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 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 plays the major role in the equinoctial asymmetry of the geomagnetic activity occurrence. During an equinox the plane of the solar coincides with that of the , 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 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 , 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 activity include processing of hourly data of 1958 -2009 from all magnetic observatories of the Northern hemisphere. It is generally accepted that the solar and geomagnetic activity were extremely low during 2009. 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 these of the same month of past and recent year. We calculated the correction for secular variation of the Earth’ magnetic field as difference between daily mean 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 Figure 2. Seasonal variations in monthly mean suppose that January geomagnetic field at the Dst, Ap, Am, AE, and PCN geomagnetic observatories of the Northern hemisphere is the most activity indices. Time intervals when these quiet. In this way we should obtain Hizm value, in units indices were available are shown. 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 calculated from data of any observatory describes local geomagnetic activity in the area of the observatory location.

297 Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia)

Assessment of geomagnetic activity during storms of different intensity Geomagnetic activity was examined during some storms of different intensity: Extreme Dstmin < -300 nT 2003, October 29-31 Severe 300 nT < Dstmin < -200 nT 1999, October 21-23 Intense -200 nT< Dstmin < -100 nT 2001, October 20-22 Moderate -100 nT< Dstmin < -50 nT 2006, April 08-10 During strong geomagnetic disturbances high latitude current systems effect on magnetometer data of observatories located at the middle latitudes. The more intensive magnetic storm, the stronger this effect. To illustrate this fact we selected the observatory chain FRD-OTT-IQA-THL (FRD-chain), where FRD and OTT is Kp-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 FRD is the ‘middle’ station and ±15° 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. Figure 3 shows variation of Hizm calculated from data of FRD-chain during selected storms. One can

Figure 3. Hizm variation at FRD-chain observatories during selected magnetic storms of different intensity in comparison with Kp-index. The dipole latitude of each observatory is shown on the left, and UT and MLT are presented on lower and upper axis correspondingly.

298 Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia)

see that the effect of high latitudinal current system on magnetometer data of Kp-index observatories became stronger during more intensive storms. So, during the extreme storm (2003, October 29-31) this effect is more appreciable at the most equatorial station FRD(48°) but during moderate storm (2006, April 08-10) it is minimum at OTT(55°). To introduce energy estimation of geomagnetic activity based on Hizm let’s divide the Northern hemisphere by 15°- 20° latitude belts and select 4 observatories located in each belt within the each 90° longitude sector. Location of selected observatories within selected belts are demonstrated in Figure 4. Using value Hizm obtained for each selected observatory we calculated Hizm2 and marked it by «+» if original Hizm was positive and «-» if it was negative. Then we calculated arithmetic mean of the Hizm2 of 4 observatories of the belt for positive values (Hv+ ) and negative ones (Hv-) separately. We suppose that Hv+ and Hv- should describe hourly energy density of the external magnetic field in the selected .belt and in some way their sign represents the energy source. To illustrate our energy estimation we present variation of Hv+ and Hv- in the selected belts during selected storms of Figure 4. Location (in dipole coordinates) of different intensity in comparison with Kp- observatories selected to compare energy estimation Hv index described above(see. Figure 5). within the different latitudinal belts during the storm of Our technology was applied to different intensity. Selected belts are shown by dotted lines. geomagnetic data of 102 observatories of the Northern hemisphere that allows to examine a relation between planetary Kp-index and local high latitude geomagnetic activity.

Conclusion Indices of geomagnetic activity introduced by our precursors over half a century ago reflect the prevailing pre-satellite era view about geomagnetic activity generation. Kp-index as quantitative estimation of planetary geomagnetic activity does not meet our current knowledge about origins of geomagnetic disturbances caused by the Interplanetary magnetic fields (IMF) and the Solar wind. Kp-index shortcomings are well-known but researchers of geomagnetic activity and magnetologists continue to use it as an indicator of the geomagnetic field state. Incorrect Kp-index assessment of real geomagnetic activity described above results in not quite correct understanding of the causes of its seasonal variation. Magnetic storms have the maximum of occurrence near vernal and autumnal equinoxes, and it introduces the peaks of the monthly Kp-index in March- April and October-November. Increasing Kp-index during magnetic storms is caused by shifting Kp-observatories from the subauraral zone to the auroral one. As is known, the peak area of the geomagnetic activity seasonal variations corresponds to the equinox periods. 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. Coronal mass ejections are associated with solar activity. And a magnetic cloud as a special type of coronal mass ejection that occurs from the surface can cause storms in the Earth's magnetosphere. It is established that negative Bz-component of the interplanetary magnetic field in near-Earth magnetic cloud (negative magnetic cloud) leads to magnetic storm generation but when positive magnetic cloud (with positive Bz-component) reaches the Earth a magnetic storm is not generated. But number of events with a positive near-Earth magnetic cloud is approximately equal to .number of events with a negative magnetic cloud. Now we understand that a magnetic storm generation is random and it’s very difficult to say why the Earth is most vulnerable to the impact of the solar activity during equinoxes. We propose to replace Kp-index with new quantitative assessment of geomagnetic activity and to introduce new estimation of a magnetic storm intensity instead of Dst-index.

299 Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia)

Figure 5. Variation of energy estimation Hv+ is (red) and Hv- (blue ) in different latitudinal belts during the storms of different intensity in comparison with Kp-index. Hv+ and Hv- are divided by 1000.

Acknowledgments.The work is supported by RFBR grant №11-0500306

References 1. Yanovsky B.M. Earth's magnetism. Leningrad, Leningrad State Univ. Publ.. , 1953 (in Russian) 2. Lyatsky W., and A. Tan, Latitudinal effect in semiannual variation of geomagnetic activity, J. Geophys. Res., 108(A5), 1204, doi:10.1029/2002JA009467, 2003

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