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Near‐Real‐Time Auroral Electrojet Index: An International Collaboration Makes Rapid Delivery of Auroral Electrojet Index

2004; American Geophysical Union; Volume: 2; Issue: 11 Linguagem: Inglês

10.1029/2004sw000116

1542-7390

Ching I. Meng, Kazue Takahashi, Manabu Kunitake, Takashi Kikuchi, T. Kamei,

Astro and Planetary Science

The ionospheric current associated with active aurora usually maximizes in the premidnight sector. In order to accurately monitor the current intensity, we need to have a magnetometer in this crucial region at any given universal time. Ideally, we would have a few dozen magnetometers uniformly distributed along the auroral zone. Unfortunately this is not realistic, as a significant fraction of the auroral zone lies over the ocean and only a small part of the land portion is suited for routine magnetometer operation. It has been suggested that at least six magnetometers, uniformly distributed in longitude, are necessary to derive meaningful AE [Davis and Sugiura, 1966]. Needless to say, the AE index derived from the stations illustrated in Figure 1 is reliable when the ionospheric currents flow along the auroral zone. At times when the currents extend beyond or move away from the auroral zone, such as during severe geomagnetic storms, data from other stations are necessary to monitor the current system. Efforts to handle this situation exist, but they have not resulted in a global index similar to AE. Presently, the AE index is derived from 12 magnetometers. With five of the 12 AE magnetometers lying in the Arctic Circle (see Table 1 and Figure 1), where hardware maintenance is a logistic challenge, it is remarkable that the AE index has been continuously produced over the past 47 years (magnetometer sites were different in early years). WDC, Kyoto University, has been responding to the demand from scientists and the space weather communities by producing AE with three different versions in terms of quality and delivery timing: final, provisional, and quick look. The final version is based on data that are free of anomalies. In the provisional version, data are cleaned but not as rigorously as for the final version. The quick look version uses data with little cleaning and sometimes from fewer stations. Up to 11 stations contribute to the quick look version, with Dixon being the one that is presently unable to deliver the data. Until 2002 there was difficulty in delivering quick look AE in a timely manner. The main problem was missing data from magnetometers in the Russian territory; data from the remaining eight stations are available with little time delay. The missing data meant that the AE index was unreliable when the Russian sites were on the nightside. There have been attempts to obtain real-time data from Russia. For example, negotiations started in 1990 to transfer near-real-time data from Tixie via satellite link. Unfortunately, this resulted in a partial success. The link was established in March 1996 only to be terminated 6 months later as the radio license expired. The history on the early efforts has been provided by Tohru Araki at http://swdcwww.kugi.kyoto-u.ac.jp/ae2/onAEindex.html. Magnetic field measurements on the ground provide a powerful means to continuously monitor the strength of ionospheric currents associated with auroral activity. This was recognized many years ago, and data acquired at longitudinally distributed stations have been used to generate the Auroral Electrojet (AE) index as a quantitative measure of the ionospheric currents. Unfortunately, the use of the index in space weather programs has been quite limited, since data from some stations were missing or became available only after a significant time delay. Especially difficult was acquiring data from the Russian sector of the auroral zone. A recent international effort, however, changed the situation dramatically. New magnetometer systems have been installed at four Russian sites, Norilsk (geographic coordinates 69.2°N, 88.0°E), Cape Chelyuskin (77.7°N, 104.3°E), Tixie (71.6°N, 129.0°E), and Pebek (70.1°N, 170.9°E), and near-real-time data from the sites are being transmitted to Japan via satellite link. Visitors to the home page of the World Data Center (WDC) for Geomagnetism, Kyoto University (http://swdcwww.kugi.kyoto-u.ac.jp), will find that the Quick Look (QL) AE index is now truly living up to its designation. Although initially intended for scientific use, the AE index, similar to the magnetospheric ring current index Dst, is a valuable parameter in space weather application. First, AE is a measure of energy flowing from the magnetosphere into the ionosphere [Østgaard et al., 2002]. Second, as AE is given at 1-min resolution, its time derivative is a good measure of geomagnetically induced currents (GIC), which affect power transmission lines and other technological networks. Third, AE is correlated with space weather quantities such as the magnetospheric magnetic field [Peredo et al., 1993], and the index is used in constructing the models for these quantities. Finally, the same data that are used for AE can be used in numerical schemes [e.g., Richmond and Kamide, 1988] that compute ionospheric parameters in a dynamic manner. For the space weather application to work properly in an operational environment, it is essential that the magnetometer data and the AE index be delivered immediately. This is not the case for most scientific research, where one can spend months and perhaps years collecting and calibrating data. During auroral activity, currents flow on a circuit that connects the magnetosphere and the ionosphere. The ionospheric parts of the currents flow mainly on the nightside in the east-west direction. The statistically determined region of strong aurora and current is called the auroral zone, which is centered at ~67° magnetic latitude and has a width of ~10°. The derivation of AE is based on the simple fact that an east–west ionospheric current produces a north-south magnetic