Space Weather Modeling Framework Research Articles

Adaptive numerical algorithms in space weather modeling

Space weather describes the various processes in the Sun–Earth system that present danger to human health and technology. The goal of space weather forecasting is to provide an opportunity to mitigate these negative effects. Physics-based space weather modeling is characterized by disparate temporal and spatial scales as well as by different relevant physics in different domains. A multi-physics system can be modeled by a software framework comprising several components. Each component corresponds to a physics domain, and each component is represented by one or more numerical models. The publicly available Space Weather Modeling Framework (SWMF) can execute and couple together several components distributed over a parallel machine in a flexible and efficient manner. The framework also allows resolving disparate spatial and temporal scales with independent spatial and temporal discretizations in the various models. Several of the computationally most expensive domains of the framework are modeled by the Block-Adaptive Tree Solarwind Roe-type Upwind Scheme (BATS-R-US) code that can solve various forms of the magnetohydrodynamic (MHD) equations, including Hall, semi-relativistic, multi-species and multi-fluid MHD, anisotropic pressure, radiative transport and heat conduction. Modeling disparate scales within BATS-R-US is achieved by a block-adaptive mesh both in Cartesian and generalized coordinates. Most recently we have created a new core for BATS-R-US: the Block-Adaptive Tree Library (BATL) that provides a general toolkit for creating, load balancing and message passing in a 1, 2 or 3 dimensional block-adaptive grid. We describe the algorithms of BATL and demonstrate its efficiency and scaling properties for various problems. BATS-R-US uses several time-integration schemes to address multiple time-scales: explicit time stepping with fixed or local time steps, partially steady-state evolution, point-implicit, semi-implicit, explicit/implicit, and fully implicit numerical schemes. Depending on the application, we find that different time stepping methods are optimal. Several of the time integration schemes exploit the block-based granularity of the grid structure. The framework and the adaptive algorithms enable physics-based space weather modeling and even short-term forecasting.

Downstream structure and evolution of a simulated CME-driven sheath in the solar corona

Context. The transition of the magnetic field from the ambient magnetic field to the ejecta in the sheath downstream of a coronal mass ejection (CME) driven shock is analyzed in detail. The field rotation in the sheath occurs in a two-layer structure. In the first layer, layer 1, the magnetic field rotates in the coplanarity plane (plane of shock normal and the upstream magnetic field), and in layer 2 rotates off this plane. We investigate the evolution of the two layers as the sheath evolves away from the Sun. Aims. In situ observations have shown that the magnetic field in the sheath region in front of an interplanetary coronal mass ejection (ICME) form a planar magnetic structure, and the magnetic field lines drape around the flux tube. Our objective is to investigate the magnetic configuration of the CME near the sun. Methods. We used the space weather modeling framework (SWMF), a 3D magnetohydrodynamics (MHD) simulation code, to simulate the propagation of CMEs and the shock driven by it. Results. We find that close to the Sun, layer 2 dominates the width of the sheath, diminishing its importance as the sheath evolves away from the Sun, in agreement with observations at 1 AU.

Self‐consistent inner magnetosphere simulation driven by a global MHD model

We present results from a one‐way coupling between the kinetic Ring Current Atmosphere Interactions Model with Self‐Consistent B field (RAM‐SCB) and the Space Weather Modeling Framework (SWMF). RAM‐SCB obtains plasma distribution and magnetic field at model boundaries from the Block Adaptive Tree Solar Wind Roe Upwind Scheme (BATS‐R‐US) magnetohydrodynamics (MHD) model and convection potentials from the Ridley Ionosphere Model within SWMF. We simulate the large geomagnetic storm of 31 August 2005 (minimum SYM‐H of −116 nT). Comparing SWMF output with Los Alamos National Laboratory geostationary satellite data, we find SWMF plasma to be too cold and dense if assumed to consist only of protons; this problem is alleviated if heavier ions are considered. With SWMF inputs, we find that RAM‐SCB reproduces well storm time magnetosphere features: ring current morphology, dusk side peak, pitch angle anisotropy, and total energy. The RAM‐SCB ring current and Dst are stronger than the SWMF ones and reproduce observations much better. The calculated field‐aligned currents (FAC) compare reasonably well with 2 h averaged pictures from Iridium satellite data. As the ring current peak rotates duskward in the storm main phase, the region 2 FACs rotate toward noon, a feature also seen in observations. Finally, the RAM‐SCB magnetic field outperforms both the dipole and the BATS‐R‐US field at Cluster and Polar spacecraft locations. This study shows the importance of a kinetic self‐consistent approach and the sensitive dependence of the storm time inner magnetosphere on plasma sheet conditions and the cross polar cap potential. The study showcases the RAM‐SCB capability as an inner magnetosphere module coupled with a global MHD model.

