spread-F Occurrence Research Articles

Role of thermospheric winds on the generation of F-region irregularities over Indonesian sector during the solar minimum year 2020

Analysis of ionosonde observations over Chumpon (10.7°N, 99.4°E, 3.3° Mag. Lat.) and Kototabang (0.2° S, 100.3° E, 10° S Mag. Lat.) during the low solar activity year 2020 reveals that the occurrence of post-sunset spread-F maximises during winter (November-February), whereas that of post-midnight spread-F maximises during summer (May-August). The thermospheric winds observed by the Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) on board the Ionospheric Connection Explorer (ICON) satellite over the Indonesian longitudes (95°E-115°E) are investigated to determine their impact on the generation of spread-F. The thermospheric winds at equatorial and low latitudes (10°N) are strongly eastward (> 50 m/s) around post-sunset hours (∼19:00 LST) in winter and equinox. The strong eastward wind is present only around midnight hours (after 22:00 LST) in summer. However, the present study reveals that the zonal wind does not contribute to the generation of plasma bubbles. Besides, the ICON-MIGHTI meridional wind at low latitude (10°N) is observed to be equatorward throughout the day during summer. Compared to the post-sunset hours, the intensity of trans-equatorial wind is stronger on the windward side (60–70 m/s) than on the leeward side (10–20 m/s) during midnight hours. The component of the equatorward meridional wind along the magnetic field lines can lift the F-layer to higher heights leading to a net decrease in the field line-integrated Pedersen conductivity thereby increasing the growth rate of the Rayleigh Taylor instability that can result in the generation of post-midnight spread F.

Equatorial spread-F forecasting model with local factors using the long short-term memory network

The predictability of the nighttime equatorial spread-F (ESF) occurrences is essential to the ionospheric disturbance warning system. In this work, we propose ESF forecasting models using two deep learning techniques: artificial neural network (ANN) and long short-term memory (LSTM). The ANN and LSTM models are trained with the ionogram data from equinoctial months in 2008 to 2018 at Chumphon station (CPN), Thailand near the magnetic equator, where the ESF onset typically occurs, and they are tested with the ionogram data from 2019. These models are trained especially with new local input parameters such as vertical drift velocity of the F-layer height (Vd) and atmospheric gravity waves (AGW) collected at CPN station together with global parameters of solar and geomagnetic activity. We analyze the ESF forecasting models in terms of monthly probability, daily probability and occurrence, and diurnal predictions. The proposed LSTM model can achieve the 85.4% accuracy when the local parameters: Vd and AGW are utilized. The LSTM model outperforms the ANN, particularly in February, March, April, and October. The results show that the AGW parameter plays a significant role in improvements of the LSTM model during post-midnight. When compared to the IRI-2016 model, the proposed LSTM model can provide lower discrepancies from observational data.Graphical

Investigation of the possible relationship between the occurrences of atmospheric gravity waves and equatorial Spread-F

Equatorial Spread F (ESF), is a manifestation linked to Atmospheric Gravity Waves (AGW). There have not been many studies to ascertain the extensive relationship between the occurrence of AGW and the occurrence of ESF. To evaluate the extent of their relationship, this study used data obtained with the aid of a satellite-borne Atmospheric Infrared Sounder (AIRS) and ionograms obtained using a Digisonde Portable Sounder (DPS-4) located at Jicamarca (geog. Lat. 11.950 ᵒS, long. 76.867 ᵒW and geomagnetic Lat. 2.27 ᵒS, Long. 4.15 ᵒW; Dip 2 ᵒN) during the year 2016. The results also suggest that whenever AGW, under the control of tidal winds, propagates into F-region heights such that both AGW and tidal wind structures act together, it could have great potential at influencing plasma instability growth rate. The result further shows that when AGW has a long enough wavelength to reach the F-region, it tends to influence the factors responsible for the occurrence of the Mixed Spread-F (MSF) type of ESF. MSF is observed to be predominant in occurrence whenever AGW and ESF occur simultaneously compared to the other two types of ESF. The coefficient of correlation between AGW and MSF ranged between 0.1 and 0.5, while for Range Spread-F and Frequency Spread-F, it ranged between ± 0.2. These levels of correlations show that AGW does not directly trigger ESF. The range of the correlation coefficient between AGW and MSF however tends to support the suggestion that AGW occurrence is capable of influencing the factors responsible for a type of ESF occurrence rather than triggering ESF occurrence altogether.

