Flux Emergence Rate in Coronal Holes and in Adjacent Quiet-sun Regions Valentyna Abramenko Big Bear Solar Observatory Lennard Fisk Lennard Fisk University of Michigan Vasyl Yurchyshyn Vasyl Yurchyshyn Big Bear Solar Observatory

Abstract Coronal holes are regions where the open magnetic flux of the Sun, the component that forms the heliospheric magnetic field, is concentrated. We determined that the rate of emergence of new We determined that the rate of emergence of new magnetic dipoles is systematically lower, by a factor of about 2, in coronal holes relative to the surrounding quiet Sun. This result is consistent with a prediction in a recent This result is consistent with a prediction in a recent model (Fisk 2005), which demonstrated that open flux tends to accumulate in regions where the rate of emergence of new magnetic flux is a local minimum. 1

QS Abramenko, Fisk, Yurchyshyn 2006 (ApJ 641,L65) 2 From EIT images we determine a boundary of a coronal hole (CH), then we project the boundary onto a full disk MDI magnetogram. For each pair of CH and Quiet Sun (QS) area, we used the same set of HR/MDI magnetograms so that, in QS and in CHs, the newly emerged dipoles had an equal opportunity to be detected. Our main goal was to study whether there exists a difference between CHs and QS in the rate of emergence of magnetic dipoles. CH

A number of dipoles that emerged during 24 hours inside an area of 200 x 200 Mm during 24 hours inside an area of 200 x 200 Mm is taken as the Dipole Emergence Rate: m = Number of dipoles Area · Time Interval 3 By comparing subsequent magnetograms we detected newly emerged dipoles. An example is shown here:

bisector m(CH) m(QS) N=34 m(CH) – the dipole emergence rate in Coronal Holes; m(QS) – the dipole emergence rate in Quiet Sun We thus obtained 34 values of the dipole emergence rate inside CHs, and, for each CH, we obtained the dipole emergence rate inside the adjacent QS area. The scattering is large, however, in all of the cases, the dipole emerging rate for CHs (m(CH)) is lower than that (m(QS)) for QS : all data points are above the bisector. 4

5 Distribution of the dipole emergence rate The peak of distribution for CHs is located at the significantly lower magnitudes than the peak for QS areas. We observe the difference of approximately two times between CHs and QS areas.

There could be concern, of course, that our low cadence might influence our conclusions To evaluate this possibility, we analyzed one pair a CH and QS observed with 5 min time cadence during 10 hours. We show here a fragment of the entire magnetogram. An area above the line is predominantly a QS, whereas the CH is below the red line (a movie is available). The dipole emergence rates became higher than those derived from our usual routine. However, the ratio did not change significantly: again we observed more dipoles emerged in QS than in the CH on an area of the same size. Therefore, our choice of time cadence does not significantly affect the conclusion that the dipole emergence rate is lower in CHs. CH QS High cadence: m(QS) m(CH) = 3.6 m(QS) m(CH) = 4.4 Low cadence: High cadence data 6

Conclusions 7 An analysis of 34 pairs of coronal holes and quiet sun areas showed that, on average, the dipole emergence rate in QS areas exceeds twice that in CHs. This implies that a coronal hole is a region with a local minimum in the rate emerging dipoles. This result is consistent with the prediction of Fisk (2005), who argues that open field lines are transported by both - (1) random convective motions and - (2) reconnection with coronal loops, and subsequent random displacement. (1) (2)

The observational evidence presented here supports the concept that reconnection of open field lines with coronal loops is an important transport mechanism on the Sun and needs to be included in models for the evolution of the solar magnetic field. In this model, the transport of open flux is coupled to the properties of the loops, which it turn are dependent on the rate of emergence of new magnetic flux. In regions where the rate of emergence of new flux is a local minimum, open flux will accumulate and form coronal holes 8

This research was conducted as a part of the Targeted Research and Technology (TR&T) program of the NASA LWS program in the framework of the activity of the Heliospheric Focus Team led by T. Zurbuchen. References: Abramenko, V.I., Fisk, L.A., Yurchyshyn, V.B. 2006, Astrophys J 641, L65. Fisk, L.A. 1996, J.Geophys.Res. 101, Fisk, L.A. 2001, J.Geophys.Res. 106, Fisk, L.A. 2005, Astrophys.J., 626, 563 Fisk, L.A., & Schwanron, N.A., 2001, Astrophys. J, 560, 425 Fisk, L.A., Zurbuchen, T.H., & Schwanron, N.A, 1999, Astrophys.J., 521, 868 Hagenaar, H.J. 2001, Astrophys.J., 555, 448 9