1 Abramenko V.I., 1 Yurchyshyn, V., 2 Linker, J., 2 Mikic, Z.1 - Big Bear Solar Observatory of NJIT; 2 – Predictive Science Inc., San Diego

Anomalous Abundance of Low-Latitude Coronal Holes in the Current Solar MinimumAnomalous Abundance of Low-Latitude Coronal Holes in the Current Solar Minimum

The area occupied by coronal holes (CHs) inside a belt of 40° around the solar equator is considerably larger in the current minimum relative to the similar phase of the previous minimum. The enhanced CH area is related to a recurrent appearance of five persistent CHs, which survived during 7-27 solar rotations. The MHD modeling showed the open magnetic fields above the most long-lived CH.

Introduction

The Multipole Power Spectrum

Distribution of Low-Latitude CHs

Luhmann, J. G., Lee, C. O., Li, Y., Arge, C. N., Galvin, A. B., Simunac, K., Russell, C. T., Howard, R. A., \& Petrie, G. 2009, Sol. Phys., 256, 285Arge, C. N., Pizzo, V. J.\ 2000, JGR, 105, Issue A5, 10465Linker, J. A., Mikic, Z., \& Schnack, D. D. 1996, Solar drivers of the interplanetary and terrestrial disturbances. ASP Conference Series, 95, Balasubramaniam, K. S., Keil, S.L., \& Smartt, R.N. (eds), 208

References

Coronal holes (CHs), in general, may occur everywhere on the solar disk, however,their spatial distribution, frequency of occurrence and evolution are governedby the solar cycle. Low-latitude CHs tend to be more often seen at the activity maximum and disappear near the activity minimum. A lack of isolated low-latitude CHs results in formation of a well defined closed magnetic configuration near the solar equator (Fig. 1) and as well as the dipole structure of the global magnetic field of the Sun.

The above is not applicable for the current extended solar minimum, when manyglobal parameters of the heliospheric magnetic field significantly differ fromwhat those measured for the past minima. Recent comprehensive analysis of the current solar minimum (Luhmann et al., 2009) based on the PFSS and WSA (Wang-Sheeley-Arge, Arge and Pizzo 2000) modeling and comparison with the STEREO and OMNI measurements of the solar wind. The model results clearly demonstrate an excess of low and mid-latitude CHs during the current minimum and show reduced magnitude of the solar polar magnetic field. The observed diminution of the polar magnetic field also implies reduction ofthe dipole component relative to the rest of the multipole components in thesolar magnetic field harmonic spectrum (Luhmann et al., 2009). This in turnmight result in re-enforcement of the multipole magnetic configuration in theglobal solar magnetic field, which can be observed as persistent CHs along thelow- and mid-latitudes on the Sun.We attempted here to take a closer look at the low-latitude CHs duringthe period of 2007-2009 by utilizing data from SOHO/EIT and STEREO-A SECCHIEUVI instruments. We paid a special attention to their recurrent properties andwe found a signature of a dominating multipole magnetic structure from the CHstracing and the analysis of the harmonic power spectrum of the solar magneticfield.

Genesis of the most persistent equatorial Coronal Hole N2

The above study of the observational data accomplished by the magnetic field modeling show that the global magnetic field of the Sun was not a simple dipoleduring this unusual extended minimum. The area occupied by low-latutute CHs is considerably larger in the current minimum than that during the similar phase of the previous activity minimum. Our conclusions confirm the results of CH modeling recently reported by Luhmann et al. (2009).

Thus, the global magnetic field of the Sun had a multipole configuration, at least in the time period between September 2006 and May 2009. The harmonic powerspectrum of the solar magnetic field, showed an unusual for a solar minimumprevalence of multipole components over the dipole component. The unusuallylarge difference between these two parameters separation is probably due to avery low (three times lower than that in the previous minimum) magnitude of thedipole component.

Conclusion Remarks