This paper presents all-sky airglow image analyses from OI 630-nm fisheye lens (FEL) photographs on the equatorial plasma bubble (EPB) observations over Taiwan and the nearby area. A FEL image calibration procedure of using single image with concentric circle and radial line control features has been described and applied to transform the raw FEL images into the expected perspective projection images. Two approximations on FEL image transformation have been proposed and evaluated to model the varying spatial resolution using polynomial and cubic spline interpolation functions. The validity and error analyses of the FEL image transformations are also demonstrated by testing a synthetic spiral coil image. For further applications, a low-cost miniature all-sky imaging system using Sony IMAX462-mode FEL camera and open-source indi-allsky operation software was set up and operated at the Lulin Mountain (23.47°N, 120.87°E; 2842 m altitude), Taiwan. The obtained field of view is from around 19.2°N to 27.8°N latitude (geomagnetic latitude 10.12°N ∼ 18.74°N) and 116.6°E to 125.2°E at an assumed airglow emission height of 250 km. We demonstrate the experimental all-sky airglow observations and obtain bifurcated and parallel plasma depletions separately during two events of post-sunset and midnight EPBs occurred before and after the spring equinox of 2025, respectively. The FEL image distortion calibration technique yields a “consistent” determination of nighttime plasma drifts from the transformed all-sky images on EPB observations and motion analyses. The derived eastward plasma drifts were higher at lower latitudes than those at higher latitudes and showed negative latitudinal gradients of the zonal plasma velocities during both events.

An improved numerical and climatological model, named the Taiwan Ionospheric Model-II (TWIM2), of global ionospheric electron density (Ne) has been investigated in this study. Based on the near-vertical Ne profiles retrieved from the FS7/COSMIC2 radio occultation observations, the TWIM2 exhibits vertically fitted oh-Chapman layers, with distinct F2, F1.5, F1, E, and D layers, and has been improved on the surface spherical harmonics approaches of the fitted Chapman-layer peak density (Nemax), peak density height (hm), and scale height (H) from geographic coordinate system to geomagnetic coordinate system. On average the mean error values from the NemaxF2 modeling in geomagnetic coordinate system are around 10 % less than those in geographic coordinate system. The result is consistent with those of surface spherical harmonic coefficient analyses and implies that the surface spherical harmonic analyses applied to three-dimensional Ne modeling in geomagnetic coordinate system are more effective. The TWIM2 results have been evaluated by the investigations of equatorial ionospheric anomaly (EIA) specification with diurnal and seasonal variations in a universal-time modeling mode or with latitudinal and longitudinal variations in a local-time modeling mode. We have applied a new proposed EIA strength (EAS) parameter, defined as the mean crest-to-trough Nemax difference, to identify and separate the post-sunset EIAs from the daytime EIAs and obtained daytime and post-sunset EIA peaks occurring at around mid-afternoon and 20.5 LT, respectively. Moreover, based on the derived TWIM2 NemaxF2-and hmF2-modeling maps at the local times with daytime or post-sunset EIA peaks, both the EAS and average low-latitude hmF2 profiles reveal wavenumber-4 longitudinal structures with similar longitudes at wave peaks. (c) 2025 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

The density variations at a constant height such as observed by the circularly orbiting ROCSAT-1 spacecraft are studied to construct a "prediction model" for the occurrences, no-occurrences, misses and false alarms of global/seasonal equatorial plasma bubble (EPB) occurrences. This global/seasonal prediction model is different from previous studies that merely provide the seasonal EPB occurrence probability at a local time region or globally. The current model uses the density increment above the seasonal mean to predict the EPB occurrences by constructing a contingency table that counts the numbers of successes, fails, misses, and false alarms in predicting the EPB occurrences. Different thresholds of density increment are used for the criteria in prediction to obtain an optimal result in the prediction model. The success of the model for predicting the global EPB occurrences or no-occurrences varies between 75% and 85% for any season between 1999 and 2004 during the moderate to high solar activity period. Studies of the causes for the misses and false alarms in the existing prediction result lead us to propose that additional observational parameter besides the density increment in the prediction model should be included. This additional parameter should be the seeding agent for the RTI process that related to the atmospheric/ ionospheric background conditions in the lower ionosphere. (c) 2024 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

