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Indian Institute of Geomagnetism (IIG)
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Recent Submissions
Diurnal UT variation of low latitude geomagnetic storms using six indices
(2021) Balan, N.; Tulasiram, S.; Manu, V.; Zhao, Lingxin; Xing, Zan-Yang; Zhang, Qing-He
(Abstract): A quasi-semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm-time) index. However, the pattern has been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT variation of the storms automatically identified in six available indices including Kyoto Dst, USGS (United States Geological Survey) Dst, SymH (symmetric-H), RC (ring current), Dcx (corrected extended Dst), and AER (Atmospheric and Environmental Research) in 50, 50, 36, 21, 5, and 7 years, respectively. The indices are derived using 4, 4, 12, 14, and 15 ground observatories (with maximum longitude separations of ∼120°, 120°, 70°, 110°, and 50°) and four DMSP (Defense Meteorology Satellite Program) satellites, respectively. The UT distribution of the storm intensity (minimum value of an index during the storm main phase) in all indices shows a striking quasi semidiurnal
type variation with maxima around 06–08 UT and 21–23 UT and minima around 03–05 UT
and 13–15 UT. Similar quasi-semidiurnal variation is also observed in the computed values of the main energy input in the ring current. The variation correlates well with the variations of the dipole tilt angles μ and θ involved in the equinoctial hypothesis and Russell-McPherron (RM) effect, respectively. These observations indicate that the quasi-semidiurnal variation is real. (Plain Language Summary) Large disturbances in the geomagnetic field lasting form several hours to several days are known as geomagnetic storms. The variations of the occurrence and intensity of the storms with solar activity and season have been understood thanks to the works of a large number of scientists. The variation of the storms with the time-of-day studied using mainly the low latitude
geomagnetic activity index Dst has shown a quasi-semidiurnal pattern. The pattern, however, has been argued as apparent due to the uneven longitude distribution of the four magnetic observatories used for deriving Dst. The present study investigates the diurnal UT variation of the storms using six available indices. The results show similar striking quasi-semidiurnal patterns in the UT distribution of the storm intensity in all indices and computed value of the main energy input in the ring current. The quasi semidiurnal
pattern also correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect. These observations indicate that the quasi-semidiurnal variation is real.
Planetary wave activity observed in atmosphere-ionosphere system over low latitudes
(2022) Jadhav, Ashish; Gurubaran, S.; Patil, Parashram T.
The coupled response of the atmosphere-ionosphere system to planetary waves propagating from below has been observed through MF and Meteor radars at different longitudes along with the ground geomagnetic data from 23 stations of Northern (NH) and Southern Hemisphere (SH) during northern winter months of January, 2015 and 2017. The focus is on delineating the quasi-2-day (Q2DW) and quasi-6-day wave signatures in the mesosphere-lower thermosphere (MLT) and in ionospheric Sq currents besides deciphering their effects on the overall neutral dynamics at low latitudes. Analysis extended to longitudinally separated stations confirms the penetration of these planetary waves into the ionosphere either directly or indirectly through interaction with other wave modes in the MLT region.
Multipoint analysis of source regions of EMIC waves and rapid growth of subpackets
(2021) Ojha, Biswajit; Omura, Yoshiharu; Singh, Satyavir; Lakhina, Gurbax S.
Electromagnetic Ion Cyclotron (EMIC) rising tone emissions are important in understanding the nonlinear wave evolution and interaction with the energetic particles. We present observations of rising tone emissions of EMIC waves by THEMIS A, D, and E spacecraft in the outer magnetosphere. These emissions with subpacket structures in the proton band were observed during the time interval 14:20 UT to 14:30 UT on September 9, 2010. The observed waves are left-handed polarized with wave normal angles lessð ´ð ´ than 30â—¦. THEMIS A was closest to the equator and was in a higher L-shell than THEMIS E and D. The smallest radial separað ´ð ´tion is ∼ 2,000 km between THEMIS E and D spacecraft.
