COMPARISON OF PC5 GEOMAGNETIC PULSATIONS ASSOCIATED WITH CIR-DRIVEN AND CME-DRIVEN STORMS. O. V. Kozyreva and N.G. Kleimenova

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1 COMPARISON OF PC5 GEOMAGNETIC PULSATIONS ASSOCIATED WITH CIR-DRIVEN AND CME-DRIVEN STORMS O. V. Kozyreva and N.G. Kleimenova Institute of Physics of the Earth RAS, B. Gruzinskaya 10, Moscow , Russia, Abstract. The dayside Pc5 pulsations activity has been analyzed during the selected succession of 10 recurrent magnetic storms (CIR-driven storms) and compared with Pc5 pulsations associated with CME-driven storms in The ULF-index, as a measure of the wave level activity in the Pc5 frequency range of 2-7 mhz, has been calculated according to the ground-based 1-min globally distributed observations in the pre-noon (03-12 MLT) and afternoon (12-18 MLT) sectors at auroral (60-70º) and polar (70-90º) geomagnetic latitudes. It was found that in all phases of the CIR-driven magnetic storms under consideration, the enhanced ULF-activity was found both at the auroral and polar latitudes. The daytime polar zone ULF intensity was of the same order or even higher (in the storm initial phase) then at the auroral zone. As usual, in the main phase of the studied CIR-driven storms, the value of the solar wind velocity was very large (more 600 km/c), which could provide the Kelvin-Helmholz instability progression and, correspondingly, the Pc5 ULF wave generation. The ULF-index, calculated for the fluctuations in the Interplanetary Magnetic Field (IMF), indicated the enhancement of the strong ULF waves in the initial and main phases of the analyzed magnetic storms. We assume that some of these fluctuations could penetrate into the open polar cap and provide an enhancement of the dayside ULF geomagnetic pulsations at the high latitudes. The comparison of the ULF-activity during the CIR-driven storms (typical for the solar activity minimum) and CME-driven storms (typical for the solar activity maximum) showed that the ULF-activity associated with the CIR-driven storms was observed at the higher latitudes then in the CME-driven storms. Introduction It is known [e.g. Tsurutani et al., 2006] that in the years of the solar activity minimum, high-speed streams (HSS) from the coronal holes are the dominant solar phenomena affecting geomagnetic activity. When HSS overtakes slow-speed stream, the interaction of the flows with different velocities forms a region of compression plasma, so called the corotating interaction region (CIR). In the maximum of the solar activity the most geoeffective structures of the solar wind, which cause the magnetic storms, are the coronal mass ejection (CME) [e.g., Yermolaev and Yermolaev, 2002; Borovsky and Denton, 2006]. For the brevity, the magnetic storms, caused by CIR, we will call CIR-storms, and the storms, caused by CME - CMEstorms. A lot of papers are devoted to the investigation of the differences between CME and CIR storms. For instance, [Borovsky and Denton, 2006] gave 21 differences between CME- and CIR-storms. They noted that in CIR-storms there are observed the more prolonged ULF pulsations related with the more prolonged time intervals of the high solar wind velocity. Some authors [e.g., Singer et al., 1977, Rostoker et al., 1998; Engebretson et al., 1998; Mathie and Mann, 2000, 2001; O' Brien et al., 2001] also showed that both the amplitude and the probability of the ULF waves in the range of the Pc5 pulsation (f~2-7 mhz) grows with an increase of solar wind velocity. In our previous works [Kozyreva and Kleimenova, 2004; 2008, 2009], we showed that the initial phase of a CME storm is characterized by excitation of long-period geomagnetic pulsations of the Pc5 6 range (T ~ 5 20 min) with the maximum amplitude in the morning sector of the polar cap. Penetration and nonlinear transformations of fluctuations in the IMF and solar wind parameters at corresponding periods in the front of the interplanetary magnetic cloud is likely the main agent exciting geomagnetic pulsations in the polar cap. A sharp decrease in the amplitudes of Pc5 pulsations is observed near the projection of the boundary of open and closed magnetic field lines. During the main phase of CME-storms, the Pc5-6 wave activity shifted into the region of the closed magnetosphere. The aim of this work is to study the global spatial distribution of dayside Pc5 geomagnetic pulsations during the different phases of the CIR-storms, developing in the period of minimum of the 23rd solar cycle.

