| 1. |
- Vaivads, Andris, et al.
(författare)
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Turbulence Heating ObserveR - satellite mission proposal
- 2016
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Ingår i: JOURNAL OF PLASMA PHYSICS. - 0022-3778. ; 82
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Tidskriftsartikel (refereegranskat)abstract
- The Universe is permeated by hot, turbulent, magnetized plasmas. Turbulent plasma is a major constituent of active galactic nuclei, supernova remnants, the intergalactic and interstellar medium, the solar corona, the solar wind and the Earth's magnetosphere, just to mention a few examples. Energy dissipation of turbulent fluctuations plays a key role in plasma heating and energization, yet we still do not understand the underlying physical mechanisms involved. THOR is a mission designed to answer the questions of how turbulent plasma is heated and particles accelerated, how the dissipated energy is partitioned and how dissipation operates in different regimes of turbulence. THOR is a single-spacecraft mission with an orbit tuned to maximize data return from regions in near-Earth space - magnetosheath, shock, foreshock and pristine solar wind - featuring different kinds of turbulence. Here we summarize the THOR proposal submitted on 15 January 2015 to the 'Call for a Medium-size mission opportunity in ESAs Science Programme for a launch in 2025 (M4)'. THOR has been selected by European Space Agency (ESA) for the study phase.
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| 2. |
- Hobara, Y., et al.
(författare)
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Cluster observations of electrostatic solitary waves near the Earth's bow shock
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A5, s. A05211-
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Tidskriftsartikel (refereegranskat)abstract
- Using a period of internal burst mode data from the Cluster Electric Field and Wave instrument a number of electrostatic solitary structures have been identified in the foot region of Earth's quasi-perpendicular bow shock. The four individual probe potential measurements are utilized to investigate the fundamental characteristics of the solitary wave structures such as wave propagation vector, propagation velocity, scale-size and potential amplitude. Two classes of waves are observed. Bipolar solitary waves typically propagate in the solar wind direction toward the shock but at a significant angle from the ambient magnetic field. Unipolar/tripolar solitary waves tend to propagate along the ambient magnetic field. The wave amplitude-scale size relation is similar to that obtained for similar structures observed in the auroral zone. The structures lie in the theoretically allowed region in width-amplitude space to be consistent with the BGK ion holes. Using a period of internal burst mode data from the Cluster Electric Field and Wave instrument a number of electrostatic solitary structures have been identified in the foot region of Earth's quasi-perpendicular bow shock. The four individual probe potential measurements are utilized to investigate the fundamental characteristics of the solitary wave structures such as wave propagation vector, propagation velocity, scale-size and potential amplitude. Two classes of waves are observed. Bipolar solitary waves typically propagate in the solar wind direction toward the shock but at a significant angle to the ambient magnetic field in contrast to most previous studies which assume parallel propagation to the ambient magnetic field. In contrast, unipolar/tripolar solitary waves tend to propagate along the ambient magnetic field. The wave amplitude-scale size relation is similar to that obtained for structures observed in the auroral zone. The structures lie in the theoretically allowed region in width-amplitude space to be consistent with the BGK (Bernstein-Greene-Kruskal) ion holes. The two classes of observed solitary waves may greatly influence the ambient plasma dynamics around the shock. The bipolar solitary waves do not exhibit a large net potential difference but may still play an important role in plasma thermalisation by particle scattering. Unipolar/tripolar solitary waves exhibit a remarkable net potential difference that may be responsible for the plasma energisation along the ambient magnetic field.
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| 3. |
- Gedalin, M., et al.