This configuration of THEMIS allows us to investigate the subpackets of rising tone EMIC waves observed simultaneously at 14:23 UT by three spacecraft. Hilbert Huang Transformation (HHT) is applied to show the variations of the instantaneous frequency and the observed wave amplitude. The direction of energy flow is determined from the analysis of the Poynting flux. There is a rapid nonlinear growth of the EMIC subpackets within one wavelength. Subpackets are dynamic in nature as their structure changes within one wave period, which is further supported by the nonlinear wave growth theory. Optimum and threshold amplitudes for the EMIC wave growth are calculated beside the nonlinear transition ð ´ð ´time (ð ´ð ´ð ‘ ð ‘ ). Observed ion energies and pitch angle spectra of the ion fluxes are consistent with the energy associated with the Landau and cyclotron resonance conditions.
Plain Language Summary Electromagnetic Ion Cyclotron (EMIC) waves are observed below the proton gyrofrequency and play an important role in the magnetospheric dynamics through the ion heating and precipitation of relativistic electrons. They often appear as a series of repetitive structures (known as subpackets) with increasing frequencies, known as rising tone emissions in their entirety. These rising tone emissions are believed to be generated near the geomagnetic equator by the anisotropic distribution of energeticð ´ð ´ ions (𠑇𠑇⟂𠑇𠑇∥ > 1). These emissions are self-sustaining after the primary linear growth of the triggering wave at lower frequencies. Although previous simulations and theory showed that the source regions of these rising tone emissions move along the magnetic field line, direct observational evidence was missing. Our article provides a case study where these emissions are observed simultaneously by three THEMIS probes in the outer magnetosphere. Adopting a multipoint observation technique, we show there are scattered source regions with an extent greater than the EMIC wavelength, and the subpacket structure changes nonlinearly within one wave period. This analysis provides crucial information about the dynamics of the fine structures of rising emissions and gives an idea about the 3D extent of subpackets.
Large-amplitude electrostatic fluctuations at the earth's magnetopause with a vortex-like distribution of hot electrons
(2022) Rufai, O. R.; Khazanov, G. V.; Singh, S. V.; Lakhina, G. S.
Large-amplitude electrostatic solitary waves (ESWs) associated with asymmetric magnetic reconnection at the Earth’s magnetopause are studied in a four-component plasma composed of a mixture of the magnetosheath and magnetosphere plasma of a cold, warm and hot electron populations, and background ions. The species are
modeled as adiabatic fluids except for the hot electrons which have a kinetic vortex-like velocity distribution. The hybrid model uses the Sagdeev pseudopotential technique to study the arbitrary amplitude ion- and electron-acoustic solitons and double layers. The numerical computations reveal that for the parameters corresponding to magnetosphere side of the ion diffusion region, only slow electron-acoustic solitons and double layer can exist. On the magnetosheath side of the ion diffusion region, only the electron-acoustic/beam solitons can exist. The electric field amplitude of the electrostatic solitary waves (ESWs) predicted by our model are consistent with the Magnetospheric Multiscale (MMS) observations.
