B. Veenadhari

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.

Low‐mid latitude D region ionospheric perturbations associated with 22 July 2009 total solar eclipse: Wave‐like signatures inferred from VLF observations

We present first report on the periodic wave-like signatures (WLS) in the D region ionosphere during 22 July 2009 total solar eclipse using JJI, Japan, very low frequency (VLF) navigational transmitter signal (22.2 kHz) observations at stations, Allahabad, Varanasi and Nainital in Indian Sector, Busan in Korea, and Suva in Fiji. The signal amplitude increased on 22 July by about 6 and 7 dB at Allahabad and Varanasi and decreased by about 2.7, 3.5, and 0.5 dB at Nainital, Busan, and Suva, respectively, as compared to 24 July 2009 (normal day). The increase/decrease in the amplitude can be understood in terms of modal interference at the sites of modes converted at the discontinuity created by the eclipse intercepting the different transmitter-receiver great circle paths. The wavelet analysis shows the presence of WLS of period ~16–40 min at stations under total eclipse and of period ~30–80 min at stations under partial eclipse (~85–54% totality) with delay times between ~50 and 100 min at different stations. The intensity of WLS was maximum for paths in the partially eclipsed region and minimum in the fully eclipsed region. The features of WLS on eclipse day seem almost similar to WLS observed in the nighttime of normal days (e.g., 24 July 2009). The WLS could be generated by sudden cutoff of the photo-ionization creating nighttime like conditions in the D region ionosphere and solar eclipse induced gravity waves coming to ionosphere from below and above. The present observations shed additional light on the current understanding of gravity waves induced D region ionospheric perturbations.

Application of lightning discharge generated radio atmospherics/tweeks in lower ionospheric plasma diagnostics

Lightning discharges during thunderstorm are the significant natural source of electromagnetic waves. They generate electromagnetic pulses, which vary from few Hz to tens of MHz, but the maximum radiated energy is confined in extremely low (ELF: 3-3000Hz) and very low (VLF: 3-30 KHz) frequency band. These pulsed signals with frequency dispersion are known as radio atmospherics or tweeks. These waves propagate through the process of multiple reflections in the earth-ionosphere waveguide over long distances with very low attenuation (2-3 dB/1000km). Since these waves are reflected by lower boundary of ionosphere, these are used extensively for probing the D-region ionosphere. D-region is important to the space weather, as well as the submarine communication and navigational aid. In this perspective the measurement of electron density profiles of the D-region is undoubtedly of great interest to both the development of reliable models and radio wave propagation. Earlier work on the tweeks is mainly focused to the theoretical considerations related to polarization, waveform analysis, and occurrence time and propagation mechanism. In this study we investigate tweeks to determine the equivalent night time electron densities at reflection height of the D-region. Distance traveled by the VLF waves from the causative lightning discharges to the receiving station has also been calculated. Tweeks recorded at a low latitude ground station of Allahabad (Geomag. lat. 16.050 N) during the night of 23 March 2007 have been used in the present analysis. Based on the analysis of the fundamental cut-off frequency of tweeks, the estimated equivalent electron density of the D-region has been found to be in the range of ~20 to 25 el/cm3 at ionospheric reflection height of ~80 to 95 km respectively. Propagation distance in Earth-Ionosphere wave guide (EIWG) from causative lightning source to experimental site varies from ~1500 to 8000 km.

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.

Coronal mass ejection-driven shocks and the associated sudden commencements/sudden impulses

Interplanetary (IP) shocks are mainly responsible for the sudden compression of themagnetosphere, causing storm sudden commencement (SC) and sudden impulses (SIs)which are detected by ground-based magnetometers. On the basis of the list of 222 IPshocks compiled by Gopalswamy et al. (2010), we have investigated the dependence ofSC/SIs amplitudes on the speed of the coronal mass ejections (CMEs) that drive the shocksnear the Sun as well as in the interplanetary medium. We find that about 91% of the IPshocks were associated with SC/SIs. The average speed of the SC/SI-associated CMEs is1015 km/s, which is almost a factor of 2 higher than the general CME speed. When theshocks were grouped according to their ability to produce type II radio burst in theinterplanetary medium, we find that the radio-loud (RL) shocks produce a much largerSC/SI amplitude (average 32 nT) compared to the radio-quiet (RQ) shocks (average 19 nT). Clearly, RL shocks are more effective in producing SC/SIs than the RQ shocks.We also divided the IP shocks according to the type of IP counterpart of interplanetaryCMEs (ICMEs): magnetic clouds (MCs) and nonmagnetic clouds. We find that theMC-associated shock speeds are better correlated with SC/SI amplitudes than thoseassociated with non-MC ejecta. The SC/SI amplitudes are also higher for MCs than ejecta.Our results show that RL and RQ type of shocks are important parameters in producing theSC/SI amplitude.

