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Simulations of Solar Wind – Magnetosheath – Magnetopause Interactions 132 CONTRIBUTIONS SIMULATIONS OF SOLAR WIND – MAGNETOSHEATH – MAGNETOPAUSE INTERACTIONS SANNI HOILIJOKI FINNISH METEOROLOGICAL INSTITUTE CONTRIBUTIONS No. 132 SIMULATIONS OF SOLAR WIND – MAGNETOSHEATH – MAGNETOPAUSE INTERACTIONS Sanni Hoilijoki Department of Physics Faculty of Science University of Helsinki Helsinki, Finland ACADEMIC DISSERTATION in theoretical physics To be presented, with the permission of the Faculty of Science of the Uni- versity of Helsinki, for public criticism in auditorium E204 at noon (12 o’clock) on May 19th, 2017. Finnish Meteorological Institute Helsinki, 2017 Supervising professor Professor Hannu E. J. Koskinen, University of Helsinki, Finland Thesis supervisor Professor Minna Palmroth, University of Helsinki, Finland Pre-examiners Professor Tuija Pulkkinen, Aalto University, Finland Professor William Lotko, Dartmouth College, USA Opponent Professor Michael W. Liemohn, University of Michigan, USA ISBN 978-952-336-018-1 (paperback) ISBN 978-952-336-019-8 (pdf) ISSN 0782-6117 Erweko Helsinki, 2017 Series title, number and report code of publication Published by Finnish Meteorological Institute Contributions 132, FMI-CONT-132 (Erik Palménin aukio 1) , P.O. Box 503 FIN-00101 Helsinki, Finland Date May 2017 Author Sanni Hoilijoki Title Simulations of solar wind – magnetosheath – magnetopause interactions Abstract This thesis investigates interactions between solar wind and the magnetosphere of the Earth using two global magneto- spheric simulation models, GUMICS-4 and Vlasiator, which are both developed in Finland. The main topic of the thesis is magnetic reconnection at the dayside magnetopause, its drivers and global effects. Magnetosheath mirror mode waves and their evolution, identification and impacts on the local reconnection rates at the magnetopause are also discussed. This thesis consists of four peer-reviewed papers and an introductory part. GUMICS-4 is a magnetohydrodynamic model solving plasma as a single magnetized fluid. Vlasiator is the world’s first global magnetospheric hybrid-Vlasov simulation model, which solves the motion of ions by describing them as velocity distribution functions, whereas electrons are described as a charge neutralizing fluid. Vlasiator is able to solve ion scale physics in a global scale simulation. However, it is computationally heavy and the global simulations are currently de- scribing Earth’s magnetosphere only in two spatial dimensions, whereas the velocity space is three dimensional. This thesis shows that magnetic reconnection at the dayside magnetopause is controlled by several factors. The impact of dipole tilt angle and sunward component of the interplanetary magnetic field on magnetopause reconnection is inves- tigated with a set of GUMICS-4 simulations. Using Vlasiator simulations, this thesis shows that local reconnection rate is highly variable even during steady solar wind and correlates well with an analytical model for 2D asymmetric recon- nection. It is also shown that the local reconnection rate is affected by local variations in the magnetosheath plasma. Fluctuations in the magnetosheath parameters near X-lines are partly generated by mirror mode waves that are observed to grow in the quasi-perpendicular magnetosheath. These results show that that the local reconnection rate at the X-lines is affected not only by the fluctuations in the inflow parameters but also by reconnection at nearby X-lines. Outflow from stronger X-lines pushes against the weaker ones and might ultimately suppress reconnection in the weaker X-lines. Magnetic islands, 2D representations of FTEs, form between X-lines in the Vlasiator simulations. FTEs propagate along the dayside magnetopause driving bow waves in the magnetosheath. The bow waves propagate upstream all the way to the bow shock causing bulges in the shock, from which solar wind particles can reflect back to the solar wind causing local foreshocks. The overall conclusion of this thesis is that the ion scale kinetic physics is important to accurately model the solar wind – magnetosheath – magnetopause interactions. Vlasiator results show a strong scale-coupling between ion and global scales: global scale phenomena have an impact on the local physics and the local phenomena may have unexpected im- pacts on the global dynamics of the magnetosphere. Neglecting the global scales in local ion scale simulations and vice versa may therefore lead to incomplete description of the solar wind – magnetosphere interactions. Publishing unit Finnish Meteorological Institute Classification (UDC) Keywords 533.9; 52; 52-17; space plasma physics; numerical simulations; 537.84 hybrid-Vlasov; magnetohydrodynamics ISSN and series title 0782-6117 Finnish Meteorological Institute