Relations of the energetic proton fluxes in the central plasma sheet with solar wind and geomagnetic activities

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, , doi: /2013ja019289, 2013 Relations of the energetic proton fluxes in the central plasma sheet with solar wind and geomagnetic activities Jinbin Cao, 1 Aiying Duan, 2,3 Henri Reme, 3,4 and Iannis Dandouras 4,5 Received 5 August 2013; revised 22 September 2013; accepted 9 November 2013; published 27 November [1] In this paper, using 9 years of Cluster data, we statistically investigate the relations of central plasma sheet energetic proton fluxes, ~30 kev to 380 kev, with the solar wind parameters and geomagnetic indexes. The energetic proton fluxes increase with increasing solar wind dynamical pressure and solar wind speed. The energetic proton fluxes are more correlated with solar wind dynamical pressure than with solar wind speed. During northward interplanetary magnetic field (IMF) Bz,energetic proton fluxes are independent of northward IMF Bz, while during southward IMF Bz, energetic proton fluxes are highly correlated with southward IMF Bz and increase with increasing IMF Bz. The response time of energetic proton flux to southward IMF Bz is between 40 and 100 min. The energetic proton fluxes are correlated with plasma sheet ion temperature. The energetic proton fluxes increase with increasing indexes of Kp, AE, and Dst. Among the three geomagnetic indexes, the central plasma sheet energetic proton fluxes are most correlated with Kp index with the largest correlation coefficient being The energetic proton fluxes are large during positive Dst index, suggesting that the sharp increase of solar wind dynamical pressure can enhance the plasma sheet energetic proton fluxes. The enhanced plasma sheet energetic proton fluxes may be important for geomagnetic storms and substorms since they can possibly directly become the source of ring current and substorm-injected energetic particles without the need of additional acceleration process in the inner magnetosphere. Citation: Cao, J., A. Duan, H. Reme, and I. Dandouras (2013), Relations of the energetic proton fluxes in the central plasma sheet with solar wind and geomagnetic activities, J. Geophys. Res. Space Physics, 118, , doi: /2013ja Introduction [2] In the plasma sheet, the ions with energies larger than thermal energy (~30 kev) are called energetic ions and the ions with energy between 100 ev and 30 kev are usually referred to as hot ions. Plasma sheet energetic particles are considered to be a significant source of the energetic particles which are injected into the inner magnetosphere during magnetospheric substorms [Birn et al., 1997; Li et al., 1998; Dandouras et al., 2009]. In addition, the plasma sheet is also one of the major particle source for the ring current population ( kev ions and electrons) [Chen et al., 1994; Jordanova et al., 1998; Kozyra et al., 1998; Wang et al., 2008]. Moreover, during the time of enhanced magnetospheric convection, the plasma sheet energetic particles can penetrate deeper inside inner magnetosphere and become the seed 1 Space Science Institute, School of Astronautics, Beihang University, Beijing, China. 2 State Key Laboratory for Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China. 3 University of Chinese Academy of Sciences, Beijing, China. 4 Institut de Recherche en Astrophysique et Planétologie, CNRS, Toulouse, France. 5 UPS, IRAP, University of Toulouse, Toulouse, France. Corresponding author: J. Cao, Space Science Institute, School of Astronautics, Beihang University, Beijing , China. (jbcao@buaa.edu.cn) American Geophysical Union. All Rights Reserved /13/ /2013JA particles of radiation belt high-energy particles since the outer boundary of radiation belt is around 6 R E [Ganushkina et al., 2011]. [3] The plasma sheet energetic protons mainly come from some local acceleration processes in the plasma sheet, which include magnetic reconnection [Wang et al., 2002; Nakamura et al., 2004], dipolarization [Delcourt and Sauvaud, 1994], and dipolarization front [Artemyev et al., 2012]. Here magnetic reconnection may be the most important source of plasma sheet energetic protons because the dipolarization front can be also produced by earthward fast flows generated by magnetic reconnection. [4] The dependence of magnetotail plasma sheet parameters on the solar wind is central to magnetospheric physics since the plasma sheet plays an important role in the transport of mass and energy from the solar wind into the