D. Pokhotelov

Dispersive ionospheric Alfvén resonator

A new model of the ionospheric Alfven resonator (IAR) including the effect of wave frequency dispersion is presented. It is shown that the shear Alfven waves in the IAR are coupled to the compressional mode through the boundary conditions at the ionosphere. This coupling results in the appearance of the Hall dispersion and subsequent shift of the IAR frequency spectrum. The excitation mechanism involving the IAR interaction with the magnetospheric convective flow is considered. It is shown that the Hall dispersion of the IAR eigenmode increases the growth rate of the feedback instability. However, for the observed values of ionospheric conductivity this effect is not very high. It is shown that the physical mechanism of the feedback instability is similar to the Cerenkov radiation in collisionless plasmas. The IAR eigenfrequencies and growth rates are evaluated for the case of exponential variation of the Alfven velocity with altitude in the topside ionosphere.

Ion distributions in the Earth's foreshock: Hybrid-Vlasov simulation and THEMIS observations

We present the ion distribution functions in the ion foreshock upstream of the terrestrial bow shock obtained with Vlasiator, a new hybrid-Vlasov simulation geared toward large-scale simulations of the Earths magnetosphere (http://vlasiator.fmi.fi). They are compared with the distribution functions measured by the multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The known types of ion distributions in the foreshock are well reproduced by the hybrid-Vlasov model. We show that Vlasiator reproduces the decrease of the backstreaming beam speed with increasing distance from the foreshock edge, as well as the beam speed increase and density decrease with increasing radial distance from the bow shock, which have been reported before and are visible in the THEMIS data presented here. We also discuss the process by which wave-particle interactions cause intermediate foreshock distributions to lose their gyrotropy. This paper demonstrates the strength of the hybrid-Vlasov approach which lies in producing uniformly sampled ion distribution functions with good resolution in velocity space, at every spatial grid point of the simulation and at any instant. The limitations of the hybrid-Vlasov approach are also discussed.

Oxygen cyclotron harmonic waves in the deep plasmasphere during magnetic storms

A new approach to the generation of storm-associated ULF waves with discrete spectra (ion cyclotron harmonic waves) observed in the equatorial plasmasphere is presented. It is proposed that the appearance of waves with phase velocities smaller than the Alfven velocity is connected with a strong dispersion of magnetosonic waves near the bi-ion frequency occurring in the presence of oxygen ions of ionospheric origin. The waves are generated by an instability involving hot oxygen ions with loss cone or ring like distributions. Such ions are found in the magnetosphere during magnetic storms. A simple analytical model of this instability is elaborated. It is shown that ULF wave observations on board Akebono satellite are in a reasonable agreement with the present theoretical approach.

High-latitude ionospheric response to co-rotating interaction region- and coronal mass ejection-driven geomagnetic storms revealed by GPS tomography and ionosondes

Positive ionospheric anomalies induced in the polar cap region by co-rotating interaction region (CIR)- and coronal mass ejection (CME)-driven geomagnetic storms are analysed using four-dimensional tomographic reconstructions of the ionospheric plasma density based on measurements of the total electron content along ray paths of GPS signals. The results of GPS tomography are compared with ground-based observations of F region plasma density by digital ionosondes located in the Canadian Arctic. It is demonstrated that CIR- and CME-driven storms can produce large-scale polar cap anomalies of similar morphology in the form of the tongue of ionization (TOI) that appears on the poleward edge of the mid-latitude dayside storm-enhanced densities in positive ionospheric storms. The CIR-driven event of 14–16 October 2002 was able to produce ionospheric anomalies (TOI) comparable to those produced by the CME-driven storms of greater Dst magnitude. From the comparison of tomographic reconstructions and ionosonde data with solar wind measurements, it appears that the formation of large-scale polar cap anomalies is controlled by the orientation of the interplanetary magnetic field (IMF) with the TOI forming during the periods of extended southward IMF under conditions of high solar wind velocity.

Wave dispersion in the hybrid-Vlasov model: Verification of Vlasiator

Vlasiator is a new hybrid-Vlasov plasma simulation code aimed at simulating the entire magnetosphere of the Earth. The code treats ions (protons) kinetically through Vlasovs equation in the six-dimensional phase space while electrons are a massless charge-neutralizing fluid [M. Palmroth et al., J. Atmos. Sol.-Terr. Phys. 99, 41 (2013); A. Sandroos et al., Parallel Comput. 39, 306 (2013)]. For first global simulations of the magnetosphere, it is critical to verify and validate the model by established methods. Here, as part of the verification of Vlasiator, we characterize the low-β plasma wave modes described by this model and compare with the solution computed by the Waves in Homogeneous, Anisotropic Multicomponent Plasmas (WHAMP) code [K. Ronnmark, Kiruna Geophysical Institute Reports No. 179, 1982], using dispersion curves and surfaces produced with both programs. The match between the two fundamentally different approaches is excellent in the low-frequency, long wavelength range which is of interest ...

