Type-1 echoes from the mid-latitude E-Region ionosphere

C. Haldoupis,D. T. Farley,K. Schlegel

doi:10.1007/s00585-997-0908-2

Abstract

Abstract. This paper presents more data on the properties of type-1 irregularities in the nighttime mid-latitude E-region ionosphere. The measurements were made with a 50-MHz Doppler radar system operating in Crete, Greece. The type-1 echoes last from several seconds to a few minutes and are characterized by narrow Doppler spectra with peaks corresponding to wave phase velocities of 250–350 m/s. The average velocity of 285 m/s is about 20% lower than nominal E-region ion-acoustic speeds, probably because of the presence of heavy metallic ions in the sporadic-E-layers that appear to be associated with the mid-latitude plasma instabilities. Sometimes the type-1 echoes are combined with a broad spectrum of type-2 echoes; at other times they dominate the spectrum or may appear in the absence of any type-2 spectral component. We believe these echoes are due to the modified two-stream plasma instability driven by a polarization electric field that must be larger than 10 mV/m. This field is similar in nature to the equatorial electrojet polarization field and can arise when patchy nighttime sporadic-E-layers have the right geometry.

Highlights

Schlegel and Haldoupis (1994) reported the first evidence of type-1 irregularities in the mid-latitude Eregion ionosphere
Type-1 backscatter is not expected at mid-latitudes where typical electric fields are only a few mV/m, well below the threshold required for excitation of the modified two-stream instability (e.g., Kelley, 1989)
For the SESCAT system, zero Doppler shift corresponds to a frequency offset of exactly 1 kHz because the transmitter and receiver local oscillator frequencies differ by 1 kHz, a technique developed by Koechler et al, (1985) that allows measurement of both positive and negative Doppler displacements

Summary

Introduction

Schlegel and Haldoupis (1994) reported the first evidence of type-1 irregularities in the mid-latitude Eregion ionosphere. The observation of Schlegel and Haldoupis (1994), implied the existence at times, of mid-latitude electric fields at least as large as 15mV/m, i.e., at least an order of magnitude larger than expected. This first mid-latitude type-1 observation was made by SESCAT in its early days of operation. SESCAT, an acronym for sporadic-E Scatter experiment, is a 50MHz continuous-wave radio Doppler system that has been operating in Crete, Greece since 1992 It monitors continuously the backscatted from a fixed E-region volume centered at about 30.8° geomagnetic latitude. We discuss further the origin of the large electric fields that we believe (Haldoupis et al, 1996) are responsible for mid-latitude type-1 wave generation

The experiment

Observed characteristics of mid-latitude type-1 echoes

The type-1 phase velocities

A mechanism for large mid-latitude electric fields

Findings

Discussion