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We study the effects of space weather on the ionosphere and low Earth orbit (LEO) satellites’ orbital trajectory in equatorial, low- and mid-latitude (EQL, LLT and MLT) regions during (and around) the notable storms of October/November, 2003. We briefly review space weather effects on the thermosphere and ionosphere to demonstrate that such effects are also latitude-dependent and well established. Following the review we simulate the trend in variation of satellite’s orbital radius ( r ), mean height ( h ) and orbit decay rate ( ODR ) during 15 October–14 November 2003 in EQL, LLT and MLT. Nominal atmospheric drag on LEO satellite is usually enhanced by space weather or solar-induced variations in thermospheric temperature and density profile. To separate nominal orbit decay from solar-induced accelerated orbit decay, we compute r , h and ODR in three regimes viz. (i) excluding solar indices (or effect), where r = r 0 , h = h 0 and ODR = ODR 0 (ii) with mean value of solar indices for the interval, where r = r m , h = h m and ODR = ODR m and (iii) with actual daily values of solar indices for the interval ( r , h and ODR ). For a typical LEO satellite at h  = 450 km, we show that the total decay in r during the period is about 4.20 km, 3.90 km and 3.20 km in EQL, LLT and MLT respectively; the respective nominal decay ( r 0 ) is 0.40 km, 0.34 km and 0.22 km, while solar-induced orbital decay ( r m ) is about 3.80 km, 3.55 km and 2.95 km. h also varied in like manner. The respective nominal ODR 0 is about 13.5 m/day, 11.2 m/day and 7.2 m/day, while solar-induced ODR m is about 124.3 m/day, 116.9 m/day and 97.3 m/day. We also show that severe geomagnetic storms can increase ODR by up to 117% (from daily mean value). However, the extent of space weather effects on LEO Satellite’s trajectory significantly depends on the ballistic co-efficient and orbit of the satellite, and phase of solar cycles, intensity and duration of driving (or influencing) solar event. [ABSTRACT FROM AUTHOR]

Cosmic ray flux in our planetary system is primarily modulated by solar activity. Radiation effects of cosmic rays on the Earth strongly depend on latitude due to the variation of the geomagnetic field strength. To study these effects we carried out a series of measurements of the radiation characteristics in the atmosphere due to cosmic rays from various places (geomagnetic latitude: ∼14.50°N) in West Bengal, India, located near the Tropic of Cancer, for several years (2012–2016) particularly covering the solar maximum in the 24th solar cycle. We present low energy (15–140 keV) secondary radiation measurement results extending from the ground till the near space (∼40 km) using a scintillator detector on board rubber weather balloons. We also concentrate on the cosmic ray intensity at the Regener-Pfotzer maxima and find a strong anti-correlation between this intensity and the solar activity even at low geomagnetic latitudes. [ABSTRACT FROM AUTHOR]

We present effects of the annular solar eclipse of 15th January 2010 on the topside ionosphere using the DEMETER satellite data. Measurements of the electron-ion density and electron temperature by the ISL (Instrument Sonde de Langmuir) and IAP (Instrument Analyseur de Plasma) instruments on board the DEMETER satellite during the eclipse time over the low latitude (±40) Indian ocean area are presented. We found the peak decrease in electron density to be about 35% and the peak decrease in ion density to be about 40% from the reference orbits at the altitude of the satellite (∼660 km). Electron and ion temperatures were found to have decreased by 200–300 K at the same altitude. Instead of simple decrease as in ion density, electron temperature showed a complex wave-like oscillation as the solar eclipse progressed. Electron density decreased to a minimum value before the maximum obscuration and starts to increase before passing through another minimum at the time of maximum obscuration. Both the minima are located at the ±10° geomagnetic latitude. Variations of electron and ion densities were found to follow the average solar illumination experienced by the satellite and its conjugate points at satellite altitude, while the electron temperature showed no such correlation. [ABSTRACT FROM AUTHOR]

