Secular trends in storm-level geomagnetic activity

Abstract

Abstract. Analysis is made of K-index data from groups of ground-based geomagnetic observatories in Germany, Britain, and Australia, 1868.0-2009.0, solar cycles 11-23. Methods include nonparametric measures of trends and statistical significance used by the hydrological and climatological research communities. Among the three observatory groups, German K data systematically record the highest disturbance levels, followed by the British and, then, the Australian data. Signals consistently seen in K data from all three observatory groups can be reasonably interpreted as physically meaninginful: (1) geomagnetic activity has generally increased over the past 141 years. However, the detailed secular evolution of geomagnetic activity is not well characterized by either a linear trend nor, even, a monotonic trend. Therefore, simple, phenomenological extrapolations of past trends in solar and geomagnetic activity levels are unlikely to be useful for making quantitative predictions of future trends lasting longer than a solar cycle or so. (2) The well-known tendency for magnetic storms to occur during the declining phase of a sunspot-solar cycles is clearly seen for cycles 14-23; it is not, however, clearly seen for cycles 11-13. Therefore, in addition to an increase in geomagnetic activity, the nature of solar-terrestrial interaction has also apparently changed over the past 141 years.

Correction of artificial jumps in the historical geomagnetic measurements of Coimbra Observatory, Portugal

“…Martini and Mursula, 2008;Love, 2011). Analysis was also made directly on K index series, leading to a similar conclusion that geomagnetic activity has increased over the past 141 yr, although not monotonically (Love, 2011;Love et al, 2012). Since solar cycle 14 in the beginning of the 20th century, a gradual increase until 1960 and then a strong dropout in the early 1960s have been reported, followed by a weaker increasing trend from around 1963 until nowadays (Martini and Mursula, 2008;Love, 2011).…”

“…Martini and Mursula, 2008;Love, 2011). Analysis was also made directly on K index series, leading to a similar conclusion that geomagnetic activity has increased over the past 141 yr, although not monotonically (Love, 2011;Love et al, 2012).…”

“…In fact, the strongest events of the geomagnetic activity take place during the declining phase of the solar activity or close to the minimum (Du, 2011). Besides, Love (2011) showed an increase of the mean level of geomagnetic activity over the past 141 yr. These features are seen not only in the Ki series itself (Figs.…”

“…Analysis was also made directly on K index series, leading to a similar conclusion that geomagnetic activity has increased over the past 141 yr, although not monotonically (Love, 2011;Love et al, 2012). Since solar cycle 14 in the beginning of the 20th century, a gradual increase until 1960 and then a strong dropout in the early 1960s have been reported, followed by a weaker increasing trend from around 1963 until nowadays (Martini and Mursula, 2008;Love, 2011). Furthermore, it has been noticed that the observed increase varies with latitude, being largest at high latitudes, smaller at low latitudes and smallest at midlatitudes (Martini and Mursula, 2008).…”

A history of solar activity over millennia

“…An extension of the geomagnetic series is available from the 1840s using the Helsinki Ak(H) index (Nevanlinna 2004a, b). Although the homogeneity of the geomagnetic series is compromised (e.g., Lukianova et al 2009;Love 2011), it still remains an important indirect index of solar activity. A review of the geomagnetic effects of solar activity can be found, e.g., in Pulkkinen (2007).…”