ISSN 00167932, Geomagnetism and Aeronomy, 2009, Vol. 49, No. 6, pp. 741–749. © Pleiades Publishing, Ltd., 2009. Original Russian Text © O.V. Kozyreva, I.N. Myagkova, E.E. Antonova, N.G. Kleimenova, 2009, published in Geomagnetizm i Aeronomiya, 2009, Vol. 49, No. 6, pp. 777–785.
Precipitation of Energetic Electrons and Pi3 Geomagnetic Pulsations at Polar Latitudes O. V. Kozyrevaa, I. N. Myagkovab, E. E. Antonovab, c, and N. G. Kleimenovaa, c a
Institute of Physics of the Earth, Russian Academy of Sciences, Bol’shaya Gruzinskaya ul. 10, Moscow, 123995 Russia b Skobeltsyn Research Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia c Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997 Russia email:
[email protected] Received May 14, 2009
Abstract—Precipitation of electrons with energies of 0.3–1.5 MeV has been analyzed based on the CORO NALF satellite data at polar latitudes of the Northern Hemisphere on December 13, 2003. The instants of electron precipitation have been compared with the groundbased observations of geomagnetic disturbances and auroras near the satellite orbit projection. It has been indicated that precipitation of energetic electrons in the highlatitude nightside sector is accompanied by the simultaneous development of baylike magnetic field disturbances on the Earth’s surface and the appearance of riometer absorption bursts and Pi3 geomag netic pulsations, and auroras. PACS numbers: 94.30.Ny, 94.30.Ms DOI: 10.1134/S0016793209060061
1. INTRODUCTION Precipitation of electrons with energies reaching several tens–hundreds of kiloelectronvolts in the polar regions were repeatedly registered during different experiments on spacecraft (see, e.g., [Fritz, 1970; Imhof et al., 1992; Kuznetsov et al., 2000]). Thus, Kuznetsov et al. [2004] indicated that precipitation of energetic electrons (E > 0.5 MeV) on the CORONASI satellite in the nightside polar cap is observed immedi ately after the substorm registered in the auroral zone and is accompanied by an increase in riometer absorp tion at polar latitudes. However, a point localization of this precipitation was not considered in these experi ments. The aim of this work is to comparatively analyze the data on electron precipitation with energies of 300 keV–1.5 MeV, observed in the polar regions onboard the CORONASF satellite, the variations in the magnetic field, riometer absorption, and auroras, registered on the Earth’s surface near the satellite orbit projection. A comparison of the available observa tional data with the calculations of the auroral oval position, performed using the OVATION (Oval Varia tion, Assessment, Tracking, Intensity, and Online Nowcasting, http://sdwww.jhuapl.edu/Aurora/) model, makes it possible to specify the localization of observed precipitation.
2. EXPERIMENTAL DATA Fluxes of electrons with energies of 300–600 keV and 0.6–1.5 MeV were registered with a semiconduc tor telescope during the experiment on the CORO NASF satellite with the polar circular orbit (an alti tude of ~500 km, an inclination of 82.5°) [Kuznetsov et al., 2002]. To study magnetic disturbances and geomagnetic pulsations, we used the data of the groundbased observations at the INTERMAGNET global network of magnetometers, GREENLAND network of Greenland stations, and IMAGE Scandinavian net work. The work also used the data of Finnish riometers (30 MHz, ftp://sgodata.sgo.fi/pub_rio/) and the data of optical registration of auroras with allsky cameras (ASCs) operating in the MIRACLE system (Magne tometers–Ionospheric Radars–Allskycameras Large Experiment, http://www.space.fmi.fi/MIRA CLE/ASC/ASC_keograms/). 3. OBSERVATIONS The present work studied in detail energetic elec tron precipitation observed on December 13, 2003, in the northern polar region. We analyzed electron flux increases at L > 8 with a maximum exceeding three standard deviations of electrons with such an energy from the background flux. That day, CORONASF 15 times crossed the polar region of the Northern
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Hemisphere, and sporadic electron precipitation at L > 8, which was not registered during the previous and the next passes, was observed only in six cases. We should note that increases in fluxes of SCR electrons in the interplanetary space were not observed that day. We consider in more detail the conditions in the interplanetary space during the period before the ana lyzed day. Figure 1a presents the variations in the parameters of IMF and the solar wind (SW) on December 3–14, 2003. Figure 1a indicates that the Dst index could not restore its prestorm level during ten days after a small (Dstmin = –66 nT) magnetic storm, the main phase of which was observed on December 5. This could be related to the fact that a highspeed SW stream, the velocity of which reached approximately 800 km/s at a low SW density (lower than 2 cm–3) three days later (on December 11), approached the Earth on December 8. Figure 1 also indicates that the magnetic field value (B) was relatively small and con stant, and the field components varied insignificantly beginning from December 11. On the considered day (December 13, 2003), the SW velocity was 750– 850 km/s, the SW density was ~2 cm–3, IMF By and Bz varied from –5 to 6 nT, and Bx varied from –6 to 0 nT. It is known that the recovery phase of classical magnetic storms, developing when