Characterizing the dayside magnetosheath using energetic neutral atoms: IBEX and THEMIS observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, , doi:.2/jgra.5353, 213 Characterizing the dayside magnetosheath using energetic neutral atoms: IBEX and THEMIS observations K. Ogasawara, 1 V. Angelopoulos, 2 M. A. Dayeh, 1 S. A. Fuselier, 1 G. Livadiotis, 1 D. J. McComas, 1,3 and J. P. McFadden 4 Received 24 January 213; revised 26 April 213; accepted 22 May 213; published 17 June 213. [1] We report on the time evolution of energetic neutral atom (ENA) emissions measured by the Interstellar Boundary Explorer (IBEX) during instances of compressed and expanded dayside magnetosheath. The ENA observations, taken during the passage of a corotating interaction region on 27 and 28 November 2, are compared with in situ observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. IBEX s field of view (6.5 full width at half maximum) covered a wide region of the dayside magnetosheath for several days, providing continuous information from that region. The high sensitivity and high-energy resolution of IBEX instruments enabled unprecedented remote-sensing diagnostics of dayside magnetosheath ENA spectra at energies between ~.1 and ~6 kev, which can be directly compared with various upstream parameters. The inferred plasma spectra from ENA observations showed characteristic suprathermal tails described by kappa distributions that correlate well with the solar wind cone angle and are in agreement with in situ observations, suggesting that the shock angle contributed to magnetosheath particle heating. Simultaneous in situ ion measurements in the dayside magnetosheath provided by THEMIS agree reasonably well with IBEX-inferred spectra, demonstrating synergy between remote IBEX ENA observations (global) and in situ measurements (local) for studying localized magnetospheric processes. Citation: Ogasawara, K., V. Angelopoulos, M. A. Dayeh, S. A. Fuselier, G. Livadiotis, D. J. McComas, and J. P. McFadden (213), Characterizing the dayside magnetosheath using energetic neutral atoms: IBEX and THEMIS observations, J. Geophys. Res. Space Physics, 118, , doi:.2/jgra Introduction [2] Since the Earth s magnetosheath lies between the magnetopause and the bow shock, the plasma components in this region are composed mainly of shocked solar wind with some leaked magnetospheric particles [e.g., Möbius et al., 1986]. As the solar wind encounters the magnetosphere, it decelerates, heats, and is deflected at the bow shock. At the subsolar magnetopause, it becomes nearly stationary as most of its flow energy is converted into thermal energy. Magnetosheath proton characteristics depend on interplanetary magnetic field (IMF) orientations with respect to the local shock normal (y BN )due to the intrinsically different properties of the bow shock during a quasi-parallel (y BN < 45 ) and quasi-perpendicular (y BN > 45 ) orientation [e.g., Burgess et al., 212]. Shock drift 1 Southwest Research Institute, San Antonio, Texas, USA. 2 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA. 3 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, USA. 4 Space Physics Research Group, University of California, Berkeley, California, USA. Corresponding author: K. Ogasawara, Southwest Research Institute, 622 Culebra Rd., San Antonio, TX 78238, USA. (keiichi.ogasawara@swri.org) 213. American Geophysical Union. All Rights Reserved /13/.2/jgra.5353 acceleration [e.g., Burgess, 1987], which operates for quasiperpendicular shocks, enables quick acceleration of the ions up to the energy determined by the scale of the shock structure and the ion gyroradii. Reflected back upstream of the shock, these energetic ions return to the magnetosheath and combine with the heated core distribution to form two-component distributions [e.g., Gosling and Robson, 1985]. For quasi-parallel shocks, first-order Fermi acceleration operates when largeamplitude waves exist upstream and downstream of the shock [e.g., Terasawa, 1981]. Higher-energy particles are expected to be produced by such Fermi acceleration processes under quasi-parallel conditions than under quasi-perpendicular shock conditions. In addition to the two-component distributions also seen in the quasi-perpendicular case, Gosling et al. [1989] reported a third component with energies > kev under quasi-parallel shock angle conditions. These multicomponent distributions are common in the magnetosheath down to the magnetopause [Fuselier et al., 1988]. Like shock acceleration, turbulence in the magnetosheath is also more intense behind the quasi-parallel shock than behind the quasi-perpendicular shock [Lucek et al., 25]. Under quasi-perpendicular conditions, temperature anisotropy in the magnetosheath produces mirror-mode waves [e.g., Lin et al., 1998] and ion-cyclotron waves [Fairfield, 1976]. Under quasi-parallel conditions and in high-beta plasmas, broadband waves 3126