This paper presents Global Navigation Satellite System radio occultation (RO) observational analyses on deducing the relationships and dependences between post‐sunset equatorial plasma bubbles (EPB) occurrences and equatorial ionospheric anomaly (EIA) strength variability. The RO data were acquired from the FormoSat‐7/Constellation Observing System for Meteorology, Ionosphere and Climate 2 (FS7/COSMIC2) Program from 2020 to 2024. In this study, we incorporate both effects from crest peak electron density ( N emax ) and crest‐to‐trough N emax ratio and propose a new EIA strength parameter defined as the mean of northern and southern crest‐to‐trough N emax differences to recognize and characterize the post‐sunset EIA features. Both seasonal–longitudinal appearances of intense post‐sunset EPB occurrences and strong EIA events occurred on more or less 30 days expanded from when and where magnetic flux tubes align with the sunset terminator at the magnetic equator but have more intense EPB and/or strong EIA days during southern (northern) hemispheric summers in the South American area (the Central Pacific area and the Africa area). It is well consistent with Tsunoda's hypothesis (Tsunoda, 1985, https://doi.org/10.1029/ja090ia01p00447 ) during the evening pre‐reversal enhancement and reveals more information on virtual day‐to‐day variability, intensities and extents of post‐sunset EPB occurrences and EIAs subject to seasonal, longitudinal, and solar cycle variabilities. Moreover, the local‐time evolutions of peak post‐sunset EIAs occurred during 19∼20 local time (LT) which is earlier than that of the obtained experimental peak (i.e., 20:20 LT) of post‐sunset EPB occurrences. We expect that the post‐sunset EIA detection could be one of the potential precursors for post‐sunset EPB occurrence.

The FormoSat-7/Constellation Observing System for Meteorology, Ionosphere, and Climate II (FS7/COSMIC2) program has acquired over three hundred thousand equatorial plasma bubble (EPB) observations from 2019 to 2023 in the equatorial and near low-latitude regions. The huge FS7/COSMIC2 database offers an opportunity to perform statistical inspections of the proposed hypothesis on seasonal versus longitudinal variability of EPB occurrence rates relevant to the Rayleigh–Taylor (R-T) instability. The detected EPBs are distributed along the magnetic equator with a half width of ~20° in geomagnetic latitude. The obtained EPB occurrence rates in local time (LT) rose rapidly after sunsets, and could be deconstructed into two overlapped Gaussian distributions resembling a major peak around 23:00 LT and a minor peak around 20:20 LT. The two groups of Gaussian-distributed EPBs in LT were classified as first- and second-type EPBs, which could be caused by different mechanisms such as sporadic E (Es) instabilities and pre-reversal enhancement (PRE) fields. The obtained seasonal–longitudinal distributions of both types of EPBs presented two diffused traces of high occurrence rates, which happened near the days and longitudes when and where the angle between the two lines of magnetic declination and solar terminator at the magnetic equator was equal to zero. Finally, we analyzed the climatological and seasonal–longitudinal variability of EPB occurrences and compared the results with the physical R-T instability model controlled by Es instabilities and/or PRE fields.

The Republic of China Satellite-1 orbiting at 600 km topside ionosphere has observed the topside ionospheric plasma flow pulsations induced by the substorm onsets. These pulsation events indicated that the plasma flow pulsations mainly oscillate in the two mutually perpendicular directions with respect to the geomagnetic field lines. The field-aligned flow as well as the ion density indicates almost no variation. This implies that the pulsation events are of Alfven wave in nature. The Hilbert-Huang transform analysis is applied to study the dominant wave frequencies and the polarization in the two perpendicular components of plasma flows (i.e., the perturbed electric/magnetic fields). The hodograms of the polarization in the Pi1 frequency is shown to be linearly polarized, while the left-handed polarization is seen in the Pi2 frequencies that are in harmonic relationship. These plasma flow pulsations in the nighttime topside ionosphere are caused by the field-line-resonance magnetic field pulsations converted from the inward propagated compressional disturbance across the nighttime magnetosphere/plasmasphere which is originated at the near-Earth magnetotail at the substorm onset.

