Introduction. Planetary atmospheres are gasses bound to astrophysical bodies by gravity. In the upper regions of an atmosphere incident photon and particle radiation can free one or two outer electrons from some atoms in the gas, "ionizing" them, producing an ionosphere as an upper boundary. Various disturbances and heating processes energize portions of the atmosphere and ionosphere, allowing some of their material to escape into space. This escape process occurs at the Earth, where Nitrogen N and Oxygen O are the two primary atmospheric and ionospheric constituents. Smaller amounts of other atoms, such as, Hydrogen H and Helium He, are also present in the ionosphere. Ionospheric composition varies significantly with altitude, changing from predominantly O and N ions at lower altitudes to mostly H and He ions at the higher outflow altitudes. A great amount of research has been devoted to the escape of H⁺, O⁺¹, and He⁺¹ ions from Earth´s grasp and their redistribution throughout the magnetosphere. N⁺¹, because of its mass´s proximity to that of O⁺¹ (14 and 16 amu, respectively) and smaller abundance, has been more difficult to detect and, therefore, not studied as thoroughly. Recently, measurements obtained in the outer Ring Current region from a mass spectrometer on the Geotail satellite have been used to show that: (1) N⁺¹ is at times the third most important energetic ion in the magnetosphere after H⁺ and O⁺¹ (see Figure 1), and (2) the ratio of energetic N⁺¹/O⁺¹ in the near-Earth magnetosphere varies significantly on 11-year Solar-Cycle time scales (see Figure 2) so that it is greatest at minimum solar activity by a factor of two.

Abundance. As shown in Figure 1, the N⁺¹ content can occasionally be comparable to the O⁺¹ content in certain regions of the magnetosphere for extended intervals. These data were measured as Geotail traveled from dawn to dusk across the quiet dayside magnetosphere during an extremely long geomagnetically quiet interval near solar activity minimum. On the average though, N⁺¹/O⁺¹ is typically between 0.1 and 0.5.

Figure 1 shows a histogram of N⁺¹ and O⁺¹ pulse height analyzed events with approximately equal peaks for the two species. In this and the next Figure, the data (histograms) are accompanied by fits to the data (red curves) from which the abundance ratios are derived. The inset shows average fluxes for this interval.

Solar Cycle Dependence. The response of O⁺¹ to solar heating is stronger than that of all other ions that escape Earth´s ionosphere, including N⁺¹, therefore the ratio N⁺¹/O⁺¹ decreases with increasing solar activity. Average values of N⁺¹/O⁺¹ decrease from about 38-40% at minimum solar activity to about 20-22% at maximum solar activity. Because of similarities between the masses, ionization potentials, and spatial distributions of Nitrogen and Oxygen, the observed differences in their escape scenarios can be used to better understand the chemical and physical processes that lead to their escape.

Figure 2. The long-term variation of the count ratio of 10-210 keV/e N⁺¹ and O⁺¹ as measured by the GEOTAIL/EPIC/STICS ion-charge-state-spectrometer is displayed with monthly averages of the International Sunspot Number R9, a direct measure of solar activity. Two insets show the mass per charge, M/Q, distribution of N⁺¹ and O⁺¹ near solar minimum (95/96) and near solar maximum (99/00). A comparable plot of flux ratios is shown below in Figure 4.

• plasma sheet and ring current mass loading by these heavy ionospheric origin ions
• acceleration processes affecting both ion species at low altitudes
• subsequent acceleration of both ion species in the plasma sheet/ring current
• variability of their concentrations in the ionosphere
• ring current source and loss processes of the dominant atmospheric constituent´s ion N⁺¹.

To our knowledge, before this study the Solar-Cycle variation of N⁺¹/O⁺¹ was unknown both at lower altitudes and in the magnetosphere.

Supplemental Information

Background. N is the dominant (80%) atom in the atmosphere and O, at about 20%, essentially makes up the rest of the air we breathe. In the ionosphere (55-1000 km altitude) the composition is reversed, O⁺¹ is the dominant ion at about 80% and N⁺¹ is now about 20%. Chappell et al. [1982], who discovered N⁺¹, and N⁺² in the plasmasphere, discuss early ionospheric N⁺¹ measurements and state " the production and loss of N⁺¹ in the ionosphere is controlled by the photoionization of N and N₂ and by the reaction of He⁺¹ + N₂ with the loss dominated by reactions of N⁺¹ with O₂ and NO (see Schunk and Raitt [1980] and Torr and Torr [1979] for details)".

Nitrogen and Oxygen are the dominant species in both the atmosphere and ionosphere. N⁺¹ is one of the four primary atomic ion species (H⁺, O⁺¹, N⁺¹, and He⁺¹) that escape from Earth´s ionosphere. H⁺ is typically the predominant ion throughout the remainder of Earth´s magnetosphere. M_N is 14 amu, M_O is 16 amu, N and O have first similar ionization potentials of 14.534 and 13.618 volts, respectively. For all their similarities, N and O seem to have very different escape scenarios, at least in terms of the final accelerated population observed in the outer ring current. They interact strongly in the ionosphere, producing the primary molecular ions in the ionosphere, N₂⁺¹, NO⁺¹, and O₂⁺¹, which have also been observed escaping Earth´s gravity during very disturbed intervals (see, e.g., Yau et al. [1993], Christon et al. [1994]), and Peterson et al. [1994]). N⁺¹/O⁺¹ ratios in the range 0.5-1.0 have previously been reported at low altitudes (see Yau et al. [1993] and Peterson et al. [1994]). Yau et al. [1991] show that minor ions, "...ions other than H+, notably O⁺¹, He⁺¹, O⁺², N⁺¹, and N⁺², are a significant (≥ 0.1) and at times dominant (≥ 0.5) component of the thermal ion population in the high-latitude polar ionosphere."

