Imaging the Equatorial Ionosphere over South America
Project: 2004
Researcher: Bettina Zapfe
The ionosphere is an ionized plasma above the Earth's
atmosphere that is subject to large diurnal and seasonal variations. In
particular, concentrations of electrons in the equatorial region are
highly variable. Movements of electrons at low latitudes were first
described by Sir. Appleton in 1946 and were called 'fountain effect'. An
electric force exists parallel to the equator that stems from surplus of
positive and negative charges at the dawn and dusk latitudes of the earth.
In addition, the Earths magnetic field is parallel to the surface of the
earth at the geomagnetic equator and near parallel at low latitudes. These
electromagnetic forces cause electrons, that increase in number after
sunrise, to rise above the geomagnetic equator. After rising the region of
high electron density moves along the magnetic field lines to both sides
of the equator to form two maxima at lower heights. It is difficult to
quantify ionospheric effects in a region of such high variability. The
Multiple Instrument Data Analysis Software, MIDAS, a tomographic imaging
software developed at Bath University, is used to model the ionosphere in
Brazil in South America. MIDAS computes a tomographic image based on the
locations of several receiving stations and a reference ionospheric model
or measured data. GPS signals have been recorded at twelve receiving
stations mainly located in southern and south-east Brazil. Two steps need
to be taken to quantify the electron concentrations and total electron
content in a horizontal plane: Firstly the quality of the image needs to
be analyzed to identify regions where tomographic images are reliable. A
reference model is used to study TEC errors in the horizontal plane and
errors in electron density in the vertical plane through the tomographic
modelling approach. Secondly, it is studied how suitable MIDAS is for
modelling the physical properties of the ionosphere. Measurement data
available and comparisons with results published in the literature allow
to access the quality of a three dimensional modelling approach with time
as additional parameter. Please find two movies below. The first shows TEC
in the Brazilian region, the second illustrates changes in electron
concentration in a vertical plane in Brazil over two days in an area of
high receiver density.
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