Proton auroral intensification induced by interplanetary shock on 7 November 2004

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010ja016239, 2011 Proton auroral intensification induced by interplanetary shock on 7 November 2004 Zhenpeng Su, 1,2 Qiu Gang Zong, 3,4 Chao Yue, 4 Yongfu Wang, 4 Hui Zhang, 5 and Huinan Zheng 1,2 Received 25 October 2010; revised 25 May 2011; accepted 31 May 2011; published 27 August [1] We report a shock induced auroral intensification event observed by the IMAGE spacecraft on 7 November The comparison of simultaneous auroral snapshots, obtained from FUV SI12 and FUV SI13 cameras onboard IMAGE spacecraft, indicates the dominance of proton precipitation (rather than electron precipitation) throughout the auroral oval region. The proton aurora in the postnoon sector showed the most significant intensification, with luminosity increasing by 5 times or more. We describe the main characteristics of interplanetary parameters observed by the ACE and Geotail satellites and plasma parameters within the mapped precipitation region detected by the Los Alamos National Laboratory satellite. The generation mechanism of postnoon proton auroral intensification is further investigated on the basis of these observations. The estimated increase of loss cone size was not enough to produce the required proton auroral precipitation enhancement. The expected oxygen band electromagnetic ion cyclotron waves (no available observation), in the highly fluctuating density region during the shock period, might contribute to the enhanced precipitation of auroral protons. Our new finding is that the shock driven buildup of 1 10 kev proton fluxes could account for the observed proton auroral intensification. Citation: Su, Z., Q. G. Zong, C. Yue, Y. Wang, H. Zhang, and H. Zheng (2011), Proton auroral intensification induced by interplanetary shock on 7 November 2004, J. Geophys. Res., 116,, doi: /2010ja Introduction [2] The interplanetary shock can strongly affect the Earth s magnetosphere. Early statistical study [Echer and Gonzalez, 2004] found that 20.6%, 35.1% and 22.3% of interplanetary shocks are followed by weak, moderate and intense storms, respectively. Substorms may occasionally occur as a direct consequence of the shock driven increase of solar windmagnetosphere dynamo efficiency [Akasofu, 1980; Zhou and Tsurutani, 2001; Yue et al., 2010]. The auroral intensifications are often triggered by interplanetary shocks [e.g., Zhou and Tsurutani, 1999; Meurant et al., 2004], which appear first at the dayside and expand longitudinally to the nightside sector within a few minutes. 1 Chinese Academy of Sciences Key Laboratory for Basic Plasma Physics, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China. 2 State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, China. 3 Center for Atmospheric Research, University of Massachusetts Lowell, Lowell, Massachusetts, USA. 4 Institute of Space Physics and Applied Technology, Peking University, Beijing, China. 5 Physics Department, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA. Copyright 2011 by the American Geophysical Union /11/2010JA [3] Several possible mechanisms [e.g., Zhou et al., 2003; Liou et al., 2007; Laundal and Østgaard, 2008] have been developed to explain the shock induced auroral intensification: field aligned acceleration, loss cone size increase and wave particle interactions. The first process may account for the discrete type aurora, while the others may lead to the diffuse type aurora. Zhou et al. [2003] suggested that fieldaligned currents are responsible for the intense discrete aurora in the poleward region of auroral oval. Such field aligned currents may be generated by some magnetopause processes under shock/pressure pulse conditions [Zhou et al., 2003; Meurant et al., 2004], e.g., magnetic field shearing, Alfvén wave generation, magnetopause perturbations, magnetic reconnection, and velocity shearing. Liou et al. [2007] found that the loss cone size increase resulting from the reduction of mirror ratio during shock compression can cause the auroral precipitation enhancement. Zhou and Tsurutani [1999], Tsurutani et al. [2001], and Zhou et al. [2003] proposed that enhanced precipitation of central plasma sheet (CPS) particles due to the pitch angle scattering by plasma waves may result in the auroral intensification after shock arrival. The electrostatic electron cyclotron harmonic (ECH) waves [e.g., Kennel et al., 1970; Lyons, 1974] and whistler mode chorus waves [e.g., Johnstone et al., 1993; Ni et al., 2008] are two candidates for the resonant scattering of diffuse auroral electrons. Su et al. [2009, 2010] simulated the resonant interaction between whistler mode chorus and CPS electrons. 1of8

