ISSN 00167932, Geomagnetism and Aeronomy, 2011, Vol. 51, No. 6, pp. 730–740. © Pleiades Publishing, Ltd., 2011. Original Russian Text © N.G. Kleimenova, O.V. Kozyreva, J. Manninen, T. Raita, T.A. Kornilova, I.A. Kornilov, 2011, published in Geomagnetizm i Aeronomiya, 2011, Vol. 51, No. 6, pp. 746–756.
HighLatitude Geomagnetic Disturbances during the Initial Phase of a Recurrent Magnetic Storm (from February 27 to March 2, 2008) N. G. Kleimenovaa, d, O. V. Kozyrevaa, J. Manninenb, T. Raitab, T. A. Kornilovac, and I. A. Kornilovc a
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Bol’shaya Gruzinskaya ul. 10, Moscow, 123995 Russia b Sodankyla Geophysical Observatory, Tähteläntie 62, Sodankyla, 99600 Finland c Polar Geophysical Institute, Apatity Division, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, 184209 Russia d Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997 Russia email:
[email protected] Received March 28, 2011
Abstract—A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a pro longed period of negative IMF Bz values, and the most intense wavelike disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with Np reaching 30 cm–3) in the solar wind under the southward IMF in the sheath region of the highspeed solar wind stream responsible for the discussed storm caused a great (the AE index is ~1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ~ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF Bz resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere. DOI: 10.1134/S0016793211060077
1. INTRODUCTION In spite of the fact that magnetic storms have been studied for more than 100 years, many important problems of the physics of their development are still unknown. There is no consensus on even such a simple problem as that related to distinguishing the phases in the development of a magnetic storm. According to wellknown classical concepts (see, e.g., (Akasofu and Chapman, 1972)), a typical magnetic storm includes three phases: initial, main, and recovery. The initial phase is characterized by positive values of the Dst index caused by increased solar wind dynamic pres sure and a dense transient in the sheath region of a highspeed solar plasma stream that approached the Earth’s orbit. The collision between the magneto sphere and bow shock causes a sudden commence ment (SC) of a magnetic storm, which is most dis tinctly registered in the dayside mid and lowlatitude sector. Intense geomagnetic disturbances and Pc5 geo magnetic pulsations (f ~ 2–7 mHz) are usually observed in the dayside polar sector immediately after the SC, when the Earth is in the region of a com
pressed turbulent front of an interplanetary magnetic cloud (see, e.g., (Schott et al., 1998, Manninen et al., 2002; Kozyreva et al., 2004)). At the same time, many researchers (e.g., Loewe and Prolss (1977)) altogether did not consider the magnetic storm initial phase, especially in the case of recurrent magnetic storms with a gradual commence ment, because they considered that the initial phase did not affect the further development of the storm main phase. However, physical processes in the Earth’s magnetosphere during the storm initial and main phases are fundamentally different and develop in different magnetospheric domains. The magnetic storm initial phase mostly develops when the IMF is northward and is accompanied by an increase in the solar wind dynamic pressure. As was mentioned in earlier works (e.g., (Akasofu and Chap man, 1972)), most intense geomagnetic variations and pulsations with periods of several minutes are observed in the dayside sector of polar latitudes during the storm initial phase, which is usually related to the direct pen etration of hydromagnetic waves from the solar wind.
