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

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, 17–19 August 2003 and 19–21 March 2001, where the interplanetary magnetic field (IMF) rotates slowly (∼10◦/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- ◦ 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 Earth’s 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 Earth’s 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 magnetosphere–ionosphere 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 R⊙, 3.0 R⊙, and 4.5 R⊙. 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

Geotail and LFM comparisons of plasma sheet climatology: 2. Flow variability

[1] We characterize the variability of central plasma sheet bulk flows with a 6-year Geotail data set and a 2-month Lyon-Fedder-Mobarry (LFM) global MHD simulation at two spatial resolutions. Comparing long databases of observed and simulated parameters enable rigorous statistical tests of the model\u27s ability to predict plasma sheet properties during routine driving conditions and represent a new method of global MHD validation. In this study, we use probability density functions (PDFs) to compare the statistics of plasma sheet velocities in the Geotail observations with those in the LFM simulations. We find that the low-resolution model grossly underestimates the occurrence of fast earthward and tailward flows. Increasing the simulation resolution inherently changes plasma sheet mass transport in the model, allowing the development of fast, bursty flows. These flows fill out the wings of the velocity distribution and bring the PDF into closer agreement with observations

Geotail and LFM comparisons of plasma sheet climatology: 1. Average values

[1] We compare the statistics of central plasma sheet properties from 6 years of Geotail observations with 2 months of Lyon-Fedder-Mobarry (LFM) global MHD simulations. This statistical validation effort represents an inherently new method of systematically characterizing and quantifying global MHD model performance. For our comparison, we identify the central plasma sheet in the observations and simulation by identical criteria and select the simulation interval to ensure statistically similar distributions of solar wind conditions in both studies. After verifying our plasma sheet selection by inspecting the magnetic signatures of both studies, we compare the resultant number densities, thermal pressures, thermal energies, and bulk flows as functions of position across the equatorial plane. We find that the LFM model successfully reproduces the gross features of the global plasma sheet in a statistical sense. However, our comparison also reveals certain systematic discrepancies between the model and the observations. The LFM predicts a plasma sheet which is too dense, too cool, and exhibits faster globally averaged bulk flows than the observed plasma sheet. By quantifying the LFM overestimate of ionospheric transpolar potential and showing that ΦPC correlates with plasma sheet flow speed, we demonstrate that 15% of the plasma sheet velocity discrepancy is reflected in a ΦPC overestimate. This statistical validation effort represents an essential first step toward the rigorous, quantitative evaluation of a global MHD model in the plasma sheet

Comparison of predictive estimates of high‐latitude electrodynamics with observations of global‐scale Birkeland currents

Two of the geomagnetic storms for the Space Weather Prediction Center Geospace Environment Modeling challenge occurred after data were first acquired by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We compare Birkeland currents from AMPERE with predictions from four models for the 4–5 April 2010 and 5–6 August 2011 storms. The four models are the Weimer (2005b) field‐aligned current statistical model, the Lyon‐Fedder‐Mobarry magnetohydrodynamic (MHD) simulation, the Open Global Geospace Circulation Model MHD simulation, and the Space Weather Modeling Framework MHD simulation. The MHD simulations were run as described in Pulkkinen et al. (2013) and the results obtained from the Community Coordinated Modeling Center. The total radial Birkeland current, ITotal, and the distribution of radial current density, Jr, for all models are compared with AMPERE results. While the total currents are well correlated, the quantitative agreement varies considerably. The Jr distributions reveal discrepancies between the models and observations related to the latitude distribution, morphologies, and lack of nightside current systems in the models. The results motivate enhancing the simulations first by increasing the simulation resolution and then by examining the relative merits of implementing more sophisticated ionospheric conductance models, including ionospheric outflows or other omitted physical processes. Some aspects of the system, including substorm timing and location, may remain challenging to simulate, implying a continuing need for real‐time specification.Key PointsPresents the first comparison between observed field‐aligned currents and models previously evaluated for space weather operational useThe model and observed integrated currents are well correlated, but the ratio between them ranges from 1/3 to 3The 2‐D current densities are weakly correlated with observations implying significant areas for improvements in the modelsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136469/1/swe20415_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136469/2/swe20415.pd

Propagation of Pi2 pulsations through the braking region in global MHD simulations

We investigate the propagation of Pi2 period pulsations from their origin in the plasma sheet through the braking region, the region where the fast flows are slowed as they approach the inner edge of the plasma sheet. Our approach is to use both the University of California, Los Angeles (UCLA) and Lyonâ Fedderâ Mobarry (LFM) global magnetohydrodynamic (MHD) computer codes to simulate the Earth’s magnetosphere during a substorm that occurred on 14 September 2004 when Pi2 pulsations were observed. We use two different MHD models in order to test the robustness of our conclusions about Pi2. The simulations are then compared with groundâ based and satellite data. We find that the propagation of the pulsations in the simulations, especially through the braking region, depends strongly on the ionospheric models used at the inner boundary of the MHD models. With respect to typical observed values, the modeled conductances are high in the UCLA model and low in the LFM model. The different conductances affect the flows, producing stronger line tying that slows the flow in the braking region more in the UCLA model than in the LFM model. Therefore, perturbations are able to propagate much more freely into the inner magnetosphere in the LFM results. However, in both models Pi2 period perturbations travel with the dipolarization front (DF) that forms at the earthward edge of the flow channel, but as the DF slows in the braking region, â 8â ¤xâ ¤â 6 RE, the Pi2 period perturbations begin to travel ahead of it into the inner magnetosphere. This indicates that the flow channels generate compressional waves with periods that fall within the Pi2 range and that, as the flows themselves are stopped in the braking region, the compressional wave continues to propagate into the inner magnetosphere.Key PointsPi2 travel with DFs until the DFs slow in the braking region and the Pi2 begin to run ahead of themIonospheric parameters strongly affect how Pi2 are able to propagateBoth the UCLA and LFM models show similar results for Pi2 propagation outside of â ¼â 7 REPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134917/1/jgra52317_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134917/2/jgra52317.pd