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. Author manuscript; available in PMC: 2022 Mar 6.
Published in final edited form as: J Geophys Res Space Phys. 2021 Sep 6;126(9):e2021JA029217. doi: 10.1029/2021ja029217

Isolated Peak of Oxygen Ion Fraction in the Post-Noon Equatorial F-Region: ICON and SAMI3/WACCM-X

Jaeheung Park 1,2, J D Huba 3, Roderick Heelis 4, Christoph Englert 5
PMCID: PMC8506983  NIHMSID: NIHMS1741790  PMID: 34650900

Abstract

In the equatorial region, the fraction of oxygen ions (O+) in the topside ionosphere contains information on the source altitude of the plasma, which is controlled, in part, by the vertical plasma motion in the F-region. Previous studies on this topic are restricted by limited coverage of local time, latitude, and season, leaving a significant knowledge gap in the distribution of the topside ionospheric composition. In this study, we statistically investigate the O+ fraction measured by ICON/IVM over all the local time sectors and seasons at low/midlatitudes. For the first time, we have found that an isolated peak in the O+ fraction emerges in the post-noon equatorial region. The peak is most prominent during equinoxes, while during solstices it is connected to the O+ fraction bulges in the local summer midlatitudes. Simulations with SAMI3 coupled with thermospheric parameters from WACCM-X reproduce the peak of the O+ fraction. The post-noon equatorial peak can be explained by the net vertical motion of plasma consisting of transports either parallel or perpendicular to geomagnetic field lines.

1. Introduction

The ionospheric F-region at low- and midlatitudes mainly consists of three ion species: O+, H+, and He+ (Johnson, 1966; Shulha, 2016). The relative dominance of these populations changes with the season, solar activity, and location (e.g., altitude and latitude), as can be seen in Stankov et al. (2003) and Shulha (2016).

In the topside ionosphere, the heavy-ion fraction generally decreases with increasing height (Ratcliffe, 1972), and since the topside ionosphere is primarily controlled by plasma transport rather than by local photochemistry, the heavy-ion fraction at a particular location bears a “fingerprint” of the convective history of the plasma (e.g., Huba et al., 2021). Specifically, spatial/temporal increases or decreases in the heavy-ion fraction at a fixed local time imply that the plasma observed at that location originates from below or above, respectively (e.g., Hanson & Sanatani, 1971; Hanson et al., 1970). For example, an enhanced fraction of metallic ions within equatorial plasma bubbles (EPBs) is cited as evidence that the plasma originates from below the observation altitude: see Hanson and Sanatani (1973), Kil (2015), Huba et al. (2020), and references therein. Similarly, Min et al. (2009, Figure 2) reported 27-day oscillations of the oxygen ions (O+) fraction at altitudes around 840 km, which were interpreted in terms of quasiperiodic vertical expansion/contraction of the ionosphere.

Figure 2.

Figure 2.

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Similar to Figure 1, but in the magnetic latitude (MLAT)-magnetic longitude (MLON) space for a limited magnetic local time (MLT) range in the afternoon (13–18 MLT). Representative signs of magnetic declination for different MLON sectors are annotated in panel (a).

In the topside ionosphere, where satellite observations are customarily conducted, O+ is frequently the most abundant ion species. Still, studies show that the O+ fractional density at a fixed altitude is a strong function of solar activity and other geophysical parameters (e.g., González et al., 2004, Figure 4; Klenzing, Simões, et al., 2011, Figure 10). At the transition height the O+ and H+ concentrations are the same, while the differences in the scale heights of the O+ and H+ make changes in the O+ fraction a sensitive function of the vertical plasma motion (e.g., Huba et al., 2021, Figure 4b).

Figure 4.

Figure 4.

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ICON/Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) wind in the direction parallel to the background magnetic field, presented in the same format as that of Figure 1. The “Red-Line” data below 300 km altitudes are used for the period between December 15, 2019 and December 31, 2020. The reddish (positive) and bluish (negative) colors respectively represent northward and southward wind parallel to the geomagnetic field.

