Ionospheric D-region Research Articles

<i>D</i>-region ion–neutral coupled chemistry (Sodankylä Ion Chemistry, SIC) within the Whole Atmosphere Community Climate Model (WACCM 4) – WACCM-SIC and WACCM-rSIC

Abstract. This study presents a new ion–neutral chemical model coupled into the Whole Atmosphere Community Climate Model (WACCM). The ionospheric D-region (altitudes ∼ 50–90 km) chemistry is based on the Sodankylä Ion Chemistry (SIC) model, a one-dimensional model containing 307 ion–neutral and ion recombination, 16 photodissociation and 7 photoionization reactions of neutral species, positive and negative ions, and electrons. The SIC mechanism was reduced using the simulation error minimization connectivity method (SEM-CM) to produce a reaction scheme of 181 ion–molecule reactions of 181 ion–molecule reactions of 27 positive and 18 negative ions. This scheme describes the concentration profiles at altitudes between 20 km and 120 km of a set of major neutral species (HNO3, O3, H2O2, NO, NO2, HO2, OH, N2O5) and ions (O2+, O4+, NO+, NO+(H2O), O2+(H2O), H+(H2O), H+(H2O)2, H+(H2O)3, H+(H2O)4, O3−, NO2−, O−, O2, OH−, O2−(H2O), O2−(H2O)2, O4−, CO3−, CO3−(H2O), CO4−, HCO3−, NO2−, NO3−, NO3−(H2O), NO3−(H2O)2, NO3−(HNO3), NO3−(HNO3)2, Cl−, ClO−), which agree with the full SIC mechanism within a 5 % tolerance. Four 3-D model simulations were then performed, using the impact of the January 2005 solar proton event (SPE) on D-region HOx and NOx chemistry as a test case of four different model versions: the standard WACCM (no negative ions and a very limited set of positive ions); WACCM-SIC (standard WACCM with the full SIC chemistry of positive and negative ions); WACCM-D (standard WACCM with a heuristic reduction of the SIC chemistry, recently used to examine HNO3 formation following an SPE); and WACCM-rSIC (standard WACCM with a reduction of SIC chemistry using the SEM-CM method). The standard WACCM misses the HNO3 enhancement during the SPE, while the full and reduced model versions predict significant NOx, HOx and HNO3 enhancements in the mesosphere during solar proton events. The SEM-CM reduction also identifies the important ion–molecule reactions that affect the partitioning of odd nitrogen (NOx), odd hydrogen (HOx) and O3 in the stratosphere and mesosphere.

22 July 2009 total solar eclipse induced gravity waves in ionosphere as inferred from GPS observations over EIA

In the present contribution we investigate the variation in the Global Positioning System (GPS) derived ionospheric Total Electron Content (TEC) over Equatorial Ionization Anomaly (EIA) region on the rare occasional astronomical phenomenon of total solar eclipse of 22 July 2009. The aim is to study and identify the wave like structure enumerated due to solar eclipse induced gravity waves in the F-region ionosphere altitude. The work is aimed to understand features of horizontal and vertical variation of atmospheric gravity waves (AGWs) properties over the Equatorial Ionization Anomaly (EIA) region in Indian low latitude region. The ionospheric observations is from the site of Allahabad (lat 25.4°N; lon. 81.9°E; dip 38.6°N) located at the fringe of eclipse totality path. The estimated vertical electron density profile from FORMOSAT-3/COSMIC GPS-RO satellite, considering all the satellite line of sight around the time of eclipse totality shows maximum depletion of 43%. The fast fourier transform and wavelet transform of GPS DTEC data from Allahabad station (Allahabad: lat 25.4N; lon. 81.9E) shows the presence of periodic waves of ∼20 to 45min and ∼70 to 90min period at F-region altitude. The shorter period correspond to the sunrise time morning terminator and longer period can be associated with solar eclipse generated AGWs. The most important result obtained is that our results along with previous result for wave like signatures in D-region ionosphere from Allahabad station show that AGWs generated by sunrise time terminator have similarity in the D and F region of the ionosphere but solar eclipse induced AGWs show higher period in the F-region compared to D-region ionosphere.