The amplitude and phase of VLF/LF radio signals are sensitive to changes in electrical conductivity of the lower ionosphere which imprints its signature on the Earth–ionosphere waveguide. This characteristic makes it useful in studying sudden ionospheric disturbances, especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbance and storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region of the ionosphere. In this paper, using the data of three propagation paths at mid-latitudes (40–54°), we analyse the trend in variation of aspects of VLF diurnal signal under varying solar and geomagnetic space environmental conditions in order to identify possible geomagnetic footprints on the D region characteristics. We found that the trend of variations generally reflected the prevailing space weather conditions in various time scales. In particular, the ‘dipping’ of mid-day signal amplitude peak (MDP) occurs after significant geomagnetic perturbed or storm conditions in the time scale of 1–2 days. The mean signal amplitude before sunrise (MBSR) and mean signal amplitude after sunset (MASS) also exhibit storm-induced dipping, but they appear to be influenced by event's exact occurrence time and the highly variable conditions of dusk-to-dawn ionosphere. We also observed few cases of the signals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic event. This effect may be related to storms associated phenomena or effects arising from sources other than solar origin. The magnitude of induced dipping (or rise) significantly depends on the intensity and duration of event(s), as well as the propagation path of the signal. The post-storm day signal (following a main event, with lesser or significantly reduced geomagnetic activity) exhibited a tendency of recovery to pre-storm day level. In the present analysis, we do not see a well-defined trend in the variation of the post-storm sunrise amplitude terminator (SRT) and sunset terminator (SST). The SRT and SST signals show more dipping in GQD-A118 propagation path but generally an increase along DHO-A118 propagation path. Thus the result could be propagation path dependent and detailed modelling is required to understand these phenomena. [ABSTRACT FROM AUTHOR]

Variability classes in the enigmatic black hole candidate GRS 1915+105 are known to be correlated with the variation of the Comptonizing Efficiency (CE) which is defined to be the ratio between the number of power-law (hard) photons and seed (soft) photons injected into the Compton cloud. Similarities of light curves of several variability classes of GRS 1915+105 and IGR 17091-3624, some of which are already reported in the literature, motivated us to compute CE for IGR 17091-3624 as well. We find that they are similar to what were reported earlier for GRS 1915+105, even though masses of these objects could be different. The reason is that the both the sizes of the sources of the seed photons and of the Comptonizing corona scale in the same way as the mass of the black hole. This indicates that characterization of variability classes based on CE is likely to be black hole mass independent, in general. [ABSTRACT FROM AUTHOR]

The upper atmosphere changes significantly in temperature, density and composition as a result of solar cycle variations, which causes severe storms and flares, and increases in the amount of absorbed solar radiation from solar energetic events. Satellite orbits are consequently affected by this process, especially those in low Earth orbit (LEO). In this paper, we present a model of atmospheric drag effects on the trajectory of two hypothetical LEO satellites of different ballistic coefficients, initially injected at h = 450 km. We investigate long-term trends of atmospheric drag on LEO satellites due to solar forcing induced atmospheric perturbations and heating at different phases of the solar cycle, and during short intervals of strong geomagnetic disturbances or magnetic storms. We show dependence of orbital decay on the severity of both solar cycle and phase and the extent of geomagnetic perturbations. The result of the model compares well with observed decay profile of some existing LEO satellites and provide a justification of the theoretical considerations used here. [ABSTRACT FROM AUTHOR]

Abstract: The Galactic black hole candidate H 1743-322 exhibited two X-ray outbursts in rapid succession: one in August 2010 and the other in April 2011. We analyze archival data of this object from the PCA instrument on board RXTE (2–25keV energy band) to study the evolution of its temporal and spectral characteristics during both the outbursts, and hence to understand the behavioral change of the accretion flow dynamics associated with the evolution of the various X-ray features. We study the evolution of QPO frequencies during the rising and the declining phases of both the outbursts. We successfully fit the variation of QPO frequency using the Propagating Oscillatory Shock (POS) model in each of the outbursts and obtain the accretion flow parameters such as the instantaneous shock locations, the shock velocity and the shock strength. Based on the degree of importance of the thermal (disk black body) and the non-thermal (power-law) components of the spectral fit and properties of the QPO (if present), the entire profiles of the 2010 and 2011 outbursts are subdivided into four different spectral states: hard, hard-intermediate, soft-intermediate and soft. We attempt to explain the nature of the outburst profile (i.e., hardness-intensity diagram) with two different types of mass accretion flow. [Copyright &y& Elsevier]

