oce (version 1.7-2)

magneticField: Earth magnetic declination, inclination, and intensity

Description

Implements the 12th and 13th generations of the International Geomagnetic Reference Field (IGRF), based on a reworked version of a Fortran program downloaded from a NOAA website (see reference 1).

Usage

magneticField(longitude, latitude, time, version = 13)

Arguments

longitude

longitude in degrees east (negative for degrees west). The dimensions must conform to lat.

latitude

latitude in degrees north, a number, vector, or matrix.

time

The time at which the field is desired. This may be a single value or a vector or matrix that is structured to match longitude and latitude. The value may a decimal year, a POSIXt time, or a Date time.

version

an integer that must be either 12 or 13, to specify the version number of the formulae. Note that 13 became the default on 2020 March 3, so to old code will need to specify version=12 to work as it did before that date.

Value

A list containing declination, inclination, and intensity.

Historical Notes

For about a decade, magneticField used the version 12 formulae provided by IAGA, but the code was updated on March 3, 2020, to version 13. Example 3 shows that the differences in declination are typically under 2 degrees (with 95 percent of the data lying between -1.7 and 0.7 degrees).

Details

The code (subroutines igrf12syn and igrf13syn) seem to have been written by Susan Macmillan of the British Geological Survey. Comments in the source code igrf13syn (the current default used here) indicate that its coefficients were agreed to in December 2019 by the IAGA Working Group V-MOD. Other comments in that code suggest that the proposed application time interval is from years 1900 to 2025, inclusive, but that only dates from 1945 to 2015 are to be considered definitive.

References

1. The underlying Fortran code for version 12 is from igrf12.f, downloaded the NOAA website (https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html) on June 7,

2. That for version 13 is igrf13.f, downloaded from the NOAA website (https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html on March 3, 2020.

3. Witze, Alexandra. “Earth's Magnetic Field Is Acting up and Geologists Don't Know Why.” Nature 565 (January 9, 2019): 143. 10.1038/d41586-019-00007-1

See Also

Other things related to magnetism: applyMagneticDeclination()

Examples

# NOT RUN {
library(oce)
# 1. Today's value at Halifax NS
magneticField(-(63+36/60), 44+39/60, Sys.Date())

# 2. World map of declination in year 2000.
# }
# NOT RUN {
data(coastlineWorld)
par(mar=rep(0.5, 4)) # no axes on whole-world projection
mapPlot(coastlineWorld, projection="+proj=robin", col="lightgray")
# Construct matrix holding declination
lon <- seq(-180, 180)
lat <- seq(-90, 90)
dec2000 <- function(lon, lat)
    magneticField(lon, lat, 2000)$declination
dec <- outer(lon, lat, dec2000) # hint: outer() is very handy!
# Contour, unlabelled for small increments, labeled for
# larger increments.
mapContour(lon, lat, dec, col='blue', levels=seq(-180, -5, 5),
           lty=3, drawlabels=FALSE)
mapContour(lon, lat, dec, col='blue', levels=seq(-180, -20, 20))
mapContour(lon, lat, dec, col='red', levels=seq(5, 180, 5),
           lty=3, drawlabels=FALSE)
mapContour(lon, lat, dec, col='red', levels=seq(20, 180, 20))
mapContour(lon, lat, dec, levels=180, col='black', lwd=2, drawlabels=FALSE)
mapContour(lon, lat, dec, levels=0, col='black', lwd=2)
# }
# NOT RUN {
# 3. Declination differences between versions 12 and 13
# }
# NOT RUN {
lon <- seq(-180, 180)
lat <- seq(-90, 90)
decDiff <- function(lon, lat) {
    old <- magneticField(lon, lat, 2020, version=13)$declination
    new <- magneticField(lon, lat, 2020, version=12)$declination
    new - old
}
decDiff <- outer(lon, lat, decDiff)
decDiff <- ifelse(decDiff > 180, decDiff - 360, decDiff)
# Overall (mean) shift -0.1deg
t.test(decDiff)
# View histogram, narrowed to small differences
par(mar=c(3.5, 3.5, 2, 2), mgp=c(2, 0.7, 0))
hist(decDiff, breaks=seq(-180, 180, 0.05), xlim=c(-2, 2),
     xlab="Declination difference [deg] from version=12 to version=13",
     main="Predictions for year 2020")
print(quantile(decDiff, c(0.025, 0.975)))
# Note that the large differences are at high latitudes
imagep(lon,lat,decDiff, zlim=c(-1,1)*max(abs(decDiff)))
lines(coastlineWorld[["longitude"]], coastlineWorld[["latitude"]])
# }