Endre søk
Begrens søket
1 - 18 of 18
Referera
Referensformat
• apa
• ieee
• modern-language-association-8th-edition
• vancouver
• Annet format
Fler format
Språk
• de-DE
• en-GB
• en-US
• fi-FI
• nn-NO
• nn-NB
• sv-SE
• Annet språk
Fler språk
Utmatningsformat
• html
• text
• asciidoc
• rtf
Treff pr side
• 5
• 10
• 20
• 50
• 100
• 250
Sortering
• Standard (Relevans)
• Forfatter A-Ø
• Forfatter Ø-A
• Tittel A-Ø
• Tittel Ø-A
• Type publikasjon A-Ø
• Type publikasjon Ø-A
• Eldste først
• Nyeste først
• Disputationsdatum (tidligste først)
• Disputationsdatum (siste først)
• Standard (Relevans)
• Forfatter A-Ø
• Forfatter Ø-A
• Tittel A-Ø
• Tittel Ø-A
• Type publikasjon A-Ø
• Type publikasjon Ø-A
• Eldste først
• Nyeste først
• Disputationsdatum (tidligste først)
• Disputationsdatum (siste først)
Merk
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
• 1.
Luleå tekniska universitet, Institutionen för system- och rymdteknik.
Ion Temperature Anisotropies in the Venus Plasma Environment2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave

Velocity distributions are a key to understanding the interplay between particles and waves in a plasma. Any deviation from a Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer MAG on-board Venus Express,  ion distributions in the plasma environment of Venus are studied. The focus lies on temperature anisotropy, that is, the difference between the ion temperature parallel and perpendicular to the background magnetic field. This study presents spatial maps of the average ratio between the perpendicular temperature $T_\perp$ and parallel temperature $T_\parallel$, both for proton and heavy ions (atomic oxygen, molecularoxygen and carbon dioxide). Furthermore average values of $T_\perp$ and $T_\parallel$ are calculated for different spatial areas around Venus. The results show that proton $T_\perp$ and $T_\parallel$ are nearly equal in the solar wind. At the bow shock and in the magnetosheath, the ratio $T_\perp/T_\parallel$ increases to provide conditions favoring mirror mode wave generation. An even higher anisotropy is found in the magnetotail with $T_\perp/T_\parallel\approx 2$ for both protons and heavy ions.

Fulltekst (pdf)
fulltext
• 2.
Luleå tekniska universitet. Swedish Institute of Space Physics, Kiruna, Sweden. Physics, Lancaster University, Lancaster, United Kingdom.
Swedish Institute of Space Physics, Kiruna, Sweden. Swedish Institute of Space Physics, Uppsala, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Department of Physics, Umeå Universitet, Umeå, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Space Research Institute, Austrian Academy of Science, Graz, Austria.
Proton Temperature Anisotropies in the Plasma Environment of Venus2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 5, s. 3312-3330Artikkel i tidsskrift (Fagfellevurdert)

Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T⊥) and parallel (T‖) to the background magnetic field. We calculate average values of T⊥ and T‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T⊥/T‖ > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near‐subsolar magnetosheath (T⊥/T‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere.

• 3.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
Solar Wind Dynamics within The Atmosphere of comet 67P/Churyumov-Gerasimenko2018Doktoravhandling, med artikler (Annet vitenskapelig)

In this thesis, we explore the dynamics of the solar wind as it perme-ates and flows through a tenuous cometary atmosphere, with a focuson the interaction observed at comet 67P/Churyumov–Gerasimenko.

Seven comets had already been visited by nine different probes when the European spacecraft Rosetta reached comet Churyumov–Gerasimenko in August 2014. The mission was however the first to orbit its host comet, which it did for a total duration of more than two years, corre-sponding to a large part of the comet’s orbit around the Sun. This en-abled to study how the dynamics of the plasma environment evolvedas the comet itself was transformed from one of the smallest obstaclesto the solar wind in the Solar System when far away from the Sun, toa well-established magnetosphere at perihelion.

