A. M. Hamza

Solar illumination as cause of the equinoctial preference for geomagnetic activity

Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earths dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind-magnetosphere-ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300–0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500–1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed. n nPrevious studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither.

Substorm onset times as derived from geomagnetic indices

[1]xa0New magnetic parameters, dAE and dAO, are introduced to determine the preferential times for substorm onsets. dAE and dAO are time derivatives of the standard AE and AO indices. By analyzing long series of dAE and dAO we reveal their seasonal and diurnal variations, which, unlike those of the original AE and AO, show reasonable similarity. The preferential periods of substorm onsets are shifted from the spring and fall equinoxes; they occur around 0900–1800 UT for the winter months with maxima of occurrence in October-November and February-March. Substorms are rarer during summer months at all UT hours, which is in agreement with the results of Newell et al. [1996] that aurora are suppressed in sunlight. The substorm suppression during summer months corresponding to large total conductance of conjugate ionospheres is consistent with one of the potential substorm triggering mechanisms, the magnetosphere-ionosphere feedback instability.

Evidence for a high-energy tail associated with foreshock field-aligned beams

[1] The reduced particle distributions of field-aligned beams observed upstream of the bow shock are examined in detail using Cluster spacecraft. We find that the reduced parallel and perpendicular distribution forms can be strongly geometry-dependent. Above a certain critical value of the angle between the local shock normal and the direction of the magnetic field, qBn, the reduced distributions are remarkably well fit by Maxwellians. We have not found any significant changes to the spread in energies for beams at higher values of qBn. When the angle qBn decreases, leading to smaller beam velocities, a highenergy tail in the distribution appears. When the tail is present, the bulk of the distribution remains Maxwellian. The development of the high-energy tail is well correlated with decreases in the beam speed (or equivalently qBn). Moreover, detailed examination of the angular distributions indicates that particles in the tails of the distributions propagate at significant pitch angles with respect to the magnetic field (are not field-aligned, as are those within the bulk of the distribution) and that these pitch angles are energy-dependent. These new observations do not fit any production mechanism expected at the shock or result from known wave-particle interactions upstream of or within the shock layer.

Substorm development as observed by Interball UV imager and 2-D magnetic array

Abstract Results of the study of two substorms from Interball auroral UV measurements and two-dimensional patterns of equivalent ionospheric currents derived from the MACCS/CANOPUS and Greenland magnetometer arrays are presented. Substorm development in 2-D equivalent ionospheric current patterns may be described in terms of the formation of two vortices in the equivalent currents: a morning vortex related to downward field-aligned current and an evening vortex related to upward field-aligned current. Poleward propagation of the magnetic disturbances during substorm expansive phase was found to be associated mainly with a poleward displacement of the morning vortex, whereas the evening vortex remained approximately at the same position. As a result, the initial quasi-azimuthal separation of the vortices was replaced by their quasi-meridional separation at substorm maximum. Interball UV images during this period showed the formation of a bright auroral border at the poleward edge of substorm auroral bulge. The auroral UV images showed also that the auroral distribution in the region between the polar border and the main auroral oval tends to have a form of bubbles or petals growing from a bright protuberant region on the equatorward boundary of the auroral oval. However, the resolution of the UV imager was not sufficient for the reliable separation of such the structures, therefore, this result should be considered as preliminary. Overlapping of the auroral UV images onto equivalent current patterns shows that the bright substorm surge was well collocated with the evening vortex whereas the poleward auroral border did not coincide with any evident feature in equivalent ionospheric currents and was located several degrees equatorward of the morning current vortex center related to downward field-aligned current. The ground-based magnetic array allowing us to obtain instantaneous patterns of equivalent ionospheric currents gives a possibility to propose a new index for substorm activity such as the magnitude of the total current between the centers of the morning and evening vortices. Such integral index would not depend on where the substorm is located and be unaffected by the migration of substorm activity poleward or equatorward.

Oblique propagation and nonlinear wave particle processes

[1]xa0The wave-particle interaction plays a fundamental role in plasma physics; it is an energy momentum exchange mechanism between particles and waves. The most fundamental examples treated in the literature address the interaction of a single wave propagating along a background magnetic field with a single ion or electron. In the present paper we propose to investigate this specific problem, but this time we take into account the fact that the electromagnetic wave propagates obliquely with respect to the background field. The obliqueness manifests itself in the appearance of a parallel component in the electric field which in turn impacts the dynamics of the charged particle it interacts with and provides a mechanism of acceleration. This parallel component of the electric field can trap or untrap particles. The parallel propagation case is recovered automatically by setting the angle of propagation with respect to the background magnetic field to zero. A simple, yet complex, dynamical system is derived and limiting cases are treated analytically while numerical integration is used to investigate the general cases. We find that physical trapping occurs for a class of initial conditions and that phase space trapping (pitch angle versus gyrophase for example) remains in some cases a signature of the dynamical system.