Using the Communications/Navigation Outage Forecasting System (C/NOFS) satellite, Stoneback et al. (2011) presented a statistical dependence of the O+ fraction that varied with local time (LT) in accord with the behavior of the vertical plasma drift during times of very low solar activity. They reported that O+ fraction maximizes in the afternoon, which was supported later by Jicamarca ISR data (Hysell et al., 2015, Figures 3 and 4).

Figure 3.

Figure 3.

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Similar to Figure 1, but presenting outward E × B drift velocity as measured by ICON/Ion Velocity Meter (IVM).

Kil et al. (2006) described global maps of O+ fraction derived during the nighttime (21 LT) from measurements by the Defense Meteorological Satellite Program (DMSP). This description, which covered a wider range of geomagnetic latitudes than was accessible from the C/NOFS satellite, showed variations in the O+ fraction that were consistent with the expected behavior of the F-region neutral wind that modulates the height of the F-peak at middle latitudes.

West and Heelis (1996) and West et al. (1997) also describe the latitude variations of the constituent ions O+, H+, and He+ in the topside ionosphere in two LT sectors at 09 LT and 21 LT. They show a dependence of the relative densities on solar activity and longitude variations that indicate the influence of both zonal and meridional winds on the latitude distribution of different species.

Heelis et al. (2009) describe the shape of the O+/H+ transition height in the equatorial region as a function of latitude and LT during a period of very low solar activity in northern summer. The results emphasize the influence of thermospheric temperature on the relative concentrations of the neutral atomic oxygen and hydrogen as well as the role of interhemispheric neutral winds that modulate the ionospheric layer height. Klenzing, Simões, et al. (2011) further investigated the LT dependence of transition height over an extended period to isolate the roles of neutral composition and dynamics on its behavior in the equatorial region.

In spite of the long history of research, there is still room for further study of the physical processes at work. Each of the previous studies mentioned above has limited data coverage in terms of latitudes (e.g., Klenzing, Simões, et al., 2011), season (Heelis et al., 2009), or local time (e.g., West et al., 1997). In this work we are able to utilize the ion composition data provided by the ICON mission to construct a seamless climatology of topside O+ concentration. Furthermore, the observations are reproduced by model simulations and interpreted in the context of ionospheric vertical drift.

In the following, Section 2 describes the main instruments and simulation tools for this study. Section 3 presents the statistical distribution of O+ fraction, which will be reproduced by model simulations and further discussed in Section 4. The final section draws conclusions and summarizes the main findings.

2. Instrument and Data Processing Methods

The Ionospheric Connection Explorer (ICON) satellite was launched into a circular orbit on October 11, 2019 from a Pegasus vehicle (Immel et al., 2018). The orbit altitude is about 600 km and the orbit inclination angle is around 27°. The mission is dedicated to elucidating the dynamics and electrodynamics in the low/midlatitude thermosphere/ionosphere system. Among the various onboard instruments, the Ion Velocity Meter (IVM) can measure ion density, composition, temperature, and full three-dimensional ion drift velocity vector (Heelis et al., 2017). This study focuses on the in-situ ion composition data, more specifically on the fraction of O+ that is derived with an accuracy of about 2% (Heelis et al., 2017, Table 4). The ion composition is obtained by voltage sweeping on the ICON/IVM grid, which can electrostatically repel the incoming ions in a mass-dependent way. By comparing the ion currents under various grid voltages, one can infer the ion composition in-situ. Readers can find detailed information on the IVM principles in Heelis et al. (2017, Equation 1) and Fang and Cheng (2013).

