E. N. Fedorov

Influence of ionospheric conductivity on mid-latitude Pc 3-4 pulsations

Diurnal variations of the parameters of the magnetospheric Alfvén resonator at different latitudes have been calculated using a semi-empirical model of the ionosphere-magnetosphere plasma distribution. The ionospheric plasma density is taken from the IRI model, the electron density at the magnetospheric equator is based on the ISEE/whistler model, and the field-aligned magnetospheric plasma distribution is calculated under the assumption of diffusive equilibrium. It is shown that for the mid-latitude ionosphere the Hall conductivity has no effect on the parameters of the magnetospheric Alfvén resonator. The calculated values of damping rates of Alfvén oscillations at middle latitudes during the dark period are too high for the “free-end” and “quarter-wave” oscillation regimes to be realized. At low latitudes quality factors are relatively high both at daytime and nighttime conditions. An expected change of a field-aligned structure of Alfvén oscillations during the transition from dayside to nightside ionospheric conditions does not occur. The analysis of the experimental data recorded at middle and low latitude stations of the “210° Magnetic Meridian” magnetometer network and station l’Aquila gives the results, compatible with the predictions of the numerical model: (a) the pulsation amplitude in a frequency band near the fundamental harmonic of the Alfvén field line resonance has the strongest dependence on the ionospheric conductivity; (b) the influence of day/night ionospheric conditions on the Pc 3 amplitudes is less at low (L ≤ 2) geomagnetic latitudes than at middle latitudes; (c) the ionospheric conductivity control of the Pc 3 amplitude at middle latitudes weakens with increasing harmonic number.

ULF wave damping in the auroral acceleration region

The width of a resonant peak in the spatial distribution of the ULF Pc5 pulsation amplitude at high latitudes is controlled by the dominant dissipation mechanism of Alfven waves in the ionosphere-magnetosphere system. Among possible mechanisms, mostly Joule dissipation and dispersive effects are considered to terminate the growth and narrowing of a resonant peak pumped by an external source. We estimate the width of the resonance which may be provided by the field-aligned electric field in a thermal, mirror-confined collisionless plasma. We consider a simple case, in which a potential drop is confined within a narrow region above the ionosphere, called the auroral acceleration region. Theoretical estimates show that the mirror-force mechanism can dominate over the ionospheric dissipation and dispersive effects in auroral regions with a sufficiently large field-aligned potential drop. Therefore ground-based monitoring of the resonance width (e.g., with the gradient method) may be used as an indicator of the occurrence of large-scale parallel electric fields in the outer magnetosphere.

Coupling between field-aligned current impulses and Pi1 noise bursts

The estimated densities of local field-aligned currents related to nighttime substorm onset or magnetic impulse events at cusp latitudes may exceed the threshold for the excitation of high-frequency turbulence in the topside ionosphere. The evolution of an ion-acoustic instability is shown to lead to quasi-oscillations around the saturation level. Consequent variations in anomalous conductivity will result in pulsed electron precipitation and noise generation in the Pi1 band. This proposed mechanism gives a natural interpretation to the experimentally observed coupling between localized magnetic disturbances, electron precipitation/acceleration, and bursts of high-frequency ULF noise. The alternative interpretations based on mechanisms of ion cyclotron instability stimulation and ionospheric Alfven resonator excitation can be applied only under specific plasma conditions.

Dispersion relation for ballooning modes and condition of their stability in the near-earth plasma

The ballooning disturbances in a finite-pressure plasma in a curvilinear magnetic field are described by a system of coupled equations for Alfvén and slow magnetosonic modes. The local dispersion relation obtained in a WKB approximation is the simplest and most evident method that can be used to characterize the properties of these disturbances. This dispersion relation is widely used to predict the possible instabilities and spectral properties of LF oscillations in the nightside magnetosphere. The formal derivation of the dispersion relation from the initial system of coupled MHD modes and the transition to different limiting cases have been traced. The behavior of dispersion curves in different oscillation branches and the possible development of instabilities and formation of regions where waves cannot propagate have been studied in detail. This made it possible to specify the results of previous works and even indicate the incorrectness in some works. In particular, it has been indicated that a fast Alfvén branch of oscillations is always stable and an aperiodic instability can originate on a slow magnetosonic oscillation branch.

MHD wave conversion in plasma waveguides

A theoretical approach to the consideration of coupling between Alfven and compressional modes in two-dimensionally inhomogeneous plasma has been developed. The conversion of a compressional mode, propagating along a high-density layer perpendicular to the magnetic field, into leaking shear Alfven waves has been considered. The wave spatial structure along the layer is described in WKB approximation. A compressional-type disturbance effectively emitts Alfven waves when a local resonant condition within the layer is fulfilled. In contrast with the common field line resonance theory, the excited Alfven modes are assumed to be running away waves but not standing oscillations. The physical situations that correspond to the considered model can be met in the outer near-equatorial magnetosphere, in the exterior cusp region, and in the other magnetospheres such as Jupiter and Saturn.

