Bibliography
|
Notice:
|
Found 12 entries in the Bibliography.
Showing entries from 1 through 12
| 2020 |
|
The k-nearest-neighbor technique is used to mine a multimission magnetometer database for a subset of data points from time intervals that are similar to the storm state of the magnetosphere for a particular moment in time. These subsets of data are then used to fit an empirical magnetic field model. Performing this for each snapshot in time reconstructs the dynamic evolution of the magnetic and electric current density distributions during storms. However, because weaker storms occur more frequently than stronger storms, the reconstructions are biased toward them. We demonstrate that distance weighting the nearest-neighbors mitigates this issue while allowing a sufficient amount of data to be included in the fitting procedure to limit overfitting. Using this technique, we reconstruct the distribution of the magnetic field and electric currents and their evolution for two storms, the intense 17–19 March 2015 “Saint Patrick s Day” storm and a moderate storm occurring on 13–15 July 2013, from which the pressure distributions can be computed assuming isotropy and by integrating the steady-state force-balance equation. As the main phase of a storm progresses in time, the westward ring current density and pressure increases in the inner magnetosphere particularly on the nightside, becoming more symmetric as the recovery phase progresses. We validate the empirical pressure by comparing it to the observed pressures from the Van Allen Probes mission by summing over particle fluxes from all available energy channels and species. Stephens, G.; Bingham, S.; Sitnov, M.; Gkioulidou, M.; Merkin, V.; Korth, H.; Tsyganenko, N.; Ukhorskiy, A; Published by: Space Weather Published on: 10/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020SW002583 storms; empirical geomagnetic field; ring current; data mining; eastward current; plasma pressure; Van Allen Probes |
| 2019 |
|
Global Empirical Picture of Magnetospheric Substorms Inferred From Multimission Magnetometer Data Magnetospheric substorms represent key explosive processes in the interaction of the Earth\textquoterights magnetosphere with the solar wind, and their understanding and modeling are critical for space weather forecasting. During substorms, the magnetic field on the nightside is first stretched in the antisunward direction and then it rapidly contracts earthward bringing hot plasmas from the distant space regions into the inner magnetosphere, where they contribute to geomagnetic storms and Joule dissipation in the polar ionosphere, causing impressive splashes of aurora. Here we show for the first time that mining millions of spaceborne magnetometer data records from multiple missions allows one to reconstruct the global 3-D picture of these stretching and dipolarization processes. Stretching results in the formation of a thin (less than the Earth\textquoterights radius) and strong current sheet, which is diverted into the ionosphere during dipolarization. In the meantime, the dipolarization signal propagates further into the inner magnetosphere resulting in the accumulation of a longer lived current there, giving rise to a protogeomagnetic storm. The global 3-D structure of the corresponding substorm currents including the substorm current wedge is reconstructed from data. Stephens, G.; Sitnov, M.; Korth, H.; Tsyganenko, N.; Ohtani, S.; Gkioulidou, M.; Ukhorskiy, A; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2019 YEAR: 2019 DOI: 10.1029/2018JA025843 Current sheet thinning; Data-mining; Magnetotail dipolarization; Storm-substorm relationship; substorm current wedge; substorms; Van Allen Probes |
| 2018 |
|
Much of plasma heating and transport from the magnetotail into the inner magnetosphere occurs in the form of mesoscale discrete injections associated with sharp dipolarizations of magnetic field (dipolarization fronts). In this paper we investigate the role of magnetic trapping in acceleration and transport of the plasmasheet ions into the ring current. For this purpose we use high-resolution global MHD and three-dimensional test-particle simulations. It is shown that trapping, produced by sharp magnetic field gradients at the interface between dipolarizations and the ambient plasma, affect plasmasheet protons with energies above approximately 10 keV, enabling their transport across more than 10 Earth radii and acceleration by a factor of 10. Our estimates show that trapping is important to the buildup of the ring current plasma pressure of injected particles; depending on the plasmasheet temperature and energy spectrum, trapped protons can contribute between 20\% to 60\% of the plasma pressure. It is also shown that the acceleration process does not conserve the particle first invariant; on average protons are accelerated to higher energies compared to a purely adiabatic process. We also investigate how trapping and energization varies for deferent ions species and show that, in accordance with recent observations, ion acceleration is proportional to the ion charge and is independent of its mass. Ukhorskiy, A; Sorathia, K.; Merkin, V.; Sitnov, M.; Mitchell, D.; Gkioulidou, M.