Geoscience Reference
In-Depth Information
For the case of zero vertical current, the vertical polarization electric field at the
magnetic equator is given by
E
z
=
(
1
+
σ
H
/σ
P
)
E
x
(4.35)
which can also be written as
E
x
κ
−
1
i
E
z
(ν
i
/
i
)
[
E
x
/(
1
+
ν
e
ν
i
/
e
i
)
]
=
/(
1
+
0
)
(4.36)
where
e
,
i
,
ν
e
, and
ν
i
are the usual electron and ion collision frequencies and
gyrofrequencies and
0
=
ν
e
ν
i
/
e
i
(see Appendix B for plots of
0
). In the
equatorial region the east-west electric field is about 0
m, and the maxi-
mum vertical polarization field (at about 105 km) is estimated from (4.35) to be
of the order of 10-15mV
.
5mV
/
/
m (see also Fig. 3.17b). The electrons are magnetized
×
and therefore the
E
B
drift under the influence of this vertical field yields an
east-west electron drift velocity on the order of 400-600m
s. The drift direc-
tion is westward during the day and eastward at night. This flow is sometimes
supersonic; that is, the electrons move faster than the acoustic velocity in the
medium.
In this section we extend material in the review by Fejer and Kelley (1980)
and ask the reader to refer to that publication for numerous references to the
early experimental work in this area. The occurrence of an anomalous scattering
region in the ionospheric E region close to the dip equator was observed ini-
tially from ionosonde records. These echoes were called “equatorial sporadic E”
(E
sq
) echoes because of their apparent similarity to the sporadic E phenomenon
(called E
s
) occurring at other latitudes. However, the characteristics and gener-
ating mechanisms of these two phenomena are now known to be quite differ-
ent. The intensity of the E
sq
is well correlated with the electrojet strength, and
VHF forward-scattering experiments showed that these echoes are field aligned
and are caused by scattering from plasma density irregularities immersed in the
electrojet. The most important results concerning the physics of the electrojet
scattering region have been obtained from VHF radar measurements performed
at the Jicamarca Radar Observatory near Lima, Peru, since 1962. Rocket obser-
vations at Thumba, India; Punta Lobos, Peru; Kwajalein Island; and Brazil have
also provided valuable information on the electrojet irregularities from electric
field and density profiles (see Fig. 3.17b). Multifrequency HF and VHF radar
observations have been performed in Central and East Africa, India, and Brazil,
and VHF/UHF in Peru.
Equatorial E-region irregularities are present during both day and night. The
irregularities are field aligned; that is, the wave number component along the
magnetic field (
k
/
. Radar echoes are observed only when
the radar wave vector is nearly perpendicular to the earth's magnetic field. The
width of the north-south angular spectrum is less than 1
◦
for 3m irregularities.
This property clearly indicates that a plasma process is occurring, since neutral
) is much less than
k
||
⊥
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