Magnetic activity indices

spaceweb@oulu.fi - last update: 13 September 1998, 1130 UT (RR)

Ground based magnetometer measurements are used to derive several indices that describe magnetic activity in the Earth 's environment.

aa, AE, Ak(Hel), Kp, Dst, PC, MT. See also ABI

aa index

See Mayaud (1973).

AE index

AE index is an auroral electrojet index obtained from a number (usually greater than 10) of stations distributed in local time in the latitude region that is typical of the northern hemisphere auroral zone (Davis and Sugiura, 1966). For each of the stations the north-south magnetic perturbation H is recorded as a function of universal time. A superposition of these data from all the stations enables a lower bound or maximum negative excursion of the H component to be determined; this is called the AL index. Similarly, an upper bound or maximum positive excursion in H is determined; this is called the AU index. The difference between these two indices, AU-AL, is called the AE index. Notice that negative H perturbations occur when stations are under an westward-flowing current. Thus the indices AU and AL give some measure of the individual strengths of eastward and westward electrojets, while AE provides a measure of the overall horizontal current strenth. Excursions in the AE index from a nominal daily baseline are called magnetospheric substorms and may have durations of tens of minutes to several hours.

You can get AE indices from the NGDC server.

Ak(Hel) index

The Ak(Hel) index measuring the geomagnetic activity was created by Nevanlinna and Ketola (1993). It has been adjusted with the aa index to form the longest uniform index of global geomagnetic activity, extending over the last 14 solar cycles (Nevanlinna and Kataja, 1993).

Kp index

The Kp index (Bartels et al., 1939) is obtained from a number of magnetometer stations at mid-latitudes. When the stations are not greatly influenced by the auroral electrojet currents, conditions are termed magnetically quiet. If the auroral zone expands equatorward, however, these stations can record the effects of the auroral electrojet current system and of the magnetospheric ring current and field-aligned currents that can connect it to the ionosphere. This occurs during so-called magnetically disturbed periods. The mid- latitude stations are rarely directly under an intense horizontal current system and thus magnetic perturbations can be dominant in either the H or D component. The Kp index utilizes both these perturbations by taking the logarithm of the largest excursion in H or D over a 3-h period and placing it on a scale from 0 to 9.

You can get Kp indices from the NGDC server in ap form. See also the Potsdam server, where Bartels diagrams are available.

Dst index

The hourly Dst index (Sugiura, 1964) is obtained from magnetometer stations near the equator but not so close that the E-region equatorial electrojet dominates the magnetic perturbations seen on the ground. At such latitudes the H (northward) component of the magnetic perturbation is dominated by the intensity of the magnetospheric ring current. Dst index is a direct measure of the hourly average of this perturbation. Large negative perturbations are indicative of an increase in the intensity of the ring current and typically appear on time scales of about an hour. The decrease in intensity may take much longer, on the order of several hours. The entire period is called a magnetic storm. During a storm it is usual to observe several isolated or one prolonged substorm signature in the AE index. Occasionally a specific high time resolution (5 min or so) version of the index has been calculated to study the relationship between storms and substorm.

Campbell (1996a,b) has recently reanalysed the Dst signatures and their connection to the ring current, and it appears that Dst is not, after all, a pure ring current index. Siscoe and Crooker (1996) have investigated the diurnal oscillation of Dst index.

You can get Dst indices from the WDC-2 Kyoto Dst index service, or from the NGDC server.

PC index

The Polar Cap index PC (Troshichev et al, 1989) measures geomagnetic disturbances at the polar cap which are due to ionospheric and field-aligned currents. It is calculated separately for both hemispheres, from only one station in each (Thule and Vostok). The PC index was designed to measure the part of the ionospheric current system that is due to magnetospheric field line convection. As this is assumed to correlate with the solar wind input, the index measures the energy inflow from the solar wind into the Earth's magnetosphere. For example, it has been shown that the index gives the same information - but much easily - as the low altitude polar satellites measuring the diameter of the polar cap!

It has been shown to agree fairly well with AE-index in the wintertime (Vassiliadis et al., 1996), although it cannot see substorms the same way. This is because the field-aligned currents are more dominant at winter time.

PC indices are available from 1975 onward. You can get them from the WDC C1, Copenhagen, or from the NGDC server.

MT index

An index for chararacterize the magnetotail configuration has been proposed by Gvozdevsky and Sergeev (1996).

ABI index

The Auroral Boundary Index, ABI, is developed by Gussenhoven et al. (1983), as the latitude of the equatorward edge of the diffuse aurora at midnight. It is routinely determined with about 30 min resolution from DMSP precipitating electron data (Madden and Gussenhoven, 1990). This boundary maps to the inner edge of the plasma sheet and is well ordered by Kp (moves to lower latitudes with increasing magnetic acitivity).

References

Bartels, J., N. H. Heck, and H. F. Johnston, The three-hour-range index measuring geomagnetic activity, J. Geophys. Res., 44, 411-, 1939.
Campbell, W. H., Geomagnetic storms, the Dst ring-current myth and lognormal distribution, J. Atmosph. Terr. Phys., 58, 1171-1187, 1996a.
Campbell, W. H., Dst is not a pure ring current index, EOS, 77, 283, 1996b.
Davis, T. N., and M. Sugiura, Auroral electrojet activity index AE and its universal time variations, J. Geophys. Res., 71, 785-, 1966.
Gussenhoven, M. S., D. A. Hardy, M. Heinemann, Systematics of the equatorward diffuse auroral boundary, J. Geophys. Res., 88, 5692-, 1983.
Gvozdevsky, B. B. and V. A. Sergeev, MT-index - A possible new index to characterize the configuration of the magnetotail, Adv. Space Res., 18, (8)51-(8)54, 1996.
Madden, D., and M. S. Gussenhoven, Auroral boundary index from 1983 to 1990, US Air Force Geophysical Laboratory, GL-TR-90-0358, 1990.
Mayaud, P. N., A hundred year series of geomagnetic data, 1868-1967, indices aa, storm sudden commencements, in IAGA Bull., 33, Int. Union of Geod. and Geophys., Paris, 1973.
Nevanlinna, H. and E. Kataja, An extension of geomagnetic activity series aa for two solar cycles (1844-1868), Geophys. Res. Lett., 20, 2703-2806, 1993.
Nevanlinna, H. and A. Ketola, Magnetic results from Helsinki magnetic-meteorological observatory, Part III, Declination 1854-1880, Geomagnetic Activity 1844-1880, Geophysical Publications, 33, Finnish Meteorological Institute, 1993.
Sugiura, M., Hourly values of equatorial Dst for IGY, pp. 945-948, in Annals of the International Geophysical Year, vol. 35, Pergamon Press, Oxford, 1964.
Siscoe, G., and N. Crooker, Diurnal oscillation of Dst: A manifestation of the Russell-McPherron effect, J. Geophys. Res., 101, 24985-24989, 1996.
Troshichev et al., Planet. Space Sci., 36, 1095-1102, 1988.
Vassiliadis, D., V. Angelopoulos, D. N. Baker, and A. J. Klimas, The relation between the northern polar cap and auroral electrojet geomagnetic indices in the wintertime, Geophys. Res. Lett., 23, 2781-2784, 1996.