Did Open Solar Magnetic Field Increase During The Last 100 Years? A Reanalysis of Geomagnetic Activity

Abstract

Abstract. Long-term geomagnetic activity presented by the aa index has been used to show that the heliospheric magnetic field has more than doubled during the last 100 years. However, serious concern has been raised on the long-term consistency of the aa index and on the centennial rise of the solar magnetic field. Here we reanalyze geomagnetic activity during the last 100 years by calculating the recently suggested IHV (Inter-Hour Variability) index as a measure of local geomagnetic activity for seven stations. We find that local geomagnetic activity at all stations follows the same qualitative long-term pattern: an increase from early 1900s to 1960, a dramatic dropout in 1960s and a (mostly weaker) increase thereafter. Moreover, at all stations, the activity at the end of the 20th century has a higher average level than at the beginning of the century. This agrees with the result based on the aa index that global geomagnetic activity, and thereby, the open solar magnetic field has indeed increased during the last 100 years. However, quantitatively, the estimated centennial increase varies greatly from one station to another. We find that the relative increase is higher at the high-latitude stations and lower at the low and mid-latitude stations. These differences may indicate that the fraction of solar wind disturbances leading to only moderate geomagnetic activity has increased during the studied time interval. We also show that the IHV index needs to be corrected for the long-term change of the daily curve, and calculate the corrected IHV values. Most dramatically, we find the centennial increase in global geomagnetic activity was considerably smaller, only about one half of that depicted by the aa index.

Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr – Part 1: A new geomagnetic data composite

“…the same time as the larger calibration glitch in the original IDV) and so wrongly concluded that there was no change at all in solar and interplanetary magnetic fields. Using other stations, Mursula et al (2004) found that the calibration of this original version of IHV was indeed poor and so Svalgaard et al (2003) revised their estimates using more stations (lowering their estimate of the calibration error in aa to 5.2 nT and so acknowledging for the first time that at least some of the drift in aa was solar in origin). However, Mursula and Martini (2006) showed that about half of this difference was actually caused by an inhomogeneity in the IHV data series, namely that Svalgaard et al (2003) had used spot values for the early years and hourly means in later years.…”

Resolving issues concerning Eskdalemuir geomagnetic hourly values

“…The significance of this work is that Eskdalemuir data are widely used in studies concerning long-term trends in magnetic field variability and inferred solar variability (for example Mursula et al, 2004;Svalgaard and Cliver, 2007b). For that reason inconsistencies and errors in the Eskdalemuir time series need to be identified and carefully corrected.…”

“…Lockwood et al, 1999) as it is the longest running index of magnetic activity available, dating back to 1868. However some records of observatory hourly values are also very long and this lead to the development and analyses of IHV indices for inferring long-term changes in the Sun and near-Earth space (Svalgaard et al, 2004;Mursula et al, 2004;Mursula and Martini, 2006;Svalgaard and Cliver, 2007a, b). The problem with the Eskdalemuir IHV series was known at this stage and attempts were made to correct it .…”

Geomagnetic response to solar and interplanetary disturbances

“…It is empirically defined but its physical meaning is not yet clear. There is a long list of studies on long-term evolution of the geomagnetic activity and its relationship with the solar variability, such as those published by Feynman & Crooker (1978), Svalgaard (1978), Cliver et al (1996), Andreasen (1997), Cliver et al (1998), Stamper et al (1999), Lockwood et al (1999), Richardson et al (2002), Mursula et al (2001), Svalgaard et al (2003), Echer et al (2004), Svalgaard et al (2004), Mursula et al (2004), Le Mouël et al (2005, Clilverd et al (2005), Svalgaard & Cliver (2005), Svalgaard & Cliver (2007), Rouillard et al (2007), Lockwood et al (2009), Feynman & Ruzmaikin (2011), Du (2011, Richardson & Cane (2012a, 2012b.…”