4. Monthly and seasonal variability

#### 4.1. Solar cycle 23

Figure 11. Monthly mean diurnal GPS TEC for cGPS stations with increasing latitude during solar cycle 24 for minimum

Figure 12. Monthly mean diurnal GPS TEC for cGPS stations with increasing latitude during solar cycle 24 for ascending

solar activity year 2009.

200 Multifunctional Operation and Application of GPS

solar activity year 2011.

Monthly diurnal mean values of TEC are plotted from November 2004 to December 2005 for all the stations between geomagnetic equator and northern crest of EIA (0–17 N geomagnetic latitude) in Figure 9 and beyond EIA region in Figure 10. The highest peak TEC values for all the sites in EIA region distinctly occur in November 2004 with a value of about 86 TECU for CSOS, 70 TECU for Bhopal to about 58 TECU for Kodaikanal. The lowest peak TEC values occur during the months of June, July, August for these sites with about 50 TECU at CSOS, 43 TECU for Bhopal to 35 TECU for Bengaluru. The highest peak TEC value during winter solstice and the lowest peak TEC value during summer solstice are due to seasonal anomaly prevalent in winter hemisphere due to increase in electrons caused by meridional neutral winds. For the sites beyond the EIA region (Figure 10), the highest peak TEC values (35–50 TECU) occur in the summer equinox months (March, April, May) and winter solstice month of November 2004 and the lowest peak TEC values (20–30 TECU) occur in January.

#### 4.2. Solar cycle 24

Monthly diurnal mean TEC values are plotted from January to December for low solar activity period of 2009 (Figure 11) and ascending solar activity period of 2011 (Figure 12). Monthly and seasonal cycle is not very pronounced during the low solar activity period of 2009 with marginally higher peak values recorded during October for IISC (35 TECU) and March for GHTU (40 TECU) and lower peaks during January (20–30 TECU) in the EIA region. For the ascending solar activity period of 2011, the monthly and seasonal variation is distinct with the highest (80 TECU) in EIA region during October and November and the lowest during January (20–30 TECU) consistent with the winter anomaly observed in the northern hemisphere.
