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A new approach to the maser emission in the solar corona  

Stephane Regnier   Submitted: 2015-08-25 03:06

The electron plasma frequency ω_pe and electron gyrofrequency Omega_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. We perform an in-depth analysis of the ω_pe/Omega_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/Omega_e is affected by the potential and non-linear force-free field models. 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/Omega_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/Omega_e are lower for non-linear force-free fields than potential fields, thus increasing the possibility of maser instability in the corona. From this new approach for estimating ω_pe/Omega_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.

Authors: S. Regnier
Projects: None

Publication Status: Published in A&A
Last Modified: 2015-08-25 16:46
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Sparkling EUV Bright Dots Observed with Hi-C  

Stephane Regnier   Submitted: 2014-02-13 07:05

Observing the Sun at high time and spatial scales is a step towards understanding the finest and fundamental scales of heating events in the solar corona. The Hi-C instrument has provided the highest spatial and temporal resolution images of the solar corona in the EUV wavelength range to date. Hi-C observed an active region on 11 July 2012, which exhibits several interesting features in the EUV line at 193Å: one of them is the existence of short, small brightenings ``sparkling" at the edge of the active region; we call these EUV Bright Dots (EBDs). Individual EBDs have a characteristic duration of 25s with a characteristic length of 680 km. These brightenings are not fully resolved by the SDO/AIA instrument at the same wavelength, however, they can be identified with respect to the Hi-C location of the EBDs. In addition, EBDs are seen in other chromospheric/coronal channels of SDO/AIA suggesting a temperature between 0.5 and 1.5 MK. Based on their frequency in the Hi-C time series, we define four different categories of EBDs: single peak, double peak, long duration, and bursty EBDs. Based on a potential field extrapolation from an SDO/HMI magnetogram, the EBDs appear at the footpoints of large-scale trans-equatorial coronal loops. The Hi-C observations provide the first evidence of small-scale EUV heating events at the base of these coronal loops, which have a free magnetic energy of the order of 1026 erg.

Authors: S. Regnier, C. E. Alexander, R. W. Walsh, A. R. Winebarger, J. Cirtain, L. Golub, K. E. Korreck, N. Mitchell, S. Platt, M. Weber, B. De Pontieu, A. Title, K. Kobayashi, S. Kuzin, C. E. DeForest
Projects: SDO-AIA

Publication Status: in press, ApJ
Last Modified: 2014-02-13 13:12
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Magnetic Field Extrapolations in the Corona: Success and Future Improvements  

Stephane Regnier   Submitted: 2013-07-16 00:06

The solar atmosphere being magnetic in nature, the understanding of the structure and evolution of the magnetic field in different regions of the solar atmosphere has been an important task over the past decades. This task has been made complicated by the difficulties to measure the magnetic field in the corona, while it is currently known with a good accuracy in the photosphere and/or chromosphere. Thus, to determine the coronal magnetic field, a mathematical method has been developed based on the observed magnetic field. This is the so-called magnetic field extrapolation technique. This technique relies on two crucial points: (i) the physical assumption leading to the system of differential equations to be solved, (ii) the choice and quality of the associated boundary conditions. In this review, I summarise the physical assumptions currently in use and the findings at different scales in the solar atmosphere. I concentrate the discussion on the extrapolation techniques applied to solar magnetic data and the comparison with observations in a broad range of wavelengths (from hard X-rays to radio emission).

Authors: S. Regnier
Projects: None

Publication Status: in press, Solar Physics
Last Modified: 2013-07-16 09:53
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Thermal shielding of an emerging active region  

Stephane Regnier   Submitted: 2012-07-18 05:10

The interaction between emerging active regions and the pre-existing coronal magnetic field is important for better understanding the mechanisms of storage and release of magnetic energy from the convection zone to the high corona. We describe the first steps of an emerging active region within a pre-existing quiet-Sun corona in terms of the thermal and magnetic structure. We used unprecedented spatial, temporal and spectral coverage from the Atmospheric Imager Assembly (AIA) and from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Starting on 30 May 2010 at 17:00 UT, we followed the emerging active region AR11076 within a quiet-Sun region for 8 hours. Using several SDO/AIA filters that cover temperatures from 50000K to 10 MK, we show that the emerging process is characterised by a thermal shield at the interface between the emerging flux and pre-existing quiet-Sun corona. The active region 11076 is a peculiar example of an emerging active region because (i) the polarities emerge in a photospheric quiet-Sun region near a supergranular-like distribution, and (ii) the polarities that form the bipolar emerging structure do not rotate with respect to each other, which indicates a slight twist in the emerging flux bundle. There is a thermal shield at the interface between the emerging active region and the pre-existing quiet-Sun region. The thermal shielding structure deduced from all SDO/AIA channels is strongly asymmetric between the two polarities of the active region, suggesting that the heating mechanism for one polarity is probably magnetic reconnection, whilst it is caused by increasing magnetic pressure for the opposite polarity.

