Abrupt Shrinking of Solar Corona in the Late 1990s

Virtanen, Ilpo; Koskela, Jennimari; Мурсула, К.

doi:10.3847/2041-8213/ab644b

Cited by 13 publications

(9 citation statements)

References 43 publications

(48 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Examples include matching total unsigned open flux to that measured in-situ in the solar wind (e.g. Lee et al, 2011;Arden, Norton, and Sun, 2014;Virtanen, Koskela, and Mursula, 2020), matching heliospheric current sheet crossings to those measured in-situ (e.g. Hoeksema, Wilcox, and Scherrer, 1983;Badman et al, 2020), or matching the locations of large open field regions with observations of coronal holes in extreme ultra-violet (EUV) images (e.g.…”

Section: Choice Of Source Surface Heightmentioning

confidence: 99%

Active Region Contributions to the Solar Wind over Multiple Solar Cycles

et al. 2021

View full text Add to dashboard Cite

Both coronal holes and active regions are source regions of the solar wind. The distribution of these coronal structures across both space and time is well known, but it is unclear how much each source contributes to the solar wind. In this study we use photospheric magnetic field maps observed over the past four solar cycles to estimate what fraction of magnetic open solar flux is rooted in active regions, a proxy for the fraction of all solar wind originating in active regions. We find that the fractional contribution of active regions to the solar wind varies between 30% to 80% at any one time during solar maximum and is negligible at solar minimum, showing a strong correlation with sunspot number. While active regions are typically confined to latitudes ±30∘ in the corona, the solar wind they produce can reach latitudes up to ±60∘. Their fractional contribution to the solar wind also correlates with coronal mass ejection rate, and is highly variable, changing by ±20% on monthly timescales within individual solar maxima. We speculate that these variations could be driven by coronal mass ejections causing reconfigurations of the coronal magnetic field on sub-monthly timescales.

show abstract

Section: Choice Of Source Surface Heightmentioning

confidence: 99%

Active Region Contributions to the Solar Wind over Multiple Solar Cycles

et al. 2021

View full text Add to dashboard Cite

show abstract

“…However, there are significant discrepancies between this assumption and physical observations of the corona. Various attempts have been made to address this problem, such as experimenting with a nonspherical source surface (e.g., Levine et al 1982), or allowing the source surface height to vary over time (Virtanen et al 2020). The latter authors showed that the PFSS open magnetic flux derived from two magnetogram data sets can match observations at 1 au, but this cannot be used for prediction since the optimum source surface radius varies in an irregular and unpredictable manner.…”

Section: Introductionmentioning

confidence: 99%

Global Coronal Equilibria with Solar Wind Outflow

Rice

Yeates

2021

ApJ

View full text Add to dashboard Cite

Given a known radial magnetic field distribution on the Sun’s photospheric surface, there exist well-established methods for computing a potential magnetic field in the corona above. Such potential fields are routinely used as input to solar wind models, and to initialize magneto-frictional or full magnetohydrodynamic simulations of the coronal and heliospheric magnetic fields. We describe an improved magnetic field model that calculates a magneto-frictional equilibrium with an imposed solar wind profile (which can be Parker’s solar wind solution, or any reasonable equivalent). These “outflow fields” appear to approximate the real coronal magnetic field more closely than a potential field, take a similar time to compute, and avoid the need to impose an artificial source surface. Thus they provide a practical alternative to the potential field model for initializing time-evolving simulations or modeling the heliospheric magnetic field. We give an open-source Python implementation in spherical coordinates and apply the model to data from solar cycle 24. The outflow tends to increase the open magnetic flux compared to the potential field model, reducing the well-known discrepancy with in situ observations.

show abstract

“…Moreover, there are indications that the photospheric and coronal magnetic structure have suddenly and systematically changed during this time period (e.g. Lukianova and Mursula, 2011;Virtanen, Koskela, and Mursula, 2020).…”

Section: Introductionmentioning

confidence: 99%

A Uniform Series of Low-Latitude Coronal Holes in 1973–2018

2021

View full text Add to dashboard Cite

Coronal holes (CHs) are regions in the solar corona characterized by plasma density lower than in the surrounding quiet Sun. Therefore they appear dark in images of the solar atmosphere made, e.g., in extreme ultraviolet (EUV). Identifying CHs on solar images is difficult since CH boundaries are not sharp, but typically obscured by magnetic structures of surrounding active regions. Moreover, the areas, shapes, and intensities of CHs appear differently in different wavelengths. Coronal holes have been identified both visually by experienced observers and, more recently, by automated detection methods using different techniques. In this article, we apply a recent, robust CH identification algorithm to a new set of homogenized EUV synoptic maps based on four EUV lines measured by the Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope (SOHO/EIT) in 1996–2018 and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) in 2010–2018 and create corresponding CH synoptic maps. We also use CHs of the hand-drawn McIntosh archive (McA) from 1973–2009 to extend the CH database to earlier times. We discuss the success of the four EUV lines to find CHs at high or low latitudes, and confirm that the combined EIT 195 Å/AIA 193 Å series applies best for both polar and low-latitude CH detection. While the polar CH detection suffers from the vantage-point limitation, the low-latitude CH areas extracted from this line correlate with the McA CH data very well. Using the simultaneous measurements between EIT and McA and EIT and AIA, we scale the different data series to the same level and form the longest uniform series of low-latitude CHs in 1973–2018. We find that, while the solar cycle maxima of low-latitude CHs in the descending phase of Solar Cycles 21–23 attain roughly similar values, the corresponding maximum during Solar Cycle 24 is reduced by a factor of two. This suggests that magnetic flux emergence is crucial for the formation of low-latitude CHs.

show abstract

Abrupt Shrinking of Solar Corona in the Late 1990s

Cited by 13 publications

References 43 publications

Active Region Contributions to the Solar Wind over Multiple Solar Cycles

Active Region Contributions to the Solar Wind over Multiple Solar Cycles

Global Coronal Equilibria with Solar Wind Outflow

A Uniform Series of Low-Latitude Coronal Holes in 1973–2018

Contact Info

Product

Resources

About