R. W. Spiro

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.

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

Abstract A model of the O+ and H+ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O+ and H+ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H+ distribution in the topside ionosphere at all local times. During the day the upward H+ flow increases with latitude to produce the light-ion trough. At night the H+ trough may be directly produced by the occurrence of the mid-latitude O+ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.

The physics of the Harang discontinuity

Absent a source of energetic ions at the flanks of the tail, the westward gradient/curvature drift of E×B-convecting plasma results in the depletion of energetic ions from the dawnside of the plasma sheet. This dawnside depletion effect means that, on average, the duskside of the plasma sheet will have higher ion temperatures, pressures, and flux tube contents, and hence, stronger westward cross-tail drift current than the dawnside. The resulting cross-tail divergence of drift current must find closure by means of Birkeland currents connecting to the ionosphere. Tailward and poleward of the inner edge region, the divergence of cross-tail current requires upward current from the ionosphere. In the ionosphere, current closure requires electric fields that are directed toward the center of the upward current, i.e., directed equatorward on the poleward side, poleward on the equatorward side. This is precisely the nature of the Harang discontinuity. The region poleward of the Harang discontinuity maps well out into the plasma sheet and provides an eastward component of E×B drift to oppose the westward gradient/curvature drift of the ions. This helps keep the flow of plasma sheet ions directed toward the inner plasma sheet, rather than toward the dusk flank of the tail, a point originally made by Atkinson. The region equatorward of the Harang discontinuity maps close to the inner edge of the plasma sheet and results in westward E×B drift, increasing the westward flow of plasma azimuthally around the duskside of the inner magnetosphere and toward the dayside magnetopause. Although this scenario can be understood qualitatively, runs were carried out using the Rice convection model (RCM) to examine the ionospheric-magnetospheric coupling implications of this dawnside depletion effect. These runs confirm the above scenario, generally. They show that dawnside ion depletion results in a band of upward Birkeland current in the central auroral zone on the nightside, similar to what has been consistently observed by Iijima and Potemra and others. These currents modify the nightside, auroral electric field distribution to produce a strong reversal in the meridional electric field, similar to the classically observed Harang discontinuity. Inclusion of dawnside ion depletion also results in a major improvement in the agreement between the RCM and observations with regard to the latitudinal distribution of Birkeland currents. Finally, the dawnside depletion effect results in a reduction of plasma sheet pressures in the near-Earth, midnight sector of the plasma sheet. However, this reduction is significantly less than that suggested by Kivelson and Spence.

Numerical simulation of torus‐driven plasma transport in the Jovian magnetosphere

The Rice convection model has been modified for application to the transport of Io-generated plasma through the Jovian magnetosphere. The new code, called the RCM-J, has been used for several ideal-MHD numerical simulations to study how interchange instability causes an initially assumed torus configuration to break up. In simulations that start from a realistic torus configuration but include no energetic particles, the torus disintegrates too quickly (∼50 hours). By adding an impounding distribution of energetic particles to suppress the interchange instability, reasonable lifetimes were obtained. For cases in which impoundment is insufficient to produce ideal-MHD stability, the torus breaks up predominantly into long fingers, unless the initial condition strongly favors some other geometrical form. If the initial torus has more mass on one side of the planet than the other, fingers form predominantly on the heavy side (which we associate with the active sector). Coriolis force bends the fingers to lag corotation. The simulation results are consistent with the idea that the fingers are formed with a longitudinal thickness that is roughly equal to the latitudinal distance over which the invariant density declines at the outer edges of the initial torus. Our calculations give an average longitudinal distance between plasma fingers of about 15°, which corresponds to 20 to 30 minutes of rotation of the torus. We point to some Voyager and Ulysses data that are consistent with this scale of torus longitudinal irregularity.

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

Abstract An attempt has been made to extend the Rice Convection Model (RCM) and to merge it with empirical models, so as to cover the entire high-latitude ionosphere. Specifically, we have modified the RCM in two ways 1. (1) by a tailward expansion of the region where coupled ionosphere-magnetosphere convection model calculations are done 2. (2) by use of ionospheric observation-based models poleward of the region where detailed convection modelling is applicable. We use a scaled and shifted Heppner-Maynard-Rich electric-field model directly for empirical extension of the ionospheric potential distribution into the polar cap and the statistical electron precipitation model of Hardy and co-workers less directly for the poleward extension of the auroral precipitation pattern. One goal in globalizing the RCM is to provide precipitation and electric field inputs for ionosphere and thermosphere modelers. We hope to provide an alternative to purely empirical precipitation and electric field models, by means of a hybrid model that is theoretical and dynamical with regard to the inner and middle plasma sheet, though still empirical with regard to the boundary plasma sheet and polar cap. We wish to avoid the statistical blurring that is a natural characteristic of empirical models and also to produce a model in which the boundaries of the precipitation and electric field patterns maintain physically consistent relationships to each other. Although the first set of runs of this globalized version of the RCM did indeed produce precipitation and electric field patterns with sharp features and with theoretically consisent relationships between boundaries, the results displayed two substantial difficulties. First, the model-predicted latitudinal width of the auroral sunward-flow region tended to be too narrow. Second, to avoid vastly unrealistic model precipitation rates, we were forced to place an artificial floor under the computed precipitation rate from the middle and outer plasma sheet. The computed auroral electron energy flux, plotted as a function of latitude, exhibited an exaggerated two-peak structure: one peak lies poleward of the coupled modeling region and is associated with the region-one Birkeland currents; the other peak lies at lower auroral latitudes within the coupled-modeling region, and is associated with the inner edge of the plasma sheet. When no floor was placed under the precipitation rate, the minimum between the two peaks was much too deep to be consistent with typical observations. The regions of excessively weak precipitation map to equatorial distances of 15 35 RE and thus to the regions of the plasma sheet that have not been included in previous self-consistent convection calculations. The most likely origin of the discrepancy is that electrons in the plasma sheet beyond 15 RE may not approximately satisfy the simple adiabatic condition p e (∝ds/B) 5 3 = constant ; there is independent evidence that plasma-sheet ions violate the analogous adiabatic condition. In neither case is it clear what physical mechanism causes the violation.