dipolar magnetosphere. Until now, a large amount of work has been dedicated to the analysis of the relations between plasma sheet parameters (plasma moment parameters) and solar wind parameters [Terasawa et al., 1997; Borovsky et al., 1998; Wing and Newell, 2002; Tsyganenko and Mukai, 2003; Kletzing et al., 2003; Wang et al., 2006, 2007; Nagata et al., 2007, 2008; Denton and Taylor, 2008; Zheng et al., 2009]. However, much less attention is given to the relations between plasma sheet energetic particles and solar wind parameters. There are only a limited number of papers concerning the relations of plasma sheet energetic electrons [Burin des Roziers et al., 2009; Luo et al., 2011] and energetic protons with solar wind parameters. 7226

2 (a) (b) show little evidence of asymmetry. Krimigis and Sarris [1979] found that the energetic electrons and protons exhibit pronounced dawn-dusk asymmetries, with electron burst intensities being largest in the dawn sector of the magnetotail while proton intensities are higher in the dusk sector. Sarafopoulos et al. [2001, 2004] studied the microinjection of the energetic particles in the plasma sheet with the Interball-tail satellite; their results suggested that the electron-ion drifts probably lead to the formation of the significant dawn-dusk species-dependent asymmetry of energetic particles within the plasma sheet. Wang et al. [2006] showed that the energetic proton fluxes (190 kev) are higher in the premidnight sector than in the postmidnight sector. [8] In this paper, using 9 years of Cluster data, we statistically investigate the relations of plasma sheet energetic proton fluxes, ~30 kev to 380 kev, with the solar wind and geomagnetic indexes (Kp, Dst, and AE indexes). It is found that the plasma sheet energetic proton fluxes increase with increasing solar wind dynamical pressure and southward IMF Bz. Besides, the energetic proton fluxes in the neutral sheet also increase with increasing Kp, Dst, and AE indexes. Figure 1. Distribution of neutral sheet crossings in the GSM XY and XZ plane. Burin des Roziers et al. [2009] investigated the relationship between energetic electrons in the central plasma sheet and the solar wind and found that energetic electron fluxes beyond geosynchronous orbit show good correlation with solar wind speed and interplanetary magnetic field (IMF) Bz, with the highest correlation coefficient within min time lag, and there is a weak negative correlation between energetic plasma sheet electrons and the solar wind density. Furthermore, Luo et al. [2011], using the data of Geotail, modeled the spatial distribution of plasma sheet energetic electron. [5] The energetic particles in the plasma sheet are also related with geomagnetic activities. Keath et al. [1976] pointed out that the electron event probabilities show a clear correlation with Kp index in both the dawn and dusk sectors of the magnetotail, but the correlation of proton events with Kp is found only in the dawn sector. Lu et al.[2007]performed a case study and found that the plasma sheet energetic ion flux is enhanced during a geomagnetic storm. [6] Imada et al. [2008] studied the distribution of the energetic/thermal particles in the plasma sheet using the Geotail data. They found the clear dependence between energetic particles and Kp index and pointed out that compared with the energetic ions, the energetic electrons are more likely to depend on Kp index. However, up to now, there is no statistical study published about the relation between plasma sheet energetic protons and Dst (AE) index. [7] The spatial distribution of the energetic particles in the plasma sheet has been discussed in many papers. Keath et al. [1976] statistically analyzed the morphology of 50 to 200 kev protons and 30 to 90 kev electrons in the magnetotail and magnetosheath using the IMP7 satellite data, and they found that the protons show a dawn-dusk asymmetry in the probability of occurrence of the events, while the electrons 2. Instruments and Data Selection Criteria 2.1. Instrumentation [9] The Cluster mission of European Space Agency is composed of four identical spacecraft in a tetrahedral formation. The four Cluster satellites have a highly elliptical polar orbit (~89 inclination) and an orbital period of 57 h, with an apogee and a perigee of about 19.6 R E and 2 4 R E, respectively. Each year from June to November, Cluster crossed tail plasma sheet, providing a good opportunity to study characteristics of plasma sheet. In this study, the plasma moment data come