September 20, 2010, 12:00 pm - 1:00 pm

September 20, 2010, 12:00 pm - 1:00 pm

Latest news about the bashful ballerina

Kalevi Mursula (University of Oulu, Finland)

The heliospheric magnetic field (HMF) is hemispherically asymmetric so that the field dominant in the northern hemisphere is weaker but has a larger area than in the south. As a consequence, the heliospheric current sheet (HCS) is shifted southwards. This asymmetry, also called the bashful ballerina, typically persists during three-year intervals in the late declining to minimum phase of the solar cycle. This pattern has been verified by ecliptic satellites to occur during solar cycles 20-22, and by geomagnetic observations during SC 16-22, so during all the time when relevant measurements exist. However, using the same low-latitude observations, we find that the HCS is considerably less asymmetric during the exceptional SC 23 than in earlier cycles. In order to further study whether the bashful ballerina occurred also during SC 23, we examine the observations by the Ulysses probe around its perihelion pass in 2007. They show that the HMF at high northern latitudes was indeed weaker than in the south, and that the HCS was shifted by the same amount, roughly two degrees southwards, as during the first perihelion pass in 1994 for SC 22. Ulysses also shows that the HCS region was considerably wider during SC 23 than during SC 22, which is likely due to the large tilt angles and weak polar fields in SC 23. Thus, the HCS is indeed southward shifted even during SC 23 but the exceptionally thick HCS hides the asymmetry from low-latitude observations. (“Bashful ballerina is dancing behind a thick HCS screen”). We also note that the exceptional properties of SC 23 agree with historical evidence that the active Sun leads to a greater asymmetry in low-latitude observations, and make a prediction on the future dancing of the ballerina.