A DATA-DRIVEN, TWO-TEMPERATURE SOLAR WIND MODEL WITH ALFVÉN WAVES

We have developed a new three-dimensional magnetohydrodynamic (MHD) solar wind model coupled to the Space Weather Modeling Framework (SWMF) that solves for the different electron and proton temperatures. The collisions between the electrons and protons are taken into account as well as the anisotropic thermal heat conduction of the electrons. The solar wind is assumed to be accelerated by the Alfvén waves. In this paper, we do not consider the heating of closed magnetic loops and helmet streamers but do address the heating of the protons by the Kolmogorov dissipation of the Alfvén waves in open field-line regions. The inner boundary conditions for this solar wind model are obtained from observations and an empirical model. The Wang–Sheeley–Arge model is used to determine the Alfvén wave energy density at the inner boundary. The electron density and temperature at the inner boundary are obtained from the differential emission measure tomography applied to the extreme-ultraviolet images of the STEREO A and B spacecraft. This new solar wind model is validated for solar minimum Carrington rotation 2077 (2008 November 20 through December 17). Due to the very low activity during this rotation, this time period is suitable for comparing the simulated corotating interaction regions (CIRs) with in situ ACE/WIND data. Although we do not capture all MHD variables perfectly, we do find that the time of occurrence and the density of CIRs are better predicted than by our previous semi-empirical wind model in the SWMF that was based on a spatially reduced adiabatic index to account for the plasma heating.

Comparison of the open‐closed separatrix in a global magnetospheric simulation with observations: The role of the ring current

The development of global magnetospheric models, such as Space Weather Modeling Framework (SWMF), which can accurately reproduce and track space weather processes has high practical utility. We present an interval on 5 June 1998, where the location of the polar cap boundary, or open‐closed field line boundary (OCB), can be determined in the ionosphere using a combination of instruments during a period encompassing a sharp northward to southward interplanetary field turning. We present both point‐ and time‐varying comparisons of the observed and simulated boundaries in the ionosphere and find that when using solely the coupled ideal magnetohydrodynamic magnetosphere‐ionosphere model, the rate of change of the OCB to a southward turning of the interplanetary field is significantly faster than that computed from the observational data. However, when the inner magnetospheric module is incorporated, the modeling framework both qualitatively, and often quantitatively, reproduces many elements of the studied interval prior to an observed substorm onset. This result demonstrates that the physics of the inner magnetosphere is critical in shaping the boundary between open and closed field lines during periods of southward interplanetary magnetic field (IMF) and provides significant insight into the 3‐D time‐dependent behavior of the Earth's magnetosphere in response to a northward‐southward IMF turning. We assert that during periods that do not include the tens of minutes surrounding substorm expansion phase onset, the coupled SWMF model may provide a valuable and reliable tool for estimating both the OCB and magnetic field topology over a wide range of latitudes and local times.

