arXiv subjects

Antoine Strugarek

Publications and source records attributed to Antoine Strugarek.

At least 55 records · Page 3Linked to original sources

arXiv2021

Modeling Solar Wind Variations over an 11-yr Cycle with Alfvén Wave Dissipation: a Parameter Study

We study the behaviour and properties of the solar wind using a 2.5D Alfvén wave driven wind model. We first systematically compare the results of an Alfvén wave (AW) driven wind model with a polytropic approach. Polytropic magnetohydrodynamic wind models are thermally driven, while Alfvén waves act as additional acceleration and heating mechanisms in the Alfvén wave driven model. We confirm that an AW-driven model is required to reproduce the observed bimodality of slow and fast solar winds. We are also able to reproduce the observed anti-correlation between the terminal wind velocity and the coronal source temperature with the AW-driven wind model. We also show that the wind properties along an eleven-year cycle differ significantly from one model to the other. The AW-driven model again shows the best agreement with observational data. Indeed, solar surface magnetic field topology plays an important role in the Alfvén wave driven wind model, as it enters directly into the input energy sources via the Poynting flux. On the other hand, the polytropic wind model is driven by an assumed pressure gradient; thus it is relatively less sensitive to the surface magnetic field topology. Finally, we note that the net torque spinning down the Sun exhibits the same trends in the two models, showing that the polytropic approach still captures correctly the essence of stellar winds.

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arXiv2020

Impact of solar magnetic field amplitude and geometry on cosmic rays diffusion coefficients in the inner heliosphere

Cosmic rays (CRs) are tracers of solar events when they are associated with solar flares, but also galactic events when they come from outside our solar system. SEPs are correlated with the 11-year solar cycle while GCRs are anti-correlated due to their interaction with the heliospheric magnetic field and the solar wind. Our aim is to quantify separately the impact of the amplitude and the geometry of the magnetic field on the propagation of CRs of various energies in the inner heliosphere. We focus especially on the diffusion caused by the magnetic field along and across the field lines. To do so, we use the results of 3D MHD wind simulations running from the lower corona up to 1 AU. The wind is modeled using a polytropic approximation, and fits and power laws are used to account for the turbulence. Using these results, we compute the parallel and perpendicular diffusion coefficients of the Parker CR transport equation, yielding 3D maps of the diffusion of CRs in the inner heliosphere. By varying the amplitude of the magnetic field, we change the amplitude of the diffusion by the same factor, and the radial gradients by changing the spread of the current sheet. By varying the geometry of the magnetic field, we change the latitudinal gradients of diffusion by changing the position of the current sheets. By varying the energy, we show that the distribution of values for SEPs is more peaked than GCRs. For realistic solar configurations, we show that diffusion is highly non-axisymmetric due to the configuration of the current sheets, and that the distribution varies a lot with the distance to the Sun with a drift of the peak value. This study shows that numerical simulations and theory can help quantify better the influence of the various magnetic field parameters on the propagation of CRs. This study is a first step towards generating synthetic CR rates from numerical simulations.

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arXiv2020

Alfvén-wave driven magnetic rotator winds from low-mass stars I: rotation dependences of magnetic braking and mass-loss rate

