Spread F occurrence and drift under the crest of the equatorial ionization anomaly from continuous Doppler sounding and FORMOSAT-3/COSMIC scintillation data
© Chum et al. 2016
Received: 4 November 2015
Accepted: 29 March 2016
Published: 8 April 2016
A relatively new method based on measurements by multipoint continuous Doppler sounding is applied to study the occurrence rate, propagation velocities, and directions of spread F structures over Tucumán, Northern Argentina, and Taiwan, both of which were under the crest of the equatorial ionization anomaly in 2014. In addition, spread F is studied globally over the same time period from the S4 scintillation index measured onboard FORMOSAT-3/COSMIC (F3/C) satellite. It is shown that the continuous Doppler sounding gives results that are consistent with S4 data and with previous optical, global positioning system (GPS), and satellite measurements. Most of the spread F events were observed from September to March, i.e., during the local summer half of the year in Tucumán, whereas in Taiwan, the highest occurrence rate was observed around equinoxes. The occurrence rate in Tucumán was about four times higher than that in Taiwan. The propagation velocities and directions were estimated from the Doppler shift spectrograms. The spread structures related to spread F propagated roughly eastward at velocities from ~70 to ~200 m s−1 during nighttime hours. The mean observed horizontal velocity was 140 m s−1 over Tucumán and 107 m s−1 over Taiwan. The local times at which the highest velocities were observed roughly correspond to local times with highest values of scintillation index S4, at ~20 to 23 LT. In addition, a comparison of measured drift velocities with neutral wind velocities predicted by models is provided. The observed velocities usually exceeded the horizontal neutral wind velocities predicted by the HWM14 model for the locations and times of observations.
Ionospheric irregularities known as spread F often prevent accurate scaling of ionograms and obtaining reliable values of F2 layer peak characteristics such as critical frequency foF2 and peak height hmF2 (McNamara et al. 2008), which limits our knowledge of the ionosphere and predictability of conditions for radio wave propagation. Moreover, equatorial and low-latitude spread F is often associated with plasma bubbles and scintillations of global positioning system (GPS) signals (Chen et al. 2006; Shi et al. 2011; Alfonsi et al. 2013), which may cause inaccuracies in position determination. It is generally accepted that equatorial spread F (ESF) and plasma bubbles result from Rayleigh–Taylor instability triggered during the uplift of the F layer owing to the prereversal enhancement of the eastward (zonal) electric field and development of the steep plasma density gradient as the bottomside ionosphere becomes depleted after sunset (Fejer et al. 1999; Stolle et al. 2006; Abdu et al. 2009a; Kelley 2009). The day-to-day variability and the roles of all factors that contribute to plasma bubble formation and spread F observation such as gravity wave (GW) seeding, neutral winds, angle between magnetic meridian, and solar terminator, however, remain enigmatic and are subjects of intense investigation (Kudeki et al. 2007; Abdu et al. 2009a, b; Cabrera et al. 2010; Hysell et al. 2014). The prereversal enhancement of the eastward electric field and hence the enhancement of the plasma vertical drift vary with longitude and season and are likely an important controlling factor for large-amplitude equatorial plasma bubble development (Huang and Hairston 2015). 30 MHz coherent backscatter radar observations at Saõ Luís have shown that evening mean upward vertical drifts are a necessary but not sufficient condition for the occurrence of topside ESF echoes (Smith et al. 2015). It is assumed that plasma bubbles are generated above the geomagnetic equator and stretch along the magnetic field lines to low latitudes (Sultan 1996; Keskinen et al. 1998; Bhattacharyya and Burke 2000). The eastward movement of the developed plasma structures at velocities usually exceeding 100 m s−1 has been experimentally confirmed by several independent studies (Terra et al. 2004; Haase et al. 2011; Chum et al. 2014). The zonal velocities derived from observations of ionospheric irregularities at night are also eastward with magnitudes decreasing from the geomagnetic equator to the equatorial ionization anomaly (EIA) crests (Kil et al. 2002). The mean zonal velocities of ionospheric irregularities at the Brazilian Saõ Luís equatorial station were larger during the December solstice and decayed during the equinoctial periods (Muella et al. 2009). Low-latitude all-sky imager observations in northern Argentina and Peru revealed at stations closer to the magnetic equator weaker (stronger) eastward plasma drifts in the postsunset (postmidnight) period (Martinis et al. 2003). Incoherent scatter radar observations at Jicamarca (Fejer et al. 2005) indicate daytime westward drifts and stronger nighttime eastward drifts under quiet conditions; nighttime perturbation drifts increase strongly with solar activity. Pacheco et al. (2011) investigated local‐time variations of zonal drifts at different latitudes, longitudes, and seasons using data from the Republic of China Satellite‐1 (ROCSAT‐1). Super-rotation of the ionosphere was observed in zonal drift measurements, particularly at lower latitudes.
