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Local particle traps in the high latitude magnetosphere and the acceleration of relativistic electrons
Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1465–1471
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Local particle traps in the high latitude magnetosphere and the acceleration of relativistic electrons E.E. Antonova a,b,, I.M. Myagkova a, M.V. Stepanova c, M.O. Riazantseva a,b, I.L. Ovchinnikov c, B.V. Mar’in a, M.V. Karavaev a a
Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia Space Research Institute RAS, Moscow, Russia c Physics Department, Universidad de Santiago de Chile, Chile b
a r t i c l e in f o
abstract
Article history: Received 31 March 2010 Received in revised form 21 June 2010 Accepted 11 November 2010 Available online 26 November 2010
Variations of electron fluxes with energies 300–600 keV in the region of quasitrapping are analyzed using data of the low orbiting Coronas-F satellite. Enhancements in the electron fluxes with energies above 300 keV are observed at the polar boundary of the outer radiation belt. Meteor-3M satellite data, OVATION and AP models of the position of the auroral oval are used to determine the position of analyzed increases in the energetic electrons with respect to the position of the auroral oval. There is a significant number of events when these increases were observed at a few consequent orbits crossing the outer radiation belt boundary. Studied increases in relativistic electron fluxes are localized at the latitudes of the auroral oval. Different mechanisms of formation of observed enhancements are discussed. The possibility of the appearance of increases due to formation of local particle traps is analyzed using Tsyganenko geomagnetic field models. The role of the formation of local particle traps at the boundary of the outer radiation belt and its possible influence to the formation of the outer radiation belt is discussed. & 2010 Elsevier Ltd. All rights reserved.
Keywords: External radiation belt Local particle traps Magnetic field models
1. Introduction A region poleward of the outer radiation belt was named the region of unstable radiation or the region of quasitrapping by Vernov et al. (1965). Comparatively large fluctuations of energetic electrons are constantly observed in this region. Well-studied feature of energetic particle motion in the region of quasitrapping is the drift shell splitting (see Shabanskiy and Antonova, 1968; Shabansky, 1968; ¨ zturk ¨ Delcourt and Sauvaud, 1999; O and Wolf, 2007). Drift shell splitting appear due to the existence of a region of space near noon named by Roederer (1970) as minimum B pockets. In this region each field line has two magnetic field minima nearly cusps in each hemisphere. It is characterized by intense chorus emissions (Tsurutani and Smith, 1977; Tsurutani et al., 2009; Verkhoglyadova et al., 2009). Resonant interactions of electrons with such kind of emissions are considered by Horne et al. (2005) as one of the reasons of energetic electron acceleration. Antonova and Shabansky (1968) showed that local closed loops Bmin ¼const (where Bmin is the minimal value of the magnetic field at the magnetic field line) around cusp can be formed. Near cusp region is considered as a local trap for energetic particles by Antonova (1996), Sheldon et al. (1998), Chen et al. (1997,
Corresponding author at: Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia. E-mail address: [email protected] (E.E. Antonova).
1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.11.020
1998), and Kirpichev et al. (1999). However, the formation of such traps continues to be under discussion. Another feature not connected to minimum B pockets appears during the analysis of the distribution of minimal values of the magnetic field at a magnetic field line Bmin ¼const near midnight in the Mead–Fairfield model (Mead and Fairfield, 1975). Fig. 1 shows an example of the contours of minimal values of the magnetic field at magnetic field line Bmin ¼ const obtained using Mead–Fairfield model (adapted from Mead and Fairfield, 1975). It is possible to see the formation of closed Bmin contours near midnight, not surrounding the Earth. It is necessary to mention that Mead and Fairfield (1975) have considered the increase in the intensity of the geomagnetic field beyond 13RE as an artifact. However, the analysis of the radial profile of Bz component of the magnetic field in Tsyganenko models (see, for example, figure 4 in the paper Antonova, 2004) shows the possibility of the formation of such Bmin contours. It is well known, that the Bmin contours specify steady-state drift trajectories of equatorial particles in the conditions of the validity of the guiding center approximation. Thus, existence of comparatively stable population of energetic particles in the high latitude magnetosphere can show the formation of local particle trap. Ukhorskiy et al. (2006) analyzed changes in the magnetic configuration during the September 7, 2002 storm using TS05 model (Tsyganenko and Sitnov, 2005). They showed that for quiet conditions there are no local extremes in the field intensity above the Earth so that the Bmin ¼const contours are equivalent to the
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appearance of the increases of fluxes of relativistic and subrelativistic electrons poleward from the outer boundary of the outer radiation belt. The enhancements in the energetic electron fluxes were observed for more than a half of crossings of the outer boundary of the outer radiation belt. For these enhancements, about a half of them were observed at two, three, or even four consequent orbits. The analysis of radial profiles of such increases shows that such profiles can be practically identical during 2–3 consecutive crossings. It was shown that analyzed increases are generally situated within the central part of the auroral oval. It was suggested that increases of fluxes of relativistic and subrelativistic electrons poleward of the outer boundary of the outer radiation belt could be connected to the formation of local particle traps in the high latitude magnetosphere. In this paper, we analyze features of such increases using data of CORONAS-F satellite and try to show that the suggestion on the formation of comparatively long lived (43–4.5 h) local particle traps is in agreement with the analysis of configuration of Bmin ¼const contours calculated using different Tsyganenko models.
