Dynamics of Ionospheric Alfvén Resonances from the End of Cycle 21 through Cycle 24 of Solar Activity

Geomagnetism and Aeronomy - The results of the study of the dynamics of ionospheric Alfvén resonances (IARs) in a range of 0–10 Hz based on data from magnetic field observations at the...     

Numerical Simulation of the High-Latitude Non-Stationary Ionospheric Alfvén Resonator During an IPDP Event

The ionospheric Alfvén resonator (IAR) was numerically simulated under non-stationary ionospheric and magnetospheric conditions of the IPDP event of December 4, 1986. The full numerical wave method was applied using height profiles of the ionospheric plasma parameters obtained from the Scandinavian EISCAT radar measurements close to the Ivalo latitude. An attempt to model the inverse problem of numerical simulation—prolongation of the electron density profiles at altitudes above the ionospheric F layer—was made on the basis of the IAR simulation in correlation with the IPDP frequency increase. The change of the IAR wave characteristics during the substorm was illustrated by height profiles of the total wave amplitude and various polarization characteristics, taking into consideration the ordinary L-mode and the extraordinary R-mode waves for parallel and non-parallel incidence with respect to the magnetic field line.     

Ionospheric Alfvén Resonator Control over the Frequency-Variable Pc1 Event in Finland on May 14, 1997

The ionospheric Alfvén resonator (IAR) control mechanism over the EMIC wave transmission to the ground is demonstrated on a selected long-term frequency-variable subauroral Pcl event. The proper ionospheric plasma data obtained from EISCAT were accessible in a wide altitude range. Applying the numerical method of simulation of a realistic inhomogeneous IAR, the problem of appearance and disappearance of the ground Pc1 signal record was clarified on the basis of coincidence between the EMIC wave frequency spectrum and the IAR fundamental frequency peak (the frequency window). A shift of the signal source field line to lower latitudes during the development of the disturbance was noticed, and the signal frequency variation on the ground was modelled in the nonstationary IAR. Variation of the IAR altitude structure in the fundamental frequency was illustrated on altitude profiles of the normalized wave magnetic field amplitude in the horizontal and vertical components. Particular conditions of L - and R -wave mode incidence were assumed. The electron density vertical profile of IAR determines the effective resonator dimensions. In this way the IAR fundamental frequency window controls the signal within the ionosphere and on the ground.     

On the latitudinal structure of the magnetohydrodynamic Alfvén resonances

According to the data of the IMAGE network of magnetometers the latitudinal profile of the amplitude of the Pc5 geomagnetic pulsations is constructed, which are excited in the Earth’s magnetosphere in the form of the resonance Alfvén magnetohydrodynamic pulsations. The approaches to the solution of two problems are studied on a specific example. The first concerns the anharmonicity of Alfvén’s resonances. The displacement of the peak of the resonance curve towards to the north with the reduction of the amplitude of the pulsations is discovered. Based on the results of measurements, the nonlinear distortion coefficient of the latitudinal profile is determined. The second problem is connected with the magnetotelluric sounding. Information about the resonance structure of the Alfvén pulsations is useful for magnetotelluric sounding. This information gives the possibility of evaluating the accuracy of the sounding with the application of the local impedance relationship and of introducing corrections if necessary.     

Observations of polar patches generated by solar wind Alfvén wave coupling to the dayside magnetosphere

A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (K_p = 7- and IMF B_z <0). The ionosondes measured electron densities of up to 9 × 10¹¹ m⁻³ in the patch center, an increase above the density minimum between patches by a factor of ≈4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF B_y > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts (flow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF B_y) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfvén waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.     

Earthquake impact on ionospheric Alfvén resonances

According to BGO data, it is discovered that ionospheric Alfvén resonances (IARs) observed as geomagnetic pulsations at frequencies of a few hertz arise in response to seismic events. The paper presents examples showing how seismic waves affect the IAR regime. Possible mechanisms of this effect are discussed.     

Excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator by modulated powerful radio waves

A review of the artificial excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator (IAR) is presented. In the framework of simplified models of the IAR and the Alfvén vortex instability, we discuss the main physical phenomena arising under the periodic heating of the ionosphere by a powerful HF radio signal with a modulation frequency F which lies in the range of short-period geomagnetic pulsations (F = 0.1–10 Hz). The amplitudes, frequency spectra, and polarization characteristics of artificial pulsations on the ground are found, and a brief comparison with experimental data is made. The Alfvén vortex instability in the IAR is analysed from the point of view of its artificial triggering. Two ways of such a triggering are discussed. The first suggests the use of two spaced transmitter antenna which produce an ionospheric current being in resonance with Alfvén vortices. Existing heating facilities are suitable for this experiment. The second method is based on the change of macroscopic parameters of the ionosphere, such as the conductivity in the instability region. This method is simple, but requires more powerful heaters.     

