| 1. |
- Richard, Louis
(author)
-
Energy Conversion and Particle Acceleration at Turbulent Plasma Jet Fronts
- 2023
-
Doctoral thesis (other academic/artistic)abstract
- High-speed plasma flows (jets) are ubiquitous phenomena in the visible Universe. When the fast plasma flow encounters the ambient plasma at rest, it forms a front where its kinetic energy is dissipated. At the jet front, charged particles gain energy from the electromagnetic fields through heating and acceleration. Plasma jets carry energy away from the most powerful sources in the visible Universe (e.g., active galactic nuclei) and transfer it to the surrounding medium. High-speed plasma flows are also common in planetary magnetospheres, including the Earth’s magnetotail. In the Earth’s magnetotail, plasma jets, called bursty bulk flows, are crucial in transporting energy to the inner magnetosphere in the (sub-)storm cycle. However, the physical mechanisms through which the jet deposits its energy into the plasma are yet to be understood. This thesis focuses on plasma jets produced by magnetic reconnection in the Earth’s magnetotail. The magnetotail is a natural laboratory to probe the plasma at the kinetic scales (10-100 km). This allows us to address some of the open questions related to plasma jet fronts and the associated energy conversion and particle acceleration. We use the four Magne-tospheric Multiscale spacecraft launched in 2015. In paper I, we focus on the global effects of the plasma jets on the Earth’s magnetotail. In the wake of a plasma jet, we show that the Earth’s magnetotail current sheet undergoes a kink-like flapping motion transporting energy across the magnetotail. In paper II, we study the ion acceleration mechanisms associated with the jet. We identify three active mechanisms depending on the relative ion energy compared with the jet size. In paper III, we challenge the picture of the jet front as a sharp two-dimensional boundary. We show that the jet front is often strongly perturbed, contrary to the commonly accepted pic-ture. In paper IV, we investigate the ion dynamics in the magnetic reconnection jets. We show that the thermal ions are rapidly scattered by the strongly curved magnetic field in the magne-totail current sheet. Finally, in paper V, we focus on the turbulence in the plasma jets. We show that the turbulence substantially contributes to the magnetic reconnection energy transfer.
|
|
| 2. |
- Lalti, Ahmad
(author)
-
Electrostatic turbulence and electron heating in collisionless shocks
- 2024
-
Doctoral thesis (other academic/artistic)abstract
- When the supersonic solar wind interacts with Earth’s magnetosphere it forms a shock wave. However, due to the low densities in space, inter-particle collisions play an insignificant role in its dynamics. Earth's bow shock is an example of a collisionless shock, ubiquitous throughout the universe. Their dynamics are complex and their physics remains an active field of research. In this thesis, we use high-resolution measurements from NASA's Magnetospheric Multiscale (MMS) spacecraft to study the plasma wave activity across Earth’s bow shock and its effects on electron heating. In Paper I we train a convolutional neural network (CNN) to identify the different plasma regions that MMS crosses. In Paper II we use the results of this CNN to compile a database of time intervals in which MMS crosses Earth’s bow shock, which we use to find suitable events to tackle the science questions of interest. In Paper III we use multispacecraft methods to properly characterize obliquely propagating whistler waves running upstream of the shock. By analyzing the ion and electron distribution functions we find that their likely source is the instability between the incoming electrons and reflected ions. Shifting our focus to Debye scale electrostatic waves, in Paper IV we develop a method to measure their 3D wave vector based on single-spacecraft interferometry. We are in the process of using this method to study the evolution of Debye scale electrostatic waves across quasi-perpendicular shocks (see Chapter 7). Finally, in Paper V we investigate the electron heating mechanism across quasi-perpendicular shocks. We find the heating mechanism to depend on the Alfvénic Mach number in the deHoffman-Teller frame . We also find that at high the heating mechanism is consistent with the stochastic shock drift acceleration mechanism.