2 Data and analysis results Ten successive recurrent storms (Fig. 1) in 2006 (the year of the minimum of the 23rd solar activity cycle) were chosen for the analysis. The list of the analyzed storms is given in the Table, where for each storm, we show the minimum of the Dst values, the solar wind density prior to the storm beginning (N 0 ) and its maximum (N max ) in the compression region, the minimum value of the solar wind speed in the compression region (V min ) and the maximum (V max ) of the velocity of HSS. This table demonstrates that in spite of the high values of the density (up to 65 cm 3) and the velocity ( km/s) of the solar wind, the values of the Dst-index of the analyzed CIR storms are low. For comparison, the strong CME storm of December 14 17, 2006, with Dst = 145 nt caused by extremely strong flares on the Sun (from December 5 13, 2006) was analyzed. Table. Characteristics of the analyzed storms Date Dst min N 0 N max V min V max nt см -3 см -3 km/s km/s Fig.1. Variations in the Kp index in 2006; ten recurrent storms (CIR storms) and one CME storm are identified In the analysis of the ground-based Pc5 activity we used the ULF-index [Kozyreva et al, 2007], characterized the intensity of the pulsations in the range of 2-7 mhz, and calculated from 1-min data of the global magnetometer network in the northern hemisphere. ULF index was calculated separately for pre-noon ( MLT) and afternoon ( MLT) sectors of two latitude zones, namely, the polar zone (geomagnetic latitudes of ) and the auroral one (geomagnetic latitudes of ). Note that such a division is conventional since with magnetic activity increasing the auroral oval shifts to lower latitudes and the observatories that formally belonged to the polar zone can shift to the auroral zone. As an example, we consider Pc5 pulsations during the CIR storm of February 18 23, Variations in the solar wind velocity and density, Bz component of the IMF, Dst and AE indices are shown in the upper part of Fig. 2. The lower part of Fig. 2 shows ULF index variations in the pre-noon ( MLT) and afternoon ( MLT) sectors for two latitude zones, namely, 70 90, this being the polar zone in our terminology, and 60 70, i.e., the auroral one. Arrows in the figure denote the ULF index maxima at the initial (interval 1), main (intervals 2), early recovery (interval 3) and late recovery (interval 4) storm phases. The global Pc5 intensity (nt/mhz 1/2 ) distribution maps (Fig. 2b) were constructed for the time intervals of the identified maxima in the geomagnetic latitude magnetic local time (MLT) coordinates. It is seen in Fig. 2b (upper plot) that the region of maximum ULF activity during the initial storm phase (interval 1) of this magnetic storm (February 19, UT) was observed in the pre-noon sector at latitudes of ~ This burst of wave activity at polar latitudes is also clearly seen in the plots of the ULF index (Fig. 2a). During the main phase of the magnetic storm (February 20 21, intervals 2), the intensity of Pc5 pulsations increased both at the polar and auroral latitudes (Fig. 2a, lower plots). The maximal ULF indices were observed in the also pre-noon sector both of polar and auroral latitudes. At that, a well defined correlation between the ULF bursts and the AE index increase was noted. It is apparent from the maps of the global distribution of Pc5 pulsations (Fig. 2b, second plot) that the region of increase in the Pc5 pulsation intensities was observed in the pre-noon sector at the geomagnetic latitudes of ~ During the recovery phase of this magnetic storm (February 22), the solar wind velocity still remained sufficiently high (~550 km/s), the

3 values of the Dst index were on the order of 15 nt, and the auroral activity continued (the AE index reached nt). During this storm phase, the maximum of Pc5 pulsation intensity was observed in the pre-noon sector at latitudes of (Fig. 2b, lower plots), i.e., at the latitudes higher than 70, as is the cases with other storm phases. Fig.2. Magnetic storm of February 18 23, 2006 (CIR storm): (a) variations in the velocity (V) and density (N) of the solar wind, Bz component of the IMF, Dst, and AE indices, as well as the ULF index for the auroral and polar zones in the pre-noon and afternoon sectors; (b) global maps of the spatial distribution of ULF activity during the initial, main, and recovery phases of this storm. The similar Pc5 pulsations distribution was found in all considered CIR-storms: the storm initial phase is characterized by the Pc5 enhancement in the pre-noon sector of the polar latitudes; the storm main phase - the Pc5 enhancement in the pre-noon and afternoon on the polar latitudes, the pulsations amplitude grows with the AE-index; in the storm recovery phase the most intensive Pc5 pulsations are observed also in the prenoon sector of the polar latitudes. Thus, during all phases of CIR-storm the most intensive Pc5 pulsations are observed on the latitudes higher than 70º and predominantly in the pre-noon time.

