1. Absence of a Long Lasting Southward Displacement of the HCS Near the Minimum Preceding Solar Cycle 24 X. P. Zhao, J. T. Hoeksema and P. H. Scherrer Stanford University D24 P110, July 15, 2008 37 th COSPAR Scientific Assembly, Montreal, Canada July 13-20, 2008

2. Abstract Definitive observational evidence of a ~10 degres southward displacement of the heliospheric current sheet (HCS) from the heliographic equator was obtained from Ulysses observations near the minimum phase preceding Solar Cycle 23. The HCS southward displacement near the minimum phase has been suggested to be a persistent pattern. We found from the PFSS model prediction that there are a 3-year lasting southward displacement of the HCS near the minimum preceding solar cycle 22 and 23 [Zhao et al., 2005]. This work shows, however, the absence of similar interval of HCS southward displacement between 2004 and 2008. Is the absence of the 3-year interval an abnormal phenomena?

3. 1. Introduction The heliospheric current sheet (HCS) separates the heliosphere into two magnetic hemispheres with opposite magnetic polarity. Definitive observational evidence of a ~10 deg southward displacement of the HCS from the heliographic equator was obtained from Ulysses observations in the rapid transit from the south to north solar poles between September 1994 and May 1995 [Simpson et al., 1996; Smith et al., 2000]. A similar southward displacement in the green-line corona was observed during solar minima of cycles 21 and 22; statistical analysis of the IMF sector duration also shows a southward displacement of the average HCS around the minimum phase of cycles 20, 21 & 22 [Mursula and Hiltula, 2003].

4. The positive-negative and north-south HCS displacements computed using a potential field - source surface model applied to Wilcox Solar Observatory (WSO) observations between 1976 and 2001 revealed the existence of ~3-year lasting HCS southward displacement near the minimum phase of solar cycles 21 and 22, consistent with the green- line corona observations and the statistical results of HMP polarity [Zhao et al., 2005]. We extend the study to April 2008 to see if the similar long lasting HCS southward displacement occurs between 2003 and 2008 and discuss whether the absence is a normal or abnormal phenomenon.

5. 2. Prediction of the hemispheric asymmetry of the heliomagnetosphere By defining the magnetic hemisphere with the same polarity as the dominant polarity in north (south) polar region as the north (south) heliomagnetic hemisphere (HH), and using the PFSS model, the effective displacement of the HCS from the Sun’s dipole equator, λ m, and from the heliographic equator, λ, can be calculated λ m = arcsin (1 – I n δΩ / 2π ) (1) λ = λ m |cos ψ| (2) Here ‘I n ’ denotes the number of solid angle element (δΩ=4π/2160 for WSO) in the north HH; ψ, tilt angle.

6. Fig. 1 Evolution of Sun’s tilt angle ψ calculated using WSO data from June 1976 to April 2008. The positive (negative) magnetic polarity is in the north polar region when ψ = 0 (180).

7. Fig 2. The difference between positive and negative solid angles corresponding to positive and negative heliomagnetic hemisphere. In 1980s, south positive area is less than north negative area; in 1990s, north positive area is greater than south negative area. Both show that the north hemisphere is more strongly developed and greater than the south hemisphere.

8. Fig. 3 Calculated λ m (dotted line) and λ (solid line). There are two 3-year intervals of southward displacement of the HCS (between 1983:03 & 1986:07 and between1992:04 & 1995:05) preceding Cycle 22 and 23. There is no such 3-year interval preceding Cycle 24 !?

9. Fig. 4 Predicted IMF radial component Br in the positive (away) and negative (toward) heliomagnetic hemisphere. Between 83:03 & 86:07, south positive Br is greater than north negative Br; between 92:04 & 95:05, south negative Br is greater than north positive Br, consistent with the HCS southward displacement. There is no such systematicly N-S asymmetry between 2005 and 2008.

10. 3. Photospheric field in north and south polar regions The southward displacement of the HCS near the minimum has been attributed to the north-south asymmetry in the Sun’s polar magnetic field. The HMF is composed mainly of lower multipole components: the dipole, the quadrupole, and the hexapole. Around sunspot minimum these lower-order multipoles are orientated basically parallel to the Sun’s rotation axis and can be approximately represented by the zonal harmonic coefficients g10, g20 and g40. The polar field represented by g10 has opposite polarity in the north and south polar regions, but the field described by g20 & g40 has the same polarity in both polar regions, giving rise to an asymmetry in the field strength. The absence of the 3-year southward displacement of the HCS preceding the cycle 24 implies the absence of the asymmetry in the field strength between the south and north polar regions.

