MAGNETIC STORM EFFECTS IN THE ATMOSPHERIC ELECTRIC FIELD VARIATIONS

MAGNETIC STORM EFFECTS IN THE ATMOSPHERIC ELECTRIC FIELD VARIATIONS N.G. Kleimenova 1, O.V. Kozyreva 1, S. Michnowski 2, M. Kubicki 2, N.N. Nikiforova 1 1 Institute of the Earth Physics RAS, Moscow, Russia, e-mail: kleimen@ifz.ru; 2 Institute of Geophysics PAS, Warsaw, Poland Abstract. The vertical component (Ez) of the atmospheric electric field variations, measured at middle (obs. Swider) and polar (obs. Hornsund) latitudes under fair-weather conditions, have been analyzed. The strong effect of sharp daytime Ez increasing (negative Ez anomalies) in the middle latitudes was found during the main phase of the strong and moderate magnetic storms. The negative Ez deviation started simultaneous with the night side geomagnetic substorm onset. The observed effects could be interpreted as a result of the influence of the strong night side ionosphere conductivity increasing, caused by substorm associated particle precipitation, to the global electrical circuit. In the polar latitudes the Ez enhancement effects of the storm initial phase have been found. Sometimes the positive Ez variations have been observed during so called polar substorm development. We also found the polar ground-based Ez enhancement coincided with similar IMF Ey variations. Introduction According to the well established concept, the integrated worldwide thunderstorm activity is considered as a main source of the electric fields in the lower atmosphere. Thunderstorm activity draws current upward from the ground. The ionosphere disperses the current globally, and it leaks back to the surface, averaging a variable potential of~100 V/m at ground level under fair weather conditions. One can see in Fig. 1 that the magnetosphere and the ionosphere represent the important part of the global electrical circuit. Variability of the vertical component of the atmospheric electric field (Ez) near the Earth surface has been investigated in many studies. The daily Ez variations are created not only by the worldwide thunderstorm activity, but different solar and geophysical phenomena can provide Fig. 1 The scheme of the global electrical circuit some influence to Ez behavior. It was suggested that solar activity influences due to ionosphere electric field disturbances may significantly control a global electric circuit state [e.g., Sao, 1967; Apsen et al., 1988, Michnowski, 1998; Bering et al., 1998; Rycroft et al., 2000; Frank-Kamenetsky et al., 2001; Nikiforova et al., 2003]. Although the ionosphere electric potential distribution and atmospheric conductivity variations depend strongly on solar wind change the response to them of the lower atmospheric electric field (Ez) and current (Jz) is still rather not known. The strongest manifestations of the solar wind interactions with the magnetosphere and ionosphere processes are especially evident at the auroral and polar latitudes. The most studies of these effects have been curried out at high and polar Arctic and Antarctic areas. Practically, magnetic storm influences, which are distinctly manifested in high latitudes, remained unknown on mid-latitude Ez variations. However, some anomalies in the middle latitude Ez behavior were found during the huge magnetic storm on October 30, 2003 [Nikiforova et al., 2005] showing a possibility to find some Ez effects associated with strong magnetic storms. This was only one event that has to be confirmed or ignored. The aim of this paper is to study possible effects of magnetic storm in atmospheric electric field (Ez) disturbances at middle (obs. Swider) and polar (obs. Hornsund) latitudes. 123

Data This study is based on the regular registrations of the vertical component of the atmospheric electric field (Ez) at mid-latitude Polish geophysical observatory Swider (geomagnetic coordinates: Φ=47.8º, Λ=96.8º). Magnetic local noon is at ~ 10 UT. The instruments and their location are reported in [Kubicki, 2001]. We used the 1 min sampling data averaged at 5 min intervals for reject the short period fluctuations associated with a local meteorological origin. We also present here one event of the Ez and Jz (vertical electric currents) observations at Polish polar station Hornsund (HOR, Ф =74.0º, Λ =110.5º) at Spitsbergen archipelago during the initial