Geoscience Reference
In-Depth Information
200 km. They can be associated also with red sprites but form before. Their
duration is less than 1 ms. The elves power is far less than the red sprites
and they can be observed only if they are viewed in profile. Viewed from
orbit, elves are projected against the stormy cell, so their observation can
be very dicult.
2.8. Noctilucent cloud
Noctilucent clouds (NLC) are the highest clouds observable in the atmo-
sphere, their average height is 83 km. The NLC are formed by water ice
and are similar to cirrus, but are thinner and blue or silver colored. Due to
the absence of winds, their evolution is very slow, if compared to the tropo-
spheric clouds. Due to their low-optical thickness, they can be observed at
sunset and dawn, when the Sun height is 6-12 below the horizon. In these
conditions their contrast with the sky is maximum. The NLC are usually
observed during the summer from regions having latitude included between
50 and 60 in both hemispheres. During one year, 10-20 NLC are visible,
thus it is a quite rare phenomenon. From space they can be only visible
from the terminator region, they are very thin, so, if observed from space,
their localization is very dicult. From the ground, in fact, they can be
observed mainly when they are low on the horizon, a condition in which
their optical thickness is higher.
From space and using a large field of view, the observation of NLC is
hampered from their faintness, which requires high-exposure times near the
terminator light.
2.9. Auroras
Auroras are caused by the interaction of charged particles of the solar
wind with the Earth's magnetic field and ionosphere atoms and molecules
(Table 2). The aurora heights span from 70 to a maximum of 1,100 km with
the higher occurrence being between 90 and 110 km. The duration of auro-
ras can span from some seconds to some days and their intensity is enough
high to pose no diculties to satellite observations.
3. Planetary Phenomena
They essentially consist in the same events we can observe on Earth,
except impact flashes on the surface of atmosphereless bodies. Of course,
these phenomena show different characteristics due to the diverse physical
environments.
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