Barometric pressure correction to gamma-ray observations and its energy dependence
(2021) Datar, Gauri; Vichare, Geeta; Chelliah, Selvaraj
Cosmic rays (CRs) have been studied extensively in the last century to understand the processes in the universe as well as in the solar system. In today's satellite era, although many observations are made from space, CR observations from the ground are still viewed as a significant tool. These observations, however, mainly detect the secondary cosmic rays (SCRs) produced via nuclear spallation processes during the interactions of the primary CR with the atmospheric nuclei. Neutron, muon, and gamma are the major components of SCRs detected on the ground. It is well known that atmospheric pressure plays a vital role in the SCR flux observed on the ground. Barometric pressure correction is standard practice for neutron monitor (NM) data. For gamma-rays, however, being massless, their pressure dependence is not intuitive. Nevertheless, the pressure affects the particles such as e±, μ±, which produce gamma rays in the cascade. Subsequently, the indirect pressure dependence of the gamma-ray flux can be anticipated. We examine this aspect in detail by studying the gamma-ray counts detected by the NaI (Tl) detector. The present study confirms that there is no correlation between the atmospheric pressure and the total counts covering the entire energy range (150 keV–10 MeV) recorded by the NaI detector. However, the scenario differs when the fluxes of different energies are investigated separately. The gamma rays of energy below ∼3 MeV are primarily due to the radioactivity originating from the ground, whereas gamma rays above 3 MeV are mainly produced in the CR cascade. It is observed that the counts of energy above 3 MeV are well anti-correlated with the atmospheric pressure. The barometric coefficient obtained here matches well with that reported by the previous studies which used anti-coincidence methods. This may indicate that the role of directly detected muons and electrons by the NaI (Tl) in the observed pressure dependence is non-significant. It is demonstrated that applying the barometric correction formula to NaI (Tl) data successfully removes the pressure dependence in the flux above 3 MeV. Therefore, we suggest that the particle flux data above 3 MeV measured by NaI (Tl) detector needs to be corrected for the local atmospheric pressure variations.
Electrostatic solitary structures in space plasmas: soliton perspective
(2021) Lakhina, Gurbax Singh; Singh, Satyavir; Rubia, Rajith; Devanandhan, Selvaraj
Occurrence of electrostatic solitary waves (ESWs) is ubiquitous in space plasmas, e.g.,
solar wind, Lunar wake and the planetary magnetospheres. Several theoretical models have been
proposed to interpret the observed characteristics of the ESWs. These models can broadly be put into
two main categories, namely, Bernstein–Green–Kruskal (BGK) modes/phase space holes models,
and ion- and electron- acoustic solitons models. There has been a tendency in the space community
to favor the models based on BGK modes/phase space holes. Only recently, the potential of soliton
models to explain the characteristics of ESWs is being realized. The idea of this review is to present
current understanding of the ion- and electron-acoustic solitons and double layers models in multicomponent space plasmas. In these models, all the plasma species are considered fluids except the
energetic electron component, which is governed by either a kappa distribution or a Maxwellian
distribution. Further, these models consider the nonlinear electrostatic waves propagating parallel to
the ambient magnetic field. The relationship between the space observations of ESWs and theoretical
models is highlighted. Some specific applications of ion- and electron-acoustic solitons/double layers
will be discussed by comparing the theoretical predictions with the observations of ESWs in space
plasmas. It is shown that the ion- and electron-acoustic solitons/double layers models provide a
plausible interpretation for the ESWs observed in space plasmas.
Terrestrial resonant oscillations during the 11 April 2012 Sumatra doublet earthquake
(2021) Nayak, Srinivas; Bagiya, Mala S.; Maurya, Satish; Hazarika, Nava Kumar; Kumar, A. S. Sunil; Prasad, D. S. V. V. D.; Ramesh, D. S.
The Earth's background free oscillations at ∼3.7 and ∼4.4 mHz resonantly couple with the atmospheric acoustic modes and thus energy cross-talk between the earth-atmosphere system is maximum at these frequencies. The present study proposes resonant coupling between the Earth's surface and atmosphere during the 11 April 2012 Sumatra doublet earthquake and offer a possible explanation to this occurrence. Following both these earthquakes, prolonged ionospheric oscillations centered at frequency of ∼4 mHz were observed in GPS (Global Positioning System) derived total electron content (TEC) towards north-northeast of the epicenters. We scrutinize these oscillations in terms of the manifestations of plausible non-tectonic and tectonic forcing mechanisms surrounding the epicentral region. Non-tectonic forcing such as the geomagnetic field coupling factor and observation geometry played a critical role in determining the amplitude anisotropy of resonant ionospheric signatures. Further, the Rayleigh waves of the first earthquake (Mw 8.6) were already characterized by an excess of energy at ∼4 mHz. We propose this could make the Mw 8.6 earthquake particularly efficient to excite the 4 mHz resonance in the atmosphere. The resonant ionospheric signatures after the second earthquake (Mw 8.2) were observed to be closely associated with the Earth's free oscillations caused by R2 Rayleigh wave train of the Mw 8.6 earthquake event. Together, all the above point to a scenario where the resonant ionospheric signatures during the Sumatra doublet event were indeed related to the seismic source. Therefore, resonant co-seismic ionospheric signatures could provide additional information on the low-frequency features of seismic ruptures.
