We present a case study of unusual spread-F structures observed by
ionosondes at two equatorial and low-latitude Brazilian stations – São Luís
(SL: 44.2
Equatorial spread F, representing small-scale to large-scale plasma irregularities, has been extensively studied for several decades. Large-scale plasma irregularities, specifically known as equatorial plasma bubbles (EPBs), are known to be associated with equatorial spread F. In the Brazilian equatorial sector, characterized by large negative magnetic declination, spread F and EPBs have high occurrence rates during local summer and equinoctial months (Abdu et al., 1981a; Sahai et al., 2000; Sobral et al., 2002). However, during low solar activity conditions, there is a class of spread-F and plasma irregularities regularly observed in distinct longitudinal sectors, such as Brazil (Candido et al., 2011), Africa (Yizengaw et al., 2013), and Asia (Nishioka et al., 2012). They are known as postmidnight plasma irregularities (PMIs), which occur mostly in the June solstice. Comprehensive reviews on postmidnight plasma irregularities and plasma irregularities have been recently published by Otsuka (2017) and Balan et al. (2018).
PMIs occur under conditions considered unfavorable for the development of the Rayleigh–Taylor (RT) instability, since at night the vertical plasma drifts are downward owing to the westward electric fields. In recent years, a variety of works have reported their occurrence both at low latitudes and the equatorial region. Otsuka and Ogawa (2009) and Nishioka et al. (2012) investigated PMIs over Indonesia and discussed their possible sources. Li et al. (2011) reported these irregularities observed over Hainan, China, during low solar activity. Candido et al. (2011) presented a study of PMIs observed over the south crest of the equatorial ionization anomaly (EIA) during low solar activity, in Cachoeira Paulista (CP), Brazil. Yokoyama et al. (2011) studied unusual patterns of echoes from coherent scatter radar data occurring around midnight during the solar minimum period. They observed two principal types of irregularities: upwelling plumes and striations similar to mesoscale traveling ionospheric disturbances (MSTIDs). They have argued that the former can be generated by RT instability (at the equatorial region) or Perkins instability (at the midlatitude region) and the later only by Perkins instability. Yizengaw et al. (2013) presented a study of PMIs over equatorial Africa and also investigated their most probable causes. Dao et al. (2016) reported the occurrence of postmidnight field-aligned irregularities (FAIs) in Indonesia during low solar activity in 2010.
Many instrumental techniques are currently providing high-quality measurements and results for ionospheric studies. Early investigations of the ionosphere observed diffuse echoes in data from measurements using ionosondes, which are high-frequency radars used for ionospheric sounding (Breit and Tuve, 1926; Booker and Wells, 1938). The term “spread F” is widely used to generically refer to the irregularities observed in equatorial and low-latitude regions. Nowadays, digital ionosondes are extensively used for ground-based sounding of the ionosphere, providing information from the E region to the peak of the F layer, over a variable range of frequencies as well as features related to the propagation of the irregularities (Reinisch et al., 2004; Batista et al., 2008; Abdu et al., 2009). Equatorial spread F has been extensively studied for several decades, and it is known to be associated with the occurrence of large-scale plasma irregularities or equatorial plasma bubbles (EPBs).
Optical imaging of thermospheric emissions, like that used in this work, is
also a useful ground-based technique for studying thermosphere–ionosphere
processes. All-sky imaging systems provide images of thermospheric emissions
(e.g., OI 630 nm, OI 777.4 nm emissions) at ionospheric heights over a large
horizontal extent. The OI 630 nm emission comes from recombination processes
between molecular oxygen and electrons and presents a volumetric emission
rate, which peaks at an altitude of
For clarity for the present study, which presents a distinct pattern of spread F from those usually observed in equatorial ionograms, we first address the current state of understanding regarding spread-F signatures in ionosonde data.
