A BRIEF INTRODUCTION

TO GEOMAGNETISM

The Earth's magnetic field is both expansive and complicated. It is generated by electric currents that are deep within the Earth and high above the surface. All of these currents contribute to the total geomagnetic field. In some ways, one can consider the Earth's magnetic field, measured at a particular instance and at a particular location, to be the superposition of symptoms of a myriad of physical processes occurring everywhere else in the world. The challenge is to untangle the rich information content of the magnetic field so that we can better understand our planet and the surrounding space environment in which it resides. Obviously, it would be a daunting task to try to summarize every single aspect of the subject of geomagnetism. Indeed, the necessity for brevity here means that our exposition will omit some relatively enormous details. Therefore, in this review for our website, we choose to concentrate on the specific phenomena that can be monitored and studied with data collected from a ground-based magnetic observatory network, such as that operated and maintained by the USGS Geomagnetism Program. The material presented here should be comprehensible by anyone having taken introductory college-level physics classes, although, having said that, we imagine that some review on the part of the reader might be required! Readers having additional curiosities about the magnetic field should consult the list of references, and the references in those references, given in our page of Further Reading. Those having specific questions are referred to our page of Frequently Asked Questions.

Magnetic-Field Components

Perhaps the most familiar demonstration of the reality of the Earth's magnetic field is the north-seeking tendency of the compass needle, a property that has been exploited by navigators for centuries. A compass is constructed, of course, such that its magnetic needle is free to rotate in the horizontal plane. But if we were to permit the compass needle to have full directional freedom, suspending the needle (say) from a thread so that it could freely obtain its orientation both horizontally and vertically, we would find that the alignment of the needle would vary continuously from one point in space to another. Furthermore, if we were to measure the force on the magnetic needle causing it to assume its preferred alignment, we would find that the strength of this force, proportional to the intensity of the magnetic field, also varies continuously with position in space. These properties can be used to map a continuous family of lines of force -- vectors having both direction and magnitude. At any one individual point on the surface of the Earth the orientation of the geomagnetic vector is conventionally described relative to a geographic coordinate system using two angles: declination, the angle between the horizontal component of the magnetic-field vector relative to true north, and inclination, the angle between the horizontal plane and the total field vector. The intensity of the magnetic field, which is independent of the orientation of the reference coordinate system, is represented by the length of the vector.

The magnetic-field components: (X,Y,Z) define the Cartesian components (north, east, down), the usual observatory components (H,D,Z) are (horizontal intensity, declination, down), the angle of the magnetic vector with respect to the horizontal plane (I) is inclination, and (F) is the total field intensity.

The Spatial Form of the Geomagnetic Field

The magnetic field of the Earth is often times described as being approximately dipolar, with field lines emanating from the south geomagnetic pole and converging at the north geomagnetic pole, as depicted in the figure below. Although this description is useful for many purposes, it is not particularly accurate. The dipolar part of the field is actually tilted by approximately 11° with respect to the rotational axis, and there are additional, non-dipolar ingredients in the geomagnetic field, all of which, when added together, are the total surficial field in all of its complex detail. As a result of this complexity, not only does the direction of the compass needle deviate from true north, but the amount of the deviation, the declination, varies as a function of geographic location; see the map below. This fact has been an historical nuisance for navigators, and, not surprisingly, it helped to motive some of the original global-scale surveys of the Earth's magnetic field. Another simple measure of the field's geometry is the position of the magnetic poles. At the north geomagnetic pole, our freely moving magnetic needle would point down, whilst at the south geomagnetic pole, the needle would point up. For these reasons, the geomagnetic poles are sometimes referred to as ‘dip poles'. The north geomagnetic pole is located in the Canadian Arctic at about 82°N latitude and 248°E longitude. The south geomagnetic pole is located in the Antarctic Ocean south of Australia at about 65°S latitude and 138°E longitude. Note that the geomagnetic poles are not antipodal, an asymmetry that is just another measure of the field's geometric complexity.

The axial-dipolar part of the Earth's magnetic field, with field lines emanating from near the south geographic pole and converging near the north geographic pole. Although the magnetic field at the Earth's surface is predominantly an axial dipole, the actual magnetic field is more complicated.

       

A time-dependent map showing the magnetic declination (D), degrees eastward, on the Earth's surface for the years 1590-1990. The geographic variation of declination is indicative of the field's complexity, with declination contours converging at the geomagnetic poles. For further information, please visit the models page to determine declination and other magnetic-field components at a particular site, visit the charts page for recent maps of the various magnetic-field components, or visit our movies page for maps of the secular variation of the Earth's magnetic field.

