Determination of interplanetary magnetic field strength, solar wind speed and EUV irradiance, 1890-2003
Abstract
A newly constructed long-term geomagnetic index, the interdiurnal variability (the IDV index; defined to be the unsigned difference between hourly averages of the H-component of the field near local midnight at a midlatitude station for consecutive days), has the useful property that its yearly averages are highly correlated with the solar wind magnetic field strength (B) and are independent of solar wind speed (V). Existing geomagnetic records allow us to construct IDV since 1890 and thus to determine solar wind B over that period. Once B is known, we use other long-term indices with known dependence on B and V to determine the variation of V since 1890. Average B during 1872-2003 was 6.4 nT with no long-term trend (other than a general correlation with the sunspot number) and average V for the interval 1890-2003 was 433 km/s also with no apparent trend. These results are confirmed using polar cap data available from 1926 to the present and magnetic observations of the Amundsen and Scott polar expeditions for years near 1900. Focusing on geomagnetic activity at local midnight hours cleanly separates the EUV-regulated regular variation (SR) of geomagnetic activity from the solar wind driven component, allowing us to determine EUV variability since 1901. Using older data, all these time series might be extended possibly back to the 1780s.
- Publication:
-
Solar Variability as an Input to the Earth's Environment
- Pub Date:
- September 2003
- Bibcode:
- 2003ESASP.535...15S
- Keywords:
-
- Interplanetary Magnetic Field;
- Solar Wind;
- Solar Irradiance;
- Extreme UV