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impulse s contribute little to the 1.� cps activity. This i s

substantiated by the fact that the curves i n Figure 4.1 show enhanced activity only between 07 - 1 7 hr T, even though the electric train system operates outside the se uours on all days of the we ek •

. 4 co . �LUS O S

It has been shown that daytime Z component micropulsation recordings are greatly influenced by two distinct type s of interference.

The signal to noise ratio at the present field station site could be improved by recording either the X or Y horizontal component of the geomagnetic field. Since the current carrying grid systems are horizontal interference is less likely to be observed on the horizontal components. In addition, the uniform conductivity of the surrounding terrain i s such that the magnitude of signals of geomagnetic origin will be greater on the hori zontal components. The ratio of vertical component signal amplitude to horizontal component signal amplitude decrease s with increasing frequency (Duffus et al, 1962) and thus this effect will be most pronounced at Pc1 frequencie s.

PART B

DISCRETE EMISSIONS IN THE VICINITY OF

. H,.., TER V

).1 I. RODUCT 0

The results presented in Chapter IV show that daytime �c1 micropulsation measurements of the vertical component of the geomagnetic field at a site near a large centre of population are limited by the high level of man-made interference. Further use of the Rolleston Field Station for recording Pc 1 pul sations during the daytime depended on whether an improved signal to noise ratio could be obtained by measuring one of the horizontal components. It was therefore decided that a portable recording system should be constructed, which, if necessary, could be moved to an

electrically quieter site.

The requirements were for a portable system th at would : ( i ) Operate from a battery supply.

(ii ) Continuously record unattended for at least three weeks.

(iii ) Detect either the X or Y component of the geomagnetic field.

(iv ) Record in the frequency range 0.3 - .5 . 0 cps with a maxim'-lm sensitivity of 1-2 m (•

Measurements of the horizontal components of the geom�gnetic field are generally conducted using either a large diameter coil (1-2m) with an air core, or a small diamter coil (2-5cm) with a high permeability ferromagnetic core. Both types of coils contain many thousands of turns. From the consideration of portability it was decided that the physically smaller co 1, with a high permeability core, was preferred.

To satisfy the power requirements of a battery supply it was necessary to design the apparatus around solid state components. A low noise transistor D.C. preamplifier, capable of amplifying signals in the range 10-16 - 10-18w induced in the detector coil , was therefore required. Two types of amplifiers were considered,

27. a chopper amplifier and a galvanometer-photocell amplifie r. The forme r proved unsatisfac tory because of the problems of noise and dri ft and the necessity for complicated filter ne tworks to eliminate radiated 50 cps interference. Conversely, the galvanome ter is itself an efficient low-pass filter and is capable of ampli fying

- 1 8

signals at levels below 1 0

w.

favour of the galvanome ter-photocell amplifier.

The convent ional me thod o f recording data on paper chart was not practicable be cause of the requirement s to resolve signals in the 1 cps range and to re cord continuously over a period of at least three weeks . Under the se conditions recording on slow-speed magne tic tape provide s the most suitable me thod for colle cting data.

This me thod is pre ferred for studying the frequency characteristics of signals since it allows the use of analogue spe ctrograph

technique s for data analysis. The frequency standard ne cessary for

the tape transport speed control was available in the form of a transistorized ele ctro-mechanical tuning fork.

In the following sections the design theory o f a dete ctor coil and a galvanome ter-photocell amplifier is developed in order to study the feasibility of constructing a re cording system with the required sensitivity and frequency re sponse. In addition, the design of a slow-speed magne tic tape re cording system is brie fly considered.

5.2 SENSITIVITY REQU I EMENTS.

The equivalent circuit of a galvanometer-pho tocell amplifier utili zing feedback and fed from a dete ctor coil containing a high pe rmeability ferromagne tic core is shown in Figure 5.1. De finitions o f the symbols used are included in the legend of the figure. In this se ction an approximate expre ssion for the minimum detec table signal will be derived ; a formal treatment of the

The maximum sensitivity of the recording system is

principally de termine d by the noise in the galvanome ter-photocell

amplifier. Two sources of noise must be considered , the thermal

noise in the input circuit and the Brownian motion of molecule s striking the galvanome ter mirror (Harris, 1 959) . Only the thermal noise will be treated here . Magnetic noise, associated with

the ferromagnetic core of the de tector , will also be negle cted. The thermal noise in the input circuit of the amplifier is given by

(5.1 ) where K is Boltzmann ' s constant , T the absolute temperature, and

b,. f the amplifier bandwidth. In the frequency range under consideration ,1 f � _fH where _fH is the upper frequency cutoff of the amplifier.

An approximate expression for the signal current in the input circuit shown in Figure _{5. 1 is obtained by assuming i2 zero and}