Czech Technical University in Prague Faculty of Nuclear Sciences and Physical Engineering Department of Physical Electronics

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1 Czech Technical University in Prague Faculty of Nuclear Sciences and Physical Engineering Department of Physical Electronics DOCTORAL THESIS STATEMENT

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3 Czech Technical University in Prague Faculty of Nuclear Sciences and Physical Engineering Department of Physical Electronics Planetary investigation by global numerical simulations: Study of Hermean environment by David Herčík Ph.D. Programme: Applied Natural Sciences Specialization: Physical Engineering Doctoral thesis statement for obtaining the academic degree of Doctor in abbreviation Ph.D. Prague, May 2013

4 The doctoral thesis was produced during full-time doctoral study at the Department of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering of the Czech Technical University in Prague. Ph.D. Candidate: Supervisor: Co-Supervisor: Opponents: David Herčík Department of Physical Electronics Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague & Astronomical Institute & Institute of Atmospheric Physics Academy of Sciences of the Czech Republic Dr. Ing. Pavel Trávníček Astronomical Institute & Institute of Atmospheric Physics Academy of Sciences of the Czech Republic Prof. Ing. Jiří Limpouch Department of Physical Electronics Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prof. Dr. Karl-Heinz Glaßmeier Institute for Geophysics and Extraterrestrial Physics Technical University Braunschweig Prof. RNDr. Petr Kulhánek, CSc. Department of Physics Faculty of Electrical Engineering Czech Technical University in Prague The doctoral thesis statement was distributed on... The defence of the doctoral thesis will be held on... at... before the Board for the Defence of the Doctoral Thesis in the Physical Engineering specialization in the meeting room No.... of the Faculty of Nuclear Sciences and Physical Engineering of the CTU in Prague. Those interested may get acquainted with the doctoral thesis concerned at the Dean Office of the Faculty of Nuclear Sciences and Physical Engineering of the CTU in Prague, at the Department for Science and Research, Břehová 7, Praha 1. Chairman of the Board for the Defence of the Doctoral Thesis in the Physical Engineering specialization Department of Physical Electronics Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague V Holešovičkách 2, Praha 8 Trojanova 13, Praha 2 iv

5 Contents 1 Introduction Motivation Problem statement Structure of the thesis Background 2 3 Overview of our approach Numerical simulations Simulations set-up Results General structure IMF orientation effects Local features In-situ data comparison Conclusions & future work Summary Contributions of the Thesis Future work Shrnutí Bibliography 25 Publications of the Author 29 v

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7 1 1 Introduction 1.1 Motivation Mercury and its ambient environment is currently under focus of many space plasma and planetary scientists, due to the new observations taken by MESSENGER mission. MESSENGER is a National Aeronautics and Space Administration (NASA) mission to Mercury after more than 30 years from Mariner 10, that made three flybys. MESSENGER was inserted into the Mercury orbit on March 18, 2011, being the first spacecraft ever to orbit Mercury. MESSENGER provides us therefore with enhanced remote as well as in-situ measurements of the planet and its environment. Another extensive mission - Bepi Colombo - to study Mercury is being prepared under cooperation of European Space Agency (ESA) in cooperation with Japan Aerospace Exploration Agency (JAXA). Mercury provides interesting environment to study various processes due to its intrinsic magnetic field, its proximity to the Sun or its magnetospheric dimensions. In-situ data are however localized only to certain position and time. In order to be able to have an insight in the global situation, numerical simulations provide a good means for such a global view. Analysis of the simulated data might reveal interesting processes and provide an indication, where to focus in the research of the real data. Comparing different simulations with various initial set up might also contribute to more general questions about solar wind - magnetospheric interaction. 1.2 Problem statement The thesis is based on the data analysis of hybrid numerical simulations of solar wind interaction with the Hermean magnetosphere. The main aim is to provide a global picture of the resulting magnetosphere, its structure, features, and highlight some processes within. Magnetosphere of Mercury is smaller than the terrestrial one, however, it seems to exhibit similar features and behaviour. Different scale (time and dimensional) plays role in the enhancement of significance of kinetic effects on the processes as well as on the global magnetospheric structure. Hybrid simulations are suitable for kinetic effects study as the proton kinetics is implemented into the code. Six simulations, varying in the orientation of the Interplanetary Magnetic Field (IMF), are analysed and compared. It shows a large influence of the IMF orientation on the magnetospheric structure. The comparison of these simulations is provided within the thesis. Via summarization of the Earth s magnetospheric features in the first part, the comparison with observed results is given. 1.3 Structure of the thesis The thesis is organized into 7 chapters as follows:

8 2 2 BACKGROUND 1. Introduction: Describes the motivation behind our efforts together with our goals and main contributions of the work. 2. Solar wind - planetary interaction: Introduces the reader to the background of the solar wind to magnetosphere interaction. It provides an insight to solar wind, its properties and origin. On the case of the Earth s magnetosphere, particular structure and known features are demonstrated. 3. Mercury: Overviews present knowledge of Mercury and its environment as derived from ground based as well as in-situ observations. 4. Numerical simulations: Provides summary of simulation approaches to study space plasma processes. Compares pros and cons of different approaches and in more detail describes the simulation code used for hybrid simulations, that are analysed. 5. Simulation parameters: Presents initial parameters of simulations being used within the study. 6. Data analysis results: Holds the main core of the thesis. Introduces into the resulting magnetospheric structure, its global topology and in relation to the Earth s magnetosphere it provides a view on particular features. 7. Conclusions: Summarizes the results of our research, suggests possible topics for further research, and concludes the thesis. 2 Background Mercury is the innermost planet in the solar system. Having the perihelion of about 0.3 AU, it is exposed to harsh conditions in terms of temperature and solar wind interaction. Mercury is an interesting object to study from several points of view. There are still several questions about Hermean properties that have to be answered, such as the inner composition that leads to the high density, which is about 5.3Mg/m 3 and could indicate the core radius to occupy large portion of the planetary radius [Solomon, 2003]. Particular value depends on the core composition, which has to be determined. Mercury is supposed to have its own intrinsic magnetic field, that has been discovered during the Mariner 10 mission [Dunne and Burgess, 1978]. Therefore, it is expected that Mercury also has an intrinsic magnetosphere, although slightly different from the terrestrial one. The differences stem, besides other aspects, from the lack of atmosphere and ionosphere at the Mercury. The lack of ionosphere and smaller spacial scales for example, result in different dynamics of the Earth s and Hermean magnetosphere. The Earth s magnetosphere

9 3 response to some change in solar wind condition is postponed by the ionospheric capability of energy storage. On the other hand, in case of Mercury, there is no ionosphere and the coupling between the solar wind changes and the inner magnetosphere are therefore not mediated and are more direct. Then the processes have about 30 times faster response than in the case of the Earth [Baumjohann et al., 2006]. Mercury is the smallest planet in the solar system (after reclassification of planet Pluto by the International Astronomical Union in 2006) with the highest eccentricity and relatively high orbit inclination (7 ). The distance from the Sun varies from up to AU, resulting in variable orbital velocity with the average of km/s. Hence one Mercury year last Earth s days. In combination with slow sidereal rotation ( hours Earth s days), Mercury performs three full rotations around its axis in two Hermean years, i.e. in two rotations around the Sun. As a consequence, one Hermean solar day last two Hermean years. Mercury has the most eccentric orbit among the solar system planets, that results in a high variability of the environmental conditions. The solar wind parameters are of course highly variable and depend a lot on the region of origin on the Sun. Typical parameters at the Earth s Orbit usually given are n = 5 cm 3, B = 5 nt, T = 10 5 K (e.g. in Baumjohann and Treumann [1996]). For Hermean orbit (taking the semimajor axis: AU), we can estimate proton density to about 40 cm 3 from the value at 1 AU taking into account that the density drops radially from the Sun with r 2. In case of temperature, the dependence is not so clear. Using data from Helios 1 and 2 as summarized e.g. in Burlaga [2001], the density varies in range n cm 3, proton temperature is T K and the IMF is B IMF nt. Mercury is exposed to more intense conditions than the Earth due to its proximity to the Sun. This results to solar irradiance more than 6.5 times larger than in case of the Earth (9.13 compared to 1.37 kwm 2 ). Besides temperature, irradiance and solar wind density, interplanetary magnetic field is another important parameter. IMF at the Hermean orbit is approximately 7 times larger in magnitude. Average direction of the IMF also differs as the position of Mercury and Earth with respect to the Parker s spiral is different. At Mercury, radial IMF component is dominant and the angular IMF component in the ecliptic plane is near zero. At the Earth s orbit, the IMF orientation makes approximately 45 angle to the radial direction. The z component, normal to the ecliptic plane, depends on the solar wind particular conditions and is usually in order of few nt above or below zero. According to the first measurements made by Mariner 10, a dipole field strength has been estimated between 170nTR 3 M (with quadrupole concerned, R M is the radius of the Mercury) [Whang, 1977] and 349nTR 3 M (with a simple dipole model) [Ness et al., 1975]. MESSENGER magnetometer [Anderson et al., 2007] measurements during Hermean fly-bys have updated the range to be between 230 and 290 ntr 3 M [Anderson et al., 2008]. Finally based on orbital observations, the magnetic field

10 4 3 OVERVIEW OF OUR APPROACH strength of Mercury is estimated to 190 ntrm 3 [Anderson et al., 2011]. Although these values are approximately 0.1% of the Earth s magnetic field strength [Solomon, 2003], the field is sufficient and convenient to give a rise to a magnetosphere similar to one encountered at the Earth. Due to the Hermean magnetic field strength, the magnetosphere is about 1/8 as large as the Earth s magnetosphere when normalized to the planetary radius [Mukai, 2004]. Therefore the nose of the bow shock (the nearest point of the bow shock to the planetary surface) is about 1.5R M from the planetary centre; in case of the Earth it is about 12R E [Slavin, 2004], where R M and R E are the radius of the Mercury and Earth, respectively. It is also probable, that at Mercury, under some solar wind conditions (with high dynamic pressure), the magnetopause would be suppressed up to the planetary surface and the solar wind particles would then be able to impact on the Hermean surface. Another interesting feature is the observed offset in the magnetic dipole position. The centre of the magnetic dipole is found to be shifted 484 ± 11 kilometres northward of the geographic equator. The tilt of the dipole is less than 3 from the rotational axis [Anderson et al., 2011]. This seemed to be consistent with the centre of mass shift estimated to be southward [Anderson et al., 1996], which would mean higher crustal thickness in the south region and indicate the core, where the magnetic field is produced, to be shifted toward north. However, this shift is not so large to explain the whole dipole shift and other effects, such as octupole and quadrupole contributions to the shift might play role [Whang, 1977]. As has been mentioned above, the magnetic field is strong enough to generate magnetospheric structure similar to one observed at the Earth with similar regions and features. Figure 1 shows sketches indicating various magnetospheric features as understood from data acquired during first two MESSENGER fly-bys. 3 Overview of our approach 3.1 Numerical simulations For a complex understanding of the whole environment around planets that are more distant and where data from in-situ measurements are rare, computer simulations are important. There are two main attitudes to simulations in plasma physics. The first one is Magnetohydrodynamics (MHD) and the second one is Particle in cell (PIC). Both kinds are in some way approximations, but either in a different way. MHD is an approximative approach to the plasma physics description. It treats plasma as a charged fluid in an external magnetic field. This provides quite a simple description of the plasma dynamics and, for example, equilibrium conditions. But this way has also a disadvantages. It neglects the processes where particles themselves are involved, such as particle wave interactions, and therefore do not take into account many energy transfers and other physically important processes.