11. Fig. 5 The difference in WSO mean field amplitude between the north and south polar caps above 55 degrees of north and south heliographic latitude. Each point denotes the rotation-averaged value of the north polar field subtracted by the rotation-averaged value of the south polar field. There are 3-year interval when south polar field is stringer than north polar field. But there is no such asymmetry in the polar field preceding cycle 24.

12. Fig 6. The evolution of the amplitude and sign of the zonal lower-order multipole components, g10, g20 and g40 between June 1976 and April 2008. The opposite sign between g10 and g20 lasts near 3-year in the earlier two intervals, but g20 shows small fluctuations around zero preceding the cycle 24, and g10 is significantly less than what in the earlier two intervals.

13. 4. Duration of two IMF sector structures The southward displacement of the HCS should affect the duration of two sector structures observed near the Earth, but the effect may be contaminated by the ICME, large-amplitude Alfvenic waves, and various stream-stream interaction. As shown in Figure 3, the southward displacement of the HCS is mostly less than 7.2 degrees, its effect may be shown in the in situ observations made in Fall and Spring seasons when the Earth reaches 7.2 degrees of solar latitude. Mursula and Hiltula [2003] have calculated the Toward sector occurrence fractions T/(T+A ) using hourly IMF data between 1967 & 2001 (here T & A denote the total number of Toward & Away sector hours for each 3-month season of Spring=Feb--Apr and Fall=Aug—Oct). They found that the IMF sector in the northern hemisphere is systematically more strongly developed, suggesting the southward displacement of the HCS. We extend the analysis to April, 2008 using IMF daily polarity of OMNI and Leif Svalgaard ( http://omniweb.gsfc.nasa.gov/ and http://www.leif.org/research/).http://omniweb.gsfc.nasa.gov/

14. Figure 7. The same as Figure 1 of Mursula and Hiltula [2003], but extending to April 2008 from 2001 and using daily IMF polarity. The evolution of the toward sector occurrence fraction before 2002 is basically the same as what Mursula and Hiltula obtained, showing that the IMF sector in the northern hemisphere is systematically more strongly developed, but there is no such asymmetry after 2003.

15. Figure 8. The same as Figure 7 but using the IMF daily polarity inferred by Dr. Leif Svalgaard. The evolution of the toward sector occurrence Fraction here is basically the same as Figure 7, implying the validation of IMF polarity data inferred by Dr. Leif Svalgaard in analyzing latitudinal Variation of dominate IMF polarity.

16. Figure 9. The same as Figure 8, but starting from the year of 1926, increasing 4 more solar cycles. The evolution of the toward sector occurrence fraction before the year of 1940 is the same as after 2003, showing the absence of the asymmetry in the development of IMF sector structures between the north and south hemisphere, i.e., the absence of the long lasting southward displacement of the HCS near the minimum phase in solar cycles 16, 17, and 23.

17. Figure 10. The time variation of the sunspot number between Janual1926 and April 2008, showing the normal evolution of the solar activity in solar cycle 23, consistent with Hathaway’s point of view that “the current minimum is not abnormally low or long” (http://science.nasa.gov/headlines/y2008/11jul_solarcycleupdate. html?list801255).

18. 6. Summary & Discussion 6.1 Using the algorithm developed for quantitative estimate of the north-south displacement of the HCS [Zhao et al., 2005] and the WSO data observed until April 2008, we find that, different from Solar cycles 21 and 22, there is no the ~3-year lasting HCS southward displacement near the minimum phase in solar cycle 23. In addition, there is no N-S asymmetry of the polar field amplitude between the two hemispheres from 2003 to 2008.

19. 6.2 The occurrence fraction of the toward sector calculated using the daily IMF polarity both observed and inferred shows the same evolution, i.e. the absence of the asymmetry of IMF sector development between northern and southern hemisphere after the year of 2003. It support our PFSS model prediction of the absence of the long lasting southward displacement of the HCS. By using the daily IMF polarity inferred by Dr. Leif Svalgaard, it is found that there was also no such asymmetry near the minimum phase in solar cycles 16 and 17, suggesting that the long lasting HCS southward displacement near the minimum phase is not a persistent pattern and thus, the absence of the long lasting southward displacement of the HCS near the minimum phase of solar cycle 23 is not an abnormal phenomenon.