4 Discussions Let us compare the space-temporal characteristics of Pc5 pulsations during CIR and CME storms. Earlier, we [Kozyreva and Kleimenova, 2008; 2009] revealed the following typical characteristics of ULF activity during strong and moderate magnetic storms initiated by the approach of interplanetary magnetic clouds to the Earth, i.e., using the terminology of the present paper, CME storms. It was shown that the initial phase of a CME storm is characterized by excitation of intense geomagnetic pulsations in the frequency range of 1 6 mhz in the daytime sector of the polar cap, whose wavelet structure and IMF oscillations are, as a rule, similar, which points to possible penetration of waves from the solar wind. During the main phase of a CME storm, the most intense pulsations were observed during the development of substorms that accompanied the main storm phase. However, the maximum amplitudes of oscillations were observed in the morning sector of the auroral zone rather than in the nighttime sector where substorms usually initiate. It was shown that the main contribution to the daytime wave activity during the main magnetic storm phase is geomagnetic pulsations of the recovery phase of these substorms. Fig. 3. The same as in Fig. 2 for the magnetic storm of December 14 16, 2006 (CME storm).

5 During the recovery phase of a CME storm, the ULF indices in the auroral zone drastically decrease, especially during a transition from an early stage of the recovery storm phase to the late one. At the early stage of recovery, at sharper positive gradients of the Dst index, maximum wave activity is observed, as earlier, in the morning sector of the auroral zone. As a rule, these are classical quasi-periodic morning Pc5 pulsations of the magnetic storm recovery phase [Troitskaya et al., 1965] of a resonant nature. Note, that during the late stage of the CME storm recovery phase at Bz > 0, inflow of energy stops and the wave processes are the result of a magnetospheric response to the energy stored during the previous main phase of the storm. Similar space-temporal characteristics of Pc5 pulsations were observed during the strong magnetic storm of December 14 16, 2006 (Figs. 3) caused by a set of powerful flares at the Sun. The conditions in the solar wind and Bz of the IMF during this storm are demonstrated in Fig. 3. The solar wind velocity was very high (~900 km/s), while the maximum density of the solar wind was low as compared to the previous CIR storm (~10 cm 3 ). In contrast to the discussed above CIR storms (Fig. 2), the solar wind velocity was high during the storm initial phase (December, 14). At the initial phase of this storm, as in the case of CIR storms (Figs. 2b), maximum wave activity was observed in the pre-noon and near noon sectors (Fig. 3a, interval 1) at latitudes of ~72 76, which is slightly lower as compared to the CIR storms considered above (~74 78 ). During the main phase of this storm (December 15), the maximal ULF indices were at least three times as high as in CIR storms. The region of the most intense Pc5 pulsations was observed, as in the case of CIR storms, in the pre-noon sector but at an earlier time (before 0900 MLT) and at latitudes lower than 70 (Fig. 3b, interval 2). The AE indices at this time were twice as high as those during the CIR storms. At the early stage of the recovery phase of this storm (December, 15), the space distribution of Pc5 pulsations (Fig. 3b, interval 3) were the same as those during the main storm phase but at latitudes of ~70. During the late stage of the recovery (Fig. 3b, interval 4), Pc5 pulsations were recorded in the pre-noon and near noon sectors of polar latitudes (Fig. 3b, interval 4) as in CIR storms. By comparing the determined space-temporal regularities of daytime Pc5 pulsations during different phases of CIR storms with the above mentioned characteristics of these oscillations during CME storms, we can conclude that the main difference is primarily attributed to the higher latitude position of the regions of increased ULF wave intensity during CIR storms. The observed regularities can be interpreted as swollen magnetospheres in the solar activity minimum even during CIR storms. Thus, the main distinction of geomagnetic pulsation during CIR and CME storms is related to the Pc5 pulsations intensity and latitudinal position of the maximum of ULF wave activity: the amplitude of Pc5 waves in CIR storms was several times lower and they were observed at higher latitudes. The most important factor in the analyzed CIR storms is the high velocity of the solar wind (>600 km/s), which can lead to development of the Kelvin Helmholtz instability in the magnetopause and enhanced generation of ULF waves in the Earth s magnetosphere due to field line resonance. An increase in the amplitude of Pc5 pulsation with an increase in the solar wind velocity was noted in many works, e.g., [Engebretson et al., 1998]. Conclusion The analysis of the daytime ULF activity during ten recurrent storms (CIR storms) in the minimum of the 23rd solar activity cycle (2006) showed that the high solar wind velocity (over 600 km/s) facilitates the generation of ULF waves in the Earth s magnetosphere, which can likely be attributed to the development of the Kelvin Helmholtz instability in the magnetopause. The most intense daytime Pc5 geomagnetic pulsations during all phases of CIR storms on the Earth s surface were mainly observed in the pre-noon sector at latitudes higher than 70, these latitudes being lower than 70 during CME storms. Comparison of the wave activity during CIR and CME storms showed that the amplitude of Pc5 pulsations during CIR storms is substantially lower and the spectral maximum is observed at lower frequencies and at higher latitudes A higher latitude position of the maximum of Pc5 pulsations intensity in CIR storms points to larger dimensions of the daytime magnetosphere during CIR storms as compared to CME storms. Acknowledgments The study was supported by the Russian Foundation for Basic Research, project no , and partly by the Program no.4 of the Presidium of the Russian Academy of Sciences.

