P. K. Purohit

Ionospheric influences on GPS signals in terms of range delay

All the transionospheric signals interact with the ionosphere during their passage through ionosphere, hence are strongly influenced by the ionosphere. One of most important ionospheric effects on the transionospheric signals is the delay both in range and time. Under this investigation we have studied the variability of ionospheric range delay in GPS signals. To accomplish this study we have used the GPS measurements at a low latitude station, IISC Bangalore (13.02◦N, 77.57◦E) during January 2012 to December 2012. We studied the diurnal, monthly as well as seasonal variability of the range delay. We also selected five intense geomagnetic storms that occurred during 2012 and investigated the variability of delay during the disturbed conditions. From our study we found the diurnal variability of the range delay is similar to the diurnal pattern observed for the Total This is the e-book version of the article, published in Russian Journal of Earth Sciences (doi:10.2205/2015ES000555). It is generated from the original source file using LaTeX’s epub.cls class. Electron Content (TEC). The delay is maximum during the month of October while lowest delay is found to occur in the month of December. During summer season the range delay in GPS signals is less while the largest delay occurs during the equinox season. The variability of delay during the geomagnetic storms of 09 March 2012, 24 April 2012, 15 July 2012, 01 October 2012 and 14 November 2012 were also studied. All these geomagnetic storms belonged to intense category. We found that the value of delay is strongly increased during the course of geomagnetic storms.

Characteristic of Ionospheric foF2 and Solar Indices during the 23rd Solar Cycle over High Latitude Station, Syowa, Antarctica

The behavior and dynamics of ionosphere are completely dependent on the solar activity. We have investigated the long term variability of ionospheric parameter foF2 with corresponding changes in the solar activity during the 23rd solar cycle. The variation of the critical frequency of ionospheric foF2 at Syowa Station Antarctica, (69°S, 39°E) is examined with four different solar activity indices characterizing the long term variability of solar activity wise Flare Index, relative sunspot number (Rz), solar flux F10.7 cm and CME occurrence index. We compared the dependency of foF2 and other solar activity indices on each other by using linear correlation investigation, and showed the qualitative similarity of the ionospheric foF2 with the solar indices. We notice that hysteresis of foF2 is lower for the growing branches of the solar cycle. The individual dissimilarity of critical frequency foF2 demonstrated the dependency on the solar cycle but this variation was different during the months, which depended on solar activity and polar ionospheric behavior. The peak to peak variation between monthly average of critical frequency foF2 and solar indices parameter is evidence for the absolute dependency for each other. The linear correlation between the solar indices and foF2 is very strong during the climbing and downward branches of the solar cycle. However the incline of their linear fits shows variations from index to index.

Evaluation of geomagnetic storm effects on the GPS derived Total Electron Content (TEC)

The geomagnetic storm represents the most outstanding example of solar wind- magnetospheric interaction, which causes global disturbances in the geomagnetic field as well as triggers ionospheric disturbances. We study the behaviour of ionospheric Total Electron Content (TEC) during the geomagnetic storms. For this investigation we have selected 47 intense geomagnetic storms (Dst ≤ -100nT) that were observed during the solar cycle 23 i.e. during 1998- 2006. We then categorized these storms into four categories depending upon their solar sources like Magnetic Cloud (MC), Co-rotating Interaction Region (CIR), SH+ICME and SH+MC. We then studied the behaviour of ionospheric TEC at a mid latitude station Usuda (36.13N, 138.36E), Japan during these storm events produced by four different solar sources. During our study we found that the smooth variations in TEC are replaced by rapid fluctuations and the value of TEC is strongly enhanced during the time of these storms belonging to all the four categories. However, the greatest enhancements in TEC are produced during those geomagnetic storms which are either caused by Sheath driven Magnetic cloud (SH+MC) or Sheath driven ICME (SH+ICME). We also derived the correlation between the TEC enhancements produced during storms of each category with the minimum Dst. We found the strongest correlation exists for the SH+ICME category followed by SH+MC, MC and finally CIR. Since the most intense storms were either caused by SH+ICME or SH+MC while the least intense storms were caused by CIR, consequently the correlation was strongest with SH+ICME and SH+MC and least with CIR.

Effect of geomagnetic storms of different solar origin on the ionospheric TEC

We have studied the behaviour of ionospheric Total Electron Content (TEC) at a mid latitude station Usuda (36.130N, 138.360E), Japan during intense geomagnetic storms which were observed during 23 solar cycle (1998-2006). For the present study we have selected 47 intense geomagnetic storms (Dst≤-100nT), for the given period, which were then categorised into four categories depending upon their solar and interplanetary sources like Magnetic Cloud (MC), Co-rotating Interaction Region (CIR), Sheath driven Interplanetary Coronal Mass Ejection (SH+ICME) and Sheath driven Magnetic cloud (SH+MC). From our study we found that the geomagnetic storms significantly affect the ionosphere having any of the solar origin. However the geomagnetic storms which are either caused by SH+MC or SH+ICME produced maximum effect in TEC.

Analysis of ionospheric parameters above active seismic regions

The aim of this paper is to examine the anomalous variations of the critical frequency (foF2), total electron content (TEC), and foEs parameters before earthquake occurred on 1 April 2014 near Iquique Chile (M = 8.2) and event on 25 October 2013 in Japan (M = 7.1). Using the bound method we analyzed the variations of foF2, TEC, foEs and NmF2 parameters. The anomalous variations in foF2, TEC, foEs and NmF2 parameters observed 1 day, 15 days, 4 days and 1 day prior to Chile event and anomalous variations in foF2, TEC and foEs observed 3 days, 3 days and 2 days prior to earthquake event of Japan. This is the e-book version of the article, published in Russian Journal of Earth Sciences (doi:10.2205/2016ES000564). It is generated from the original source file using LaTeX’s ebook.cls class.

