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The NWRA Effective Sunspot-Number Index

The effective sunspot number (SSNe) index is one of several similar indexes which use the state of the global ionosphere to monitor the integrated EUV output of the sun. It is determined by fitting a global model of the ionospheric F2-layer critical frequency (foF2) to a set of foF2 observations so that the average difference between model and observations is zero. The fitting is done by adjusting the smoothed sunspot number (SSN) used to drive the model until the fit criterion is met. The SSN value that achieves this is designated the effective SSN for that set of observations.

While the solar EUV flux is a, if not the, primary driver for the global foF2 status, an important secondary driver is geomagnetic activity through its impact on the global distribution of the neutral atmosphere, winds in the neutral atmosphere, and bulk movement of ionospheric plasma by electric fields generated during periods of geomagnetic activity. This can have strong and complex impacts on SSNe calculated from a model fit to foF2 observations. From a purely engineering standpoint, the SSNe is still useful as a “best estimate” of an SSN value to use for assessing radio propagation conditions. However, the strong impact of geomagnetic storms on SSNe makes this parameter more difficult to interpret in terms of solar variability.

The SSNe index presented on the NWRA website is calculated using the URSI coefficient-based model for foF2 commonly referred to as the URSI-88 model. The foF2 data used are provided from a global network of ionospheric sounders currently collected at the National Centers for Environmental Information (NCEI). An SSNe index is generated once per hour for four different set of observed foF2: (1) all observations equatorward of 50 degrees geomagnetic latitude over the past 24 hours (SSNe24), (2) all observations equatorward of 50 degrees geomagnetic latitude over the past 6 hours (SSNe06), (3) all observations between +15 and +50 degrees geomagnetic latitude (NH SSNe24) over the past 24 hours, and (4) all observations between -15 and -55 degrees geomagnetic latitude (SH SSNe24) over the past 24 hours.

References for the SSNe index are as follows:

  • Secan, J.A. and P.J. Wilkinson, Statistical studies of an effective sunspot number, Radio Science, 32, 1717-1724, 1997. [A comparison of various effective sunspot-number indices and discussion of the NWRA SSNe calculation.]
  • Thompson, R. L., User requirements of aerospace propagation-environment modeling and forecasting, in Operational Modeling of the Aerospace Propagation Environment, AGARD-CP-238, edited by C. Soicher, 1978. [A brief discussion of the original SSNe calculated by the USAF.]

NOTE: The foF2 model the SSNe estimate is calculated from was based on the V1.0 sunspot number series. Thus all SSNe values are inherently tied to the V1.0 scaling. In order to compare SSNe with both the V2.0 sunspot series and sunspot number estimates calculated from the 10.7cm solar radio flux using the V2.0 algorithm, the SSNe values need to be divided by 0.7 (the scaling relationship between V1.0 and V2.0). On this site we will annotated clearly when the SSNe has been scaled to V2.0. If no annotation is given, the SSNe values shown (in a list or graph) use the V1.0 scaling.

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