2. Instrumentation and data analysis Fig. 1. The calculated using Mead–Fairfield model (Mead and Fairfield, 1975) contours of minimal values of the magnetic field on the magnetic field line for Kp o2.
contours of a dipole field. During storm main phase, an increase in ring current intensity yields local extremes in the field intensity, which changes the topology of Bmin contours, resulting in contours that close on themselves but do not encompass the Earth. In the paper of Ukhorskiy et al. (2006) such magnetic field regions are referred to as ‘‘magnetic drift path islands’’. It is demonstrated that during analyzed storm electrons could stay on closed dayside drift orbits for as long as 11 h. Ukhorskiy et al. (2006) suggest that the topological changes in the field accompanied by formation and disappearance of magnetic drift path islands can have strong impact on the outer electron belt. Local, not surrounding the Earth, contours of Bmin can transform into the traps for particles if particle Larmor radius is much smaller than the radius of curvature of magnetic field line and if the energy of the particle is much larger than the amplitude of fluctuations of electrostatic potential, multiplied by particle charge. The appearance of particle traps in the high latitude magnetosphere can be checked by analyzing energetic particle fluxes with energies much larger than the variations of electrostatic potential multiplied by particle charge (4100–200 keV). When considering the motion of energetic electrons with energy above 300 keV in the magnetosphere of the Earth, one can neglect drifts under the large-scale and middle-scale electric fields in the first approximation. The Larmor radius of energetic ions is comparable with the field line curvature, and drift approximation is not valid in this case. That is why only data on energetic electrons can be used. Increases in the energetic electron fluxes at the boundary of the outer radiation belt and beyond this boundary have been observed in many experiments (see Fritz, 1968, 1970; Imhof et al., 1979, 1992, 1997; Imhof, 1988, etc.). For instance, increases in the electron fluxes with energies of 100 keV poleward to the outer radiation belt boundary observed on board the TIROS/NOAA satellite are clearly seen in figure 1 in the paper of Yahnin et al. (1997). However, to date no detailed analysis of the measurements at low altitudes at consequent crossings of the same region has been done. Myagkova et al. (2010) analyzed the data from the CORONAS-F satellite near the outer boundary of the outer radiation belt and compared them with the Meteor-3M auroral observations and OVATION model of the auroral particle fluxes. They showed the
The CORONAS-F satellite was launched on July 31, 2001, to a circular polar orbit with an inclination of 82.51, an altitude of 500 km and a period of 95 min (see Kuznetsov et al., 2002, http://www.izmiran.rssi.ru/projects/CORONAS/F/lcoronas-f.html). The Cosmic Ray Monitor (CRM) instrument measured the differential proton fluxes with energies of 1–90 MeV and electrons with energies of 0.3–12 MeV. The electrons with energies from 300 keV to 12 MeV were detected using a semiconductor telescope. This telescope consisted of two semiconductor detectors having a thickness of 0.05 and 2.0 mm and a CsI crystal having a thickness of 1.0 cm, surrounded by a plastic scintillator 0.5 cm in thickness, which operated in the anti-coincidence regime. Electrons were detected in 5 energy ranges: 0.3–0.6, 0.6–1.5, 1.5–3, 3–6 and 6–12 MeV. The telescope aperture was 231, it was oriented in the antisolar direction. The scientific information about energetic particle fluxes is available between July 14, 2002 and June 26, 2005. During the analyzed period, the CORONAS-F satellite altitude was gradually decreasing from 500 to 370 km. The increases in electron fluxes poleward of the outer radiation belt were considered as significant when their maximum was located at L48, and the flux value in the maximum exceeded three standard deviations of the background electron flux in the polar cap for a given energy. During the analyzed period, the Coronas-F satellite was located in the region of high latitudes during approximately 15% of the total operation time. This region is located above the outer radiation belt boundary in the region of the auroral oval and polar caps. The events of solar cosmic rays (SCR) electrons, observed in the polar caps, were explicitly excluded from the analysis. The verification of the hypothesis on the existence of local particle traps requires the analysis of conjugate observations (nearly the same fluxes have to be observed in both hemispheres). The satellite CORONAS-F can be localized exactly at the same magnetic field line in L–B space (where L–B are McIlwain coordinates) one time per day only. Nevertheless, nearly the same profiles of the increases of energetic electron fluxes were observed for shorter periods (up to four consecutive orbits, i.e. 6 h). It gave a possibility to compare only the forms of the time profiles in energetic electron fluxes during consecutive orbits. An example of increases in the intensity of electron fluxes at the high-latitude region, observed in both hemispheres on July 26, 2007, is shown in Fig. 2. The black arrow at the figure corresponds to the increase of the flux in the southern hemisphere southward from the outer
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Fig. 2. Electron fluxes with energies 300–600 keV measured by the CORANAS-F satellite on July 26, 2003. Arrows show the position of peaks in electron fluxes to pole from the outer boundary of outer radiation belt for both northern (white) and southern (black) hemispheres.
Fig. 3. Variation of shown in Fig. 2 electron fluxes with the McIlwain L parameter for both southern (thin line) and northern (thick line) hemispheres on July 26, 2003.
radiation belt, while the white arrow corresponds to the increase in the flux northward from the outer radiation belt in the northern hemisphere. It can be seen, that for this event, the increase is observed in the morning sector with the maximum situated at L 12RE, polarward from the stronger and more durable enhancements, which correspond to the satellite crossing of the outer radiation belt at L¼4–4.5. Fig. 3 shows the variation of the electron fluxes with the L value for the both southern (thin line) and northern (thick line) hemispheres. As it can be seen the main peaks of the external radiation belt are located practically at the same L value, and the increases in fluxes poleward from the outer radiation belt look very similar. Such a similarity shows the conjugasy of observed peaks. The difference of the intensity is related to the difference in the orientation of the detector axis with respect to the magnetic field line. The similarity in variations of the particle fluxes is even more pronounced for measurements at consecutive orbits for
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Fig. 4. Dependence of the electron fluxes intensity on the L shell for three consequent passages in the high-latitude region of the Southern Hemisphere.