For the period between December 15, 2019 and December 31, 2020 we investigate the statistical distribution of the O+ fraction as a function of magnetic latitude (MLAT), magnetic longitude (MLON), magnetic local time (MLT), and seasons. All the magnetic coordinates used for the observation data are based on the quasidipole system (e.g., Laundal & Richmond, 2017; Richmond, 1995) referenced to the observation altitude plus the nominal Earth’s radius of 6,371.2 km. Three Lloyd seasons are used to investigate seasonal variations, which are defined as follows. An abbreviated June solstice consists of months from May to August, while the December solstice is composed of November-December and January–February. All the other months are categorized as the combined equinoxes. To avoid possible effects of strong geomagnetic activity (e.g., Hajra et al., 2017), we exclude days with the daily maximum Kp index exceeding 3+. Also, data points that have plasma density equal to or larger than 1 × 107 cm−3 are discarded.

Besides the ion composition data, three-dimensional ion drift measured by ICON/IVM is used to interpret the statistical results. In the following sections, we use the terms, “outward/inward” drift to signify E × B drift toward higher/lower L-shells, respectively: the drift is inherently perpendicular to background magnetic field. For ion motions in the vertically upward/downward directions, which are not necessarily perpendicular to magnetic field lines, we use the terms, “upward/downward.”

Another ICON instrument, the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI), can measure thermospheric wind velocity vectors between 90 and 300 km altitude (Englert et al., 2017; Harding et al., 2017). Among these data, “Red-Line” winds representing the midthermosphere will be used in Section 4 in interpreting the O+ fractions measured by ICON/IVM.

To put the ICON/IVM data into a broader context, we conduct model simulations using the “Sami3 is Also a Model of the Ionosphere” (SAMI3: see Huba et al., 2000, 2008 for example) developed at the U.S. Naval Research Laboratory (NRL). The code is based on three-dimensional conducting fluid equations (e.g., continuity, velocity, and temperature equations) for the ionosphere. In this study, we use SAMI3 with thermospheric inputs (e.g., density, velocity, and temperature of neutral particles) from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X). The lower boundary of thermospheric parameters from WACCM-X is nudged to the Goddard Earth Observing System 5 (GEOS-5) state. WACCM-X outputs representing the thermospheric status are one-way coupled to SAMI3 while plasma parameters of WACCM-X are not used for SAMI3 simulations. Details of the simulation setup are given in Huba et al. (2021, Section 3).

3. Results

Figure 1 presents the O+ fraction measured by the ICON/IVM from December 15, 2019 to December 31, 2020, as a function of MLT (magnetic local time; horizontal axis) and MLAT (magnetic latitude; vertical axis). Each panel from top to bottom shows data from combined equinoxes, June solstice, and December solstice, in that order. Bluish colors signify a low fraction of O+ in the total plasma density, while reddish colors imply the dominance of O+. Contour lines are given to guide the readers’ eyes. The general climatology evident in Figure 1 can be summarized as follows. First, the O+ fraction is higher during daytime than at night, with the latter mostly below 50%. Second, during solstices (bottom two panels) the local summer hemisphere exhibits the higher fraction of O+ than the winter hemisphere, for both the dayside and night-side. This is generally attributed to the average summer-to-winter interhemispheric transport caused by the prevailing neutral wind (e.g., Burrell et al., 2011; Kil et al., 2006), which in the summer hemisphere lifts oxygen-rich plasma to higher altitudes. Third, during the equinoxes a peak of O+ fraction emerges at the equator from noon to evening (approximately from 1200 MLT to 2100 MLT): refer to the red horizontal arrow. The peak is latitudinally and zonally isolated by the surrounding regions where the fraction of O+ is lower. Similar peaks can be identified during solstices, but they are displaced to the summer side of the magnetic equator and appear connected (or not clearly separated) to the regions of high-fractional O+ (red) in the summer hemisphere at the highest latitudes accessed by the satellite (MLAT ~ ±35°).

Figure 1.

Figure 1.

