Analysis of Near-Earth plasma-sheet cumulative magnetic flux transport and its’ comparison for several different runs using the Rice Convection Model

Interaction between Earth’s magnetotail and its inner magnetosphere plays an important role in the transport of mass and energy in the ionosphere–magnetosphere coupled system. A number of first-principles models are devoted to understanding the associated dynamics. However, running these models, including both magnetohydrodynamic models and kinetic drift models, can be computationally expensive when self-consistency and high spatial resolution are required. In this study, we exploit an approach of building a parallel statistical model, based on the long short-term memory (LSTM) type of recurrent neural network, to forecast the results of a first-principles model, called the Rice Convection Model (RCM). The RCM is used to simulate the transient injection events, in which the flux-tube entropy parameter, dawn-to-dusk electric field component, and cumulative magnetic flux transport are calculated in the central plasma sheet. These key parameters are then used as initial inputs for training the LSTM. Using the trained LSTM multivariate parameters, we are able to forecast the plasma sheet parameters beyond the training time for several tens of minutes that are found to be consistent with the subsequent RCM simulation results. Our tests indicate that the recurrent neural network technique can be efficiently used for forecasting numerical simulations of magnetospheric models. The potential to apply this approach to other models is also discussed.

An influence of long-lasting and gradual magnetic flux transport on fate of magnetotail fast plasma flows: An energetic particle injection substorm event study

A comprehensive ring current model (CRCM) has been developed that couples the Rice Convection Model (RCM) and the kinetic model of Fok and coworkers. The coupled model is able to simulate, for the first time using a self‐consistently calculated electric field, the evolution of an inner magnetosphere plasma distribution that conserves the first two adiabatic invariants. The traditional RCM calculates the ionospheric electric fields and currents consistent with a magnetospheric ion distribution that is assumed to be isotropic in pitch angle. The Fok model calculates the plasma distribution by solving the Boltzmann equation with specified electric and magnetic fields. To combine the RCM and the Fok model, the RCM Birkeland current algorithm has been generalized to arbitrary pitch angle distributions. Given a specification of height‐integrated ionospheric conductance, the RCM component of the CRCM computes the ionospheric electric field and currents. The Fok model then advances the ring current plasma distribution using the electric field computed by the RCM and at the same time calculates losses along particle drift paths. We present the logic of CRCM and the first validation results following the H+ distribution during the previously studied magnetic storm of May 2, 1986. The H+ fluxes calculated by the coupled model agree very well with observations by AMPTE/CCE. In particular, the coupled model is able to reproduce the high H+ flux seen on the dayside at L ∼ 2.3 that the previous simulation, which employed a Stern‐Volland convection model with shielding factor 2, failed to produce. Though the Stern‐Volland and CRCM electric fields differ in several respects, the most notable difference is that the CRCM predicts strong electric fields near Earth in the storm main phase, particularly in the dusk‐midnight quadrant. Thus the CRCM injects particles more deeply and more quickly.

[1] To understand the processes responsible for the formation and structure of plasma sheet and ring current particles, we have used THEMIS and Geotail data to investigate statistically the distributions of ions and electrons from the midtail to the inner magnetosphere and compared them with results from the Rice convection model (RCM). The observed distributions show clear magnetic local time (MLT) asymmetries in the thermal energy and energy fluxes of plasma sheet particles but many more MLT symmetric ring current particles. Our RCM runs include both self-consistent electric and magnetic fields and realistic MLT-dependent outer particle sources. Starting with no initial particles, particles released from the RCM outer sources move along electric and magnetic drift paths and change energy adiabatically. Comparison of the observation with the simulation indicates that the particles along the open drift paths can account for the observed plasma sheet populations and that the observed significant MLT variations are a combined result of species- and energy-dependent drift and location-dependent source strength. The simulated energy and spatial distributions of the particles within closed drift paths are found to be consistent with the observed ring current particles. These ring current particles are originally plasma sheet particles which became trapped along closed paths due to temporal variations of drift paths. The good agreement in key features of the spatial distributions of thermal energy and energy fluxes between the RCM and observations clearly indicates that electric and magnetic drift transport and the associated energization play dominant roles in plasma sheet and ring current dynamics.

