Stephane Regnier

Northumbria University · Department of Mathematics, Physics and Electrical Engineering

Oscillatory reconnection is a relaxation process in magnetized plasma, with an inherent periodicity that is exclusively dependent on the properties of the background plasma. This study focuses on the seismological prospects of oscillatory reconnection in the solar corona. We perform three sets of parameter studies (for characteristic coronal values...

Oscillatory reconnection is a relaxation process in magnetised plasma, with an inherent periodicity that is exclusively dependent on the properties of the background plasma. This study focuses on the seismological prospects of oscillatory reconnection in the solar corona. We perform three sets of parameter studies (for characteristic coronal values...

Oscillatory reconnection can manifest through the interaction between the ubiquitous MHD waves and omnipresent null points in the solar atmosphere and is characterized by an inherent periodicity. In the current study, we focus on the relationship between the period of oscillatory reconnection and the strength of the wave pulse initially perturbing...

Oscillatory reconnection can manifest through the interaction between the ubiquitous MHD waves and omnipresent null points in the solar atmosphere and is characterized by an inherent periodicity. In the current study, we focus on the relationship between the period of oscillatory reconnection and the strength of the wave pulse initially perturbing...

Oscillatory reconnection (a relaxation mechanism with periodic changes in connectivity) has been proposed as a potential physical mechanism underpinning several periodic phenomena in the solar atmosphere, including, but not limited to, quasi-periodic pulsations (QPPs). Despite its importance, however, the mechanism has never been studied within a h...

Oscillatory reconnection (a relaxation mechanism with periodic changes in connectivity) has been proposed as a potential physical mechanism underpinning several periodic phenomena in the solar atmosphere including, but not limited to, quasi-periodic pulsations (QPPs). Despite its importance, however, the mechanism has never been studied within a ho...

Coronal loops form the basic building blocks of the magnetically closed solar corona yet much is still to be determined concerning their possible fine-scale structuring and the rate of heat deposition within them. Using an improved multi-stranded loop model to better approximate the numerically challenging transition region, this article examines s...

Coronal loops form the basic building blocks of the magnetically closed solar corona yet much is still to be determined concerning their possible fine-scale structuring and the rate of heat deposition within them. Using an improved multi-stranded loop model to better approximate the numerically challenging transition region, this paper examines syn...

Alfvénic waves have gained renewed interest since the existence of ubiquitous propagating kink waves were discovered in the corona. It has long been suggested that Alfvénic waves play an important role in coronal heating and the acceleration of the solar wind. To this effect, it is imperative to understand the mechanisms that enable their energy to...

Alfv\'enic waves have gained renewed interest since the existence of ubiquitous propagating kink waves were discovered in the corona. {It has long been suggested that Alfv\'enic} waves play an important role in coronal heating and the acceleration of the solar wind. To this effect, it is imperative to understand the mechanisms that enable their ene...

Correction to: Solar Phys (2018) 293:7510.1007/s11207-018-1278-5 Please find in this correction document the correct versions of abstract, Sect. 3.1 and Figs. 3 and 12.

Within the framework of 3D resistive magnetohydrodynamic, we simulate the formation of a plasma jet with the morphology, upward velocity up to 130 km s ⁻¹ , and time-scale formation between 60 and 90 s after beginning of simulation, similar to those expected for type II spicules. Initial results of this simulation were published in paper by, e.g. G...

Within the framework of 3D resistive MHD, we simulate the formation of a plasma jet with the morphology, upward velocity up to 130 km/s and timescale formation between 60 and 90 s after beginning of simulation, similar to those expected for Type II spicules. Initial results of this simulation were published in Paper (e.g., González-Avilés et al. 20...

Within the framework of resistive MHD, implementing the C7 equilibrium atmosphere model and a 3D potential magnetic field realistic configuration, we simulate the formation of a plasma jet with the morphology, upward velocity up to 130 km/s and timescale formation between 60 and 90 s after beginning of simulation, similar to those expected for Type...

Within the framework of resistive MHD, implementing the C7 equilibrium atmosphere model and a 3D potential magnetic field realistic configuration, we simulate the formation of a plasma jet with the morphology, upward velocity up to 130 km/s and timescale formation between 60 and 90 s after beginning of simulation, similar to those expected for Type...

Within the framework of resistive MHD, implementing the C7 equilibrium atmosphere model and a 3D potential magnetic field realistic configuration, we simulate the formation of a plasma jet with the morphology, upward velocity up to 130 km/s and timescale formation between 60 and 90 s after beginning of simulation, similar to those expected for Type...

Taking the 32 storm sudden commencements (SSCs) listed by ISGI during 2002 as a starting point, we performed a multi-criterion analysis based on observations (propagation time, velocity comparisons, sense of the magnetic field rotation, radio waves) to associate them with solar sources. We identified their effects in the interplanetary medium, and...