Investigation on high velocity plasmas and field aligned currents at high latitudes
(2020) Akhila, J.C.K.; Kumar, C. P. Anil
The interaction of high velocity plasma with Earth’s magnetic field is fundamental and offer many questions on high latitude electrodynamics. The problems associated with influence of electric field and Field Aligned Current (FAC) generation is investigated with the aid of spherical cap harmonic analysis at 830 Mag. Lat. in southern hemispheres. The investigation is done on the cases with different Interplanetary Magnetic Field (IMF) conditions after the earth directed solar events. The helio-plasma parameters viz., density, velocity, energy, electron temperature are also noted during the field aligned current studies. It seems that, due to external magnetic field influence polarization of plasma electric field take place (reorientation of the convective cells). It happens with different orientation as per the magnitude and direction of By and Bz component and the horizontal currents. It is noted that the FAC value also depends on kinetic energy of the plasma streams and conductivity of external loading. As the plasma decelerates by force Jsw X Esw, the resultant current may extend along the field lines. Increases in the FAC density are seemed to be proportional to the transmission function.
Quantitative assessment of protons during the solar proton events of September 2017
(2021) Pandya, Megha; Veenadhari, B.
(Abstract)
We present multi-spacecraft observations of the proton fluxes spanning from 1.5 to 433 MeV for the largest solar proton event of solar cycle 24, i.e., September 7 and 10, 2017. In September 2017, M5.5 flare on September 4, X9.3 flare on September 6 and X8.2 flare on September 10 gave rise to solar proton event when observed by near-Earth spacecrafts. On September 7 and September 10, 2017, a strong enhancement in the proton intensities was observed by Advanced Composition Explorer (ACE) and WIND at L1 and Van Allen Probes, GOES-15 and POES-19 in the Earth's inner magnetosphere. Below geosynchronous orbit, Van Allen Probes and POES-19 show that no significant proton flux was observed with eneErgies  25 MeV on September 4, while the fluxes peaked 3 to 7-times during September 7 and by 25 times during the third proton flux event on September 10, 2017. Van Allen Probe-A observation shows that the closest distance that solar proton fluxes could approach the EaErth is L∼4.4 for 102.6 MeV energiEes on 10th September 2017, while lower energy protons i.e., 25 MeV are observed deepE up to L∼3.4 E on 11th September 2017. POES-19 observations show that there is no particular magnetic local time (MLT) dependence of the solar proton flux and is symmetric everywhere at high and low latitudes. The measurements from multiple spacecrafts located in the different regions of the Earth's magnetosphere show that the increased level of solar proton flux population persisted for ∼2 days. Thus, we quantify the temporal flux variability in terEms of L -value, energy and MLT.
(Plain Language Summary) During a solar energetic particle (SEP) event, energetic electrons and ions flood the heliosphere causing severe damage to satellites, radio communication and humans in space. The Earth's magnetic field controls the dynamics of these particles to near-Earth space. One such unique event was observed in September 2017 for which the energy spectra and quantification of the proton fluxes spanning from 1.5 to 433 MeV using multi-satellite observations is studied. This was the largest proton event of the solar cycle 24 with three M-class and four X-class flares were observed by
near-Earth spacecrafts. Proton fluxes were quantified at different locations like L1 point, geostationary orbit, inner magnetosphere, and low altitudes. The extent of flux enhancements, its access into the Earth's magnetosphere, MLT dependence and time to reach maximum fluxes are computed and compared before and after the SEPs arrived. We show that the multiple spacecraft observations are the key tool to quantify the temporal flux variability in terms of L-value, energy and MLT.
