140 research outputs found
Magnetospheric Cavity Modes Driven by Solar Wind Dynamic Pressure Fluctuations
We present results from Lyon-Fedder-Mobarry (LFM) global, three-dimensional
magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere
interaction. We use these simulations to investigate the role that solar wind
dynamic pressure fluctuations play in the generation of magnetospheric
ultra-low frequency (ULF) pulsations. The simulations presented in this study
are driven with idealized solar wind input conditions. In four of the
simulations, we introduce monochromatic ULF fluctuations in the upstream solar
wind dynamic pressure. In the fifth simulation, we introduce a continuum of ULF
frequencies in the upstream solar wind dynamic pressure fluctuations. In this
numerical experiment, the idealized nature of the solar wind driving conditions
allows us to study the magnetospheric response to only a fluctuating upstream
dynamic pressure, while holding all other solar wind driving parameters
constant. The simulation results suggest that ULF fluctuations in the solar
wind dynamic pressure can drive magnetospheric ULF pulsations in the electric
and magnetic fields on the dayside. Moreover, the simulation results suggest
that when the driving frequency of the solar wind dynamic pressure fluctuations
matches one of the natural frequencies of the magnetosphere, magnetospheric
cavity modes can be energized.Comment: 2 figure
Global MHD modeling of resonant ULF waves: Simulations with and without a plasmasphere
We investigate the plasmaspheric influence on the resonant mode coupling of magnetospheric ultralow frequency (ULF) waves using the Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamic (MHD) model. We present results from two different versions of the model, both driven by the same solar wind conditions: one version that contains a plasmasphere (the LFM coupled to the Rice Convection Model, where the Gallagher plasmasphere model is also included) and another that does not (the stand-alone LFM). We find that the inclusion of a cold, dense plasmasphere has a significant impact on the nature of the simulated ULF waves. For example, the inclusion of a plasmasphere leads to a deeper (more earthward) penetration of the compressional (azimuthal) electric field fluctuations, due to a shift in the location of the wave turning points. Consequently, the locations where the compressional electric field oscillations resonantly couple their energy into local toroidal mode field line resonances also shift earthward. We also find, in both simulations, that higher-frequency compressional (azimuthal) electric field oscillations penetrate deeper than lower frequency oscillations. In addition, the compressional wave mode structure in the simulations is consistent with a radial standing wave oscillation pattern, characteristic of a resonant waveguide. The incorporation of a plasmasphere into the LFM global MHD model represents an advance in the state of the art in regard to ULF wave modeling with such simulations. We offer a brief discussion of the implications for radiation belt modeling techniques that use the electric and magnetic field outputs from global MHD simulations to drive particle dynamics
Magnetohydrodynamic Modeling of Three Van Allen Probes Storms in 2012 and 2013
Coronal mass ejection (CME)-shock compression of the dayside magnetopause has been observed to cause both prompt enhancement of radiation belt electron flux due to inward radial transport of electrons conserving their first adiabatic invariant and prompt losses which at times entirely eliminate the outer zone. Recent numerical studies suggest that enhanced ultra-low frequency (ULF) wave activity is necessary to explain electron losses deeper inside the magnetosphere than magnetopause incursion following CME-shock arrival. A combination of radial transport and magnetopause shadowing can account for losses observed at radial distances into L=4.5, well within the computed magnetopause location. We compare ULF wave power from the Electric Field and Waves (EFW) electric field instrument on the Van Allen Probes for the 8 October 2013 storm with ULF wave power simulated using the LyonâFedderâMobarry (LFM) global magnetohydrodynamic (MHD) magnetospheric simulation code coupled to the Rice Convection Model (RCM). Two other storms with strong magnetopause compression, 8â9 October 2012 and 17â18 March 2013, are also examined. We show that the global MHD model captures the azimuthal magnetosonic impulse propagation speed and amplitude observed by the Van Allen Probes which is responsible for prompt acceleration at MeV energies reported for the 8 October 2013 storm. The simulation also captures the ULF wave power in the azimuthal component of the electric field, responsible for acceleration and radial transport of electrons, at frequencies comparable to the electron drift period. This electric field impulse has been shown to explain observations in related studies (Foster et al., 2015) of electron acceleration and drift phase bunching by the Energetic Particle, Composition, and Thermal Plasma Suite (ECT) instrument on the Van Allen Probes
Effects of electrojet turbulence on a magnetosphere-ionosphere simulation of a geomagnetic storm
