The Need for Detailed Ionic Composition of the Near-Earth Plasma

HeliophysicsHELIO 2050 2050 White Papers (2021) R. Ilie4094.pdf

THE NEED FOR DETAILED IONIC COMPOSITION OF THE NEAR-EARTH PLASMA

An impressive body of work has been devoted to the escape of H+, He+ and O+ ions from Earth’s ionosphere, and their circulation and redistribution throughout the terrestrial magnetosphere (Schunk and Raitt, 1980; Schunk and Sojka, 1997; Winglee et al., 2002; Glocer et al., 2009; Ilie et al., 2015; Ilie and Liemohn, 2016). However, the transport and energization of N+, in addition to that of O+, has not been considered by most studies, even though a number of direct and indirect measurements made to describe the ionic composition of the ionosphere-magnetosphere system, have established that N+ is a significant ion species in the ionosphere and its presence in the magnetosphere is significant. A wide range in the magnitude oftheN+ to O+ density ratio has been reported, with significant variations not only with geomagnetic activity, but also with solar cycle, season, time of day, latitude, etc (Mall et al., 2002; Christon et al., 2002). These variations suggest that N+ and O+, while relatively close in mass, they obey different chemical and energization processes, and possibly follow different paths of energization. In addition, observations from the WIND STICS ISIS 2 spacecraft (Hoffman et al., 1974) indicated that not only N+, but also molecular ions were observed in the Earth’s magnetosphere Geotail STICS AMPTE and ionosphere. Measurements from CCE Explorer 31 Akebono SMS the Arase satellite (Miyoshi et al., MMS ISIS-2 IMS DE-1 RIMSS e-POP 2018) showed the existence of molec- OGO-6 OGOGO-2-2 IMSS Sputnik3 IMS AE-C ular ions in the Earth’s ring cur- RMS IMS rent region, even under moderate geomagnetic storm conditions. The 20 21 22 23 24 Low-Energy Particle experiments-Ion mass analyzer (LEPi) instrument (Asamura et al., 2018) on board the Figure 1. Sunspot number from 1958 to 2020 (black lines) in- Arase satellite clearly identified the dicative of solar cycles 19 through 24 (numbers in grey). Over- energies of the molecular ions above ∼ plotted are the “nitrogen measuring”” missions and their corre- 12 keV during most magnetic storms, sponding operating altitude (perigee to apogee) vs. time. and the average energy density ratio of the molecular ions to O+ is ∼ 3% (Seki et al., 2019). Furthermore, recent observations based on the CASSIOPE Enhanced Polar Outflow Probe (e-POP) mission data report on the presence + + + of cold atomic N , and molecular N2 and NO densities in both ion up-flows and down-flows. Interestingly enough, the densities of N+ can contribute up to 10-50% of the plasma density, at all times, independent of geomagnetic activity (Yau et al., 2019). Albeit limited, observations of outflowing nitrogen ions have been reported by early missions, spanning a wide range of altitudes, from ∼ 200 km for the Sputnik III spacecraft, to millions of km for the WIND spacecraft (see Figure 1 for their solar cycle coverage). However, most space missions lacked the possibility to reliably separate the N+ from O+ owning to their very close masses, and relatively few currently active ion spectrometers in space are capable of separating N+ from O+, therefore the observational record of its existence and significance has been overlooked. Although these observations show the importance of heavy ions in the high-altitude ionosphere and magnetosphere, the mechanisms responsible for accelerating the ionospheric heavy ions from eV to keV energies are still largely unknown. Their transport and acceleration in the ionospheric

1 HeliophysicsHELIO 2050 2050 White Papers (2021) R. Ilie4094.pdf

outflow, as well as the relative abundance of the molecular ions in the low-altitude ionosphere, is still unknown. Recently, numerical simulations using the 7iPWOM Lin et al. (2020), which solves for the trans- + + + + − + + + port of H , He ,O ,N , and e , and includes three static minor ion species, NO ,N2 and O2 , showed that N+ ions play a key role in the ionospheric outflow for all conditions. Accounting for the presence of N+ ions in the lower ionosphere and their transport outwards drastically improved the polar wind solution. Furthermore, as it leads to an improved solution for He+ ions and captures their seasonal variations. Figure 2, from Lin et al. (2020) shows the simulation results from eight different sets of numerical experiments. The upward transport of these ions in the Earth’s polar ionosphere is solved for using both the 7iPWOM (solid line) and the 3iPWOM (dashed line) which only considers H+, He+,O+, and e−, and comparison with the appropriate data (dotted line) from OGO-6 and AE-C for various solar flux and seasonal conditions is presented. Understanding plasma composition requires to ultimately include a variety of ions that are currently known, although neglected, to be present in the low altitude ionosphere. Spatial and temporal variations in the heavy ion composition can provide unique insights into the dynamics of the terrestrial magnetosphere, as heavy ions alter the plasma mass density, the Alfvén velocity, they modulate wave-particle interactions and alter the development and decay of the ring current and radiation belt (e.g. Bashir and Ilie, 2018).

Summer Noon Summer Midnight Winter Noon Winter Midnight Appropriate knowl- O+ edge of the ionic N+ H+ composition will He+ guide the development of instru-

SOLAR MAXIMUM mentation and space missions, capable of distinguishing between O+ and N+ ions, since the differences between O+ and N+ trans-

SOLAR MINIMUM port and energization are not quantified, nor un- derstood, at this 3iPWOM 7iPWOM DATA time. Further- Figure 2. Polar wind densities for H+, He+,O+, and N+ ions as predicted by 7iP- more, understand- WOM (solid line) and 3iPWOM (dashed line) under various solar and seasonal condi- ing the differen- tions. The dotted lines show the data from AE-C (solar minimum) or OGO-6 (solar tial transport of maximum). Figure from Lin et al. (2020) nitrogen and oxygen ions, together with molecular ions, throughout the ionosphere-magnetosphere region can also provide knowledge of: i.Exospheric morphology, which at this time is unknown. Several models to predict the exosphere density have been developed throughout the years; however, aside form their lack of temporal variation, there is strong disagreement between all models. ii.Ionospheric physics, since ionosphere-exosphere-magnetosphere coupling is controlled by the ionization at the topside ionosphere, which is strongly dependent on external driving. iv. Planetary atmospheric evolution, by providing hints about variation in the nitrogen budget with solar activity over ge- ological scales time periods. The atmospheric escape via the polar wind is highly affected by the ionospheric composition, as the escape of the nitrogen ions three gigayears ago increases by ∼ 3

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orders of magnitude compared to its present value (Kislyakova et al., 2020).

References

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