Observatories in India
(2007) Lakhina, G. S.; Alex, S.
Nonresonant instability of kinetic Alfven waves with κ-electrons
(2020) Barik, K. C.; Singh, S. V.; Lakhina, G. S.
A nonresonant instability of kinetic Alfvén waves (KAWs) is studied in a three-component plasma system consisting of background cold ions, an ion beam, and hot electrons with a κ-distribution. The nonresonant KAW instability is produced by the combined sources of ion beam and velocity shear. It is found that the wave excitation by velocity shear alone will give rise to purely growing KAWs, whereas the ion beam velocity alone as a source cannot excite the waves for the considered plasma parameters. It is also observed that the combined sources of ion beam and velocity shear can excite the KAWs in nonresonant instability with finite wave frequency (the mode is not a purely growing mode). Also note that κ-electrons restrict the wave propagation very close to 90°, whereas the Maxwellian electrons permit the wave to propagate a few degrees away from 90°. It is inferred that the presence of κ-electrons shrinks the wave-unstable region of a KAW’s nonresonant instability. The coupling between KAWs and ion-acoustic waves occurs at a lower value of βi for Maxwellian electrons as compared to κ-electrons.
Ground-based GNSS and C/NOFS observations of ionospheric irregularities over Africa: a case study of the 2013 St. Patric's Day geomagnetic storm
(2021) Amaechi, P. O.; Oyeyemi, E. O.; Akala, A. O.; Messanga, H. E.; Panda, S. K.; Seemala, Gopi K.; Oyedokun, J. O.; Fleury, R.; Amory-Mazaudier, C.
In this paper, the variations of ionospheric irregularities have been studied using C/NOFS, ground-based GNSS and magnetometer measurements in Africa during the St. Patrick geomagnetic storm of March 17, 2013. The latitudinal distribution of irregularities was examined using GNSS-ROTI maps covering longitude 25°–45°E. Longitudinal characteristics were also investigated along with equatorial plasma bubbles (EPBs) and vertical drift velocity (Vz) from 12 to 21 March 2013. The results show postsunset irregularities from 12°S–27°N with the stronger ones confined within 1°S–7°S and 12°N–22°N in the prestorm period. The observed pre-reversal enhancement (PRE) with Vz varying from
22.51–59.47 m/s between 20.26 and 20.86 LT corresponded with the occurrence of EPBs. PRE greater than 40 m/s nevertheless, supported long lasting depletions. During the main phase, prompt penetration electric field enhanced the PRE thus, extended the latitudinal range of irregularities to 31°N. It also induced a long duration EPB along 15°E and several depletions over the Eastern sector. During the recovery phase, storm time wind drove a conspicuous asymmetry in the morphology of the postsunset anomaly. This corresponded with the reduction in the latitudinal extent and strength of irregularities. Westward/eastward disturbance dynamo electric field inhibited/triggered irregularities in the postsunset/ postmidnight period on 18 March over the Eastern sector. The difference in the drift accounted for the longitudinal variations of irregularities before the storm. During the main phase however, irregularities were present (reduced) over the Eastern (Atlantic/Western) sectors. This difference might have been related to the changes in the wind inferred from the anomaly shape.
Large geomagnetically induced currents at equator caused by an interplanetary magnetic cloud
(2022) Nilam, B.; Tulasiram, S.
