Heliophysics 2050 White Papers (2021) 4083.pdf

White Paper for Heliophysics 2050

Kinetic effects of solar driving on magnetospheres Li-Jen Chen, Michael Collier, John Dorelli, Shing Fung, Daniel Gershman, Judith Karpen, Robert Michell, Jonathan Ng, Douglas Rowland, Marilia Samara, David Sibeck, Shan Wang

Heliophysics Science Division NASA Goddard Space Flight Center (Contact email: [email protected])

September 2020

In this white paper, we discuss kinetic effects of solar driving on magnetospheres and interconnected science topics envisioned for future major research directions in heliophysics. We recommend (1) promotion of global particle simulations to address how these kinetic effects impact the evolution of magnetized planets and bodies in the heliosphere, past and present; and (2) advancing NASA’s high end computing to exascale to provide the critical ground for (1). An underlying connection between collisionless shocks, turbulence, and magnetic reconnection leads to complex interactions in the geospace environment, as revealed by global kinetic simulations in 2D [Karimabadi et al., 2014] and 3D (Figure 1). Ions reflected off the segment of the bow shock where the surface normal is quasi-parallel to the interplanetary magnetic field lead to generation of intense turbulence. This turbulence takes the forms of nonlinear waves that penetrate into the inner magnetosphere [Takahashi et al., 2016], and steepened magnetic structures [e.g., Schwartz et al., 1992; Chen et al., 2020] resulting in plasma heating, particle acceleration, and reconnection. At the shock, the turbulence causes the formation of high speed jets that penetrate the bow shock and bombard the magnetopause a few times per hour leading to dayside reconnection [Hietala et al., 2018]. These large-scale structures are generated by the kinetic turbulence arising from interaction of the incomping solar wind with reflected ions, and is beyond the reach of any fluid modeling. Turbulence gives rise to intense current sheets what are conducive to reconnection both at the bow shock and extending deep into the downstream magnetosheath [Karimabadi et al., 2014; Retinò et al., 2007; Phan et al., 2018]. Kinetic scale current sheets form and reconnect at the foreshock and bow shock transition layer, as recently predicted by fully kinetic simulations [Bessho et al., 2019, 2020], 3D global particle simulations [to be published], and observed by the MMS spacecraft [Wang et al., 2019a, 2020; Gingell et al., 2020]. The turbulence stirred up and amplified at the bow shock enters the magnetosheath and the cusp when the cusp is behind the quasi-parallel bow shock (Figure 1). This turbulence induces responses in the coupled magnetosphere-ionosphere-thermosphere (MIT) system. For example, energetic particles produced in the turbulence (as well as those of the solar origin) have direct access of the cusp region [Trattner et al., 2001], and the amplified fields produce intense poynting flux into the cusp. Another example is the foreshock low-frequency electromagnetic

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waves impacting the magnetosphere and causing particle scattering and precipitation into the ionosphere [e.g., Wang et al., 2019b].

Interconnected topics: Here we list example topics for major future investigations.  Collisionless shock physics: magnetic field amplification, particle acceleration, cross shock dissipation, 3D shock structures and dynamics, wave excitation and interaction with particles  Evolution of magnetized bodies: how does kinetic turbulence due to reflected particles at the bow shock affect the evolution of magnetized planets and bodies in the universe, past and present?  Turbulent reconnection: the nature of reconnection varies with scales. Reconnection in electron-scale turbulence precludes ions from participating in the dynamics, and hence the energy conversion process and evolution in kinetic turbulence call for new examination.  Impact on the magnetosphere-ionosphere-thermosphere: What are the MIT responses to the shock turbulence both in terms of the amplified fields and energized particles?  Impact on the magnetotail: Turbulence affects the dayside reconnection rate which influences the magnetic flux circulation, and hence magnetotail dynamics. Magnetotail substorm onset, for example, is initiated at kinetic scale current sheets formed due to increasing magnetic fluxes from the dayside.

