Evolution of the outer radiation belt during the November 1993 storms driven by corotating interaction regions

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi:10.1029/2006ja012148, 2007 Evolution of the outer radiation belt during the November 1993 storms driven by corotating interaction regions Y. Miyoshi, 1 A. Morioka, 2 R. Kataoka, 1,3 Y. Kasahara, 4 and T. Mukai 5 Received 31 October 2006; revised 21 February 2007; accepted 27 February 2007; published 18 May 2007. [1] Evolution of energetic electron fluxes, related solar wind conditions, and relevant plasma waves in the inner magnetosphere are examined during the two corotating interaction region (CIR)-driven magnetic storms in November 1993. In this paper we focus on the fact that the flux of the outer radiation belt electrons increased significantly during the 3 November storm, while it did not increase above the prestorm level during the 18 November storm. The recovery phase of the 3 November storm is associated with the prolonged substorm activity; continuous injections of hot and subrelativistic electrons, and enhanced chorus wave activity which can accelerate subrelativistic electrons to MeV energies by means of wave-particle interactions. In contrast, the recovery phase of the 18 November storm is associated with reduced substorm activity, weak injections of hot and subrelativistic electrons, and low chorus wave activity. These differences in the recovery phase can be related to the southward offset of interplanetary magnetic field (IMF) in the high-speed coronal hole stream, which is influenced by the IMF sector polarity via the Russell-McPherron effect (dipole tilt effect associated with the IMF polarity). Citation: Miyoshi, Y., A. Morioka, R. Kataoka, Y. Kasahara, and T. Mukai (2007), Evolution of the outer radiation belt during the November 1993 storms driven by corotating interaction regions, J. Geophys. Res., 112,, doi:10.1029/2006ja012148. 1. Introduction [2] The outer radiation belt changes dramatically in association with magnetic storms. The flux decreases during the main phase, but flux enhancements do not always occur during the recovery phase. Reeves et al. [2003] found that only about half the storms increase the fluxes of relativistic electrons, one quarter decrease them, and the other quarter produce little or no change in the fluxes, which are independent of the strength of a storm. They also suggested that effects of storms on radiation belt fluxes are determined by a delicate and complicated balance between acceleration and loss of particles. [3] The acceleration process of MeV electrons in the outer radiation belt is an important topic in the magnetospheric physics. The adiabatic acceleration by radial diffusion has been considered as the fundamental acceleration process [e.g., Schultz and Lanzerotti, 1974], while recent observational studies have shown that the internal acceleration process by wave-particle interactions with whistler mode 1 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 2 Planetary Plasma and Atmospheric Research Center, Tohoku University, Sendai, Japan. 3 Now at Institute of Physics and Chemical Research (RIKEN), Saitama, Japan. 4 Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan. 5 Japan Aerospace Exploration Agency, Kanagawa, Japan. Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JA012148 chorus also influences the flux enhancement in the outer belt [e.g., Meredith et al., 2002; Miyoshi et al., 2003; Horne et al., 2005a; Iles et al., 2006]. [4] Previous studies suggested that the solar wind speed is a primary parameter in determining the evolution of the outer radiation belt [e.g., Paulikas and Blake, 1979; O Brien et al., 2001]. A solar wind speed of 600 km/s is a useful threshold for identifying the strong flux enhancement using only the solar wind speed [Weigel et al., 2003]. A recent statistical study by Miyoshi and Kataoka [2005] showed that the high-speed solar wind is not a sufficient condition for the flux enhancement. They discussed that the magnetic field fluctuations within fast coronal hole stream following the corotating interaction regions (CIRs) play an important role in the flux enhancement in the outer belt. Lyons et al. [2005] indicated that the enhancement of both relativistic electrons and the chorus waves that drive an acceleration process are associated with the enhanced convection driven by Alfvénic fluctuations within the coronal hole stream. Kim et al. [2006] suggested, however, that rapid and small amplitude fluctuations of the southward component of the interplanetary magnetic field (IMF), IMF Bz, have only a small impact on the flux enhancement of the outer belt. [5] The purpose of this paper is to show an excellent example to understand differences of such storms with and without flux enhancements of the outer belt, comparing two particular CIR-driven storms which occurred in November 1993. The enhancement of relativistic electrons in the outer belt was strong during the recovery phase of the 3 November storm, and the enhanced flux remained above the prestorm level for a number of days (for a review of this 1of7