Modulation of total electron content by ULF Pc5 waves

An intriguing effect was found while analyzing the small-scale variations of total electron content (TEC) derived from global positioning system (GPS) signals. We found a response in TEC variations to intense global Pc5 pulsations with periods of a few millihertz covering the corrected geomagnetic latitudes ~58°–75° during the recovery phase of the strong magnetic storms on 31 October 2003. Earlier studies demonstrated that the GPS-TEC technique is a powerful method to study the propagation pattern of transient disturbances in the ionosphere, generated by seismic or internal gravity waves. This technique has turned out to be sensitive enough to ULF waves as well. During periods with intense Pc5 geomagnetic wave activity, distinct pulsations with the same periodicity were found in the TEC data from high-latitude GPS receiving stations in Scandinavia. Wavelet and cross-spectral analysis showed a high coherence (~0.9) between the periodic geomagnetic and TEC variations. Moreover, the relative amplitude of TEC periodic fluctuations ΔTEC/TEC was about or even larger than the relative amplitude of geomagnetic variations ΔB/B. So far, the effect of TEC modulation by Pc5 waves is not well understood and is still a challenge for the MHD wave theory. Various possible modulation mechanisms have been estimated, but no mechanism has been firmly identified.

Ballooning modes and their stability in a near-Earth plasma

As a possible trigger of the substorm onset, the ballooning instability has been often suggested. The ballooning disturbances in a finite-pressure plasma immersed into a curved magnetic field are described with the system of coupled equations for the Alfven and slow magnetosonic modes. The spectral properties of ballooning disturbances and instabilities can be characterized by the local dispersion equation. The basic system of equations can be reduced to the dispersion equation for the small-scale in transverse direction disturbances. From this relationship the dispersion, instability threshold, and stop-bands of the Alfvenic and slow magnetosonic modes have been determined. The field-aligned structure of unstable mode is described with the solution of the eigenvalue problem in the Voigt model. We have also analyzed in a cylindrical geometry an eigenvalue problem for the stability of ballooning disturbances with a finite scale along the plasma inhomogeneity. The account of a finite scale in the radial direction raises the instability threshold as compared with that in the WKB approximation.

ULF Waves in the Topside Ionosphere: Satellite Observations and Modeling

Low-orbiting observations at satellites with high-accuracy magnetometers onboard (Oersted, CHAMP, ST5) have provided the possibility to examine the ULF wave (Pc3, Pi2, Pc1) structure in the topside ionosphere. Pc3 waves were detected very clearly in the compressional component of the satellite magnetic data, whereas on the ground, their signature was found in the H component. The relationships between the Pc3 wave compressional magnetic component above the ionosphere and the ground response can be produced by two different mechanisms: (a) an incident Alfven wave generates an evanescent fast mode as a result of its interaction with the anisotropically conducting ionosphere; (b) transport of ULF wave energy from a distant source towards the ionosphere predominantly occurs via the fast mode. Numerical modeling and analytical estimates have been applied to the interpretation of Pc3 waves observed by CHAMP in the upper ionosphere and by ground stations at mid-latitudes. The observed ratio between the compressional component in space and the ground signal corresponds better to the scenario of direct fast mode transmission to the ground. To interpret simultaneous observations of low-latitude Pi2 pulsations at CHAMP and on the ground a simple analytical model which couples an incident compressional wave above the equatorial ionosphere with the ground response can be used. The amplitude and phase relationships between Pi2 signals in the upper ionosphere and on the ground at conjugate stations Tihany and Hermanus (L∼1.8) match the theoretical predictions.

Ionospheric propagation of magnetohydrodynamic disturbances from the equatorial electrojet

The propagation features of magnetohydrodynamic (MHD) disturbances along the ionosphere are considered from a theoretical point of view. Special attention is paid to the possibility of ionospheric propagation of disturbances produced by variations in the equatorial electrojet. The possible MHD modes within the E layer include oscillations that have damping scales of about effective ionospheric skin depth or less. The existence of a large-scale compressional surface mode at small inclinations of the geomagnetic field along the E layer is shown. The propagation of this mode, called the gyrotropic surface mode, takes place in a diffusive-like way along the E layer. Its damping scale is much greater than the ionospheric skin depth, and it may reach several hundred to thousand kilometers. The apparent propagation velocity of the gyrotropic mode at the near-equatorial latitudes is determined by height-integrated Cowling conductivity; its value is estimated to be about 20 – 100 km/s for the Pc3–4 frequency range. We suggest that the equatorial electrojet may contribute to the ULF geomagnetic variations observed at near-equatorial stations.

Alfven wave modulation of the auroral acceleration region

We consider the interaction of Alfven waves with the auroral acceleration region (AAR). The AAR is characterized by an electric potential drop that supports a field-aligned upward current and the acceleration of precipitating electrons. An Alfven wave incident on the AAR from the magnetosphere partially reflects back and partially penetrates into the AAR. The rate of wave reflection/transmission is estimated to be critically dependent on the wave transverse scale. Magnetospheric Alfven waves penetrating into the AAR can produce oscillatory variations of the field-aligned potential drop, thus constituting a new mechanism of ULF modulation of electron acceleration. Estimates of the potential drop modulation by Alfven waves are provided within the “thin” AAR approximation, which is valid for a wide range of wave and plasma parameters. The proposed mechanism will produce nearly simultaneous ULF magnetic and riometric variations at auroral latitudes. Occurrence of the AAR-associated resonator in the auroral topside ionosphere between the bottom boundary of the AAR and the E-layer may cause oscillatory frequency dependence of electron acceleration modulations in the range around fractions of a Hz. Another feature of the mechanism considered is the critical dependence of the ratio between the magnetic and riometric signals on the transverse scale of the disturbance. The predicted effects are to be searched for in the simultaneous data of IRIS multi-beam riometers and magnetometers.