Observations of E-region irregularities at mid-latitudes

摘要

We discuss observations of E-region irregularities at mid-latitudes during the current and prior solar cycle. Our observations have been conducted using the Millstone Hill UHF Radar, the Manastash Ridge Radar, and the ISIS Array. The Millstone Hill UHF system is an incoherent scatter radar which observes at 440 MHz (68 cm) and is capable of detecting irregularities at large magnetic aspect angles. The Manastash Ridge Radar and ISIS array are passive radar systems which typically operate using FM radio signals (～3 m) and have widely illuminated fields of view and both bi-static and multi-static geometries. Meter scale E-region irregularities commonly occur during active geomagnetic conditions. They are produced by strong electric fields in conjunction with density gradients and influenced strongly by the plasma composition and temperature environment. The geophysical drivers for the irregularities most often occur at mid-latitudes during significant geomagnetic events (e.g. Kp > 6) but can be associated with a variety of phenomena. These include structuring of the plasma sphere boundary layer, auroral current systems, and natural and artificial transient events. We will discuss observations to date of the irregularities and briefly describe the radar techniques which have been applied. Following this we will describe the meso-scale k-space dependence of the irregularities for the plasma spatial scales associated with our observational systems. Evolution of the irregularities in time and space will be discussed in the context of specific geophysical events. The relationship of the irregularities to magnetosphere ionosphere coupling will be briefly highlighted using satellite data to provide context. Finally, we will describe the prospects for new measurements, open questions which it may be possible to address experimentally, and the potential for meso-scale observation of irregularity events.