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3 Multiscale Dynamics in the High‐Latitude Ionosphere

       Yukitoshi Nishimura1,2, Yue Deng3, Larry R. Lyons2, Ryan M. McGranaghan4,5, and Matthew D. Zettergren6

       1 Department of Electrical and Computer Engineering and Center for Space Physics, Boston University, Boston, MA, USA

       2 Department of Physics, The University of Texas at Arlington, Arlington, TX, USA

       3 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

       4 Science Division, Atmosphere and Space Technology Research Associates, Louisville, CO, USA

       5 NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

       6 Department of Physical Sciences and Center for Space and Atmospheric Research, Embry‐Riddle Aeronautical University, Daytona Beach, FL, USA

      ABSTRACT

      This chapter reviews recent findings of multi-scale structures and dynamics in the high-latitude ionosphere, particularly on meso-scale (10s‐100s km) processes, as well as their roles in cross-regional interaction processes. Localized and transient structures often occur at the cusp, polar cap, and auroral oval, and their magnitudes can be comparable or larger than those of large-scale background. However, their properties and coupling are not well understood, and specification of their structures and variabilities are critical for numerical modeling. The meso-scale covers a myriad processes and phenomena, including poleward-moving auroral forms (PMAFs), polar cap patches, auroral arcs, poleward boundary intensifications (PBIs), streamers, substorm, surges, diffuse aurora, and related flow channels, field-aligned currents (FACs) and precipitation/conductance. Small‐scale features also play important roles in the creation and behavior in multi-scale dynamics. While those structures are localized, they have net effects on large-scale dynamics, and can influence surrounding regions by propagating over long distances. An approach to quantify meso-scale precipitation contributions using the THEMIS all‐sky imagers is presented, and we show that meso-scale precipitation has a substantial (25‐50%) contribution, indicating critical importance of multi-scale processes for understanding Geospace processes. The current state and necessity of specification for advancing understanding of multi-scale coupling processes are discussed.

3.1 INTRODUCTION

Geospace coupling involves mass, momentum and energy transport across regions and scales, and thus it inherently poses cross‐regional and multiscale nature. Global processes have been relatively well understood thanks to a large amount of data accumulated over decades, a growing observation network, global simulations, and data science approaches. During geomagnetic storms, solar wind energy reaching the magnetosphere is some tens of TW, and ~1% of solar wind energy is supplied to the magnetosphere‐ionosphere (M‐I) system (Feldstein et al., 2003). Of the energy reaching the M‐I system, ~50% is supplied to the ionosphere as Poynting flux or field‐aligned currents (FACs) and dissipates as Joule heating. Precipitation into the atmosphere carries ~25% of energy. The rest (~25%) goes into the ring current, primarily in a form of bursty injections from the magnetotail rather than slow convection. Similar energy partition is seen during substorms even with weaker solar‐wind energy input (Ostgaard et al., 2002a, 2002b). These numbers emphasize the importance of ionosphere processes for energy dissipation processes.

Owing to accumulation of large database and data science, such large‐scale energetics in the high‐latitude ionosphere can be specified nearly routinely. For example, data assimilation incorporating SuperDARN, AMPERE, and a statistical precipitation model specifies global convection, FAC, Poynting flux, and conductance distributions (e.g., Cousins et al., 2015). However, as Figure 3.1a shows, statistical averaging of precipitation only provides a smooth large‐scale (> ~1,000 km horizontal) oval structure, which is vastly different from actual structured and dynamically varying aurora. The energy flux distribution reconstructed from the THEMIS all‐sky imager (ASI) network (Fig. 3.1b) depicts a number of mesoscale (tens to hundreds km horizontal) precipitation structures during a substorm (see section 3.4). Narrow field‐of‐view (FOV) imaging has even resolved small‐scale (< ~10 km horizontal) intense precipitation structures and conductance gradients (Fig. 3.1c). While it is difficult to measure kinetic scale structures (< ~1 m), recent PIC simulations have predicted the existence of meter‐scale turbulent density structures created by Farley‐Buneman instability (Oppenheim & Dimant, 2013). The process is nonlinear and there are net changes in conductance due to the density structures. The instability also increases temperature over the scale size of flows (Schlegel & St. Maurice, 1981). The mesoscale, small‐scale, and kinetic‐scale structures can have comparable or larger magnitudes than those at larger scales, and thus those potentially affect larger‐scale dynamics. While studies on how global processes are affected by small‐scale processes are limited, Wiltberger et al. (2017) incorporated the kinetic turbulence conductance formulated by Oppenheim and Dimant (2013) into the coupled MHD simulation and evaluated effects of kinetic turbulence. They showed that the inclusion of kinetic turbulence processes led to a conductance increase by 20% and a better agreement with measured D_st. The cross‐polar‐cap potential decreased by 13%, and this may substantially improve estimation of cross‐polar‐cap potential, which tends to be overestimated in MHD simulations. Thus, the multiscale processes may play a substantial role in understanding ionosphere processes.

Figure