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In the magnetic storm main phase, which develops when the IMF turns southward and the solar wind dynamic pressure decreases, the main geomagnetic disturbances are as a rule observed in the dusk and nighttime sectors of auroral latitudes. Even proceed ing from these differences, it is reasonable to distin guish the magnetic storm initial phase as an indepen dent storm development phase. The storm initial phase duration varies from several minutes to 8–24 h, depending on the solar wind characteristics and the dimensions of the dense plasma region in the sheath region of a highspeed stream (in the case of the so called CIR storms) or a magnetic cloud (CME storms). The aim of this work is to study the complex of phe nomena, such as geomagnetic pulsations in different frequency ranges, riometer absorption, VLF emis sions, and auroras in the initial phase of a small recur rent magnetic storm that occurred on February 27– March 2, 2008, at a solar activity minimum. The dif ference between this and other typical magnetic storms consisted in that its initial phase developed under a prolonged period when IMF Bz was negative, and the most intense wavelike disturbances during the storm initial phase were observed in the dusk and nighttime sectors of the magnetosphere rather than in the daytime sector. 2. OBSERVATION RESULTS AND DISCUSSION It is well known that a solar activity minimum is characterized by moderate and weak (small values of the Dst index) recurrent magnetic storms caused by highspeed solar wind streams from coronal holes (CIR storms). A long 27day series including 14 mag netic storms (the considered storm is the third event in this series) was observed in 2008. 2.1. We consider highlatitude geomagnetic wave like disturbances in the initial phase of the discussed storm (from February 27 to March 2, 2008). This storm occurred when a highspeed solar wind stream from a large recurrent coronal hole crossed the Earth, which was observed on February 25 in the Western Hemisphere of the visible solar disk. The specific fea ture of this magnetic storm consisted in that the initial phase, observed under small negative values of IMF Bz, was relatively prolonged, as a result of which an intense substorm developed in the nighttime magneto sphere. The hourly variations in the parameters of the IMF (Вх, By, and Bz) and the solar wind (velocity V, density Np, and dynamic pressure Р) are shown in Fig. 1a (OMNI data, http://nssdcftp.gsfc.nasa.gov/spacecraft_data/omni/) for the period from February 24 to March 5. The vari ations in the Dst and AE indices are also presented in the figure. It is clear that this storm occurred after a rather prolonged magnetically quiet period when IMF Bz was negative. The storm commencement is related GEOMAGNETISM AND AERONOMY
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to an abrupt increase in the solar wind density on Feb ruary 27. The 1min variations in the IMF and solar wind parameters during the magnetic storm initial phase on February 27 are shown in more detail in Fig. 1b, which indicates that the storm initial phase was characterized by a considerable increase in the dynamic pressure (up to 8 nPa) and by abrupt increases in the solar wind velocity. A sudden change in the solar wind dynamic pressure at about 15 UT caused impulse Si (SC?), which was observed on the Earth’s surface as a positive impulse in the closed magnetosphere and as a negative impulse at polar latitudes. 2.2. A dense transient with Np reaching 30 cm–3 was registered in the solar wind near the Earth’ orbit at 1630–1730 UT, and a substorm with a maximal value of the AE index of about 1250 nT started in the mag netosphere at ~1845 UT (Fig. 1b). In the nightside magnetosphere, the substorm was simultaneously observed in a large interval of geomagnetic latitudes (Φ ~ 56–76°) on the Scandinavian profile of IMAGE stations. These data are shown in Fig. 2a. Comparing the variations in the X and Z components of the field, we can conclude that an explosive substorm started at about 1845 UT between the BJN and SOR observato ries closer to SOR. Then, a disturbance started, very rapidly propagating northward, which can be inter preted as a rapid disturbance motion from the explo sion source toward the magnetotail. The poleward displacement of magnetic activity was accompanied by the excitation of geomagnetic pul sations in the 2–7 mHz band (the Рс5–Pi3 range) at polar latitudes (BJN–LYR), which is shown in Fig. 2b. It was previously established (e.g., (Manninen et al., 2002; Kozyreva et al., 2004; Kozyreva and Kleimen ova, 2010)) that wave activity is maximal in the day time sector of polar latitudes during the magnetic storm initial phase, which results from the penetration of wave turbulence from the