TOWARD A REALISTIC THERMODYNAMIC MAGNETOHYDRODYNAMIC MODEL OF THE GLOBAL SOLAR CORONA

In this work we describe our implementation of a thermodynamic energy equation into the global corona model of the Space Weather Modeling Framework (SWMF), and its development into the new Lower Corona (LC) model. This work includes the integration of the additional energy transport terms of coronal heating, electron heat conduction, and optically thin radiative cooling into the governing magnetohydrodynamic (MHD) energy equation. We examine two different boundary conditions using this model; one set in the upper transition region (the Radiative Energy Balance model), as well as a uniform chromospheric condition where the transition region can be modeled in its entirety. Via observation synthesis from model results and the subsequent comparison to full sun extreme ultraviolet (EUV) and soft X-Ray observations of Carrington Rotation (CR) 1913 centered on Aug 27, 1996, we demonstrate the need for these additional considerations when using global MHD models to describe the unique conditions in the low corona. Through multiple simulations we examine ability of the LC model to asses and discriminate between coronal heating models, and find that a relative simple empirical heating model is adequate in reproducing structures observed in the low corona. We show that the interplay between coronal heating and electron heat conduction provides significant feedback onto the 3D magnetic topology in the low corona as compared to a potential field extrapolation, and that this feedback is largely dependent on the amount of mechanical energy introduced into the corona.

Validation of SWMF magnetic field and plasma

[1] The Space Weather Modeling Framework (SWMF), using the coupled BATSRUS model, the Rice Convection Model, and an ionosphere electrodynamics solver, is used to simulate 10 space weather events. The simulations are completed in near real time using a limited amount of computational resources. Satellite-specific magnetic field, plasma energy-density spectra, and integrated particle density and temperature are compared against in situ measurements to validate the coupled system. Results are examined both qualitatively and quantitatively to assess model performance. The results show that the system is adept at reproducing large-scale magnetic field variations and predicts plasma temperature and density at values near to the measured mean. Smaller-scale features, such as dipolarizations of the magnetic field at geosynchronous locations, are not reproduced well. Lack of cold particle sources in the inner magnetosphere causes the particle results to suffer. This work is part of a larger effort to thoroughly validate the SWMF.

Distortions of the magnetic field by storm-time current systems in Earth's magnetosphere

Abstract. Magnetic field and current system changes in Earth's inner magnetosphere during storm times are studied using two principally different modeling approaches: on one hand, the event-oriented empirical magnetic field model, and, on the other, the Space Weather Modeling Framework (SWMF) built around a global MHD simulation. Two storm events, one moderate storm on 6–7 November 1997 with Dst minimum about −120 nT and one intense storm on 21–23 October 1999 with Dst minimum about −250 nT were modeled. Both modeling approaches predicted a large ring current (first partial, later symmetric) contribution to the magnetic field perturbation for the intense storm. For the moderate storm, the tail current plays a dominant role in the event-oriented model results, while the SWMF results showed no strong tail current in the main phase, which resulted in a poorly timed storm peak relative to the observations. These results imply that the the development of a ring current depends on a strong force to inject the particles deep into the inner magnetosphere, and that the tail current is an important external source for the distortions of the inner magnetospheric magnetic field for both storms. Neither modeling approach was able to reproduce all the variations in the Bx and By components observed at geostationary orbit by GOES satellites during these two storms: the magnetopause current intensifications are inadequate, and the field-aligned currents are not sufficiently represented. While the event-oriented model reproduces rather well the Bz component at geostationary orbit, including the substorm-associated changes, the SWMF field is too dipolar at these locations. The empirical model is a useful tool for validation of the first-principle based models such as the SWMF.

