Coronal Heating in Active Regions as a Function of Global Magnetic Variables

Fisher, G. H.; Longcope, D. W.; Metcalf, Thomas R.; Pevtsov, Alexei A.

doi:10.1086/306435

Cited by 150 publications

(230 citation statements)

References 39 publications

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“…The pressure distribution, which is higher in the core and lower in the halo, is also in agreement with the Ðnding that pressure in active regions is correlated with higher magnetic Ñux concentrations (Golub et al 1980). The high core and cooler halo temperatures also show a correlation with the conÐn-ing magnetic Ðeld strength and conÐguration, a point that was previously discussed by Falconer et al (1997) and Fisher et al (1998).…”

Section: Discussionsupporting

confidence: 89%

“…correlates with magnetic Ðelds in active regions, while Fisher et al (1998) found a strong correlation between the magnetic Ñux and soft X-ray intensity, implying that the magnetic Ðeld is a primary quantity as regards heating. Recent results have also shown that simple scaling laws may not be universally applicable, not holding for some loop classes such as cool transition region loops (Oluseyi et al 1999a(Oluseyi et al , 1999b or Ñaring loops (Garcia 1998), while and Aschwanden, Nightengale, & Alexander (2000a) have also highlighted inconsistencies associated with scaling law relationships.…”

Section: Introductionmentioning

confidence: 94%

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The Extreme‐Ultraviolet Structure and Properties of a Newly Emerged Active Region

Gallagher

Phillips

Lee

et al. 2001

ApJ

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The structure and properties of a newly emerged solar active region (NOAA Active Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308È381 and 513È633 wavelength ranges, while EIT recorded full-disk EUV images in A A the He II (304 Fe IX/X (171 Fe XII (195 and Fe XV (284 bandpasses. Electron density mea-A ), A ), A ), A ) surements from Si IX, Si X, Fe XII, Fe XIII, and Fe XIV line ratios indicate that the region consists of a central high-density core with peak densities of the order of 1.2 ] 1010 cm~3, which decrease monotonically to D5.0 ] 108 cm~3 at the active region boundary. The derived electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatures, and the constant pressure assumption was found to be an unrealistic simpliÐcation. Indeed, the active region is found to have a high-pressure core (1.3 ] 1016 cm~3 K) that falls to 6.0 ] 1014 cm~3 K just outside the region. CDS line ratios from di †erent ionization stages of iron, speciÐcally Fe XVI (335.4and Fe XIV A ) (334.4 were used to diagnose plasma temperatures within the active region. Using this method, peak A ), temperatures of 2.1 ] 106 K were identiÐed. This is in good agreement with electron temperatures derived using EIT Ðlter ratios and the two-temperature model of Zhang et al. The high-temperature emission is conÐned to the active region core, while emission from cooler (1È1.6) ] 106 K lines originates in a system of loops visible in EIT 171 and 195 images. Finally, the three-dimensional geometry of the A active region is investigated using potential Ðeld extrapolations from a Kitt Peak magnetogram. The combination of EUV and magnetic Ðeld extrapolations extends the "" core-halo ÏÏ picture of active region structure to one in which the core is composed of a number of compact coronal loops that conÐne the hot, dense, high-pressure core plasma while the halo emission emerges from a system of cooler and more extended loops.

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Section: Discussionsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 94%

The Extreme‐Ultraviolet Structure and Properties of a Newly Emerged Active Region

Gallagher

Phillips

Lee

et al. 2001

ApJ

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“…Böhm-Vitense 1992). The coronal X-ray radiative flux density is proportional to the average surface magnetic flux density (Fisher et al 1998). Hence, the L X /L bol ratio is a lower limit of the ratio between the surface magnetic flux and the outer convective flux.…”

Section: Discussionmentioning

confidence: 99%

“…In particular, the relationship between the coronal radiative flux density and the average surface magnetic flux density is nearly linear for solar active regions and for entire stars (e.g. Fisher et al 1998;Schrijver & Zwaan 2000) over 12 orders of magnitude in absolute magnetic flux (Pevtsov et al 2003). The X-ray emission from late-type stars in open clusters exhibits two kinds of dependences on stellar rotation (e.g.…”

