First observation of presunset ionospheric F region bottom-type scattering layer (original) (raw)
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Progress in Earth and Planetary Science, 2018
Typical equatorial spread-F events are often said to occur during post-sunset, equinox conditions in most longitude sectors. Recent studies, however, have found an unexpected high occurrence of ionospheric F-region irregularities during June solstice, when conditions are believed to be unfavorable for the development of plasma instabilities responsible for equatorial spread-F (ESF). This study reports new results of a multi-instrumented investigation with the objective to better specify the occurrence of these atypical June solstice ESF in the American sector and better understand the conditions prior to their development. We present the first observations of June solstice ESF events over the Jicamarca Radio Observatory (11.95°S, 76.87°W, ∼ 1 • dip latitude) made by a 14-panel version of the Advanced Modular Incoherent Scatter Radar system (AMISR-14). The observations were made between July 11 and August 4, 2016, under low solar flux conditions and in conjunction with dual-frequency GPS, airglow, and digisonde measurements. We found echoes occurring in the pre-, post-, and both pre-and post-midnight sectors. While at least some of these June solstice ESF events could have been attributed to disturbed electric fields, a few events also occurred during geomagnetically quiet conditions. The late appearance (22:00 LT or later) of three of the observed events, during clear-sky nights, provided a unique opportunity to investigate the equatorial bottomside F-region conditions, prior to ESF, using nighttime airglow measurements. We found that the airglow measurements (630 nm) made by a collocated all-sky camera show the occurrence of ionospheric bottomside F-region perturbations prior to the detection of ESF echoes in all three nights. The airglow fluctuations appear as early as 1 hour prior to radar echoes, grow in amplitude, and then coincide with ESF structures observed by AMISR-14 and GPS TEC measurements. They also show some of the features of the so-called large-scale wave structures (LSWS) that have been detected, previously, using other types of observations and have been suggested to be precursors of ESF. The bottomside fluctuations have zonal spacings between 300 and 500 km, are aligned with the magnetic meridian, and extend at least a few degrees in magnetic latitude.
Daytime Equatorial Spread F‐Like Irregularities Detected by HF Doppler Receiver and Digisonde
Space Weather-the International Journal of Research and Applications, 2021
The ionosphere is an indispensable medium for the propagation of communication and navigation signals. It has been extensively studied over the years to understand its properties and variability under different conditions. Spread F (SF) is a common feature of the ionosphere that is caused by irregularities in electron density distribution in the F-layer. Radio signals incident on these irregular plasma structures are scattered, thereby causing diffuse spreading in the F-layer trace of ionograms. Bottomside spread F was first reported by Booker and Wells (1938) and a number of mechanisms are understood to be responsible for its occurrence and development. Post-sunset SF is most commonly attributed to increased upward vertical plasma drift (also known as the pre-reversal enhancement [PRE]) in response to an enhanced eastward electric field. Due to the absence of a highly conducting E-layer at night, plasma instabilities can develop more easily at elevated F-layer heights and can subsequently grow via the Rayleigh-Taylor instability (RTI). This phenomenon is even more common at equatorial latitudes because the geometry of the geomagnetic field at these latitudes facilitates E B plasma drifts (Sreehari et al., 2006). Apart from the PRE associated with post-sunset F-region electrodynamics, other mechanisms known to contribute to F-layer plasma instabilities include zonal winds, atmospheric gravity waves (AGWs) which manifest in the ionosphere as traveling ionospheric disturbances (TIDs), and trans-equatorial thermospheric winds (Abdu, 2012). The role of the RTI in the development of the equatorial spread F (ESF) was first suggested by