Spread-F occurrences and relationships with foF2 and h′F at low- and mid-latitudes in China
KeywordsIonospheric irregularities Spread-F occurrence percentage foF2 threshold for FSF Relationship between h′F and RSF
the frequency spread-F
the range spread-F
the mixed spread-F
the critical frequency of the F2-layer
the virtual height of the bottom-side F-layer
equatorial ionization anomaly
the monthly average data of 10.7 cm radio flux
pre-reversal electric field
traveling planetary wave ionospheric disturbance
high solar activity
low solar activity
China Research Institute of Radio-wave Propagation
In the middle to late 1930s, ionospheric irregularities and the manner in which their electrodynamic mechanisms affected ionospheric behaviors began to attract the interest of many researchers (Abdu et al. 1981a, b, 1998, 2009; Booker and Wells 1938; Bowman 1974, 1990; Chandra and Rastogi 1970; Chou and Kuo 1996; de Jesus et al. 2013; Ossakow 1981; Xiong et al. 2012). Ionospheric irregularities appear as scattered echoes in high-frequency (HF) band ionograms that are known as spread-F events. Spread-Fs can manifest as frequency spread-Fs (FSF) that are broadened traces that mark reflections from the ionosphere along the frequency axis, or as range spread-Fs (RSF) that are along the vertical height axis. Many ground-based instruments (optical, ionosondes, and radar) and space-borne platforms (rockets and satellites) have been employed to explore the spread-F phenomenon over the past seven decades. These efforts have deepened our knowledge on spread-Fs showing that they vary with respect to latitude, local time, season, and solar and magnetic activity (Alfonsi et al. 2013; Banola et al. 2005; Chou and Kuo 1996; Deng et al. 2013; Huang et al. 1993; Scherliess and Fejer 1999). Different mechanisms have been proposed to explain spread-F occurrences and their development (Bowman 1990; Fejer et al. 1999; Fukao et al. 2004); among these, the primary mechanism in equatorial regions is the generalized Rayleigh–Taylor (R–T) instability mechanism. The R–T instability mechanism suggests that pre-reversal electric field enhancements (PRE) during the evening cause a rapid uplift of the ionosphere’s F-layer (Fejer et al. 1999; Fukao et al. 2004; Manju et al. 2007; Sukanta et al. 2017; Xiong et al. 2012; Upadhayaya and Gupta 2014). Relationships between spread-Fs and other ionospheric parameters, particularly the F2-layer (foF2) and h′F variations with the occurrence of spread-Fs, have also been statistically examined (Rungraengwajiake et al. 2013; Joshi et al. 2013; Madhav Haridas et al. 2013; de Abreu et al. 2014a, b, c, 2017; Abadi et al. 2015; Manju and Madhav Haridas 2015; Smith et al. 2015; Liu and Shen 2017). In addition, the effects of seasonal, solar, and magnetic activity variabilities on the h′F threshold have also been investigated (Manju et al. 2007; Manju and Madhav Haridas 2015; Madhav Haridas et al. 2013; Stoneback et al. 2011; Narayanan et al. 2014, 2017).
Devasia et al. (2002) first introduced the concept of threshold height (h′Fc) as a critical parameter controlling the day-to-day equatorial spread-F (ESF) variability. Past studies have revealed the dependence of the h′Fc on seasonal variations and solar and magnetic activity for the occurrence of ESFs and found the occurrences to be irrespective of the magnitude and polarity of meridional winds (Jyoti et al. 2004; Manju et al. 2007). Rungraengwajiake et al. (2013) presented a comparative study of the correlation between h′F and RSF occurrences in Thailand, and the results showed that high RSF occurrences mostly happened during equinoctial months that corresponded to rapid increases in the monthly mean h′F after sunset. Joshi et al. (2013) found that the h′F plays a key role in determining the R–T instability growth rate. Madhav Haridas et al. (2013) presented the effects of seasonal and solar activity variations of the h′Fc on ESF occurrences in India and found that substantial increases in the h′Fc varied with magnetic activity during every season.
Similar studies in Brazil have been presented (de Abreu et al. 2014a, b, c) to show that the occurrence of ESFs are closely related to daily variations of the h′F near the equator. During periods of low solar activity (LSA), the 250 km h′F altitude acted as the h′Fc for the generation of spread-Fs, while the 300 km h′Fc was during periods of high solar activity (HSA). An investigation using measurements from multiple instruments over the American sector showed that spread-Fs were often observed the nights before and during storms near the equator, in which the foF2 was less than 8 MHz and the h′F was lower than 300 km (de Abreu et al. 2017).
