NASA’s Interstellar Boundary Explorer (IBEX) mission aims to discover the global interaction between solar winds and the local interstellar medium (McComas et al., 2009). IBEX can also provide energy-resolved terrestrial hydrogen (H) energetic neutral atom (ENA) observations due to the following observation configurations. First, the IBEX spacecraft follows a highly elliptical and nearly equatorial orbit around Earth. Due to this elongated orbit, IBEX routinely views Earth’s magnetosphere from side-viewing vantage points from March to June (spring) and September to December (fall) (see Figure 1 in Hart et al., 2021). Second, the spacecraft spins with a Sun-pointing rotation axis, and its ENA instruments have a ∼7° full-width-half-max (FWHM) ENA acceptance angle (McComas et al., 2009). This configuration results in its field-of-view (FOV) covering a swath of the sky perpendicular to the Sun-pointed spin-axis. Finally, IBEX is equipped with two single-pixel ENA instruments that measure neutral atoms in the energy ranges of ∼0.01–2 keV in eight energy steps of IBEX-Lo (Fuselier et al., 2009) and ∼0.3–6 keV in six energy steps of IBEX-Hi (Funsten et al., 2009). The energy ranges of IBEX correspond to the energies of the major plasma populations in the magnetosphere (Chappell et al., 2008), enabling it to provide energy-resolved observations of terrestrial ENAs, which are produced by charge exchange between plasma ions and ambient neutral atoms.

For the past decade, IBEX has provided not only heliospheric ENA observations but also imaging data of distant magnetospheric regions and energy-resolved observations of terrestrial ENAs. IBEX-Hi terrestrial ENA observations have been analyzed to study terrestrial ENA sources, including the Earth’s subsolar magnetopause (Fuselier et al., 2010; Fuselier et al., 2020), the terrestrial plasma sheet (Dayeh et al., 2015; Fuselier et al., 2015; McComas et al., 2011), magnetospheric cusps (Petrinec et al., 2011), and the dayside magnetosheath (Ogasawara et al., 2013). Previous literature shows that IBEX terrestrial ENA observations can provide a unique global viewing perspective of the magnetosphere, complementing in situ measurements and, thus, enhancing the understanding of magnetospheric plasma regions and processes on both micro and macro scales.

Plasma populations with energy ranging from 0.1 eV to hundreds of eV are found in the magnetosphere (Chappell et al., 2008). These ions can exchange electrons with ambient exospheric H atoms (∼0.01 eV), becoming charge-exchange-induced H ENAs that can escape the Earth’s gravitational influence and be measured by the IBEX-Lo ENA instrument. Studying the escaping neutral atoms is key to understanding the role of non-thermal atoms in Earth’s atmospheric loss (Shizgal and Arkos, 1996). The IBEX-Lo terrestrial ENA observation provides a unique tool for investigating low-energy terrestrial ENAs that escape the Earth’s atmosphere. However, the IBEX-Lo terrestrial ENA observations have not yet been explored. This study examines whether the IBEX-Lo terrestrial H ENA observations provide the in situ measurements of charge-exchange-induced H ENAs generated in the inner exosphere (<4 R_E).

This paper is organized as follows. Section 2 describes the IBEX-Lo ENA dataset and the data selection and analysis methodology. Section 3 discusses the IBEX 15 eV ENA observations and compares the observed fluxes with simulated fluxes obtained from a simple ENA flux model. Finally, Section 4 presents our summary.

In this study, we first investigated the physical properties of outward H fluxes measured by the IBEX-Lo instrument. The outward H fluxes are likely charge-exchange-induced H ENAs produced within the inner exosphere. We analyzed differential H fluxes derived from 35 1-h cadence swaths over 45 days during Case 2009 and 39 1-h cadence swaths over 89 days during Case 2013. Figure 3 illustrates the counts of outward H atoms measured within the restricted NEP angle range for selected 1-h cadence swaths during the restricted time intervals (upper panel), the corresponding signal-to-noise ratios (middle panel), and the differential fluxes of outward H atoms calculated by Equation 2 (bottom panel) as a function of the geocentric radius. For statistical reliability, we excluded fluxes with signal-to-noise ratios below 1.5 in the bottom panel. Additionally, fluxes that measured closer than 28 R_E in 2013 were omitted to avoid possible contamination from magnetosheath ions near the bow shock after our filtration process described in Section 2. Blue squares represent the values from Case 2009, while red circles correspond to values from Case 2013.

