The Echo 7 sounding rocket experiment injected electron beams on central tail field lines near L = 6.5. Numerous injections returned to the payload as “conjugate echoes” after mirroring in the southern hemisphere. We compare field line lengths calculated from measured conjugate echo bounce times and energies to predictions made by integrating electron trajectories through various magnetospheric models: the Olson‐Pfitzer Quiet and Dynamic models and the Tsyganenko‐Usmanov model. Although Kp at launch was 3−, quiet time magnetic models best fit the echo measurements. Geosynchronous satellite magnetometer measurements near the Echo 7 field lines during the flight were best modelled by the Olson‐Pfitzer Dynamic Model and the Tsyganenko‐Usmanov model for Kp = 3. The discrepancy between the models that best fit the Echo 7 data and those that fit the satellite data was most likely due to uncertainties in the small‐scale configuration of the magnetospheric models. The field line length measured by the conjugate echoes showed some temporal variation in the magnetic field, also indicated by the satellite magnetometers. This demonstrates the utility an Echo‐style experiment could have in substorm studies.
The interaction with the ionospheric plasma of two separately controllable electron beams injected from the Echo 6 sounding rocket in the auroral zone has been studied using orthogonal electric double probes and other instruments on a separate plasma diagnostics payload. The gun pulses produced a myriad of large electric probe signals covering the entire ion gyro range between dc and 1250 Hz. In particular a signal driven at the 1‐kHz gun frequency and a naturally occurring resonance at 840 Hz near the 799‐Hz proton gyrofrequency were studied. The 1‐kHz signal decreased by a factor of 100 between 45 and 90 m from the beam field line but was detectable in the 1‐mV/m range at 120 m, confirming previous findings that pulsed electron beams may serve as ELF wave sources in space. The 840‐Hz resonance was excited by a beam injected parallel to the magnetic field but could be quenched by the addition of a transverse beam in what appears to be a case of controlled damping. The 840‐Hz and 1‐kHz signals were superposed on large‐amplitude fluctuations and were extracted using power spectral analysis. The data contain a large variety of ion gyro range frequencies which are under study. The paper discusses the interpretation of the electric probe responses and large turbulent payload floating potential effects produced by the hot plasma generated by the beams.
Particle beams released from space vehicles provide a unique way to study many problems in basic plasma physics in the ionosphere and magnetosphere. Electron beams, for example, may act as probes or tracers in the large‐scale magnetosphere but also generate waves, plasma heating, and current systems in the local ionosphere. Such experiments are a natural extension of laboratory plasma physics to a regime where there are no walls, but where a high degree of control may still be achieved. The first electron beam experiment in space was the Hess Artificial Aurora Experiment [Hess et al., 1971], which was launched in 1969 from the NASA Wallops Island Range, Wallops Island, Va., not only to generate auroral streaks but to map magnetic field lines between conjugate regions above the Earth's surface. This was followed in 1970 by ECHO 1, also from Wallops. Since that time, a multitude of electron beam experiments have been carried out on sounding rockets, and some at orbiting altitudes on the Space Shuttle. In the ECHO program, six additional experiments have been flown, culminating in ECHO 7, which is the subject of this paper and the last of the series. The history of the subject up to 1980 has been covered in a review paper [Winckler, 1980]. Particle beams in space plasmas were the subject of an extensive symposium in 1981 [Grandal, 1982]. This symposium includes a review of ECHO results [Winckler, 1982]. Abe et al., [1988] have analyzed ELF wave generation observed during the previous flight, ECHO 6, and referenced recent papers under the ECHO program.
Following two upward injections of energetic electrons (38 keV and 26 keV) from the Echo 4 rocket-borne electron accelerator (January 31, 1976, Poker Flat, Alaska), artificial auroral streaks were detected by ground-based low-lightlevel television. The streaks were recognized recently by using newly developed imageprocessing techniques. They were delayed relative to the injections by 2.06 s and 2.42 s, respectively. These measurements of the bounce times of electrons of known energy constitute two accurate determinations of a high-latitude field line length (106+_1X106 m). The delays are only 4-5% longer than calculated using a dynamic model of the geomagnetic field (Olson and Pfitzer, 1982). Other field models yielded shorter bounce times. Since the delays were in the inverse ratio of the relativistic velocities calculated for the nominal beam energies, it is concluded that the potential of the payload remained below i kV during 45 mA injections at an altitude of 210 km. The echo streaks showed little dispersion in either time or space, indicating that the portion of the beam returning to the northern hemisphere loss cone remained collimated and nearly monoenergetic. But there was a 70% loss in the return flux. A diligent search failed to locate similar echoes from the more powerful injections employed in the Echo 5 and Echo 7 rocket experiments, suggesting flux losses of at least 98% and 92%, respectively. The losses are thought to be due to pitch angle scattering out of the loss cone as the electrons traverse the equatorial region but could also be due to collective beam plasma interactions. The lower loss rate in Echo 4 is likely related to the presence of only weak diffuse aurora during that flight. UT Launch Apogee, Energy, Current,
ELF electric disturbances in the frequency range from DC to 1 kHz produced in the auroral ionosphere by the injection of powerful electron beams have been studied by orthogonal probes on a free‐flying plasma diagnostics payload. Frequency spectrograms have been produced for various pitch angles, pulsing characteristics and other properties of the injected beams. The large‐scale DC ionospheric convection electric field has also been measured, as well as the auroral particle precipitation, the visual auroral forms and the ionospheric parameters. The convection field was strongly attenuated as the experiment system moved northward across discrete arcs but recovered to a higher value north of the arcs where the particle precipitation ceased. The production of ELF waves by beam injection was also strongly reduced in the arcs, and like the DC field, recovered to an even greater intensity north of the arcs. This behavior did not extend into the lower hybrid or whistler mode region. The DC field decrease has been explained by the high Pederson conductivity caused by auroral electron precipitation in the E‐region below the vehicle. It is postulated that the observed ELF waves are in the Alfven and drift modes and are generated by the positive vehicle potential during beam injection. More effective vehicle neutralization in the auroral arcs lowers the vehicle potential and, therefore, reduces wave production. Most features of the experimental observations can be qualitatively explained by this mechanism.
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