Digisonde Observations Research Articles

Response of equatorial ionosphere to the September 2017 geomagnetic storm: a case study using digisonde observations at Fortaleza

Response of equatorial ionosphere to the September 2017 geomagnetic storm: a case study using digisonde observations at Fortaleza

A Simplified Method of True Height Analysis to Estimate the Real Height of Sporadic E Layers

AbstractThe sporadic E (Es) layer virtual height and the ordinary wave critical frequency are routinely measured ionogram parameters, used to characterize the Es altitude occurrence and intensity. It has become common practice to take the real height to be about equal to by assuming that signal propagation delays in the E region plasma below the layer are small and can be neglected. Although this applies for nighttime, during daytime it may overestimate significantly. The present paper relies on true height analysis theory to devise a simplified method and propose an algorithm that can estimate Es real heights reasonably well. The method relies on and ionosonde measurements and E region electron density profiles obtained from the International Reference Ionosphere model. The algorithm is applied to a typical set of Digisonde observations to compute and examine real height variations and functional dependencies. Whereas ≃ at nighttime, during daytime there are notable − differences taking values less than 10 km for most of the observed layers. During the early morning and early afternoon hours, however, when weak layers appear at upper heights, the virtual to real height differences become larger reaching 20–25 km. The method proposed here for the estimation of can be easily applied to improve the accuracy of the results of sporadic E layer studies.

Evaluation of the performance of F-layer peak height models used in IRI-2016 over the Indian equatorial and low latitudes

The performance of different height models (BSE-1979, AMTB-2013, and SHU-2015) in the International Reference Ionosphere (IRI-2016) is evaluated over the Indian equatorial and low latitude sectors. The impact of deviations in the modelled hmF2 on the estimation of electron density profiles and the Total Electron Content (TEC) is examined using the three different hmF2 methods in IRI-2016. For this study digisonde observations over Trivandrum (8.5 °N, 76.9 °E) and Ahmedabad (23.0 °N, 72.5 °E) along with GPS observations over Colombo (6.8 °N, 79.8 °E) and Ahmedabad during a high solar activity year 2014 are considered. A comparison of electron density profiles between digisonde and IRI-2016 at both the equatorial station, Trivandrum, and the low latitude station, Ahmedabad, revealed significant deviations with higher deviations in the post sunset hours at the equator. The three height methods have shown significant differences in their predictions of hmF2 at both locations. At the equator, the SHU-2015 method shows better estimates closer to the measurement values, than the other two methods except at Pre-Reversal Enhancement (PRE) time, during which, the AMTB-2013 method shows better performance. At the low latitude station, the newly added methods (SHU-2015 and AMTB-2013) do not indicate any improvements in estimating the hmF2. The availability of three different height methods in IRI-2016 is exploited to understand the modifications if any, in the ionospheric height structure and TEC for different hmF2 methods and the corresponding results are presented. As the variations in hmF2 are used to derive vertical drift on many occasions in the absence of incoherent scatter radar, this study is significant in assessing the relative efficacy of the three height models and hence useful to understand various ionospheric processes over the low/equatorial latitudes.

Identification of Large-Scale Travelling Ionospheric Disturbances (LSTIDs) Based on Digisonde Observations

In this paper we analyze Digisonde observations obtained in the European region to specify the effects of large-scale travelling ionospheric disturbances (LSTIDs) on the ionospheric characteristics that define the conditions in the bottomside ionosphere. While this type of disturbances affects all frequency ranges in the F region, the most pronounced effect is detected in the foF2 critical frequency, where the density is the highest. During LSTID activity, a significant uplifting of the F2 layer is observed to accompany an oscillation pattern in the foF2. Concurrent variations in the height of the peak electron density hmF2 and the corresponding scale height, Hm are also observed. These findings are used to propose a new methodology for the identification of LSTIDs, comprising a combination of different criteria. The efficiency of the proposed methodology is tested at middle latitudes during geomagnetically quiet and disturbed intervals as well as during time periods of lower atmosphere forcing affecting the ionosphere.

