**Updated: **
20 August 2014

# Geomagnetic Data from Australia

## Geomagnetic Indices

Geomagnetic indices have been developed to characterise the variability of the Earth's magnetic field, in all its complexity, in a single number. There are many such indices that each characterise a different aspect or time-frame of the dynamic geomagnetic field. The most widely used of these is the *K* index, a quasi-logarithmic index of geomagnetic activity relative to an estimated undisturbed or regular quiet day variation for the recording site. Links to sites with other magnetic indices and information can also be accessed from this page.

*k* index

The *k* index is a quasilogarithmic index of geomagnetic activity relative to an assumed quiet day curve for the recording site. *k* is a code from 0-9 that characterizes magnetic activity (0 being the least active field and 9 the most active field) over a 3 hour period.

*k* indices from Canberra, Gnangara, Gingin and Mawson observatories are available online.

An estimated *k* index for Learmonth is available from IPS.

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*aa* index

The *aa* index is three hourly index of geomagnetic activity determined from the *k* indices scaled at two antipodal subauroral stations: Canberra Australia, and Hartland England.

For each three hour interval, *k* indices measured at the two stations are converted back into amplitude. A three hour *aa* index is the mean of the northern and southern values, weighted to account for the small differences in the latitudes of the two stations.

The index is calculated at the International Service for Geomagnetic Indices (ISGI), France.

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### Other Indices

The International Services of Geomagnetic Indices (ISGI) is in charge of elaboration and dissemination of magnetic indices. Any magnetic indices not found on this website can be accessed through the ISGI site.

*kpa* index
- Is the coded value corresponding to
*aa* index, given in the same scale as the *kp* index.

*am* index
- Is a three hourly index derived using data from a network of 20 subauroral stations evenly distributed in longitude in the northern and southern hemispheres. For each three hourly interval,
*k* indices for each station are converted back into amplitude. The mean of these values, weighted to account for differences in position, are calculated separately for the northern and southern hemispheres giving *an* and *as*, respectively. A three hour *am* index is the mean of *an* and *as*.

*kpm* index
- Is the coded value corresponding to the
*am* index, given in the same scale as the *kp* index.

*ae* index
- Is and auroral electrojet index. It is the difference between
*au* and *al* indices. Large deviances from a nominal daily baseline in the *ae* index are called magnetospheric substorms. This index provides an overall measure of the horizontal current strength in the northern auroral zone. The base values are average Sq estimates based on the 5 international quietest days (IQD) of the month.

*au* and *al* indices
- The horizontal component of the magnetic field at 10 (or greater) stations distributed longitudinally in the northern hemisphere auroral zone, is superposed on a magnetogram. The lower bound, or maximum negative excursion is called the
*al* index. Similarly the upper bound or maximum positive excursion is the *au* index. *au-al* = *ae*. When a westward current flows the horizontal component of the geomagnetic field decreases (negative perturbation). Thus, the *al* index gives a measure of the westward electrojects. Likewise, when an eastward current flows the horizontal component of the geomagnetic field increases (positive perturbation). Thus, the *al* index gives a measure of the eastward electrojects.

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### IQD/IDD

International Quiet Days (IQD's) are the days where the geomagnetic variations are a minimum in each month.

International Disturbed Days (IDD's) are 5 days in each month where the geomagnetic variations are maximum.

The classification of days is relative only to the month of calculation. The average disturbance level in one month may differ from another.

IQDs and IDDs are derived at GFZ-Potsdam.

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