A relatively steady band of ELF hiss has been detected by the Ogo 5 search coil magnetometer on almost every passage through the plasmasphere; except for an anomalous region on the dayside at high geomagnetic latitudes, the emissions terminate abruptly at the plasmapause, and we therefore refer to them as 'plasmaspheric hiss.' A preliminary statistical study of the properties of the observed whistler mode turbulence has yielded the following characteristics' the waves are band limited with a sharp lower-frequency cutoff and a more diffuse upper-frequency cutoff; power spectra show a well-defined maximum near a few hundred hertz, the peak intensities generally ranging between 10 -7 and 10 -5 v"Hz; the wave energy is spread over a bandwidth of a few hundred hertz, and cerresponding wide band amplitudes are 5-50 my; the waves are highly turbulent in nature and show little tendency of definite polarization. The above properties remain essentially constant throughout the plasmasphere. Observed properties of the hiss are con. sistent with generation at all local times in • restricted L range just within the plasmapause. Waves subsequently propagate on complex paths to fill the plasmasphere. The most probable generation mechanism is cyclotro n resonant instability wit h lo.w-energy electrons that continually diffuse inward from the outer radiation zone. At lower L, hiss resonates with higherenergy electrons, and thus the electrons are scattered in pitch angle and hence lost to the atmosphere throughout the 'slot' between the inner and outer radiation belts. Extremely low frequency whistler mode wavesare an important constituent of the magnetosphere, since they can resonate with radiation belt electrons and thus induce pitch angle scattering and a concomitant precipitational loss to the atmosphere. To date, the most complete in ritu measurements of such waves have been made on the Ogo satellites [.Dunckel and Helliwell, 1969; Russell et al., 1969]. Valuable information has also been obtained from loweraltitude polar-orbiting satellites [Taylor and Gurnett, 1968; Gurnett et al., 1969; Barrington, 1971; Muzzio and Angerami, 1972], but an extrapolation of such observations deep into the magnetosphere is complicated by the susceptibility of the waves to internal reflection [Ki-
General technique for modeling the position and shape of planetary bow waves are reviewed. A three-parameter method was selected to model the near portion (i.e., x' > -1Roo) of the Venus, earth, and Mars bow shocks and the results compared with existing models using 1 to 6 free variables. By limiting consideration to the forward part of the bow wave, only the region of the shock surface that is most sensitive to obstacle shape and size was examined. In contrast, most other studies include portions of the more distant downstream shock, thus tending to reduce the planetary magnetosphere in question to a point source and constrain the resultant model surfaces to be paraboloid or hyperboloid in shape to avoid downstream closure. It was found by this investigation that the relative effective shapes of the near Martian, Cytherean, and terrestrial bow shocks are ellipsoidal, paraboloidal, and hyperboloidal, respectively, in response to the increasing bluntness of the obstacles that Mars, Venus, and earth present to the solar wind. The position of the terrestrial shock over the years 1965 to 1972 showed only a weak dependence on the phase of the solar cycle after the effects of solar wind dynamic pressure on magnetopause location were taken into account. However, the bow wave of Venus was considerably more distant around solar maximum in 1979 than at minimum in 1975-6 suggesting a solar cycle variation in its interaction with the solar wind. Finally, no significant deviations from axial symmetry were found when the near bow waves of the earth and Venus were mapped into the aberrated terminator plane. This finding is in agreement with the predictions of gas dynamic theory which neglects the effects of the IMF on the grounds of their smallness. Farther downstream where the bow wave position is being limited by the MHD fast mode Mach cone, an elliptical cross section is expected and noted in the results of other investigations. INTRODUCTIONAmong the major early discoveries of the space program was the presence of a bow shock upstream of the earth (e.g., Spreiter and Alksne [1970], Dryer [1970], and references therein). Given the large collisional mean free path of solar wind particles (i.e., h -,• ! AU) it might have been expected at the time that the magnetosphere would represent a small (i.e. ,'--, 10 -3 •.) scattering center as opposed to an obstacle deflecting fluid through the formation of a standing bow wave. Thus planetary bow shocks comprise some of our earliest, and perhaps most striking, observational evidence for the existence of the microscale plasma processes that allow the solar wind to exhibit bulk fluid properties on spatial scales much smaller than the physical dimensions of the planets. Further, the thickness of the transition layer within which a portion of the plasma flow energy is converted to internal energy, turbulence, waves, and suprathermal particles has been found to be small in comparison with the shock stand-off distance. Hence, the bow wave may, for some purposes, b• considered a mathematical d...
