Abstract
Abstract. Radar observations of the E- and F-region ionosphere from the Arecibo Observatory made during moderately disturbed conditions are presented. The observations indicate the presence of patchy sporadic E (Es) layers, medium-scale traveling ionospheric disturbances (MSTIDs), and depletion plumes associated with spread F conditions. New analysis techniques are applied to the dataset to infer the vector plasma drifts in the F region as well as vector neutral wind and temperature profiles in the E region. Instability mechanisms in both regions are evaluated. The mesosphere–lower-thermosphere (MLT) region is found to meet the conditions for neutral dynamic instability in the vicinity of the patchy Es layers even though the wind shear was relatively modest. An inversion in the MLT temperature profile contributed significantly to instability in the vicinity of one patchy layer. Of particular interest is the evidence for the conditions required for neutral convective instability in the lower-thermosphere region (which is usually associated with highly stable conditions) due to the rapid increase in temperature with altitude. A localized F-region plasma density enhancement associated with a sudden ascent up the magnetic field is shown to create the conditions necessary for convective plasma instability leading to the depletion plume and spread F. The growth time for the instability is short compared to the one described by [Perkins(1973)]. This instability does not offer a simple analytic solution but is clearly present in numerical simulations. The instability mode has not been described previously but appears to be more viable than the various mechanisms that have been suggested previously as an explanation for the occurrence of midlatitude spread F.
Highlights
The relative importance of plasma and neutral drivers in the midlatitude ionosphere is being debated in the contexts of sporadic E (Es) layers, medium-scale traveling ionospheric disturbances (MSTIDs), and plasma density irregularities associated with midlatitude spread F
Patchy Es layers, MSTIDs, and midlatitude spread F occurred during the night of the to the morning of July 2015, over Arecibo during moderately disturbed, low solar flux conditions
We focus our analysis on two aspects of irregularities in the Arecibo dataset: the patchy sporadic E layers and the spread F plume
Summary
The relative importance of plasma and neutral drivers in the midlatitude ionosphere is being debated in the contexts of sporadic E (Es) layers, medium-scale traveling ionospheric disturbances (MSTIDs), and plasma density irregularities associated with midlatitude spread F. The billowy appearance of the layers in incoherent scatter radar observations like those presented by Miller and Smith (1978) and Smith and Miller (1980) points to neutral shear (dynamical) instability as the cause (Larsen, 2000; Bernhardt, 2002; Hysell et al, 2004; Bernhardt et al, 2006; Larsen et al, 2007; Hysell et al, 2009) This premise is consistent with results from Larsen (2002) and Hecht et al (2004), and others which suggest that the mesosphere lower thermosphere region is frequently dynamically if not convectively unstable. Kelley and Miller (1997) and Kelley (2011) argued that MSTIDs are induced merely by gravity waves rather than by Perkins instability and that they propagate in the direction for which dissipation is the weakest (see Ogawa et al, 2009) This could be called the “Perkins stability” effect, whereby the midlatitude ionosphere exerts a damping force unless Perkin’s criteria are met, in which case it is neutral. The data, tools, and simulation are presented and evaluated in a common context
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