On the Onset Time and Background Ionospheric Conditions of Postsunset Equatorial Plasma Bubble

AbstractOnset time and conditions of the postsunset equatorial plasma bubble (EPB) observed by the Gadanki Ionospheric Radar Interferometer are presented and discussed in the light of collocated digisonde observations of background ionosphere. Observations show two important findings: (a) onset time of EPB is linked with the sunset terminator and it varies between the low latitude E region sunset and apex F region sunset with a tendency of EPB onset to be close to low latitude E region sunset, and (b) higher height of the bottomside F layer (by 50–60 km) and vertical plasma drift (by 10–30 m s−1) observed over Gadanki are found to be associated with the EPBs growing in a narrow longitude zone of ±1° centered on Gadanki than those outside this zone. The early onset of EPB coinciding with the low latitude E region sunset is attributed to the early predominance of the F region dynamo leading to prereversal enhancement (PRE) of zonal electric field, consistent with theoretical expectation. Delay in the onset of EPB is attributed to the effect of flux tube integrated Pedersen conductivity owing to low latitude sporadic E activity and the F region meridional neutral wind that could change the F region plasma density along the flux tube through field‐aligned movement of plasma. Observations also show that ionosonde observations from a single location cannot be used to predict the growth of EPB at other location, including those close to the station but away from the station by about 120 km.

Morphology and Evolution of Plasma Density Enhancement Events in the Equatorial Ionospheric F Region on 8 February and 4 November 2018

AbstractIn this study, we used observations from multiple instruments (all‐sky imager [ASI], digisonde, and Beidou geostationary satellite global positioning system receivers) to show the occurrence and evolution of two plasma density enhancement structures (PDESs) during the night of 8 February and 4 November 2018. The PDESs were elongated from northwest to southeast. Their zonal scales were about 200–400 km and longitudinal ranges were about 800–1,400 km. Within the PDES, the plasma density can be enhanced by about two times over the background density. The PDESs appeared before midnight and gradually dissipated after midnight. Plasma density depletion structures appeared within the PDES at the later stage of PDES. Each PDES event lasted for over 4 hours. The morphology and evolution of the PDESs presented in this paper are different from those reported in previous studies. In addition, based on the thermosphere‐ionosphere‐electrodynamics general circulation model simulations and digisonde observations, we proposed a possible cause for the PDESs in the events, in which the occurrence and evolution of PDES are related to the variations of thermospheric meridional winds in the equatorial region.

Migrating and non-migrating tidal influences on the high occurrence of post-midnight spread F over Ascension Island during solar minimum

The digisonde observations over Ascension Island (8°S, 14°W, dip latitude: 16°S) reveal that the occurrence of post-midnight spread F (or more appropriately delayed spread F) is more when compared to that of the post-sunset spread F during solar minimum years. While the monthly variations of the percentage of occurrence (PO) of post-sunset spread F (18:00–22:00) do not now show any consistent seasonal variation, the PO of the post-midnight spread F reveals a seasonal variation with a high PO during austral summer (November-February) and minimum PO during austral winter (May-August) and it shows a large interannual variability. The high PO during September 2019 may be associated with the austral sudden stratospheric warming (SSW). In the year 2020, the PO is higher (>60%) in November-February. When the PO of the post-midnight spread F is larger, there is a strong semi-diurnal variation of peak electron density height of F-layer (hmF2) with a maximum height at noon and midnight hours, suggesting a possible semi-diurnal tidal influence. It is also found that the occurrence of spread F just after sunset hours is less over Ascension Island and it does not show any clear seasonal variation. The space-time spectral analysis of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) of TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) satellite reveals the dominant presence of migrating semi-diurnal tide (SW2) at 10°N during January-February 2020. During November-December 2020, though the SW2 tidal amplitudes are weaker, there is a dominant presence of the eastward propagating non-migrating diurnal (DE3) tide. The thermospheric winds measured by MIGHTI (Michelson Interferometer for Global High-resolution Thermospheric Imaging) instrument of ICON (Ionospheric Connection Explorer) mission show equatorward winds in the late evening hours during November-February 2020. The migrating diurnal tide (DW1) dominates in MIGHTI zonal winds at F-region heights, whereas SW2, as well as the eastward propagating semi-diurnal tide with zonal wavenumber 2 (SE2), tidal amplitudes are comparable to DW1 in the meridional wind. It is suggested that the semi-diurnal variation in the meridional winds turning the winds equatorward during late evening hours can cause eastward electric field that can lift the F layer to higher heights and thereby favouring the Rayleigh-Taylor Instability (RTI) to act, which can cause the post-midnight spread F. It is also shown that the growth rate can be sufficiently positive around midnight hours to cause the RTI to act.