from the HIA (Hot Ion Analyzer) instrument of Cluster Ion Spectrometry [Rème et al., 2001], which can provide three-dimensional velocity distribution of ions in the energy range 5 ev to 32 kev. The magnetic field data are from Flux Gate Magnetometer [Balogh et al., 2001], which can provide magnetic field measurements with a sampling rate up to about Hz. The data of energetic protons are from the IIMS (Imaging Ion Mass Spectrometer) instrument of the RAPID (Research with Adaptive Particle Imaging Detectors) instrument [Wilken et al., 1997]. The RAPID instrument measures ion distributions in the energy range from 30 to 1500 kev for protons and 0.1 kev/nucleon to 1500 kev for heavier ions. We use proton data in IIMS/ HSPCT (Hydrogen Spectrum Product) mode. HSPCT is a spin-averaged product producing a nearly omnidirectional product for protons in eight energy channels. HSPCT energy channels E1 E8 have the following energy ranges in kev: E1: , E2: , E3: , E4: , E5: , E6: , E7: , and E8: >4007. However, the data from the last four channels are sparse. Therefore, we only use the E1 E4 data from IIMS/HSPCT. Since there are no HSPCT data for C1 from 2007 to 2009 and for all four spacecraft in 2008, all Cluster data used in this study are from C1 from 2001 to 2006 and from C3 in the years of 2007 and All Cluster data have 4 s time resolution. The solar wind data are from the OMNI database (available from NASA s Space Physics Data Facility gov), a compilation of values from various satellites, 7227

3 Figure 2. Distribution of the energetic proton differential fluxes of the four energy channels averaged over an area of 1 1 R 2 E in the GSM XY plane. Some regions are left blank due to insufficient data samples. shifted to the subsolar point 10 R E from the Earth. We use the 1 min resolution data here Data Selection Criteria [10] First, we choose the time intervals during which the Cluster satellites are located inside the plasma sheet using the following criteria: (1) the plasma β > 0.3 and (2) the spacecraft did not observe fast flows with V > 150 km/s for at least 20 min. The second criterion is used to exclude the influence of the fast flows in plasma sheet because these fast ion flows can produce remarkable variations of local plasma parameters [Baumjohann et al., 1990; Angelopoulos et al., 1994; Shiokawa et al., 1998; Nakamura et al., 2002; Slavin et al., 2002; Cao et al., 2008; Ma et al., 2009]. [11] Second, we confine the positions of the spacecraft as in Angelopoulos et al. [1994] and Cao et al. [2006]: (1) 7228

4 ALL CPS crossings Dawnside CPS crosssings [13] The above three criteria result in a total of 3468 central plasma sheet (CPS) data points. We define T NS =(t i 1 + t i +t i +1 + t i+2 )/4 and J=(J(t i 1 )+J(t i )+J(t i+1 )+J(t i+2 ))/4 as the CPS center crossing time and the corresponding flux of the energetic protons, respectively. The data of OMNI are obtained by searching the time point T omni which is the closest to T NS. Figure 3. Scatterplot of the energetic proton fluxes of four energy channels J E (E1 E4) versus CPS ion temperature T i for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. X GSM < 10 R E, which guarantees the database outside the hinge point; (2) Y GSM < 15 R E, which basically excludes the events outside magnetopause; and (3) to eliminate the mantle crossings, we remove the data in the region Y 2 GSM þ 1=2 Z2 GSM > 10RE and Z GSM > 6 R E. [12] Third, we determine the time point at which the spacecraft crossed the center of plasma sheet using the reversal characteristic of the magnetic field, namely, the criterion B x (t i ) B x (t i +1 ) < 0(B x is the x component of the magnetic field, t i and t i+1 are two successive measurement times). In addition, multiple crossing events of the center of plasma sheet within one orbit are also removed so as to avoid the influence of plasma sheet flapping [Lui et al., 1978; Zhang et al., 2002; Sergeev et al., 2003; Runov et al., 2005; Duan et al., 2013]. 