IONOSPHERIC RESPONSE TO SOLAR PHENOMENA AND THUNDERSTORMS

Abstract

The region lying from 65-70 km to about 1000 km altitude above the earth surface containing free electrons and ions is known as ionosphere. The equatorial ionosphere is formed primarily by the ionization of neutral gas atoms/molecules present in the upper atmosphere by exposure to solar radiation. The free thermal electrons produced by high-energy photoelectron collision with neutral atoms, form more than 90% of the flux in the ionosphere. Many modern technological systems are affected by ionospheric phenomena. The ionospheric weather plays an important role in communication, navigation, exploration of near earth space and even exploration of deeper part of earth's interior. Natural electromagnetic fields generated by the interaction of solar wind, magnetosphere and ionosphere as well as generated by thunderstorms and lightning/sprites are affected by the ionospheric weather. The natural electromagnetic fields in the frequency range from 104 Hz to 10"4 Hz are used as a source in magnetotelluric method of geo-exploration. Understanding the role of ionosphere in generating, propagating and attenuating these fields are crucial for an effective exploration program using magnetotelluric currents. The weather disturbances like thunderstorms alter the ion-electron production rate and hence change the ionospheric temperature and density. The thunderstorms are the main source of lightning and sprites. The sprites propagating from cloud top to the ionosphere generate the radiations from ULF to VLF, which in turn propagate still upwards and heat the local plasma. Further, the solar phenomena such as solar flares, coronal mass ejection etc. also play an important role in changing the ionospheric temperatures and ion density. Recently it has been observed that the electromagnetic flux associated with the seismic activity also affects the ionospheric phenomena. Present work is devoted to the study of ionospheric electron and ion temperature variations in the low latitude F2 region (altitude ranges 425-625 km) over India during the period from 1995 to 1998 (which covers the 1995-96 solar minimum year). The average diurnal, seasonal and latitudinal behavior of electron and ion temperatures has been studied in details. The study also includes the ionospheric response to the sunrise, solar flares and active thunderstorms. The data for these studies have been obtained by Retarding Potential Analyzer (RPA) payload aboard SROSS-C2 satellite during the period from 1995 to 1998. Thunderstorm data were obtained from India Meteorological Department (IMD) and that on Solar flares from National Geophysical Data Center (NGDC), Boulder, Colorado, U. S. A. The International Reference Ionosphere (IRI) model values for the same period were used for comparison. The present thesis is a systematic presentation of the work done on the behavior of ionospheric temperatures, the diurnal, seasonal and latitudinal variations and their response to thunderstorms and solar phenomena. Some studies on tropospheric aerosols in relation to meteorological parameters have also been carried out and the same has been included as an appendix. A brief history of ionospheric studies has been presented in Chapter-I. Work done on the variations in ionospheric parameters using another satellite data has been reviewed. A Japanese satellite (Hinotori) which had a near circular orbit of -600 km provided an ideal database for the study of the temporal and spatial variation of electron temperature in the topside ionosphere. The descriptions of the solar phenomena, thunderstorms, lightning/sprites and their electromagnetic fields have also been given in this chapter. Chapter-ll deals with the details of data used and their sources. It gives a brief description of RPA payload aboard SROSS-C2 satellite used in the measurement of ionospheric temperatures. The SROSS-C2 was launched by ISRO on May 4, 1994. It was successfully operated continuously for seven years and on July 12, 2001 returned to the earth. The IRI model has also been discussed briefly. Diurnal, seasonal and latitudinal variation of ionospheric temperatures of the topside F region ionosphere over India during the solar minimum year (1995- 96) at an average altitude of ~500 km have been presented in Chapter-Ill. The measured electron and ion temperature data have been analysed for three different seasons; summer (May, June, July and August), winter (November, December, January and February) and equinoxes (March, April, September and October) during the solar minimum year to study the diurnal, seasonal and latitudinal behaviour of electron and ion temperatures. This study reveals that the electron and ion temperatures have lowest values during night hours and show in variations during daytime in all seasons. The daytime electron temperature shows atleast two peak values with different magnitudes. The peak during sunrise hours is relatively sharp and high in magnitude and the peak during sunset hours are diffused and lower in magnitude. Similar variations in the ion temperature have also been observed. However, their amplitude and sharpness are smaller in comparison with the behaviour of electron temperature. The predicted values by IRI-95 model show a good agreement with electron and ion temperatures during night hours. The daytime electron temperature variation and sunrise peak values are under estimated by IRI-95 model and unable to produce secondary peaks during sunset. The electron and ion temperatures show a positive correlation with latitude during daytime over the geographical region under study. However, no latitudinal dependence has been observed during night hours at an altitude of -500 km. The sunrise effect on the ionospheric electron and ion temperatures as measured by the SROSS-C2 satellite during the period from 1995 to 1998 (which cover the solar minimum year of 1995-96) for specific location of Bhopal (23.16° N, 77.36° E) and Chennai (13.04° N, 80.19° E) in India have been presented in Chapter-IV. It has been found that the electron and ion temperatures are minimum during the local night hours and maximum at the local sunrise time. The nighttime electron temperature (Te) varies nearly from 800 to 1000 K and rises sharply at sunrise to 3000 K or more. The nighttime ion temperature (T) varies from nearly 600 to 800 K and also rises at sunrise to about 2000 K or more. IV Chapter-V has been dedicated to the study of the effect of thunderstorms on ionospheric temperatures. The study reveals that there is a consistent enhancement of electron and ion temperatures recorded during active thunderstorms over the normal day temperatures. It is worth mentioning that in the present analysis the data were selected in such a way that the effect of diurnal, seasonal, latitudinal, longitudinal and altitude variations are minimized. It has been concluded that the ionospheric temperature anomalies are directly related to the thunderstorm events. The enhancements in ionospheric electron and ion temperatures have been attributed to the lightning/sprites activity and the associated phenomena such as the radiations from ULF to VLF range, which in turn, may propagate still upward and heat the local plasma in the F region ionosphere. The effect of solar flares on ionospheric electron and ion temperatures over the topside F region has been presented in Chapter-VI. This study reveals that there is a consistent enhancement of the ionospheric electron and ion temperatures recorded during solar flares. The enhancement in case of electron temperature is slightly higher than the enhancement in ion temperature. The data selection and analysis shows that the enhancement of ionospheric temperatures can be related to the solar flares. The effect of solar flare on the nightside ionospheric temperatures has not been detected in the altitude range from 425 to 625 km. Chapter-VII summarizes the work done in the present thesis and the conclusions have been drawn on the basis of our studies. An Appendix has also been added to the thesis, which describes the work done on aerosol studies. It reviews the studies on aerosol generation, distribution and different mechanisms involved in aerosol formation and their relation to meteorological parameters. A brief description of the instrument used for aerosols study has also been given. The study of aerosols number concentration in relation to some meteorological parameters (relative humidity, temperature, wind speed and rainfall) during the period from April to July 1999 at Roorkee for different size ranges viz. 0.3-0.5 jam, 0.5-1.0 jam, 1.0-2.0 jxm and 2.0-5.0 |am has been presented.