Abstract
Abstract. The Auroral Electrojet (AE) indices, which are composed of four indices (AU, AL, AE, and AO), are calculated from the geomagnetic field data obtained at 12 geomagnetic observatories that are located in geomagnetic latitude (GMLAT) of 61.7°–70°. The indices have been widely used to study magnetic activity in the auroral zone. In the present study, we examine magnetic local time (MLT) dependence of geomagnetic field variations contributing to the AU and AL indices. We use 1-min geomagnetic field data obtained in 2003. It is found that both AU and AL indices have two ranges of MLT (AU: 15:00–22:00 MLT, ~06:00 MLT; and AL: ~02:00 MLT, 09:00–12:00 MLT) contributing to the index during quiet periods and one MLT range (AU: 15:00–20:00 MLT, and AL: 00:00–06:00 MLT) during disturbed periods. These results are interpreted in terms of various ionospheric current systems, such as, Sqp, Sq, and DP2.
Highlights
The Auroral Electrojet (AE) indices were originally introduced by Davis and Sugiura (1966) as a measure of global electrojet activity in the auroral zone
Tomita production of 2.5 min values was made at the Goddard Space Flight Center for the period from September 1964 to June 1968. After these early publications the index was regularly issued by the World Data Center A for Solar-Terrestrial Physics (WDC-A for STP) in Boulder, Colorado, which published 2.5 min values for the years 1966 to 1974 and 1.0 min values for 1975 and the first 4 months of 1976. When it became difficult for the WDC-A for STP to continue the production of the AE index, the WDC-C2 for Geomagnetism, which is operated by the Data Analysis Center for Geomagnetism and Space Magnetism, Faculty of Science, Kyoto University, took over the job and started to produce the AE index from the International Magnetospheric Study period (1978–1979)
During the selected quiet periods, the 1-min H values are compared among the 12 AE observatories at a given UT time, and we identify which 2 stations contributed to the AU and AL indices, respectively
Summary
The Auroral Electrojet (AE) indices were originally introduced by Davis and Sugiura (1966) as a measure of global electrojet activity in the auroral zone. Stations in the sunlit hemispheres contribute to low-amplitude AU and AL indices and their peak contribution is from 06:15 MLT for AU and 11:15 MLT for AL In these previous studies, the number of the AE observatories was less than 12 (i.e., 7 and 11, respectively) and the time resolution is larger than 1 min (i.e., 2.5-min), both of which are not relevant to the present AE index derivation. After the paper by Allen and Kroehl (1975) was published, more than 35 years have passed; and the number of reseachers using the AE index has been increasing because the real time AE index has become available from http://wdc.kugi.kyoto-u.ac.jp/ae realtime/index.html.
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