Improving the twilight model for polar cap absorption nowcasts

• Published in: Space Weather
• Main Authors: Rogers, N. C., Kero, A., Honary, F., Verronen, P. T., Warrington, E. M., Danskin, D. W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union (AGU), Wiley 2016
• Subjects: Kilpisjärvi; Canopus; Turunen; Lapland
• Online Access: http://onlinelibrary.wiley.com/doi/10.1002/2016SW001527/abstract; http://hdl.handle.net/2381/38479; https://doi.org/10.1002/2016SW001527

We acknowledge the efforts of Don Wallis who was principally responsible for the scientific operation of the CANOPUS riometer array and the NORSTAR team for providing the riometer data used in this study, which are available from ftp://aurora.phys.ucalgary.ca/data/riometer/. SGO riometer measurements are available from http://www.sgo.fi/Data/Riometer/rioData.php, and IRIS (Kilpisjärvi) riometer data are available from http://spears.lancs.ac.uk/data/request.html. NRCan riometer data are provided as a data set in the supporting information and are also available on request from [e-mail]. We also acknowledge the U.S. National Geophysical Data Centre for providing GOES satellite data, available from http://satdat.ngdc.noaa.gov/sem/, and Kp geomagnetic and solar activity indices, available from ftp://ftp.ngdc.noaa.gov. Dst geomagnetic indices were provided by the World Data Centre for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/dstdir/). During solar proton events (SPE), energetic protons ionize the polar mesosphere causing HF radio wave attenuation, more strongly on the dayside where the effective recombination coefficient, αeff, is low. Polar cap absorption models predict the 30 MHz cosmic noise absorption, A, measured by riometers, based on real-time measurements of the integrated proton flux-energy spectrum, J. However, empirical models in common use cannot account for regional and day-to-day variations in the daytime and nighttime profiles of αeff(z) or the related sensitivity parameter, m = A / sqrt(J). Large prediction errors occur during twilight when m changes rapidly, and due to errors locating the rigidity cutoff latitude. Modeling the twilight change in m as a linear or Gauss error-function transition over a range of solar-zenith angles (χl < χ < χu) provides a better fit to measurements than selecting day or night αeff profiles based on the Earth-shadow height. Optimal model parameters were determined for several polar cap riometers for large SPEs in 1998–2005. The optimal χl parameter was found to be most variable, with smaller values (as low as 60°) postsunrise compared with presunset and with positive correlation between riometers over a wide area. Day and night values of m exhibited higher correlation for closely spaced riometers. A nowcast simulation is presented in which rigidity boundary latitude and twilight model parameters are optimized by assimilating age-weighted measurements from 25 riometers. The technique reduces model bias, and root-mean-square errors are reduced by up to 30% compared with a model employing no riometer data assimilation. This work was funded by the UK Engineering and Physical Sciences Research Council grants EP/K007971/1 and EP/K008781/1. A. Kero was supported by the European Regional Development Fund (Regional Council of Lapland, decision number A70179). The work of P.T. Verronen was funded by the Academy of Finland project 276926 (SECTIC: Sun-Earth Connection Through Ion Chemistry). The authors wish to thank Esa Turunen (SGO) for helpful discussions and assistance. Operational support for the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS)/NORSTAR instrument array was provided by the Canadian Space Agency Peer-reviewed Publisher Version