L. Rastätter

Geospace Environment Modeling 2008–2009 Challenge: Ground magnetic field perturbations

helps the users of the modeling products to better understand the capabilities of the models and to choose the approach that best suits their specific needs. Further, metrics!based analyses are important for addressing the differences between various modeling approaches and for measuring and guiding the progress in the field. In this paper, the metrics!based results of the ground magnetic field perturbation part of the Geospace Environment Modeling 2008‐2009 Challenge are reported. Predictions made by 14 different models, including an ensemble model, are compared to geomagnetic observatory recordings from 12 different northern hemispheric locations. Five different metrics are used to quantify the model performances for four storm events. It is shown that the ranking of the models is strongly dependent on the type of metric used to evaluate the model performance. None of the models rank near or at the top systematically for all used metrics. Consequently, one cannot pick the absolute“winner”: the choice for the best model depends on the characteristics of the signal one is interested in. Model performances vary also from event to event. This is particularly clear for root!mean!square difference and utility metric!based analyses. Further, analyses indicate that for some of the models, increasing the global magnetohydrodynamic model spatial resolution and the inclusion of the ring current dynamics improve the models’capability to generate more realistic ground magnetic field fluctuations.

Global MHD modeling of the impact of a solar wind pressure change

[1] A sudden increase in the solar wind dynamic pressure compresses the magnetosphere and launches compressional waves into the magnetosphere. The global response of the magnetosphere, including the ionosphere and the location of the field-aligned current (FAC) generation, to a step increase in the solar wind density has been studied using a global three-dimensional adaptive MHD model. As the density increase propagated along the flanks of the magnetopause, a two-phased response was seen in the ionosphere. The first response was an increase in FACs near the polar cap. For this response we found the location of FACs to lie just inside the magnetosphere. The second response was an increase in FACs at lower latitudes. The increase in FACs was in the same direction as region 1 currents. For the second response we found the location of FACs to fall well within the magnetosphere.

CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for systematic assessment of ionosphere/thermosphere models: NmF2, hmF2, and vertical drift using ground‐based observations

[1] Objective quantification of model performance based on metrics helps us evaluate the current state of space physics modeling capability, address differences among various modeling approaches, and track model improvements over time. The Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Electrodynamics Thermosphere Ionosphere (ETI) Challenge was initiated in 2009 to assess accuracy of various ionosphere/thermosphere models in reproducing ionosphere and thermosphere parameters. A total of nine events and five physical parameters were selected to compare between model outputs and observations. The nine events included two strong and one moderate geomagnetic storm events from GEM Challenge events and three moderate storms and three quiet periods from the first half of the International Polar Year (IPY) campaign, which lasted for 2 years, from March 2007 to March 2009. The five physical parameters selected were NmF2 and hmF2 from ISRs and LEO satellites such as CHAMP and COSMIC, vertical drifts at Jicamarca, and electron and neutral densities along the track of the CHAMP satellite. For this study, four different metrics and up to 10 models were used. In this paper, we focus on preliminary results of the study using ground-based measurements, which include NmF2 and hmF2 from Incoherent Scatter Radars (ISRs), and vertical drifts at Jicamarca. The results show that the model performance strongly depends on the type of metrics used, and thus no model is ranked top for all used metrics. The analysis further indicates that performance of the model also varies with latitude and geomagnetic activity level.

Hall‐MHD modeling of near‐Earth magnetotail current sheet thinning and evolution

The formation of thin current sheets is a common feature of the late substorm growth phase in the magnetotail of the Earth. The location and the structure of the evolving thin current sheet determine the onset and the dynamic phase of magnetic substorms. To determine the formation and further evolution of thin current sheets in the near-Earth magnetotail, we employ a two-fluid model of electrons and ions, the Hall-MHD. We start from a two-dimensional tail-equilibrium model and apply a plasma inflow from the lobes to simulate the presubstorm loading process of the tail. Our results confirm recent 2.5-dimensional particle and hybrid simulations which have shown thin current sheet formation, with the majority of the new current supported by the electrons. The Hall-MHD simulations are extended to a current sheet thinning model with three-dimensional driving conditions and different magnetic field boundary conditions. We investigated the changes to the tail magnetic and current structure brought about by the cross-tail magnetic field components generated by Hall electric fields. For all these cases, the additional current is carried by the electrons, with very small effects of magnetic field and driving boundary conditions.

