Daniel T. Welling

Comparative study of ring current development using empirical, dipolar, and self‐consistent magnetic field simulations

[1] The effects of nondipolar magnetic field configuration and the feedback of a self-consistently computed magnetic field on ring current dynamics are investigated during a double-dip storm with minima SYM-H = -90 nT at ~2000 UT, 20 November, and SYM-H = -127 nT at ~1000 UT, 21 November 2002. We use our kinetic ring current-atmosphere interactions model with self-consistent magnetic field (RAM-SCB) to study the redistribution of plasma in the inner magnetosphere after its fresh injection from the plasma sheet. The kinetic model is fully extended to nondipolar magnetic (B) field geometry and two-way coupled with an Euler-potential-based equilibrium model that calculates self-consistently the three-dimensional magnetic field in force balance with the anisotropic ring current distributions. The ring current source population is inferred from LANL geosynchronous satellite data; a superdense plasma sheet observed during the second storm main phase contributes significantly to ring current buildup. We find that the bounce-averaged velocities increase while the bounce-averaged geocoronal hydrogen densities decrease on the nightside when a nondipolar B field is used. A depression of the ring current fluxes and a confinement of the ring current close to Earth are thus observed on the nightside as geomagnetic activity increases. In contrast to the dipolar case, the proton anisotropy increases considerably in the postnoon sector, and the nondipolar simulations predict the excitation of intense EMIC waves at large L shells. The total ring current energy and |Dst| index calculated with the self-consistent B field are in best agreement with observations, being smaller compared to the dipolar calculations but larger than the empirical B field predictions.

Including gap region field‐aligned currents and magnetospheric currents in the MHD calculation of ground‐based magnetic field perturbations

[1] Many high‐latitude modeling studies utilize the horizontal ionospheric Hall current in calculating ground‐based magnetic perturbations, but low‐latitude and midlatitude studies should include current systems such as the magnetospheric, field‐aligned, and Pedersen currents. Recently, by including all these current systems, a more precise ground‐ based perturbation calculator has been implemented in the Space Weather Modeling Framework. Using this new method, ground‐based perturbations generated by different current systems are analyzed at low, middle, and high latitudes. As a result of the current systems, MLT‐UT maps of ground‐based perturbations are studied. Furthermore, nine storms events are simulated at more than 20 low‐latitude and midlatitude magnetometer locations and compared with observational ground‐based perturbations. These studies show that for specifying the northward component of the ground magnetic perturbations, the inclusion of magnetospheric, field‐aligned, and Pedersen current is important and improves the prediction significantly over the prediction only considering the Hall current in the calculation. The improvement is the most during the storm main phase. However, for the vertical and eastward components of the perturbations, which were typically smaller than the northward component, the inclusion of these current systems actually made the specifications worse because the ring current in the model rotates more toward the dayside than in reality.

Outflow in global magnetohydrodynamics as a function of a passive inner boundary source

Numerous studies of the terrestrial magnetosphere that use global magnetohydrodynamic codes have found that the models inner boundary can act as a significant source of plasma, even if the radial velocity about the boundary is held at zero. Though inherent in many models, this “de facto outflow” is poorly understood. This work uses the Block Adaptive Tree Solar Wind Roe-type Upwind Scheme MHD model to investigate the behavior of this type of outflow as a function of boundary conditions and solar wind drivers. It is found that even for temporally and spatially constant boundary conditions, the mass is accelerated away from the body in a dynamic manner. Fluxes organize into cusp, polar cap, and auroral zone concentrations. Pressure gradient forces appear predominantly responsible for cusp and polar cap outflow, while the Lorentz force, resulting from field-aligned current systems, is the strongest driver of outflow in other regions. Integrated fluxes probed just outside of the inner boundary vary linearly as a function of cross polar cap potential and solar wind dynamic pressure. The resulting dynamics strongly resemble patterns found in in situ measurements, while net fluences agree within an order of magnitude. Two free parameters, inner boundary mass density and composition, can strongly affect results. Accounting for these unknowns is likely best left to physics-based or empirical specifications of outflow. Despite this, such outflow appears to be an acceptable proxy.