field perturbation on the ground. The perturbation is largest directly underneath the current, so it is necessary to place magnetometers close to the location of the current for the index to be meaningful. Davis and Sugiura [1966] defined AE to be the difference in nanotesla between the upper and lower envelopes of the northward perturbations observed at stations distributed along the auroral zone. For intense auroras the ionospheric current is of the order of 106 A, corresponding to values of AE exceeding 1000 nT. The temporal resolution of the AE index is 1 min, which is sufficient for tracking the enhancement of auroral intensity. The coupled ionosphere-magnetosphere current system is strongly controlled by the solar wind plasma and the interplanetary magnetic field. The influence of these external parameters on the radiation belt and the ionosphere is currently a central theme in solar-terrestrial physics and space weather programs. The AE index provides a quantitative measure of energy dissipated at the ionospheric end of the coupled Sun-Earth system. While spacecraft observations provide global images of aurora and in-situ measurements of plasma and fields in space, these observations are not continuous and the temporal resolution of the data depends on the type of instrumentation and the spacecraft location. The solution to the data problem required a new approach. In 1999 an initial discussion took place between scientists from the United States and Japan who were concerned about the delay in the AE index production. Shortly afterward, the discussion evolved into a team effort including Russian scientists. A project team was formed to install and operate a near-real-time magnetometer system for AE. The participants were from six organizations: the WDC, Kyoto University; the National Institute of Information and Communications Technology (NICT) of Japan; Institute for Dynamics of Geospheres (IDG), Russian Academy of Sciences; Arctic and Antarctic Research Institute (AARI), St. Petersburg; Geophysical Institute (GI), University of Alaska; and the Johns Hopkins University Applied Physics Laboratory (JHU/APL). The magnetometers and the transmitters were installed at Pebek in April 2001, at Tixie in February 2002, at Norilsk in March 2002, and at Cape Chelyuskin in August 2002. At each station the installation was followed a few months later by transmission of data to Japan by satellite link. Figure 2 shows the data stream from the four magnetometers. At each station the magnetometer system consists of a vector fluxgate magnetometer for high time resolution geomagnetic measurement, a proton magnetometer for absolute measurement of the magnitude (F), and a fluxgate theodolite for absolute measurement of the declination (D) and inclination (I), a data-collecting platform (DCP) with radio transmitter, a personal computer for data logging, and another personal computer for system control and data display. Each DCP transmits data every 12 min. The data transmitted by the DCPs are relayed by the GMS and downlinked to the Meteorological Satellite Center of the Japan Meteorological Agency (JMA) by cooperation of the Observations Department and the Meteorological Satellite Center of JMA. From there the data are sent to NICT through the Japan Weather Association. The data reach NICT in about 15 min and this is our operational definition of “near-real-time.” Figure 3 illustrates the importance of data from the Russian stations. The upper panel shows a superposed 12-hour plot of the H (northward) components of the magnetic field from the 11 stations for the quick look AE index. The components are the difference from a baseline value defined for each station. The red traces indicate data from the three Russian stations Cape Chelyuskin (CCS), Tixie Bay (TIK), and Pebek (PBK). The lower panel shows estimated AE, denoted AEest, which is the difference between the upper and lower envelopes of the field variations. In accordance with the upper panel, a red line is used for results using all 11 stations and a black line for results without the three Russian stations. The official AE index is defined in the same manner except that the baseline for H is defined differently. In this example it is clear that the two AEest values differ significantly from 1500 to 1900 UT. Without data from the three Russian stations the enhancement of the auroral current peaking at 1700 UT appears to be only about half in magnitude. Having successfully started data acquisition from the four stations, our next target is to install the same system at Dixon (73.5°N, 80.6°E) and Amderma (69.5°N, 61.4°E), which are included in Table 1 and Figure 1. Once these stations become operational, the quick look AE index will become almost as reliable as the final AE index. The following members contributed to the magnetometer project: Oleg Troshichev, Sasha Janzhura (AARI); Julius Zetzer (IDG); Toyohisa Kamei, Tohru Araki, Toshihiko Iyemori, Masahito Nosé (WDC); Takashi Kikuchi, Manabu Kunitake, Tsutomu Nagatsuma (NICT); Roger Smith (GI); Ching Meng, Kazue Takahashi (JHU/APL). Jesper W. Gjerloev provided the original for Figure 1. Data from stations outside Russia that appear in Figure 3 were provided by SGU, Sweden; USGS, USA; CGS, Canada; DMI, Denmark; and U. Iceland, through the INTERMAGNET and WDC data service. The preparation of this article was in part supported by space weather research grants to JHU/APL: NSF ATM-0318556 and NASA NAG5-13509. Ching I. Meng is a researcher at the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland. Kazue Takahashi is a researcher at the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland. Manabu Kunitake is a researcher at the National Institute of Communications and Information Technology, Tokyo, Japan. Takashi Kikuchi is a researcher at the National Institute of Communications and Information Technology, Tokyo, Japan. Toyohisa Kamei is an instructor at the World Data Center for Geomagnetism, Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, Kyoto, Japan.