Journal of Geophysical Research: Space Physics

Transcription

1 RESEARCH ARTICLE Special Section: Dayside Magnetosphere Interaction Key Points: One-to-one correspondences between magnetopause transients observed by MMS and throat auroras observed on the ground are identified Auroral observation indicates that the transients reflect localized indentations, but not back-and-forth motions, of the magnetopause The transients observed here are associated with earthward flow enhancements Supporting Information: Supporting Information S1 Movie S1 Movie S2 Correspondence to: D.-S. Han, Citation: Han, D.-S., Liu, J.-J., Chen, X.-C., Xu, T., Li, B., Hu, Z.-J., et al. (2018). Direct evidence for throat aurora being the ionospheric signature of magnetopause transient and reflecting localized magnetopause indentations. Journal of Geophysical Research: Space Physics, org/ Received 1 NOV 2017 Accepted 22 MAR 2018 Accepted article online 26 MAR American Geophysical Union. All Rights Reserved. Direct Evidence for Throat Aurora Being the Ionospheric Signature of Magnetopause Transient and Reflecting Localized Magnetopause Indentations De-Sheng Han 1,2, J.-J. Liu 2, X.-C. Chen 2,T.Xu 3,B.Li 2, Z.-J. Hu 2, H.-Q. Hu 2, H.-G. Yang 2, S. A. Fuselier 4,5, and C. J. Pollock 6 1 State Key Laboratory of Marine Geology, School of Ocean and Earth Science, Tongji University, Shanghai, China, 2 SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai, China, 3 National Key Laboratory of Electromagnetic Environment, China Research Institute of Radiowave Propagation, Qingdao, China, 4 Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA, 5 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA, 6 Denali Scientific, Healy, AK, USA Abstract Magnetopause transients, observing as brief entries into the magnetosheath by satellites, are commonly observed in the vicinity of the magnetopause and have been explained by several possible mechanisms. However, satellite observations alone are insufficient to determine the dynamics and context of transients. Throat auroras are characterized as north-south aligned discrete auroral forms extending from the equatorward edge of the discrete auroral oval that are only observed near dayside convection throat region and have been suggested as the ionospheric signature of localized magnetopause indentations. Using coordinated observations from the Magnetospheric Multiscale Mission (MMS) and ground-based all-sky imagers, we show apparent one-to-one correspondences between transients observed by MMS near the subsolar magnetopause and throat auroras observed on the ground. The correspondence is valid not only for typical throat aurora with larger spatial scale but also for these with tiny scales. We even notice that the transient durations observed by satellite are approximately proportional to the width (east-west extension) of the throat aurora. These results provide direct evidence that throat auroras are ground signatures for the magnetopause transients. With the aid of auroral observations, we suggest that these transients reflect localized magnetopause indentations but are not produced by motion of the entire magnetopause. We also found that most transients observed here are associated with earthward flow enhancements, which indicates that high-speed jets in the magnetosheath could be a driver for producing these transients. Plain Language Summary We present observational evidence that some of the magnetopause transient observed near subsolar point by satellite reflect magnetopause indentations and can be well displayed by auroral observation on the ground. 1. Introduction Optical auroras observed on the ground have been classified into two broad categories, that is, discrete and diffuse auroras, which are different in both morphological and physical properties (Akasofu, 1974). Morphologically, the discrete auroras can be in forms of arc, ray, band, or curl that are characterized by intense luminosity and clear boundaries (e.g., Sandholt et al., 1998), whereas the representative properties of diffuse aurora are weak and homogeneous luminosity in the auroral region (Akasofu, 1974; Lui et al., 1973). Physically, the source particles for discrete auroras can be from either open or closed field line regions and are generally on field lines with acceleration (e.g., Li et al., 2013), whereas the source particles for diffuse auroras are from the central plasma sheet and are involved in wave-particle interactions (e.g., Meng et al., 1979; Ni et al., 2014). Discrete auroras observed near magnetic local noon (MLN) have been confirmed to be caused by particles originated from the magnetosheath and are on the open field lines (Lockwood, 1997; Sandholt et al., 1998), so the equatorward boundary of the discrete auroral oval has been taken as the open-closed field line boundary near MLN (Lockwood & Moen, 1996; Moen et al., 1996). Thus, any deformation of the equatorward edge of the auroral oval should be mapped to a deformation of the open-closed field line boundary at the magnetopause. Recently, a particular auroral form, manifested as a localized equatorward extension of the equatorward edge of the east-west aligned persistent midday auroral oval, has been observed near the dayside convection HAN ET AL. 1