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018 YEAR: 2018 DOI: 10.1029/2018JA025370 injections; plasma pressure; ring current; trapping; Van Allen Probes |
| 2017 |
|
Ion acceleration at dipolarization fronts in the inner magnetosphere During geomagnetic storms plasma pressure in the inner magnetosphere is controlled by energetic ions of tens to hundreds of keV. Plasma pressure is the source of global storm time currents, which control the distribution of magnetic field and couple the inner magnetosphere and the ionosphere. Recent analysis showed that the buildup of hot ion population in the inner magnetosphere largely occurs in the form of localized discrete injections associated with sharp dipolarizations of magnetic field, similar to dipolarization fronts in the magnetotail. Because of significant differences between the ambient magnetic field and the dipolarization front properties in the magnetotail and the inner magnetosphere, the physical mechanisms of ion acceleration at dipolarization fronts in these two regions may also be different. In this paper we discuss a new acceleration mechanism enabled by stable trapping of ions at the azimuthally localized dipolarization fronts. It is shown that trapping can provide a robust mechanism of ion energization in the inner magnetosphere even in the absence of large electric fields. Ukhorskiy, A; Sitnov, M.; Merkin, V.; Gkioulidou, M.; Mitchell, D.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2016JA023304 |
|
Acceleration at Dipolarization Fronts in the Inner Magnetosphere During geomagnetic storms plasma pressure in the inner magnetosphere is controlled by energetic ions of tens to hundreds keV. Plasma pressure is the source of global storm-time currents, which control the distribution of magnetic field and couple the inner magnetosphere and the ionosphere. Recent analysis showed that the buildup of hot ion population in the inner magnetosphere largely occurs in the form of localized discrete injections associated with sharp dipolarizations of magnetic field, similar to dipolarization fronts in the magnetotail. Because of significant differences between the ambient magnetic field and the dipolarization front properties in the magnetotail and the inner magnetosphere, the physical mechanisms of ion acceleration at dipolarization fronts in these two regions may also be different. In this paper we discuss a new acceleration mechanism enabled by stable trapping of ions at the azimuthally localized dipolarization fronts. It is shown that trapping can provide a robust mechanism of ion energization in the inner magnetosphere even in the absence of large electric fields. Ukhorskiy, A; Sitnov, M.; Merkin, V.; Gkioulidou, M.; Mitchell, D.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017 YEAR: 2017 DOI: 10.1002/2016ja023304 |
| 2015 |
|
Global Storm-Time Depletion of the Outer Electron Belt The outer radiation belt consists of relativistic (>0.5 MeV) electrons trapped on closed trajectories around Earth where the magnetic field is nearly dipolar. During increased geomagnetic activity, electron intensities in the belt can vary by ordersof magnitude at different spatial and temporal scale. The main phase of geomagnetic storms often produces deep depletions of electron intensities over broad regions of the outer belt. Previous studies identified three possible processes that can contribute to the main-phase depletions: adiabatic inflation of electron drift orbits caused by the ring current growth, electron loss into the atmosphere, and electron escape through the magnetopause boundary. In this paper we investigate the relative importance of the adiabatic effect and magnetopause loss to the rapid depletion of the outer belt observed at the Van Allen Probes spacecraft during the main phase of March 17, 2013 storm. The intensities of >1 MeV electrons were depleted by more than an order of