To characterize the physical properties of the outward H fluxes, we applied a power-law function, J = c·r ^-k, to fit the measured fluxes. In Figure 3, the dashed lines represent the power-law fits, with power indices k = 0.79 ± 0.49 for Case 2009 (blue) and k = 0.96 ± 0.70 for Case 2013 (red). These power-law fits reveal three key findings. First, the fluxes are on the order of 10⁶ cm⁻² s⁻¹ sr⁻¹ keV⁻¹, as shown in the bottom panel of Figure 3. The average fluxes were (2.10 ± 0.89)×10⁶ cm⁻² s⁻¹ sr⁻¹ keV⁻¹ in 2009 and (2.16 ± 0.87) × 10⁶ cm⁻² s⁻¹ sr⁻¹ keV⁻¹ in 2013. Second, the outward H fluxes gradually decrease with increasing geocentric distance, as indicated by the power indices of 0.79 and 0.96. Although these indices have relatively large uncertainties, their possible range remains positive, confirming that the flux intensity decreases with increasing distance. To further validate this trend, we applied the power-law function to fit the measured fluxes with signal-to-noise ratios above 2.0 and taken within 45 R_E. This yielded positive power indices of 0.73 ± 0.62 for 2009 and 0.88 ± 0.76 for 2013, reinforcing the observed decrease in the flux intensity. Lastly, there is no significant difference in the flux intensities between the 2009 and 2013 spring seasons. Figure 4 shows the F10.7 indices obtained from the OMNIWeb database, where solid lines indicate hourly averages during the 2009 (red) and 2013 (blue) spring seasons. In addition, red squares and blue circles denote the averaged values over the restricted time intervals in the 2009 (red) and 2013 (blue) seasons, respectively. Although the F10.7 indices confirm higher solar radio fluxes in 2013 than in 2009, our results suggest that the outward H fluxes are not influenced by the solar radio flux.

To confirm the inner exosphere as the possible source of the outward H fluxes, we consider a simple model for ENAs produced in the inner exosphere. The ENA differential flux ( J ENA ) at an observer location ( r → sc ) and the ion differential flux ( J ion ) are related by the following equation: J ENA E , r → sc = ∫ J ion E , r → ⋅ σ E ⋅ n H r → d r (3) where σ(E) is the charge-exchange cross-section, n H r → is the exospheric neutral H density at r → , and the integral is computed along the LOS crossing the inner exosphere. We simplify this relation under the following assumptions: (1) the ion flux depends only on energy and not on position, i.e., J ion E , r → = J ion E ; (2) the exospheric density decreases as n H r → = n H 0 r 0 / r 3 , where r ₀ = 10 R_E; and (3) there is no loss after the ENA is created, i.e., no loss term in Equation 3. Thus, Equation 3 simplifies as follows: J ENA E , r → sc = J ion E ⋅ σ E ⋅ n H 0 r 0 3 ⋅ ∑ r → r − 3 Δ r (4)

Figure 5A shows the schematic diagram of the simple model for the expected ENA fluxes produced in the inner exosphere. The gray dashed circle represents a sphere with a 4 R_E geocentric radius, the black solid circle indicates the Earth, and the blue dots illustrate example integral points. We consider the situation where the IBEX spacecraft is located along the Y _GSE axis at distances ranging from 25 R_E to 50 R_E. For each location of the IBEX spacecraft, we consider the angle ζ between the Y _GSE axis and an LOS (red solid line in Figure 5A). The restricted angle ζ _lim is defined as the angle at which the LOS passes through the surface of a 1 R_E sphere. We compute the integral along the LOS in the inner exosphere when ζ > ζ _lim, but we perform the integral only in the dawn region when ζ < ζ _lim as the dusk side is blocked by the Earth.

