Sporadic E ionization layers observed with radar imaging and ionospheric modification

Abstract

Sporadic E ionization layers have been observed in the daytime subauroral ionospheric E layer by a 30 MHz radar in Alaska. The radar detects coherent backscatter from meter-scale field-aligned plasma density irregularities. The irregularities were generated by ionospheric modification-by the emission of strong HF electromagnetic waves directly beneath the layers-making the layers visible to the radar. Aperture-synthesis methods are used to generate imagery of the layers from the radar data. The layers are patchy, with patches organized along fronts spaced by tens of kilometers and propagating slowly toward the southwest. Similar, naturally occurring layers are commonly observed at middle latitudes at night in the absence of ionospheric modification. That the patchy layers can be found at high magnetic latitudes during the day argues that they are most likely produced through the interaction of the ionospheric layer with neutral atmospheric waves and instabilities. Attenuation of the radar echoes when the HF emission frequency exceeded the third harmonic of the electron gyrofrequency was observed and is discussed.

VHF Radar Images of Artificial Field‐Aligned Ionospheric Irregularities in the Subauroral E Region

“…Their behavior is not obviously related to the heater cycling and also appears to be indicative of natural ionospheric phenomena. The striations are reminiscent of those analyzed by Hysell et al, ) and associated with drifting, patchy sporadic E layers of the kind commonly found at middle latitudes. Their appearance at subauroral latitudes was novel and constrained the candidate mechanisms that could be responsible.…”

First wind shear observation in PMSE with the tristatic EISCAT VHF radar

“…In addition to the observational findings, also hydrodynamic models show that turbulence can initiate Kelvin‐Helmholtz instability and lead to strong radar echoes from electron irregularities in the presence of heavy ions and charged dust [ Hill et al, ]. It is also worthwhile to note that in other atmospheric studies, the influence of wind shear is observed even at higher altitude, e.g., at sporadic E layers [ Hysell et al, ].…”

Global distribution of neutral wind shear associated with sporadic E layers derived from GAIA

“…As some of the previous studies have suggested, atmospheric tides are at least one of the factors controlling the neutral wind shears. It should be noted, however, that rocket and lidar measurements indicate that both wind shears with the small vertical scale sizes as well as the large shear magnitudes are inconsistent with predictions of tidal theory [ Larsen et al ., ; Williams et al ., ; Hysell et al ., ; Kurihara et al ., ; Hysell et al ., , ]. Figure illustrates the global distribution of the vertical shear of diurnal components of zonal neutral winds at five different altitudes for the June–August period.…”

“…In addition to the neutral wind shear generated by atmospheric tides, which has the vertical scale of about 10 km, smaller‐scale wind shear with a vertical scale of a few kilometers or less must be considered. Rocket and radar observations have suggested that strong wind shear with smaller scales less than a few kilometers play an important role in the formation of E s layer rather than large‐scale wind shear produced in the present model [ Larsen et al ., ; Williams et al ., ; Hysell et al ., ; Kurihara et al ., ; Hysell et al ., , ]. Such small‐scale shear is considered to be associated with dissipation of gravity waves [ Liu , ] or interaction of gravity waves and tides [ Yue et al ., ].…”

“…However, since the wind theory is at least one of the factors which control E s OR, it is meaningful to derive the global distribution of E s OR from a physical model of the atmosphere and the ionosphere, which self‐consistently gives neutral wind shears associated with atmospheric waves originating from the lower atmosphere. It must be kept in mind that there is a possibility that strong wind shear with smaller scales less than a few kilometers play an important role in the formation of E s layer rather than large‐scale wind shear produced in the model [ Larsen et al ., ; Williams et al ., ; Hysell et al ., ; Kurihara et al ., ; Hysell et al ., , ].…”