one hemisphere. An example of such kind of events observed on April 5–6, 2004 is shown in Fig. 4. It is possible to see very high similarity of particle fluxes during crossing of the outer radiation belt at L 4–4.5 and the increased particle flux in such peak poleward from the outer radiation belt. Table 1 summaries the details of particle peak location. The analyzed events take place during a small magnetic storm with minimum Dst¼ 74 nT. According to the Kyoto data base (http://swdcdb.kugi.kyoto-u/ac/jp), the first event on April 5, 2004 at 22:48 UT occurred when the Dst¼ 74 nT and AE¼740 nT, the second one (April 6, 2004 at 00:23 UT) with the Dst¼–57 nT, AE¼490 nT, and the third one (01:56 UT) with the Dst¼–47 nT, and AE¼845 nT. High-energy measurements from the CORONAS-F satellite were complemented by the auroral particle measurements from the Meteor-3M auroral satellite (http://smdc.sinp.msu.ru/), by the OVATION model of the auroral oval made using the DMSP satellite measurements (http://sd-www.jhuapl.edu/Aurora/), and by the AP-Model (http://pgia.ru/lang/en/webapps/). These data were used to determine the position of analyzed increases in the energetic electrons with respect to the position of the auroral oval. Fig. 5 shows precipitating electron fluxes in the auroral zone obtained by the Meteor-3M satellite on April 06, 2003 nearly at the same time, when the increase of energetic electrons was identified at 01:56 UT. The difference in the MLT location between the CORONAS-F and Meteor-3M satellites was less than 1 h. The arrow shows the L-value corresponding to the position of the analyzed increase, localized inside the auroral oval. The same result was obtained using the OVATION model and the AP-Model. The same analysis applied to all events of increases in the high energy electron fluxes shown in Fig. 4 showed that all of them are situated within the central part of the auroral oval.
3. Contours of the minimum values of the geomagnetic field and formation of local particle traps The April 5–6, 2004, event was selected for further analysis, as it demonstrates very good coincidence in the form of the increase of energetic electron fluxes poleward from the outer boundary of the outer radiation belt during three consecutive orbits and taking into account the significant progress in the development of the magnetic field models for storm-time periods. The geomagnetic field
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Table 1 Coordinates of the event on 05–06 April, 2004. Date, time (hour and part of hour)
Position inside the region
L
LT
MLT
Altitude (km)
Latitude (grad)
Longitude (grad)
Magnetic latitude (grad)
Magnetic longitude (grad)
05.04.2004, 22.8137 05.04.2004, 22.8293 05.04.2004, 22.8410
Outer boundary Center Inner boundary
12.9 9.9 8.07
7.3 8.2 8.6
6.8 7.3 7.6
417.0 417.4 417.6
80.2 77.6 75.4
127.4 140.2 146.5
69.1 67.0 65.2
171.9 164.1 159.0
06.04.2004, 0.3544 06.04.2004, 0.3700 06.04.2004, 0.3818
Outer boundary Center Inner boundary
13.6 10.4 8.4
7.0 8.0 8.5
7.4 7.9 8.2
416.8 417.2 417.4
80.7 78.2 76.0
100.2 114.5 121.5
69.4 66.9 64.8
175.4 177.8 173.6
06.04.2004, 1.9030 06.04.2004, 1.9147 06.04.2004, 1.9225
Outer boundary Center Inner boundary
13.2 10.6 9.2
7.3 8.0 8.3
8.4 8.7 8.8
417.0 417.3 417.4
80.2 78.2 76.8
81.2 91.2 96.1
69.5 67.1 65.6
166.3 170.2 172.3
pressure at the equatorial plane with multiple peaks in a number of cases. Transverse current has westward direction if the plasma pressure gradient is directed to the Earth and eastward direction if the plasma pressure gradient is directed from the Earth. Local current loops and corresponding local Bmin ¼const contours can appear in such a case.
4. Discussion and conclusions
Fig. 5. Auroral electron precipitating fluxes measured by the Meteor-3M satellite on April 6, 2003.
configuration for the April 5–6, 2004, event was analyzed using the Tsyganenko 2001 model (Tsyganenko, 2002a, b). The location of the inner and outer borders and of the maximum value of the electron fluxes was mapped into the equatorial plane (see Fig. 6a–c). It is possible to see that the structure is mainly mapped into the geomagnetic tail up to 35RE. Fig. 6d shows the corresponding distribution of Bmin ¼const mapped into the equatorial plane. It is possible to see clearly that there are contours closed inside the tail (see Fig. 6d, black lines). Although, the obtained mapping cannot be considered as a proof that the appearance of observed increases in Fig. 4 is due to particle trapping inside shown in Fig. 6d local particle traps, it demonstrates the possibility of such scenario. To check the robustness of the results shown in Fig. 6d, we calculated the contours of the minimal value of the geomagnetic field using the TS05 (Tsyganenko and Sitnov, 2005) and the widely used Tsyganenko-96 (Tsyganenko and Stern, 1996) models (see Fig. 7a and b). It is possible to see, that the position of closed contours depends strongly on the model. This difference can be connected to the overstretching of the Tsyganenko-96 model. This overstretching becomes very clear in the process of comparison of measured plasma pressure gradients and field-aligned current mapping produced by Xing et al. (2009) (see figure 4 from this paper). Nevertheless, all three versions of Tsyganenko models show the presence of closed Bmin ¼const contours near midnight just as in case of the Mead–Fairfield model (see Fig. 1). It is necessary to note that afore-mentioned geomagnetic field models have been created by averaging numerous satellite observations of the geomagnetic field. Therefore, such models can only reproduce averaged magnetic field configuration. Many experimental results (see, for example, Antonova et al., 2003; Kozelova et al., 2008) show an inhomogeneity of radial profile of plasma
