S. Tulasi Ram

Duskside enhancement of equatorial zonal electric field response to convection electric fields during the St. Patrick's Day storm on 17 March 2015

The equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13°N and 25.98°S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand-Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM-A and SWARM-C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field Bz.

Strong thermospheric cooling during the 2009 major stratosphere warming

Thermospheric density simultaneously observed by the CHAMP and GRACE satellites in both the pre?dawn and afternoon local time sectors undergoes significant decrease across both hemispheres during the major stratospheric sudden warming (SSW) in January 2009. This decrease is largest in the equatorial region near the subsolar latitude, reaching ?30% at 325 km, and 45% at 475 km altitude in the afternoon sector. This large density drop demonstrates a substantial cooling of about 50 Kelvin in the equatorial upper thermosphere. Furthermore, the cooling varies clearly with longitude in terms of magnitude and the timing of the maximum cooling. Thermosphere cooling can have important impact on the ionosphere, as indicated by simultaneous plasma observations. Though many questions remain about what causes the cooling, our results open a new perspective for investigating the global coupling of the lower and upper atmosphere during SSWs.

Equatorial electrodynamics and neutral background in the Asian sector during the 2009 stratospheric sudden warming

Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes ?25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere.

Statistics of geomagnetic storms and ionospheric storms at low and mid latitudes in two solar cycles

[1] The statistics of occurrence of the geomagnetic storms, and ionospheric storms at Kokubunji (35.7°N, 139.5°E; 26.8°N magnetic latitude) in Japan and Boulder (40.0°N, 254.7°E; 47.4°N) in America are presented using the Dst and peak electron density (Nmax) data in 1985–2005 covering two solar cycles (22–23) when 584 geomagnetic storms (Dst ≤− 50 nT) occurred. In addition to the known solar cycle and seasonal dependence of the storms, the statistics reveal some new aspects. (1) The geomagnetic storms show a preference for main phase (MP) onset at around UT midnight especially for major storms (Dst ≤− 100 nT), over 100% excess MP onsets at UT midnight compared to a uniform distribution. (2) The number of positive ionospheric storms at Kokubunji (about 250) is more than double that at Boulder, and (3) the occurrence of the positive storms at both stations shows a preference for the morning‐noon onset of the geomagnetic storms as expected from a physical mechanism of the positive storms. (4) The occurrence of negative ionospheric storms at both stations follows the solar cycle phases (most frequent at solar maximum) better than the occurrence of positive storms, which agrees with the mechanism of the negative storms.

Characteristics of large‐scale wave structure observed from African and Southeast Asian longitudinal sectors

The spatial large-scale wave structure (LSWS) at the base of F layer is the earliest manifestation of seed perturbation for Rayleigh-Taylor instability, hence, found to play a deterministic role in the development of Equatorial Plasma Bubbles (EPBs). Except for a few case studies, a comprehensive investigation has not been conducted on the characteristics of LSWS because of the complexity involved in detecting the LSWS, particularly, in spatial domain. In this scenario, a comprehensive study is carried out, for the first time, on the spatial and temporal characteristics of LSWS observed in spatial domain over African and Southeast Asian longitudinal sectors during the year 2011. The observations indicate that these wave structures can be detected a few degrees west of E region sunset terminator and found to grow significantly at longitudes past the sunset terminator. The phase fronts of these spatial structures are found to align with the geomagnetic field (B→) lines over a latitudinal belt for at least 5−6° (~500–600 km) centered on dip equator. The zonal wavelengths of these structures are found to vary from 100 to 700 km, which is consistent with the earlier reports, and the EPBs were consistently observed when the amplitudes of LSWS were grown to sufficient strengths. These results would provide better insights on the underlying physical processes involved in excitation of LSWS in terms of important roles being played by E region electrical loading and polarization electric fields induced via spatially varying dynamo current due to neutral wind perturbations associated with atmospheric gravity waves.

Explicit characteristics of evolutionary‐type plasma bubbles observed from Equatorial Atmosphere Radar during the low to moderate solar activity years 2010–2012

Using the fan sector backscatter maps of 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S geographic latitude, 100.3°E geographic longitude, and 10.4°S geomagnetic latitude), Indonesia, the spatial and temporal evolution of equatorial plasma bubbles (EPBs) were examined to classify the evolutionary-type EPBs from those which formed elsewhere and drifted into the field of view of radar. A total of 535 EPBs were observed during the low to moderate solar activity years 2010–2012, out of which about 210 (~39%) are of evolving type and the remaining 325 (~61%) are drifting-in EPBs. In general, both the evolving-type and drifting-in EPBs exhibit predominance during the postsunset hours of equinoxes and December solstices. Interestingly, a large number of EPBs were found to develop even a few minutes prior to the apex sunset during equinoxes. Further, the occurrence of evolving-type EPBs exhibits a clear secondary peak around midnight (2300–0100 LT), primarily, due to higher rate of occurrence during the postmidnight hours of June solstices. A significant number (~33%) of postmidnight EPBs generated during June solstices did not exhibited any clear zonal drift, while about 14% of EPBs drifted westward. Also, the westward drifting EPBs are confined only to June solstices. The responsible mechanisms for the genesis of fresh EPBs during postmidnight hours were discussed in light of equatorward meridional winds in the presence of weak westward electric fields.