Figure 3 compares the estimated Kp dependence of N⁺¹ from this study to measurements of other energetic (0.9-15.9 keV/e) atomic ions from an earlier study of near-geosynchronous altitude data [Young et al,. 1982]. The shaded area covers the range of estimated N⁺¹ values derived from our N⁺¹/O⁺¹ count ratios. The heavy dashed line shows the average estimated N⁺¹ value over the Kp range sampled. The light dashed line is an extrapolation of the heavy dashed line to higher Kp values.

Average Geomagnetic Activity Dependence. Both N⁺¹ and O⁺¹ fluxes respond to geomagnetic disturbance. In Figure 3, we scale the N⁺¹ to the O⁺¹ regression curve from [Young et al,. 1982], using our measured N⁺¹/O⁺¹ ratio in order to estimate the geomagnetic dependence of N⁺¹. The comparison is not necessarily direct because we have not yet converted our count data to densities. However, the flux/density ratio will be similar to the count ratio as a result of the similar N⁺¹ and O⁺¹ efficiencies in STICS (see e.g., Figure 2 of Christon et al. [1994]). The Kp dependence of N⁺¹/O⁺¹ and estimated N⁺¹ from this study is consistent with values reported by Gloeckler and Hamilton [1987] for the ring current. On the other hand, our results are the reverse of the low altitude N⁺¹/O⁺¹ values of about 0.1 (quiet) and 0.5-1.0 (disturbed) reported by Yau et al. [1991], suggesting that acceleration processes at higher altitudes are important in differentiating these species´ magnetospheric populations. This difference remains to be confirmed and investigated further.

Geomagnetic Activity Dependence During a Major Storm. During major geomagnetic storms the flux of N⁺¹ is sometimes the third highest in the outer ring current (see Figure 4). Its density contribution is then also third highest.

Figure 4. The long-term variation of the flux ratio of 10-210 keV/e N⁺¹ and O⁺¹ as measured by the GEOTAIL/EPIC/STICS ion-charge-state-spectrometer is displayed for comparison with Figure 2. The inset shows the differential fluxes of the principal ion components of the outer ring current during a major geomagnetic storm at 1200-1400 UT on 10.Jan.1997. O⁺¹ and N⁺¹ are the second and third most important ion species in the ring current from 1115-2200 UT on 10.Jan.1997.

Summary. We have found that ionospheric origin N⁺¹ flux is an important constituent of the accelerated, end-products of ionospheric outflow, the outer ring current. N⁺¹ is as important (populous) as O⁺¹ during certain quiet geomagnetic conditions. At the minimum, N⁺¹ is probably at least as important to dynamic magnetospheric processes as the more often studied, but lighter, ion He⁺¹. In the outer ring current/quasi-trapping region, we find that N⁺¹/O⁺¹ has average values of about 38-40% at solar minimum activity and about 20-22% at solar maximum activity. We estimate that N⁺¹ increases with increasing geomagnetic disturbance levels in a manner similar to H⁺, that is, the N⁺¹ correlation with geomagnetic activity is positive, but weaker than that of O⁺¹.

A paper by Christon et al. showing these results is in preparation for submission to one of the American Geophysical Union journals.

References

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Christon, S.P., et al., Energetic Atomic and Molecular Ions of Ionospheric Origin Observed in Distant Magnetotail Flow-Reversal Events, Geophys. Res. Lett., 21, 3023-3026, 1994.

Gloeckler, G. and D.C. Hamilton, AMPTE Ion Composition Results, Physica Scripta., T18, 71-84, 1987.

Peterson, W.K., et al., On the Sources of Energization of Molecular Ions at Ionospheric Altitudes, J. Geophys. Res., 99, 23257-23274, 1995.

Schunk, R.W. and W.J. Raitt, Atomic Nitrogen and Oxygen Ions in the Daytime High-Latitude F Region, J. Geophys. Res., 77, 6104, 1980.

Young, D.T., H. Balsiger, and J. Geiss, Correlations of Magnetospheric Ion Composition with Geomagnetic and Solar Activity, J. Geophys. Res., 87, 9077-9096, 1982.

Torr, D.G. and M.R. Torr, Minor Ion Composition in the Polar Ionosphere, Geophys. Res. Lett., 18, 345-348, 1991.

Yau, A.W., B.A. Whalen, and E. Sagawa, Determination of the Sources and Sinks of N⁺ in the Thermosphere, Geophys. Res. Lett., 18, 345-348, 1991.

Yau, A.W., et al., EXOS D (Akebono) Observations of Molecular NO⁺¹ and N₂⁺¹ Upflowing Ions in the High-Altitude Auroral Ionosphere, J. Geophys. Res., 98, 11205-11224, 1993.

EPIC was designed and built by teams at the Johns Hopkins University Applied Physics Laboratory and at the University of Maryland.