ESF has been studied by ground-based measurements using optical airglow cameras, radars, ionosondes, and GPS receivers (Haase et al. 2011; Fejer et al. 1999; Shi et al. 2011) and in situ measurements using satellite observations. Either direct in situ measurements of plasma density depletions (Huang et al. 2001; Park et al. 2005) or their magnetic signatures owing to the diamagnetic effect of plasma can be used to study the properties and distributions of plasma bubbles by satellites (Luhr et al. 2003; Stolle et al. 2006).
In this paper, we present horizontal velocities of spread F structures observed by multipoint continuous Doppler sounding over Tucumán, Northern Argentina (27°S, 65°W, inclination of magnetic field I = 27°), and over Taiwan (24°N, 121°E, inclination of magnetic field I = 35°) and we compare the values with neutral wind velocities obtained from recent experimental model HWM14 (Drob et al. 2015). The occurrence rates of spread F structures observed by Doppler sounding are compared with the new data from global radio occultation measurements of GPS signal scintillation onboard the FORMOSAT-3/COSMIC (F3/C) satellites. Both regions of Doppler sounding are located under the crest of the EIA; Tucumán is under the southern crest, whereas Taiwan is under the northern crest. The time shift between these regions is about 12 h. Although continuous Doppler sounding was used to investigate wave-like disturbances in the ionosphere in the 1960s (Davies et al. 1962; Davies and Baker 1966), its application to ESF studies is relatively new and rare. To the best of our knowledge, Chum et al. (2014) were the first to use Doppler shift measurements for spread F investigations. They presented systematic analysis of propagation velocities of spread F structures based on multipoint continuous Doppler sounding over Tucumán from December 2012 to November 2013. Their results can be summarized as follows. Spread F occurred at night generally in the local summer half of the year and propagated roughly eastward at typical velocities of ~100–160 m s−1. This result is consistent with those other reports based on airglow, ionosonde, and GPS measurements (Terra et al. 2004; Haase et al. 2011; Alfonsi et al. 2013). The advantage of continuous Doppler sounding compared with optical measurements is its independence of tropospheric weather conditions. The Doppler shift spectrograms provide a relatively simple and rapid qualitative overview of the spread F occurrence; however, its relatively low accuracy of velocity determination is a disadvantage.