Most of our efforts tackle the early part of this transformation, when the creation of new-born cometary ions starts to induce significant disturbances to the incident flow. During this stage, a kinetic descrip-tion of the interaction is necessary, as the system of interest cannot be reduced to a hydrodynamic problem. This contrasts with the situation closer to the Sun, where a fluid treatment can be used, at Churyumov–Gerasimenko as well as at previously visited comets.

Rosetta was not a mission dedicated to plasma studies, however. It directly translates into a limited spatial coverage of the cometary plasma environment, which by its nature extends over several spatial scales. An approach solely based on the analysis of in-situ data cannot properly address the major questions on the nature and physics of the plasma environment of Churyumov–Gerasimenko. This thesis there-fore largely exploits the experimental–analytical–numerical triad of approaches. In Chapters 3 and 4 we propose simple models of the ion dynamics and of the cometary plasma environment, and these are tested against experimental and numerical data. Used together,they give a global description of the solar wind ion dynamics through the cometary atmosphere, that we explore in the 2-dimensional and 3-dimensional cases (Chapter 5). In Chapter 6, we propose a view onthe interaction and its fluid aspects when closer to the Sun.

Fulltekst (pdf)
fulltext
• 4.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna. Aalto University, School of Electrical Engineering, Department of Electronics and Nanoengineering, Finland. Technische Universität Braunschweig, Institute for Geophysics and Extraterrestrial Physics, Germany. Jet Propulsion Laboratory, California Institute of Technology, USA.
The birth and growth of a solar wind cavity around a comet: Rosetta observations2017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, nr Suppl. 2, s. S369-S403Artikkel i tidsskrift (Fagfellevurdert)

The Rosetta mission provided detailed observations of the growth of a cavity in the solar wind around comet 67P/Churyumov–Gerasimenko. As the comet approached the Sun, the plasma of cometary origin grew enough in density and size to present an obstacle to the solar wind. Our results demonstrate how the initial slight perturbations of the solar wind prefigure the formation of a solar wind cavity, with a particular interest placed on the discontinuity (solar wind cavity boundary) passing over the spacecraft. The slowing down and heating of the solar wind can be followed and understood in terms of single particle motion. We propose a simple geometric illustration that accounts for the observations, and shows how a cometary magnetosphere is seeded from the gradual steepening of an initially slight solar wind perturbation. A perspective is given concerning the difference between the diamagnetic cavity and the solar wind cavity.

Fulltekst (pdf)
fulltext
• 5.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna. LPC2E, CNRS, Orléans. Swedish Institute of Space Physics, Kiruna. Swedish Institute of Space Physics, Kiruna. Swedish Institute of Space Physics, Kiruna. Swedish Institute of Space Physics, Kiruna. LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC Univ. Paris. IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC Univ. Paris. Technische Universität Braunschweig, Institute for Geophysics and Extraterrestrial Physics.
The root of a comet tail: Rosetta ion observations at comet 67P/Churyumov–Gerasimenko2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikkel-id A21Artikkel i tidsskrift (Fagfellevurdert)

Context.The first 1000 km of the ion tail of comet 67P/Churyumov–Gerasimenko were explored by the EuropeanRosettaspacecraft,2.7 au away from the Sun.Aims.We characterised the dynamics of both the solar wind and the cometary ions on the night-side of the comet’s atmosphere.Methods.We analysed in situ ion and magnetic field measurements and compared the data to a semi-analytical model.Results.The cometary ions are observed flowing close to radially away from the nucleus during the entire excursion. The solar windis deflected by its interaction with the new-born cometary ions. Two concentric regions appear, an inner region dominated by theexpanding cometary ions and an outer region dominated by the solar wind particles.Conclusions.The single night-side excursion operated byRosettarevealed that the near radial flow of the cometary ions can beexplained by the combined action of three different electric field components, resulting from the ion motion, the electron pressuregradients, and the magnetic field draping. The observed solar wind deflection is governed mostly by the motional electric field−uion×B.

