Over the past decade a new approach to geomagnetic field modelling has been developed jointly between the magnetics groups at Goddard Space Flight Center (NASA/GSFC) and the Danish Space Research Institute (DSRI). This approach, known as "comprehensive modelling" was developed to overcome the problem of separation spatial and temporal variations from observed magnetic field at ground and satellite levels.

Earth's magnetic field is composed of contributions from many sources. By far the most dominant of these fields is of core origin, accounting for over 97 per cent of the field observed at the Earth's surface and ranging in intensity from about 30000 nT at the equator to about 60000 nT at the poles. According to geodynamo theory, inductive interactions between the fluid motion of the liquid outer core and the geomagnetic field not only modify the source current so as to induce secular variation (SV) of the field, but sustain it against long-term decay caused by magnetic diffusion and Ohmic dissipation of the current source. The timescale of this field is of the order of centuries with typical SV magnitudes at the core mantle boundary (CMB) of a few thousand nT/yr.

The lithosphere is a rheological classification for that outer layer of the Earth, which is rigid and the crust is the petrologically distinct upper portion of this. The lithosphere contains regions for which the temperature is below the Curie point of magnetite and other magnetic minerals. As a result, it can have magnetization that is either induced by the present-day ambient field or frozen into the rocks at their last time of cooling below the Curie temperature, i.e. remanent magnetization. Fields from the lithosphere are of amplitude up to several thousand nT at the surface and at aircraft altitude, and up to about 20 nT at satellite altitudes (300-800 km).The length-scales of lithospheric fields range from extremely broad, e.g. continent ocean magnetization contrast, to very local,although only features larger than a few hundred kilometres may be resolved at satellite altitude.

The diagram below shows the internal(core&lithospheric) magnetic field sources. Click on the appropriate source (grey box) to see the current CM4 product.

The solar-quiet (Sq) magnetic field variation is a manifestation of an ionospheric current system. At middle and low altitudes heating by solar radiation on the dayside and cooling on the nightside of the atmosphere generates tidal winds that move ionospheric plasma through the geomagnetic field, inducing electric fields and currents in the dynamo region between 100-140 km in height. The current system remains relatively fixed to the Earth Sun line and produces regular, broad-scale daily variations that are seen directly in the magnetograms of geomagnetic quiet days, hence the name Sq. On disturbed days there is an additional variation, which includes superimposed magnetic storm signatures. Because the geomagnetic field is strictly horizontal at the dip equator, there is an enhancement of the effective conductivity, which results in an enhanced eastward current, called the equatorial electrojet (EEJ), flowing along a few hundred kilometers wide band centred on the dayside dip equator. In addition, auroral electrojets (AEJ) flow in the auroral belt and vary in amplitude with different levels of magnetic activity. At the Earth' s surface, the Sq fields are of the order of 10-50 nT, depending upon component, latitude, season, solar activity and time of day; the magnetic signature of the EEJ can be about 5-10 times that of Sq; and that of the AEJ can vary widely from a few 10s nT during quiet periods to several thousand nT during major magnetic storms.

The field originating in the Earth s magnetosphere is a result primarily of the ring-current and the currents on the magnetopause and in the magnetotail. Currents flowing on the outer boundary of the magnetospheric cavity, known as the magnetopause, cancel the Earth s field outside and distend the field within the cavity. This produces an elongated tail in the antisolar direction within which sheet currents are established in the equatorial plane. The interaction of these currents with the radiation belts near the Earth produces a ring-current in the dipole equatorial plane, which partially encircles the Earth, but achieves closure via field-aligned currents (FACs) into and out of the ionosphere. These resulting broad-scale fields have magnitudes of the order of 20-30 nT near the Earth during magnetically quiet periods, but can increase to several hundred nT during disturbed times. If displacement currents are neglected, then the current densities associated with these external fields are solenoidal and therefore must flow along closed circuits. Given the complex nature of the conductivity structure in the near-Earth region, circuit closure is sometimes achieved through currents that couple the various source regions.

At high latitudes, the auroral ionosphere and magnetosphere are coupled by currents that flow along the Earth's magnetic field lines. The fields from these FACs have magnitudes that vary with the magnetic disturbance level. However, they are always present (of the order of 30-100 nT during quiet periods and up to several thousand nT during substorms). Fields from these currents have been detected in surface data in the Y (east) component of the magnetic field at low latitudes, with difficulty, but are mostly mapped using magnetometers aboard near-Earth-orbiting satellites. There are also currents that couple the Sq currents systems in the two hemispheres that flow, at least in part, along magnetic field lines. Detection of these has been reported by Olsen (1997a) using data from the Magsat satellite. The associated magnetic fields are generally 10 nT or less. Finally, there exists a meridional current system that is connected to the EEJ with upward-directed currents at the dip equator and field-aligned downward-directed currents at lowlatitudes (within 15 degrees of the dip equator). Fields from this current system have been detected by magnetic measurements taken on rocket and from those taken by Magsat. In the latter case, the EEJ coupling currents resulted in fields of about 15-40 nT in the Magsat data at dusk local time.

The diagram below shows the external(ionospheric, magnetospheric & toroidal) magnetic field sources. Click on the appropriate source (grey box) to see the current CM4 product.

Animation Sources of the Earth's Magnetic Field (AVI file 15 MB)