Observations of stellar rotation show that low-mass stars lose angular momentum during the main sequence. We simulate the winds of Sun-like stars with a range of rotation rates, covering the fast and slow magneto-rotator regimes, including the transition between the two. We generalize an Alfvén-wave driven solar wind model that builds on previous works by including the magneto-centrifugal force explicitly. In this model, the surface-averaged open magnetic flux is assumed to scale as $B_\ast f^{\rm open}_\ast \propto {\rm Ro}^{-1.2}$, where $f^{\rm open}_\ast$ and ${\rm Ro}$ are the surface open-flux filling factor and Rossby number, respectively. We find that, 1. the angular momentum loss rate (torque) of the wind is described as $τ_w \approx 2.59 \times 10^{30} {\rm \ erg} \ \left( Ω_\ast / Ω_\odot \right)^{2.82}$, yielding a spin-down law $Ω_\ast \propto t^{-0.55}$. 2. the mass-loss rate saturates at $\dot{M}_w \sim 3.4 \times 10^{-14} M_\odot {\rm \ yr^{-1}}$, due to the strong reflection and dissipation of Alfvén waves in the chromosphere. This indicates that the chromosphere has a strong impact in connecting the stellar surface and stellar wind. Meanwhile, the wind ram pressure scales as $P_w \propto Ω_\ast^{0.57}$, which is able to explain the lower-envelope of the observed stellar winds by Wood et al. 3. the location of the Alfvén radius is shown to scale in a way that is consistent with 1D analytic theory. Additionally, the precise scaling of the Alfvén radius matches previous works which used thermally-driven winds. Our results suggest that the Alfvén-wave driven magnetic rotator wind plays a dominant role in the stellar spin-down during the main-sequence.

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arXiv2020

Tearing instability and periodic density perturbations in the slow solar wind

In contrast with the fast solar wind, that originates in coronal holes, the source of the slow solar wind is still debated. Often intermittent and enriched with low FIP elements -- akin to what is observed in closed coronal loops -- the slow wind could form in bursty events nearby helmet streamers. Slow winds also exhibit density perturbations which have been shown to be periodic and could be associated with flux ropes ejected from the tip of helmet streamers, as shown recently by the WISPR white light imager onboard Parker Solar Probe (PSP). In this work, we propose that the main mechanism controlling the release of flux ropes is a flow-modified tearing mode at the heliospheric current sheet (HCS). We use MHD simulations of the solar wind and corona to reproduce realistic configurations and outflows surrounding the HCS. We find that this process is able to explain long ($\sim 10-20$h) and short ($\sim 1-2$h) timescales of density structures observed in the slow solar wind. This study also sheds new light on the structure, topology and composition of the slow solar wind, and could be, in the near future, compared with white light and in situ PSP observations.

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arXiv2020

Could star-planet magnetic interactions lead to planet migration and influence stellar rotation ?

The distribution of hot Jupiters, for which star-planet interactions can be significant, questions the evolution of exosystems. We aim to follow the orbital evolution of a planet along the rotational and structural evolution of the host star by taking into account the coupled effects of tidal and magnetic torques from ab initio prescriptions. It allows us to better understand the evolution of star-planet systems and to explain some properties of the distribution of observed close-in planets. To this end we use a numerical model of a coplanar circular star-planet system taking into account stellar structural changes, wind braking and star-planet interactions, called ESPEM (Benbakoura et al. 2019). We find that depending on the initial configuration of the system, magnetic effects can dominate tidal effects during the various phases of the evolution, leading to an important migration of the planet and to significant changes on the rotational evolution of the star. Both kinds of interactions thus have to be taken into account to predict the evolution of compact star-planet systems.

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arXiv2020

From Stellar Coronæ to Gyrochronology: a theoretical and observational exploration

Context. Stellar spin-down is the result of a complex process involving rotation, dynamo, wind and magnetism. Multi-wavelength surveys of solar-like stars have revealed the likely existence of relationships between their rotation, X-ray luminosity, mass-losses and magnetism. Those impose strong constraints on the corona and wind of cool stars. Aims. We aim to provide power-law prescriptions of the mass-loss of stars, of their magnetic field, and of their base coronal density and temperature that are compatible with their observationally-constrained spin-down. Methods. We link the magnetic field and the mass-loss rate from a wind torque formulation in agreement with the distribution of stellar rotation periods in open clusters and the Skumanich law. Given a wind model and an expression of the X-ray luminosity from radiative losses, we constrain the coronal properties by assuming different physical scenarii linking closed loops to coronal holes. Results. We find that the magnetic field and the mass loss are involved in a one-to-one correspondence constrained from spin-down considerations. We show that a magnetic field depending on both the Rossby number and the stellar mass is required to keep a consistent spin-down model. The estimates of the magnetic field and the mass-loss rate obtained from our formalism are consistent with statistical studies as well as individual observations and give new leads to constrain the magnetic field-rotation relation. The set of scaling-laws we derived can be broadly applied to cool stars from the PMS to the end of the MS, and allow for a stellar wind modelling consistent with all the observational constraints available to date.

