Decay of Solar Wind Turbulence behind Interplanetary Shocks

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Turbulence in the solar wind from inertial to kinetic scales

Solar wind, a stream of supersonic plasma emanating from the solar corona, serves as an ideal laboratory for a study of high Reynolds number plasma flows. Turbulent processes that govern the dynamics of the so-called inertial range, i.e., the spatial scales smaller than energy injection scales but larger than the scales where the dissipation processes set in, have been studied for decades. At present, it is believed that the large-scale free energy in a form of kinetic and magnetic fluctuations is transferred via turbulent cascade into smaller scales, where kinetic effects become dominant and heating takes place. In order to understand dissipation processes, high-cadence measurements of solar wind parameters are necessary. The bright monitor of the solar wind (BMSW) instrument on board the Spektr-R spacecraft provides such data, and in tandem with high-cadence measurements of the magnetic field from the Wind spacecraft, we are able address the nature of the sub-ion scale fluctuations. The thesis focus on three interconnected topics, (a) what changes are induced by the passage of a collisionless IP shock in the framework of turbulence, (b) study of a decay of the turbulent energy downstream an IP shock, and (c) identifying the dominant mode of the sub-ion scale fluctuations

Cosmic background radiation

Astronomický ústav UKAstronomical Institute of Charles UniversityFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Statistické zpracování družicových dat gama záblesků družice Fermi

Gamma-ray bursts (GRBs) are one of the phenomena that still puzzle the astrophysi- cists. Due to their extreme luminosities, they are visible in cosmological distances. They could provide a tool for understanding the early Universe. This thesis focuses on the estimation of the total energy released by the GRBs, their luminosities and how these quantities depend on the redshift. For a sample of 28 long GRBs with known redshifts, a dependence of their total energies and luminosities on the redshift is found. The thesis further studied if this dependence is either an observational bias or a real astrophysical phenomenon. Firstly, proper use of k-correction revealed that it has lit- tle effect on the redshift dependence of these quantities. Secondly, a new approach has been proposed to investigate the redshift dependence of the luminosity function. Thirdly, selection effect has been found, when investigating the sample of bursts with known redshifts, implying a caution when combining data from different sources. All these efforts show that the observational bias can still explain the redshift dependence

A Novel Method for Estimating the Intrinsic Magnetic Field Spectrum of Kinetic-Range Turbulence

Understanding plasma turbulence below the ion characteristic scales is one of the key open problems of solar wind physics. The bulk of our knowledge about the nature of the kinetic-scale fluctuations comes from the high-cadence measurements of the magnetic field. The spacecraft frame frequencies of the sub-ion scale fluctuations are frequently around the Nyquist frequencies of the magnetic field sampling rate. Thus, the resulting ‘measured’ time series may significantly differ from the ‘true’ ones. It follows that second-order moments (e.g., power spectral density, PSD) of the signal may also be highly affected in both their amplitude and their slope. In this paper, we focus on the estimation of the PSD slope for finitely sampled data and we unambiguously define a so-called local slope in the framework of Continuous Wavelet Transform. Employing Monte Carlo simulations, we derive an empirical formula that assesses the statistical error of the local slope estimation. We illustrate the theoretical results by analyzing measurements of the magnetic field instrument (MFI) on board the Wind spacecraft. Our analysis shows that the trace power spectra of magnetic field measurements of MFI can be modeled as the sum of PSD of an uncorrelated noise and an intrinsic signal. We show that the local slope strongly depends on the signal-to-noise (S/N) ratio, stressing that noise can significantly affect the slope even for S/N around 10. Furthermore, we show that the local slopes below the frequency corresponding to proton inertial length, 5≳kλpi>1, depend on the level of the magnetic field fluctuations in the inertial range (Pin), exhibiting a gradual flattening from about −11/3 for high Pin toward about −8/3 for low Pin

Turbulence in the solar wind from inertial to kinetic scales

Solar wind, a stream of supersonic plasma emanating from the solar corona, serves as an ideal laboratory for a study of high Reynolds number plasma flows. Turbulent processes that govern the dynamics of the so-called inertial range, i.e., the spatial scales smaller than energy injection scales but larger than the scales where the dissipation processes set in, have been studied for decades. At present, it is believed that the large-scale free energy in a form of kinetic and magnetic fluctuations is transferred via turbulent cascade into smaller scales, where kinetic effects become dominant and heating takes place. In order to understand dissipation processes, high-cadence measurements of solar wind parameters are necessary. The bright monitor of the solar wind (BMSW) instrument on board the Spektr-R spacecraft provides such data, and in tandem with high-cadence measurements of the magnetic field from the Wind spacecraft, we are able address the nature of the sub-ion scale fluctuations. The thesis focus on three interconnected topics, (a) what changes are induced by the passage of a collisionless IP shock in the framework of turbulence, (b) study of a decay of the turbulent energy downstream an IP shock, and (c) identifying the dominant mode of the sub-ion scale fluctuations

