Longitude Sector Research Articles

On the variation in the ionospheric response to geomagnetic storms with time of onset

AbstractRecent observations from Immel and Mannucci (2013) have indicated that geomagnetic storms cause larger enhancements in the ionospheric plasma density and total electron content (TEC) in the American sector than anywhere else on the planet. This suggests that the presence of a UT storm onset effect is important for correctly understanding the impact, longitudinal structure, and timing of geomagnetic storms. Using the Global Ionosphere‐Thermosphere Model (GITM), we conduct a modeling experiment of the August 2011 geomagnetic storm by modifying the storm arrival time (UT) in Earth's daily rotation and examining the subsequent system response. We find that the simulations reflect the recent studies indicating that the strongest enhancements of TEC are in the American and Pacific longitude sectors of storms with onsets between 1600 UT and 2400 UT. The underlying mechanisms of the strong TEC increases during storm times in these longitude sectors are also examined. Some of the resulting TEC structures may be explained by changes in the [O]/[N2] ratio (especially in the high latitudes), but it is unable to explain all of the variability in the equatorial regions. Storm time neutral winds and vertical ion motions coupled to Earth's asymmetrical geomagnetic topology appear to be driving the longitude sector variability due to UT storm onset times.

Comparison of quiet time vertical plasma drifts with global empirical models over the Indian sector: Some insights

The threshold vertical plasma drifts and their polarities are inferred from E-region irregularities reported earlier using different experiments conducted from India during geomagnetically quiet time. In addition, the hourly variations of magnetometer data sets are used in conjunction with an equatorial electrojet model (Anandarao, 1976) to deduce the vertical drifts around noon time (10:00–140:00 LT). These results are then compared with the vertical drifts presented by empirical models (Scherliess and Fejer, 1999; Fejer et al., 2008a) corresponding to the 60°E longitude sector. In general, the vertical drifts presented by empirical models are consistent with those inferred from snapshot measurements of E-region irregularities at different local times of the day except around sunrise hours. Further, the vertical drifts presented by Fejer et al. (2008a) model match fairly well with seasonally averaged vertical drifts deduced (within 1σ variation) using magnetometer data. A time difference is noticed between occurrence of pre-reversal enhancement in vertical drifts over India reported earlier using different techniques and Scherliess and Fejer (1999) model. Probable reason for the time difference is discussed. The occurrence characteristic of afternoon equatorial counter electrojet in June solstice during low solar epoch is consistent with the drifts obtained from empirical models. The seasonally averaged vertical drifts during nighttime reported earlier using ionosonde/HF radar experiments are not consistent with the presence of streaming plasma waves on a few occasions. Further, nocturnal vertical drifts are systematically under-estimated and probable reason for this is discussed.

Forecasting scintillation activity and equatorial spread F

AbstractWhen transionospheric radio waves propagate through an irregular ionosphere with plasma depletions or “bubbles,” they are subject to sporadic enhancement and fading, which is referred to as scintillation. Communication and navigation systems may be subject to these detrimental effects if the scintillation is strong enough. It is critical to have knowledge of the current ionospheric conditions so that system operators can distinguish between the natural radio environment and system‐induced failures. In this paper we briefly describe the Forecasting Ionospheric Real‐time Scintillation Tool UHF scintillation forecasting technique, which utilizes the observed characteristic parameter h′F from a ground‐based, ionospheric sounder near the magnetic equator. The prereversal enhancement in vertical E × B drift velocity after sunset is the prime driver for creating plasma depletions and bubbles. In addition, there exists a “threshold” in the h′F value at 1930 LT, h′Fthr, such that, on any given evening, if h′F is significantly above h′Fthr, then scintillation activity is likely to occur, and if it is below h′Fthr, scintillation activity is unlikely to occur. We use this technique to explain the lack of scintillation activity prior to the Halloween storm in October 2003 in the Peruvian longitude sector. In addition, we have carried out a study which forecasts the occurrence or nonoccurrence of equatorial spread F (ESF), on a night‐to‐night basis, in five longitude sectors. The overall forecasting success is greater than 80% for each of the five longitude sectors.

