We propose a 3D model of small-scale density cavities stimulated by an auroral field-aligned current and an oscillating field-aligned current of kinetic Alfvén waves. It is shown that when the field-aligned current increases so that the electron drift velocity exceeds a value of the order of the electron thermal velocity, the plasma becomes unstable to the formation of cavities with low density and strong electric field. The condition of instability is associated with the value of the background magnetic field. In the case of a relatively weak magnetic field (where the electron gyro-radius is greater than the ion acoustic wavelength), the current instability can lead to the formation of one-dimensional cavities along the magnetic field. In the case of a stronger magnetic field (where the ion acoustic wavelength is greater than the electron gyro-radius, but still is less than the ion gyro-radius), the instability can lead to the formation of 3D density cavities. In this case, the spatial scales of the cavity, both along and across the background magnetic field, can be comparable, and at the earlier stage of the cavity formation they are of the order of the ion acoustic wavelength. Rarefactions of the cavity density are accompanied by an increase in the electric field and are limited by the pressure of bipolar electric fields that occur within them. The estimates of typical density cavity characteristics and the results of numerical solutions agree with known experimental data: small-scale structures with a sufficiently strong electric field are observed in the auroral regions with strong field-aligned current.
We use Cluster and THEMIS simultaneous observations to study the spatial distributions of a shear BY field in the Plasma Sheet (PS) of the Earth's magnetotail at -31 RE < X < -9 RE. The best correlation between the BY field in the PS (BY_PS) and the Y-component of the Interplanetary Magnetic Field (IMF) (BY_IMF) was observed during the quiet PS periods when high speed plasma flows were not detected. During active PS periods the correlation between the BY_PS and BY_ IMF was poor. The analysis of spatial distribution of the BY field along the direction perpendicular to the Current Sheet (CS) plane showed the presence of one of the following configurations, which can be self-consistently generated in the CS: 1) the "quadrupole" distribution of the BY field usually associated with the Hall current system in the vicinity of X-line and 2) the symmetrical "bell-shaped" distribution formed due to the BY amplification near the neutral plane of the CS. Multipoint observations revealed the transient appearance of the "quadrupole" BY distribution during the periods of X-line formation in the mid-tail. This distribution was observed during a few minutes within, at least, 12 RE from the estimated X-line position. On the contrary, the symmetrical "bell-shaped" distribution is more localized in the radial direction and, generally, has a larger observation time (up to ∼10 min). Thus, the internal CS perturbations caused either by the Hall currents related to reconnection or by the peculiarities of the local quasi-adiabatic ion dynamics sufficiently affect the shear BY field existing in the magnetotail due to the partial IMF penetration.
The possibility of initiation of electric discharges by a crossbow bolt (projectile) moving in the electric field of a cloud of negatively charged water droplets has been demonstrated for the first time. Over one hundred of discharges have been produced. For each event, a high-speed video camera recorded the images of upward positive leaders developing from both the nearby grounded sphere and the projectile, followed by the return-stroke-like process. Corresponding currents were measured and integrated photos of the events were obtained. The results can help to improve our understanding of lightning initiation by airborne vehicles and by a vertical conductor rapidly extended below the thundercloud in order to trigger lightning with the rocket-and-wire technique.
In this study generalised Lamb waves in a nonisothermal atmosphere have been examined theoretically. Our results suggest that the pressure component of the Lamb wave decreases exponentially upwards near the layer with an extremum of sound speed. We find that local wave disturbances of the wave pressure component are formed in the resonance layer at which the horizontal phase velocity is equal to the sound speed. These resonance layers are the reason for a filtration of atmospheric disturbances. Such filtration is an obstacle to the acoustic-gravity wave propagation up to the ionosphere.