Characteristics of the Effective Scale Height in the Topside Ionosphere Extracted From Swarm A and Digisonde Observations: Preliminary Results

AbstractThe ionospheric scale height is a key parameter that defines the shape of the electron density profile. Given that its direct calculation requires knowledge of the physical and chemical state of the plasma, the concept of the effective scale height was proposed to facilitate its application. It is defined as the scale height that fits the electron density profiles with the α‐Chapman function. Its main characteristics, including the diurnal and altitudinal variation are still under investigation. This paper focuses on the specification of its key characteristics at middle latitudes based on coincident Digisonde and Swarm A satellite data. Based on the assumption that the topside electron density profile is approximated with the α‐Chapman model of variable scale height, the effective scale height at the Swarm A altitude is calculated and compared with the effective scale height at . A general conclusion is that the topside effective scale height at ∼450 km exhibits a diurnal variation with highest values in the early morning and evening sectors, while at it gets its maximum values at noon. At night, the effective scale height tends to increase with altitude, however in the daytime, its behavior is more complex. On a local scale, evidence for connection between spread F irregularities and high variability in the topside effective scale height is reported. These preliminary results indicate the potential to further exploit Swarm and Digisonde data for the development of a model for the effective scale height, able to support realistic electron density reconstruction models.

Onset Conditions and Features of Equatorial F Region Irregularities: New Insight From Collocated Digisonde and Radar Observations From Gadanki

AbstractIn this paper, we study the onset conditions and features of equatorial F region irregularities linked with equatorial plasma bubbles (EPBs) using observations made simultaneously by using a DPS‐4D digisonde and the Gadanki Ionospheric Radar Interferometer (GIRI), both collocated at Gadanki. Importantly, we have employed specific analysis tools on DPS‐4D observations, providing range‐time displays of radio frequency and signal‐to‐noise ratio (SNR) of the reflected/backscattered echoes and the angle‐of‐arrival of the echoes, to characterize the echoes and to study the ambient and disturbed states of the ionosphere. Observations clearly show noticeably different background conditions, in terms of the height of the F layer base, electron density gradient and vertical drift, for the freshly generated and drifting EPBs. The zonal wavelengths of the pre‐sunset large‐scale wave structures (LSWS) observed using the DPS‐4D, however, are found to show a close connection with EPB spacings for freshly generated and drifting EPBs, consistent with earlier findings. The satellite traces were observed just prior to the equatorial spread F (ESF) and were found to be associated with the bottomside upwellings. Comparison of the range‐time displays of radio frequency and SNR of the DPS‐4D observations and the height‐time variations of SNR of the GIRI observations demonstrates that the relatively weak echoes in the DPS‐4D observations, which represent the ESF echoes, correlate fairly well temporally with the plume structures observed by the GIRI. The GIRI observations also reproduce the bottomside upwellings observed by the DPS‐4D. We propose that the bottomside upwellings are due to the growth of the LSWS and these bottomside upwellings are instrumental for the growth of the Rayleigh‐Taylor instability generating EPBs. Finally, the new aspects of the digisonde observations are discussed with regard to their usefulness in understanding and forecasting EPBs/ESF.

Tidal influence on the longitudinal variabilities of the post-midnight spread F during September 2019

