[1] We report measurement of whistler-mode chorus by the four Cluster spacecraft at close separations. We focus our analysis on the generation region close to the magnetic equatorial plane at a radial distance of 4.4 Earth's radii. We use both linear and rank correlation analysis to define perpendicular dimensions of the sources of chorus elements below one half of the electron cyclotron frequency. Correlation is significant throughout the range of separation distances of 60 -260 km parallel to the field line and 7 -100 km in the perpendicular plane. At these scales, the correlation coefficient is independent for parallel separations, and decreases with perpendicular separation. The observations are consistent with a statistical model of the source region assuming individual sources as gaussian peaks of radiated power with a common half-width of 35 km perpendicular to the magnetic field. This characteristic scale is comparable to the wavelength of observed waves.

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Plasma wave and plasma measurements from the Dynamics Explorer 1 (DE 1) spacecraft are used to investigate an intense broadband spectrum of low‐frequency, < 100 Hz, electric and magnetic noise observed at low altitudes over the auroral zones. This noise is detected by DE 1 on essentially every low‐altitude pass over the auroral zone and occurs in regions of low‐energy, 100 ev to 10 keV, auroral electron precipitation and field‐aligned currents. The electric field is randomly polarized in a plane perpendicular to the static magnetic field. Correlation measurements between the electric and magnetic fields show that the perpendicular (∼ north‐south) electric field fluctuations are closely correlated with the perpendicular (east‐west) magnetic field fluctuations and that the Poynting flux is directed downward, toward the earth. The total electromagnetic power flow associated with the fluctuations is large, approximately 108 W. Two general interpretations of the low‐frequency noise are considered: first, that the noise is produced by static fields imbedded in the ionosphere and, second, that the noise is due to Alfven waves propagating along the auroral field lines. For the static interpretation the ratio of the magnetic to electric field strengths at the base of the ionosphere is determined by the Pedersen conductivity, B/(µ0E) = Σp, whereas for the Alfven wave interpretation it is determined by the Alfven index of refraction, cB/E = nA. Measurements show that the magnetic to electric field ratio decreases rapidly with increasing height. This height dependence is in strong disagreement with the static model if the magnetic field lines are assumed to be equipotentials (E∥ = 0). At present, no satisfactory model is available for comparison with the data if an electrostatic potential drop is assumed to exist along the magnetic field (E∥ ≠ 0). The Alfven wave model is in good agreement with the general form of the height dependence of the magnetic to electric field ratio but disagrees in certain details. The cB/E ratio tends to decrease with increasing frequency and is usually somewhat larger than the computed value of the Alfven index of refraction. Some of these difficulties could be accounted for by reflections at the base of the ionosphere or propagation at large angles to the magnetic field (kinetic Alfven waves). For both the static model and the Alfven wave model the source must be located at high altitudes, since the average Poynting flux is always directed downward, even at radial distances up to 2 RE.

Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4500 km during magnetic conjunctions. The measured electric fields are usually nearly perpendicular to the magnetic field lines. The north-south meridional electric fields are "projected" to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE 1 and DE 2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths of less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high-and low-altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE 1. A Fourier transform of the east-west magnetic field component measured on the high-altitude satellite is found to be nearly identical to the Fourier transform of the north-south electric field measured on the low-altitude satellite, with a constant ratio. This ratio is proportional to the ionospheric conductivity. The experimental measurements are found to agree with a steady state theory which postulates that there are parallel potential drops associated with the variations in the perpendicular electric fields. It is assumed that there is a linear relationship between the field-aligned current and the total parallel potential drop and that the fieldaligned currents close through Pedersen currents in the ionosphere. The theory predicts that the ratio between the low-and high-altitude electric fields varies with the wavelength. Below a "critical" wavelength the electric field is not effectively transmitted to low altitudes. Owing to the good agreement between the theory and observations, it is concluded that the linear relationship between the current density and potential drop is a valid approximation. . 1/--3 , ß ß i ! ß ß ß i ß ! ß

A general theoretical treatment of energetic oxygen ion conic formation through cyclotron resonance with magnetospheric electromagnetic plasma turbulence is presented. With suitable assumptions, there exists a similarity regime in which the process may be profitably characterized by two parameters v o and o, corresponding roughly to the velocity scale and pitch angle of the ion distribution. These may be independently determined from the wave and particle observations of a conic event, as is illustrated here using typical auroral passes of the Dynamics Explorer 1 satellite. The predictions of the theory are found to be in excellent agreement with the observations. INTRODUCTIONThis article is concerned with a class of the energetic ion populations found within the Earth's ionosphere-magnetosphere system which are generically referred to as ion conics. The etymology of the term begins with its application to ion populations where the greatest concentration of ions lies on a cone in velocity space, but with the proliferation of both observations and theoretical models the correct application of the term has become considerably less restricted. Observationally, the term now includes a veritable zoo of ion observation events, with a variety not solely attributable to differences in instrumentation; and theoretically, there are now a number of reasonably plausible mechanisms which might be engaged to create ion populations suitable for inclusion in this zoo. The challenge to the experimentalist is to construct events sufficiently complete in their observations to allow a discrimination between theories, and the challenge to the theorist is to collect a subset of the events into a class which may be understood through the invocation of a single theory. With the aim of meeting this challenge and for the immediate goals of this paper, we are inclined to make the following definition:Definition. An "ion cyclotron resonance heated conic," which we shall abbreviate as "ICRH conic," is an ion population of ionospheric origin which was energized primarily through an ion cyclotron resonance interaction with electromagnetic plasma turbulence. This is a resonant wave-particle interaction where the ions are extracting energy from that portion of the electric field turbulence which includes their cyclotron frequency. The result of this extraction is a net gain for the ions in their energy of motion perpendicular to the magnetic field orientation.Unfortunately, this still results in a wide range of morphologies for the ion population, which is primarily due to the geometry in which this process takes place and the point of observation. In this paper we shall be concerned with geometries where the source of the ions lies in the ionosphere, the turbulent spectrum exists along an entire (auroral) field line, and the point of observation lies in the magnetosphere. There clearly are many other possible applications for the heating mechanism. The term "auroral" appears in parentheses because although the events we shall consider are indee...