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A new phenomenon was found at the polar edge
of the auroral oval in the postmidnight-morning sectors: field-aligned (FA)
high-energy upward electron beams in the energy range 20–40 keV at altitudes
about 3RE, accompanied by bidirectional electron FA beams of keV
energy. The beam intensity often reaches more than 0.5·103
electrons/s·sr·keV·cm2, and...
Context in source publication
Context 1
... velocity x À369 km/sX Value of Kp at the moment of observations was 1+, Dst index was À8 nT. Case 3: pass 250 (see Fig. 3). At 03:55 UT on 28 October, 1996 both TOF analyzers indicated a weak (less than 10 electronsas Á sr Á keV Á cm 2 beam of October 27, 1996). For details see Fig. 1 20±40 keV electrons a pitch angles about 130 AE 20 , with almost no counts above the threshold around 90 of pitch angle. At 04:03 UT the TOF instruments detected a 3-min ...
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Citations
The NVK ONCH experiment is a part of the Auroral Probe wave complex and is aimed at measurements of the electric and magnetic components of the electromagnetic field in the 8 Hz-20 kHz frequency range. The instrument description, its characteristics, interaction with other wave complex equipment, and first results obtained during the first stage of the satellite operation are presented.
The first results of a statistical analysis of short term medium energy (≈1 keV) particle bursts, observed by Interball-2 at high invariant latitudes (up to 81°) above the nightside northern hemisphere, are presented. These particle bursts are believed to be detected during crossings of the plasma flux tubes which contain auroral forms detached from the nightside auroral oval. Those auroral forms may persist up to ten minutes or more, and they are detected by the onboard particle instruments as a rather short term (time of detection is approximately several minutes) bursts of particles. For a period of October and November 1996, the author has collected a data set of 228 such particle bursts. A comparison of the author's data set with the IMF and solar wind data available from the ISTP program revealed the following: 1) high latitude particle bursts were observed on 30% of all Interball-2 passes above the nightside northern hemisphere; 2) burst occurrence probability does not depend strongly on the IMF and global magnetic activity conditions. The author suggests that most of the observed particle bursts originated from the plasma sheet (PS) or plasma sheet boundary layer (PSBL). This implies a large scale PS/PSBL interchange instability, which can occur intermittently at any magnetic activity level and results in plasma filaments formation on field lines generally ascribed to the tail lobe. Therefore, the poleward auroral oval boundary can often be highly dynamic and filamentary.
It is commonly believed that Alfvénic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the velocity shear associated with the background electric field can significantly affect the observable properties of Alfvén waves. In particular, Alfvén waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin-Helmholtz instabilities, which are non-Alfvénic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Freja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the Kelvin-Helmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.
Ion data acquired by the Interball-Auroral satellite during crossings of the poleward boundary of the auroral oval in the 2200-0300 MLT sector at altitudes of ~2.5-3 Earth's radii reveal the frequent occurrence of thermal and superthermal H+ ion outflows. These events are strongly correlated with suprathermal electron fluxes and broadband electromagnetic ULF waves. The pitch angle distributions give evidence of transverse heating occurring in a latitudinally narrow layer at the boundary between the polar cap and the plasma sheet boundary layer, over a broad altitude range extending up to the satellite altitude. The distributions evolve with latitude, exhibiting fluxes maximizing at pitch angles close to 90° at the poleward edge of the outflow structure and at pitch angles closer to the upward field-aligned direction at lower latitudes. The data analysis suggests that ion cyclotron resonance interaction with ULF electromagnetic turbulence can account for the observed heating, even if we cannot totally exclude that transverse velocity shears and nonresonant stochastic transverse acceleration sometimes contribute to the ion energization in view of the dc electric field fluctuations commonly observed at the same times. During the expansion phase of substorms the region of transverse heating at the poleward boundary of the discrete auroral oval exhibits a latitudinal structure characterized by an alternate occurrence of latitudinally narrow regions of intense and weak ion fluxes. These latitudinal variations are associated with magnetic fluctuations at a frequency of ~2×10-2Hz, interpreted in terms of hydromagnetic Alfvén waves. Equatorward of the heating region, the energy spectrograms recorded during the same events exhibit an energy-latitude dispersion signature with energy decreasing as latitude decreases. This dispersion is the result of the velocity filter effect due to the large-scale convection and of the poleward motion of the ion heating source associated with the poleward motion of the high-latitude edge of the active auroral region. The poleward edge of the low-energy ion structure marked by a sharp latitudinal gradient of the ion flux appears as a reliable midaltitude criterion for identifying the poleward boundary of the soft electron layer lying at the high-latitude edge of the plasma sheet boundary layer.