• 6.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna.
LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC Univ. Paris. Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna.
Dawn-dusk asymmetry induced by the Parker spiral angle in the plasma dynamics around comet 67P/Churyumov-Gerasimenko2018Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 478, nr 2, s. 1570-1575Artikkel i tidsskrift (Fagfellevurdert)

When interacting, the solar wind and the ionised atmosphere of a comet exchange energy and momentum. Our aim is to understand the influence of the average Parker spiral configuration of the solar wind magnetic field on this interaction. We compare the theoretical expectations of an analytical generalised gyromotion with Rosetta observations at comet 67P/Churyumov-Gerasimenko. A statistical approach allows one to overcome the lack of upstream solar wind measurement. We find that additionally to their acceleration along (for cometary pick-up ions) or against (for solar wind ions) the upstream electric field orientation and sense, the cometary pick-up ions are drifting towards the dawn side of the coma, while the solar wind ions are drifting towards the dusk side of the coma, independent of the heliocentric distance. The dynamics of the interaction is not taking place in a plane, as often assumed in previous works.

• 7.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna.
LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC Univ. Paris. IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC Univ. Paris. LPC2E, CNRS, Orléans. Laboratory for Atmospheric and Space Physics (LASP), University of Colorado Boulder. Umeå University, Department of Physics. Swedish Institute of Space Physics, Kiruna. Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna.
Solar wind dynamics around a comet: A 2D semi-analytical kinetic model2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, artikkel-id A35Artikkel i tidsskrift (Fagfellevurdert)

Aims.We aim at analytically modelling the solar wind proton trajectories during their interaction with a partially ionised cometaryatmosphere, not in terms of bulk properties of the flow but in terms of single particle dynamics.Methods.We first derive a generalised gyromotion, in which the electric field is reduced to its motional component. Steady-stateis assumed, and simplified models of the cometary density and of the electron fluid are used to express the force experienced byindividual solar wind protons during the interaction.Results.A three-dimensional (3D) analytical expression of the gyration of two interacting plasma beams is obtained. Applying it to acomet case, the force on protons is always perpendicular to their velocity and has an amplitude proportional to 1/r2. The solar winddeflection is obtained at any point in space. The resulting picture presents a caustic of intersecting trajectories, and a circular regionis found that is completely free of particles. The particles do not lose any kinetic energy and this absence of deceleration, togetherwith the solar wind deflection pattern and the presence of a solar wind ion cavity, is in good agreement with the general results of theRosettamission.Conclusions.The qualitative match between the model and thein situdata highlights how dominant the motional electric field isthroughout most of the interaction region for the solar wind proton dynamics. The model provides a simple general kinetic descriptionof how momentum is transferred between these two collisionless plasmas. It also shows the potential of this semi-analytical modelfor a systematic quantitative comparison to the data.

• 8.
Swedish Institute of Space Physics, Kiruna.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna. Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna. Swedish Institute of Space Physics, Kiruna. Swedish Institute of Space Physics, Kiruna. Swedish Institute of Space Physics, Kiruna. Technische Universität Braunschweig, Institute for Geophysics and Extraterrestrial Physics.
Cometary ion dynamics observed in the close vicinity of comet 67P/Churyumov-Gerasimenko during the intermediate activity period2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 613, s. 1-8Artikkel i tidsskrift (Fagfellevurdert)

Aims.Cometary ions are constantly produced in the coma, and once produced they are accelerated and eventually escape the coma.We describe and interpret the dynamics of the cometary ion flow, of an intermediate active comet, very close to the nucleus and in theterminator plane.Methods.We analysed in situ ion and magnetic field measurements, and characterise the velocity distribution functions (mostly usingplasma moments). We propose a statistical approach over a period of one month.Results.On average, two populations were observed, separated in phase space. The motion of the first is governed by its interactionwith the solar wind farther upstream, while the second one is accelerated in the inner coma and displays characteristics compatiblewith an ambipolar electric field. Both populations display a consistent anti-sunward velocity component.Conclusions.Cometary ions born in different regions of the coma are seen close to the nucleus of comet 67P/Churyumov–Gerasimenko with distinct motions governed in one case by the solar wind electric field and in the other case by the position relative tothe nucleus. A consistent anti-sunward component is observed for all cometary ions. An asymmetry is found in the average cometaryion density in a solar wind electric field reference frame, with higher density in the negative (south) electric field hemisphere. Thereis no corresponding signature in the average magnetic field strengt

• 9.
Luleå tekniska universitet, Institutionen för system- och rymdteknik.
Europa's Hydrogen Corona in a Large Set of HST Lyman-Alpha Images2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave

Far-ultraviolet (FUV) spectral images of Jupiter's moon Europa were obtained by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 20 occasions between the years 1999 and 2015. In this thesis these data are analyzed to look for Lyman-alpha emissions from a hydrogen corona. This hydrogen corona was recently discovered in absorption, also from HST Lyman-alpha images but with Europa in transit of Jupiter, and the aim of this study is to confirm the existence of the corona also in emission. Europa's thin atmosphere is dominated by molecular oxygen, mainly produced by radiolysis and sputtering of the icy surface. Atomic hydrogen, the main target for this study, is produced by sputtering from the surface and the dissociation of H2 and H2O. It quickly escapes the gravity of Europa. To study the hydrogen corona in the spectral STIS images the data need to be processed to remove the other Lyman-alpha contributions to the image. These other contributions include emissions from the geocorona, emissions from the interplanetary medium (IPM), dark current in the detector and sunlight reflected from the surface of Europa. To estimate the contribution to the image from the hydrogen corona, a basic model of the expected emissions from the corona is developed. By fitting this model to the processed STIS data values of the hydrogen density and the surface Lyman-alpha albedo of the moon are obtained. The results confirm the presence of a hydrogen corona, with varying densities between the different observations but generally about twice as large as the results from the previous study. The uncertainty for the results is however large and there is a clear correlation between hydrogen density and background level in the image, for which the reason is poorly understood. No hemispheric variability or connections to the true anomaly of the moon are found, but the hydrogen density seems to be increasing during the time of the observations. The results for the albedo is consistent with previous results, indicating a lower albedo on the leading than on the trailing hemisphere.

Fulltekst (pdf)
fulltext
• 10.
Luleå tekniska universitet, Institutionen för system- och rymdteknik.
Simulation Study of Electrostatic Analyser Designs2018Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
• 11.
Luleå tekniska universitet, Institutionen för system- och rymdteknik.
Cometary ion dynamics with Rosetta Ion Composition Analyzer2019Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave

For centuries, comet sightings have fascinated us and we have strived to understand their nature. The knowledge of the behavior and composition of comets would help in understanding the formation of the Solar System, as they are believed to be the oldest objects in it. Cometary research, however, is in a developing stage because from an estimated trillion of comets, we have studied one through extended in-situ in-orbit measurements. Previous research has now established that comets become visible when they approach close to the Sun while their surface volatile material is sublimed by the solar radiation. The neutral atmosphere thus created is also ionized by the solar radiation, resulting in creation of positive cometary ions that are picked up and accelerated by the solar wind electric and magnetic fields. The fields influence the trajectories of these accelerated ions, causing variations in their flow angles as a function of their energy, a mechanism called energy - angle dispersion. Dispersion has only been studied for specific cases so far.

In this work, the nature of the energy - angle dispersion is statistically examined using scientific data from the Rosetta mission, which orbited the Comet 67P/Churyumov-Gerasimenko from August 2014 to September 2016. One of the instruments onboard Rosetta, the Ion Composition Analyzer (ICA), measured the three-dimensional 360◦ × 90◦ energy and mass distribution of positive ions around the comet. In this work, the ICA data is used to identify dispersion events, their properties and trends using data analysis and image processing techniques at different temporal resolutions. The results are analyzed against the data from physical simulations, models and instruments onboard Rosetta. With the detailed statistical and quantitative analysis of the evolution of the energy - angle dispersion, it is found that the dispersion events are quite coherent over time scales of a few days and that the dispersion is very dynamic in nature. An understanding of this dispersion of accelerated cometary ions is key to understand the cometary ion dynamics.

• 12.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
Back-tracing of water ions at comet 67P/Churyumov–Gerasimenko2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave

This paper examines the neutral coma of comet 67P/Churyumov–Gerasimenko by using measurements of charged particles (water ions) and tracing them back to their place of ionisation. The measurements were taken from Rosetta’s Ion Composition Analyser. The simulations made use of an existing program which traces particles forward, which was changed to trace particles backwards, with new conditions for terminating the simulation.

