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A doubling of the Sun's coronal magnetic field during the past 100 years


The solar wind is an extended ionized gas of very high electrical conductivity, and therefore drags some magnetic flux out of the Sun to fill the heliosphere with a weak interplanetary magnetic field1,2. Magnetic reconnection—the merging of oppositely directed magnetic fields—between the interplanetary field and the Earth's magnetic field allows energy from the solar wind to enter the near-Earth environment. The Sun's properties, such as its luminosity, are related to its magnetic field, although the connections are still not well understood3,4. Moreover, changes in the heliospheric magnetic field have been linked with changes in total cloud cover over the Earth, which may influence global climate5. Here we show that measurements of the near-Earth interplanetary magnetic field reveal that the total magnetic flux leaving the Sun has risen by a factor of 1.4 since 1964: surrogate measurements of the interplanetary magnetic field indicate that the increase since 1901 has been by a factor of 2.3. This increase may be related to chaotic changes in the dynamo that generates the solar magnetic field. We do not yet know quantitatively how such changes will influence the global environment.

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Figure 1: Annual means of a, the geomagnetic activity index, 〈aa 〉, and b, Sargent's recurrence index, 〈I 〉. I is defined for the j th 27-day Carrington rotation period as where c is the correlation coefficient between two consecutive 27-day intervals of 12-hourly aa values13,23.
Figure 2: Time series of observed annual means and corresponding best-fit predicted values for 1964–96.
Figure 3: The total solar magnetic flux emanating through the coronal source sphere12, F s.
Figure 4: Schematic illustration of how the IMF B sw emerges from holes in the solar atmosphere (coronal holes) and is dragged to Earth by the solar wind.

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The data used are stored and made available via World Data Centre C1 for STP at RAL, which is funded by the UK Particle Physics and Astronomy Research Council and, until 1 April 1999, by the National Radio Propagation Programme of the Radiocommunications Agency. We also thank the many scientists who have contributed data to the WDC.

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Correspondence to M. Lockwood.

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Lockwood, M., Stamper, R. & Wild, M. A doubling of the Sun's coronal magnetic field during the past 100 years. Nature 399, 437–439 (1999).

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