Statistical Analysis of SF Occurrence in Middle and Low Latitudes Using Bayesian Network Automatic Identification

Spread-F (SF) is one of the most important types of the ionospheric irregularities as it causes ionospheric scintillation which can severely affect the performance and reliability of communication, navigation, and radar systems. The ionosonde provides the most effective and economical way to study the ionosphere and SF. However, the manual identification of SF from an ionogram is boring and hard work. To automatically identify SF on the ionogram and extend the study of SF into the middle and low latitudes of East Asia, this paper presents a statistical analysis of SF in this region, based on the naïve Bayesian classifier. The results showed that the accuracy of automatic identification reached up to 97% on both the validation datasets and test datasets composed of Mohe, I-Cheon, Jeju, Wuhan, and Sanya ionograms, suggesting that it is a promising way to automatically identify SF on ionograms. Based on the classification results, the statistical analysis shows that SF has a complicated morphology in the middle and low latitudes of East Asia. Specifically, there is a peak of occurrence of SF in the summer in I-Cheon, Jeju, Sanya, and Wuhan; however, the Mohe station has the highest occurrence rate of SF in December. The different seasonal variations of SF might be due to the different geographic local conditions, such as the inland-coastal differences and formation mechanism differences at these latitudes. Moreover, SF occurs more easily in the post-midnight hours when compared with the pre-midnight period in these stations, which is consistent with the previous results. Furthermore, this paper extracts the frequency SF (FSF) index and range SF (RSF) index to characterize the features of SF. The results shows that the most intense FSF/RSF appeared in the height range of 220–300 km/1–7 MHz in these stations, although there are different magnitude extensions on different season in these regions. In particular, strong spread-F (SSF) reached its maximum at the equinox at Sanya, confirming the frequent SSF occurrence at the equinox at the equator and low latitudes. These results would be helpful for understanding the characteristics of SF in East Asia.

Latitudinal Differences in Spread F Characteristics at Asian Longitude Sector during the Descending Phase of the 24th Solar Cycle

By using ionosonde data recorded at Chiang Mai (18.8° N, 98.9° E, magnetic latitude is 9.1° N), Puer (22.7° N, 101.1° E, magnetic latitude is 12.9° N), and Leshan (29.6° N, 103.7° E, magnetic latitude is 19.8° N), the statistical features of different types of spread F (SF) occurrence at low and middle latitudes were analyzed in this study. The results showed that the SF occurrence had obvious local time, latitude, and SF-type variations. The range spread F (RSF) occurrence in equinox months decreased with the increase in latitude, while the frequency spread F’s (FSF) occurrence rate in the summer months increased and the onset time of FSF became earlier when the latitude increased. The generation of SF depends on the SF type. A plasma bubble excited by the generalized Rayleigh–Taylor instability (GRT) at the equator is more likely to produce RSF, while nighttime medium-scale traveling ionospheric disturbances (MSTIDs) induced by Perkins instability at middle latitudes is the main reason for the generation of FSF.

Modeling Post-Sunset Equatorial Spread-F Occurrence as a Function of Evening Upward Plasma Drift Using Logistic Regression, Deduced from Ionosondes in Southeast Asia

The occurrence of post-sunset equatorial spread-F (ESF) could have detrimental effects on trans-ionospheric radio wave propagation used in modern communications systems. This problem calls for a simple but robust model that accurately predicts the occurrence of post-sunset ESF. Logistic regression was implemented to model the daily occurrence of post-sunset ESF as a function of the evening upward plasma drift (v). The use of logistic regression is formalized by y^=1/[1+exp(−z)], where y^ represents the probability of post-sunset ESF occurrence, and z is a linear function containing v. The value of v is derived from the vertical motion of the bottom side of the F-region in the evening equatorial ionosphere, which is observed by the ionosondes in Southeast Asia. Data points (938) of v and post-sunset ESF occurrence were collected in the equinox seasons from 2003 to 2016. The training set used 70% of the dataset to derive z and y^ and the remaining 30% was used to test the performance of y^. The expression z=−2.25+0.14v was obtained from the training set, and y^≥0.5 (v ≥ ~16.1 m/s) and y^<0.5 (v < ~16.1 m/s) represented the occurrence and non-occurrence of ESF, respectively, with an accuracy of ~0.8 and a true skill score (TSS) of ~0.6. Similarly, in the testing set, y^ shows an accuracy of ~0.8 and a TSS of ~0.6. Further analysis suggested that the performance of the z-function can be reliable in the daily F10.7 levels ranging from 60 to 140 solar flux units. The z-function implemented in the logistic regression (y^) found in this study is a novel technique to predict the post-sunset ESF occurrence. The performance consistency between the training set and the testing set concludes that the z-function and the y^ values of the proposed model could be a simple and robust mathematical model for daily nowcasting the occurrence or non-occurrence of post-sunset ESFs.