phase of the widely discussed strong magnetic storm on July 15, 2000 (Bastille Day event). Only data, fulfilling the criteria of so called fair weather conditions lasting all 24 hours during the given day, have been used in our analysis. The fair weather conditions request the absence of rain, drizzle, snow, hail, fog, lower cloudiness, local and distant thunderstorms, wind velocity exceeding 6 m/c, negative Ez values. Such long-lasting periods are usually seldom occurred at Świder, especially in summer and autumn; so there are not more ~ 40-60 fair weather days in the year (i.e., ~12-15% of the total observations). Observations Middle latitudes (obs. Swider). To distinguish magnetic storm effects in atmospheric electricity it is very important to establish the middle latitude Ez diurnal variations under quiet geomagnetic conditions, observed under fair-weather conditions. An average ground-level electric field diurnal curve (known as Carnegie curve ) with a minimum near 03-05 UT and a maximum near 18-20 UT, occurred due to longitudinal distribution of the global thunderstorm activity (Fig.2., right panel). The Ez data of tree magnetically quiet days with Kp < 2 are shown in Fig. 1 (left panel). The solid red line demonstrates the established average Ez values. The strong daily variations are seen with two enhancements: before local noon (at 06-10 UT corresponds to 08-12 MLT) and in the local evening (14-18 UT corresponds to 16-20 MLT). One can see that the average diurnal Ez variations roughly match the Carnegie curve, but demonstrate many short-lasting differences up to 50-80 mv/ms. The influence of the worldwide thunderstorm activity in Asia and Africa clearly seen, however, the effect of American center is very poor. The strongest magnetospheric and ionospheric disturbances are usually observed during the main phase of a magnetic storm, thus we suppose that the storm main phase would produce some effects in Ez variation even in the middle latitudes due to significant change of the ionosphere conductivity which is an important sector of the global electrical circuit. In 2000-2004 we could found only 14 magnetic storms, observed under Fig. 2 Ez variations at Swider under quiet geomagnetic conditions (Kp<2) and the distribution of the global thunderstorm activity fair weather periods lasting all 24 hours during the given day. As a magnetic storm characteristic we used the Dst-index, and the 1-min sampled solar wind and IIMF data collected in the OMNI base. As an example, the very strong magnetic storm on March 31, 2001 is shown in Fig.3. As the fair weather magnetic quiet day we applied the Ez variations on the previous day (30 March) because the solar wind and IMF parameters were in very quiet state. However, according to Ez observations, the morning hours (01-08 UT) of this day could not be attributed to the clear fair weather conditions. The main phase of this magnetic storm was characterized by strong (up to ~2000 nt) night side substorms recorded at 124

Fig. 3 Dst-index, solar wind electric field (Em) and velocity (V), and IMF Bx, By. Bz components during the magnetic storm on March 30-31, 2001; the right panel - Ez observations (solid line) at Swider, quiet Ez (thin line); the estimated their divergence, and magnetograms from middle latitude station BEL and auroral latitude stations CMO and SOD. auroral stations College (CMO, Ф=64.7º, Λ=263º) and Sodankyla (SOD, Ф=63.8º, Λ=108º) as it is shown in lower part of the right panel of Fig. 3. The obs. CMO is located at opposite to obs. Swider side of the Earth, thus the magnetic local noon at Swider (~10 UT) approximately correspondent to the magnetic local midnight at CMO (~11 UT). Accurately at the time of substorms occurrence, the dayside negative Ez deviations from the quiet level was observed at Swider. Two more examples of the magnetic storm effect in mid-latitude Ez variations are presented in Fig. 4 for October 13-14, 2000 and May 23-24, 2000 data. The strong negative estimated Ez deviations from magnetically quiet Ez level are clearly seen. The daytime deflection signatures of the Ez at Swider started simultaneously with the magnetospheric substorm onset at the night sector (obs. College - CMO). The similar negative Ez deviations were observed during the main phases of all 14 magnetic storms