The Doppler shift measurements have relatively high time resolution of ~10 s and complement the ionosonde measurements. These measurements can be used among others for the monitoring of spread F, which is a severe limitation for radio propagation at low latitudes. This paper builds on the previous work and methods described by Chum et al. (2014), and its motivation is as follows. The continuous Doppler sounder measurement data were previous applied to investigations of ESF only once for one station (Tucumán) using one year of data (Chum et al. 2014). In the present study, we use two stations, Tucumán under the southern crest of EIA near the South Atlantic magnetic anomaly (Hartmann and Pacca 2009) and Taiwan under the northern crest of EIA in southeastern Asia, to obtain statistical information on ESF behavior at two significantly different locations, and we add one more year of data. In addition, the spread F occurrence and its global distribution are studied from F3/C satellite data, particularly from the occultation measurements of the GPS signal scintillation. This investigation is similar to a local study of spread F and scintillations over Sanya by Zhang et al. (2015), who showed that spread F and large scintillations occur simultaneously. As the second objective, we verify that the new method of spread F velocity measurements based on continuous Doppler sounding gives values consistent with previously published results based on different methods of measurement and that these values properly describe the reported longitudinal differences. In the past, this was only partly achieved for one station (Tucumán). In the present study, spread F occurrences and velocities are additionally studied over Taiwan and Tucumán in 2014, where the multipoint continuous Doppler sounding systems (CDSSs) have been installed, primarily to examine the wave activity in the low-latitude ionosphere. (The Doppler sounder installed in Taiwan in late 2013 has provided reliable data only since 2014.) Details on velocity distributions and occurrence rates of spread F structures in these locations are provided in “Occurrences and velocities over Taiwan and Tucumán and global distribution of spread F from S4 measurements by F3/C” section, and a comparison with previous reports is given in the first part of “Comparison with other measurements and relation to neutral winds from HWM14 model” section. The third goal of the current work is comparison of spread F velocities with neutral winds obtained from the recent neutral wind model HWM14 (Drob et al. 2015). This comparison is motivated by the theoretical prediction that the plasma drifts in the nighttime equatorial F2 region should approach neutral wind velocities (Kelley 2009). The measurements of spread F/plasma depletion drift velocities thus can serve as additional information in the evaluation of empirical wind models at these locations. This comparison is partly inspired by the work of England and Immel (2012), who reported that the previous HWM07 model shows discrepancies with drift velocities of equatorial and low-latitude plasma depletions observed by the far ultraviolet imager on the IMAGE spacecraft.
Measurements and data analysis
Continuous Doppler sounding considers that the sounding radio wave experiences frequency shift, known as Doppler shift, if the ionospheric layer from which the wave reflects moves or if processes occur that lead to plasma compression or rarefaction (Davies et al. 1962; Chum et al. 2012, and references therein). The CDSS, used in both Tucumán and Taiwan, consists of three transmitters known as Tx1, Tx2, and Tx3 that form a roughly equilateral triangle with sides of about 100 km. The sounding frequencies of the transmitters are mutually shifted by ~4 Hz at the specific region so that all of the sounding paths corresponding to different transmitter–receiver pairs can be easily displayed in one Doppler shift spectrogram. The geographical coordinates of the system installed in Tucumán have been presented in Fig. 2 in Chum et al. (2014) and include transmitters Tx1 (26.943°S, 65.707°W), Tx2 (26.563°S, 64.550° W), Tx3 (27.499°S, 64.874°W), and receiver Rx (26.840°S, 65.230°W). The coordinates of the system deployed in Taiwan are Tx1 (23.897°N, 121.551°E), Tx2 (24.341°N. 120.778°E), Tx3 (24.816° N, 121.727°E), Rx0 (24.972°N, 121.192°E), and Rx1 (23.955°N, 120.927°E). The transmitted power was only ~1 W, and the sounding frequencies were 4.63 MHz in Tucumán and 6.57 MHz in Taiwan. The reflection heights were determined from nearby ionospheric sounders.
Another point worth mentioning is that the OSSs were observed with time delays between different sounding paths. These time delays, together with known separations of the transmitters, can be used to estimate the propagation velocities and directions of OSSs. More precisely, the separations of reflection points, which are assumed to be the midway between the individual transmitters and receiver, were used in these calculations. It is necessary to stress that it was not possible to determine the time delays reliably for a number of OSS events, e.g., we were not able to determine the time delays for the first OSS event in Fig. 1 observed from ~01:00 to 01:30 UT. The time delays are usually measured as time differences between the beginnings of the OSSs recorded on different sounding paths. The determination of these beginnings is performed manually by clicking in the Doppler shift spectrogram in MATLAB software; an example of the determined OSS beginning times is represented by magenta asterisks in Fig. 2. It is therefore partly subjective, and the estimated average error of the time determination is 1 min. This error propagates into the error estimates, or uncertainties, of the calculated velocities and azimuths.
