ALBERT

Beschreibung: We present two separate time series of the near-Earth heliospheric magnetic field strength (B) based on geomagnetic data and sunspot number (SSN). The geomagnetic-based B series from 1845-2013 is a weighted composite of two series that employ the interdiurnal variability index; this series is highly correlated with in situ spacecraft measurements of B (correlation coefficient, r = 0.94; mean square error, MSE = 0.16 nT 2 ). The SSN-based estimate of B, from 1750-2013, is a weighted composite of eight time series derived from two separate reconstruction methods applied to four different SSN time series, allowing determination of the uncertainty from both the underlying sunspot records and the B-reconstruction methods. The SSN-based composite is highly correlated with direct spacecraft measurements of B and with the composite geomagnetic B time series from 1845-2013 (r = 0.91; MSE = 0.24 nT 2 ), demonstrating that B can accurately reconstructed by both geomagnetic and sunspot-based methods. The composite sunspot and geomagnetic B time series, with uncertainties, are provided as supplementary electronic material.

Beschreibung: This is Part 2 of a study of the near-Earth heliospheric magnetic field strength, B, since 1750. Part 1 [ Owens et al ., 2016] produced composite estimates of B from geomagnetic and sunspot data over the period 1750-2013. Sunspot-based reconstructions can be extended back to 1610, but the paleo-cosmic ray (PCR) record is the only data set capable of providing a record of solar activity on millennial timescales. The process for converting 10 Be concentrations measured in ice cores to B is more complex than with geomagnetic and sunspot data and the uncertainties in B derived from cosmogenic nuclides (~20% for any individual year) are much larger. Within this level of uncertainty, we find reasonable overall agreement between PCR-based-B and the geomagnetic- and SSN-based series. This agreement was enhanced by excising low-values in PCR-based B attributed to high-energy solar proton events. Other discordant intervals, with as yet unspecified causes remain included in our analysis. Comparison of three-year averages centred on sunspot minimum yields reasonable agreement between the three estimates, providing a means to investigate the long-term changes in the heliospheric magnetic field into the past even without a means to remove solar proton events from the records.

Beschreibung: We use cosmic radiation records (neutron monitor and the cosmogenic radionuclides, 10 Be and 14 C) as a proxy to compare the solar activity during the extended solar minimum 2006-9, with that during the Grand Solar Minima and Maxima that occurred between 1391-2010. The inferred cosmic ray intensities during the Spoerer, Maunder, and Dalton Grand Minima were significantly greater than those during 2006-9. The onset phases of the three Grand Minima extended over between two and five Schwabe (sunspot) cycles, the cosmic ray intensity at the Schwabe minima increasing from a value approximating that of 2006-9, to substantially higher values later in the Grand Minimum. The minimum estimated strengths of the heliospheric magnetic field near Earth during the Grand Minima were 2.4nT (Spoerer); 〈2.0nT(Maunder) and 2.6 nT (Dalton), compared to 3.9nT in 2009. We conclude that the periods of highest solar activity during the Maunder Minimum approximated those near the sunspot minima between 1954 and 1996. The average ratio of the maximum to minimum estimated HMF in the six Schwabe cycles in the Maunder Minimum is 1.54 (range 1.30-1.85) compared to 1.52 (1.31-1.63) for the modern epoch suggesting similar operation of the solar dynamo in both intervals. The onset phase of the Maunder Minimum extending over five Schwabe cycles, and the large increase in cosmic ray flux (and decrease in estimated heliospheric magnetic field), leads us to speculate that the magnetohydrodynamic amplification in the solar dynamo exhibits a relaxation time well in excess of the 11 year period of the Schwabe cycle.