Ideal magnetohydrodynamics is known to be inaccurate for the Earth's inner magnetosphere, where transport by gradient‐curvature drift is nonnegligible compared to E × B drift. Most theoretical treatments of the inner plasma sheet and ring current, including the Rice Convection Model (RCM), treat the inner magnetospheric plasma in terms of guiding center drifts. The RCM assumes that the distribution function is isotropic, but particles with different energy invariants are treated as separate guiding center fluids. However, Peymirat and Fontaine [1994] developed a two‐fluid picture of the inner magnetosphere, which utilizes modified forms of the conventional fluid equations, not guiding center drift equations. Heinemann [1999] argued theoretically that for inner magnetospheric conditions the fluid energy equation should include a heat flux term, which, in the case of Maxwellian plasma, was derived by Braginskii [1965]. We have now reconciled the Heinemann [1999] fluid approach with the RCM. The fluid equations, including the Braginskii heat flux, can be derived by taking appropriate moments of the RCM equations for the case of the Maxwellian distribution.The physical difference between the RCM formalism and the Heinemann [1999] fluid approach is that the RCM pretends that particles suffer elastic collisions that maintain the isotropy of the distribution function but do not change particle energies. The Heinemann [1999] fluid treatment makes a different physical approximation, namely that the collisions maintain local thermal equilibrium among the ions and separately among the electrons. For some simple cases, numerical results are presented that illustrate the differences in the predictions of the two formalisms, along with those of MHD, guiding center theory, and Peymirat and Fontaine [1994].

The main goal of this paper is to estimate the errors involved in applying a quasi‐static convection model such as the Rice Convection Model (RCM) or its equilibrium version (RCM‐E), which neglect inertial currents, to treat the injection of fresh particles into the inner magnetosphere in a substorm expansion phase. The approach is based on the idea that the dipolarization process involves earthward motion of a bubble that consists of flux tubes that have lower values of the entropy parameter than the surrounding medium. Our tests center on comparing MHD simulations with RCM‐ and RCM‐E‐like quasi‐static approximations, for cases where the bubble is considered to be a thin ideal‐MHD filament. Those quasi‐static solutions miss the interchange oscillations that are often a feature of the MHD results. RCM and, to a lesser extent, RCM‐E calculations tend to overestimate the westward electric field at the ionospheric footprint of the bubble and underestimate its duration. However, both get the time integral of the E × B drift velocity right as well as the net energization of the particles in the filament. The quasi‐static approximation is most accurate if its computed value of the braking time of the bubble's earthward motion is long compared to the period of the relevant interchange oscillation. Comparison of MHD filament simulations of interchange instability with corresponding RCM calculations suggests a similar validity criterion. For plasma sheet conditions, the quasi‐static approximation is typically best if the background medium has lowβ, worst if it consists of highly stretched field lines.

Extension of convection modeling into the high-latitude ionosphere: some theoretical difficulties

The Rice Convection Model (RCM) is an established first‐principles physics model in which the field‐aligned current density is computed by the Vasyliunas equation. Its slow‐flow assumption neglects the inertial term in the MHD momentum equation, which is a major obstacle to using the RCM to model some of the fluid dynamics of bursty bulk flows in the plasma sheet. This paper describes the RCM‐I, which represents an effort to approximately add inertial effects in the RCM by correcting the expression for field‐aligned currents. That inertial current is calculated under the assumption that the mass on each field line is concentrated near the equatorial plane. RCM‐I results are presented with magnetic fields calculated in two different ways: A static statistics‐based model and an MHD code. In both cases, the bubble flow velocities are much smaller and more realistic than those calculated from the traditional RCM. An additional promising feature is that the injection of a low‐entropy bubble produces interchange (braking) oscillations and buoyancy waves that radiate away from the original bubble, forming multiple flow vortices. None of these features could be produced by traditional RCM simulations. Comparison simulations also suggest that gradient and curvature drifts have substantial effect on oscillation of bubbles and tend to damp buoyancy waves. We also note that substantial work will be needed in the future to further improve the pressure distribution by inertializing an anisotropic version of the RCM and to understand the effects of neglecting the magnetosphere‐ionosphere communication time.

Flow bursts are a major component of transport within the plasma sheet and auroral oval (where they are referred to as flow channels), and lead to a variety of geomagnetic disturbances as they approach the inner plasma sheet (equatorward portion of the auroral oval). However, their two-dimensional structure as they approach the inner plasma sheet has received only limited attention. We have examined this structure using both the Rice Convection Model (RCM) and ground-based radar and all sky imager observations. As a result of the energy dependent magnetic drift, the low entropy plasma of a flow burst spreads azimuthally within the inner plasma sheet yielding specific predictions of subauroral polarization stream (SAPS) and dawnside auroral polarization stream (DAPS) enhancements that are related to the field-aligned currents associated with the flow channel. Flow channels approximately centered between the dawn and dusk large-scale convection cells are predicted to give significant enhancements of both SAPS and DAPS, whereas flow channel further toward the dusk (dawn) convection cell show a far more significant enhancement of SAPS (DAPS) than for DAPS (SAPS). We present observations for cases having good coverage of flow channels as they approach the equatorward portion of the auroral oval and find very good qualitative agreement with the above RCM predictions, including the predicted differences with respect to flow burst location. Despite there being an infinite variety of flow channels’ plasma parameters and of background plasma sheet and auroral oval conditions, the observations show the general trends predicted by the RCM simulations with the idealized parameters. This supports that RCM predictions of the azimuthal spread of a low-entropy plasma sheet plasma and its associated FAC and flow responses give a realistic physical description of the structure of plasma sheet flow bursts (auroral oval flow channels) as they reach the inner plasma sheet (near the equatorward edge of the auroral oval).

