Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmae43a..04r&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #AE43A-04
Physics
3304 Atmospheric Electricity, 3324 Lightning, 3329 Mesoscale Meteorology, 3394 Instruments And Techniques
Scientific paper
Most of the vertical profiles of the electric fields in thunderstorms come from low-mass balloon-borne electric field meters (EFM) based on the design originally developed by Winn and Byerley (Q. J. Royal Met. Soc., 101, 979-94, 1975). This instrument uses a pair of aluminum spheres as sensing electrodes, which spin about the horizontal axis with a frequency of about 2.5~Hz. The charge induced on the spheres from the vertical component electric field, Ez, varies sinusoidally with this frequency. The spin frequency is measured with a mercury switch. The horizontal axis rotates about the vertical with a frequency of about 0.1~Hz, which introduces a modulation of the sinusoidal signal due to the horizontal field Eh. A Hall-effect sensor is used to detect the rotation by sensing the Earth's magnetic field. Data (induced charge, mercury switch, Hall sensor) are telemetered using a 400~MHz radiosonde transmitter, with the spheres acting as the transmitter antenna. The viscosity of the mercury in the mercury switch varies with temperature, which results in phase variations between switch closures and the absolute vertical orientation of the spheres, and this information is lost completely when the mercury freezes at about -40°C. The Hall-effect sensor confirms the spin and rotation, and some success had been achieved in separating Ez from Eh. The 1-D Hall-effect sensor may be used with especially clean data to calculate Ex and Ey from Eh. The demodulation of the horizontal components is subject to several approximations and is noisy in all but the best cases. The nulls of the dipole antenna formed by the rotating spheres sometimes result in noisy data, and telemetry is sometimes lost completely when the balloon travels far from the ground-based receiving station. We have recently redesigned the EFM package to overcome these problems. The new EFM uses a MEMS two-axis accelerometer to determine vertical orientation, and a three-axis flux-gate magnetometer to sense the Earth's magnetic field. Combining these data allows calculation of the absolute orientation of the sensor, from which ěc{E} can be determined. The data are telemetered as in the older design, but are also recorded on-board on a Secure Digital (SD) card, which provides a full error-free record for those EFMs which are recovered. An increased data rate allows better separation of Ez from Eh, and an increased rotation frequency results in better time resolution for Ex and Ey. A GPS beacon on a separate radiosonde gives the instrument's 3D location during flight; this information is also used to help retrieve the instrument after the flight. A mechanical redesign made the EFM much more robust for better survival in high-wind launches and results in faster rebuild after flights.
Apostolakopoulos I.
Bruning Eric
Fredrickson S.
Kennedy Diane
Krehbiel Paul R.
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