2 originatingin the ion foreshock region dominate within the magnetosheath [Engebretson et al., 1991], where ion-cyclotron and mirror-mode waves are also observed [Anderson and Fuselier, 1993]. Turbulence in the magnetosheath may also contribute to local plasma heating via stochastic acceleration through scattering and dissipation [e.g., Giacalone et al., 1992], which also produces distributions that are distinct from the Maxwell-Boltzmann particle distributions, featuring a high-energy tail. [3] Thus, although magnetosheath particle spectra are determined by a complicated combination of shock acceleration and wave-particle interactions, the upstream shock angle is the key to evaluating acceleration processes in the magnetosheath. However, because of the bow shock s three-dimensional shape (asymptotically, at the nose, a paraboloid by revolution), downstream conditions vary from place to place in the magnetosheath even under static solar wind conditions, making it difficult to compare upstream (solar wind) and downstream (magnetosheath) plasma distributions without multispacecraft measurements in a favorable configuration that seldom occurs. In addition, since plasma properties differ considerably from the magnetopause to the bow shock, it is very difficult to describe particle energization of the magnetosheath using a single spectrum. [4] Remote-sensing energetic neutral atom (ENA) observations of magnetosheath spectra from the Interstellar Boundary Explorer (IBEX) are unique and useful not only because of the needed energy coverage for the magnetosheath particles but also because of the full and spot-on spatial coverage around the nose of the magnetopause that allows us to compare upstream parameters directly to shock conditions without having to include spacecraft location. We seek to evaluate the dominant parameter controlling magnetosheath particle distributions by linking solar wind conditions with spectral properties originating from the dayside magnetosheath using remote-sensing ENA observations and in situ measurements. Although the integration time required for ENA observations is much longer than that for in situ measurements, the global imaging of the magnetosheath ENA spectra will add a view to the in situ measurement in terms of the global energy conservations in this region. [5] ENAs from the dayside magnetosheath were first measured by the low-energy neutral atom (LENA) imager [Moore et al., 2] on the Imager for Magnetopause to Aurora: Global Exploration (IMAGE) spacecraft [Collier et al., 21; Taguchi et al., 24; Collier et al., 25; Hosokawa et al., 28]. These results were limited by an upper energy of ~3 ev, however, and thus provided little information on the actual magnetosheath population in energy spectra, which extends over several kev because Collier et al. [21] used negative ions sputtered at LENA s conversion surface as a proxy to infer higher-energy ENAs. More recently, the IBEX mission [McComas et al., 29a], primarily targeted for imaging the interaction region between the solar wind and the local interstellar medium, has observed other ENA emissions from the magnetospheric cusps [Petrinec et al., 211], the Earth s nightside plasma sheet [McComas et al., 211], and ENAs reflected directly from the Moon s surface [McComas et al., 29b; F. Allegrini et al., submitted, 213]. IBEX has two high-sensitivity single-pixel ENA cameras with a.1 to 6 kev energy range [Funsten et al., 29; Fuselier et al., 29]. It has already measured subsolar magnetopause emissions from the magnetosheath during static upstream conditions [Fuselier et al., 2] and during the disturbance from a coronal mass ejection [McComas et al., 212]. Since ENAs are not bound to the local magnetic field, they follow ballistic trajectories, maintaining the velocity distributions of their progenitor ions. The shocked solar wind stagnates at the nose of the magnetopause and produces a fairly uniform phase space distribution of plasma ions. This makes it possible to observe ENAs even from the GSE-Y direction, i.e., even from a direction where the bulk velocity effect of the solar wind (predominantly along GSE-X) is significantly reduced. In addition, the relatively large geocoronal neutral hydrogen density at the magnetopause (especially near the nose) produces the brightest emissions in the magnetosheath since that location is closest to the Earth and geocoronal intensities in the sheath are large there compared to other sheath locations. Therefore, it is possible to investigate magnetosheath spectra around the magnetopause using remotely observed ENA emissions. [6] Using simultaneous observations of remote ENA emissions (IBEX) and in situ plasma (Cluster) in the dayside magnetosheath, Fuselier et al. [2] quantified the charge exchange process and estimated the neutral hydrogen density at the nose of the magnetosphere. In this paper, we characterize the energy spectra of plasmas in the dayside magnetosheathusingibexandinsitumeasurementsfrom the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft [Angelopoulos, 28]. In particular, we will describe the heating processes dominant in the magnetosheath and correlate the spectral properties with upstream conditions taken from NASA Goddard Space Flight Center s OMNI data set as available through OMNIWeb. [7] The THEMIS mission consists of five spacecraft (probes TH-A, TH-B, TH-C, TH-D, and TH-E) carrying plasma and wave instruments. Electrostatic analyzers measure the distribution