The effect of smoothed solar wind inputs on global modeling results

This study investigates the role of fluctuations in the solar wind parameters in triggering a magnetic storm and assesses the storm simulation ability of the Space Weather Modeling Framework (SWMF) through a model–data comparison. The event of 22 September 1999 is examined through global magnetosphere simulations, using as input Advanced Composition Explorer (ACE) observations (4 min temporal resolution) along with running averages of this data with windows of 60, 120, and 180 min. It is noted that for this storm the model produces a two phase, fast then slow recovery phase due to a sudden drop in plasma sheet density during the interval of southward interplanetary magnetic field (IMF). Also, smoothing the input with a window larger than 60 min changes the entire magnetosphere and reduces the plasma sheet density and pressure, therefore a less intense storm develops. It is worth mentioning that the main phase (measured from Storm Sudden Commencement to minimum Dst) for this magnetic storm lasted about 3 h. This explains the change in the Dst profile for the 120 and 180 min averaged input. Averaging only IMF Bz or solar wind density reveals that all input parameters are important for the development of the storm, but Bz is the most significant. Also, comparison with Dst predictions (using the formula of O'Brien and McPherron (2000)) are presented and discussed. For all cases studied, there are no significant differences for Cross Polar Cap Potential (CPCP) in both hemispheres, while the nightside plasma sheet density shows a sharp drop when the input is averaged over 60 min or more. Our results indicate that the magnetosphere responds nonlinearly to the changes in the energy input, suggesting the need for a threshold in the amount of energy transferred to the system in order for the ring current to develop. Further increase of the energy input leads to a saturation limit where the inner magnetosphere response is no longer affected by any additional amount of energy contained within high‐frequency oscillations, because the magnetospheric system acts as a low‐pass filter on the interplanetary magnetic field.

Operational Space Weather Entering a New Era

U.S. operational space weather maturing because of two competing factors. On one hand, directed agency funding at about $1 billion for model development over the past decade has brought modeling maturity to !ve broad Sun-toEarth domains, i.e., Sun, heliosphere, magnetosphere, ionosphere, and thermosphere. On the other hand, agency funding for transitioning these models into operations has been a small fraction of the level provided for model development. is situation has le# implementation of operational space weather largely unfunded and woefully undirected, with the exception of a few U.S. Air Force Weather Agency projects. A new vision needed so that operational space weather can help solve 21st-century challenges. e current paradigm for operational space weather began in the 1990s, when the idea was to build complete systems at single facilities. ese rapid prototyping centers were envisioned as a means for implementing new operational systems, but this path was unsuccessful. Although increased funding for operational space weather would have bene!ted the community, this did not happen, and in the lopsided funding pro!le, operational space weather was forced to develop an unusual but robust architecture. is architecture consists of distributed networks—automated systems of models, data streams, and algorithms at multiple, geographically dispersed facilities linked by operational servers. A central server manages the network and uses a database enabling remote nodes to deposit output data and access input data at their geophysical or measurement cadences and latencies. e emergence of distributed networks represents a paradigm shi# from 1995, when the National Space Weather Program (NSWP) strategic plan was !rst published. At that time, very few models existed that could be even considered for use in operations. Model development increased due to the funding surge, and by 2000 seventy-three models characterizing 15 space environment domains were listed in the NSWP implementation plan (2nd edition). During the past decade some of these models were implemented operationally and became important components of distributed networks. Early examples were the Magnetospheric Spec!cation Model (MSM; 2000), SOLAR2000 (S2K; 2001), and High Accuracy Satellite Drag Model (HASDM; 2004). More recently, systems of coupled models and data streams have been created, including Global Assimilation of Ionospheric Measurements (GAIM; 2006), HakamadaAkasofuFry (HAF; 2007), and Communication Alert and Prediction System (CAPS; 2008). Coupled model/ data systems that are currently being implemented include the JacchiaBowman 2008 (JB2008), the Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS), U.S. Geological Survey Dst, ENLIL with Cone, and Ionosphere Operational System (IONS; i.e., the commercial version of GAIM at the Utah Science Technology and Research (USTAR) Center for Space Weather). Of added note that the Space Weather Modeling Framework (SWMF) and the Center for Integrated Space Weather Modeling (CISM) have developed multi domain systems to represent the entire Sunto-Earth environment, in which some of the models have been running for years. e need to test coupled systems existed even without funding, and the lack of funds drove small businesses, universities, and the multiagency Community Coordinated Modeling Center to create innovative, cost-e$ective solutions. Basically, these organizations had to !nd cheaper ways to accomplish testing and operational implementation. e result was the development of distributed networks to close the Technology Readiness Level (TRL) 7–9 gap. TRL the concept used by agencies and companies to assess the maturity of evolving technologies. TRL 7 models and data can operate in stand-alone mode within the context of an operational environment. TRL 8 signi!es the completion of a fully functional system prototype, and TRL 9 describes a fully delivered, operational system. e distributed network systems at TRL 7–9 are linked in a way that allows developers to maintain versioning and proprietary control over their models, permits data streams to be integrated at their own measurement cadence, results in testing %exibility, enables rapid