The magnetotail dipolarization during substorms is a mesoscale dynamical phenomenon, well documented by in-situ spacecraft observations, numerical simulations, and ground-based optical measurements. The magnetic energy release during substorms results in significant plasma heating and the formation of strong temperature and density gradients behind the dipolarization front. Such gradients are the source of many plasma instabilities that are responsible for further transport of energy to smaller spatial scales. In this study, we investigate an example of in-situ and optical observations of the nonlinear stage of one class of such instabilities, associated with the formation of sub-ion scale magnetic holes in the equatorial dipolarized magnetotail. THEMIS in-situ measurements demonstrate that these holes propagate dawnward and are associated with strong gradients of electron thermal pressure (~100-200 km in scale-size). Optical observations of THEMIS all sky imagers show comparably small-scale structures (accounting for magnetic mapping), just poleward of the active region associated with the dipolarization front and plasma flows. Comparison of in-situ and optical observations confirms their association and agreement in propagation direction. Our results demonstrate that sub-ion (electron dominated) magnetic field structures can be a source of small-scale aurora, and thus can host magnetosphere-ionosphere interactions at small scales.
Some aspects of the theory of generation of magnetospheric chorus are discussed. An original approach to solving the problem of oblique chorus generation near the Gendrin angle is outlined and partially realized within the framework of a beam pulsed amplifier mechanism. Parameters of the resonance electron beam in the chorus excitation region are determined theoretically. A short electromagnetic pulse amplification is calculated by means of a linear approach. Some important properties of the oblique chorus emissions, such as the location of the excitation region, frequency band, wave vector direction, group velocity direction, temporary dynamics, and energy of particles and waves are explained.
Trans-ionospheric propagation of the VLF electromagnetic wave from an altitude of 800 km to the Earth's surface is considered using the model of stratified media. The numerical solution of the wave equations for the mid-latitude ionosphere model conditions is found. The wave field in the lower ionosphere is calculated using the full-wave approach. The wave field in the upper ionosphere is calculated using the matrix method of perturbations for a slightly inhomogeneous plasma. Energy reflection coefficient and the horizontal magnetic field amplitude of the wave on the ground surface are calculated. Peculiarities of the wave reflection and transmission at different times of the day are analyzed. The obtained results are important for studying the ELF/VLF emission phenomena observed both onboard the satellites and in ground-based observatories.
In the theory of wave propagation in near-Earth space, there is a long-standing but still unsolved problem —
this is the problem of whistler exit to the ground after passing through the magnetospheric trajectory. Snell’s
law forbids exit of a wave to the ground if the angle between the wave normal and the vertical is outside the
penetration cone, which is almost always true, at least for middle and low latitudes. However, whistlers are
often recorded at ground stations, where they were first discovered. Most of the previous theoretical works
considered the exit of whistlers to the ground as a stationary problem, taking the wave field in the form
E = Re[𝐄_0(ℎ) exp(𝑖𝜅𝜉 − 𝑖𝜔𝑡)] with a constant frequency 𝜔 and a constant wave number 𝜅, where ℎ and 𝜉 are
the vertical and horizontal coordinates, respectively. Taking the field in this form assumes that the parameters
of the medium are time independent and change only in the vertical direction. However, those are necessary,
but not sufficient conditions for using the above given form of the wave field. It is clear that the real problem
is not stationary, and the wave packet incident on the ionosphere from above is limited both in vertical and
horizontal directions. Investigation of the whistler wave packet reflection from the ionosphere and exit to the
ground as a non-stationary two-dimensional problem is the subject of this work.
In this work, values of the fractal dimension and the connectivity index characterizing the structure of Hall conductivities on the night side of the auroral ionosphere are derived in general form. Restrictions imposed on fractal structure of the ionospheric conductivity are analyzed in terms of the percolation of the ionospheric Hall currents. It is shown that the structure of ionospheric Hall conductivities can be described as asymptotically path connected fractal. This result is supported by analysis of typical structure observed in auroral electron precipitation which are also the main source of ionization on the night side of the ionosphere. It is demonstrated that crossing the precipitation region in the direction perpendicular to the multiple arcs system, one should observe the structure of the precipitation which looks like a generalized Cantor set.