The manufacture and testing of the "core" optical substrates for the Laser Interferometer Gravitational-wave Observatory (LIGO) are described in this paper. These substrates are for use in long baseline Michelson interferometers with Fabry Perot cavities up to 4 lan in length in each arm. The optical surfaces of the substrates (250 mm diameter by up to 100 nun thick) are specified either flat or curved, with radii of curvature varying between 7 and 15 km and tolerance bands on the radius equivalent to variations in the sag (over 200 mm) of twenty nanometres or so. Very strict tolerances were also placed on the astigmatism of the surfaces and the surface errors in two spatial frequency bands, one at low frequencies ("waviness")and another at high frequencies ("roughness"). In some cases the radius of the wavefront emerging from the substrate was also specified (for a collimated test beam).

A joint Discussion Meeting of the Royal Astronomical Society and the Royal Irish Academy, held on January 11th, 1991, commemorated the establishment of some early magnetic observatories, discussed recent research using global geomagnetic data and described the present status of magnetic observatories in the United Kingdom. The observatory and instruments at the Dublin magnetic observatory; the origins of the Greenwich magnetic observatory, and why it eventually had to be resited; and the history of the Munich magnetic observatory formed the historical part of the proceedings. Current research topics discussed were the geomagnetic secular variation and deep Earth structure and dynamics; fluid flow patterns near the top of the core; the origin of the annual variation of the geomagnetic field; results of an analysis of monthly means from some British observatories; a new theory of the geomagnetic daily variation; and the interactions between ionospheric science and geomagnetism. The present-day observatory scene was described in terms of the information that can be derived from the almost 40 year series of data from Hartland magnetic observatory; of the methods used to process data from the three UK magnetic observatories, which nowadays are operated automatically and remotely; and (a look into the future) of a new project. INTERMAGNET, which aims to make available, in near real time, data from the world-wide network of magnetic observatories.

In answer to Patrick Taylor in Forum (Eos, April 16, 1991) I will not pursue the question further if it is defeated democratically in Vienna in August.Two years after Exeter the proposal is no longer a “shock to the system” of the scientific establishment. It has been put formally to IAGA, and the Executive has not moved to oppose it. Instead IAGA recognizes the merit, the legality, and the operational viability of the principle I have raised by proposing an amendment which is completely acceptable to me. The turnaround reflects opinion in the international community.

Accurate measurements from standard observatories and repeat stations will continue to be essential for modelling the geomagnetic field for navigation and scientific purposes. The technology of data base management increasingly calls for the use of digitally recording magnetometer systems at observatories, and suggests that absolute and repeat station observations should be made with compatible equipment. The cost of such equipment is reducing and its reliability is increasing. Digital equipment relieves operators of the tedious scalings which often result in backlogs of years in the presentation of data to the community. There is, therefore, every reason to recommend that observatories should buy and operate digital magnetometers and adopt digital processing systems.

ARGOS, an instrument and recording system for performing standard geomagnetic observatory functions at the three U.K. observatories, is described. Operations are controlled by a minicomputer at each observatory communicating by modem through the public telephone system to a central computer in Edinburgh. A fluxgate magnetometer provides 10-s samples of the variation field at + 1 nT resolution. These values are filtered to produce 1-min values (centred on the minute) which in turn are used to compute hourly mean values. These and other derivatives of the raw data are stored in the observatory computer and transmitted to Edinburgh daily by operator command, where they are transferred to a data file which can be accessed by users via the Joint Academic Network computer network (JANET). Observatory data are available to users by this means within 24 h and can be made available in near real time by special arrangement. ARGOS performs standardization measurements of the values of the field components remotely using a proton magnetometer employing standard techniques for absolute observations. Comparison of these Baseline Reference Measurements with manual absolute observations shows them to be acceptable for baseline adoption. In the first year of operation it has been established that ARGOS produces data which are of comparable quality to the classical standards expected from the U.K. observatories. Data loss has been less than 1%. Further automation of routine procedures (e.g. magnetogram plotting, editing, baseline adoption and the adjustment of minute values day by day) will be the focus of attention in the next 2 years.

At the 1989 International Association of Geomagnetism and Aeronomy meeting in Exeter, U.K., a proposal was presented to support geomagnetic observatories in the Third World by charging commercial users of the International Geomagnetic Reference Field a price for its use. The proposal guarantees research scientists free access to IGRF. Seven British companies have contracted to pay $1 million as an expression of the concern of the commercial world to maintain a high standard of accuracy in IGRF. The IAGA executive committee rejected the proposal and at the final plenary session, that decision was upheld 29 to 14 in a scientific vote of all delegates present, a poll representing about 5% of the IAGA population. The main objection to the proposal was that the sale by contract would create an infringement of scientific liberty.