Impact of multiconstellation satellite signal reception on performance of satellite‐based navigation under adverse ionospheric conditions

AbstractApplication of multiconstellation satellites to address the issue of satellite signal outages during periods of equatorial ionospheric scintillations could prove to be an effective tool for maintaining the performance of satellite‐based communication and navigation without compromise in accuracy and integrity. A receiver capable of tracking GPS, Global Navigation Satellite System (GLONASS), and Galileo satellites is operational at the Institute of Radio Physics and Electronics, University of Calcutta, Calcutta, India, located near the northern crest of the equatorial ionization anomaly in the Indian longitude sector. The present paper shows increased availability of satellites combining GPS, GLONASS, and Galileo constellations from Calcutta compared to GPS‐only scenario and estimates intense scintillation‐free (S4 < 0.6) satellite vehicle look angles at different hours of the postsunset period 19:00–01:00 LT during March 2014. A representative case of 1 March 2014 is highlighted in the paper and overall statistics for March 2014 presented to indicate quantitative advantages in terms of scintillation‐free satellite vehicle look angles that may be utilized for planning communication and navigation channel spatial distribution under adverse ionospheric conditions. The number of satellites tracked and receiver position deviations has been found to show a good correspondence with the occurrence of intense scintillations and poor user receiver‐satellite link geometry. The ground projection of the 350 km subionospheric points corresponding to multiconstellation shows extended spatial coverage during periods of scintillations (0.2 < S4 < 0.6) compared to GPS.

Role of IMF By in the prompt electric field disturbances over equatorial ionosphere during a space weather event

AbstractOn 7 January 2005 (Ap=40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However, the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during 14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bz condition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during 15:00–15:15 UT, despite IMF Bz remaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF By component under southward IMF Bz condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high‐latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF By are fundamentally important to understand the prompt penetration effects over low latitudes.

Assessing ionospheric response during some strong storms in solar cycle 24 using various data sources

AbstractWe present an analysis of a regional ionospheric response during six strong storms (−200 nT ≤Dst≤−100 nT) that occurred in 2012 for the geographic latitudinal coverage of 10°S–40°S within a longitude sector of 10°E–40°E. Although these storms occurred during the same solar activity period and were all coronal mass ejection driven, their impacts and associated features on the ionosphere have been found different due to different contributing factors to their driving mechanisms. With the exception of one case, the rest of the storm periods were characterized by positive storm effects during the main and (or) recovery phases with varying physical mechanisms including low‐latitude electrodynamics, neutral composition changes, and traveling ionospheric disturbances (TIDs). The common result to all the analyzed strong storms was the presence of large‐scale TIDs during the storm main phases. Using total electron content data derived from the Global Navigational Satellite System (GNSS) observations and radio occultation (RO) electron density data on a regional scale, we have attempted to investigate meridional and vertical propagation of TIDs simultaneously during the strong storms. We have showed that it is possible to identify vertical motion of TIDs using RO data in cases when equatorward TIDs, as revealed by GNSS total electron content data, are present. RO results were compared to ionosonde data, and both data sources gave vertical velocities below 100 m/s of the associated TIDs.

Midlatitude ionospheric changes to four great geomagnetic storms of solar cycle 23 in Southern and Northern Hemispheres

