Magnetic Field Extrapolations into the Corona: Success and Future Improvements

Régnier, Stéphane

doi:10.1007/s11207-013-0367-8

Cited by 51 publications

(34 citation statements)

References 258 publications

(202 reference statements)

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“…Magnetic field extrapolations are well-known techniques for describing the 3D nature of the coronal magnetic fields (see review by Régnier 2013). Under coronal conditions, the magnetic forces dominate the pressure gradient and gravity, and so we regard the corona above active regions as being well described by the force-free approximation (see e.g.…”

Section: Potential and Non-linear Force-free Modelsmentioning

confidence: 99%

A new approach to the maser emission in the solar corona

Régnier

2015

A&A

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Aims. The electron plasma frequency ω pe and electron gyrofrequency Ω e are two parameters that allow us to describe the properties of a plasma and to constrain the physical phenomena at play, for instance, whether a maser instability develops. In this paper, we aim to show that the maser instability can exist in the solar corona. Methods. We perform an in-depth analysis of the ω pe /Ω e ratio for simple theoretical and complex solar magnetic field configurations. Using the combination of force-free models for the magnetic field and hydrostatic models for the plasma properties, we determine the ratio of the plasma frequency to the gyrofrequency for electrons. For the sake of comparison, we compute the ratio for bipolar magnetic fields containing a twisted flux bundle, and for four different observed active regions. We also study how ω pe /Ω e is affected by the potential and non-linear force-free field models. Results. We demonstrate that the ratio of the plasma frequency to the gyrofrequency for electrons can be estimated by this novel method combining magnetic field extrapolation techniques and hydrodynamic models. Even if statistically not significant, values of ω pe /Ω e ≤ 1 are present in all examples, and are located in the low corona near to photosphere below one pressure scale-height and/or in the vicinity of twisted flux bundles. The values of ω pe /Ω e are lower for non-linear force-free fields than potential fields, thus increasing the possibility of maser instability in the corona. Conclusions. From this new approach for estimating ω pe /Ω e , we conclude that the electron maser instability can exist in the solar corona above active regions. The importance of the maser instability in coronal active regions depends on the complexity and topology of the magnetic field configurations.

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Section: Potential and Non-linear Force-free Modelsmentioning

confidence: 99%

A new approach to the maser emission in the solar corona

Régnier

2015

A&A

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“…This allows the problem to be uniquely solved as a spherical harmonic decomposition, resulting in a full 3D magnetic field model between the photosphere and source surface. The solution is steady state and represents a low energy bound on more general force free models, (Régnier 2013).…”

Section: Introductionmentioning

confidence: 99%

Magnetic Connectivity of the Ecliptic Plane within 0.5 au: Potential Field Source Surface Modeling of the First Parker Solar Probe Encounter

Badman

Bale

Oliveros

et al. 2020

ApJS

135

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We compare magnetic field measurements taken by the FIELDS instrument on Parker Solar Probe (PSP) during it's first solar encounter to predictions obtained by Potential Field Source Surface (PFSS) modeling. Ballistic propagation is used to connect the spacecraft to the source surface. Despite the simplicity of the model, our results show striking agreement with PSP's first observations of the heliospheric magnetic field from ∼0.5 AU (107.5 R ) down to 0.16 AU (35.7 R ). Further, we show the robustness of the agreement is improved both by allowing the photospheric input to the model to vary in time, and by advecting the field from PSP down to the PFSS model domain using in situ PSP/SWEAP measurements of the solar wind speed instead of assuming it to be constant with longitude and latitude. We also explore the source surface height parameter (R SS ) to the PFSS model finding that an extraordinarily low source surface height (1.3 − 1.5R ) predicts observed small scale polarity inversions which are otherwise washed out with regular modeling parameters. Finally, we extract field line traces from these models. By overlaying these on EUV images we observe magnetic connectivity to various equatorial and mid-latitude coronal holes indicating plausible magnetic footpoints and offering context for future discussions of sources of the solar wind measured by PSP.

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“…However, as also pointed out by the authors, such results might strongly depends on the quality or reliability of the coronal magnetic field reconstructions. Currently there are many methods available for NLFFF extrapolations from the vector magnetograms, but different methods seem to produce rather inconsistence results between each other (e.g., DeRosa et al 2009;Régnier 2013;Aschwanden et al 2014;Duan et al 2017;Wiegelmann et al 2017). Thus any results based on any single NLFFF code must be taken with cautions, and independent studies with different codes are required for a better inspection.…”

Section: Introductionmentioning

confidence: 99%

A Study of Pre-flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes

Duan

Jiang

et al. 2019

ApJ

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Magnetic flux ropes (MFRs) are thought to be the central structure of solar eruptions, and their ideal MHD instabilities can trigger the eruption. Here we performed a study of all the MFR configurations that lead to major solar flares, either eruptive or confined, from 2011 to 2017 near the solar disk center. The coronal magnetic field is reconstructed from observed magnetograms, and based on magnetic twist distribution, we identified the MFR, which is defined as a coherent group of magnetic field lines winding an axis with more than one turn. It is found that 90% of the events possess pre-flare MFRs, and their three-dimensional structures are much more complex in details than theoretical MFR models. We further constructed a diagram based on two parameters, the magnetic twist number which controls the kink instability (KI), and the decay index which controls the torus instability (TI). It clearly shows lower limits for TI and KI thresholds, which are n crit = 1.3 and |T w | crit = 2, respectively, as all the events above n crit and nearly 90% of the events above |T w | crit erupted. Furthermore, by such criterion, over 70% of the events can be discriminated between eruptive and confined flares, and KI seems to play a nearly equally important role as TI in discriminating between the two types of flare. There are more than half of events with both parameters below the lower limits, and 29% are eruptive. These events might be triggered by magnetic reconnection rather than MHD instabilities.

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Magnetic Field Extrapolations into the Corona: Success and Future Improvements

Cited by 51 publications

References 258 publications

A new approach to the maser emission in the solar corona

A new approach to the maser emission in the solar corona

Magnetic Connectivity of the Ecliptic Plane within 0.5 au: Potential Field Source Surface Modeling of the First Parker Solar Probe Encounter

A Study of Pre-flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes

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