A Case Study of Transversely Heated Low‐Energy Helium Ions by EMIC Waves in the Plasmasphere

The Van Allen Probe A spacecraft observed strong ∼0.5‐Hz helium (He+) band and weak ∼0.8‐Hz hydrogen (H+) band EMIC waves on April 17, 2018, at L = ∼4.5–5.2, in the dawn sector, near the magnetic equator, and close to the plasmapause. We examined low‐energy ion fluxes observed by the Helium Oxygen Proton and Electron (HOPE) instrument onboard Van Allen Probe A during the wave interval and found that low‐energy He+ flux (<10 eV) enhancements occur nearly simultaneously with He‐band and H‐band EMIC wave power enhancements in a direction mostly perpendicular to the background magnetic field without significant low‐energy H+ and O+ flux variations. We suggest that cold He+ ions (<1 eV) are preferentially and transversely heated up 10 eV through the interaction with EMIC waves inside the plasmasphere. The low‐Earth orbit spacecraft observed localized precipitations of energetic protons in the upper ionosphere at subauroral latitudes near the magnetic field footprint of Van Allen Probe A. Our observations provide a clear evidence that EMIC waves play an important role in the overall dynamics in the inner magnetosphere, contributing to the high‐energy particle loss and low‐energy particle energization.

Studies of EMIC waves including cold heavy ions (He + and O + ) reported that the heavy ions play a significant role in determining the spectral and propagation properties of EMIC waves even though in small concentrations of cold heavy ions (∼5%-10% for He + and ∼1%-2% for O + ) (Kozyra et al., 1984;Mauk et al., 1981;Roux et al., 1982;Young et al., 1981). In the presence of such cold heavy ions, EMIC waves can be generated in three distinct bands: a H-band between the local H + gyrofrequency (  waves is peaked in the dawnside sector at L = ∼10-12 (Anderson et al., 1992a(Anderson et al., , 1992bKeika et al., 2013;G.-J. Kim et al., 2016;Min et al., 2012). Since low frequencies in O-band waves are overlapped with a frequency of background magnetic field oscillations, there are few observations of O-band EMIC waves (e.g., Fraser et al., 1980;Sakaguchi et al., 2013).
EMIC waves interact with energetic protons and relativistic radiation belt electrons. These resonant wave-particle interactions cause pitch angle scattering that leads to subsequent precipitation of energetic protons and relativistic electrons into the atmosphere. The EMIC wave scattering is currently considered one of the leading loss processes of energetic protons and relativistic electrons in the magnetosphere. Theoretical studies of the early 1970s suggested resonant pitch angle scattering of protons and electrons by EMIC waves (Lyons & Thorne, 1972;Thorne & Kennel, 1971). Evidence for EMIC wave-induced scattering has been reported from the observations of polar-orbiting POES and METOP spacecraft in the upper ionosphere (e.g., Bingley et al., 2019;Clilverd et al., 2007;Hendry et al., 2016;Hyun et al., 2014;K.-H. Kim et al., 2016b;Liu et al., 2018;Miyoshi et al., 2008;Rodger et al., 2008;Sandanger et al., 2009;Yuan et al., 2012;Zhang et al., 2016). Simultaneous appearance of isolated proton auroras and Pc1 geomagnetic pulsations at subauroral latitudes provides evidence for wave-particle interactions with EMIC waves generated near the plasmapause .
Previous numerical studies suggested that EMIC waves can preferentially heat cold He + ions in the direction perpendicular to the ambient magnetic field (e.g., Gendrin & Roux, 1980;Mauk, 1982aMauk, , 1982bOmidi et al., 2010;Omura et al., 1985;Qian et al., 1990). Earlier observations at geosynchronous orbit showed the acceleration of cold He + ions up to suprathermal energies (tens of eV to a few hundred eV) (Mauk & McPherron, 1980;Roux et al., 1982;Young et al., 1981). AMPTE/CCE observations in the outer magnetosphere (L ∼ 6-9) also reported that cold He + ions are transversely heated to average temperatures of about 35 eV and sometimes above 100 eV through the interaction with EMIC waves (Anderson & Fuselier, 1994). Zhang et al. (2011) using data from the Cluster spacecraft in the outer magnetosphere (L ∼ 8-14) showed that the He + ions resonantly interacted with EMIC waves and were perpendicularly energized to energies up to 1 keV. Although the EMIC wave-associated He + ion heating has been reported using the data obtained in the outer magnetosphere, there are few spacecraft observations for cold He + ion heating by EMIC waves in the inner magnetosphere (L < 6) (Yuan et al., 2016).