It is possible to suggest a number of explanations for the formation of the increases of energetic electrons to the pole from the outer boundary of the outer radiation belt. Nevertheless, it is necessary to keep in mind that in approximately thirty percent of events of these increases were observed for two, three, or even four consecutive orbits with nearly the same radial profiles during quite and disturbed conditions. First explanation is the existence of a region of very small pitchangle diffusion near the boundary of the outer radiation belt. Observed increases in such a case simply show the distribution of particle fluxes at low altitudes and do not correspond to distribution at the equatorial plane. It is interesting to mention, that in this case the outer boundary of the outer radiation belt should be localized at much larger distances as ordinarily suggested. However, nearly equatorial observations do not support this scenario. In particular, the dynamics of the outer edge of the ring current in the vicinity of the geomagnetic equator was analyzed by Pissarenko et al. (1998) using data of the Interball/Tail Probe satellite. It was shown that at the outer boundary of the ring current local increases (bursts) of electron fluxes with energies of 40–150 keV and 0.3–1.0 MeV (called by the authors ‘‘islands’’) could be observed. Simultaneously ‘‘bunches’’ of magnetospheric plasma were observed. Therefore, the experimental results in the vicinity of the equatorial plane show a possibility of appearance of local increases in the precipitating electron fluxes not only due to changes in the pitch-angle diffusion regime. Unfortunately, the nearly equatorial observations by a single satellite do not allow answering the question about the steady-state character of the observed structures. Nevertheless, Myagkova et al. (2010) showed that the dynamics of the enhancements in the energetic electron fluxes registered at the outer radiation belt boundary can be studied using the data from low altitude satellites, which cross the same region approximately every 95 min, giving an opportunity to register these structures at two or even more consecutive orbits, which correspond to the structure lifetime 495 min. Second explanation is connected to the hypothesis of energetic electron acceleration by chorus emissions. As was mentioned in the Introduction the mechanism of electron acceleration by chorus waves is rather popular now. However, Kennel and Petschek (1966)
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Fig. 6. (a)–(c) The geomagnetic field lines corresponding to the location of the inner boundary, position of the maximal flux and the outer boundary of the large latitude particle peak for the April 5–6, 2004, event, (d) the distribution of Bmin ¼ const at the equatorial plane from the Tsyganenko 2001 geomagnetic field model.
Fig. 7. Contours of the constant minimum values of the geomagnetic field obtained using (a) the TS05 model and (b) Tsyganenko-96 model for the April 6, 2004, event.
stressed that whistler waves cannot be an acceleration mechanism of energetic electrons since there is little diffusion in energy. Analyzing the results obtained by Tsurutani and Smith (1977), Tsurutani et al., (2009) and Verkhoglyadova et al. (2009) it is possible to suggest that at least part of the observed increases by Myagkova et al. (2010) could be explained by the mechanism of electron interactions with the region of increased chorus waves.
Nevertheless, in this case rather strong pitch angle diffusion produced by chorus waves should be effectively compensated by slower particle acceleration. The action of such mechanism requires the existence of comparatively stable regions of increased chorus emissions in the geomagnetic tail at geocentric distances till 35Re according to our mapping (see Fig. 6). To check this hypothesis it is necessary to measure simultaneously energetic
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particle fluxes and electromagnetic emissions. Unfortunately, the solar observatory Coronas-F has no wave observations and it should be an object of future studies. The third scenario involves the existence of magnetic storms, during which energetic electrons are often injected into the inner magnetosphere. Nevertheless, this approach does not allow explaining the appearance of increased fluxes at large L during quiet geomagnetic conditions. It is also difficult to explain the quasistationary forms of increases during comparatively long time intervals ( 44.5 h). The fourth scenario-formation of quasi-stable particle traps discussed in this paper explains the appearance of quasistationary forms of increases in the electron fluxes (formation of local radiation belts). However, it is necessary to continue these studies. It is important to stress that analyzed increases of energetic electron fluxes are two orders of magnitude lower than fluxes in maxima of the external radiation belt. Nevertheless, their comparatively frequent registration and apparent stability can be an important factor, which can change some aspects of the analysis of the formation of large fluxes of relativistic electrons during magnetic storms. It is necessary to stress that the discussed scenario does not contradict the hypothesis of turbulent acceleration of energetic electrons. Inversely, the formation of local particle traps significantly increases the effectiveness of the process of turbulent acceleration as the time of wave-particle interaction can be greatly increased. High level of turbulence at different frequency ranges is a permanent feature of auroral and plasma sheet observations. Turbulent electron acceleration in discussed particle traps can be very effective as drift trajectories of accelerated particles do not cross magnetopause and particles can be accumulated in the trap. It is possible to select events when analyzed increases are observed only in one side of the polar cap and is not observed at the other side during the polar cap crossings. This supports the azimuthal asymmetry of the observed phenomena. Inward transport of such electrons produce the azimuthally asymmetric boundary conditions for the outer radiation belt which will be necessary to take into account in the modern models of outer radiation belt formation such as described in Shprits et al. (2008a, b). The results obtained can be considered as one of the first steps on the study of the possibility of the formation of local traps for energetic electrons in high latitude magnetosphere. Nevertheless, even now it is clear that comparatively stable electron populations with the same parameters as the outer radiation belt particles do exist at the latitudes of the auroral oval and even taking into consideration that the proposed here local magnetic traps could not be the unique mechanism of its formation, these populations should be considered during the analysis of the processes at the latitudes of the auroral oval as a part of the solution of the problem of outer radiation belt formation.
Acknowledgements The authors acknowledge the creators of OVATION and AP models of auroral oval position. The study was partially supported by FONDECYT Grants 1070131, MK-1579.2010.2, and RFBR Grant 10-05-00247-a.
