Science Themes

A central theme of the CGS science investigation, enabled by the MAGE model, is the importance of mesoscale processes and cross-scale coupling in emergent global-scale dynamics.

There are many examples of such mesoscale processes acting throughout stormtime geospace. Some examples are indicated with yellow arrows in the image at the top. They include, but are not limited to:

  • Build-up of the ring current by bursty bulk flows and localized energetic particle injections.

  • Polar cap density structures, such as tongues of ionization and patches, that present a major space weather hazard due to their detrimental effect on communications. They also present a major scientific and modeling challenge as part of the global plasma circulation process involving the ionosphere, the plasmasphere and the inner magnetosphere, sometimes referred to as the geospace plume (Foster et al., 2020)

  • Kelvin-Helmholtz waves on the magnetospheric boundary. These waves drive ultra-low frequency oscillations in the inner magnetosphere that are important for shaping the response of radiation belts to geospace storms. These waves are also believed to be a major mechanism for solar wind plasma entry into the magnetosphere (Wing et al., 2014)

  • Atmospheric gravity waves exert forcing on and precondition the ionosphere and thermosphere (Qian and Yue, 2017), and thus can affect their responses to geospace storms. At the same time, storm-time effects in the ionosphere-thermosphere system introduce changes in the circulation and may affect the breaking and deposition of energy and momentum by gravity waves and tides. Thus it is important to investigate the change of global mean flow and lower atmosphere waves, and their interaction during storms and of the physical mechanisms that determine this interaction (Hagan et al., 2015; Pedatella, 2016).

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A quintessential example of a mesoscale process with global-scale consequences in geospace is the bursty bulk flows on the nightside of the magnetosphere, in the magnetotail (see picture above). These azimuthally localized (mesoscale here means 1-3 Earth radii in the cross-tail direction), intermittent flows cumulatively may account for a large portion of the entire earthward plasma and magnetic flux transport in the magnetotail. They are also believed to contribute significantly to the formation of the ring current via injection of energetic ions into the inner magnetosphere. The ring current, in turn, drives the so-called "Region 2" magnetic field-aligned currents into the ionosphere where they connect to the "Region 1" currents driven by magnetospheric processes at higher latitudes. The mutual closure of these currents in the ionosphere occurs across the so-called electrojets, which are the most intense electrical currents in the ionosphere. It is in these auroral regions where strong Joule heating and momentum transfer between the ions and neutrals occur. As the storm develops, the thermosphere is significantly stirred up. Changes in high-latitude neutral temperature and winds are transmitted to lower latitudes through non-linear, coupled dynamical processes, causing global-scale variations in neutral temperature, winds and composition. This alters global ionospheric plasma densities through chemistry and plasma transport. Changes in plasma densities are also produced by energetic particle precipitation at different altitudes, depending on their energy spectra. In other regions, significant plasma density depletions occur due to stormtime wind circulation and thermospheric composition changes. All this results in complex stormtime ionospheric conductivity changes that enforce electrodynamic feedback on the magnetosphere. Furthermore, Joule heating, soft electron precipitation, and plasma transport cause upwelling of ionospheric ions that enables mass coupling with the magnetosphere by providing the source for ion outflow. Lastly, in the storm recovery phase, the so-called flywheel effect due to the large inertia of neutrals keeps driving wind-generated currents, providing feedback on the magnetosphere for hours. Through all of these changes in the conductivities, mass outflow and neutral wind patterns, the ionosphere-thermosphere system exerts influence on the magnetospheric dynamics closing this complex feedback loop.

This chain of processes demonstrates the strong interaction of the geospace regions, from the magnetotail to the upper atmosphere, initiated and mediated by the mesoscale plasmasheet flows. Simulating this complex chain of coupled phenomena demands incorporation of all of the involved domains, while resolving both the physics and the spatiotemporal scales of the key processes at play.

The MAGE model is designed with exactly these requirements in mind and is thus built to solve the complex problems of stormtime geospace dynamics.

References

Foster, J.C., Erickson, P.J., Walsh, B.M., Wygant, J.R., Coster, A.J. and Zhang, Q.‐H. (2020). Multi‐Point Observations of the Geospace Plume. In Dayside Magnetosphere Interactions (eds Q. Zong, P. Escoubet, D. Sibeck, G. Le and H. Zhang). doi:10.1002/9781119509592.ch14

Hagan, M. E., K. Häusler, G. Lu, J. M. Forbes, and X. Zhang (2015), Upper thermospheric responses to forcing from above and below during 1–10 April 2010: Results from an ensemble of numerical simulations, J. Geophys. Res. Space Physics, 120, 3160–3174, doi:10.1002/2014JA020706.

Lu, G., Richmond, A. D., Lühr, H., and Paxton, L. (2016), High‐latitude energy input and its impact on the thermosphere, J. Geophys. Res. Space Physics, 121, 7108– 7124, doi:10.1002/2015JA022294.

Pedatella, N. M. (2016), Impact of the lower atmosphere on the ionosphere response to a geomagnetic superstorm, Geophys. Res. Lett., 43, 9383–9389, doi:10.1002/2016GL070592.

Qian, L., and J. Yue (2017), Impact of the lower thermospheric winter-to-summer residual circulation on thermospheric composition, Geophys. Res. Lett., 44, doi:10.1002/2017GL073361.

Wing, S., Johnson, J. R., Chaston, C. C., Echim, M., Escoubet, C. P., Lavraud, B., ... & Wang, C. P. (2014). Review of solar wind entry into and transport within the plasma sheet. Space Science Reviews, 184(1-4), 33-86. doi: 10.1007/s11214-014-0108-9