Figure 1. Yohkoh soft X-ray images of the Sun (a) on 3 November (DOY 307) and (b) on 15 November (DOY 319) 1993 showing a northern hemisphere-centered coronal hole with a away from the Sun (positive sign) sector polarity in the IMF and a southern hemisphere-centered coronal hole with a toward (negative sign) sector polarity. The white solid lines outline the coronal hole. storm, see Knipp et al. [1998]). However, no such enhancement was observed during the 18 November storm, even though the maximum solar wind speed during both these storms exceeded the threshold level of 600 km/s. These two storms were chosen for the case study because of the availability of comprehensive data sets of in situ particles and plasma waves in the inner magnetosphere, the solar wind data, and solar observations. Using these comprehensive data sets, we investigate the difference in the evolution of the outer belt during the two events, and we discuss why the high-speed stream of the 18 November storm did not cause flux enhancements in the outer radiation belt. 2. Results [6] Figures 1a and 1b show the solar soft X-ray images observed by the Yohkoh satellite on 3 November and 15 November, respectively. The dark regions outlined with solid white lines indicate coronal holes. According to the coronal magnetic field map obtained from Wilcox Solar Observatory (http://soi.stanford.edu/wso/), the magnetic field pointed away from the northern coronal hole on 3 November, while the magnetic field pointed toward the southern coronal hole on 15 November. Taking into account the approximate propagation time of 2 3 days from the Sun to the Earth, it can be concluded that the high-speed streams from these two coronal holes produced the CIRs to drive the two storms as shown below in detail. [7] Figure 2 shows the solar wind and IMF conditions derived from the low-energy particle experiment (LEP) [Mukai et al., 1994] and magnetic field experiment (MGF) data [Kokubun et al., 1994] measured by Geotail along with the indices of geomagnetic activity. During this period, Geotail was located about 200 R E tailward of the Earth, mainly in the magnetosheath in the vicinity of the magnetopause. The data gaps in Figure 2 occurred during the time interval when Geotail was in the magnetosphere, as identified by Eastman et al. [1998] and Christon et al. [1998]. Stream interfaces indicated by vertical lines in Figure 2 are identified from the deflection in the east/west flow. The observed large-scale solar wind variations during the two storms show a typical structure of CIRs, a stream interface corresponds to an increase in the solar wind speed and temperature and a decrease in the density [e.g., Kataoka and Miyoshi, 2006], although McAllister et al. [1998] suggested that the 3 November storm was possibly associated with small-scale coronal mass ejection embedded within the CIR. The existence of stream interfaces confirms the fact that the two CIRs at the leading edges of high-speed streams emanating from the coronal holes (Figure 1) arrived at the Earth and caused the two geomagnetic storms. After the density enhancement of the 3 November storm, the solar wind speed increased to about 800 km/s accompanied by an increase in the ion temperature. Within the CIR, the minimum IMF Bz that produced the main phase of the storm was about 20 nt. During the 18 November storm, the maximum solar wind speed was about 650 km/s, while the maximum solar wind density was larger than that of the 3 November storm. IMF fluctuations were observed within the coronal hole streams in both events. [8] The IMF sector polarity was different during these two events. Figure 2f shows the azimuth angle 8 of the IMF in the GSE coordinate [Russell, 1971], which is used to identify the away sector of 90 < 8 < 180 and toward sector of 90 < 8 <0. The sector polarity changed from toward to away in the initial phase of the 3 November storm and stayed away during the recovery phase of the storm. During the 18 November storm the sector polarity again changed from away to toward in the initial phase and stayed toward throughout the storm. These variations of the sector polarity were consistent with the coronal field map from the Wilcox Solar Observatory. A somewhat large scatter in the 8 angles as observed during the period is not unusual considering the fact that Geotail located downstream of the shock [see Kataoka et al., 2005]. The magnetosheath data obtained from Geotail is capable as a proxy of large-scale solar wind structures such as CIRs, coronal hole streams, and sector polarity as shown in Figure 2. Note also that the direct solar wind observation by IMP8 had significant data gaps during the whole time period. The minimum Dst for the 3 November storm was 119 nt, while that for the 18 November storm was 83 nt. The maximum AE index for the 3 and 18 November storms were about 2300 nt and 1500 nt, 2of7