The Geodynamo

The Earth is, of course, extremely complicated; it consists of many different interacting parts. But broadly speaking the Earth below our feet is stratified in radius, being composed of a solid-iron inner core, a liquid-iron outer core, and an electrically-insulating, rocky over-lying mantle; see the figure below. The main part of the Earth's magnetic field is generated by electric currents sustained by a dynamo situated in the core, and the study of the form and long-term behavior of the geomagnetic field can be used to discover how the geodynamo works. Paleomagnetic measurements of rocks indicate that the Earth has possessed a magnetic field for at least 3.5 billion years, and yet, without some sort of regenerative process to offset the inevitable ohmic dissipation of electric currents, the geomagnetic field would vanish in about 15,000 years. Therefore, the dynamo in the core must be regenerative, and it is generally thought that this regenerative process relies on the principles of magnetic induction. In effect, the core is a naturally occurring electric generator, where convection kinetic energy, driven by chemical differentiation and the heat of internal radioactivity, is converted into electrical-magnetic energy. More specifically, electrically-conducting fluid flowing across magnetic-field lines induces an electric current, and this generated current supports its own associated magnetic field. Depending on the geometrical relationship between the fluid flow and the magnetic field, the generated magnetic field can reinforce the pre-existing magnetic field, in which case the dynamo is said to be ‘self-sustaining'.

The anatomy of the Earth. The mantle has a radius (a) of 6371 km, the inner core has a radius (b) of 1215 km, and the outer core has a radius (c) of 3485 km.

The details of exactly how the dynamo works are not entirely resolved. Hence, the theory is the subject of on-going research. Nonetheless, a reasonably coherent qualitative understanding exists of how the core's fluid motion sustains the Earth's magnetic field. We know that the dynamo is governed by dynamical, nonlinear mathematical equations, somewhat akin to the equations of meteorology and oceanography, but with the additional complication presented by the magnetic field itself. From a purely kinematic standpoint dynamo action relies on the so-called ‘alpha-omega' process, in which core fluid motion, influenced by the Coriolis force, consists of a combination of differential rotation and convective, turbulent helical motion. These two motional ingredients work together to reinforce the magnetic field and, thereby, offset the destructive affects of ohmic dissipation; see the figure below. The alpha-omega process successfully describes how it is that the magnetic field can be amplified, but it is the dynamics that ultimately determine the field's strength: the field grows until a rough balance is attained between the Coriolis and the Lorentz forces.

The alpha-omega dynamo cycle: Consider an initial dipolar poloidal field, such as in (a). The omega-effect consists of (b,c) differential rotation, wrapping the magnetic field around the rotational axis, thereby creating (d) a quadrupolar toroidal field magnetic field inside the core. A closure of the dynamo cycle requires a bit of symmetry breaking, brought about by the alpha-effect, whereby (e) helical upwelling and downwelling creates loops of magnetic field. These loops coalesce (f) to reinforce the original dipolar field.

Field Variation Over Historical Timescales

Although the magnetic field is sustained by the dynamo within the Earth's core, part of the field threads its way through the mantle and up to the surface, where it has influence on compasses and where it can be measured by magnetometers. The same convective motion that drives the geodynamo also causes the field, measured at the surface, to be time-dependent over historical timescales, a phenomenon known as ‘secular variation'. In fact, magnetic models and charts must be periodically updated to accommodate the continual secular variation of the field. Secular variation can also be clearly seen in long-timescale, magnetic records collected at ground-based observatories. In the figure below we show annual-mean declinations take from over a century's worth of data collected from the various USGS Geomagnetism Program observatories. For making long-term studies of the secular variation it is important that magnetic measurements such as these be made relative to a stable reference. In the case of the observatories, this means that the same stable absolutes pier must be used year-after-year. Of course, for practical reasons, occasionally, an observatory pier must be moved, and sometimes even an entire observatory must be moved, and in those cases the secular variation data exhibit a baseline step-offset due to differences in the local underlying crustal magnetization. In a sense, the figure below not only shows a history of the magnetic field over the past century within the United States, but it also shows a history of the Geomagnetism Program itself.

A stackplot of the annual-mean declination (D), measured in degrees east, at the USGS magnetic observatories as a function of year. The field at the surface, and the declination in particular, has a complex form, and, as a result, the secular variation of the declination is complex as well. Note, for example, that the declination variation at San Juan (SJG) is significantly different from that of Honolulu (HON). Also note the occasional offset, such as in the Honolulu, San Juan, or Fredericksburg (FRD)/Cheltenham (CLH) annual-mean data, due to change of the absolutes pier or moving of the observatory.

The Earth's Ionosphere and Diurnal-Field Variation

The magnetic field, generated in the core and measured at the surface, continues upward through the ionosphere, the electrically-conducting, ionized layer of the Earth's upper atmosphere. The ionosphere extends in height from about 90 km to about 600 km, and it is electrically conducting because ultraviolet radiation from the Sun is absorbed by the electrons of nitrogen and oxygen molecules in the atmosphere. This absorption causes electrons to be dislodged from their molecular orbits, thereby producing free negative charges (electrons) and free positive charges (ions). As the Earth rotates underneath the Sun, periodic differential heating of the atmosphere causes it to expand on the day-side and contract on the night-side. Superimposed upon this variation is an atmospheric tide, similar to the oceanic tide and driven most substantially by the rotation of the Earth under the gravitational field of the Moon. The combination of these periodic forces drive winds in the ionosphere, and with the resulting fluid motion across magnetic-field lines, electric currents are induced. These currents support their own magnetic fields, and thus a diurnal perturbation in the magnetic field is generated. The diurnal-field variation can be measured at the Earth's surface, and with an array of magnetometers it is possible to map the electric currents in the ionosphere. What remains a matter of current research, however, is a dynamically-consistent mapping of the fluid motion sustaining these electric currents in the ionosphere.