|
|
| 3. |
- Steinvall, Konrad
(author)
-
Electrostatic plasma waves associated with collisionless magnetic reconnection : Spacecraft observations
- 2022
-
Doctoral thesis (other academic/artistic)abstract
- Magnetic reconnection is a fundamental plasma process where changes in magnetic field topology result in explosive energy conversion, plasma mixing, heating, and energization. In geospace, magnetic reconnection couples the Earth’s magnetosphere to the solar wind plasma, enabling plasma transport across the magnetopause. On the sun, reconnection is responsible for coronal mass ejections and flares, which can affect everyday life on Earth, and it influences the evolution of the solar wind. Although collisionless magnetic reconnection has been studied for a long time, some fundamental aspects of the process remain to be understood. One such aspect is if/how plasma waves affect the process. Simulations and spacecraft observations of magnetic reconnection have shown that plasma waves are ubiquitous during reconnection. Particularly interesting are simulation results which show that electrostatic waves can affect the rate at which reconnection occurs, but this has not yet been experimentally verified. The recently launched Magnetospheric Multiscale (MMS) mission was designed to investigate the smallest scales of collisionless magnetic reconnection, making it an excellent mission to study small-scale waves as well. In this thesis, we use MMS to study electrostatic waves associated with magnetic reconnection in geospace. Our first two studies are devoted to the properties of electron holes (EHs), believed to play an important role in collisionless reconnection. Using MMS, we analyze EHs in unprecedented detail, and compare their properties to theory and previous studies. Importantly, we find evidence of EHs radiating whistler waves in the reconnection separatrices, a process which might modulate the reconnection rate. In our third study, we show that the presence of cold ions at the reconnecting magnetopause can lead to the growth of the ion-acoustic instability. This instability leads to dissipation and cold ion heating. The fourth study compares different techniques for determining the velocity of electrostatic waves. Accurate velocity estimates are important, since they are needed to understand how the wave interacts with the plasma. Finally, in our fifth study, we calibrate the E-field measurements made in the solar wind by the Solar Orbiter spacecraft, to aid future studies of solar wind processes, including magnetic reconnection.
|
|
| 4. |
- Eriksson, Elin, 1989-
(author)
-
Electron energization in near-Earth space : Studies of kinetic scales using multi-spacecraft data
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Plasma, a gas of charged particles exhibiting collective behavior, is everywhere in the Universe. The heating of plasma to millions of degrees and acceleration of charged particles to very high energies has been observed in many astrophysical environments. How and where the heating and acceleration occur is in many cases unclear. In most astrophysical environments, plasma consists of negative electrons and positive ions. In this thesis we focus on understanding the heating and acceleration of electrons. Several plasma processes have been proposed to explain the observed acceleration. However, the exact heating and acceleration mechanisms involved and their importance are still unclear. This thesis contributes toward a better understanding of this topic by using observations from two multi-spacecraft missions, Cluster and the Magnetospheric MultiScale (MMS), in near-Earth space.In Article I we look at magnetic nulls, regions of vanishing magnetic field B believed to be important in particle acceleration, in the Earth's nightside magnetosphere. We find that nulls are common at the nightside magnetosphere and that the characterization of the B geometry around a null can be affected by localized B fluctuations. We develop and present a method for determining the effect of the B fluctuation on the null's characterization.In Article II we look at a thin (a few km) current sheet (CS) in the turbulent magnetosheath. Observations suggest local electron heating and beam formation parallel to B inside the CS. The electron observations fits well with the theory of electron acceleration across a shock due to a potential difference. However, in our case the electron beams are formed locally inside the magnetosheath that is contrary to current belief that the beam formation only occurs at the shock.In Article III we present observations of electron energization inside a very thin (thinner than Article II) reconnecting CS located in the turbulent magnetosheath. Currently, very little is know about electron acceleration mechanisms at these small scales. MMS observe local electron heating and acceleration parallel to B when crossing the CS. We show that the energized electrons correspond to acceleration due to a quasi-static potential difference rather than electrostatic waves. This energization is similar to what has been observed inside ion diffusion regions at the magnetopause and magnetotail. Thus, despite the different plasma conditions a similar energization occurs in all these plasma regions.In Article IV we study electron acceleration by Fermi acceleration, betatron acceleration, and acceleration due to parallel electric fields inside tailward plasma jets formed due to reconnection, the so called tailward outflow region. We show that most observations are consistent with local electron heating and acceleration from a simplified two dimensional picture of Fermi acceleration and betatron acceleration in an outflow region. We find that Fermi acceleration is the dominant electron acceleration mechanism.