The analysis of digisonde ionograms of Ascension Island (7.9°S, 14.4°W geographic; dip latitude: 16°S) for the geomagnetically quiet days shows a high percentage of occurrence (PO) of post-midnight spread F (PMSF) in September 2019. However, Jicamarca (11.95°S, 76.87°W, dip latitude: 1.10°N) digisonde observations do not show any significant increase in the PO. Besides, the wavelet spectrum of the peak height of the F-layer (hmF2) shows a strong semi-diurnal (12 h) variation over Ascension Island, whereas diurnal (24 h) variation is dominant in Jicamarca. The high PO of PMSF and the semi-diurnal variation indicates the lower atmospheric tidal forcing. The two-dimensional spectral analysis of the space-borne SABER (Sounding of the Atmosphere by Broadband Emission Radiometry) temperature measurements shows that the migrating semi-diurnal (SW2) tidal amplitude exceeds the migrating diurnal (DW1) tidal amplitude at 10°N, which is close to the dip equator in the Ascension Island longitude, whereas the DW1 dominates at 10°S over the dip equatorial station, Jicamarca. The SW2 dominated tidal spectrum can drive semi-diurnal variation in the zonal electric field. Though the presence of the F-layer at lower heights and the westward electric field might have prevented the growth of the Rayleigh-Taylor Instability (RTI) after sunset, the dominant semi-diurnal variation of the zonal electric field can become eastward at a later time and lift the F-layer to higher heights creating favourable conditions for the growth of the RTI to generate the spread F around midnight.

Comparison between Swarm measured and IRI-2016 ,IRI-Plas 2017 modeled electron density over low and mid latitude region

In the present paper, we investigate the electron density observations from the Langmuir probes (LPs) onboard low Earth orbiting (LEO) Swarm satellites. To accentuate its employability in model improvements, we compared the extracted F2 region critical electron density (foF2) and its corresponding altitude (hmF2) with the Digisonde observations and IRI-Plas 2017 model estimations at eight worldwide Digisonde locations during the year 2020. We attempted to visualize the discrepancy in magnitude between Swarm observation and IRI-Plas 2017 by independent simulation and then optionally ingesting the Digisonde foF2 and hmF2 parameters into the model. The results show that during nighttime, Swarm LP measured electron density model overestimates the IRI-Plas 2017 modeled electron density for all the stations. To confirm the above results, a comparison analysis has been performed for the Ne-Swarm A and Swarm B observations with that of IRI-2016 simulated output keeping the topside sub model as NeQuick along with other default options. The results show that Ne from Swarm A and Swarm B is higher than IRI-2016 estimations during the nighttime with the ratio ranging from 1 to 5 whereas during the daytime the ratio remains 0 to 1. This confirms that Swarm Ne observations have significantly higher values during nighttime over global region, reinforcing further studies on the surged nighttime variables. It has been also observed that there is no latitudinal disperency between Swarm and IRI-Plas 2017 modeled or IRI-2016 modeled observations.

On the Nature of F‐Region Bottom‐Type Echoing Layer Over Gadanki

AbstractBottom‐type echoing layers observed from Gadanki are presented and analyzed. The analysis is based on observations made using a substantial database of the 30 MHz Gadanki Ionospheric Radar Interferometer (GIRI) during 2017–2019, collocated digisonde observations, and a few days of overlapping volume observations made using the GIRI and the 53 MHz Advanced Indian MST Radar (AIR) . The echoes commence well before the E‐ and F‐region sunsets and last as late as 23:00 Local Time. The echoing layers occur in the height range of 220–300 km with a thickness of 10–15 km and the Doppler spectra are narrow with spectral width confined to 10 m s−1. The echoing layers are very common during the entire observational period and do not show any noticeable seasonal dependence. The zonal velocities of the irregularities responsible for the radar echoes are close to zero irrespective of the height of the echoing region. The echoes are stronger at 30 MHz than at 53 MHz indicating strong Bragg scale dependence. The height of the echoing layer agrees very well with the height of the F‐region peak electron density (hmF2). Also, early commencement of the echoing layer is found to be associated with low hmF2 and corresponding high electron density (foF2). While the results indicate toward the wind‐driven interchange instability as their origin, the early onset of the echoing layer and its relationship with the background ionospheric parameters call for explanation. The results are discussed in the light of current understanding on the bottom‐type echoing layer.

