Automated Identification of Coronal Holes from Synoptic EUV Maps

Abstract

Coronal holes (CH) are regions of open magnetic field lines in the solar corona and the source of fast solar wind. Understanding the evolution of coronal holes is critical for solar magnetism as well as for accurate space weather forecasts. We study here the extreme ultraviolet (EUV) synoptic maps at three wavelengths (195Å/193Å, 171Å and 304Å) measured by Solar and Heliospheric Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) instruments. The two datasets are first homogenized by scaling the SDO/AIA data to the SOHO/EIT level by means of histogram equalization. We then develop a novel automated method to identify CHs from these homogenized maps by determining the intensity threshold of CH regions separately for each synoptic map. This is done by identifying the best location and size of an image segment, which optimally contains portions of coronal holes and the surrounding quiet Sun allowing us to detect the momentary intensity threshold. Our method is thus able to adjust itself to the changing scale size of coronal holes and to temporally varying intensities. To make full use of the information in the three wavelengths we construct, a composite CH distribution, which is more robust than distributions based on one wavelength. Using the composite CH dataset we discuss the temporal evolution of CHs during the solar cycles 23 and 24.

On Solutions of the PFSS Model With GONG Synoptic Maps for 2006–2018

“…The changes in the CH area, shown in Figure , have a similar trend as the observed CH changes shown in Figure 3 in Lowder et al () and Figure 10 in Hamada et al (). For example, the results shown in Figure with the PFSS model and those in Lowder et al () and Hamada et al () obtained using images of the solar corona agree that northern and SP CHs show asymmetric evolution and a time lag between their formation. This should not be confused with periodic variations of the CH area in the polar regions which are caused by the lack of their coverage due to the tilt of the Sun's axis with respect to the ecliptic plane.…”

“…Since the interplanetary magnetic field represents the coronal magnetic field which is carried out by the solar wind, this implies a complex dependence of regions which are the source of the solar wind on solar activity. It is known from observations of the solar corona that CHs, which are a source of solar wind and interplanetary magnetic field, change in size and location as the solar cycle progresses (see, e.g., Hamada et al, , Harvey & Recely, , Lowder et al, , ). During the solar minimum, they occupy large polar regions of the Sun, while during the period of high solar activity usually smaller CHs appear at the lower latitudes.…”

“…Thus, the CH boundaries can be difficult to see in the satellite images of the solar disc. Furthermore, observations of CHs at different wavelengths can give different results for their size and shape (Caplan et al, ; Hamada et al, ; de Toma, ). Moreover, since the Sun's rotation axis is not perpendicular to the ecliptic plane (7.25° tilt), every 6 months one of the poles is not visible.…”

New Homogeneous Dataset of Solar EUV Synoptic Maps from SOHO/EIT and SDO/AIA

“…Synoptic maps provide a convenient way to display the evolution and/or distribution of various physical quantities and features on the entire solar surface. For example, synoptic maps have been used to study coronal holes (CH) (Karna et al, 2015;Golubeva and Mordvinov, 2017;Hamada et al, 2018), photospheric and coronal magnetic fields (Virtanen and Mursula, 2016), and long-lived active regions (ARs). Solar synoptic maps can also be used to study the nonuniform structure of the EUV corona in both longitude and latitude, reflecting the nonuniform distribution of the large-scale magnetic field (Benevolenskaya, Kosovichev, and Scherrer, 2001).…”

Comparing Two Intervals of Exceptionally Strong Solar Rotation Recurrence of Galactic Cosmic Rays

“…The convection effect by high-speed SW is the likely source of the strong periodicity in cosmic rays. Lowder et al (2017) and Hamada et al (2018) found a difference in the evolution of northern and southern CHs during the SC 23 and SC 24. In Interval B the polar CHs were not as large as in Interval A, but there were low-latitude CHs (Figure 10), even with transequatorial extensions (e.g., Dunzlaff et al, 2008;Gómez-Herrero et al, 2009), which caused the narrow peak in power spectrum density of 27.5 days in GCR.…”