AbstractThis paper presents an investigation of ionospheric response to great (Dst ≤−350 nT) geomagnetic storms that occurred during solar cycle 23. The storm periods analyzed are 29 March to 2 April 2001, 27–31 October 2003, 18–23 November 2003, and 6–11 November 2004. Global Navigation Satellite System, total electron content (TEC), and ionosonde critical frequency of F2 layer (foF2) data over Southern Hemisphere (African sector) and Northern Hemisphere (European sector) midlatitudes were used to study the ionospheric responses within 15°E–40°E longitude and ±31° to ±46° geomagnetic latitude. Midlatitude regions within the same longitude sector in both hemispheres were selected in order to assess the contribution of the low‐latitude changes especially the expansion of equatorial ionization anomaly (EIA) also called the dayside ionospheric superfountain effect during these storms. In all storm periods, both negative and positive ionospheric responses were observed in both hemispheres. Negative ionospheric responses were mainly due to changes in neutral composition, while the expansion of the EIA led to pronounced positive storm effects at midlatitudes for some storm periods. In other cases (e.g., 29 October 2003), penetration electric fields, EIA expansion, and large‐scale traveling ionospheric disturbances were found to be present during the positive storm effect at midlatitudes in both hemispheres. An increase in TEC on the 28 October 2003 was because of the large solar flare with previously determined intensity of X45 ±5.

Longitudinal variation in the ionosphere‐plasmasphere system at the minimum of solar and geomagnetic activity: Investigation of temporal and latitudinal dependences

AbstractWe use the Global Self‐consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) as the first‐principle calculation of the physical system state, the quick‐run ionospheric electron density model (NeQuick) as the climatology background, and the International Reference Ionosphere‐based Real‐Time Assimilative Model for a global view of the ionospheric weather during a quiet period of the December 2009 solstice. The model computations are compared to the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation profiles, CHAMP and Gravity Recovery and Climate Experiment in situ densities, and GPS total electron content (TEC). It is shown that the plasma density in the ionosphere is generally larger in the American/Atlantic longitudinal sector at any local time. The high‐latitude density enhancements are visible in the GSM TIP output at different altitudes but are not reproduced by the NeQuick empirical model. Given that observational data confirm an existence of the high‐latitude areas where ionospheric densities are elevated in the altitude range between 300 and 480 km, we conclude that the NmF2 maximum in the GSM TIP output can be trusted. Indeed, such high‐latitude NmF2, ionospheric electron content, and TEC maxima in the American longitude sector form on the proper places as shown by the GSM TIP data, COSMIC and GPS observations. According to our results, the high‐latitude maximum of NmF2 (1) manifests itself only when the integration over LT or UT of the global maps for 22 December 2009 includes nighttime, i.e., supporting an argument of its close association with the Weddell Sea Anomaly, and (2) also appears in the Ne distribution at altitudes above the F2 peak.

Conjugate hemisphere ionospheric response to the St. Patrick's Day storms of 2013 and 2015 in the 100°E longitude sector

AbstractThe effects of the St. Patrick's Day geomagnetic storms of 2013 and 2015 in the equatorial and low‐latitude regions of both hemispheres in the 100°E longitude sector is investigated and compared with the response in the Indian sector at 77°E. The data from a chain of ionosondes and GPS/Global Navigation Satellite Systems receivers at magnetic conjugate locations in the 100°E sector have been used. The perturbation in the equatorial zonal electric field due to the prompt penetration of the magnetospheric convective under shielded electric field and the over shielding electric field gives rise to rapid fluctuations in the F2 layer parameters. The direction of IMF Bz and disturbance electric field perturbations in the sunset/sunrise period is found to play a crucial role in deciding the extent of prereversal enhancement which in turn affect the irregularity formation (equatorial spread F) in the equatorial region. The northward (southward) IMF Bz in the sunset period inhibited (supported) the irregularity formation in 2015 (2013) in the 100°E sector. Large height increase (hmF2) during sunrise produced short‐duration irregularities during both the storms. The westward disturbance electric field on 18 March inhibited the equatorial ionization anomaly causing negative (positive) storm effect in low latitude (equatorial) region. The negative effect was amplified in low midlatitude by disturbed thermospheric composition which produced severe density/total electron content depletion. The longitudinal and hemispheric asymmetry of storm response is observed and attributed to electrodynamic and thermospheric differences.

