The Nature of Solar Polar Rays

Abstract
We use time series observations from the SOHO and Yohkoh spacecraft to study solar polar rays. Contrary to our expectations, we find that the rays are associated with active regions on the sun and are not features of the polar coronal holes. They are extended, hot plasma structures formed in the active regions and projected onto the plane of the sky above the polar coronal holes. We present new observations and simple projection models that match long-lived polar ray structures seen in limb synoptic maps. Individual projection patterns last for at least 5 solar rotations.

Fig.1 Coronal limb synoptic map at 1.0 solar radii from EIT 195/171 (FeXII/FeIX,X) images in the period January 1996 to June 1998. The Day of Year (DOY) is indicated along the lower x-axis. The dates are marked along the upper x-axis. The polar angle (PA) is on the y-axis. Zero limb angle corresponds to the north pole (NP), while the east limb is at 90 degree (EL). To display the entire southern (180 degree) and northern (0 degree and 360 degree) polar holes, the polar angle is extended by 90 degree. The heliographic latitude is plotted with a solid curve.

Fig.2 A portion of the synoptic chart from EIT 284 A from DOY=210 to DOY=350, corresponding to July to December 1996. The dashed curve shows Eq. (1) with theta_0=60, the dotted curve corresponds to theta_0=10. The notations on the right side of the figure have the same meaning as those in Fig.1. NP is Northern Pole, EL is East Limb.

Fig.3 Sample model image in which the active region is located at 30 degree longitude and the north pole of the sun is tilted towards Earth by 7.25 degree(i.e. B_0=+7.25 degree). Image has north to the top, East to the left, and the grid spacing is 10 degree.

Fig.4 Simulated limb synoptic map made from the model of Figure 3. A rotational resolution of 1 degree was used. About 5 solar rotations are included here. The panels show a) B_0=+7.25 degree and b) B_0=-7.25 degree. Note the reversal in the sense of the asymmetry of the brightness of polar rays.