In this study, a low-cost, software-defined Global Positioning System (GPS) and Satellite-Based Augmentation System (SBAS) Reflectometry (GPS&SBAS-R) system has been built and proposed to measure ocean-surface wave parameters on board the research vessel New Ocean Researcher 1 (R/V NOR-1) of Taiwan. A power-law, ocean-wave spectrum model has been used and applied with the Small Perturbation Method approach to solve the electromagnetic wave scattering problem from rough ocean surface, and compared with experimental seaborne GPS&SBAS-R observations. Meanwhile, the intensity scintillations of high-sampling GPS&SBAS-R signal acquisition data are thought to be caused by the moving of rough surfaces of the targeted ocean. We found that each derived scintillation power spectrum is a Fresnel-filtering result on ocean-surface elevation fluctuations and depends on the First Fresnel Zone (FFZ) distance and the ocean-surface wave velocity. The determined ocean-surface wave speeds have been compared and validated against nearby buoy measurements.

We develop a new class of the multiscale finite element method (MsFEM) to solve the convection-diffusion problems. In the proposed framework, we decompose the solution function space into two parts in MsFEM with locally adaptive bubble function enrichment (LABFE). The first part is the one that the multiscale basis functions can resolve, and the second part is an unresolved part that is taken care of by a set of bubble functions. These bubble functions are defined similarly to construct multiscale basis functions. We exchange the local-global information through updated local boundary conditions for these bubble functions. The new multiscale solution recovered from the solution of global numerical formulation provides feedback for updating the local boundary conditions on each coarse element. As the approach iterates, the quality of MsFEM-LABFE solutions improves since these multiscale basis functions with bubble function enrichment are expected to capture the multiscale feature of the approximate solution more accurately. However, the most expansive part of the algorithm is reconstructing the bubble functions on each coarse element. To reduce the overhead of the bubble function reconstruction, we update the local bubble function only when the intermediate multiscale solution is not well resolved within the region corresponding to the sharp local gradient or the discontinuity of the solution. We illustrate the effectiveness of the proposed method through some numerical examples for convection-diffusion benchmark problems.

High-rate radio occultation (RO) in COSMIC-2 (FORMOSAT7) enables us to investigate the finer details of the ionosphere owing to the measurements being made at a significantly high spatiotemporal resolution, which was unthinkable a decade ago. In the vertical plane, local-time ionospheric wavenumber-4 (WN4) structures display tilted phase-fronts over the equatorial ionization anomaly (EIA) belt. The longitudinal extent of a tilted WN4 phase-front approximates the zonal wavelength of nonmigrating DE3 tide in the local-time frame of reference, i.e., ~900. The WN4-filtered (residual) component indicates a greater tilt (when visible), with a larger longitudinal extent of a wavenumber structure in the vertical plane. The WN4 structure over the EIA crest region is found to be out of phase (in phase) with respect to that over the EIA trough region during daytime (nighttime), which also depended on the altitude under consideration. Intriguingly, above 400 km, the WN4 structures in the EIA crest and trough regions are seen to be in phase with each other at all local times. The phenomenon of the “longitudinal co-location” of WN4 over the EIA crest and trough regions at altitudes above ~400 km at all local times remains unexplained. Results also highlight that the formation of WN4 is governed by a complex interplay of direct forcing of nonmigrating tides and the zonal electric field whose characteristics within the EIA belt vary drastically with latitude and altitude under consideration.

In this study, a multi-station and multi-instrument system, developed for ionospheric scintillation and equatorial spread-F (ESF) specification in the Taiwan-Philippines sector, is outlined. The issues related to the scintillation and ESF event observed on Oct. 26, 2021, and at magnetic quiet conditions are presented and discussed. We first indicate the existence of a plasma bubble in the Taiwan-Philippines sector using the FormoSat-7 / Constellation Observing System for Meteorology, Ionosphere and Climate-2 (FS7/COSMIC2) GPS/GLONASS radio occultation (RO) observations. We verify the latitudinal extent of the tracked plasma bubble using the recorded ionograms from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) located at Hualien (23.89°N, 121.55°E, dip latitude 17°N), Taiwan. We further discuss the spatial and temporal variabilities of two-dimensional vertical scintillation index VS4 maps based on the simultaneous GPS L1-band signal measurements from 133 ground-based receivers located in Taiwan and the surrounding islands. We also operate two high-sampling software-defined GPS receivers and characterize the targeted plasma irregularities by carrying out spectrum analyses of the received signal. As a result, the derived plasma irregularities moved eastward and northward, and smaller irregularity scale higher the spectral index and stronger scintillation intensity at lower latitudes on the aimed irregularity feature.