solar wind. In contrast to this, the wave activity at frequencies of 2–7 mHz was maximal in the nighttime sector in the discussed time interval of the storm initial phase (19–20 UT). To confirm this, in Fig. 2c, we present a map of the global spatial distribution of geomagnetic pulsation ampli tudes at 1900–1945 UT, constructed in the geomag netic latitude–geomagnetic local time coordinates. It is evident that the most intense pulsations were observed at geomagnetic latitudes higher than 70° in the nightside magnetosphere (~19–04 MLT). A comparison of the groundbased observations with the IMF variations (Fig. 2d) indicated that intense pulsations in the By and Bz components were registered after 1930 UT in the IMF and before ~1930 UT on the Earth’s surface (Fig. 2b). Consequently, we can assume that the source of the excited oscillations reg istered on the Earth’s surface before 1930 UT was probably located in the remote magnetotail rather than in the IMF. 2011
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The position of the SOD, SOR, and BJN projec tions at 19 UT was calculated according to the T89 Tsyganenko model (http://modelweb.gsfc.nasa.gov/ models/cgm/t89.html). The calculations indicated that SOD was projected in the dusk magnetosphere onto the point with the coordinates X = –6.1 Re and Y = +5.6 Re; SOR, onto the point with Х = –15.2 Re and Y = +12.0; and BJN, deep in the magnetotail onto the point with X = –42.8 and Y = +19.5. It was useful to consider the registration data on the THEMIS satel lites in order to understand the situation in the magne totail in the discussed time interval. Unfortunately, at 18–21 UT, the THD and THE THEMIS satellites were located too close to the Earth, and the THC and
THB satellites were in the tail lobes at a large azi muthal distance (larger than 10 Re). The THA satel lite was in the dusk sector within the magnetosphere in the region with the coordinates X = –(4–9) Re, Y = +(4–5) Re and Z = –(1–0)Re, i.e., it was close to the SOD projection. The THA satellite was at distances of X = –5.3, –6.4 Re at 19 and 20 UT, respectively. Figure 3 presents the magnetic field variations detected on THA at 18–20 UT. Intense bursts of pul sations in the Bx and By field components were observed on this satellite at 1850 UT, i.e., after the onset of the explosive substorm. Such a delay is not surprising, since the satellite was deeper in the mag netosphere at that time than the SOD projection and
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the explosion region in the magnetotail. Judging by the ground data, the substorm source was supposedly at a distance of about 20 Re (Fig. 2a). The intense geo magnetic pulsations, which were observed at ~1900– 1930 UT at polar latitudes on the Earth (Fig. 2b), were not registered on THA, apparently, because the satel lite was closer to the Earth than the source of these oscillations. In the dayside magnetosphere, a considerable burst of Pc5 geomagnetic pulsations in all field components was registered at 1950–2020 UT (the discussed time interval) on the GOES11 geostationary satellite (Fig. 4a). On geostationary satellites near the equato rial plane, the Hp, He, and Hn components corre spond to the fieldaligned, radial, and azimuthal directions, respectively. Consequently, oscillations in the magnetosphere had both toroidal and poloidal GEOMAGNETISM AND AERONOMY
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field components. Similar Pc5 bursts were simulta neously observed on the Earth’s surface in the prenoon sector at the YKC observatory (Φ = 69.0° and Λ = 293.3°), which was located near the GOES11 satellite projection on the Earth’s surface (Fig. 4b), and in the nearmidnight time at SOD (Fig. 4c). Consequently, these oscillations were global; their amplitude in the dayside sector was almost three times as large as in the nightside sector. The global character of the oscilla tions is evident on the map of the pulsation amplitude planetary distribution at 1945–2030 UT presented in Fig. 4d. In the magnetic storm main phase, daytime global Pc5 pulsations were previously observed during only individual great magnetic storms, e.g., on March 24, 1991 (Fujitani et al., 1993; Schott et al., 1998). 2.3. During this storm, specialists from Finland registered ground VLF emissions at a remote point 2011
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band. At 2000 UT, the chorus spectrum abruptly changed toward higher frequencies, which could be caused by a displacement of the chorus source to lower L shells. Note that such a displacement could have been caused by an insignificant abrupt change in the solar wind velocity and density (i.e., dynamic pressure), which was registered at about 20 UT near the magnetospheric boundary according to the OMNI data (Fig. 1b).