3. Observations [14] The spatial distributions of the 3468 CPS crossings on the GSM XY and XZ planes are shown in Figure 1. From Figure 1a, one can see that there is an obvious dawn-dusk asymmetry of the CPS crossing distributions, which shows that about 67% of the events are observed in the dawn sector. This asymmetry comes from the fact that the fast ion flows in the near-earth plasma sheet drift duskward while moving toward the Earth, and thus, Cluster observed more fast ion flows in the dusk sector than in the dawn sector. These CPS crossings accompanied by fast ion flows are removed from our CPS crossing list according to the above mentioned selection criteria, resulting in the dawn-dusk asymmetry of the CPS crossing distributions. [15] Figure 2 shows a 2-D distribution of the energetic proton differential fluxes of the four energy channels averaged over 1 1 R E 2 in the GSM XY plane. The energetic proton fluxes show an obvious dawn-dusk asymmetry with higher energetic proton fluxes on the duskside (Y GSM > 0). The dawn-dusk asymmetry of energetic protons is energy dependent and become weaker when the energy increases. This dawn-dusk asymmetry distributions are consistent with the previously published results and are considered to result from the westward diamagnetic drift of ions [Imada et al., 2008; Sarafopoulos et al., 2001, 2004; Wang et al., 2006]. In the central plasma sheet, ions are subject to earthward electric drift and westward magnetic drift. For energetic protons discussed here, the drift is dominated by the westward magnetic drift. This makes the dawnside tail accessible mostly by protons from the dawn flank and the duskside tail accessible mostly by protons from the tail. Since the energetic proton drift is dominated by the westward magnetic drift [Wang et al., 2006] and the energetic protons elsewhere in the tail cannot drift to the dawn flank, the energetic protons in the dawn flank area can only be produced by some local accelerating processes (for example, magnetic reconnection and dipolarization front) [Fu et al., 2011; Artemyev et al., 2012] which occur in the dawn flank area. Therefore, the number of the energetic protons from the dawn flank is much smaller than that from the tail, allowing the energetic proton fluxes in the duskside tail to be much larger than those in the dawnside tail. In addition, Table 1. Correlation Coefficients Between J E and D p /V x for Dawnside Samples at Different Time Delays (TD) TD (min) CC(D p,j E1 ) CC(D p,j E2 ) CC(D p,j E3 ) CC(D p,j E4 ) CC(V x,j E1 ) CC(V x,j E2 ) CC(V x,j E3 ) CC(V x,j E4 )

5 CAO ET AL.: CPS ENERGETIC PROTONS CC= 0.49 CC= 0.65 CC= 0.31 CC= 0.41 temperature, and the correlation coefficients (CC) between JE1 and Ti are 0.48 for all CPS crossings and 0.50 for dawnside CPS crossings. The median energetic proton flux JE1 increases with increasing ion temperature Ti. For other three energy channels, the median energetic proton fluxes still increase with increasing ion temperature. However, the dependence of energetic proton fluxes on hot ion temperature becomes weaker and weaker with increasing energy. For example, the correlation coefficients between JE4 and Ti are 0.16 for all CPS crossings and 0.23 for dawnside CPS crossings Relations Between Solar Wind Parameters and Energetic Proton Fluxes in the Central Plasma Sheet Relation Between Solar Wind Parameters and Energetic Proton Fluxes in the Central Plasma Sheet [18] Table 1 shows the correlation coefficients between JE and solar wind parameters (Dp and Vx) for dawnside samples CC= 0.28 JE1(cm-2sr-1s-1keV-1) 10 6 CC= 0.39 ALL CPS crossings Dawnside CPS crosssings CC= 0.20 CC= 0.30 CC= 0.04 CC= 0.20 CC= 0.06 CC= 0.13 CC= 0.05 CC= CC= 0.28 JE2(cm-2sr-1s-1keV-1) 10 5 CC= energetic protons can obtain more or less energy in the drift path from the tail due to gradually increasing magnetic field. These two effects are combined together and result in the dawn asymmetry seen in Figure Relation Between Energetic Proton Flux and Ion Temperature in the Central Plasma Sheet [16] In this study, we perform our statistical study for two groups of CPS crossings: (1) all the CPS crossings and (2) dawnside CPS crossings in the region 16 RE XGSM 12 RE and 15 RE YGSM 10 RE. [17] Figure 3 shows the scatterplot of CPS energetic proton fluxes of the four energy channels versus ion temperature for two CPS crossing groups. The ion temperature is provided by the instrument HIA. As seen in Figures 3a1 and 3a2, the energetic proton fluxes are moderately correlated with ion JE4(cm-2sr-1s-1keV-1) Figure 4. Scatterplot of the energetic proton fluxes of the four energy channels JE (E1 E4) versus solar wind dynamical pressure Dp for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. JE3(cm-2sr-1s-1keV-1) VSW (km/s) VSW (km/s) Figure 5. Scatterplot of the energetic proton fluxes of the four energy channels JE (E1 E4) versus solar wind velocity Vx for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. 7230

