J. E. Borovsky

Dominant role of the asymmetric ring current in producing the stormtime Dst

Three storms are examined to determine the contribution to the Dst* index from the symmetric and asymmetric (partial) components of the ring current. The storms (September 24–25, 1998, October 18–19, 1998, and May 14–15, 1997) all have a similar solar wind trigger (an initial shock followed by a coronal mass ejection with southward interplanetary magnetic field) and placement in the solar cycle (rising phase). The near-Earth ion distribution function is simulated for each storm using a kinetic transport model. The use of a McIlwain magnetospheric electric field description improves the simulation results over the Volland-Stern field used previously. It is found that most of the main phase magnetic field depression is due to the asymmetric component of the ring current (≥80% at the Dst* minimum for the three storms). Note that this is a minimum asymmetric ring current contribution, because the closed-trajectory ions may also be spatially asymmetric. Ions in the partial ring current make one pass through the inner magnetosphere on open drift paths that intersect the dayside magnetopause. Changes in the density of the inner plasma sheet are transmitted directly along these open drift paths. For a steady convection field, an increase in the source population produces a decrease (more intense perturbation) in Dst*, while a decrease produces a Dst* recovery. As the storm recovery proceeds, a decrease in the electric field results in a conversion of open to closed drift paths, forming a trapped, symmetric ring current that dominates Dst*. The mostly H+ composition of the ring current for all three storms rules out the possibility of differential charge exchange being the cause of the fast and slow decay timescales, confirming that outflow is the main loss of ring current-generated Dst* during the early phase decay. The slow decay timescale in the late recovery, however, is dominated by charge exchange with the hydrogen geocorona. The symmetric-asymmetric ring current is also placed in the context of the solar wind and plasma sheet drivers.

Plasma sheet access to geosynchronous orbit

One years worth of magnetospheric plasma analyzer data from three Los Alamos geosynchronous satellites are used for a statistical study of proton and electron fluxes at geosynchronous orbit and their dependence on local time (LT) and geomagnetic activity level as measured by Kp. When displayed as a function of LT and Kp, the fluxes exhibit distinct boundaries, which are shown to be consistent with a combination of a global pattern of particle drift through the magnetosphere and loss processes mainly due to charge exchange of the ions and auroral precipitation of the electrons. A Hamiltonian energy conservation approach combined with the (U, B, K) coordinate transformation introduced by Whipple [1978] is used to calculate the theoretical position of the separatrix between open and closed drift trajectories (Alfven layer) as a function of particle species, energy, local time, and geomagnetic activity level. The comparison of the theoretical boundaries with the observations confirms the predictions of plasma sheet access to the geosynchronous region. The analysis also provides independent statistical support for previously derived relationships between Kp and the strength of the global convection electric field.

Characteristic plasma properties during dispersionless substorm injections at geosynchronous orbit

The substorm-associated behavior of the thermal plasma (30eV < E < 40keV) in the plasma sheet is examined by means of a superposed epoch analysis, using a full year of data from a spacecraft in geosynchronous orbit. The zero epoch time is taken to be substorm onset as indicated by a dispersionless energetic particle injection observed on the same spacecraft. Five classes of injection events are found to be well ordered by their average local times. These range from pure ion injections ∼3 hours prior to local midnight, to ion injections followed a few minutes later by an electron injection ∼2 hours before midnight, to simultaneous ion and electron injections close to midnight, to electron injections that are followed by an ion injection ∼1 hour postmidnight, and finally to pure electron injections ∼2 hours postmidnight. The thermal electrons show a significant increase in temperature (from ∼1 to ∼2keV) and pressure at substorm onset, while the density and the thermal ion signatures (below ∼30keV) are typically weak and may even vary with local time. However, energetic ions (above ∼30keV), which contribute significantly to the total ion pressure, show clear flux enhancements, leading to a rise in the total temperature from ∼10 to ∼16keV. Preexisting perpendicular anisotropies in the thermal electrons are reduced during the substorm growth phase but become enhanced again after onset, sometimes after a brief period of parallel anisotropy. Similar anisotropy signatures are found for the thermal ions, although somewhat less pronounced.