In this study, we have analyzed magnetic field and low-energy particles data acquired by the Radiation Belt Storm Probes-A spacecraft from L = ∼4.5-5.2. We observed EMIC waves just inside the plasmapause near the magnetic equator and a clear correlation between the EMIC wave activity and low-energy (<10 eV) He + flux enhancement. We suggest that cold He + ions are heated up to 10 eV through the interaction with EMIC waves and discuss which frequency band of EMIC waves contributes to heating the cold He + ions in the plasmasphere. Using the magnetic field and energetic proton data obtained from the low-Earth orbit spacecraft in the upper ionosphere, we also show localized precipitation of energetic protons occurring near the magnetic footprint of the EMIC wave's source region.

Data Sets
In this study, we use the magnetometer data (64 samples per second) obtained by EMFISIS (Electric and Magnetic Field Instrument Suite and Integrated Science) instrument package (Kletzing et al., 2013) onboard the Van Allen Probe A to examine the properties of EMIC waves observed on April 17, 2018. The Van Allen Probe A magnetic field data are exactly resampled at 0.1 s intervals after interpolation and presented in a mean-field-aligned (MFA) coordinate system. In this system, ˆz e is in the direction of the mean magnetic field, which is defined by taking 5 min boxcar running averages of the 0.1 s data; ˆy e (eastward) is parallel to  z e r, where r is the radial vector pointing from the center of the Earth toward the spacecraft; and ˆx e (radially outward) is given by ˆx e = ŷ z e e . To be consistent, the perturbed parallel magnetic field component (δB z ) is defined by B z (0.1 s averages) minus B z (5 min averages). The EMFISIS experiment includes a plasma wave spectrum analyzer, which provides information on the electron number density (Kurth et al., 2015). We have used the electron density to identify the plasmapause location. The Van Allen Probe A particle data used in this study include low-energy (<52 keV) ion fluxes measured by the Helium Oxygen Proton Electron (HOPE) instrument (Funsten et al., 2013). We also use the NOAA-15 spacecraft observations from the Medium Energy Proton and Electron Detector (MEPED) (Evans & Greer, 2004) in Sun-synchronous near-polar orbits at ∼850 km altitude to examine the precipitating energetic protons into atmosphere due to pitch angle scattering associated with EMIC waves.

EMIC Wave Observations in the Inner Magnetosphere
Figures 1a-1c show the magnetic field dynamic spectra of the transverse (δB x and δB y ) and compressional (δB z ) field components of Van Allen Probe A from 10:50 UT to 12:20 UT on April 17, 2018. The spacecraft positions (the magnetic shell parameter L, magnetic local time [MLT], and magnetic latitude [MLAT]) are shown at the bottom using a centered dipole. During the time period Van Allen Probe A was very close to the magnetic equator (|MLAT| ≤ ∼0.5°) on the inbound leg, moving from L = 5.4 to L = 4.1, within a few hours of local dawn (MLT = 5.8-7.3 h). In the inner magnetosphere (L < 6) near the dawnside magnetic equator, Van Allen Probe A observed wave activity in all field components, but strong spectral intensity appears in the transverse δB x and δB y components in the frequency band between the local helium (   wave generation, the EMIC waves in Figure 2a may be observed during the time of decay phase of EMIC waves rather than growth and saturation phases (e.g., Fu et al., 2016). The anisotropies in Figure 2b were calculated using the definition given by Chen et al. (1998):

Low-Energy He + Flux Enhancement During the EMIC Wave Event
where f is the particle phase space distribution function, and α 0 is equatorial pitch angle of the particle. We where J is the measured differential proton flux, v is the proton velocity, and m is the proton mass.