6 Table 2. Correlation Coefficients Between J E and Northward/ Southward IMF Bz for Dawnside Samples at Different Time Delays (TD) TD (min) CC(IMF Bz > 0,J E2 ) CC(IMF Bz > 0,J E2 ) CC(IMF Bz > 0,J E3 ) CC(IMF Bz > 0,J E4 ) CC(IMF Bz < 0,J E2 ) CC(IMF Bz < 0,J E2 ) CC(IMF Bz < 0,J E3 ) CC(IMF Bz < 0,J E4 ) at different time delays between solar wind parameters and CPS energetic proton fluxes. It can be found that the correlation coefficients between J E and D p are larger than the correlation coefficients between J E and V x. The correlation coefficients between J E and D p decrease slowly with increasing time delays and are largest at a time delay between 0 and 20 min. For all four energy channels, the correlation coefficients between J E1 and D p are largest. The correlation coefficients between J E and V x display similar features. [19] Figure 4 shows the scatterplot of the energetic proton fluxes of the four energy channels versus solar wind dynamical pressure D p for two CPS crossing groups. For all four energy channels, the median energetic proton fluxes J E increase with increasing solar wind dynamical pressure. The correlation coefficients between J E1 and D p are 0.49 for all CPS crossings and 0.65 for dawnside CPS crossings. When D p > 4nPa,the lower limit of energetic proton fluxes J E1 increases rapidly with increasing solar wind dynamical pressure and the upper limit of energetic proton fluxes J E1 remains almost unchanged. [20] We further study the correlations of energetic proton fluxes with solar wind velocity and ion number density. Figure 5 shows the scatterplot of energetic proton fluxes of the four energy channels versus solar wind flow velocity. Although the median energetic proton fluxes J E still increase slowly with increasing V x,thecorrelationcoefficients between J E and V x are small. For the first energy channel, the correlation coefficient between J E1 and V x is only 0.2 for all samples and 0.30 for duskside samples. For the other three energy channels, the correlation coefficients are even smaller than 0.2. The energetic proton fluxes of the four energy channels are also poorly correlated with solar wind ion number density and the correlation coefficients are all smaller than [21] Therefore, among solar wind dynamical pressure, velocity, and ion number density, the energetic proton fluxes are most correlated with solar wind dynamical pressure. This indicates that the compression of magnetosphere can lead to the increase of energetic proton fluxes. There are two possible mechanisms that may contribute to the above mentioned positive correlation between solar wind dynamical pressure and CPS energetic proton fluxes. First, the increase of solar wind dynamical pressure can cause the compression of the magnetosphere [Shue et al., 1998; Lin et al., 2010], resulting into the increase of magnetic field magnitude in the tail plasma sheet [Keika et al., 2008]. Since the compression of plasma sheet is adiabatic [Miyashita et al., 2010], the increase of magnetic field magnitude in the plasma sheet can produce adiabatic acceleration of energetic protons. Second, the increase of solar wind dynamical pressure will lead to the increase of plasma sheet ion temperature [Terasawa et al., 1997; Borovsky et al., 1998; Miyashita et al., 2010]. This is because the compression of plasma sheet by increasing solar wind dynamical pressure is an adiabatic process. Therefore, the increase of solar wind dynamical pressure will cause both the density and the temperature of plasma sheet to rise [Borovsky et al., 1998]. According to the result presented in Figure 3, large ion temperatures are on average accompanied by large energetic proton fluxes Relation Between IMF Bz and Energetic Proton Fluxes in the Central Plasma Sheet [22] Wang et al. [2006, 2007] showed that the plasma sheet hot ions react differently to northward and southward IMF Bz conditions. Here we further show how the plasma sheet energetic Figure 6. Scatterplot of the energetic proton fluxes of the four energy channels J E (E1 E4) versus northward IMF Bz (Bz > 0) for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. 7231

