SpreadFand ionospheric currents

Abstract

It is shown that the widely used equilibrium in which the nighttime F region is supported by E×B drifts is unstable if, in addition to the supporting eastward field, a north‐south electric field component exists. The instability, which takes the form of rising and falling sheets of ionization, has a growth rate γ ≈ 3 × 10−4 νin−1 sec−1, where νin is ion‐neutral collision frequency at the F region peak. These conclusions are based on a set of new moment equations that govern the time evolution of the Pedersen conductance and the plasma content of a flux tube. It is argued that temperate‐ and high‐latitude spread F are a result of this instability. The new moment equations also predict: (1) that there is a maximum height‐integrated Pedersen current that the F region can carry; and (2) that barium clouds deform in agreement with observations of recent releases.

Decameter mid-latitude sporadic-E irregularities in relation with gravity waves

“…It is known that the main source of spread-F echoes is the modulation of the F layer by medium-scale gravity waves (Fejer and Kelley, 1980;Bowman, 1990). The initiation of mid-latitude F-region instability by gravity waves was studied theoretically by Huang et al (1994), who extended the instability mechanism ®rst proposed by Perkins (1973). Some correlation between spread-F and Es scattering seems to be observed in our data.…”

A direction‐dependent instability of sporadic‐E layers in the nighttime midlatitude ionosphere

“…In this paper, we show that this equilibrium is in fact unstable, at night, to electrodynamical forces that arise when plane wave perturbations in altitude or density are imposed on the E s layer. We show that the growth rate of the instability depends on the azimuthal alignment of the phase fronts of the plane-wave distortion, a feature that is reminiscient of the Perkins instability [Perkins, 1973]. The dependence on azimuth is consistent with observations of structure in E s layers that show the same preferred alignment.…”

Airglow observations over the equatorial ionization anomaly zone in Taiwan

“…The evening vertical drift, or the pre-reversal electric field increases with solar flux (Abdu et al, 1983;Fejer et al, 1999), which could lead to greater rate of occurrence of ESF and EPB during the periods of higher solar activity (Sahai et al, 2000;Sobral et al, 2002). Growth rate for Perkins instability (Perkins, 1973), which is used to explain the generation of MSTID (Kelley and Fukao, 1991), is inversely proportional to ion-neutral collision frequency. When the solar activity is low, the smaller ion-neutral collision frequency could give a significantly higher growth rate for the Perkins instability (Kelley and Fukao, 1991), thus favoring more occurrence of MSTID than high solar activity period.…”