Here, we report a rare observation of an extremely large and rapid change of geomagnetic field (dB/dt), a proxy for the geomagnetically induced currents (GICs), at the equator caused by a sudden drop in solar wind density at the front boundary of a magnetic cloud (MC) during the great 31 March 2001 storm. The horizontal component at the Indian equatorial station, Tirunelveli, recorded a sharp decline of ∼350 nT in just 5 min with a peak dB/dt exhibiting a concerning value of 136 nT/min, a possible GIC risk to the electric power systems. The responsible physical mechanisms were examined through magneto-hydrodynamic model simulations and found that a prompt penetration of strong westward-overshielding electric fields and ionospheric currents at the equator play a dominant role. This study provides some new insights into the extent of extreme geomagnetic field changes that can occur at the equatorial region due to solar wind density reduction at MC, which can have potential impacts on the electric power grid systems.
Plain Language Summary: The Earth, a planet with an intrinsic magnetic field bubble around it, is immersed in a hot and energetic solar wind plasma that continuously emanated from the Sun. The sudden disturbances on the Sun, such as coronal mass ejections, induce transient structures in the solar wind which, when directed earthward, can cause severe disturbances in the Earth's (Geo) magnetic field. The rapid changes in the geomagnetic field induce electric fields at the conducting surface of Earth, which can cause strong electrical currents, known as geomagnetically induced currents (GICs), that flow through the long conducting structures, like electric power transmission lines, long pipelines, etc., on the ground. The enhanced GICs are of serious threat to the electric power and pipeline grids. The elevated GICs are most popularly known to occur at high latitudes during severe geomagnetic storms. This study reports a rare observation of a large and rapid change in geomagnetic field indicating the strong GICs at the geomagnetic equator due to a magnetic cloud structure in solar wind and reveals the underlying physical processes.
Association of ionospheric signatures to various tectonic parameters during moderate to large magnitude earthquakes: case study
(2021) Sunil, A. S.; Bagiya, Mala S.; Bletery, Quentin; Ramesh, D. S.
The sudden ground movement associated with Mw > 6.5 earthquakes is considered a potential source of ionospheric electron density perturbations over the fault region. Coseismic ground displacement is a function of various seismic source parameters such as moment magnitude, focal depth, and focal mechanism etc. We study here the distinct effects of vertical ground displacement, moment magnitude and focal depth on coseismic ionospheric perturbation (CIP) amplitudes during moderate-tolarge earthquakes. We analyze GPS-total electron content variations during 59 dip-slip earthquakes that occurred in the last 20 years. Our study reveals that though CIP amplitudes are primarily controlled by moment magnitude, they are also sensitive to the earthquake focal depth. To understand the influence of focal depth on the displacement field and therefore on CIP amplitudes, we present a simple synthetic test, for a depth range of 0–200 km, highlighting that the maximum vertical ground displacement decreases logarithmically with increasing focal depth while the volume (i.e., integrated vertical ground displacement) of uplifted/subsided material varies very marginally. We conclude that CIP is sensitive to the wavelength of co-seismic vertical displacement field and that seismic energy propagation to the overlying atmosphere during deep earthquakes is not adequate to generate detectable CIP.
Plain Language Summary A small part of energy released during earthquakes transfer to the atmosphere in terms of mechanical waves. For large magnitude earthquakes (generally Mw > 6.5), the atmospheric amplification of such seismically induced waves can generate disturbances in ionospheric electron density termed as Coseismic Ionospheric Perturbations (CIP). The moment magnitude of earthquakes mainly controls the amplitude of ionospheric perturbations. In this study, the effect of focal depth on CIP amplitudes are investigated. The analysis of 59 dip-slip earthquakes shows that deep earthquakes generate smaller CIP amplitudes. The sudden ground movement during earthquakes is responsible for the transfer of seismic energy to the atmosphere. Our study shows that CIP amplitudes are not only sensitive to earthquake magnitude but also to their focal depth. Though the integrated displacement field is insensitive to the earthquake depth, its distribution is very different, concentrated in a narrow area for shallow earthquakes, spread over a wide region for deep earthquakes. CIP amplitudes appear to scale with the maximum values of vertical surface displacement rather than their average. Thus, the energy transfer is more efficient during shallow earthquakes.