7 mentioned that the response time inferred here is only confined to plasma sheet energetic protons. In fact, the response times of different parameters in the magnetosphere to southward IMF Bz are different. For example, the response time of subsolar geomagnetic field to a sudden southward turning of IMF Bz is only 6 min [Yu and Ridley, 2009; Li et al., 2013]. [23] Figures 6 and 7 show the scatterplots of the energetic proton fluxes of the four energy channels versus northward and southward IMF Bz, respectively, for a time delay of 80 min that corresponds to the peak correlation coefficient between J E1 and southward IMF Bz. As seen in Figure 6, during northward IMF Bz (IMF Bz > 0), the energetic proton fluxes are almost independent of IMF Bz, the median values do not obviously increase with increasing IMF Bz, and the absolute value of correlation coefficients for all four energy Figure 7. Scatterplot of the energetic proton fluxes of the four energy channels J E (E1 E4) versus southward IMF Bz (Bz < 0) for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. protons respond to the northward and southward IMF Bz. Table 2 shows the correlation coefficients between J E and southward/northward IMF Bz for dawnside samples at different time delays between IMF Bz and CPS energetic proton fluxes. It can be seen that the energetic proton flux is closely correlated with southward IMF Bz for a time delay between 40 and 100 min. The correlation coefficient between J E1 and southward IMF Bz has a peak at a time delay of min. The absolute value of the maximum correlation coefficient between J E1 and southward IMF Bz reaches The negative correlation coefficient means that statistically, the energetic proton flux increases with increasing IMF Bz(<0). On the other hand, the energetic proton flux seems to be not related with northward IMF Bz. The correlation coefficient between J E and northward IMF Bz is smaller than 0.15 and also fluctuates around 0. It should be Figure 8. Scatterplot of the energetic proton fluxes of the four energy channels J E (E1 E4) versus Kp index for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. 7232

8 Figure 9. Scatterplot of the energetic proton fluxes of the four energy channels J E (E1 E4) versus AE index for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. channels is all below However, during southward IMF Bz (IMF Bz < 0), the median energetic proton fluxes increase obviously with the increase of absolute value of IMF Bz, and the maximum absolute value of correlation coefficient reaches Wang et al. [2006] once reported that the overall fluxes for the high-energy (50 kev and 190 kev therein) ions are larger during southward IMF Bz than those during northward IMF Bz. In our study, the energetic proton fluxes during weak southward IMF Bz for 2 nt< IMF Bz < 0in Figure 7 are comparable to those during weak northward IMF Bz for 0 < IMF Bz < 2 nt in Figure 6. The increase of energetic proton fluxes for enhanced southward IMF Bz makes the average energetic proton fluxes during southward IMF Bz larger than the average energetic proton fluxes during northward IMF Bz. In fact, IMF Bz not only controls the energetic proton fluxes as presented here but also controls the energetic electron fluxes [Burin des Roziers et al., 2009; Luo et al., 2011]. It is worth noting that the behavior of energetic proton fluxes during northward IMF Bz is different from that of energetic electron fluxes during northward IMF Bz presented in Figure 7 of Luo et al. [2011]. In Luo et al. [2011], the energetic electron fluxes decrease with increasing IMF Bz during northward IMF Bz. [24] The correlation between energetic proton fluxes and southward IMF Bz can be explained by the following physical process. The enhanced southward IMF Bz increases the input of solar wind energy into magnetosphere. The inputted energy of solar wind will be stored in the tail lobe in the form of magnetic energy. When more and more solar wind energy is stored in the tail lobe, the magnetic field magnitude in the lobe will increase. The increasing magnetic field in the lobe causes the thinning of the plasma sheet and the increase of ion temperature in the plasma sheet [Miyashita et al., 2010], eventually leading to the increase of energetic proton fluxes in the central plasma sheet. In addition, the thinning of plasma sheet implies that the oppositely directed field lines in the northern and southern plasma sheets become closer to each other, resulting in the increase of probability of magnetic field line reconnection. More magnetic reconnection will certainly produce more energetic protons. This mechanism can also be used to explain the enhancement of energetic plasma sheet electrons during times of southward interplanetary magnetic field (IMF), which has been reported in previous papers [Burin des Roziers et al., 2009; Luo et al., 2011]. [25] Comparing Tables 1 and 2, we found that the time delays of CC(J E, D p )andcc(j E,IMFBz) are different. The correlation coefficients