Geoscience Reference
In-Depth Information
techniques leading to the release of a number of several key editions of the model.
The latest version of the IRI model, IRI-2012 (Bilitza et al.
2011
), will include signif-
icant improvements not only for the representations of electron density, but also for
the description of electron temperature and ion composition. These improvements
are the result of modeling efforts, since the last major release, IRI-2007 (Bilitza and
Reinisch
2007
). IRI is an empirical model based on most of the available data sources
for the ionospheric plasma. The data sources of IRI include the worldwide network
of ionosondes, which is monitoring the ionospheric electron densities at and below
the F-peak since more than fifty years, the powerful incoherent scatter radars which
measure plasma temperatures, velocities, and densities throughout the ionosphere,
at eight selected locations, the topside sounder satellites which provide a global dis-
tribution of electron density from the satellite altitude down to the F-peak, in situ
satellite measurements of ionospheric parameters along the satellite orbit, and finally
rocket observations of the lower ionosphere. Since IRI is an empirical model it has
the advantage of being independent from the advances achieved in the theoretical
understanding of the processes that shape the ionospheric plasma. Nevertheless such
an empirical model has a disadvantage of being strongly dependent on the underlying
data base. Therefore regions and time periods not well covered by the data base will
result a lower reliability of the model in that area (Bilitza et al.
2011
).
The vertical electron density profile within IRI is divided into six sub-regions:
the topside, the F2 bottom-side, the F1 layer, the intermediate region, the E region
valley, the bottom-side E and D region. The boundaries are defined by characteristic
points such as F2, F1, and E peaks. The strong geomagnetic control of the F region
processes is taken into account for the analysis of the global electron density behavior
(Feltens et al.
2010
).
IRI has a wide range of applications. Among these applications, IRI has played
an important role in geodetic techniques as well. In several studies IRI has been
used as a background ionosphere in order to validate the reliability and accuracy
of an approach for obtaining ionospheric parameters from geodetic measurements
(e.g. Hernández-Pajares et al.
2002
). Another field which IRI has helped geodetic
techniques is with interpolating in areas with no or few available GPS measurements
(e.g. Orús et al.
2002
).
4.1.4 GAIM Model
In 1999 the Multidisciplinary University Research Initiatives (MURI) sponsored
by the U.S. Department of Defense developed the Global Assimilative Ionospheric
Model (GAIM). The GAIM model is a time-dependent, three-dimensional global
assimilation model of the ionosphere and neutral atmosphere (JPL
2011
). GAIM
uses a physical model for the ionosphere/plasmasphere and for assimilating real-
time measurements, it uses the Kalman filter approach. Within GAIM the ion and
electron volume densities are solved numerically using the hydrodynamic equations
for individual ions. The model is physical-based or first-principles based and includes
state of the art optimization techniques providing the capability of assimilating differ-