2 Figure 1. Interplanetary parameters observed by ACE (black lines) and Geotail (green lines) satellites on 7 November 2004: (a) southward magnetic field B z, (b) magnetic field strength B, (c) solar wind ion density N, (d) solar wind bulk speed V x in GSE coordinates, and (e) solar wind dynamic pressure P. The time of ACE is shifted 30 min according to the solar wind speed, and the vertical line marks the interplanetary shock front. The modeling results showed that chorus waves can effectively scatter kev electrons into the loss cone, and the kev electron pitch angle distribution can evolve into the frequently observed pancake type distribution during substorms. The resonant scattering of diffuse auroral protons is often attributed to the electromagnetic ion cyclotron (EMIC) waves. Anderson and Hamilton [1993] suggested that magnetospheric compression can increase the temperature anisotropy of hot protons and then stimulate the EMIC waves. Fuselier et al. [2004] investigated the magnetospheric plasma properties in the geosynchronous region where transient proton aurora maps to, and found that the EMIC wave growth rate tends to increase after the shock compression. Zhang et al. [2008] presented the simultaneous multisatellite observations of shock induced proton aurora and EMIC waves on 21 January 2005, which provided a direct evidence of the link between EMIC waves and proton aurora. [4] In general, both diffuse and discrete auroral emissions are excited principally by electron precipitation. However, for certain time and spatial regions, the dominant auroral precipitation is the proton component [Lui et al., 1977; Frey et al., 2001; Hubert et al., 2003; Meurant et al., 2004]. In this study, we report a special event of shock induced auroral intensification, with the dominance of proton precipitation (rather than electron precipitation) throughout the auroral oval region, observed by the Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft on 7 November The most significant auroral intensification occurred in the postnoon sector. The generation mechanism of postnoon proton auroral intensification shall be determined based on the interplanetary parameters observed by the Advanced Composition Explorer (ACE) and Geotail satellites, and plasma parameters within the mapped precipitation region 2of8

3 Figure 2. Auroral snapshots obtained by (a d) FUV SI12 and (e h) FUV SI13 onboard IMAGE spacecraft on 7 November Note here that the airglow correction has been made. The aurora is shown in geomagnetic latitude/mlt coordinates. In each image the magnetic pole is at the center of cycles, magnetic local noon is at the top, dusk is on the left, and dawn is on the right. The image cadence is about 2 min. The arrival time of interplanetary shock was about 18:27:30 UT. detected by the Los Alamos National Laboratory (LANL) spacecraft. 2. Observations [5] A strong interplanetary shock [Zong et al., 2009] driven by a coronal mass ejection encountered the Earth s magnetosphere on 7 November The interplanetary parameters of this event observed by ACE and Geotail satellites are shown in Figure 1. The shock arrival times recorded by ACE and Geotail were about 17:54 and 18:24UT. The corresponding position of ACE and Geotail spacecrafts were (241.2, 24.45, 16.09)R E and (19.16, 13.62, 2.67)R E in the GSE coordinates. Clearly, the observations of ACE (timeshifted) and Geotail were generally consistent with each other. Following the interplanetary shock, there was a magnetic field strengthening from 20 to 40 nt, a solar wind density jump from less than20 to more than 40 cm 3, a solar wind speed increase from 500 to 700 km/s, and a dynamic pressure enhancement from less than 10 to 30 npa. The intense ( 40 nt) northward interplanetary magnetic field following shock front lasted for nearly 2 hours. [6] Figure 2 shows the shock induced auroral intensification detected by the IMAGE FUV SI12 and FUV SI13 imagers on 7 November FUV SI12 images the Dopplershifted Lyman a emission emitted by the hydrogen atoms resulting from charge exchanges between precipitating protons and atmospheric constituents [Mende et al., 2003]. FUV SI13 images the atomic oxygen emission at nm that are excited by secondary electrons produced by both precipitating electrons and protons [Mende et al., 2003]. It is obvious that the auroral luminosity observed by FUV SI12 was 3 5 times higher than that observed by FUV SI13 throughout the auroral oval region, indicating that proton (instead of electron) was the dominant auroral precipitation component in this event. Hence, we will concentrate on the proton aurora in this study. The proton auroral brightening appeared first in the postnoon sector at about 18:28 UT, and then expanded longitudinally to the nightside sector within two minutes. The most significant proton auroral intensification occurred in the postnoon sector. The postnoon proton aurora possessed the luminosity 2 kr during the preshock time period. It reached the peak intensity 10 kr at 18:28 UT, and dimmed gradually during the postshock time period. [7] In this event, the LANL satellite was in the postnoon sector, where the most intense precipitation mapped to. The plasma parameters observed by LANL are plotted in Figure 3. Both before and after the shock, the cold proton density n cp fluctuated significantly. After the arrival of shock, the hot (>0.1 kev) proton density n p increased by a 3of8

