SPRG Seminars
September 16, 2008:
"Long-term Variations of Open Flux in the Solar Corona"
Leif Svalgaard
The geomagnetic record allows us to infer the strength of the Interplanetary Magnetic Field, | B |, at Earth for the past ~175 years. We find B to be 4.5 + 0.28 sqrt (SSN) nT, where SSN is the sunspot number. We interpret the SSN-dependent part to be closed flux related to CMEs and flare ejecta, effectively riding on top of a constant minimum of open B of 4.5 nT. At each solar minimum as SSN goes to near zero, the field strength B approaches the same constant value of 4.5 [0.5] nT (plus a small SSN-related residual if the SSN didn't go all the way to zero), corresponding to a nearly constant open flux of ~4x10 14 Wb. We review the evidence (and the growing consensus) for this startling conclusion. As the sun's polar fields vary considerably from cycle minimum to cycle minimum, it seems that the Heliospheric field is not determined by the polar fields, contrary to what is commonly held. As the open flux apparently has stayed close to constant over the past ~175 years, it means that it, in particular, did not double during the past century. In fact, the IMF during the current cycle 23 is very much the same as it was during cycle 13 a century ago. The above conclusions are consistent with GCR-based determinations of B under the assumption that transients play a major role in GCR modulation.