Scale Dependence of Medium-Scale Traveling Ionospheric Disturbances: Perkins Instability and Es Layer Seeding

Abstract:

Plasma density structures and associated irregularities in the nighttime midlatitude ionosphere are frequently observed as frontal structures elongated from northwest to southeast (NW-SE) in the Northern Hemisphere with a wavelength of 100-200 km, also known as medium-scale traveling ionospheric disturbances (MSTIDs). The MSTIDs and the coupling process between the E and F regions are studied with a midlatitude ionosphere electrodynamics coupling (MIECO) model which can simulate two instability mechanisms: Perkins instability in the F region and sporadic-E (Es)-layer instability in the E region. Using the MIECO model, MSTID structure is reproduced from random perturbation on an Es layer in a wide horizontal coverage. A typical wavelength of ~150 km, larger amplitude, and smaller MSTID's tilt angles at lower latitudes are consistent with observations. It is shown that the polarization process in the E region driven by neutral winds is essentially important for the full development of MSTIDs as well as the seeding of NW-SE perturbation in the F region. The upgraded version of MIECO model comprises both hemispheres where electric field is solved by integrating conductivities in both hemispheres, and all other parameters such as plasma density and neutral wind are independent. In order to study the scale dependence of MSTIDs, the initial perturbation scale on the Es layer is given from 20 km to 320 km range. The results show that shorter scale perturbation tends to saturate faster both at the E and F regions, whereas the Es-layer instability does not work effectively at very long wavelength mode. As a result, the typical wavelength of MSTIDs (100-200 km) can be spontaneously generated without scale-dependent forcing.