Storm‐time penetration electric fields and their effects

The ‘convection’ electric field, set up in the magnetosphere by the interaction of the solar wind plasma flowing around the Earths magnetic field, projects along magnetic field lines to low altitudes where it drives the high-latitude ionospheric convection. During active times, ionospheric electric fields are thought to originate from two sources: a disturbed wind dynamo and electric fields that penetrate from high latitudes. In the latter, when the magnetospheric convection is enhanced following a southward turning of the interplanetary magnetic field (IMF), the initial high-latitude electric field will penetrate to the equatorial latitudes.

Generation of region 1 current by magnetospheric pressure gradients

The Rice Convection Model (RCM) is used to illustrate theoretical possibilities for generating region 1 Birkeland currents by pressure gradients on closed field lines in the Earths magnetosphere. Inertial effects and viscous forces are neglected. The RCM is applied to idealized cases, to emphasize the basic physical ideas rather than realistic representation of the actual magnetosphere. Ionospheric conductance is taken to be uniform, and the simplest possible representations of the magnetospheric plasma are used. Three basic cases are considered: (1) the case of pure northward Interplanetary Magnetic Field (IMF), with cusp merging assumed to create new closed field lines near the nose of the magnetosphere, following the suggestion by Song and Russell (1992); (2) the case where Dungey-type reconnection occurs at the nose, but magnetosheath plasma somehow enters closed field lines on the dawnside and duskside of the merging region, causing a pressure-driven low-latitude boundary layer; and (3) the case where Dungey-type reconnection occurs at the nose, but region 1 currents flow on sunward drifting plasma sheet field lines. In case 1, currents of region 1 sense are generated by pressure gradients, but those currents do not supply the power for ionospheric convection. Results for case 2 suggest that pressure gradients at the inner edge of the low-latitude boundary layer might generate a large fraction of the region 1 Birkeland currents that drive magnetospheric convection. Results for case 3 indicate that pressure gradients in the plasma sheet could provide part of the region 1 current.

Numerical simulation of plasma transport driven by the Io torus

The Rice Convection Model (RCM) has been modified to a form suitable for Jupiter (RCM-J) to study plasma interchange motion in and near the Io plasma torus. The net result of the interchange is that flux tubes, heavily loaded with torus plasma, are transported outward, to be replaced by tubes containing little low-energy (< 1 keV) plasma. The process is numerically simulated in terms of time evolution from an initial torus that is longitudinally asymmetric and with gradually decreasing density outward from Ios orbit. In the simulations, the nonlinear stage of the instability characteristically exhibits outreaching fingers of heavily-loaded flux tubes that lengthen at an accelerating rate. Our principal finding is that the primary geometrical form of outward transport of torus plasma in Jupiters magnetosphere is through long, outward-moving fingers of plasma. In the simulations, the fingers mainly form in the active sector of the Io torus (the heavier side of the asymmetric torus), and they are spaced longitudinally roughly 20° apart.

Numerical Modeling of the Ring Current and Plasmasphere

Over the last 25 years, considerable scientific effort has been expended in the development of quantitative models of the dynamics of Earths inner magnetosphere, particularly on studies of the injection of the storm-time ring current and of dynamic variations in the shape and size of the plasmasphere. Nearly all modeling studies of ring-current injection agree that time-varying magnetospheric convection can produce approximately the ion fluxes that are observed in the storm-time ring current, but the truth of that assumption has never been demonstrated conclusively. It is not clear that the actual variations of convection electric fields are strong enough to explain the observed flux increases in ~100 keV ions at the peak of the storm-time ring current. Observational comparisons are generally far from tight, primarily due to the paucity of ring-current measurements and to basic limitations of single-point observations. Also, most of the theoretical models combine state-of-the-art treatment of some aspects of the problem with highly simplified treatment of other aspects. Even the most sophisticated treatments of the sub-problems include substantial uncertainties, including the following: (i) There is still considerable theoretical and observational uncertainty about the dynamics of the large-scale electric fields in the inner magnetosphere; (ii) No one has ever calculated a force-balanced, time-dependent magnetic-field model consistent with injection of the storm-time ring current; (iii) The most obvious check on the overall realism of a ring-current injection model would be to compare its predicted Dst index against observations; however, theoretical calculations of that index usually employ the Dessler-Parker-Sckopke relation, which was derived from the assumption of a dipole magnetic field and cannot be applied reliably to conditions where the plasma pressure significantly distorts the field; (iv) Although loss rates by charge exchange and Coulomb scattering can be calculated with reasonable accuracy, it remains unclear whether wave-induced ion precipitation plays an important role in the decay of the ring current. However, considerable progress could be made in the next few years. Spacecraft that can provide images of large regions of the inner magnetosphere should eliminate much of the present ambiguity associated with single-point measurements. On the theoretical side, it will soon be possible to construct models that, for the first time, will solve a complete set of large-scale equations for the entire inner magnetosphere. The biggest uncertainty in the calculation of the size and shape of the plasmasphere lies in the dynamics and structure of the electric field. It is still not clear how important a role interchange instability plays in determining the shape of the plasmapause or in creating density fine structure.

Modeling Inner Magnetospheric Electric Fields: Latest Self-Consistent Results

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.