Magnetospheric MHD Resonances and ULF Pulsations - Ultra and Extremely Low Frequency Electromagnetic Fields - page 257

It should be emphasized that the mechanism we have treated above is not unique
because other mechanisms of 1=f noise such as MHD waves and solar activity
might also be operative.

6.4.9 Electric Field Pulsations at Fair-Weather Conditions

In this section we consider reasonably steady status of the Earth's electric field. This
implies that no processes of charge separation are taking place in the atmosphere
(Chalmers 1967 ). Here we focus our attention alone on short-term ULF electric
field pulsations at fair-weather conditions in lower atmosphere. The study of the
pulsations of both the atmospheric electric field and charge density is indicative
of the existence of certain relation between these pulsations and the turbulent
stirring of charged particles in surface air as well as the drift of space charge
(e.g., see the papers by Ogden and Hutchinson 1970 ; Yerg and Johnson 1974 ;
Anderson 1982 ; Hoppel et al. 1986 ;Anisimovetal. 1999 ). It is believed that the
charge density behaves like a passive air-entraining admixture and the electric field
spectra are therefore controlled by the neutral-gas turbulence that can drag the aero-
electric structures in the near-surface atmospheric layer. In contrast to the traditional
problem of the atmospheric turbulence which deals with the fluctuations of mass
velocity and gas temperature, the electric field fluctuations are nonlocal values since
they depend on the spatial distribution of atmospheric charges around observation
point. Thus, the electric field and charge density pulsations in the lower atmosphere
are a significant indicator of atmospheric dynamics at fair-weather conditions.
Anisimov et al. ( 2002 ) have reported that at the frequencies of 0:01-0:1 Hz the
spectral density, .E/ , of the electric field pulsations in the surface atmospheric
layer obeys the power law .E/ / f n . Under the fair-weather and fog conditions,

the spectral index n varies in the range of 1.23-3.36 with the most probable value
from 2:25 to 3:0. The study of the temporal variations has shown that the structured
pulsations alternate with unstructured variations of the electric field. The spectral
index of the structured pulsation lies within interval 2.03-3.36 whereas the spectra
of the unstructured variations is characterized by n D 1:23-2:89. Furthermore, these
latter variations have small amplitude and energy.
The structured pulsations are thought to be due to the aero-electric structures
flying at a low altitude, which is of the order of the structure size. As would be
expected, the main energy of the aero-electric pulsation is concentrated in the near-
surface atmospheric layer. The theory predicts the spectral density of such electric
variations to be .E/ / f 11=3 .

The unstructured variations can be resulted from the distant submesoscale aero-
electric structures, which move along with mean atmospheric air flow. If the
turbulent pulsations in these structures lie in the inertial subrange, the Kolmogorov
theory predicts that the power spectrum of the pulsations is proportional to f 5=3 .
The spatial horizontal size of such structures is estimated as 0.5-1 km and their
lifetime is not less than 10-20 min.