Investigation of the morphology and Wait’s parameter variations of the low-latitude D region ionosphere using the multiple harmonics of tweeks

Recording the tweeks with a maximum up to eight harmonics using the receiver installed at Tay Nguyen University (12.65° N, 108.02° E) during 2013–2014, we investigated the morphology of the nighttime D-region ionosphere. Tweeks were recorded on 5 quiet nights per month. The results show that the mean reflection height in 2014 (Rz=79.3) is lower by 3.3km than that in 2013 (Rz=64.9). The reflection height at low latitudes is higher than that at high latitudes. The mean reference height h′ in 2013 is higher about 0.9km than that in 2014 and the mean sharpness factor β in 2013 is higher by 0.07km−1 than that in 2014. The short-term variation of reflection heights for tweeks with harmonics m=1–3 and sunspot number have the negative correlation coefficients. However, the correlations between them are not clear. On some nights, from 19:00–21:00LT, the reflection height temporal variability shows a moderate to strong negative correlation with the tweek occurrence. This suggests that the reflection height variation may be caused by QE fields generated by lightning discharges. The variations of tweek reflection heights observed during 2013–2014, at low latitudes could be significantly caused by the ionization effect by Lyman-α and Lyman-β coming from geocorona, variation of neutral density, particle precipitations, and by direct energy coupling between lightning and lower ionosphere.

Влияние вспышек на Солнце в марте 2012 г. на профили проводимости высокоширотной нижней ионосферы

The ionospheric D-layer affects the electromagnetic waves propagated in the Earth – ionosphere waveguide. It is known that the propagation velocity of atmospherics – electromagnetic pulses from lightning discharge depends on the conductivity profile of the lower ionosphere. In this paper the authors have considered the influence of solar flares in March 2012 on the propagation velocity of atmospherics and thus the state of the high-latitude lower ionosphere. The possibility to estimate the conductivity profiles of the daytime ionosphere under disturbed and undisturbed geomagnetic conditions using the measurements of the propagation velocity of atmospherics along the high-latitude path has been demonstrated. Ключевые слова: волновод Земля – ионосфера, D-слой, скорость распространения, атмосферик, солнечная вспышка.

Behaviour of electron content in the ionospheric D-region during solar X-ray flares

One of the most important parameters in ionospheric plasma research, also having a wide practical application in wireless satellite telecommunications, is the total electron content (TEC) representing the columnal electron number density. The F-region with high electron density provides the biggest contribution to TEC while the relatively weakly ionized plasma of the D-region (60 km { 90 km above Earth's surface) is often considered as a negligible cause of satellite signal disturbances. However, sudden intensive ionization processes, like those induced by solar X-ray flares, can cause relative increases of electron density that are significantly larger in the D-region than in regions at higher altitudes. Therefore, one cannot exclude a priori the D-region from investigations of ionospheric influences on propagation of electromagnetic signals emitted by satellites. We discuss here this problem which has not been sufficiently treated in literature so far. The obtained results are based on data collected from the D-region monitoring by very low frequency radio waves and on vertical TEC calculations from the Global Navigation Satellite System (GNSS) signal analyses, and they show noticeable variations in the D-region's electron content (TECD) during activity of a solar X-ray ?are (it rises by a factor of 136 in the considered case) when TECD contribution to TEC can reach several percent and which cannot be neglected in practical applications like global positioning procedures by satellites.