A linear eigenmode analysis shows that small-scale static electric field structures commonly found in the auroral ionosphere are natural generators of low-frequency ( ω≤Ωi, where Ωi is the ion cyclotron frequency) electromagnetic waves with broadbanded frequency spectra. These waves can account for the observed upward Poynting flux and distinctive polarization signatures in the ionosphere. The ratio of wave electric to magnetic field strength can be much larger than the Alfvén velocity. The local Doppler shift is important for facilitating resonant energization of ions and electrons.
Measurements from the S3-3, Viking, Freja and FAST satellites in auroral
and magnetospheric plasmas revealed various types of large amplitude,
small-scale, moving plasma structures such as weak double layers, "ion
holes", caviton-like and soliton-like features, and knoidal waves. In a
series of papers in 1995 and 1997 the authors developed a nonlinear MHD
theory of moving collisionless and collisional plasma structures of
arbitrary amplitude which describes the waveforms and spatial and
temporal evolution of such stationary and quasi-stationary small-scale
structures. It was shown that such structures can exist in certain
parameter ranges of auroral and magnetospheric plasmas. Their
amplitudes, waveforms, and velocities were derived, and shown to be
consistent with the characteristics of nonlinear plasma structures
observed from Viking. The authors studied the stabilization of plasma
instabilities and formation of stationary plasma structures as
influenced by various physical processes in auroral and magnetospheric
plasmas. The processes considered are collisions (in particular, Coulomb
or effective collisions in turbulent plasmas), viscosity, dispersion,
non-linearity, gradients of electron pressure, and ponderomotive force,
their rôle in stabilization of instabilities and in the formation
of stationary moving structures. Numerical modeling was made of some
evolution scenarios of these processes in magnetospheric plasmas.
Possibilities are discussed of detection and diagnostics of nonlinear
quasi-stationary small-scale moving structures by the Interball
satellites, and their comparisons with the theoretical models.
The INTERBALL-2 satellite (AURORAL PROBE) was launched on August 29, 1996, about a year after the INTERBALL-1 and MAGION-4 pair already worked on a higher orbit. It performed comprehensive measurements of magnetospheric plasmas and fields mainly at altitudes 2 – 3 RE during more than two years. Some of the main findings from these measurements are briefly reviewed, in particular: 1. Discovery of long-duration high-energy electron dispersion events above the polar edge of the post-midnight auroral oval. 2. Discovery of high-energy (> 20 keV, i.e. super-auroral energy) upward moving field-aligned electron beams accompanied by bi-directional electrons ∼ 1 keV, also in these auroral regions. 3. Mass-resolved velocity-dispersed ion structures (VDIS) extended down to superthermal energies injected in the plasma flux tubes at the auroral oval polar border in a wide range of local times. 4. Mid-altitude ion distribution functions in the cusp showing a combination of downward moving, and mirror reflected, magnetosheath ions, with upward moving thermal and superthermal H+, He+, and O+ ions of the “cleft ion fountain” starting from eV energy range. 5. Thermal and superthermal plasma outflows of the classic polar wind type (only H+ and He+, no O+) as contrasted to conditions when additional acceleration mechanisms are operating which accelerate thermal heavy ions also (O+, etc.).
