عنوان مقاله [English]
The Earth’s ionosphere is one of the important layers of the atmosphere, starting from 60 kilometers extending up to about 1000 kilometers. Even though the layer contains less than 1% of the total mass of the atmosphere; however, it has very important effects on the solar radiation and transmission of radio waves. The ionosphere is formed under effect of solar extreme ultraviolet (EUV), solar X-ray radiation, and electron precipitation of solar winds. The lower atmosphere also contributes to the variability of the ionosphere. In other words, ionosphere is under effects of both lower atmosphere dynamics from below and solar radiation from above boundary. Therefore, the amount of changes of particles in the ionosphere depend largely on many parameters such as time, radiation pattern, Sun-Earth geometry, ion chemistry, and solar activity. Sun’s variability is most important origin of the ionosphere long term change, so that the amount of change in the ionosphere layers depends largely on the time and mode of radiation, the Earth-Sun status, and the solar activity. Variety of periodic and nonperiodic variations should be considered in the ionosphere, which makes serious impacts on satellite and ground communication, precise navigation and radio broadcasting. In this research, the relationship between solar activity and total electron content (TEC) is investigate with gridded global TEC data.
Total electron content (TEC) data are important ionosphere parameter that can be derived from time delay of radio wave transmitted from satellite to ground base station. The maps of TEC are given with the resolution of 5o in longitude and with the resolution of 2.5o in latitude, 12 times every day (one map in every two hours at UTC time). In the other word, each IONEX file includes 13 maps in which one map has overlap with next day. Longitude ranges from -180 to 180 degrees that includes 73 points resolution. Latitude ranges from -87.5 to 87.5 degrees that indicates 71 points with the resolution of 2.5o. We use F10.7 index for determination of solar activity. This parameter is indicative of radio emittion of sun in 10.7 cm radio wave. This index has a good correlation with sun spot number and nowadays it is used in many research as solar activity parameter.
In this investigation, we used 19-years data IONEX for the period 1999 – 2017 for both 23 and 24 solar cycles. At first, we calculated zonal mean of data (in all longitudes, for every latitude) every day, then for every month and finally for every year. We had the mean data of TEC for every day, month and year of these 19 years. In the 23 solar cycle that began in August 1996 and continued to December 2008, minimum amount of TEC was 14 TECU in 2008, and maximum amount was about 57 TECU in 2000 and 2002. In this solar cycle, the time gap between minimum and maximum was 6 to 8 years. In the current solar cycle, solar cycle 24, with minimum amount of TEC was 14 TECU in 2009 and with maximum 44 TECU in 2014. The time gap between both extremes was about 5 years. In all years, maximum amount of TEC was in low and middle latitude, and minimum was in high latitude. Results indicated that maximum TEC was in southern hemisphere in December, January and February. In June, July and August, maximum TEC is located in northern hemisphere. Maximum amount of TEC was in March, April and October, and minimum was in June, July and August. It seems that maximum position and value depend on solar declination, Earth-Sun position and geometry.