Geoscience Reference
In-Depth Information
The receivers make it possible to record the lightning-generated low-frequency
electromagnetic waves (5
15 kHz) which propagate via the waveguide
Earth
-
ionosphere
over distances of several thousands kilometers. Estimates have been
obtained of the errors of observations at the network ZEUS (from the viewpoint of
reliability of detection, localization, and characteristic of lightning strikes) by
comparing with the data of independent observations. Such comparisons have been
made for three regions: the eastern coast of the USA/north-western sector of the
Atlantic Ocean, Africa, and Spain. Data for comparison came from the results of
observations made with the use of the lightning sensor mounted on the TRMM
satellite to study precipitation in the tropics, as well as data of the Spanish network
for lightning observations. Results of the comparisons have shown that the errors of
localization (determination of coordinates) of lightning vary within 40
400 km at
distances up to 5,000 km and farther. Within the territory on which the network is
located the errors do not exceed 40 km.
The formation of positive charges in the lower parts of clouds revealed by
calculations with the use of the so-called tripole model of thunderclouds, was
veri
-
ed by results of direct balloon soundings and remote sensing from the data of
ground measurements of the electric
field. However, since the sources of these
charges have not been understood clearly, Mo et al. (2002) undertook direct aircraft
measurements of positive charges in the lower parts of clouds with the use of two
aircraft
flying close by.
Data of observations obtained near New Mexico (USA) on 10 August 1997
demonstrated (at least, in some cases) that near the bottoms of clouds (at an altitude
of about 3.4 km) there were located positive charges formed, apparently, under the
in
fl
uence of lightning strikes. The charge recorded in one of the cases at an altitude
of about 4 km was
fl
1.25 C, which agrees with the data of balloon and ground
measurements of electric
*
fields prescribed
from the data of aircraft observations near the idealized charge dipole with the
instant introduction of the positive charge gave results which agree well with
observations. The observational data indicate that at a ripe stage of formation of the
horizontal distribution of the charge near the cloud bottom, this distribution can be
very complicated and is characterized by a combination of the contacting regions
with opposite charges.
Rakov and Tuni (2003) studied an adequacy of numerical modeling of the
lightning electric
fields. Numerical modeling with electric
field at a great distance using the model of the transmission line
(TL) and modi
ed model MTLE taking into account an exponential decrease of
current with altitude depending on polar angle (elevation) and the rate of propa-
gation of the opposite charge. The shape of the wave of the latter was approximated
by a step function. The same presentation was used for the TL model, whereas in
the case of the MTLE model it was supposed that the electric
field increases
instantly to the level corresponding to TL and then decreases exponentially. The
exponential decrease with altitude (in the case of MTLE) results in a considerable
decrease of the electric
field intensity during about 1 microsecond after reaching a
maximum, especially at low values of the polar angle and high rate of propagation.
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