Abstract: The Galactic microquasar GRS 1915+105 exhibits at least seventeen types of variability classes. Intra and inter class transitions are reported to be observed within seconds to hours. Since the observation was not continuous, these classes appeared to be exhibited in a random order. Our goal is to predict a sequence of these classes. In this paper, we compute the ratio of the photon counts obtained from the power-law component and the blackbody component of each class and call this ratio as the ‘Comptonizing efficiency’ (CE) of that class. We sequence the classes in the ascending order of CE and find that this sequence matches with a few class transitions observed by RXTE satellite and IXAE instruments on board IRS-P3. A change in CE corresponds to a change in the optical depth of the Compton cloud. Our result implies that the optical depth of the Compton cloud gradually rises as the variability class becomes harder. [Copyright &y& Elsevier]

Abstract: During the total solar eclipse of 2009, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2kHz. It was observed that in several places, the signal amplitude is enhanced while in other places the amplitude is reduced. We simulated the observational results using the well known Long Wavelength Propagation Capability (LWPC) code. As a first order approximation, the ionospheric parameters were assumed to vary according to the degree of solar obscuration on the way to the receivers. This automatically brought in non-uniformity of the ionospheric parameters along the propagation paths. We find that an assumption of 4km increase of lower ionospheric height for places going through totality in the propagation path simulate the observations very well at Kathmandu and Raiganj. We find an increase of the height parameter by km for the VTX-Malda path and km for the VTX-Kolkata path. We also present, as an example, the altitude variation of electron number density throughout the eclipse time at Raiganj. [Copyright &y& Elsevier]

The study of secondary particles produced by the cosmic-ray interaction in the Earth's atmosphere is very crucial as these particles mainly constitute the background counts produced in the high-energy detectors at balloon and satellite altitudes. In the present work, we calculate the abundance of cosmic-ray generated secondary particles at various heights of the atmosphere by means of a Monte Carlo simulation and use this result to understand the background counts in our X-ray observations using balloon-borne instruments operating near the tropical latitude (geomagnetic latitude: ~ 14.50 ° N). For this purpose, we consider a 3D description of the atmospheric and geomagnetic field configurations surrounding the Earth, as well as the electromagnetic and nuclear interaction processes using Geant4 simulation toolkit. Subsequently, we use a realistic mass model description of the detector under consideration, to simulate the counts produced in the detector due to secondary cosmic-ray particles. [ABSTRACT FROM AUTHOR]

Earthquake preparation processes may start 1–30 days before its actual occurrence. Measurements of outgoing longwave radiation (OLR) and detection of the presence of atmospheric gravity waves (AGW) in very low frequency (VLF) radio signals can be used as tools to identify such processes. We studied these signals monitored prior to a recent major earthquake that occurred in Nepal at southeast of Kodari on May 12, 2015 at 12:50 pm local time (07:05 UTC) with Richter scale magnitude of M  = 7.3 and depth 10 km (6.21 miles). It was preceded by another major earthquake on April 25, 2015 with magnitude M  = 7.9. First, to study the effects of seismic events on OLR, we used NOAA/IR daily (two degree gridded) data from April 16 to May 30, 2015 and followed the method of Eddy field calculation mean to find pre-seismic anomalies. We found singularities in Eddy field OLR curves around the earthquake epicenter starting 3 days prior to the earthquake days and disappearance of such singularities after the events. Such singularities can be associated with a large amount of energy released by the earthquakes. Second, we analyzed very low frequency (VLF) data recorded at Ionospheric and Earthquake Research Centre (IERC) of Indian Centre for Space Physics transmitted from JJI (22.2 kHz) station of Japan. We looked for the presence of atmospheric gravity waves in the ionosphere which can be considered as an important factor in finding seismo-ionospheric correlations. We performed both fast Fourier transform (FFT) and wavelet analysis on the signal and found significant presence of such waves (periods of almost 1 h) four days before the earthquake. [ABSTRACT FROM AUTHOR]

