Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia) Kp- INDEX AND LOCAL HIGH-LATITUDINAL GEOMAGNETIC ACTIVITY A.E. Levitin, L.I. Gromova, S.V. Gromov, L.A. Dremukhina Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation , Troitsk, 142090, Russia, e-mail : [email protected] Abstract. Nowadays the geomagnetic activity seasonal variations are described using monthly data of the global Kp index averaged for several years. As is known, the peak area of the geomagnetic activity seasonal variations corresponds to the equinox periods (maximums), while the two minimums are found at the solstices. It is supposed that the formation of activity areas in the solar regions between 10° and 30° of the N and S geliographical latitudes plays the major role in the equinoctial asymmetry of the geomagnetic activity occurrence. During an equinox the plane of the solar equator coincides with that of the Earth, and in this period the Earth is most vulnerable to the impact of the solar activity areas. We establish the correlation of the ground based magnetometer measurements used to derive Kp index and the data of the high latitude observatories used for AE-index calculating during magnetic storms. We consider inconsistent calculations on the base of Kp index, as far as it doesn’t describe the global geomagnetic activity during magnetic storms, but it describes local high latitude magnetic disturbances in these periods when the polar oval shifts equatorwards and Kp-observatories indicate the increase of the auroral current system intensity. Magnetic storms have the maximum of occurrence near vernal and autumnal equinoxes, and it induces the peaks of the monthly Kp index in March-April and October-November. As we suppose, it would be more accurate to estimate the geomagnetic activity using the data of the magnetic observatories located at the different latitudes of the Northern hemisphere from the equator to the polar cap. For data processing and calculating of the geomagnetic activity we apply a new technology that allows to give quantitative assessment of the current local geomagnetic activity and of the magnetic storm intensity. Introduction Indices of geomagnetic activity K, Kp, AE(AU, AL), Dst were introduced over half a century ago. They reflected the prevailing pre-satellite era view about generation of the external magnetic field activity. To examine geomagnetic variations researchres was able to use observatory geomagnetic data only, and magnetic measurements was stored on photo paper and magnetic tape. They did not know much about interaction of the Earth’ magnetosphere with interplanetary space, physical processes of generation of magnetospheic storms and substorms, processes in the magnetospheric tail. Kp was introduced as a index of geomagnetic activity by Bartels in 1938. Kp scale has the values 0 to 9. The values 0 - 3 are in accord with the quiet state of geomagnetic activity, 4 is in accord with disturbed ones, and 5 - 9 correspond to magnetic storms of different intensity that is described by Dst-index. Global (planetary) three-hour-range index Kp is the mean standardized K-index from 13 subauroral Kp-observatories. By applying the conversion tables, K- indices (and Kp-index correspondingly) are determined from geomagnetic data of Kp-observatories by the special service in GeoForschungsZentrum (GFZ), Potsdam, Germany (http://www-app3.gfz- potsdam.de/kp_index/). In [Yanovsky, 1953].one can find the next description of geomagnetic activity ‘Value characterizing geomagnetic field variation during some time interval by level of its disturbance is called magnetic activity or magnetic disturbance’. Namely, if the geomagnetic field of high amplitude doesn’t change dramatically during any time period geomagnetic activity is described as zero one. Most surprising for quantitative assessment of geomagnetic activity is that we realize geomagnetic activity indices shortcomings which doesn’t allow to describe realistic temporal geomagnetic situation but we continue to use them. Below we discuss these shortcomings of Kp-index and propose new technology to quantitatively e assess geomagnetic activity. Shortcomings of Kp-index of planetary geomagnetic activity Kp-index is used for in the study of magnetism to describe planetary geomagnetic activity. Global three-hour-range index Kp is the mean standardized K-index from 13 subauroral Kp-observatories. The K-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0-9 with 1 indicating quiet state of the geomagnetic field and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval. 295 Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia) The conversion from maximum fluctuation in nT to K-index varies from observatory to observatory (see Table 1).Observatories at higher geomagnetic latitudes routinely experience wider magnetic field fluctuations than lower-latitudes observatories therefore, the magnetic field amplitude range corresponding to K-index =9 at these higher latitudes is wider. A table of conversion assigned to each observatory giving the limits, or range, corresponding to each of the ten values of K. Table 1.Amplitude of horizontal component of Earth's magnetic field assigned corresponding to К = 9 in depending on geomagnetic latitude Ф Ф 64° - 90° 65° - 80° 60° - 65° 55° - 60° 48° - 54° 30° - 47° 0° - 30° H, nT 2500 2000 1000 - 1800 600 - 1500 550 350 300 Observatories at higher geomagnetic latitudes routinely experience wider magnetic field fluctuations than lower-latitudes observatories therefore, the magnetic field amplitude range corresponding to K =9 at these higher latitudes is wider. A table of conversion assigned to each observatory giving the limits, or range, corresponding to each of the ten values of K. But this conversion tables don’t depend on season of the year. The Kp-index is derived from a number of magnetometer stations at mid-latitudes. When the stations are not greatly influenced by the auroral electrojet currents, conditions are termed magnetically quiet. If the auroral zone expands equatorward, however, these stations can record the effects of the auroral electrojet current system and of the magnetospheric ring current and field-aligned currents that can connect it to the ionosphere. Thus, during magnetically disturbed periods (magnetic storms) Kp-index reflects auroral, not the planetary, activity. Annual variation of geomagnetic activity based on Kp-index decribes annual distribution of number of magnetic storms not realistic geomagnetic activity one. Here we should have in mind that geomagnetic activity should be defined as activity integrated during the time period when there are no any storms. This situation occurs during 90% of the time of year. Besides, real dynamics of geomagnetic activity should be described by the temporal variation of the external magnetic filed in all near-Earth’s space and it could not be estimated only basing on data of Kp-stations located mostly in the Northern hemisphere. We studied seasonal variation of geomagnetic activity based on monthly distribution of index Kp (Ap). The planetary Ap-index actually is calculated as a running average of Kp-index of eight 3-hour periods. Figure 1 shows average monthly distribution of Ap-index in period 1970–2002. One can see that the peaks of monthly Kp index in March-April and October-November is caused by the most frequent occurrence of magnetic storms and extreme ones near vernal and autumnal equinoxes. Figure 1. Kp-index histogram by month based on selected of Kp-index maximum amplitude over 1970- 2002. 296 Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia) As it is known the planetary Kp-index is derived from the data of subauroral observatories. It is considered that each observatory is located far from any magnetospheric sources and it doesn’t responsive to geomagnetic disturbances in the auroral zone of the high latitudes. Thus, it doesn’t describe geomagnetic activity in the region where the most high-amplitude geomagnetic disturbance occur, and doesn’t estimate total geomagnetic activity over the Earth. Figure 2 demonstrates seasonal variations in monthly mean Dst, Ap, Am, AE, and PCN geomagnetic activity indices [Lyatsky, 2003]. From Figure 2 one can see that high latitude geomagnetic disturbances described by indices AE and PC are more intensive than geomagnetic activity described by index Kp(Ap on the figure). The peak area of the seasonal variations of index AE(PC) corresponds to the summer (winter) periods (maximums) because the ionospheric conductivity in the Northern Hemisphere is the largest in summer and the smallest in winter. Moreover magnetic variations of Kp- index observatories should be similar to those of auroral observatories as onecan see in Figure 3 shown variations of quantitative assessment of geomagnetic activity at some observatories during the magnetic storms of different intensity in comparison with Kp-index . 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 external sources. Our technology of the quantitative assessment of geomagnetic