Assessing the performance of community‐available global MHD models using key system parameters and empirical relationships

Global magnetohydrodynamic (MHD) modeling is a powerful tool in space weather research and predictions. There are several advanced and still developing global MHD (GMHD) models that are publicly available via Community Coordinated Modeling Centers (CCMC) Run on Request system, which allows the users to simulate the magnetospheric response to different solar wind conditions including extraordinary events, like geomagnetic storms. Systematic validation of GMHD models against observations still continues to be a challenge, as well as comparative benchmarking of different models against each other. In this paper we describe and test a new approach in which (i) a set of critical large-scale system parameters is explored/tested, which are produced by (ii) specially designed set of computer runs to simulate realistic statistical distributions of critical solar wind parameters and are compared to (iii) observation-based empirical relationships for these parameters. Being tested in approximately similar conditions (similar inputs, comparable grid resolution, etc.), the four models publicly available at the CCMC predict rather well the absolute values and variations of those key parameters (magnetospheric size, magnetic field, and pressure) which are directly related to the large-scale magnetospheric equilibrium in the outer magnetosphere, for which the MHD is supposed to be a valid approach. At the same time, the models have systematic differences in other parameters, being especially different in predicting the global convection rate, total field-aligned current, and magnetic flux loading into the magnetotail after the north-south interplanetary magnetic field turning. According to validation results, none of the models emerges as an absolute leader. The new approach suggested for the evaluation of the models performance against reality may be used by model users while planning their investigations, as well as by model developers and those interesting to quantitatively evaluate progress in magnetospheric modeling.

Buildup of the ring current during periodic loading‐unloading cycles in the magnetotail driven by steady southward interplanetary magnetic field

During prolonged intervals of negative interplanetary magnetic field (IMF) B z the magnetosphere often enters a state in which quasi-periodic, large-amplitude oscillations of energetic particle fluxes are observed at the geosynchronous orbit. We use the global magnetosphere MHD code BATS-R-US output during a long period of steady southward IMF B z to drive the Fok Ring Current Model. Previous simulations of such events demonstrated flat behavior of the energetic particle fluxes after the initial injection. Periodical north/south IMF turning was required to reproduce oscillations in particle fluxes at geosynchronous orbit. In the present study we use a global magnetosphere MHD code that reproduces fast magnetotail reconnection rates observed in kinetic simulations. This results in periodical loading-unloading cycles in the magnetotail even for steady southward B z and can explain quasi-periodic oscillations of geosynchronous energetic particle fluxes. The total proton energy within geosynchronous orbit exhibits overall growth in time due to quasi-steady convection and oscillates due to injection through inductive electric field caused by multiple dipolarization. The flux oscillation amplitude is stronger in- the outer regions of the ring current although the regions close to the geosynchronous orbit experience substantial perturbations as well.

CalcDeltaB: An efficient postprocessing tool to calculate ground‐level magnetic perturbations from global magnetosphere simulations

Ground magnetic field variations can induce electric currents on long conductor systems such as high-voltage power transmission systems. The extra electric currents can interfere with normal operation of these conductor systems; and thus, there is a great need for better specification and prediction of the field perturbations. In this publication we present CalcDeltaB, an efficient postprocessing tool to calculate magnetic perturbations ΔB at any position on the ground from snapshots of the current systems that are being produced by first-principle models of the global magnetosphere-ionosphere system. This tool was developed during the recent “dB/dt” modeling challenge at the Community Coordinated Modeling Center that compared magnetic perturbations and their derivative with observational results. The calculation tool is separate from each of the magnetosphere models and ensures that the ΔB computation method is uniformly applied, and that validation studies using ΔB compare the performance of the models rather than the combination of each model and a built-in ΔB computation tool that may exist. Using the tool, magnetic perturbations on the ground are calculated from currents in the magnetosphere, from field-aligned currents between magnetosphere and ionosphere, and the Hall and Pedersen currents in the ionosphere. The results of the new postprocessing tool are compared with ΔB calculations within the Space Weather Modeling Framework model and are in excellent agreement. We find that a radial resolution of 1/30RE is fine enough to represent the contribution to ΔB from the region of field-aligned currents.