A statistical study on the occurrence characteristics of global low-to-midlatitude topside ionospheric density enhancements (plasma blobs) has been carried out using the ROCSAT-1 (Republic of China Satellite-1) data during high solar activity period of 1999 to 2004. The latitudinal distribution of plasma blobs indicates that the occurrence rates increase from the dip latitude +/- 15 degrees to midlatitudes that appear to complement the latitudinal distribution of equatorial plasma bubbles (EPBs). For the seasonal distribution, it is found that the maximum occurrence rate appears during the June solstice in both northern and southern hemispheres. The local-time distribution indicates a maximum occurrence rate around midnight, and maintains a high occurrence rate until about 04 h in local time in contrast to a rapid decrease after midnight for the EPB occurrences. Negative correlation with the solar activity is found for the occurrence of plasma blobs that is opposite to the positive correlation with the solar activity for the EPB occurrences. All these different and contrasting occurrence characteristics between the low-to-midlatitude plasma blob occurrence and the EPBs imply that the occurrence mechanism for the low-to-midlatitude plasma blobs should have little relationship to that of EPBs. From the fact that an upward/outward plasma drift and the increase of O+ species are observed inside the plasma blob, an eastward polarization electric field should directly appear at topside ionosphere for the occurrence of plasma blobs. This polarization electric field should be mapped from the nighttime midlatitude Es layer instability process where the polarization electric field is generated. (C) 2022 COSPAR. Published by Elsevier B.V.

In this study, a multi-station and multi-instrument system, organized and proposed for ionospheric scintillation and equatorial spread-F (ESF) specification and their associated motions in the Taiwan–Philippines sector, is outlined. The issues related to the scintillation and ESF event observed on 26 October 2021, at magnetic quiet conditions are presented and discussed. We first indicate the existence of a plasma bubble in the Taiwan–Philippines sector by using the FormoSat-7/Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (FS7/COSMIC2) GPS/GLONASS radio occultation observations. We verify the latitudinal extent of the tracked plasma bubble using the recorded ionograms from the Vertical Incidence Pulsed Ionospheric Radar located at Hualien, Taiwan. We further discuss the spatial and temporal variabilities of two-dimensional vertical scintillation index VS4 maps based on the simultaneous GPS L1-band signal measurements from 133 ground-based receivers located in Taiwan and the surrounding islands. We also operate two high-sampling, software-defined GPS receivers and characterize the targeted plasma irregularities by carrying out spectrum analyses of the received signal. As a result, the derived plasma irregularities moved eastward and northward. Furthermore, the smaller the irregularity scale, the higher the spectral index and the stronger the scintillation intensity were at lower latitudes on the aimed irregularity feature.

Nighttime F-region field-aligned irregularities (FAIs) associated with equatorial plasma bubbles (EPBs) are impacted by terrestrial factors, such as solar irradiance and geomagnetic activity. This paper examines the impact of the planetary-scale periodic variability of terrestrial processes on EPB activity. Continual observations of the Equatorial Atmosphere Radar (EAR) have been utilized to derive the intra-seasonal variability of nighttime F-region FAIs in the context of the terrestrial factors mentioned above. A periodicity analysis using wavelet and Lomb–Scargle (LS) spectral analysis indicated significant amplitudes of the long-period planetary-scale variability in the F-region FAI signal-to-noise ratio (SNR), 10.7 cm flux, and geomagnetic indices, as well as a shorter period of variability. Interestingly, a careful inspection of the time series indicated the planetary-scale variability of F-region FAIs to be reasonably out of phase with the periodic geomagnetic variability. EPB occurrence and the FAI signal-to-noise ratio presented a systematic decrease with an increase in the level of geomagnetic activity. Non-transient quiet-time geomagnetic activity has been found to suppress both the occurrence as well as the strength of F-region FAIs. The impacts of planetary-scale geomagnetic activity appear to be non-identical in the summer and equinoctial EPBs. The results highlight the importance of periodic terrestrial processes in driving the planetary-scale variability of EPBs.

