Scientific objectives of my researching activity are interactions between plasmas and neutral particles in the polar ionosphere/thermosphere, in particular, energy-dissipation processes associated with auroral activities. Two recent projects are presented here.
PROJECT // 01
3D-current system and thermospheric winds near auroral arc
Aurora is emissions from excited particles by energetic electrons came from the magnetosphere downward (to the earth) along the geomagnetic field line. Since motion of the electron is current (but with opposite direction), we have tried to understand aurora and related physics with electromagnetic dynamics and current system. The current system plays important roles for energy dissipations as Joule heating and for momentum transfer as Lorentz force. These energies and forces are dissipated into the thermosphere, accelerating winds and changing the temperature and density.
Panel shown above is a snap shot of auroral arc taken with a digital camera on the ground (so that is a horizontal pattern of the arc). A cartoon is overlaid as an example of the current system near the arc. In the bright part of the arc, the field-aligned current (FAC) flows upward due to downward motion of the energetic electrons. In the dark part slightly above the arc (in this case poleward), FAC flows downward due to upward motions of ionospheric electrons. Horizontal currents (or Pedersen current) flows between them to close the current circuit in the ionosphere due to motion of ions by ExB force (E: electric field, B: magnetic field). Since ions of the Pedersen current move in partially-ionized plasmas, the ions collide with neutral particles many times. So momentum energy of ions is converted to thermal energy and momentum energy of the thermosphere. These energy conversions cause various kinds of signatures in the upper atmosphere. Physical mechanisms of these signatures are not well understood yet.
PROJECT // 02
pulsating aurora and energy dissipations in the thermosphere
Pulsating aurora (PsA) is a typical auroral morphology appeared after substorm onset. The emission intensity oscillates at periods of 3-10 seconds and ~0.3 seconds with sometimes drifting eastward. The emission-intensity oscillation is caused by fluctuations of high-energetic electrons (> 10 keV). The eastward drifting is a mirror motion of the magnetospheric dynamics. While understanding of the magnetosphere has been integrated by many researching activities, there are many outstanding questions about the ionospheric phenomena.
In general the Pedersen current in the pulsating aurora (PsA) is not large to heat up the thermosphere by the Joule heating process. So many researchers have thought that thermosphere in PsA does not change very much. However, our measurements with a Fabry-Perot interferometer found significant wind perturbations in PsA. Of particular interest is location that the perturbations are found only in a darker area appeared in PsA.