V. A. Pilipenko

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 waveguides in space plasma

The waveguide properties of two characteristic formations in the Earth’s magnetotail—the plasma sheet and the current (neutral) sheet—are considered. The question of how the domains of existence of different types of MHD waveguide modes (fast and slow, body and surface) in the (k, ω) plane and their dispersion properties depend on the waveguide parameters is studied. Investigation of the dispersion relation in a number of particular (limiting) cases makes it possible to obtain a fairly complete qualitative pattern of all the branches of the dispersion curve. Accounting for the finite size of perturbations across the wave propagation direction reveals new additional effects such as a change in the critical waveguide frequencies, the excitation of longitudinal current at the boundaries of the sheets, and a change in the symmetry of the fundamental mode. Knowledge of the waveguide properties of the plasma and current sheets can explain the occurrence of preferred frequencies in the low-frequency fluctuation spectra in the magnetotail. In satellite observations, the type of waveguide mode can be determined from the spectral properties, as well as from the phase relationships between plasma oscillations and magnetic field oscillations that are presented in this paper.

Longitudinal Structure of Ballooning MHD Disturbances in a Model Magnetosphere

Large-scale toroidal Pc5 pulsations are commonly treated as Alfven oscillations of a magnetic field line. According to observations, their longitudinal structure is described well by theory. At the same time, the longitudinal structure of azimuthal small-scale poloidal Pc5 pulsations is virtually unknown. These pulsations are associated with ballooning disturbances described by a system of coupled equations for Alfvenic and slow magnetosonic (SMS) modes. In this work, the Voigt model is used to describe the equilibrium finite-pressure plasma configuration in an inhomogeneous magnetosphere plasma in a curved magnetic field. Spectral characteristics and the spatial structure of natural ballooning modes are calculated for this model. The model calculations demonstrate the possibility of different longitudinal scales for transverse and longitudinal magnetic components of oscillations near the top of the field line.

Correspondence between the ULF wave power spatial distribution and auroral oval boundaries

Аннотация. Пространственное распределение волновой активности диапазона Pс5 во время магнитных бурь сопоставляется с положением границ аврорального овала. Полярная и экваториальная границы аврорального овала определялись по данным УФ-наблюдений на спутнике IMAGE из базы данных Британской Антарктической Службы или по модели OVATION, где использовались данные спутников DMSP. Показано, что в начальную фазу бури основной пик спектральной мощности широкополосных Рс5-колебаний отображается внутрь аврорального овала. Во время восстановительной фазы, максимум спектральной мощности узкополосных Рс5-волн в утреннем и вечернем секторах находился внутри аврорального овала или вблизи его экваториальной границы. Этот экспериментальный результат подтверждает ранее обнаруженные эффекты: пространственно-временные вариации мощности Pс5-волн в утреннем/дополуденном секторе тесно связаны с динамикой аврорального электроджета и магнитосферных продольных токов. При этом квази-монохроматические Pс5-волны демонстрируют типичные резонансные особенности в широтной структуре амплитуды и фазы. Таким образом, положение аврорального овала или его экваториальной границы является предпочтительной широтой для возбуждения магнитосферного альвеновского резонатора. Этот эффект не учитывается современными теориями Рс5-волн, но он может быть значимым для построения более адекватных моделей.

On the magnetic precursor of the Chilean earthquake of February 27, 2010

Some recent publications reported on an anomalous geomagnetic disturbance that was observed three days before the strongest Chilean earthquake on February 27, 2010. The present paper analyzes in detail the data from magnetic station, photometers, and riometers in Canada, Chile, and Antarctica. The analysis unambiguously shows that the supposedly anomalous geomagnetic disturbance was not related to seismic activity and was caused by a standard isolated substorm.

On the Possibility of Reflection of Alfvén Waves in a Curvilinear Magnetic Field

The propagation of Alfvén waves in a plasma immersed in a curvilinear magnetic field is investigated by using a 2D model. The waves are described by a 1D equation that formally coincides with the equation for the case of a quasi-uniform straight magnetic field with a modified Alfvén velocity that takes into account the longitudinal dependence of the Lame coefficients. It is shown that toroidal and poloidal Alfvén modes depend differently on the magnetic-field geometry. In the case of a 2D plane-parallel configuration of the magnetic field, poloidal modes are efficiently reflected from regions where the magnetic field lines sharply converge or diverge. This effect can result in the formation of open-field-line Alfvén quasi-resonators.