|
|
| 5. |
- Johlander, Andreas, 1990-
(author)
-
Ion dynamics and structure of collisionless shocks
- 2016
-
Licentiate thesis (other academic/artistic)abstract
- Shock waves are responsible for slowing down and heating supersonic flows. In collisionless space plasmas, shocks are able to accelerate particles to very high energies. We study injection of suprathermal ions at Earth’s quasi- parallel shock using high time resolution data from the Cluster spacecraft. We find that solar wind ions reflect off short large-amplitude magnetic structures (SLAMSs) and are subsequently accelerated by the convection electric field. We also use data from the closely-spaced Magnetospheric MultiScale (MMS) spacecraft to compare competing non-stationarity processes at Earth’s quasi- perpendicular bow shock. Using MMS’s high cadence plasma measurements, we find that the shock exhibits non-stationarity in the form of ripples.
|
|
| 6. |
- Johlander, Andreas, 1990-
(author)
-
Ion dynamics and structure of collisionless shocks in space
- 2019
-
Doctoral thesis (other academic/artistic)abstract
- Shock waves form when supersonic flows encounter an obstacle. Like in regular gases, shock waves can form in a plasma - a gas of electrically charged particles. Shock waves in plasmas where collisions between particles are very rare are referred to as collisionless shock waves. Collisionless shocks are some of the most energetic plasma phenomena in the universe. They are found for example around exploded supernova remnants and in our solar system where the supersonic solar wind encounters obstacles like planets and the interstellar medium. Shock waves in plasmas are very efficient particle accelerators though a process known as diffusive shock acceleration. An example of particles accelerated in shock waves are the extremely energetic galactic cosmic rays that permeate the galaxy. This thesis addresses the physics of collisionless shocks using spacecraft observations of the Earth's bow shock, particularly understanding the ion dynamics and shock structure for different shock conditions. For this we have used data from ESA's four Cluster satellites and NASA's four Magnetospheric Multiscale (MMS) satellites. The first study presents Cluster measurements from the quasi-parallel bow shock, where the angle between the magnetic field and the shock normal is less than 45 degrees. We study the first steps of acceleration of solar wind ions at short large-amplitude magnetic structures (SLAMS). We observe nearly specularly reflected solar wind ions upstream of a SLAMS. By gyration in the solar wind, the reflected ions are accelerated to a few times the solar wind energy. The second and third study are about shock non-stationarity using MMS measurements from the quasi-perpendicular shock, where the angle between the magnetic field and the shock normal is greater than 45 degrees. In the second study we show that the shock is non-stationary in the form of ripples that propagate along the shock surface. In the third study we study closer in detail the dispersive properties of the ripples and find that whether a solar wind ion will be reflected at the shock is dependent on where it impinges on the rippled shock. In the fourth study we quantify the conditions for ion acceleration shocks by using MMS measurements from many encounters with the bow shock. We find that the quasi-parallel shock is efficient with up to 10% of the energy density in energetic ions. We also find that at quasi-parallel shocks, SLAMS can restrict high-energy ions from propagating upstream and convect them back to the shock, potentially increasing acceleration efficiency.
