April 9, 2010, 12:00 pm - 1:00 pm

April 9, 2010, 12:00 pm - 1:00 pm

The 27-Day Recurrence Index: Its Development and Use

Howard Sargent (NOAA (retired))

By the Nineteen Seventies, high speed streams in the solar wind had been identified as causing recurrent geomagnetic activity. The 27-day Recurrence Index was developed, then, to identify 'seasons' in the eleven-year sunspot cycle when recurrence would be a useful tool for forecasting geomagnetic activity, days in advance. The creation of this index is discussed, as well as the appearance of (and characteristics of) such 'seasons'. Recently, several authors have used the index as proxy data for aspects of solar output stretching back to 1868. The Index has recently been brought up-to-date. Improvements are discussed which should make the index easier to use.