Long Hops by Mark Denny

Author:Mark Denny
Language: eng
Format: epub
Publisher: University of Hawaii Press
Published: 2016-10-15T00:00:00+00:00

FIGURE 4.8. Geomagnetic declination. This data is for the year 2010. Lines are contours of constant declination angle, as labeled. Image courtesy of NOAA.

Thus the geomagnetic field changes measurably in both strength and direction as we move from point to point over the surface of the earth. However, this is not all: the geomagnetic field varies over time as well as over space. During the past two thousand years, the strength of the earth’s magnetic field has fallen by a third; there has been an easily measurable change since geomagnetic field strengths were first recorded systematically in the eighteenth century. The magnetic north pole has moved over time. In the early twentieth century, it was drifting across northern Canada at the rate of about 10 km per year; in recent decades, this rate has increased to 40 km per year. Soon magnetic north will leave Canada and enter Siberia. Magnetic south is also moving (unlike the geographic North and South Poles, magnetic north and south are not at opposite ends of the earth; also, their movements are not synchronized).

The changes in our geomagnetic field do not just occur at the ends of the earth. At any given position on the surface, current measurements (which are exquisitely sensitive) show that magnetic field strength and direction both change daily. The field strength changes are on the order of 10 nT per day, and the rate of change itself varies a lot due to solar activity. Magnetic storms are disturbances in our geomagnetic field due to solar flares (coronal mass ejections) that give rise to huge magnetic fields that can reach out from the sun as far as our Earth; they influence the magnetosphere16 (hence the aurora borealis and aurora australis) and so influence our geomagnetic field. These magnetic storms can cause rapid changes of, typically, 100 nT in surface magnetic field strength.

Over time, the geomagnetic field changes in direction as well as strength. The field lines change direction by amounts that accumulate to the extent that new geomagnetic maps are issued every few years—the map of figure 4.8 is now out of date.

I repeat, for future reference, that the daily—indeed, sometimes hourly—changes in geomagnetic field strength at any point on the surface of the earth are typically 10 nT, mostly due to solar activity. Occasionally, very weak transient magnetic signals can originate from the earth. Tsunamis involve the flow of massive volumes of seawater; seawater is a conductor, so when it moves it generates a magnetic field. The magnetic anomalies that arise from tsunamis are about 1 nT—very feeble but detectable by sensitive instruments.

One type of change in the magnetic field of our planet is profound: the polarity of the bar magnet (fig. 4.7a) flips—reverses—every few hundred thousand years. That is, north and south poles switch positions. This surprising effect is now understood by physicists; it is chaotic—the timing of the flips is seemingly random, occurring on average every 300,000 years or so, but the spread is very large—we haven’t had a flip for 780,000 years.

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