11 3.1 Numerical simulations 5 (a) Northward IMF (b) Southward IMF Figure 1: Global structure of the Hermean magnetosphere. Comparing two different IMF situations as observed by the NASA mission MESSENGER. These schematic images overviews magnetospheric properties and processes and highlight several features and phenomena. Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. PIC solves more proper equations for plasma than for MHD model. It considers the motion of particles. However, some approximation is also included. Because of the extremely high computation requirements that are increased with the number of particles, the separate particles are replaced by the so called super-particles. In a simulation box these virtual particles represent a specified number of real particles. The code then performs an iterative process. At first, it moves the particles according to the initial external forces that influence particles through a Lorenz force. Then new positions and velocities of the particles are computed and new currents are determined. Electric currents then contribute to the electric and magnetic fields and this circle is iterated further on. The main problem of full particle-in-cell simulations is time and timescales, because electrons have different timescale than ions. Electrons, being approx times lighter than protons have processes much faster and the time step shall therefore differ from the ion time step. This feature then limits using full PIC codes in large simulations. A way of resolving such an issue is using hybrid simulation, that treats ions as particle-in-cell and electrons as charged fluid according to MHD. This allows us to study also kinetic effects of ions, that are in the macroscopic view more pronounced. For the purposes of this study, we are using numerical hybrid simulations based on code presented in Travnicek et al. [2007], Travnicek et al. [2009], and Travnicek et al. [2010]. For the simulation, 3D hybrid code has been used, with implementation based on Matthews [1994]. Electrons are managed as a mass-less, isothermal fluid and protons are treated by PIC method as a collision-less plasma. That means electronwave interactions are neglected. Due to the lower mass and hence lower inertia, the

12 6 3 OVERVIEW OF OUR APPROACH electrons are much faster in reacting to changes of external fields and, therefore, they can be taken as a background fluid. Protons are the interesting particles that drive the system. In hybrid simulation, so called super-particles are used to substitute for particular number of ions, having respective mass and charge, but being one particle in the simulation code. 3.2 Simulations set-up For the analysis of the Hermean environment, data from hybrid simulations are used. The simulation runs are updated global hybrid simulations based on older model introduced in Travnicek et al. [2007] and described in more detail in Travnicek et al. [2010]. The model uses a hybrid code as mentioned before. A magnetized body, represented by a magnetic dipolar field, is inserted in a simulation box. Interplanetary magnetic field is superposed to this magnetic field at the initiation of the simulation. Macro-particles, representing the solar wind, are constantly injected from the left box boundary. The simulation box coordinates could be denoted as Hermean centric Solar Wind with origin in the Hermean planetary centre, X -direction along the solar wind flow, Z-direction along the magnetic axis towards geographic north pole, and Y-direction completing the right-hand coordinate system. We are using a scaled down model of Mercury with a magnetic moment M = 100, 000 B sw d 3 psw4π/µ 0, where B sw is the magnitude of the solar wind magnetic field, d psw = c/ω ppsw is the proton inertial length in the solar wind, c is the speed of light, ω ppsw is the solar wind proton plasma frequency. The solar wind magnetic field B sw corresponds to 20 nt. The d psw is equivalent to Larmor radius of proton with Alfvén speed d psw v Asw /w gpsw. The downscaling preserves a stand-off magnetopause distance R mp predicted from the pressure balance between the solar wind ram pressure P ram,sw and the magnetospheric pressure: R mp = [Beq/(2µ 2 0 P ram,sw )] 1/6 R M, where B eq is the magnetic field at the equator of the planet and before downscaling corresponds to 195 nt. Following recent observation of MESSENGER [Anderson et al., 2011], the dipolar magnetic field is shifted towards the north pole by 0.2 R M. For the simulations we use a 3-D simulation box with mesh points distributed equidistantly along the three (Cartesian) dimensions with the spatial resolution x = 0.4 d psw, y = z = d psw. The planetary radius is R M = d psw, which gives R M spatial resolution, taking into account one cell dimensions of d psw. The simulated planet centre is located at the distance 5.19 R M in the solar wind flow direction. Macro-particles are advanced with the time step t = 0.02 ωgpsw, 1 where ω gpsw is the solar wind proton gyro-frequency; whereas the electromagnetic fields are advanced with the finer time resolution t B = t/20. The magnetic field is initialized with a superposition of the homogeneous IMF B sw and a dipolar planetary magnetic field B M. The IMF orientation B sw =

