Study of dynamical coupling of the atmosphere-ionosphere system at low latitudes

Abstract

The earth’s atmosphere is generally divided into four distinct layers depending on its temperature profile, namely Troposphere, Stratosphere, Mesosphere and thermosphere. On the other hand, the region of atmosphere between 60 km and 1000 km altitude is termed as ionosphere. Ionosphere consists of weakly ionised plasma and the plasma density is typically below 0.01% of the total neutral particle density at any altitude. The atmosphere and ionosphere of the earth is dynamically coupled to each other with the help of tides, waves etc. When tidal winds blow across the geomagnetic field lines, they generate electric fields in the ionosphere. The magnitude of the wind and conductivity of the ionosphere controls the magnitude and direction of the electric field in the ionosphere. These electric fields can rearrange ionospheric plasma with altitude and latitude. Such rearrangement of plasma density can in turn, modify the background wind flow through ion drag. On the other hand, wind in the ionosphere can carry the plasma along the field lines and modify their latitudinal and altitudinal arrangements. Another important component of the atmosphere-ionosphere coupled system is the atmospheric waves. These waves could transport energy and momentum from lower altitudes into the ionosphere and thereby, could perturb the plasma density and background electric field in the F region to a large extent. One of the remarkable low latitude, nighttime phenomena is the equatorial spread-F (ESF). It is now well accepted that Generalized Rayleigh-Taylor (RT) instability could generate ESF in the presence of proper seed mechanism. Since, presence of ESF disrupts the trans-ionospheric communications heavily; to attain prediction capability of this phenomenon is highly desired. Though, seasonal and solar cycle variation of ESF occurrence pattern and its characteristics are now well understood, prediction of the day to day variability still remains a challenging research problem. It is envisaged that better understanding of the iv seeding mechanism could help to resolve this problem. The role of a seeding mechanism (i.e. large scale wavelike structure (LSWS)) and the pre-reversal enhancement (PRE) phenomenon in the generation process of ESF during deep solar minimum period is studied in this thesis. For this study, ionosonde data obtained from dip equatorial station Tirunelveli is utilized. LSWS is manifested in the ionograms as satellite traces (STs). Our analysis reveals that in around 70% cases appearance of STs is followed by ESF. This indicates that presence of STs does not trigger generation of ESF on all occasions as has been claimed by several other researchers (Tsunoda, 2005; Abdu et al., 2014). Thus, there could be some additional factors responsible for the generation of ESF apart from LSWS. The average time interval between the first appearance of STs and ESF signatures in the ionograms is found to be ~30 min. Therefore, STs could be utilized as a precursor of ESF. We also observed that PRE does not play a significant role in the ESF generation process during the solar minimum period. Another important component of the atmosphere-ionopshere coupled system is the gravity waves (GWs). GWs can perturb the neutral and plasma densities significantly and change the mean flow pattern at the ionospheric altitudes (Vadas and Liu, 2013). GWs have been suggested to be the probable seeding mechanism of ESF for the last few decades (e.g. Kelley, 1981). Also, it is presumed that the inter-depletion distance between the ESF plume structures are most likely associated with GWs (Makela and Miller, 2008, Narayanan et al., 2012). In addition, generation of LSWS is also attributed to zonally propagating GWs in recent reports (Tsunoda, 2010a and references therein). Therefore, investigation of GWs in the ionosphere is crucial to understand the dynamics of atmosphere-ionosphere system and the day to day variability of ESF characteristics. All sky images of OI 630 nm emission obtained from Indian dip equatorial station Tirunelveli during the period of January 2013 to January 2015 is utilized in this thesis to look out for probable wave signatures in the bottomside ionosphere. v Two types of wave signatures were noticed in this work, namely quasi-periodic waves and single band of enhanced intensity (SBEI). In total, 11 cases of quasi-periodic waves and three cases of SBEI features were observed. Phase speed, wavelength and time period of the quasi-periodic waves were estimated to be in the range of 70-160 m/s, 130-575 km and 25-75 min, respectively. While phase speed and scale size of the SBEI features were found to be in the range of 150-250 m/s and 230-470 km, respectively. Both of these wave signatures predominantly propagated towards north-northwest or south-southeast direction. No zonal propagation of the wave signatures was observed in our study. The quasi-periodic waves are interpreted as signature of either primary or secondary GWs propagating in the thermosphere. The exact generation mechanism of the SBEI features is not yet known. We also suggested that neutral density perturbations caused by the GWs may alter rates of the different chemical reactions associated with the OI 630 nm emission and thereby, induce quasi-periodic waves in the OI 630 nm intensity in the dip equatorial region. During geomagnetic storms, dynamics and electrodynamics of the low latitude atmosphere-ionosphere system is perturbed significantly (Abdu, 1997; Buonsanto, 1999). The main sources of these perturbations are the prompt penetration electric field (PPEF) and the disturbed dynamo electric field (DDEF). We have presented response of the atmosphere-ionosphere system to the severe storm of 17 March 2015 as a last component of this thesis. Also, we have reported evolution of different characteristics of equatorial plasma bubbles (EBPs) as observed in the all sky airglow images obtained from the Indian dip equatorial (Tirunelveli) and low latitude (Kolhapur) region during this storm. EPBs were observed to drift eastward during 14:30–16:30 UT at different dip latitude sectors in accordance with the quiet time drift pattern of the EPBs. However, a drift reversal of the EPBs was observed at 9°N at 16:10 UT presumably under the effect of the disturbed dynamo. The drift reversal initially occurred at higher dip latitudes and afterwards it appeared at lower dip latitudes. In addition, a longitudinal variation in the EPB drift values was also observed. vi Another important finding during this storm was the presence of large westward tilt of the EPBs. The maximum tilt observed on 17 March 2015 was two to ten times larger than the maximum tilts observed on other days during January-May, 2015. Presence of latitudinal gradient and longitudinal variation in the drift velocities as well as the occurrence of drift reversal over a relatively short period of time are attributed as the main cause of the large westward tilt observed on this day. The most interesting finding of this study was the observation of asymmetry in the tilt of the EPBs at conjugate points. Such asymmetry in the structures of the EPBs has not been reported previously. The exact reason for such asymmetry is not known yet. In this work we have also analysed the east and west wall dynamics of the EPBs on 17 March 2015 and compared the results with the observation on two quiet days, namely, 22 February 2015 and 22 April 2015.