Magnetic Direction Finding Research Articles

Effect of surface conductivity on the peak magnetic field radiated by first return strokes in cloud‐to‐ground lightning

The effect of surface conductivity on the peak magnetic field radiated by the first return stroke in cloud‐to‐ground lightning was investigated by comparing the peak magnetic fields from return strokes that struck water with those that struck land. The data were obtained from a network of three gated, wideband magnetic direction finders (DFs) at the NASA Kennedy Space Center during the summer of 1985. Two geographical areas that were equidistant from two of the direction finders were compared where the flash distances ranged from approximately 40 to 60 km. An unbiased data set was obtained by correcting site errors, equalizing differences in sensor gain, eliminating directional biases in DF triggering, and keeping differences in signal attenuation over the two surfaces to a minimum. When a statistical analysis was performed on the frequency distributions of the signal amplitudes, there was no statistically significant difference in the peak amplitudes of first return strokes over land (λ = 8.2 × 10−3 mho m−1) and over water (λ = 4 mho m−1). Therefore we infer that the conductivity of the underlying surface does not significantly affect the magnitude of the peak magnetic field, and hence the peak current, in the first return stroke of a cloud‐to‐ground lightning flash.

東北地域における磁界による落雷位置標定システムの高性能化

東北地域における磁界による落雷位置標定システムの高性能化

Calibration of a magnetic direction finding network using measured triggered lightning return stroke peak currents

Peak currents from 18 triggered lightning return strokes lowering negative charge have been measured at the NASA Kennedy Space Center, Florida. These strokes are correlated with simultaneous measurements by six Lightning Location and Protection (LLP) Inc. wideband magnetic direction finders (DFs) in Georgia and Florida operating at high gain from 1985 through 1988. The following have been determined: (1) The normalized signal strength amplitudes, in LLP units, can be used to estimate the peak current in lightning return strokes by the relation, I(kA) = 2.3+0.19(Normalized Amplitude), where the amplitude is range‐normalized to 100 km. An approximation of the form, I(kA) = 0. 2(Normalized Amplitude) is also valid, introducing errors of typically less than 6 %. (2) Range‐normalized signal strength amplitudes, in LLP units, for individual DFs have been examined as a function of the measured return stroke peak currents. It is found that a DFs normalized amplitude has a higher correlation with peak currents, the further the DF site is from the triggered lightning. The correlation coefficients range from 0.53 to 0.93, At a DF site the standard deviation of the normalized amplitudes from the best fit line to the measured peak currents are independent of the DF site's distance from the triggered lightning, averaging 28 LLP units, which corresponds to 6 kA. (3) Signal strength amplitudes, which are proportional to the peak magnetic radiation fields, have been examined for seven triggered lightning return strokes. The signal strength amplitudes decay with distance, D, as D−1.13. (4) An evaluation of the error in the mean range‐normalized signal strength amplitude caused by lightning location errors reveals that the mean amplitude is typically in error by less than 2% from the value calculated using the exact location of the triggered flash.

Lightning location and storm severity display system

A unique lightning location and lightning storm severity assessment display system designed and developed at the Detroit Edison Company is described. The design is driven by the need to provide a real time lightning storm severity information display for electric utility operations that includes storm warning, storm tracking, storm dispatching, and storm restoration resource planning. Its application results in an estimated annual savings of $250000. The methodology for correlation equations and their accuracy developed over three years (1985-7) of lightning storm activity are reported relating cloud-to-ground strikes from a magnetic direction finding network to important storm management factors for six independent geographic areas (operating divisions) within southeastern Michigan. >

A parametric estimation of systematic errors in networks of magnetic direction finders

We consider the problem of estimating systematic errors in a network of magnetic direction finders using bearing data from a historical record containing a large number of lightning flash observations. It is clear that this problem is linked with the flash location estimation problem and that the two are statistically confounded in the sense that the effect of one cannot be separated from the other. We formalize mathematically a connection between the two problems to arrive at a self‐consistent system. In this paper we parameterize the site errors as two‐cycle sinusoidal functions and show that the estimation equations can be described by a model which separates into linear and nonlinear parameters. We take advantage of this separation to decouple large‐dimensional equations into smaller dimensional equations. Through these equations we show the link between the site error and flash location estimation problems. Validation of our modeling procedures is done by heuristic arguments and analysis of residuals. The latter reveals that not all of the residuals have been explained by sinusoidal site errors but that some residuals might result from the geometry of the network. This appears to be an important problem and is presently under study. Also, there is a need to test the site error estimation algorithm developed in this paper against some ground truth.

Lightning Direction Finding Systems and their Developments

The detection and location of lightning by magnetic direction finding system (MDFS) and lightning position and tracking system (LPATS) are major meteorological techniques in recent years. The two systems have been critically discussed and the irrelative advantages and deficiencies are focused.Lightning is a major meteorological hazard. In addition to the protection of general public, access to real time lightning ground strike data could play a vital role in forest Are measurement, computer and communication network operation, explosives and toxic material handling, aircraft refueling and maintenance and repair of utility transmission and distribution systems. Even the nuclear plant operations may be considerably impacted by lightning [1]. Ruat et al [2] have shown the value of lightning data as a component of severe thunderstorm research. The needs for accurate location of lightning ground strikes are hence well documented.This note summarize the efforts to evaluate a new lightning location technology wi...

