Recently published data from Reeves et al. (2011) on the fluxes of 1.8–3.5 MeV electrons at geostationary orbit are subjected to Error Reduction Ratio (ERR) analysis in order to identify the parameters that control variance of these fluxes. ERR shows that it is the solar wind density not the velocity that controls most of the variance of the energetic electrons fluxes. High fluxes are observed under the conditions of low density in absolute majority of cases. Under the condition of fixed density the dependence of fluxes upon the velocity is the following: fluxes increase with the velocity reaching some saturation level. Both the level of saturation and the value of the velocity where it is achieved decrease with the increase of solar wind density.

A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as ‘equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes ‘ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations.

In the two decades prior to the launch of Cluster, collisionless shocks at which the magnetic field in the unshocked plasma is nearly perpendicular to the shock normal ('quasi-perpendicular shocks') received considerable attention. This is due, in part, to their relatively clean, laminar appearance in the time series data. The tendency of the magnetic field to bind particles together owing to their (perpendicular) gyromotion gives rise to this appearance, which facilitated deeper studies into the collisionless processes responsible for the overall thermalization of the principle plasma populations as well as the acceleration of an energetic non-thermal component. Despite the considerable effort, key questions remained unanswered or re-

When the interplanetary magnetic field is oriented such that the angle between the upstream magnetic field and the nominal bow shock normal is small (θ Bn < 45 • ), a much more complex shock is observed than in the quasi-perpendicular case. Historically, this has made interpreting single spacecraft data more difficult, so that for a long time the quasi-parallel shock remained relatively poorly understood. The difficulties arise, as we now understand, because the supercritical quasi-parallel shock is a spatially extended and inhomogeneous transition, with smaller lengthscale features cyclically reforming within it.

[1] The methodology based on the Error Reduction Ratio (ERR) determines the causal relationship between the input and output for a wide class of nonlinear systems. In the present study, ERR is used to identify the most important solar wind parameters, which control the fluxes of energetic electrons at geosynchronous orbit. The results show that for lower energies, the fluxes are indeed controlled by the solar wind velocity, as was assumed before. For the lowest energy range studied here (24.1 keV), the solar wind velocity of the current day is the most important control parameter for the current day's electron flux. As the energy increases, the solar wind velocity of the previous day becomes the most important factor. For the higher energy electrons (around 1 MeV), the solar wind velocity registered 2 days in the past is the most important controlling parameter. Such a dependence can, perhaps, be explained by either local acceleration processes due to the interaction with plasma waves or by radial diffusion if lower energy electrons possess higher mobility. However, in the case of even higher energies (2.0 MeV), the solar wind density replaces the velocity as the key control parameter. Such a dependence could be a result of solar wind density influence on the dynamics of various waves and pulsations that affect acceleration and loss of relativistic electrons. The study also shows that statistically the variations of daily high energy electron fluxes show little dependence on the daily averaged B z , daily time duration of the southward IMF, and daily integral R B s dt (where B s is the southward component of IMF).