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Local particle traps in the high latitude magnetosphere and the acceleration of relativistic electrons E.E. Antonova a,b,, I.M. Myagkova a, M.V. Stepanova c, M.O. Riazantseva a,b, I.L. Ovchinnikov c, B.V. Mar’in a, M.V. Karavaev a a
Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia Space Research Institute RAS, Moscow, Russia c Physics Department, Universidad de Santiago de Chile, Chile b
a r t i c l e in f o
abstract
Article history: Received 31 March 2010 Received in revised form 21 June 2010 Accepted 11 November 2010 Available online 26 November 2010
Variations of electron fluxes with energies 300–600 keV in the region of quasitrapping are analyzed using data of the low orbiting Coronas-F satellite. Enhancements in the electron fluxes with energies above 300 keV are observed at the polar boundary of the outer radiation belt. Meteor-3M satellite data, OVATION and AP models of the position of the auroral oval are used to determine the position of analyzed increases in the energetic electrons with respect to the position of the auroral oval. There is a significant number of events when these increases were observed at a few consequent orbits crossing the outer radiation belt boundary. Studied increases in relativistic electron fluxes are localized at the latitudes of the auroral oval. Different mechanisms of formation of observed enhancements are discussed. The possibility of the appearance of increases due to formation of local particle traps is analyzed using Tsyganenko geomagnetic field models. The role of the formation of local particle traps at the boundary of the outer radiation belt and its possible influence to the formation of the outer radiation belt is discussed. & 2010 Elsevier Ltd. All rights reserved.
Keywords: External radiation belt Local particle traps Magnetic field models
1. Introduction A region poleward of the outer radiation belt was named the region of unstable radiation or the region of quasitrapping by Vernov et al. (1965). Comparatively large fluctuations of energetic electrons are constantly observed in this region. Well-studied feature of energetic particle motion in the region of quasitrapping is the drift shell splitting (see Shabanskiy and Antonova, 1968; Shabansky, 1968; ¨ zturk ¨ Delcourt and Sauvaud, 1999; O and Wolf, 2007). Drift shell splitting appear due to the existence of a region of space near noon named by Roederer (1970) as minimum B pockets. In this region each field line has two magnetic field minima nearly cusps in each hemisphere. It is characterized by intense chorus emissions (Tsurutani and Smith, 1977; Tsurutani et al., 2009; Verkhoglyadova et al., 2009). Resonant interactions of electrons with such kind of emissions are considered by Horne et al. (2005) as one of the reasons of energetic electron acceleration. Antonova and Shabansky (1968) showed that local closed loops Bmin ¼const (where Bmin is the minimal value of the magnetic field at the magnetic field line) around cusp can be formed. Near cusp region is considered as a local trap for energetic particles by Antonova (1996), Sheldon et al. (1998), Chen et al. (1997,
Corresponding author at: Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia. E-mail address: [email protected] (E.E. Antonova).
1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.11.020
1998), and Kirpichev et al. (1999). However, the formation of such traps continues to be under discussion. Another feature not connected to minimum B pockets appears during the analysis of the distribution of minimal values of the magnetic field at a magnetic field line Bmin ¼const near midnight in the Mead–Fairfield model (Mead and Fairfield, 1975). Fig. 1 shows an example of the contours of minimal values of the magnetic field at magnetic field line Bmin ¼ const obtained using Mead–Fairfield model (adapted from Mead and Fairfield, 1975). It is possible to see the formation of closed Bmin contours near midnight, not surrounding the Earth. It is necessary to mention that Mead and Fairfield (1975) have considered the increase in the intensity of the geomagnetic field beyond 13RE as an artifact. However, the analysis of the radial profile of Bz component of the magnetic field in Tsyganenko models (see, for example, figure 4 in the paper Antonova, 2004) shows the possibility of the formation of such Bmin contours. It is well known, that the Bmin contours specify steady-state drift trajectories of equatorial particles in the conditions of the validity of the guiding center approximation. Thus, existence of comparatively stable population of energetic particles in the high latitude magnetosphere can show the formation of local particle trap. Ukhorskiy et al. (2006) analyzed changes in the magnetic configuration during the September 7, 2002 storm using TS05 model (Tsyganenko and Sitnov, 2005). They showed that for quiet conditions there are no local extremes in the field intensity above the Earth so that the Bmin ¼const contours are equivalent to the
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appearance of the increases of fluxes of relativistic and subrelativistic electrons poleward from the outer boundary of the outer radiation belt. The enhancements in the energetic electron fluxes were observed for more than a half of crossings of the outer boundary of the outer radiation belt. For these enhancements, about a half of them were observed at two, three, or even four consequent orbits. The analysis of radial profiles of such increases shows that such profiles can be practically identical during 2–3 consecutive crossings. It was shown that analyzed increases are generally situated within the central part of the auroral oval. It was suggested that increases of fluxes of relativistic and subrelativistic electrons poleward of the outer boundary of the outer radiation belt could be connected to the formation of local particle traps in the high latitude magnetosphere. In this paper, we analyze features of such increases using data of CORONAS-F satellite and try to show that the suggestion on the formation of comparatively long lived (43–4.5 h) local particle traps is in agreement with the analysis of configuration of Bmin ¼const contours calculated using different Tsyganenko models.
