Horseshoes in the High-Latitude Ionosphere

Abstract

In the high-latitude ionosphere, predicting transport of polar cap patches is important because of their impact on radio communications and navigation systems. Lagrangian coherent structures (LCSs) are barriers to transport in nonlinear time-varying flow fields, found by computing the local maximum finite-time Lyapunov exponent (FTLE). We propose that LCSs are barriers governing patch formation. In this work, we compute and visualize the LCSs in high-latitude ionospheric convection by computing the FTLE field with the Ionosphere-Thermosphere Algorithm for Lagrangian Coherent Structures (ITALCS). The Weimer 2005 high-latitude electric potential model and the 12th-generation International Geomagnetic Reference Field (IGRF-12) are used to generate the (Formula presented.) drift field at each gridpoint in the ionosphere. The (Formula presented.) drifts are used as the input to ITALCS. Time-varying structures are detected in two-dimensional ionospheric drifts at high latitudes based on locally maximum forward time FTLE values for both geomagnetically stormy and quiet periods. Typically, the dominant structure is shaped like the letter “U,” or a “horseshoe,” oriented with the curved portion of the “U” on the dayside around local noon. The LCSs during the geomagnetically stormy period have more complex topology and shift equatorward compared to the LCSs during quiet times. Analysis of a polar cap patch observed on 17 March 2015 with the Multi-Instrument Data Analysis System indicates that a necessary condition for its formation and transport is that storm enhanced density exist poleward of the LCS.