IMF Bz is positive, is characterized by the cessation of nighttime geomag netic disturbances and the generation of Pc5 geomag netic pulsations in the morning and dayside sectors of the magnetosphere. In the discussed case, substorm activity during the storm recovery phase did not cease upon the completion of the main phase and was observed for a rather long time (during several days) on December 5–14, 2003 (Fig. 1b). Figure 1b presents the variations in the AE index. It is clear that auroral activity was rather high during that period, and the AE value reached 1500 nT on the studied day (December 13). On December 13, precipitation of electrons with energies of 300–600 MeV was most intense during two satellite passes: 1734–1743 and 2213–2223 UT. The projections of the trajectories of these passes, when the northern polar region was crossed by CORONASF, are presented in Fig. 2a in geographic coordinates. It is evident that the Spitsbergen ground observatories (NAL, LYR, and NOR) are located near the projec tion of intense bursts of precipitating electrons at polar latitudes. Figure 2a also shows the auroral oval posi tion based on the OVATION model. Figure 2b presents the data of observations of the magnetic field X component at the highlatitude observatories of the IMAGE Finnish network as well as the keograms of auroras at NAL and LYR. The mid dle panel of Fig. 2b presents the riometer data at HOR. Gray bands mark the satellite pass instants. In both cases auroras were observed at NAL and LYR, and auroras were most intense at LYR lowerlatitude observatory (Φ = 75°).
These satellite passes have a number of differences. Electrons precipitated at higher latitudes during the first pass. According to the OVATION model, this region corresponds to the polar cap; however, the ASC camera data at NAL and LYR indicate that auroras developed near the poleward edge of the auroral oval. Note that the OVATION model gives the integral image of the auroral oval position during a certain period (30–60 min), and the obtained pattern can be very approximate and far from the real one (see, e.g., [Marjin et al., 2006]). The auroral oval is not static, and its shape and position constantly vary. During the first satellite pass, polar baylike distur bances (polar substorms) were registered at LYR and HOR observatories (Fig. 2b) at the IMAGE profile of ground stations, which was located at that time in the evening sector of the magnetosphere (~2000 MLT, MLT ~ UT + 2.5), whereas magnetic disturbances were absent at auroral latitudes. In the second case, polar baylike magnetic distur bances were observed immediately after the magneto spheric storm, registered in the auroral zone (SOD and SOR), and resulted from the substorm auroral expansion. Precipitation of energetic electrons on the CORONASF satellite were more intense in this case than in the first case and were observed closer to the position of the auroral oval boundary according to the OVATION data. Note that intense precipitation of electrons with energies of not only 300–600 keV but also 600–1500 keV were registered near the polar oval edge during the 1125–1133 UT pass (these data are not presented in Fig. 2). Figure 3a presents the results of a wavelet analysis of geomagnetic pulsations at frequencies of 1–8 mHz, accompanying baylike magnetic disturbances at SOR and NAL observatories and precipitation of energetic electrons on the CORONASF satellite. In the first event, pulsations and baylike disturbances were observed only at HOR and NAL polar observatories. In the second case, geomagnetic pulsations were also registered at auroral latitudes, and the pulsation burst was most intense at LYR observatory in the 1–2 mHz band. The results of a wavelet analysis of pulsations in the riometer absorption and in the magnetic field at HOR are compared in Fig. 3b. The wavelet structure is similar, which most probably indicates that the pulsa tions have a common source. 4. DISCUSSION OF RESULTS The events of December 13, 2002, studied above are of a certain interest since they make it possible to additionally elucidate the problem of acceleration of relativistic electrons. As a result of a detailed analysis of the satellite and groundbased data on the events observed on December 13, 2003, we established that intense precipitation of energetic electrons, registered during two passes of the CORONASF satellite
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(1734–1743 and 2213–2223 UT), were observed simultaneously with baylike disturbances of the geo magnetic field, bursts of geomagnetic pulsations and riometer absorption, and auroras at highlatitude observatories, located near the satellite orbit projec tion. Thus, the observational data indicate that the processes of energetic electron acceleration are closely related to substorm activity and geomagnetic pulsa tions. During the analyzed events, the outer boundary of auroral precipitation was localized in the region that is the polar cap in average (which is confirmed by com paring with the OVATION model). However, it is well known that auroral bulge expands toward the pole to comparatively high latitudes during the magneto spheric storm expansion phase (see, e.g., [Elphinstone et al., 1995]), is confirmed by the observational results obtained above. This is also confirmed by the keo grams of auroras and