between J E and D p are largest at a time delay between 0 and 20 min. On the other hand, the correlation coefficient between J E and southward IMF Bz has a peak at a time delay of min. The difference between two kinds of correlation coefficients is not surprising because solar wind dynamical pressure and southward IMF Bz influence the CPS energetic proton fluxes via two different physical processes. The increase of solar wind dynamical pressure compresses the magnetotail at a time scale of several minutes [Keika et al., 2008; Li et al., 2011]. Therefore, the response time of energetic proton fluxes to solar wind dynamical pressure is very short and at a time scale of several minutes. In contrast, for southward IMF Bz, the increase of lobe magnetic field, a consequence of the increase of solar wind energy stored in the tail lobe, is not a transient process but an energy accumulation process. That the correlation coefficient between southward IMF Bz and J E peaks at a time delay of min means that the response time of CPS energetic proton fluxes to IMF Bz is min. This conclusion is consistent with the min response time of CPS energetic electron fluxes to the IMF Bz [Luo et al., 2011] Relations Between Geomagnetic Indexes and Energetic Proton Fluxes in the Central Plasma Sheet [26] Figure 8 shows the scatterplot of CPS energetic proton fluxes of the four energy channels versus Kp index. The energetic proton fluxes have a strong dependence on Kp index. The correlation coefficient between J E1 and Kp is 0.82 for dawnside samples and 0.65 for all samples. The median energetic proton fluxes increase with increasing Kp index for all energy channels. This result is not surprising. In fact, the 7233

9 CAO ET AL.: CPS ENERGETIC PROTONS [28] Figure 10 shows the scatterplot of CPS energetic proton fluxes of the four energy channels versus Dst index. For negative Dst values, the median energetic proton fluxes increase with decreasing Dst (or increasing absolute value of Dst), suggesting that strong geomagnetic storms are usually accompanied with large CPS energetic proton fluxes. The enhanced energetic proton fluxes in the plasma sheet can provide effective seed source for storm time ring current. This is important because without need of additional acceleration mechanism in the inner magnetosphere, they can directly become ring current ions only if they can drift to ring current regions. Therefore, it is possible that the energetic protons in the magnetotail can play an important role in the formation of storm time ring current. Another important new thing in Figure 10 is that for positive Dst index, energetic proton fluxes increase with increasing Dst index. The positive Dst index usually represents the sudden storm commencement caused by the sudden increase of solar wind dynamical pressure. Therefore, this result is consistent with the conclusion drawn in Figure 4 that the energetic proton fluxes increase with increasing solar wind dynamical pressure. Previous studies once show that the increase of solar wind dynamical pressure can cause the increase of energetic particles in the inner magnetosphere [Dandouras et al., 2009; Fu et al., 2012]. Our study here indicates that the increase of solar wind dynamical pressure can cause the increase of energetic particles in the tail plasma sheet. Figure 10. Scatterplot of the energetic proton fluxes of the four energy channels J E (E1 E4) versus Dst index for (a) all CPS crossings and (b) dawnside CPS crossings. The red line denotes the median of energetic proton fluxes. increasing Kp index represents an enhanced magnetospheric convection [Thomsen, 2004]. The enhanced convection will lead to the thinning of plasma sheet and increasing plasma sheet ion temperature [Wang et al., 2004]. The increasing plasma sheet ion temperature is accompanied with the increase of CPS energetic proton fluxes as seen in Figure 3. [27] Figure 9 shows the scatterplot of CPS energetic proton fluxes of the four energy channels versus AE index. The energetic proton fluxes have also a correlation with AE index, which is strongest for the first energy channel E1 and weakest for the fourth energy channel E4. For the first energy channel E1, the median energetic proton flux increases with increasing AE index. The reason for this correlation is that during substorms, the plasma sheet ion temperature increases [Christon et al., 1991; Birn et al., 1997], which results into the increase of CPS energetic proton fluxes. 