Figure 2. Solar wind parameters observed from the Geotail satellite. Two stream interfaces are indicated by vertical lines. (a) Solar wind density, (b) speed, (c) ion temperature, (d) magnetic field strength, (e) GSM z-component of the magnetic field. The red (green) color indicates the positive (negative) IMF Bz. (f) Azimuth angle of the IMF in the GSE coordinates. The red (green) color indicates the away (toward) sector polarity. (g) AE, (h) Kp, and (i) Dst indices. respectively. The recovery phase of the 3 November storm lasted for about 1 week and was associated with prolonged and enhanced magnetic activity as monitored by the AE index, while the intense AE activity was seen only around the main phase and the early recovery phase for the 18 November storm. The Dst index can also be seen to have quickly recovered after the main phase of the 18 November storm. The behavior of the AE index shows that the recovery phase of the 3 November storm is so-called high-intensity, longduration, continuous AE activities (HILDCAA) [Tsurutani and Gonzalez, 1987; Tsurutani et al., 2006]. [9] A significant difference between these two storms can be found in the observations of relativistic electrons, hot and subrelativistic electrons, and plasma waves which have been considered as related parameters for the flux enhancement [e.g., Horne and Thorne, 1998; Horne, 2002; Meredith et al., 2003; Miyoshi et al., 2003]. Figure 3a shows the variation in relativistic electrons (>2 MeV) at geosynchronous orbit as observed by GOES-07. Typical storm time variations are observed with an initial depletion in the flux followed by a recovery. During the 3 November storm, the flux recovered to the prestorm level starting from late main phase. The flux exceeded 10 4 /cm 2 s str at the day after the 3of7

Figure 3. (a) Electron flux (>2.0 MeV) at geosynchronous orbit measured by the GOES-07 satellite. (b) 135 kev electron flux measured by the LANL 1989-046 satellite. (c) Electron flux (>2.5 MeV) measured by the Akebono satellite. (d) Electron flux (30 100 kev) at the dawnside measured by the NOAA-12 satellite. (e) Electric field of 0.1 1.0 fce_eq at the dawnside measured by the Akebono satellite. The red line in each figure represents the empirical plasmapause location. (f) Dst index. arrival of the coronal hole stream and then gradually increased and remained high level until 13 November (DOY 317), when the next solar wind disturbance arrived at the Earth. The flux did not show any significant increase above the prestorm levels for the 18 November storm. The maximum flux during the recovery phase of this storm was less than 10 3 /cm 2 s str, which was comparable to the prestorm values. [10] Since subrelativistic electrons are considered to be the seed population for producing relativistic electrons [e.g., Baker et al., 1998; Miyoshi et al., 2003; Meredith et al., 2003], the difference in their flux variations may affect the evolution of the outer belt. Subrelativistic electron flux in the energy range of 135 kev measured by LANL 1989-046 are shown in Figure 3b. The flux enhancements associated with substorm injections were observed during both of these storms. The injections continued during the recovery phase of the 3 November storm with a higher level of flux, while flux enhancements were observed only in the early recovery phase (19 November, DOY 323) for the 18 November storm. [11] Figures 3c and 3d show the L*-time diagrams of relativistic (>2.5 MeV) electron flux measured by the Akebono radiation monitor (RDM) [Takagi et al., 1993], and dawnside hot (30 100 kev) electron flux measured by 4of7

the NOAA12 MEPED [Raben et al., 1995], respectively. Figure 3e is the L*-time diagram of the amplitude of whistler mode plasma waves measured by the Akebono very low frequency experiment (VLF) [Kimura et al., 1990]. In order to eliminate local electrostatic cyclotron harmonic waves, we calculated the wave amplitude in the frequency range 0.1 < f/fce_eq < 1.0 where fce_eq is the equatorial gyrofrequency. The frequency-time diagram of VLF data (not shown) confirmed that the waves in this frequency range are of the chorus emissions seen outside plasmapause [Miyoshi et al., 2003]. Akebono was in the dawn-dusk meridian over the southern hemisphere at altitudes above 3000 km during this period. Note that the whistler mode amplitudes shown in Figure 3e were obtained on the dawnside where strong chorus waves can be seen during enhanced magnetic activity [e.g., Meredith et al., 2001]. The altitude of the NOAA12 satellite was about 850 km in a Sunsynchronous orbit along the 7.5 19.5 LT meridian. To reduce the effect of variations due to the Earth s magnetic field, we have restricted our analysis of the NOAA data to the northern hemisphere between geographic longitudes of 60 and 305 east. For a detailed description of data analysis, see Miyoshi et al. [2003]. Roederer-L (L*) was used in this study [Roederer, 1970], which was evaluated from the IGRF [Macmillan and Maus, 2005] and the Tsyganenko 1989 magnetic field model [Tsygananeko, 1989]. The red line in each panel indicates the positions of the empirical plasmapause at 0700 MLT parameterized by the Kp index [O Brien and Moldwin, 2003]. [12] During the recovery phase of the 3 November storm, a strong flux enhancement in the relativistic electrons (>2.5 MeV) was observed (Figure 3c). This increase started from L* 3.5, and then this region of large flux spread over the entire outer belt. The integral electron flux gradually increased during the recovery phase and exceeded 10 5 /cm 2 s str. The high level of flux persisted for several days, at least until 12 November (DOY 316). After 12 November (DOY 316), the flux decreased to 10 2 /cm 2 sstr at L* > 5. In contrast, during the recovery phase of the 18 November storm, the flux recovered to 10 4 /cm 2 sstr around L*=3 4.5 which was below prestorm levels. [13] Hot electrons of a few tens of kev provide a source of free energy for whistler mode chorus waves [e.g., Meredith et al., 2002; Miyoshi et al., 2003]. From the main phase to the recovery phase of the 3 November storm, hot electrons (30 100 kev) from the plasma sheet were intermittently injected into the outer belt (Figure 3d). These injections began during the main phase and persisted into the recovery phase. In contrast, particle injections occurred only during the main phase of the 18 November storm and were absent during the recovery phase. [14] Since the whistler mode chorus waves can be a plausible driver for the acceleration of relativistic electrons [e.g., Summers et al., 1998; Summers and Ma, 2000; Summers et al., 2002; Meredith et al., 2003; Miyoshi et al., 2003; Lyons et al., 2005; Horne et al., 2005b; Shprits et al., 2006; Omura and Summers, 2006], it is expected that the activity of the plasma waves will affect the evolution of the outer belt. Corresponding to the difference in the injection of hot electrons between the two storms, there was a pronounced difference in chorus wave activity (Figure 3e). During the main phase of the 3 November storm, intense whistler mode chorus waves can be seen to be generated outside of the plasmapause, and then waves were detected in the entire outer belt during the recovery phase. Around 10 November (DOY 314), i.e., during the late recovery phase, the intense waves were still observed. However, chorus wave activity was low during the recovery phase for the 18 November storm. 3. Summary and Discussion [15] Combining the observations from the previous section, we discuss the major conditions to make the difference in the flux evolution of the outer belt between these two storms. Miyoshi et al. [2003] examined the 3 November storm in detail and concluded that nonadiabatic acceleration by means of wave-particle interactions was the main reason for the flux enhancement in the outer belt: continuous injection of hot electrons (Figure 3d) generates whistler mode chorus waves (Figure 3e), which accelerate the injected subrelativistic electrons (Figure 3b) to relativistic energies (Figure 3c) outside the plasmapause. This process is also consistent with observations from Meredith et al. [2002] and Lyons et al. [2005]. From this point of view, a difference in the evolution of the flux in the outer belt during these two storms can be attributed to the hot/subrelativistic electron injections. In our hypothesis the continuous convection and/or intermittent injections during prolonged and enhanced AE activities in the 3 November storm would have produced intense plasma waves and enriched seed population, while the weak and limited injections during the 18 November storm did not produce such a situation. [16] Substorm activity influences the injections from the plasma sheet, so the difference in them during these two storms affected the evolution of flux in the outer belt. Prolonged substorm activities were present during the recovery phase of the 3 November storm, while the substorm activity was very weak during the recovery phase of the 18 November storm. It is natural to consider that Alfvénic fluctuations within a coronal hole stream are a driver for socalled HILDCAA (and HILDCAA-like) event during CIRdriven storms [e.g., Tsurutani and Gonzalez, 1987; Tsurutani et al., 2006]. As shown in Figure 2, the two storms were driven by CIRs, and magnetic field fluctuations were observed in both coronal hole streams, but there is the northward offset of the IMF Bz during the 18 November storm. In fact, the average amplitude of Bz for 72 hours after the stream interface crossing is 0.34 nt in the 3 November storm and +0.77 nt in the 18 November storm. Note that during the recovery phase of the 18 November storm the toward sector in fall produces an offset in the positive IMF Bz component in the GSM frame, thus suppressing geomagnetic activity due to so-called the Russell-McPherron effect (dipole tilt effect associated with the IMF polarity) [Russell and McPherron, 1973]. This mechanism works in an opposite sense during the 3 November storm. Therefore one possible explanation is that the IMF sector polarity controls the substorm activity during the recovery phase due to the Russell-McPherron effect, thus causing the difference in the flux evolution of the outer radiation belt. [17] One may simply conclude that the difference of the solar wind speed is just significant to explain the difference of the flux evolution in the outer belt because the maximum 5of7

