Automated Detection of EUV Polar Coronal Holes During Solar Cycle 23

TL;DR

This paper introduces a new automated perimeter tracing method for detecting polar coronal holes in EUV images, enabling analysis of their evolution during solar cycle 23 and revealing that their area decreased by about 15% from cycle start.

Contribution

The paper presents a novel perimeter tracing technique that minimizes line-of-sight effects and derives polar hole areas from EUV images, providing new insights into coronal hole evolution during solar cycle 23.

Findings

Polar hole areas decreased by about 15% from 1996 to 2007.

The method allows for organic emergence of the polar rotation period as 33 days.

Polar magnetic fields decreased by approximately 40% during the cycle.

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 SOHO over solar cycle 23 to measure the perimeter of polar coronal holes as they appear on the limbs. Perimeter tracing minimizes line-of-sight obscurations caused by the emitting plasma of the various wavelengths by taking measurements at the solar limb. Perimeter tracing also allows for the polar rotation period to emerge organically from the data as 33 days. We have called this the Harvey rotation rate and count Harvey rotations starting 4 January 1900. From the measured perimeter, we are then able to fit a curve to the data and derive an area within the line of best fit. We observe the area of the northern polar hole area in 1996, at the beginning of solar cycle 23, to be about 4.2% of the total solar surface area and about 3.6% in 2007. The area of the southern polar hole is observed to be about 4.0% in 1996 and about 3.4% in 2007. Thus, both the north and south polar hole areas are no more than 15% smaller now than they were at the beginning of cycle 23. This compares to the polar magnetic field measured to be about 40% less now than it was a cycle ago.