Comparative study of a substorm event by satellite observation and model simulation

A substorm event has been simulated for the first time by using SWMF (Space Weather Modeling Framework) developed by the University of Michigan. The model results have been validated using Geotail and Cluster satellite observations. The substorm onset occurs at 22:08 UT on September 28 2004, as identified from FUV WIC observations on the NASA IMAGE spacecraft. SWMF can couple effectively the magnetosphere, inner magnetosphere and ionosphere processes and is driven by the solar wind and IMF (Interplanetary Magnetic Field) parameters, which are measured by ACE satellite and time delayed to the upstream boundary of the model. It shows that (1) SWMF can predict well the large-scale variations of the magnetospheric magnetic field and ionospheric currents during the substorm event; and (2) the accuracy of the time delay of the solar wind from ACE to the outer boundary of the model has great effects on the model results. Finally, the substorm trigger mechanism has been discussed and the way of improvement of the model has been pointed out.

Modeling ionospheric outflows and their impact on the magnetosphere, initial results

Ionospheric outflow has been shown to be a significant contributor to the plasma population of the magnetosphere during active geomagnetic conditions. We present the results of new efforts to model the source and effects of out‐flowing plasma in the Space Weather Modeling Framework (SWMF). In particular, we develop and use the Polar Wind Outflow Model (PWOM), a field‐aligned, multifluid, multifield line polar wind code to simulate the ionospheric outflow. The PWOM is coupled to the ionosphere electrodynamics and global magnetosphere components of the SWMF, so we can calculate the outflow and its resulting impact on magnetospheric composition and dynamics. By including the outflow as part of a coupled system, we study the consequences of outflow on the larger space environment system. We present our methodology for the magnetosphere‐ionosphere coupling, as well as the effect of outflow on the magnetosphere during two geomagnetic storms. Moreover, we explore the use of multispecies MHD to track the resulting plasma composition in the magnetosphere. We find that, by including ionospheric outflow during geomagnetic storms, we can reduce the RMS error in the simulated magnetic field as compared with various GOES satellites by as much as 50%. Additionally, we find that the outflow causes a strong decrease in Dst and in the cross‐polar cap potential.

Solar – Terrestrial Simulation in the STEREO Era: The 24 – 25 January 2007 Eruptions

The SECCHI instruments aboard the recently launched STEREO spacecraft enable for the first time the continuous tracking of coronal mass ejections (CMEs) from the Sun to 1 AU. We analyze line-of-sight observations of the 24 – 25 January 2007 CMEs and fill the 20-hour gap in SECCHI coverage in 25 January by performing a numerical simulation using a three-dimensional magneto-hydrodynamic (MHD) code, the Space Weather Modeling Framework (SWMF). We show how the observations reflect the interaction of the two successive CMEs with each other and with the structured solar wind. We make a detailed comparison between the observations and synthetic images from our model, including time-elongation maps for several position angles. Having numerical simulations to disentangle observational from physical effects, we are able to study the three-dimensional nature of the ejections and their evolution in the inner heliosphere. This study reflects the start of a new era where, on one hand, models of CME propagation and interaction can be fully tested by using heliospheric observations and, on the other hand, observations can be better interpreted by using global numerical models.