Two types of simulations were made. The ﬁrst type is referred to as ”one-day simulations”. In these, simulations are made using data from a single occasion, with nine occasions studied per selected day. The days were selected so that the spacecraft was in diﬀerent positions in relation to the comet. The second is referred to as the ”full-hemisphere” simulation. In this simulation, data from all usable days are used to produce an image of the hemisphere facing the Sun.

The full-hemisphere simulation suﬀers from lack of simultaneous measurements, and indeed it is impossible to obtain in-situ measurements at all positions at once. Both simulations could be improved using more precise models, which could not be done within the allotted time of this work.

Fulltekst (pdf)
fulltext
• 13.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
Mass Loading of Space Plasmas2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave

The solar wind interaction with an icy comet is studied through a model problem. A hybrid simulation is done of a box with evenly distributed water ions and protons, where initially the water ions are stationary, and protons move with the speed of the solar wind. The purpose of the thesis is to investigate the interaction between the two species through the convective electric field, and focus is on early acceleration of pick-up ions, and deflection of the solar wind. It is relevant to the cometary case, because it enables study of the physics of this interaction, without involving other mechanisms, such as bow shock, magnetic field pile-up and draping. The species are found to exchange kinetic energy similar to a damped oscillator, where the dampening is caused by kinetic energy being transferred to the magnetic field. At early times, i.e. times smaller than the gyration time for the water ions, the solar wind does not lose much speed when it is deflected. For comparable number densities, the solar wind can be deflected more than 90° at early times, and loses more speed, and water ions are picked up faster. The total kinetic energy of the system decreases when energy builds up in the magnetic field. The nature of the energy exchange is strongly dependent on the number density ratio between water ions and protons. A density instability with behaviour similar to a plasma beam instability forms as energy in the magnetic field increases, and limits the amount of time the simulation preserves total energy, for the particular hybrid solver used. There is a discussion on the structure of the density instability, and it is compared to cometary simulations.

Fulltekst (pdf)
fulltext
• 14.
IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France.
I Physikalisches Institut, Universität zu Köln, Köln, Germany. Max-Planck-Institut für Radioastronomie, Bonn, Germany . Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic .
High-spectral resolution M-band observations of CO Rot-Vib absorption lines towards the Galactic center2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, artikkel-id A44Artikkel i tidsskrift (Fagfellevurdert)

Context. In the near- to mid-infrared wavelength domain, bright continuum sources in the central parsec of the Galactic center (GC) are subject to foreground absorption. These sources therefore represent ideal probes of the intervening material that is responsible for the absorption along the line of sight.

Aims. Our aim is to shed light on the location and physics of the absorbing clouds. We try to find out which of the gaseous absorbing materials is intimately associated with the GC and which one is associated with clouds at a much larger distance.

Methods. We used the capabilities of CRIRES spectrograph located at ESO Very Large Telescope in Chile to obtain absorption spectra of individual lines at a high spectral resolution of R = 50 000, that is, 5 km s−1. We observed the 12CO R(0), P(1), P(2), P(3), P(4), P(5), P(6), P(7) and P(9) transition lines, applied standard data reduction, and compared the results with literature data.

Results. We present the results of CRIRES observations of 13 infrared sources located in the central parsec of the Galaxy. The data provide direct evidence for a complex structure of the interstellar medium along the line of sight and in the close environment of the central sources. In particular we find four cold foreground clouds at radial velocities vLSR of the order of −145, −85, −60, and −40 ± 15 km s−1 that show absorption in the lower transition lines from R(0) to P(2) and in all the observed spectra. We also find in all sources an absorption in velocity range of 50–60 km s−1, possibly associated with the so-called 50 km s−1 cloud and suggesting an extension of this cloud in front of the GC. Finally, we detect individual absorption lines that are probably associated with material much closer to the center and with the sources themselves, suggesting the presence of cold gas in the local region.