functions of ions between.5 and 25 kev over 4p steradian, providing high time resolution (>3 s) plasma moments and angular particle distributions [McFadden et al., 28]. The FGM 3-D fluxgate magnetometer measures the magnetic field and its fluctuations up to 64 Hz [Auster et al., 28]. From September to November 2, THEMIS probes conducted a dayside science phase, providing a unique opportunity to make conjugate observations with IBEX.Inthisstudy,weutilized magnetic field vectors, omnidirectional particle spectra, and plasma moments computed every 3 s onboard TH-D and TH-E. We chose IBEX orbit 3 (27 28 November, 2) because of the wide variation in solar wind parameters during the passage of a corotating interaction region (CIR). The Dst index was not significantly enhanced during that time. Therefore, the contribution of energetic particles leaking from the magnetosphere was expected to be minimal. 2. Methodology 2.1. THEMIS and IBEX Orbital Conjunction [8] Figure 1 shows the trajectories of the TH-D and TH-E during IBEX orbit 3 from 12 UT on day of year (DOY) 331 to UT on DOY 333, 2 in GSE coordinates. The field of view (FOV) is a 6.5 full width at half 3127

3 15 (a) (b) IBEX Orbit Z GSE [R E ] 333 Y GSE [R E ] IBEX Orbit X GSE [R E ] X GSE [R E ] Figure 1. (a) Ecliptic and (b) meridian projections of TH-D (dark green) probe, TH-E (green) probe, and IBEX (red) locations in GSE coordinates from 12 UT on DOY 331 to UT on DOY 333 in 2. The entire trajectory of IBEX during Orbit 3 is also shown with markers on UT for each day. Possible magnetopause locations [Shue et al., 1998] and bow shock locations [Chao et al., 22] during intervals 1 4 in Figure 3 are shown with translucent colors pink and blue, respectively. maximum (FWHM) cone fixed in the GSE-X plane that scans a great circle in the GSE-Y-Z plane during each spacecraft spin. During a 1.5 day survey by IBEX (IBEX location indicated by a thick red line), TH-D and TH-E, whose separation distance ranged from 2 to 6 km during this time, traversed the dayside magnetosheath two times on the dawn-north side. IBEX was in the far upstream duskside region and observed the dayside magnetosheath from near the equatorial plane. As indicated in the figure, the magnetopause (blue) and the bow shock (pink) were at approximately 8 11 R E and 15 R E, respectively, at the nose position Conversion of ENA Flux to Proton Flux [9] ENA and ion fluxes are related by the following function for IBEX s viewing geometry used here [e.g., Fuselier et al., 2]: Z J ENA ðe; x; zþ ¼ J ion ðe; x; y; zþsðeþn H ðx; y; zþdy; (1) where J ENA (E,x,z) is the observed ENA flux projected in the GSE-XZ plane, J ion (E,x,y,z) is the local ion flux, and s(e) is the energy-dependent charge-exchange cross section between energetic proton and ambient hydrogen atoms (p +* +H! H * + p + ), calculated from an empirical formula [Lindsay and Stebbings, 25]. The exospheric neutral hydrogen density (n H ) is calculated based on a spherical harmonics model [Zoennchen et al., 211] obtained by the TWINS1-LAD measurement assuming isotropic Lyman a resonant scattering. Zoennchen et al. [211] indicated that the uncertainty of their model at R E would be % and did not recommend its use at 8 R E. However, we applied this model for the following reasons: (1) it is the latest extensive analytic model under solar minimum conditions that can be easily extrapolated to the nose magnetosheath region; (2) the result agrees fairly well with our measurements (see also Figures 5 and 7); and (3) we used the shape of the distributions for our detailed analysis, and n H only affects the intensity and is independent of the particle energy (equation (1)). In order to calculate the line density, integration was carried out within to R E in GSE-Y. Figure 2 shows the calculated column density mapped by applying this method to the region from 7.5 to 15 R E. Then we applied IBEX angular response [Funsten et al., 29] (see the figure for a weighting function) to average the line densities within the instrument FOV. Since the exospheric density decreases rapidly with increasing distance from the nose (e.g., ~1/R 2.9 by Rairden et al. [1986]), the properties of observed ENAs would be weighted to those closer to the Earth rather than to those in the FOV center. [] By applying this model to our data set at the nose of the magnetosheath, we calculated a flux ratio of ENAs to solar wind protons of (1.9.4) 4 (see Figures 4e and 6e). These values are consistent with the 3 to 4 estimate from IMAGE/LENA observations under average solar wind conditions [Collier et al., 21] and the estimate at 1 kev from comparison of IBEX ENA observations to Cluster in situ measurements [Fuselier et al., 2] Kappa Distributions [11] As described in section 1, the plasma distribution in the magnetosheath is usually characterized by its energetic 3128