Upper atmospheric remote sensing using ELF – VLF lightning generated tweek and whistler sferics

Tweek and whistler atmospherics (sferics) recorded at a low latitude station, Suva (18.2oS, 178.3oE, geomag. lat. 22.2oS, L = 1.17) Fiji, in the South Pacific region, during September 2003–July 2004 and March–December 2006 were analyzed to probe the nighttime D-region ionosphere and plasmasphere. From the first harmonic analysis of the selected 1063 tweeks, the path integrated ionospheric reflection height was estimated mostly between 75–97 km with median and standard deviation of 83.6 and 4.1 km, respectively. The path integrated mean electron density was estimated to vary from 24.4–31.5 cm–3 over the reflection heights. The whistler recorded on 20 September 2006, a magnetically quite day with maximum three hourly Kp value of 2o, most likely propagated in the ducted mode to this station. The whistler spectrum was used to calculate the equatorial magnetic field and the equatorial electron density at L = 1.3 which are in agreement with low latitude whistler observations.

Seasonal variations of nighttime D-region ionosphere in 2013 solar maximum observed from a low-latitude station

We present the observation of tweek atmospherics with harmonics m = 1–8 during the solar maximum year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65° N, 108.02° E). The analysis of 33,690 tweeks on ten international quiet days during 2 months each season, summer (May, August), winter (February, November), and equinox (March, September), shows that tweeks occur about 51 % during summer, 22 % during winter, and 27 % during equinox. The D-region ionosphere is more sharply bounded for harmonics m = 5–6 around an altitude of 85.5 km. The environment of the D-region is more inhomogeneous during winter and equinox seasons. The mean electron density varies from 28.4–225 cm −3, which corresponds to the harmonics m = 1–8 at the mean reflection height of 81.5–87.7 km. The results reveal that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The equivalent electron density profile of the nighttime D-region ionosphere using tweek method during summer, equinox, and winter seasons shows lower values of electron density by 12–58 %, 3–67 %, and 24–76 % than those obtained using the International Reference Ionosphere (IRI-2012) model.

Воздействие магнитных бурь на нижнюю ионосферу в авроральной зоне

The results of the paper were based upon data on magnetic storms for a five-year period (2008–2012) such as vertical sounding data, magnetometer and riometer data from the Sodankylä station, and satellite data. The main efforts were directed to reveal the relationship between the level of absorption in the ionospheric D-region and magnetic field intensity in a disturbance bay. We detected thresholds in the level of absorption when ionospherically reflected signals were absent in the vertical sounding. Another task of the study was to reveal the relationship between maximum frequencies of Es layers and magnetic field intensity for the magnetic storm period. We established differences between the mentioned regularities for daytime and nighttime periods.

The three-dimensional global numerical model CHARM-I: The incorporation of processes in the ionospheric D-region

We describe the three-dimensional numerical global photochemical model CHARM-I (CHemical Atmospheric Research Model with Ions) and the results of numerical calculations of global distributions of neutral and charged atmospheric trace gases (in the height range of up to 90 km), such as ozone, nitrogen oxides, electrons, and positive and negative ions. This model is an improved version of the CHARM three-dimensional photochemical model of neutral components with additional reactions with the involvement of ions (a total of 200 photochemical reactions). The model incorporates UV-radiation fluxes on the Lyman-α line and galactic cosmic rays as ionizing factors. The neutral components are calculated with the method of “chemical families” and the concentrations of charged components are calculated by the electroneutrality condition at each time step. The spatial transport of chemically active species is described in the model by the Prather scheme. The developed model makes it also possible to take into account solar flares and particle precipitations in the ionospheric D-region.

Remote sensing of D-region ionosphere using multimode tweeks

Lightning discharges radiate electromagnetic waves in a wide frequency range, with maximum energy in extremely low frequency/very low frequency band. A part of the radiated extremely low frequency/very low frequency wave energy is trapped in the Earth–ionosphere waveguide and travels thousands of kilometers in different modes with lower attenuation. Amplitude, frequency and phase of these waves are used to study the less explored D-region ionosphere at lower latitudes. Extremely low frequency/very low frequency observations are recorded continuously by automatic whistler detector setup installed at low-latitude Indian station Lucknow (Geom. lat. 17.6°N; long. 154.5°E). In total, 149 cases of tweeks having modes ranging from 3 to 6 have been recorded by automatic whistler detector during December 2010 and analyzed. Result shows that the propagation distance in the Earth–ionosphere waveguide lies between 1.1 and 9.4 Mm. The electron density in the lower D-region varies between 25 and 150 cm−3. The upper boundary of the waveguide varies between 80 and 95 km. The reported results are in good agreement with the earlier measurements at different latitudes and longitudes.