It should also be noted that in the initial phase of development of a geomagnetic field-aligned equatorial plasma bubble, which gives rise to spread F, the bubbles may have large upward velocities that often exceed the horizontal velocity in magnitude. This upward velocity could substantially affect the Doppler shift and hence the tilt method. However, this initial development is realized at very low latitudes close to the geomagnetic equator, whereas our measurements were located in the EIA crest region sufficiently far from this area, as described in “Background” section. Thus, we can assume that the irregularity movement is predominantly horizontal in the region of our measurements. Therefore, the application of the tilt method for estimating the horizontal velocities of spread F structures is justified. The fact that the horizontal movement is dominant in our observations is also supported by the examples in Figs. 1 and 2. In these figures, the OSSs reach both positive and negative values of Doppler shift in a roughly symmetrical manner with respect to the signals in time intervals in which the spread features were not observed. The potential influence of vertical motion is also partly compensated by the fact that we used differential and not absolute values of Doppler shifts in Eq. (1). It should be noted that a constant offset of Doppler shift should not affect the results of our measurements. However, the tilt method could lead to incorrect results if measurements are conducted close to the geomagnetic equator when instability develops and plasma bubbles are created.
The GPS signal scintillations were obtained from the radio occultation measurements onboard the F3/C satellites. The Constellation Observing System for Meteorology, Ionosphere, and Climate mission (COSMIC) is formed by six microsatellites at 800 km low Earth orbit (LEO) and a 72° orbital inclination angle. The radio GPS occultation experiment (GOX) performs radio occultation observations in both the atmosphere and the ionosphere, and the radio scintillation of the GPS L1 band C/A code (1.575 GHz) is calculated and recorded as the S4-index, which is a standard deviation of the received power normalized by its mean value (http://cdaac-www.cosmic.ucar.edu/cdaac/doc/documents/s4_description.pdf). An average of 2000–3000 occultation scintillation profiles is recorded per day in the scintillation files (data type scnLv1) and can be obtained from Taiwan Analysis Center for COSMIC (TACC) and the COSMIC Data Analysis and Archive Center (CDAAC).
To specify the location and time of the most intensive scintillation, the maximum value S4max on each profile is determined. According to the time, location, and altitude of the observation, each S4max is assigned to its proper grid in a three-dimensional (3D) map. A median value of S4max is then computed in a specific region in the 3D map over a specific time period, which in our case was the year 2014.
Occurrences and velocities over Taiwan and Tucumán and global distribution of spread F from S4 measurements by F3/C
Figure 13 shows the dependence of S4max on altitude in various forms. Figure 13a, b shows S4max values as a function of local time and altitude for Tucumán and Taiwan, respectively. Figure 13c, d shows the same dependence as Fig. 13a, b but for the locations at the geomagnetic equator, which are magnetically conjugated with Tucumán and Taiwan, respectively. Figure 13e, f presents the S4max values as a function of season and altitude for Tucumán and Taiwan, respectively. It is obvious that the altitude range of ~200–300 km, which is the region of the largest electron densities, dominantly contributes to scintillations. Another peak appearing near ~100 km in Taiwan corresponds to the Es layer. The occurrence of the Es layer was significantly lower in Tucumán. These results are consistent with the study of Arras et al. (2008), who showed that the Es layer occurs relatively rarely over South America likely because of the South Atlantic anomaly.
Another point worth mentioning is that the scintillation reached maximum at about the same local time at the geomagnetic equator and under the crest of the EIA, as shown by comparisons of Fig. 13a, c and b, d. This result could indicate that the spread F and possibly plasma bubbles evolve very quickly along the magnetic field lines as suggested by Bhattacharyya and Burke (2000). However, this statistical study based on S4max indices cannot provide reliable information on the ESF time evolution; dedicated investigations based on detailed case studies and simultaneous multipoint observations are needed verify this reaction.