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near the Sodankyla–Kannuslehto (KAN) observato ries (geographic coordinates ϕ = 67.74° N and λ = 26.27° E, geomagnetic coordinates Φ = 64.2° and Λ = 107.9°, L = 5.3). An analysis of the VLF observations indicated that a short burst of auroral hiss, accompa nied by pulsed bursts of Pi1B geomagnetic pulsations, was observed at the onset of the substorm (~1845 UT) (Fig. 5). An allsky camera (ASC) at SOD registered the development of an auroral breakup at that time (the top part of Fig. 5a). An analysis of VLF wave polarization indicated that clockwise circular polar ization was typical of VLF hiss. According to theoret ical concepts (e.g., (Yearby and Smith, 1994)), this results from the fact that the exit of VLF waves from the ionosphere is located near the surface registration point and apparently coincides with the region of the auroral breakup development in this case. The gener ation of Pi1B geomagnetic pulsation bursts is usually related to auroral electron precipitation into the iono sphere during the substorm expansion phase (Troitskaya and Kleimenova, 1972), when field aligned currents directed from the ionosphere are enhanced. Approximately 50 min after the auroral hiss burst, a burst of VLF hiss was registered on the Earth’s surface (the middle part of Fig. 5b). An analysis of the polar ization of these waves indicates that both clockwise and counterclockwise polarization is registered for hiss, which can be caused by the simultaneous arrival of signals from different distances. The chorus inten sity was maximal at 1935–2000 UT in the ~0.5–1.5 kHz
In addition, a beginning of Pc1 geomagnetic pulsa tions in the 0.6–1.5 Hz band with a spreading dynamic spectrum was registered at 20 UT at SOD (the lower part of Fig. 5c), which indicates that the nonlinear stage of the ion cyclotron instability of energetic pro tons developed in the Earth’s magnetosphere (Feigin et al., 2009). It is well known that Pc1 pulsations can be generated in the magnetosphere in the regions of the dusk plasmapause plume or cold plasma clouds detached from the plasmapause. However, the dis cussed time interval corresponds to the nighttime rather than dusk localtime hours. Spasojevic et al. (2003) indicated that density variations due to the corotation of plasma plumes with the plasmasphere could also be observed in the nighttime at negative IMF Bz in the plasmapause spatial structure at L ~ 4.5–5.5. Precisely such a situation was possibly observed in our case. Chorus emissions are typical of the dawn sector of the magnetosphere, and it has long been known (e.g., (Kleimenova et al., 1970; Smith et al., 1996)) that these emissions are closely related to substorms that develop in the nightside magnetosphere. However, in the discussed case, the burst of intense chorus was observed in the premidnight time, which is atypical of “classical” chorus emissions. We assumed that chorus emissions were simultaneously generated in a long longitudinal interval in this case. Such an assumption is confirmed by the fact that chorus simultaneously appeared on the Yakutsk meridian (private communi cation by V. A. Mullayarov), i.e., in the dawn sector (~04 MLT). Many works (e.g., (Bortnik and Thorne, 2007)) indicate that the generation of chorus in the magneto sphere results in pitch angle scattering and precipita tion of energetic particles, which is registered as an increase in riometer absorption on the Earth’s surface. Figure 6 presents the data of riometer observations at three auroral stations (L ~ 5) spaced in longitude: Sodankyla (SOD, Φ = 63.80°, Λ = 107.7°), Tixie Bay (TIX, Φ = 65.3°, Λ = 196.3°), and Gakona (GAK, Φ = 63.1°, Λ = 267.2°). S. N. Samsonov politely presented data from the Tixie Bay observatory for us. When the dis cussed chorus burst was generated (~20 UT), these sta tions were located in the nighttime (SOD, ~23 MLT), early morning (TIX, ~04 MLT), and prenoon (GAK, ~10 MLT) sectors of the magnetosphere. It is clear that the time variations in riometer absorption are in good agreement at all stations, and this is especially evident at about 20 UT. Therefore, we can assume that
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VLF chorus emissions were simultaneously generated in a very long longitudinal interval of auroral latitudes (from the premidnight to prenoon hours of geomagnetic local time) in the magnetosphere at 1930–2030 UT. Note that Pc5 pulsations were globally generated precisely in this time interval.