Solar flare induced ionospheric D-region perturbation as observed at a low latitude station Agra, India

The results of solar flare induced D-region perturbation studies along a short great circle path (GCP=6690 km) lying entirely in the low and equatorial latitude region are presented. We use SoftPAL receiver at Agra (Geograph. lat. 27.2°N, long. 78°E), India and monitor NWC signal (f=19.8 kHz) transmitted from Australia. We analyze the data for the year 2011 and find that the results of amplitude and phase perturbations, time delay, zenith angle independence, and electron density variation in the lower ionosphere are consistent with those observed along similar paths at low and high latitudes. The new work includes; (i) the distribution of peak X-ray flares in the mixed solar cycle period 2011 responsible for clear and measurable sudden phase anomalies (SPAs) is different from that in minimum solar cycle period, though the cut off hardening factor is the same; (ii) the phase anomalies are evaluated in terms of X-ray fluence (J/m2); (iii) the perturbation due to X-class of flare is used to calculate the electron densities in 70–60 km height range which are found to be 60–80 % lower than those in the polar region where X-ray flare is followed by solar proton event.

ВОЗМУЩЕНИЕ ИОНОСФЕРЫ МОЩНЫМ НЕПРЕРЫВНЫМ ПРОХОДЯЩИМ РАДИОИЗЛУЧЕНИЕМ: РЕЗУЛЬТАТЫ ЧИСЛЕННОГО МОДЕЛИРОВАНИЯ

Numerical simulations of disturbances in the electron temperature and density arising in the ionosphere from high-power continuous radio transmissions at various frequencies, power levels, and polarizations have been performed. It has been demonstrated that huge disturbances arise in the ionospheric D region electron temperature. In the E and F regions, the nocturnal electron temperature is also significantly disturbed. The electron density is disturbed much less than the electron temperature at all altitudes. Key words: ionosphere, high-power radio transmissions, numerical simulations, disturbances in the electron temperature and density Manuscript submitted 06.11.2014 Radio phys. radio astron. 2015, 20(1): 48-63 REFERECES 1. GUREVICH, A. V. and SHVARTSBURG, A. B., 1973. Nonlinear Theory of Radio waves Propagation in the Ionosphere. Moscow, Russia: Nauka Publ. (in Russian). 2. GUREVICH, A. V., 1978. Nonlinear Phenomena in the Ionosphere. New York, Heildelberg, Berlin: Springer-Verlag. https://doi.org/10.1007/978-3-642-87649-3 3. CHERNOGOR, L. F., 2014. Physics of High-Power Radio Emissions in Geospace: Monograph. Kharkiv, Ukraine: V. N. Karazin Kharkiv National Universine Publ. (in Russian). 4. GINZBURG, V. L. and GUREVICH, A. V., 1960. Nonlinear phenomena in a Plasma located in an alternating electromagnetic field. Usp. Fiz. Nauk. vol. 70, is. 2, pp. 201–246 (in Russian). DOI: https://doi.org/10.3367/UFNr.0070.196002a.0201 5. GINZBURG, V. L. and GUREVICH, A. V., 1960. Nonlinear phenomena in a Plasma located in an alternating electromagnetic field. Usp. Fiz. Nauk. vol. 70, is. 3, pp. 393–428 (in Russian). DOI: https://doi.org/10.3367/UFNr.0070.196003a.0393 6. GINZBURG, V. L., 1960. Propagation of Electromagnetic Waves in Plasmas. Moscow, Russia: Nauka Publ. (in Russian). 7. GUREVICH, A. V., 2007. Nonlinear effects in the ionosphere. Usp. Fiz. Nauk. vol. 177, is. 11, pp. 1145–1177 (in Russian). DOI: https://doi.org/10.1070/PU2007v050n11ABEH006212 8. GUREVICH , A. V. and SHLYUGER, I. S., 1975. Investigations of nonlinear phenomena with a powerful radio pulse in the lower ionosphere. Izv. Vysh. Uchebn. Zaved. Radiofizika. vol. 43, no. 9, pp. 1237–1260 (in Russian). 