solar wind speed for the 3 November storm (800 km/s) was faster than that for the 18 November storm (650 km/s) as shown in Figure 2. However, note that the flux enhancement on the 18 November storm is almost nothing comparing with the prestorm level, and such a situation itself is unusual considering the solar wind speed faster than 600 km/s based on the statistical study of Weigel et al. [2003] in which the solar wind speed faster than 600 km/s is basically a sufficient condition to enhance the outer belt flux. Therefore it is not unreasonable to consider here that the suppression of the flux evolution on the 18 November storm may be caused by other parameters than the solar wind speed. Iles et al. [2002] showed that the main requirements for significant enhancements in the outer radiation belt during the recovery phase were fast solar wind speed and an IMF Bz that is fluctuating around zero or more predominantly southward. They also found that predominantly northward IMF during the recovery phase limits the recovery of the outer belt flux. Their results are consistent with this study. The statistical study to confirm the importance of the IMF for the flux enhancement will be reported in future. [18] Finally, on the basis of our findings, we discuss a general association between magnetic storms and flux enhancements at the outer portion of the outer belt. Meredith et al. [2003] found that flux enhancements can occur at prolonged substorm activity even in the absence of a magnetic storm. A subsequent modeling study showed that the chorus emission enhanced during prolong substorm activity can cause flux enhancements in the outer belt regardless of the presence or absence of a magnetic storm [Summers et al., 2004]. Kim et al. [2006] showed statistically that the intense flux enhancement at geosynchronous orbit occurs regardless of whether or not a magnetic storm takes place. Lyons et al. [2005] suggested that a magnetic storm is not necessary for the relativistic electron enhancement. Considering these results, it is expected that the effective solar wind structure for flux enhancements at the outer portion of the outer belt would be different from that for driving magnetic storms. Our results indicate that the large-amplitude magnetic field fluctuations with the southward offset within the coronal hole stream are important for the flux enhancement in the outer belt via continuous substorm activities in the magnetosphere. The intense southward IMF, which is the driver for the main phase of geomagnetic storms through evolution of ring current ions, produces a net reduction of the flux in the outer belt. We suggest that the southward IMF associated with magnetic field fluctuations within the coronal hole stream tends not to be effective enough to drive intense ring current evolutions but still plays an important role in energy transfer from the solar wind to the magnetosphere to produce the relativistic electrons at the outer portion of the outer belt. Therefore on the basis of this idea, it can be reasonably understood that the flux enhancement at the outer portion of the outer belt by magnetic field fluctuations within the coronal hole stream is independent of the strength of a magnetic storm as shown by Reeves et al. [2003]. 4. Conclusions [19] Using the comprehensive data sets, we studied differences in the flux evolution of MeV electrons in the outer belt during the two storms in November 1993. The recovery phase of the 3 November storm was associated with the prolonged and enhanced substorm activity, continuous hot and subrelativistic electron injections, and resultant intense chorus activity. The injected hot electrons provide free energy for the excitation of plasma waves that can accelerate subrelativistic electrons to MeV energies by means of nonadiabatic acceleration process. In contrast, the recovery phase of the 18 November storm was associated with reduced substorm activity, weak injections of hot and subrelativistic electrons, and low chorus wave activity. This case study suggests that the difference in the IMF sector polarity via the Russell-McPherron effect (dipole tilt effect associated with the IMF polarity) produces the different activities in substorms during the two storms, leading to the difference in the evolution of the flux in the outer radiation belt. [20] Acknowledgments. We would like to thank C. Farrugia and H. 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