Ionospheric conductance plays an important role in regulating the response of the magnetosphereâionosphere system to solar wind driving. Typically, models of magnetosphereâionosphere coupling include changes to ionospheric conductance driven by extreme ultraviolet ionization and electron precipitation. This paper shows that effects driven by the FarleyâBuneman instability can also create significant enhancements in the ionospheric conductance, with substantial impacts on geospace. We have implemented a method of including electrojet turbulence (ET) effects into the ionospheric conductance model utilized within geospace simulations. Our particular implementation is tested with simulations of the LyonâFedderâMobarry global magnetosphere model coupled with the Rice Convection Model of the inner magnetosphere. We examine the impact of including ETâmodified conductances in a case study of the geomagnetic storm of 17 March 2013. Simulations with ET show a 13% reduction in the cross polar cap potential at the beginning of the storm and up to 20% increases in the Pedersen and Hall conductance. These simulation results show better agreement with Defense Meteorological Satellite Program observations, including capturing features of subauroral polarization streams. The fieldâaligned current (FAC) patterns show little differences during the peak of storm and agree well with Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) reconstructions. Typically, the simulated FAC densities are stronger and at slightly higher latitudes than shown by AMPERE. The inner magnetospheric pressures derived from TsyganenkoâSitnov empirical magnetic field model show that the inclusion of the ET effects increases the peak pressure and brings the results into better agreement with the empirical model.This material is based upon work supported by NASA grants NNX14AI13G, NNX13AF92G, and NNX16AB80G. The National Center for Atmospheric Research is sponsored by the National Science Foundation. This work used the XSEDE and TACC computational facilities, supported by National Science Foundation grant ACI-1053575. We would like to acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation. We thank the AMPERE team and the AMPERE Science Center for providing the Iridium derived data products. All model output, simulation codes, and analysis routines are being preserved on the NCAR High-Performance Storage System and will be made available upon written request to the lead author of this publication. (NNX14AI13G - NASA; NNX13AF92G - NASA; NNX16AB80G - NASA; National Science Foundation; ACI-1053575 - National Science Foundation
Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations
Low altitude field-aligned current densities ob-
tained from global magnetospheric simulations are compared
with two-dimensional distributions of Birkeland currents at
the topside ionosphere derived from magnetic field observa-
tions by the constellation of Iridium satellites. We present the
analysis of two magnetic cloud events, 17â19 August 2003
and 19â21 March 2001, where the interplanetary magnetic
field (IMF) rotates slowly (âŒ10âŠ/h) to avoid time-aliasing in
the magnetic perturbations used to calculate the Birkeland
currents. In the August 2003 event the IMF rotates from
southward to northward while maintaining a negative IMF
By during much of the interval. During the March 2001
event the IMF direction varies from dawnward to southward
to duskward. We find that the distributions of the Birkeland
current densities in the simulations agree qualitatively with
the observations for northward IMF. For southward IMF,
the dayside Region-1 currents are reproduced in the simu-
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the ionospheric grids in the simulations and the observations is shown to have only secondary effect on the magnitudes of the Birkeland currents. The electric potentials in the simu- lation for southward IMF periods are twice as large as those obtained from measurements of the plasma drift velocities by DMSP, implying that the reconnection rates in the simulation are too large.
Keywords. Ionosphere (Electric fields and currents; Ionosphere-magnetosphere interactions; Modeling and forecasting)
1 Introduction
Global magnetohydrodynamic (MHD) models are the most comprehensive numerical tool for studying the coupling of energy and momentum of the solar wind into the Earthâs magnetosphere and ionosphere. A particular advantage of global MHD simulations is the ability to provide continu- ous temporal and spatial coverage of key physical parame- ters over the entire simulation volume. For this reason, MHD simulations have become one of the principal tools for study- ing space weather events such as the interaction of the Earthâs magnetosphere with coronal mass ejections (CMEs) (Ridley et al., 2002) as well as magnetic storms (Slinker et al., 1998; Goodrich et al., 1998) and substorms (Lyon et al., 1998; Lopez et al., 1998; Wiltberger et al., 2000). Since the simula- tion results are frequently used to interpret physical processes in the magnetosphereâionosphere system, assessing their ac- curacy by comparison with observations is an important task. A number of such studies have been carried out in the past us- ing space-based (Frank et al., 1995; Raeder et al., 1997) and ground-based observations (Ridley et al., 2001), or a com- bination thereof (Fedder et al., 1998; Slinker et al., 1999). However, interpreting the discrepancies between model and observations is not straightforward because the observational
lation, but appear on average 5 served location, while the nightside Region-1 currents and the Region-2 currents are largely under-represented. Com- parison of the observed and simulated Birkeland current dis- tributions, which are intimately related to the plasma drifts at the ionosphere, shows that the ionospheric convection pat- tern in the MHD model and its dependence on the IMF ori- entation is essentially correct. The Birkeland total currents in the simulations are about a factor of 2 larger than observed during southward IMF. For Bz\u3e0 the disparity in the total current is reduced and the simulations for purely northward IMF agree with the observations to within 10%. The dispar- ities in the magnitudes of the Birkeland currents between the observations and the simulation results are a combined effect of the simulation overestimating the ionospheric electric field and of the Iridium fits underestimating the magnetic pertur- bations