magnitude over the entire radial extent of the belt in less than 6 hours after the sudden storm commencement. For the analysis we used three-dimensional test-particle simulations of global evolution of the outer belt in the Tsyganenko-Sitnov (TS07D) magnetic field model with an inductive electric field. Comparison of the simulation results with electron measurements from the MagEIS experiment shows that magnetopause loss accounts for most of the observed depletion at L>5, while at lower L shells the depletion is adiabatic. Both magnetopause loss and the adiabatic effect are controlled by the change in global configuration of the magnetic field due to storm-time development of the ring current; a simulation of electron evolution without a ring current produces a much weaker depletion. Ukhorskiy, A; Sitnov, M.; Millan, R.; Kress, B.; Fennell, J.; Claudepierre, S.; Barnes, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2014JA020645 dropout; Geomagnetic storms; magnetopause loss; Radial Transport; Radiation belt; ring current; Van Allen Probes |
|
The structure of storm-time currents in the inner magnetosphere, including its innermost region inside 4RE, is studied for the first time using a modification of the empirical geomagnetic field model TS07D and new data from Van Allen Probes and THEMIS missions. It is shown that the model, which uses basis-function expansions instead of ad hoc current modules to approximate the magnetic field, consistently improves its resolution and magnetic field reconstruction with the increase of the number of basis functions and resolves the spatial structure and evolution of the innermost eastward current. This includes a connection between the westward ring current flowing largely at inline image and the eastward ring current concentrated at inline image resulting in a vortex current pattern. A similar pattern coined \textquoteleftbanana current\textquoteright was previously inferred from the pressure distributions based on the energetic neutral atom imaging and first-principles ring current simulations. The morphology of the equatorial currents is dependent on storm phase. During the main phase, it is complex, with several asymmetries forming \textquoterightbanana currents\textquoteright. Near Sym-H minimum, the \textquoterightbanana current\textquoteright is strongest, is localized in the evening-midnight sector, and is more structured compared to the main phase. It then weakens during the recovery phase resulting in the equatorial currents to become mostly azimuthally symmetric. Stephens, G.; Sitnov, M.; Ukhorskiy, A; Roelof, E.; Tsyganenko, N.; Le, G.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2015 YEAR: 2015 DOI: 10.1002/2015JA021700 eastward current; empirical geomagnetic field; magnetic storm; ring current; Van Allen Probes |
| 2014 |
|
Rotationally driven zebra stripes in Earth s inner radiation belt Structured features on top of nominally smooth distributions of radiation-belt particles at Earth have been previously associated with particle acceleration and transport mechanisms powered exclusively by enhanced solar-wind activity1, 2, 3, 4. Although planetary rotation is considered to be important for particle acceleration at Jupiter and Saturn5, 6, 7, 8, 9, the electric field produced in the inner magnetosphere by Earth\textquoterights rotation can change the velocity of trapped particles by only about 1\textendash2 kilometres per second, so rotation has been thought inconsequential for radiation-belt electrons with velocities of about 100,000 kilometres per second. Here we report that the distributions of energetic electrons across the entire spatial extent of Earth\textquoterights inner radiation belt are organized in regular, highly structured and unexpected \textquoteleftzebra stripes\textquoteright, even when the solar-wind activity is low. Modelling reveals that the patterns are produced by Earth\textquoterights rotation. Radiation-belt electrons are trapped in Earth\textquoterights dipole-like magnetic field, where they undergo slow longitudinal drift motion around the planet because of the gradient and curvature of the magnetic field. Earth\textquoterights rotation induces global diurnal variations of magnetic and electric fields that resonantly interact with electrons whose drift period is close to 24 hours, modifying electron fluxes over a broad energy range into regular patterns composed of multiple stripes extending over the entire span of the inner radiation belt. Ukhorskiy, A; Sitnov, M.; Mitchell, D.; Takahashi, K; Lanzerotti, L.; Mauk, B.; Published by: Nature Published on: 01/2014 YEAR: 2014 DOI: 10.1038/nature13046 |
| 2013 |
|
Dynamics of Radiation Belt Particles This paper reviews basic concepts of particle dynamics underlying theoretical aspect of radiation belt modeling and data analysis. We outline the theory of adiabatic invariants of quasiperiodic Hamiltonian systems and derive the invariants of particle motion trapped in the radiation belts. We discuss how the nonlinearity of resonant interaction of particles with small-amplitude plasma waves, ubiquitous across the inner magnetosphere, can make particle motion stochastic. Long-term evolution of a stochastic system can be described by the Fokker-Plank (diffusion) equation. We derive the kinetic equation of particle diffusion in the invariant space and discuss its limitations and associated challenges which need to be addressed in forthcoming radiation belt models and data analysis. Published by: Space Science Reviews Published on: 11/2013 YEAR: 2013 DOI: 10.1007/s11214-012-9938-5 |
|
Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) The Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on the two Van Allen Probes spacecraft is the magnetosphere ring current instrument that will provide data for answering the three over-arching questions for the Van Allen Probes Program: RBSPICE will determine \textquotedbllefthow space weather creates the storm-time ring current around Earth, how that ring current supplies and supports the creation of the radiation belt populations,\textquotedblright and how the ring current is involved in radiation belt losses. RBSPICE is a time-of-flight versus total energy instrument that measures ions over the energy range from \~20 keV to \~1 MeV. RBSPICE will also measure electrons over the energy range \~25 keV to \~1 MeV in order to provide instrument background information in the radiation belts. A description of the instrument and its data products are provided in this chapter. Mitchell, D.; Lanzerotti, L.; Kim, C.; Stokes, M.; Ho, G.; Cooper, S.; UKHORSKIY, A; Manweiler, J.; Jaskulek, S.; Haggerty, D.; Brandt, P.; SITNOV, M; Keika, K.; Hayes, J.; Brown, L.; Gurnee, R.; Hutcheson, J.; Nelson, K.; Paschalidis, N.; Rossano, E.; Kerem, S.; Published by: Space Science Reviews Published on: 11/2013 YEAR: 2013 DOI: 10.1007/s11214-013-9965-x |
|
Rapid acceleration of protons upstream of earthward propagating dipolarization fronts [1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (Bz), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in Bz. We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it. Ukhorskiy, A; Sitnov, M.; Merkin, V.; Artemyev, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2013 YEAR: 2013 DOI: 10.1002/jgra.50452 |
| 2008 |
|
Radial transport in the outer radiation belt due to global magnetospheric compressions Earth\textquoterights outer radiation belt is populated by relativistic electrons that produce a complex dynamical response to varying geomagnetic activity. One fundamental process defining global state of the belt is radial transport of electrons across their drift shells. Radial transport is induced by resonant interaction of electron drift motion with ULF oscillations of electric and magnetic fields and is commonly believed to be a diffusive process. The goal of this paper is the analysis of radial transport due to typical ULF fluctuations in the inner magnetospheric fields. For this purpose a test-particle approach is used in the guiding center approximation. In particular we consider ULF oscillations due to global magnetospheric compressions. It is shown that typical pressure variations induce large-scale fluctuations in magnetic and inductive electric fields that produce a substantial impact on relativistic electrons. Electron motion becomes stochastic due to overlap of electron populations trapped in the vicinities of drift resonances with adjacent harmonics of the field spectrum. It is shown that in spite of the underlying stochasticity the radial diffusion limit is not fully attainable in the outer radiation belt. This is attributed to the fact that phase correlations in electron motion do not have time to decay due to finite size of the system. As a result collective motion of the outer belt electrons can exhibit large deviations from radial diffusion. Solution of the full Liouville\textquoterights equation is required for accurate description of radial transport in the belt. Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 11/2008 YEAR: 2008 DOI: 10.1016/j.jastp.2008.07.018 |
1