13 3.2 Simulations set-up 7 (B x, B y, B z ), B sw = 1, is different for different simulations runs. Table 1 summarises 6 simulation runs differences in the IMF orientation. The orientation of IMF for all cases is indicated in Figure 2. EIMF-P SIMF-P NIMF-P EIMF-S NIMF-S SIMF-S Planetward IMF Sunward IMF IMF φ [ ] IMF θ [ ] Table 1: Orientation of IMF for all six studied simulation cases. A graphical overview is given in Figure 2. IMF φ is the angle between solar wind flow (+X ) direction and IMF magnetic field vector in the noon-midnight (X Z) meridian plane. IMF θ is the angle between solar wind flow (+X ) direction and IMF magnetic field vector in the equatorial (X Y) plane. Figure 2: Overview of IMF orientation (blue arrows) for presented simulation runs. Panel a) shows situation in noon-meridian plane for simulations with northward and southward IMF. Panel b) shows equatorial plane for simulation with equatorial IMF. At t = 0 the simulation box is loaded with 70 macro-particles in each cell outside the planet for all simulations, representing the solar wind Maxwellian isotropic protons with the density n p = n psw and the bulk speed v p = (4v Asw, 0, 0). This plasma flow is continuously injected from the left boundary of the simulation box at X = 5.19R M. The ratio of proton to magnetic pressure in the solar wind is β psw = 0.5. An overview of the simulation parameters is given in Table 2. At boundaries, we use open boundary conditions, i.e., macro-particles freely leave the simulation box on all sides. Macro-particles hitting the planetary surface (set at R M = d psw ) are removed from the simulation. We keep B/ t = 0 in the interior of the planet and E/ r = 0. An additional resistivity layer η 0.8 exp ( h 2 /h 2 0), where h is radial distance from the surface and h 0 = 3 d psw, is applied near the planet s surface. We have also injected H + ions with density of the order of n p 10 4 n psw isotropically from Mercury s surface with velocity v p 0.05 v Asw normal to the surface. For both simulations we also use a flat resistivity η = 0.1µ 0 vasw 2 /ω gpsw in the entire simulation box, where v Asw is the Alfvén velocity in the solar wind.

14 8 4 RESULTS The simulation unit settings corresponds to conditions of slow solar wind with speed 450 km/s and density of 15 cm 3. Presented study compares six simulation runs with same initial set-up except the IMF orientation. These cases have been selected in order to be able to compare the influence of change of the external conditions to the magnetospheric structure and processes therein. The six cases are denominated according to the IMF orientation as follows: EIMF-P means magnetic field parallel to the equatorial plane with planetward (-P) component. EIMF-S means magnetic field parallel to the equatorial plane with sunward component (-S). SIMF-P (SIMF-S) denominates southward IMF with planetward (sunward) component respectively and NIMF-P (NIMF-S) means northward IMF with planetward (sunward) component respectively (see Table 1 and Figure 2 for an overview). Parameter Value Number of cells N x N y N z Spatial resolution x [d psw ] 0.4 Spatial resolution y = z [d psw ] 1.0 Size of the system L x = N x x [d psw ] 376 Size of the system L y = L z = N y y [d psw ] 400 Mercury s radius R M [d psw ] Temporal resolution (time step) t [ωgpsw] Time sub-stepping for fields t B [ω 1 gpsw] gpsw] 80 Duration of simulation [ω 1 Number of macro-particles per cell 70 Total number of macro-particles Solar wind velocity v psw [v Asw ] 4.0 Strength of IMF [nt] 20 Mercury s magnetic moment M [ntrm 3 ] 195 n psw, B sw, v Asw, d psw, ω gpsw = 1 (in simulation units) Table 2: An overview of basic common parameters of simulation runs used for data analysis. n psw is proton density in the solar wind, B sw is magnetic field magnitude in the solar wind, v Asw is the Alfvén speed in the solar wind. Dipole centre is shifted by 1/5 R M towards north. 4 Results The thesis, as discussed above, is based on six simulation cases for different IMF orientation. This approach enables us to study general effects of the IMF on the magnetosphere. Some effects can be compared to the Earth s magnetosphere, others are