Using simulated data-driven, 3D resistive MHD simulations of the solar atmosphere, we show that 3D magnetic reconnection may be responsible for the formation of jets with characteristics of Type II spicules. We numerically model the photosphere-corona region using the C7 equilibrium atmosphere model. The initial magnetic configuration is a 3D poten...

The signatures of energy release and energy transport for a kink-unstable coronal flux rope are investigated via forward modelling. Synthetic intensity and Doppler maps are generated from a 3D numerical simulation. The CHIANTI database is used to compute intensities for three Hinode/EIS emission lines that cover the thermal range of the loop. The i...

Determining the mechanisms responsible for the heating of the coronal plasma and maintaining and accelerating the solar wind are long standing goals in solar physics. There is a clear need to constrain the energy, mass and momentum flux through the solar corona and advance our knowledge of the physical process contributing to these fluxes. Furtherm...

We study the propagation of a fast magnetoacoustic wave in a 3D magnetic field created from two magnetic dipoles. The magnetic topology contains an X-line. We aim to contribute to the overall understanding of MHD wave propagation within inhomogeneous media, specifically around X-lines. We investigate the linearised, 3D MHD equations under the assum...

The nonlinear force-free field (NLFFF) model is often used to describe the solar coronal magnetic field, however a series of earlier studies revealed difficulties in the numerical solution of the model in application to photospheric boundary data. We investigate the sensitivity of the modeling to the spatial resolution of the boundary data, by appl...

The electron plasma frequency $\omega_{pe}$ 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 perf...

The acoustic resonator is an important model for explaining the three-minute oscillations in the chromosphere above sunspot umbrae. The steep temperature gradients at the photosphere and transition region provide the cavity for the acoustic resonator, which allows waves to be both partially transmitted and partially reflected. In this paper, a new...

[...] redshifts are predominantly observed in the core of active regions, while blueshifts are observed at the edge of active regions. [...] Using a nanoflare heating model for multi-stranded coronal loops (Sarkar and Walsh, 2008, 2009), we reproduce the above Dopplershift observations using spectral lines covering a broad range of temperature (fro...

Observing the Sun at high time and spatial scales is a step toward understanding the finest and fundamental scales of heating events in the solar corona. The high-resolution coronal (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 regio...

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 g...

Plasma flows within prominences/filaments have been observed for many years and hold valuable clues concerning the mass and energy balance within these structures. Previous observations of these flows primarily come from H-alpha and cool EUV lines (e.g., 304A) where estimates of the size of the prominence threads has been limited by the resolution...

Variations in the propagation of globally-propagating disturbances (commonly called "EIT waves") through the low solar corona offer a unique opportunity to probe the plasma parameters of the solar atmosphere. Here, high-cadence observations of two "EIT wave" events taken using the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dyna...

The interaction between emerging active regions and the pre-existing coronal magnetic field is important to understand better the mechanisms of storage and release of magnetic energy from the convection zone to the high corona. We are aiming at describing the first steps of the emergence of an active region within a pre-existing quiet-Sun corona in...

We observed active region 10943 located at Sun center on February 20, 2007 and we characterized its interactions with active region 10942 by using a multi-instrument comparison between Hinode/XRT and STEREO/SECCHI images, magnetic field maps from SoHO/MDI, and Hinode/EIS spectra from which we built maps of the Doppler shift and the intensity of the...

Upflows observed at the edges of active regions have been proposed as the source of the slow solar wind. In the particular case of Active Region (AR) 10942, where such an upflow has been already observed, we want to evaluate the part of this upflow that actually remains confined in the magnetic loops that connect AR10942 to AR10943. Both active reg...

The potential field is a quite simple assumption to model the magnetic field of the solar corona as it relies only on the vertical component of the magnetic field measured on the photosphere. However, with the always increasing amount of line-of-sight magnetic field, this assumption is often the best way to approximate the corona magnetic field and...

Context. The Solar Dynamics Observatory (SDO) was launched in February 2010 and is now providing an unprecedented view of the solar activity at high spatial resolution and high cadence covering a broad range of temperature layers of the atmosphere. Aims: We aim at defining the structure of a polar crown cavity and describing its evolution during th...

[..] The change in the magnetic configuration due to the increase/decrease of electric current for different force-free models (potential, linear and nonlinear force-free fields) has never been studied in detail before. Here we focus especially on the evolution of the free magnetic energy, the location of the excess of energy, and the distribution...

In this review, I focus on the structure and evolution of the coronal magnetic fields modelled from observations. The development of instruments measuring the photospheric and chromospheric magnetic fields with a high spatial and time resolutions allows us to improve the modeling of the coronal fields based on extrapolation and evolution techniques...

A data pipeline for the distribution of SDO data products has been developed throughout a number of countries in the US, Europe and Asia. The UK node within this pipeline is at the University of Central Lancashire (UCLan), where a data center has been established to host a rolling AIA and HMI archive, aimed at supplying data to the country's large...

