail has been detected during many subsequent new Moon periods.
An
Adobe Acrobat pdf version of our Discovery
paper of the
tail is here. Other publications are here.
1. Viewing angle has the greatest effect on the apparent brightness of the tail feature. It is brightest during the time of New moon and rapidly fades either side of that time. This effect is shown in the large brightness variation of the tail seen during the new Moon period of November 1998 in the figure below.
2. Intrinsic causes - more interestingly, the tail can also vary in brightness due to meteoric activity, and possibly due to solar activity. An example was during March 2000 when meteor activity was low..
