Geoscience Reference
In-Depth Information
Red sprites appear as luminous globes red in color with a quite low-
surface intensity. Their frequency is about 1% with respect to the common
lightning. Red sprites occur over the top of stormy cells and can be single
or multiple. They can reach a height of 95 km with a maximum brightness
around 65-75 km. The duration of a red sprite is of few milliseconds. The
optical peak power is about 5-25 MW, while the dimensions are of the order
of 10-20 km. If observed from a height
q
(in km), the apparent magnitude,
forapowerof10
7
W in the area of maximum luminosity is
m
RSprites
=5log(
q
−
70)
−
20
.
8
.
(5)
A red sprite viewed from a height of 400 km has a total apparent magni-
tude of
8. The angular dimension will be about 4
◦
. A hindrance to the
observation of red sprites from space can be due to the fact that they tend
to be projected on the stormy cell and then to be hidden by the lightning
dazzle.
−
2.6.
Blue jets
Much more enigmatic are the
blue jets
, high height luminous phenomena
whose origin is still unknown. Blue jets appear as luminous blue-colored
beams coming from the top of a stormy cell.
They have a conical form with an aperture of about 15
◦
. Their veloc-
ity is about 100 km/s and reach 40-50 km in height, then their luminosity
decreases and they disappear. The phenomenon duration is of the order
of 0.1 s, while the peak power is 10
4
W. If observed from orbit, the total
apparent magnitude in function of the height is
m
BJets
=5log(
q
−
20)
−
13
.
3
.
(6)
At 400 km a blue jet has an apparent magnitude of about
0.4. The prob-
lems arising when these phenomena are observed from space are analogous
to those related to the red sprites: they are geometrically projected against
the stormy cell, and then the emitted radiation is dominated by the light
coming from the lightning. In any case, if the sensor limit magnitude is +6,
the observing height cannot be larger than 1,000 km.
−
2.7.
Elves
Elves are produced by the interaction between the mesosphere (between
75 and 100 km) and the electromagnetic fields originated by very powerful
lightning. They have a flat and circular shape with diameters of about