Authors: S. Regnier
Projects: SDO-AIA,SDO-HMI

Publication Status: Accepted in A&A
Last Modified: 2012-07-18 09:32
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A new look at a polar crown cavity as observed by SDO/AIA  

Stephane Regnier   Submitted: 2011-07-18 08:03

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: S. Regnier, R. W. Walsh, C. E. Alexander
Projects: SDO-AIA

Publication Status: Accepted for publication in A&A
Last Modified: 2011-07-18 10:58
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Magnetic Energy Storage and Current Density Distributions for Different Force-Free Models  

Stephane Regnier   Submitted: 2011-07-18 06:18

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: S. Regnier
Projects: None

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2011-07-18 10:59
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Relationship between photospheric currents and coronal magnetic helicity for force-free bipolar fields  

Stephane Regnier   Submitted: 2009-03-16 21:19

The origin and evolution of the magnetic helicity in the solar corona are not well understood. For instance, the magnetic helicity of an active region is often about 1042 Mx^2 (1026 Wb^2), but the observed processes whereby it is thought to be injected into the corona do not yet provide an accurate estimate of the resulting magnetic helicity budget or time evolution. The variation in magnetic helicity is important for understanding the physics of flares, coronal mass ejections, and their associated magnetic clouds. To shed light on this topic, we investigate here the changes in magnetic helicity due to electric currents in the corona for a single twisted flux tube that may model characteristic coronal structures such as active region filaments, sigmoids, or coronal loops. For a bipolar photospheric magnetic field and several distributions of current, we extrapolated the coronal field as a nonlinear force-free field. We then computed the relative magnetic helicity, as well as the self and mutual helicities. Starting from a magnetic configuration with a moderate amount of current, the amount of magnetic helicity can increase by 2 orders of magnitude when the maximum current strength is increased by a factor of 2. The high sensitivity of magnetic helicity to the current density can partially explain discrepancies between measured values on the photosphere, in the corona, and in magnetic clouds. Our conclusion is that the magnetic helicity strongly depends on both the strength of the current density and also on its distribution. Only improved measurements of current density at the photospheric level will advance our knowledge of the magnetic helicity content in the solar atmosphere.

Authors: S. Regnier
Projects: None

Publication Status: to appear in A&A
Last Modified: 2009-03-17 07:15
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Coronal Alfvén speeds in an isothermal atmosphereI. Global properties  

Stephane Regnier   Submitted: 2008-09-05 08:17

Estimating Alfvén speeds is of interest in modelling the solar corona, studying the coronal heating problem and understanding the initiation and propagation of coronal mass ejections (CMEs). We assume here that the corona is in a magnetohydrostatic equilibrium and that, because of the low plasma eta, one may decouple the magnetic forces from pressure and gravity. The magnetic field is then described by a force-free field for which we perform a statistical study of the magnetic field strength with height for four different active regions. The plasma along each field line is assumed to be in a hydrostatic equilibrium. As a first approximation, the coronal plasma is assumed to be isothermal with a constant or varying gravity with height. We study a bipolar magnetic field with a ring distribution of currents, and apply this method to four active regions associated with different eruptive events. By studying the global properties of the magnetic field strength above active regions, we conclude that (i) most of the magnetic flux is localized within 50 Mm of the photosphere, (ii) most of the energy is stored in the corona below 150 Mm, (iii) most of the magnetic field strength decays with height for a nonlinear force-free field slower than for a potential field. The Alfvén speed values in an isothermal atmosphere can vary by two orders of magnitude (up to 100000 km s-1). The global properties of the Alfvén speed are sensitive to the nature of the magnetic configuration. For an active region with highly twisted flux tubes, the Alfvén speed is significantly increased at the typical height of the twisted flux bundles; in flaring regions, the average Alfvén speeds are above 5000 kms~and depart strongly from potential field values. We discuss the implications of this model for the reconnection rate and inflow speed, the coronal plasma eta and the Alfvén transit time.