9. BRYUNELLI, B. E. and NAMGALADZE, A. A., 1987. Physics of the ionosphere. Moscow, Russia: Nauka Publ. (in Russian). 10. SCHUNK, R. W. and NAGY, A. F., 2000. Ionospheres: Physics, Plasma Physics, and Chemistry. – Cambridge: Cambridge University Press. https://doi.org/10.1017/CBO9780511551772 11. GOKOV, A. M., MARTYNRNKO, S. I., MISYURA, V. A., PIVEN', L. A., SOMOV, V. G., CHERNOGOR. L. F. and SHEMET, A. S., 1980. A study of natural and artificially disturbed by high-power radio emission ionosphere by high-frequency ionospheric sounding. In: The 2nd Conference on Polar Ionosphere and Magnetosphere-Ionosphere Coupling Proceedings. Norilsk, Russia, pp. 32–33 (in Russian). 12. Gokov, A. M., Martynrnko, S. I., Misyura, V. A., Piven', L. A., Somov, V. G., Fedorenko, Yu. P., CHERNOGOR. L. F. and SHEMET, A. S., 1982. Electron density profiles in the nighttime high-latitude lower ionosphere, artificially disturbed by high-power radio waves. Geomagnetizn i Aeronomiya. vol. 22, no. 15, pp. 748–752 (in Russian). 13. MARTYNRNKO, S. I., MISYURA, V. A., PIVEN', L. A., SOMOV, V. G., CHERNOGOR. L. F. and SHEMET, A. S., 1983. Disturbance of the inhomogeneous structure and regulary parameters in the middle and high-latitude lower ionosphere by continuous and periodic radiation. In: The Suzdal Symposium on Effects of high-power artificial radio emission on Earth ionosphere u1047 Proceedings. Sept 1983, Suzdal, Russia, pp. 94–95 (in Russian). 14. CHERNOGOR, L. F., 1985. Disturbance of the inhomogeneous structure in the lower ionosphere as a result of powerful radio emission. Radiophysics and Quantum Electronics. vol. 28, no. 1, pp. 12–20. DOI: https://doi.org/10.1007/BF01037452 15. CHERNOGOR, L. F., 1991. Diagnostics of Lower Ionosphere with Powerful Nonstationary Radio-Frequency Radiation. In: The III Suzdal URSI Symposium on Modification of the Ionosphere by Powerful Radiowaves (ISIM – 3) Proceedings. Suzdal, Russia, pp. 167–168. 16. HOLT, O., BREKKE, A., HANSEN, T., KOPKA, H. and STUBBE, P., 1985. HF modification of the auroral D-region detected by a partial reflection experiment. J. Atmos. Terr. Phys. vol. 47, no. 6, pp. 537–545. DOI: https://doi.org/10.1016/0021-9169(85)90036-4 17. KERO, A., BOSINGER, T., POLLARI, P., TURUNEN, E. and REITVELD, M. 2000. First EISCAT measurement of electron-gas temperature in the artificially heated D-region ionosphere. Ann. Geophys. vol. 18, no. 9, pp. 1210–1215. DOI: https://doi.org/10.1007/s00585-000-1210-8

Contribution of solar hydrogen Lyα line emission in total ionization rate in ionospheric D-region during the maximum of solar X-flare

The solar Ly? line emission can be considered as the dominant source of ionization processes in the ionospheric D-region at altitudes above 70 km during unperturbed conditions. However, large sudden impacts of radiation in some other energy domains can also significantly influence the ionization rate and, in this paper, we present a study on the contribution of Ly? radiation to the ionization rate when the ionosphere is disturbed by solar X-flares. We give relevant analytical expressions and make calculations and numerical simulations for the low ionosphere using data collected by the VLF receiver located in Serbia for the VLF radio signal emitted by the DHO transmitter in Germany.

