Measurement of atmospheric electric fields
The local voltage in a space charge cloud (for example of charged mist or dust) can be measured with a voltage follower probe or with an electrostatic fieldmeter acting as a potential probe.The electric field at the sensing aperture of an earthed fieldmeter E depends on the effective diameter of the fieldmeter d and the local potential V, present before the fieldmeter was introduced as: E = f V / d The factor f is near unity. The fieldmeter needs to be several diameters away from nearby surfaces.
This approach is very appropriate for measurement and long term continuous monitoring of atmospheric electric fields.
The ambient atmospheric electric field Ea
is conveniently measured using an electrostatic fieldmeter mounted on a pole a known distance above ground level. In this arrangement the fieldmeter is being used as a probe of the local potential at its mounting height. This mounting arrangement is simple to implement, avoids anxieties about ground level dust and debris (and insects) entering the fieldmeter sensing aperture and gives useful enhancement to the basic fieldmeter sensitivity.A fieldmeter at a mounting height h (m) in an ambient electric field E
a (V m-1) acts as a potential probe. Thus V = Ea h - where V is the local potential (V) at the height h. The sensitivity of a fieldmeter mounted well clear of nearby surfaces as a potential probe is close to V = Ef d - where Ef is the electric field (V m-1) at the fieldmeter sensing aperture responsible for the fieldmeter reading and d the effective sensing head diameter (m). Hence the ambient electric field is obtained from measurement of the electric field at the fieldmeter sensing aperture as:E
a = Ef d /hThere will be a contribution to the electric field measured dependent on the alignment of the sensing aperture relative to the ambient electric field. If for example the two field components are in directions to add, then the atmospheric field can be derived as:
E
a = Ef d / (h (1-d/h))As d/h is normally small the influence of this effect is small.
The actual sensitivity of measurements can be checked in-situ by applying a calibration voltage to the whole fieldmeter assembly. This gives the fieldmeter reading as a function of local voltage - so the local ambient atmospheric electric field is obtained knowing the mounting height of the sensing aperture. Appication of a modest level alternating potential to the fieldmeter assembly can be used for continuous monitooring of the operational health of the observation system. This ensures confidence in observations during operation in adverse environmental conditions.
An appropriate fieldmeter for measuring and monitoring atmospheric electric fields is a JCI 131 electrostatic fieldmeter. This has been designed and constructed for reliable operation even in very wet environments and with driving rain.
The relation of local ambient atmospheric electric field values to the charge, altitude and distance characteristics of thunderclouds may be modelled with Spreadsheet calculations treating the cloud charges, and their image charges below the ground plane, as dipoles.
Where the fieldmeter is mounted other than above a large plane ground area well clear of any buildings or earthy projections there will be need to ?interpret? electric field measurements in relation to the geometric arrangement of the surroundings. This can be done with computer modelling calculations - but this may be difficult and lacking conviction in complex three dimensional arrangements. One approach to tackle this problem is to normalise readings in relation to otherwise known ambient atmospheric electric field values with, for example, a clear sky situation - when the ambient field is typically around 100V m
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John Chubb Instrumentation,
Unit 30, Lansdown Industrial Estate, Gloucester Road, Cheltenham, GL51 8PL, UK
Tel:+44 (0)1242 573347 Fax: +44 (0)1242 251388
email: jchubb@jci.co.uk
Page Update: 02/01/2000 Ôø? John Chubb Instrumentation.