|
|
| 7. |
- Lindberg, Martin
(author)
-
Electron Heating and Acceleration at Earth’s Collisionless Bow Shock
- 2024
-
Doctoral thesis (other academic/artistic)abstract
- Cosmic rays are ultra-relativistic particles traveling near the speed of light permeating the galaxy. Collisionless shock waves with their ubiquity throughout the universe and excellent capability of accelerating charged particles offer an explanation to the origin of cosmic rays. It is well established that the particles are predominately accelerated at young supernova remnant shocks through a mechanism called Diffusive Shock Acceleration (DSA). However, this theory only applies if the particles already have a relativistic starting energy. Therefore, the charged particles must be pre-accelerated up to relativistic energies by some unknown mechanism(s) before being injected into the cosmic ray acceleration process. This is known as the injection problem and a lot of effort has been put into resolving it over the past decades. This thesis will use spacecraft data from NASA's Magnetospheric Multiscale (MMS) mission to study electron acceleration at Earth's collisionless bow shock. In particular, we will study what mechanisms are able to accelerate electrons from solar wind thermal energies (~20 eV) up to mildly relativistic energies 10-100 keV. Paper III and Paper IV set out to study energetic electron events observed at Earth's bow shock by MMS. In Paper III, we investigate the most promising candidate for a solution to the long-standing electron injection problem, the Stochastic Shock Drift Acceleration (SSDA) mechanism. SSDA successfully describes a mechanism for electrons to be accelerated up to mildly relativistic energies. However, only one previous observation of the theory exists. Building on that study, we provide further evidence in favor of the theory by showing good agreement between predictions and observations. Observational evidence of an alternative electron acceleration mechanism is presented in Paper IV. The observation displays an increase in electron flux up to ~60 keV, and inconsistent features with the SSDA mechanism. The event exhibits bi-directional electron pitch angle distributions which are generally associated with magnetic bottles and are rarely observed around Earth's bow shock. The evidence led us to propose a two-step acceleration process where field-aligned electron beams are injected into a shrinking magnetic bottle configuration caused by either a shock surface deformation or a bent upstream magnetic field line intersecting the shock surface at two different locations. Papers I and II are directed more toward the heating of electrons at collisionless shocks. The studies investigate electron entropy generation at collisionless shocks and its dependence on shock parameters. Paper I states and deals mostly with the (instrumental) challenges of calculating entropy using the MMS spacecraft data. The close relation between entropy and irreversible heating is then discussed and used to classify different heating mechanisms at the shock. We show that the electron entropy generation at Earth's bow shock depends strongly on the upstream electron plasma beta and Alfvén Mach number. In the absence of collisions, the exact generation of entropy across collisionless shocks is an open question. Early theoretical studies suggest that particle-particle collisions are replaced by plasma wave-particle interaction. In Paper II, we build on the result from Paper I, by performing a statistical study of electron entropy change across Earth's bow shock and try to answer what plasma wave modes are important for entropy generation.