On the reduced geoeffectiveness of solar cycle 24: a moderate storm perspective

The moderate and intense geomagnetic storms are identified for the first 77 months of solar cycle 23 and 24. The solar sources responsible for the moderate geomagnetic storms are indentified during the same epoch for both the cycles. Solar cycle 24 has shown nearly 80 % reduction in the occurrence of intense storms where as it is only 40 % in case of moderate storms when compared to previous cycle. The solar and interplanetary characteristics of the moderate storms driven by CME are compared for solar cycle 23 and 24 in order to see reduction in geoeffectiveness has anything to do with the occurrence of moderate storm. Though there is reduction in the occurrence of moderate storms, the Dst distribution does not show much difference. Similarly the solar source parameters like CME speed, mass and width did not show any significant variation in the average values as well as the distribution. The correlation between VBz and Dst is determined and it is found to be moderate with value of 0.68 for cycle 23 and 0.61 for cycle 24. The magnetospheric energy flux parameter epsilon (Ɛ) is estimated during the main phase of all moderate storms during solar cycles 23 and 24. The energy transfer decreased in solar cycle 24 when compared to cycle 23. These results are significantly different when all geomagnetic storms are taken in to consideration for both the solar cycles.

Characteristics of tweeks radio atmospherics observed in Indian low latitude region using AWESOME VLF receiver

In the present work tweeks recorded at Indian low latitude ground station Allahabad (Geomag. lat. 16.05° N) during April 2007 to March 2008 have been used to study the occurrence characteristics, distance & reflection height in the Earth Ionospheric waveguide. Results shows that summer seasons has highest tweek occurrence ∼63% and lowest for winter season ∼18%. The tweek recorded having maximum mode n=6, tweeks with n=2 are more dominant with occurrence ∼70%. Tweeks with n>3 are more common during early night & maximum tweeks occurs in late night. Average distance traveled by maximum tweeks is in range of ∼4000–8000 km.

Waves-like signatures in the D-region ionosphere generated by solar flares

The present study reports presence of periodic Wave-Like Signatures (WLS) in the D-region ionosphere detected using NWC, Australia, VLF navigational transmitter signal (19.8 kHz) observed at Allahabad, an Indian low latitude station. The observed WLS are associated with series of solar flares which includes 12 C, 3 M and 2 X class flares occurred during the month of May 2013. Significant variations are observed on NWC-VLF amplitude and phase due to different classes of flares which occurred at different solar zenith angles. The wavelet analysis of VLF amplitude on control day reveals presence of WLS with periods 40-180 minutes during day/night and night/day transition times which are probably generated due to passage of dusk and dawn solar terminator. Flare day WLS are observed with period varying 90-200 minutes and are remarkably different in their period, occurrence duration/time and amplitude depending on the class and occurrence time of flare and are more prominent during morning and evening times when D-region is in developing stage. The WLS on flare day are probably generated by solar flare induced gravity waves which may cause periodic changes in temperature, electron density, and plasma conductivity in the ionosphere. The present observations seem to shed additional light on the current understanding of gravity wave induced D-region dynamics.

VLF perturbations associated earthquake precursors using subionospheric VLF signals

Summary form only given. Statistical results on the detection of lower ionospheric perturbations due to recent great earthquakes with M > 5.5 during the year 2009 in India, China, Bhutan and Indonesian regions, have been presented. The amplitude of two navigational transmitters NWC (19.8 kHz) in Australia and JJI (- kHz) in Japan received at Indian low latitude station Allahabad (24.8° N, 81.9° E) have been utilized. EQs selected are those whose epicenters fall in the wave sensitive area between transmitter and receiver defined by 5th Fresnel zone. The EQs are termed as JJI-EQs and NWC-EQs depending on their epicenter location along JJI-Allahabad and NWC-Allahabad paths. To find out the earthquake related precursory signatures in sub-ionospheric signals the terminator time (TT) and nighttime amplitude fluctuation (NF) methods have been utilized. The TT and NF methods show significant sub-ionospheric perturbations for NWC EQs. Shift in evening TT was observed few days to weeks prior to the NWC EWs. NF method reveals significant precursory signatures with fluctuations over 2σ (σ =slandered deviation) criterion from few days to weeks before both NWC and JJI EQs. The gravity wave associated with EQs seem responsible for the strong nighttime fluctuations whereas the changes in TTs are associated with the electric field and extra ionization due to radon gas associated with this EQs which change the ionospheric reflection height.