2 throat region. This auroral form has been named throat aurora (Han et al., 2015). Coordinated low-altitude satellite observations show that throat auroras are caused by magnetosheath-like particles and are suggested to be associated with localized magnetopause deformations (Han et al., 2016). These deformations are supposed to be indented structures on the magnetopause and thus called indentations in this paper. Throat auroras have observational properties as follows (Han, Hietala, et al., 2017). (1) They appear as approximate north-south aligned arcs extending from the equatorward edge of the east-west aligned persistent midday auroral oval, and their orientation is actually consistent with the direction of ionospheric convection flow. (2) They can be very large in spatial scale, for example, as long as ~3.5 in longitudinal direction, which is equivalent to ~3.0 Earth radii (R E ) in the equatorial plane. (3) Their daily occurrence rate is no less than 50%. (4) Their magnetic local time (MLT) coverage can be wide; for instance, they can simultaneously occur from prenoon (~1100 MLT) to postnoon. (5) Their occurrence show less dependence on the interplanetary magnetic field (IMF) Bz but shows clear dependence on the IMF cone angle. These observational properties imply that if the throat aurora indeed corresponds to magnetopause indentation, many detailed properties of the indentation, such as occurrence, spatial scales, and dynamic properties, can be inferred from the auroral observations. On the other hand, early studies suggested that magnetopause transients may correspond to indentations on the magnetopause (Kawano et al., 1992; Sanny et al., 1996). The transients are characterized as brief entries into the magnetosheath when satellites are in the vicinity of the magnetopause and can be explained either by back-and-forth motion of the entire magnetopause due to variation of solar wind dynamic pressure (Shue et al., 1997) or by satellite passing through a localized magnetopause indentation due to various mechanisms such as wave structures at the magnetopause (Sibeck, 1992; Zhang et al., 2010). Actually, satellite observations alone are insufficient to determine the dynamics and context of transients. Therefore, if throat aurora can be confirmed to be the ionospheric signature of a magnetopause transient, the continuous and two-dimensional auroral observations will provide us with much information for studying the transients. In this study, using coordinated observations from Magnetospheric Multiscale (MMS) satellites and auroral observation at Yellow River Station (YRS), we will show direct evidence that throat auroras are indeed correspondent to magnetopause transients. The results are important not only for understanding throat aurora and transients but also for studying the solar wind-magnetosphere-ionosphere coupling. 2. Data and Method Ground-based optical auroral data are obtained at Yellow River Station at Ny-Ålesund, Svalbard (magnetic latitude N) using all-sky imagers equipped with charge coupled device (CCD) resolution of pixels and with band-pass filters at nm (green line) and nm (red line). The temporal resolution of aurora observation is 10 s. Satellite data in the vicinity of the magnetopause are obtained from NASA s MMS mission. Measurements of the magnetic field from the Fluxgate Magnetometer (FGM) (Russell et al., 2016), of plasma moments from the Fast Plasma Investigation (Pollock et al., 2016), and of the composition-resolved velocity-space distribution of ions from the Hot Plasma Composition Analyzer (HPCA) (Young et al., 2016) are used in this study. The HPCA provides accurate measurements of the ion species of H +,He 2+,He +, and O + in the energy range from 10 ev to 40 kev, among which the ions of He 2+ are used as an indicator for judging the particles from the magnetosheath in this paper because they exclusively originate from the solar wind. The most representative property of throat aurora is the equatorward extension of an auroral form from the equatorward edge of the persistent east-west aligned auroral oval. This can be seen from the typical case shown in Figure 1 and from the cases given in Han et al. (2015, 2016) and Han, Hietala, et al. (2017). The cases examined in this study own typical property of throat aurora, but most of them are in smaller spatial scales. In January 2016, when MMS reach their apogees near the subsolar magnetopause, YRS was located in the same local time sector as the satellites. This provides a good opportunity for investigating the conjugate observations between satellite and ground. Under such conditions, the footprints of satellite have very high possibility to be inside the field of view of the all-sky camera. The problem is how to get a reliable correspondence between the in situ and ground observations through a method of field line mapping. In this study, we used T01 (Tsyganenko, 2002) model for estimating the footprint of the magnetic field line that threads MMS1. HAN ET AL. 2