Authors: S. Regnier, E. R. Priest, A. W. Hood
Projects: None

Publication Status: To be published in A&A
Last Modified: 2008-09-05 08:30
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Subject will be restored when possible  

Stephane Regnier   Submitted: 2008-05-09 04:44

With the recent launch of the Hinode satellite our view of the nature and evolution of quiet-Sun regions has been improved. In light of the new high resolution observations, we revisit the study of the quiet Sun's topological nature. Topology is a tool to explain the complexity of the magnetic field, the occurrence of reconnection processes, and the heating of the corona. This Letter aims to give new insights to these different topics. Using a high-resolution Hinode/SOT observation of the line-of-sight magnetic field on the photosphere, we calculate the three dimensional magnetic field in the region above assuming a potential field. From the 3D field, we determine the existence of null points in the magnetic configuration.} From this model of a continuous field, we find that the distribution of null points with height is significantly different from previous studies. In particular, the null points are mainly located above the bottom boundary layer in the photosphere (54%) and in the chromosphere (44%) with only a few null points in the corona (2%). The density of null points (expressed as the ratio of the number of null points to the number of photospheric magnetic fragments) in the solar atmosphere is estimated to be between 3% and 8% depending on the method used to identify the number of magnetic fragments on the observed photosphere. This study reveals that the heating of the corona by magnetic reconnection at coronal null points is unlikely. Our findings do not rule out the heating of the corona at other topological features. We also reveal the topological complexity of the chromosphere as strongly suggested by recent observations from Hinode/SOT.

Authors: S. Regnier, C. E. Parnell, A. L. Haynes
Projects: Hinode/SOT

Publication Status: Accepted in A&A Letter
Last Modified: 2008-09-23 21:02
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Subject will be restored when possible  

Stephane Regnier   Submitted: 2008-05-09 04:43

With the recent launch of the Hinode satellite our view of the nature and evolution of quiet-Sun regions has been improved. In light of the new high resolution observations, we revisit the study of the quiet Sun's topological nature. Topology is a tool to explain the complexity of the magnetic field, the occurrence of reconnection processes, and the heating of the corona. This Letter aims to give new insights to these different topics. Using a high-resolution Hinode/SOT observation of the line-of-sight magnetic field on the photosphere, we calculate the three dimensional magnetic field in the region above assuming a potential field. From the 3D field, we determine the existence of null points in the magnetic configuration.} From this model of a continuous field, we find that the distribution of null points with height is significantly different from previous studies. In particular, the null points are mainly located above the bottom boundary layer in the photosphere (54%) and in the chromosphere (44%) with only a few null points in the corona (2%). The density of null points (expressed as the ratio of the number of null points to the number of photospheric magnetic fragments) in the solar atmosphere is estimated to be between 3% and 8% depending on the method used to identify the number of magnetic fragments on the observed photosphere. This study reveals that the heating of the corona by magnetic reconnection at coronal null points is unlikely. Our findings do not rule out the heating of the corona at other topological features. We also reveal the topological complexity of the chromosphere as strongly suggested by recent observations from Hinode/SOT.

Authors: S. Regnier, C. E. Parnell, A. L. Haynes
Projects: Hinode/SOT

Publication Status: Accepted in A&A Letter
Last Modified: 2008-09-23 21:02
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Subject will be restored when possible  

Stephane Regnier   Submitted: 2007-09-07 08:41

To understand the physics of solar flares, including the local reorganisation of the magnetic field and the acceleration of energetic particles, we have first to estimate the free magnetic energy available for such phenomena, which can be converted into kinetic and thermal energy. The free magnetic energy is the excess energy of a magnetic configuration compared to the minimum-energy state, which is a linear force-free field if the magnetic helicity of the configuration is conserved. We investigate the values of the free magnetic energy estimated from either the excess energy in extrapolated fields or the magnetic virial theorem. For four different active regions, we have reconstructed the nonlinear force-free field and the linear force-free field corresponding to the minimum-energy state. The free magnetic energies are then computed. From the energy budget and the observed magnetic activity in the active region, we conclude that the free energy above the minimum-energy state gives a better estimate and more insights into the flare process than the free energy above the potential field state.

