Region Field-aligned Irregularities Research Articles

On the Linkage Between Daytime E Region Field‐Aligned Irregularities and Sporadic E Layer at Low Latitude

AbstractWe investigate the generation of low latitude daytime E region field‐aligned irregularities (FAIs) and focus on whether the FAIs are exclusively linked with sporadic E (Es) and whether the FAIs are driven by wind‐driven gradient‐drift instability (GDI). For this investigation, we use observations of FAIs made by the 30 MHz Gadanki Ionospheric Radar Interferometer (GIRI) and Es made by the DPS‐4D digital ionosonde, both collocated at Gadanki. We have also examined simultaneous GIRI and Ionospheric Connection Explorer wind observations to substantiate our results. Results show that daytime E region FAIs are closely linked with Es activity and FAIs are not formed when Es is absent. FAIs are formed when blanketing frequency of Es (fbEs) exceeds frequency of the E layer. Both signal‐to‐noise ratio (SNR) and spectral width of the FAIs echoes display close relationship with top frequency of Es (ftEs). Further, SNR and spectral width of the FAIs echoes display a near‐linear relationship with minimum deviation when Es activity is moderate and show dispersion from near‐linear relationship when Es activity is strong. Zonal drift of the FAIs are predominantly eastward displaying both positive and negative vertical shear. Results also show that Es activity in terms of fbEs and ftEs is found to be closely related to the vertical shear in the zonal drift of the FAIs. Considering the height region of FAIs being highly collisional, we argue that zonal drifts are dominated by zonal wind and vertical shear in the zonal neutral wind is responsible for the formation/maintenance of Es layer and the eastward neutral wind is primarily responsible for the generation of the FAIs through the GDI.

On the Spatial Structure and Zonal Drift of Low Latitude E Region Irregularity Patches Over Hainan

AbstractAll‐sky radar measurements provide a unique capability to resolve the spatial structure of E region irregularities over a large zonal region. With the all‐sky radar interferometry observations at Ledong (18.4°N, 109.0°E) in Hainan, China, the spatial structure and zonal drift of low latitude E region field‐aligned irregularities (FAIs) over a large region are statistically investigated for the first time. It is revealed that the E region FAIs, including both the continuous and quasi‐periodic (QP) types shown in radar range‐time‐intensity maps, occurred most frequently in summer. The continuous type was observed being generated locally without obvious zonal drift. The QP type generally covered ∼40–500 km zonally, and consisted of up to 9 (peaking at 3–4) FAI patches separated by ∼20–130 km (peaking at ∼60 km). The spatially separated FAI patches predominantly drifted westward at the speed ∼50–200 m/s. Besides the Kelvin‐Helmholtz instability, gravity waves were surmised to be a major source for causing the spatial structures of low latitude QP type E region FAIs. Neutral wind was suggested to play an important role for their zonal drifts.

On the high percentage of occurrence of type-B 150-km echoes during the year 2019 and its relationship with mesospheric semi-diurnal tide and stratospheric ozone

• Anomalously high occurrence of 150-km radar echoes over Kototabang during September 2019. • In the years 2017 and 2019, SW2 dominates DW1 due to the larger stratospheric ozone, when compared to other years. • Dominant SW2 along with solar minimum conditions may lead to large echo occurrence through interchange instability. The Kototabang (0.20°S, 100.3°E, 10.36°S dip latitude) Equatorial Atmosphere Radar (EAR) observations of valley region field-aligned irregularities (FAI) reveal maximum percentage of occurrence (PO) of high signal to noise ratio (greater than 3 dB) type-B 150-km echoes during boreal summer (June-August) and winter (December-January) of solar moderate to minimum years 2016–2019. The PO is anomalously high in the solar minimum year 2019 and particularly during the September equinox when a major austral sudden stratospheric warming (SSW) event has occurred. Its possible relation with mesospheric tides and stratospheric ozone is investigated. The space–time spectral analysis of upper mesospheric temperature information obtained from the spaceborne radiometer instrument Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) reveals that migrating semi-diurnal tide (SW2) is dominant during June-August, when the PO of the 150-km echoes maximizes, whereas migrating diurnal tide (DW1) is dominant during equinox months. It also reveals quasi-biennial variability of the DW1 tide probably due to similar variability of the stratospheric ozone. It is suggested that the DW1 tides generated due to solar radiation absorption by stratospheric ozone and tropospheric water vapour can have phases opposite to each other, leading to the suppression of DW1 tide during September 2017 and 2019 resulting in the relative dominance of SW2 tide over DW1 tide. The meridional wind shear associated with the relatively dominant SW2 tide results in an interchange instability developed on the gradient of daytime descending ion layer along with solar minimum conditions leads to plasma irregularities responsible for these echoes. The large eastward propagating diurnal tide with zonal wavenumber 3 (DE3) observed during June-September 2019 is unlikely to cause the instability, due to its weak meridional component.

