The Effect of Magnetic Substorms on Near-Ground Atmospheric Current E
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The effect of magnetic substorms on near-ground atmospheric current E. Belova, S. Kirkwood, H. Tammet To cite this version: E. Belova, S. Kirkwood, H. Tammet. The effect of magnetic substorms on near-ground atmospheric current. Annales Geophysicae, European Geosciences Union, 2000, 18 (12), pp.1623-1629. hal- 00316832 HAL Id: hal-00316832 https://hal.archives-ouvertes.fr/hal-00316832 Submitted on 1 Jan 2000 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Ann. Geophysicae 18, 1623±162942001) Ó EGS ± Springer-Verlag 2001 The eect of magnetic substorms on near-ground atmospheric current E. Belova1, S. Kirkwood1, H. Tammet2 1 MRI Atmospheric Research Programme, Swedish Institute of Space Physics, Box 812, Kiruna 98128, Sweden 2 Institute of Environmental Physics, University of Tartu, 18 UÈ likooli Street, Tartu, 50090, Estonia Received: 9March 2000 / Revised: 13 September 2000 / Accepted: 5 October 2000 Abstract. Ionosphere-magnetosphere disturbances at high latitudes, e.g. magnetic substorms, are accompa- 1 Introduction nied by energetic particle precipitation and strong variations of the ionospheric electric ®elds and currents. The global electric circuit, in which atmospheric currents These might reasonably be expected to modify the local ¯ow from the ground to the ionosphere in low-latitude atmospheric electric circuit. We have analysed air±earth regions, and then spread all over the globe and return to vertical currents 4AECs) measured by a long wire the ground through the fair weather regions, has been antenna at Esrange, northern Sweden during 35 geo- widely studied over the last 30 years. A review on the magnetic substorms. Using superposed epoch analysis topic was recently published by Bering et al. 41998). The we compare the air-earth current variations during the three main generators, or energy sources, of this circuit 3 h before and after the time of the magnetic X- are believed to be thunderstorms, the ionospheric component minimum with those for corresponding local dynamo 4tides) and the solar wind/magnetosphere times on 35 days without substorms. After elimination dynamo 4Roble and Tzur, 1986). Thunderstorms, which of the average daily variation we can conclude that the occur mostly at equatorial and subequatorial latitudes eect of substorms on AEC is small but distinguishable. maintain the potential dierence of 150±600 kV 4Muhl- It is speculated that the AEC increases observed during eisen, 1977) between the ground and the ionosphere. about 2 h prior to the geomagnetic X-component Tides in the ionosphere lead to the appearance of a minimum, are due to enhancement of the ionospheric horizontal potential dierence of 5±15 kV between high electric ®eld. During the subsequent 2 h of the substorm and low latitudes at ionospheric heights 4Richmond, recovery phase, the dierence between ``substorm'' and 1986). Solar wind-magnetosphere coupling results in an ``quiet'' atmospheric currents decreases. The amplitude additional ionospheric potential drop of 40±100 kV of this ``substorm'' variation of AEC is estimated to be across the polar caps. less than 50% of the amplitude of the diurnal variation After the discovery of the latter two generators, the in AEC during the same time interval. The statistical question arose of how and to what extent the atmo- signi®cance of this result was con®rmed using the Van spheric currents near the ground are in¯uenced by the der Waerden X-test. This method was further used to ionospheric horizontal electric ®eld. There are many show that the average air-earth current and its ¯uctu- papers devoted to the theoretical problem of mapping ations increase during late expansion and early recovery the ionospheric electric ®eld to the ground and solving it phases of substorms. for dierent cases and approaches 4e.g., BostroÈ m and Fahleson, 1974; Dejnakarintra et al., 1985; Roble and Hays, 1979). In general, downward mapping of a Key words: Ionosphere 4electric ®elds and currents) ± horizontal electric ®eld leads to a decrease of the Magnetospheric physics 4storms and substorms) ± horizontal component with decreasing height and its Meteorology and atmospheric dynamics 4atmospheric conversion to vertical electric ®eld near ground. Calcu- electricity) lations by Park 41976) and Roble and Hays 41979) showed that the magnetospheric generator could pro- duce perturbations of 20% in the air-earth current 4AEC) at high latitudes. Recently Tinsley et al. 41998) Correspondence to: E. Belova have reported a signature of large-scale ionospheric e-mail: [email protected] electric potential in the vertical electric ®eld data from 1624 E. Belova et al.: The eect of magnetic substorms on near-ground the South Pole