Geoscience Reference
In-Depth Information
are: Zaire, Sudan, Cuba, Mexico (Sierra Madre), India (Himalaya), and
South-East Asian countries.
A typical lightning is composed by a series of electron discharges, in
average four. The duration of each discharge is 35 ms, while the duration
of the series spans from 0.1 to 0.25 s, but can reach 2-3 s. In the electric
discharge the temperature of the atmospheric gas can reach 20,000 K, while
the peak optical power is in the range 7
10
9
W.
9
The spectrum is
black body like with a peak emission around 144 nm. On the Earth every
time about 2,000 stormy cells exist, producing about 100 discharges per
second.
From the Pogson relation between flux density and magnitude, and
using the above-mentioned typical values, we can derive the magnitude dif-
ference between two bodies having different fluxes. Considering as reference
flux the optical solar constant
F
1
(1
.
83
10
7
-3
×
×
10
2
W
/
m
2
), the magnitude
m
1
is
×
26
.
74). The apparent visual magni-
tude of a lightning, having an optical power of 3
−
the apparent Sun visual magnitude (
×
10
9
W and observed from
aheight
q
(km) on the Earth's surface, is:
m
lightning
= 5 log(
q
)
−
27
.
0
.
(4)
Assuming a 400 km height, the apparent peak magnitude of a lightning
is
12
.
5). Then, a light-
ning can be revealed without diculty through the clouds and on the back-
ground of anthropic lights.
The observation of lightning from space differs in a substantial way
from meteor observation not only for the peak magnitude, but because the
electric discharges occur in short time in the same position.
−
14, larger than the Moon apparent magnitude (
−
2.5.
Sprites
The terrestrial electric activity involves not only the troposphere but also
the highest atmospheric levels up to the ionosphere. As already seen, a
cloud-to-ground lightning transports negative charges from the cloud to
the ground. Sometimes, the positive cloud top can lose its charge which
discharges on the ground. When this occurs, the isolated negative region
can create an intense electric field within the ionosphere. This field accel-
erates the electrons towards the ionosphere and, hitting the atmospheric
molecules, they excite their energetic levels. When these molecules return
to the fundamental state, they emit visible radiation. This is the origin of
the so-called
red sprites
.
