Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 292

in which an off-equatorial plasma is unstable due to gravitational currents. We
take up other midlatitude instabilities in the next section.
With the advent of GPS observations a remarkable structure has been found to
develop during magnetic storms. These storm-enhanced density (SED) features,
one of which is presented in Fig. 6.14c, exhibit total electron content values
of nearly 200 10 16 cm − 2 . They typically develop in the late afternoon and

form a channel of high density at midlatitudes, which becomes entrained in the
convection and moves into the auroral oval and polar cap (Foster et al., 2002).
The origin seems to be the dayside equator where high eastward electric fields
continually pump solar-produced plasma up and into the equatorial fountain
(Vlasov et al., 2003). We have already discussed prompt penetrating electric
fields (PPE) and their effect at the equator in Chapter 4. SED events are very
likely a midlatitude effect of both zonal and meridional electric fields penetrating
to low latitudes. Meridional PPE fields are more difficult to study using the ISR
technique, although Fig. 3.23 shows how a shift to B z north decreases an auroral
zonelike penetrating meridional field overMillstone Hill ( L

×

2). Rowland and
Wygant (1998) have used double probe electric field instruments (see Appendix
A) in the CRESS satellite for this purpose. They showed that as Kp increases,
the radial component of the electric field penetrates deeper and deeper into the
mid- to low latitudes. Note that the radial electric field in the equatorial plane
maps to a meridional component in the ionosphere. For a Kp of 5 or more, this
component is affected for L values as low as 2.

= 3 .

6.6 Midlatitude F-Region Plasma Instabilities

In this section the plasma physics of the midlatitude F-region ionosphere is dis-
cussed. Many of the processes are similar to those discussed already in Chapter 4
and thus need only be briefly reintroduced. E-region processes are presented in
Section 6.7.

6.6.1 F-Region Plasma Instabilities in the Equatorial Anomaly
(Equatorial Arc) Region
The plasma bubbles discussed in Chapter 4 are electrodynamically produced, so
they involve uplift of the entire flux tube. Indeed, the radar map in Fig. 4.4 and
the all-sky image from Christmas Island in Fig. 4.7a show that the depletions
are field aligned. These flux tubes reach sufficiently high altitudes that they map
into the equatorial anomaly. Just such an effect is shown in Fig. 6.15a, an all-
sky camera airglow photograph taken in the anomaly region over Ascension
Island (geographic position 8 ◦ S, 14 ◦ W). The dark bands correspond to plasma
depletions near 300 km altitude, where the airglow originates. They extend from
horizon to horizon in the north-south direction and are 50-100 km in east-west
horizontal size. When mapped to the equatorial plane along magnetic field lines,