4 signal of FUV SI12 can be approximately given as [Mende et al., 2003] I ¼ F bðheiþ ð1þ where I is the photo electron counts; F represents the total proton number flux (protons/cm 2 /s); hei denotes the mean energy of precipitating protons; the parameter b is a function of the mean energy hei, and its value depends on the atmospheric models and the instrument parameters (e.g., passband and gain). In the dayside auroral oval, the mean energy of precipitating protons may be assumed to be unchanged (1 2 kev) [Mende et al., 2003]. As shown in Figure 2, the proton aurora in the postnoon sector showed the most significant intensification, whose luminosity increased by a factor of 5 or more. Hence, 5 times enhancement in total precipitating proton flux F was required to produce the observed proton auroral intensification. The generation mechanism shall be further pursued based on the observations. For such diffuse type proton auroral intensification, loss cone size increase and resonant scattering by EMIC waves are two frequently invoked mechanisms Loss Cone Size Increase [9] We first evaluate the contribution of loss cone size increase to the proton auroral intensification. The solid angle $ subtended by the ploss ffiffiffiffiffiffiffiffiffiffiffiffiffi cone can be computed by $ = 2(1 cos a L )(a L = arcsin B e =B m is the equatorial loss cone pitch angle, where B e and B m are the magnetic field at the equator and mirror point). The magnetic field at the mirror point is assumed to be unchanged. The equatorial magnetic field may be estimated by the balance between ram and magnetic pressures. The p3 ffiffi times increase in ram pressure (Figure 1) would result in 3 times increase in equatorial magnetic field B e. In the equatorial mapped precipitation region (L 6.6), the ratio between the postshock and preshock loss cone size can be approximately obtained: $ post =$ pre ¼ 1 cos L; post = 1 cos L; pre 2 L; post = L; pre Figure 3. Plasma parameters in the mapped precipitationregionobservedbylanl spacecraft on 7 November 2004: (a) cold proton density n cp, (b) hot proton density n p, (c) hot proton perpendicular temperature T?, (d) hot proton parallel temperature T k, and (e) hot proton temperature anisotropy A = T? /T k 1. The shock front is marked by the vertical dashed line. When the shock arrived, the satellite position was Radius = 6.60R E, MLT = 15.65, MLAT = Its corresponding ionospheric footprint was MLT = 15.65, MLAT = 67. factor of 4, while both the perpendicular temperature T? and parallel temperature T k of hot protons decreased by a factor of 3 4. The shock arrival also made the temperature anisotropy A = T? /T k 1 decrease from 0.4 to less than Generation Mechanism [8] The observed auroral emissions depend on the differential flux of precipitating protons and on the atmospheric composition [Frey et al., 2001; Mende et al., 2003]. The B e; post =B e; pre p ffiffi 3 : Following the previous work [Liou et al., 2007], the variation of total proton flux can be given as (with the assumption of an isotropic pitch angle distribution in the loss cone) p F post =F pre $ post =$ pre ffiffi 3 : ð3þ It is clear that the loss cone size increase alone could not fully explain the observed proton auroral intensification Resonant Scattering by EMIC Wave [10] We next examine the potential contribution of resonant scattering by EMIC waves to the proton precipitation enhancement. In a multi ion (H +,He +,O + )plasma,emic waves typically occur in three distinct bands below the hydrogen (H + ), helium (He + ) and oxygen (O + ) ion gyrofrequencies [Summers et al., 2007a]. The hot proton temperature anisotropy provides a free energy source of EMIC instability. As shown in Figure 3, hot proton anisotropy A did not show an increase but a notable decrease after shock, contrary to the ð2þ 4of8