Finite amplitude electron-acoustic waves in the electron diffusion region
(2021) Rufai, Odutayo R.; Khazanov, George V.; Singh, S.V.
The electron-acoustic solitons are studied with two temperature electrons (a hot trapped having vortex-like velocity distribution and a warm adiabatic fluid) and stationary ions. The theoretical model is based on the observations of a mixture of the hot, tenuous magnetospheric and warm, dense magnetosheath plasma particles associated with the asymmetric magnetic reconnection at the Earth’s magnetopause by Magnetospheric Multiscale (MMS). Using the reductive perturbation technique, the model supports the existence of nonlinear electron-acoustic structures derived from the mKdV-like equation. The electron-acoustic waves propagate at supersonic speeds above the electron sound speed. The results are applied to observations of electric field structures in the electron diffusion region (EDR).
Ionospheric disturbances over the Indian sector during 8 September 2017 geomagnetic storm: plasma structuring and propagation
(2021) Alfonsi, L.; Cesaroni, C.; Spogli, L.; Regi, M.; Paul, A.; Ray, S.; Lepidi, S.; Mauro, D. Di; Haralambous, H.; Oikonomou, C.; Shreedevi, P. R.; Sinha, A. K.
The series of X and M class flares and associated coronal mass ejections that occurred on the first days of September 2017 induced significant perturbations on the low-latitude ionospheric electrodynamics. On 8 September in the Indian sector, the storm caused a severe modification of the equatorial electrojet (EEJ) with a consequent variation of the ionospheric structuring and dynamics. In our analysis, we propose an original method to isolate and identify EEJ variations from geomagnetic data and we detect the presence of equatorial plasma bubbles (EPB) from L-band total electron content
(TEC) data in order to understand their movement. Our results provide evidence of independent EPBs appearance freshly generated and inherited from a migrating plasma structure. The EPB (or EPBs) occurring in the south of India is/are freshly generated just above the magnetic equator, and is/are likely triggered by the sudden increase of EEJ just before the local sunset, acting as a pre-reversal enhancement. The EPB appearing in the North-East Indian region is associated with a migrating structure, resulting in a northward movement with a velocity of about 650 m/s, possibly testifying the passage of a large-scale traveling ionospheric disturbance. The occurrence of severe post-sunset scintillations in the northeastern sector suggests a possible cascade process forming small-scale irregularities from the migrating EPB.
Plain Language Summary At low latitudes, the ionosphere, the upper part of the atmosphere rich in free electrons, shows a peculiar distribution of its electron density with two maxima around the geomagnetic equator and a minimum above it. When a geomagnetic storm occurs, this configuration can be deeply modified. The way and the persistence of this modification is a matter of study and, to date, still unpredictable. Our study aims to contribute to the advancement of the knowledge in the field presenting a detailed reconstruction of the ionospheric response over India to the geomagnetic storm occurred in early September 2017. We have detected an uneven distribution of free electrons that shows clear evidence of two deep minima (called bubbles): the first one originated elsewhere and then transported over the considered region, the other one freshly and locally produced. Regional analysis of the ionospheric response to the geomagnetic activity can help to improve the space weather forecasting capabilities, supporting the development of alerts and mitigation tools for the users of communication and navigation systems.
Interplay of diverse atmospheres
(2021) Gawali, Praveen
Enhanced gravity wave activity in the mesosphere lower thermosphere region over Tirunelveli as a response to tropospheric convective event
(2023) Krishnapriya, K.; Sathishkumar, S.; Sridharan, S.