Alfvén solitons in the Earth's ionosphere and magnetosphere

Nonlinear equations of the Kadomtsev-Petviashvili type are derived for Alfven and magnetosonic waves propagating at a small angle to the Earths magnetic field. This represents a generalization of the derivative nonlinear Schrodinger equation to the two-dimensional case. The theory is applied to explain the observations of stable Alfven-like solitons by means of the Aureol 3 satellite during the Russian seismic impact experiments. It may also be useful for theoretical interpretations of the chevron structures of the “pearls” that are occasionally observed in the Earths magnetosphere.

Ionospheric response to the corotating interaction region–driven geomagnetic storm of October 2002

Unlike the geomagnetic storms produced by coronal mass ejections (CMEs), the storms generated by corotating interaction regions (CIRs) are not manifested by dramatic enhancements of the ring current. The CIR-driven storms are however capable of producing other phenomena typical for the magnetic storms such as relativistic particle acceleration, enhanced magnetospheric convection and ionospheric heating. This paper examines ionospheric plasma anomalies produced by a CIR-driven storm in the middle- and high-latitude ionosphere with a specific focus on the polar cap region. The moderate magnetic storm which took place on 14–17 October 2002 has been used as an example of the CIR-driven event. Four-dimensional tomographic reconstructions of the ionospheric plasma density using measurements of the total electron content along ray paths of GPS signals allow us to reveal the large-scale structure of storm-induced ionospheric anomalies. The tomographic reconstructions are compared with the data obtained by digital ionosonde located at Eureka station near the geomagnetic north pole. The morphology and dynamics of the observed ionospheric anomalies is compared qualitatively to the ionospheric anomalies produced by major CME-driven storms. It is demonstrated that the CIR-driven storm of October 2002 was able to produce ionospheric anomalies comparable to those produced by CME-driven storms of much greater Dst magnitude. This study represents an important step in linking the tomographic GPS reconstructions with the data from ground-based network of digital ionosondes.

Excitation of helium cyclotron harmonic waves during quiet magnetic conditions

A general approach to the generation of ion cyclotron harmonic waves observed on board the Akebono satellite in the deep plasmasphere is presented. It is shown that during quiet magnetic conditions the development of the hydrodynamic cyclotron instability with growth rate γ ∝ ni1/2, where ni is the number density of the hot heavy ions, is suppressed by the field-aligned inhomogeneity of the dipole magnetic field. The instability is, in this case, controlled by the weak resonant interaction of the waves and the trapped particles with growth rate γ ∝ ni. The waves are generated by a kinetic instability involving hot helium ions with a ring-like distribution. Such ions are present in the magnetosphere during quiet magnetic conditions. A simple analytical model of this instability accounting for the inhomogeneity of the ambient magnetic field is used. It is shown that the ULF wave observations during quiet times on board the Akebono satellite are in a reasonable agreement with the present theoretical approach.

Substructures within a dipolarization front revealed by high‐temporal resolution Cluster observations

The dipolarization front (DF), usually observed near the leading edge of a bursty bulk flow (BBF), is thought to carry an intense current sufficient to modify the large-scale near-Earth magnetotail current system. However, the physical mechanism of the current generation associated with DFs is poorly understood. This is primarily due to the limitations of conventional plasma instruments which are unable to provide a sufficient number of unaliased 3-D distribution functions on the timescale of the DF, which usually travels past a spacecraft in only a few seconds. It is thus almost impossible to unambiguously determine the detailed plasma structure of the DF at the usual temporal resolution of such instruments. Here we present detailed plasma measurements using the Cluster Plasma Electron and Current Experiment and Cluster Ion Spectrometry-Composition and Distribution Function ion data for an event during which it was possible to observe the full pitch angle distribution at a cadence of 1/4 s. The observations clearly show details of plasma substructure within the DF, including the presence of field-aligned electron beams. In this event, the current density carried by the electron beam is much larger than the current obtained from the curlometer method. We also suggest that the field-aligned current around the DF obtained from the curlometer method may have been misinterpreted in previous studies. Our results imply that the nature of the DF current system needs to be revisited using high-resolution particle measurements, such as those observations shortly to be available from the Magnetospheric Multiscale mission.

Exo‐plasmaspheric refilling due to ponderomotive forces induced by geomagnetic pulsations

The effect of exo-plasmaspheric refilling due to ponderomotive forces induced by geomagnetic pulsations is considered. It is shown that two maxima of high-density cold plasma can appear on field lines near the day side magnetospheric boundary, located symmetrically with respect to the equator. When moving away from the noon meridional plane the plasma density distribution along the field line undergoes a smooth transition from two off-equatorial maxima to one maximum at the equator. We calculate the plasma condensation due to the ponderomotive forces in this region. On the other hand, the satellite data have shown that exoplasmaspheric Pc1 pulsations have a maximum in the noon-dusk sector. These facts bring us to a conclusion that the formation of high-density cold plasma outside plasmasphere in the magnetospheric trough in the noon-dusk sector may be accelerated by ponderomotive effects of Pc1 pulsations.