Dependence of the IMF sector structure on the solar dipole tilt angle

Abstract

[1] The quasiperiodic variations of the interplanetary magnetic field (IMF) sector structure at the ACE satellite (L1) are investigated using power spectrum analysis. It appears that the two-sector structure of the IMF dominates during the solar maximum, lasting 3 years from 2000 through 2003, and the four-sector structure dominates during the solar activity minimum. A possible explanation is introduced for this phenomenon in which the IMF sector structures vary with the tilt angle of the heliospheric current sheet (HCS) with respect to the solar rotation axis. During the solar activity maximum, the HCS is perpendicular to the ecliptic plane. Therefore the spacecrafts at L1 point are expected to pass through the HCS only twice in one solar rotation, which gives rise to the observed two-sector structure of the IMF. On the other hand, during the solar activity minimum, the HCS is nearly parallel to the ecliptic plane, and the polarity of IMF Bx changes several times in one solar rotation due to the wavy nature of the HCS, which may lead to the observed four-sector structure.

Where are the solar magnetic poles?

“…The misalignment must also have an impact on the solar wind (Pizzo & Gosling 1994;Riley et al 1996), the interplanetary magnetic field and the Earth's magnetosphere and the impact the Sun has on them on a monthly temporal scale. In fact, strong inclinations of the heliospheric current sheet (HCS) have already been reported in the past (Norton et al 2008;Hundhausen 1977;Wang 1993;Du et al 2008) which may be related to the existence of an inclined solar magnetic dipole. Quasiperiodic 27-day variations of the interplanetary magnetic field (±5nT) and solar wind speed (400 km s −1 ) have been detected (Fang & Forbes 2012;Neugebauer et al 2000) and can be interpreted as the result of a tilt angle of the HCS with respect to the solar rotation axis.…”

Global thermospheric density variations caused by high‐speed solar wind streams during the declining phase of solar cycle 23

“…Accordingly, the deformations of the HCS are most pronounced at solar minimum and become less significant at solar maximum [e.g., Hoeksema et al , 1982; Riley et al , 2002]. The wavy HCS in alignment with the ecliptic plane at solar minimum may lead to multiple IMF sector crossings and possibly result in the periodicities observed in solar wind and geomagnetic activity [ Nayar et al , 2001; Du et al , 2008]. Nevertheless, Tsurutani et al [1995] reported that there is no obvious dependence of the occurrence of CIR‐storms on the HCS crossing polarity in the solar minimum year 1974.…”