First Results on E Region Irregularities From a 53 MHz Radar Experiment From Haringhata, India

AbstractWe present the first results on E region field‐aligned irregularities (FAIs) observed using a 53 MHz radar experiment conducted from Ionosphere Field Station, Haringhata (geographic latitude 22.93°N; geographic longitude 88.5°E; magnetic dip angle 35.2°N) of University of Calcutta, a location just on the Tropic of Cancer and below the Equatorial Ionization Anomaly. Results show that the E region FAI echoes, which occur with signal intensity as high as 35 dB above noise, are mostly confined to altitudes of 90–120 km with daytime FAI being confined to 100 km. The FAI echoes are often found to occur in the form of descending echoing region that includes continuous, quasi periodic and discrete patches of echoes. The FAI echoes display large day‐to‐day variations and tend to occur more during the morning (06–09 IST) and evening (18–21 IST) with relative less occurrence during the noon and midnight. Doppler spectra are type‐2 in nature with mean Doppler velocity in the range of −100–120 m s−1 and spectral width limited to 100 m s−1. While these characteristics are very similar to those of off‐equatorial low latitudes and mid‐latitudes, comparison of these results with those concurrently observed at Gadanki reveals remarkable difference in the echo occurrence, day‐to‐day variability and Doppler velocity, in spite of the small difference in ionospheric pierce points between these two stations. These results are discussed and the prospective of these observations in understanding the E region FAI over a convective active meteorological region is outlined.

Potential Application of 205-MHz Stratosphere–Troposphere Wind Profiling Radar in Ionospheric Studies: Preliminary Results

A state-of-the-art 205-MHz wind profiling radar designed for measuring both the horizontal and vertical wind components from 315 m to beyond 20 km has been installed at the Cochin University of Science and Technology (CUSAT), India (10.04°N, 76.33°E, dip angle ~7.1°N). Subsequently, the radar was operated with special configuration to probe the ionosphere. After a series of experiments, the radar was successfully configured to receive ionospheric reflections from about 90 to 500 km range covering the E and F layers, with high resolution (45 m for the 90-110 km region). The received echoes are identified as signatures of field-aligned irregularities (FAIs) of the E and F regions of the ionosphere. The E region echoes were observed at an altitude range of 90-110 km. Both continuous and quasi-periodic structures were identified. Further analysis from the spectrum shows that the E region FAIs are Type 2 in nature. The night time spread-F observed so far is of either bottom type or bottom side in nature. This letter portrays the scope of employing 200 MHz range of very high frequency (VHF) band for ionospheric observations, and the technical details and the initial results of the experiment conducted with this stratosphere-troposphere (ST) Radar.