Reply to comment by Kil et al. on “The night when the auroral and equatorial ionospheres converged”

AbstractOne of the goals of the Martinis et al. (2015) (M15) paper was to stimulate interest in the topic of “severe events during moderate geomagnetic storms”—and we succeeded. We welcome the Comment by Kil et al. (2016) (K16) who identified weak signatures related to medium‐scale traveling ionospheric disturbances (MSTIDs) and tried to argue that airglow depletion signatures of equatorial spread F (ESF) were not present. We agree with K16 in that MSTIDs were present to the north of the all‐sky imager, but only during the early observations. They are easily distinguished from the ESF‐related dark structures observed at the same time to the south of the imager. We stress some observational facts that K16 overlooked or erroneously used to support their case. M15 offered evidence and discussion evidence and discussion of a dynamical pattern—both in space and time domains—and the counter arguments by K16 should have dealt with the same places and same times. We also point out that the GPS data shown by K16 do not provide any information below ~30° geographic latitude in the longitude sector covered by the all‐sky imager. This is the region where the ESF structures appeared first and grew to reach higher latitudes. We believe that the issues addressed by K16 do not provide conclusive evidence to rule out the interpretation given by M15 on the nature of the airglow depletions observed at McDonald Observatory.

Atmospheric inertia-gravity waves retrieved from level-2 data of the satellite microwave limb sounder Aura/MLS

Abstract. The temperature profiles of the satellite experiment Aura/MLS are horizontally spaced by 1.5° or 165 km along the satellite orbit. These level-2 data contain valuable information about horizontal fluctuations in temperature, which are mainly induced by inertia-gravity waves. Wave periods of 2–12 h, horizontal wavelengths of 200–1500 km, and vertical wavelengths of 6–30 km efficiently contribute to the standard deviation of the horizontal temperature fluctuations. The study retrieves and discusses the global distributions of inertia-gravity waves in the stratosphere and mesosphere during July 2015 and January 2016. We find many patterns that were previously present in data of TIMED/SABER, Aura/HIRDLS, and ECMWF analysis. However, it seems that Aura/MLS achieves a higher vertical resolution in the gravity wave maps since the maps are derived from the analysis of horizontal fluctuations along the orbit of the sounding volume. The zonal mean of the inertia-gravity wave distribution shows vertical modulations with scales of 10–20 km. Enhanced wave amplitudes occur in regions of increased zonal wind or in the vicinity of strong wind gradients. Further, we find a banana-like shape of enhanced inertia-gravity waves above the Andes in the winter mesosphere. We find areas of enhanced inertia-gravity wave activity above tropical deep convection zones at 100 hPa (z ∼ 13 km). Finally, we study the temporal evolution of inertia-gravity wave activity at 100 hPa in the African longitude sector from December 2015 to February 2016.

Seasonal and solar cycle effects on TEC at 95°E in the ascending half (2009–2014) of the subdued solar cycle 24: Consistent underestimation by IRI 2012

TEC measured at Dibrugarh (27.5°N, 94.9°E, 17.5°N Geomag.) from 2009 to 2014 is used to study its temporal characteristics during the ascending half of solar cycle 24. The measurements provide an opportunity to assess the diurnal, seasonal and longterm predictability of the IRI 2012 (with IRI Nequick, IRI01-corr, IRI 2001topside options) during this solar cycle which is distinctively low in magnitude compared to the previous cycles. The low latitude station Dibrugarh is normally located at the poleward edge of the northern EIA. A semi-annual variation in GPS TEC is observed with the peaks occurring in the equinoxes. The peak in spring (March, April) is higher than that in autumn (September, October) irrespective of the year of observation. The spring autumn asymmetry is also observed in IRI TEC. In contrast, the winter (November, December, January, February) anomaly is evident only in high activity years. TEC bears a distinct nonlinear relationship with 10.7cm solar flux (F10.7). TEC increases linearly with F10.7 up to about 125sfu beyond which it tends to saturate. The correlation between TEC and solar flux is found to be a function of local time and peaks at 10:00LT. TEC varies nonlinearly with solar EUV flux similar to its variation with F10.7. The nonlinearity is well captured by the IRI. The saturation of TEC at high solar activity is attributed to the inability of the ionosphere to accommodate more ionization after it reaches the level of saturation ion pressure. Annual mean TEC increased from the minimum in 2009 almost linearly till 2012, remains at the same level in 2013 and then increased again in 2014. IRI TEC shows a linear increase from 2009 to 2014. IRI01-corr and IRI-NeQuick TEC are nearly equal at all local times, season and year of observation while IRI-2001 simulated TEC are always higher than that simulated by the other two versions. The IRI 2012 underestimates the TEC at about all local times except for a few hours in the midday in all season or year of observation. The discrepancy between model and measured TEC is high in spring and in the evening hours. The consistent underestimation of the TEC at this longitude by the IRI may be attributed to the inadequate ingestion of F region data from this longitude sector into the model and exclusion of the plasmaspheric content.