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A statistical distribution of the oxygen ion (O+) fraction as a function of magnetic latitude (MLAT) (ordinate) and magnetic local time (MLT) (abscissa). Reddish and bluish color represents the dominance of O+ and other ions, respectively. Each panel from the top corresponds to one Lloyd season: (a) combined equinoxes, (b) June solstice, and (c) December solstice. A horizontal red arrow in each panel annotates the post-noon equatorial peak of the O+ fraction. The contour lines are redundant but added to guide the readers’ eyes.

Figure 2 is similar to Figure 1, but only postnoon data (13–18 MLT; where the equatorial peak of O+ fraction appears in Figure 1) are used to describe the longitude distribution of the O+ fraction. Regions of positive, negative, and zero magnetic declination near the magnetic equator are indicated in the top panel. During the equinoxes a clear longitude variation in the O+ fraction is seen that is not aligned with magnetic declinations.

The longitude variation, which is seen near the magnetic equator during equinox, becomes much weaker during the solstices when the maximum in the O+ fraction is displaced to the summer side of the magnetic equator. During the northern summer solstice, strong interhemispheric asymmetry in the O+ fraction is seen in the region of positive magnetic declination: there the summer-to-winter meridional wind and the westward zonal wind expected during the daytime (e.g., Drob et al., 2015, Figure 3) would act in concert. This may be contrasted with a minimum inter-hemispheric asymmetry in the O+ fraction seen in the same longitude region during the northern winter solstice when the meridional and zonal wind components in the magnetic meridian are expected to oppose each other (e.g., Burrell & Heelis, 2012).

Overall, the distributions of the O+ fraction in Figure 2a (equinox) are not well aligned with the magnetic declination. Therefore, field-aligned plasma transport by neutral wind cannot fully explain the distribution of the O+ fraction. Other effects should participate in controlling the O+ fraction profiles during equinoxes.

4. Discussions

4.1. Effects of Vertical Plasma Transport on O+ Fraction: ICON/IVM and ICON/MIGHTI Observations

In the previous sections, we demonstrated that a peak of O+ fraction emerges in the post-noon equatorial ionosphere. As mentioned in Section 1, heavy (e.g., oxygen) ion fraction in the topside ionosphere is known to decrease with altitude (e.g., Ratcliffe, 1972). In order for ICON/IVM, which stays nearly at a fixed altitude, to encounter a localized enhancement of O+ fraction, a net upward movement of lower-altitude (i.e., more oxygen-rich) plasma is required: see, for example, Kil et al. (2006, Section 2).

However, Figures 1 and 2 already showed that field-aligned transport induced by thermospheric neutral winds cannot fully explain the postnoon equatorial peak of the O+ fraction and the related, nonmonotonic MLAT profile. We expect that more drivers come into play in determining the O+ fraction at ICON altitudes (or equivalently, vertical motion of plasma parcels).

At the equator, due to the small magnetic inclination, the dominant vertical plasma motion is that associated with the zonal electric field. The E × B drift would contribute to net upward plasma motion (i.e., enhancing O+ fraction) during the day and to net downward drift (i.e., decreasing O+ fraction) at night: see Fejer et al. (2008) for example. Though outward E × B drift is shortly enhanced near sunset (Pre-Reversal Enhancement: PRE), this effect would be feeble during solar minimum years of 2019–2020 (e.g., Fejer et al., 1991). These two features of E × B drift are reproduced by the SAMI3/WACCM-X simulations in Huba et al. (2021, Figure 3e). It is notable that the daytime outward E × B drift maximizes before noon (e.g., Fejer et al., 2008; Huba et al., 2021) while the O+ fraction in Figures 1 and 3 peaks around dusk. This MLT offset can be interpreted as follows. Net vertical “displacement” of a plasma parcel, which is expected to reflect the O+ fraction, results from the integration of vertical velocity along the trajectory (i.e., not the instantaneous velocity) as the parcel approximately corotates with the Earth (e.g., Klenzing, Rowland, et al., 2011). The importance of such a drift “history” in shaping topside ion composition was also mentioned by Hysell et al. (2009), which can explain the MLT offset between the vertical drift and ion composition near the equator.