A mechanism of Pc 1 pearl formation based on the Alfvén sweep maser

A self-consistent model for the generation of Pc 1 pearl emissions based on the nonlinear coupling between the magnetospheric and ionospheric resonators for Alfvén waves is considered. Formation of pearls is attributed to the pulsating regime of the Alfvén sweep maser with nonlinear selective mirrors. Such mirrors are formed by the conjugate ionospheres: their reflection coefficient has an oscillatory frequency dependence due to eigenmodes of the ionospheric Alfvén resonator. Nonlinear magnetosphere/ionosphere feedback is provided by the dependence of the value and frequency of the reflection maxima on the flux of energetic protons precipitated into the ionospheres in the course of Alfvén wave generation in the magnetosphere. A nonlinear soliton-like solution of this model is found which corresponds to a single wave packet having the positive frequency drift and oscillating between the conjugate ionospheres. Properties of this solution are shown to explain many observational characteristics of Pc 1 pearls, such as their morningside predominance, correlation with low magnetic activity, spatio-temporal and spectral patterns.     

Formation of accelerated ion flows in Alfvén disturbances of the magnetotail

This paper shows that there exists a mechanism of longitudinal plasma acceleration which is inherent in the process of the process of resonant conversion of a fast magnetosonic wave freely propagating along the magnetic field into an Alfvén wave. This mechanism is caused by the Ampere force arising due to the interaction between the poloidal component of the current of the compressible disturbance and the generated toroidal disturbance. It is shown that plasma acceleration takes place at the stage of increase in the Alfvén wave amplitude and that the accelerated flow retains its velocity when the process of resonant conversion is over. We describe spatiotemporal structures of plasma flows arising with the transformation of fast magnetosonic waves into Alfvén waves. An interpretation of the presence of fast ion flows in the magnetotail as a consequence of the action of the plasma acceleration mechanism considered in this work is proposed.     

Characteristics of Arc-Polarized Alfvén Waves in the Near-Earth Solar Wind

Geomagnetism and Aeronomy - The arc-polarized Alfvén waves observed in the near-Earth region of the heliosphere are studied with instruments of the WIND spacecraft in 1995–2011....     

Characteristics of the Energy Transfer by Alfvén Waves in the Solar Atmosphere

Geomagnetism and Aeronomy - The propagation of linear Alfvén waves with periods of 10–200 s from the photosphere to the solar chromosphere under the conditions of an isothermal...     

Effect of the ponderomotive force caused by Alfvén waves on a background plasma in the dayside magnetosphere

The effect of the ponderomotive force on the background plasma modification near magnetic holes, which form at the dayside magnetospheric boundary under the action of the solar wind, has been studied. It was shown that this effect results in a substantial increase in a nonlinear plasma density disturbance. The dependence of the ponderomotive force on the magnetospheric parameters (the magnetic longitude, distance from the Earth’s surface, ratio of the wave frequency to the proton gyrofrequency, and ionospheric ion cyclotron wave amplitude) has been studied. Nonlinear plasma density disturbances will be maximal in the region of magnetic holes, which are located in the dayside magnetosphere at λ ~ 0°?30° geomagnetic longitudes (λ = 0° corresponds to noon), where the effect of the solar wind pressure is maximal. A similar effect is also observed in the dependence of a nonlinear plasma density disturbance on other magnetospheric parameters.     

Interaction between the Alfvén wave and turbulent sheet

The interaction between the Alfvén wave and turbulent sheet (TS) with an anomalous conductivity has been considered. High frequency turbulence causes the appearance of not only anomalous field-aligned plasma conductivity but also cross-field conductivity. Alfvén waves can be partially reflect from TS, be absorbed in this sheet, and pass through TS. When field-aligned conductivity is predominant, the relative effectiveness of these processes strongly depends on a cross-field wave scale. If TS is thin as compared to the Alfvén wavelength, the resistive Alfvén wave (λ _A ) characterized by the field-aligned resistivity and Alfvén velocity above the sheet is the characteristic parameter responsible for the wave-sheet coupling. A comparison of the loss, estimated using the analytical relationships for a thin sheet and numerically calculated based on the complete formulas for a sheet with a finite thickness, indicates that the approximation of a thin sheet results in reasonable estimates at all wave scales except very small ones. The developed model has been applied to the interpretation of the results of the works on Pi2 pulsation damping during the substorm expansion phase, which indicated that the damping decrement increases at large substorm amplitudes. The estimates indicate that this increase in damping is related to the appearance of anomalous resistivity in the case when field-aligned currents exceed the threshold values necessary for excitation of high frequency turbulence.     

Spatial structure of standing wave electromagnetic fields at the lower harmonics of the ionospheric Alfvén resonator

During the multiband wave Pc1 event on March 7, 2001 the EISCAT UHF and VHF incoherent scatter radars operated simultaneously covering an exceptionally wide altitude range of the ionosphere ～90—2000 km. This made possible to test the ionospheric Alfvén resonator (IAR) model over a large altitude range. The three lowest IAR eigenfrequencies, where the most of the Pc1 pulsation signal bands occur, were selected for the spatial analysis of the standing wave electromagnetic fields, applying the full-wave numerical simulation method. The altitude spread of amplitude maxima and nodes together with polarization characteristics of oscillation maxima in the horizontal plane are presented. The comparison of the standing wave oscillations on the altitude profile with the signal amplitude observed on the ground is also presented.     

