Turbulent Heating and Anisotropy in the Solar Wind: A Numerical Study PDF
Preview Turbulent Heating and Anisotropy in the Solar Wind: A Numerical Study
Springer Theses Recognizing Outstanding Ph.D. Research Victor Montagud-Camps Turbulent Heating and Anisotropy in the Solar Wind A Numerical Study Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. 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More information about this series at http://www.springer.com/series/8790 Victor Montagud-Camps Turbulent Heating and Anisotropy in the Solar Wind A Numerical Study Doctoral Thesis accepted by Paris-Sud University, Orsay, France 123 Author Supervisor Dr. Victor Montagud-Camps Dr. RolandGrappin Department ofSurface andPlasma Science Laboratoire dePhysiquedes plasmas CharlesUniversity EcolePolytechnique Prague,Czech Republic Palaiseau, France ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-030-30382-2 ISBN978-3-030-30383-9 (eBook) https://doi.org/10.1007/978-3-030-30383-9 ©SpringerNatureSwitzerlandAG2019 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. 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ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland ’ Supervisor s Foreword Turbulence is a widespread phenomenon that can be found in a great variety of naturalflows.Imposedlarge-scaleflows,asrotation,orshearflows,arewell-known ways to deeply alter the properties of isotropic turbulence, in particular, eddy ani- sotropy and turbulent dissipation. The solar wind flow is a particularly interesting example,combiningtwoimportantconstraints:alarge-scaleradialflowandamean magnetic field, which in a conductive plasma leads to two-symmetry axis in com- petitionduringtheturbulentcascade. Thisthesis contributesinseveralwaystothe understanding of turbulence in the solar wind, in the framework of magnetohy- drodynamics(MHD).Theresultsareimportantforotherastrophysicalflowsaswell. The several findings presented in this thesis include the first direct numerical simulations showing a strong slow down of cooling during transport by the solar wind, the slow down being comparable to the one observed. This demonstration deals with eddy anisotropies compatible with the one observed in slow winds. A second study, valid for eddy anisotropies compatible with fast winds, contains the revelation that the flow cross helicity, characteristic of fast winds, strongly enhancestheimprintofexpansionontheanisotropyofeddies,butatthesametime maintainsastrongturbulentdissipation.Commonwisdomwould,inthiscase,have predicted the reverse, namely that cross helicity should decrease turbulent dissi- pation. Finally, the obtained slow down of cooling during transport is found to be comparable in slow and fast winds, which matches the solar wind situation. The thesis also includes a thorough study of the phenomenology applied to the problem of turbulent heating (or more properly turbulent slow down of cooling) during solar wind transport. This phenomenology has been applied recently to the question of solar wind acceleration, allowing to show that, in the acceleration region close to the Sun, one should take into account compressibility in the tur- bulent cascade and not only use incompressible models. v vi Supervisor’sForeword This thesis contains remarkable results, it uses both numerical and phe- nomenological approaches to make several valuable contributions to the field of astrophysicalturbulentflows.Forallitsachievements,itconstitutesagoodaddition to the Springer Theses series. Palaiseau, France Dr. Roland Grappin July 2019 Abstract Physical phenoma based on non-linear processes pose some of the most thrilling andcomplexproblemstotreatinphysics.Amongthoseproblems,fluidturbulence is of great relevance due to its presence in multiple physical contexts, from geo- physicstoastrophysics.Inthelatter,turbulencedevelopsinfluidsmadeofionized particles whose movements are coupled to the electromagnetic fields that they create and to those already present in the medium. Such fluids are known as plasmas. The Sun, a plasma itself, continuously ejects its atmosphere in all directions, reaching all the solar system at velocities of several hundreds kilometers per sec- ond.ThisplasmaisknownastheSolarWindand,duetoitsavailabilityforin-situ measurements, is one of the most studied examples of turbulence in astrophysical plasmas. Thisthesishasforgoalthestudyofturbulentanisotropyandturbulentheatingin thesolarwindbetween0.2and1Astronomicalunits(Earth’sorbit)withtheaidof direct numerical simulations. Both issues can be considered as inverse problems: the majority of in-situ measurements of the solar wind have been done at 1 astronomical unit and despite knowing most of the