Investigation of Low Latitude Spread-F Triggered by Nighttime Medium-Scale Traveling Ionospheric Disturbance

In the current study, we investigated the mechanism of medium-scale traveling ionospheric disturbance (MSTID) triggering spread-F in the low latitude ionosphere using ionosonde observation and Global Navigation Satellite System-Total Electron Content (GNSS-TEC) measurement. We use a series of morphological processing techniques applied to ionograms to retrieve the O-wave traces automatically. The maximum entropy method (MEM) was also utilized to obtain the propagation parameters of MSTID. Although it is widely acknowledged that MSTID is normally accompanied by polarization electric fields which can trigger Rayleigh–Taylor (RT) instability and consequently excite spread-F, our statistical analysis of 13 months of MSTID and spread-F occurrence showed that there is an inverse seasonal occurrence rate between MSTID and spread-F. Thus, we assert that only MSTID with certain properties can trigger spread-F occurrence. We also note that the MSTID at night has a high possibility to trigger spread-F. We assume that this tendency is consistent with the fact that the polarization electric field caused by MSTID is generally the main source of post-midnight F-layer instability. Moreover, after thorough investigation over the azimuth, phase speed, main frequency, and wave number over the South America region, we found that the spread-F has a tendency to be triggered by nighttime MSTID, which is generally characterized by larger ΔTEC amplitudes.

Spread-F occurrence during geomagnetic storms near the southern crest of the EIA in Argentina

This work presents, for the first time, the analysis of the occurrence of ionospheric irregularities during geomagnetic storms at Tucumán, Argentina, a low latitude station in the Southern American longitudinal sector (26.9°S, 294.6°E; magnetic latitude 15.5°S) near the southern crest of the equatorial ionization anomaly (EIA). Three geomagnetic storms occurred on May 27, 2017 (a month of low occurrence rates of spread-F), October 12, 2016 (a month of transition from low to high occurrence rates of spread-F) and November 7, 2017 (a month of high occurrence rates of spread-F) are analyzed using Global Positioning System (GPS) receivers and ionosondes. The rate of change of total electron content (TEC) Index (ROTI), GPS Ionospheric L-band scintillation, the virtual height of the F-layer bottom side (h'F) and the critical frequency of the F2 layer (foF2) are considered. Furthermore, each ionogram is manually examined for the presence of spread-F signatures.The results show that, for the three events studied, geomagnetic activity creates favorable conditions for the initiation of ionospheric irregularities, manifested by ionogram spread-F and TEC fluctuation. Post-midnight irregularities may have occurred due to the presence of eastward disturbance dynamo electric fields (DDEF). For the May storm, an eastward over-shielding prompt penetration electric field, (PPEF) is also acting. A possibility is that the PPEF is added to the DDEF and produces the uplifting of the F region that helps trigger the irregularities. Finally, during October and November, strong GPS L band scintillation is observed associated with strong range spread-F (SSF), that is, irregularities extending from the bottom-side to the topside of the F region.

Longitudinal differences in the statistical characteristics of ionospheric spread-F occurrences at midlatitude in Eastern Asia

Spread-F is known as the electron density inhomogeneous structures in F layer of ionosphere and can usually be classified as frequency spread-F (FSF) and range spread-F (RSF). Few studies have reported on the statistical characteristics of spread-F occurrences at midlatitudes in Eastern Asia, particularly the comparison of spread-F occurrences between China and Japan. In this paper, we used spread-F data recorded by ten ionosondes located between 25°N and 45°N from 1997 to 2016, to investigate the longitudinal differences in the statistical characteristics of spread-F occurrence and the probable mechanism for its occurrence at midlatitudes in Eastern Asia. Variations in the spread-F occurrences with the solar and geomagnetic activities, season and local time are presented. The main conclusions are as follows: (a) the occurrence percentage of FSF is higher than that of RSF, of which the former is anti-correlated with the solar or geomagnetic activities; (b) higher FSF occurrence percentages mostly appeared during summer, while RSF occurred more frequently in winter near 45°N latitude such as Urumqi, Changchun and Wakkanai; (c) the maximum of the FSF occurrence percentages mostly appeared between 01:00 and 02:00 LT approximately, whereas that of RSF appeared near 00:00 LT; (d) the spread-F occurrence percentages in the coastal or marine areas are higher than those in the inland region between 35°N and 45°N latitudes; however, this phenomenon is not obvious at lower latitudes; and (e) both the mean occurrences of FSF and RSF reach the minimum around 31°N latitude. These above results are helpful for understanding variations in spread-F occurrence at midlatitudes in Eastern Asia.