15 4.1 General structure 9 results of local kinetic processes and are connected to environments with smaller magnetospheric scales (compared to local Larmor radius). 4.1 General structure The solar wind interaction with the Hermean magnetic field results in the terrestriallike magnetosphere with various features. In all the presented simulation runs typical magnetospheric features appear. Terrestrial-like magnetosphere with bow shock ahead of the planet is formed, magnetosheath with plasma flowing around the planet along the magnetospheric boundary - the magnetopause. The inner magnetosphere exhibits a plasma belt around the planet, that is connected to the current sheet in the tail region. The six cases, that differ in orientation of the IMF, are compared with focus on global general magnetospheric structure and the effect of different IMF orientation on its features. The resulting structures are illustrated on the proton density plots displayed in noon-meridian plane, plane of magnetic equator, and dawn-dusk meridian plane. The magnetic equator plane we define as a plane parallel to the equatorial plane shifted towards north by 0.2 R M, which corresponds to Hermean magnetic dipole shift observed by MESSENGER [Anderson et al., 2011] as well as presented already by Whang [1977] based on Mariner 10 data. Figure 3 shows situation for three cases with planetward and Figure 4 for three cases with sunward IMF direction. Equatorial IMF is shown on panels a), d), and g) in both Figures. Southward IMF display panels b), e), and h). Panels c), f), and i) show northward IMF case. Clear effect of the IMF orientation is the foreshock location. Foreshock region is adjacent to the quasi-parallel bow shock, i.e. the region, where angle between the IMF vector and normal of the bow shock boundary is lower than 45. In foreshock region, backscattered ions from the bow shock travel along the IMF field lines and interact with incoming solar wind. This interaction gives rise to plasma waves. Such turbulent region could be observed at expected locations in all the six simulation cases. The quasi-parallel bow shock region is observable also in the density data, where the shock is disrupted and enhanced proton density fluctuations appear. For equatorial IMF case (EIMF-P and EIMF-S), the foreshock is at the dawn side (Y > 0) of the planet ahead of the bow shock. Let us first focus on Figure 3. The location of the foreshock implies bow shock and magnetosheath asymmetry. For EIMF-P case, this kind of asymmetry arise in the equatorial plane, where in the dawn sector (see panel a)) the bow shock is disrupted and adjacent magnetosheath turbulent. Also the magnetosheath thickness is different. The magnetosheath thickness we define as a distance from the bow shock to magnetopause in the direction along the bow shock normal. In the dawn sector, the magnetosheath is thinner than in the dusk sector.

16 10 4 RESULTS Figure 3: Structure of the magnetosphere for simulations with planetward oriented IMF (EIMF-P, SIMF-P, and NIMF-P). Colour shows cuts in the proton density normalized to the solar wind proton density. Top panels show data at magnetic equator plane (parallel with equatorial plane, but shifted by 0.2 R M towards north). Middle panels show noon-midnight meridian plane and bottom panels show dawn-dusk meridian plane. Panels a), d), and g) show proton density plots from simulation with IMF in the equatorial plane, panels b), e), and h) form simulation SIMF-P (southward IMF), and panels d), f), and i) from simulation NIMF-P (northward IMF).

17 4.1 General structure 11 Figure 4: Structure of the magnetosphere for simulations with sunward oriented IMF (EIMF-S, SIMF-S, and NIMF-S). Colour shows cuts in the proton density normalized to the solar wind proton density. Top panels show data at magnetic equator plane (parallel with equatorial plane, but shifted by 0.2 R M towards north). Middle panels show noon-midnight meridian plane and bottom panels show dawn-dusk meridian plane. Panels a), d), and g) show proton density plots from simulation with IMF in the equatorial plane, panels b), e), and h) form simulation SIMF-P (southward IMF), and panels d), f), and i) from simulation NIMF-P (northward IMF).

18 12 4 RESULTS The magnetosheath also differs in the stand-off distance from the planet, i.e., the distance between the sub-solar point at the magnetopause and the planetary surface. The dayside magnetosphere is largest for NIMF case (panels c) and f)), that represents closed magnetosphere. On the contrary, smallest dayside magnetosphere and therefore smallest stand-off distance is in SIMF case (panels b) and e)). In that case, the dayside magnetospheric field lines are anti-parallel with IMF and magnetic reconnection at the subsolar region is expected to take place. Such open magnetospheric configuration effectively diminishes the dayside magnetosphere, while the magnetospheric field line reconnects with IMF forming a pair of open magnetic fluxtubes, that are transported towards tail via the magnetosheath convection. On the Earth s dayside magnetosphere a boundary layer has been observed. Observations suggest that low latitude boundary layer (LLBL) preferentially forms in case of northward IMF [Walker et al., 1995]. Such a result is consistent with simulated data as NIMF-P simulation shows boundary layer adjacent to the dayside magnetopause, while for simulation SIMF-P the boundary layer is not so clearly visible. Moreover, the stand-off distance is small even for the NIMF-P case, and the possible observed boundary layer likely merges with the ring current region. Tail lobes have also different structure depending on the IMF orientation. SIMF-P simulation shows thick plasma mantle in the northern lobe (panel e) in the Figure 3). Plasma mantle is a magnetopause boundary layer found at the Earth for high latitude magnetospheric lobe. Northward IMF case also posses plasma mantle in the northern lobe, however significantly thinner. Observations at the Earth s magnetosphere show, that plasma mantle appears mainly for southward IMF, while for northward IMF the mantle almost vanishes [Hultqvist et al., 1996]. These observations are in accordance with simulated results for Mercury. As has been mentioned above, the LLBL is preferentially formed in the northward IMF case. Solar wind entering the cusp region is therefore likely supplying boundary layer on the dayside for northward IMF and nightside at southward IMF [Hultqvist et al., 1996]. As suggested in Sauvaud and Nemecek [2003] the LLBL and plasma mantle could be therefore the same feature, having also similar properties, resulted from similar processes and originated in the cusp region. Another feature similar to the Earth s magnetosphere is the tail current sheet. In all studied cases the current sheet is formed. In case of NIMF-P simulation, the current sheet is thicker and more steady than in case of southward IMF (SIMF-P). In all planetward IMF simulations, the tail is shifted towards north. On the contrary, simulations with sunward IMF direction (Figure 4) show the shift in the current sheet southward. In other words, one lobe (northern for sunward and southern for planetward IMF) is larger than the other. This feature seems to be driven, by the B x component of the IMF. The equatorial EIMF case is a transition between NIMF and SIMF conditions. As shown above, the resulting structure and particular magnetospheric features exhibit