Integration of the radiation belt environment model into the space weather modeling framework

We have integrated the Fok radiation belt environment (RBE) model into the space weather modeling framework (SWMF). RBE is coupled to the global magnetohydrodynamics component (represented by the Block-Adaptive-Tree Solar-wind Roe-type Upwind Scheme, BATS-R-US, code) and the Ionosphere Electrodynamics component of the SWMF, following initial results using the Weimer empirical model for the ionospheric potential. The radiation belt (RB) model solves the convection–diffusion equation of the plasma in the energy range of 10 keV to a few MeV. In stand-alone mode RBE uses Tsyganenko's empirical models for the magnetic field, and Weimer's empirical model for the ionospheric potential. In the SWMF the BATS-R-US model provides the time dependent magnetic field by efficiently tracing the closed magnetic field-lines and passing the geometrical and field strength information to RBE at a regular cadence. The ionosphere electrodynamics component uses a two-dimensional vertical potential solver to provide new potential maps to the RBE model at regular intervals. We discuss the coupling algorithm and show some preliminary results with the coupled code. We run our newly coupled model for periods of steady solar wind conditions and compare our results to the RB model using an empirical magnetic field and potential model. We also simulate the RB for an active time period and find that there are substantial differences in the RB model results when changing either the magnetic field or the electric field, including the creation of an outer belt enhancement via rapid inward transport on the time scale of tens of minutes.

Three‐dimensional MHD Simulation of the 2003 October 28 Coronal Mass Ejection: Comparison with LASCO Coronagraph Observations

We numerically model the coronal mass ejection (CME) event of 2003 October 28 that erupted from AR 10486 and propagated to Earth in less than 20 hr, causing severe geomagnetic storms. The magnetohydrodynamic (MHD) model is formulated by first arriving at a steady state corona and solar wind employing synoptic magnetograms. We initiate two CMEs from the same active region, one approximately a day earlier that preconditions the solar wind for the much faster CME on the 28th. This second CME travels through the corona at a rate of over 2500 km s−1, driving a strong forward shock. We clearly identify this shock in an image produced by the Large Angle Spectrometric Coronagraph (LASCO) C3 and reproduce the shock and its appearance in synthetic white-light images from the simulation. We find excellent agreement with both the general morphology and the quantitative brightness of the model CME with LASCO observations. These results demonstrate that the CME shape is largely determined by its interaction with the ambient solar wind and may not be sensitive to the initiation process. We then show how the CME would appear as observed by wide-angle coronagraphs on board the Solar Terrestrial Relations Observatory (STEREO) spacecraft. We find complex time evolution of the white-light images as a result of the way in which the density structures pass through the Thomson sphere. The simulation is performed with the Space Weather Modeling Framework (SWMF).

The August 24, 2002 coronal mass ejection: when a western limb event connects to earth

AbstractWe discuss how some coronal mass ejections (CMEs) originating from the western limb of the Sun are associated with space weather effects such as solar energetic particles (SEPs), shocks or geo-effective ejecta at Earth. We focus on the August 24, 2002 coronal mass ejection, a fast (~2000 km s−1) eruption originating from W81. Using a three-dimensional magneto-hydrodynamic simulation of this ejection with the Space Weather Modeling Framework (SWMF), we show how a realistic initiation mechanism enables us to study the deflection of the CME in the corona and the heliosphere. Reconnection of the erupting magnetic field with that of neighboring streamers and active regions modify the solar connectivity of the field lines connecting to Earth and can also partly explain the deflection of the eruption during the first tens of minutes. Comparing the results at 1 AU of our simulation with observations by the ACE spacecraft, we find that the simulated shock does not reach Earth, but has a maximum angular span of about 120°, and reaches 35° West of Earth in 58 hours. We find no significant deflection of the CME and its associated shock wave in the heliosphere, and we discuss the consequences for the shock angular span.

The Brightness of Density Structures at Large Solar Elongation Angles: What Is Being Observed bySTEREOSECCHI?

