摘要:

The source location of the smooth high‐frequency (SHF) radio emissions from Uranus has been determined using a technique differing from those applied previously. Specifically, by fitting the signal dropouts which occurred as Voyager traversed the hollow center of the emission pattern to a symmetrical cone centered on the source magnetic field direction at the cyclotron frequency, a southern‐hemisphere (nightside) source was found at approximately 56° S, 219° W. The half‐angle for the hollow portion of the emission pattern was found to

ISSN:0094-8276    

DOI:10.1029/GL016i005p00341

年代:1989

数据来源: WILEY

摘要:

Large scale one‐dimensional hybrid simulations with resistive electrons have been carried out of a quasi‐parallel (θBn= 30°) high Mach number collisionless shock. The shock initially appears stable, but then exhibits cyclic behavior. For the magnetic field the cycle consists of a period when the transition from upstream to downstream is steep and well defined, followed by a period when the shock transition is extended and perturbed. This cyclic shock solution results from upstream perturbations, caused by backstreaming gyrating ions, convecting into the shock. The cyclic re‐formation of a sharp shock transition can allow ions, at one time upstream because of reflection or leakage, to contribute to the shock thermalization. The simulations are simplistic in a number of ways for the quasi‐parallel shock, but their results suggest a model which may explain qualitatively several features of obs

ISSN:0094-8276    

DOI:10.1029/GL016i005p00345

年代:1989

数据来源: WILEY

摘要:

The Alfvén shock structure is obtained by a two‐fluid simulation code including dissipations due to resistivity, viscosity and heat conductivity. The hodogram of the tangential magnetic field component of the simulated Alfvén shock is S‐shaped with a 180° angular rotation. This is the first simulation result to show the S‐shaped hodogram associated with Alfvén shocks. The S‐shaped hodograms have been observed at the magnetopause [Berchem and Russell, 1982]. On the basis of this result, we propose that the magnetopause is an Alfvén shock where and when the hodogram is S‐shaped and rotates 180°. The upstream portion of the hodogram is always right‐hand polarized; the downstream portion is always left‐handed. The thickness of the Alfvén shock transition layer is about a few ion inertial lengths, which tends to increase with decreasing shock normal angle θbn. For θbn= 80°, the thickness is estimate

ISSN:0094-8276    

DOI:10.1029/GL016i005p00349

年代:1989

数据来源: WILEY

摘要:

Theoretical considerations and observational evidence suggest that the polar cap (which marks the boundary between closed and open field lines) can expand rapidly enough during the growth phase of a substorm to release geomagnetically trapped particles from previously closed drift shells and thus enable them to escape into the tail. This scenario requires induced electric fields sufficient to yieldE · J0 there at all times. In support of our concept we cite observations which suggest that the moving separatrix can overtake convecting nightside plasma during intervals of polar‐cap expansion, and we illustrate the occurrence of this phenomenon with a simple model which suggests that closed nightside field lines containing trapped particles can become open and thus be evacuated of their particle populations. Such evacuation seems to account for the occurrence of energetic‐particle bursts in the tail (for which impulsive energization is therefore unnecessary) and for some of the plasma flows that occur in the region adjacent to the plasma‐sheet boundary layer. Expansion of the dayside portion of the polar cap should contribute to the transition of particles from closed to open field lines there and thus should increase the fluxes of magnetospheric ions in the magn

ISSN:0094-8276    

DOI:10.1029/GL016i005p00353

年代:1989

数据来源: WILEY

摘要:

We investigate a subharmonic‐resonant interaction between ions and magnetohydrodynamic waves propagating perpendicular to the background magnetic field,Bo. This interaction occurs at frequencies which are subharmonics of the cyclotron frequency of ions (Ωi/n, wherenis an integer, 2,3,4,...). We have found that this interaction becomes quite effective if the wave magnetic field δBexceeds ∼10% ofBo. Further, when δBexceeds ∼50% ofBo, ions have stochastic orbits in the wave and are accelerated with quite high efficiency. This subharmonic resonant interaction can play quite significant roles in the ion heating/acceleration processes in space plasmas, where large amplitude waves (δB/Bo≳ 0.1) are not unusual. Possible observational features of the subharmonic resonance which we suggest are: (1) “absorption lines” in the broad background spectrum of compressible geomagnetic pulsations, and (2) enhancement of energetic ion fluxes relating to the large fluctuation of the magnetic

ISSN:0094-8276    

DOI:10.1029/GL016i005p00357

年代:1989

数据来源: WILEY

摘要:

In a large laboratory plasma the current pattern set up by moving electrodes has been measured. It is observed that the current flows oblique to the magnetic fieldB→oat an angle given by the electrode speed acrossB→oand the current penetration alongB→owhich is controlled by whistler waves. The current pattern, characterized as “whistler wings”, occurs irrespective of whether the electrodes collect electrons/ions or emit fast electron beams. These results are relevant to active experiments in space involving electrodynamic tethers, beam injections and large space

ISSN:0094-8276    

DOI:10.1029/GL016i005p00361

年代:1989

数据来源: WILEY

摘要:

The CHARGE‐2 sounding rocket flight was designed primarily to measure the vehicle potential of an electron‐emitting payload in the mid‐latitude ionosphere. The experiment was separated into two sections, the larger section carried a 1 keV electron gun and was referred to as the mother vehicle. The smaller section, referred to as the daughter, was connected to the mother by an insulated, conducting tether, and was deployed to a distance of up to 426 m across the geomagnetic field. In addition to performing electron beam experiments, the mother vehicle contained a high voltage power supply capable of applying up to 450 V and 28 mA to the daughter through the tether. Steady state potentials of up to 560 V were measured for the mother vehicle during beam emissions of up to 36 mA. The daughter collected currents up to 6.5 mA and attained potentials of up to 1000 V relative to the background ionosphere. The CHARGE‐2 experiment was unique in that for the first time a comparison can be made of the current collection between an electron beam‐emitting vehicle and a non‐emitting vehicle at high potentials (400 V to 1000 V). The daughter current collection agreed well with the Parker‐Murphy model, while the mother current collection always exceeded the Parker‐Murphy limit and even exceeded the Langmuir‐Blodgett predicted current below 240 km. The additional current collection of the mother is attributed to beam‐plasma interactions. This additional source of collected current may be very important for successful electron beam emission at alt

ISSN:0094-8276    

DOI:10.1029/GL016i005p00365

年代:1989

数据来源: WILEY

摘要:

The first observations of HF heater stimulated electromagnetic emissions induced in the low‐latitude ionospheric plasma above Arecibo, Puerto Rico, are reported. Many systematic spectral features of the emissions bear a close resemblance to those observed in ionospheric modification experiments in the auroral zone and scale in accordance with detailed theory. This proves that these sideband emissions are not dependent on specific geophysical conditions but are due to fundamental interaction processes in the ionospheric plasma. We also report the discovery of unique, short‐lived HF sideband emissions that are less systematic than those observed previously and accompanied by a quenching of the HF enhanced plasma and ion lines in the Arecibo 430 MHz incoherent scatter radar spec

ISSN:0094-8276    

DOI:10.1029/GL016i005p00369

年代:1989

数据来源: WILEY

摘要:

Neutral gas temperatures inferred from topside plasma scale heights of electron density distributions obtained from observations by US and USSR Mars missions imply a much higher dependence on solar activity (expressed by the 10.7 cm radio fluxF10.7) than that found for Venus. This dependence, however, does not appear to be consistent with the observed solar cycle dependence of ionospheric peak plasma densities. The reason for this discrepancy seems to lie in the fact, that photochemical equilibrium applies only to altitudes below 170 km, whereas topside scale heights are usually derived for a much greater altitude range and thus may be modified by transport and other processes. If scale heights are obtained by matching a Chapman‐layer to plasma density profiles near the ionosphere peak, the derived neutral temperatures show a much weaker dependence onF10.7, in fact one essentially the same as for Venus. Thus, the response of the upper atmosphere of Mars, at least near the ionospheric peak, appears to be virtually identical to that of Venu

ISSN:0094-8276    

DOI:10.1029/GL016i005p00373

年代:1989

数据来源: WILEY

摘要:

The 1988 spring Antarctic ozone depletion was observed by TOMS to be substantially smaller than in recent years. The minimum polar total ozone values declined only 15% during September 1988 compared to nearly 50% during September 1987. At southern midlatitudes, exceptionally high total ozone values were recorded beginning in July 1988. The total integrated southern hemispheric ozone increased rapidly during the Austral spring, approaching 1980 levels during October. The high midlatitude total ozone values were associated with a substantial increase in eddy activity as indicated by the standard deviation in total ozone in the zonal band 30°–60°S. The standard deviation also correlates with the QBO cycling of the tropical winds. Mechanisms through which the increased midlatitude eddy activity could disrupt the formation of the Antarctic ozone hole are briefly discus

ISSN:0094-8276    

DOI:10.1029/GL016i005p00377

年代:1989

数据来源: WILEY