6 References Borovsky, J.E. and M.H. Denton (2006), Differences between CME-Driven Storms and CIR-Driven Storms, J. Geophys. Res., 111, A07S08. Engebretson, M.J., K.H. Glassmeir, and M. Stellmacher (1998), The Dependence of High_Latitude Pc5 Power on Solar Wind Velocity and Phase of High_Speed Solar Wind Streams, J. Geophys. Res., 103, Ermolaev, Yu.I. and M.Yu. Ermolaev (2002), On Certain Statistical Interactions between Solar, Interplanetary, and Geomagnetospheric Disturbances in , Kosm. Issled., 40 (1), Kozyreva, O., V. Pilipenko, M.J. Engebretson, K. Yumoto, J. Watermann, and N. Romanova (2007), In Search of a New ULF Wave Index: Comparison of Pc5 Power with Dynamics of Geostationary Relativistic Electrons, Planet. Space Sci., 55, Kozyreva, O.V. and N.G. Kleimenova (2008), Estimation of Storm-Time Level of Day-Side Wave Geomagnetic Activity Using a New ULF Index, Geomagn. Aeron., 48(4) Kozyreva, O.V. and N.G. Kleimenova (2009), Variations in the ULF Index of Geomagnetic Pulsations during Strong Magnetic Storms, Geomagn. Aeron., 49(4), Kozyreva, O.V., N.G. Kleimenova, and J.-J. Shott (2004), Geomagnetic Pulsations at the Initial Phase of a Magnetic Storm, Geomagn. Aeron., 44(1), Mathie, R.A. and I.R. Mann (2000), A Correlation between Extended Intervals of ULF Wave Power and Storm-Time Geosynchronous Relativistic Electron Flux Enhancements, Geophys. Res. Lett., 27, Mathie, R.A. and I.R. Mann (2001), On the Solar Wind Control of Pc5 ULF Pulsation Power at Mid- Latitudes: Implications for MeV Electron Acceleration in the Outer Radiation Belt, J. Geophys. Res., 106, O'Brien, T.P., R.L. McPherron, D. Sornette, G.D. Reeves, G.D. Friedel, and H.H. Singer (2001), Which Magnetic Storms Produce Relativistic Electrons at Geosynchronous Orbit?, J. Geophys. Res., 106, Rostoker, G., S. Skone, and D.N. Baker (1998), On the Origin of Relativistic Electrons in the Magnetosphere Associated with Some Geomagnetic Storms, Geophys. Res. Lett., 25, Singer, H.J., C.T. Russell, M.G. Kivelson, E.W. Greenstadt, and J.V. Olson (1977), Evidence for the Control of Pc 3.4 Magnetic Pulsations by the Solar Wind Velocity, Geophys. Res. Lett., 4(9), Troitskaya, V.A., M.V. Mel nikova, O.V. Bol shakova, D.A. Rokityanskaya, and G.A. Bulatova (1965), Fine Structure of Magnetic Storms, Izv. Akad. Nauk SSSR, Ser Fiz., 49(1), Tsurutani, B.T., W.D. Gonzalez, A.L.C. Gonzalez, F.L. Guarnieri, N. Gopalswamy, M. Grande, Y. Kamide, Y. Kasahara, G. Lu, I. Mann, R. McPherron, F. Soraas, and V. Vasyliunas (2006), Corotating Solar Wind Streams and Recurrent Geomagnetic Activity: A Review, J. Geophys. Res., 111, A07S01.