4. Summary and Conclusions [29] In this paper, using 9 years of Cluster data, we statistically investigate the relations of plasma sheet energetic proton fluxes with the solar wind and geomagnetic indexes (Kp, Dst, andae indexes). The results indicate that the CPS energetic proton fluxes increase with increasing solar wind dynamical pressure and solar wind speed. The CPS energetic proton fluxes are more correlated with solar wind dynamical pressure than with solar wind speed. The CPS energetic proton fluxes are independent of northward IMF Bz but highly correlated with southward IMF Bz. The CPS energetic proton fluxes increase with increasing IMF Bz. The analysis of the dependence of correlation coefficients on time delay shows that the response time of CPS energetic proton fluxes to southward IMF Bz is between 40 and 100 min, and the response time of CPS energetic proton fluxes to solar wind dynamical pressure is between 0 and 20 min, [30] The CPS energetic proton fluxes are correlated with geomagnetic activities and increase with increasing geomagnetic indexes of Kp, AE, and Dst. Among the three geomagnetic indexes, the CPS energetic proton fluxes are most correlated with Kp index with the largest correlation coefficient being The CPS energetic proton fluxes are large during positive Dst index. Since positive Dst indexes are usually accompanied with sudden storm commencement originating from sudden compression of magnetosphere, the enhanced CPS energetic proton fluxes during positive Dst index may suggest that the sharp increase of solar wind dynamical pressure can enhance the plasma sheet energetic proton fluxes. [31] Although the CPS energetic proton fluxes are correlated with both solar wind parameters and geomagnetic indexes, the physical meanings of the correlations of CPS energetic proton fluxes with solar wind parameters and geomagnetic indexes are different. The correlations of CPS energetic proton fluxes with solar wind parameters mean that solar wind dynamical 7234

10 pressure and southward IMF Bz can control to some extent the CPS energetic proton fluxes. However, the correlations of CPS energetic proton fluxes with geomagnetic indexes only mean that increasing CPS energetic proton fluxes and geomagnetic activities are related with each other and they may be parallel products of the variations of solar wind parameters. For example, larger Kp index represents enhanced magnetospheric convection [Thomsen, 2004]. Enhanced magnetospheric convection generally occurs during the period of southward IMF Bz. The enhanced southward IMF Bz causes not only the increase of energetic proton flux but also the enhancement of magnetospheric convection (i.e., the increase of Kp index). Therefore, the correlation between plasma sheet energetic protons and Kp actually originates from the correlation between plasma sheet energetic protons and southward IMF Bz. [32] As mentioned in section 1, previous studies have revealed many features of plasma sheet energetic electrons. Some features are similar to those of plasma sheet energetic protons, such as the dawn-dusk asymmetry [Sarafopoulos et al., 2001; Wang et al., 2006; Imada et al., 2008] and the correlation with solar wind velocity, southward IMF Bz, and Kp index [Asnes et al., 2008; Burin des Roziers et al., 2009; Luo et al., 2011]. However, there are some differences between the features of plasma sheet energetic electrons and protons. For energetic electrons, the correlation with solar wind dynamical pressure is similar to the correlation with solar wind velocity. For energetic protons, the correlation with solar wind dynamical pressure is much stronger than the correlation with solar wind velocity. Also, plasma sheet energetic electrons are correlated with northward IMF Bz and have a tendency to increase with northward IMF Bz. However, energetic protons are independent of northward IMF Bz. The reasons behind these differences are still not fully clear. [33] The enhanced plasma sheet energetic proton fluxes are important to geomagnetic storms and substorms. First, during geomagnetic storms and substorms, enhanced magnetospheric convection electric field can make plasma sheet energetic ions drift closer to the Earth, even inside geosynchronous orbit [Korth et al., 1999; Ding et al., 2010; Cao et al., 2011; Jiang et al., 2011]. These energetic protons can possibly directly become the source of ring current and substorm-injected energetic particles without the need of additional acceleration process in the inner magnetosphere. 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