under consideration. These negative dayside Ez anomalies appeared simultaneously with night side substorms developing. Thus, for the first time it was found that during the main phase of a magnetic storm a strong daytime negative Ez deviations could be observed at middle latitudes simultaneously with magnetic substorm development at night side of the auroral zone. There were no local magnetic perturbations as it is shown by geomagnetic records at mid-latitude station Belsk (BEL). Polar latitudes (obs. Hornsund). In the polar region, the interaction of the solar wind and the Earth's magnetic field leads to polar convection enhancement driven by horizontal dawn-to-dusk electric fields across the polar cap. For structures larger than ~500 km, this polar cap potential drop can produce significant vertical electric fields at ground level [Park, 1976]. We present here some experimental results of the possible contribution of magnetospheric sources to the high latitude atmospheric electric field variations, based on the Ez and Jz observations on Polish polar station Hornsund (HOR, Ф =74.0º, Λ =110.5º).The position of this station depending on geomagnetic 125

Fig. 4 The same as in Fig. 3(right panel) for magnetic storms on October 13-14, 2000 and May 23-24, 2000 activity may be mapped or inside of the auroral oval, or in the polar cap region, i.e. under the open geomagnetic field lines (Fig. 5). Kozyreva et al. (2004) reported that the storm initial phase as well as its sudden commencement (SC), caused by a passage of the compression front edge of an interplanetary magnetic cloud, manifests itself mainly as wave magnetic disturbances at dayside polar cap latitudes. We studied a response of Ez and Jz variations, observed at Hornsund, to the SC and the initial phase of the large magnetic storm on July 15, 2000. This storm was associated with the powerful coronal mass ejection (CME). The observations (Fig. 6) demonstrate the occurrence of the very strong (up to ~ 800 V/m) positive bursts of Ez and Jz following the sudden jump of the solar wind dynamic pressure and the IMF B corresponding to the bow shock impact (magnetic storm sudden commencement - SC. The positive Ez Fig. 5 The location of Hornsund relatively to auroral oval position (OVIATION data) 126 and Jz enhancement was observed under the significant cross-polar cap potential drop. The amplitude of Ez

Fig. 6 Left panel - Dst variations and IMF data from Geotail; right panel - the maps of ionosphere convection, cross-polar cap potential drop, and Ez and Jz data at Hornsund during the initial phase of the magnetic storm on July 15, 2000. and Jz started to decrease with the cross-polar cap potential increasing. Discussion A magnetic storm main phase is usually accompanied by high latitude geomagnetic substorms that manifest large magnetosphere-ionosphere disturbances seen in energetic particle precipitations and ionosphere potential configuration changes. These disturbances seem to be able to affect lower atmosphere global electric circuit changes. As a result the total resistance of the ionosphere part of the global electric circuit decreases [e.g., Apsen et al., 1988, Michnowski, 1998; Bering et al., 1998]. A large number of high latitude experimental investigations of magnetosphere effects in atmospheric electricity support this suggestion. According to many authors [e.g., Apsen et al., 1988; Nikiforova et al., 2003] magnetospheric substorms and visible auroras at high latitudes stations are accompanied by negative night side Ez variations. Moreover, in solar wind induced changes can be involved by more direct effects of the deep penetration of the interplanetary electric field into middle and low latitude ionosphere. Both factors can be responsible for the up to now unknown a middle latitude response of the electric field on magnetic substorms. In presented study we found the strong negative Ez disturbances at dayside middle latitude station Swider associated with strong magnetic storm. We suppose that the considered effect of the storm main phase in mid-latitude atmospheric electricity is a result of large scale change in the total conductivity of the global electric circuit. Another possible source of this Ez disturbances might be the penetration of the interplanetary electric field (Ey=V*Bz) deep