Contributions ISBN Language Pages 978-952-336-018-1 (paperback) English 151 978-952-336-019-8 (pdf) Julkaisun sarja, numero ja raporttikoodi Julkaisija Ilmatieteen laitos Contributions 132, FMI-CONT-132 ( Erik Palménin aukio 1), PL 503 00101 Helsinki Julkaisuaika Toukokuu 2017 ___________________________________________________________________________________________________________ Tekijä Sanni Hoilijoki Nimeke Simulaatioita aurinkotuulen ja Maan magnetosfäärin vuorovaikutuksista Tiivistelmä Aurinkotuuli koostuu plasmasta eli suuresta määrästä sähköisesti varattuja hiukkasia, joita Aurinko jatkuvasti syöksee ympärilleen. Nämä varatut hiukkaset kantavat Auringon magneettikenttää mukanaan planeettojen väliseen avaruuteen. Aurinkotuulen sähkömagneettiset kentät vuorovaikuttavat Maan magneettikentän kanssa muodostaen Maan magnetosfäärin. Vaihtelut aurinkotuulen koostumuksessa, nopeudessa ja magneettikentän suunnassa ja voimakkuudessa aiheuttavat häiriöitä Maan magneetttikentässä. Näitä häiriöitä kutsutaan avaruussääksi. Jotta avaruussään rajuihin muutoksiin voidaan varautua, täytyy aurinkotuulen ja Maan magnetosfäärin vuorovaikutus tuntea. Tärkein vuorovaikutusmekanismi on magneettinen rekonnektio eli magneettisten kenttäviivojen uudelleenkytkeytyminen, jota tapahtuu magnetopausilla eli pinnalla, joka erottaa Maan magnetosfäärin aurinkotuulesta. Rekonnektio avaa Maan kenttäviivoja ja päästää energiaa ja plasmaa virtaamaan sisään ja ulos Maan magnetosfääristä. Tämä väitöskirja koostuu neljästä vertaisarvioidusta julkaisusta ja johdannosta, joissa tutkitaan aurinkotuulen ja Maan magnetosfäärin vuorovaikutusta käyttäen kahta laskennallista mallia. Toinen malli, GUMICS-4, perustuu magnetohydrodynamiikkaan eli plasman kuvailuun yhtenä magneettisena nesteenä. Toinen malli, Vlasiator, on maailman ainoa koko Maan magnetosfääriä kuvaava malli, jossa käytetään hybridi-Vlasov -menetelmää. Vlasiator mallintaa protonien käyttätymisen ratkaisemalla niiden nopeusjakaumien kehityksen, mutta elektronit se mallintaa nesteenä. Tämä metodi mahdollistaa ioniskaalan fysiikan mallintamisen, mutta on laskennallisesti huomattavasti raskaampi kuin plasman kuvaaminen yhtenä nesteenä, mistä johtuen Vlasiator-simulaatiot kuvaavat tällä hetkellä Maan magnetosfääriä ainoastaan kahdessa tilaulottuvuudessa. Ensimmäisessä julkaisussa tutkitaan magneettikentän aaltoja, joita kutsutaan peilimoodiksi, alueessa, joka on Maan magnetopausin ja sen keulashokin väliin jäävä pyörteinen välivyöhyke (magnetosheath). Toisessa julkaisussa tutkitaan GUMICS-mallilla, miten Maan dipolikentän kallistuskulma ja aurinkotuulen magneettikentän auringonsuuntainen komponentti vaikuttavat sijaintiin, missä magneettista rekonnektiota tapahtuu. Kolmannessa julkaisussa rekonnektioita tutkitaan käyttäen Vlasiatoria. Vaikka simulaatioissa aurinkotuulen parametrin pidetään vakioina, havaitaan, että magneettisen rekonnektion tahti vaihtelee huomattavasti. Julkaisussa näytetään myös, että mangetosheathin peilimoodeilla on vaikutusta rekonnektioon mangetopausilla. Tämä vaiheteleva rekonnektio johtaa magneettisten saarien muodustumiseen, joiden eteneminen ajaa magnetosheathissa aaltoja, joita tutkitaan tämän väitöskirjan neljännessä julkaisussa. Nämä aallot etenevät ylävirtaan kohti Maan keulashokkia ja aiheuttavat shokissa pullistuman, josta aurinkotuulen hiukkaset voivat kimmota takaisin aurinkotuuleen aiheuttaen paikallisia esishokkeja. Tämä väitöskirja näyttää, että ioniskaalan fysiikan kuvailu on tärkeää, jotta aurinkotuulen ja Maan magnetosfäärin vuorovaikututsta voidaan mallintaa tarkasti. Nestekuvailulla ei kyetä mallintamaan esimerkiksi esishokin muodostumista eikä magnetosheathin peiliaaltoja, jotka vaikuttavat rekonnektioon. Vlasiator-mallin tulokset osoittavat, että suuren mittakaavan ilmiöllä on vaikutusta paikallisiin pienen ioniskaalan ilmiöihin ja vastaavasti paikallisilla ilmiöllä voi olla odottamattomia vaikutuksia globaaliin dynamiikkaan. Satelliitit pystyvät tuottamaan havaintoja vain hyvin rajatulta alueelta ja tämän vuoksi magnetosfäärin mallintaminen ja mallien kehittäminen on erityisen tärkeää. Julkaisijayksikkö Uudet havaintomenetelmät Luokitus (UDK) Asiasanat 533.9; 52; 52-17; avaruusplasmafysiikka; numeeriset simulaatiot; 537.84 magnetohydrodynamiikka; hybrid-Vlasov -malli ISSN ja avainnimike 0782-6117 Finnish Meteorological Institute Contributions ISBN Kieli Sivumäärä 978-952-336-018-1 (nidottu) Englanti 151 978-952-336-019-8 (pdf) Contents Preface iii Research articles and the author’s contributions v Terminology & Variables vii 1 Introduction 1 2 Space physics background 3 2.1 Plasma . 3 2.1.1 Plasma description . 3 2.1.2 Shocks and sheath regions . 8 2.1.3 Magnetic reconnection . 9 2.2 Earth’s Magnetosphere . 12 2.2.1 Structure . 12 2.2.2 Dynamics . 15 2.3 Current spacecraft missions . 16
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