دم ِ سودیومی
Fr.: queue de sodium
1) A kind of → cometary tail appearing in some
→ comets, such as → Hale-Bopp.
Sodium tails arise from the very strong → fluorescence
of their sodium atom → D lines
in the visible. They are rapidly accelerated
to high velocities by the Sun, forming a very straight tail distinct from the
→ ion tail.
The release mechanism of sodium from comets is still a matter of debate.
Also called → neutral tail.
2) An enormous, comet-like tail of sodium gas stretching out behind the Moon,
at least 800,000 km (many hundreds, and perhaps thousands, of lunar radii). The tail
is due to the strong influence of the Sun's → radiation pressure
on sodium atoms (→ D line) present in the
→ lunar exosphere. Near
→ new Moon phase, the extended lunar sodium tail can
be observed as it sweeps over the Earth
and is gravitationally focused into a visible sodium "spot" in the
→ anti-solar direction.
It is too faint to be detected by the human eye.
→ sodium; → tail.