Effects of a high‐density plasma sheet on ring current development during the November 2–6, 1993, magnetic storm

The growth and recovery of the November 2–6, 1993 magnetic storm was simulated using a drift-loss ring current model that was driven by dynamic fluxes at geosynchronous orbit as an outer boundary condition. During the storm main phase, a high-density plasma sheet was observed by the Los Alamos National Laboratory geosynchronous satellites to move into and flow around the inner magnetosphere over a period of ∼12 hours [Borovsky et al., 1997; this issue] during the storm main phase. Densities at the leading edge of this structure reached 3 cm−3 as compared with more typical values <1 cm−3. The factor of 3 change in the plasma sheet density from quiet to active times produced a factor of 3 enhancement in the strength of the simulated ring current. In addition, a short-timescale recovery in the Dst index at 1600 UT on November 4 was driven by changes in the outer boundary condition and appeared even in the absence of collisional losses. An overshoot in the minimum Dst* occurred in the simulated ring current compared with observed values at ∼0200 UT on November 4 and is taken as evidence of a loss process not included in the ring current-atmosphere interaction model (RAM). The storm onset was associated with a compression of the entire dayside magnetopause to within geostationary orbit starting at 2307 UT and continuing for a half hour. It is suggested that a possible additional loss may have resulted as ions drifted to the compressed dayside magnetopause. In fact such losses were found in another simulation of the inner magnetosphere for the same storm by Freeman et al. [1996]. The energy supplied to the inner magnetosphere, relative to the total energy input during this magnetic storm, was examined by comparing two widely used energy input functions, the e parameter [Akasofu, 1981] and the F parameter [Burton et al., 1975] against energy input to the ring current model based on geosynchronous plasma observations at the outer boundary. It is found that the e parameter [Akasofu, 1981] overestimates the ring current energy input compared to the drift-loss model by almost an order of magnitude during the main phase. However, the integrated energy input from e, over the 4 day interval of the storm, is in very good agreement with the total energy input inferred from observations. On the other hand, F more closely approximates the magnitude of the ring current energy input alone as calculated in the drift-loss model. An energy budget is constructed for the storm that shows energy inputs from the solar wind and energy dissipation due to ring current buildup and decay, auroral electron precipitation, Joule heating, ion precipitation, and energy storage in the magnetotail in reasonable balance. The ring current energy input accounts for only 15% of the total dissipated energy in this storm interval. A more complete energy budget that extends to November 11, 1993, was compiled by Knipp et al. [this issue].

Plasma sheet turbulence observed by Cluster II

Cluster fluxgate magnetometer (FGM) and ion spectrometer (CIS) data are employed to analyze magnetic field fluctuations within the plasma sheet during passages through the magnetotail region in the summers of 2001 and 2002 and, in particular, to look for characteristics of magnetohydrodynamic (MHD) turbulence. Power spectral indices determined from power spectral density functions are on average larger than Kolmogorovs theoretical value for fluid turbulence as well as Kraichnans theoretical value for MHD plasma turbulence. Probability distribution functions of the magnetic fluctuations show a scaling law over a large range of temporal scales with non-Gaussian distributions at small dissipative scales and inertial scales and more Gaussian distribution at large driving scales. Furthermore, a multifractal analysis of the magnetic field components shows scaling behavior in the inertial range of the fluctuations from about 20 s to 13 min for moments through the fifth order. Both the scaling behavior of the probability distribution functions and the multifractal structure function suggest that intermittent turbulence is present within the plasma sheet. The unique multispacecraft aspect and fortuitous spacecraft spacing allow us to examine the turbulent eddy scale sizes. Dynamic autocorrelation and cross correlation analysis of the magnetic field components allow us to determine that eddy scale sizes fit within the plasma sheet. These results suggest that magnetic field turbulence is occurring within the plasma sheet resulting in turbulent energy dissipation.

Variability of the ring current source population

The relationship between the strength of the storm-time ring current and the available density in the presumed source region, the plasma sheet, is examined for 23 storms with Dst * (minimum Dst corrected for magnetopause currents) ranging from -50 to -164 nT. A good correlation is found between the plasma-sheet density at geosynchronous orbit and the minimum Dst * , The minimum Dst * is also well correlated with the eastward interplanetary electric field measured upstream by the Wind spacecraft, in agreement with previous studies, but the dependence of Dst * on plasma-sheet density is unrelated to its dependence on the electric field. The best correlation is between the minimum Dst * and the product of the plasma-sheet density and the eastward interplanetary electric field. These results are consistent with a scenario in which the intensity of the storm-time ring current is determined by a combination of source strength (plasma-sheet density) and injection strength (interplanetary electric field).