A minimum in the latitude variation of spread-F at March equinox

This short paper presents the statistics of spread-F occurrence at four low-mid latitude locations (17.0° N, 21.2° N, 26.5° N and 36.5° N mag. lat.) in Japan longitude sector (~ 135° E) in March to April 2002, 2003 and 2006 (high, medium and low solar activity). The location of a spread-F minimum predicted by theoretical models is identified for the first time. The spread-F minimum occurs at the poleward side of the equatorial ionisation anomaly crest and shifts equatorward from ~ 25° N mag. lat. at high solar activity to below 17° N at low solar activity. The corresponding spread-F maximum occurring on the poleward side of the minimum also shifts equatorward from ~ 35° N (or beyond) at medium solar activity to 20°–25° N at low solar activity. The spread-F occurrence increases with decreasing solar activity at all locations especially at higher latitudes (> ~ 25° N) where there is almost no occurrence at high solar activity.

Spread-F occurrences and relationships with foF2 and h\u2032F at low- and mid-latitudes in China

Ionospheric irregularities are an important phenomenon in scientific studies and applications of radio-wave propagation. Spread-F echoes in ionograms are a type of high-frequency band irregularities that include frequency spread-F (FSF), range spread-F (RSF), and mixed spread-F (MSF) events. In this study, we obtained spread-F data from four ionosondes at low- and mid-latitudes near the 120°E chain in China during the 23rd solar cycle. We used these data to investigate spread-F occurrence percentages and variations with local time, season, latitude, and solar activity. The four ionosondes were located at Haikou (HK) (20°N, 110.34°E), Guangzhou (GZ) (23.14°N, 113.36°E), Beijing (BJ) (40.11°N, 116.28°E), and Changchun (CC) (43.84°N, 125.28°E). We also present possible correlations between spread-Fs and other ionospheric parameters, such as the critical frequency of the F2-layer (foF2) and the virtual height of the bottom-side F-layer (h′F). In particular, we investigated the possible threshold of the foF2 affecting the FSF and the relationship between the h′F and the RSF. The main conclusions are as follows: (a) the FSF occurrence percentages were anti-correlated with solar activity at all four sites; meanwhile, RSF occurrence rates increased with the increase in solar activity at HK, but not at the other three sites; (b) FSF occurrence rates were larger at the mid-latitudes than expected, while FSFs occurred more often after midnight; (c) the highest FSF occurrence rates mostly appeared during the summer months, while RSFs occurred mostly in the equinoctial months of 2000–2002 at HK and GZ; (d) a lower foF2 was suitable for FSF events; nevertheless, h′F and RSF occurrences satisfied the parabolic relationship; (e) the foF2 thresholds for FSFs were 15, 14, 7.6, and 7.8 MHz at HK, GZ, BJ, and CC, respectively. The h′Fs occurring between 240 and 290 km were more favorable for RSF occurrences. These results are important for understanding ionospheric irregularity variations in eastern Asia and for improving space weather modeling and forecasting capabilities .

Effects of sporadic E-layer characteristics on spread-F generation in the nighttime midlatitude ionosphere: A climatological study

