Electrostatic instability excited by an electron beam trapped in the magnetic mirror of the magnetosphere

A dispersion relation which takes into account the nonuniformity of the magnetic field as well as the plasma density along the field lines is obtained for an electrostatic wave propagating parallel to the magnetic field. This dispersion relation is solved for a particular case in which a group of electrons with a monochromatic distribution in magnetic moment is mixed with a low energy plasma. Such electrons are shown to excite ion acoustic waves carried by the low-energy plasma component near multiples of the bounce frequency of these electrons. The theoretical results are applied to explain electrostatic oscillations with a period of approximately fifty seconds observed in the high energy electron fluxes at synchronous altitude.     

Hydromagnetic waves associated with possible flux transfer events

Magnetic fluctuations in the hydromagnetic frequency band 0 to 0.05 Hz are examined at magnetospheric cusp latitudes during two times when ionospheric signatures of possible flux-transfer events were evident in the data. Ultralow frequency power is found to be very broad band in the range 0.02–0.05 Hz and to be more narrowly confined at a frequency 0.0025 Hz. At lower latitudes, the higher frequency (broad-band) power excites narrower-band field line resonances at the fundamental frequency of the respective field line — a standing Alfvén wave. The narrow-band power in the lower frequency band (period around 400 s) is approximately that expected for a field line resonance on a closed field line near the magnetopause; it also corresponds approximately to the width of the convected field-aligned current filament as observed on the ground. The reconnection process at the dayside magnetopause evidently plays an important role in the generation of low-frequency (0.008 Hz) hydromagnetic energy in the dayside magnetosphere, energy which can produce Alfvén waves deeper in the magnetosphere.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

High resolution scans of Comet Kohoutek in the vicinity of 5015, 5890, and 6563 Å

High resolution scans were made of Comet Kohoutek (1973f) using the McMath solar telescope at Kitt Peak National Observatory. The data were taken on January 1 and 4, 1974 UT, just after the comet perihelion. Hα emission (～4.1 × 10²⁷ photon sec^?1) was observed from the head of the Comet. An upper limit on the He I(5015) radiation was determined to be less than 2% of the observed Hα emission. The Na D1/D2 line intensities on both nights were approximately 0.5, indicating an optically thin emission region.     

The planetary scale distribution of telluric currents and the effect of the equatorial electrojet: An investigation by canonical GDS

The planetary scale distribution of electrical currents in the Earth is still largely unknown. The role of the oceans for long period (hours to days) inducing electromagnetic fieldsB _e of external origin has been investigated by several authors, while the role of telluric current channelling, from the planetary viewpoint, is still far from a satisfactory understanding. Canonical geomagnetic depth sounding (GDS) analysis can yield locally a direction parallel to the strike of a telluric current density flowing in a region around the recording site and which also has the property of being the most relevant source for the internal origin fieldB _i observed at the given site at the given frequency. The use of such local information from 64 geomagnetic observatories is discussed here in a study to infer evidences of (a) the role of the polarization properties ofB _e and (b) the role of the telluric current channelling within conductivity anomalies relevant to the planetary scale circuitry. The results show clear evidence of the influence of the equatorial electrojet on the polarization ofB _e in a latitudinal band between ±(15°–20°) latitude. There is also evidence that theB _e associated with the equatorial electrojet produces telluric currents which flow at a much shallower depth than the skin depth to be expected in the case of a plane Earth. This implies that the Parkinson planes in these regions reflect the conductivity structure underground more than the polarization ofB _e due to the equatorial electrojet. Further, it clearly appears that some regular planetary scale pattern of telluric currents plays a more significant role than current channelling within some conductivity anomalies of fixed strike close to some geomagnetic observatories. Finally, the number of observatories used in this study appears to be insufficient to deduce any information concerning a seasonal evolution of the telluric current pattern on a planetary scale.     

Ocean cable measurements of the tsunami signal from the 1992 Cape Mendocino earthquake

The movement of the seawater across the earth's magnetic field produces a large-scale motional electric field. Using the Point Arena, California, to Hanauma Bay, Hawaii, unpowered HAW-1 cable, we have studied the geopotential across this distance to look for possible tsunami-induced fields that might have been produced following the April 1992 Cape Mendocino earthquake. We have used a ten-day interval prior to and including the earthquake as a reference for geopotential signals and for geomagnetic activity. We have also used geomagnetic data from Point Arena, Honolulu and Boulder as reference data. The results of the analyses show that there are tsunami-related effects in the cable geopotential data. These are (a) larger voltage prediction errors (residuals) for the interval following the main shock; (b) enhanced (compared to the 10d reference interval) geopotential spectral power following the main shock: two enhancements are larger than geomagnetically-induced spectral power enhancements in the same time interval; and (c) strong evidence for an 30 min echo in the cable geopotential signal following the main shock.     

Intensity variations in plasma flow at the dawn magnetopause

Voyager 1, exiting the earth's magnetosphere along the dawn meridian at a velocity of ~11 km/sec, measured strong tailward flows of ions (E30keV) immediately outside the magnetopause. These flows are found to originate sunward of the dawn meridian and to exhibit significant variabilities on the time scale of 400 msec. The variations are not related to changes in the magnetosheath magnetic fields and are likely produced up-stream by the leakage of magnetosphere protons or by a magnetopause particle energization process. The intensities of the dawn meridian ion flows are greater in the magnetosheath than in the magnetosphere. The flows appear to penetrate inside the dawn magnetosphere to a depth 0.1 R>_E, less than an ion gyroradius.     

Hydromagnetic waves as a cause of a SAR arc event

A study is made of the hydromagnetic wave activity observed on the ground during the sub-auroral red (SAR) arc event of 17–18 December 1971. The available wave energy flux in the magnetosphere, inferred from the observed wave amplitude on the ground using the present understandings of wave localization and ionosphere wave attenuation is sufficient to produce the SAR arc. This finding supports kinetic Alfvén wave heating as a production mechanism for SAR arc optical emissions.     

Electrical conductivity structure in the lower crust

The subject is reviewed, notwithstanding the existence of a number of disagreeing and/or controversial results found in the literature. First, a brief critical reexamination of the methodology is presented. Second, it is attempted to put the results, or partial conclusions by different authors, using different methods and referring to different geographical regions, into a working scheme. This is done by investigating, as far as possible, the relationships between the electrical conductivity information and other types of geophysical and geological information for each geographical area investigated. It appears almost impossible to draw general conclusions that hold for the entire Earth. Conclusions are given for those areas with some very well-defined geomorphological characters. Unfortunately, the available investigations still appear to give a poor coverage of several types of geographic areas with specific tectonic characteristics, and certainly the scientific coverage by electromagnetic methods of investigation cannot be compared with those available today from seismological methods. Investigating the electrical conductivity structure of the lower crust certainly opens relevant heuristic possibilities, but there appears to be a great need both for a refinement in the basic methodology, and for a better coverage of the investigated areas.