W. Lotko

Multiscale electrodynamics of the ionosphere-magnetosphere system

(1) In this paper we investigate how the parameters of the ionosphere and the low-altitude magnetosphere mediate the formation and spatiotemporal properties of small-scale, intense electromagnetic structures commonly observed by low-altitude satellites in the auroral and subauroral magnetosphere. The study is based on numerical modeling of a time-evolving, nonlinear system that describes multiscale electrodynamics of the magnetosphere-ionosphere coupled system in terms of field-aligned currents, both quasi- static and Alfvenic. Simulations show that intense electric fields and currents with a perpendicular size of 10-20 km at 120 km altitude can be generated by a large-scale, slowly evolving current system interacting with a weakly conducting ionosphere, even without a resonant cavity in the magnetosphere. These structures form in the strong gradient in the ionospheric conductivity that develops at the boundary between the large- scale upward and downward currents when the background ionospheric Pedersen conductivity, SP, is low but higher than the Alfven conductivity, SA =1 /m0vA, above the ionosphere. When SPSA the ionosphere can generate electromagnetic waves with perpendicular sizes less than 10 km. These waves can be trapped inside the cavity of the classical ionospheric Alfven resonator, and their amplitude can be significantly amplified there by the ionospheric feedback instability. INDEX TERMS: 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 2704 Magnetospheric Physics: Auroral phenomena (2407); 2753 Magnetospheric Physics: Numerical modeling; 2411 Ionosphere: Electric fields (2712); 2437 Ionosphere: Ionospheric dynamics; KEYWORDS: magnetosphere-ionosphere interaction, ionospheric feedback instability, field-aligned current, Alfven wave

Discrete auroral arc, electrostatic shock and suprathermal electrons powered by dispersive, anomalously resistive field line resonance

Particle and field data from a 4100-km-altitude satellite pass through a 1.3-mHz field line resonance, identified by ground-based optical, magnetic and radar signatures, are compared with results from a two-fluid MHD-gyrokinetic simulation, including dispersively and resistively sustained parallel electric fields. It is shown that the resonance powers spatially adjacent up- and down-going suprathermal-electron fluxes, a 10-km-scale auroral arc and an imbedded electrostatic shock. Alfven wave dispersion and anomalous plasma resistivity are key elements in the interpretation of the event.

Magnetosphere Sawtooth Oscillations Induced by Ionospheric Outflow

Numerical simulations show that a magnetospheric disturbance is caused by an influx of O+ ions from the ionosphere. The sawtooth mode of convection of Earth’s magnetosphere is a 2- to 4-hour planetary-scale oscillation powered by the solar wind–magnetosphere–ionosphere (SW-M-I) interaction. Using global simulations of geospace, we have shown that ionospheric O+ outflows can generate sawtooth oscillations. As the outflowing ions fill the inner magnetosphere, their pressure distends the nightside magnetic field. When the outflow fluence exceeds a threshold, magnetic field tension cannot confine the accumulating fluid; an O+-rich plasmoid is ejected, and the field dipolarizes. Below the threshold, the magnetosphere undergoes quasi-steady convection. Repetition and the sawtooth period are controlled by the strength of the SW-M-I interaction, which regulates the outflow fluence.

Effects of causally driven cusp O + outflow on the storm time magnetosphere-ionosphere system using a multifluid global simulation

densities in the inner magnetosphere can increase the strength of the ring current, reducing Dst and inflating the magnetosphere. This effect is mostly found for the less energetic outflow specification. O + outflow is found to reduce the access of solar wind ions to the inner magnetosphere, which, through the MI coupling in LFM reduces the precipitating electron power, conductance and field‐aligned currents. The effect outflow has on the cross polar cap potential (CPCP) depends upon two competing factors. The reduction in Region I currents when outflow is present appears to increase the CPCP whilst the inflation of the magnetosphere due to an enhanced ring current decreases the CPCP. Citation: Brambles, O. J., W. Lotko, P. A. Damiano, B. Zhang, M. Wiltberger, and J. Lyon (2010), Effects of causally driven cusp O + outflow on the storm time magnetosphere‐ionosphere system using a multifluid global simulation, J. Geophys. Res., 115,

Large Alfvén wave power in the plasma sheet boundary layer during the expansion phase of substorms

Observations by the Polar satellite of large Poynting flux in the plasma sheet boundary layer at geocentric distances of 4 to 6 RE and between 22 and 3 hrs magnetic local time were correlated with H-bay signatures from ground magnetometer records. We provide evidence that large Poynting fluxes occur during the substorm expansion phase. The Poynting fluxes exceeded 1 ergs/cm²s (125 ergs/cm²s when mapped to 100 km), were dominantly directed toward the ionosphere, and were associated with Alfven waves. These observations demonstrate the importance of Alfven wave power as a means of energy transport from the distant magnetotail to the ionosphere during the most dynamic phase of substorms.