We obtain the 15-eV ion flux from the in situ ion flux observations of the HOPE mass spectrometer aboard the RBSP during the spring season of Case 2013 (days 88–177, 2013). The RBSP was orbiting within a 6 R_E geocentric radius and close to the X _GSE–Y _GSE plane. Figure 5B shows the ion differential fluxes (black dots) at 15 eV, with the red dots representing the ion fluxes measured during the specific intervals corresponding to the restricted time intervals of Case 2013 for the outward H fluxes from the inner exosphere (Table 1). Since the ion fluxes in the restricted time intervals are widely distributed, as seen in the bottom panel of Figure 5B, we compute the mean of the distribution, which corresponds to the center of the peak in the Gaussian function. The red dashed curve represents the fit curve for the ion flux distribution. The mean ion flux is J ¯ ion = 6.0 × 10 5 cm^-2 s⁻¹ sr⁻¹ keV⁻¹ at 15 eV, with one-sigma uncertainties ranging from 2.8 × 10⁵ cm⁻² s⁻¹ sr⁻¹ keV⁻¹ to 1.1 × 10⁶ cm⁻² s⁻¹ sr⁻¹ keV⁻¹, representing the lower and upper limits of the 15 eV ion fluxes. We use the charge-exchange cross-section computed by Lindsay and Stebbings (2005): σ = 4.07 × 10 − 15 cm² at E = 15 eV. Finally, we consider the expected ENA fluxes for two cases of the exospheric density at 10 R_E: n _H0 = 10 cm^-3 and 50 cm⁻³ because the neutral density at 10 R_E ranges from 4 to 59 cm⁻³ (Connor et al., 2021).

Figure 6 shows the expected ENA fluxes (green solid line for n _H0 = 10 cm⁻³ and cyan solid line for n _H0 = 50 cm^-3) and the measured outward H fluxes (blue squares for Case 2009 and red circles for Case 2013). The shaded regions represent the uncertainty in the expected ENA fluxes, arising from the lower and upper limits of the ion fluxes. The blue and red dashed lines indicate the power-law fit curves for the measured fluxes. We also computed the power-law fit curves for the expected fluxes, with the estimated power index k = 0.27 ± 0.01. The expected ENA fluxes exhibit two features consistent with the measured outward H fluxes. First, the expected ENA fluxes slightly decrease as the geocentric distance increases. This decrease in the intensity is reasonable because the LOS length in the sphere of 4 R_E varies depending on the spacecraft’s location. When the spacecraft is closer to Earth, the LOS slopes are slightly steeper than when the spacecraft is further away. The steeper slope causes longer integral distances in the sphere of 4 R_E and increases the probability of ENA production. Second, the upper limit of the expected ENA fluxes of n_H0 = 50 cm^-3 is of the same order of magnitude as the measured outward H fluxes. This suggests that the measured outward H fluxes may include ENAs generated in the inner exosphere.

The discrepancy between the observed and expected H ENAs may arise from the simplicity of our model. We assumed that the ion flux does not depend on location, that the exospheric density decreases as 1/r ³, and that the ENAs are created in the sphere of 4 R_E. However, the ion flux exhibits large variability in magnitude, as seen in the bottom panel of Figure 5B, and the exospheric density does not exactly follow the inverse cube law in the inner exosphere. The IBEX-Lo FOV covers a larger sphere than the inner exosphere when the spacecraft was further than 25 R_E geocentric distance. Furthermore, we only used the ion flux measurements from the 2013 season as no 15 eV ion flux observations were available for the 2009 season. As a result, the expected ENA fluxes calculated by the simple model did not fully capture the ENAs produced during the solar minimum. On the other hand, the measured outward H fluxes could be overestimated due to contamination by any energetic particles even though we removed most of the contaminating sources. Despite these limitations, the two features described above support the conclusion that the measured outward H fluxes are closely related to the ENAs produced in the inner exosphere.