Evidence of Mid- and Low-Latitude Nighttime Ionospheric $E$ –$F$ Coupling: Coordinated Observations of Sporadic $E$ Layers, $F$ -Region Field-Aligned Irregularities, and Medium-Scale Traveling Ionospheric Disturbances

We present the observational evidence of $E$ - and $F$ -region field-aligned irregularities (FAIs), nighttime medium-scale traveling ionospheric disturbances (MSTIDs), and a sporadic $E$ ( $E_{\text {S}}$ )-layer using the Wuhan very-high-frequency (VHF) coherent scatter radar, Wuhan Global Navigation Satellite System (GNSS) network, and Wuhan ionosonde. We observed simultaneously E and F FAIs by VHF radar and the $E_{\text {S}}$ -layer and spread- $F$ by Wuhan ionosonde. We also observed MSTIDs using the Wuhan GNSS network in a southwestward direction of propagation and horizontal propagation velocity of less than 180 m/s, for a period of ~33 min. Simultaneous observations of an $E_{\text {S}}$ -layer and FAIs in the $E$ -region and FAIs in the $F$ -region suggested the existence of an electrodynamic coupling between the $E$ and $F$ regions in the mid- and low-latitude nighttime ionosphere. A polarized electric field associated with nighttime MSTIDs can generate the uplift movements of the $F$ -region’s electron density. This uplift effect consequently can excite the gradient drift instability (GDI) with an accompanying enhanced vertical gradient of the $F$ -layer’s electron density. Our results indicated that the $F$ -region MSTIDs excited by Perkins instability might be further modulated by the $E \times B$ effect and subsequently can evolve to spread-F like FAIs. Our observational investigation provided the first evidence of the full dynamics and links among different ionospheric disturbances in a mid- and low-latitude nighttime region of China.

Altitude development of postmidnight F region field‐aligned irregularities observed using Equatorial Atmosphere Radar in Indonesia

AbstractFor the first time, vertical rise velocities of postmidnight field‐aligned irregularities (FAIs) at low geomagnetic latitudes have been examined near the June solstice by using two‐dimensional maps of F region FAI echoes observed with the Equatorial Atmosphere Radar in Indonesia for 3 years starting in May 2010. We found 15 freshly growing FAIs at postmidnight between May and August during the 3 years. The rise velocities of FAIs are smaller at postmidnight than at postsunset, and most postmidnight FAIs do not exceed an altitude of 450 km. Based on the rise velocities, a lower limit for the creation time of the postmidnight FAIs is estimated to be between 21:30 LT and 02:00 LT for 14 of the 15 events, indicating that this class of FAIs is distinct from the postsunset FAIs.

Occurrence climatology of F region field‐aligned irregularities in middle latitudes as observed by a 40.8 MHz coherent scatter radar in Daejeon, South Korea

AbstractA new 40.8 MHz coherent scatter radar was built in Daejeon, South Korea (36.18°N, 127.14°E, dip latitude: 26.7°N) on 29 December 2009 and has since been monitoring the occurrence of field‐aligned irregularities (FAIs) in the northern middle latitudes. We report on the occurrence climatology of the F region FAIs as observed by the Daejeon radar between 2010 and 2014. The F region FAIs preferentially occur around 250–350 km at 18:00–21:00 local time (postsunset FAI), around 350–450 km near midnight (nighttime FAI), around 250–350 km before sunrise (presunrise FAI), and around 160–300 km after 05:00 local time (postsunrise FAI). The occurrence rates of nighttime and presunrise FAIs are maximal during summer, though the occurrence rates of postsunset and postsunrise FAIs are maximal during the equinoxes. FAIs rarely occur during local winter. The occurrence rate of F region FAIs increases in concert with increases in solar activity. Medium‐scale traveling ionospheric disturbances (MSTIDs) are known as an important source of the F region FAIs in middle latitudes. The high occurrence rate of the nighttime FAIs in local summer is consistent with the high occurrence rate of MSTIDs in that season. However, the dependence of the FAI activity on the solar cycle is inconsistent with the MSTID activity. The source of the F region FAIs in middle latitudes is an open question. Our report of different types of FAIs and their occurrence climatology may provide a useful reference for the identification of the source of the middle latitude FAIs.

