Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr – Part 3: Improved representation of solar cycle 11

Abstract

Abstract. Svalgaard (2014) has recently pointed out that the calibration of the Helsinki magnetic observatory's H component variometer was probably in error in published data for the years 1866-1874.5 and that this makes the interdiurnal variation index based on daily means, IDV(1d), (Lockwood et al., 2013a), and the interplanetary magnetic field strength derived from it (Lockwood et al., 2013b), too low around the peak of solar cycle 11. We use data from the modern Nurmijarvi station, relatively close to the site of the original Helsinki Observatory, to confirm a 30 % underestimation in this interval and hence our results are fully consistent with the correction derived by Svalgaard. We show that the best method for recalibration uses the Helsinki Ak(H ) and aa indices and is accurate to ±10 %. This makes it preferable to recalibration using either the sunspot number or the diurnal range of geomagnetic activity which we find to be accurate to ±20 %. In the case of Helsinki data during cycle 11, the two recalibration methods produce very similar corrections which are here confirmed using newly digitised data from the nearby St Petersburg observatory and also using declination data from Helsinki. However, we show that the IDV index is, compared to later years, too similar to sunspot number before 1872, revealing independence of the two data series has been lost; either because the geomagnetic data used to compile IDV has been corrected using sunspot numbers, or vice versa, or both. We present corrected data sequences for both the IDV(1d) index and the reconstructed IMF (interplanetary magnetic field). We also analyse the relationship between the derived near-Earth IMF and the sunspot number and point out the relevance of the prior history of solar activity, in addition to the contemporaneous value, to estimating any "floor" value of the near-Earth interplanetary field.Keywords. Geomagnetism and palaeomagnetism (time variations, secular and long term; instruments and technique) -interplanetary physics (interplanetary magnetic fields)

Solar wind control of ionospheric equivalent currents and their time derivatives

“…The effect of IMF strength is more significant, as the ovals are larger (especially wider in latitude) in Figure than in Figure . This is in agreement with Finch et al [] and Lockwood et al [, , , ]. IMF B y affects most clearly the dayside configuration of < J eq >, as expected [e.g., Weimer , , ].…”

Near‐Earth heliospheric magnetic field intensity since 1750: 1. Sunspot and geomagnetic reconstructions

“…The second approach is that recently undertaken by Lockwood and coworkers [ Lockwood et al ., , , , ], hereafter LEA2013, which uses IDV from daily means, IDV(1d), of H . This definition is equivalent to that for the Bartels' u index [ Mayaud , ], except that IDV(1d) is based on a single station at any one time rather than on a network of stations and IDV(1d) corrects for the effects of secular change in the geomagnetic latitudes of the stations used.…”

“…The second approach is that recently undertaken by Lockwood and coworkers [Lockwood et al, 2013a[Lockwood et al, , 2013b[Lockwood et al, , 2014a[Lockwood et al, , 2014b, hereafter LEA2013, which uses IDV from daily means, IDV(1d), of H. This definition is equivalent to that for the Bartels' u index [Mayaud, 1980], except that IDV(1d) is based on a single station at any one time rather than on a network of stations and IDV(1d) corrects for the effects of secular change in the geomagnetic latitudes of the stations used. While IDV mitigates the effect of the regular diurnal magnetic variation by considering only data for the midnight hour, IDV(1d) does so by averaging over all 24 h a day.…”

“…The Lockwood et al study reinvigorated the field of long‐term solar variability. After ~15 years of lively research based on sunspot data [e.g., Solanki et al ., , ; Wang et al ., ; Tapping et al ., ; Balmaceda et al ., ; Schwadron et al ., ; Vieira and Solanki , ; Vieira et al ., ; Owens and Lockwood , ; Clette et al ., ; Lockwood and Owens , ; Lockwood et al ., ; Yeo et al ., ; Clette and Lefèvre , ; Svalgaard and Schatten , ; Usoskin et al ., ] and the geomagnetic activity record [e.g., Svalgaard et al ., ; Mursula et al ., ; Svalgaard et al ., ; Svalgaard and Cliver , ; Lockwood et al ., ; Mursula and Martini , ; Svalgaard and Cliver , ; Rouillard et al ., ; Svalgaard and Cliver , ; Lockwood et al ., ; Svalgaard and Cliver , ; Lockwood and Owens , ; Lockwood et al ., , , , ; Svalgaard , ], an emerging consensus on the reconstruction of B based on geomagnetic data has been forged for the last century [ Svalgaard and Cliver , ; Lockwood and Owens , ]. As noted by Lockwood and Owens [], “This is a significant development because individually, each [method] has uncertainties introduced by instrument calibration drifts, limited numbers of observatories, and the strength of the correlations employed.”…”

An empirical model of heliospheric cosmic ray modulation on long-term time scale

“…In this model / 0 , a 0 , n and b are free parameters. In order to find the best-fit parameters that minimise the mean square differences between the observed and the modelled /, we performed a non-linear model fitting using the annual modulation potential values, calculated from ground based cosmic ray observations (Usoskin et al 2005(Usoskin et al , 2011, the OSF reconstructed by Lockwood et al (2013aLockwood et al ( , 2013bLockwood et al ( , 2014aLockwood et al ( , 2014b and the tilt angle as described in Section 2. The polarity used for the fitting is defined as p = 1 for positive polarity periods, p = À1 for negative polarity periods and p = 0 for the years when polarity reversal occurs.…”