Section: Introductionmentioning

confidence: 99%

X-ray emission regimes and rotation sequences in M 35

Gondoin

2013

A&A

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Context. Late-type stars in young open clusters show two kinds of dependence of their X-ray emission on rotation. They also tend to group into two main sub-populations that lie on narrow sequences in diagrams where their rotation periods are plotted against their (B − V) colour indices. A correlation between these two regimes of X-ray emission and the rotation sequences has been recently observed in the M 34 open cluster. Sun-like M 34 stars also show a drop of their X-ray to bolometric luminosity ratio by about one order of magnitude at a Rossby number of about 0.3. Aims. The present study looks for similar connections between X-ray activity and rotation in an other open cluster. The aim is to consolidate a model of X-ray activity evolution on the main sequence and to provide observational constraints on dynamo processes in the interiors of late-type stars. Methods. The paper compares XMM-Newton measurements of X-ray stellar emission in M 35 with X-ray luminosity distributions derived from rotation period measurements assuming either an X-ray regime transition at a critical Rossby number or a correlation between X-ray emission regimes and rotation sequences. Results. This second hypothesis could account for the low number of M 35 stars detected in X-rays. A model of X-ray activity evolution is proposed based on the correlation. One major output is that the transition from saturated to non-saturated X-ray emission occurs at Rossby numbers between about 0.13 and 0.4 for each star depending on its mass and initial period of rotation on the ZAMS. This prediction agrees with observations of stellar X-ray emission in M 34. It explains the large range of X-ray luminosities observed among Sun-like stars in young open clusters. Conclusions. I conclude that the correlation between X-ray emission regimes and rotation sequences could be a fundamental property of the early evolution of stellar magnetic activity on the main sequence. I argue that the angular momentum redistribution mechanism(s) responsible for the transition between rotation sequences could result in a changing mixture of dynamo processes occurring side by side, possibly including a turbulent dynamo at high rotation rate and an interface-type dynamo whose efficiency decreases progressively at later stage of stellar evolution when rotation dies away.

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“…To make sure that the approximately linear relationship between SXR flux and magnetic field is not due to a natural proportionality of the extensive quantities with the magnetic area considered, which may dominate over the relatively smaller variations of the intensive quantities, the intrinsic physical relations between intensive quantities (Δ and¯), i.e., flux densities should be analysed. In the study by Fisher et al (1998) SXT DN was transformed into flux values by assuming a fixed of 3 MK, therefore the dependence of the instrumental response function was not taken into account. This may not have been important for that study, however, as the analysed ARs were approximately in the same evolutionary stage, i.e., their was probably similar.…”

Section: Evolution Of Multi-wavelength Emitted Fluxesmentioning

confidence: 99%

Evolution of Active Regions

Driel-Gesztelyi

Green

2015

Living Rev. Sol. Phys.

252

131

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The evolution of active regions (AR) from their emergence through their long decay process is of fundamental importance in solar physics. Since large-scale flux is generated by the deepseated dynamo, the observed characteristics of flux emergence and that of the subsequent decay provide vital clues as well as boundary conditions for dynamo models. Throughout their evolution, ARs are centres of magnetic activity, with the level and type of activity phenomena being dependent on the evolutionary stage of the AR. As new flux emerges into a pre-existing magnetic environment, its evolution leads to re-configuration of small-and large-scale magnetic connectivities. The decay process of ARs spreads the once-concentrated magnetic flux over an ever-increasing area. Though most of the flux disappears through small-scale cancellation processes, it is the remnant of large-scale AR fields that is able to reverse the polarity of the poles and build up new polar fields. In this Living Review the emphasis is put on what we have learned from observations, which is put in the context of modelling and simulation efforts when interpreting them. For another, modelling-focused Living Review on the sub-surface evolution and emergence of magnetic flux see Fan (2009). In this first version we focus on the evolution of dominantly bipolar ARs. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

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Coronal Heating in Active Regions as a Function of Global Magnetic Variables

Cited by 150 publications

References 39 publications

The Extreme‐Ultraviolet Structure and Properties of a Newly Emerged Active Region

The Extreme‐Ultraviolet Structure and Properties of a Newly Emerged Active Region

X-ray emission regimes and rotation sequences in M 35

Evolution of Active Regions

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