Abstract
The imaging of fast-moving electron-density structures in the polar cap presents a unique set of challenges that are not encountered in other ionospheric imaging problems. GPS observations of total electron content in the polar cap are sparse compared to other regions in the Northern Hemisphere. Furthermore, the slow relative motion of the satellites across the sky complicates the problem since the velocity of the plasma can be large in comparison and traditional approaches could result in image blurring. This paper presents a Kalman-filter based method that incorporates a forward projection of the solution based on a model plasma drift velocity field. This is the first time that the plasma motion, rather than just integrations of electron density, has been used in an ionospheric imaging algorithm. The motion is derived from the Weimer model of the electric field. It is shown that this novel approach to the implementation of a Kalman filter provides a detailed view of the polar cap ionosphere under severe storm conditions. A case study is given for the October 2003 Halloween storm where verification is provided by incoherent scatter radars.
Highlights
Imaging the electron density in the Earth’s ionosphere presents a particular challenge at high latitudes. This is for two reasons: firstly the electron density can be moving at a large enough velocity to cause ‘blurring’ (Watermann et al, 2002) and secondly the ground-based datasets result in a more sparse geometrical coverage than at lower latitudes. This results in two requirements for an imaging algorithm beyond those needed for imaging the mid-latitude ionosphere
While much of the early literature on ionospheric imaging deals with the incomplete and biased geometry of the measurements (e.g., Yeh and Raymund, 1993), this paper introduces a new algorithm that addresses the motion of the plasma and the sparsity of data in the high-latitude regions
In this paper we did not aim to address the science; the storm is being used as an extreme example of plasma convection to demonstrate a technique to image the polar cap ionosphere
Summary
Imaging the electron density in the Earth’s ionosphere presents a particular challenge at high latitudes This is for two reasons: firstly the electron density can be moving at a large enough velocity to cause ‘blurring’ (Watermann et al, 2002) and secondly the ground-based datasets result in a more sparse geometrical coverage than at lower latitudes. Spencer et al (2004) showed that a reformulation of the Kalman filter approach using orthogonal profiles with electron-density gradients rather than absolute values can be used successfully in imaging the electron density over the mainland U.S.A. This paper introduces a new a priori constraint for ionospheric imaging; the incorporation of a model of the plasma motion into a Kalmanfilter based ionospheric imaging technique. Herent scatter radars: the European Incoherent Scatter Radar (EISCAT, 69.6°N, 19.2°E) and the EISCAT Svalbard Radar (78.2°N, 15.8°E)
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