A low-cost transportable software-based global positioning system reflectometry (GPS-R) scheme, which can measure sea-surface wave frequency, period, and speed, is proposed and described. We designed and implemented an appropriate software receiver to acquire and track GPS-R L1-band C/A code signals in near real time. At Lanyu, Taiwan, a research platform has been built with two software-based GPS-R receivers overlooking the seas in the east-northeast and southwest directions. Additionally, we propose applying the maximum entropy method for the spectral analyses of recorded time series of GPS-R signal acquisition data and derive the mean frequencies, periods, and speeds of random sea-surface waves. The derived sea-surface wave frequencies have been compared and validated against buoy measurements and Weather Research and Forecasting (WRF) model data around Lanyu Island. The results show that the buoy wave heights and the modeled WRF wave heights have linear correlation coefficients of 0.64 and 0.47, respectively, with the GPS-R wave frequency measurements. The observed coastal sea area has a maximum horizontal distance of approximately 20 km from station Lanyu (22.037°N, 121.559°E). Thus, the corresponding mapping products of the sea-surface wave period and speed are presented with wave propagation footprints of the first Fresnel zone sizes.

A large density irregularity structure in latitudinal extent was observed by the sun‐synchronous orbiting FORMOSAT‐5 satellite at 720 km topside ionosphere in the South American sector on November 24, 2018 during a magnetically quiet time. The observed density irregularity structure has a latitudinal extent of ∼30° across the dip equator, and a longitudinal width of ∼6°. Unlike previously studied equatorial plasma bubble structures with the airglow images that indicate hemispheric magnetic conjugate similarity, the gross feature of this large density irregularity structure indicates a north‐south hemispheric asymmetry in density variation. We use the Hilbert‐Huang Transform analysis to study the irregularity structure in different scales of density variations. It is found that the amplitude of density variations in the scale sizes from 22.5 to 450 km all indicate the hemispheric asymmetry. However, the spectral property of density variations in the two hemispheres is almost identical to each other. This implies that the density irregularity structure which originates from the same Rayleigh‐Taylor (RT) instability process has preserved the spectral characteristics inside the magnetic flux tube. Therefore, the cause that fails to preserve the magnetic conjugate property in the density irregularity structure comes from the fact that the background ionospheric density distribution has a north‐south hemispherical asymmetry in the beginning of the RT instability process. This hemispheric asymmetrical density distribution is caused by the seasonal variation of density distributions in the two hemispheres and the prevailing inter‐hemispheric neutral wind during solstices in the South American sector.

The FormoSat-3/ Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) has been proven a successful mission on performing active limb sounding of the ionosphere using the GPS radio occultation (RO) technique. The follow-on program called FS7/COSMIC2 is in progress with satellite launched on 25 June of 2019 and includes six low-Earth-orbit (LEO) satellites at 24°-inclination and ~720-km orbits to receive multi-channel (1.5GHz and 1.2GHz) GPS and GLONASS satellite signals. The FS7/COSMIC2 can provide about 5,000 GNSS RO observations per day which are increased by a factor of about 5 comparing to FS3/COSMIC and within the region from the geographic equator to the latitude at 40°. We process 1-Hz amplitude data and obtain complete limb-viewing profiles of the undersampling-S4 scintillation index to study global F-layer irregularity morphology. There are a few percent of FS3/COSMIC and FS7/COSMIC2 GPS/GNSS RO observations having >0.09 undersampling S4max values on average. However, seven identified areas Central Pacific Area, South American Area, African Area, European Area, Japan Sea Area, Arctic Area and Antarctic Area have been designated to have a much higher percentage of strong limb-viewing L-band scintillations. Generally, the F-layer scintillation climatology, namely, its variations with each identified zone, altitude, season, and local time have been documented. The large dataset from the FS3/COSMIC and FS7/COSMIC2 programs enable statistical studies on equatorial and low-latitude ionospheric irregularity and their models.

The basic theory and experimental results of amplitude scintillation from GPS/GNSS radio occultation (RO) observations on sporadic E (Es) layers are reported in this study. Considering an Es layer to be not a “thin” irregularity slab on limb viewing, we characterized the corresponding electron density fluctuations as a power-law function and applied the Ryton approximation to simulate spatial spectrum of amplitude fluctuations. The scintillation index S4 and normalized signal amplitude standard deviation S2 are calculated depending on the sampling spatial scale. The theoretical results show that both S4 and S2 values become saturated when the sampling spatial scale is less than the first Fresnel zone (FFZ), and S4 and S2 values could be underestimated and approximately proportional to the logarithm of sampled spatial wave numbers up to the FFZ wave number. This was verified by experimental analyses using the 50 Hz and de-sampled FormoSat-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) GPS RO data in the cases of weak, moderate, and strong scintillations. The results show that the measured S2 and S4 values have a very high correlation coefficient of >0.97 and a ratio of ~0.5 under both complete and undersampling conditions, and complete S4 and S2 values can be derived by dividing the measured undersampling S4 and S2 values by a factor of 0.8 when using 1-Hz RO data.

