Coronal Holes and Open Magnetic Flux over Cycles 23 and 24

Lowder, Chris; Qiu, Jiong; Leamon, Robert J.

doi:10.1007/s11207-016-1041-8

Cited by 73 publications

(70 citation statements)

References 32 publications

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“…Authors such as Lowder et al (2017) using the OMNI database and Owens et al (2008) using historical heliospheric spacecraft, obtain values in agreement with our ACE 27-day averages. However, another source of disagreement in the open flux may come from these observed values.…”

Section: The Open Flux Problemsupporting

confidence: 91%

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. I. The Solar Wind Torque during Sunspot Cycles 23 and 24

Finley

See

2018

ApJ

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The rotational evolution of cool stars is governed by magnetised stellar winds which slow the stellar rotation during their main sequence lifetimes. Magnetic variability is commonly observed in Sun-like stars, and the changing strength and topology of the global field is expected to affect the torque exerted by the stellar wind. We present three different methods for computing the angular momentum loss in the solar wind. Two are based on MHD simulations from Finley & Matt (2018), with one using the open flux measured in the solar wind, and the other using remotely-observed surface magnetograms. Both methods agree in the variation of the solar torque seen through the solar cycle and show a 30 − 40% decrease from cycle 23 to 24. The two methods calculate different average values, 2.9 × 10 30 erg (open flux) and 0.35 × 10 30 erg (surface field). This discrepancy results from the already wellknown difficulty with reconciling the magnetograms with observed open flux, which is currently not understood, leading to an inability to discriminate between these two calculated torques. The third method is based on the observed spin-rates of Sun-like stars, which decrease with age, directly probing the average angular momentum loss. This method gives 6.2 × 10 30 erg for the solar torque, larger than the other methods. This may be indicative of further variability in the solar torque on timescales much longer than the magnetic cycle. We discuss the implications for applying the formula to other Sun-like stars, where only surface field measurements are available, and where the magnetic variations are ill-constrained.

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Section: The Open Flux Problemsupporting

confidence: 91%

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. I. The Solar Wind Torque during Sunspot Cycles 23 and 24

Finley

See

2018

ApJ

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“…There is a good agreement between our results and those of Lowder, Jiong, and Leamon (2017) (their Figure 3 PCH area is clearly larger in 2005, while the two years have roughly equally large northern PCH areas in Lowder, Jiong, and Leamon (2017). Moreover, while we find a small but clear non-zero northern PCH area in 1999, it is practically zero in Lowder, Jiong, and Leamon (2017). However, we find a significant difference for the LLCH areas between the two studies.…”

Section: Temporal Evolution Of Detected Coronal Holessupporting

confidence: 90%

“…The fractional CH areas depicted in Fig. 10 and their overall temporal evolution agree fairly well with the recent study by Lowder, Jiong, and Leamon (2017), who also used SOHO/EIT and SDO/AIA data to identify coronal holes. There is a good agreement between our results and those of Lowder, Jiong, and Leamon (2017) (their Figure 3 PCH area is clearly larger in 2005, while the two years have roughly equally large northern PCH areas in Lowder, Jiong, and Leamon (2017).…”

Section: Temporal Evolution Of Detected Coronal Holessupporting

confidence: 86%

“…However, we find a significant difference for the LLCH areas between the two studies. Lowder et al (2017) indicate that LLCH areas increased rather steadily from 1997 to 2003 without a significant decrease between these years, i.e., around sunspot maximum of cycle 23. Contrary to this, our results suggest local maxima in LLCH area in 1999LLCH area in , 2002LLCH area in and 2003, and a rather long local minimum in 2000-2001. There is a notable difference in the size of LLCH area especially in 2000-2001 between these two CH series.…”

Section: Temporal Evolution Of Detected Coronal Holesmentioning

confidence: 89%

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Automated Identification of Coronal Holes from Synoptic EUV Maps

et al. 2018

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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.

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“…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.…”

Section: Resultsmentioning

confidence: 99%

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

Nikolić

2019

Space Weather

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The potential field source surface (PFSS) model is widely used to derive the magnetic field of the solar corona. The only free parameter in the PFSS model is the radius of the so-called source surface, where magnetic field lines are forced to open. The radius of this surface is typically set to 2.5 solar radii in research and operational PFSS numerical models. Here, using Global Oscillation Network Group (GONG) synoptic maps of the photospheric field, solutions of the PFSS model for various heights of the source surface are investigated for 2006-2018. In particular, numerically derived open solar magnetic flux and coronal holes are examined. Solutions of the PFSS model based on GONG synoptic maps are particularly important since they are often used to drive operational space weather forecast models. Comparisons between observations and numerical results in this paper suggest that the radius of the source surface is significantly lower than 2.5 solar radii during the active phase of solar cycle 24. The fact that the source surface location depends on the solar activity suggests that relations which associate solar wind properties with the coronal magnetic field in the PFSS-based solar wind modes should be revisited. Furthermore, although the correction of the polar magnetic field is part of GONG synoptic map production pipeline, the results suggest that better treatment of polar fields is needed to cover observational gaps. The issue with the polar fields in GONG maps is particularly pronounced in recent years.

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Coronal Holes and Open Magnetic Flux over Cycles 23 and 24

Cited by 73 publications

References 32 publications

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. I. The Solar Wind Torque during Sunspot Cycles 23 and 24

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. I. The Solar Wind Torque during Sunspot Cycles 23 and 24

Automated Identification of Coronal Holes from Synoptic EUV Maps

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

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