Turbulent upwelling of the mid‐latitude ionosphere: 2. Theoretical framework

Abstract

In a companion paper, data from the MU radar have been presented which show that during sunspot minimum conditions in Japan the mid‐latitude ionosphere is sometimes characterized by regions of rapid and turbulent upwelling. In this paper we explore possible mechanisms for these events. The most likely process seems to be the instability of the equilibrium which occurs when the mid‐latitude plasma is supported against gravity either by an eastward electric field component or by a southward neutral wind, as was proposed by Perkins (1973). We show, for example, that the growth rate determined by Perkins is considerably higher in sunspot minimum conditions than at sunspot maximum for comparable altitudes of the ionospheric F layer. The growth rate is not very large, however, and we argue here that the observed structures must evolve from preexisting undulations of the bottomside of the F region which are generated by gravity waves. That is, the gravity waves create finite amplitude structures which are amplified by the plasma instability. An intriguing feature of the gravity wave role in this process is that the echoing patches detected by the MU radar and the height bands detected by the Arecibo radar, which we believe to be related phenomena, all seem to propagate to the west. This is the same direction reported for the angle‐of‐arrival measurements of classic mid‐latitude spread F by a number of researchers using ionosonde techniques. Since the MU radar detects the upwelling regions from nonthermal 3‐m irregularities there must be mechanism to create such tracers. We propose that secondary structures are created at intermediate scales via the E×B instability operating on the dome of the upwelling structure and by the neutral wind‐driven process on either the west or the east wall of the structure, depending on the direction of the zonal wind. The 3‐m waves are then created in a cascade process which brings energy into a range of k space in which the structures are linearly damped. Finally, we discuss the fine structure of the echoing patches and suggest several plausible mechanisms, two of which involve E region coupling and one which deals with the vertical structure of the gravity wave seeding process.

Airglow observations over the equatorial ionization anomaly zone in Taiwan

“…The occurrence of MSTID, on the other hand, is found to be frequent during low solar activity years (Candido et al, 2008), when the conditions favor the Perkins instability, which is one suggested mechanism of generating MSTID's, with seeding by atmospheric gravity waves to provide sufficient growth rate (Kelley and Fukao, 1991;Garcia et al, 2000). However, several observations of MSTID occurrence have been reported in the solar maximum period of 1998-2000 over the mid-latitude region (Shiokawa et al, 2003).…”

“…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.…”

“…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.…”

“…The ion-drag force is given by Nm i ν in (U − V ), where N is the ion density, m i is the mass of ion, ν in ion neutral collision frequency, U and V are, respectively, the velocities of neutral and ion (Kelley, 1991). Assuming that the velocities do not change, the ion-drag force at the time of MSTID observation in 2006 is about 16 times less than the corresponding value in the solar maximum year 2000.…”

Imaging observations of nighttime mid-latitude F-region field-aligned irregularities by an MU radar ultra-multi-channel system

“…The Perkins instability cannot explain how 3-m-scale FAIs are generated, because they are supposed to be secondary waves generated after non-linear evolution of primary waves. Kelley et al (1991) proposed that interchange-type instability could be excited by the electron density gradient associated with MSTIDs and the neutral wind. In this case, a certain slope of MSTIDs with ∇n e ·u<0, where n e and u are the electron density and the neutral wind velocity, respectively, would be unstable, and the other slope would be stable.…”