Daytime E region field‐aligned irregularities observed during a solar eclipse

AbstractThe driving mechanism of the midlatitude field‐aligned irregularities (FAIs) has been in dispute for many years. The experimental observations were carried out during the solar eclipse of 22 July 2009 in Wuhan, China, to study the possibility of the wave‐driven irregularities. A high‐frequency coherent scatter radar was used to detect the E region irregularities. An ionosonde was applied to record the trace of gravity waves in ionosphere. The E region FAIs occurred at the end of the solar eclipse with fluctuant Doppler. At the same time, the oscillations on the fEs (maximum reflecting frequency of Es) curve and in the Doppler velocity of the echoes from the Es layer were also recorded. The data analysis and comparison show that the gravity waves and the FAIs occurred at the same time with the in‐phase variations in amplitude and phase. Thus, the solar eclipse and the gravity waves may play important roles in the occurrence of the irregularities. A schematic diagram of one‐period gravity wave is used to explain the possible gravity wave‐driven mechanism and the Doppler fluctuation in the irregularities. The daytime FAI in midlatitude is a rare phenomenon, even in the condition of solar eclipse. There are only four cases of the E region FAIs observed during a solar eclipse, including our observations. The unique feature of our observations is the synchronized oscillations in the irregularities and in the Es layer, which will help address the outstanding question of the source of the midlatitude E region FAIs.

First results on low‐latitude E and F region irregularities obtained using the Gadanki Ionospheric Radar Interferometer

AbstractA 30 MHz radar has recently been established at Gadanki (13.5°N, 79.2°E; 6.5°N magnetic latitude) to make unattended observations of the ionospheric field‐aligned irregularities (FAI). This radar, called the Gadanki Ionospheric Radar Interferometer (GIRI), has been designed to have scanning capability of 100° in the east‐west plane perpendicular to Earth's magnetic field and interferometry/imaging system to study drifts and spatial distribution of plasma irregularities at both large and small scales. In this paper, we present the first results on the E and F region FAI made using the scanning capability of the GIRI. Daytime observations of E region FAI show type 2 echoes with velocities predominantly upward northward (downward‐southward) at altitudes >100 km (<100 km) and westward (eastward) in the forenoon (afternoon) with signature of tidal wind field. F region irregularities show bottom‐type, bottomside and plume structures with close resemblance to those observed over the magnetic equator. Observations made with the east‐west scanning capability have been used to study the origin, evolution, and drift of the FAI for the first time from Gadanki. Eastward drifts are estimated to be 90–210 m s−1 during 20–24 LT. Upward velocity as large as 500 m s−1 has been observed in the initial phase of the plume structures. Intriguingly, downward velocity as large as 60 m s−1 has also been observed in the plumes, displaying descending pattern, observed in the early evening hours. These results are presented and discussed in the light of current understanding of low‐latitude plasma irregularities, and future prospects of GIRI are outlined.

Comparison of SuperDARN irregularity drift measurements and F-region ion velocities from the resolute bay ISR

A number of studies have addressed the principal assumption used by the SuperDARN network of HF radars that the scatter from F region field-aligned irregularities has a Doppler shift given by the cosine component of the EXB plasma drift. However, the slopes of the best-fit line to the measured points have consistently been low, perhaps implying contamination from irregularities with smaller amplitude drifts. This work is motivated by testing the same assumption on a more optimal experimental setting: using the north face of the Resolute Bay incoherent scatter radar (RISR-N), which provides high resolution magnitude and direction measurements of EXB drift in a region where the ionospheric flow is mostly uniform relative to auroral latitudes, thereby reducing echo mixing due to spatio-temporal structuring. We compared the EXB drift measured by RISR-N to the line-of-sight Doppler velocities measured PolarDARN which is composed of Rankin Inlet and Inuvik HF radars, both having a field of view over Resolute Bay. An aggregate scatter plot of all the echoes observed during a 5-day period in early May 2011 contains two distinct groups of echoes. The first group is hypothesized to be from the E region because the echoes appear above a threshold EXB drift and at small flow angles, which are characteristics of primary Farley–Buneman waves. The Doppler velocity of the second group of echoes increase linearly with the EXB drift and is identified here as F region echoes. A special joint fit showed a slope of 0.85 for the F region echoes. During the observation period, the F region electron density has large variations between 1 and 10×1011, in particular due to the polar cap patches. Considering a representative F region electron density of 5×1011 m−3, which has a refraction index of 0.86 at 12.5MHz (for PolarDARN frequencies) and the fact that the measured velocity is the product of the actual velocity and the refractive index, the expected slope is 0.86, in agreement with our measurement. The fit for E region Doppler velocities shows a saturation speed at 170m/s, while the data were spread between 100 and 300m/s. Although the saturation speed is somewhat lower than the ion acoustic speed, it is acceptable considering earlier work attributing similarly low velocities to large aspect angles at the scattering altitudes.