reaching at times 30% of the usual low- latitude electric ®eld. The geomagnetic substorm is a common phenome- non at high latitudes, occurring near local magnetic midnight. It can be detected as a variation of the horizontal component of the geomagnetic ®eld recorded on the ground. The signature is in the form of a magnetic bay generally lasting several hours 4Rostoker et al., 1980). The distribution of the ionospheric electric ®eld and conductivity are known to change in charac- teristic ways during substorms, so substorms could provide an additional ionospheric source of large enough horizontal scale to contribute to the local atmospheric electric current. In a series of papers, Tinsley and Heelis 41993) and Tinsley et al. 41994) have suggested that the global electric circuit may provide a link between solar variability and climate changes. They considered, as Fig. 1. Daily variations of air-earth current measured at Esrange for one example, the eect of solar wind magnetic sector two days: August 15, and November 18, 1998 crossings on tropospheric vertical electric ®eld. Magnet- ic substorms may also be associated with changes in the interplanetary magnetic ®eld, i.e. with a change to time variations of the signal dier greatly and change southward direction 4Rostoker et al., 1980). Hence, an from day to day and from hour to hour. in¯uence of geomagnetic substorms on air-earth current We measure the local air-earth current, which is could provide one more channel for solar-atmospheric determined by the potential dierence between the interaction. ground and the ionosphere as well as the pro®le of Ruhnke 41969) and Ruhnke et al. 41983) have atmospheric conductivity. The conductivity increases discussed the advantages of using a long wire horizontal approximately exponentially with altitude, which means antenna to collect AEC to detect the global variations of that the atmospheric resistance resides mainly at trop- atmospheric electricity. Our goal is to study the eect of ospheric heights and near the ground. As a consequence the ionospheric substorm generator on the AEC using the AEC is very sensitive to variations in local wind, measurements carried out at ground level in northern cloudiness and other local meteorological conditions. Sweden with this type of antenna. Other reasons for local variations can include cell- structured convection, mechanical vibration of the antenna and other instrumental and recording factors 2 Experimental setup 4Tammet, 1991). As a result there is great variability in current amplitude. Observations of the fair weather current were made The redistribution of the ionospheric electric ®eld using a long wire antenna 4100 m long) located near the during a magnetic substorm generally occurs in a region top of a hill at Esrange 468°N, 21°E), Sweden. The near magnetic midnight but extending over roughly 60° diameter of the antenna wire is 2 mm and the antenna is in longitude, and between 57°±70° of latitude 4Weimer, placed at about 2 m height parallel to the ground 1999), i.e. in a small part of the auroral oval. We expect surface. It collects the atmospheric vertical currents that the eect of a substorm should be much less than ¯owing above it from an eective area of about 200 m2 that due to the solar/magnetosphere generator 4around 4Ruhnke, 1969; Tammet et al., 1996). The current is fed 25% from the undisturbed level for low latitudes) and, to an ampli®er and recorded via an analogue-to-digital of course, much less than that due to the thunderstorm converter by a computer. During 1998 and early 1999 generator. From Fig. 1 it can be concluded that we have only 1-min averaged data were recorded. Measurements to separate such small variations from the strong have been made since August 1998. The measurement of disturbances caused by many other reasons before we total current collected by the antenna can be used to can identify the eects of substorms. The method we estimate the atmospheric current density by dividing the use, appropriate for such a kind of separation, is current by the eective area of the antenna as, for superposed epoch analysis 4SEA). instance, described by Tammet et al. 41996). The signature of a magnetic substorm on the ground is generally a decrease of the horizontal component of the magnetic ®eld in a ``bay'' form. As key times 4t =0) 3 Geomagnetic substorm eect for superposed epoch analysis we used the times when the north-south component of the geomagnetic ®eld Usually the air-earth currents show rather strong detected by the Kiruna magnetometer 435 km west of variability. We discuss the reasons for this later. To Esrange) reached a minimum during the substorm. We illustrate, we show the daily variations of AEC for included only substorms occurring near local magnetic August 15, and November 18, both 1998 4in Fig.