Earth Planets Space, Vol. 61 (No. 7), pp. e25-e28, 2009E-LETTER
Yasuhiro Minamoto and Yosuke Taguchi
Kakioka Magnetic Observatory, Japan Meteorological Agency, 595 Kakioka, Ishioka-shi, Ibaraki-ken, Japan
(Received April 15, 2009; Revised July 2, 2009; Accepted July 3, 2009; Online published August 3, 2009)
Solar activity was quite low from January 2008 to February 2009, ever since the start of Solar Cycle 24. In 2008, the number of days in a year during which there were no sunspots was the fourth-largest since 1842. However, it is likely that the sunspot number would not have been able to reveal the solar inactivity in the beginning phase of cycle 24 because it hit the lower limit (zero) on more than two-thirds of the days of 2008. Geomagnetic data covers the longest span of time next to sunspot number, but its value has never reached that limit. The K-index, which is a geomagnetic index, from 2008 to the beginning of 2009, has been recorded at several observatories as being the smallest in the history of observation. The Kp-index, which characterizes the geomagnetic planetary activity, has also significantly decreased. Taking into account the relation between the Kp-index during the minimum phase of the solar cycle and sunspot numbers of the following maximum, it has been suggested that the peak solar activity in cycle 24 will be quite low. If solar activity in cycle 24 continues to be quiet, geomagnetic data are expected to be one of the key tools for monitoring space climate.
Key words: Solar activity, geomagnetic, K-index, Kp-index, solar cycle, sunspot number, space climate, KAKIOKA.