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arXiv2019

The dynamo-wind feedback loop: Assessing their non-linear interplay

Though generated deep inside the convection zone, the solar magnetic field has a direct impact on the Earth space environment via the Parker spiral. It strongly modulates the solar wind in the whole heliosphere, especially its latitudinal and longitudinal speed distribution over the years. However the wind also influences the topology of the coronal magnetic field by opening the magnetic field lines in the coronal holes, which can affect the inner magnetic field of the star by altering the dynamo boundary conditions. This coupling is especially difficult to model because it covers a large variety of spatio-temporal scales. Quasi-static studies have begun to help us unveil how the dynamo-generated magnetic field shapes the wind, but the full interplay between the solar dynamo and the solar wind still eludes our understanding. We use the compressible magnetohydrodynamical (MHD) code PLUTO to compute simultaneously in 2.5D the generation and evolution of magnetic field inside the star via an alpha-Omega dynamo process and the corresponding evolution of a polytropic coronal wind over several activity cycles for a young Sun. A multi-layered boundary condition at the surface of the star connects the inner and outer stellar layers, allowing both to adapt dynamically. Our continuously coupled dynamo-wind model allows us to characterize how the solar wind conditions change as a function of the cycle phase, and also to quantify the evolution of integrated quantities such as the Alfvén radius. We further assess the impact of the solar wind on the dynamo itself by comparing our results with and without wind feedback.

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arXiv2019

Stellar Physics with High-Resolution UV Spectropolarimetry

Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution. UV spectropolarimetry is proposed on missions of various sizes and scopes, from POLLUX on the 15-m telescope LUVOIR to the Arago M-size mission dedicated to UV spectropolarimetry.

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arXiv2019

Detecting volcanically produced tori along orbits of exoplanets using UV spectroscopy

We suggest to use the Hubble Space Telescople (HST) follow-up observations of the TESS targets for detecting possible plasma tori along the orbits of exoplanets orbiting M dwarfs. The source of the torus could be planetary volcanic activity due to tidal or electromagnetic induction heating. Fast losses to space for planets orbiting these active stars can lead to the lost material forming a torus along the planetary orbit, similar to the Io plasma torus. We show that such torus would be potentially detectable by the HST in the UV.

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arXiv2019

Exoplanet host-star properties: the active environment of exoplanets

The primary objectives of the ExoplANETS-A project are to: establish new knowledge on exoplanet atmospheres; establish new insight on influence of the host star on the planet atmosphere; disseminate knowledge, using online, web-based platforms. The project, funded under the EU's Horizon-2020 programme, started in January 2018 and has a duration ~3 years. We present an overview of the project, the activities concerning the host stars and some early results on the host stars.

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arXiv2018

Does magnetic field modify tidal dynamics in the convective envelope of Solar mass stars?

The energy dissipation of wave-like tidal flows in the convective envelope of low-mass stars is one of the key physical mechanisms that shape the orbital and rotational dynamics of short-period planetary systems. Tidal flows, and the excitation, propagation, and dissipation of tidally-induced inertial waves can be modified by stellar magnetic fields (e.g., Wei 2016, 2018, Lin and Ogilvie 2018). It is thus important to assess for which stars, at which location of their internal structure, and at which phase of their evolution, one needs to take into account the effects of magnetic fields on tidal waves. Using scaling laws that provide the amplitude of dynamo-generated magnetic fields along the rotational evolution of these stars (e.g., Christensen et al. 2009, Brun et al. 2015), combined with detailed grids of stellar rotation models (e.g., Amard et al. 2016), we examine the influence of a magnetic field on tidal forcing and dissipation near the tachocline of solar-like stars. We show that full consideration of magnetic fields is required to compute tidal dissipation, but not necessarily for tidal forcing.