Measurements of early and late time plasmasphere refilling as observed from geosynchronous orbit

Measurements of the cold ion density at geosynchronous orbit obtained by Los Alamos magnetospheric plasma analyzers over the course of 7 years are used to estimate the rate of plasmaspheric refilling at early times in the refilling process (≤24 hours) and at late times (up to several days during intervals of prolonged geomagnetic quiet). While the measured refilling rates are highly variable, we find that plasmasphere refilling at geosynchronous orbit occurs as a two-step process: for early times the refilling rate is ∼0.6–12 cm−3 d−1, while for later times the refilling rate rises up to 10–50 cm−3 d−1. These results are consistent with a model of plasmasphere refilling which uses Coulomb collisions as a dominant trapping mechanism [see Wilson et al. 1992].

The fate of the outer plasmasphere

Both the solar wind and the ionosphere contribute to Earths magnetospheric plasma environment. However, it is not widely appreciated that the plasmasphere is a large reservoir of ionospheric ions that can be tapped to populate the plasma sheet. We employ empirical models of high-latitude ionospheric convection and the geomagnetic field to describe the transport of outer plasmasphere flux tubes from the dayside, over the polar cap and into the magnetotail during the early phases of a geomagnetic storm. We calculate that this process can give rise to high densities of cold plasma in the magnetotail lobes and in the near-Earth plasma sheet during times of enhanced geomagnetic activity, and especially during storms. This model can help explain both polar cap ionization patches and the presence of cold flowing ions downtail.

Magnetosphere preconditioning under northward IMF: Evidence from the study of coronal mass ejection and corotating interaction region geoeffectiveness

[1] Motivated by recent observations and simulations of the formation of a cold and dense plasma sheet in the tail of the magnetosphere under northward interplanetary magnetic field (IMF) and of the direct influence of the plasma sheet density on the ring current strength, this paper aims at (1) highlighting how the coupling of these effects may lead to a preconditioning of the magnetosphere under northward IMF and (2) performing first tests of the validity of this hypothesis. We have analyzed superposed epoch time series of various parameters to investigate the response of the magnetosphere (as indicated by the Dst index) to the passage of coronal mass ejections (CMEs) and corotating interaction regions (CIRs). We first focused on the difference between the measured Dst signature and that predicted by a semiempirical Dst model. For both CME- and CIR-driven storms the superposed epoch results show that the model Dst predictions tend to underestimate the actual storm strength (by up to 10–30%) for events that are preceded by a substantial interval of northward IMF, as opposed to those with no such preceding northward IMF. We also analyzed Los Alamos geosynchronous spacecraft data for these events. The average density and temperature measured at storm onset are substantially higher and slightly lower, respectively, for the cases with preceding northward IMF intervals. These results suggest that solar wind structures may be more geoeffective if preceded by a northward IMF interval and they are consistent with the hypothesis of a preconditioning by a cold, dense plasma sheet. A colder and denser plasma sheet may lead to a stronger ring current when that plasma is convected inward during the main phase of an ensuing storm.

Properties of asymmetric magnetic reconnection

Properties of magnetic reconnection are investigated in two-dimensional, resistive magnetohydrodynamic (MHD) simulations of current sheets separating plasmas with different magnetic field strengths and densities. Specific emphasis is on the influence of the external parameters on the reconnection rate. The effect of the dissipation in the resistive MHD model is separated from this influence by evaluating resistivity dependence together with the dependence on the background parameters. Two scenarios are considered, which may be distinguished as driven and nondriven reconnection. In either scenario, the maximum reconnection rate (electric field) is found to depend on appropriate hybrid expressions based on a magnetic field strength and an Alfven speed derived from the characteristic values in the two inflow regions. The scaling compares favorably with an analytic formula derived recently by Cassak and Shay [Phys. Plasmas 14, 102114 (2007)] applied to the regime of fast reconnection. An investigation of the ...