The aim of this study is to examine the effects of Es-layer characteristics on spread-F generation in the nighttime midlatitude ionosphere. The Es-layer parameters and spread-F appearance of the 23rd solar cycle (1996–2008) are recorded by the Kokubunji ionosonde. The Es-layer parameters are foEs (critical frequency of Es-layer), fbEs (blanketing frequency of Es-layer), and Δf (≡foEs-fbEs). In order to completely explore the effects, the pre-midnight and post-midnight data are classified by seasons, solar activities, and geomagnetic conditions. Results show that the spread-F occurs more frequently in post-midnight and in summer. And, the occurrence probabilities of spread-F are greater, when the solar activity is lower. For the occurrence probabilities of spread-F versus foEs and Δf under geomagnetic quiet-conditions, the trend is increasing, when the associated probabilities are significant. These indicate that the spread-F occurrence increases with increasing foEs and/or Δf. Further, the increasing trends demonstrate that polarization electric fields generated in Es-layer would be helpful to generate spread-F, through the electrodynamical coupling of Es-layer and F-region. Moreover, this electrodynamical coupling is efficient not only under quiet-conditions but under disturbed-conditions, since the significant increasing trend can also be found under disturbed-conditions. Regarding the occurrence probabilities of spread-F versus fbEs, the evident trends are not in the majority. This implies that fbEs might not be a major factor for the spread-F formation.

Occurrence statistics of Spread F at Ilorin

This study is the first of its kind aimed at the statistical study of the occurrence of Equatorial Spread-F (ESF) using ionosonde data obtained with the aid of Digisonde (DPS-4) during the year 2010 at Ilorin (Geo. Lat. 8.470N, Long. 4.680E, and Dip. Lat. 7.8670S): an equatorial station in the African sector. The study revealed that the ionosphere over Ilorin exhibit three types of equatorial spread-F i.e. frequency spread F (FSF), range spread F (RSF) and mixed frequency (MSF). ESF is observed to be a post-sunset and nighttime phenomenon; occurring within the hours of 2000LT and 0700LT with the range of hmF2 being between 230 and 405 km. ESF grows during the post-sunset periods and decays towards the sunrise hour. ESF is observed to be monthly/seasonal dependent. Occurrence of ESF is observed to be predominant during the equinox months. The results further show that RSF type is the most prevalent at this station.

Equinoctial spread-F occurrence at low latitudes in different longitude sectors under moderate and high solar activity

A comparative study aimed to investigate the equatorial and low-latitude spread-F occurrences for moderate solar activity (MSA) and high solar activity (HSA), was carried out considering concurrent observations made in some ionospheric stations, which identify three separate longitudinal sectors: Chiang Mai (CGM; 18.8° N, 98.9° E, mag. Lat. 13.2° N) and Chumphon (CPN; 10.7° N, 99.4° E, mag. Lat. 3.2° N), Thailand; Palmas (PAL; 10.2° S, 311.8° E, mag. Lat. 0.9° S) and São José dos Campos (SJC; 23.2° S, 314.1° E, mag. Lat. 14.0° S), Brazil; Tucumán (TUC; 26.9° S, 294.6° E, mag. Lat. 16.8° S), Argentina.Spread-F phenomena recorded during the equinoctial months of September and October 2010, March and April 2011, for MSA, March and April 2014, September and October 2014, for HSA, were classified in two different modes: range spread-F (RSF) and frequency spread-F (FSF). The satellite trace (ST) occurrence was also investigated as possible precursor of spread-F events. When comparing the results of equatorial (CPN and PAL) and low-latitude (CGM, SJC, and TUC) stations, some common features independently of the solar activity emerge: (1) a prevalence of RSF signatures is observed in the time interval 20:00–03:00 LT, while FSF occurrences prevail in the time interval 03:00–06:00 LT; (2) STs are confirmed to be a possible precursor of RSF occurrences.For HSA, at equatorial latitudes, spread-F occurrences in the Thai sector (CPN) are higher than those observed in the Brazilian sector (PAL).When comparing the results of low-latitude stations of CGM, SJC, and TUC some unusual aspects characterizing the morphology of spread-F occurrences emerge: (1) contrary to the Thai and Argentine sectors, in the Brazilian sector (SJC), RSF and FSF appearances in September, for HSA, are observed with relatively long persistence times between about 03:00-06:00 LT and 01:00-03:00 LT respectively, while balanced RSF and FSF occurrences with short persistence times are observed for months for MSA; (2) a prevalence of FSF at CGM during the first half of September for MSA, never observed in the Brazilian and Argentine areas.During years of LSA and MSA common morphological aspects are found at CGM and SJC, that is a predominance of FSF, with the lowest persistence times characterizing SJC. This suggests that the low-latitude behaviour of spread-F occurrences, under different levels of solar activity, at least in the longitude sectors here analysed, can be to a some extent generalized.