Comparison with other measurements and relation to neutral winds from HWM14 model
It is interesting to compare the occurrence rates of OSS events with the occurrence rates of plasma bubbles observed by satellites. The fact that OSSs are more frequent in Tucumán than in Taiwan and that they mostly occur from September to March in that region is consistent with the occurrence rates of plasma bubbles as derived from magnetic signatures on the CHAMP satellite orbiting at altitudes of ~450–380 km (Stolle et al. 2006). Similar results were obtained from direct plasma density measurements onboard DMSP F9 and F10 satellites orbiting at ~840 km altitude (Huang et al. 2001) and with recent measurement of prereversal enhancement of ion vertical drift by the C/NOFS satellite (Huang and Hairston 2015). Two factors likely contributing to the higher occurrence rates of OSS events and spread F events in Tucumán than those in Taiwan include a weaker Earth magnetic field in Tucumán, which is close to the South Atlantic magnetic anomaly, and a longitudinal distribution of prereversal enhancements of the equatorial electrojet. The seasonal distribution of OSSs in Tucumán is also consistent with the occurrence of the spread F and scintillation reported for Tucumán (e.g., Alfonsi et al. 2013; Ezquer et al. 2003). For Taiwan, Lee et al. (2013) reported a different seasonal distribution of spread F. On the basis of data obtained during the solar minimum in 1996, they found that the spread F occurrence peaks from May to August, i.e., in the local summer half of the year. However, the observation of plasma bubbles by the CHAMP satellite (Stolle et al. 2006) in 2001–2004 and the measurements of prereversal enhancement of vertical plasma drift (Huang and Hairston 2015) are closer to our findings such that spread F is more frequently observed around equinoxes. Our results concerning the propagation velocities are consistent with previous reports based on optical and GPS total electron content measurements for South America (e.g., Haase et al. 2011) and for Taiwan (Huang 1990; Liu et al. 2011). Our results are based on larger numbers of events and cover the entire year. Pacheco et al. (2011) studied the super-rotation and zonal ion drift from the ROCSAT-1 satellite orbiting at an altitude of ~600 km from November 1999 to December 2003, which corresponds to the solar maximum. Their results are consistent with our results in that they observed the largest nighttime eastward drifts in the American sector during the northern winter; similar behavior was shown in the neutral winds. They observed the lowest nighttime eastward drifts from May to August.
GW seeding as a potential mechanism for the initiation of ESF and plasma bubble development has been discussed in many experimental and theoretical papers (e.g., Abdu et al. 2009b; Cabrera et al. 2010; Hysell et al. 2014, and references therein). Unfortunately, our observations are under the crest of the EIA rather than at the geomagnetic equator, where the initiation is expected, and the GW seeding in the bottomside ionosphere should be important. Hence, we were unable to draw conclusions from our observations. For completeness, we note only that we usually did not observe significant GW wave activity prior to the OSS events in the Doppler shift spectrograms. We observed noticeable unusual waves that preceded about 10 % of the OSS events, as shown in the example in Fig. 1. The waves propagated roughly eastward in all cases. Their observed propagation velocities obtained by slowness search, which is the method described by Chum et al. (2014), were slightly larger or the same as the propagation velocities of the OSSs. More precisely, they fit the propagation velocities of the OSSs within the estimated uncertainties. This result indicates that their horizontal velocities in the wind frame were very small, at close to zero or on the order of ~10 m s−1, if we assume that OSSs propagate with neutral winds. Therefore, we were not able to distinguish whether we observed real GWs or simply undulations in the vicinity of spread F irregularities that were generated together with spread F. The observations of noticeable wave-like perturbations that preceded about ~10 % of the OSS events can be briefly summarized as follows. (1) If present, the waves occur before the first OSS event at the given night, i.e., relatively soon after sunset, and they sometimes merge with the first OSS events. They have not been observed before the OSS events that occurred late at night. (2) The amplitudes, or Doppler shifts, of waves are usually larger on sounding paths that are closer to the equator (e.g., Fig. 1). We note that the distances between the transmitters are about 100 km; hence, the distances between the reflection points are about 50 km (“Measurements and data analysis” section). (3) It is not possible to accurately determine the velocity and propagation direction for all observations. However, the propagation characteristics of GWs for which the propagation analysis is feasible are similar to propagation of the related OSSs. It should also be noted in this respect that GWs usually do not propagate eastward, particularly in Tucumán (Chum et al. 2014). It is therefore probable that we observed only undulation related to the OSSs rather than real GWs.