Abadi et al. (2015) studied the influences of the h′F on the latitudinal extension of ionospheric irregularities in Southeast Asia. Their results suggested that the latitudinal extension of plasma bubbles was mainly controlled by the PRE magnitude and h′F peak values during the initial phases of the ESF. Manju and Madhav Haridas (2015) investigated the h′Fc for the occurrences of ESFs during equinoxes and showed that the equinoctial asymmetry of the h′Fc increases with solar activity. Aside from the studies mentioned above, there are few reports that consider the effect of the foF2 threshold on the generation of spread-F events. Liu and Shen (2017) conducted a case study during a severe geomagnetic storm near 120°E in China and showed that the spread-F was suppressed near Sanya and Wuhan during the storm’s main phase when the frequency spread over 14 MHz, and the suppression was sustained for several hours. This helped us to understand the possible onset causes of the day-to-day spread-F variability.
Stoneback et al. (2011) investigated the local time distribution of meridional (vertical) drifts during the prolonged solar minimum. They found that the downward drifts across sunset and the upward drifts across midnight were also consistent with the delay in the appearance of ionospheric irregularities after midnight. Narayanan et al. (2014) studied the relationship between the occurrence of satellite traces (STs) in ionograms and the formation of ESFs using observations from an Indian dip equatorial station during solar minimum conditions. They found that the ST occurred later in the night as well implying that the PRE was not the cause of the ST during these times. Additionally, they also found that the STs were not followed by ESFs in about 30% of the cases indicating that large-scale wave-like structures (LSWS) do not trigger ESFs on all occasions. Narayanan et al. (2017) also found that the plasma bubbles were generated without strong PREs when the ion-neutral collision frequencies possibly dropped significantly during the unusually low solar activity conditions of 2008. Abdu et al. (2006) found that the existence of significant planetary wave (PW) influences on plasma parameters at E- and F-region heights over the equatorial latitudes using airglow, radar, and ionospheric sounding observations. A direct consequence of the PW scale oscillations in the evening electric field is its role in the quiet time day-to-day variability of the ESF/plasma bubble occurrences and intensities.
We limited our focus to spread-F occurrences and their relationships with foF2 and h′F that affected spread-F occurrences during a complete solar cycle in the low- and mid-latitudes over China. The International Reference Ionosphere-2012 (IRI-2012) model includes the monthly mean spread-F occurrences for predicting in the Brazilian longitude sector but not for Chinese sector. Therefore, the studies of spread-F occurrence statistics in China are part of an on-going effort to develop the spread-F occurrence prediction abilities to improve the IRI model. In the present study, we focused on the characteristics and correlations between spread-F occurrences and the foF2 and h′F. Furthermore, we also present the thresholds of the foF2 as they relate to the generation of FSFs.
Data and analysis
Details of the digital ionosonde sites used in the investigation
Geog. Lat. (°N)
Geog. Long. (°E)
Geom. Lat. (°N)
Geom. Long. (°E)
Results and discussion
Nocturnal, seasonal, and solar activity variations on spread-F occurrences
Abdu et al. (2003) showed that RSF events are associated with developed or developing plasma bubble events, while FSF events are associated with narrow-spectrum irregularities that occur near the peak of the F-layer. These results suggest that the upward velocity of plasma bubbles have a strong seasonal connection with the maximum values observed during the summer. Variations of FSF and RSF except for those during the 2000–2002 solar maximum period are mainly consistent with these studies. Rungraengwajiake et al. (2013) showed that FSF events appear later than RSF events on average and that FSFs remain until morning, while RSFs almost disappear by around 04:00 LT. The results shown in Figs. 5 and 7 are slightly different, which may be partly attributed to the effects of geomagnetic activity. Figure 2 shows the geomagnetic activity during the equinoxes in 2001 and 2002. It is possible these activities caused the RSFs to occur mainly during equinoxes at HK and GZ in 2001 and 2002. The peak FSF occurrence rate appeared later at GZ than at HK, which is well correlated with the manner in which fresh bubbles start from the latter station and then expand to high latitudes. The average FSF occurrence percentage mostly peaks from 24:00 LT to 02:00 LT at HK and from 03:00 LT to 05:00 LT at GZ. The average RSF occurrence percentages mostly peaked from 21:00 LT to 23:00 LT at HK and from 24:00 LT to 02:00 LT at GZ during periods of HSA. Meanwhile, RSF occurrence rates were higher at HK and GZ than those at BJ and CC; FSF occurrence rates were higher at HK, BJ, and CC than at GZ. These results support the hypothesis that solar and geomagnetic activity affects seasonal and longitudinal variations of spread-Fs.