Generally, the occurrence of F region irregularities associated with Equatorial Plasma Bubbles is considered difficult to predict due to its day-to-day variability. Recent investigation shows that the dominant period of variability of EPB can be >25 days (quasi 27 day), presumably associated with solar rotation period. VHF scintillation recorded from Pingtung, Taiwan in 2015 indicated the co-existence of planetary scale variability (4-8 days), 10-15 days variability and quasi 27 day variability. Lomb-Scargle power spectrum of S4 time series indicated that the larger period variability (quasi 27 days) is much more dominant than the other scales (which includes day-to-day variability). Interestingly, the dominant period of S4 variability corresponded exactly with the dominant period of high-latitude geomagnetic variability. Continuous operation of Equatorial Atmosphere Radar (EAR) in 2012 also indicated similar observation. Dominant period of variability of EPB in EAR observations corresponded with the high-latitude geomagnetic variability. These observations reveal that EPB are controlled predominantly by geomagnetic activity through large period variability (quasi- 27 days).

FORMOSAT-5 satellite was launched into a sun-synchronous orbit at 720 km altitude with 98.28 degrees inclination on 25 August 2017. The onboard scientific payload, Advanced Ionospheric Probe (AIP) is capable of measuring topside ionospheric ion density, cross-track flow velocities, ion composition and temperature, and electron temperature. Initial observations of nighttime midlatitude ionospheric density and vertical flow velocity variations at 2230 LT sector during a few quiet magnetic days in December 2017 are studied here. Longitudinal density variations in the equatorward edge of midlatitude ionospheric trough (MIT) region are noticed. Accompanied with this density variation, the vertical flow velocities also behave differently. Although the density difference has been stated due to zonal wind effect related to the declination of the geomagnetic field lines, the vertical flow velocity variation seems to play the opposite role. All these density and vertical flow observations in the northern winter hemisphere can only be explained by the longitudinal differences in the diffusion velocity coming down from the protonsphere (plasmasphere). In addition, the hemispheric asymmetry in the vertical flow velocity can also be explained by the interaction between the topside ionosphere and the protonsphere. The observed vertical flow variations near MIT at different longitudes should present a new potential tool for the study of MIT formation. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

The nature of intraseasonal (within a season) variability of the nighttime ionospheric irregularities has been studied utilizing the observation of the recently installed VHF scintillation receiver from Pingtung, Taiwan. Investigation is based on observations made in 2015 (high-moderate solar epoch) and 2017 (low solar epoch). Detailed investigation has revealed coexistence of planetary-scale variability (3-7 days), 10- to 16-day variability, and > 25-day variability in 2015. Interestingly, the Lomb-Scargle frequency domain analysis indicated the variability of larger timescales to display somewhat greater amplitudes. The virtual height of the F layer, h'F, over Sanya, also indicated intraseasonal periodicities similar to S4 periodicities observed at Pingtung. Observations have also revealed that, equatorial plasma bubbles (EPBs) over Pingtung tended to occur on contiguous nights as compared to isolated occurrence, and > 60% of the EPB nights were part of two or more (up to 6) nights of contiguous occurrence. Spectral analysis revealed a quasi-27-day variability in the ap and AE geomagnetic indices presumably associated with the solar rotation period. The time period of geomagnetic variability corresponded with the period of dominant S4 variability, indicating that the effect of geomagnetic activity on the low-latitude ionosphere can be periodic in nature. In 2017 (low solar flux), even with a very low occurrence of ionospheric irregularities, planetary-scale variability was observed. The dominant period of S4 variability in the autumnal equinox was highly correlated with the periodic geomagnetic activity. Results indicate that a deeper understanding of the periodic intraseasonal variability can be imperative for extended range forecast/prediction of ionospheric irregularities.