19 4.2 IMF orientation effects 13 intermediate properties between the two extreme cases, which is also supporting the self-consistency of the simulation model. Interesting feature is however visible on the dawn-dusk meridian plane plots for both equatorial simulations (panel g) in Figure 3 for EIMF-P and Figure 4 for EIMF-S). The magnetosphere seems to be tilted along the X -axis, i.e., along the Sun-planet line, in the negative direction when looking towards the Sun for EIMF-P and in positive direction in case of EIMF-S. All the features are turned, current sheet is not aligned with equatorial plane, but is tilted, as well as magnetospheric lobes and even cusps could be located off the noon-midnight meridian plane. This effect is known from Earth observations and is caused by the By component (east-west) of the IMF. The origin of the magnetospheric tilt is in the magnetic curvature force [Hultqvist et al., 1996]. As discussed above, the resulting features of the solar wind to Hermean magnetosphere interaction are strongly IMF dependent. In following section a summary of observed effects of the IMF orientation is provided. 4.2 IMF orientation effects Figure 5: Sketch showing different IMF orientation and effects on reconnection locations. Panel a) shows duskward (By < 0) and panel b) dawnward (By > 0) IMF orientation in a dawn-dusk meridian plane looking from the Sun. Panel c) shows planetward (Bx > 0), panel d) sunward, panel e) northward, and panel f) southward orientation of the IMF in noon-midnight meridian plane.

20 14 4 RESULTS The appearance and location of the features described above are driven by the IMF orientation. Here below is a summary of main effects with possible explanation. For simplicity, we have focused on cases with IMF directed along the main axes. These cases are sketched in Figure 5. The effects are summarized and explanation provided in following table. IMF effects summary Duskward IMF (B y < 0) Dawnward IMF (B y > 0) anti-clockwise rotation of the magnetosphere when looking from the Sun clockwise rotation when looking from the Sun Effect caused by the reconnection at the dayside magnetopause and resulting magnetic curvature force straightening the field lines (see Figure 5 panels a) and b)). The effect is known at the Earth. Planetward IMF (B x > 0) Sunward IMF (B x < 0) tail current sheet shifts northwards plasma mantle in northern lobe tail current sheet shifts southwards plasma mantle in southern lobe The tail shift is likely caused by the reconnection at the lobe (northern or southern), that removes a magnetic flux from the lobe and therefore weakens the magnetic pressure in the respective lobe. Plasma mantle location is also caused by the reconnection location and by the fact, that the configuration results in solar wind having predominant access to northern or southern lobe for different IMF orientation. These effects seem not to be described in available literature. See Figure 5 panels c) and d). Northward IMF (B z > 0) Southward IMF (B z < 0) dayside magnetopause boundary layer appearance larger dayside magnetosphere plasma mantle appearance smaller dayside magnetosphere

21 4.3 Local features 15 Both effects result from reconnection at dayside or in tail (see Figure 5 panels e) and f ). During the Northward IMF, the reconnection takes place in the near tail, forming a dayside magnetic field line that populates dayside boundary. For southward IMF the reconnection takes place at the dayside, reducing the dayside magnetosphere and the field line convection populates predominantly the tail lobes. The fact that dayside boundary layer and plasma mantle results from same processes is mentioned in literature, however it is still not widely accepted. 4.3 Local features The thesis focuses on several particular features of the magnetosphere, describes their properties and processes within. It attempts to characterize, e.g., magnetosheath, magnetopause boundary layer, current sheet or plasma belt. Focusing on the properties and also effects of the IMF orientation. As one of the main results, we can mention a magnetosheath asymmetry. When comparing southward (SIMF-P) and northward (NIMF-P) IMF orientation with planetward component, clear magnetosheath asymmetry is visible in the equatorial plane. Figure 6 shows magnetosheath in the equatorial plane for both mentioned simulation cases with indicated bow shock (red line) and magnetopause (green line). On the left side, tables showing average values for respective simulation and side of the magnetosheath are shown. The magnetosheath is asymmetric not only in geometry, but also in plasma parameters. For SIMF-P, the dawn side has lower average magnetic field, lower bulk proton velocity, but higher proton temperature and higher proton β than the dusk side. In case of northward IMF simulation (NIMF-P) it is vice-versa. Detailed analysis also showed, that the asymmetry has further consequences in the processes. We have studied mirror waves generation within the magnetosheath. The growth and propagation of the waves also exhibit asymmetry behaviour driven by IMF orientation. In case of SIMF-P case, the mirror waves appear on the dawn side of the magnetosheath, while in case of NIMF-P the waves appear on the dusk side. It means each time it appears on the side of the magnetosheath with higher proton β and lower velocities. The dawn-dusk asymmetry in the magnetosheath has been recently observed by MESSENGER in terms of Kelvin-Helmholtz waves. Sundberg et al. [2012] reported, the K-H waves appearing in the post-noon and dusk region of the magnetopause. The asymmetry seen in the simulations might also explain such observations, while the K- H instability is generated for high velocity shear. As shown above, there is quite strong asymmetry in the velocity flow for dusk and dawn region, that might be a favourable aspect. A hypothesis of the origin of the magnetosheath asymmetry is provided within the thesis. The effect seems to have origin in local kinetic processes connected to Larmor

