L. V. Medford

Large-scale electric field measurements on the Earth's surface : a review

There exist only a few reported measurements of quasi-stationary (near dc) electric potentials over very large spatial scales (hundreds of kilometers or more) on the Earths surface. Such measurements have typically been made using unpowered submarine telecommunications cables. The measurements pose unique experimental challenges and require careful procedures to avoid data contamination by electrode contact potentials and local ground currents. In addition, there are possible interpretational problems from pervasive, poorly understood, low-frequency electric fields induced by ocean water motion through the Earths stationary magnetic field. Nevertheless, estimates of the magnitude of the electric field computed from large-scale potential difference measurements, made principally to date in the Pacific Ocean, can be used to place a limit on the size of the toroidal magnetic field at the core-mantle boundary under certain conditions on the Earths electrical conductivity profile. Thus, large-scale electric potential measurements can serve as an adjunct probe of the Earths dynamo process in addition to measurements of the poloidal magnetic field and its secular changes made at and above the surface of the Earth. A review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in magnitude.

Earth potential over 4000 km between Hawaii and California

Measurements of the value of the large-scale (4050 km) mean earth potential between California and Hawaii are reported as determined over a one year interval in 1990–91. The mean electric field measured during quiet geomagnetic intervals, 0.183 ± 0.056 mV/km, is about six times larger than that reported for a shorter interval over a slightly longer distance in the Atlantic [Lanzerotti et al., 1985], but of the same sign. The present results are also about three standard deviations from a null value, in contrast to the previous results, which were consistent with a null mean. If the mean potential is entirely attributable to leakage of a poloidal electric field from the core-mantle boundary, then under some models of the mantle conductivity one can conclude that the deduced toroidal magnetic field intensity in the core would be of the same order as the poloidal magnetic field at the core-mantle boundary. The measured potential gradient is smaller than that which might be expected from thermoelectric emfs at the core-mantle boundary. The potential could also correspond to a south to north quasi-steady water flow of ∼0.6 cm/sec.

Low-Power Portable Geophysical Data Acquisition System And Its Use in Geomagnetic Measurements

A low-power portable data acquisition system presently in use for geomagnetic measurements is described. The system is composed of a data-processing system containing a low-power microprocessor, a 9-track digital tape recorder, and a rechargeable battery pack. The magnetometer is a low-power three axis fluxgate design. Under program control the data processing system keeps track of time of day and date, samples three analog magnetometer outputs at intervals of either 0.4 or 2 s, digitizes the data to 15-bit resolution, and, depending upon relative magnetic activity, decides upon data compression to increase the tape storage capacity. It also monitors and records internal voltages and provides self-checking functions which may be monitored through a visual readout on the control panel. The system is mounted in a rugged, weather-tight carrying case suitable for use outdoors with minimal protection. The system, including magnetometer, uses 1.6-W power and can store 5.7 Mbytes of data.

Ocean Cable Measurements of the Tsunami Signal from the 1992 Cape Mendocino Earthquake

The movement of the seawater across the earths 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.

Possible measurements of small‐amplitude tid's using parallel, unpowered telecommunications cables

The authors report the observation of {approximately}40 min quasiperiodic variations during local daytime conditions in the differences between the voltages measured on two parallel (separation distance {approximately}100 km) unpowered telecommunications cables between California and Hawaii (length of each {approximately}4,000 km). The events are inferred to be produced by small changes in magnitude and/or position of the Sq ionosphere current system, the focus of which passes on average daily over the cables. The changes in the current system are probably produced by medium-scale traveling ionospheric disturbances (TIDs). This dual cable system appears to provide a very sensitive technique for measuring small amplitude, medium-scale mid-latitude changes in ionospheric currents, especially those produced by TIDs.

Space weather: Response of large‐scale geopotentials to an interplanetary magnetic cloud

The interaction of solar wind disturbances with the Earths magnetosphere can produce disturbances, and at times complete disruptions, of technological systems on the Earth and in the space around the Earth. This brief report shows the changes induced in the large-scale geopotentials of the Earth (as provided from measurements across transoceanic cables) produced by a well-documented interplanetary magnetic cloud event. The study of such a well-measured event can be used to begin to make empirical space weather phenomena more quantitative. We show that geopotentials at low geomagnetic latitudes can be used to infer the time derivative of the near-equatorial magnetic disturbance index, Dst. At low geomagnetic latitudes, a peak geopotential of about 4 mV/km is found to correspond to a time rate of change of this index of about 50 nT/hr. Further, we show that in this event increases in the near-equatorial geopotential are linearly related to the energy input to the magnetosphere from the solar wind as given by the e parameter [e.g., Akasofu, 1979]. We find that an increase in the geopotential of about 4 mV/km corresponds to an energy input of about 2.8 × 1011 W for the event analyzed here.

Inferred quasi‐steady ionospheric neutral winds and electrical currents at 79° south latitude in austral summer conditions

An analysis of the range of geomagnetic variations as measured at Arrival Heights and South Pole stations in the Antarctic over an 8-year interval during the present solar cycle shows that there are quasi-DC level shifts in some of the magnetic components during austral summer conditions when referenced to austral winter. The most dramatic of these level shifts in the signals occurs in the D-component (west-east magnetic component) at Arrival Heights (geomagnetic latitude ∼79°S). The onsets of the D-component shifts occur during a period that lasts one to several days in late austral spring. Such geomagnetic variations would not appear to arise from magnetosphere processes coupling to the ionospheric E-region. The inferred quasi-steady ionospheric current is estimated to be of the order of ∼105 amps above Arrival Heights. It is speculated that neutral wind dynamics with quasi-steady E-region (∼110–115 km) neutral wind speeds of ∼10 m/sec directed east to west above AH are responsible for generating the austral summer electrical currents in the E-region.