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Ionospheric Scintillation

Irregularly structured ionospheric regions can cause diffraction and scattering of trans-ionospheric radio signals. When received at an antenna, these signals present random temporal fluctuations in both amplitude and phase. This is known as ionospheric scintillation. Ionospheric scintillation may cause problems such as signal power fading, phase cycle slips, receiver loss of lock, etc., and degrade the quality of satellite navigation systems.

Figure 1 The scintillation of the satellite signals is due to the random fluctuations of the refractive index which distort the original wave front, giving rise to a random phase modulation of a wave. If the satellite and/or the ionosphere move relative to the receiver, temporal variations of amplitude and phase are recorded on the ground. The fluctuations in the refractive index are due to the irregularities in the ionosphere!

The ionospheric irregularities

The ionosphere can deviate from the expected behaviour, as for instance modelled by the Klobuchar model. This is the case when the ionosphere includes irregularities in which the electron density differs significantly from the “ambient” plasma. These irregularities can cause diffraction effects, i.e. scintillations, on the signals passing through them. The formation, evolution and dynamics of such irregularities are ruled by the interplay between the geomagnetic field, the Interplanetary Magnetic Field (IMF) and the solar wind (that is the emission of energetic particles coming from the Sun).

Figure 2 typical comet shape of the geomagnetic field lines (compressed in front of the Sun and tailored in the back front), caused by the interaction between the solar wind and geomagnetic field. The geomagnetic field acts as a shield protecting the terrestrial atmosphere from the solar wind. The weak points of the shield are around the poles where the solar wind particles have direct access

Over the equator the geomagnetic field lines are almost horizontal in respect to the Earth surface, while at polar and auroral latitudes they are almost vertical. This strong characterization in relation to the geomagnetic field configuration makes these regions the most affected by the formation of the ionospheric irregularities potentially dangerous for positioning systems.

Figure 3 The occurrence of L band scintillation reported during high and low solar activity (Basu, S. et al., J. Atmos. Terr. Phys, v.64, pp. 1745-1754, 2002)

The scintillation indices

The widely used ionospheric scintillation indices S4 and σΦ give an indication of the intensity of amplitude and phase scintillation affecting GNSS receivers.
The amplitude scintillation index is called S4 and it is the standard deviation of the intensity of the received signal over a certain interval, typically over 60 seconds.

Figure 4 One hour behaviour of S4 for all satellites in view (in different colors) recorded during a geomagnetic storm that occurred at the end of October 2003. ISACCO data are accessible from the eSWua system.

The phase scintillation index is called sF and it is the standard deviation of the detrended phase of the carrier frequency over a certain interval, typically over 60 seconds.

Figure 5 One hour behaviour of σΦ for all satellites in view (in different colors) recorded during a geomagnetic storm that occurred at the end of October 2003. ISACCO data are accessible from the eSWua system.

The ionospheric scintillation monitoring

To monitor transient effects like scintillations it is necessary to rely on high rate sampling GPS receivers. Space weather must include scintillations real time monitoring, this is the reason why INGV is managing a network of specially modified dual-frequency GPS receivers for Ionospheric Scintillation and TEC Monitoring (GISTM) since 2003 (ISACCO project).
The GISTM system is based on a Novatel GSV4004B receiver with the AJ System firmware, able to provide scintillation indices from L1 frequency and TEC from L1 and L2 (minute data) and 50 Hz (20 ms) raw data. Currently the network counts on three GISTM receivers at Svalbard (Norway), and two in Antarctica at MZS (74.7° S, 164.1° E) and on the Antarctic plateau at Concordia Station (Dome C, 75.1° S, 123.4° E). The first GISTM was deployed in September 2003 at the Italian Arctic station “Dirigibile Italia” in Ny Alesund (79.9° N, 11.9° E, Svalbard, Norway).

Figure 6 The GISTM acquisition system at Concordia Station (Antarctica)
Figure 7 The GISTM antenna installation at Mario Zucchelli Station (Antarctica)
Figure 8 Scintillation studies supported by the multi-instrument approach. This figure shows the snapshot of topographic reconstruction at different times of which scintillation phase peaks are overlapped (white dots). From these images we have further confirmation of the crucial role played by TEC gradients on scintillation effects as they are observed on the edge of regions of high TEC. (De Franceschi G., L. Alfonsi, V. Romano, M. Aquino, A. Dodson, C. N. Mitchell, A. W. Wernik , Journal of Atmospheric and Solar-Terrestrial Physics, 2008)

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