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The equatorial plasma bubbles, once developed, grow nonlinearly into topside ionosphere, and simultaneous secondary instabilities lead to the development of shorter scale irregularities. The altitudinal growth and generation of smaller scale irregularities determine the spatio‐temporal occurrence and the intensity of ionospheric scintillations at wide spectrum of radio waves and have significant implications on the GNSS/Satellite Based Augmentation Systems. In this letter, we present a unique equatorial plasma bubble observation from equatorial atmosphere radar that provides hitherto undisclosed evidence for the smaller (3‐m) scale irregularities initially developing at higher altitudes and subsequently developing to lower altitudes in a narrow funnel‐like structure. The responsible mechanisms for early development of shorter scale irregularities in the topside and their subsequent development at lower altitudes are discussed in light of difference between the time scales of altitudinal growth and cascading rate of secondary instabilities through high‐resolution bubble model simulations. Key Points: Unusual observation of EPB with smaller scale irregularities appears initially at top and dilatory development in narrow funnel region laterWe suggest that rapid vertical rise of bubble due to large background electric field surpassed the rate of cascading secondary instabilitiesHypothesis is tested with the available background ionospheric conditions and high‐resolution bubble model simulations [ABSTRACT FROM AUTHOR]

Three different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high‐latitude field‐aligned currents and reconfiguration of high‐latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high‐frequency radar observations. Key Points: Three sharp and intense PPEF disturbances under steady southward IMF Bz phase of St. Patricks's Day geomagnetic stormA sharp and strong westward PPEF disturbance due to sharp duskward to dawnward reversal of IMF By barring any significant change in BzSW‐Pdyn and Substorm induced a strong eastward PPEF, equatorial super fountain and quick rejuvenation and symmetric redistribution of EIA [ABSTRACT FROM AUTHOR]

The existence of an additional stratification in the daytime equatorial ionospheric F region (the F3 layer) was known since the 1940s. However, its characteristics and the underlying physical mechanism have been uncovered only recently. In this paper, we present and discuss the F3 layer characteristics observed by six ionosondes distributed over equatorial and low latitudes (−20° to +25° dip latitudes) in the Brazilian longitude sector during the strongest geomagnetic storm (DstMin = −223 nT) of solar cycle 24, the St. Patrick's Day storm of 17 March 2015. Two eastward prompt penetration electric field (PPEF) events, as seen in equatorial electrojet, occurred during the main phase of the storm on 17 March 2015, a strong one (~100 nT) at around ~1200 UT and a weak one (~50 nT) at around ~1725 UT. Local time variations in the F3 layer occurrence and ionospheric base height (h′F), peak height (hmF), and peak electron density (Nmax) are investigated. Notably, the F3 layer occurred at all six locations, more distinctly during the stronger PPEF event. The large latitudinal extend in the occurrence of the F3 layer in opposite hemispheres (−20° to +25° dip latitudes) covering the equatorial ionization anomaly crests observed for the first time is interpreted in terms of the combined effect of the super plasma fountain generated by the eastward PPEF and storm time equatorward neutral wind. Key Points: F3 layer occurred over a large latitudinal extent from 20°S to 25°N dip latitudes during 17 March 2015 storm‐induced PPEF phasesThe PPEF‐generated superfountain led to the F3 layer occurrence earlier at the equator and later at higher latitudes in both hemispheresF3 layer was more distinct and stronger and lasted longer in the Northern Hemisphere due to the storm time asymmetric equatorward neutral winds [ABSTRACT FROM AUTHOR]

Abstract: The electrodynamics and neutral dynamics of the equatorial ionosphere undergo significant changes during geo-magnetically disturbed periods as a result of the high and low latitude electro-dynamical coupling. These changes may sometimes trigger the plasma instabilities that lead to the onset of spread-F and scintillations even at the L-band frequencies. The response of the equatorial ionosphere–thermosphere system over the Indian sector to moderate geomagnetic storms of 11th Feb 2004 and 9th Mar 2004 has been studied using the ground based measurements from the Indian equatorial and low latitude region. A large vertical drift of the equatorial F-layer followed by nearly simultaneous onset of Spread-F on ionograms and scintillations at VHF (244MHz) and L-band (1.5GHz) frequencies, have been observed during the early evening periods that correspond to the time of rapid decrease (−25 to −30nT/h) in Sym-H index due to prompt penetration of eastward electric fields into equatorial and low latitudes. The Spread-F and scintillations continued to exist for longer durations up to post-midnight to pre-dawn hours, possibly due to the combined action of prompt penetration and long-lived Disturbance Dynamo electric fields. Due to prolonged geomagnetic activity during the subsequent days (morning-to-noon hours), strong reversal in the Equatorial ElectroJet current is observed and the Equatorial Ionization Anomaly (EIA) is significantly suppressed under the influence of westward disturbance dynamo electric fields. The post-sunset enhancements in the vertical drift of the equatorial F-layer are also reduced significantly from their quiet day patterns and the subsequent occurrence of Spread-F and scintillations is inhibited. [Copyright &y& Elsevier]

The vertical E × B drift is very important parameter as its day to day variability has great influence on the variability in the low latitude F-region ion and electron density distributions. The measurements of vertical ion velocity from the first Republic of China Satellite (ROCSAT-1) provide a unique data base for the development of possible relationship between vertical E × B drifts and ground based magnetometer observation. An attempt has been made to derive quantitative relationship between F-region vertical E × B drifts measured by ROCSAT-1 (600 km) and ground measured equatorial electrojet for the solar maximum period 2001–2003 for Indian and Japanese sectors. The results consistently indicate existence of linear relationship between the measured vertical E × B drifts at topside F-region and EEJ for both the sectors, with a moderate to high correlation coefficients. The linear relationship between ROCSAT-1 measured E × B drifts and EEJ for Indian and Japanese sectors has been compared with a similar relationship with Jicamarca Unattended Long-term Ionosphere Atmosphere Radar (JULIA) measured E × B drifts (150 km echos) and EEJ strength from Peruvian sector during 2003. It has been found that ROCSAT-1 measured E × B drifts shows linear relationship with EEJ, however, exhibits a larger scatter unlike JULIA radar observed E × B drifts. This may be attributed to the large height difference as ROCSAT-1 measures E × B drifts at 600 km altitude and the EEJ is E-region (110 km) phenomenon. [ABSTRACT FROM AUTHOR]