into the magnetosphere [Huang et al., 2005]. The polar cap Ez effects of the magnetic storm initial phase might be results of the change in the ionosphere plasma convection due to strong solar wind dynamic pressure increasing on the front edge of the interplanetary magnetic cloud. However, we could not ignore an enhancement of the polar cap (zone 3) field aligned currents (zone 3 FACs), associated with strong increasing of the positive Bz IMF. As a support of that assumption could be the fact that during the previous magnetic storm SC, caused by bow shock with very similar solar wind dynamic pressure jump, but accompanied by negative Bz IMF, an enhancement of Ez and Jz at Hornsund did not observed. 127

Summary For the first time there was found the effect of the main phase of a magnetic storm on the daytime middle latitude Ez variations being any local magnetic activity. Various strong (~100-300 V/m) deflection signatures of the Ez variations (negative Ez disturbances) relative to the quiet magnetic daily level have been observed at Swider in the daytime simultaneously with magnetospheric substorm onset at the night sector (obs. College). This result could be account for a significant substorm associated solar wind interplanetary electric field penetration from polar cap regions into the day side ionosphere influence. Such affects appear to be so intensive that they could be seen even in the middle and low latitude day sector of the Earth. During storm initial phase there was strong daytime Ez and Jz enhancement observed at polar latitudes. The cross polar cap potential was dropped at this time. References Apsen, A.G., Kh.D. Kanonidi, S.P. Chernishova, D.N. Chetaev, and V. M. Sheftel (1988), Magnetospheric effects in atmospheric electricity, 150 pp. M., Nauka. Bering, E.A., A.A. Few, and J.R. Bennnbrook (1998), The global electric circuit, Phys. Today, 51 (10), 24-30. Frank-Kamenetsky, A.V., O.A. Troshichev, G.B. Burns, and V.O. Papitashvili (2001), Variations of the atmospheric electric field in the near-pole region related to the interplanetary magnetic field, J. Geophys. Res., 106, 179-190. Huang, C-S., J.C Foster., and M.C. Kelley (2005) Long-duration penetration of the interplanetary electric field to the low-latitude during the main phase of magnetic storms, J. Geophys. Res., 110, A11309, doi: 10.1029/2005JA011202. Kleimenova, N.G., S. Michnowski, N.N. Nikiforova, and O.V. Kozyreva (1995) Long-period geomagnetic pulsations and fluctuations of the atmospheric electric field intensity at the polar cusp latitudes, Geomagn. Aeron., 35(4), 469-477. Kleimenova, N.G., S. Michnowski, N.N. Nikiforova, and O.V. Kozyreva (1998) Variations of atmospheric electric field vertical component at the evening sector of polar latitudes (obs.hornsund), Geomagn. Aeron., 38(6), 149-156. Kozyreva O.V., N.G. Kleimenova, and J.-J. Schott (2004), Geomagnetic pulsations in a storm initial phase, Geomagn. Aeron., 44(1), 37-46. Kubicki, M. (2001), Results of atmospheric electricity and meteorological observations S. Kalinowski geophysical observatory at Świder, Publ. Inst.Geophysics Polish Acad. Sci., D-56 (333), 3-7. Michnowski, S. (1998), Solar wind influences on atmospheric electricity variables in polar regions, J.Geophys.Res., 103(D12), 13939-13948. Nikiforova, N.N., N.G. Kleimenova, O.V. Kozyreva, M. Kubicki, and S. Michnowski (2003), Influence of auroral-latitude precipitation of energetic electrons on variations in the atmospheric electric field at polar latitudes (Spitsbergen Archipelago), Geomagn. Aeron., 43(4), 29-35. Nikiforova, N.N., N.G. Kleimenova, O.V. Kozyreva, M. Kubicki, and S. Michnowski (2005), Unusual atmosphere electric field variations during the main phase of the huge magnetic storm of October 30, 2003 at the Polish mid-latitude station Swider, Geomagn.Aeron., 45, 148-152. Park, C.G. (1976), Solar magnetic sector effects on the vertical atmospheric electric field at Vostok, Antarctica, Geophys. Res. Lett., 3, 475-478. Rycroft, M.J., S. Israelsson, and C. Price (2000), The global atmospheric electric circuit, solar activity and climate change, J. Atmos. Terr. Phys., 62, 1563-1576. Sao, K. (1967), Correlation between solar activity and the atmospheric potential gradient in the Earth s surface in the polar regions, J. Atmos. Terr. Phys., 29, 213-215. 128