4 15 4. (k > 1.5) [see, e.g., Livadiotis and McComas, 29] and kappa temperature (T): Z GSE [R E ] X GSE [R E ] tail departed from a single Maxwell-Boltzmann distribution such as secondary Maxwell-Boltzmann distribution or power law distribution. In our study, we applied kappa distributions to evaluate the core and the tail portion of the distribution simultaneously within the limited energy bins of the IBEX data. Before the formulation of kappa distributions is discussed, the bulk velocity effect requires explanation. For the interval examined here, the OMNI solar wind speed was km/s. Many magnetosheath flow models (magnetogasdynamics model [Spreiter and Stahara, 198], magnetohydrodynamics models [Erkaev et al., 1999; Walsh et al., 212], and analytical models from in situ measurements [Soucek and Escoubet, 212]) predict that the ratio of the magnetosheath bulk speed to the solar wind speed will range from.1 to.2 around the nose region. With this solar wind speed and ratio in the magnetosheath, the plasma energy equivalent to the bulk velocity ranges from to 4 ev. The bulk velocity in the sheath does not affect the spectral shape in the energy range (>5 ev). It does, however, impact lower energies in the IBEX-Lo data. Although we only use data to fit the kappa distribution in this study, the extrapolation of the distribution to lower energies is meaningful and can be used as a diagnostic of the flow at the nose of the magnetopause and the extensive region around it. [12] A kappa distribution f(e) without a bulk velocity term is described by the following formula with the kappa index 1..1 H Column density [x 11 cm -2 ] Weight (w) 6.5 (FWHM) FOV (Funsten et al., 29) Figure 2. (left) The exospheric hydrogen column density, calculated from the density profile by extrapolating from TWINS Lymann-alpha measurement using spherical surface harmonics [Zoennchen et al., 2] and integrating between R E in the GSE-Y direction. (right) angular response from Funsten et al. [29] simplified by a triangular function. We applied this angular response as a weight function to calculate the average column density for each 2 bin in the GSE-Z direction. To focus on the nose region in the magnetosheath, the IBEX FOV in this analysis is limited to 5 R E in the GSE-Z direction with 2 resolution by the time sequence of the IBEX direct event (DE) data. f ðþ/ e 1 e k 1 1 þ : (2) k 3=2 k B T [13] Since equation (2) is in the form of velocity space density, we converted velocity space density to differential flux J(E) with units of [cm 2 sr 1 s 1 ev 1 ] by using the following relationship: f ðþ¼ e m2 JE ð Þ 2e 2 E ; (3) where E denotes the energy in electron volts (ev), m denotes the particle mass, and e denotes the elementary charge. A kappa distribution has a core distribution similar to a Maxwell-Boltzmann distribution at lower energies and an energetic tail similar to a power law distribution with a power of k at higher energies (E k B T). In addition, the kappa distribution asymptotically approaches a Maxwell-Boltzmann distribution, representing the thermal equilibrium state, at the higher k index. By showing a further departure of higher-energy populations from the Maxwell-Boltzmann distribution, the lower k index indicates a harder spectrum, i.e., one with higher contribution of energetic particles. 3. Observations 3.1. Event Overview [14] Figure 3 summarizes the entire event profile from 12: UT on Day 331 until : UT on Day 332 in 2. We defined four intervals in this range based on differences in upstream region properties and the ENA count rate: interval 1 turbulent, lower ENA count rate; interval 2 turbulent, higher ENA count rate; interval 3 quiet, higher solar wind pressure, quasi-radial IMF field; and interval 4 quiet, moderate solar wind pressure, quasi-parallel IMF field. As seen in the Figure 3b-2, a gradual increase in the solar wind speed with several pressure pulses was observed during this CIR event. Along with the turbulent phase of the solar wind (intervals 1 and 2), the IMF had a sector structure as often seen during CIRs (Figure 3b-3). [15] Figure 3a shows the integrated count rates based on observations projected in the GSE-X-Z plane. The count rate was averaged in 96 spins within a 2 bin in the GSE-Z direction and 6.5 bin in the GSE-X direction. Then it accumulated to project in a plane considering the coverage area as a weighing function. As seen in Figure 3b-1, the IBEX count rate increased and decreased, responding to changes in the solar wind in each interval (numbered 1 to 4). The ENA count rate from the nose region for each energy bin ranged from 5 to 3 in 96 spin bins (~25 min). We selected this period to keep the IBEX FOV between 8 R E and 15 R E, where we expected magnetosheath emissions to occur. In addition to the spatial limitations on our data, the energy range covers the characteristic energy range of magnetosheath particles where the particle fluxes in the magnetosphere are relatively low, as seen in the THEMIS in situ data (Figures 4a and 4b). Thus, in our study we assume that the contribution of ENAs of magnetospheric origin is limited. The fact that the strongest emission was observed 3129