We discuss features of coronal mass ejections (CMEs) that are specific to heliospheric observations at large elongation angles. Our analysis is focused on a series of two eruptions that occurred on 2007 January 24-25, which were tracked by the Heliospheric Imagers (HIs) onboard STEREO. Using a three-dimensional (3-D) magneto-hydrodynamic simulation of these ejections with the Space Weather Modeling Framework (SWMF), we illustrate how the combination of the 3-D nature of CMEs, solar rotation, and geometry associated with the Thomson sphere results in complex effects in the brightness observed by the HIs. Our results demonstrate that these effects make any in-depth analysis of CME observations without 3-D simulations challenging. In particular, the association of bright features seen by the HIs with fronts of CME-driven shocks is far from trivial. In this Letter, we argue that, on 2007 January 26, the HIs observed not only two CMEs, but also a dense corotating stream compressed by the CME-driven shocks.

A Simulation of a Coronal Mass Ejection Propagation and Shock Evolution in the Lower Solar Corona

We present a detailed simulation of the evolution of a moderately slow coronal mass ejection (CME; 800 km s−1 at 5 R☉, where R☉ is solar radii) in the lower solar corona (2-5 R☉). The configuration of the Sun's magnetic field is based on the MDI data for the solar surface during Carrington rotation 1922. The pre-CME background solar wind is generated using the Wang-Sheeley-Arge (WSA) model. To initiate a CME, we inserted a modified Titov-Demoulin flux rope in an active region near the solar equator using the Space Weather Modeling Framework (SWMF). After the initiation stage (within 2.5 R☉), the CME evolves at a nearly constant and slow acceleration of the order of 100 m s−2, which corresponds to an intermediate-acceleration CME. Detailed analysis of the pressures shows that the thermal pressure accounts for most of the acceleration of the CME. The magnetic pressure contributes to the acceleration early in the evolution and becomes negligible when the CME moves beyond ~3 R☉. We also present the evolution of the shock geometry near the nose of the CME, which shows that the shock is quasi parallel most of the time.

Validation of the space weather modeling framework using ground‐based magnetometers

Geomagnetically induced currents (GICs) can disrupt power grid operations, causing significant interference for many people; therefore, predictions of ground‐based magnetic perturbations and their time derivatives are quite important. This study quantifies how well the University of Michigan's Global MHD code predicts approximately 150 ground‐based magnetometer traces for a number of storm‐time intervals. It is found that in order to accurately represent the magnetic perturbation, Biot‐Savart integrals over the entire hemisphere are needed, especially when calculating the vertical component. The 4 May 1998 storm is examined in detail. The code compares well with some stations, quantified by a normalized root mean squared error and cross correlation, while missing even the general trends for other stations. When multiple magnetometer station perturbations are averaged together, the model does an adequate job in the north and vertical components but reverses the trends in the eastward component. The code does significantly better when comparing an AL‐like index but does not have as strong a variation as the actual data. Comparison of dB/dt in a wide window rather than simultaneously shows better model performance in capturing events but worse in yielding false alarms. It is further found that the MHD code models the magnetic perturbations better in the summer hemisphere than in the winter hemisphere.

Storm time dynamics of auroral electrojets: CHAMP observation and the Space Weather Modeling Framework comparison

Abstract. We investigate variations of the location and intensity of auroral currents during two magnetic storm periods based on magnetic field measurements from CHAMP separately for both hemispheres, as well as for the dayside and nightside. The corresponding auroral electrojet current densities are on average enhanced by about a factor of 7 compared to the quiet time current strengths. The nightside westward current densities are on average 1.8 (2.2) times larger than the dayside eastward current densities in the Northern (Southern) Hemisphere. Both eastward and westward currents are present during the storm periods with the most intense electrojets appearing during the main phase of the storm, before the ring current maximizes in strength. The eastward and westward electrojet centers can expand to 55° MLat during intense storms, as is observed on 31 March 2001 with Dst=−387 nT. The equatorward shift of auroral currents on the dayside is closely controlled by the southward IMF, while the latitudinal variations on the nightside are better described by the variations of the Dst index. However, the equatorward and poleward motion of the nightside auroral currents occur earlier than the Dst variations. The Space Weather Modeling Framework (SWMF) can capture the general dynamics of the storm time current variations. Both the model and the actual data show that the currents tend to saturate when the merging electric field is larger than 10 mV/m. However, the exact prediction of the temporal development of the currents is still not satisfactory.