22 16 4 RESULTS rotation of protons and energy dissipation on the bow shock by reflected protons. Reflected protons play significant role in energy dissipation in case of super-critical shocks. When the Larmor radius is large in respect to the global magnetospheric scales, reflected protons gain different amount of energy on dawn and dusk side due to curvature of the bow shock a and Larmor rotation orientation. This difference seems to be significant in case of Mercury and results in a bow shock structure, that has shifted sub-solar point. The whole geometry of the bow shock is changed as if the solar wind would be flowing from different direction. Such shift results in aforementioned asymmetric magnetosheath. Figure 6: On the right, colour plots of proton density on the magnetic equator plane (0.2 R M northward of the geographic equator plane). Panel a) shows data from simulation SIMF-P and panel b) from simulation NIMF-P. White solid lines show projection of 3D flow lines started at the displayed plane -3 R M ahead of the planet. The red solid line indicates the bow shock boundary and the green line indicates the magnetopause. On the left, tables show average parameters in the magnetosheath region in the equatorial plane. 4.4 In-situ data comparison Another local feature studied within the thesis is a plasma belt. Plasma belt region is known to be formed around the Earth, containing ring current and Van Allen radiation belts. It was an open question, whether such feature possess also Mercury. Hermean intrinsic magnetic field is sufficient to form magnetospheric structure similar to terrestrial, but significantly smaller. Therefore the whole potential plasmasphere

23 4.4 In-situ data comparison 17 region has been expected to be below 1 Hermean radii (R M ) [Russell et al., 1988]. Recently, numerical simulations [Travnicek et al., 2010; Yagi et al., 2010] as well as data from MESSENGER [Schriver et al., 2011] shown formation of the ring current with quasi-trapped population possible. We have focused on the plasma belt for simulation case NIMF-P and tried to investigate its properties and compare the simulation results with in-situ data. The simulation data show distinguished plasma belt, that merges at the dayside with the magnetopause boundary layer. The plasma belt is well confined in a magnetic shell. It exhibits higher temperature and lower bulk velocities than outer plasma, e.g., in the tail current sheet. The increase in temperature signifies a trapping mechanism and heating of the trapped protons. The trapping has been confirmed by the observation of proton velocity distribution functions within the plasma belt region. At the dayside, there is a clear loss-cone distribution with angles in accord with theoretical values. On the nightside, the loss-cone distribution is not so clearly visible due to constant injections of solar wind protons via tail current sheet to the plasma belt region supplying the proton population. However due to small scales, the trapping mechanism is not strong enough to enable full ring current flow. Protons are only quasi-trapped for shorter time in the magnetosphere. The westward drift of the protons is present, but after short path (around quarter of the orbit around the planet) particles escape the trapping. Therefore we speak about quasi-trapped particles in the magnetosphere of Mercury. For the confirmation of the plasma belt features and also for confirmation of the model settings and representativity of the real conditions we have selected one MES- SENGER orbit and compared data. The orbit is from (day 182) in particular from T06:27:52 till T09:44:52. It is almost in the noonmidnight plane with the perihermion on the nightside around 45 of northern latitude. This orbit crosses the cusp region on the dayside and enables us to study plasma ring current on the nightside. We have chosen simulation case (NIMF-P) according to matching initial parameters to the solar wind condition at Mercury during observations. In particular IMF from MESSENGER MAG data has been: B x = 17.33nT, B y = 0.04nT, B z = 4.15nT. These values have been calculated as average from selected MAG data before bow shock crossing. The IMF was therefore north-ward with angle of between the IMF direction and the sub-solar line. Simulation NIMF-P has IMF set-up corresponding to B x = 18.79nT, B y = 0.0nT, B z = 6.84nT with 20 between the IMF direction and the sub-solar line. These parameters match the real situation and the simulation results have been therefore compared to the real measurements. Figure 7 shows data along selected trajectory. Panel a) shows density along the virtual MESSENGER trajectory through the simulated data. Panel b) shows in blue magnetic field measured by MAG, in red magnetic field from the simulation along the same trajectory, and in background magnetic field fluctuations computed from

24 18 4 RESULTS MAG measurements. Panel c) shows data acquired by the FIPS instrument and gives spectrogram of high energetic ions (FIPS SCAN mode). All panels have highlighted region from 7:28 to 7:46 UTC, that corresponds to density increase in the inner magnetosphere indicating ring current region. This density increase is accompanied by the diamagnetic decrease visible in magnetic data (simulated as well as measured). Moreover FIPS data indicate a presence of plasma and high energetic ions. These energetic ions are indicating quasi-trapped particles. The presence of such particles have been already reported by Schriver et al. [2011] MESSENGER data (MAG and FIPS) indicate crossing of various magnetospheric regions: bow shock, magnetopause, boundary layer, cusp, plasma belt. These features as well as the overall profile in the simulation and in-situ data show nice match. This comparisons indicate correct simulation setup and gives credit also to other results presented within the thesis. Figure 7: Comparison of MESSENGER data from part of the orbit of day 182 in 2011 and simulated data along the same trajectory through the NIMF-P dataset. Panel a) shows simulated proton density. Panel b) shows simulated magnetic field intensity in red and in blue real magnetic field measurements of the MESSENGER MAG instrument. Background shows power spectral density of the measured magnetic field. Panel c) shows ions energies as measured by the MESSENGER FIPS instrument. Region bordered by solid green line indicates part of the trajectory, where MESSENGER was apparently crossing the plasma belt. Red dashed line indicates estimated bow shock location and dashed blue lines indicate estimated locations of in and out bound magnetopause.