5 X X X X Figure 3. (a) Count rate maps of hydrogen ENAs integrated over.5 6 kev for intervals 1 4 as indicated in the figures below. Estimated locations of the bow shock and magnetopause based on averaged solar wind conditions during each interval are also indicated. (b-1) Time profiles for count rates in ESA-3 ( kev) and ESA-6 ( kev) per 96 spins (~24 min) from the subsolar magnetosheath. (b-2) Time profiles of OMNI solar wind speed and solar wind dynamic pressure. (b-3) Time profiles of OMNI IMF B magnitude and IMF x, y, andz components in GSE. (b-4) Time profiles of SYM-H index and Dst index. only from the nose region of the magnetosheath supports this assumption, even if the ENA intensity varied in time. In the GSE-Z direction, we chose the ENA fluxes that originated between 5 and 5 R E for later analysis. Importantly, the SYM-H (and Dst) index (Figure 3b-4) showed no enhancement of the ring current as usually observed during a storm time. Therefore, the magnetopause position and the magnetosheath particle population were not affected significantly by ring current growth Intervals 1 and 2: Dynamic Solar Wind Phase [16] Figure 4 shows a summary plot of key THEMIS features with the observations and upstream conditions for intervals 1 and 2. The energy-time spectrograms (Figures 4a and 4b) as well as the temperature and magnetic field strength (Figure 4d) indicate that during this period, THEMIS probes made multiple crossings of the boundaries between the solar wind (typically ~18: UT, Day 331), the magnetosheath (e.g., ~19: UT, Day 331), and the magnetosphere (~23: UT, Day 331). Figure 4c shows the calculated magnetopause and bow shock positions at the subsolar point using empirical models by Shue et al. [1998] and Chao et al. [22], respectively. The dynamic movement of the magnetosheath during these intervals is clearly seen in this figure. The motion basically is in agreement with the observation of the different regions on the THEMIS space probes. The IBEX FOV with FWHM is also shown by blue curves in Figure 4c. By comparing the magnetopause and bow shock locations with the IBEX FOV, we determined the observation regions. We discuss this FOV effect later when comparing the in situ to remotely observed spectra. 313

6 TH D TH E (a) 5-(b) 5-(c) 2 4-(a) 4-(b) 3 [/s/cm 2 /str/kev] THEMIS R [Re] THEMIS Temp. ENA Flux [/s/cm 2 /str] Sheath Temp. SW Cone Angle [deg] IBEX FOV (FWHM) TD-Temp TE-Temp IBEX IBEX TD-R TE-R Chao 2 TD- B TE- B Shue 98 18: 21: : 3: 6: 9: UT; DOY , 2 (Orbit 3) 4-(c) 4-(d) 4-(e) 4-(f) 4-(g) IV Emp. Model [Re] Themis B [nt] Proton Flux [/s/cm 2 /str] Kappa IMF B z [nt] Figure 4. Energy-time spectrogram for (a) TH-D and (b) TH-E. (c) Empirical models of the bow shock [Chao et al., 22] and the magnetopause [Shue et al., 1998] locations at the subsolar nose with TH-D and TH-E radial locations and IBEX FOV in FWHM. (d) Time profiles of ion temperature and total magnetic field measured by THEMIS probes. (e) Time profile of IBEX-integrated ENA flux ( kev) averaged in 24 spins (~1 h) and estimated proton flux at the magnetosheath. (f) Calculated kappa and temperature by fitting kappa distribution to the 1 h averaged IBEX ENA spectra. (g) Time profiles of solar wind cone angle and IMF Bz for intervals 1 and 2. [17] Figures 4e and 4f show the integrated ENA flux in the energy range as well as the converted magnetosheath fluxes and the fitted kappa parameters (k, T) calculated by equations (1) (3). All kappa values that exceed 7 cannot be discriminated with the Maxell-Boltzmann distributions, considering energy resolutions and statistical errors in the observations. Therefore, the values of kappa over 7 are all counted in the value (k 7) to show that the plasma distribution is near to the thermal equilibrium. The data were averaged over 24 spins (~1 h) to obtain enough counting statistics to validate the fitting procedure. The spectral shape (k) and temperature were more stable in interval 2 than in interval 1 except for one outlier period starting from 6 UT and ending in 7 UT. In this specific period, the distribution function was closer to a Maxwell- Boltzmann distribution than a kappa distribution, where the IBEX FOV offsets to cover the upstream region rather than the magnetosheath. The spectral index (k ~ 5) indicates that a particle heating occurred during interval 2, producing the energetic particle populations exceeding that of the Maxwell-Boltzmann distributions typical to the solar wind. Conversely, the temperature was lower in interval 1. The temperature measured by THEMIS probes in the magnetosheath for interval 1 corresponded well to the inferred temperature at the corresponding time during the intervals marked as 5-(a), 5-(b), and 5-(c), which ranged from to 3 ev. Since the gap between the integral flux of ENAs and protons was limited, the effect of the different neutral hydrogen 3131

