Geoscience Reference
In-Depth Information
4.3.4 Irregular Temporal Variations
Ionospheric storms are large scale disturbances of the ionospheric structure and
dynamics caused by a Coronal Mass Ejection (CME) from the sun. The storm is usu-
ally initiated by a huge solar flare followed by several coronal mass ejections on the
subsequent days. The strong enhancements of the solar wind energy generate large
perturbations in the high-latitude ionosphere and thermosphere, resulting in signif-
icant variability of the plasma density, which commonly propagates towards lower
latitudes (e.g. Förster and Jakowski (
2000
), and Ho et al. (
1996
)). The ionospheric
storm can increase the TEC by more than 10 Total Electron Content Units (TECU)
(Feltens et al.
2009
).
Traveling ionospheric disturbances (TID) are wave shape plasma density fluctu-
ations propagating through the ionosphere at different range of velocities and fre-
quencies. TID are observable in most of the ionospheric measurements (e.g. Faraday
rotation, VLBI, GPS, and Incoherent Scatter-Radar). TID can change the value of the
TEC in the region of their occurrence in the range of several percent (Schaer
1999
).
Three types of TID are defined: large- (LSTID), medium- (MSTID) and small-scale
(SSTID). LSTID present a period of 30-180min and move at about 300m/s. LSTID
are related to geomagnetic activities and the Joule effect in high latitudes, producing
thermospheric waves towards lower latitudes. MSTIDmove slower (50-300m/s) and
have shorter periods (ranging from 10min to 1h). The origin of MSTID is related to
meteorological phenomena like neutral winds or solar terminators, which produce
atmospheric gravity waves revealing TID at different ionospheric heights. Table
6
lists the main classifications of TID. According to Hobiger (
2005
), TID occur mostly
in midlatitudes and are more evident close to solar maximum (Hernández-Pajares
et al.
2006
).
Scintillations describe the irregular variations of the amplitude or the phase of a
radio signal received after passing through, or being reflected by the ionosphere. A
strong scintillation can typically last for periods of up to several hours in the evening,
broken up with varying intervals of time with no scintillation. The most severe scintil-
lation effects are observed at and near the equatorial regions (Goodman and Aarons
1990
). However, the times of strong scintillation effects are generally limited to
approximately 1h after local sunset to local midnight except some exceptions. The
occurrence of strong scintillation is closely related to the sunspot number. During
the years of maximum solar activity, strong scintillation effects can be observed
in the equatorial and low-latitude regions. In the months from September through
March, chances are high for significant scintillation in the American, African, and
Indian longitude regions. However, in the Pacific region, scintillation effects maxi-
Ta b l e 6
Traveling ionospheric disturbances
Classification
LSTID
MSTID
SSTID
Horizontal wavelentgh (km)
>
1000
>
100
>
10
Period (min)
30-180
10-60
>
1
Phase velocity (ms
−
1
)
300-1000
100-300
-
