NASA’s Jet Propulsion Laboratory, Pasadena, California
An analysis of solar, solar wind, and geomagnetic data during the recent solar cycle minimum (2008) has recently been undertaken. It was discovered that the lowest value of the Ap index [this index is an averaged planetary A index based on data from a set of specific magnetometer stations] in the history of its recording occurred during 2009, about 1 year after solar minimum. The investigators were able to determine that it was the decrease of solar and interplanetary forcing that was causing the anomalous effect. The coronal holes, which emit high-speed solar wind plasma streams, were located at middle (solar) latitudes during this epoch of the solar cycle. Thus the full brunt of the solar wind and its embedded Alfvén waves missed the Earth and its magnetosphere, lessening the effect of the solar wind-magnetosphere interaction.
Interplanetary space data and solar information gathered from NASA satellites were used to solve the problem of the anomalously low Ap index. It should be noted that two of the authors (B.T. Tsurutani and W.D. Gonzalez) using archival data are also responsible for reporting the highest known geomagnetic activity on Earth. A magnetic storm that had a ring current intensity of about 1760 nT occurred during 1-2 September 1859.