The first time observations of low-latitude ionospheric irregularities by VHF radar in Hainan

Sanya VHF radar (18.4°N, 109.6°E, dip latitude 12.8°N) at Hainan Island is the first coherent backscatter radar for sounding low-latitude ionospheric irregularities in the mainland of China. In this paper, we present the first results of low-latitude ionospheric E and F region irregularities using the radar data during the period from February 2009 to March 2010. The Doppler velocity of radar echoes from E region field aligned irregularities (FAIs) was about several tens of meters per second, while the Doppler spectral width was appreciably larger than the velocity, and could reach one hundred meters per second, indicating that the observed E region FAIs belonged to type 2 irregularities. The observations of range time intensity (RTI) maps of FAIs showed that E region irregularities most often occurred at night within the altitude range 85–115 km, and were rarely observed at afternoon hours. The percentage occurrence of E region FAIs maximized during spring months (Feb.–May) with a peak value over 80%. The heights at which the strongest echo related FAIs occurred were mainly around 100 km, lower than h’Es and the difference is mostly 10–20 km. December solstice seemed to be the minimum period of occurrence when the FAI echoes were commonly detected at a narrow altitude range 90–100 km. Moreover, simultaneous radar and GPS observations during spread F events in the pre-midnight hours of solar minimum revealed that significant GPS L band scintillations coincided with the appearance of F region plasma plume structures, which extended up to 450 km in altitude.

Seasonal and Local Time Variations ofE-Region Field-Aligned Irregularities Observed with 30.8-MHz Radar at Kototabang, Indonesia

A VHF backscatter radar with operating frequency 30.8 MHz has been operated at Kototabang (0.20°S, 100.32°E; dip latitude 10.36°S), Indonesia, since February 2006. We analyzedE-region field-aligned irregularities (FAIs) observed by this radar through a year of 2007 and found that theE-region FAI observed at Kototabang can be classified into two groups. One is “descending FAI”. Altitude of the FAI echo region descends with time from 102 km to 88 km altitude during 0700–1000 and 1900–0000 LT in June solstice season. The other is “low-altitude FAI”, which is observed in an altitude range from 88 to 94 km mainly during nighttime. The observed Doppler velocity show distinct local time and altitude dependence. The seasonally averaged zonal velocity above (below) approximately 94 km altitude is westward (eastward) during daytime and eastward (westward) during nighttime. Meridional/vertical velocity perpendicular to the geomagnetic fields is upward during daytime and downward during nighttime. The direction of the FAI velocity above approximately 94 km altitude is consistent with that of the backgroundE×Bplasma drifts reported previously.

Neutral wind-driven gradient drift instability in the low-latitude daytime E region

[1] We present strong evidence for the wind-driving, daytime E region field-aligned irregularities (FAIs) on the basis of Sanya VHF radar observations on 29 June 2009. The observed sporadic E (Es) and echoing layers show a clear diurnal tidal descending trend and a gravity-wave-scale oscillation with a period of 40–50 min, which are consistent with the diurnal tidal height evolution seen from the meteor wind observations. The most important finding is that the daytime continuous FAIs are closely related to the negative vertical zonal wind shear, which also forms the dense ion layer. Associated observations suggest that the FAIs are generated on both sides of the Es layer. The echoes form two layers with 10 km vertical distance, about half the vertical wavelength of the diurnal tide. Our results, for the first time, demonstrate that the daytime E region continuous FAIs can be formed by the wind-driven gradient drift instabilities. We suggest that the latitudinal variation of the daytime E region FAI morphologies is caused by the dip angle dependence of the instability growth rate. Continuous echoes observed elsewhere at low latitudes and midlatitudes may also be attributed to the wind effects.