Longitudinal variations of thermospheric composition at the solstices

AbstractO/N2, measured by the Global Ultraviolet Imager on board the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite, has large longitudinal variations at the solstices, which are simulated well in upper atmosphere general circulation models. These longitudinal variations are caused by the displacement of the Earth's magnetic poles from the geographic ones. The location of a magnetic pole affects the latitude at which the winds, driven by heating in summer, converge in the subauroral region of the winter hemisphere. In the magnetic pole's longitude sector, this convergence occurs at relatively low latitudes, which results in the maximum values of O/N2 also occurring at relatively low latitudes. These latitudes have a relatively small solar zenith angle, contributing to a strong winter anomaly. In the zonally opposite longitude sector, maximum values of O/N2 occur at relatively high latitudes because the summer‐to‐winter wind convergence also occurs at relatively high latitudes. These high latitudes have a relatively large solar zenith angle, so ionization is weak, contributing to a weak winter anomaly. Therefore, the displacement between the magnetic and geographic poles not only results in a strong longitudinal variation of O/N2 but also results in a strong longitudinal variation of the ionosphere winter anomaly.

Unusual behavior of the low‐latitude ionosphere in the Indian sector during the deep solar minimum in 2009

AbstractIn this paper we carry out a comparative study of the daytime (7–18 LT) behavior of the near‐equatorial ionospheric F region at the end of the long deep solar minimum (2009) with respect to that of the previous normal solar minimum (1995) in the Indian longitude sector using ionosonde observations of F layer parameters, radar observations of E × B drift, and the IRI‐2012 (International Reference Ionosphere‐2012) model. We investigate the F2 and F3 layer behaviors separately. The results reveal that the peak frequencies of the F layer (fpeak), F2 layer (foF2), and F3 layer (foF3) in 2009 are consistently lower than those in 1995. Maximum difference in fpeak/foF2/foF3 between 2009 and 1995 observations is found in the equinoxes followed by winter and summer. The annual mean, seasonal mean, and 10 day mean peak electron density (corresponding to fpeak) in 2009 were lower than those in 1995 by as much as 34%, 46%, and 65%, respectively. Solar rotation effect is less conspicuous in 2009 than in 1995, consistent with the solar rotation signature in F10.7. Observations also show considerable amount of equinoctial asymmetry in electron density, which is found to be closely linked with the corresponding asymmetry in the vertical E × B drift. Seasonal mean peak electron densities of the F layer (corresponding to fpeak) and the F2 layer (corresponding to foF2) observed during the deep solar minimum of 2009 were smaller than those corresponding to IRI‐2012 model foF2 by as much as 45% and 50%, respectively, underlining the need to incorporate the data collected during the long deep minimum in the IRI model. The unusually weak ionosphere observed in 2009 is discussed in terms of the direct effect of the low solar EUV flux in 2009 as compared to 1995 and its indirect effects on ionospheric electric field, thermospheric composition (or O/N2 ratio), and thermospheric neutral winds.