Figure1: (Left) Turbulence stirred up and amplified at the quasi-parallel bow shock penetrates into the magnetosheath, reaches the magnetopause and into northern cusp (from a global hybrid simulation, unpublished), triggering magnetopause reconnection and plasma heating as well as precipitation into the cusp. (Middle) The shock turbulence in the simulation exhibits properties similar to those observed by MMS (Right) in terms of magnetic field, plasma density enhancements and wave polarization (Bz leads By by ~90 degrees).

Exascale 3D global particle simulations will provide critical grounds for a host of future scientific investigations to thrive. A primary challenge in understanding kinetic turbulence and its global impact is its multi-scale nature, spanning from particle gyroradii, plasma skin depths, to macro scales of the magnetosphere. The example simulation shown in Figure 1 utilizes a major fraction of a NASA HEC supercomputer, and it only models the dayside dynamics and

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only for the short time of the first impact (only considering the solar driving and ignoring most of the responses of the MIT system). To get to true global modeling of kinetic turbulence thus requires advancing the current NASA high end computing by orders of magnitude to exascale (perhaps by partnering with DOE: exascaleproject.org). For scientific topics envisioned and not yet envisioned in the realm of global particle simulations, exascale computing will enable transformative leaps. In conclusion, we recommend the agency to cultivate an environment to support global particle simulations to unfold the new science dimensions not accessible by any fluid approach. Advancing NASA’s high end computing to exascale will enable multi-scale and multi- physics science in the decades to come.

References: Bessho, N., L.-J. Chen, S. Wang, et al. (2019), Fully kinetic simulations of magnetic reconnection in the Earth's quasi-parallel bow shock, Geophysical Research Letters, 46. Bessho, N., L.-J. Chen, S. Wang (2020), Magnetic reconnection and kinetic waves generated in the Earth’s quasi-parallel bow shock, Phys. Plasmas, in press. Chen, L.-J., et al. (2020), Solitary magnetic structures at quasi-parallel collisionless shocks: drift- driven amplification, submitted to Geophysical Research Letters. Gingell I., et al. (2020). Statistics of reconnecting current sheets in the transition region of earth's bow shock. Journal of Geophysical Research: Space Physics, 125, e2019JA027119. Hietala, H., et al. (2018). In situ observations of a magnetosheath high-speed jet triggering magnetopause reconnection. Geophysical Research Letters, 45, 1732–1740. Karimabadi, H., et al. (2014), The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas, Phys. Plasmas, 21(6), 062308 Phan, T. D., et al. (2018). Electron magnetic reconnection without ion coupling in Earth’s turbulent magnetosheath. Nature, 557, 202–206. Retinò, A., et al. (2007). In situ evidence of magnetic reconnection in turbulent plasma. Nature Physics, 3, 236–238. Schwartz, S. J., et al. (1992), Observations of short large-amplitude magnetic structures at a quasi-parallel shock, J. Geophys. Res., 97, 4209–4227 Takahashi, K., et al. (2016), Propagation of ULF waves from the upstream region to the midnight sector of the inner magnetosphere, J. Geophys. Res. Space Physics, 121, 8428– 8447 Trattner, K. J., et al. (2001), Origins of energetic ions in the cusp, J. Geophys. Res., 106( A4), 5967– 5976 Wang, S., L.-J. Chen, N. Bessho, et al. (2019a), Observational evidence of magnetic reconnection in the terrestrial bow shock transition region, Geophys. Res. Lett., 46. 562– 570 Wang, B., et al (2019b). The 2‐‐ D structure of foreshock driven field line resonances observed by THEMIS satellite and ground‐ based imager conjunctions. Journal of Geophysical Research: Space Physics, 124, 6792–6811. Wang, S., L.-J. Chen, N. Bessho, et al. (2020), Ion-scale current structures in Short Large- Amplitude Magnetic Structures, The Astrophys. J., 898:121

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