Data for secondary-electron production from ion precipitation at Jupiter IV: Simultaneous and non-simultaneous target and projectile processes in collisions of Sq+ + H2 (q=116)

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Abstract

Results of calculations describing secondary-electron production and other inelastic processes in collisions of 1 to 25,000 keV/u Sq+ (q=116) with H2 are presented. These data complement previous results for the same processes in Oq+ (q=18) + H2 (Schultz et al. 2017, 2019) and for Hq+ (q=1,0,1) (Schultz et al. 2020). Used in ion-transport and secondary-electron simulations, these data provide a description of the atomic processes driven by these principal ions precipitating into the upper atmosphere of Jupiter, enabling better understanding the coupling of the Jovian magnetosphere, ionosphere, and atmosphere.

Introduction

During the flybys of Jupiter in 1979, NASA’s Voyager spacecraft detected a significant ion population within the Jovian system. These observations are consistent with physical chemistry models for the region between six and nine Jovian radii inferring the presence of O+, S2+, O2+, S3+, and S+ with abundances of 15%–22%, 10%–19%, 4%–8%, 4%–6%, and 1%–5%, respectively. These ion fractions, given relative to the local electron number, decline with increasing radial distance, and are the results of very recent re-analysis of the original data [1]. These abundances are supported by data from the subsequent observations made when the Ulysses probe passed by Jupiter in 1992 to gain an orbital assist, by the Galileo mission to Jupiter (1995–2003), by the Cassini spacecraft on its way to Saturn (2000), and by the Juno spacecraft that has recently arrived at Jupiter (2016-). They originate from Jupiter’s Galilean satellites, principally volcanic Io, with a population also possible from the entrainment of the solar wind by Jupiter’s magnetic field.

These ions, present in the magnetosphere, also precipitate into the Jovian atmosphere as evidenced by the existence of both north and south polar X-ray auroras identified [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] as originating from de-excitation emission following charge transfer between the precipitating ions and molecules of the upper atmosphere, principally molecular hydrogen. In this process low-charge-state, heavy ions are accelerated by Jupiter’s prodigious magnetic field to 100’s of keV/u energies, strip to high-charge states in collisions with H2, slow down in their passage through the atmosphere, and produce secondary electrons, dissociation of H2, and photon emission from H2 and from the precipitating ions. These atomic processes heat the atmospheric molecules and contribute to the atmospheric ion and electron currents. In particular, this physical chain of events links the sources of these ions, the Jovian moons and possibly the solar wind, with the magnetosphere, ionosphere, and atmosphere.

Unique observations being made by Juno of the polar regions in which these ions precipitate along magnetic field lines, made possible by its unique highly elliptical polar orbit with perijove very near the planet, have motivated the production of the data required to simulate the physics of the ion and secondary-electron currents, photon emission, and molecular dissociation associated with oxygen [12], [13] and hydrogen [14] precipitation.

As noted from in situ observations, data for heavy ions was of principal importance for oxygen owing to its high abundance, but next in importance is data for sulfur. For example, as evidenced by analysis of X-ray observations from Earth’s orbit, sulfur is required in addition to oxygen to model the precipitation–emission process [9], [10]. To illustrate this, Fig. 1 shows a synthetic photon-emission spectrum based on previous work [10] for equal abundances of 1.2 MeV/u O+ and S+ that were accelerated by Jupiter’s magnetic field, passed through the upper atmosphere stripping on the gas to high charge state, and emitted photons including in the X-ray waveband following the capture of electrons from H2. The sulfur emission fills in emission between about 250 and 550 eV in the gap formed by oxygen emission dominant at lower and high photon energy. Therefore, here we provide data not otherwise available for secondary-electron emission in collisions of 1–25,000 keV/u Sq+ + H2 (q=116) along with other inelastic processes required to model the charge and energy evolution of the ions along their passage through Jupiter’s upper atmosphere.

