Characteristics of Low-latitude Coronal Holes near the Maximum of Solar Cycle 24

Hofmeister, Stefan J.; Veronig, Astrid; Reiss, Martin A.; Temmer, Manuela; Vennerstrøm, S.; Vršnak, Bojan; Heber, B.

doi:10.3847/1538-4357/835/2/268

Cited by 53 publications

(66 citation statements)

References 33 publications

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“…Since we aim to analyze small-scale magnetic features within coronal holes, we use the magnetograms at full resolution, meaning that we do not rescale the magnetograms to the plate scale of the AIA 193 Å images in order to avoid interpolation effects. The mean magnetic field density of each pixel of the magnetograms is further corrected for the assumption of a radial mean magnetic field density following Hofmeister et al (2017). We project the coronal holes as extracted from the AIA 193 Å images to the magnetograms, and derive their photospheric mean magnetic field strength B CH , unbalanced, i.e., presumably open, magnetic flux Φ CH , total unsigned magnetic flux Φ CH,us , and percentaged unbalanced magnetic flux Φ CH,pu = Φ CH /Φ CH,us .…”

Section: Datasets and Methodsmentioning

confidence: 99%

“…For simplicity, throughout the article, we only give the absolute values of the magnetic properties of the coronal holes. Again, for more details on the computations, we refer to Hofmeister et al (2017).…”

Section: Datasets and Methodsmentioning

confidence: 99%

“…However, differences were found in the three-dimensional magnetic field topology. In coronal, holes, the magnetic elements are the footpoints of open-to-interplanetary-space magnetic funnels, i.e., special clusters of vertical magnetic fibers, which are thought to be the small-scale source regions of high-speed solar wind streams (Hassler et al 1999;Tu et al 2005) and contain 80 % of the unbalanced magnetic flux of the coronal holes (Hofmeister et al 2017;Heinemann et al 2018). Further, the closed loops within coronal holes are almost all low-lying not even reaching the corona, whereas in quiet Sun regions these closed loops entirely cover the corona (Wiegelmann and Solanki 2004).…”

Section: Introductionmentioning

confidence: 99%

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Photospheric magnetic structure of coronal holes

Hofmeister

Utz

Heinemann

et al. 2019

A&A

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In this study, we investigate in detail the photospheric magnetic structure of 98 coronal holes using line-of-sight magnetograms of SDO/HMI, and for a subset of 42 coronal holes using HINODE/SOT G-band filtergrams. We divided the magnetic field maps into magnetic elements and quiet coronal hole regions by applying a threshold at ±25 G. We find that the number of magnetic bright points in magnetic elements is well correlated with the area of the magnetic elements (cc=0.83 ± 0.01). Further, the magnetic flux of the individual magnetic elements inside coronal holes is related to their area by a power law with an exponent of 1.261 ± 0.004 (cc=0.984 ± 0.001). Relating the magnetic elements to the overall structure of coronal holes, we find that on average (69 ± 8) % of the overall unbalanced magnetic flux of the coronal holes arises from long-lived magnetic elements with lifetimes >40 hours. About (22 ± 4) % of the unbalanced magnetic flux arises from a very weak background magnetic field in the quiet coronal hole regions with a mean magnetic field density of about 0.2 to 1.2 G. This background magnetic field is correlated to the flux of the magnetic elements with lifetimes of >40 h (cc=0.88 ± 0.02). The remaining flux arises from magnetic elements with lifetimes <40 hours. By relating the properties of the magnetic elements to the overall properties of the coronal holes, we find that the unbalanced magnetic flux of the coronal holes is completely determined by the total area that the long-lived magnetic elements cover (cc=0.994 ± 0.001).Article number, page 2 of 21

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Section: Datasets and Methodsmentioning

confidence: 99%

Section: Datasets and Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Photospheric magnetic structure of coronal holes

Hofmeister

Utz

Heinemann

et al. 2019

A&A

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“…The CH field was predominantly comprised of positive-polarity magnetic field. The average magnetic flux density was ∼11 G, which is typical of CHs for the declining phase of the solar cycle during which the measurements were made (Harvey et al 1982;Hofmeister et al 2017). A number of small bipoles emerged and/or decayed into the positive-polarity CH throughout the observing period.…”