Planetary‐scale wave observations over a low‐latitude E region using simultaneous observations of VHF radar and ionosonde over Sanya (18.34°N, 109.62°E)

The planetary‐scale oscillations of the field‐aligned irregularities (FAI) and the sporadic E (Es) layers in the low‐latitude E region were studied in terms of the observations from 8 June to 22 August 2009 using coherent backscatter radar and a digital ionosonde installed at Sanya (18.34°N, 109.62°E), Hainan Island, China. Quasi 2 day waves were observed both in the meridional wind and in daytime backscatter echo occurrence. Simultaneous observations on sporadic E layers in daytime showed a planetary‐scale wave signature in the low‐latitude region for the first time. Close correlation between echo occurrence and the mean critical frequency (foEs) was found when the quasi 2 day planetary wave (QTPW) existed. These results suggest that the Es plays an important role in determining the long‐term variability of the low‐latitude E region FAI. To test the nonlinear interactions between tides and QTPW, the hourly meridional and zonal winds are analyzed. The subsidiary waves with anticipated frequencies and their height evolutions provide evidence of tide‐QTPW interactions. Bispectral analysis on the winds further validates the strong nonlinear interactions between tides and the QTPW in the meridional wind. Our results suggest that the mechanism proposed by Pancheva et al. [2003] is a reasonable explanation for the planetary‐scale variability of the low‐latitude E region FAI and Es. The different responses of the Es layers and the E region FAIs on the QTPW between day and night can be attributed to diurnal variations in ion densities in the region between 100 and 160 km.

Gadanki radar observations of F region field‐aligned irregularities during June solstice of solar minimum: First results and preliminary analysis

In this paper we present the first results of F region field‐aligned irregularities (FAI) made during the summer of low solar condition using the Gadanki mesosphere‐stratosphere‐troposphere radar. FAI echoes were observed on all 20 nights of radar observations and were mostly confined to the postmidnight hours. Echo morphology is found to be very different from the equinoctial postsunset features reported earlier from Gadanki. Echo SNRs are lower by 25 dB than their equinoctial postsunset counterparts but are quite comparable to those of the equinoctial decaying FAI during the postmidnight hours. The Doppler velocities, which lie in the range of ±100 m s−1, are predominantly upward‐northward during 0000–0300 LT and downward‐southward afterward, in contrast to those observed as predominantly downward‐southward associated with the decaying equinoctial postmidnight F region FAI. Spectral widths of the summer echoes, which are well below 50 m s−1 and are very similar to those of the decaying equinoctial irregularities, represent the presence of weak plasma turbulence. Simultaneous observations made using a collocated ionosonde show no ionogram trace during 2200–0530 LT except for a few cases of weak spread F events. Concurrent ionosonde observations made from magnetic equatorial location Trivandrum also show very similar results. The observations are discussed in the light of current understanding on the postmidnight occurrence of F region irregularities in the summer of low solar condition.