Assessment of IRI-2016 hmF2 model options with digisonde, COSMIC and ISR observations for low and high solar flux conditions

The attempt of this work is to evaluate the three options (BSE-1979, AMTB-2013, SHU-2015) embedded in IRI-2016 for the F2 layer peak height (hmF2) estimation under low and high solar flux levels. We compare the model hmF2 with digisonde observations from Qaanaaq, Milllstone Hill, Ramey, Jicamarca and Port Stanley as well as Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric radio occultation data over the five stations spanning from northern hemisphere to southern hemisphere. In the case of Millstone Hill, we also use incoherent scatter radar (ISR) measurements. For the comparison with digisonde data, SHU-2015 is the most accurate option for reproducing hmF2 at low and middle latitude stations (Milllstone Hill, Ramey, Jicamarca) with the smallest Root-Mean-Square-Error and the highest correlation with observations. At high latitude (Qaanaaq and Port Stanley), BSE-1979 becomes the best option. This is true for both low and high solar flux conditions. Similar results are obtained for the comparison between ISR and IRI-2016 hmF2 model options over Millstone Hill, where SHU-2015 option has the smallest deviations from the observation data in both low and high solar activity conditions. As for the comparison with COSMIC data, SHU-2015 edges out the other two options under low solar activity level with better performance at three stations. There is no optimal option under high solar activity level, which is to our surprise, cause SHU-2015 was developed based on a large amount of radio occultation data. At Jicamarca, all options show dramatic deviations from COSMIC observations, showing even negative correlations. We propose that the performance of the three options do not vary much with solar condition, while they differ a lot with location, for example, latitude.

On the Type‐A and Type‐B 150‐km Radar Echoes

AbstractType‐A (weaker and wider Doppler spectra) and Type‐B (stronger and narrower Doppler spectra) echoes are inherent to the puzzling 150‐km echoing phenomenon. In this paper, we investigate the characteristics and possible origin of the Type‐A and Type‐B 150‐km echoes using high‐resolution 53‐MHz radar and Digisonde observations from Gadanki. Results show that the Type‐A echoes dominate the echoing phenomenon and they always precede the Type‐B echoes. Intriguingly, the Type‐B echoes are fundamentally not narrower than the Type‐A echoes as was originally thought. The primary reason that distinguishes the Type‐B echoes from the rest is the confinement of a few high PSD values at the center of the spectra, resulting in narrow spectral width. Spectral spreads of both types of echoes are found to increase with echo signal‐to‐noise ratio (SNR), and in fact, the spectral spreads of the Type‐B echoes are higher than those of the Type‐A echoes. Observations also show spectral evolution displaying the occurrence of Type‐A and Type‐B echoes in tune with the quasiperiodicity in the echo strength (SNR). We propose that both types of echoes primarily are of common origin, presumably linked with naturally enhanced incoherent scattering. Looking at the simultaneously observed short period variations in the lower F region electron density, we propose a gravity wave induced pathway as a plausible mechanism accounting for echo intensity modulation as well as spectral transition.

F3 Layer Characteristics Revealed by the Jicamarca Incoherent Scatter Radar: First Results

AbstractIn this paper, we present a systematic study on the characteristics and evolution of the F3 layer using the Jicamarca incoherent scatter radar (ISR) observations made during 2014–2018. These results are also compared with those obtained from collocated digisonde observations in an effort to understand the F3 layer in more detail. The purpose of the comparison is twofold: first is to see how the digisonde observations compare with those of ISR, and second is to see to what extent our understanding on the F3 layer is observationally complete. The basic characteristics of the F3 layers observed by digisonde and ISR are found to compare well. When the F3 layer becomes topside ledge, which was missed by the digisonde, ISR observations provided vital information in characterizing such F3 layers. Importantly, the ISR capability to observe the F3 layer irrespective of its density being lower or higher than that of the F2 layer could help understand the evolution of F3 layer. The occurrence statistics show two distinct maxima, one centered on 11:00 LT and the other centered on 18:00 LT quite consistent with earlier report. While the first peak is found to be consistent with model prediction, we suggest that the second peak is governed by quiet time height varying zonal electric field linked with sunset electrodynamics. Results presented and discussed here based on ISR observations are first of its kind and reveal the behavior and evolution of the F3 layer not reported before.

