5. Summary

For the first time an in-depth study of ionosphere variability in low and mid-latitude region using TEC estimates from 24 cGPS stations for a 14-year (2002–2016) period is carried out. The cGPS data covers peak (2002–2004), descending phase (2005–2008) of solar cycle 23 and minimum (2008–2010), ascending phase (2011–2016) of current solar cycle 24.

Inter-annual variability of GPS-TEC depicts the peak, descending phase of solar cycle 23 and minimum, ascending phase of solar cycle 24. Maximum TEC values are observed during 2002– 2004 and minimum TEC values are observed during 2008–2010. GPS TEC indicates a distinct daily, monthly, semi-annual and annual cycle. Sensitivity of TEC to solar activity is prominent in EIA region compared to mid-latitudes in northern hemisphere. TEC values recorded are consistent with large-scale electrodynamics associated with the equatorial electrojet (EEJ), plasma fountain, equatorial ionization anomaly (EIA), equatorial wind and temperature anomaly, which affect the ionosphere variability at equatorial and low latitude regions. The high variability of equatorial and low latitude ionosphere are due to the perfect horizontal alignment of the geomagnetic field lines at the dip equator and the shifting between the geographic and geomagnetic equator.

month of March. Maximum spread of diurnal maxima is observed in crest of EIA during the equinox month of September. The results indicate that the variability of diurnal TEC in low-

Ionosphere Variability in Low and Mid-Latitudes of India Using GPS-TEC Estimates from 2002 to 2016

http://dx.doi.org/10.5772/intechopen.74172

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Monthly diurnal mean TEC values are the highest in November and the lowest in the months June to August for solar cycle 23 and increase with latitude in the EIA region. This is due to the winter anomaly observed in the EIA region of northern hemisphere and is consistent with previous studies. Beyond EIA region, the high values are observed in the summer equinox months and November and minimum values occur during January. For the current solar cycle 24, the monthly and seasonal variability is marginal for the low solar activity year (2009) when compared to 2011. In the EIA region, the highest values are recorded during October-November and the lowest during January for ascending phase (2011) of current solar cycle 24. The seasonal and monthly variation is random depending upon the intensity

In summary, the temporal and spatial variability of equatorial, low and mid-latitude ionosphere reported using the GPS-TEC estimated from new GPS data during 2002–2016 are broadly consistent with previous studies globally and specific to the Indian subcontinent. When compared to previous studies, present study with longer data span and spatial spread gives significant insights into the randomness of day-to-day variability of ionosphere as detailed above. This high and random variability of TEC is due to the changes associated with solar activity, intensity of the sun radiation and zenith angle at which they impinge the earth's atmosphere. TEC variability on quiet days depends on the changes in Earth's magnetic field and EEJ strength. In equatorial and low-latitude region of Indian subcontinent there is intense east–west electric current (EEJ) due to neutral winds and the plasma flow associated with the EIA plays a significant role in the day-to-day variability of diurnal TEC. Ionosphere is also affected by solar and geomagnetic storms, solar eclipse, seismic disturbances, volcanic eruptions, tsunamis, and so on. Indian Space Research Organisation in collaboration with Airports Authority of India developed a model to predict TEC in the Indian region which can be used to provide TEC maps. They have used GAGAN (GPS Aided Geo Augmented Navigation) ground network of 18 stations for this model and predict TEC between 8 and 30 N latitude and 60–100 E longitude. Since the present study uses a new set of cGPS data for a 14 year period, benchmarking ISRO ionosphere model with the current data and combining with the current TEC estimates would give an opportunity to develop precise ionosphere models and maps for this region. In addition these GPS-TEC estimates can be used to model the spatial and temporal variability of the low and mid latitude ionosphere specific to Indian subcontinent. GPS TEC study has several applications in varied fields such as precise positioning, navigation, seismo-ionosphere coupling, propagation of radio waves and solar-terrestrial events.

This is a CSIR-4PI ARiEES contribution. We thank the anonymous reviewers for their time and

latitude region is highly random as it is caused by several factors as detailed earlier.

of solar cycle and seasons in each year.

Acknowledgements

effort.