the data on riometer absorption. The effect of auroral bulge expansion to high lati tudes is closely related to a change in the magnetic configuration during the substorm expansion phase. Acceleration and injection of particles during dipolar ization of magnetic field lines results in the develop ment of the eastward transverse current [Antonova et al., 1999; Akasofu, 2003]. As a result, the projection of a fixed region of the highlatitude magnetosphere shifts poleward [Antonova and Ganyushkina, 1999]. In this case the magnetic field in geostationary orbit can exceed the dipole field value, which results in hyperdipolarization obtained in [DeForest and McIl wain, 1971] and subsequently confirmed in [Kuz netsov et al., 2000, 2004], where the development of substorms in the polar cap after strongly disturbed periods is related to the phenomenon of hyperdipolar ization. The coincidence of the region of energetic electron precipitation localization on CORONASF with the region of intense riometer absorption indicates that the observed events can be projected into the region of quasitrapping in spite of a relatively highlatitude character of the observed events. In the region of quasitrapping, drift trajectories of particles with large pitch angles do not cross the magnetospheric bound ary [Antonova and Shabanskii, 1968]. According to [Lazutin, 2004], the nightside region of quasitrap ping, located between the stable trapping boundary and the magnetotail, is filled with hot plasma and auroral particles with energies of several tens–hun dreds of kiloelectronvolts during a disturbed period. During this period, the outer boundary of the quasi
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trapping region has a sharp boundary (suddenly ceases) simultaneously for electrons and protons in a wide range of energies [Lazutin, 2004]. It is usually considered that transverse currents in the quasitrap ping region are closed by the currents at the magneto pause and are part of the magnetotail current system. However, we can indicate [Antonova, 2001; Antonova et al., 2009] that transverse currents in this region are closed within the magnetosphere, branching to high latitudes in daytime hours, i.e., being the highlatitude continuation of the ring current. The considered events are registered during the recovery phase of a small magnetic storm at very large values of the SW velocity (up to 900 km/s), which is typical of the polar cap substorm development [Ser geev et al., 1979]. Despirak et al. [2008] statistically indicated that the westward electrojet center is observed at geomagnetic latitudes higher than 75° at SW velocities higher than 500 km/s; i.e., the substorm activity region shifts poleward. The considered increases in the energetic electron fluxes are localized outside the outer boundary of the outer radiation belt [Myagkova et al., 2008]. In spite of the fact that this phenomenon was repeatedly observed (see, e.g., [Kuznetsov et al., 2004; Myagkova et al., 2008; Lazutin and Kuznetsov, 2008]), a unified expla nation of the nature of the observed increases has not yet been found. However, we should note that the explanation of the nature of subrelativistic electron precipitation in polar regions can be of key importance in understanding electron acceleration processes in the Earth’s magnetosphere. The problem of energetic electron acceleration in the Earth’s magnetosphere is still far from being finally solved [Shprits et al., 2008a, 2008b]. We have a popular mechanism by which electrons are accelerated in the magnetospheric plasma during the phase of magnetic storm recovery in the presence of prolonged substorm activity due to cyclotron resonance of electrons with whistler waves in the region of chorus emissions [Horne et al., 2005; Summers et al., 2007]. At high SW velocities, which takes place during the considered events, intense ULF Pc5 pulsations can originate due to the development of the Kelvin–Helmholtz instabil ity on the flanks of the magnetosphere [Engebretson et al., 1998; Posch et al., 2003]. In [Baker et al., 1998; Pilipenko, 1990], it was indicated that intense ULF waves in the range 1–8 mHz in the dayside magneto sphere can effectively accelerate electrons to relativis tic energies. O’Brien et al. [2001] indicated that pre cipitating electron fluxes closely correlate with Pc5
Fig. 2. (a) The projection of the CORONASF satellite orbit during two passes: 1734–1743 and 2213–2223 UT over the northern polar region in geographic coordinates, the intensity of the flux of precipitating electrons with energies of 300–600 keV and 0.6– 1.5 MeV, and the auroral oval position according to the OVATION model. (b) The keograms of auroras at NAL and LYR obser vatories, the riometer data at HOR observatory, and the X components of the magnetic field at highlatitude observatories of the IMAGE Finnish network. GEOMAGNETISM AND AERONOMY
