1. Ionosonde-Based Indices for Improved Representation of Solar Cycle Variation in IRI Steven Brown, Dieter Bilitza Department of Physics and Astronomy, George Mason University, Fairfax, Virginia ● IG Index, “Global Sunspot Number” is an ionospheric solar index based on foF2 from ionospheric measurements by ionosondes across the globe and CCIR model predictions. ● The International Reference Ionosphere (IRI) model, the international standard for ionospheric parameter specification, currently uses the IG index for foF2 solar cycle variation specification. Key question: Can we improve the description of ionospheric variation across both hemispheres using an IG index calculated over a given hemisphere? Goal: Develop two hemispheric indices in place of IG index Introduction StationGLAT(°N)GLON(°E)StationGLAT(°N)GLON(°E) Ashkhabad37.958.3Canberra *-35.3149.1 Boulder40.0-105.3Christchurch-43.6172.8 Del'ebre40.80.3Grahamstown-33.326.5 Irkutsk52.5104Hobart-42.9147.3 Kokubunji *26.3127.8Norfolk-29.0168.0 Chilton/Slough*51.5-0.6Port Stanley *-51.7-57.8 Rome41.812.5 Figure 1: Comparison of 12-month running means of IG, IG 12 (black), sunspot number, R 12 (Red) and F10.7, F10.7 12 (blue). F10.7 12 is shifted down 50 units for plotability. The three indices show a similar ~12 year solar cycle variation, but are most different from one another during periods of high and low solar activity. Unlike F10.7 12,, both R 12, and IG 12 decreases more during the 2007-2010 deep solar minimum than any other solar minimum period. NOTE: Largest differences between the three indices occurs during solar maxima and the 2007-2010 deep solar minima. Data and Methods Figure 2: Map of ionosonde stations used for the comparison of the new IG indices using foF2 linear regression analysis. Solid blue circles mark stations used for IG index comparisons. Also, the black crosses mark the stations used for the calculation of new IG indices. Data and Methods Continued Figure 3: Stacked plot of foF2 data, model and model terms. (Upper) Plot of Rome ionosonde station foF2 (black) monthly daytime data for with model fit (red) and the residual variation, model-data (blue). (Lower) Plot of model terms shifted for plotability. The scaling for each term is the same as the upper plot. Terms included are the linear solar term, IG 12 (black); solar semi-annual cross term, IG 12 *SA (purple); solar annual cross term, IG 12 *Annual (green); semi-annual term, SA (blue); and annual term, “Annual” (red). NOTE: The solar linear and cross-terms are the largest in the model, thus the most significant. Results ● 12-Month running means of IG N and IG S are similar to IG 12 ~11 year solar cycle variation. ● IG 4 and IG 13 (not shown) 12-month running means also similar to IG 12 ● Differences in indices are most apparent in the monthly values of IG N and IG S, most notably during high solar activity periods Figure 4: Comparison of the calculated indices, IG N and IG S, with IG 12. (Left) Plot of IG N and IG 12 indices. Monthly values of IG N (connected black dots) are plotted with its the 12-month running mean, IG N 12 (solid black curve). IG 12 is plotted as a solid red curve. The difference, IG N 12 – IG 12 (blue), corresponds with the right axis. (Right) The right plot demonstrates the same, but with IG S in place of IG N. Figure 5: Comparison of foF2 (black), model fit (red) and residual (blue) with Rome ionosonde station data as a function of for the different IG indices: IG 12 (upper left), IG 13 (upper right), IG S (lower left), and IG N (lower right). For each plot, the IG index specified in the axis corresponds with the IG index used for model fit. RMS% of the residual variation is printed on each plot. NOTE: Residual variation for indice values larger than 100. ● At Rome (GLAT=41.8 0 ), with the exception of IG S, foF2 model representation with calculated indices (IG 13, IG N ) better describe the month-to-month foF2 variation than IG 12, hence the lower RMS% ● Residual variation for index values greater than 100 (high solar activity) seem to influence value of RMS% ● At Rome, using IG N results in lowest RMS%, 2.2% lower than using IG 12. ● IG 13 reduces RMS% across both hemispheres, IG 4 does not. ● IG N and IG S reduce %RMS the most (by 1-2.5%) and are most effective in their respective hemispheres ● Our analysis shows reductions in RMS% are lost once a 12- month running mean is applied to the indices. ● Monthly values of the hemispheric indices capture more of foF2 monthly variation in model representation than IG 12 ● Hemispheric indices better describe foF2 variation specific to a particular region of the globe, such as the winter anomaly. ● IG 13 reduces %RMS more than IG 4, this suggests increasing the number of stations used for index calculation improves foF2 solar cycle representation Future work ● Repeat index calculation using URSI foF2 model vs. CCIR ● Integrate indices into IRI and evaluate model performance. ● Develop regularly updated daily IG index Conclusion/Future Work Bilitza, D., Brown, S. a., Wang, M. 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