The space weather
The term "Space weather" refers to the conditions on the Sun and in the solar wind, magnetosphere, ionosphere and thermosphere that affect the performance and the reliability of technological systems on Earth and in space. Space Weather processes can include changes in the interplanetary magnetic field, coronal mass ejection from the sun, and disturbances in the Earth’s magnetic field. With the increased sunspot activity still going on, interest in the possibility of severe space weather exits. Many organizations around the world are now actively engaged in monitoring, modeling and predicting the space weather. Violent eruption in the sun’s outer atmosphere on November 8, 2000, spewed billions of tons of charged particles towards the earth and caused severe turbulence in the space around the earth (James, 2001). One of the consequences of severe space weather is often the disruption to satellite communications and even disruption of power grid on the Earth. GPS navigation accuracy is greatly affected by the space weather, as it is particularly sensitive to changes in the electron density in the ionosphere and described in the following section. The ionosphere under the purview of the GPS studies is shown in Fig.1.
Fig.1. The Earth ’s ionospheric environment within the purview of GPS measurements
Ionosphere and its estimation
The UV radiation from sun ionizes a fraction of Earth’s neutral atmosphere. This ionized environment, which envelops the Earth at the height of 70-1000 km and forms an interface between the atmosphere and space is called ionosphere. At mid-latitudes, the electron density, temperature and magnetic field strength are 10
5 per cc, 10
3 K and 10
4. nT respectively (Baumjohann and Treumann, 1977).
The ionosphere is divided into D (50-90 km), E (90-140km ) and F ( 140-600 km) layers based on the electron density and ion composition, and their variability. The electron density is maximum at F layer and the density profile with height is shown in Fig.2 (in red color). During daytime the F layer bifurcates in F1 and F2 layers. During night, the D and F1 layers vanish and the E region becomes much weaker. Beyond 1000 Km, the ionosphere thinning into the plasmasphere (or protono- sphere), magnetosphere and subsequently the inter-planetary plasma (Fig 1). Study of the ionospheric behavior is of scientific interest as it can facilitate improving ionospheric communication. Some of the traditional methods such as Incoherent Scatter Radars (also called Thomson-Scattering-Sounding) and Ionosondes (ionospheric sounder) and satellite based techniques such as Faraday rotation and GPS are used for ionospheric estimation.
As the ionosphere is dispersive in nature, the delay in the GPS signals is proportional to the inverse of the squared frequency and directly proportional to the refractive index of Total Electron Content (TEC) i.e. the free electrons in a column of 1 m2 cross sectional area centered on the signal path (Fig. 2). The TEC in turn depends on the geographic latitude, longitude, local time, season, geomagnetic activity and viewing direction. By forming the linear combination of the GPS measurement on L1 and L2 frequencies, the TEC can be estimated, either by using GPS carrier phase or pseudo-ranges observables. The TEC estimated by the carrier phase values will be less noisy than that from pseudo-ranges, but biased by the carrier phase integer ambiguity. The TEC from pseudo-ranges is absolute and used to fix the absolute level of the carrier phase TEC (called phase leveling). Some of these aspects have been reviewed by Klobuchar (1991). And the methods used to obtain the TEC are given by (Lanyi and Roth, 1988).
Fig. 2 The ionosphere is represented as thin shell. The variation of electron
density shown in red color and the peak value represent the F layer
(show as yellow line) (Fedrizzi et al, 2002)