The magnetic field of the Earth originates in the motions of the fluid core of the planet. By good fortune it is predominantly dipolar and so has a simple topography. The core motions cause the field to change with time, and once charted it has to be charted again and again. Mathematical models can be made which represent the geomagnetic field to an accuracy of about one degree globally. Higher accuracy demands advanced techniques such as satellite surveys, and also attention to the detail of external fields caused by solar activity which disturb the main field, restricting the accuracy of mapping and producing magnetic deviations from the mean value from day to day. Understanding such effects is essential in preparing magnetic reference data for navigation.Though now losing ground to other methods, magnetic compasses still have a valuable role to play in many situations, so long as the limitations are fully appreciated and taken into account. Modern technology in standard geomagnetic observatories offers ways of updating a datum very accurately in automatic navigational systems and even of forecasting disturbances, and may bring magnetic reference back to the forefront of many navigational systems.

TheK index was developed by Bartels and is a measure of geomagnetic activity. The important feature of this index is that it is a measure of solar activity superimposed on the regular daily variation.

The normalized rate of occurrence of dayside Pc 3,4 pulsations from L=2.4 to 4.3 has a strong enhancement for low cone angles of the interplanetary magnetic field. When the angle of the IMF to the earth sun line, θBX, is 15° or less the occurrence rate is 7‐8 times the average rate at L=2.4 to 2.8 and 2.2 to 3.5 times the average rate at L=4 to 4.3. These waves disappear when the IMF is nearly at right angles to the sun‐earth‐line. This absence of pulsations occurs over the widest range of angles at lowest L‐values. These observations are consistent with a source originating in the waves upstream of the subsolar bow shock which are transported by convection to the magnetopause where they couple to oscillations of magnetospheric field lines. Since the index of refraction of the magnetospheric plasma decreases with decreasing radial distance, except at the plasmapause, inward propagating waves should be refracted away from the radial direction. Thus, to reach low L‐values the waves should couple near the stagnation point and propagate nearly radially inwards. Upstream waves should be convected to the stagnation point for only a limited range of θBX. However, to reach higher L‐values the coupling may be at later local times where cross streamline propagation can bring waves from a larger range of θBX. The streamline geometry and its connection to the foreshock region is illustrated for a variety of IMF orientations using a simple approximation to the magnetosheath flow field.

A long period (∼6 min) hydromagnetic wave was observed near L∼4 in the southern hemisphere at both Halley Bay and Siple, Antarctica, to have a substantially larger amplitude than that observed in the northern hemisphere conjugate areas. At Halley Bay, as well as at Siple, the wave appeared quite monochromatic. Data from a station array conjugate to Siple in the northern hemisphere showed the wave to be localized near L∼3.4. Whistler wave determinations of the equatorial electron densities show the wave to occur inside the plasmasphere. The wave period and localization region are not readily reconcilable with a cold, pure hydrogen plasma. The density of heavy ions that would be required within the plasmasphere to explain the observations appears excessive from present understanding of magnetosphere plasmas.

Magnetometer arrays operated in Europe during the IMS by the Institute of Geological Sciences (Edinburgh) and the Institut für Geophysik (Gottingen) are described. Some of the analytical procedures available for pulsation research are described as is the format and availability of digital data.

Observations of dPi's at two conjugate stations (Halley Bay and St. Anthony, Newfoundland) show that differences occur in their time of arrival at the two stations. Arrival time difference is interpreted as the difference in the time taken for the initial disturbance caused by the impulse which generates Pi oscillation to travel from its origin in the magnetosphere to the ends of the field line. The observations imply that the initial impulse occurs preferentially in the southern hemisphere. Hodograms of the horizontal polarization of dPi's are examined for the symmetry properties expected from the ‘vibrating string’ analogy. They show that an odd mode travelling wave results from the initial impulse (a fact in accord with earlier evidence about the initial movement of the distension field). In some of the events examined the oscillation settles to an even mode standing wave after a period of time which can be related to the travel of field line guided waves along the total length of the field line.

A brief examination of micropulsations recorded simultaneously at Mid-latitude stations (the U.K. array) and at a station near the dip equator was conducted during an expedition to study the eclipse of 30 June 1973. No eclipse effects were detected. However, it is demonstrated that night time events (particularly Pi 2's anddPi's) are highly correlated between mid latitudes and the Equator, whereas day time events in the Pc3 range show little or no correspondence.

An application of the ‘vibrating string’ analogy to the horizontal polarisation characteristics of Pi2's at two conjugate stations reveals an initial travelling wave, with predominantly ODD mode characteristics, which settles rapidly to a standing wave with predominantly EVEN mode characteristics. This rationalization is in agreement with several earlier observations of Pi generation and may be the basis of quantitative measurements of magnetosphere perturbations using conjugate pulsation recordings.