Analyzing Scintillation Using Ionospheric Asymmetry Index

Existence of random electron-density fluctuations cause scintillation of radio waves on trans-ionospheric links. Ionospheric irregularities that account for scintillation may vary significantly in spatial range and drift with the background plasma at speeds of 45 m/sec to more than 130 m/sec. These patchy irregularities may occur several times during night, e.g. in equatorial region, with the patches move through the ray paths of the GNSS satellite signals. These irregularities are often characterized as either `large scale' (which can be as large as several hundred km in East-West direction and many times that in the North-South direction) or `small scale' (which can be as small as 1m). In normal solar activity conditions, the mid-latitude ionosphere is not much disturbed. However, during severe magnetic storms, the aurora oval extends towards the equator and the equator anomaly region may be stretched towards poles extending the scintillation phenomena into mid-latitudes. In such stormy conditions, the predicted total electron content (TEC) may deviate largely from the true value of the TEC both at low and mid-latitudes due to which GNSS applications may be strongly degraded. This work is an attempt to analyze ionospheric scintillation using ionospheric asymmetry index. The asymmetry index is based on trans-ionospheric propagation between GPS and LEO in radio occultation (RO) scenario, using background ionospheric data provided by MIDAS. Using MIDAS, we attempt to simulate one of the recent geomagnetic storms (NOAA scale G4) occurred over low/mid-latitudes. The storm started on 26 September 2011 at UT 18:00 and lasted until early hours of 27 September 2011. It was observed that the level of asymmetry was significantly increased during the main phase of the geomagnetic storm. This happened due to a significant change in ionization, which in turn produced large gradients along occulted ray path in the ionosphere. A very good correlation was found between the evaluated ionospheric asymmetry index and the S4 scintillation index.