• 15.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics.
How does O+ outflow vary with solar wind conditions?2019Doktoravhandling, med artikler (Annet vitenskapelig)

The entire solar system including Earth is enveloped in a region of space where the Sun’s magnetic field dominates, this region is called the heliosphere. Due to this position in the heliosphere, a strong coupling exists between the Sun and our planet. The Sun continuously ejects particles, the solar wind, which is composed mainly of protons, electrons as well as some helium and heavier elements. These high energetic particles then hit the Earth and are partly deflected by the Earth’s magnetosphere (the region around Earth governed by the geomagnetic field). Depending on the strength of the solar wind hitting our planet, the magnetosphere is disturbed and perturbations can be seen down to the lower atmosphere.

The upper atmosphere is affected by short wave-length solar radiation that ionise the neutral atoms, this region is referred to as the ionosphere. In the ionosphere, some of the heavier ion populations, such as O+, are heated and accelerated through several processes and flow upward. In the polar regions (polar cap, cusp and plasma mantle) these mechanisms are particularly efficient and when the ions have enough energy to escape the Earth’s gravity, they move outward along open magnetic field lines. These outflowing ions may be lost into interplanetary space.

Another aspect that influences O+ ions are disturbed magnetospheric conditions. They correlate with solar active periods, such as coronal holes or the development of solar active regions. From these regions, strong ejections emerge, called coronal mass ejections (CMEs). When these CMEs interact with Earth, they produce a compression of the magnetosphere as well as reconnection between the terrestrial magnetic field lines and the interplanetary magnetic field (IMF) lines, which very often leads to geomagnetic storms. The energy in the solar wind as well as the coupling to the magnetosphere increase during geomagnetic storms and therefore the energy input to the ionosphere. This in turn increases the O+ outflow. In addition, solar wind parameter variations such as the dynamic pressure or the IMF also influence the outflowing ions.

Our observations are made with the Cluster mission, a constellation of 4 satellites flying around Earth in the key magnetospheric regions where we usually observe ion outflow. In this thesis, we estimated O+ outflow for different solar wind parameters (IMF, solar wind dynamic pressure) and extreme ultraviolet radiations (EUV) as well as for extreme geomagnetic storms. We found that O+ outflow increases exponentially with enhanced geomagnetic activity (Kp index) and about 2 orders of magnitude during extreme geomagnetic storms compared to quiet conditions. Furthermore, our investigations on solar wind parameters showed that O+ outflow increases for high dynamic pressure and southward IMF, as well as with EUV radiations. Finally, the fate of O+ ions from the plasma mantle were studied based on Cluster observations and simulations. These results confirm that ions observed in the plasma mantle have sufficient energy to be lost in the solar wind.

Fulltekst (pdf)
fulltext
• 16.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics, Kiruna, Sweden.
O+ outflow during geomagnetic storms observed by Cluster satellites2018Licentiatavhandling, med artikler (Annet vitenskapelig)

The region of space dominated by the Sun's magnetic field is called the heliosphere. It envelops the entire solar system including Earth. Therefore, a strong coupling exists between the Sun and our planet. The Sun continuously ejects particles, the solar wind, and when these high energy particles hit Earth, the magnetosphere (the region around the Earth governed by the geomagnetic field) is affected. When the solar wind is enhanced this disturbs the magnetosphere and perturbations can be seen also in ground-based observations.

The upper atmosphere is subjected to solar radiation that ionise the neutral atoms and molecules, this region is referred to as the ionosphere. In the ionosphere, some of the heavier ion populations, such as O+, are heated and accelerated through several processes and flow upward. In the polar regions these mechanisms are particularly efficient and when the ions have enough energy to escape the Earth's gravity, they move outward along open magnetic field lines and may be lost into interplanetary space. Ion outflow in general has already been well studied, however, ion outflow under extreme magnetospheric conditions has not been investigated in detail.

Disturbed magnetospheric conditions correlate with solar active periods, such as coronal holes or the development of solar active regions. From these regions, strong ejections called coronal mass ejections (CMEs) emerge. When these extreme events interact with Earth, they produce a compression of the magnetosphere as well as reconnection between the terrestrial magnetic field lines and the interplanetary magnetic field (IMF) lines, which most of the time leads to geomagnetic storms. The amounts of incoming solar particles and energy increase during geomagnetic storms and we also observe an increase in the O+ outflow.