The role of ring current particle injections: Global simulations and Van Allen Probes observations during 17 March 2013 storm

We simulate substorm injections observed by the Van Allen Probes during the 17 March 2013 storm using a self-consistent coupling between the ring current model RAM-SCB and the global MHD model BATS-R-US. This is a significant advancement compared to previous studies that used artificially imposed electromagnetic field pulses to mimic substorm dipolarization and associated inductive electric field. Several substorm dipolarizations and injections are reproduced in the MHD model, in agreement with the timing of shape changes in the AE/AL index. The associated inductive electric field transports plasma sheet plasma to geostationary altitudes, providing the boundary plasma source to the ring current model. It is found that impulsive plasma sheet injections, together with a large-scale convection electric field, are necessary to develop a strong ring current. Comparisons with Van Allen Probes observations show that our model reasonably well captures dispersed electron injections and the global Dst index.

Statistical analysis of the geomagnetic response to different solar wind drivers and the dependence on storm intensity

Geomagnetic storms start with activity on the Sun that causes propagation of magnetized plasma structures in the solar wind. The type of solar activity is used to classify the plasma structures as being either interplanetary coronal mass ejection (ICME) or corotating interaction region (CIR) driven. The ICME-driven events are further classified as either magnetic cloud (MC) driven or sheath (SH) driven by the geoeffective structure responsible for the peak of the storm. The geoeffective solar wind flow then interacts with the magnetosphere producing a disturbance in near-Earth space. It is commonly believed that a SH-driven event behaves more like a CIR-driven event than a MC-driven event; however, in our analysis this is not the case. In this study, geomagnetic storms are investigated statistically with respect to the solar wind driver and the intensity of the events. We use the Hot Electron and Ion Drift Integrator (HEIDI) model to simulate the inner magnetospheric hot ion population during all of the storms classified as intense (Dstmin ≤ −100 nT) within solar cycle 23 (1996–2005). HEIDI is configured four different ways using either the Volland-Stern or self-consistent electric field and either event-based Los Alamos National Laboratory (LANL) magnetospheric plasma analyzer (MPA) data or a reanalyzed lower resolution version of the data that provides spatial resolution. Presenting the simulation results, geomagnetic data, and solar wind data along a normalized epoch timeline shows the average behavior throughout a typical storm of each classification. The error along the epoch timeline for each HEIDI configuration is used to rate the models performance. We also subgrouped the storms based on the magnitude of the minimum Dst. We found that typically the LANL MPA data provide the best outer boundary condition. Additionally, the self-consistent electric field better reproduces SH- and MC-driven events throughout most of the storm timeline, but the Volland-Stern electric field better reproduces CIR-driven events. Contrary to what we expect, examination of the HEIDI model results and solar wind data shows that SH-driven events behave more like MC-driven events than CIR-driven storms.

Geomagnetically induced currents: science, engineering, and applications readiness

This paper is the primary deliverable of the very first NASA Living With a Star Institute Working Group, Geomagnetically Induced Currents (GIC) Working Group. The paper provides a broad overview of the current status and future challenges pertaining to the science, engineering, and applications of the GIC problem. Science is understood here as the basic space and Earth sciences research that allows improved understanding and physics-based modeling of the physical processes behind GIC. Engineering, in turn, is understood here as the “impact” aspect of GIC. Applications are understood as the models, tools, and activities that can provide actionable information to entities such as power systems operators for mitigating the effects of GIC and government agencies for managing any potential consequences from GIC impact to critical infrastructure. Applications can be considered the ultimate goal of our GIC work. In assessing the status of the field, we quantify the readiness of various applications in the mitigation context. We use the Applications Readiness Level (ARL) concept to carry out the quantification.

Long‐lived plasmaspheric drainage plumes: Where does the plasma come from?

