Alfven Waves In Magnetosphere Research Articles

Dependence of the Pc4 magnetic pulsation parameters on the radiated power of the EISCAT HF heating facility

The interaction between the Earth’s ionosphere and magnetosphere in a situation when artificial disturbances are generated in the F region of the auroral ionosphere with the EISCAT/Heating facility is studied. An experiment was performed in the daytime when the facility effective radiated power changed in a stepwise manner. Wavelike disturbances with periods of (130–140) s corresponding to Pc4 pulsations were simultaneously registered by the method of bi-static backscatter and with ground magnetometers. The variations in the Doppler frequency shift were correlated with the changes in the facility power. Incoherent scatter radar measurements at a frequency of 930 MHz (Tromso) and numerical calculations were used in an analysis. It has been indicated that the ionospheric drift of small-scale artificial ionospheric irregularities was modulated by magnetospheric Alfven waves. The possible effect of powerful HF radioemission on the Alfven wave amplitude owing to the modification of the magnetospheric resonator ionospheric edge reflectivity and the generation of an outgoing Alfven wave above the region where the ionospheric conductivity is locally intensified has been considered.

Energy budget of Alfven wave interactions with the auroral acceleration region

We consider the interaction of Alfven waves with the auroral acceleration region (AAR), which is characterized by a mirror resistance and a related electric potential drop that supports a field‐aligned upward current and the acceleration of electrons. An Alfven wave incident on the AAR from the magnetosphere partially reflects back and partially penetrates into the AAR. An analytical treatment of the interaction of Alfven waves with the combined magnetosphere‐AAR‐topside ionosphere‐E layer system immersed into a converging dipole‐like magnetic field has been made within the “thin” AAR approximation. The rate of wave reflection/transmission is estimated to be critically dependent on the wave transverse scale. Incident Alfven waves with spatial scales about the Alfven damping scale λA are absorbed most effectively in the AAR. Magnetospheric Alfven waves penetrating into the AAR can produce oscillatory variations of the field‐aligned potential drop and field‐aligned electron acceleration. Modeling the spatial spectrum of an Alfven burst by a power‐law dependence ∝ k−p in the interval between low and high cutoff wave numbers indicates that the estimated rate of total wave power absorbed by the AAR is significant, up to 30–50% depending on p, when the ionospheric projection of the low cutoff wave number is on the order of the scale λA.

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.

Self‐excitation of auroral arcs in a three‐dimensionally coupled magnetosphere‐ionosphere system

This paper presents the first full three‐dimensional dynamic simulation of auroral arc formation. The magnetospheric and ionospheric dynamics are represented by one‐fluid magnetohydrodynamic equations and two‐fluid weakly ionized plasma equations, respectively. The feedback coupling between magnetospheric Alfven waves and ionospheric density waves results in a spontaneous generation of longitudinally elongated striations of field‐aligned currents and ionospheric electron densities, which compare very well with many features of quiet auroral arcs.

The effect of non-uniform ionospheric conductivity on standing magnetospheric Alfven waves

We look at time-dependent normal mode solutions to the Alfven wave equation in a uniform magnetic field, between planar ionospheres. In particular, the effect of sharp gradients in ionospheric conductivity on the spatial and temporal structure of the waves is considered. We show that the electric field of the wave must always be perpendicular to any conductivity discontinuities present, and that this is achieved by the generation of circularly polarized Alfven waves at the discontinuity. The results are applied to an ionospheric strip of high conductivity; this being relevant to Pi2s.