Our observations are made with the Cluster mission, a constellation of 4 satellites flying around Earth in the key magnetospheric regions where ion outflow is usually observed. In this thesis, we estimate O+ outflow under disturbed magnetospheric conditions and for several extreme geomagnetic storms. We find that O+ outflow lost into the solar wind increases exponentially with enhanced geomagnetic activity (Kp index) and increases about 2 orders of magnitude during extreme geomagnetic storms.

Fulltekst (pdf)
ASchillings_Lic
Fulltekst (pdf)
fulltext
• 17.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Swedish Institute of Space Physics.
Department of Physics, Umeå University, Umeå, Sweden. Belgian Institute for Space Aeronomy, Brussels, Belgium. Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement. Department of Physics, Umeå University, Umeå, Sweden. Research Institute for Sustainable Humanosphere, Kyoto University, Japan, Gokasho, Uji, Kyoto. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Swedish Institute of Space Physics, Kiruna, Sweden. EISCAT Scientific Association, Kiruna, Sweden.
The fate of O+ ions observed in the plasma mantle and cusp: particle tracing modelling and Cluster observations2019Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576Artikkel i tidsskrift (Fagfellevurdert)
• 18.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
Electron heating in collisionless shocks observed by the MMS spacecraft2018Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave

Shock waves are ubiquitous in space and astrophysics. Shocks transform directed particle flow energy into thermal energy. As the major part of space is a collisionless medium, shocks in space physics arises through wave-particle interactions with the magnetic field as the main contributor.The heating processes are scale dependent. The large scale processes governs the ion heating and is well described by magnetohydrodynamics. The small scale processes governs the electron heating lies within the field of kinetic plasma theory and is still today remained disputed. A step towards the answer for the small scale heating would be to measure the scale, in order to relate it to a known instability or other small scale processes.The multi-spacecraft NASA MMS spacecraft carries several high resolute particle and field instruments enabling almost instantaneous 3D particle measurements and accurate measurements of the magnetic field. Also the separation between the four MMS spacecraft is as small as < 8km for a certain mission phase. This allows for new approaches when determining the scale which for shocks has not been possible before when using data from previous multi-spacecraft missions with spacecraft separation much larger. The velocity of the shock is large compared to the spacecraft,thus the shock width cannot be directly measured by each spacecraft. Either a constant velocity has to be estimated or we could use gradients of a certain parameter between the spacecraft as the shock flows over them. The usage of gradients is only possible with MMS as all the spacecraft could for MMS be within the shock simultaneously. The change for a parameter within the shockis assumed to be linear between the spacecraft and measurements. It is also assumed that the gradient of the parameter maximizes in the shock normal direction. Using these assumptions two methods have been developed. They have the same working principles but are using two or four spacecraft for linear estimation at each measurement point. From the gradient and parametric data the shock ramp width could then be found. The parameter used in this thesis is the electron temperature. The methods using one, two and four spacecraft were tested using electron temperature data from different shock crossings. Two problems with the gradient methods were found from the results, giving false data for certain time spans. To avoid these problems, the scale of the electron temperature gradient was determined for roughly half the shock ramp. It was found using the two and four spacecraft methods that an assumption of constant velocity for the shock speed is an uncertain assumption. The shock speed varies over short time scales and in the shock crossings analysed the constant velocity estimations were generally overestimated. From the two and four spacecraft methods roughly half of the temperature rise in the shock ramp occurred over 10.8km or 12.4 lpe. This is almost a factor of two greater than previous scale estimates using Cluster data and a multi-spacecraft timing method for shock speed estimation.From the results it is concluded that the methods when using gradients between spacecraft has some restrictions. They can only be used for MMS data, requires quasi-perpendicular high Mach number and will give false results if the temperature is disturbed by interacting hot plasma clouds. However, even though we have these limitations for the tested gradient methods, they were found better and more reliable compared to previous methods for shock scaling.

Fulltekst (pdf)
fulltext
1 - 18 of 18
Referera
Referensformat
• apa
• ieee
• modern-language-association-8th-edition
• vancouver
• Annet format
Fler format
Språk
• de-DE
• en-GB
• en-US
• fi-FI
• nn-NO
• nn-NB
• sv-SE
• Annet språk
Fler språk
Utmatningsformat
• html
• text
• asciidoc
• rtf