O<Superscript>+</Superscript>-Reduced Models of the High Latitude Ionosphere and the Ionospheric Alfvén Resonator in Broadband Pc1 Events

The significance of the O⁺-ion density altitude profile of the outer ionosphere for determination of the Ionospheric Alfvén Resonator (IAR) lower harmonic structure has been demonstrated. The O⁺-reduced and exponentially extrapolated ionosphere models at high altitudes are generally acceptable for the IAR interpretation of subauroral broadband Pc1 events. Instantaneous ionospheric plasma data based on simultaneous EISCAT (CP-1, CP-7) measurements should be most suitable for the interpretation of different pulsation events. The limited applicability of the averaged International Reference Ionosphere (IRI) models has also been demonstrated.     

Non-stationary Alfvén resonator: vertical profiles of wave characteristics

A Pc1/IPDP event recorded by the Finnish search coil magnetometers on 15 December 1984 was analyzed in a companion paper (Mursula et al., 2000. Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events. J. Atmos. Solar-Terr. Phys. 62(4), 299–309) using numerical simulations of the ionospheric Alfvén resonator (IAR). EISCAT incoherent scatter radar data were used to determine the vertical profiles of ionospheric plasma parameters. In this paper, the detailed altitude profiles of several wave characteristics at the IAR eigenfrequency are computed up to 1000 km height, including, e.g., the real normalized amplitude of the magnetic wave field component, ellipticity and orientation of the polarization ellipse in the horizontal plane. We also calculate the altitude profile of the energy flux density (Poynting vector). These features illustrate in detail the ionospheric effects on the wave spectral structure in a non-stationary IAR, and their significance in the formation of the Pc1/IPDP signal on the ground.     

Periodic Restructuring of Auroral Arcs as an Indicator of Alfvén Resonance in the Region of Substorm Onset

Using optical data from observatories of the Polar Geophysical Institutes, as well all-sky TV observations at Canadian stations of ground support for the THEMIS satellite mission, we clarify whether Alfvén resonance should necessarily be present in the region of subsequent substorm onset. If this is true, the diversion of magnetospheric cross-tail current to the ionosphere, which leads to substorm onset, may be due to resonant Alfvén (or flapping) oscillations that increase in duration. This possibility is believed to indicate optically the presence of Alfvén resonance via periodic restructuring of the preonset auroral arc 3–15 min before onset at T ₀ . At the latitudes of the observatories included in this study, auroral restructuring occurs as repetitive poleward excursions of the preonset arc (the periods of excursions are 1–3 min) and can be readily explained by the theory of Alfvén resonance. It is shown that this feature, while typically observed in strong substorm events, may be lacking for weaker substorms. As proved by conjugate satellite observations, the lack of auroral restructuring in the latter case may result from the weakness of the involved Alfvén resonance, which is still present but not accompanied by large field-aligned currents sufficient for visualization in the ionosphere of the apparent propagation of oscillation phase across the resonance layer.     

Range finding of Alfvén oscillations and direction finding of ion-cyclotron waves by using the ground-based ULF finder

A new approach to the problem of direction and distance finding of magnetospheric ULF oscillations is described. It is based on additional information about the structure of geoelectromagnetic field at the Earths surface which is contained in the known relations of the theory of magnetovariation and magnetotelluric sounding. This allows us to widen the range of diagnostic tools by using observations of Alfvén oscillations in the PC 3–5 frequency band and the ion-cyclotron waves in the PC 1 frequency band. Preliminary results of the remote sensing of the magnetosphere at low-latitudes using the MHD ranger technique are presented. The prospects for remote sensing of the plasmapause position are discussed.     

Observations on the effects of barite on the gill tissues of the suspension feeder Cerastoderma edule (Linné) and the deposit feeder Macoma balthica (Linné)

Barite, an important component of offshore drilling muds, is shown to adversely affect the ctenidia of the suspension feeding bivalve, Cerastoderma edule and the deposit feeder, Macoma balthica. SEM observations showed that exposure to barite caused cilia to shorten and coagulate, and, in some extreme cases, cause the disintegration of the gill structure itself. Using a simple ciliary condition index (CCI) the impact of the barite was quantified and damage rates expressed. The bivalves were treated with daily doses of 1, 2, and 3-mm depth equivalents of barite. In all treatments significant damage to the gills was recorded although, in the case of the 1-mm dose rate, this did not occur for 4 days. In the other two treatments, damage was apparent within a day of exposure with 100% mortality occurring within 12 days. Macoma balthica appeared slightly more tolerant of exposure to barite than C. edule.     

Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events

We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.