Inferred electric field variability in the polarization jet from Millstone Hill E region coherent scatter observations

Abstract

We investigate the fine-scale structure of the subauroral electric field in the vicinity of the polarization jet and subauroral ion drift (SAID) using coherent E region backscatter observed with the Millstone Hill 440 MHz UHF radar during a moderately disturbed period (Kp = 5) on 13 November 1998. We use a combination of data obtained at high time and spatial resolution and radar response modeling, incorporating magnetic aspect angle sensitivity and radar antenna beam shape. The modeled radar response function depends not only on the magnetic and radar pointing geometry but also on the altitude and extent of the irregularity layer. In addition to modeling of the radar system response, an optimal, regularized deconvolution technique is used to resolve fine-scale spatial structure from the relatively tong pulse length used. At the 440 MHz Millstone radar frequency, coherent backscattered power is a linear function of electric field amplitude, and we are able to relate changes in the range variation of deconvolved power to spatial/temporal structure in the electric field. We observe the polarization jet to have superimposed multiple instances of narrow (0.1°) intense SAID configuration electric fields having lifetimes as short as 1.5 min and gradients on their equatorward edge as large as 4 mV/m per kilometer. Simultaneous DMSP satellite observations of westward ion drift across our experimental field-of view confirms our interpretion of the radar data in terms of a subauroral polarization jet structure spanning over 3° latitude moving poleward at 250-400 m/s velocity (cross-L-shell) past the radar beam with superimposed intense SAID-like electric fields.