Ionospheric D-region temperature relaxation and its influences on radio signal propagation after solar X-flares occurrence

In this paper our attention is focused on relations between radio signal propagation characteristics and temperature changes in D-region after solar X-flare occurrence. We present temperature dependencies of electron plasma frequency, the parameter that describes medium conditions for propagation of an electromagnetic wave, and the refractive index which describes how this wave propagates. As an example for quantitative calculations based on obtained theoretical equations we choose the reaction of the D-region to the solar X-flare occurred on May 5th, 2010. The ionospheric modelling is based on the experimental data obtained by low ionosphere observations using very low frequency radio signal.

Modeling of high frequency radio wave absorption on oblique soundings during a solar X-ray flare

High frequency radio wave absorption induced by Solar Ultra-Violet (UV) and X-ray flux is investigated. The influence of the solar flare observed on 11 April 2013 on the structure of oblique sounding ionograms in the Arctic region of Russia is considered. An adjustable model of the ionosphere developed for high frequency (HF) propagation problems was employed for this purpose. The simulation algorithm has been designed to accept a large variety of ionospheric conditions. On the basis of the SWPC D-region Absorption model the absorption effects in the ionosphere at sub-auroral latitudes of the Earth were calculated. This approach does not require knowledge of the electron density and electron collision frequency profiles of the D-region ionosphere. The oblique ionograms simulated with the absorption effect and ionograms provided by Russian network of ionospheric observations deployed in Arctic region exhibit quite a good resemblance.

Space weather effects on the low latitude D-region ionosphere during solar minimum

The effects of the solar flares and the geomagnetic storms (disturbance storm time (Dst) < −50 nT) during December 2006 to 2008, a period during the unprecedented solar minimum of solar cycles 23 and 24, have been examined on sub-ionospheric very low frequency (VLF) signals from NWC (19.8 kHz), NPM (21.4 kHz), VTX (18.2 kHz), and NLK (24.8 kHz) transmitters monitored at Suva (18.2° S, 178.4° E), Fiji. Apart from the higher class solar flares (C to X), a solar flare of class B8.5 also produced enhancements both on the amplitude and phase. The amplitude enhancements in NLK, NPM, and NWC signals as a function of peak solar flare X-ray flux in decibel (dB; relative to 1 μW/m2) shows that the relationship curve is steeper and quite linear between the flare power levels of 0 to 15 dB; below 0 dB, the curve gets less steep and flattens towards −5 dB flare power level, while it also gets less steep above 15 dB and almost flattens above 20 dB. In general, the level of amplitude enhancement for NLK signal is higher than that for NPM and NWC signals for all solar flares. The enhancement in the amplitude and phase of VLF signals by solar flares is due to the increase in the D-region electron density by the solar flare-produced extra ionization. The modeling of VLF perturbations produced by B8.5 and C1.5 classes of solar flares on 29 January 2007 using LWPC (Long Wave Propagation Capability) V2.1 codes show that reflection height (H') was reduced by 0.6 and 1.2 km and the exponential sharpness factor (β) was raised by 0.010 and 0.005 km−1, respectively. Out of seven storms with Dst < −50 nT, only the intense storm of 14 to 16 December 2006 with a minimum Dst of −145 nT has shown a clear reduction in the signal strength of NWC and NPM sub-ionospheric signals due to storm-induced reduction in the D-region electron density.