On the generation of proton beams in fast solar wind in the presence of obliquely propagating Alfvén waves

[1]xa0Over the past few decades, satellite investigations have revealed that the solar wind contains a plethora of distribution function signatures (proton beam, anisotropic cores) requiring models departing from the conventional fluid theory and necessitating the inclusion of wave-particle interactions. Numerous theoretical models and explanations have been proposed, but several questions concerning time and heliocentric evolution as well as the formation of beam components and anisotropic cores remain unanswered. We propose a simple solution for the generation of beam components, as a result of physical trapping due to obliquely propagating Alfven modes, based on a previously reported dynamical system describing the wave interaction with a single ion in the absence of dissipation mechanisms. We have found that beams with significant densities (7%–8%) and with velocities of the order of the Alfven speed can be generated. The beams properties, including density, velocity, and temperature, are dependent on the wave propagation angle. The results are in qualitative agreement with the observations and could provide an explanation for the evolution of nonthermal distribution functions observed in fast solar wind and other space plasmas regimes.

Possible Role of ion Demagnetization in the Plasma Sheet in Auroral arc and Substorm Generation

Ion demagnetization in the plasma sheet causes the formation of field-aligned current that can trigger a magnetosphere-ionosphere coupling feedback instability, which may play an important role in substorm and auroral arc generation. Since field-aligned currents close ionospheric currents, their magnitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric conductivity, which in turn stimulates an increase in the field-aligned currents. When the magnitude of these currents becomes sufficiently large for the acceleration of precipitating electrons, a feedback mechanism becomes possible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Demagnetization of hot ions in the plasma sheet leads to the motion of magnetospheric electrons through a spatial gradient of ion population. Field-aligned currents, because of their effect on particle acceleration and the magnitude of ionospheric conductivity, can also lead to another type of instability associated with the breaking of the earthward convection flow into convection streams. The growth rate of this instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km. This instability is able to explain many features of auroral arcs, including their conjugacy in opposite hemispheres. However, it cannot explain very narrow (less than 1 km) arcs.

Investigating high‐latitude ionospheric turbulence using global positioning system data

Statistical properties of the amplitude and phase of GPS L1 signals sampled at 50u2009Hz are investigated to understand the turbulent behavior of the polar region ionosphere. Wavelet detrended amplitude and phase data are used to construct the probability distribution function (PDF) of the amplitude and phase fluctuations of the signal. Turbulent behavior of the ionosphere is quantified using the skewness and kurtosis of the PDF. It is found that these two independent moments are related through a parabolic relationship, which was also reported in the case of turbulent neutral fluids and turbulent laboratory plasmas.

On the Field‐Aligned Beam Thermal Energy

[1]xa0The parallel and perpendicular reduced distribution functions of field-aligned beams (FABs) observed upstream of the Earths bow shock using the Cluster spacecrafts are examined. A previous study revealed that FABs, observed in oblique shock geometries, exhibit reduced distribution functions with high-energy tails. A selection of FABs with weak-energy tails are considered, and the associated reduced distributions are fit with Maxwellians. First, we have found that the FABs full width at half maximum (FWHM), σ∥ and σ⊥ derived from the fit, are linearly correlated with the solar wind speed (or equivalently to solar wind temperature). Moreover, the parallel beam σ∥ has a very weak dependence upon the beam parallel speed which reflects the shock geometry; we have found that σ∥∼0.23Vsw. In contrast, we have found that the perpendicular beam σ⊥, in the range of beam speeds investigated, depends on the shock geometry. These new results indicate that the parallel σ∥ is essentially controlled by the solar wind while the shock geometry plays, along with the solar wind, a role in the perpendicular σ⊥. These results also put some strong constraints on theoretical models as far as field-aligned beam production mechanisms are concerned. One potential explanation for the significant perpendicular broadening of the FAB distribution reported in this study could be the presence of kinetic Alfven (or/and whistler) turbulence at the shock.

Eastward convection jet at the poleward boundary of the nightside auroral oval

The statistical study of the azimuthal convection flow in the midnight sector, as measured by the Saskatoon and the Stokkseyri SuperDARN radars, reveals the existence of an enhanced eastward convection stream around the poleward boundary of the auroral oval. The stream occupies two to three degrees and is located at geomagnetic latitudes 72–75°, which is indeed significantly poleward of the position of the center of the auroral oval. Poleward of the eastward convection stream, a westward convection stream is detected by the Saskatoon radar though not evident in the Stokkseyri radar measurements. The existence of the eastward convection stream at the poleward edge of the nightside auroral oval is very consistent with earlier results from the Akebono spacecraft. Such plasma flows are the source of possible plasma instabilities in the ionospheric E and F-regions.