Multimodel comparison of the ionosphere variability during the 2009 sudden stratosphere warming

AbstractA comparison of different model simulations of the ionosphere variability during the 2009 sudden stratosphere warming (SSW) is presented. The focus is on the equatorial and low‐latitude ionosphere simulated by the Ground‐to‐topside model of the Atmosphere and Ionosphere for Aeronomy (GAIA), Whole Atmosphere Model plus Global Ionosphere Plasmasphere (WAM+GIP), and Whole Atmosphere Community Climate Model eXtended version plus Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (WACCMX+TIMEGCM). The simulations are compared with observations of the equatorial vertical plasma drift in the American and Indian longitude sectors, zonal mean F region peak density (NmF2) from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites, and ground‐based Global Positioning System (GPS) total electron content (TEC) at 75°W. The model simulations all reproduce the observed morning enhancement and afternoon decrease in the vertical plasma drift, as well as the progression of the anomalies toward later local times over the course of several days. However, notable discrepancies among the simulations are seen in terms of the magnitude of the drift perturbations, and rate of the local time shift. Comparison of the electron densities further reveals that although many of the broad features of the ionosphere variability are captured by the simulations, there are significant differences among the different model simulations, as well as between the simulations and observations. Additional simulations are performed where the neutral atmospheres from four different whole atmosphere models (GAIA, HAMMONIA (Hamburg Model of the Neutral and Ionized Atmosphere), WAM, and WACCMX) provide the lower atmospheric forcing in the TIME‐GCM. These simulations demonstrate that different neutral atmospheres, in particular, differences in the solar migrating semidiurnal tide, are partly responsible for the differences in the simulated ionosphere variability in GAIA, WAM+GIP, and WACCMX+TIMEGCM.

The response of equatorial electrojet, vertical plasma drift, and thermospheric zonal wind to enhanced solar wind input

AbstractIn this study we used observations from the CHAMP and ROCSAT‐1 satellites to investigate the solar wind effects on the equatorial electrojet (EEJ), vertical plasma drift, and thermospheric zonal wind. We show that an abrupt increase in solar wind input has a significant effect on the low‐latitude ionosphere‐thermosphere system, which can last for more than 24 h. The disturbance EEJ and zonal wind are mainly westward for all local times and show most prominent responses during 07–12 and 00–06 magnetic local time (MLT), respectively. The equatorial disturbance electric field is mainly eastward at night (most prominent for 00–05 MLT) and westward at daytime with small amplitudes. In this study we show for the first time that the penetration electric field is little dependent on longitude at both the day and night sides, while the disturbance zonal wind is quite different at different longitude sectors, implying a significant longitudinal dependence of the ionospheric disturbance dynamo. Our result also indicates that the F region equatorial zonal electric field reacts faster than E region dynamo, to the enhanced solar wind input.

On the mutual relationship of the equatorial electrojet, TEC and scintillation in the Peruvian sector

AbstractThis paper presents the interrelationship between the equatorial electrojet (EEJ) strength, Global Positioning System (GPS)‐derived total electron content (TEC), and postsunset scintillation from ground observations with the aim of finding reliable precursors of the occurrence of ionospheric irregularities. Mutual relationship studies provide a possible route to predict the occurrence of TEC fluctuation and scintillation in the ionosphere during the late afternoon and night respectively based on daytime measurement of the equatorial ionosphere. Data from ground based observations in the low latitudes of the west American longitude sector were examined during the 2008 solar minimum. We find a strong relationship exists between the noontime equatorial electrojet and GPS‐derived TEC distributions during the afternoon mediated by vertical E × B drift via the fountain effect, but there is little or no relationship with postsunset ionospheric scintillation.