Continuous ground-based monitoring of Very Low Frequency (VLF) transmitter signals is an efficient remote sensing tool for studying of the lower ionosphere (60–90 km). Here, we present the use of VLF radio data to study short-period (∼min–hrs) atmospheric gravity waves and long-period (∼days) planetary waves. We analyse VLF data from several receiving stations obtained by ICSP-VLF network during the total solar eclipse of July, 2009 to show the existence of short-period atmospheric gravity waves. We find dominant wave periods range from 10 min to 1 h around the time of maximum eclipse phase which could be associated with atmospheric gravity waves excited due to the eclipse. We also analyse VLF amplitude data of 2007 received at ICSP, Kolkata from VTX (18.2 kHz) transmitter for planetary wave-type oscillations in the mesosphere–lower ionosphere system. Fourier and wavelet analysis show presence of periodic structures with periodicity in the range of 5–27 days. We compare VLF planetary spectrum with spectrum obtained from total column density of Ozone and mesospheric average temperature data which may indicate vertical coupling between the stratosphere and ionosphere in winter to early spring time. [ABSTRACT FROM AUTHOR]

To examine the quality and propagation characteristics of the Very Low Frequency (VLF) radio waves in a very long propagation path, Indian Centre for Space Physics, Kolkata, participated in the 27th Indian scientific expedition to Antarctica during 2007–2008. One Stanford University made AWESOME VLF receiving system was installed at the Indian Antarctic station Maitri and about five weeks of data were recorded successfully from the Indian transmitter VTX and several other transmitting stations worldwide. The quality of the signal from the VTX transmitter was found to be very good, consistent and highly stable in day and night. The signal shows the evidences of the presence of the 24 h solar radiation in the Antarctic region during local summer. Here we report the both narrow band and broadband VLF observations from this site. The diurnal variations of VTX signal (18.2 kHz) are presented systematically for Antarctica path and also compared the same with the variations for a short propagation path (VTX-Kolkata). We compute the spatial distribution of the VTX signal along the VTX-Antarctica path using the most well-known LWPC model for an all-day and all-night propagation conditions. The calculated signal amplitudes corresponding to those conditions relatively corroborate the observations. We also present the attenuation rate of the dominant waveguide modes corresponding to those propagation conditions where the effects of the Antarctic polar ice on the attenuation of different propagating waveguide modes are visible. [ABSTRACT FROM AUTHOR]

Abstract: A ground based Very Low Frequency (VLF) radio receiver of Indian Centre for Space Physics located at Khukurdaha (, monitored the VLF signal at 19.8kHz from the NWC station during a partial solar eclipse (maximum obscuration 75%) which took place on January 15, 2010. The receiver and the transmitter were on two opposite sides of the annular eclipse belt. During the same period, a solar flare also occurred and it was partly blocked by the lunar disk. Thus the resultant signal was perturbed both by the eclipse and by the flare. The deviation of the signal from the normal value was obtained by subtracting from the average diurnal signal on days bracketing the eclipse. The deviation was analysed. We compare the data from GOES-14, HINODE and RHESSI satellites during the event. We got a clear depression in the data during the period of the partial eclipse. Most interestingly, there was also a flaring activity in the sun which reached its peak (C-type) just after the time when the eclipse was near maximum. By superposing the lunar disk on the image obtained by HINODE mission, we pin-point the time frame of blocking hard and soft X-rays. We extract the time variation of the electron density profile in the D-region of the ionosphere due to occulted solar flare from the combined effect of the eclipse and the flare. We also compare the results with a normal solar flare. [Copyright &y& Elsevier]

Ionospheric-magnetospheric transition region and the time correlation of particle rate fluctuation and earthquake has been a subject of interest for various authors for the last few decades. Van Allen Radiation Belt (VAB) is a zone of energetic charged particles originated from solar wind that are captured by Earth's magnetic field lines. There are several Low Earth Orbital (LEO) satellites to observe count rate of energetic particles in the VAB. In the present study, we used Medium Energy Proton and Electron Detector (MEPED) instrument data which is on-board the NOAA-15 satellite. This study presents a comparison of the effects of a land earthquake and an ocean earthquake on the observed particle bursts (PBs). The transport of electromagnetic emissions of seismic origin from the ground to the magnetosphere through the ionosphere depends on various land and atmospheric variables. One of the primary parameters is the ground conductivity. The conductivity of ocean surface is much higher compared to that of the land surface and thus, an ocean earthquake is expected to impact the ionosphere-magnetosphere region more than a land earthquake. With this aim, we considered one land earthquake on January 3, 2016 and an ocean earthquake on December 6, 2016. The data were taken from the NOAA website and analyzed for the entire months of January and December, 2016. For the January 3 (land) earthquake, PBs were found to be accumulated only around the earthquake day, being maximum on the day of the earthquake and with complete absence of such events on days away from the event day. For the December 6 (ocean) earthquake, the effects were found to be post-seismic in nature and existed for days away from the earthquake day. Also the maximum particle count rate recorded on the day of the December 6 earthquake was found to be significantly high compared to the January 3 earthquake. We conclude that the difference in ground conductivity of land and ocean surface is the decisive factor behind such differences in the observed effects. [ABSTRACT FROM AUTHOR]

