The objective of a geomagnetic observatory is to record continuously,
and over the long term, the time variations of the magnetic field vector and
the accurate absolute standard of the measurements. At Valentia Observatory,
absolute magnetic observations have been made since 1888 and the variations in
the earth's magnetic field have been continuously recorded since 1953.
Variations in the Earth's Magnetic Field
The variations in declination (D), horizontal intensity (H) and vertical
intensity (Z) are continuously recorded at the Observatory using tri-axial
As only the variations, rather than the full field, are recorded a value, called
the base-line value, has to be added to obtain the final absolute value of
the component of the field. Inclination, the angle at which the magnetic
the earth's field pass through a horizontal plane on the surface of the earth
(I), the total intensity (F), the north and east components of the vector,
referred to as X and Y respectively, are computed from the recorded
The outputs from the fluxgate magnetometers are connected to data loggers.
The data loggers convert the analogue outputs from the magnetometers to digital
and store them. Data are transferred via a modem to a PC data base where
they are edited and processed. Editing includes base-line adjustments,
corrections etc. Data are put into the correct format for forwarding to the
World Data Centres, for catering for special requests and for Magnetic Yearbook
Absolute Magnetic Measurements
Absolute magnetic measurements are made three times weekly for base-line determination.
Measurements are made at precise times and the values obtained are compared
with simultaneous instantaneous values from the variometers. From these comparisons
base-line values for the variometers are determined so that the true value
of the elements of the magnetic field can be derived from the variometer data
for any time.
A Zeiss 010B non-magnetic theodolite with a fluxgate sensor is used
for the measurement of Declination (D) and Inclination (I). An Elsec
proton magnetometer is
used to determine the absolute value of Total Force (F).
Horizontal force (H) is computed from F.CosI and Vertical force (Z) from F.SinI.
Regular Variations of the Magnetic Field
The regular variations of the earth's magnetic field are related to the
rotation and/or orbital movements of the earth, sun and moon. The most
prominent is the
solar daily variation or diurnal variation. There is a lunar daily variation
whose magnetic effect is small and as the lunar tides are not fixed to the solar
day they can be separated only statistically from the solar effect.
Another periodic variation seen in magnetic data is the yearly variation,
has a small amplitude.
In magnetic activity, a period of 27 days can also be seen. The period
corresponds to the rotation of the sun's latitude zone where the most
active areas and sunspots
The main field of the earth is that part of the field which has its origin
in the core. The changes of the main field are called secular variation.
changes in secular variation are small but definite. In the 100 years 1900 -
1999 declination has changed from 21° 30' west to 7° 44' west at Valentia.
During the same period horizontal intensity has changed from 17765nT to 19196nT.
Vertical intensity was 45084nT in 1900 and decreased to 43842nT in 1936 and increased
from that year to 44528nT in 1999. The knowledge of the secular variation is
essential in updating magnetic charts.
During strong magnetic storms, auroras may be seen at unusually low latitudes
and radio propagation may be severely disturbed. As a result of severe storms,
electric power lines can suffer from induced currents, satellites have suffered
from enhanced density of particles from the earth's expanding atmosphere and
disturbances in radio communications have been experienced.
Magnetic data are plotted on charts from which magnetic disturbances are identified
and classified and listings forwarded to World Data Centres (ISGI; International
Service of Geomagnetic Indices).
The Observatory serves as a base station for and conducts national magnetic
A comprehensive survey of Ireland was completed during 1964-1965 as Ireland's
contribution to the World Magnetic Survey and involved a programme of observations
at 57 stations. Since then, the Observatory has made magnetic survey observations
at a number of "repeat stations" to determine the absolute values of
the magnetic elements at certain locations.
Observations of Declination (D) Inclination (I) and Total Force (F)
at the repeat stations.
Isogonic Maps (lines of equal declination) and Isodynamic Maps (lines
of equal total, horizontal and vertical intensities) are produced from
data and can be updated at frequent intervals.
"The Secular Variation of Magnetic Declination Map for Ireland" is
updated regularly for the Ordnance Survey of Ireland.
Magnetic survey data are used to update lines of equal declination
on the aviation maps produced by the Irish Aviation Authority (IAA).
The magnetic declination values for IAA reference points are provided
Compass calibration circles at airports are surveyed and calibrated.
Compasses are calibrated regularly for the Air Corps and occasionally
for private aviators.
Data are provided on request to private companies and research bodies.
Magnetic data are sometimes needed in the investigation of power
failures and difficulties in telecommunications.
Data are generally supplied free of charge to schools, colleges
and research students as well as to other government agencies,
and on a fee basis
to commercial institutions.