2. Instrumentation and data analysis Fig. 1. The calculated using Mead–Fairfield model (Mead and Fairfield, 1975) contours of minimal values of the magnetic field on the magnetic field line for Kp o2.
contours of a dipole field. During storm main phase, an increase in ring current intensity yields local extremes in the field intensity, which changes the topology of Bmin contours, resulting in contours that close on themselves but do not encompass the Earth. In the paper of Ukhorskiy et al. (2006) such magnetic field regions are referred to as ‘‘magnetic drift path islands’’. It is demonstrated that during analyzed storm electrons could stay on closed dayside drift orbits for as long as 11 h. Ukhorskiy et al. (2006) suggest that the topological changes in the field accompanied by formation and disappearance of magnetic drift path islands can have strong impact on the outer electron belt. Local, not surrounding the Earth, contours of Bmin can transform into the traps for particles if particle Larmor radius is much smaller than the radius of curvature of magnetic field line and if the energy of the particle is much larger than the amplitude of fluctuations of electrostatic potential, multiplied by particle charge. The appearance of particle traps in the high latitude magnetosphere can be checked by analyzing energetic particle fluxes with energies much larger than the variations of electrostatic potential multiplied by particle charge (4100–200 keV). When considering the motion of energetic electrons with energy above 300 keV in the magnetosphere of the Earth, one can neglect drifts under the large-scale and middle-scale electric fields in the first approximation. The Larmor radius of energetic ions is comparable with the field line curvature, and drift approximation is not valid in this case. That is why only data on energetic electrons can be used. Increases in the energetic electron fluxes at the boundary of the outer radiation belt and beyond this boundary have been observed in many experiments (see Fritz, 1968, 1970; Imhof et al., 1979, 1992, 1997; Imhof, 1988, etc.). For instance, increases in the electron fluxes with energies of 100 keV poleward to the outer radiation belt boundary observed on board the TIROS/NOAA satellite are clearly seen in figure 1 in the paper of Yahnin et al. (1997). However, to date no detailed analysis of the measurements at low altitudes at consequent crossings of the same region has been done. Myagkova et al. (2010) analyzed the data from the CORONAS-F satellite near the outer boundary of the outer radiation belt and compared them with the Meteor-3M auroral observations and OVATION model of the auroral particle fluxes. They showed the
The CORONAS-F satellite was launched on July 31, 2001, to a circular polar orbit with an inclination of 82.51, an altitude of 500 km and a period of 95 min (see Kuznetsov et al., 2002, http://www.izmiran.rssi.ru/projects/CORONAS/F/lcoronas-f.html). The Cosmic Ray Monitor (CRM) instrument measured the differential proton fluxes with energies of 1–90 MeV and electrons with energies of 0.3–12 MeV. The electrons with energies from 300 keV to 12 MeV were detected using a semiconductor telescope. This telescope consisted of two semiconductor detectors having a thickness of 0.05 and 2.0 mm and a CsI crystal having a thickness of 1.0 cm, surrounded by a plastic scintillator 0.5 cm in thickness, which operated in the anti-coincidence regime. Electrons were detected in 5 energy ranges: 0.3–0.6, 0.6–1.5, 1.5–3, 3–6 and 6–12 MeV. The telescope aperture was 231, it was oriented in the antisolar direction. The scientific information about energetic particle fluxes is available between July 14, 2002 and June 26, 2005. During the analyzed period, the CORONAS-F satellite altitude was gradually decreasing from 500 to 370 km. The increases in electron fluxes poleward of the outer radiation belt were considered as significant when their maximum was located at L48, and the flux value in the maximum exceeded three standard deviations of the background electron flux in the polar cap for a given energy. During the analyzed period, the Coronas-F satellite was located in the region of high latitudes during approximately 15% of the total operation time. This region is located above the outer radiation belt boundary in the region of the auroral oval and polar caps. The events of solar cosmic rays (SCR) electrons, observed in the polar caps, were explicitly excluded from the analysis. The verification of the hypothesis on the existence of local particle traps requires the analysis of conjugate observations (nearly the same fluxes have to be observed in both hemispheres). The satellite CORONAS-F can be localized exactly at the same magnetic field line in L–B space (where L–B are McIlwain coordinates) one time per day only. Nevertheless, nearly the same profiles of the increases of energetic electron fluxes were observed for shorter periods (up to four consecutive orbits, i.e. 6 h). It gave a possibility to compare only the forms of the time profiles in energetic electron fluxes during consecutive orbits. An example of increases in the intensity of electron fluxes at the high-latitude region, observed in both hemispheres on July 26, 2007, is shown in Fig. 2. The black arrow at the figure corresponds to the increase of the flux in the southern hemisphere southward from the outer
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Fig. 2. Electron fluxes with energies 300–600 keV measured by the CORANAS-F satellite on July 26, 2003. Arrows show the position of peaks in electron fluxes to pole from the outer boundary of outer radiation belt for both northern (white) and southern (black) hemispheres.
Fig. 3. Variation of shown in Fig. 2 electron fluxes with the McIlwain L parameter for both southern (thin line) and northern (thick line) hemispheres on July 26, 2003.
radiation belt, while the white arrow corresponds to the increase in the flux northward from the outer radiation belt in the northern hemisphere. It can be seen, that for this event, the increase is observed in the morning sector with the maximum situated at L 12RE, polarward from the stronger and more durable enhancements, which correspond to the satellite crossing of the outer radiation belt at L¼4–4.5. Fig. 3 shows the variation of the electron fluxes with the L value for the both southern (thin line) and northern (thick line) hemispheres. As it can be seen the main peaks of the external radiation belt are located practically at the same L value, and the increases in fluxes poleward from the outer radiation belt look very similar. Such a similarity shows the conjugasy of observed peaks. The difference of the intensity is related to the difference in the orientation of the detector axis with respect to the magnetic field line. The similarity in variations of the particle fluxes is even more pronounced for measurements at consecutive orbits for
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Fig. 4. Dependence of the electron fluxes intensity on the L shell for three consequent passages in the high-latitude region of the Southern Hemisphere.