Solar wind driving of magnetospheric ULF waves: Field line resonances driven by dynamic pressure fluctuations

[1] Several observational studies suggest that solar wind dynamic pressure fluctuations can drive magnetospheric ultralow‐frequency (ULF) waves on the dayside. To investigate this causal relationship, we present results from Lyon‐Fedder‐Mobarry (LFM) global, three‐dimensional magnetohydrodynamic (MHD) simulations of the solar wind–magnetosphere interaction. These simulations are driven with synthetic solar wind input conditions where idealized ULF dynamic pressure fluctuations are embedded in the upstream solar wind. In three of the simulations, a monochromatic, sinusoidal ULF oscillation is introduced into the solar wind dynamic pressure time series. In the fourth simulation, a continuum of ULF fluctuations over the 0–50 mHz frequency band is introduced into the solar wind dynamic pressure time series. In this numerical experiment, the idealized solar wind input conditions allow us to study only the effect of a fluctuating solar wind dynamic pressure, while holding all of the other solar wind driving parameters constant. We show that the monochromatic solar wind dynamic pressure fluctuations drive toroidal mode field line resonances (FLRs) on the dayside at locations where the upstream driving frequency matches a local field line eigenfrequency. In addition, we show that the continuum of upstream solar wind dynamic pressure fluctuations drives a continuous spectrum of toroidal mode FLRs on the dayside. The characteristics of the simulated FLRs agree well with FLR observations, including a phase reversal radially across a peak in wave power, a change in the sense of polarization across the noon meridian, and a net flux of energy into the ionosphere.

Dispersive field line resonances on auroral field lines

The formation of dispersive Alfven resonance layers is investigated using a three-dimensional, two-fluid, magnetically incompressible model, including electron inertia and finite pressure. The equations are solved in {open_quotes}box{close_quotes} geometry with uniform magnetic field bounded by perfectly conducting ionospheres. Field line resonance (FLR) is stimulated within a density boundary layer with gradient transverse to ambient B; a parallel gradient in the Alfven speed is also included. Numerical results show that the resonance amplitude is largest on the magnetic shell with eigenfrequency matching the frequency of the surface wave propagating on the density boundary layer. Efficient coupling between the resonant Alfven wave and surface wave produces a relatively narrow FLR spectrum, even when the driver is broadbanded. Effective coupling to the external driver occurs only for long-wavelength azimuthal modes. It is shown that the parallel inhomogeneity limits radiation of dispersive Alfven waves by the FLR. The results provide new insights into low-altitude satellite observations of auroral electromagnetic fields and the formation of discrete auroral arcs. 61 refs., 9 figs.

Coupling between density structures, electromagnetic waves and ionospheric feedback in the auroral zone

This paper presents results from a numerical study of nonlinear interactions between ultra-low-frequency (ULF) electromagnetic waves and the magnetospheric-ionospheric plasma at high latitudes. The study is motivated by observations of density cavities in the ionosphere in regions of downward field-aligned current adjacent to auroral arcs. The role of active ionospheric feedback in the development of intense, small-scale electromagnetic waves with frequencies of 0.1-1 Hz is considered, together with the effects of the waves on the ion dynamics. The numerical results are based on a reduced two-fluid MHD model that self-consistently describes shear Alfven waves, ion parallel dynamics, and effects of ionospheric E-region activity and magnetosphere-ionosphere feedback instability. Numerical simulations performed in dipole magnetic field geometry with realistic parameters of the ambient plasma show that, under some conditions, ionospheric feedback gives rise to intense ULF electromagnetic waves, which, via the ponderomotive force, produce density cavities in the bottomside ionosphere (between the E- and F-region peaks) and an associated upwelling of the topside ionosphere. The simulated magnitude and spatial and temporal scales of the cavities match the corresponding parameters of cavities observed with ground-based radars.

Small-scale electric fields in downward auroral current channels

[1] The origin and spatiotemporal properties of small-scale, intense electric fields and currents commonly observed by low-altitude, polar-orbiting satellites in the vicinity of discrete auroral arcs are investigated numerically. It is shown that these electromagnetic structures can be generated by the development of ionospheric feedback instability inside the resonant cavity formed by the ionospheric E-layer and an auroral acceleration region. The major factor regulating feedback unstable dynamics is the downward current channel of a large-scale, slowly evolving auroral current system interacting with the ionosphere. The downward current lowers the instability threshold by depleting the E-region plasma density and conductivity, thereby allowing the development of a large perpendicular electric field in the E-layer. The downward field-aligned current also produces a collisionless resistive layer in the lower magnetosphere where the parallel drift speed of current-carrying electrons exceeds a critical threshold for onset of microinstability. This resistive layer provides a highly reflective upper boundary for the

The fine structure of dispersive, nonradiative field line resonance layers

The fine structure of dispersive Alfven wave resonance layers extending along magnetic field lines from northern to southern auroral ionospheres is investigated using a magnetically incompressible, linear, two-fluid model. The model includes effects of finite electron inertia (at low altitude) and finite electron pressure (at high altitude). Plasma parameters are chosen so that refraction by the parallel inhomogeneity causes the dispersive Alfven wave to become trapped in the resonance layer. The parallel and perpendicular structure of these nonradiative, dispersive resonance layers is computed for the first four odd harmonics. A significant enhancement of the perpendicular and parallel components of the electric field near the ionosphere is found. The instantaneous potential drop along the magnetic field is sufficient to accelerate electrons up to several keV. The thickness of field line resonance layers in the ionosphere is estimated to be less than 5 km. The results suggest that dispersive resonance layers produce subkilometer-scale, multiple, discrete auroral arcs.