EXTREMELY QUIET 2009 STATE OF GEOMAGNETIC FIELD AS A REFERENCE LEVEL FOR AMPLITUDE OF THE LOCAL GEOMAGNETIC DISTURBANCES

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1 EXTREMELY QUIET 2009 STATE OF GEOMAGNETIC FIELD AS A REFERENCE LEVEL FOR AMPLITUDE OF THE LOCAL GEOMAGNETIC DISTURBANCES A.E. Levitin, L.I. Gromova, S.V. Gromov, L.A. Dremukhina Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk, 12190, Russia, Россия, levitin@izmiran.ru Abstract. We present a new technology of calculating the external geomagnetic field hourly amplitudes applicable for any hour and any day of the whole survey period of any observatory proceeding from the ground based magnetometer measurements. The calculated values may be used to find the amplitudes of the most magneto-quiet hour (day, month, year) and the most magneto-disturbed one for the whole measurement period. These data allow to estimate and to draw maps of the recent geomagnetic activity and the activity during specific geophysical events in the past at any point on the Earth. For the description of the past and recent geomagnetic activity we consider this method more opportune than that of AE(AU, AL), Kp, Dst indices, out-of-date and not without drawback, since introduced more than half a century ago. A detailed description of the method is presented with examples, for estimation of the local geomagnetic activity, its seasonal variations and calculation of Dst variation during the magnetic storms. Introduction. Extremely quiet 2009 state of the geomagnetic field may be used as a reference level to create new technology of quantitative assessment of geomagnetic activity which could be more opportune than that of AE(AU, AL), Kp, Dst indices, out-of-date and not without drawback, since introduced more than half a century ago. Nowadays geomagnetic activity is described by special geomagnetic indices. They were introduced taking into account the experimental fact that the intensity of geomagnetic variation depends on the observatory location. According to spatial-temporal structure of geomagnetic variations, the following areas can be distinguished: the polar cap, the auroral ova (auroral zone)l, subauroral latitudes, the middle and low latitudes. Correspondingly, all indices can be divided into 3 groups: I) Indices presenting local geomagnetic disturbances (so-called K-indices) which describe the geomagnetic field disturbances at any observatories and depend on their latitudinal location. II) Indices describing global geomagnetic field disturbances, the foremost being the planetary Kp-index derived from the data of subauroral observatories. It is considered that each observatory is located far from any magnetospheric source and at the same time is responsive to any one. III) Indices that should reflect the magnetic disturbance intensity caused by different sources: Dst-index should follow the dynamics of the magnetospheric ring current; AE, AU, AL indices should describe auroral electrojet dynamics; PC-index should indicate the electric field over the polar caps (cross-polar cap potential). These indices were induced to quantify the temporal scale of geomagnetic variation amplitudes and became very popular. Though rather rough, they have not changed for ten years and are used by scientists and engineers as real estimation the scale and temporal dynamics of geomagnetic activity. Up to now, instead of a concept of real geomagnetic activity as total energy of geomagnetic variations generated by magnetospheric and ionospheric current systems in the near Earth s space, a concept of geomagnetic activity as temporal variations of the indices (first of all, of so-called planetary Kp-index) exists. Below we demonstrate new technology of quantitative assessment of local geomagnetic activity proceeding from the ground-based magnetometer measurements of magnetic observatory Moscow. For the period we calculated hourly values of H-component of the external geomagnetic field at Moscow observatory, based on hourly values of H-component of extremely quiet 2009 as a reference level, and estimated correction for secular variation of the Earth magnetic field at the observatory location. Thus we are able to assess local current geomagnetic activity and that for any hour (day, month, year) of the observatory measurements, to study seasonal variations of geomagnetic activity, evaluate the external magnetic field contribution to annual values of H-component of the Earth s Main magnetic field which are used to estimate the rate of change of Earth's magnetic field. 290

2 Quantitative assessment of geomagnetic activity Hizm To introduce quantitative assessment of geomagnetic activity Hizm we chose H-component of the geomagnetic field recorded by observatories of the Northern hemisphere because it is more affected by external sources. Our technology of the quantitative assessment of geomagnetic activity include processing of hourly data of from any magnetic observatories. It is generally accepted that the solar and geomagnetic activity were extremely low during Extremely quiet 2009 state of the geomagnetic field is assumed as a reference level of the geomagnetic activity for past and recent magnetic observations. For any month of 2009 we selected the most magneto-quiet day. Hourly H-component amplitudes of the selected day is subtracted from those of the same month of past and recent years. We calculated the correction for secular variation of the Earth magnetic field as difference between daily means of the most quiet January days of two adjacent years. Our correction is rather close to the correction for secular variation calculated as difference annual H-component amplitudes of two adjacent years but we suppose that January geomagnetic field at the observatories of the Northern hemisphere is the most quiet. In this way we should obtain Hizm value, in units of nt, for any hour of any day of any month of any year for the period of any observatory measurements. We suppose that quantitative assessment. Hizm Table 1. Houly data of Moscow observatory submitted to World Data Center for STP, Moscow, ( and quantitative assessment of geomagnetic activity Hizm. Observatory Tablur UT code, year, base month, (tab. element, day of base) the month Hourly mean values (tab.value) of the geomagnetic field components submitted to WDC. For D we have that hourly value (degrees) = tab.base + tab.value/600 For H, Z we have that hourly value (nanoteslas) = tab.base*100 + tab.value MOS8903D MOS8903D MOS8903D MOS8903H MOS8903H MOS8903H MOS8903Z MOS8903Z MOS8903Z Quantitative assessment of geomagnetic activity Hizm Hourly mean values of the H-component of the magnetic field MOS8903H MOS8903H MOS8903H Hourly mean values of the H-component of the quietest day of March 2009 as the reference level MOS0903H Deviation of H-component of the magnetic field from the quiet level adjusted for the secular variation (Hizm) K-index for every three-hourly interval. MOS8903Hizm K-индекс 9 MOS8903Hizm K-индекс 9 6 MOS8903Hizm K-индекс 3 291