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arXiv2018

Simulations of solar wind variations during an 11-year cycle and the influence of north-south asymmetry

We want to study the connections between the magnetic field generated inside the Sun and the solar wind impacting Earth, especially the influence of north-south asymmetry on the magnetic and velocity fields. We study a solar-like 11-year cycle in a quasi-static way: an asymmetric dynamo field is generated through a 2.5-dimensional (2.5-D) flux-transport model with the Babcock-Leighton mechanism, and then is used as bottom boundary condition for compressible 2.5-D simulations of the solar wind. We recover solar values for the mass loss rate, the spin-down time scale and the Alfvén radius, and are able to reproduce the observed delay in latitudinal variations of the wind and the general wind structure observed for the Sun. We show that the phase lag between the energy of the dipole component and the total surface magnetic energy has a strong influence on the amplitude of the variations of global quantities. We show in particular that the magnetic torque variations can be linked to topological variations during a magnetic cycle, while variations in the mass loss rate appear to be driven by variations of the magnetic energy.

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arXiv2017

On differential rotation and overshooting in solar-like stars

We seek to characterize how the change of global rotation rate influences the overall dynamics and large scale flows arising in the convective envelopes of stars covering stellar spectral types from early G to late K. We do so through numerical simulations with the ASH code, where we consider stellar convective envelopes coupled to a radiative interior with various global properties. As solar-like stars spin down over the course of their main sequence evolution, such change must have a direct impact on their dynamics and rotation state. We indeed find that three main states of rotation may exist for a given star: anti-solar-like (fast poles, slow equator), solar-like (fast equator, slow poles), or a cylindrical rotation profile. Under increasingly strict rotational constraints, the latter profile can further evolve into a Jupiter-like profile, with alternating prograde and retrograde zonal jets. We have further assessed how far the convection and meridional flows overshoot into the radiative zone and investigated the morphology of the established tachocline. Using simple mixing length arguments, we are able to construct a scaling of the fluid Rossby number $R_{of} = \tildeω/2Ω_* \sim \tilde{v}/2Ω_* R_*$, which we calibrate based on our 3-D ASH simulations. We can use this scaling to map the behavior of differential rotation versus the global parameters of stellar mass and rotation rate. Finally, we isolate a region on this map ($R_{of} \gtrsim 1.5-2$) where we posit that stars with an anti-solar differential rotation may exist in order to encourage observers to hunt for such targets.

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arXiv2016

Age dependence of wind properties for solar type stars: a 3d study

Young and rapidly rotating stars are known for intense, dynamo generated magnetic fields. Spectropolarimetric observations of those stars in precisely aged clusters are key input for gyrochronology and magnetochronology. We use ZDI maps of several young K-type stars of similar mass and radius but with various ages and rotational periods, to perform 3D numerical MHD simulations of their coronae and follow the evolution of their magnetic properties with age. Those simulations yield the coronal structure as well as the instant torque exerted by the magnetized, rotating wind on the star. As stars get older, we find that the angular momentum loss decreases with $Ω^3$, which is the reason for the convergence on the Skumanich law. For the youngest stars of our sample, the angular momentum loss show signs of saturation around $8Ω_{\odot}$, which is a common value used in spin evolution models for K-type stars. We compare these results to semi-analytical models and existing braking laws. We observe a complex wind speed distribution for the youngest stars with slow, intermediate and fast wind components, which are the result of the interaction with intense and non axisymmetric magnetic fields. Consequently, in our simulations, the stellar wind structure in the equatorial plane of young stars varies significantly from a solar configuration, delivering insight about the past of the solar system interplanetary medium.

