Aeronomy and astrophysics A southern high-latitude geomagnetic index: AES-80 C.G. MACLENNAN, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 P. B ALLATORE, 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 eomagnetic measurements obtained at McMurdo Sta- G tion (Arrival Heights) and at two of the U.S. automatic geophysical observatories (AGO-1; AGO-4) are being com- bined 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 Hemi- sphere 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 sub- storms (Baumjohann 1986). It is calculated as the difference between the upper (AU) and the lower (AL) envelope of mag- netograms from 12 observatories located at northern geomag- netic latitudes between 60° and 70° and rather uniformly distributed over all longitudes. Because of the land mass dis- tribution in Antarctica, it is impossible to have ground obser- vatories located uniformly at geomagnetic latitudes between 60° and 70°S (see figure 1). Unlike in the Northern Hemisphere, Figure 1. Map of Antarctica, with the stations used to calculate the however, it is possible in Antarctica to have reasonable ground AES-80 index shown as filled circles. coverage at 80°S geomagnetic latitude. Data used for calculating the AES-80 parameter are the geomagnetic north-south (H) components at 1-minute resolu- days of each month and was subtracted from the data. After tion from the five antarctic stations shown as filled circles in this subtraction, the H lower (ALS-80) and the H upper (AUS- figure 1, all of which are located at corrected geomagnetic 80) envelopes and their difference were calculated (AES-80 = coordinates approximately 80°S (McMurdo, AGOs P1 and P4, AUS-80 –ALS-80) at 1-minute resolution, in a manner similar Casey, and Dumont D'Urville). [Information about the AGO to that used to produce the AE index. stations can be found in Rosenberg and Doolittle (1994).] The To investigate relationships between AES-80 and AE and time period analyzed in this article covers May and June 1994. the Northern Hemisphere Kp index, Dst, and the IMF-Bz, cor- Where only geographically oriented components (X, Y) were relation coefficients have been calculated separately for each available (Casey and Dumont D'Urville), the geomagnetic H 2-hour universal time (UT) time range. Hourly averages were component was calculated via the equation H = Xcos(θ)+ used for all the indices except Kp; the same Kp value was used Ysin(θ), where θ is the angle between the geographic and geo- for each of the 3 hours to which it corresponds. The results are magnetic direction computed via the GEOCGM program plotted in figure 2, where correlation coefficients for AU and (developed by N.E. Papitashvili and V.O. Papitashvili, and AUS-80 are shown in the top panels, for AL and ALS-80 in the described in Gustafsson, Papitashvili, and Papitashvili 1992) middle panels, and for AE and AES-80 in the bottom panels. available from the National Aeronautics and Space Adminis- The number of data points in each correlation interval for Kp tration/Goddard Space Flight Center database. For each sta- and Dst is between 114 and 122. For the correlations with the tion, the average value of H was calculated for the two quietest IMF-Bz component, the number of data points is much fewer ANTARCTIC JOURNAL – REVIEW 1997 193 (between 50 and 70) due to lim- ited satellite data availability. (One-hour average IMP-8 data from the U.S. National Space Sci- ence Data Center database are used.) The best correlation is found with the Kp index, indicat- ing 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 sta- tions contributing to AES-80, it is Figure 2. Correlations vs. time of geomagnetic indices with Kp, Dst, and IMF-Bz component; the top expected that, in general, iono- panels show correlation coefficients for AU (·) and AUS-80 (*), the middle panels show correlation coeffi- spheric currents detected by AES- cients for AL (·) and ALS-80 (*), the bottom panels for AE (·) and AES-80 (*); each point is shown at the 80 should be of lower intensity center of the 2-hour interval to which it refers. 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 maxi- mum 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 cur- rents 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, indi- cating a good relationship between the two indices. The main Figure 3. Average values for Kp (·) and Dst (*) for the data binned into difference is in the AUS-80 parameter in the time range about the AES/AE ratio ranges indicated on the abscissa. (nT denotes nan- 6–22 UT, when AUS-80 shows features that differ from AU with oteslas.) respect to their correlations with Kp; these may be related to seasonal dependence or to the distribution of the contributing tively, and T. Kamei from World Data Center-C2 in Kyoto for stations and require further study. providing the official auroral indices data. M.J. Engebretson Further, more detailed discussions of the relationships of acknowledges support from National Science Foundation the AES-80 index to global geomagnetic activity are contained grant OPP 95-29177 to the University of Maryland and by sub- in two papers that have been submitted for publication. contract to Augsburg College. The U.S. investigators acknowl- The authors thank J. Bitterly of the Ecole et Observatoire edge the support of the Office of Polar Programs, National de Physique du Globe, Strasbourg, France, and G. Burns of the Science Foundation, for upper atmosphere physics programs Australian Antarctic Division, Hobart, Australia, for the mag- at McMurdo and the AGOs. This research was supported in netometer data from Dumont D'Urville and Casey, respec- part by the Italian Antarctic Research Program (PNRA). ANTARCTIC JOURNAL – REVIEW 1997 194 References Baumjohann, W. 1986. Merits and limitations of the use of geomag- Gustafsson, G., N.E. Papitashvili, and V.O. Papitashvili. 1992. A revised netic indices in solar-wind magnetosphere coupling studies. In Y. corrected geomagnetic coordinate system for epochs 1985 and Kamide and J.A. Slavin (Eds.), Solar-wind magnetosphere coupling. 1990. Journal of Atmospheric and Terrestrial Physics, 54, 1609–1631. Tokyo: Terra Scientific. Rosenberg, T.J., and J.H. Doolittle. 1994. Studying the polar ionosphere Davis, T.N., and M. Sugiura. 1966. Auroral electrojet activity index AE and magnetosphere with automatic geophysical observatories: and its universal time variations. Journal of Geophysical Research, The U.S. program in Antarctica. Antarctic Journal of the U.S., 29(5), 71, 785–801. 347–349. Long-period hydromagnetic waves at very high geomagnetic latitudes L.J. LANZEROTTI, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 A. SHONO and H. FUKUNISHI, Department of Astrophysics and Geophysics, Tohoku University, Sendai, Japan C.G. MACLENNAN, and L.V. MEDFORD, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 agnetospheric phenomena occurring at the highest M geomagnetic latitudes (above approximately 70°) are produced by plasma physics phenomena on the dayside mag- netopause and along the boundary of the geomagnetic tail (e.g., Troitskaya and Bol’shakova 1977; McHarg and Olson 1992). Unfortunately, measurements at very high geomagnetic latitudes, whether made in space or on the ground, present difficult challenges—for many different reasons. The plasma phenomena are most often highly localized in space and quite highly time variable on scales ranging from electron and ion gyrofrequencies to bulk plasma flow velocities. The Polar Experiment Network for Geophysical Upper-atmosphere Investigations (PENGUIN) program, using automatic geo- physical observatories (AGOs) that are designed for the harsh antarctic environment, was devised specifically to place geo- physical investigations at very high latitudes (the 1994 review issue of Antarctic Journal contains initial reports from the AGO program).