Frank R. Toffoletto

Interaction between direct penetration and disturbance dynamo electric fields in the storm-time equatorial ionosphere

[1] The direct penetration of the high-latitude electric field to lower latitudes, and the disturbance dynamo, both play a significant role in restructuring the storm-time equatorial ionosphere and thermosphere. Although the fundamental mechanisms generating each component of the disturbance electric field are well understood, it is difficult to identify the contribution from each source in a particular observation. In order to investigate the relative contributions of the two processes, their interactions, and their impact on the equatorial ionosphere and thermosphere, the response to the March 31, 2001, storm has been modeled using the Rice Convection Model (RCM) and the Coupled Thermosphere-Ionosphere-Plasmasphere-Electrodynamics (CTIPe) model. The mid- and low-latitude electric fields from RCM have been imposed as a driver of CTIPe, in addition to the high latitude magnetospheric sources of ion convection and auroral precipitation. The high latitude sources force the global storm-time wind fields, which act as the driver of the disturbance dynamo electric fields. The magnitudes of the two sources of storm-time equatorial electric field are compared for the March 2001 storm period. During daytime, and at the early stage of the storm, the penetration electric field is dominant; while at night, the penetration and disturbance dynamo effects are comparable. Both sources are sufficient to cause significant restructuring of the low latitude ionosphere. Our results also demonstrate that the mid- and low-latitude conductivity and neutral wind changes initiated by the direct penetration electric field preferentially at night are sufficient to alter the subsequent development of the disturbance dynamo.

Simulated magnetopause losses and Van Allen Probe flux dropouts

Three radiation belt flux dropout events seen by the Relativistic Electron Proton Telescope soon after launch of the Van Allen Probes in 2012 (Baker et al., 2013a) have been simulated using the Lyon-Fedder-Mobarry MHD code coupled to the Rice Convection Model, driven by measured upstream solar wind parameters. MHD results show inward motion of the magnetopause for each event, along with enhanced ULF wave power affecting radial transport. Test particle simulations of electron response on 8 October, prior to the strong flux enhancement on 9 October, provide evidence for loss due to magnetopause shadowing, both in energy and pitch angle dependence. Severe plasmapause erosion occurred during ~ 14 h of strongly southward interplanetary magnetic field Bz beginning 8 October coincident with the inner boundary of outer zone depletion.

Opening the cusp

Defining the equatorward boundary of the cusp region in the ionosphere as the projection of the merging line from the magnetopause, we use a quantitative, geometrically realistic model to show how the local time span of the cusp increases with increasing merging rate for southward interplanetary magnetic field. Since the merging rate is fixed by the magnitude of the magnetic field component normal to the model magnetopause and then normalized to the cross-polar-cap potential, the result gives the variation of cusp local time span as a function of potential. From 0 kV up to 60 kV, the cusp expands from a point at noon to 2 hours on either side. Nearly tripling the voltage to 170 kV adds one more hour to each side, yielding a span from 0900 to 1500 LT. As an example of broad local time span during magnetically active periods, we present spacecraft observations of cusp particles during the great storm of March 1989 that cover more than 8 hours of local time. An important aspect of the result is the demonstration that a merging line of fixed length on the magnetopause, as assumed in the model, maps to a projected length in the ionosphere that increases as the funnel-shaped cusp opens. This behavior contrasts with earlier models that have cleft rather than cusp geometry, where the projected merging line length is proportional only to its length on the magnetopause. The model results are used to construct the footprint of a flux transfer event caused by time variations of the merging rate, uniform along the length of the merging line. The cusp geometry distorts the field lines mapped from the magnetopause to yield footprints with dawn and dusk protrusions into the region of closed magnetic flux.

Injection of a bubble into the inner magnetosphere

[1] Magnetic reconnection in the magnetotail or other forms of current sheet disruption are believed to produce plasma bubbles, which consist of flux tubes that have lower entropy content PV 5/3 than their surroundings. We present an initial Rice-Convection-Model-based simulation of the injection of a bubble into the inner magnetosphere and explore the consequences on ring current formation. As the bubble moves into the inner magnetosphere, region-1-sense Birkeland currents form along its eastward and westward edges while strong westward electric field and earthward flow form inside it; gradient/curvature drift causes it to drift westward. The simulations indicate that the presence of a bubble results in an increase in the peak particle pressure in the ring current region. Results are presented from several computer experiments to determine sensitivities to assumptions.

A nonsingular model of the open magnetosphere

We present a modified version of the Toffoletto and Hill (1989) open magnetosphere model that incorporates a tail-like interconnection field with a discontinuity to represent the slow-mode expansion fan that defines the high-latitude tail magnetopause. (The interconnection field is defined as the perturbation on an initially closed magnetosphere model to make it open.) The expansion fan controls the open field line region in the tail, and the intersection of the fan with the tail current sheet is, by design, the x line. The new interconnection field allows greater control of the tail field structure; in particular, it enables us to eliminate the nightside mapping singularity that occurs in previous models when the interplanetary magnetic field is nonsouthward. Also, in contrast to earlier models, the far tail x line extends farther downstream on the flanks than in the center of the tail, consistent with observations.