Turbulence ve slunečním větru: od inerciální ke kinetické oblasti

Sluneční vítr, proud supersonické plazmy ze sluneční korony, je ideálním prostředím ke studiu toků plazmatu s vysokými hodnotami Reynoldsova čísla. Turbulentní procesy, které řídí dynamiku tzv. inerciální oblasti turbulence-tj. na škálách menších než charakteristické rozměry největších turbulentních vírů, ale větších než charakteristické disipační škály-byly studovány po celá desetiletí. V současnosti se předpokládá, že volná energie obsažena ve fluktuacích na největších škálách ve formě magnetické a kinetické energie se přenáší prostřednictvím turbulentní kaskády do fluktuací na malých škálách, kde začnou dominovat kinetické efekty vedoucí k zahřátí plazmatu. Abychom pochopili procesy způsobující disipaci na malých škálách, je nutno měřit parametry plazmatu s vysokou kadencí. Přístroj Bright monitor of the solar wind (BMSW) na družici Spektr-R je schopen měřit s časovým rozlišením 30 Hz a společně s rychlými měřeními magnetického pole družicí Wind, můžeme analyzovat fluktuace na charakteristických iontových škálách. Práce se zaměřuje na tři vzájemně provázaná témata spojená s turbulencí, (a) jaké změny vyvolává průchod meziplanetárních rázových vln na charakter fluktuací, (b) studium rozpadu energie v oblasti za rázovou vlnou a (c) identifikaci dominantního plazmatického módu fluktuací na iontových škálách.Solar wind, a stream of supersonic plasma emanating from the solar corona, serves as an ideal laboratory for a study of high Reynolds number plasma flows. Turbulent processes that govern the dynamics of the so-called inertial range, i.e., the spatial scales smaller than energy injection scales but larger than the scales where the dissipation processes set in, have been studied for decades. At present, it is believed that the large-scale free energy in a form of kinetic and magnetic fluctuations is transferred via turbulent cascade into smaller scales, where kinetic effects become dominant and heating takes place. In order to understand dissipation processes, high-cadence measurements of solar wind parameters are necessary. The bright monitor of the solar wind (BMSW) instrument on board the Spektr-R spacecraft provides such data, and in tandem with high-cadence measurements of the magnetic field from the Wind spacecraft, we are able address the nature of the sub-ion scale fluctuations. The thesis focus on three interconnected topics, (a) what changes are induced by the passage of a collisionless IP shock in the framework of turbulence, (b) study of a decay of the turbulent energy downstream an IP shock, and (c) identifying the dominant mode of the sub-ion scale fluctuations.Katedra fyziky povrchů a plazmatuDepartment of Surface and Plasma ScienceMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

A Novel Method for Estimating the Intrinsic Magnetic Field Spectrum of Kinetic-Range Turbulence

Understanding plasma turbulence below the ion characteristic scales is one of the key open problems of solar wind physics. The bulk of our knowledge about the nature of the kinetic-scale fluctuations comes from the high-cadence measurements of the magnetic field. The spacecraft frame frequencies of the sub-ion scale fluctuations are frequently around the Nyquist frequencies of the magnetic field sampling rate. Thus, the resulting ‘measured’ time series may significantly differ from the ‘true’ ones. It follows that second-order moments (e.g., power spectral density, PSD) of the signal may also be highly affected in both their amplitude and their slope. In this paper, we focus on the estimation of the PSD slope for finitely sampled data and we unambiguously define a so-called local slope in the framework of Continuous Wavelet Transform. Employing Monte Carlo simulations, we derive an empirical formula that assesses the statistical error of the local slope estimation. We illustrate the theoretical results by analyzing measurements of the magnetic field instrument (MFI) on board the Wind spacecraft. Our analysis shows that the trace power spectra of magnetic field measurements of MFI can be modeled as the sum of PSD of an uncorrelated noise and an intrinsic signal. We show that the local slope strongly depends on the signal-to-noise (S/N) ratio, stressing that noise can significantly affect the slope even for S/N around 10. Furthermore, we show that the local slopes below the frequency corresponding to proton inertial length, 5≳kλpi>1, depend on the level of the magnetic field fluctuations in the inertial range (Pin), exhibiting a gradual flattening from about −11/3 for high Pin toward about −8/3 for low Pin