7 THEMIS-D THEMIS-E THEMIS-D THEMIS-E THEMIS-D THEMIS-E (a) DOY331 18:29-2:24 UT 6 (b) DOY331 21:22-22:19 UT 6 (c) DOY332 :14-1:12 UT Energy Energy Energy Figure 5. Energy spectra obtained by TH-D and TH-E compared with the spectrum calculated from the measurement. Each IBEX spectrum is averaged for (a) 18:29 UT to 2:24 UT, DOY 331; (b) 21:22 UT to 22:19 UT, DOY 331; and (c) :14 UT to 1:12 UT, DOY 332 as also suggested by the dashed red borders in Figure 4. TH-D and TH-E spectra are averaged using only the data obtained from their magnetosheath transit. densities was relatively small in this static model. Figure 4g shows the solar wind cone angle and IMF Bz. The solar wind cone angle used here is defined as the geometric angle between the IMF vector and the GSE-X unit vector, describing the shock angle at the subsolar point of the Earth s bow shock. [18] Figure 5 compares the energy spectra obtained from the THEMIS in situ observations with those obtained from remote-sensing observations. We separated and chose THEMIS probe magnetosheath spectral data for analysis based on plasma temperature and magnetic field. As indicated in Figure 4 with dashed red lines, we focused on three periods with constant simultaneous coverage of the magnetosheath from THEMIS and IBEX. In general, the averaged THEMIS sheath spectra agree well with the observations through corresponding periods. In Figure 5a, the spectra corresponded well (within the error bar differences) in intensity, shape, and position of the shoulder when IBEX was mainly observing the magnetosheath (see Figure 4c). Figure 5b shows a greater difference between spectra than Figure 5a due to lack of coverage in the magnetosheath: The IBEX FOV covered the upstream region rather than the magnetosheath for half the period seen in Figure 4c. However, some characteristic features of these spectra were common, e.g., the position of the knee and the slope of the higher-energy tail. In Figure 5c, the IBEX FOV covered mainly the upstream region, which resulted in an even greater difference between TH-E and IBEX spectra. The TH-D spectra were also strongly biased by magnetospheric spectra because the probe was in the magnetosheath for only a short time Intervals 3 and 4: Static Solar Wind Phase [19] Figure 6 summarizes the parameters shown in Figure 4 but for intervals 3 and 4. Since the upstream conditions were quieter than in the other intervals, the positions of the magnetopause and the bow shock (Figure 6c) were more or less stable except for the transition from interval 3 to interval 4. Therefore, the IBEX FOV could constantly cover the subsolar magnetosheath in these intervals. The THEMIS probes spent considerable time in the magnetosheath, especially in interval 3 (Figures 6a and 6b). Under almost constant IMF Bz = conditions, the spectral index (k) and temperature seemed to reflect the solar wind cone angle variations. The local sheath temperature measured by THEMIS probes agreed with the temperature calculated from data (around 25 ev in interval 3). In response to the quiet conditions during this time, the total ENA fluxes became more stable than those obtained in intervals 1 or 2. [2] Figure 7 compares the energy spectra obtained from THEMIS probe in situ observations with remotesensing observations for intervals 3 and 4 under the same conditions as in Figure 5. Corresponding periods for Figures 7a 7c are indicated by dashed red lines in Figure 6. In all cases, the spectra agree well with THEMIS probe spectra in intensity and shape. Due to the better coverage of the magnetosheath by the IBEX FOV, spectra lined up better in Figure 7 than in Figure 5. However, there was a slight but systematic gap between in situ fluxes and ENAbased fluxes with a maximum difference of a factor of 2 at.7 kev (Figure 7a). There are three potential explanations for this gap: (1) the exosphere density model had a large error bar in this location [Zoennchen et al., 211]; (2) the static model of the exosphere ignored the dynamic process that continuously changed the dayside hydrogen density by charge exchange [Zoennchen et al., 2]; and (3) the original proton distribution in the magnetosheath varied even within the same line of sight [Spreiter and Stahara, 198] and had a temperature anisotropy [e.g., Fuselier et al., 1994]. 4. Discussion 4.1. IBEX-Lo and Comparison With Kappa Fitting [21] Figure 8 shows average magnetosheath proton fluxes inferred from the and IBEX-Lo instruments for each interval. Since IBEX-Lo had approximately an order of magnitude lower sensitivity than, the error bars are relatively large even after ~6 h integrations. However, the flux in the overlapping energy range agreed well. For all four intervals, kappa distributions were applied and fitted to the data. Then the IBEX-Lo results were compared to the trend of the fitted functions at lower energies. 3132

8 TH D TH E (a) 7-(b) 4 7-(c) 6-(a) 6-(b) 9 3 [/s/cm 2 /str/kev] THEMIS R [Re] THEMIS Temp. ENA Flux [/s/cm 2 /str] Sheath Temp. SW Cone Angle [deg] 2 18 IBEX FOV (FWHM) 16 TD-R TE-R Chao TD-Temp TE-Temp IBEX IBEX TD- B TE- B 18 6-(c) Shue (d) 2 12: 15: 18: 21: : UT; DOY 332, 2 (Orbit 3) Figure 6. Energy-time spectrogram for (a) TH-D and (b) TH-E. (c) Empirical models of the bow shock [Chao et al., 22] and the magnetopause [Shue et al., 1998] locations at the subsolar nose with TH-D and TH-E radial locations and IBEX FOV in FWHM. (d) Time profiles of ion temperature and total magnetic field measured by THEMIS probes. (e) Time profile of IBEX-integrated ENA flux ( kev) averaged in 24 spins (~1 h) and estimated proton flux at the magnetosheath. (f) Calculated kappa and temperature by fitting kappa distribution to the 1 h averaged IBEX ENA spectra. (g) Time profiles of solar wind cone angle and IMF Bz during intervals 3 and 4. 6-(e) 6-(f) 6-(g) 7 6 IV Emp. Model [Re] Themis B [nt] Proton Flux [/s/cm 2 /str] Kappa IMF Bz [nt] THEMIS-D THEMIS-E THEMIS-D THEMIS-E THEMIS-D THEMIS-E (a) DOY332 13:41-15:36 UT 6 (b) DOY332 16:33-18:28 UT 6 (c) DOY332 18:28-2:24 UT Energy Energy Energy Figure 7. Energy spectra obtained by TH-D and TH-E compared with spectra calculated from measurement. Each IBEX spectrum is averaged for (a) 13:41 UT to 15:36 UT, DOY 332; (b) 16:33 UT to 18:28 UT, DOY 332; and (c) 18:28 UT to 2:24 UT, DOY 332 as also suggested by the dashed red borders in Figure 6. TH-D and TH-E spectra are averaged using the data only obtained from their magnetosheath transit. 3133