one hemisphere. An example of such kind of events observed on April 5–6, 2004 is shown in Fig. 4. It is possible to see very high similarity of particle fluxes during crossing of the outer radiation belt at L 4–4.5 and the increased particle flux in such peak poleward from the outer radiation belt. Table 1 summaries the details of particle peak location. The analyzed events take place during a small magnetic storm with minimum Dst¼ 74 nT. According to the Kyoto data base (http://swdcdb.kugi.kyoto-u/ac/jp), the first event on April 5, 2004 at 22:48 UT occurred when the Dst¼ 74 nT and AE¼740 nT, the second one (April 6, 2004 at 00:23 UT) with the Dst¼–57 nT, AE¼490 nT, and the third one (01:56 UT) with the Dst¼–47 nT, and AE¼845 nT. High-energy measurements from the CORONAS-F satellite were complemented by the auroral particle measurements from the Meteor-3M auroral satellite (http://smdc.sinp.msu.ru/), by the OVATION model of the auroral oval made using the DMSP satellite measurements (http://sd-www.jhuapl.edu/Aurora/), and by the AP-Model (http://pgia.ru/lang/en/webapps/). These data were used to determine the position of analyzed increases in the energetic electrons with respect to the position of the auroral oval. Fig. 5 shows precipitating electron fluxes in the auroral zone obtained by the Meteor-3M satellite on April 06, 2003 nearly at the same time, when the increase of energetic electrons was identified at 01:56 UT. The difference in the MLT location between the CORONAS-F and Meteor-3M satellites was less than 1 h. The arrow shows the L-value corresponding to the position of the analyzed increase, localized inside the auroral oval. The same result was obtained using the OVATION model and the AP-Model. The same analysis applied to all events of increases in the high energy electron fluxes shown in Fig. 4 showed that all of them are situated within the central part of the auroral oval.
3. Contours of the minimum values of the geomagnetic field and formation of local particle traps The April 5–6, 2004, event was selected for further analysis, as it demonstrates very good coincidence in the form of the increase of energetic electron fluxes poleward from the outer boundary of the outer radiation belt during three consecutive orbits and taking into account the significant progress in the development of the magnetic field models for storm-time periods. The geomagnetic field
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Table 1 Coordinates of the event on 05–06 April, 2004. Date, time (hour and part of hour)
Position inside the region
L
LT
MLT
Altitude (km)
Latitude (grad)
Longitude (grad)
Magnetic latitude (grad)
Magnetic longitude (grad)
05.04.2004, 22.8137 05.04.2004, 22.8293 05.04.2004, 22.8410
Outer boundary Center Inner boundary
12.9 9.9 8.07
7.3 8.2 8.6
6.8 7.3 7.6
417.0 417.4 417.6
80.2 77.6 75.4
127.4 140.2 146.5
69.1 67.0 65.2
171.9 164.1 159.0
06.04.2004, 0.3544 06.04.2004, 0.3700 06.04.2004, 0.3818
Outer boundary Center Inner boundary
13.6 10.4 8.4
7.0 8.0 8.5
7.4 7.9 8.2
416.8 417.2 417.4
80.7 78.2 76.0
100.2 114.5 121.5
69.4 66.9 64.8
175.4 177.8 173.6
06.04.2004, 1.9030 06.04.2004, 1.9147 06.04.2004, 1.9225
Outer boundary Center Inner boundary
13.2 10.6 9.2
7.3 8.0 8.3
8.4 8.7 8.8
417.0 417.3 417.4
80.2 78.2 76.8
81.2 91.2 96.1
69.5 67.1 65.6
166.3 170.2 172.3
pressure at the equatorial plane with multiple peaks in a number of cases. Transverse current has westward direction if the plasma pressure gradient is directed to the Earth and eastward direction if the plasma pressure gradient is directed from the Earth. Local current loops and corresponding local Bmin ¼const contours can appear in such a case.
4. Discussion and conclusions
Fig. 5. Auroral electron precipitating fluxes measured by the Meteor-3M satellite on April 6, 2003.
configuration for the April 5–6, 2004, event was analyzed using the Tsyganenko 2001 model (Tsyganenko, 2002a, b). The location of the inner and outer borders and of the maximum value of the electron fluxes was mapped into the equatorial plane (see Fig. 6a–c). It is possible to see that the structure is mainly mapped into the geomagnetic tail up to 35RE. Fig. 6d shows the corresponding distribution of Bmin ¼const mapped into the equatorial plane. It is possible to see clearly that there are contours closed inside the tail (see Fig. 6d, black lines). Although, the obtained mapping cannot be considered as a proof that the appearance of observed increases in Fig. 4 is due to particle trapping inside shown in Fig. 6d local particle traps, it demonstrates the possibility of such scenario. To check the robustness of the results shown in Fig. 6d, we calculated the contours of the minimal value of the geomagnetic field using the TS05 (Tsyganenko and Sitnov, 2005) and the widely used Tsyganenko-96 (Tsyganenko and Stern, 1996) models (see Fig. 7a and b). It is possible to see, that the position of closed contours depends strongly on the model. This difference can be connected to the overstretching of the Tsyganenko-96 model. This overstretching becomes very clear in the process of comparison of measured plasma pressure gradients and field-aligned current mapping produced by Xing et al. (2009) (see figure 4 from this paper). Nevertheless, all three versions of Tsyganenko models show the presence of closed Bmin ¼const contours near midnight just as in case of the Mead–Fairfield model (see Fig. 1). It is necessary to note that afore-mentioned geomagnetic field models have been created by averaging numerous satellite observations of the geomagnetic field. Therefore, such models can only reproduce averaged magnetic field configuration. Many experimental results (see, for example, Antonova et al., 2003; Kozelova et al., 2008) show an inhomogeneity of radial profile of plasma