5 Figure 4. Bounce averaged pitch angle diffusion coefficients hd aa i as functions of equatorial pitch angle a and energy E k with different ion compositions. Two typical values of cold plasma density (a c) 5 cm 3 and (d f) 30 cm 3 are tested. No resonance occurred in the blank region (hd aa i = 0). previous observations [Anderson and Hamilton,1993;Fuselier et al., 2004]. However, the growth rate of EMIC waves also depends on other parameters (e.g., the hot plasma beta b and the cold plasma density n c )[Fuselier et al., 2004]. The upper frequency limit w ul of expected EMIC waves can be approximately calculated by Horne and Thorne [1993]! ul A A þ 1 W H þ ¼ 0:09W H þ ¼ 0:36W He þ ¼ 1:44W O þ; ð4þ A¼0:1 where the value of A is given by the observation from LANL satellite (Figure 3) immediately following the shock compression, and W H +, W He + and W O + represent the gyrofrequencies of H +,He + and O +, respectively. Obviously, only the He + and O + bands EMIC waves might occur in this event. [11] Here we will calculate the bounce averaged pitch angle diffusion coefficients hd aa i for the assumed He + and O + bands EMIC waves following the previous works [Summers, 2005; Summers et al., 2007a, 2007b]. The diffusion coefficient code was developed by Xiao et al. [2009, 2011]. The dipole geomagnetic field model is adopted at L 6.6. Considering the significant fluctuations of cold plasma density (see Figure 3), two typical values of cold plasma density 5 cm 3 and 30 cm 3 are tested. Three types of ion composition are examined: 0.85H He O +,0.70H He O + and 0.60H He O +. The minimum resonant energy is then determined by the upper frequency limit of each band [Summers et al., 2007a]. It is found that the He + band EMIC waves with the upper frequency limit w ul can not resonate with the protons below 50 kev. For brevity, only the diffusion coefficient of O + band EMIC waves, as a function of equatorial pitch angle a and kinetic energy E k, is shown in Figure 4. The O + band EMIC emissions are assumed to have a wave amplitude 1 nt, a frequency range W O +,a central frequency 0.92W O +, a frequency width 0.07W O +, and a latitude distribution region l < 15. The upper frequency limit of O + band EMIC has been set to be close the W O + in order to lower the minimum resonant energy. Clearly, the minimum resonant energy of EMIC waves in the high density (30 cm 3 ) region is much less than that in the low density (5 cm 3 ) region, and the increase of fractional O + number density can reduce the minimum resonant energy. For the simulation cases in the low and high density regions with ion position 0.60H He O +, the minimum resonant energies are about 4.0 and 0.7 kev. Note that the usually assumed value of mean energy for precipitating protons in the dayside auroral oval region is about 1 2 kev [Mende et al., 2003]. 5of8

6 present proton auroral intensification. Figure 5 shows the spin averaged proton flux j observed by the Magnetospheric Plasma Analyzer (MPA) onboard LANL spacecraft. A significant( 50 times) enhancement of kev proton flux occurred after the shock. The filling of loss cone due to the pitch angle scattering by some waves would not produce such significant enhancement of spin averaged proton fluxes at kev. Since the LANL has no measurements of magnetic field, the proton flux in the loss cone can not be directly observed. The precipitating proton flux variation in this event is further estimated as follows. These hot protons are assumed to obey the bi Maxwellian distribution f f / n p T 1=2 k exp E k cos2 T? T k E k sin 2 T? ; ð5þ Figure 5. Evolution of the spin averaged proton flux j observed by MPA instrument onboard LANL spacecraft on 7 November The vertical dashed line marks the shock front. [12] The effectiveness of all diffusive mechanisms can be evaluated by the strong diffusion limit D sd =4a 2 L /t B with the particle bounce period t B [Lyons, 1974; Schulz and Lanzerotti, 1974]. Under the strong diffusion, the waves are able to scatter the particles across the loss cone in less than a quarter of the bounce period (2 6 minutes for 1 10 kev protons) [Lyons, 1974; Schulz and Lanzerotti, 1974]. The proton precipitation timescale ( minutes) corresponding to the strong diffusion is close to the observed timescale of proton auroral intensification, indicating that strong diffusion is required for such diffusive mechanism. In the energy range 1 10 kev, the strong diffusion limit is about s 1.The current obtained pitch angle diffusion coefficients near the loss cone are about s 1 in the low density region, and s 1 in the high density region. In order to reach the strong diffusion, the amplitude of expected EMIC waves must increase to nt in the low density region, and 3 nt in the high density region. Note that the changes in other spectral characteristics of EMIC waves will not significantly increase the values of pitch angle diffusion coefficients near the loss cone. The observed amplitudes of EMIC waves are usually less than 10 nt [e.g., Fraser et al., 1996; Bräysy et al., 1998; Erlandson and Ukhorskiy, 2001]. Hence, the expected oxygen band EMIC waves in the high density region might be able to effectively scatter the auroral protons into the