It is also interesting to note that we did not observe a systematic decrease in the Doppler shift before the OSSs events, which could indicate uplift of the F layer, particularly that of the reflecting level for the sounding frequency f 0. However, as previously noted, our observations are under the crest of EIA and not under the geomagnetic equator, where the initiation of instability is expected.
Continuous Doppler sounding was applied to study the occurrence rates and propagation velocities of OSSs in Tucumán and Taiwan. The observed OSSs can be related to the equatorial/low-latitude spread F and roughly eastward propagation in both Tucumán and Taiwan. The observed horizontal velocities of the OSSs were in the range of ~70 to ~200 m s−1. The mean velocities over Tucumán, at ~140 m s−1, were higher than those over Taiwan, at ~107 m s−1. In addition, the occurrence rate of the OSS events in Tucumán was about four times higher than that in Taiwan.
It was shown that spread F occurred at the equatorial region at very low latitudes in Jicamarca during the nights of OSS observations in Tucumán and in Sanya during the nights of OSS observations in Taiwan. It was found that spread F usually occurs longer at these equatorial stations.
The GPS signal scintillation data obtained by radio occultation measurements onboard the F3/C satellite are consistent with the occurrences of OSSs in the Doppler data and show a similar seasonal and local time dependence in both Tucumán and Taiwan. The largest values of S4max scintillation index were observed at about the same local times (~20:00 to ~23:00 LT) as the highest velocities and occurrence rates of OSSs observed by Doppler. In addition, global maps of the S4max index and the dependence of S4max on altitude were presented. The main contributions to the GPS signal scintillations, i.e., the highest values of S4max, were observed at the altitudes of the F2 layer around the geomagnetic equator in the region of EIA.
The propagation velocities at both locations were usually larger than the horizontal velocities of neutral winds estimated by the HWM14 experimental model. This might indicate that the HWM14 is not suitable enough for determining plasma drifts at the crest of EIA during the spread F conditions, at least over Tucumán and Taiwan.
JC wrote most of the paper and performed the analysis of the Doppler measurements. JYL and SPC contributed by examining the F3/C scintillation data and related analysis and the ionosonde data in Taiwan. MAC is responsible for the operation of Doppler sounding in Tucumán and worked with RGE to analyze the ionosonde data in Tucumán. JL helped with the text of the paper, particularly with the introduction and comparison with previous works. DB suggested the comparison with neutral winds. JB, JF, and FH helped with the Doppler shift measurements and data analysis. All authors read and approved the final manuscript.
The Doppler data are available at http://datacenter.ufa.cas.cz/under the link to the Spectrogram archive. The IRI-2012, http://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html, is acknowledged for providing the electron density profile from which the reflection heights were obtained. The NASA National Space Science Data Center, http://nssdcftp.gsfc.nasa.gov/models/, is acknowledged for providing the source code of the IGRF magnetic field model. The DPS4 portable sounder database is acknowledged for providing ionograms from Jicamarca and Sanya. The support under Grants 15-07281J and P209/12/2440 by the Czech National Foundation, Project M100421201 of the Czech Academy of Sciences, Project MOST104-2923-M-008-002-MY3 Granted to National Central University, Taiwan, and Projects PICT2011-1008 (FONCyT) and PIUNT-UNT 26/E508, Argentina, are acknowledged. J.E. Ise, J.I. Cangemi, and Y. Chen are acknowledged for the maintenance of the Doppler system in Tucumán and Taiwan. V. Truhlik is acknowledged for help with the HWM14 model.
The authors declare that they have no competing interests.
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