Total electron content at equatorial and low-, middle- and high-latitudes in African longitude sector and its comparison with IRI-2016 and IRI-PLAS 2017 models

Ionosphere plays crucial role in satellite communication as well as the modern space-based positioning, navigation and timing applications. Progressive exploitation of ground and space-based observations across the African region has relatively strengthened the understanding and modeling of spatiotemporal ionospheric variations over the territory. However, it is also equally important to understand the latitudinal behaviour of ionosphere over the African longitude that would reinforce the ionospheric delay error modelling practices for an improved radio communication along the satellite pass. Hence, the aim of this research is to investigate the diurnal, seasonal and latitudinal variations of global positioning system derived total electron content (GPS-TEC) and to assess the performance of the recent version of empirical international reference ionosphere (IRI-2016) and plasmaspheric IRI (IRI-PLAS 2017) models during the ascending phase of solar cycle 24 (2012). The study is also supported by the integrated ionospheric TEC (IONO-TEC) extracted from the Digisonde observations in the vicinity of GPS receiver locations. The 3 GPS stations considered in this study are at a) an equatorial/ low-latitude location Federal University of Technology, Yola (FUTY; 9.35°N, 12.50°E, 4.91°S dip), b) a mid-latitude location Ankara (ANKR; 39.89°N, 32.76°E, 57.62°N dip), and a high-latitude location Tromso (TRO1; 69.66°N, 18.94°E, 78.17°N dip) in the northern hemisphere. Observations show the magnitude of TEC generally decreases with increasing latitude. While the average diurnal maximum of GPS-TEC at equatorial and low-latitude follow an almost equal level of magnitude during March and September equinoxes, the observations at middle and high latitude stations presents the highest magnitudes during the September equinox. Moreover, we noticed obvious higher magnitude of GPS-TEC than IONO-TEC at the three stations suggesting the discrepancies could be corresponding to the topside modeling and plasmaspheric contributions. The other prominent feature is the pattern of diurnal TEC being dome shaped at middle and high latitude locations whereas the equatorial and low latitude location manifested a noon bite-out characteristic in the IONO-TEC, IRI-2016, and IRI-PLAS outcomes. The comparative analysis of IRI-2016 and IRI-PLAS 2017 estimated TEC with the GPS-TEC show a clear overestimation of IRI-PLAS 2017 outcomes irrespective of seasons and latitudinal locations whereas the IRI-2016 outcomes either overestimated or underestimated the GPS-TEC. Hence, this study reveals that IRI-2016 performs better predictions compared to IRI-PLAS 2017 at all the three latitudinal locations. Such a latitudinal comparison across the African-European longitudinal sector may complement towards the efforts for improving the regional as well as global ionospheric model performances.

Optical observation of plasma bubbles and comparative study of multiple methods of observing the ionosphere over China

Ionospheric plasma bubbles and scintillation phenomena often occur at low latitudes in China. The all-sky airglow imager located at Fuke station in Hainan (19.5°N, 109.2°E) is part of the Meridian project in China. In this work, we used mainly the observation data obtained by the imager and a digisonde to study plasma bubbles. Optical image processing—including image enhancement, azimuth correction and image projection—was performed on the original airglow observation images, and clear high-resolution images of the bubbles were obtained. Based on these, the morphological features, spatial and temporal variations, spatial scale, drift velocity of the plasma bubbles in the F region of the ionosphere were investigated and analyzed. The plasma bubbles appeared as long, dark stripes on the bright airglow emission background. Their large-scale structures often bifurcated vertically, and were usually tilted westward with north-south extension along the magnetic field lines. The east-west scale of the plasma bubbles was usually greater than 200 km and these increased gradually with time. Moreover, the distance between the adjacent plasma bubbles also increased over time, and was in the 100–400 km range. Plasma bubbles generally drifted eastward with an average drift velocity of approximately 90 m/s. The occurrence and duration of the plasma bubbles were compared with digisonde observations, and it was shown that plasma bubbles lasted for nearly four hours. Additionally, the main characteristics of the plasma bubbles were obtained using both optical and radio observations and the study of which can be beneficial for monitoring and predicting ionospheric scintillations.

