We report new modeling results that show the formation of an electron hole in the topside equatorial ionosphere. The reduction in the electron density occurs in the altitude range 1500 - 2500 km at geomagnetic equatorial latitudes. The electron hole is shown in Fig. 1 which is a colored contour plot of the logarithm of the electron density as a function of geographic latitude versus altitude at time 1900 LT. The hole shows up as a closed, dark blue contour at about 2000 km. The hole is produced by transhemispheric O+ flows that collisionally couple to H+ and transport it to lower altitudes, thereby reducing the electron density at high altitudes. The transhemispheric O+ flows are caused by an interhemispheric pressure anisotropy that can be generated by the neutral wind, primarily during solstice conditions. The formation of the electron hole has a seasonal and longitudinal dependence. This result has been found with a new low-latitude ionospheric model that has been developed at the Naval Research Laboratory: SAMI2 (Sami2 is Another Model of the Ionosphere).
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We present the results of three simulations that highlight the mechanism by which the high altitude electron hole is formed. The parameters used are the following:
- Day: 173 (June 22)
- Year: 1999
- Longitude: 293.4 E
(Arecibo)
- Ap: 2
- F10.7: 180
- F10.7A: 180
The zonal electric field is set to zero for these simulations. The reason for this is to focus on the hole formation mechanism; the zonal electric field does not cause the hole to form, although it does affect the time at which the hole forms. The simulation results are presented as MPEG animations.
1. Normal Ion-Ion Collision Frequency
The first simulation
(MPEG)
shows the time evolution of the electron
density as a function of latitude and altitude. Also shown is
the oxygen ion velocity, denoted by `wind flags'. Typical ion velcoities
above 1000 km are a few 100 m/sec.
The electron hole does not form until after
sunset (2000 LT) when the ionosphere falls. During the collapse
of the ionosphere a residual transhemispheric oxygen ion
flow persists in the altitude range 1500 - 2000 km that
`sweeps out' hydrogen ions. The hole persists until about
0100 LT.
2. Reduced Ion-Ion Collision Frequency
The second simulation
(MPEG)
is identical to that described above except
that the ion-ion collision frequency is reduced by a factor
of 10.
This acts to decouple the oxygen and hydrogen ions so
that the transhemispheric oxygen ion flows should be much
less effective in `picking up' hydrogen ions. In fact, this is
the case as seen in the simulation animation. The transhemispheric
oxygen ion flows are larger than in Simulation 1 because of
the reduced hydrogen ion drag, but no electron hole forms because
the hydrogen ions are not `swept out.'
3. Normal Ion-Ion Collision Frequency (Equinox Conditions)
The third simulation
(MPEG)
tests the hypothesis that if there
is no asymmetry in the ion pressure with respect to the
geomagnetic equator along the magnetic flux tube
then there should be no strong
transhemispheric flows and, hence, no electron hole.
The parameters used are the same in Simulation 1 but that
the geographic longitude is 23.4 E and the day is 265.
The geomagnetic equator is roughly the same as the
geographic equator, and the sun is overhead at the equator.
In this simulation we see that the ionosphere is reasonably
symmetric about the equator and that the oxygen ion flows
are much smaller than in simulation 1. The ion velocity
at high altitude is only 10's m/sec during the day.
After sunset the ionosphere falls but no hole forms as
anticipated.
Huba, J.D., G. Joyce, and J.A. Fedder, The Formation of an Electron Hole in the Topside Equatorial Ionosphere, Geophys. Res. Lett., 27, 181, 2000
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