The major magnetic storm of 4–5 June 1991 was well observed with the Combined Release and Radiation Experiment (CRRES) satellite in the duskside inner magnetosphere and with three Defense Meteorological Satellite Program (DMSP) spacecraft in the polar ionosphere. These observations are compared to results from the Rice Convection Model (RCM), which calculates the inner magnetospheric electric field and particle distribution self‐consistently. This case study, which uses the most complete RCM runs to date, demonstrates two significant features of magnetospheric storms, the development of subauroral polarization streams (SAPS) and plasma‐sheet particle injection deep into the inner magnetosphere. In particular, the RCM predicts the electric field peak near L = 4 that is observed by the CRRES satellite during the second injection. The RCM calculations and DMSP data both show SAPS events with similar general characteristics, though there is no detailed point‐by‐point agreement. In the simulation, SAPS are generated by the deep penetration of plasma sheet protons to L < 4 and Earthward of the plasma sheet electrons. Similarly, the vast majority of the ions that make up the storm‐time ring current came from the plasma sheet; most of the particles that made up the prestorm quiet‐time ring current escaped through the dayside magnetopause during ring current injection. The RCM demonstrates the capability of plasma sheet ions to reach all ring current orbits and predicts the location of the injected particles (both ions and electrons) reasonably well. However, it overpredicts the ion flux in the inner magnetosphere.

The Rice Convection Model (RCM) is an established physical model of the inner and middle magnetosphere that includes coupling to the ionosphere. It uses a many-fluid formalism to describe adiabatically drifting isotropic particle distributions in a self-consistently computed electric field and specified magnetic field. We review a long-standing effort at Rice University in magnetospheric modeling with the Rice Convection Model. After briefly describing the basic assumptions and equations that make up the core of the RCM, we present a sampling of recent results using the model. We conclude with a brief description of ongoing and future improvements to the RCM.

Magnetospheric substorms: An inner-magnetospheric modeling perspective

To understand the large‐scale plasma sheet thermodynamics, we have used Geotail data and a formula for estimating flux tube volume to investigate statistically the equatorial distributions of P5/3 and n for slow flowing plasma in the nightside plasma sheet and compare them with the physical bases of ideal MHD and the Rice Convection Model (RCM). We have examined the distributions under three conditions: (1) weak convection and low AE, (2) enhanced convection and low AE, and (3) enhanced convection and high AE. The overall n decreases significantly with increasing convection or AE, while the overall P5/3 remains similar. We found that P5/3 drops significantly earthward along the estimated electric drift paths near midnight, inconsistent with ideal MHD. Examination of Pk5/3 and the electric and magnetic drift paths of different energy invariants, where Pk is the partial pressure of a specific energy invariant, shows that the strong duskward drift of the above thermal‐energy particles due to magnetic drift, together with there being significantly fewer higher‐energy particles from the dawn flank than from the tail, results in the strong earthward decrease of P5/3. We also found that d(Pk5/3)/dt = 0 and d(nk)/dt = 0 along the electric and magnetic drift paths used in the RCM are good approximations for slow flowing pressure‐bearing plasma sheet ions. The distributions of n indicate that low‐energy plasma becomes dominant when convection and AE are low, and their transport from the flanks may be affected by nonadiabatic processes not included in ideal MHD or the RCM.

