Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

Abstract

Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide nowcasting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assimilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, E t . Measurements from 14 riometers during 94 solar proton events (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) are assimilated by optimising the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as E t are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a 3-h lag in the geomagnetic index K p in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many highlatitude riometers now provide absorption measurements in near-real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters.

Examination of the relationship between riometer-derived absorption and the integral proton flux in the context of modeling polar cap absorption

“…CME arrival times were identified based on the sudden commencement (SC) time reported by the International Service on Rapid Magnetic Variations hosted by the Observatorio del Ebro (http://www.obsebre.es/php/geomagnetisme/variaciorap.php). We note that similar considerations were taken by Rogers and Honary []. To our knowledge there is currently no model which incorporates the impacts of auroral absorption.…”

“…Rogers and Honary [] point out that data sets used to develop the D‐RAP model in Sellers et al . [] and Sauer and Wilkinson [] are limited to only a handful of events for a single high‐latitude station and propose optimizing the parameters in equation based on a larger data set.…”

“…Rogers and Honary [] demonstrate the use of their technique for real‐time absorption modeling. For such an application they propose optimizing m d and m n using equation for consecutive instantaneous maps.…”

“…Data for two SPEs and associated PCA events in January and March 2012 are presented for consideration. The impact of relationships derived from these events is gauged by incorporating these relationships into the current D‐RAP absorption modeling [ Sauer and Wilkinson , ] and optimized D‐RAP modeling [ Rogers and Honary , ] techniques and comparing measurement‐derived and modeled absorption values.…”

Near real‐time input to a propagation model for nowcasting of HF communications with aircraft on polar routes

“…Consequently, new data‐assimilative models have been developed in which the parameters of the PCA models are optimized in real time using a weighted nonlinear regression to riometer measurements, and these will be included in the propagation predictions. Details are not included here, and the reader is referred to Rogers and Honary [] and Rogers et al [].…”