Performance of IRI‐2012 model during a deep solar minimum and a maximum year over global equatorial regions

AbstractPresent paper inspects the prediction capability of the latest version of the International Reference Ionosphere (IRI‐2012) model in predicting the total electron content (TEC) over seven different equatorial regions across the globe during a very low solar activity phase 2009 and a high solar activity phase 2012. This has been carried out by comparing the ground‐based Global Positioning System (GPS)‐derived VTEC with those from the IRI‐2012 model. The observed GPS‐TEC shows the presence of winter anomaly which is prominent during the solar maximum year 2012 and disappeared during solar minimum year 2009. The monthly and seasonal mean of the IRI‐2012 model TEC with IRI‐NeQ topside has been compared with the GPS‐TEC, and our results showed that the monthly and seasonal mean value of the IRI‐2012 model overestimates the observed GPS‐TEC at all the equatorial stations. The discrepancy (or over estimation) in the IRI‐2012 model is found larger during solar maximum year 2012 than that during solar minimum year 2009. This is a contradiction to the results recently presented by Tariku (2015) over equatorial regions of Uganda. The discrepancy is found maximum during the December solstice and a minimum during the March equinox. The magnitude of discrepancy in the IRI‐2012 model showed longitudinal dependent which maximized in western longitude sector during both the years 2009 and 2012. The significant discrepancy in the IRI‐2012 model observed during the solar minimum year 2009 could be attributed to larger difference between F10.7 flux and EUV flux (26–34 nm) during low solar activity period 2007–2009 than that during high solar activity period 2010–2012. This suggests that to represent the solar activity impact in the IRI model, implementation of new solar activity indices is further required for its better performance.

An oblate beaming cone for Io‐controlled Jovian decameter emission

AbstractWe investigate the emission cone of the Io‐controlled Jovian decameter radiation from the layout of the sources in the central meridian longitude (CML)‐Io phase diagram where the occurrence of the radiation is plotted versus the central meridian longitude (CML) and the phase of the satellite Io. Four zones of enhanced probability are revealed in this diagram and named Io‐controlled sources Io‐A, Io‐B, Io‐C, and Io‐D. We propose an angular representation of the CML‐Io phase diagram in a coordinate system linked to the local magnetic field in the radio source and the gradient of the magnetic field. The angular distribution of the sources in such a diagram clearly shows that the radio emission is radiated in a hollow cone which is not axisymmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The use of elliptic coordinates allows us to compute the variable opening angle and the flattening of the cone in both Jovian hemispheres. Finally, it comes from our investigation that the flattening of the emission cone allows the theoretical existence of a Jovian active longitude sector much more easily than in the case of an axisymmetrical cone, and the zones of maximum theoretical amplification correspond to the areas of high occurrence probability observed in the CML‐Io phase diagram.

Observation of TEC perturbation associated with medium‐scale traveling ionospheric disturbance and possible seeding mechanism of atmospheric gravity wave at a Brazilian sector

AbstractIn the present study, we document daytime total electron content (TEC) disturbances associated with medium‐scale traveling ionospheric disturbances (MSTIDs), on few chosen geomagnetically quiet days over Southern Hemisphere of Brazilian longitude sector. These disturbances are derived from TEC data obtained using Global Navigation Satellite System (GNSS) receiver networks. From the keograms and cross‐correlation maps, the TEC disturbances are identified as the MSTIDs that are propagating equatorward‐eastward, having most of their average wavelengths longer in latitude than in longitude direction. These are the important outcomes of the present study which suggest that the daytime MSTIDs over Southern Hemisphere are similar to their counterparts in the Northern Hemisphere. Another important outcome is that the occurrence characteristics of these MSTIDs and that of atmospheric gravity wave (AGW) activities in the thermosphere are found to be similar on day‐to‐day basis. This suggests a possible connection between them, confirming the widely accepted AGW forcing mechanism for the generation of these daytime MSTIDs. The source of this AGW is investigated using the Geostationary Operational Environmental Satellite system (GOES) and Constellation Observing System for Meteorology, Ionosphere, and Climate satellite data. Finally, we provided evidences that AGWs are generated by convection activities from the tropospheric region.