3 Figure 1. A typical throat aurora observed at Yellow River Station by all-sky camera on 7 December 2007 shown (left) in original observation and (right) after mapping to the geomagnetic coordinates. The input parameters of T01 model include Dst index, solar wind dynamic pressure (DynP), and IMF Bz and By components. The method of field line mapping used in this study includes three steps as follows. (1) using T01 model and default inputs to trace the MMS s footprint into the ionosphere; (2) To plot the footprints on auroral images and to visually check if the transients observed by MMS are concurrent to some aurora structures within some reasonable time lags; and (3) even though we can find some concurrent cases, the footprints are always far from the concurrent auroral structures. Therefore, we tried different input parameters with constant values for the model until optimized correspondences are obtained (for Case 1: DynP = 6.5, Dst = 6, By = 15, and Bz = 4.5; for Case 2: DynP = 2.5, Dst =0,By = 8, and Bz = 3). We should note that all of these inputs are artificial values determined through trial and refinement. Because our experimental results show good correspondences between MMS and ground observations, we argue that the footprint estimation method used here is valid and gives credible results. In Figure 2, the orbits of MMS satellites in the X-Y plane and the estimated footprints in the northern ionosphere for observations on 10 and 8 January 2016 are presented. For these two cases, the footprints of the satellite were just inside the field of view of Yellow River Station. The same observations and the same footprint estimation results for the Case 2 have been used to study the particle properties associated with two types of diffuse auroras by Han, Li, et al. (2017). 3. Observations 3.1. Event on 10 January 2016 In Figure 3, the top three panels show MMS1 measurements from 09:00 UT to 10:00 UT on 10 January 2016 of the magnetic field Bx, By, and Bz components in GSM coordinates from Fluxgate Magnetometer, of the energy spectrogram of the He 2+ ions from HPCA, and of the ion bulk velocity in GSE coordinates from Fast Plasma Investigation, respectively. Figure 2 shows that MMS1 was close to the subsolar magnetopause during this time period. Vertical dashed lines labeled by a to i in the top panel of Figure 3 indicate some selected moments. The auroral images corresponding to these moments are shown in the bottom by considering a time lag of 30 s. On each auroral image, the trajectory of the footprint is shown by a yellow curve started with a red circle. A red triangle on the yellow curve indicates the MMS1 s position that is mapped into the ionosphere at the moment when this auroral image was recorded. In Figure 3, the MMS1 s observations before and after the moment a, that is, 09:10 UT, show magnetosheath and magnetospheric properties, respectively. For example, the magnetic field Bz component with magnitude of ~70 nt was predominantly observed after moment a, which is a clear signature of the magnetosphere and is apparently different from the observations before this moment. This implies that HAN ET AL. 3

4 Figure 2. MMS1 orbit in GSE coordinates X-Y plane and the estimated footprint in the northern ionosphere for two cases observed on (left panels) 10 January and (right panels) 8 January The gold-shaded segment represents the science region of interest marked by MMS mission. MMS1 should just enter into the magnetosphere at this moment. Considering the MMS1 s footprint at this moment, we expect that it should be at just equatorward edge of the discrete aurora oval, because the equatorward edge of the discrete auroral oval near local noon has been regarded as the open-closed field line boundary of the magnetopause (Lockwood & Moen, 1996). In Figure 3, we see that the MMS1 s footprint, as indicated by the red triangle on the auroral image a, is just at the equatorward edge of the aurora oval, which is consistent with the above expectation and provides support for the field line mapping method used in this study. From a to b, several transients were observed by MMS1. Although we can find some auroral activities associated with these transients, one-to-one correspondence between aurora and these transients is not easily demonstrated. This is because when the footprint is close to the edge of the field of view, the corresponding zenith angle of observation is large. Under such conditions, many auroral structures will be superposed on the line of sight, so that they are not easily discriminated. For regions c and d, both the magnetic field and energy spectrum show that MMS1 was inside the magnetosphere. Correspondingly, the footprints of MMS1 on the auroral images of c and d are located equatorward of the equatorward edge of the auroral oval boundary. From e to h, MMS1 observed that the magnetic Bz component changed from positive (~70 nt) to negative and the He 2+ energy flux clearly increased, which is a typical transient event. Such events have been often explained by back-and-forth motion of the entire magnetopause. However, from auroral images e to h, we clearly see that an auroral form extended from the equatorward edge of the auroral oval and swept over the footprint of MMS1, whereas the entire equatorward boundary of the auroral oval did not show clear motion at all. At the same time, the ion bulk flow shows a clear decrease in the V x component, which indicates an earthward flow enhancement. This process can be seen from the movie provided in supporting HAN ET AL. 4

5 Journal of Geophysical Research: Space Physics 2+ Figure 3. (top panel) The magnetic field, (second panel) the energy spectrogram of the He ions, and (third panel) the ion bulk velocity observed by MMS1 on 10 January Auroral images correspondent to moments a to i are shown in the bottom. On each auroral image, the red triangle indicates the MMS1 s position that is mapped into the ionosphere at the moment when the auroral image was recorded. Corresponding to the transient observed from e to h, an earthward flow enhancement was observed and a typical throat swept over the footprint of the satellite. information S1. We argue that here we provide a credible correspondence between a transient observed near the subsolar magnetopause and a typical throat aurora observed on the ground Events on 8 January 2016 Figure 4 shows observations from 0840 UT to 0940 UT on 8 January 2016 in the same format as Figure 3. During this time period, MMS1 was also in vicinity of the subsolar magnetopause and the magnetic field observations indicate that the satellite first entered into the magnetosphere at ~08:45 UT. Vertical dashed lines labeled by letters from a to l indicate some time periods when satellite observed strong (at a, c, e, g, and k) and weak (at b, d, f, h, and l) He2+ energy fluxes. Corresponding with all of the strong energy flux moments, auroral enhancements have been observed close to the satellite s footprints as marked out by red arrows on the auroral images. Those auroral enhancements do not appear at the weak energy fluxes HAN ET AL. 5

6 Journal of Geophysical Research: Space Physics Figure 4. Top Three panels are in the same format as Figure 3 for observations on 8 January Auroral images correspondent to moments a to l are shown in the bottom. Corresponding to the transients observed at moments a, c, e, g, and k, auroral structures extending from the auroral oval toward low latitude (throat auroras with small spatial scales) were observed close to the footprint of the satellite, as indicated by the red arrows. Please note that the transient durations are proportional to width (east-west extension) of the throat auroras and that earthward flows are observed at a, c, e, and k. moments of b, d, f, h, and l. Compared with moments a, c, and g, the auroral enhancement at moment k is slightly shifted from the footprint of MMS. This may be due to the errors introduced by the constant inputs for the footprint tracing model. Note that all of the auroral enhancements, as indicated by the red arrows, are consistent with the typical properties of throat aurora, that is, an auroral form extending equatorward from the equatorward edge of the auroral oval, so they are all throat auroras with small spatial scales. Most interestingly, in Figure 5, we approximately estimate the transient durations (as indicated by the black horizontal bars plotted under the transients) observed by MMS distributed with the width (east-west extension) of the throat aurora (as indicated by the red bars plotted under the auroral structures). In Figure 5, the widths of throat auroras have been mapped into the ionosphere. The red line shows the fitted results for the transient durations and the auroral widths. We note that the transient durations are approximately proportional to the width of the throat auroras. In addition, clear earthward flow enhancements were observed associated with the strong energy flux moments at a, c, e, and k, except at g when the magnetic field shows clear magnetospheric property. The apparent one-to-one correspondence between the variations of He2+ energy flux observed at MMS1 and the auroral activity observed on the ground for this time period can be found from the supporting information Movie S2, which is replotted from the same data as supporting information Movie S9 in Han, Li, et al. (2017) with different elements. HAN ET AL. 6

