Properties of bipolar active regions through two and a half solar cycles

VILLAR, P.I.; LÓPEZ FUENTES, M.C.; MANDRINI, C.H.; DÉMOULIN, P.

Buenos Aires

Congreso; 45 Reunión Anual de la Asociación Argentina de Astronomía; 2002

Asociación Argentina de Astronomía

Hale´s Law (Hale & Nicholson, 1925, ApJ, 62, 270) describes the orientation of the bipolar active regions (BARs) that are observed in the solar photosphere. Generally, BARs are oriented almost parallel to the east-west direction, with the preceeding polarity (and, therefore, the following one) having different sign in both hemispheres and changing from one solar cycle to the other. Joy´s Law (Hale et al. 1919, ApJ, 49, 153) states that BARs are generally seen with their preceedig polarity closer to the equator; this law defines the tilt angle as the angle formed by the principal axis of the bipole with the horizontal direction. Furthermore, the tilt angle grows as latitude increases. Many numerical models simulating the emergence of flux tubes through the convective zone are able of reproducing this law. Some of these demonstrate that the tilt of BARs is originated by the effect of the Coriolis Force which acts on the flux tubes on their way up to the photosphere. Others, predict a relation between the tilt angle (varphi), the latitude (lambda) and the magnetic flux (Phi) of BARs (Fan, Fisher & McClymont, 1994, ApJ, 436, 907). In this relation the tilt is proportional to (Phi)alpha with alpha=0.25. In this context, we generated a big database containing values of the most relevant parameters of the magnetic field and other characteristics of BARs. This was done using the synoptic magnetograms of the Kitt Peak National Solar Observatory, which has all the magnetograms from 1975 to the present year. The database contains information about the flux (above different values of the magnetic field), the radius and the area of the polarities, the flux weighted mean longitude and latitude and the tilt angle of each BAR. Through a statistical analysis, we determined the functional dependence of the tilt angle on the latitude and tested the law proposed by Fan, Fisher & McClymont for the tilt angle, latitude and magnetic flux. Consequently, we found that the tilt angle holds a linear dependence with the sin(lambda) and the value of alpha is 0.22. Our results constrain the validity of theoretical and numerical models of the solar dynamo and flux tube emergence.