Browsing by Author "Barnard, L."
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- Near-Earth heliospheric magnetic field intensity since 1750: 1. Sunspot and geomagnetic reconstructionsPublication . Owens, M. J.; Cliver, E.; McCracken, K. G.; Beer, J.; Barnard, L.; Lockwood, M.; Rouillard, A.; Passos, Dário; Riley, P.; Usoskin, I.; Wang, Y-M.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 to 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=0.16nT(2)). The SSN-based estimate of B, from 1750 to 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 to 2013 (r=0.91; MSE=0.24nT(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 supporting information.
- Near-Earth heliospheric magnetic field intensity since 1750: 2. Cosmogenic radionuclide reconstructionsPublication . Owens, M. J.; Cliver, E.; McCracken, K. G.; Beer, J.; Barnard, L.; Lockwood, M.; Rouillard, A.; Passos, Dário; Riley, P.; Usoskin, I.; Wang, Y-M.This is Part 2 of a study of the near-Earth heliospheric magnetic field strength, B, since 1750. Part 1 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 paleocosmic ray (PCR) record is the only data set capable of providing a record of solar activity on millennial timescales. The process for converting Be-10 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 (similar to 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 sunspot number-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 3year averages centered 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.