It is possible to suggest a number of explanations for the formation of the increases of energetic electrons to the pole from the outer boundary of the outer radiation belt. Nevertheless, it is necessary to keep in mind that in approximately thirty percent of events of these increases were observed for two, three, or even four consecutive orbits with nearly the same radial profiles during quite and disturbed conditions. First explanation is the existence of a region of very small pitchangle diffusion near the boundary of the outer radiation belt. Observed increases in such a case simply show the distribution of particle fluxes at low altitudes and do not correspond to distribution at the equatorial plane. It is interesting to mention, that in this case the outer boundary of the outer radiation belt should be localized at much larger distances as ordinarily suggested. However, nearly equatorial observations do not support this scenario. In particular, the dynamics of the outer edge of the ring current in the vicinity of the geomagnetic equator was analyzed by Pissarenko et al. (1998) using data of the Interball/Tail Probe satellite. It was shown that at the outer boundary of the ring current local increases (bursts) of electron fluxes with energies of 40–150 keV and 0.3–1.0 MeV (called by the authors ‘‘islands’’) could be observed. Simultaneously ‘‘bunches’’ of magnetospheric plasma were observed. Therefore, the experimental results in the vicinity of the equatorial plane show a possibility of appearance of local increases in the precipitating electron fluxes not only due to changes in the pitch-angle diffusion regime. Unfortunately, the nearly equatorial observations by a single satellite do not allow answering the question about the steady-state character of the observed structures. Nevertheless, Myagkova et al. (2010) showed that the dynamics of the enhancements in the energetic electron fluxes registered at the outer radiation belt boundary can be studied using the data from low altitude satellites, which cross the same region approximately every 95 min, giving an opportunity to register these structures at two or even more consecutive orbits, which correspond to the structure lifetime 495 min. Second explanation is connected to the hypothesis of energetic electron acceleration by chorus emissions. As was mentioned in the Introduction the mechanism of electron acceleration by chorus waves is rather popular now. However, Kennel and Petschek (1966)
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Fig. 6. (a)–(c) The geomagnetic field lines corresponding to the location of the inner boundary, position of the maximal flux and the outer boundary of the large latitude particle peak for the April 5–6, 2004, event, (d) the distribution of Bmin ¼ const at the equatorial plane from the Tsyganenko 2001 geomagnetic field model.
Fig. 7. Contours of the constant minimum values of the geomagnetic field obtained using (a) the TS05 model and (b) Tsyganenko-96 model for the April 6, 2004, event.
stressed that whistler waves cannot be an acceleration mechanism of energetic electrons since there is little diffusion in energy. Analyzing the results obtained by Tsurutani and Smith (1977), Tsurutani et al., (2009) and Verkhoglyadova et al. (2009) it is possible to suggest that at least part of the observed increases by Myagkova et al. (2010) could be explained by the mechanism of electron interactions with the region of increased chorus waves.
Nevertheless, in this case rather strong pitch angle diffusion produced by chorus waves should be effectively compensated by slower particle acceleration. The action of such mechanism requires the existence of comparatively stable regions of increased chorus emissions in the geomagnetic tail at geocentric distances till 35Re according to our mapping (see Fig. 6). To check this hypothesis it is necessary to measure simultaneously energetic
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particle fluxes and electromagnetic emissions. Unfortunately, the solar observatory Coronas-F has no wave observations and it should be an object of future studies. The third scenario involves the existence of magnetic storms, during which energetic electrons are often injected into the inner magnetosphere. Nevertheless, this approach does not allow explaining the appearance of increased fluxes at large L during quiet geomagnetic conditions. It is also difficult to explain the quasistationary forms of increases during comparatively long time intervals ( 44.5 h). The fourth scenario-formation of quasi-stable particle traps discussed in this paper explains the appearance of quasistationary forms of increases in the electron fluxes (formation of local radiation belts). However, it is necessary to continue these studies. It is important to stress that analyzed increases of energetic electron fluxes are two orders of magnitude lower than fluxes in maxima of the external radiation belt. Nevertheless, their comparatively frequent registration and apparent stability can be an important factor, which can change some aspects of the analysis of the formation of large fluxes of relativistic electrons during magnetic storms. It is necessary to stress that the discussed scenario does not contradict the hypothesis of turbulent acceleration of energetic electrons. Inversely, the formation of local particle traps significantly increases the effectiveness of the process of turbulent acceleration as the time of wave-particle interaction can be greatly increased. High level of turbulence at different frequency ranges is a permanent feature of auroral and plasma sheet observations. Turbulent electron acceleration in discussed particle traps can be very effective as drift trajectories of accelerated particles do not cross magnetopause and particles can be accumulated in the trap. It is possible to select events when analyzed increases are observed only in one side of the polar cap and is not observed at the other side during the polar cap crossings. This supports the azimuthal asymmetry of the observed phenomena. Inward transport of such electrons produce the azimuthally asymmetric boundary conditions for the outer radiation belt which will be necessary to take into account in the modern models of outer radiation belt formation such as described in Shprits et al. (2008a, b). The results obtained can be considered as one of the first steps on the study of the possibility of the formation of local traps for energetic electrons in high latitude magnetosphere. Nevertheless, even now it is clear that comparatively stable electron populations with the same parameters as the outer radiation belt particles do exist at the latitudes of the auroral oval and even taking into consideration that the proposed here local magnetic traps could not be the unique mechanism of its formation, these populations should be considered during the analysis of the processes at the latitudes of the auroral oval as a part of the solution of the problem of outer radiation belt formation.
Acknowledgements The authors acknowledge the creators of OVATION and AP models of auroral oval position. The study was partially supported by FONDECYT Grants 1070131, MK-1579.2010.2, and RFBR Grant 10-05-00247-a.
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