Figure 3 presents the outward E × B velocity (i.e., ion drift velocity perpendicular to background magnetic field and toward outer L-shells), as observed by ICON/IVM during the same period as shown in Figure 1. Note that ICON/IVM data currently available have issues in velocity between 0200 and 1100 local time (Huba et al., 2021) because of (a) low plasma density and low O+ fraction during the solar-minimum post-midnight conditions (e.g., Heelis et al., 2017, Figure 6) and (b) photo-electron contamination from instrument grids (Huba et al., 2021). Therefore, regions between 1900–2400 MLT and 0000–1200 MLT are conservatively shadowed in Figures 3 and 5 in order to avoid misinterpretations. Except for the shadowed regions, ICON/IVM data support the above-mentioned properties of daytime outward E × B velocity. The drift is generally outward, with the magnitude larger in the near-noon than in the postnoon sector. Post-sunset enhancement of the outward E × B velocity is weak, as expected for the period of low solar activity (e.g., Huba et al., 2021, Figure 3e). Note that the MLAT dependence of the outward drift is monotonic or flat, with the crest sitting around the equatorial region. We cannot find conspicuous off-equatorial peaks of outward E × B velocity in Figure 3. Hence, the outward E × B velocity alone cannot fully explain the postnoon equatorial peak of O+ fraction.

Figure 6.

Figure 6.

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(a) O+ fraction simulated by SAMI3/Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) for December 23, 2019, in the same format as that of Figure 1c, (b) similar to panel (a), but without meridional wind input to SAMI3, and (c) similar to panel (a), but without E-field input to SAMI3.

Figure 5.

Figure 5.

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Similar to Figure 1, but presenting radially outward (not necessarily perpendicular to geomagnetic field) drift velocity as measured by ICON/Ion Velocity Meter (IVM).

At midlatitudes, the situation concerning upward/downward ion drift becomes more complex because two more drivers play significant roles: field-aligned ambipolar diffusion and neutral wind (Chen et al., 2014; Saha et al., 2021, Equation 1). The ambipolar diffusion is upward during daytime and downward at night (Chen et al., 2014). The concomitant vertical drift is stronger at higher |MLAT| due to the field-aligned nature of the diffusion: that is, the geomagnetic dip angle is larger in magnitude at higher |MLAT|. Hence, we expect that the ambipolar diffusion significantly contributes to the daytime midlatitude regions exhibiting a high fraction of O+ and the resultant non-monotonic MLAT profiles. Note that during equinox the horizontal wind diverges from the equator during the day and converges at night (e.g., Drob et al., 2015), and actual ICON/MIGHITI data (Englert et al., 2017; Harding et al., 2017) also conform to this trend. Figure 4 presents ICON/MIGHTI wind parallel to the background magnetic field in the same format as that of Figure 1. The “Red-Line” data below 300 km altitudes are used for the period between December 15, 2019 and December 31, 2020. Note that the “parallel” wind is derived from horizontal wind data transformed into the local field-aligned coordinate system under the assumption of zero vertical wind. For details of the wind derivation, readers are referred to the document, “ICON Data Product 2.2: Cardinal Vector Winds” (ftp://icon-science.ssl.berkeley.edu/pub/Documentation/ICON_L2-2_MIGHTI_Vector-Wind_v03.pdf). The reddish and bluish colors represent northward and southward wind parallel to the magnetic field, respectively. As can be seen in Figure 4, the daytime thermospheric wind is generally poleward for all the seasons, with the summer-to-winter trend superposed during solstices (Figures 4b and 4c). As a result, wind-induced downward transport of plasma during the day may counteract the upward ambipolar diffusion. However, the net effect (=ambipolar diffusion + wind-induced transport) is expected to be generally upward in the daytime midlatitude ionosphere, which can enhance O+ fraction there, according to the Arecibo data in Fejer (1993, Figures 4 and 5). This is evidenced by the actual ICON/IVM data in the following paragraph.

