New method in computer simulations of electron and ion densities and temperatures in the plasmasphere and low-latitude ionosphere
A new theoretical model of the Earth’s low- and mid-latitude ionosphere and plasmasphere has been developed. The new model uses a new method in ionospheric and plasmaspheric simulations which is a combination of the Eulerian and Lagrangian approaches in model simulations. The electron and io...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2003-07-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/21/1601/2003/angeo-21-1601-2003.pdf |
| Summary: | A new theoretical model
of the Earth’s low- and mid-latitude ionosphere and plasmasphere has been
developed. The new model uses a new method in ionospheric and plasmaspheric
simulations which is a combination of the Eulerian and Lagrangian approaches in
model simulations. The electron and ion continuity and energy equations are
solved in a Lagrangian frame of reference which moves with an individual parcel
of plasma with the local plasma drift velocity perpendicular to the magnetic
and electric fields. As a result, only the time-dependent, one-dimension
electron and ion continuity and energy equations are solved in this Lagrangian
frame of reference. The new method makes use of an Eulerian computational grid
which is fixed in space co-ordinates and chooses the set of the plasma parcels
at every time step, so that all the plasma parcels arrive at points which are
located between grid lines of the regularly spaced Eulerian computational grid
at the next time step. The solution values of electron and ion densities <i>Ne</i>
and <i>Ni </i>and temperatures <i>Te</i> and <i>Ti</i> at the Eulerian
computational grid are obtained by interpolation. Equations which determine the
trajectory of the ionospheric plasma perpendicular to magnetic field lines and
take into account that magnetic field lines are "frozen" in the
ionospheric plasma are derived and included in the new model. We have presented
a comparison between the modeled <i>N</i>mF2 and <i>h</i>mF2 and <i>N</i>mF2
and <i>h</i>mF2 which were observed at the anomaly crest and close to the
geomagnetic equator simultaneously by the Huancayo, Chiclayo, Talara, Bogota,
Panama, and Puerto Rico ionospheric sounders during the 7 October 1957
geomagnetically quiet time period at solar maximum. The model calculations show
that there is a need to revise the model local time dependence of the
equatorial upward <i><b>E × B</b></i> drift velocity given by Scherliess and
Fejer (1999) at solar maximum during quiet daytime equinox conditions.
Uncertainties in the calculated <i>Ni , Ne , Te</i> , and <i>Ti</i> resulting
from the difference between the NRLMSISE-00 and MSIS-86 neutral temperatures
and densities and from the difference between the EUV97 and EUVAC solar fluxes
are evaluated. The decrease in the NRLMSISE-00 model [O]/[N<sub>2</sub>] ratio
by a factor of 1.7–2.1 from 16:12 UT to 23:12 UT on 7 October brings the
modeled and measured <i>N</i>mF2 and <i>h</i>mF2 into satisfactory agreement.
It is shown that the daytime peak values in <i>Te</i> , and <i>Ti </i>above the
ionosonde stations result from the daytime peak in the neutral temperature. Our
calculations show that the value of <i>Te</i> at F2-region altitudes becomes
almost independent of the electron heat flow along the magnetic field line
above the Huancayo, Chiclayo, and Talara ionosonde stations, because the
near-horizontal magnetic field inhibits the heat flow of electrons. The
increase in geomagnetic latitude leads to the increase in the effects of the
electron heat flow along the magnetic field line on <i>Te</i> . It is found
that at sunrise, there is a rapid heating of the ambient electrons by
photoelectrons and the difference between the electron and neutral temperatures
could be increased because nighttime electron densities are less than those by
day, and the electron cooling during morning conditions is less than that by
day. This expands the altitude region at which the ion temperature is less than
the electron temperature near the equator and leads to the sunrise electron
temperature peaks at <i>h</i>mF2 altitudes above the ionosonde stations. After
the abrupt increase at sunrise, the value of <i>Te</i> decreases, owing to the
increasing electron density due to the increase in the cooling rate of thermal
electrons and due to the decrease in the relative role of the electron heat
flow along the magnetic field line in comparison with cooling of thermal
electrons. These physical processes lead to the creation of sunrise electron
temperature peaks which are calculated above the ionosonde stations at <i>h</i>mF2
altitudes. We found that the main cooling rates of thermal electrons are
electron-ion Coulomb collisions, vibrational excitation of N<sub>2</sub> and O<sub>2</sub>,
and rotational excitation of N<sub>2</sub>. It is shown that the increase in
the loss rate of O<sup>+</sup>(<sup>4</sup>S) ions due to the vibrational
excited N<sub>2</sub> and O<sub>2</sub> leads to the decrease in the calculated
<i>N</i>mF2 by a factor of 1.06–1.44 and to the increase in the calculated <i>h</i>mF2,
up to the maximum value of 32 km in the low-latitude ionosphere between –30
and +30° of the geomagnetic latitude. Inclusion of vibrationally excited N<sub>2</sub>
and O<sub>2</sub> brings the model and data into better agreement.<br><br><b>Key words. </b>Ionosphere (equatorial
ionosphere; electric fields and currents, plasma temperature and density; ion
chemistry and composition; ionosphere-atmosphere interactions; modeling and
forecasting) |
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| ISSN: | 0992-7689 1432-0576 |