Schematic diagram of the electric-current pattern in the ionosphere driven by diurnal heating from the Sun. Note that the current is concentrated on the day side, consisting of two oppositely oriented circuits.

A stackplot of the diurnal variation in the horizontal-field intensity (H) as measured by USGS observatory magnetometers during magnetically quiet conditions in early January 2003. The data have been adjusted to account for different observatory site longitudes. Note that the high-latitude observatories of Barrow (BRW) and College (CMO) show a higher level of activity, which is normal, even during quiet conditions. The regular diurnal magnetic-field variation is seen clearly as mid- and lower-latitude observatories, and is generated by large-scale electric currents in the Earth's ionosphere.

The Earth's Magnetosphere

The magnetosphere is the outer part of the Earth's magnetic field, a region in the near-Earth space environment where the shape and behavior of the geomagnetic field is governed by the Sun. Of course, the Sun is a highly dynamic presence within the solar system. It has its own dynamo, generating a somewhat tangled magnetic field that extends out into interplanetary space. The Sun also emits a wind of electrically-charged particles, a plasma that flows outwards into space and which carries with it the heliomagnetic field. Because of the pressure exerted by the solar wind on the geomagnetic field, the magnetosphere is compressed on the day side and elongated on the night side of the Earth, such as depicted in the schematic figure below. In dimension along the equatorial plane, the day-side magnetosphere, the boundary of which is called the ‘magnetopause', is about 10 Earth radii from the surface of the Earth, while the length of the ‘magnetotail' varies greatly, being very approximately 100 Earth radii in length. Since the solar wind is supersonic, having a velocity relative to the Earth that is faster than the speed of sound within the plasma, there is a shockwave that precedes the Earth in its passage through the solar wind.

A schematic diagram depicting the magnetosphere in the near-Earth space environment. Note the southward orientation of the interplanetary magnetic field, and the reconnective process with the geomagnetic field that follows as the solar wind carries the interplanetary field past the Earth.

Magnetic Storms

Most of the time the dynamical situation in the magnetosphere is quasi-stationary, with little effect on (say) ground-based magnetometer measurements. Occasionally, however, the magnetosphere is extremely time-dependent, having an inductive enhancement of magnetospheric electric currents and their associated magnetic fields. This is manifest in ground-based magnetometer measurements as rapid, erratic variation. In analogy with atmospheric meteorology, such periods of magnetic activity are known as ‘magnetic storms', and they are triggered by the interplanetary conditions established by the Sun. The pressure balance between the solar wind and the geomagnetic field is delicate, and perturbations in solar-wind velocity can cause the magnetosphere to oscillate. Even more dramatically, occasionally the Sun emits a sudden gust of solar wind, a so-called ‘coronal mass ejection'. If this impacts upon the magnetosphere then a magnetic storm can follow. Alternatively, magnetic storms can also be caused by a process called ‘magnetic reconnection'. This is important in two places, in the magnetopause and in the magnetotail, and it is illustrated in the figure above. Let's first consider the reconnective process at the magnetopause. In the complicated flow of the solar wind, the interplanetary magnetic field can take on virtually any orientation, but sometimes it assumes a southward orientation. In this orientation the interplanetary field has the opposite orientation of the Earth's magnetic field at the magnetopause. This juxtaposition of oppositely-oriented field lines is an unstable situation, one which mathematically is identical to a shear-flow instability in fluid mechanics. With a little bit of diffusion, brought about by electrical resistance, the interplanetary magnetic field can connect onto the Earth's field. Then, particles from the Sun, which would otherwise be deflected by the Earth's field, can stream along these continuously-connected field lines and enter into the magnetospheric cavity. Further advection of the interplanetary field lines by the solar wind peels back the geomagnetic field, and the tail of the magnetosphere is further stretched as well. In the equatorial plane of the magnetotail, and with this stretching of the field lines, we have a second place where neighboring field lines have opposite orientation, see, once again, the figure above. As before, with a little diffusion we obtain reconnection, causing part of the magnetic field, and the plasma in which it is embedded, to break off from the rest of the geomagnetic field and float down the tail. It is rather like the boudinage familiar from extensional geological formations! Simultaneously, the inner field lines recoil and accelerate the plasma particles within the magnetosphere, giving rise to an enhancement of electric currents. All of this, of course, describes a highly dynamic process, which causes the magnetic field measured at the Earth's surface to become extremely active.

A stackplot of horizontal-field intensity (H) as measured by USGS observatory magnetometers during the 'great storm' of March 1989. This storm induced large quasi-direct currents in power grids, causing system-wide power blackouts in Quebec and numerous power-equipment problems in the northeastern United States. The same storm also caused radio-communication blackouts, and, because of an expansion of the ionosphere, increased the atmospheric drag on satellites, thereby altering their orbits.