Dungey (1956). The RTI develops when a heavy fluid rests on a lighter one. In such a state, gravity or other triggers can cause the heavier liquid to become unstable. Ossakow (1979) and references therein discussed the role of the F-layer peak and electron density scale length in the linear growth of the RTI. A higher F-layer peak and smaller electron density scale length are both favorable conditions for the linear growth of the RTI. In the mathematical formulation by Huang et al. (1993), it was shown that AGWs can directly seed the RTI. Huang et al. (1993) showed that AGWs with speed 5-20 ms −1 and wavelength in the order of 100 km Abstract Daytime equatorial spread F (ESF) is not as common as nighttime ESF due to the presence of a highly conducting E-layer during the daytime which counteracts the development of F-layer plasma irregularities. This study presents two rare daytime ESF-like events which occurred over an interval ∼2 h and were detected by the HF Doppler receiver located in Lagos (LAG: geographic: 3.27°E, 6.48°N; dip latitude −1.72°) and the Lowell Digisonde at Ilorin (ILR; 4.68°E, 8.50°N; dip latitude −1.25°), managed by Lowell GIRO Data Center (LGDC). Analysis of the first event revealed ∼30 min periodic oscillations in iso-heights of ionospheric electron density. Shorter period (∼15 min) oscillations appeared simultaneously in HF Doppler measurements and these oscillations lasted nearly 3 h. Close inspection of the ionograms from ILR during this interval (1500-1800 UT) showed the occurrence of small-scale spreading in the F-layer trace which varied in altitude as the disturbance progressed. Computation of the linear growth rate of the collisional Rayleigh-Taylor instability showed that the plasma instability was seeded by a traveling ionospheric disturbance (TID). The characteristics of the second event suggest that horizontal stratifications in plasma density distribution at the reflecting ionospheric layer were responsible for the spread F traces in the ionograms. Analysis of GPS TEC data from Nigeria during these events revealed the presence of wave structures consistent with TIDs.
Recent case studies on the precursor signatures of equatorial spread-F (ESF) have shown a one-to-one correspondence between the large-scale wave structures (LSWS) and ESF development at equatorial latitude. In this study, the LSWS and the onset and development of the ESF are investigated over Sanya (18°N, 109°E), a station located at 13° north of the magnetic equator, during both geomagnetic quiet and disturbed conditions in September–October 2011. High-time-resolution ionograms from Digisonde Portable Sounder (DPS-4D) provided the satellite trace measurements that were used to indicate the occurrence of the LSWS. The development of local ESF activity was identified using GPS scintillation and VHF coherent radar echo measurements from the same site, together with the range type spread-F (RSF) in ionograms. Additionally, the Sanya VHF radar five-beam scanning measurements in east–west direction were used to characterize the longitudinal difference in establishing the initial conditions for ESF development. Correlative studies between the LSWS and ESF activities during the observational period offer consolidated evidence that the LSWS is a necessary precursor for the ESF development. It is shown that the LSWS and ESF have nearly a one-to-one relationship when the F layer undergoes an abrupt post-sunset rise (PSSR), revealing that the magnitude of the pre-reversal enhancement in zonal electric field (PRE) that elevates the F layer to a high enough altitude is an important parameter controlling the generation of post-sunset ESF. However, in the absence of the PSSR, the ESF and GPS scintillation did not always occur following the appearance of LSWS. Sometimes the LSWS events preceded the generation of bottom type spread-F (BSF) that did not develop vertically into ESF and radar plumes. This result may indicate that under inexpressive, weak, or even moderate PRE conditions, the appearance of the LSWS alone may not be sufficient to produce the post-sunset F region irregularities responsible for ionospheric scintillations. More factors, other than the LSWS, could play crucial roles favoring the growth of ESF instabilities responsible for ionospheric scintillations.