2.4. The results of optical (ASC) observations in northern Scandinavia indicate that intense auroras were observed from 1840 to 2100 UT during the initial phase of the discussed storm. After the breakup observed at SOD at 1844 UT (the middle frame in the Fig. 5a), auroras started rapidly moving northward and
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intense auroras appeared at the NAL polar observatory (Φ = 76.1°) at about 19 UT according to keograms (these data are not presented here). Intense auroras were observed at NAL up to ~1930 UT, i.e., in the same time interval when intense geomagnetic pulsa tions in the 2–7 mHz band (Fig. 2b) were registered at polar latitudes (HOR–NAL). TV observations of auroras were also performed at the Loparskaya observatory (LOP, Φ = 64.94°, Λ = 113.6°) at that time. An analysis of these observations, performed using a special datafiltering program developed by I. A. Kornilov (Kornilov et al., 2008), made it possible to study the fine time features of auro ras. Figure 7a presents several characteristic TV frames demonstrating the time variable regimes of optical pul sations. Initial TV (unfiltered) ASC frames are shown in the upper raw, and these frames after filtering are pre sented below. It is evident that only filtering made it possible to observe the changes in the regime of pulsat ing auroras developing under diffuse luminosity. Figure 7a indicates that largescale pulsating patches filled the entire field of view of the TV camera and moved southward at 1915:29–1923:15 UT. The pulsating structures gradually changed their forms, GEOMAGNETISM AND AERONOMY
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decreased (1932:08 UT), and were subsequently trans formed into pulsating arcs, which were oriented from east to west and drifted southward (frame 2010:19). At that time, the emission frequency band up to ~2.2 kHz gradually increased at SOD in the VLF chorus (Fig. 5b), and global Pc5 pulsations were registered in the magnetic field. At 2022:08 UT, the auroras started intensifying again in the northern field of view of the TV camera. This corresponds to the appearance of a pulsed burst of 2–7 mHz geomagnetic pulsations at the SOR observa tory (Φ = 67.2°). Then, the pulsating auroral arcs shifted southward (frames 2026:35–2045:48 UT). Such a change in the regime of the pulsating auroras (as well as the appearance of VLF chorus) is more typ ical of dawn hours than of dusk ones. The results of the conducted photometric measure ments with the following digital filtering make it pos sible to trace the dynamics of the optical pulsation periods. We selected two ranges of periods: 20–60 s (Fig. 7b) and 20–360 s (Fig. 7c). The vertical arrows mark the instants corresponding to the TV frames in Fig. 7a. The region of photometric measurements is marked by an open square within the TV frame field of 2011
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view in the upper left corner in Fig. 7. The magnetic pulsations registered at the Lovozero observatory (LOZ, Φ = 64.22°, Λ = 114.6°) in the same range of periods are presented below each plot for optical pul sations. The most intense optical pulsations in the 20–60 s range of periods were observed at ~1920–2030 UT (Fig. 7b); i.e., when a burst of VLF chorus was regis tered at SOD (Fig. 5b). Higher frequency oscillations were registered at that time in the magnetic field. The spectrograms of geomagnetic pulsations, obtained using a fluxgate magnetometer at SOD (Fig. 5c), con firmed that the VLF chorus emissions were accompa nied by Pi1C geomagnetic pulsations (as is usually observed), which were most intense at ~1920– 2030 UT. An enhancement of geomagnetic pulsations at this time interval is also observed in Fig. 7b. The same situation was also detected in the lower frequency range (Fig. 7c). Irregular variations with periods of about 10 and 5 min were observed in the optical and magnetic pulsations, respectively. Thus, the observations indicated that there was no similarity between the variations in the optical and magnetic pulsations in the ranges of 20 to 60s (Fig. 7b) and 20 to 360s (Fig. 7c) periods. However, we note that it is difficult to anticipate a good correlation between these phenomena, since optical pulsations are a local phe nomenon, pulsating auroras are caused by pulsating fluxes of precipitating electrons with energies of 10– 50 keV; at the same time, ionospheric currents, the spatiotemporal characteristics of which can substan tially differ from the parameters characterizing precip itating electron fluxes, make the decisive contribution to magnetic pulsations. 3. CONCLUSIONS An analysis of the wavelike geomagnetic distur bances during the initial phase of a weak recurrent magnetic storm (from February 27 to March 2, 2008) indicated that, in contrast to typical magnetic storms, the most intense wavelike disturbances during the storm initial phase were observed in the dusk and nightside sectors of the magnetosphere rather than in the dayside sector as is observed in the majority of cases. An analysis of the observations at stations spaced in longitude indicated that Pc5 pulsations, bursts of precipitating energetic electrons (riometer absorption), and VLF chorus were simultaneously registered in a wide sector of longitudes—from premidnight to prenoon hours. Such a situation apparently originated because negative IMF Bz values, under which a dense transient (with Np reaching ~30 cm–3) in the sheath region of a highspeed solar wind stream resulted in the develop ment of a strong magnetospheric storm with AE of GEOMAGNETISM AND AERONOMY
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