Assessment of ionospheric variability from IRI-2016, SPIM-2017, and IGS-GIM using Digisonde and GPS observations over Cyprus

The present study focuses on ionospheric F-layer critical parameters (foF2/hmF2) and Total Electron Content (TEC) by exploiting a long-term data (2009 to 2016) from Digisonde and Global Positioning System (GPS) receiver observations over a lower-middle latitude European location Nicosia, Cyprus (geographic 35° N, 33° E; magnetic dip. 32° N). Also, it adheres to evaluating the performances of global empirical standard International Reference Ionosphere (IRI-2016), Standard Plasmasphere Ionosphere Model (SPIM-2017/IRI-Plas 2017), and interpolated TEC from the IGS-Global Ionospheric Maps (GIMs) by comparing with the Digisonde and GPS observations. Our results confirm a noteworthy departure of estimated foF2/hmF2 and TEC in SPIM-2017, even larger than that of IRI-2016 as regards to the Digisonde and GPS observations, irrespective of season, solar activity, and geomagnetic activity conditions. This contradicts the general perception on an ideal behavior of the empirical climatological model over the middle latitude region, inferring that the transient region between the low and middle latitude does suffer from noticeable uncertainties. Also, we did not notice any convincing response of the storm sub-model in IRI except a rare flipping in its magnitude and absolutely no alteration in SPIM model estimations to the onset of geomagnetic storms over the region despite the storm sub-model was kept ‘on’. This suggests a revisit into the underlying storm sub-model database in the models. Interestingly, the ingestion of Digisonde recorded foF2/hmF2 and GPS-TEC into the SPIM-2017 model results in a remarkable convergence of the parameters towards the local observations even during active geomagnetic storm conditions. It ascertains the flexibility in SPIM-2017 towards plausible improvements in the model estimations by ingesting GPS-TEC. Above all, such a scheme is in line with efforts towards reliable ionospheric error mitigations in navigation, communication, and satellite applications.

MELISSA: System description and spectral features of pre‐ and post‐midnight F‐region echoes

AbstractMost of the low‐latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (∼75°W). The deployment of the 30 MHz FAPESP‐Clemson‐INPE (FCI) coherent backscatter radar in the magnetic equatorial site of São Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (∼45°W). However, despite being operational for several years (2001–2012), FCI only made observations during daytime and pre‐midnight hours, with a few exceptions. Here, we describe an upgraded system that replaced the FCI radar and present results of full‐night F‐region observations. This radar is referred to as Measurements of Equatorial and Low‐latitude Ionospheric irregularities over São Luís, South America (MELISSA), and made observations between March 2014 and December 2018. We present results of our analyses of pre‐ and post‐midnight F‐region echoes with focus on the spectral features of post‐midnight echoes and how they compare to spectra of echoes observed in the post‐sunset sector. The radar observations indicate that post‐midnight F‐region irregularities were generated locally and were not a result of “fossil” structures generated much earlier in time (in other longitude sectors) and that drifted into the radar field‐of‐view. This also includes cases where the echoes are weak and that would be associated with decaying equatorial spread F (ESF) structures. Collocated digisonde observations show modest but noticeable F‐region apparent uplifts prior to post‐midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F‐region conditions leading to ESF development.

On the Genesis of Postmidnight Equatorial Spread F: Results for the American/Peruvian Sector

AbstractPrevious studies of the Earth's low‐latitude ionosphere using in situ measurements made by sensors on the Communication/Navigation Outage Forecasting System (C/NOFS) satellite showed an unexpected predominance of equatorial spread F (ESF) events in the postmidnight sector during June and December solstice months of the 2008–2009 deep solar minimum. It has been suggested that these events might have been driven by the unusual behavior of the equatorial plasma drifts, which showed an abnormal upward peak around midnight during the same period. We use coherent backscatter radar (Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere ‐ JULIA) measurements made at the Jicamarca Radio Observatory (11.95°S, 76.87°W, ∼1∘ dip lat) in Peru to better understand the origin of the ESF irregularities observed by C/NOFS. The radar observations show that ESF events during December solstice start in the postsunset sector. These ESF events and the conditions for their development are shown to continue through midnight hours. The predominance of postmidnight irregularities on C/NOFS observations during December solstice is caused by the slow vertical development of the ESF structures, which only reach the topside near midnight in most cases. On June solstice, however, JULIA observations show that ESF started predominately in the midnight to postmidnight sector. Collocated digisonde observations provide additional insight on F‐region conditions leading to these ESF events.