GPS-TEC values increase from geomagnetic equator to the crest of EIA region (17 N geomagnetic latitude) after which they gradually decrease toward mid-latitudes in the northern hemisphere. Latitude variability of ionosphere is more pronounced during the high solar activity years (2002–2004) when compared to low solar activity years (2008–2010). Diurnal peak TEC value has longer duration between 0 and 9 N geomagnetic latitude. Diurnal maxima have pronounced peaks and diurnal minima is observed for longer duration in the northern crest of EIA region and beyond. Ionosphere variability with longitude is observed for longitude difference of 19 E and above during the ascending phase of current solar cycle 24. Normally, solar radiation strikes the atmosphere more obliquely with increasing latitude decreasing its intensity and production of free electrons, whereas near the geomagnetic equator its strikes horizontally with eastward electric field during day and westward during night. This causes plasma diffusion along magnetic field lines at approximately 15 geomagnetic latitudes forming crests on both the hemispheres (EIA region). Hence, TEC increases gradually from geomagnetic equator to the EIA crest, beyond which it decreases toward the mid-latitude regions. Intensity of EIA and its latitude of crest development vary with the strength of EEJ, season and solar activity. Our study indicates that the northern crest of EIA region extends up to about 17–18 N geomagnetic latitude in Indian region.

Diurnal variability of ionosphere depends on the intensity of solar activity, season and strength of geomagnetic field with high TEC values recorded in 2004 and 2011 and low values in 2009. Day-to-day variability is more pronounced for the high solar activity years when compared to low solar activity years. Maxima occurs during midday (7–13 h UT) with longer duration for geomagnetic latitudes between 0 and 9 N and pronounced peaks for greater than 9 N. Minima occurs after midnight (20–24 h UT) between 0 and 9 N geomagnetic latitude whereas it is for longer duration (15–24 h UT) for northern crest of EIA region and beyond. Day-to-day variability of maxima is more pronounced in the crest of EIA regions (9–18 N geomagnetic latitude). Day-to-day variability of diurnal TEC is high and random during December 2004 and September 2011 due to seismo-ionosphere disturbance caused by 2004 Sumatra and 2011 Sikkim earthquake. Also anomalous day-to-day TEC variability is observed for Gujarat stations (MABU, KHAV, BELP) in 2011 which needs further detailed study. Diurnal maxima and minima vary significantly during the equinox and solstice of summer and winter seasons with lower values during summer solstice in EIA region and higher values during equinox and winter solstice. Beyond EIA (>18 N), maxima with pronounced peak occurs in the equinox month of March. Maximum spread of diurnal maxima is observed in crest of EIA during the equinox month of September. The results indicate that the variability of diurnal TEC in lowlatitude region is highly random as it is caused by several factors as detailed earlier.

Monthly diurnal mean TEC values are the highest in November and the lowest in the months June to August for solar cycle 23 and increase with latitude in the EIA region. This is due to the winter anomaly observed in the EIA region of northern hemisphere and is consistent with previous studies. Beyond EIA region, the high values are observed in the summer equinox months and November and minimum values occur during January. For the current solar cycle 24, the monthly and seasonal variability is marginal for the low solar activity year (2009) when compared to 2011. In the EIA region, the highest values are recorded during October-November and the lowest during January for ascending phase (2011) of current solar cycle 24. The seasonal and monthly variation is random depending upon the intensity of solar cycle and seasons in each year.

In summary, the temporal and spatial variability of equatorial, low and mid-latitude ionosphere reported using the GPS-TEC estimated from new GPS data during 2002–2016 are broadly consistent with previous studies globally and specific to the Indian subcontinent. When compared to previous studies, present study with longer data span and spatial spread gives significant insights into the randomness of day-to-day variability of ionosphere as detailed above. This high and random variability of TEC is due to the changes associated with solar activity, intensity of the sun radiation and zenith angle at which they impinge the earth's atmosphere. TEC variability on quiet days depends on the changes in Earth's magnetic field and EEJ strength. In equatorial and low-latitude region of Indian subcontinent there is intense east–west electric current (EEJ) due to neutral winds and the plasma flow associated with the EIA plays a significant role in the day-to-day variability of diurnal TEC. Ionosphere is also affected by solar and geomagnetic storms, solar eclipse, seismic disturbances, volcanic eruptions, tsunamis, and so on. Indian Space Research Organisation in collaboration with Airports Authority of India developed a model to predict TEC in the Indian region which can be used to provide TEC maps. They have used GAGAN (GPS Aided Geo Augmented Navigation) ground network of 18 stations for this model and predict TEC between 8 and 30 N latitude and 60–100 E longitude. Since the present study uses a new set of cGPS data for a 14 year period, benchmarking ISRO ionosphere model with the current data and combining with the current TEC estimates would give an opportunity to develop precise ionosphere models and maps for this region. In addition these GPS-TEC estimates can be used to model the spatial and temporal variability of the low and mid latitude ionosphere specific to Indian subcontinent. GPS TEC study has several applications in varied fields such as precise positioning, navigation, seismo-ionosphere coupling, propagation of radio waves and solar-terrestrial events.