CME front and severe space weather

Thanks to the work of a number of scientists who made it known that severe space weather can cause extensive social and economic disruptions in the modern high-technology society. It is therefore important to understand what determines the severity of space weather and whether it can be predicted. We present results obtained from the analysis of coronal mass ejections (CMEs), solar energetic particle (SEP) events, interplanetary magnetic field (IMF), CME-magnetosphere coupling, and geomagnetic storms associated with the major space weather events since 1998 by combining data from the ACE and GOES satellites with geomagnetic parameters and the Carrington event of 1859, the Quebec event of 1989, and an event in 1958. The results seem to indicate that (1) it is the impulsive energy mainly due to the impulsive velocity and orientation of IMF Bz at the leading edge of the CMEs (or CME front) that determine the severity of space weather. (2) CMEs having high impulsive velocity (sudden nonfluctuating increase by over 275 km s−1 over the background) caused severe space weather (SvSW) in the heliosphere (failure of the solar wind ion mode of Solar Wind Electron Proton Alpha Monitor in ACE) probably by suddenly accelerating the high-energy particles in the SEPs ahead directly or through the shocks. (3) The impact of such CMEs which also show the IMF Bz southward from the leading edge caused SvSW at the Earth including extreme geomagnetic storms of mean DstMP < −250 nT during main phases, and the known electric power outages happened during some of these SvSW events. (4) The higher the impulsive velocity, the more severe the space weather, like faster weather fronts and tsunami fronts causing more severe damage through impulsive action. (5) The CMEs having IMF Bz northward at the leading edge do not seem to cause SvSW on Earth, although, later when the IMF Bz turns southward, they can lead to super geomagnetic storms of intensity (Dstmin) less than even −400 nT.

On the fresh development of equatorial plasma bubbles around the midnight hours of June solstice

Using the 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia, the nocturnal evolution of Equatorial Plasma Bubbles (EPBs) were examined during the moderate solar activity years 2011-2012. While the evolution of EPBs were mostly (86%) confined to post-sunset hours (1900 – 2100 LT) during equinoxes, in contrast, the majority of EPBs (~71%) in June solstice found evolve around the midnight hours (2200 – 0300 LT). The mechanisms behind the fresh evolution of summer time midnight EPBs were investigated, for the first time, through SAMI2 model simulations with a realistic input of background ExB drift variation derived from CINDI IVM on board C/NOFS satellite. The term-by-term analysis of linear growth rate of RT instability indicates that the formation of high flux tube electron content height gradient (KF) (steep vertical gradient) region at higher altitudes is the key factor for the enhanced growth rate of RT instability. The responsible factors are discussed in light of relatively weak westward zonal electric field in the presence of equatorward neutral wind and bottom side recombination around the midnight hours of June solstice. The effects of neutral winds and weak westward electric fields on the uplift of equatorial F layer were examined separately using controlled SAMI2 simulations. The results indicate that relatively larger linear growth rate is more likely to occur around midnight during June solstice because of relatively weak westward electric field than other local times in the presence of equatorward meridional wind.

Estimation of interplanetary electric field conditions for historical geomagnetic storms

Ground magnetic measurements provide a unique database in understanding space weather. The continuous geomagnetic records from Colaba-Alibag observatories in India contain historically longest and continuous observations from 1847 to present date. Some of the super intense geomagnetic storms that occurred prior to 1900 have been revisited and investigated in order to understand the probable interplanetary conditions associated with intense storms. Following Burton et al. (1975), an empirical relationship is derived for estimation of interplanetary electric field (IEFy) from the variations of Dst index and ΔH at Colaba-Alibag observatories. The estimated IEFy values using Dst and ΔHABG variations agree well with the observed IEFy, calculated using Advanced Composition Explorer (ACE) satellite observations for intense geomagnetic storms in solar cycle 23. This study will provide the uniqueness of each event and provide important insights into possible interplanetary conditions for intense geomagnetic storms and probable frequency of their occurrence.

Electrodynamic disturbances in the Brazilian equatorial and low‐latitude ionosphere on St. Patrick's Day storm of 17 March 2015

The St. Patricks Day storm of 17th March, 2015 has a long lasting main phase with the Dst reaching a minimum of -223 nT. During the main phase, two strong prompt penetration electric field (PPEF) phases took place; first with the southward turning of IMF Bz around ~1200 UT and the second with the onset of a substorm around ~1725 UT leading to strong equatorial zonal electric field enhancements. The consequent spatio-temporal disturbances in the ionospheric Total Electron Content (TEC) and the resultant modifications in the Equatorial Ionization Anomaly (EIA) over the Brazilian longitudinal sector are investigated in detail. The simultaneous measurements from a large network of GPS receivers, Ionosonde and magnetometers over the Brazilian longitudinal sector are used for this study. In the presence of enhanced zonal electric field, the equatorial F2-layer peak (hmF2) experienced a rapid uplift without any significant change in the base height (h′F); while the F2 layer is redistributed into F2 and F3 layers. The enhanced zonal electric filed due to PPEF led to the strong super fountain effect under which, the anomaly crest departed poleward to ~40o S latitude. In the presence of westward and equatorward wind surge over Brazil with the co-existing disturbance dynamo fields, strong hemispheric asymmetry is seen in the storm time response of EIA during both the PPEF phases.