(författare)
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Growth of filaments and saturation of the filamentation instability
- 2010
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 17:3
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Tidskriftsartikel (refereegranskat)abstract
- The filamentation instability of counterstreaming beams is a nonresonant hydrodynamic-type instability whose growth rate is a smooth function of the wavelength (scale). As a result, perturbations with all unstable wavelengths develop, and the growth saturates due to the saturation of available current. For a given scale, the magnetic field at saturation is proportional to the scale. As a result, the instability develops in a nearly linear regime, where the unstable modes stop growing as soon as the saturation of the corresponding wavelength is reached. At each moment there exists a dominant scale of the magnetic field which is the scale that reached saturation at this particular time. The smaller scales do not disappear and can be easily distinguished in the current structure. The overall growth of the instability stops when the loss of the streaming ion energy because of deceleration is comparable to the initial ion energy.
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| 4. |
- Gedalin, M., et al.
(författare)
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Relativistic filamentary equilibria
- 2011
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Ingår i: Journal of Plasma Physics. - : Cambridge University Press. - 0022-3778 .- 1469-7807. ; 77, s. 193-205
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Tidskriftsartikel (refereegranskat)abstract
- Plasma filamentation is often encountered in collisionless shocks and inertial confinement fusion. We develop a general analytical description of the two-dimensional relativistic filamentary equilibrium and derive the conditions for existence of potential-free equilibria. A pseudopotential equation for the vector-potential is constructed for cold and relativistic Maxwellian distributions. The role of counter-streaming is explained. We present single current sheet and periodic current sheet solutions, and analyze the equilibria with electric potential. These solutions can be used to study linear and nonlinear evolution of the relativistic filamentation instability.
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| 5. |
- Bates, I., Balikhin, M., Alleyne, H. and Andre, M.
(författare)
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Minimum-variance free determination of magnetosheath wave propagation vectors.
- 2001
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Ingår i: : Les Woolliscroft Memorial Conference Sheffield Space Plasma Meeting: Multipoint Measurements versus Theory. Proceedings (SP-492). - : ESA.
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The direction of propagation of waves in the plasma is important when studying space plasma turbulence and the linear and nonlinear processes that occur. The usual procedure of determining this propagation direction involves minimum variance analysis. Thi
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| 6. |
- Dimmock, Andrew P., et al.
(författare)
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Modeling the Geomagnetic Response to the September 2017 Space Weather Event Over Fennoscandia Using the Space Weather Modeling Framework : Studying the Impacts of Spatial Resolution
- 2021
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Ingår i: Space Weather. - : American Geophysical Union (AGU). - 1542-7390. ; 19:5
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Tidskriftsartikel (refereegranskat)abstract
- We must be able to predict and mitigate against geomagnetically induced current (GIC) effects to minimize socio-economic impacts. This study employs the space weather modeling framework (SWMF) to model the geomagnetic response over Fennoscandia to the September 7-8, 2017 event. Of key importance to this study is the effects of spatial resolution in terms of regional forecasts and improved GIC modeling results. Therefore, we ran the model at comparatively low, medium, and high spatial resolutions. The virtual magnetometers from each model run are compared with observations from the IMAGE magnetometer network across various latitudes and over regional-scales. The virtual magnetometer data from the SWMF are coupled with a local ground conductivity model which is used to calculate the geoelectric field and estimate GICs in a Finnish natural gas pipeline. This investigation has lead to several important results in which higher resolution yielded: (1) more realistic amplitudes and timings of GICs, (2) higher amplitude geomagnetic disturbances across latitudes, and (3) increased regional variations in terms of differences between stations. Despite this, substorms remain a significant challenge to surface magnetic field prediction from global magnetohydrodynamic modeling. For example, in the presence of multiple large substorms, the associated large-amplitude depressions were not captured, which caused the largest model-data deviations. The results from this work are of key importance to both modelers and space weather operators. Particularly when the goal is to obtain improved regional forecasts of geomagnetic disturbances and/or more realistic estimates of the geoelectric field.
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| 7. |
- Walker, S. N., et al.