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

Effect of auroral substorms on the ionospheric range spread-F enhancements at high southern midlatitudes using real time vertical-sounding ionograms

A comprehensive study has been undertaken on the effect of magnetic substorm onsets (as deduced from the auroral hourly electrojet AE-index ) on the occurrence of high midlatitude (or sub-auroral latitude) ionospheric range spread-F (Sr). Unlike the previous reports real-time ionograms were used in this analysis thus eliminating ambiguities stemming from the correlating secondary evidence of spread-F with auroral substorms. The Australian southernmost ionosonde station Hobart (51.6°S geom.) proved to be uniquely suitable for the task as being sufficiently close to the southern auroral zone. Sr was assigned in km to each hourly nighttime ionogram at two sounding frequencies: Sr1 (at 2MHz) and Sr2 (at 6MHz) for four months in 2002: January and June (representing southern summer and winter solstices), and March and September (representing autumn and vernal equinoxes). It is evident that the southern winter solstitial period (June) is associated with high endemic midlatitude spread-F activity. All other seasons are closely linked with temporal sequences of enhanced spread-F activity following substorm onsets.For the first time it was possible not only find a simultaneous occurrence pattern of these diverse phenomena but to deduce numerical characteristics of the response of midlatitude ionosphere to the global auroral stimulus. Excellent case events, hitherto unpublished, are shown illustrating the presence of the AE peaks (in nT) being ahead of Sr peaks (in km) by a time shift ∆t (in h). Sr1 magnitude showed a significant correlation with the magnitudes of the preceding AE with a correlation coefficient (r) of 0.51 (probability of the occurrence by chance less than 0.01). Sr2 peaks were more sensitive to auroral disturbances but were not correlated with the AE magnitude variations. The time shift (∆t) was on average 4h with a standard deviation of 3h.The general pattern in the occurrence of magnetic substorms and spread-F is very similar. A number of corresponding peaks of the AE-index and Sr fluctuations were identified. The sub-auroral ionosphere response tends to last longer than the initiating auroral disturbance (hysteresis effect). The simultaneous quiescent periods in the auroral and sub-auroral ionospheres have been encountered on at least 14 days.

Automatically identification of Equatorial Spread-F occurrence on ionograms

F-region large-scale irregularities, also called plasma bubbles, are one of the most interesting equatorial ionospheric phenomena. These irregularities are generated in the equatorial region and afterwards extend to lower latitudes. They are one of the important topics of investigation in equatorial ionosphere electrodynamics and, therefore, are subject to intense theoretical and experimental research. The ionosonde is the most used scientific equipment to study the ionosphere and the F-region. With advancement of digital ionosonde, it is now possible to carry out an ionospheric sounding with a cadence of 5min or even with 1-minute cadence. To analyse a large amount of ionograms, more sophisticated tools are needed. Thus, development of algorithms to identify and analyse different aspects of ionograms has become very important to space science researchers. Multiple echoes recorded on ionograms are the signature of these irregularities in the ionograms, usually called Spread-F. Spread-F is classified into three types: range, frequency, and mixed. Thus, automatic identification of Spread-F is important in ionospheric studies, because studies usually involve the analysis and interpretation of large numbers of ionograms. The main objective of this paper is to present a new computational tool, based on fuzzy relation, designed to automatically identify the occurrence of Spread-F in ionograms. The test was conducted in ionograms recorded in the Brazilian sector (São José dos Campos (23.2°S, 45.9°W, dip latitude 17.6°S-low latitude) and Palmas (10.2°S, 48.2°W, dip latitude 5.5°S-near the magnetic equatorial)). The automatic identification of Spread-F occurrence was compared with those obtained manually and good agreement was found.

OI 630.0 nm all-sky image observations of medium-scale traveling ionospheric disturbances at geomagnetic conjugate points

