The cold ion drag and its possible impact on the dayside magnetospheric convection

Cold ions (energy < a few 100's of eV) of ionospheric and plasmaspheric origin are ubiquitous in the magnetosphere. In situ observations of the very lowest energy portion of these populations (< ~10eV) are far from trivial since their energies tend to be comparable with sensitivity thresholds of particle detectors and/or the electrostatic potential of the spacecraft. The Fast Plasma Investigation (FPI) spectrometers onboard the Magnetospheric Multiscale Mission (MMS) are the first particle instruments to resolve the spatial / temporal variations of these populations at 150 milliseconds (S. Smith et al., 2019).

Cold ions are predicted to play a noticeable part in the dayside reconnection process by mass-loading the reconnection site and thus reducing the reconnection rate (J. Birn et l., 2008; J. Borovsky et al., 2008). Since these ions are more difficult to demagnetize, their contribution to local plasma kinetics can also modify reconnection triggering conditions while delaying the reconnection onset and introducing a new characteristic plasma scale associated with the cold ion diffusion region (S. Toledo-Redondo et al, 2016). Overall, the cold ions are believed to form a negative-feedback loop to the driving of the system (J. Borovsky, 2014): the stronger the solar wind driving, the more rapidly cold and dense plasma is transported toward the reconnection site where it suppresses and slows down the field line merging. The life span of the cold ions is typically much longer than the duration of a single magnetic storm (B. Walsh et al., 2013; J. Borovsky et al., 2014, resulting in a continuing presence of this population and its stabilizing effect on the dayside reconnection region over a broad range of geomagnetic disturbance levels.

In this talk, we present a new physical mechanism through which cold ions (predominantly protons) can provide a negative feedback on the Dungey substorm cycle. Using the data from MMS FPI, we demonstrate that the bulk acceleration of cold ion populations surrounding a magnetospheric flux transfer event (FTE) can create a significant convective drag draining energy from recently reconnected flux tubes and slowing down the dayside convection. The magnitude of the predicted cold ion drag is estimated for a representative set of FTE events detected during the first dayside phase MMS by several complementary techniques assuming laminar or turbulent ion motions. We find that the kinetic energy transfer describing bulk energization of cold ion populations by the ExB drift by an FTE can be comparable with the energy of the moving flux tube. Depending on the cold ion density in the dayside magnetosphere and the solar wind driving conditions, the FTE motion can be affected to varying degrees and in some cases be considerably deflected and/or decelerated.

The presented drag mechanism could play a significant role in the substorm convection cycle by increasing the duration of the loading phase of the substorm response and reducing the geoeffectiveness of the solar wind driver. It can also introduce a strong additional nonlinearity into the solar wind - magnetospsphere - ionosphere system not accounted for by existing coupling models, and enhance the dawn-dusk asymmetry of the loading process impacted by the geometry of the plasmaspheric drainage plume.