In discussing EMIC wave's generation, it is important to determine where the wave's generation location is with respect to the plasmapause because the cold plasma density plays a significant role in determining the spectral properties of EMIC waves (e.g., G.-J. Kozyra et al., 1984;Young et al., 1981). Data from the plasma wave experiment on Van Allen Probe A are used to estimate the electron number density (N e ) based on the relationship between the density and characteristic frequencies in the spectrum: the plasma frequency cutoff of continuum radiation and the upper hybrid resonance (Kurth et al., 2015). To infer the location of the plasmapause, we have examined the plasma wave spectra data provided on NASA/GSFC Space Physics Data Facility Coordinated Data Analysis Web (https://cdaweb. gsfc.nasa.gov) and found a clear upper hybrid resonance frequency (data not shown here). The electron plasma frequency (f pe ) was extracted from the narrow-band emission at the upper hybrid resonance frequency, and the electron number density was calculated from f pe using The corresponding number density N e is plotted in Figure 2c. N e indicates that Van Allen Probe A crossed the plasmapause, which is defined to be an electron density change by a factor of 5 or more over an L distance smaller than 0.5 (e.g., Carpenter & Anderson, 1992), around 10:55-11:10 UT on the inbound leg at L = ∼5.2-5.3 before the onset of EMIC wave activity, representing that wave generation is not associated with crossing the plasmapause and that the EMIC waves are generated just inside the plasmapause.
Figures 2d-2f show the energy-time spectrograms of the omnidirectional differential fluxes of H + , He + , and O + ions measured by the HOPE instrument on Van Allen Probe A. The fluxes of different ion species are plotted in the same range. The HOPE H + data show enhanced fluxes of plasma sheet protons (<1 keV) from 10:50 UT to 11:15 UT when the spacecraft encountered the inner edge of the plasmapause. It has been reported that the inner edge of the plasma sheet coincides with the plasmapause location (e.g., Engebretson et al., 2015;Kwon et al., 2015). The HOPE H + data also show weak fluxes in the energy range above 1 keV. They exhibit a band structure with two populations, one between 1 and 5 keV and the other between 20 keV and the instrument energy upper limit of 52 keV, and penetrated to the plasmasphere. This structure is similar to ion nose structures, which are a common feature in the inner magnetosphere, reported by Ferradas et al. (2016). The spectrogram of He + shows repeatedly enhanced fluxes in the low-energy population below 10 eV from 11:15 UT to 11:35 UT. These low-energy He + flux enhancements occur nearly simultaneously with the intense He-band and weak H-band wave activity interval. No enhanced flux was found in the HOPE O + data for the interval. Figure 3 shows the EMIC wave power integrated over the frequency range of 0.8-1.1 Hz for H-band wave and 0.25-0.7 Hz for the He-band wave, the difference between the observed magnetic field magnitude and model magnetic field magnitude (ΔB T ), and He + fluxes for pitch angles 72°, 54°, 36°, and 18°. We used the magnetic field model developed by Tsyganenko and Sitnov (2005) (TS05) at the position of Van Allen Probe A. For the input parameters of the model TS05, solar wind dynamic pressure P sw = 1.0 nPa, interplanetary magnetic field (IMF) B y = 0 nT, IMF B z = 0 nT, and Dst = 0 nT were used. The vertical dashed lines indicate the center of the interval of each He + flux enhancement. As expected in Figure 2, the quasiperiodic appearance of low-energy He + flux enhancements after 11:10 UT has a clearly associated enhancement in He-band EMIC wave power greater than 10 −1 nT 2 . There are enhancements in wave power lower than 10 −1 nT 2 at 11:41 UT, 11:49 UT, and 11:56 UT. For these low wave power enhancements there is no corresponding He + flux increase. It is likely that lower energy He + ion (1-10 eV) enhancements are associated with He-band waves with power greater than 10 −1 nT 2 . The H-band wave power modulation is also correlated with the He + flux enhancement even though its power is much smaller than the He-band wave power. Figures 3c-3f show that the low-energy He + flux has no distribution at 18° pitch angle and that the enhanced He + flux observed in the EMIC wave interval corresponds to large pitch angle distributions. That is, the flux is mostly enhanced in a direction perpendicular to the magnetic field. ΔB T is slightly reduced for the interval when the lower energy helium ions appear. The reduced ΔB T has no time-dependent oscillations corresponding to the flux