Emission of alfvén waves from a nonuniform MHD waveguide

The efficiency of the wave energy loss from a nonuniform MHD waveguide due to the conversion of the trapped magnetosonic waveguide modes into runaway Alfvén waves is estimated theoretically. It is shown that, if the waveguide parameters experience a jumplike change along the waveguide axis, the interaction between the waveguide modes and Alfvén waves occurs precisely at this “jump.” This effect is incorporated into the boundary conditions. A set of coupled integral equations with a singular kernel is derived in order to determine the transmission and reflection coefficients for the waveguide modes. The poles in the kernels of the integral operators correspond to the surface waves. When the jump in the waveguide parameters is small, analytic expressions for the frequency dependence of the transformation coefficients are obtained by using a model profile of the Alfvén velocity along the magnetic field. For the jump characterized by the small parameter value ε=0.3, the wave-amplitude transformation coefficient can amount to 5–10%. Under the phase synchronization condition (when the phase velocities of the waveguide modes on both sides of the jump are the same), the wave-energy transformation coefficient is much higher: it increases from a fraction of one percent to tens of percent. The transformation of fast magnetosonic waves into Alfvén waves is resonant in character, which ensures the frequency and wavelength filteringof the emitted Alfvén perturbations.

Characteristics of the variability of a geomagnetic field for studying the impact of the magnetic storms and substorms on electrical energy systems

There are numerous models of geomagnetically induced currents in which the role of the main sources is allotted to the variations in the intensity of the auroral electrojet inducing the currents flowing along the latitude. Based on this it is believed that magnetic disturbances mainly threaten technological systems that are elongated in the longitudinal (W–E) direction. In this work, we make an attempt to employ new characteristics to describe the variability of the geomagnetic field during the geomagnetic storm of March 17, 2013. These characteristics, calculated from the data of the IMAGE magnetometer network stations, are compared to the records of the induced currents in the power lines on the Kola Peninsula and in Karelia. The vector technique revealed a considerably lower variability of the horizontal component of the geomagnetic field compared to its derivative. Quantitative estimates of the variability supported the fact that the variations of the field occur on a commensurate scale both in magnitude and direction. These results cannot be accounted for by the simple model of the extended ionospheric current and demonstrate the importance of allowing for small-scale current structures (with the spatial scales of a few hundred km) in the calculations of the geomagnetically induced currents. Our analysis shows that the geomagnetically induced currents are not only hazardous for the technological objects oriented in the longitudinal (W–E) direction but also for those elongated meridionally.

Equatorward Propagating Auroral Arcs Driven by ULF Wave Activity: Multipoint Ground and Space Based Observations in the Dusk Sector Auroral Oval

Observations of multiple equatorward propagating arcs driven by a resonant Alfven wave on closed field lines are presented. Data sets from the European Incoherent Scatter Svalbard Radar (ESR) and Meridian Scanning Photometer in Longyearbyen, All-Sky Camera in Ny Alesund, ground magnetometer data in Svalbard, and Defense Meteorological Satellite Program (DMSP) F16 satellite were utilized to study the arc structures. The arcs had an equatorward phase propagation of ~0.46 km s−1 and were observed in the dusk ionosphere from 1800 to 2030 magnetic local time. Analysis of the optical data indicates that the Alfven wave had a frequency of 1.63 mHz and an azimuthal wave number, m ~ −20 (the negative sign indicating a westward propagation). Inverted-V electron populations associated with field-aligned currents of between 0.5 and 0.8 μA m−2 are observed by DMSP F16 inside the arc structures. In addition to electron density enhancements associated with the arcs, the ESR data show elevated ion temperatures in between the arcs consistent with electric field enhancements and ionospheric heating effects. The combination of ESR and DMSP F16 data indicates that the wave energy was dissipated through ionospheric Joule and/or ion frictional heating and acceleration of particles into the ionosphere, generating the auroral displays. The fine-scale structuring, in addition to the propagation direction and scale size, would suggest that the auroral features are the signatures of a field line resonance driven by an interaction with a compressional fast mode wave propagating earthward from the magnetotail.

Global stability of the ballooning mode in a cylindrical model

Ballooning disturbances in a finite-pressure plasma in a curvilinear magnetic field are described by the system of coupled equations for the Alfvén and slow magnetosonic modes. In contrast to most previous works that locally analyzed the stability of small-scale disturbances using the dispersion relationship, a global analysis outside a WKB approximation but within a simple cylindrical geometry, when magnetic field lines are circles with constant curvature, is performed in the present work. This model is relatively simple; nevertheless, it has the singularities necessary for the formation of the ballooning mode: field curvature and non-uniform thermal plasma pressure. If the disturbance finite radial extent is taken into account, the instability threshold increases as compared to a WKB approximation. The simplified model used in this work made it possible to consider the pattern of unstable disturbances at arbitrary values of the azimuthal wavenumber (ky). Azimuthally large-scale disturbances can also be unstable, although the increment increases with decreasing azimuthal scale and reaches saturation when the scales are of the order of the pressure nonuniformity dimension.