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Abstract: A new method for automated detection of polar coronal holes is presented.This method, called perimeter tracing, uses a series of 171, 195, and 304 \AA\full disk images from the Extreme ultraviolet Imaging Telescope EIT on SOHOover solar cycle 23 to measure the perimeter of polar coronal holes as theyappear on the limbs. Perimeter tracing minimizes line-of-sight obscurationscaused by the emitting plasma of the various wavelengths by taking measurementsat the solar limb. Perimeter tracing also allows for the polar rotation periodto emerge organically from the data as 33 days. We have called this the Harveyrotation rate and count Harvey rotations starting 4 January 1900. From themeasured perimeter, we are then able to fit a curve to the data and derive anarea within the line of best fit. We observe the area of the northern polarhole area in 1996, at the beginning of solar cycle 23, to be about 4.2% of thetotal solar surface area and about 3.6% in 2007. The area of the southern polarhole is observed to be about 4.0% in 1996 and about 3.4% in 2007. Thus, boththe north and south polar hole areas are no more than 15% smaller now than theywere at the beginning of cycle 23. This compares to the polar magnetic fieldmeasured to be about 40% less now than it was a cycle ago.



Author: Michael S. Kirk, W. Dean Pesnell, C. Alex Young, Shea A. Hess Webber

Source: https://arxiv.org/



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