(författare)
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Lower hybrid waves at the shock front : a reassessment
- 2008
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Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 26:3, s. 699-707
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Tidskriftsartikel (refereegranskat)abstract
- The primary process occur-ring at a collisionless shock is the redistribution of the bulk upstream energy into other degrees of freedom. One part of this process results in the acceleration of electrons at the shock front. Accelerated electrons are observed at the terrestrial and other planetary shocks, comets, and their effects are observed in astrophysical phenomena such as supernova remnants and jets in the form of X-ray bremsstrahlung radiation. One of the physical models for electron acceleration at supercritical shocks is based on low-hybrid turbulence due to the presence of reflected ions in the foot region. Since lower hybrid waves propagate almost perpendicular to the magnetic field they can be simultaneously in resonance with both the unmagnetised ions (omega=V(i)k(perpendicular to)) and magnetised electrons (omega=V(e)k(parallel to)). In this paper, Cluster observations of the electric field are used to study the occurrence of lower hybrid waves in the front of the terrestrial bow shock. It is shown that the lower hybrid waves exist as isolated wave packets. However, the very low level of the observed lower hybrid turbulence is too small to impart significant energisation to the electron population.
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| 8. |
- Boynton, R. J., et al.
(författare)
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System Identification of Local Time Electron Fluencies at Geostationary Orbit
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 125:11
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Tidskriftsartikel (refereegranskat)abstract
- The electron fluxes at geostationary orbit measured by Geostationary Operational Environmental Satellite (GOES) 13, 14, and 15 spacecraft are modeled using system identification techniques. System identification, similar to machine learning, uses input-output data to train a model, which can then be used to provide forecasts. This study employs the nonlinear autoregressive moving average exogenous technique to deduce the electron flux models. The electron fluxes at geostationary orbit are known to vary in space and time, making it a spatiotemporal system, which complicates the modeling using system identification/machine learning approach. Therefore, the electron flux data are binned into 24 magnetic local time (MLT), and a separate model is developed for each of the 24 MLT bins. MLT models are developed for six of the GOES 13, 14, and 15 electron flux energy channels (75 keV, 150 keV, 275 keV, 475 keV, >800 keV, and >2 MeV). The models are assessed on separate test data by prediction efficiency (PE) and correlation coefficient (CC) and found these to vary by MLT and electron energy. The lowest energy of 75 keV at the midnight sector had a PE of 36.0 and CC of 59.3, which increased on the dayside to a PE of 66.9 and CC of 81.6. These metrics increased to the >2 MeV model, which had a low PE and CC of 63.0 and 81.8 on the nightside to a high of 80.3 and 90.8 on the dayside.
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| 9. |
- Liemohn, Michael W., et al.
(författare)
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Model Evaluation Guidelines for Geomagnetic Index Predictions
- 2018
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Ingår i: Space Weather. - 1542-7390. ; 16:12, s. 2079-2102
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Tidskriftsartikel (refereegranskat)abstract
- Geomagnetic indices are convenient quantities that distill the complicated physics of some region or aspect of near‐Earth space into a single parameter. Most of the best‐known indices are calculated from ground‐based magnetometer data sets, such as Dst, SYM‐H, Kp, AE, AL, and PC. Many models have been created that predict the values of these indices, often using solar wind measurements upstream from Earth as the input variables to the calculation. This document reviews the current state of models that predict geomagnetic indices and the methods used to assess their ability to reproduce the target index time series. These existing methods are synthesized into a baseline collection of metrics for benchmarking a new or updated geomagnetic index prediction model. These methods fall into two categories: (1) fit performance metrics such as root‐mean‐square error and mean absolute error that are applied to a time series comparison of model output and observations and (2) event detection performance metrics such as Heidke Skill Score and probability of detection that are derived from a contingency table that compares model and observation values exceeding (or not) a threshold value. A few examples of codes being used with this set of metrics are presented, and other aspects of metrics assessment best practices, limitations, and uncertainties are discussed, including several caveats to consider when using geomagnetic indices.
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| 10. |
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