An examination of the noise present between 1 and 1000 Hz at the magnetic equator with the OGO 3 search coil magnetometer has revealed a previously unobserved class of signals existing only in the outer plasmasphere. These waves are propagating nearly perpendicular to the magnetic field, probably within less than 1° of perpendicular to it; they exist only between about twice the proton gyrofrequency and half the lower hybrid resonant frequency. The waves are confined to a region within about 2° of the equator and therefore have a large amplitude gradient along the magnetic field lines. They are resonant with harmonics of the electron bounce frequency and have sufficient amplitude (∼10 mγ rms) to cause the observed pitch‐angle diffusion of electrons mirroring near the equator.
Wave normals of chorus in the outer magnetosphere have been determined for the first time from data obtained with the Ogo 5 search coil magnetometer. These measurements combined with simultaneous information concerning geomagnetic field, plasma density, and the electron energy and pitch angle distributions provide a consistent picture of the generation, propagation, and subsequent damping of chorus in agreement with theory. Specifically, the data are consistent with chorus generation within 25° of the equatorial plane on the dayside and within 2° on the nightside. Chorus is generated by a Doppler‐shifted cyclotron resonance with electrons between 5 and 150 keV but only when the pitch angle distribution is peaked at 90° to Bˆ and the anisotropy exceeds a critical value. In the source, wave normals are contained within an unstable generation cone of 20° half angle about Bˆ . Away from the source the data indicate that both unducted and ducted propagation of chorus can occur at different times. The ducted chorus wave normals are contained within a cone with a half angle of 40° about Bˆ out to magnetic latitudes of 50°, the limit of the measurements. Such ducts must have relative density enhancements of about 50% above background. In the case of unducted chorus the size of the wave normal cone is approximately constant, and its axis remains close to the magnetic meridian plane. However, within that plane the axis direction varies continuously from one parallel to Bˆ (θ = 0°)to one perpendicular to Bˆ pointing away from the earth (θ = 90°). When θ > 25°, attenuation becomes strong and very little chorus was detected with wave normals beyond θ = 60°. Little evidence for chorus reflection, which can occur when θ = 90°, was found. This lack of evidence is probably due to the fact that the wave normal measurement was not successful with weak signals.
The magnetic noise in the magnetosphere in the frequency range from 10 to 800 Hz has been extensively measured by the spectrum analyzers of the search coil magnetometer on OGO 3. This paper is a statistical study of the spatial extent and frequency of occurrence of noise at the higher end of this passband, at which frequencies noise above the detector thresholds is most common within the magnetosphere. Steady noise and noise bursts are found to constitute two distinct populations. Both the local‐time and magnetic latitude distribution of both classes of signals are investigated. When the magnetic latitude distributions are extrapolated downward to 1000‐km altitudes, the results are consistent with previous satellite observations at these low altitudes. However, the equatorial distributions cannot be inferred by simply projecting the magnetic noise measured at low altitudes onto the equator along flux tubes. The in situ measurements cannot determine the exact location of the source of all the noise observed. However, it is found that steady noise is definitely generated near 45° magnetic latitude on the dayside of the magnetosphere for L values from 6 to 10 and that bursts are generated near the equator above L = 6 from 0400 to 1800 local time. The latter observation can be used to explain the generation of both auroral microbursts and chorus as seen on the ground by means of whistler mode wave growth at the equator supported by a pitch‐angle anisotropy maintained by the loss cone.
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