loss cone, but the observed cold plasma density showed a significant fluctuation both before and after the shock (see Figure 3) Shock Driven Buildup of Proton Flux [13] We propose a new scenario that shock driven enhancement of 1 10 kev proton flux could account for the which is an appropriate model for magnetospheric protons with energies kev to tens of kev [e.g., Anderson and Hamilton, 1993]. The density n p, perpendicular temperature T?, parallel temperature T k during the preshock and postshock time periods are listed in Table 1. It is straightforward to compute the preshock and postshock differential proton fluxes j / E k f at the given energy channel E k and pitch angle a. The shock driven enhancement factor of 1, 3, 5, 7 and 9 kev proton fluxes are found to be about 18, 10, 6, 3 and 2 within the loss cone, respectively. These variation scales are comparable with those of spin averaged proton fluxes. The ratio between the postshock and preshock total number fluxes of precipitating protons (1 10 kev) can be approximately obtained F post F pre Z L 0 Z L 0 Z 10 1 Z 10 1 j post sin de k d 6; j pre sin de k d where the preshock and postshock loss cone size a L = arcsin (L 3/2 (4 3/L) 1/4 ) L=6.6 = 2.46 is assumed to be constant, excluding the variation of a L estimated by equation (2). The numerical estimates above support our scenario that the enhancement of 1 10 kev proton flux driven by interplanetary shock was able to cause the observed proton auroral intensification. 4. Summary [14] We report a shock induced auroral intensification event observed by the IMAGE FUV imagers on 7 November The auroral luminosity observed by FUV SI12 was 3 5 times higher that observed by FUV SI13 throughout the auroral oval region, indicating that the dominant component of auroral precipitation was proton instead of electron in this event. The proton auroral intensification appeared first at the postnoon sector and expanded azimuthally toward nightside Table 1. Preshock and Postshock Plasma Parameters n p (cm 3 ) T? (kev) T k (kev) Pre Post ð6þ 6of8

7 within two minutes, similar to the previous observations [e.g., Zhou and Tsurutani, 1999]. The postnoon proton aurora presented the most significant intensification, with the luminosity increasing by 5 times more. [15] We further investigate the interplanetary parameters (observed ACE and Geotail satellites) and plasma parameters within the mapped precipitation region (detected by LANL satellite). Numerical estimates are conducted based on these observations in order to determine the generation mechanism of present postnoon proton auroral intensification. Thepincrease ffiffi of loss cone size is found to be able to only produce 3 times enhancement of proton auroral precipitation. The pitch angle diffusion by the expected oxygen band EMIC waves (no available observation) in the high density region might be able to effectively scatter the 1 10 kev auroral protons into the loss cone, but the observed cold plasma density fluctuated significantly both before and after shock. Our new finding is that the shock driven buildup of 1 10 kev proton fluxes could account for the postnoon proton auroral brightening in this event. The LANL satellite only provided the observations on electron fluxes. The lack of local measurements of magnetic field and electric field makes it difficult to identify the accurate buildup mechanism. Such buildup of 1 10 kev proton fluxes may be partially attributed to the adiabatic acceleration of low energy protons due to shock compression, but it seems to be invalid for the protons with pitch angle a = 0. We are still working on other alternative explanations for the buildup of hot proton fluxes. [16] Acknowledgments. We acknowledge the CDAweb, ACE, Geotail, IMAGE, and LANL MPA teams for the use of observation data. This work is partly supported by the National Natural Science Foundation of China grants Work at the University of Colorado was supported by funding from NASA and from the Natural Science Foundation. 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8 and DMSP observations, J. Geophys. Res., 108(A4), 8019, doi: / 2002JA Zong, Q., et al. (2009), Energetic electron response to ULF waves induced by interplanetary shocks in the outer radiation belt, J. Geophys. Res., 114, A10204, doi: /2009ja Z. Su and H. Zheng, Chinese Academy of Sciences Key Laboratory for Basic Plasma Physics, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui , China. mail.ustc.edu.cn; Y. Wang and C. Yue, Institute of Space Physics and Applied Technology, Peking University, Beijing , China. H. Zhang, Physics Department, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Dr., Fairbanks, AK 99775, USA. Q. G. Zong, Center for Atmospheric Research, University of Massachusetts Lowell, Lowell, MA 01854, USA. 8of8