Figure 5 is similar to Figure 1, but shows net upward ion velocity (not necessarily perpendicular to background magnetic field). The parameter, “ICON_L27_Ion_Velocity_Z” in the ICON/IVM data files, is used after sign inversion. We expect that this velocity reflects the net combination of outward E × B velocity, ambipolar diffusion, and wind-induced field-aligned transport mentioned above. The net upward ion velocity in Figure 5 exhibits nonmonotonic MLAT dependence, with the peaks at the equator (marked with red horizontal arrows) and at higher latitudes beyond ±20°. Furthermore, the equatorial peak is displaced toward the summer hemisphere during solstices. The nonmonotonic MLAT profiles are generally more conspicuous near the noon than in the postnoon sector. As discussed in a previous paragraph, we need to consider the MLT offset (or time delay) between instantaneous velocity and time-integrated vertical plasma displacement (or O+ fraction). Then, the non-monotonic MLAT profiles of net upward velocity in Figure 5 can explain the isolated peak of O+ fraction in the postnoon equatorial region (as well as its offset toward the summer hemisphere) in Figure 1. Finally, we note that the equatorial peak of vertical drift in Figure 5 extends to later MLTs during solstices (Figure 5b and 5c) than during equinoxes (Figure 5a). This trend also agrees with that of the isolated peak of O+ fraction, which extends toward later MLTs during solstices (Figures 1b and 1c) than during equinoxes (Figure 1a).

As the ion drift data of ICON/IVM are still under intensive calibration and improvement, the quality issues at night- and morning sides will be alleviated in a near future. More quantitative discussions on the relationship between the O+ fraction and vertical transport will be deferred to future works.

4.2. SAMI3 Simulations

In this subsection, we investigate whether the post-noon equatorial peak of the O+ fraction is reproduced by the physics-based SAMI3 model, coupled to WACCM-X. A SAMI3/WACCM-X simulation for December 23, 2019 is presented in Figure 6a, with the same format as that of Figure 1c. This date was chosen because the simulation results have been thoroughly validated against observations by Huba et al. (2021). As this date is near the December solstice, Figure 6a can be directly compared to Figure 1c.

Overall, the SAMI3/WACCM-X results in Figure 6a agree with the observed behavior of the O+ fraction in Figure 1. An equatorial peak emerges in the afternoon and evening, which is isolated from the two midlatitude peaks at |MLAT| > 20°. This trend can also be found for all the three seasons in Figure 1 although the isolation is only weakly seen during solstices. The equatorial peak in Figure 6a is offset toward the summer (Southern) hemisphere, and the O+ fraction is higher irrespective of MLT in the summer hemisphere than in winter: these characteristics conform to the solstitial results in Figures 1b and 1c. Therefore, we can conclude that the SAMI3/WACCM-X simulations reproduce all the main features of the O+ fraction as observed by ICON/IVM.

As a small discrepancy, the equatorial peak of the O+ fraction is shifted toward later MLTs in the SAMI3/WACCM-X simulation (Figure 6a) than in the ICON/IVM data (Figure 1c). For example, the crest of the equatorial peak is mainly located before 1800 MLT in Figure 1c while it is later in Figure 6a. This MLT offset can be attributed to the fact that the simulated E × B drift is slightly more outward than observations in the afternoon and evening, as shown by Huba et al. (2021, Figure 3). This excessive outward drift will cause the fraction of O+ to increase continuously even after sunset. However, the qualitative agreement between simulations (Figure 6a) and observations (Figure 1) is good. Lastly, we note that the SAMI3/WACCM-X is for a single day which may not represent the seasonal average in detail.