Comment on "Westward electric field penetration to the dayside equatorial ionosphere during the main phase of the geomagnetic storm on 22 July 2009" by V. Sreeja et al.

[1] Sreeja et al. [2011] have claimed that the “geomagnetic storm that occurred during the period from 21 to 25 July 2009 is anomalous because the storm main phase developed during northward interplanetary magnetic field (IMF)” and “westward electric field penetration to the dayside equatorial ionosphere during the main phase of the storm.” However, as discussed subsequently, the so‐called “anomalous” feature of geomagnetic storm is ill‐fully claimed due to their incorrect IMF Bz data. Since Sreeja et al. [2011] have made serious claims such as the geomagnetic storm of 21–25 July 2009 is anomalous which would have adverse impact on the future scientific investigations, it is highly necessary to clarify. [2] For example, Figures 1a–1d show the variation of interplanetary magnetic field (IMF Bz), interplanetary electric field (IEF) (IEFy = −Vsw × Bz), symmetric ring current (Sym‐H) index and the strength of equatorial electrojet (EEJ), respectively. The 1 min time resolution data of IMF Bz and IEFy are obtained from the Atmospheric Composition Explorer (ACE). The Sym‐H index (1 min values) data is obtained from the World data center for Geomagnetism, Kyoto University (http://wdc.kugi.kyoto‐u.ac.jp/). The strength of equatorial electrojet (1 min values) is obtained from the ground‐based magnetometers at Tirunelveli (8.7°N geog.latitude, 77.8°E geog.longitude and 0.6°N dip.latitude) and Alibagh (18.5°N geog.latitude, 72.9°N geog.longitude and 12.9°N dip.latitude) in India. The IMF Bz and IEFy parameters have been shifted to account for the delay from ACE location to the bow shock (http://omniweb.gsfc.nasa. gov/html/HROdocum.html). It could be clearly seen from Figure 1 that the IMF Bz is southward continuously from several hours before the onset of main phase at 0154 UT (0724 IST) of 22 July 2009. However, unlike Sreeja et al. [2011] claimed, the IMF Bz continues to be southward till 0544 UT (1114 IST). During the same period (0154 to 0544 UT) the ring current continues to be strengthened as evidenced by the variation of Sym‐H index which reached to a maximum negative excursion of −89 nT at 0544 UT. This clearly shows that the main phase of the storm is developed mainly under the southward orientation of the IMF Bz which is a well‐known phenomenon. Therefore, the principal finding of anomalous geomagnetic storm where the main phase is claimed to be developed during the northward IMF Bz is not true. While comparing the time variations of IMF Bz and IEFy with those from Sreeja et al. [2011, Figure 1], it can be clearly noticed that the IMF Bz and IEFy data shown in their figure is incorrect where the IMF Bz is turned northward around ∼0700 IST. Also in section 3.1 of Sreeja et al. [2011], they have claimed that “the IMF Bz is observed to be remain northward for a very long duration (∼8 h) from 0700 IST to 1600 IST.” This is incorrect as can be seen from Figure 1 that the IMF Bz is northward only for a period of 83 min from 0544 to 0707 UT (1114–1237 IST) and later turned southward from 0707 to 0849 UT (1237–1419 IST). Therefore, the anomalous main phase of the geomagnetic storm which developed under northward IMF Bz is not true and primarily arise due to incorrect data of IMF Bz used by Sreeja et al. [2011]. Though the authors have stated in sections 2 and 3.1 that the 1 min data of IMF Bz obtained from the Website of Atmospheric Composition Explorer spacecraft, it is observed that the data shown by Sreeja et al. [2011, Figure 1] do not match with the IMF data of ACE. Further, Sreeja et al. [2011] have stated that they have estimated a convection time delay of ∼40 min from an average solar wind velocity of 560 km/s. However, it is noticed that solar wind velocity never reached to a value of 560 km/s during the entire period of this storm event. Instead, the solar wind velocity varies between ∼300 and 370 km/s during the main phase and varies between ∼370 and 500 km/s during the recovery phase of the storm period. These publicly available data of ACE solar wind and magnetic parameters were also published in earlier studies [e.g., Fok et al. 2010]. In their reply to our comment, Sreeja et al. [2011] insisted that they have estimated the time delay based on the solar wind speed of ∼560 km/s on 24 July. However, their estimation of time delay is inappropriate because the geomagnetic storm is actually developed on 22 July. Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto, Japan. Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan. Space Environment Research Center, Kyushu University, Fukuoka, Japan. Indian Institute of Geomagnetism, Navi Mumbai, India. Solar‐Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan.