This module allows you to determine the magnetic field at the surface of the Earth due to an infinite line current (or multiple line currents) in the ionosphere above. Such a narrow “line” in the ionosphere is known as an electrojet. The surface of the Earth is taken to be flat, and the Earth itself to be insulating, but an infinitely conducting layer can be placed at any depth from 0 to 200 km. The currents in this sheet may be turned on and off with a check box.
The location of the observer, the orientation of the current flow, and the components of the magnetic field are all specified in geographic coordinates in which X is north, Y is east, and Z is down. This is the traditional coordinate system used in geomagnetism. An alternative that is sometimes used is the HDZ coordinate system, where Z is the same as in the XYZ system, H is along the projection of the average magnetic field on the surface of the Earth, and D is perpendicular to H and Z, and is roughly eastward. Originally, the D coordinate was an angle measuring the direction of the horizontal projection of the field away from the average direction.
The current flows in the horizontal plane above the observer at an altitude of 50 to 150 km. The latitude of the center of the current is adjustable from 0° to 90°. Since geomagnetic phenomena are usually aligned in some direction other than strictly east-west geographic, the orientation of the current in the horizontal plane (the azimuth) can be rotated. For a 0° azimuth the current flows from west to east geographic. For a 180° azimuth the current flows from east to west. This is the direction of the usual westward electrojet due to substorms. A 90° azimuth current flows due south and a -90° azimuth current flows due north.
The width of the current can be altered keeping the current centered at its specified location. The program places a wire every 0.1 over the interval specified and splits the total current evenly among these wires. The strength of the current can be specified in steps of 0.1 MA from 0 to 10 MA.
The default mode of the program is to have no currents induced in the Earth. However, an infinitely conducting layer may be switched on. This layer shields the interior of the Earth from the magnetic field produced by the ionospheric current. This shielding is equivalent to the introduction of a mirror current flowing parallel to the ionospheric current at a depth below the conducting layer equal to the sum of the depth of the conducting layer and the height of the ionospheric current. If the ionospheric current is narrow and the conducting sheet is at the surface of the Earth, the X component of the field at the surface of the Earth is doubled and the Z component goes to zero everywhere for an east-west current. If the sheet is at a depth of 200 km so that the mirror current is at 400 km, there is a small enhancement of the X and Z components over the non-conducting sheet case. For a broad sheet of current flowing east-west the conducting sheet will double the north-south field if it is at the surface and increase it 50% if at 200 km depth.
The north-south extent of the plot can be changed by specifying the minimum and maximum geographic latitudes. To measure the three components of the field at any latitude, move the pointer to that latitude and click the mouse. The measurement will appear in the boxes to the left. Click Calculate to redraw the graphs after changing any of the parameters. The vertical scale of the three graphs is kept the same so that the variations of the three components will be displayed in their proper relative proportions.