Lightning phenomenology in the Tampa Bay area

A lightning locating system employing two wideband, gated magnetic direction finders was used to study the negative cloud‐to‐ground lightning in 111 storms that occurred on 8 days during August 1979 in the Tampa Bay area of Florida. The storms were classified as (1) single‐peak storms—spatially isolated groupings of lightning whose cloud‐to‐ground flashing rate vs. time curves exhibited a single peak; (2) multiple‐peak storms—spatially isolated groupings of lightning with multiple‐peak flashing rates; and (3) storm systems—two or more related single‐peak and/or multiple‐peak storms. The following parameters are given for single‐peak storms, multiple‐peak storms, and storm systems: duration, area, number of ground flashes, mean ground flash density, mean ground flashing rate, and maximum ground flashing rate averaged over a 5‐min interval. Ground flash counts are corrected for the location system detection efficiency, which varied from about 75% at close range to about 55% at 100 to 150 km. The mean duration of single‐peak storms, multiple‐peak storms, and storm systems was 41, 77, and 130 min, respectively; the mean area 103, 256, and 900 km2, respectively; and the mean number of lightning flashes to ground 73, 270, and 887, respectively. The mean ground flash density for single‐peak storms, multiple‐peak storms, and storm systems was 18×10−3, 15×10−3, and 9.7×10−3 km−2 min−1, respectively; and the mean ground flashing rate 1.7, 3.4, and 6.8 min−1, respectively. The highest maximum ground flashing rate averaged over a 5‐min interval for all single‐peak storms, multiple‐peak storms, and storm systems was 17, 22, and 32 min−1, respectively, while the mean of the maximum flashing rates was 3.7, 7.3, and 14 min−1, respectively. The relation between storm duration D in minutes and number of ground flashes Ng for single‐ and multiple‐peak storms can be described by log10 Ng = 0.014D + 1.2.

Errors in magnetic direction finding due to nonvertical lightning channels

We present theory to show that errors in lightning magnetic direction finding due to nonvertical source orientations are minimized by detecting the field from the lowest possible channel section. For 99 cloud‐to‐ground lightning channels photographed in Arizona, 45 exhibited a bottom few hundred meters within about 8° of the vertical, the standard deviation of the distribution of lower‐channel tilt angles being about 18°. These experimental results, coupled with theory, suggest that if a wideband gated direction finder is used, polarization errors due to nonvertical sources can be kept less than 1° for lightning beyond 10 km.

Lightning location and lower‐ionospheric height determination from two‐station magnetic field measurements

Lightning locations in two maritime nocturnal thunderstorms, one occurring over the Atlantic Ocean, the other over the Gulf of Mexico, were determined using wide band magnetic direction finding from Gainesville and Kennedy Space Center, Florida. Within experimental error, all lightning was found to occur within National Weather Service radar reflectivity patterns. From the lightning locations and measured arrival times of first and second sky waves at Kennedy Space Center, ionospheric reflecting heights for both sky waves were determined. Lightning distance and ionospheric height were also computed using the single‐station technique recently described by Kinzer [1974]. Kinzer's technique is shown to be very inaccurate for one of the two storms studied because effective reflecting heights for first and second sky waves were different. During a 15‐min interval, the Atlantic storm produced an average of about 6 flashes per minute, an average of about 3 strokes per flash, and a mean initial electric field peak, normalized to 100 km, of 5.4 V/m for first strokes and 3.6 V/m for subsequent strokes. During a 45‐min interval the Gulf storm produced an average of about 5 flashes per minute, an average of about 4 strokes per flash, and a mean initial peak field, normalized to 100 km, of 10.2 V/m and 5.5 V/m for first and subsequent strokes, respectively.

Magnetic Direction Finding: Evidence for Its Use in Migratory Indigo Buntings

The orientational capabilities of caged migratory indigo buntings were studied under differing magnetic field conditions. When tested in a situation allowing minimal exposure to visual cues but in the presence of the normal geomagnetic field, the birds demonstrated a significant orientation in the appropriate migratory direction (to the north). When the horizontal component of the magnetic field was deflected clockwise 120 degrees by activation of Helmholtz coils surrounding the cage, the orientation of the buntings shifted accordingly (clockwise to geographic east-southeast). These results suggest that indigo buntings are not only able to detect the geomagnetic field, but also can use this information in the finalization of their migratory direction.

Test of the Principle of Operation of a Wideband Magnetic Direction Finder for Lightning Return Strokes

The principle of operation of a wideband crossed-loop magnetic-field direction finder is studied by comparing the bearing determined from the NS and EW magnetic fields at various times up to 155 microsec after return stroke initiation with the TV-determined lightning channel base direction. For 40 lightning strokes in the 3 to 12 km range, the difference between the bearings found from magnetic fields sampled at times between 1 and 10 microsec and the TV channel-base data has a standard deviation of 3-4 deg. Included in this standard deviation is a 2-3 deg measurement error. For fields sampled at progressively later times, both the mean and the standard deviation of the difference between the direction-finder bearing and the TV bearing increase. Near 150 microsec, means are about 35 deg and standard deviations about 60 deg. The physical reasons for the late-time inaccuracies in the wideband direction finder and the occurrence of these effects in narrow-band VLF direction finders are considered.

A Gated, Wideband Magnetic Direction Finder for Lightning Return Strokes

A magnetic direction finder has been developed which utilizes only the initial few microseconds of wideband return stroke waveforms to provide accurate directions to the channel bases of lightning discharges to ground. Bearing errors are minimized because, near the ground, most channels tend to be straight and vertical with no large branches or horizontal sections. Tests on a number of lightning storms at distances of 10 to 100 km indicate the angular resolution is in the range 1-2 deg, with little or no systematic dependence on azimuth or distance.