Response of the mid-latitude D-region ionosphere to the total solar eclipse of 22 July 2009 studied using VLF signals in South Korean peninsula

In this paper, we analyze VLF signals received at Busan to study the the D-region changes linked with the solar eclipse event of 22 July 2009 for very short (∼390km) transmitter–receiver great circle path (TRGCP) during local noon time 00:36–03:13UT (09:36–12:13KST). The eclipse crossed south of Busan with a maximum obscuration of ∼84%. Observations clearly show a reduction of ∼6.2dB in the VLF signal strength at the time of maximum solar obscuration (84% at 01:53UT) as compared to those observed on the control days. Estimated values of change in Wait ionospheric parameters: reflection height (h′) in km and inverse scale height parameter (β) in km−1 from Long Wave Propagation Capability (LWPC) model during the maximum eclipse phase as compared to unperturbed ionosphere are 7km and 0.055km−1, respectively. Moreover, the D-region electron density estimated from model computation shows 95% depletion in electron density at the height of ∼71km. The reflection height is found to increase by ∼7km in the D-region during the eclipse as compared to those on the control days, implying a depletion in the Lyman-α flux by a factor of ∼7. The present observations are discussed in the light of current understanding on the solar eclipse induced D-region dynamics.

Plasma irregularities in the D-region ionosphere in association with sprite streamer initiation.

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Although the streamer nature of sprites has been generally accepted, how these filamentary plasmas are initiated remains a subject of active research. Here we present observational and modelling results showing solid evidence of pre-existing plasma irregularities in association with streamer initiation in the D-region ionosphere. The video observations show that before streamer initiation, kilometre-scale spatial structures descend rapidly with the overall diffuse emissions of the sprite halo, but slow down and stop to form the stationary glow in the vicinity of the streamer onset, from where streamers suddenly emerge. The modelling results reproduce the sub-millisecond halo dynamics and demonstrate that the descending halo structures are optical manifestations of the pre-existing plasma irregularities, which might have been produced by thunderstorm or meteor effects on the D-region ionosphere.

First results from imaging riometer installed at Indian Antarctic station Maitri

Cosmic noise absorption (CNA) measurred by imaging riometer, is an excellent tool to passively study the high latitude D-region ionospheric conditions and dynamics. An imaging riometer has been installed at Indian Antarctic station Maitri (geographic 70.75°S, 11.75°E; corrected geomagnetic 63.11°S, 53.59°E) in February 2010. This is the first paper using the imaging riometer data from Maitri. The present paper introduces the details of this facility, including its instrumentation, related CNA theory and its applications. Sidereal shift of around 2 hours in the diurnal pattern validates the data obtained from the newly installed instrument. Moreover, the strength of cosmic noise signal on quiet days also varies with months. This is apparently due to solar ionization of D-region ionosphere causing enhanced electron density where collision frequency is already high. The main objective of installing the imaging riometer at Maitri is to study magneotspheric–ionospheric coupling during substorm processes. In the current study, we present two typical examples of disturbed time CNA associated with storm-time and non-storm time substorm. Results reveal that CNA is more pronounced during storm-time substorm as compared to non-storm time substorm. The level of CNA strongly depends upon the strengthening of convectional electric field and the duration of southward turning of interplanetary magnetic field before the substorm onset.

Characteristic of Tweek Atmospherics Observed in Mid-latitude using AWESOME VLF Receiver

This study presents the analysis of tweek atmospherics received by AWESOME VLF receiver at station of Gakona (62.71°N, 143.99°W) during four months observation from January to April 2011. Tweek which originates from lightning discharge are used to monitor the nighttime D-region ionosphere using the fundamental cut-off frequency to measure the variations of the lower ionosphere’s reflection height, the equivalent electron density at the reflection height and the propagation distance travel by tweeks. In this study, a total of 1316 tweeks are analyzed and from the analysis, it shows that equinox’s season has the highest tweek occurrence compared to winter season in March and April. The maximum harmonic (m) of t weeks is found to be up to fourth (m = 4) and tweeks with mode number one (m = 1) are more dominantly occurred. Our observations indicate that the equivalent electron densities for tweeks varies from 22-27 eL/cm3 in the altitude ranged of 75 to 91 km and demonstrate that these ELF/VLF signals travel considerable distances up to 6700 km from the causative lightning discharges. The ionospheric parameters for three locations (high, middle and low latitude respectively) were compared and the results show that they are almost consistent for all the locations.