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

$${\text{P}}\left( y, m, h \right) = \frac{{n \left( {y, m, h} \right)}}{{N\left( {y,m,h} \right)}} \times 100{\text{\% }}$$
$$p_{i} = \frac{{m}_{i} }{{\mathop \sum \nolimits_{i} {m}_{i} }}$$
$$\mathop \sum \limits_{i} {p}_{i} = 1$$






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Earth, Planets and Space

volume 70 , Article number: 59 ( 2018 )
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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
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.
The China Research Institute of Radio-wave Propagation (CRIRP) constructed and operated a network of long-running ionospheric observation sites that cover mainland China. In this study, we extracted simultaneous spread-F data from four digital ionosondes 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). In addition, we also determined the data characteristic of the foF2 and h′F at these sites to reveal possible correlations between spread-F occurrences and the foF2 and h′F. No data were recorded in December 1997 and from May to December 1999 at CC, because the ionosonde was being repaired. The observational site details are shown in Table 1 .
The HK and GZ sites lie near the north crest of the equatorial ionization anomaly (EIA) zone. The EIA zone is where the fountain effect phenomena and the equatorial electrojet often interact resulting in complicated ionospheric physical processes. BJ and CC are located at the mid-latitudes in China. According to previous studies, ionospheric irregularities greatly depend on solar activity, local time, season, latitude and longitude, and geomagnetic disturbances (Abdu et al. 1981a , b , 1983 , 1998 , 2009 ; Booker and Wells 1938 ; Bowman 1974 ; Chandra and Rastogi 1970 ; Maruyama 1988 ; Xiong et al. 2012 ). To discuss the correlations between spread-Fs and solar and geomagnetic activities, we show the monthly mean 10.7 cm radio flux (F10.7) and ap index during the 23rd solar cycle in Fig. 1 that covers the epochs of the LSA and HSA. We used a 3-hourly ap index to identify geomagnetically quiet and disturbed days. If the maximum value of the 3-hourly ap index for a day was greater than 12, the day was considered as a disturbed day (Narayanan et al. 2017 ). Figure 2 shows the daily max ap indices from 2000 to 2005. Further, it can be seen from the figure that there were more geomagnetically disturbed days during the vernal equinox and autumn equinoxes in 2001 and 2002.
Monthly averaged 10.7 cm solar flux (F10.7) (y axis: F10.7/sfu) and ap index from 1997 to 2008 denoting the solar activity
Daily max ap indices from 2000 to 2005
Ionogram data were collected using type TYC-1 ionosondes, which are designed and manufactured by the CRIRP (Xu et al. 2001 ). Ionograms were recorded at 1-h intervals for a frequency range from 1 to 32 MHz. We distinguished two types of spread-F, FSF, and RSF for detailed study. We used the percentage of spread-F occurrences to describe the spread-F statistical features, which is defined as follows:
where y , m, and h represent the year, month, and local time (LT), respectively; n is the number of spread-F occurrences that appear at the same local time but during different days of a single month, and N is the total number of days for a given year and local time. Spread-Fs typically appeared after sunset and lasted until the subsequent sunrise; thus, the percentage of spread-F occurrences from 18:00 LT to 06:00 LT is the topic of interest in this study. Occurrences of FSF and RSF were compared with monthly medians of the foF2 and h′F to find the correlations between foF2 and h′F for the generation of spread-Fs. The FSF, RSF, foF2, and h′F were differentiated by manually analyzing the ionograms. The foF2 and h′F can sometimes be measured, but sometimes cannot be obtained when a spread-F occurs. The foF2 and h′F cannot be obtained during a strong spread-F (SSF). SSFs are a type of spread-F that can be identified when there is strong diffusion on the frequency and height axis of an ionogram. Figure 3 shows a SSF event in Haikou on March 26, 2012. The observations presented in this manuscript contain data when the foF2 and h′F values were reliably scaled during a spread-F. To examine their seasonal variations, we grouped the data into the following four seasonal bins: summer (May, June, July and August), vernal equinox (March and April), autumn equinox (September and October), and winter (January, February, November and December) (Maruyama and Matuura 1984 ; Maruyama et al. 2009 ; Sripathi et al. 2011 ; Xiao and Zhang 2001 ).
A SSF event in Haikou on March 26, 2012
The monthly mean of the FSF occurrence rates varied with local time and are presented separately in Fig. 4 for Haikou, Guangzhou, Beijing, and Changchun. It can be found that the FSF occurrences frequently appeared after midnight. Also, the FSF occurrences observed at different sites exhibited distinct local time distribution patterns. Previous studies have also observed this trend (Zhang et al. 2015 ; de Jesus et al. 2010 , 2012 , 2016 ). The FSF occurrence rates at HK, BJ and CC were higher than GZ. The maximum FSF occurrence rate was ~ 80% and occurred in July 1997 at HK, in August 2008 at BJ and in June 2006 at CC. The LSA yielded high FSF occurrence percentages at all four sites. The relationship between the FSF and solar activity was approximate to a negative correlation. The seasonal variation of the FSF occurrence rates observed at the four sites is shown in Fig. 5 a–d. We found that FSFs occurred mostly during the summer at HK and the occurrence rate was lower between 1999 and 2002. FSF occurrence rates were higher during the autumn equinox than during the vernal equinox between 2000 and 2001 at HK. FSFs occurred mostly during the summer at GZ, however, scarcely occurred in 2002 and 2008. Statistically, the FSFs started at approximately 21:00 LT and lasted until 05:00 LT at HZ and CC. However, FSFs started at about 23:00 LT and lasted until 05:00 LT at GZ and BJ, with post-midnight FSFs as the most commonly observed.
Monthly mean FSF occurrence percentages at the four sites
Seasonal variation of the FSF occurrences
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