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arXiv2016

On the Stability of a General Magnetic Field Topology in Stellar Radiative Zones

This paper provides a brief overview of the formation of stellar fossil magnetic fields and what potential instabilities may occur given certain configurations of the magnetic field. In particular, a purely magnetic instability can occur for poloidal, toroidal, and mixed poloidal-toroidal axisymmetric magnetic field configurations as originally studied in Tayler (1973), Markey & Tayler (1973), and Tayler (1980). However, most of the magnetic field configurations observed at the surface of massive stars are non- axisymmetric. Thus, extending earlier studies of the axisymmetric Tayler instability in spherical geometry (Goossens, 1980), we introduce a formulation for the global change in the potential energy contained in a convectively-stable region given an arbitrary Lagrangian perturbation, which permits the inclusion of both axisymmetric and non-axisymmetric magnetic fields. With this tool in hand, a path is shown by which more general stability criterion can be established.

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arXiv2015

From solar to stellar corona: the role of wind, rotation and magnetism

Observations of surface magnetic fields are now within reach for many stellar types thanks to the development of Zeeman-Doppler Imaging. These observations are extremely useful for constraining rotational evolution models of stars, as well as for characterizing the generation of magnetic field. We recently demonstrated that the impact of coronal magnetic field topology on the rotational braking of a star can be parametrized with a scalar parameter: the open magnetic flux. However, without running costly numerical simulations of the stellar wind, reconstructing the coronal structure of the large scale magnetic field is not trivial. An alternative -broadly used in solar physics- is to extrapolate the surface magnetic field assuming a potential field in the corona, to describe the opening of the field lines by the magnetized wind. This technique relies on the definition of a so-called source surface radius, which is often fixed to the canonical value of 2.5Rsun. However this value likely varies from star to star. To resolve this issue, we use our extended set of 2.5D wind simulations published in 2015, to provide a criteria for the opening of field lines as well as a simple tool to assess the source surface radius and the open magnetic flux. This allows us to derive the magnetic torque applied to the star by the wind from any spectropolarimetric observation. We conclude by discussing some estimations of spin-down time scales made using our technique, and compare them to observational requirements.

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arXiv2014

The effect of magnetic topology on thermally-driven winds: towards a general formulation of the braking law

Stellar winds are thought to be the main process responsible for the spin down of main-sequence stars. The extraction of angular momentum by a magnetized wind has been studied for decades, leading to several formulations for the resulting torque. However, previous studies generally consider simple dipole or split monopole stellar magnetic topologies. Here we consider in addition to a dipolar stellar magnetic field, both quadrupolar and octupolar configurations, while also varying the rotation rate and the magnetic field strength. 60 simulations made with a 2.5D, cylindrical and axisymmetric set-up and computed with the PLUTO code were used to find torque formulations for each topology. We further succeed to give a unique law that fits the data for every topology by formulating the torque in terms of the amount of open magnetic flux in the wind. We also show that our formulation can be applied to even more realistic magnetic topologies, with examples of the Sun in its minimum and maximum phase as observed at the Wilcox Solar Observatory, and of a young K-star (TYC-0486- 4943-1) whose topology has been obtained by Zeeman-Doppler Imaging (ZDI).

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arXiv2014

Modelling the Corona of HD 189733 in 3D

The braking of main sequence stars originates mainly from their stellar wind. The efficiency of this angular momentum extraction depends on the rotation rate of the star, the acceleration profile of the wind and the coronal magnetic field. The derivation of scaling laws parametrizing the stellar wind torque is important for our understanding of gyro-chronology and the evolution of the rotation rates of stars. In order to understand the impact of complex magnetic topologies on the stellar wind torque, we present three- dimensional, dynamical simulations of the corona of HD 189733. Using the observed complex topology of the magnetic field, we estimate how the torque associated with the wind scales with model parameters and compare those trends to previously published scaling laws.

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