We report on the high resolution and high cadence observations of the birth of an active region in a supergranular cell. We use SDO/AIA EUV images to derive the evolution of the thermal properties, and SDO/HMI continuum images and line-of-sight magnetograms to study the time evolution of the magnetic polarities forming a pore. We restrict our study...

There is considerable interest in accurate modelling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data, and the nonlinear force-free model is often used. However, recent studies using Hinode data have demonstrated that this modelling fails in basic ways, with the failure attributable to the departure of the...

The new space instrumentation on board the Hinode satellite allows us to study in details the dynamic evolution of the quiet-Sun magnetic field from the photosphere to the corona. Based on high resolution magnetograms Hinode/SOT/SP, we investigate the effects of photospheric motions on the evolution of coronal structures and open field lines. Obser...

Nonlinear force-free solutions for the magnetic field in the solar corona constructed using photospheric vector magnetic field boundary data suffer from a basic problem: the observed boundary data are inconsistent with the nonlinear force-free model. Specifically, there are two possible choices of boundary conditions on vertical current provided by...

Nonlinear force-free field (NLFFF) modeling seeks to provide accurate representations of the structure of the magnetic field above solar active regions, from which estimates of physical quantities of interest (e.g., free energy and helicity) can be made. However, the suite of NLFFF algorithms have failed to arrive at consistent solutions when appli...

Aims. 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 $10^{42}$ Mx$^2$ ($10^{26}$ 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 ma...

The POLAR Investigation of the Sun (POLARIS) mission uses a combination of a gravity assist and solar sail propulsion to place a spacecraft in a 0.48 AU circular orbit around the Sun with an inclination of 75° with respect to solar equator. This challenging orbit is made possible by the challenging development of solar sail propulsion. This first e...

Nonlinear force-free field (NLFFF) models are thought to be viable tools for investigating the structure, dynamics and evolution of the coronae of solar active regions. In a series of NLFFF modeling studies, we have found that NLFFF models are successful in application to analytic test cases, and relatively successful when applied to numerically co...

Nonlinear force-free field (NLFFF) modeling promises to provide accurate representations of the structure of the magnetic field above solar active regions, from which estimates of physical quantities of interest (e.g., free energy and helicity) can be made. However, the suite of NLFFF algorithms have so far failed to arrive at consistent solutions...

Estimating Alfven 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 beta, one may decouple the magnetic forces from press...

Nonlinear force-free extrapolations are used to determine the 3D nature of the coronal magnetic field. The nonlinear force-free fields require knowledge of the vector magnetic field as boundary conditions at the photospheric or chromospheric level. The development of new vector magnetographs such as Hinode/SOT and SDO/HMI will certainly enhance the...

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 configur...

Context. 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.Aims. Topology is a tool to explain the complexity of the magnetic field, the occurrence of reconnection pro...

We investigate the values and distribution of the Alfvén speed in the solar corona. We assume e that the coronal magnetic field is force-free and the plasma is isothermal and in hydrostatic equilibrium. Firstly we consider a bipolar configuration in order to study the effect of parameters such as the pressure scale-height and the density at the bas...

The Hinode satellite was launched in 2006 with unprecedented high spatial and temporal resolution revealing the detailed nature of the quiet Sun. Based on the new data recorded by Hinode/SOT, we revisit the magnetic topology of the quiet Sun. It has been found, using point source models, that approximately one null point exists for each source with...

The magnetic topology is a key element to understand the physics of solar eruptions. It is well-known that magnetic reconnection leading to the fast release of magnetic energy occurs at topological elements such as null points, separators, quasi-separatrix layers or hyperbolic flux tubes. We investigate the possible change of topology for different...

Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of pre-existing atmospheric magnetic flux subject to surface plasma flows, or...

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 fo...

We present two methods for deriving the velocity field in magnetized regions of the Sun's photosphere. As a preliminary step, we introduce a Fourier-based local correlation tracking (LCT) routine that we term "FLCT." By explicitly employing the observation made by Démoulin & Berger (2003), that results determined by LCT applied to magnetograms invo...

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 in...

We report on theoretical modeling of an eruptive event observed by SoHO/SUMER in Hi Lyβ 1025.72Å, CII 1037.00 Å and O VI 1037.64 A emission of the quiet solar atmosphere. We are using two different theoretical approaches. The first one is based on the application of the cloud model to a loop hanging in the solar atmosphere and being irradiated by i...

The active region 8210 observed on May 1st 1998 has produced numerous C-class flares. To understand the physical process associated with these eruptive events, we describe the evolution of AR 8210 using a time series of nonlinear force-free equilibria. We study the time evolution of the magnetic configurations in terms of the storage and the releas...

We compare six algorithms for the computation of nonlinear force-free (NLFF) magnetic fields (including optimization, magnetofrictional, Grad–Rubin based, and Green's function-based methods) by evaluating their performance in blind tests on analytical force-free-field models for which boundary conditions are specified either for the entire surface...

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 magnetogram...

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 helici...