enhancements. Thus, we suggest that the low-energy He + flux enhancement is not driven by long-period external force variations. Figure 4a repeats the wave power from Figure 3a for easy comparison. Figure 4b shows proton (  On the basis of the ratio, we find that the helium flux in the low-energy range is more intensified than the proton flux when the strong EMIC waves appear. This indicates that the EMIC waves strongly affect low-energy He + ions rather than the H + ions. Figure 4d shows the electron number density N e obtained from EMFISIS and the number density of low-energy He + (  He N ), which is a summation of the density calculated for each energy step from 1 to 10 eV of the HOPE instrument using the formula given by Goldstein et al. (2014) and Sarno-Smith et al. (2015): where the quantity J i = J He+ (E i ) is the He + differential flux at the median of each energy channel E i between 1 and 10 eV, ΔE i is the range of each energy channel, and  Van Allen Probe A are plotted in Figures 5e and 5f, respectively. The positions of Van Allen Probe A in MLAT are the magnetic footprints calculated using the Tsyganenko model (1989). During the interval of EMIC waves observed just inside the plasmapause near the magnetic equator by Van Allen Probe A (Figure 5a), NOAA-15 spacecraft passed the Southern Hemisphere and observed an isolated peak of the precipitating protons in the differential flux for the 39 and 115 keV proton channels at 11:18:46 UT when the NOAA-15 was located at MLAT = -63.1° and MLT = 5.8 h. The isolated peak region (marked by solid circles in Figures 5e and 5f) is located well equatorward of the isotropic zone, in which the 39-keV trapped and precipitating fluxes are comparable, encountered by NOAA-15 around 11:21 UT as moving higher (auroral) latitudes and close to the projected location (MLAT = -66.5° and MLT = 6.2 h) of Van Allen Probe A with separation of ΔMLAT = ∼3.4° and ΔMLT = ∼0.4 h. The small separations in MLAT and MLT indicate that the precipitating energetic protons at NOAA-15 are due to pitch angle scattering associated with EMIC waves at Van Allen Probe A. (e.g., Hyun et al., 2014;K.-H. Kim et al., 2016b;Miyoshi et al., 2008;Yuan et al., 2012. The flux ratio of precipitating protons to trapped protons at the isolated peak is 0.29 for the 39-keV channel and 0.19 for the 115-keV channel, indicating that lower energetic protons are scattered more efficiently than higher energetic protons through EMIC waves .

Discussion
Previous observations reported that there is a close correlation between the occurrence of strong EMIC waves and low-energy He + heating. We make a brief comparison of previous and our observations. Young et al. (1981) observed an enhancement of suprathermal He + ions (tens of eV to a few hundred eV) ∼10-50 min after the onset of the EMIC waves with frequencies in the vicinity of the He + gyrofrequency at geosynchronous orbit. They showed that EMIC waves are clearly associated with the abundance of cold He + and suggested that the enhancement in He + concentration is a necessary condition to promote EMIC wave growth. Roux et al. (1982) reported that 68% of the geosynchronous EMIC wave events were accompanied by strongly or weakly enhanced He + fluxes in the suprathermal energy range. They also found that cold He + ions can be heated in the direction perpendicular to the background magnetic field up to a few hundred eV when EMIC waves were detected. Anderson and Fuselier (1994) provided clear evidence for a correlation between EMIC waves and perpendicular heated ion distribution by using AMPTE/CCE observations in the outer magnetosphere above geosynchronous orbit and showed that cold He + ions are preferentially and transversely energized to average temperatures of about 35 eV and sometimes above 100 eV through the interaction with EMIC waves. There have been several numerical studies on the preferential heating of cold He + ions by EMIC waves (e.g., Fu et al., 2016;Gendrin & Roux, 1980;Mauk, 1982aMauk, , 1982bOmidi et al., 2010;Omura et al., 1985;Qian et al., 1990).
Unlike previous observations using the data in the outer magnetosphere we report EMIC wave-associated He + heating inside the plasmasphere. Our observations provide a clear correlation between the EMIC wave activity and low-energy (1-10 eV) He + flux enhancement (Figure 3). Comparing the ratio of the helium (   (Figure 4). We also showed that  He J is mostly enhanced in a direction perpendicular to the magnetic field rather than in a field-aligned direction (Figure 3). These observations indicate that the cold He + ions below 1 eV are preferentially and transversely heated up to 10 eV by the EMIC waves.
