C.G. MACLENNAN, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
P. BALLATORE, Istito de Fisica della Spazio Interplanetario, Consiglio Nazionale della Ricerche, c.p. 27, 00044 Frascati, Rome, Italy
M.J. ENGEBRETSON, Department of Physics, Augsburg College, Minneapolis, Minnesota 55454
L.J. LANZEROTTI, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
Geomagnetic measurements obtained at McMurdo Station (Arrival Heights) and at two of the U.S. automatic geophysical observatories (AGO-1; AGO-4) are being combined with measurements made at Casey and Dumont D'Urville to construct a Southern Hemisphere geomagnetic index for the geomagnetic latitude 80°S. The calculation of the index is modeled on the calculation of the Northern Hemisphere auroral electrojet index AE and is thus called the AES-80 index.
The AE index was developed to monitor geomagnetic activity at auroral zone latitudes in the Northern Hemisphere (Davis and Sugiura 1966) and indicates the level of auroral electrojet currents and in particular the occurrence of substorms (Baumjohann 1986). It is calculated as the difference between the upper (AU) and the lower (AL) envelope of magnetograms from 12 observatories located at northern geomagnetic latitudes between 60° and 70° and rather uniformly distributed over all longitudes. Because of the land mass distribution in Antarctica, it is impossible to have ground observatories located uniformly at geomagnetic latitudes between 60° and 70°S ( see figure 1). Unlike in the Northern Hemisphere, however, it is possible in Antarctica to have reasonable ground coverage at 80°S geomagnetic latitude.
Data used for calculating the AES-80 parameter are the geomagnetic north-south (H) components at 1-minute resolution from the five antarctic stations shown as filled circles in figure 1, all of which are located at corrected geomagnetic coordinates approximately 80°S (McMurdo, AGOs P1 and P4, Casey, and Dumont D'Urville). [Information about the AGO stations can be found in Rosenberg and Doolittle (1994).] The time period analyzed in this article covers May and June 1994. Where only geographically oriented components (X, Y) were available (Casey and Dumont D'Urville), the geomagnetic H component was calculated via the equation H = Xcos(q)+ Ysin(q), where q is the angle between the geographic and geomagnetic direction computed via the GEOCGM program (developed by N.E. Papitashvili and V.O. Papitashvili, and described in Gustafsson, Papitashvili, and Papitashvili 1992) available from the National Aeronautics and Space Administration/Goddard Space Flight Center database. For each station, the average value of H was calculated for the two quietest days of each month and was subtracted from the data. After this subtraction, the H lower (ALS-80) and the H upper (AUS-80) envelopes and their difference were calculated (AES-80 = AUS-80 -ALS-80) at 1-minute resolution, in a manner similar to that used to produce the AE index.
To investigate relationships between AES-80 and AE and the Northern Hemisphere Kp index, Dst, and the IMF-Bz, correlation coefficients have been calculated separately for each 2-hour universal time (UT) time range. Hourly averages were used for all the indices except Kp; the same Kp value was used for each of the 3 hours to which it corresponds. The results are plotted in figure 2, where correlation coefficients for AU and AUS-80 are shown in the top panels, for AL and ALS-80 in the middle panels, and for AE and AES-80 in the bottom panels. The number of data points in each correlation interval for Kp and Dst is between 114 and 122. For the correlations with the IMF-Bz component, the number of data points is much fewer (between 50 and 70) due to limited satellite data availability. (One-hour average IMP-8 data from the U.S. National Space Science Data Center database are used.) The best correlation is found with the Kp index, indicating that both AE and AES-80 are in good agreement with activity on a planetary level. In addition, there is good agreement between AE and AES-80 independent of time (bottom panels). For the AL and AU components, this agreement is less satisfactory; in particular, the correlation of Kp with AUS-80 (top left panel) between about 6 and 22 UT is low.
Because the average position of the auroral electrojet is at a lower latitude than that of stations contributing to AES-80, it is expected that, in general, ionospheric currents detected by AES-80 should be of lower intensity than those monitored by AE. To determine whether there are times when the southern index measures a perturbation higher than classical AE, we calculated the hourly ratios between AES-80 and AE to find the percentage of the maximum currents detected by the AE index that are also measured by the southern index. These ratios have been binned into a number of ranges, and the average values of Kp and Dst have been computed separately for the data in each interval. Figure 3 plots the Kp and Dst values vs. the ratio bins for the AES-80/AE ratio. One can see that the quietest periods correspond to data with AES-80/AES greater than 1, implying that when the auroral oval is sufficiently contracted poleward, generally under conditions of Bz positive, AES-80 measures auroral currents better than AE.
The correlations of AES-80 with Kp, Dst, and Bz in figure 2 indicate that AES-80 can be considered as a global average geomagnetic activity level indicator. The correlations are very similar to those obtained for the AE index at all UT times, indicating a good relationship between the two indices. The main difference is in the AUS-80 parameter in the time range about 6-22 UT, when AUS-80 shows features that differ from AU with respect to their correlations with Kp; these may be related to seasonal dependence or to the distribution of the contributing stations and require further study.
Further, more detailed discussions of the relationships of the AES-80 index to global geomagnetic activity are contained in two papers that have been submitted for publication.
The authors thank J. Bitterly of the Ecole et Observatoire de Physique du Globe, Strasbourg, France, and G. Burns of the Australian Antarctic Division, Hobart, Australia, for the magnetometer data from Dumont D'Urville and Casey, respectively, and T. Kamei from World Data Center-C2 in Kyoto for providing the official auroral indices data. M.J. Engebretson acknowledges support from National Science Foundation grant OPP 95-29177 to the University of Maryland and by subcontract to Augsburg College. The U.S. investigators acknowledge the support of the Office of Polar Programs, National Science Foundation, for upper atmosphere physics programs at McMurdo and the AGOs. This research was supported in part by the Italian Antarctic Research Program (PNRA).
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Gustafsson, G., N.E. Papitashvili, and V.O. Papitashvili. 1992. A revised corrected geomagnetic coordinate system for epochs 1985 and 1990. Journal of Atmospheric and Terrestrial Physics , 54, 1609-1631.
Rosenberg, T.J., and J.H. Doolittle. 1994. Studying the polar ionosphere and magnetosphere with automatic geophysical observatories: The U.S. program in Antarctica. Antarctic Journal of the U.S., 29(5), 347-349.