GEM‐CEDAR challenge: Poynting flux at DMSP and modeled Joule heat

Poynting flux into the ionosphere measures the electromagnetic energy coming from the magnetosphere. This energy flux can vary greatly between quiet times and geomagnetic active times. As part of the Geospace Environment Modeling-coupling energetics and dynamics of atmospheric regions modeling challenge, physics-based models of the 3-D ionosphere and ionospheric electrodynamics solvers of magnetosphere models that specify Joule heat and empirical models specifying Poynting flux were run for six geomagnetic storm events of varying intensity. We compared model results with Poynting flux values along the DMSP-15 satellite track computed from ion drift meter and magnetic field observations. Although being a different quantity, Joule heat can in practice be correlated to incoming Poynting flux because the energy is dissipated primarily in high latitudes where Poynting flux is being deposited. Within the physics-based model group, we find mixed results with some models overestimating Joule heat and some models agreeing better with observed Poynting flux rates as integrated over auroral passes. In contrast, empirical models tend to underestimate integrated Poynting flux values. Modeled Joule heat or Poynting flux patterns often resemble the observed Poynting flux patterns on a large scale, but amplitudes can differ by a factor of 2 or larger due to the highly localized nature of observed Poynting flux deposition that is not captured by the models. In addition, the positioning of modeled patterns appear to be randomly shifted against the observed Poynting flux energy input. This study is the first to compare Poynting flux and Joule heat in a large variety of models of the ionosphere.

Magnetosheath variations during the storm main phase on 20 November 2003: Evidence for solar wind density control of energy transfer to the magnetosphere

[1] Energy transfer from the solar wind into the magnetosphere and ionosphere is controlled by the southward magnetic field in the magnetosheath which under normal high Mach number conditions is about four times the solar wind southward field. In a low Mach number regime, however, the magnetosheath compression is diminished by a low solar wind density when the magnetic field remains steady. When magnetic clouds with extremely strong magnetic field cause severe geomagnetic storms under such low Mach number conditions, the density control of the energy transfer is expected to be important in understanding ring current evolution. Here we show evidence for such a density effect using in-situ observation by the GOES and Cluster spacecraft in the magnetosheath during the main phase of the super storm on 20 November 2003. Results from a global magnetohydrodynamic (MHD) simulation with an embedded ring current model also support this density effect. Citation: Kataoka, R., D. H. Fairfield, D. G. Sibeck,

Modeling and analysis of solar wind generated contributions to the near-Earth magnetic field

Solar wind generated magnetic disturbances are currently one of the major obstacles for improving the accuracy in the determination of the magnetic field due to sources internal to the Earth. In the present study a global MHD model of solar wind magnetosphere interaction is used to obtain a physically consistent, divergence-free model of ionospheric, field-aligned and magnetospheric currents in a realistic magnetospheric geometry. The magnetic field near the Earth due to these currents is analyzed by estimating and comparing the contributions from the various parts of the system, with the aim of identifying the most important aspects of the solar wind disturbances in an internal field modeling context. The contribution from the distant magnetospheric currents is found to consist of two, mainly opposing, contributions from respectively the dayside magnetopause currents and the cross-tail current. At high latitudes the field-aligned component is of partidular interest in connection with internal field-modelling. In the altitude regime of 400–800 km (typical for low Earth orbit satellites) the ionospheric currents are found to contribute significantly to the disturbancance, and account for more than 90% of the field-aligned disturbance. The magnetic disturbance field from field-aligned currents (FACs) is basically transverse to the main field, and they therefore contribute with less than 2% to the disturbance in total field intensity. Inhomogeneity in ionospheric conductance is identified as the main cause of main-field disturbance in the field-aligned direction. These disturbances are associated with the ionospheric Pedersen currents, and may introduce systematic errors in internal field models.