Geoscience Reference
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been difficult to distinguish this phenomenon from electrojet-related plasma waves on the
basis of RTI information alone.
The cause of the echoes and the underlying source of free energy have not been identified.
Recent simulations suggest that the marginal magnetization of the ions at these altitudes is
significant and that the irregularities may be due to a class of collisional drift waves. Radar
imaging will play a key role in the ongoing investigation of these irregularities.
Fig. 4. Radar images of so-called “150-km echoes” observed in the afternoon of Mar. 8, 2005.
The maximum unaliased Doppler velocity is 780 m/s. Signal-to-noise ratios shown span
7-25 dB.
Figure 4 presents radar images associated with the so-called “150-km” echoes (Chau, 2004;
Kudeki & Fawcett, 1993; Royrvik & Miller, 1981). These daytime echoes are enigmatic, having
regular and striking but unexplained patterns in RTI representations. The Doppler shifts of the
echoes are known to match the background line-of-sight
E
×
B
drift, implying that dielectric
plasma polarization probably does not play a significant role in irregularity production. The
spectra, in fact, conform in many ways to expectation for incoherent scatter, both looking
perpendicular to
B
and obliquely to
B
, and part of the 150-km echoes constitute an ion-line
enhancement Chau et al. (2009). The most obvious source of free energy for the irregularities
is photoelectron production, which peaks nearby, but the mechanisms at work have yet to be
articulated.
The 150-km echoes are weak compared to echoes from other equatorial plasma density
irregularities. The image in Figure 4 reveals that the echoes are not homogeneous or
beam filling but are instead spatially (and temporally) intermittent. Over time, the spatial
organization of the echoes in the imagery varies abruptly in a way that does not convey the
sense of proper motion.
Lastly, Figure 5 shows images of large-scale waves in the daytime equatorial electrojet (Farley,
1985; Farley & Balsley, 1973; Kudeki et al., 1982). Coherent scatter from the electrojet is the
strongest radar target in the upper atmosphere at VHF frequencies and is produced by a
combination of gradient-drift and Farley-Buneman instability. Here, large-scale gradient drift
waves with wavelengths of 1-2 km can be seen propagating westward under the influence
of a sheared zonal electron
E
×
B
flow associated with a Cowling conductivity. Echoes come
from gradient drift wave turbulence and from small-scale, secondary Farley-Buneman waves,
with large-telltale Doppler shifts. At night, the flow and the propagation direction reverse,
and the wavelength of the dominant large-scale waves increases.