Medium‐scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido radar, all‐sky imager, and GPS network and their relation to concurrent sporadic E irregularities

We present midlatitude medium‐scale traveling ionospheric disturbances (MSTIDs) observed with a Super Dual Auroral Radar Network (SuperDARN) HF radar at around 10 MHz in Hokkaido, Japan, in combination with a 630‐nm all‐sky imager and a GPS network (GEONET) that provides total electron content (TEC) data. MSTIDs propagating southward from high latitudes are detected at first with the HF radar and then with the imager and GEONET. We analyze two MSTID events, one in winter (event 1) and the other in summer (event 2), to find that MSTIDs appear simultaneously, at least, at 55°–25°N. It is shown that nighttime MSTIDs propagate toward the southwest over a horizontal distance of about 4000 km, and daytime MSTIDs do so toward the southeast. Daytime radar echoes are due to ground/sea surface (GS) scatter, while nighttime echoes in event 1 return from 15‐m‐scale F region field‐aligned irregularities (FAIs) and those in event 2 are due to GS scatter. Doppler velocities of the nighttime F region FAI echoes in event 1 are negative (motion away from the radar) within strong echo regions and are positive (motion toward the radar) within weak echo regions. This fact suggests that the strong (weak) echoes return from suppressed (enhanced) airglow/TEC areas, in line with previous observations over central Japan. The nighttime MSTIDs in events 1 and 2 are often accompanied by concurrent coherent echoes from FAIs in sporadic E (Es) layers. The Es echo areas in event 2 rather coincide with suppressed airglow/TEC areas in the F region that are connected with the echo areas along the geomagnetic field, indicating the existence of E and F region coupling at night.

Seasonal variation of low-latitude E-region plasma irregularities studied using Gadanki radar and ionosonde

Abstract. In this paper, we present seasonal variation of E region field-aligned irregularities (FAIs) observed using the Gadanki radar and compare them with the seasonal variation of Es observed from a nearby location SHAR. During daytime, FAIs occur maximum in summer and throughout the day, as compared to other seasons. During nighttime, FAIs occur equally in both summer and winter, and relatively less in equinoxes. Seasonal variations of Es (i.e. ftEs and fbEs) show that the daytime activity is maximum in summer and the nighttime activity is maximum in equinoxes. No relation is found between FAIs occurrence/SNR and ftEs/fbEs. FAIs occurrence, however, is found to be related well with (ftEs−fbEs). This aspect is discussed in the light of the present understanding of the mid-latitude Es-FAIs relationship. The seasonal variations of FAIs observed at Gadanki are compared in detail with those of Piura, which show a significant difference in the daytime observations. The observed difference has been discussed considering the factors governing the generation of FAIs.

30 MHz radar observations of artificial E region field-aligned plasma irregularities

Abstract. Artificial E region field aligned irregularities (FAIs) have been observed during heating experiments at the HAARP facility using a new 30 MHz coherent scatter radar imager deployed near Homer, Alaska. Irregularities were observed during brief experiments on three quiet days in July and August, 2007, when the daytime E region critical frequency was close to 3 MHz. Irregularities were consistently generated and detected during experiments with O-mode HF pumping on zenith with a 1-min on, 1-min off CW modulation. The scattering cross sections, rise, and fall times of the echoes were observed as well as their spectral properties. Results were found to be mainly in agreement with observations from other mid- and high-latitude sites with some discrepancies. Radar images of the irregularity-filled volume on one case exhibited clear variations in backscatter power and Doppler shift across the volume. The images furthermore show the emergence of a small irregularity-filled region to the south southwest of the main region in the approximate direction of magnetic zenith.

Disruption of E region echoes observed by the EAR during the development phase of equatorial spread F: A manifestation of electrostatic field coupling

Simultaneous observations of E‐ and F‐ region irregularities made using the Equatorial Atmosphere Radar (EAR) are presented to study the coupling between the E‐ and F‐ region. The observations clearly suggest that the disruption of E region field aligned irregularities (FAI) over the EAR is closely associated with the development phase of equatorial plasma bubble. The observed coupling effects on the low latitude E region FAI during the development phase of equatorial bubble are found to be quite pronounced over the EAR. This is consistent with the fact that the E region over the EAR connects the F region over the equator, where strong polarization electric fields are generated in association with equatorial plasma bubble phenomenon. Relatively weak vertical downward electric field and large eastward polarization electric field associated with plasma density depletion in the evening equatorial ionosphere, which map to low latitude E region over the EAR, are proposed to be responsible for the inhibition of growth of irregularities.