25 19 5 Conclusions & future work 5.1 Summary Mercury, named after Roman messenger of gods, has been observed already in classical era by ancient Greeks, although then mistaken for two stars one appearing in the morning and second after dusk. Its proximity to the Sun hindered the ground based observation as well as later space based missions. First in-situ measurements were provided by NASA mission Mariner 10, followed by more than 30 years of vacuum interrupted by enhanced ground based observations. Recently another NASA mission MESSENGER has been inserted into the Hermean orbit on 18 March 2011 providing us with unique opportunity to study Hermean environment, that is interesting from many aspects. Mercury have an intrinsic magnetic field, that is smaller than on Earth, however sufficient for magnetosphere creation. The structure and processes seem to be terrestrial like, nevertheless different scales and conditions might result in altered properties of magnetospheric features. In-situ data are invaluable for basic space research. For the global view of the solar wind to magnetospheric interaction, numerical simulations play significant role. Global hybrid simulations, that are used in presented study provide global overview of the magnetospheric structure as well as an insight into local phenomena. The thesis comprises concise introduction into magnetospheric creation in general and provides an insight on the features at the Earth magnetosphere for later comparison. The main part of the work is based on data analysis of six simulation cases of the solar wind interaction with magnetized body representative of Mercury and its environmental conditions. Full 3D hybrid simulations reveal global structure and its response to different orientation of the Interplanetary Magnetic Field (IMF). The comparison of particular features of the magnetosphere is given and the explanation of described effect is suggested. Some of the features are well known from the Earth s magnetosphere, some seem to be yet not well described in available literature. This is, e.g., the case of shift of the tail current sheet in north/south direction depending on the B x (sunward/anti-sunward) component of IMF. Another difference is the role of kinetic processes, that seem to be much more significant at Mercury than at the Earth. Local Larmor radius being comparably large to magnetospheric scale at Mercury, as the simulations suggests, results in a global influence on the bow shock and magnetosheath shape. Other local phenomena, such as mirror wave generation and propagation are also studied. Presented work presents a complex summary of the solar wind to Hermean magnetic field interaction and might enlighten some of interesting features and processes within the Hermean magnetosphere.

26 20 5 CONCLUSIONS & FUTURE WORK 5.2 Contributions of the Thesis So far the knowledge of the Hermean magnetosphere and its features stems from insitu observations and Magnetohydrodynamic (MHD) simulations. In-situ measurements provide only limited data (in time, space, and amount) and MHD simulations global structure generated by fluid dynamics. Hybrid simulations enable us to estimate also the kinetic effect contribution to the local as well as global features. Results of the thesis comprise a global view of the interaction of Mercury with solar wind as well as properties of local phenomena and structures within the Hermean magnetosphere. One of the main contributions to the topic is a global structure of the Hermean magnetosphere. The thesis shows the response of Hermean own magnetic field to different solar wind conditions in terms of Interplanetary Magnetic Field (IMF) orientation. A summary of dependencies of particular magnetospheric feature or behaviour depending on the IMF orientation is provided. Some of these dependencies are well known from Earth observations. Dayside magnetosphere is such an example, where reconnection between IMF and magnetospheric field lines occurs. The reconnection rate, however, depends on the B z component of the IMF. When B z is directed southward (B z < 0), then the reconnection is more likely and we called this configuration open magnetosphere. In opposite case (northward IMF, B z > 0), the reconnection is rare and we call the configuration closed magnetosphere. Such behaviour is well described at the Earth s magnetosphere. We have observed same feature in case of Mercury. Another example is the rotation of the magnetospheric structure along the solar wind flow direction, that depend on the B y component of the IMF. However, as the analysis includes different IMF cases and full 3D results, it enables us to study the response to IMF change in a global view. Therefore, there are also results, not mentioned in the available literature (known to the author). This is for example the plasma mantle location and shift in northward or southward direction depending on direction of B x IMF component. As has been shown on the studied cases and as has been attempted to explain theoretically, positive/negative B x (directed from the sun in the solar wind flow direction) cause the tail current sheet to shift northwards/southwards respectively. The shift is likely caused by the difference in reconnection rate at northern/southern lobe, which results into different magnetic pressure in the lobes. Such behaviour is likely to occur in the Earth s magnetosphere as well and it might be an interesting subject for future study comparing in-situ measurements. Several local phenomena are then described in the thesis with a focus on comparison to the Earth s magnetosphere. Structures like bow shock, boundary layer, current sheet or plasma belt are analysed in detail and its properties compared in the six simulation cases. There are some interesting features to be highlighted as part of the main contributions of the thesis. At first, a magnetosheath asymmetry observed in the data shall be mentioned. The asymmetry appears in plane perpendicular to the IMF vector orientation. It is geometrical asymmetry as we as asymmetry in plasma parameters of