Long-lived (weeks) plasmaspheric drainage plumes are explored. The long-lived plumes occur during long-lived high-speed-stream-driven storms. Spacecraft in geosynchronous orbit see the plumes as dense plasmaspheric plasma advecting sunward toward the dayside magnetopause. The older plumes have the same densities and local time widths as younger plumes, and like younger plumes they are lumpy in density and they reside in a spatial gap in the electron plasma sheet (in sort of a drainage corridor). Magnetospheric-convection simulations indicate that drainage from a filled outer plasmasphere can only supply a plume for 1.5–2 days. The question arises for long-lived plumes (and for any plume older than about 2 days): Where is the plasma coming from? Three candidate sources appear promising: (1) substorm disruption of the nightside plasmasphere which may transport plasmaspheric plasma outward onto open drift orbits, (2) radial transport of plasmaspheric plasma in velocity-shear-driven instabilities near the duskside plasmapause, and (3) an anomalously high upflux of cold ionospheric protons from the tongue of ionization in the dayside ionosphere, which may directly supply ionospheric plasma into the plume. In the first two cases the plume is drainage of plasma from the magnetosphere; in the third case it is not. Where the plasma in long-lived plumes is coming from is a quandary: to fix this dilemma, further work and probably full-scale simulations are needed.

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 (SWPC) Geospace Environment Modeling (GEM) challenge [cf. Pulkkinen et al., 2013] 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 (CCMC). 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.

Nowcast model for low‐energy electrons in the inner magnetosphere

We present the nowcast model for low-energy (<200 keV) electrons in the inner magnetosphere, which is the version of the Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) for electrons. Low-energy electron fluxes are very important to specify when hazardous satellite surface-charging phenomena are considered. The presented model provides the low-energy electron flux at all L shells and at all satellite orbits, when necessary. The model is driven by the real-time solar wind and interplanetary magnetic field (IMF) parameters with 1 h time shift for propagation to the Earths magnetopause and by the real time Dst index. Real-time geostationary GOES 13 or GOES 15 (whenever each is available) data on electron fluxes in three energies, such as 40 keV, 75 keV, and 150 keV, are used for comparison and validation of IMPTAM running online. On average, the model provides quite reasonable agreement with the data; the basic level of the observed fluxes is reproduced. The best agreement between the modeled and the observed fluxes are found for <100 keV electrons. At the same time, not all the peaks and dropouts in the observed electron fluxes are reproduced. For 150 keV electrons, the modeled fluxes are often smaller than the observed ones by an order of magnitude. The normalized root-mean-square deviation is found to range from 0.015 to 0.0324. Though these metrics are buoyed by large standard deviations, owing to the dynamic nature of the fluxes, they demonstrate that IMPTAM, on average, predicts the observed fluxes satisfactorily. The computed binary event tables for predicting high flux values within each 1 h window reveal reasonable hit rates being 0.660–0.318 for flux thresholds of 5 ·104–2 ·105 cm−2 s−1 sr−1 keV−1 for 40 keV electrons, 0.739–0.367 for flux thresholds of 3 ·104–1 ·105 cm−2 s−1 sr−1 keV−1 for 75 keV electrons, and 0.485–0.438 for flux thresholds of 3 ·103–3.5 ·103 cm−2 s−1 sr−1 keV−1 for 150 keV electrons but rather small Heidke Skill Scores (0.17 and below). This is the first attempt to model low-energy electrons in real time at 10 min resolution. The output of this model can serve as an input of electron seed population for real-time higher-energy radiation belt modeling.

The global structure and time evolution of dayside magnetopause surface eigenmodes

Theoretical work and recent observations suggest that the dayside magnetopause may support its own eigenmode, consisting of propagating surface waves which reflect at the northern and southern ionospheres. These magnetopause surface eigenmodes (MSEs) are a potential source of magnetospheric ultralow-frequency (ULF) waves with frequencies less than 2 mHz. Here we use the Space Weather Modeling Framework to study the magnetospheric response to impulsive solar wind dynamic pressure increases. Waves with 1.8 mHz frequency are excited whose global properties are largely consistent with theoretical predictions for MSE and cannot be explained by other known ULF wave modes. These simulation results lead to two key findings: (1) MSE can be sustained in realistic magnetic field geometries with nonzero flow shear and finite current layer thickness at the magnetopause and (2) MSE can seed the growth of tailward propagating surface waves via the Kelvin-Helmholtz instability.