This editorial highlights recent data and model results for Space Weather readers. One of the ways that American Geophysical Union (AGU) features some of the most interesting journal articles is Editors' Highlights. As of late 2017 these are now hosted on the Eos website: https://eos.org/editor-highlights, giving them new prominence. Editors contribute a short summary of the highlighted article in language that will appeal to general Eos readers and include a “teaser” line and visually attractive figure from the manuscript. Doing so helps to share scientific content beyond a journal's readership to a broader audience. However, occasionally a situation arises, wherein an Editor sees a manuscript worthy of additional notoriety, but the summary simply needs more technical wording than might be appropriate for general readers. Below, I am highlighting two such manuscripts that illustrate advances in Space Weather (SWE). Based on analysis of total electron content estimated from Global Positioning System (GPS) and Defense Meteorological Satellite Program spacecraft measurements, Katamzi-Joseph et al. (2017) report the occurrence of ionospheric depletions (plasma bubbles) at anomalously high, European latitudes (42°N geographic) during two intense geomagnetic storms in 2000 and 2001. Somewhat counterintuitively, such depletions may develop in the postsunset sector where plasma density has been previously enhanced. GPS signals often scintillate passing through these bubbles. The authors suggest that conditions supporting rapid upward plasma drift near the equator can extend to the midlatitude ionosphere during storm time, making these regions susceptible to navigation signal scintillation during intense geomagnetic storms. They estimate a maximum apex height of the postsunset plasma depletions as ~4,000 km. The data for the two intense storms show that the evening enhancement of the equatorial ionization anomaly extended to middle-latitude Europe during the time of the plasma bubble occurrence. The authors propose an eastward penetrating electric field-associated with the interplanetary Bz southward turning as a possible mechanism for plasma enhancement and subsequent middle-latitude plasma bubbles. Although the midlatitude ionosphere is generally considered safe from radio signal scintillation effects, this study confirms that there may be no safe zone from space-based signal scintillation during intense storms. (See Mendillo et al., 2018; Wei et al., 2015, and references, within these for previous examples and further impacts.) The Space Weather Prediction Center (SWPC) now runs subsets of the models that comprise the Space Weather Modeling Framework (SWMF). See http://www.swpc.noaa.gov/news/announcing-geospace-model-version-15. Haiducek et al. (2017) simulated a month of geospace response for January 2005 using SWMF with the University of Michigan global magnetohydrodynamic code Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) coupled to the Rice Convection Model (RCM) inner magnetosphere and the Ridley Ionosphere Model. The interval included three large, previously simulated, and well-studied geospace storms. The authors launched the simulation three times. The first launch was with a standard 0.25 Earth radii (RE) SWMF resolution, corresponding to the operational version at SWPC. Then a higher (0.125 RE) resolution grid was applied with and without the RCM inner magnetosphere model. Various indices of geomagnetic activity (Kp, Sym-H, and AL) and the cross polar cap potential (CPCP), which is a proxy for geospace interaction with the solar wind, were calculated from the simulation and compared with observations and an empirical model that estimates CPCP. The SWMF model predicted the Sym-H (proxy for ring current) index very well. The Kp geomagnetic index and the CPCP are well predicted during storm conditions, but these indicators are overpredicted during quiet time. The model tends to underpredict the magnitude of the auroral electrojet, AL index. The Generally, the grid resolution has limited influence for the accuracy of the predictions except for auroral activity. However, the use of the inner magnetosphere model is essential. For the CPCP there is little effect from turning off the RCM. The Sym-H, AL, and Kp indexes, however, are very different when the inner magnetosphere model was turned off. This study is a significant step in showing how well one of the new SWPC models performs during storm conditions and in detailing the sensitivity in model driving. Importantly, the authors provide access to the model output for others to use. As an applications journal, SWE content covers data, model description and validation, and aspects of engineering that challenge even our technically savvy readers. So there are times when technical terms and acronyms might be problematic for a short highlight aimed at a wider readership. You can help SWE Editors promote and publicize as much content as possible by providing illustrative graphics with easy-to-follow captions and by writing plain language summaries of your manuscripts. In turn your Editors will work to highlight the best work. No data were created for this editorial.

This paper presents some of the latest results of self-consistent numerical modeling of large-scale inner-magnetospheric electric fields obtained with the Rice Convection Model (RCM). The RCM treats plasma drifts, electric fields, and currents in the inner magnetosphere self-consistently in the quasi-static (slow-flow) approximation under the assumption of isotropic pitch-angle distribution. Event simulations of the magnetic storm of March 31, 2001 are used with two newly available RCM input models: an empirical model of the storm-time magnetospheric magnetic field, and an empirical model of the plasma sheet. Results show that the effect of severe distortion of the magnetic field during very large magnetic storms improves the ability of the RCM to predict the location of Sub-Auroral Polarization Stream (SAPS) events, although there is not perfect agreement with observations. Weakening of shielding by region-2 Birkeland currents during times of severe magnetic field inflation also improves comparison of the RCM-computed plasmapause location with data. Results of simulations with plasma boundary sources varying in response to measured solar wind inputs show that the plasma sheet may become interchange unstable under certain geomagnetic conditions.