properties of the solar wind within the inner heliosphere, information about the properties of solar wind tur- bulence is in general only available close to Earth’s orbit. As a consequence, we need to infer the initial conditions of the solar wind turbulence at 0.2 AU and compare the results of the simulations to its arrival at 1 AU. From the simulations that match the observations at 1 AU and the radial evolution of other plasma properties, we deduce what are the physical mechanisms in both cases. Inthefirstpartofthisthesis,wewillintroducethecontextonwhichthisworkis framed. A general overview of the solar wind and the lower layers of the helio- sphere will be followed by an introduction of the modeling of plasmas to end up with description of the equations that is used in our simulations, the Expanding BoxModelequations.Then,wewilldefineturbulence,andthetechniquesthathave been developed to study this physical phenomenon. We then explain how the measurements of plasma and turbulence properties in the solar wind have been done. vii viii Abstract The following part is dedicated to our study of turbulence anisotropy. At large scales, in-situ measurements of the turbulence at 1 AU show two preferred direc- tions to transfer coherent energy via nonlinear couplings, the perpendicular to the meanmagneticfieldinthecaseofslowwindsandtheparalleltothesameaxisfor fast winds. We willrevisit apreviouspaper inthis area byVerdiniand Grappin in 2016thatalreadytreatedthisproblem.Inourcase,wewillexplorethevariationof physical parameters that were neglected and have proven to be decisive in the development of turbulent anisotropy. We will present several models of initial turbulentanisotropythatmanagetoreproducetheobservationsat1AUandwewill discuss the suitability of each one. Inthethirdpart,wefocusonturbulentheatinginfastandslowwinds.Between 0.3and1AU,protontemperature ofthesolarwindhasbeenobservedtodiminish as a power law of heliospheric distance. An adiabatic expansion of the solar wind would lead to a faster decrease than what is observed. This has suggested that turbulencecanbealocalsourceofheatingresponsibleforthetemperaturegradient flatter than the adiabatic cooling predictions. Here, we will show a 1D analytical model of hydrodynamic turbulence in and expanding in an expanding box, developed to give an idea on how this parameter can change turbulence and the temperaturegradient.WewillshowtheresultsfoundusingtheEBMequationsina 3D numerical box. First, we will see how the temperature gradient can be repro- duced in the case of slow winds. The extension of this result to fast winds will be reached right after. In thefourth and final part, we acknowledge thelimitations encountered by our simulations and the physics that have been neglected. We address the absence of temperature anisotropy, and multiple particle species, specially electrons, whose contribution to the studied problems have been neglected. In the case of the tem- perature anisotropy, we make a proposition of future work based on the modifi- cationoftheEBMequationstoaccountforprotontemperatureanisotropy.Forthe electrons and other species, instead of proposing new modifications to EBM, we discuss alternative models of solar wind that confer to electrons the main role in solar wind dynamics. Acknowledgements Thepresentdocumentistheresultof3yearsofworkasaPh.D.studentofRoland Grappin and Filippo Pantellini. I would like to thank them for their mentoring and scientific discussions that we have had along these years, but also for our day-to-day conversations accompanied with tea or coffee. I also want to thank Roland, the Doctoral School of Astronomy and Astrophysics of Paris Area (ED127) and the Department of Physics of Paris-Sud University for submitting my thesis to the Springer Theses programme. I would also like to express my gratitude to Andrea Verdini, who has always managed to find some time to answer my many doubts and questions. I am thankful to Viviane Pierrard and to Franck Plunian, referees of my thesis, whose attentive reading of my work and their corrections have allowed me to improve my manuscript. Itisnotpossibleformenottomentionmyfriends,whohavebeenbymysideall along these years. I would also like to acknowledge all the help and support that I have received frommyformerlaboratory,theLaboratoiredePhysiquedesPlasmasandtothank all its members: the administrative and technical teams, the direction and the sci- entific groups, specially the space plasma, numerical simulations and turbulence groups. Last,butnotleast,tomyfamily:Iwouldneverbeabletothankyouenoughall that you have done for me, but I will always try, thank you! ix
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