Journal of Geophysical Research, 2012
An atypical nighttime spread-F structure is observed on ionograms at or above the F 2 trace, near the crest of the ionospheric equatorial ionization anomaly (EIA) region. This ionospheric atypical spread-F phenomenon was observed using two closed spaced ($115 km) ionospheric soundings stations located in Sao Jose dos Campos (23.21 S, 45.97 W) and Cachoeira Paulista (22.70 S, 45.01 W), Brazil, in a low-latitude station (near the southern crest of the EIA region), during nighttime, low solar activity, and quiet geomagnetic conditions. This structure, in the initial phase, appears in the ionogram as a faint spread-F trace above or at the F 2-layer peak height. After a few minutes, it develops into a strong spread-F trace, and afterwards, it moves to altitudes below the F 2-layer peak heights. Finally, the atypical nighttime F-layer trace structure may remain for a while between the F-layer bottom side and peak height or can move to an altitude above the F-layer peak height, and then it disappears. In order to have a comprehensive view of the ionospheric environment characterizing the phenomenon under study, complementary data from six GPS station were used to investigate the ionosphere environment conditions, during both events. The six GPS stations used in this study are distributed from near the equatorial region to low latitudes and provide evidence that the atypical nighttime spread-F structures are not related with large scale equatorial irregularities (plasma bubbles).
Annales Geophysicae Discussions, 2018
We present a case study of unusual spread-F structures observed by ionosondes at two equatorial and low latitude Brazilian stations-Sao Luis (SL: 44.2° W, 2.33° S, dip angle: −6.9°) and Fortaleza (FZ: 38.45°W, 3.9° S, dip angle: −16°). The irregularity structures observed from midnight to post-midnight hours of moderate solar activity (F10.7 < 97) have characteristics different from typical post-sunset equatorial spread-F. The spread-F traces first appeared at or above the F-layer peak and gradually became well-formed mixed spread-F. They also appeared as plasma depletions in the 630.0 nm airglow emissions made by a wide-angle imager located at nearby low latitude station Cajazeiras (CZ: 38.56° W, 6.87° S, dip angle:-21.4°). The irregularities appeared first over FZ and later over SL, giving evidence of an unusual westward propagation or a horizontal plasma advection. The drift mode operation available in one of the ionosondes (a Digital Portable Sounder, DPS-4) has enabled us to analyze the horizontal drift velocities and directions of the irregularity movement. We also analyzed the neutral wind velocity measured by a Fabry-Perot interferometer (FPI) installed at CZ and discussed its possible role on the development of the irregularities. during low solar activity conditions, there is a class of spread-F/plasma irregularities regularly observed in distinct longitudinal sectors. They are known as post-midnight plasma irregularities (PMIs), which occur mostly in June solstice. A recent review of plasma irregularities is provided by Balan et al. (2018). PMIs occur under conditions considered not favorable for the development of the Rayleigh-Taylor (RT) instability, since that at night the vertical plasma drifts are downward, owing to the westward electric fields. In recent years, a variety of works have reported their occurrence both at low latitudes and equatorial region. Otsuka et al. (2009) and Nishioka et al. (2012) investigated PMIs over Indonesia and discussed their possible sources. Li et al. (2011) reported these irregularities observed over Hainan, China during low solar activity. Candido et al. (2011) presented a study of PMIs observed over the south crest of the equatorial ionization anomaly (EIA) during low solar activity, in CP, Brazil. Yokohama et al. (2011) studied unusual patterns of echoes from coherent scatter radar data occurring around midnight during the solar minimum period. They observed two principal types of irregularities: the upwelling plumes and MSTID-like striations. They have argued that the former can be generated by both the RT instability (at equatorial region) or to Perkins instability (at mid-latitude region) and the later only by the Perkins instability. Yizengaw et al. (2013) presented the study of the PMIs over equatorial Africa, and also investigated their most probable causes. Dao et al. (2017) reported in a very interesting work the occurrence of postmidnight field-aligned irregularities (FAIs) in Indonesia during low solar activity in 2010. Many instrumental techniques are currently providing high-quality measurements and results for ionospheric studies. Early investigations of the ionosphere referred to the diffuse echoes seen in data from measurements using ionosondes, which are high-frequency radars
Journal of Geophysical research: Space Physics, 2022
In 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.