7 Auroral width (Km) Discussion In this study, we show apparent one-to-one correspondence between magnetopause transient and throat aurora. This correspondence is valid not only for the typical throat aurora event presented with a larger spatial scale (Figure 3) but also for the smaller-scale ones (Figure 4). We notice that the transient durations observed by satellite are proportional to the width of the throat aurora (Figure 5) and that most throat auroras are associated with earthward flow enhancements. These results are important to improve our understanding on both throat aurora and magnetopause transients Implications for Throat Aurora Transient duration (s) Previous studies have shown that discrete auroras observed near MLN map to the magnetosheath (Lockwood, 1997) and thus the equatorward edge of the discrete auroral oval has been regarded as the Figure 5. The transient durations observed by MMS (as indicated by the black horizontal bars at a, c, e, and k in upper panel of Figure 4) distributed with the widths of the throat auroras (as indicated by the red bars plotted under the auroral open-closed field line boundary. Because throat auroras are characterized as auroral forms extending from the equatorward edge of structures in images of a, c, e, and k in bottom panel of Figure 4). The widths of throat auroras have been mapped into the ionosphere. the discrete auroral oval, they have been assumed to correspond to indentations in the magnetopause boundary. This assumption is supported by simultaneous low-altitude and ground observations, which indicate that throat auroras are caused by magnetosheath particles locally penetrated into the magnetosphere (Han et al., 2016). However, direct evidence for the correspondence between throat aurora and the magnetopause processes has not been reported and how the throat auroras are generated is not well understood. For considering the generation of throat aurora, Han, Hietala, et al. (2017) examined the observational properties in detail. It was found that the occurrence rate of throat aurora shows less dependence on the IMF Bz component but clearly decreases with increase of the IMF cone angle. This is very similar with the occurrence of high-speed jet (HSJ) in the magnetosheath dependent on the IMF cone angle. The HSJs are typically observed as localized earthward flow enhancements with ~1 Earth radius (R E ) in size (Plaschke et al., 2009, 2016) and have typical dynamic pressure enhancements of up to a factor of 15 compared to the ambient magnetosheath (Archer & Horbury, 2013). Such energetic HSJs can reach the magnetopause and cause large-amplitude yet localized indentations of the magnetopause boundary (Shue et al., 2009), which just meets the expectation of throat aurora. Considering these factors, Han, Hietala, et al. (2017) proposed that throat auroras may be the ground signature of magnetopause indentations that are most likely caused by HSJs impacting on the magnetopause. Here we show apparent one-to-one correspondence between throat aurora and magnetopause transients. We also found that all of these throat auroras are associated with earthward flow enhancements except at moment g in Figure 4. Around the moment g, the magnetic components show clear magnetospheric properties and the energy flux of He 2+, that is, particles from the magnetosheath, is not as strong as at moments of a, c, e, and k. This can be understood as that MMS was just immediately inside the indented magnetopause structure where the earthward flow has been dissipated. Therefore, we argue that our results provide strong evidence that throat auroras in these cases are mapping to magnetopause indentations that are most likely caused by HJSs impacting on the magnetopause. In addition, because the HSJs are locally generated in the magnetosheath but not in the upstream solar wind (Hao et al., 2016, 2017; Hietala et al., 2009), the commonly observed throat auroras strongly indicate that transient structures locally generated in the magnetosheath could cause frequent and drastic disturbances in the geospace Implications on Magnetopause Transients We suggest that the observational results are also important for understanding the transients. Magnetopause transients, observing as a short-time entry into the magnetosheath, can be frequently observed when a satellite is traveling inside the magnetosphere and close to the magnetopause (Kawano et al., 1992). These transients can be explained either by back-and-forth motion of the entire magnetopause due to variation of solar wind dynamic pressure (Shue et al., 1997) or by satellite passing through a localized magnetopause indentation. The magnetopause motion in response to variation of solar wind dynamic pressure can be easily HAN ET AL. 7

8 identified (Shue et al., 1998), but generation of the localized magnetopause indentations has been a controversial issue for many years. At least, four possible mechanisms have been proposed including the Kelvin-Helmholtz instability (Junginger & Baumjohann, 1988), impulsive plasma penetration (Lemaire, 1977), boundary waves driven by solar wind, or locally generated, dynamic pressure variations (Sibeck, 1992), or unsteady magnetopause reconnection (Russell & Elphic, 1979; Song et al., 1994). In fact, it is still controversial whether the transients are caused by flux transfer event (FTE) (Song et al., 1994, 1996) or whether they are produced by boundary surface waves due to pressure pulses (Sibeck, 1992; Sibeck & Newell, 1995, 1996). However, satellite observations alone are insufficient to determine the dynamics and context of transients. Here we observe apparent one-to-one correspondences between transients and