This paper presents a medium-scale traveling ionospheric disturbance (MSTID) occurrence detected through the OI 630.0nm emission all-sky images, obtained by ground-based imaging systems installed at close geomagnetic conjugate locations: one at the Southern Space Observatory-SSO/CRS/INPE-MCTI, (29.4°S, 53.8°W), in Sao Martinho da Serra, RS, Brazil, and another at the Arecibo Observatory (18.3°N, 66.7°W), in Puerto Rico. The images obtained show the optical signature of MSTIDs (low intensity regions of 630.0nm airglow emission), propagating simultaneously in both hemispheres, during geomagnetically quiet conditions (Kp<3). Observations using digital ionosonde installed at Falkland Islands (51.4°S, 57.5°W) and Ramey, Puerto Rico (18.5°N, 67.1°W) show an abrupt upward motion of the ionospheric F region plasma and the occurrence of frequency-type spread-F that could be related to the MSTIDs observed optically. Using Global Positioning System (GPS) receiver data, we also obtained the rate of the Total Electronic Content (TEC) change (ROT) for two geomagnetically conjugated sites-Santa Maria (29.7°S, 53.7°W) and Virgin Islands (17.7° N, 64.7°W). The analysis of TEC allows us to identify an irregular ionization in the F layer associated with the passage of an MSTID. The possible effects of the South America Magnetic Anomaly (SAMA) on the electrodynamic processes that control the inter-hemispheric coupling that give origin to the conjugate MSTIDs are highlighted and discussed.

Effects of sporadic E-layer characteristics on spread-F generation in the nighttime ionosphere near a northern equatorial anomaly crest during solar minimum

This study is to know how the characteristics of sporadic E-layer (Es-layer) affect the generation of spread-F in the nighttime ionosphere near the crest of equatorial ionization anomaly during solar minimum. The data of Es-layer parameters and spread-F are obtained from the Chungli ionograms of 1996. The Es-layer parameters include foEs (critical frequency of Es-layer), fbEs (blanketing frequency of Es-layer), and Δf (≡foEs−fbEs). Results show that the nighttime variations of foEs and fbEs medians (Δf medians) are different from (similar to) that of the occurrence probabilities of spread-F. Because the total number of Es-layer events is greater than that of spread-F events, the comparison between the medians of Es-layer parameters and the occurrence probabilities of spread-F might have a shortfall. Further, we categorize the Es-layer and spread-F events into each frequency interval of Es-layer parameters. For the occurrence probabilities of spread-F versus foEs, an increasing trend is found in post-midnight of all three seasons. The increasing trend also exists in pre-midnight of the J-months and in post-midnight of all seasons, for the occurrence probabilities of spread-F versus Δf. These demonstrate that the spread-F occurrence increases with increasing foEs and/or Δf. Moreover, the increasing trends indicate that polarization electric fields generated in Es-layer assist to produce spread-F, through the electrodynamical coupling of Es-layer and F-region. Regarding the occurrence probabilities of spread-F versus fbEs, the significant trend only appears in post-midnight of the E-months. This implies that fbEs might not be a major factor for the spread-F formation.

The occurrence of equatorial spread-F at conjugate stations in Southeast Asia

In this study, the probability of equatorial spread-F (ESF) occurrence at the conjugate stations in Southeast Asia: Chiangmai station (CMU), Thailand, and Kototabang station (KTB), Indonesia, and near the magnetic equator, Chumphon station (CPN), Thailand, is presented. We analyze the ionogram data recorded by the Frequency Modulated Continuous Wave (FM/CW) ionosondes for the periods of minimum solar activity from September 2008 to April 2009 and in the equinoctial months (March and April) from 2006 to 2013. The spread-F signatures are manually categorized into three types: the frequency spread-F (FSF), the range spread-F (RSF) and the mixed spread-F (MSF) and the monthly average percentage of the occurrence of each ESF type is presented. The results show that the percentage of RSF occurrence at CPN, which is located around the magnetic equator, is higher than at other stations and the RSF mostly occurs during the equinoctial months. On the other hand, the FSF occurrence at CMU and KTB, that are located in the northern and southern hemispheres, respectively, are higher than at CPN. The RSF occurrence typically has the peaks before midnight, while the maximum occurrence rate of FSF is after midnight. Furthermore, the RSF onsets normally precede the FSF onsets by about 1–2h. As the solar activity levels go up, the percentages of RSF occurrence increase, but the percentages of FSF tends to decrease. In addition, we compare the statistics of observed RSF occurrence with the prediction of the IRI-2012 model. The results show that the IRI model overestimates the observed RSF occurrence at all stations during most of the solar activity levels and seasons, except in June 2008 (21:00–03:00LT), March 2011 (23:30–01:00LT) and March 2013 (01:00–02:30LT) when the IRI model underestimates our observations. However, the IRI model gives closer probability of RSF occurrence to our observed values at CPN, especially in equinoctial months and during the periods of medium solar activity. This work is important for an improvement of the IRI model in the prediction of the spread F occurrence probability in the low-latitude region.