Authors: S. Regnier and E. R. Priest
Projects: None

Publication Status: Accepted ApJ Letters
Last Modified: 2007-09-07 10:19
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Nonlinear force-free models for the solar corona I. Two active regions with very different structure  

Stephane Regnier   Submitted: 2007-03-29 04:51

With the development of new instrumentation providing measurements of solar photospheric vector magnetic fields, we need to develop our understanding of the effects of current density on coronal magnetic field configurations. The object is to understand the diverse and complex nature of coronal magnetic fields in active regions using a nonlinear force-free model. From the observed photospheric magnetic field we derive the photospheric current density for two active regions: one is a decaying active region with strong currents (AR8151), and the other is a newly emerged active region with weak currents (AR8210). We compare the three-dimensional structure of the magnetic fields for both active region when they are assumed to be either potential or nonlinear force-free. The latter is computed using a Grad-Rubin vector-potential-like numerical scheme. A quantitative comparison is performed in terms of the geometry, the connectivity of field lines, the magnetic energy and the magnetic helicity content. For the old decaying active region the connectivity and geometry of the nonlinear force-free model include strong twist and strong shear and are very different from the potential model. The twisted flux bundles store magnetic energy and magnetic helicity high in the corona (about 50 Mm). The newly emerged active region has a complex topology and the departure from a potential field is small, but the excess magnetic energy is stored in the low corona and is enough to trigger powerful flares.

Authors: S. Regnier, E. R. Priest
Projects: None

Publication Status: accepted to be published in A&A
Last Modified: 2007-03-29 10:44
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Nonlinear force-free extrapolation: numerical methods and applications  

Stephane Regnier   Submitted: 2007-01-15 03:31

To model 3D coronal magnetic fields, we use different assumptions: the potential field, the linear force-free field and the nonlinear force-free field. The latter assumption requires the knowledge of the three components of the magnetic field at the bottom boundary (photosphere or chromosphere). The recent development of new spectro-polarimetric instruments allows a more accurate and more systematic measurement of the three components of the magnetic field. Before we can make use of the full potential of these instruments, we need to review our knowledge on nonlinear force-free modelling and the solar physics that can be done with those computations. We will summarise the different numerical methods used to determine the coronal magnetic field, and we will review the physical processes and properties derived from the computed magnetic configurations (e.g., magnetic reconnection, energy storage, source of energetic particles).

Authors: Stephane Regnier
Projects: None

Publication Status: To appear in Mem. S. A. It.
Last Modified: 2007-01-15 10:21
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Evolution of magnetic fields and energetics of flares in active region 8210  

Stephane Regnier   Submitted: 2006-02-06 08:06

To better understand eruptive events in the solar corona, we combine sequences of multi-wavelength observations and modelling of the coronal magnetic field of NOAA AR 8210, a highly flare-productive active region. From the photosphere to the corona, the observations give us information about the motion of magnetic elements (photospheric magnetograms), the location of flares (e.g., Hα , EUV or soft X-ray brightenings), and the type of events (Hα blueshift events). Assuming that the evolution of the coronal magnetic field above an active region can be described by successive equilibria, we follow in time the magnetic changes of the 3D nonlinear force-free ({em nlff}) fields reconstructed from a time series of photospheric vector magnetograms. We apply this method to AR 8210 observed on May 1, 1998 between 17:00 UT and 21:40 UT. We identify two types of horizontal photospheric motions that can drive an eruption: a clockwise rotation of the sunspot, and a fast motion of an emerging polarity. The reconstructed {em nlff} coronal fields give us a scenario of the confined flares observed in AR 8210: the slow sunspot rotation enables the occurence of flare by a reconnection process close to a separatrix surface whereas the fast motion is associated with small-scale reconnections but no detectable flaring activity. We also study the injection rates of magnetic energy, Poynting flux and relative magnetic helicity through the photosphere and into the corona. The injection of magnetic energy by transverse photospheric motions is found to be correlated with the storage of energy in the corona and then the release by flaring activity. The magnetic helicity is derived from the magnetic field and the vector potential of the {em nlff} configuration is computed in the coronal volume. The magnetic helicity evolution shows that AR 8210 is dominated by the mutual helicity between the closed and potential fields and not by the self helicity of the closed field which characterizes the twist of confined flux bundles. We conclude that for AR 8210 the complex topology is a more important factor than the twist in the eruption process.