throat auroras, which shed new light on understanding the transients by taking the advantages of the continuous and two-dimensional observations of aurora. First, from the supporting information Movie S1 and Figure 3, we can clearly see that the transient is correspondent to the throat aurora sweeping over the footprint of MMS and the motion of MMS is much slower than throat aurora. We also should notice that the equatorward edge of the discrete auroral oval keeps at the same latitude during this time period. Based on this, we can definitely conclude that the transient was not produced by back-and-forth motion of the magnetopause but by a localized magnetopause indentation. This, for the first time, clearly depicts the relative motion between the magnetopause indentation and the satellite. Most importantly, because the duration of the transient is proportional to the width of the throat aurora (Figure 5), the spatial scales of the indentations might also be able to be estimated from the aurora observation. Besides the one-to-one correspondence shown here, the statistical studies also provide evidence for the linkage between transient and throat aurora. First, they are both commonly observed and show recurrence with period from several minutes to tens of minutes (Han, Hietala, et al., 2017; Song et al., 1994). Second, although FTEs have a tendency to occur predominantly during periods of southward IMF and FTE signatures are often observed during transient events, statistical studies show little tendency for the transients to occur during periods of southward IMF (Kawano et al., 1992). These are all comparable with the observational properties of throat aurora, because throat auroras often contain poleward moving aurora forms (PMAFs) (Chen et al., 2017; Han et al., 2016). PMAFs have been generally accepted as the ground signature of FTE (Moen et al., 1996) and have tendency to occur under southward IMF (Xing et al., 2012), whereas throat aurora show little tendency to occur during southward IMF (Han, Hietala, et al., 2017). All of these results provide strong evidence that throat auroras are definite ground signatures for the magnetopause transients that are all correspondent to magnetopause indentations. Thus, the properties of throat aurora, such as occurrence, local time distribution, spatial scale, and dynamic properties, can be used to infer the properties of magnetopause indentations. Based on the observational properties, Han, Hietala, et al. (2017) suggest that generation of throat aurora should be affected by factors both inside and outside the magnetosphere. Here we show that the transients observed in Figures 3 and 4 are mostly associated with earthward flow enhancements, which indicates that the outside factor, such as pressure pulses proposed by Sibeck (1992), should be a necessary driver for producing the throat aurora, namely, the transient or the magnetopause indentation. These pressure pulses, as discussed above, are most likely associated with the HSJs generated in the magnetosheath, while observational results of throat aurora containing PMAFs (Chen et al., 2017; Han et al., 2016) strongly imply that FTEs may occur on the indented magnetopause boundary during HSJ impacting on the magnetopause. Recently, Hietala et al. (2018) provide evidence that HSJ can trigger magnetic reconnection. 5. Conclusion We observe apparent one-to-one correspondences between transients and throat aurora and even notice that the transient duration is approximately proportional to the width of the throat aurora. These observational results indicate that all the transients observed here are associated with localized magnetopause indentations but not caused by back-and-forth motion of the entire magnetopause. Based on the observational properties of throat aurora, we argue that pressure pulses with limited spatial scale in the magnetosheath, namely, the HSJs, are necessary for producing the transients, while the previously reported FTE signatures observed during transients can be explained by that reconnections occurred on the indented magnetopause boundary. HAN ET AL. 8

9 Acknowledgments This research was supported by the NSFC grants , , , , and Auroral observation at YRS is supported by CHINARE, and the data can be obtained through org.cn/aurora/dataquery. MMS data are obtained from MMS science data center. Research at Southwest Research Institute is conducted under the NASA MMS contract. References Akasofu, S. I. (1974). Discrete, continuous and diffuse auroras. Planetary and Space Science, 22(12), Archer, M. O., & Horbury, T. S. (2013). Magnetosheath dynamic pressure enhancements: Occurrence and typical properties. Annales Geophysicae, 31(2), Chen, X. C., Han, D. S., Lorentzen, D. A., Oksavik, K., Moen, J. I., & Baddeley, L. J. (2017). Dynamic properties of throat aurora revealed by simultaneous ground and satellite observations. Journal of Geophysical Research: Space Physics, 122, JA Han, D.-S., Chen, X.-C., Liu, J.-J., Qiu, Q., Keika, K., Hu, Z.-J., et al. (2015). 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