The recent upgrade of Medium-Frequency (MF) wind radar at Tirunelveli (8.7⠰N, 77.8⠰E) has improved the height and time resolution of the wind measurements, which are utilized in the present study to examine high frequency (in periods 20–60 min) gravity waves (GW) in the mesosphere and lower thermosphere (MLT) region. We observe, in addition to the dominant wave activity during equinox months, dominant episodes of high-frequency GW activities in the meridional winds during the times 31 May–4 June and 25–27 June 2019. Using the perturbation ellipse method, we infer the direction of propagation of the GW and it is found to be in the north–south plane. The GW activity in the MLT region exhibits anti-correlation with NOAA outgoing longwave radiation (OLR) and positive correlation with rainfall rates indicating the latent heat release due to tropical convection as the possible source of the GW. Besides, the presence of dynamical instability is inferred from the calculations using the radar wind and the space borne SABER (Sounding of Atmosphere using Broadband Emission Radiometry) temperature data suggesting a possible causality of convectively generated GWs dissipation.
Theory of ion holes in plasmas with flat-topped electron distributions
(2023) Aravindakshan, Harikrishnan; Vasko, Ivan Y.; Kakad, Amar; Kakad, Bharati; Wang, Rachel
Coherent bipolar electric field structures with negative unipolar potentials are widely observed in space plasmas. These bipolar structures are often found to be ion Bernstein Greene Kruskal (BGK) modes or ion holes. Most theoretical models of ion holes assume them to be stationary with respect to the background plasma that follows either Maxwellian or kappa-type distribution. In this paper, we present a new theoretical model of ion holes where the structures are non-stationary, and electrons follow flat-topped distribution. We use the classical BGK approach to derive the inequality separating allowed and forbidden simultaneous values of amplitude and spatial width of ion holes. The model reveals that the parametric space for the existence of ion holes decreases with their speed. We applied the developed model to the largest available dataset of ion holes obtained from the magnetospheric multiscale spacecraft observations in the Earth's bow shock region.
A case of anomalous electric field perturbations in the equatorial ionosphere during post-sunset hours: Insights
(2023) Kumar, A.; Chakrabarty, D.; Fejer, B. G.; Reeves, G. D.; Rout, D.; Sripathi, S.; Seemala, G. K.; Sunda, S.; Yadav, A. K.
During a weak geomagnetic storm (Ap = 15) on 24 December 2014, the penetration electric field perturbations over the Indian dip equatorial sector are found to be anomalous on a number of occasions during post-sunset hours. The event is anomalous as the magnitude and polarity of penetration electric fields do not obey the existing paradigm. The penetration electric field perturbations are investigated using the vertical drifts derived from the CADI (Canadian Advanced Digital Ionosonde) measurements at Tirunelveli (8.7° N, 77.7° E, dip angle: 1.7°). During this event, we observed post-sunset vertical drift of ∼42 ms−1 not only at 1810 LT but also ∼36 ms−1 at ∼2100 LT which is anomalous. Interestingly, the dawn-dusk component of interplanetary electric field (IEFy) is relatively less ( < 2 mV/m) at ∼2100 LT compared to the interval 1930-2030 LT (IEFy ∼3 mV/m). Despite that, the vertical drift observed over Tirunelveli is very close to zero or nominally upward during 1930-2030 LT. In addition, the downward drift just after 2130 LT on this night is found to be exceptionally large ( ∼-60 ms−1). By combining vertical total electron content over the Indian sector with the OI 630.0 nm airglow intensity from Mt. Abu, chain of magnetometer and Los Alamos National Laboratory (LANL) geosynchronous satellite particle measurements, it is suggested that the anomalous penetration electric field perturbations on this night arise from the effects of IMF By and substorm.
Plain Language Summary Variation in the zonal electric field in the equatorial ionosphere during post-sunset hours is important to understand the plasma distribution over low latitudes and also generation of plasma irregularities. The changes in the ionospheric conditions over low/equatorial latitudes have implications for communication and navigational applications. Therefore, if ionospheric electric field over equatorial ionosphere behaves anomalously during space weather events, it will be difficult to model the low latitude ionosphere for scientific understanding and practical applications. In this investigation, we show that the less studied Y-component of interplanetary magnetic field and substorm can significantly modulate the ionospheric electric field giving rise to anomalous response
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