Authors: S. Regnier, R. C. Canfield
Projects: None

Publication Status: Accepted 01/30/2006, A&A
Last Modified: 2006-02-06 08:06
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Self and Mutual Magnetic Helicities in Coronal Magnetic Configurations  

Stephane Regnier   Submitted: 2005-08-01 06:34

Together with the magnetic energy, the magnetic helicity is an important quantity used to describe the nature of a magnetic field configuration. In the following, we propose a new technique to evaluate various components of the total magnetic helicity in the corona for an equilibrium reconstructed magnetic field. The most meaningful value of helicity is the total relative magnetic helicity which describes the linkage of the field lines even if the volume of interest is not bounded by a magnetic surface. In addition if the magnetic field can be decomposed into the sum of a closed field and a reference field (following Berger 1999), we can introduce three other helicity components: the self helicity of the closed field, the mutual helicity between the closed field and the reference field, and the vacuum helicity (self helicity of the reference field). To understand the meaning of those quantities, we derive them from the potential field (reference) and the force-free field computed with the same boundary conditions for three different cases: a single twisted flux tube derived from the extended Gold-Hoyle solutions, a simple magnetic configuration with three balanced sources and a constant distribution of the force-free parameter, and the AR 8210 magnetic field observed from 17:13 UT to 21:16 UT on May 1, 1998. We analyse the meaning of the self and mutual helicities: the self and mutual helicities correspond to the twist and writhe of confined flux bundles, and the crossing of field lines in the magnetic configuration respectively. The main result is that the magnetic configuration of AR 8210 is dominated by the mutual helicity and not by the self helicity (twist and writhe). Our results also show that although not gauge invariant the vacuum helicity is sensitive to the topological complexity of the reference field.

Authors: Regnier, S., Amari, T., Canfield, R.C.
Projects:

Publication Status: Accepted in A&A
Last Modified: 2005-08-01 06:35
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3D magnetic configuration of the Hα filament and X-ray sigmoid in NOAA AR 8151  

Stephane Regnier   Submitted: 2004-06-24 10:02

We investigate the structure and relationship of an Hα filament and an X-ray sigmoid observed in active region NOAA 8151. We first examine the presence of such structures in the reconstructed 3D coronal magnetic field obtained from the non-constant α force-free field hypothesis using a photospheric vector magnetogram (IVM, Mees Solar Observatory). This method allows us to identify several flux systems: a filament (height 30 Mm, aligned with the polarity inversion line (PIL), magnetic field strength at the apex 49 G, number of turns 0.5-0.6), a sigmoid (height 45 Mm, aligned with the PIL, magnetic field strength at the apex 56 G, number of turns 0.5-0.6) and a highly twisted flux tube (height 60 Mm, magnetic field strength at the apex 36 G, number of turns 1.1-1.2). By searching for magnetic dips in the configuration, we identify a filament structure which is in good agreement with the Hα observations. We find that both filament and sigmoidal structures can be described by a long twisted flux tube with a number of turns less than 1 which means that these structures are stable against kinking. The filament and the sigmoid have similar absolute values of α and Jz in the photosphere. However, the electric current density is positive in the filament and negative in the sigmoid: the filament is right-handed whereas the sigmoid is left-handed. This fact can explain the discrepancies between the handedness of magnetic clouds (twisted flux tubes ejected from the Sun) and the handedness of their solar progenitors (twisted flux bundles in the low corona). The mechanism of eruption in AR 8151 is more likely not related to the development of instability in the filament and/or the sigmoid but is associated with the existence of the highly twisted flux tube (~ 1.1-1.2 turns).

Authors: S. Regnier, T. Amari
Projects: None

Publication Status: A&A (in press)
Last Modified: 2004-06-24 10:02
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Abstracts by Author
A new approach to the maser emission in the solar corona
Sparkling EUV Bright Dots Observed with Hi-C
Magnetic Field Extrapolations in the Corona: Success and Future Improvements
Thermal shielding of an emerging active region
A new look at a polar crown cavity as observed by SDO/AIA
Magnetic Energy Storage and Current Density Distributions for Different Force-Free Models
Relationship between photospheric currents and coronal magnetic helicity for force-free bipolar fields
Coronal Alfvén speeds in an isothermal atmosphere I. Global properties
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Nonlinear force-free models for the solar corona I. Two active regions with very different structure
Nonlinear force-free extrapolation: numerical methods and applications
Evolution of magnetic fields and energetics of flares in active region 8210
Self and Mutual Magnetic Helicities in Coronal Magnetic Configurations
3D magnetic configuration of the Halpha filament and X-ray sigmoid in NOAA AR 8151

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