To gain more information on pitch angle distribution characteristics of low-energy He + ions during the wave activity, we plot differential He + fluxes from selected energy channels below 10 eV, as a function of time and pitch angle, with the EMIC wave power in Figure 6. The thick white bar in Figure 6d indicates the interval when the He-band wave power is over 10 −1 nT 2 (see Figure 3). As noted above, there are strong fluxes during the interval when the H-band and He-band wave powers are enhanced. They occurred over a range of pitch angles from 45° and 120°, representing that the He + flux has asymmetric pitch angle distributions about a pitch angle at 90°. For 1.0 and 1.5-eV He + ions, the flux intensity above 90° pitch angle is larger than that below 90° pitch angle, and there is a flux gap near at 90° pitch angle. This pitch angle distribution is similar to previous observations of X-type He + distributions reported by Anderson and Fuselier (1994), who suggested that the X-type distribution is due to a gyroresonant interaction at a point off the magnetic equator where the wave frequency approached the local He + gyrofrequency. However, the 3.0-eV He + ions have strong flux at 90° pitch angle without a gap. Inspection of the selected low-energy He + channels indicates that pitch angle distributions of He + ions heated by EMIC waves show different features at different energy channels.
There are previous theoretical studies of cold He + heating by EMIC waves in the band below or above  He f (Gendrin & Roux, 1980;Omidi et al., 2010;Omura et al., 1985;Qian et al., 1990). Gendrin and Roux (1980) suggested that He-band waves could energize cold He + ions by gyroresonant interaction ( where k is the wave number and v ∥ is the parallel velocity of the He + ion). Fu et al. (2016) examined the excitation and saturation of EMIC waves using hybrid simulations. In their simulations, cold protons are weakly heated in the direction perpendicular to the background magnetic field when the generated EMIC waves saturate, and cold helium ions have been heated by more than 40 times in the perpendicular direction at the end of the simulation. These simulation results provide that EMIC waves contribute to the high-energy particle scattering and low-energy particle heating. Although Fu et al. (2016) suggest that the low-energy KIM ET AL.

10.1029/2020JA028560
10 of 16 Figure 6. (a-d) Differential He + fluxes from selected energy channels below 10 eV, as a function of time and pitch angle.
particle heating is due to the cyclotron resonance at  He f , they did not discuss which band waves of EMIC waves are more effective contributor to the cold He + ion heating.
AMPTE/CCE observations in the outer magnetosphere have shown that low-energy He + ion heating occurs more frequently during periods of strong H-band EMIC wave activity rather than He-band wave activity interval (Anderson & Fuselier, 1994). For H-band waves, the gyroresonant heating of cold He + ions would occur off the equator because the helium gyrofrequency increases with latitude in the real magnetosphere as the waves generated in the equatorial region propagate toward higher magnetic latitudes. These waves are left-handed from the cutoff frequency up to the crossover frequency and converted into the right-handed mode at the local crossover frequency (Roux et al., 1982;Young et al., 1981). The waves should penetrate the stop band between the cutoff frequency and the helium gyrofrequency to reach the helium gyrofrequency. Thus, they are observed as a right-hand mode near the equatorial region after reflecting at the bi-ion frequency above the helium gyrofrequency (Kim & Johnson, 2016;Roux et al., 1982;Thorne & Horne, 1993). Supporting evidence for the resonant heating of He + was provided from the ATS-6 satellite observations at geosynchronous orbit off the magnetic equator (MLAT = ∼10°) (Mauk & McPherron, 1980). The evidence includes that the He + ion fluxes are gyrophase bunched by the EMIC wave events for which frequencies normalized to the local proton gyrofrequency were 0.1 (He-band) and 0.33 (H-band), representing that He + ions are heated by H-band and/or He-band EMIC waves. The ATS-6 satellite observations provided that the He-band waves were left-handed and the H-band waves were right-handed.
