Long dance of the bashful ballerina

Hiltula, T.; Мурсула, К.

doi:10.1029/2005gl025198

Cited by 25 publications

(25 citation statements)

References 15 publications

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“…This pattern was later confirmed for several earlier cycles before satellite observations, using HMF sector information extracted from ground-based magnetic field observations (Hiltula & Mursula 2006). Accordingly, the dance of the bashful ballerina has been verified by Earth-based observations for cycles 16-22.…”

Section: Introductionmentioning

confidence: 58%

The last dance of the bashful ballerina?

Мурсула

Virtanen

2010

A&A

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Aims. The heliospheric magnetic field (HMF) has long been hemispherically asymmetric so that the field in the northern hemisphere is weaker and the area larger than in the south. This asymmetry, also called the bashful ballerina, has existed during roughly three-year intervals of the late declining to minimum phase of solar cycles 16-22. We study the HMF and its hemispheric asymmetry during the exceptional solar cycle 23. Methods. We use NASA National Space Science Data Center OMNI database, which contains all solar wind and HMF observations at the Earth's orbit, and coronal field predictions by Wilcox Solar Observatory. We present a new method to study the global hemispheric asymmetry by using the power n of the radial decrease of the radial field from the coronal source surface to 1 AU. Results. We find that the HMF is exceptional at low latitudes in solar cycle 23: while the typical latitudinal variation was attained in the north in 2008, it did not take place in the south until Spring 2009. Thus, the Rosenberg-Coleman rule is abnormally delayed or broken for the first time in 50 years. The n-values verify the clear northern dominance in cycles 21-22. However, the low-latitude observations depict a considerably smaller asymmetry in cycle 23, although Ulysses observations at high latitudes show an equally large asymmetry in 2007 and in 1994-1995. We argue that the weak low-latitude visibility of the asymmetry in cycle 23 is due to the exceptionally weak polar fields, leading to large tilt angle and a wide current sheet. Conclusions. We note that the exceptional properties of cycle 23 (weak dynamo, large tilt, small asymmetry) agree with the long-term evolution of hemispheric asymmetry viewed at the Earth. The active Sun is seen as more asymmetric at the Earth than the quiet Sun because the polar coronal holes with unipolar fields extend closer to the equator, allowing their asymmetry to be viewed even at low latitudes. We suggest that, after the period of weak activity and small asymmetry at 1 AU that started with cycle 23, the hemispheric asymmetry will again, with the increasingly active cycles, become better visible at 1 AU but the asymmetry will be oppositely oriented, including a northward shifted current sheet, and larger areas but weaker intensities in the south. Thus, the ballerina should no longer be systematically bashful for some 100-150 years.

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Section: Introductionmentioning

confidence: 58%

The last dance of the bashful ballerina?

Мурсула

Virtanen

2010

A&A

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“…The asymmetry around zero of the radial (Figure 6(a)) and tangential (Figure 6(b)) components for the SC 23 period is an indication of the recently observed unbalanced ecliptic plane magnetic field polarity. Such unbalanced field polarities are observed quite commonly (e.g., Hiltula and Mursula, 2006) and can be envisioned as a signature of greater heliospheric divergence of a particular sign of the coronal open fields. Figure 6(a) shows that the asymmetry in the distribution of the radial field values switched between SC 22 and 23.…”

Section: Observations: Imf At 1 Aumentioning

confidence: 99%

“…Figure 6(a) shows that the asymmetry in the distribution of the radial field values switched between SC 22 and 23. When the fields switched polarity between the two solar cycles, the heliospheric current sheet maintained its southward cone shape; that is, the solar ballerina remained "bashful" (see Hiltula and Mursula, 2006, for a discussion).…”

Section: Observations: Imf At 1 Aumentioning

confidence: 99%

Effects of the Weak Polar Fields of Solar Cycle 23: Investigation Using OMNI for the STEREO Mission Period

et al. 2009

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The current solar cycle minimum seems to have unusual properties that appear to be related to weak solar polar magnetic fields. We investigate signatures of this unusual polar STEREO field in the ecliptic near-Earth interplanetary magnetic field (IMF) for the STEREO period of observations. Using 1 AU OMNI data, we find that for the current solar cycle declining phase to minimum period the peak of the distribution for the values of the ecliptic IMF magnitude is lower compared to a similar phase of the previous solar cycle. We investigate the sources of these weak fields. Our results suggest that they are related to the solar wind stream structure, which is enhanced by the weak polar fields. The direct role of the solar field is therefore complicated by this effect, which redistributes the solar magnetic flux at 1 AU nonuniformly at low to mid heliolatitudes.

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“…Satellite measurements of the HMF at 1 AU since 1960s show that the southward shift of the HCS, the bashful ballerina phenomenon, occurs systematically in the declining to minimum phase of the solar cycle (Mursula & Hiltula 2003). Daily polarities of the HMF extracted from geomagnetic measurements at high latitudes since 1920s indicate that the southward shift has been a persistent pattern since cycle 16 (Hiltula & Mursula 2006). The analysis of the coronal magnetic field, obtained using the photospheric magnetic field measured at WSO and the PFSS model, verified the southward shift of the HCS by a few degrees during about three years of the declining to minimum phase of SC21 and SC22 (Zhao et al 2005).…”

Section: Introductionmentioning

confidence: 99%

Photospheric and coronal magnetic fields in six magnetographs

Virtanen

Мурсула

2017

A&A

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Aims. We study the long-term evolution of photospheric and coronal magnetic fields and the heliospheric current sheet (HCS), especially its north-south asymmetry. Special attention is paid to the reliability of the six data sets used in this study and to the consistency of the results based on these data sets. Methods. We use synoptic maps constructed from Wilcox Solar Observatory (WSO), Mount Wilson Observatory (MWO), Kitt Peak (KP), SOLIS, SOHO/MDI, and SDO/HMI measurements of the photospheric field and the potential field source surface (PFSS) model. Results. The six data sets depict a fairly similar long-term evolution of magnetic fields and the heliospheric current sheet, including polarity reversals and hemispheric asymmetry. However, there are time intervals of several years long, when first KP measurements in the 1970s and 1980s, and later WSO measurements in the 1990s and early 2000s, significantly deviate from the other simultaneous data sets, reflecting likely errors at these times. All of the six magnetographs agree on the southward shift of the heliospheric current sheet (the so-called bashful ballerina phenomenon) in the declining to minimum phase of the solar cycle during a few years of the five included cycles. We show that during solar cycles 20-22, the southward shift of the HCS is mainly due to the axial quadrupole term, reflecting the stronger magnetic field intensity at the southern pole during these times. During cycle 23 the asymmetry is less persistent and mainly due to higher harmonics than the quadrupole term. Currently, in the early declining phase of cycle 24, the HCS is also shifted southward and is mainly due to the axial quadrupole as for most earlier cycles. This further emphasizes the special character of the global solar field during cycle 23.

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Long dance of the bashful ballerina

Cited by 25 publications

References 15 publications

The last dance of the bashful ballerina?

The last dance of the bashful ballerina?

Effects of the Weak Polar Fields of Solar Cycle 23: Investigation Using OMNI for the STEREO Mission Period

Photospheric and coronal magnetic fields in six magnetographs

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