9 8 IBEX-Lo 8 IBEX-Lo (a) Interval 1 T = 175 ev k = (b) Interval 2 T = 484 ev k = IBEX-Lo 8 IBEX-Lo (c) Interval 3 T = 34 ev k = Energy Energy (d) Interval 4 T = 41 ev k = 3.76 Figure 8. Combined magnetosheath proton spectra using IBEX-Lo and data averaged for four intervals: (a) interval 1, (b) interval 2, (c) interval 3, and (d) interval 4. The kappa distributions are fitted only to the data. It is clear that the data could be well fitted by kappa distributions. Moreover, the IBEX-Lo results also seemed to generally follow the same distribution functions. They apparently have secondary distributions for higher energies especially for Figures 8a, 8b, and 8d. Both these secondary distributions and the core distributions have been commonly found in the magnetosheath (see section 1), which may explain the slight disagreement of the IBEX-Lo data at lower energies. Nonetheless, to avoid complexity, we applied a single kappa distribution to each spectrum. Then we utilized the kappa index as a measure of the proportion of higher-energy particles. Among the four intervals, intervals 1 and 3 had the lowest kappa, indicating that they had the highest number of energetic ions. In interval 1, the magnetosheath experienced turbulent upstream conditions due to which stochastic acceleration could be driven by scattering and dissipation in accordance with plasma wave turbulence. In interval 3, the quasi-radial configuration of the IMF (see Figure 6g) forms a quasi-parallel shock in the subsolar position, resulting in active contributions from shock acceleration or downstream turbulence typical of parallel shock. These features will be discussed further by comparison with the upstream conditions. Frequency 5 (a) 1.5/2. 2./ /3. 3./ /4. Kappa 4./4.5 Mean=3.5 Median= /5. 5./ /6. 6./6.5 Sheath Temp..8.4 (b) Kappa Figure 9. Properties of fitted parameters for 1 h averaged spectra from data using kappa distributions: (a) occurrence frequency of k and (b) distribution of the magnetosheath temperature as a function of k. The number of samples in this analysis is 3 (n = 3). 3134

10 SW Cone Angle [deg] (a) (b) R=.67 R=.62 1 P=.13 P=.23 R= P= (d) (e) (c) (f) IMF Bz [nt] R=.63 R=.49 P=.28 P=.19 R= P= Kappa Sheath Temp. T 5 Figure. Scatter plots of the 1 h averaged solar wind cone angles and magnetosheath ion spectral parameters calculated based on the IBEX observations and OMNI data: (a, d) kappa, (b, e) temperature, and (c, f) temperature deviation (see equation (5)). The color with corresponding numbers in the bottom left side in each plot indicates the four different intervals as defined in Figure 3. [22] The caveat to use IBEX-Lo data is that we have to reject intervals when the high-energy ion flux (from the Earth s bow shock) is high because one of the high-voltage power supply to reject energetic (> kev) ions is not operational. Although we carefully used a background monitor to choose the data, this issue does not affect our study because the fluxes from the Earth are orders of magnitude higher than the very low fluxes of heliosheath ENAs [Fuselier et al., 2]. Moreover, the fact that the fluxes compare well between and IBEX-Lo confirms that the back ground issue is not a big problem in our results Kappa Fitting Evaluation and Comparison With the Heliosheath [23] Figure 9a shows the distribution of k values corresponding to the 1-h averaged spectra (as shown in Figures 4f and 6f), and the resulting k and T relationship is shown in Figure 9b. Most k values were greater than 2, with the median equal to 3.3 (n = 3). This value is similar to or only slightly harder than kappa distributions found in the solar wind [e.g., Collier et al., 1996], the expected source of the magnetosheath particles. The difference between ENA spectra originating in the heliosheath and the magnetosheath observed by the same instrument,, is interesting because they are mainly composed of plasmas downstream of a fast shock with a strong shock acceleration effect. Our magnetosheath measurement showed that the kappa indices of the terrestrial magnetosheath were distributed within the near-equilibrium region (2.5 < k 1) [Livadiotis and McComas, 2], whereas the far-equilibrium states (1.5 < k 2.5) were found in the inner heliosheath [Livadiotis and McComas, 212]. The difference could be explained by considerable populations of pickup ions for the heliosheath protons [Livadiotis et al., 213] in addition to the core population of shocked solar wind behind the termination shock. [24] As clearly seen in Figure 9b, kappa can be correlated with temperature using a relation that adheres to a generalized polytropic law including all the nonequilibrium thermodynamic