Coupling of the solar driven prolonged and transient processes to the equatorial and low latitude ionosphere

Abstract

A brief summary of important and new findings is listed below: • Studied the local time response of the geomagnetic storms on the ESF irregularities over Indian region under geomagnetic storms based on Aarons criteria. The analysis of the results suggests that occurrence of ESF irregularities is always not followed the Aarons criteria. • The role of different seasons on the Aarons criteria also suggests that there is no consistent pattern emerged in the favor of Aarons criteria. The results ascertain that occurrence of ionospheric irregularities are not depends on the Dst index during the magnetic storms but results suggest that they can be affected by local and other solar and geomagnetic parameters such as the solar wind and high latitude parameters. • The analysis on the role of low latitude Es layers on ESF irregularities based Hyderabad station Es layers under the geomagnetic storms suggest that low latitude Es layers could play important role in the ESF occurrence. It is found that whenever Es-layers are present (absent) at Hyderabad, v low latitude station, suppression (generation) of ESF irregularities at Tirunelveli. So, results suggest that there could be role of low latitude Es-layers on these plasma irregularities apart from the high latitude auroral electric fields, magnetospheric electric fields and winds/waves. • The observations showed a remarkable increase of virtual height where (h’F (km)) is raised to as high as ∼560 km over Tirunelveli (magnetic equator) with vertical drift of ∼70 m/s at ∼19:30 IST due to addition of direct penetration of storm time eastward electric fields which exactly coincided with local time of pre-reversal enhancement (PRE) and caused intense ESF irregularities in the ionosondes and L-band scintillations in the GPS receivers at wide latitudes in Indian longitude during St. Patrick’s day storm which falls under equinox condition. • Nocturnal thermospheric meridional winds as estimated using two ionosondes during this storm night suggest the equatorward surge of gravity waves with period of ∼2 hrs. Suppression of anomaly crest on the subsequent day of the storm suggests the complex role of disturbance dynamo electric fields and disturbance wind effects. • Simultaneous penetration of under-shielding and over-shielding electric fields observed at chain of stations namely Tirunelveli, Hyderabad and Allahabad at ∼19:30 UT and 20:30 UT (local midnight) on the 22nd June that fall under summer geomagnetic storm that caused abrupt decrease of virtual height (h’F (km)) to ∼200 km due to the strong westward prompt penetration electric field (PPEF) and increase to ∼500 km due to eastward PPEFs simultaneously over the all three stations. Suppression of spread-F is observed during westward penetration over Indian sector simultaneously super plasma bubbles were observed in European longitudes where the equatorial type bubbles were detected at midlatitudes. • The strong F3 layers observed simultaneously at Tirunelveli/Hyderabad in association with IMF Bz which is distinctly different than other days under this summer storm. Appearance and Disappearance of Esq type layers at equator during EEJ and CEJ events are linked to storm time electric fields. • During summer storm, early morning spread-F is noticed at Hyderabad/Allahabad which is devoid of equatorial type spread-F at Tirunelveli suggesting possibility of TID propagation to low latitudes to cause mid latitude type spread F generation at low latitudes vi • The observations of decrease of delta TEC in the northern hemisphere and increase of TEC in the southern hemisphere on 23rd June 2015 during this summer storm do suggest that these results are similar to model simulations. On 24th June 2015, strong negative storm effect over Allahabad/Hyderabad due to disturbance dynamo and positive storm effect in Tirunelveli is observed due to CEJ effect which again due to DD effect. • The oscillatory behavior in the foF2, h’F (km) and TEC is noticed during both main and recovery phases in the summer storm. Periodogram analysis of these variations revealed presence of shorter periods (<2 hours) while larger periods (>2 hours). Based on the periodogram analysis, it is suggested that shorter period fluctuations as seen in foF2 are primarily caused by PPEF fluctuations, while larger period fluctuations are mainly caused by Disturbance winds or its Electric Fields (DDEFs). • During winter storm, twin PREs are observed around ∼18:00 and 22:00 IST associated with upward vertical drift of ∼30 and 20 m/s respectively, under the influence of undershielding and overshielding electric field effects. Simultaneously height enhancement is noticed at ALD and HYD stations due to the direct penetration of electric field from high to low latitude ionosphere. • During summer solstice storm, early morning spread-F observations are observed at Hyderabad/Allahabad which is devoid of equatorial type spread-F at Tirunelveli indicates possibility of mid latitude type spread F generation at low latitudes triggered due to TID propagation. • The strong DD electric field effect during the pre-sunrise hours produced remarkable height enhancement in virtual height (h’F) that reached upto ∼600 km ∼05:30 IST with upward vertical velocity of ∼50 m/sec which suddenly reduced to normal quiet time value at 06:30 IST with downward velocity of ∼-50 m/s over equator. During same time, enhancement/reduction in the virtual height at HYD (∼420 km) and ALD (∼410 km) stations is noticed as compared to other quiet days. • Sub-storm effect on the equatorial F-layer vertical plasma drift is noticed with its velocity is oscillating between ∼50 m/sec during ∼17:30-11:30 IST on 20-21 Dec. The wavelet and cross wave late analysis shows the relationship between solar wind and ionospheric parameters. The drifts and solar wind parameters (solar wind velocity, IEFy and AE) show the shorter periods vii (<1 hrs) and longer (>1) dominant periods. The phase relationship analysis between drift and SW shows the dominant periods (∼40 min) in same phase, drift and IEFy (sim ∼15 and 60 min periods, in same phase) indicate the PP effect (undersheilding). • The observations of decrease of delta TEC in the northern hemisphere and increase of TEC in the southern hemisphere during summer season shows strong negative ionospheric storm effect over Allahabad/Hyderabad due to disturbance dynamo and positive storm effect in Tirunelveli is observed due to CEJ effect which again due to DD effect. Observations showed positive ionospheric storm (increase of delta TEC) during the winter solstice in the northern hemisphere. As per the current understanding trans-equatorial winds from summer hemisphere reduces N2 molecules in the northern hemisphere to increase the electron density at low latitudes. However, increase of delta TEC in the summer hemisphere during this storm. • F3 layer observations near the EIA crest region at Allahabad in the after-noon hours in the winter storm were found to be associated with gravity waves, with ∼48 min periodicity, downward phase propagation velocity ∼46 m/s, vertical wavelength ∼130 km and horizontal wavelength ∼435 km. • It is known that geomagnetic storms are pronounced during high solar activity periods than low solar activity periods. But studies during recent prolonged low solar activity period suggest that equatorial and low latitude ionosphere is also significantly affected by recurrent geomagnetic activity due to high stream solar wind emanating from coronal holes. Accordingly, the thesis investigates the possible role of recurrent geomagnetic activity vis-a-vis lower atmospheric forcing on the periodic oscillations of the ionospheric density during low solar activity period as obtained using ionosonde at Tirunelveli during deep solar minimum year 2008. The analysis of ground based observations suggest the periodic oscillations in the ionospheric density, TEC and EEJ strength at 9 and 13 day which are quite similar to that of the periods seen in solar wind or Kp index. The Lomb-Scargle (L-S) periodogram shows the existence of sub-harmonic periods in the ionosphere. The results presented in this thesis suggest that these periodic oscillations could be arising mainly from the solar origin but not due to lower atmospheric coupling.