Section: Magnetic Field Observationsmentioning

confidence: 99%

Coronal Elemental Abundances in Solar Emerging Flux Regions

Baker

Brooks

Driel-Gesztelyi

et al. 2018

ApJ

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The chemical composition of solar and stellar atmospheres differs from the composition of their photospheres. Abundances of elements with low first ionization potential (FIP) are enhanced in the corona relative to high-FIP elements with respect to the photosphere. This is known as the FIP effect and it is important for understanding the flow of mass and energy through solar and stellar atmospheres. We used spectroscopic observations from the Extreme-ultraviolet Imaging Spectrometer on board the Hinode observatory to investigate the spatial distribution and temporal evolution of coronal plasma composition within solar emerging flux regions inside a coronal hole. Plasma evolved to values exceeding those of the quiet-Sun corona during the emergence/early-decay phase at a similar rate for two orders of magnitude in magnetic flux, a rate comparable to that observed in large active regions (ARs) containing an order of magnitude more flux. During the late-decay phase, the rate of change was significantly faster than what is observed in large, decaying ARs. Our results suggest that the rate of increase during the emergence/early-decay phase is linked to the fractionation mechanism that leads to the FIP effect, whereas the rate of decrease during the later decay phase depends on the rate of reconnection with the surrounding magnetic field and its plasma composition.

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“…CH structures are of low plasma density, and therefore of reduced emission, which makes them observable as dark regions in the extreme ultraviolet (EUV) and X−ray wavelength range (e.g., Schwenn 2006). The identification and extraction is usually based on intensity-threshold methods (Krista & Gallagher 2009;Rotter et al 2012;Reiss et al 2014;Hofmeister et al 2017). The observed CH areas are found to be well correlated with the solar wind speed measured in-situ at 1 AU (Krieger et al 1973;Nolte et al 1976;Vršnak et al 2007;Rotter et al 2012;Hofmeister et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Three-phase Evolution of a Coronal Hole. I. 360° Remote Sensing and In Situ Observations

Heinemann

Temmer

Hofmeister

et al. 2018

ApJ

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We investigate the evolution of a well-observed, long-lived, low-latitude coronal hole (CH) over 10 solar rotations in the year 2012. By combining EUV imagery from STEREO-A/B and SDO we are able to track and study the entire evolution of the CH having a continuous 360 • coverage of the Sun. The remote sensing data are investigated together with in-situ solar wind plasma and magnetic field measurements from STEREO-A/B, ACE and WIND. From this we obtain how different evolutionary states of the CH as observed in the solar atmosphere (changes in EUV intensity and area) affect the properties of the associated high-speed stream measured at 1AU. Most distinctly pronounced for the CH area, three development phases are derived: a) growing, b) maximum, and c) decaying phase. During these phases the CH area a) increases over a duration of around three months from about 1 · 10 10 km 2 to 6 · 10 10 km 2 , b) keeps a rather constant area for about one month of > 9 · 10 10 km 2 , and c) finally decreases in the following three months below 1 · 10 10 km 2 until the CH cannot be identified anymore. The three phases manifest themselves also in the EUV intensity and in in-situ measured solar wind proton bulk velocity. Interestingly, the three phases are related to a different range in solar wind speed variations and we find for the growing phase a range of 460 − 600 km s −1 , for the maximum phase 600 − 720 km s −1 , and for the decaying phase a more irregular behavior connected to slow and fast solar wind speed of 350 − 550 km s −1 .

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Characteristics of Low-latitude Coronal Holes near the Maximum of Solar Cycle 24

Cited by 53 publications

References 33 publications

Photospheric magnetic structure of coronal holes

Photospheric magnetic structure of coronal holes

Coronal Elemental Abundances in Solar Emerging Flux Regions

Three-phase Evolution of a Coronal Hole. I. 360° Remote Sensing and In Situ Observations

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