To determine the properties of EMIC waves, which appeared coincidentally with low-energy He + flux enhancement, we examine the polarization and ellipticity of transverse magnetic field perturbations δB x and δB y during the interval shown in Figure 3, and the results are plotted in Figure 7. We used the method described by Fowler et al. (1967) for the polarization analysis. The degree of polarization ( Figure 7c) and ellipticity (Figure 7d) are defined in the two-dimensional spectral matrix constructed from the Fourier transform of the δB x and δB y time series. The ellipticity in Figure 7d is shown only for the frequency at which the degree of polarization is higher than 80%. The ellipticity is defined to be negative (positive) if the magnetic field perturbation rotates clockwise (counterclockwise) in the δB x -δB y plane, that is, corresponding to left-handed (right-handed) polarization. The degree of polarization is enhanced in the band occupied by the He-band waves, but the high degree of polarization (>80%) is not shown in the entire band. The ellipticity corresponding to the high degree of polarization is mostly below zero, indicating left-handed polarization. The H-band wave does not show the high degree of polarization for the wave activity interval. This observation leads us to conclude that the Heband EMIC waves are the effective contributor to the cold He + ion heating. Young et al. (1981) observed that EMIC waves are clearly associated with the abundance of low-energy ( ≤110 eV) He + ions and suggested that the enhanced He + concentration promotes EMIC wave growth. To examine whether the enhanced low-energy He + ions observed at Van Allen Probe A play some sort of catalytic role in the growth of EMIC waves, we calculate the growth rate of EMIC waves for a plasma composition assumed to be cold protons (89%), cold helium ions (10%), and energetic protons (1%). The energetic proton populations are considered to be T ⊥ /T ∥ = 2.0 for two parallel energies of 10 and 33 keV. The observed values of the cold plasma density (N e = 165 cm −3 ) and background magnetic field intensity (B T = 210 nT) for the interval of 11:24-11:25 UT at Van Allen Probe A are used for the calculation. It should be noted that 1% energetic protons assumed here is overestimated when the proton fluxes above 10 keV are compared with proton and helium fluxes below 10 eV in the HOPE data (see Figures 2 and 4).
In Figure 8a, the calculated growth rates are plotted against the frequency normalized to  H f . The magnitude of the peak growth rate in the He-band is significantly greater than that in the H-band for the parallel energy of 33 keV. This is consistent with the observations of strong He-band and weak H-band EMIC waves near the magnetic equator. The calculated frequency is 0.1  H f at the peak growth rate. As the parallel energy of the energetic protons is reduced to 10 keV, the He-band wave growth rate decreases, and the peak in the growth rate is shifted to higher normalized frequency of 0.14  H f . The peak growth rate of the H-band for 10 keV is comparable to that for 33 keV with a small shift toward higher frequency. It is likely that the growth rate of the H-band wave is hardly affected by parallel energies of energetic protons. Figure 8b shows the result considering 0.4% warm He + ions, 88.6% cold protons, 10% cold helium ions, and 1% energetic protons. B T , N e , and the energy and anisotropy factor of the energetic protons are the same as those used to calculate the result shown in Figure 8a. The warm He + population consists of the enhanced low-energy He + ions at temperatures of 10 eV and 0.7 cm −3 observed at Van Allen Probe A. The temperature anisotropy T ⊥ /T ∥ for the warm He + ions is assumed to be 5 because the enhanced He + flux is mostly observed in the direction to the background magnetic field. The wave growth rate considering the presence of 0.4% warm He + ions is nearly identical to the result calculated with only cold ions and energetic protons, indicating that the low-energy He + ions do not promote wave growth. Thus, we suggest that the enhancement of the low-energy He + ions (∼10 eV) is due to preferential heating of cold He + (<1 eV) by EMIC waves (Mauk, 1982a;Qian et al., 1990).

Conclusions
Our case study showed a clear correlation between the EMIC wave activity and low-energy He + flux enhancement in the direction perpendicular to the ambient magnetic field, providing evidence that cold He + ions (<1 eV) are transversely heated up to 10 eV through the interaction with EMIC waves. The EMIC waves were observed at Van Allen Probe A with a strong spectral intensity in the He-band and weak intensity in the H-band when the spacecraft was just inside the plasmapause near the magnetic equator. Previous theoretical studies suggested that cold He + ions can be heated by H-band and/or He-band EMIC waves. In our case it is likely that He-band waves play a key role in heating the cold He + ions.
The case event in our study shows that the He + ions heated up to 10 eV. This energy is lower than that in the previous studies. Since the present study is based on only one event observed on a single Van Allen Probe A pass inside the plasmasphere, we cannot generalize the quantitative argument for the question: What is a critical factor for the EMIC wave-driven He + heating? To answer the question, we need to observe the EMIC wave-associated He + heating events at various latitudes, longitudes, and radial distances and under various background plasma and magnetic field conditions. Currently, we statistically examine the relationship between EMIC He-band waves and cold ion heating, and the results will be presented in a related publication.

Data Availability Statement
The Van Allen Probe A EMFISIS data were obtained from the University of Iowa (http://emfisis.physics. uiowa.edu), and the Van Allen Probe A HOPE data were obtained from the Van Allen Probes ECT website (https://www.rbsp-ect.lanl.gov/science/DataDirectories.php/). NOAA-15 data are available at https:// cdaweb.gsfc.nasa.gov.