3 00 MOS Hizm, nt March, 13-1, UT Figure 1. Variation of the quantitative assessment of geomagnetic activity Hizm calculated from Moscow observatory data for interval of the extreme magnetic storm of 1989, March It is shown K-index for every 3-hourly interval. calculated from data of any observatory, describes of local geomagnetic activity in the area of the observatory location. At first we applied our technology to geomagnetic data of observatory Moscow and calculated Hizm(Mos) for every hour of every day of every month for every year of when observatory measurements were are available. Usually observatory data are collected by World Data Center for STP as tables of hourly measurements of the geomagnetic field components and three-hourly K-index derived from these measurements. One can find digital WDC format for observatory hourly mean values on Table 1 demonstrates Moscow observatory data for interval of the extreme magnetic storm of 1989, March 13-1, submitted to WDC and quantitative assessment of geomagnetic activity Hizm calculated from the data according to our technology. Variation of the geomagnetic activity assessment of Hizm(Mos) and 3-hourly K-index for the same period are shown in Figure 1. One can see that K-index does not provide good description of amplitudes of the geomagnetic disturbance during extreme storm presented by Hizm variation. It is related to the minimum peak-tomaximum peak fluctuations of the horizontal magnetic field component observed on a magnetometer during a three-hour interval relative to a quiet day, and K-index bases on two measurements during tree-hourly interval only. 292

4 Our technology was applied to geomagnetic data of 102 observatories of the Northern hemisphere that allows to study local geomagnetic activity and its spatial and temporal variations. Dst-index and assessment of geomagnetic activity during storms of different intensity Dst-index was introduced to estimate a magnetic storm intensity, and it is derived using the data from the four magnetic observatories. It was assumed that a magnetic storm is generated by the ring current located in the Earth equatorial plane. For this reason Dst-observatories were chosen so that their locations are sufficiently distant from the auroral and equatorial electrojets and that they are distributed in longitude as evenly as possible. But the ring current spatial configuration is not so simple, and a magnetic storm should be generated by different magnetospheric and magnetospheric-ionospheric current systems including the high-latitudinal ones. According to the ring current model used by the authors of Dst-index during a magnetic storm the intensity of geomagnetic disturbance should decrease from the equator to the high latitudes with decreasing cosine of latitude and the geomagnetic disturbance at Dst-observatories should be more intensive that these at observatories located at higher latitudes. We selected the observatory chain (SJG-chain) FUQ-SJG-FRD-OTT-IQA-THL, where SIG is Dstindex - observatory, FRD and OTT are Kp-index observatories located at the middle latitudes, and THL and IQA are high latitude observatories. Besides, all observatories are located among 30 dipole longitude sector (it is assumed that SJG is the middle station and ±1 is counted from its dipole longitude) therefore UT at time when given observatory of the chain is at midnight (MLT) may differ by no more than 2 hours. Geomagnetic activity was examined during some storms of different intensity: Extreme Dstmin < -300 nt 2003, October Severe 300 nt < Dstmin < -200 nt 1999, October Intense -200 nt < Dstmin < -100 nt 2001, October Moderate -100 nt < Dstmin < -0 nt 2006, April Figure 2 shows variation of Hizm calculated from observatory data of SJG-chain during selected storms and variation of product Dst(t) by cosine of corresponding observatory dipole latitude. One can see that at more equatorward stations these variations are more similar and at more polarward stations those are more different from each other. Hence at more low latitudes dominating disturbance should be generated by the ring current but at more higher latitudes it should be generated by other current systems including highlatitudinal ones which should shift equatorward during a storm. The variations hold similar up to more high latitudes during more weak geomagnetic disturbances, and their similarity is broken at more low latitudes during more strong disturbances. This is a reason to suppose that Dst-index in not suitable for describing geomagnetic activity. In fact to estimate geomagnetic activity it should be necessary to calculate energy of geomagnetic variations. For this aim we plan to use observatory data and our technology of quantitative assessing geomagnetic activity. Conclusion New technology of processing the observatory magnetic measurements using extremely quiet 2009 state of the geomagnetic field as a reference level of average hourly data allows to get very useful information about local geomagnetic activity in the area of an observatory location. It is quantitative assessment of geomagnetic activity calculated as hourly amplitudes of the external geomagnetic field, in units of nt, for any hour of any day of any month of any year for the period of any observatory measurements. The calculated values may be used to find the amplitudes of the most magneto-quiet hour (day, month, year) and the most magneto-disturbed one for the whole measurement period and an user would be able to estimate current geomagnetic activity as well as that during monitoring interval of specific geophysical events. If we applied new technology of quantitative assessment of geomagnetic activity to magnetic measurements of all observatories, these data should allow to estimate and to draw maps of the recent geomagnetic activity at any point on the Earth. We should describe the past and recent geomagnetic activity more accurately than with AE(AU, AL), Kp, Dst indices, since introduced more than half a century ago. Acknowledgments. The work is supported by RFBR grant

5 Figure 2. Hizm variation (solid black) along SJG-chain during storms of different intensity and variation of product Dst-index by cosine of corresponding observatory dipole latitude (blue). The dipole latitude of each observatory is shown on the left, UT and MLT are presented on lower and upper axis correspondingly. 29