Liu and Shen (2017) found that the disturbance of electric fields could also contribute to the occurrence of spread-Fs, especially at low-latitude stations. The disturbed electric fields and the disturbance winds are also the probable factors that promote the spread-F along with the gravity-driven R–T instability. In addition, the electric field disturbances can also generate spread-Fs through R–T instability only (de Jesus et al. 2010; Wang et al. 2014; Wan and Xu 2014; Mo et al. 2017). The disturbance of the dynamo driven by enhanced global thermospheric circulation resulting from energy input at high latitudes is another factor for promoting spread-Fs (de Jesus et al. 2010; Liu and Shen 2017). Therefore, it can be seen that there are many possible mechanisms for spread-F occurrences, and more in-depth analysis is needed.
Nocturnal, seasonal, and solar activity variations on foF2 and h′F
The possible foF2 threshold for FSFs and the relationship between the h′F and RSF
Figure 13 shows the post-sunset h′F variations compared with the RSF occurrence rates at the four sites. The red point is the sample value. The blue line is the fit curve. The RSF occurrence rate and the h′F satisfy the parabolic relationship. When the probability of the RSF was ~ 25% of the maximum probability of occurrence, we treated that virtual height value as the threshold value. The h′F occurring between 240 and 290 km is more favorable for RSF occurrence by calculation, which is different from the relationship between foF2 and FSF. Figures 6, 10, and 13 indicate that the higher occurrence rates of RSFs are well correlated with higher post-sunset h′F peaks (Rungraengwajiake et al. 2013). Previous studies observed spread-Fs in the equatorial region on nights when the h′F was below 300 km (Abadi et al. 2015; Manju and Madhav Haridas 2015; Liu and Shen, 2017; de Abreu et al. 2017). Our results also support this conclusion. In addition, Devasia et al. (2002), Jyoti et al. (2004) and Manju et al. (2007) obtained an h′F threshold for the spread-F occurrences in their studies in India. Devasia et al. (2002) found a threshold of about ~ 300 km for the cases in their study. Our results also show that when the virtual height is greater than 300 km, the probability of an RSF is very small. Jyoti et al. (2004) showed a linear relationship between solar activity and the h′F threshold. Manju et al. (2007) investigated the dependence of the h′F threshold on seasonal and solar activity for magnetically quiet conditions and proposed the important role of neutral dynamics in controlling the day-to-day ESF variability. Abadi et al. (2015) found that latitudinal extension of plasma bubbles was mainly controlled by the h′F peak value during the initial phase of an ESF. Manju and Madhav Haridas (2015) showed that the equinoctial asymmetry of the h′Fc increases with solar activity. In this article, the correlation between the h′F threshold and the seasonal and solar activities are not involved, and we will also focus on this content. The new idea presented from our study is the correlation between RSF occurrences and the h′F, which are different from previous research results. In a follow-up study, we will examine the relationship between the h′F threshold for RSFs and the solar and geomagnetic activities and equinoctial asymmetry.
The correlation RSF occurrence percentages with rapidly increasing post-sunset monthly mean h′F values substantiated the role of the PRE enhancement on RSF onsets. Traveling planetary wave ionospheric disturbance (TPWID)-type oscillations (de Abreu et al. 2014a, c; Fagundes et al. 2009) in the modulation of the virtual height in the F-region increased during sunset hours. Meridional wind velocities corresponding to the post-sunset h′F for each spread-F event have been considered. Buonsanto and Titheridge (1987) found that the hmF2 dropped from 13:00 to 18:00 LT during the solar maximum periods because of the meridional wind. These results also indicate that the spread-F is a complex phenomenon, which implies that other possible factors can be ascribed to spread-F occurrences. The atmosphere ionosphere coupling process has been proposed as a contributing factor for spread-F development. Therefore, the connections between spread-F occurrence characteristics and the foF2 and h′F magnitudes deserve detailed investigation by additional theoretical and observational research. The foF2 and h′F thresholds also require further investigation using observations from different regions and under different solar activity conditions.