As shown in the previous work for heavy ions [13], it is necessary to treat electronic transitions that occur on the projectile (impacting ions) and the target (H2) simultaneously, as opposed to independently. In particular, it was shown that treating projectile and target processes as non-simultaneous (NSIM) resulted in a deficit of energy loss at intermediate collision energies (50 keV/u to 2 MeV/u) as evidenced by comparison with reference data for the stopping power computed in an ion transport simulation using the NSIM-processes data. In fact, this deficit increases with increasing projectile nuclear charge. Therefore in the present work we consider the simultaneous (SIM) projectile and target processes as follows:

Calculation of the full range of these processes, projectile charge states, and impact energies is presently only achievable using the classical trajectory Monte Carlo (CTMC) method, as described in the previous work for oxygen [12], [13] and hydrogen [14]. In brief, this method is a simulation of the atomic collision in which trajectories of the projectile, target nuclei, and electrons are calculated using classical mechanics but with the initial electron orbits chosen so that the properties of the ensemble mimic the appropriate quantum mechanical binding energies and radial and momentum distributions. CTMC models are employed adapted from those developed to treat H2 [15], [16] (target processes) and multi-electron atoms and ions [17] (projectile processes). Comparisons of the results of this method with measurements and other theoretical treatments of the spectrum of secondary-electron emission have been made, for example, for proton [18] and He+ [19] impact of H; fluorine ions colliding with H2 [20]; C+ [21], F9+ [22], and highly charged gold [23] ion-impact of He; and carbon ion-impact of neon [24].

The CTMC calculations are used to produce the impact-parameter-dependent inelastic probabilities for transitions on the projectile and target independently, which are then combined as described in previous work [13] to produce the SIM-process cross sections. Because the target processes have a larger range as a function of impact parameter, this model partitions the integral cross section for the target processes into fractions occurring with and without simultaneous projectile processes. For example, the SI NSIM process is partitioned into the SIM processes SI, SI+SPEX, SI+DPEX, SI+SS, and SI+DS, as described in detail in Ref. [13].

In the following sections we provide a description of the tabulations of results for the present calculations (Section 2), results of the ion-transport simulation demonstrating the overall validity of the data through comparison with the accepted value and measurements of the stopping power (Section 3), and provide conclusions drawn from the work (Section 4).

Section snippets

Integral cross sections and average energy losses

For both ion-transport and secondary-electron simulations, the integral cross sections for each of the processes (e.g., SI, SI+SPEX, TEX, DI+SS) are needed. These cross sections represent the probability that a particular process takes place, giving the evolution of the charge state of the ion during its passage through the atmosphere, and also gives the mean free path between each such event. Therefore in Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10

Ion-transport simulation results: Stopping power, equilibrium charge state, and ion-charge-state populations

As noted in previous work, the scarcity, or here, the non-existence, of measurements to test the very wide range of data for inelastic processes required for ion-transport and secondary-electron simulations precludes a direct test of the present comprehensive set of inelastic cross sections and energy losses. However, as before, we have performed an ion-transport simulation, using the present integral cross sections and average energy losses, which allows the calculation of the stopping power

Conclusions

We have presented data for inelastic processes in 1–25,000 keV/u Sq+ + H2 (q=116) that complement those previously computed for 1–25,000 keV/u Oq+ + H2 (q=18) enabling modeling of transport of these heavy ions through molecular hydrogen gas and of the secondary electrons produced. We have utilized the model previously developed treating electronic transitions on both the projectile and target simultaneously in order to most realistically reproduce the energy loss at intermediate energies.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

DRS and TEC gratefully acknowledge support from the NASA Planetary Atmospheres program, USA through grant NNX14AG79G.

References (30)

  • SchultzD.R. et al.

    At. Data Nucl. Data Tables

    (2017)
  • ReinholdC.O. et al.

    Nucl. Instrum. Methods B

    (1991)
  • L.P. Dougherty, K.M. Bodisch, F. Bagenal, J. Geophys. Res. Space Phys., 122, 8257–8276,...
  • HoranyiM. et al.

    J. Geophys. Res.

    (1988)
  • CravensT.E. et al.

    J. Geophys. Res.

    (1995)
  • KharchenkoV. et al.

    J. Geophys. Res.

    (1998)
  • LiuW. et al.

    Ap. J.

    (1999)
  • CravensT.E. et al.

    J. Geophys. Res.

    (2003)
  • KharchenkoV. et al.

    Geophys. Res. Lett.

    (2006)
  • KharchenkoV. et al.

    J. Geophys. Res.

    (2008)
  • HuiY.-W. et al.

    Astrophys. J. Lett.

    (2009)
  • HuiY.-W. et al.

    J. Geophys. Res.

    (2010)
  • OzakN. et al.

    J. Geophys. Res.

    (2010)
  • SchultzD.R. et al.

    At. Data Nucl. Data Tables

    (2018)
  • SchultzD.R. et al.

    At. Data Nucl. Data Tables

    (2018)
  • View full text