|
|
| 8. |
- Retinò, Alessandro, 1974-
(author)
-
Magnetic Reconnection in Space Plasmas : Cluster Spacecraft Observations
- 2007
-
Doctoral thesis (other academic/artistic)abstract
- Magnetic reconnection is a universal process occurring at boundaries between magnetized plasmas, where changes in the topology of the magnetic field lead to the transport of charged particles across the boundaries and to the conversion of electromagnetic energy into kinetic and thermal energy of the particles. Reconnection occurs in laboratory plasmas, in solar system plasmas and it is considered to play a key role in many other space environments such as magnetized stars and accretion disks around stars and planets under formation. Magnetic reconnection is a multi-scale plasma process where the small spatial and temporal scales are strongly coupled to the large scales. Reconnection is initiated rapidly in small regions by microphysical processes but it affects very large volumes of space for long times. The best laboratory to experimentally study magnetic reconnection at different scales is the near-Earth space, the so-called Geospace, where Cluster spacecraft in situ measurements are available. The European Space Agency Cluster mission is composed of four-spacecraft flying in a formation and this allows, for the first time, simultaneous four-point measurements at different scales, thanks to the changeable spacecraft separation. In this thesis Cluster observations of magnetic reconnection in Geospace are presented both at large and at small scales. At large temporal (a few hours) and spatial (several thousands km) scales, both fluid and kinetic evidence of reconnection is provided. The evidence consist of ions accelerated and transmitted across the Earth’s magnetopause. The observations show that component reconnection occurs at the magnetopause and that reconnection is continuous in time. The microphysics of reconnection is investigated at smaller temporal (a few ion gyroperiods) and spatial (a few ion gyroradii) scales. Two regions are important for the microphysics: the X-region, around the X-line, where reconnection is initiated and the separatrix region, away from the X-line, where most of the energy conversion occurs. Observations of a separatrix region at the magnetopause are shown and the microphysics is described in detail. The separatrix region is shown to be highly structured and dynamic even away from the X-line.Finally the discovery of magnetic reconnection in turbulent plasma is presented by showing, for the first time, in situ evidence of reconnection in a thin current sheet found in the turbulent plasma downstream of the quasi-parallel Earth’s bow shock. It is shown that turbulent reconnection is fast and that electromagnetic energy is converted into heating and acceleration of particles in turbulent plasma. It is also shown that reconnecting current sheets are abundant in turbulent plasma and that reconnection can be an efficient energy dissipation mechanism.
|
|
| 9. |
- Rosenqvist, Lisa, 1973-
(author)
-
Energy Transfer and Conversion in the Magnetosphere-Ionosphere System
- 2008
-
Doctoral thesis (other academic/artistic)abstract
- Magnetized planets, such as Earth, are strongly influenced by the solar wind. The Sun is very dynamic, releasing varying amounts of energy, resulting in a fluctuating energy and momentum exchange between the solar wind and planetary magnetospheres. The efficiency of this coupling is thought to be controlled by magnetic reconnection occurring at the boundary between solar wind and planetary magnetic fields. One of the main tasks in space physics research is to increase the understanding of this coupling between the Sun and other solar system bodies. Perhaps the most important aspect regards the transfer of energy from the solar wind to the terrestrial magnetosphere as this is the main source for driving plasma processes in the magnetosphere-ionosphere system. This may also have a direct practical influence on our life here on Earth as it is responsible for Space Weather effects. In this thesis I investigate both the global scale of the varying solar-terrestrial coupling and local phenomena in more detail. I use mainly the European Space Agency Cluster mission which provide unprecedented three-dimensional observations via its formation of four identical spacecraft. The Cluster data are complimented with observations from a broad range of instruments both onboard spacecraft and from groundbased magnetometers and radars.A period of very strong solar driving in late October 2003 is investigated. We show that some of the strongest substorms in the history of magnetic recordings were triggered by pressure pulses impacting a quasi-stable magnetosphere. We make for the first time direct estimates of the local energy flow into the magnetotail using Cluster measurements. Observational estimates suggest a good energy balance between the magnetosphere-ionosphere system while empirical proxies seem to suffer from over/under estimations during such extreme conditions.Another period of extreme interplanetary conditions give rise to accelerated flows along the magnetopause which could account for an enhanced energy coupling between the solar wind and the magnetosphere. We discuss whether such conditions could explain the simultaneous observation of a large auroral spiral across the polar cap.Contrary to extreme conditions the energy conversion across the dayside magnetopause has been estimated during an extended period of steady interplanetary conditions. A new method to determine the rate at which reconnection occurs is described that utilizes the magnitude of the local energy conversion from Cluster. The observations show a varying reconnection rate which support the previous interpretation that reconnection is continuous but its rate is modulated.Finally, we compare local energy estimates from Cluster with a global magnetohydrodynamic simulation. The results show that the observations are reliably reproduced by the model and may be used to validate and scale global magnetohydrodynamic models.
|
|
| 10. |
|
|