Solar Transients Disturbing the Mid Latitude Ionosphere during the High Solar Activity

We investigate the effect of solar transients on the mid latitude ionosphere during the high solar activity period of solar cycle 23 i.e 2003 and 2004. A mid latitude station, Guangzhou (23.1N, 113.4E) was selected to carry out the investigation. The ionospheric behaviour at the selected station is characterized by considering the critical frequency of F2 layer (foF2) obtained by using the ground based Ionosonde observations. Then we selected two types of solar transients viz. solar flares and Coronal Mass Ejections (CMEs). To quantify the effect of solar flares we have considered the X-ray flux (1-8 A) and EUV flux (26-34nm). Similarly to quantify the effect of CMEs, we have considered the geomagnetic storms, because during high solar activity the geomagnetic storms are caused by CMEs. From our analysis we conclude that during the geomagnetic storms the value of foF2 decreases as compared to quiet days thereby showing a negative effect. On the contrary we found that during solar flares there is sudden and intense increase in foF2. We also performed a correlation analysis to access the magnitude of association between changes in flux values and peak values of Dst during flares and storms with the corresponding changes and peak values of foF2. We found that a strong correlation exists between the enhancements/decrements in foF2 and enhancements in flux values and Dst. We conclude, while geomagnetic activity suppresses ionospheric activity the flares enhance the same.

Evaluation of long term solar activity effects on GPS derived TEC

The solar activity hence the solar radiance follows a long term periodic variability with eleven years periodicity, known as solar cycle. This drives the long term variability of the ionosphere. In the present problem we investigate the long term behaviour of the ionosphere with the eleven year cyclic solar activity. Under the present study we characterize the ionospheric variability by Total Electron Content (TEC) using measurements made by Global Positioning System (GPS) and solar cycle variability by various solar activity indices. We make use of five solar activity indices viz. sunspot number (Rz), solar radio Flux (F10.7 cm), EUV Flux (26-34 nm), flare index and CME occurrences. The long term variability of these solar activity indices were then compared and correlated with the variability of ionospheric TEC, at a mid latitude station, Usuda (36.13N, 138.36E), of Japan, during the solar cycle 23 and ascending phase of cycle 24. From our study, we found that long term changes in the ionospheric TEC vary synchronously with corresponding changes in the solar activity indices. The correlation analysis shows that all the solar activity indices exhibit a very strong correlation with TEC (R =0.76 -0.99). Moreover the correlation between the two is stronger in the descending phase of the solar cycle. The correlation is found to be remarkably strongest during the deep minimum of the solar cycle 24 i.e. between 2007- 2009. Also we noticed a hysteresis effect exists with solar radio flux (F10.7 cm) and solar EUV flux (26-34 nm). This effect is absent with other parameters.

Morphological Investigation of Disturbed Ionosphere during Intense Geomagnetic Storms

Geomagnetic Storms are the disturbed magnetic conditions, influenced and induced by Interplanetary Magnetic Field and the Charged Particles motion around the Earth, respectively, in Geospace. As the ionosphere is woven by the earths magnetic field it responds to the change in that. During the geomagnetic storms the filled-in plasma between the magnetic field lines, geomagnetic flux tubes, redistributes itself in effect of the magnetic field forcing. In the present study we have done the investigation of the morphology of the ionosphere over the mid and high latitude regions during intense Geomagnetic Storms. We got fairly convincing results; in three cases decrease of the critical frequency of F2 layer (foF2) and in one case enhancement of the critical frequency of F2 layer (foF2) at mid and high latitudes.

Ionospheric response to annular and partial solar eclipse of 29 April 2014 in Antarctica and Australian Regions

An annular and partial solar eclipse was observed on 29 April 2014 over Australian and Antarctic regions. In this study we have analyzed the ionospheric response of this solar eclipse event. We have done a comprehensive study to find out the changes that occurred in various ionospheric parameters during the solar eclipse event over Australia and Antarctic region. We selected four Australian stations Brisbane (27.5◦S, 152.9◦E), Canberra (35.3◦S, 149.1◦E), Hobart (42.9◦S, 147.3◦E) and Perth (31.955◦S, 115.859◦E) as well as one Antarctic station Mawson (70.6455◦S, 131.2573◦E). We have studied the changes in the E and F ionospheric layers using the ground based observations at these stations. From our analysis we found that there occurred a decrease in the critical frequencies of sporadic E (foEs) and F (foF2) layers during the time eclipse was in progress at all the four Australian stations while as at Antarctic the value of foF2 recorded an enhancement. At the same time an increase in the corresponding heights of these layers (h′Es , h′F2) was also observed. This is the e-book version of the article, published in Russian Journal of Earth Sciences (doi:10.2205/2015ES000548). It is generated from the original source file using LaTeX’s epub.cls class.

The statistical investigation of amplitude Scintillations at Indian high latitude Station Maitri, Antarctica

We have investigated the occurrence characteristics of ionospheric scintillations, using dual frequency GPS receiver, installed and operated at Indian scientific base station Maitri (71.45S and 11.45E) Antarctica, during December 2009 to December 2010. The scintillation morphology is described in terms of S4 Index. The scintillations are classified into four main categories as Weak (0.2 1.0). From the analysis we found that the percentage of weak, moderate, strong and saturated scintillations were 96%, 80%, 58% and 7% respectively. The maximum percentage of all types of scintillation was observed in the summer season, followed by equinox and the least in winter season. As the year 2010 was a low solar activity period, consequently the maximum occurrences of scintillations were those of weak and moderate and only four cases of saturated scintillation were observed.