parameters (e.g., kappa index, density, and temperature). A similar trend was observed in the inner heliosheath [Livadiotis et al., 211]. To further understand the effect of upstream conditions on temperature by differentiating this universal trend, the k T relation was fitted by a linear function T(k). Then the deviation from that function (ΔT i ) was calculated in the analysis described in section 4.3. The formulae are expressed as follows: TðkÞ ¼ a k þ d; (4) ΔT i ¼ T i Tðk i Þ; (5) where a and d denote the slope and the intercept of T(k) and i identifies the number of each data point Statistical Analysis [25] Based on qualitative implications of the kappa fitting results, we acquired correlations between major solar wind parameters averaged at the same time. We found two noticeable trends: the solar wind cone angle and IMF Bz. The solar wind cone angle is related to the type of bow shock (parallel or perpendicular), and the IMF Bz is related to the reconnection rate at the nose of the magnetopause which might enhance magnetospheric particle contributions. In this analysis, we used data points with <25% standard deviation during a 1 h average (n = 22). Figure summarizes the results. The R and P in the plots show the correlation coefficient and the significance by t test evaluation, respectively. These figures indicate hysteresis starting from the square 3135

11 point with connected lines in each point. A different color is used for each interval as shown in the figure. As clearly seen in Figure a, the correlation between solar wind cone angle and kappa is statistically significant with a correlation coefficient of.67 and a P-value of.1, indicating that the parallel configuration at the bow shock contributes to the production of the energetic portion of the magnetosheath particles. This correlation has two potential explanations: (1) The parallel field at the bow shock enabled easy access of the protons between upstream and downstream, driving first-order Fermi acceleration. This mechanism could produce harder spectra than caused by shock drift acceleration in the perpendicular configuration of the bow shock. (2) The turbulence in the magnetosheath especially in the case of parallel upstream conditions caused a stochastic heating of the local plasmas via resonance, diffusion, and scattering with plasma waves. [26] The correlation between IMF Bz and kappa was significant as well (Figure d), but it could also be explained by the existence of two independent distributions for IMF Bz north and south. Thus, it is difficult to draw conclusions from this analysis alone because the only example of the strong northward IMF occurred under perpendicular shock conditions. As seen in Figures a and b, or Figures d and e, the trend between the magnetosheath temperature and the kappa indices is similar. Although we cannot exclude the possibility of a mechanism to control the kappa and temperature in a correlated way, this relation may only be the result of polytropic relations of kappa distributions, since ΔT has almost no correlation with the solar wind cone angle or the IMF Bz as seen in Figures c and f. 5. Conclusions [27] ENA spectral analysis from the dayside magnetosheath leads to four conclusions: [28] 1. The remotely sensed 1 h average spectra calculated from IBEX ENA observations of the dayside magnetosheath correspond well with in situ measurements of THEMIS probes over a fairly wide range of solar wind parameters if we carefully limit the field of view to cover the nose region in the magnetosheath. This enabled us to evaluate global downstream bow shock conditions and their correlations with upstream conditions, the three-dimensional shape of the bow shock, and bulk flow variations within the magnetosheath. [29] 2. The observations indicate that the kappa distribution could be applied to calculated magnetosheath distributions, resulting in agreement with IBEX-Lo observations. The computed kappa values indicate the existence of an energetic non-maxwellian tail in the distribution functions. This is consistent with the earlier in situ observations in the magnetosheath plasma where more than one component in the distribution function was regularly found. [3] 3. The calculated kappa values are consistent with previous solar wind observations but differ from outer heliospheric spectra. [31] 4. The appearance of the energetic tail correlates with the solar wind cone angle, showing a strong effect of the shock angle at the dayside nose position to the production of suprathermal particles. The result indicates that a parallel (perpendicular) shock configuration produces more power-law-like (Maxwell-Boltzmann-like) plasmas within the magnetosheath. [32] This study focuses on the IMF clock and cone angles to identify contributions to particle distributions from shock acceleration and the magnetic field that merge at the nose of the magnetosheath. In the future, we will carry out a larger statistical analysis to determine additional correlations of magnetosheath spectra with upstream conditions to control the emergence of the suprathermal particles using 4 years of data. [33] Acknowledgments. 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