There are various plasma populations in the terrestrial magnetosphere. The protons of plasmasphere origin are at a temperature of approximately 2,000–10,000 K (0.1 to a few eV), whereas exospheric H is at approximately 750–1,250 K (Shizgal and Arkos, 1996). The combination of polar wind flow and a centrifugal acceleration can give the ions energies above 10 eV, which can send them back into the magnetotail (lobal wind, 10–300 eV). In the magnetotail, the curvature drift across the tail through the cross-tail potential can add energies of 1 keV and more (plasma sheet, 0.5–5 keV) (Chappell et al., 2008). The amount of energy gained by these particles of plasmasphere origin and the resulting flow paths can lead to two separate plasma populations: the warm plasma cloak and the ring current. The warm plasma cloak is a bidirectional, field-aligned distribution of ions with an energy range of a few eV to hundreds of eV. Ions with these characteristics are found outside the plasmasphere across the night side and can extend eastward from the plasmapause to the magnetopause. The ring current is composed of more accelerated ions with an energy range of 3–30 keV that moves westward by the curvature drifts. These magnetospheric plasma populations can become charge-exchange-induced ENAs, which could be measured by the IBEX-Lo instrument. In this study, we found that the outward H fluxes measured by the IBEX-Lo are closely associated with the charge-exchange-induced H ENAs. Since these plasma populations have different dynamics and behaviors, we anticipate that the observations of outward H fluxes show temporal and spatial evolutions depending on an observation season and a side-viewing vantage point if they contain the terrestrial ENAs. Because the full energy range of the IBEX-Lo instrument overlaps with the energy ranges of the magnetospheric plasma populations, we anticipate that the energy-resolved ENA observations of IBEX-Lo could show different energy spectra depending on what the instrument observed. We plan to continue researching the dynamics and energy spectra of the outward H fluxes as follow-up projects.

In this study, we first investigated the IBEX-Lo 15 eV ENA observations when the inner exosphere entered the IBEX FOV during two spring seasons: 11 April–26 May 2009 (Orbit 25–30, DOY 101–146) and 29 March–27 June 2013 (Orbit 201–210, DOY 88–178). We used the ENA histogram data obtained from the IBEX Raw Data Release (https://ibex.princeton.edu) and defined the restricted time intervals and angle ranges where the H fluxes escaping from the inner exosphere (<4 R_E) could be detected by the IBEX-Lo instrument. Then, we estimated the outward H fluxes in the outer exosphere (25 ∼ 50 R_E). The measured outward H fluxes are characterized by three key findings: (1) the fluxes vary in the order of 10⁶ cm⁻² s⁻¹ sr⁻¹ keV⁻¹, (2) the outward H fluxes gradually decrease with increasing geocentric distance, and (3) there is no significant difference in flux intensities between the 2009 and 2013 spring seasons.

Additionally, we computed the expected H ENA fluxes produced in the inner exosphere by solving Equation 4 using ion flux data from HOPE on RBSP and a simplified exospheric density model inversely proportional to the cube of the geocentric distance. In this calculation, we assume that the ion flux depends only on energy and not on position as well as there is no loss after the ENA is created. Both the measured outward H fluxes and the expected ENA fluxes reveal two similar features: (a) a slight decrease in the intensity with increasing geocentric distance and (b) intensities on the order of 10⁶ cm⁻² s⁻¹ sr⁻¹ keV⁻¹. These two features strongly suggest that the observed outward H fluxes are associated with the H ENAs produced in the inner exosphere. We also found no significant difference in the intensities of the outward H fluxes measured in different solar radio flux conditions (quiet in 2009 versus more active in 2013). However, further statistical analysis is required to fully understand the variation of outward H fluxes over the solar cycle, which is left for future work.