We have searched for transient and variable radio sources in the field of Galactic micro-quasar Cygnus X-1 near 1.4 GHz (L band) using data from the Karl G. Jansky Very Large Array. We used twenty years of data between 1983 and 2003. We found a source NVSS J1957+35 showing transient behavior. The source was also mentioned earlier in NVSS and WENSS catalog but its transient nature was not reported earlier. The source is located 23.8 arcminutes far from Cygnus X-1. It is detected many times during the span of our study and it varied between less than 1.9 mJy (3 σ) to 201 mJy. NVSS J195754+353513 also showed high intra-day variability. In one occasion, the source rose from ∼15 mJy to ∼170 mJy within 700 s. We detected circularly polarized emission from the source for a limited number of cases with fractional circular polarization varies between 0.14 and 0.17. 2MASS J19575420+3535152 may be the near-infrared counterpart of the source. We compared the properties of the source with other Galactic transient sources having similar properties. The nature of the source is still unknown. We discussed the possible nature of the source. [ABSTRACT FROM AUTHOR]

This paper reports the first extensive study of the existence and effects of interstellar hydrogen bonding. The reactions that occur on the surface of the interstellar dust grains are the dominant processes by which interstellar molecules are formed. Water molecules constitute about 70% of the interstellar ice. These water molecules serve as the platform for hydrogen bonding. High level quantum chemical simulations for the hydrogen bond interaction between 20 interstellar molecules (known and possible) and water are carried out using different ab-intio methods. It is evident that if the formation of these species is mainly governed by the ice phase reactions, there is a direct correlation between the binding energies of these complexes and the gas phase abundances of these interstellar molecules. Interstellar hydrogen bonding may cause lower gas abundance of the complex organic molecules (COMs) at the low temperature. From these results, ketenes whose less stable isomers that are more strongly bonded to the surface of the interstellar dust grains have been observed are proposed as suitable candidates for astronomical observations. [ABSTRACT FROM AUTHOR]

We present perturbations due to seismo-ionospheric coupling processes in propagation characteristics of sub-ionospheric Very Low Frequency (VLF) signals received at Ionospheric & Earthquake Research Centre (IERC) (Lat. 22.50°N, Long. 87.48°E), India. The study is done during and prior to an earthquake of Richter scale magnitude M = 7.3 occurring at a depth of 18 km at southeast of Kodari, Nepal on 12 May 2015 at 12:35:19 IST (07:05:19 UT). The recorded VLF signal of Japanese transmitter JJI at frequency 22.2 kHz (Lat. 32.08°N, Long. 130.83°E) suffers from strong shifts in sunrise and sunset terminator times towards nighttime starting from three to four days prior to the earthquake. The signal shows a similar variation in terminator times during a major aftershock of magnitude M = 6.7 on 16 May, 2015 at 17:04:10 IST (11:34:10 UT). These shifts in terminator times is numerically modeled using Long Wavelength Propagation Capability (LWPC) Programme. The unperturbed VLF signal is simulated by using the day and night variation of reflection height ( h ′ ) and steepness parameter ( β ) fed in LWPC for the entire path. The perturbed signal is obtained by additional variation of these parameters inside the earthquake preparation zone. It is found that the shift of the terminator time towards nighttime happens only when the reflection height is increased. We also calculate electron density profile by using the Wait’s exponential formula for specified location over the propagation path. [ABSTRACT FROM AUTHOR]