A novel metric for coronal MHD models
[1] In the interest of quantitatively assessing the capabilities of coronal MHD models, we have developed a metric that compares the structures of the white light corona observed with SOHO LASCO C2 to model predictions. The MAS model is compared to C2 observations from two Carrington rotations during solar cycle 23, CR1913 and CR1984, which were near the minimum and maximum of solar activity, respectively, for three radial heights, 2.5 Râ, 3.0 Râ, and 4.5 Râ. In addition to simulated polarization brightness images, we create a synthetic image based on the field topology along the line of sight in the model. This open-closed brightness is also compared to LASCO C2 after renormalization. In general, the model\u27s magnetic structure is a closer match to observed coronal structures than the model\u27s density structure. This is expected from the simplified energy equations used in current global corona MHD models
An event study to provide validation of TING and CMIT geomagnetic middle-latitude electron densities at the F2 peak
[1] The coupled thermosphere-ionosphere magnetosphere (CMIT) model and the Thermosphere Ionosphere Nested Grid (TING) model have been run to simulate the 15 May 1997 interplanetary coronal mass ejection\u27s (ICME) effects on the Earth\u27s ionosphere and thermosphere. Comparisons were made between these model runs, the IRI-2007 model, and geomagnetic middle-latitude ionosonde data (NmF2) from the World Data Center to determine how well the models simulated the event and to understand the causes of model-data disagreement. The following conclusions were drawn from this study: (1) skill scores were more often negative than positive on average; (2) the best and the worst skill scores occurred on the recovery day; (3) the line plots comparing models to data look better than the skill scores might suggest; (4) skill scores are significantly affected by timing issues and large, short-duration variability; (5) skill scores give an indication of the relative ability of one model relative to another, rather than an absolute statement of model accuracy; (6) the models capture negative storm effects better than they capture positive storm effects; (7) the TING model captured many short duration features seen in the data at high middle latitude stations that result from changes in the size of the auroral oval; (8) CMIT overestimates the energy driving changes in NmF2, whereas TING provides approximately the correct energy input as a result of the saturation effects on potential that are included in TING; and (9) both TING and CMIT electron densities decreased too rapidly after sunset
Predicting magnetopause crossings at geosynchronous orbit during the Halloween storms
[1] In late October and early November of 2003, the Sun unleashed a powerful series of events known as the Halloween storms. The coronal mass ejections launched by the Sun produced several severe compressions of the magnetosphere that moved the magnetopause inside of geosynchronous orbit. Such events are of interest to satellite operators, and the ability to predict magnetopause crossings along a given orbit is an important space weather capability. In this paper we compare geosynchronous observations of magnetopause crossings during the Halloween storms to crossings determined from the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the magnetosphere as well to predictions of several empirical models of the magnetopause position. We calculate basic statistical information about the predictions as well as several standard skill scores. We find that the current Lyon-Fedder-Mobarry simulation of the storm provides a slightly better prediction of the magnetopause position than the empirical models we examined for the extreme conditions present in this study. While this is not surprising, given that conditions during the Halloween storms were well outside the parameter space of the empirical models, it does point out the need for physics-based models that can predict the effects of the most extreme events that are of significant interest to users of space weather forecasts
Geospace Environment Modeling 2008â2009 Challenge: Geosynchronous magnetic field
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94577/1/swe422.pd
Posteruptive phenomena in coronal mass ejections and substorms: Indicators of a universal process?
[1] We examine phenomena associated with eruptions in the two different regimes of the solar corona and the terrestrial magnetosphere. We find striking similarities between the speeds of shrinking magnetic field lines in the corona and dipolarization fronts traversing the magnetosphere. We also examine the similarities between supra-arcade downflows observed during solar flares and bursty bulk flows seen in the magnetotail and find that these phenomena have remarkably similar speeds, velocity profiles, and size scales. Thus we show manifest similarities in the magnetic reconfiguration in response to the ejection of coronal mass ejections in the corona and the ejection of plasmoids in the magnetotail. The subsequent return of loops to a quasi-potential state in the corona and field dipolarization in the magnetotail are physical analogs and trigger similar phenomena such as downflows, which provides key insights into the underlying drivers of the plasma dynamics
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