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geomagnetic pulsations during the storm recovery phase. It is interesting to note (see, e.g., Paquette et al., 1994; Weatherwax et al., 1997]) that bursts of geomagnetic pulsations in the band of several mega hertz, accompanying by precipitation of auroral elec trons, can be observed in the nightside polar cap at a high SW velocity (according to the riometer observa tions); in this case the spectra of variations in riometer absorption coincided with the spectra of geomagnetic pulsations. All proposed acceleration mechanisms require a comparatively large acceleration time, which can hardly be related to the development of the substorm expansion phase. The process of nonlinear interaction with chorus modes proposed in [Trakhtengerts et al., 2003; Demekhov et al., 2006] is the exception. How ever, other acceleration processes can also be consid ered in the case of development of the magnetospheric substorm expansion phase. Myagkova et al. [2008] registered increases in the fluxes of energetic electrons during several successive crossings of the outer boundary of the outer radiation belt, which was related to the formation of local traps for energetic electrons outside the trapping region. In contrast to the events discussed in [Myagkova et al., 2008], increases studied above were observed only dur ing one pass. Such increases could be caused by a local particle release during the development of wavelike disturbances in the course of a magnetospheric sub storm, if large fluxes of energetic particles constantly GEOMAGNETISM AND AERONOMY
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existed in the quasitrapping region outside the boundary identified as the outer boundary of the outer radiation belt according to the observations at low alti tudes (i.e., at the equator this boundary would be localized poleward of the boundary fixed at low alti tudes). It is difficult to implement this scenario at an experimentally registered anisotropy [Imhof et al., 1991] of the energetic particle distribution function at the outer radiation belt boundary. Therefore, the observational data probably testify to a local accelera tion in the region of intense auroras, riometer absorp tion, and intense geomagnetic pulsations during a magnetospheric substorm. First of all, we should note that intense induction electric fields, resulting in the formation of a seed elec tron population with an energy of ~100 keV, con stantly registered in the equator plane during the sub storm expansion phase, develop during this phase. However, the estimates (e.g., the results of modeling performed in [Birn et al., 1997]) indicate that it is dif ficult to relate acceleration of electrons to relativistic energies to particle motion in induction fields during dipolarization of magnetic field lines. Dipolarization is accompanied by the development of a wide spec trum of waves with large amplitudes (which is con firmed by the above results of a wavelet analysis of geo magnetic pulsations). Intense auroras registered dur ing the studied events result from the formation of electron beams with energies about several kiloelec tronvolts, which in turn cause intense HF oscillations. 2009
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The appearance of tails in the distribution functions during the development of turbulence is well known from the results of numerous laboratory and satellite observations. This process can be rather rapid at a high level of turbulence and can be locally adjusted to the region of intense turbulence. Several indications, cor responding to this scenario, are observed in the studied cases. However, it is necessary to continue an analysis. Finally, we can note that the studied cases of increases in the fluxes of energetic electrons at high latitudes during a magnetospheric substorm cast a new light on the problem of acceleration of such electrons since these cases testify to the operation of relatively fast acceleration processes, related to the development of a magnetospheric substorm, localized in the space. 5. CONCLUSIONS (1) Based on the CORONASF satellite data, we found out sporadic precipitation of energetic electrons (E > 300 keV) at polar latitudes near the assumed pole ward boundary of the auroral oval in the nightside sec tor of the northern polar region on December 13, 2003. (2) For the first time, we indicated that such pre cipitation is accompanied by the simultaneous devel opment of baylike disturbances in the magnetic field and by the appearance of burst of riometer absorption and Pi3 geomagnetic pulsations as well as auroras on the Earth’s surface near the satellite orbit projection. (3) The wavelet structure of geomagnetic and riom eter pulsations at frequencies of 1–10 mHz was simi lar, which can indicate that these pulsations have a common source. However, it is still unclear what pro cess is primary: precipitating electrons with energies of several kiloelectronvolts are modulated by geomag netic pulsations or pulsating fluxes of these electrons result in the generation of waves, observed on the Earth’s surface as geomagnetic pulsations, in the ion osphere. ACKNOWLEDGMENTS This work was partially supported by the Presidium of the Russian Academy of Sciences, program 16. REFERENCES S.I. Akasofu, “A Source of Auroral Electrons and the Mag netospheric Substorm Current Systems Process,” J. Geophys. Res. 108A, 8006 (2003). E. E. Antonova and N. Yu. Ganyshkina, “Auroral Bulge Formation as the Result of Isoline Mapping of Mag netic Flux Tube Volume,” Adv. Space Res. 23 (10), 1667–1670 (1999). A. E. Antonova and V. P. Shabanskii, “On the Motion of Charged Particles in the Geomagnetic Field,” Izv. Akad. Nauk SSSR 22 (11), 1802–1808 (1968).
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