Design criteria for an angle resolved electron spectrometer of novel toroidal geometry

Abstract A charged particle analyser based on the use of polar trajectories in a toroidal sector is described in which a large number of particle emission angles are detected simultaneously. An analysis of the trajectories of particles in this class of analyser is presented and we develop transfer matrices which enable the imaging properties of such devices to be determined analytically. Expressions for the axial and radial magnification and for the energy resolving power are also derived. The analysis and the derived properties are compared with a numerical simulation and with all available published data.

Global MHD modeling of resonant ULF waves: Simulations with and without a plasmasphere

Abstract We investigate the plasmaspheric influence on the resonant mode coupling of magnetospheric ultralow frequency (ULF) waves using the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic (MHD) model. We present results from two different versions of the model, both driven by the same solar wind conditions: one version that contains a plasmasphere (the LFM coupled to the Rice Convection Model, where the Gallagher plasmasphere model is also included) and another that does not (the stand‐alone LFM). We find that the inclusion of a cold, dense plasmasphere has a significant impact on the nature of the simulated ULF waves. For example, the inclusion of a plasmasphere leads to a deeper (more earthward) penetration of the compressional (azimuthal) electric field fluctuations, due to a shift in the location of the wave turning points. Consequently, the locations where the compressional electric field oscillations resonantly couple their energy into local toroidal mode field line resonances also shift earthward. We also find, in both simulations, that higher‐frequency compressional (azimuthal) electric field oscillations penetrate deeper than lower frequency oscillations. In addition, the compressional wave mode structure in the simulations is consistent with a radial standing wave oscillation pattern, characteristic of a resonant waveguide. The incorporation of a plasmasphere into the LFM global MHD model represents an advance in the state of the art in regard to ULF wave modeling with such simulations. We offer a brief discussion of the implications for radiation belt modeling techniques that use the electric and magnetic field outputs from global MHD simulations to drive particle dynamics.

RCM‐E simulation of bimodal transport in the plasma sheet

Plasma sheet transport is bimodal, consisting of both large-scale adiabatic convection and intermittent bursty flows in both earthward and tailward directions. We present two comparison simulations with the Rice Convection Model—Equilibrium (RCM-E) to investigate how those high-speed flows affect the average configuration of the magnetosphere and its coupling to the ionosphere. One simulation represents pure large-scale slow-flow convection with time-independent boundary conditions; in addition to the background convection, the other simulation randomly imposes bubbles and blobs through the tailward boundary to a degree consistent with observed statistical properties of flows. Our results show that the bursty flows can significantly alter the magnetic and entropy profiles in the plasma sheet as well as the field-aligned current distributions in the ionosphere, bringing them into much better agreement with average observations.

On the contribution of plasma sheet bubbles to the storm time ring current

Particle injections occur frequently inside 10 RE during geomagnetic storms. They are commonly associated with bursty bulk flows or plasma sheet bubbles transported from the tail to the inner magnetosphere. Although observations and theoretical arguments have suggested that they may have an important role in storm time dynamics, this assertion has not been addressed quantitatively. In this paper, we investigate which process is dominant for the storm-time ring current buildup: large-scale enhanced convection or localized bubble injections. We use the Rice Convection Model—Equilibrium (RCM-E) to model a series of idealized storm main phases. The boundary conditions at 14-15 RE on the night side are adjusted to randomly inject bubbles to a degree roughly consistent with observed statistical properties. A test particle tracing technique is then used to identify the source of the ring current plasma. We find that the contribution of plasma-sheet bubbles to the ring current energy increases from ~20% for weak storms to ~50% for moderate storms and levels off at ~61% for intense storms; while the contribution of trapped particles decreases from ~60% for weak storms to ~30% for moderate and ~21% for intense storms. The contribution of non-bubble plasma-sheet flux tubes remains ~20% on average regardless of the storm intensity. Consistent with previous RCM and RCM-E simulations, our results show that the mechanisms for plasma-sheet bubbles enhancing the ring current energy are: (1) the deep penetration of bubbles and (2) the bulk plasma pushed ahead of bubbles. Both the bubbles and the plasma pushed ahead typically contain larger distribution functions than those in the inner magnetosphere at quiet times. An integrated effect of those individual bubble injections is the gradual enhancement of the storm time ring current. We also make two predictions testable against observations. First, fluctuations over a time scale of 5~20 minutes in the plasma distributions and electric field can be seen in the central ring current region for the storm main phase. We find that the plasma pressure and the electric field EY there vary over about 10%~30% and 50%~300% of the background values, respectively. Second, the maximum plasma pressure and magnetic field depression in the central ring current region during the main phase are well correlated with the Dst index.

RCM‐E simulation of a thin arc preceded by a north‐south‐aligned auroral streamer

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager data have recently revealed a repeatable sequence that occurs during many auroral substorms, in which a newly formed thin arc is preceded by an equatorward propagating streamer. The paper aims at modeling this sequence using the Rice Convection Model–Equilibrium. The simulation shows a thin arc arising when a plasma sheet bubble with its PV5/3 reduced to the transition region value arrives at the magnetic transition region. The modeled thin arc consists of two parts: the one east of the streamer is the result of the bubble pushing high PV5/3 flux tubes ahead of it, strengthening the upward region 2 current, and the one west of the streamer is associated with westward drifting bubble particles, sliding along the transition region. The model predicts that (1) the westward and eastward leading edges of the thin arc propagate azimuthally at a speed of ~0.5–2.7 km/s and (2) the streamer-induced thin arc is accompanied by classic signatures of bubble injections.