Summary and conclusions
The FSF occurrence rates increased during years of LSA at all four sites. FSFs mainly occurred during the summer months, while RSFs occurred mostly in the equinoctial months between 2000 and 2002 at HK and GZ. Post-midnight FSFs were the most observed type of spread-F events. The typical FSF onset time was about 21:00 LT, and the FSFs normally lasted until 05:00 LT, while the RSFs occurred 2–3 h earlier at HK and GZ during periods of HSA.
The foF2 and h′F peak values come mainly before midnight at low latitudes, while h′F peak values appeared after midnight at mid-latitudes during periods of HSA.
Lower foF2 values were appropriate for FSF events; nevertheless, h′F and RSF occurrences satisfied the parabolic relationship. Most FSF events occurred when the foF2 was below 15 and 14 MHz at HK and GZ, and below 7.6 and 7.8 MHz at BJ and CC. The h′Fs occurring between 240 and 290 km were more favorable for RSF occurrences, which differ from the foF2. However, some questions remain unresolved and further studies are in progress.
Our studies of FSFs and RSFs in China are useful and have the potential to be included in the future IRI model. However, even after such studies of spread-F onsets and growth conditions, some uncertainties remain. This requires further efforts to understand the spread-F phenomenon at different locations. Soon, long irregularity data coverage over the China sector will be studied. More ionospheric parameters will be compared with local time and seasonal spread-F variations to amplify knowledge of the involved physical mechanisms.
WN designed the study, analyzed the data, and wrote the manuscript. GLX and ZZW contributed related analysis on data from HK and GZ. DZH and LLK helped with the text of the paper, particularly with the introduction and comparison with previous works. All coauthors contributed to the revision of the draft manuscript and improvement of the discussion. All authors read and approved the final manuscript.
Ning Wang, is currently a Ph.D. student at Xidian University. She also is an Associate Professor at the China Research Institute of Radiowave Propagation. She has authored and coauthored 8 patents and over 15 journal articles. Her current research interests are in ionospheric irregularities and ionosphere radiowave propagation. Dr. Linxin Guo is currently a Professor and Head of the School of Physics and Optoelectronic Engineering Science at Xidian University, China. He has been a Distinguished Professor of the Changjiang Scholars Program since 2014. He has authored and coauthored 4 books and over 300 journal articles. Dr. Zhenwei Zhao is currently a Professor and Chief engineer at the China Research Institute of Radiowave Propagation. His current positions include: Chairman of the ITU-R SG3 in China; Head of the Chinese Delegation of ITU-R SG3; Lead expert for the Asia-Pacific Space Cooperation Organization (APSCO). Dr. Zonghua Ding is currently an Associate Professor at the China Research Institute of Radiowave Propagation. His current research interests are in ionosphere and ionosphere radiowave propagation. Dr. Leke Lin is currently a Professor at the China Research Institute of Radiowave Propagation. He has participated in the activities of the ITU-R study group 3 and has submitted about 40 contributions to the ITU-R SG3.
The authors acknowledge the Data Center of the China Research Institute of Radio-wave Propagation for help with ionogram scaling and classification. The authors would like to thank Dr. Shuji Sun and Dr. Tong Xu for proofreading this manuscript. The authors would also like to thank the anonymous referee for the useful comments and suggestions for improving the paper.
The authors declare that they have no competing interests.
Availability of data and materials
Regretfully, the data used in this manuscript cannot be shared because they belonged to the China Research Institute of Radio-wave Propagation (CRIRP).
Consent for publication
Written informed consent was obtained from study participants for participation in the study and for the publication of this report and any accompanying images. Consent and approval for publication was also obtained from Xidian University and China Research Institute of Radio-wave Propagation.
Ethics approval and consent to participate
This research was supported by the National Natural Science Foundation of China (Grant No. 41604129) and the National Key Laboratory Foundation of Electromagnetic Environment (Grant Nos. A171501016, A171601003, A161601002, and B041605003). The funds from Grant No. 41604129 were used for data collection and analysis. The funds from Grant Nos. A171501016, A171601003, A161601002, and B041605003 were used for manuscript preparation.
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