In the preceding subsection, we suggested that neither one among the E × B drift, neutral wind, and ambipolar diffusion can solely explain the non-monotonic |MLAT| profile of O+ fraction at the postnoon MLT, while their combination does. Now we give additional support to this claim by controlled SAMI3/WAC-CM-X simulations.

Figure 6b is similar to Figure 6a, but without the meridional wind input from WACCM-X to SAMI3. The isolated peak of O+ fraction does appear, but the magnitude is weaker than in Figure 6a, especially before 1900 MLT. Also, the offset of the O+ fraction peak toward the summer (Southern) hemisphere almost disappears.

Figure 6c is also similar to Figure 6a, but without the E-field input. The agreement with Figure 6a appears better than that between Figures 6b and 6a, which implies the importance of meridional wind in generating the O+ fraction peak. However, the near-equatorial postnoon peak is not as well isolated from the summer-side bulge as in Figure 6a.

In Figures 6b and 6c, we control the wind or E-field, both of which are external inputs to SAMI3. We do not endeavor to modify the ambipolar diffusion, for which we need to manipulate physics equations in SAMI3: that would be too unrealistic. Nevertheless, ICON drift and wind data in Figures 3–5 as well as the SAMI3/WACCM-X simulations in Figure 6 support that realistic drivers of plasma upward drift (i.e., E-field, neutral wind, and ambipolar diffusion) are essential for generating the isolated peak of O+ fraction in the post-noon equatorial F-region.

5. Summary and Conclusion

Analyzing ICON/IVM data for solar-minimum years between 2019 and 2020, we have constructed the first-ever statistics of low/midlatitude oxygen ion (O+) fraction covering the whole LT sectors and seasons. The observation results are reproduced by SAMI3/WACCM-X simulations. The main findings can be summarized as follows:

  1. Postnoon equatorial peak of O+ fraction: A peak of O+ fraction isolated in MLAT and MLT emerges in the postnoon equatorial region. The peak is most prominent during combined equinoxes while solstitial peaks are connected to the high-fraction regions in the local summer midlatitudes.

  2. Longitude dependence: The longitudinal distribution of O+ fraction does not follow that of geomagnetic declinations. This feature implies that drivers other than horizontal neutral wind participate in shaping the O+ fraction profiles.

  3. Vertical ion transport: Net vertical (including field-aligned and perpendicular) ion drift measured by ICON/IVM exhibits non-monotonic dependence on |MLAT| around noon, resembling that of the O+ fraction. Considering the time delay between instantaneous velocity and net vertical displacement, we largely attribute the postnoon equatorial peak of O+ fraction to the vertical ion transport having non-monotonic |MLAT| dependence.

  4. SAMI3/WACCM-X simulation: SAMI3/WACCM-X for a solar-minimum December season reproduces (a) the postnoon equatorial peak of the O+ fraction and (b) the higher fraction in the summer hemisphere than in the winter. The good agreement between ICON/IVM and SAMI3/WACCM-X again supports that the postnoon equatorial peak of O+ fraction is produced under the realistic neutral winds and plasma drifts.

Key Points:

  • Using ICON data, we report the statistics of the low/midlatitude F-region O+ fraction with an extensive coverage of local time (LT), magnetic latitude (MLAT), and season

  • A latitudinally and zonally isolated peak of the oxygen ion (O+) fraction emerges at the post-noon equatorial region

  • Simulations by SAMI3 using WACCM-X thermospheric fields reproduce the peak of the observed O+ fraction

Acknowledgments

J. Park is grateful to J.-M. Choi at NCKU, Taiwan for valuable discussions on the usage of ICON/IVM data. ICON is supported by NASA’s Explorers Program through contracts NNG12FA45C and NNG12FA42I.

Data Availability Statement

The ICON data are openly available on the ICON website (http://icon.ssl.berkeley.edu/Data).

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The ICON data are openly available on the ICON website (http://icon.ssl.berkeley.edu/Data).

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