It is currently accepted that there are two main spread-F types: range- and
frequency-type spread-F traces (Abdu et al., 1998). The range type of spread F,
often associated with the occurrence of medium- and large-scale
irregularities, including EPBs, is comprised of trace patterns with the
echoes spread in range and with the onset beginning at the lower-frequency
end of the F-layer trace in ionograms. During the spread-F season in Brazil
between October and March, evening pre-reversal enhancement in the zonal
electric field, and therefore in the F-layer vertical drift, attains large
values, and the range type of spread F is observed in equatorial ionograms, followed
by their appearance at the crest region of the EIA, which is located around
Cachoeira Paulista (CP: 22.4
Some studies have pointed out that frequency-type spread F can sometimes be associated with patches of ionization propagating eastward (MacDougall et al., 1998). However, other spread-F patterns are frequently observed in solstices in distinct longitudinal sectors as reported in Brazilian (Candido et al., 2011; MacDougall et al., 2011), Asian (Yokoyama et al., 2011), African (Yizengaw et al., 2013), and Peruvian (Zhan et al., 2018) sectors. Also, it is known that both frequency and range spread-F types can appear simultaneously as a mixed spread-F pattern. In this work, we present a case study on an unusual (anomalous) spread-F and plasma irregularity–depletion pattern observed over the equatorial region. We use the term “unusual” in the sense that the observed features are distinct from those typically observed for spread F associated with post-sunset spread F, as described above. Although the unusual type of spread F has been recognized since early studies of the equatorial ionosphere (Munro and Heisler, 1956; Heisler, 1958; Calvert and Cohen, 1961; Bowman, 2001), this is the first time that it is reported for the Brazilian equatorial region with simultaneous airglow observations, which reveal important ionospheric characteristics not available when using only ionosonde data. Earlier studies extensively reported the occurrence of anomalies in F-layer traces, such as cusps, F2 forking, and their possible association with TIDs. Calvert and Cohen (1961) presented a comprehensive study of the distinct spread-F patterns. They concluded that the distinct configurations or shapes of spread F were associated with scattering in the vertical east–west plane from field-aligned irregularities and that the spread-F pattern depends on the position relative to the ionosonde and the scale sizes of the irregularities.
We analyzed ionograms from two digisondes (DPS-4) operated at two Brazilian
equatorial sites, São Luís (SL; 44.2
The airglow images of the OI 630 nm emission used in this study were
measured by a Portable Ionospheric Camera and Small-Scale Observatory
(PICASSO) wide-angle imaging system deployed at Cajazeiras (CZ:
6.87
FPIs are optical instruments that measure the spectral line shape of the 630.0 nm emission at around 250 km of altitude and are very useful to study thermospheric winds from Doppler shifts in the emission's frequency. For more details on the FPI technique, see Fisher et al. (2015) and references therein. Investigation of the departures of the background wind system can be useful to explain possible sources of the F-uplifts associated with late-time RT instability. For this purpose, we analyzed the behavior of the neutral winds over the equatorial region taken from a ground-based FPI installed in CZ.
We present a case study of a spread-F event that occurred in the June
solstice of 2011 during a geomagnetically quiet (
Sequence of ionograms obtained on 25–26 July 2011 at São Luís (SL) from 00:40 to 03:10 LT and over Fortaleza (FZ), Brazil, from 22:00 to 01:30 LT. The spread F shows an unusual pattern, with oblique echoes. The color scale in FZ ionograms indicates that echoes are coming from the east and propagating westward.
An important point to consider is the local ionospheric background in
which the spread F occurred. The F-layer parameters,
F-layer parameters
On the other hand, over FZ, where heights are lower than at SL, we observe
stronger wave-like oscillations in both
Figure 3 shows a sequence of four images of the OI 630.0 nm emission collected on 25–26 July 2011 at Cajazeiras (CZ: center of the frame), Brazil. The images are projected over a geographic map of Brazil assuming an emission altitude of 250 km. The sites of FZ and SL are also indicated in the images for reference. Between 23:12 and 01:26 LT at least two depletions (dark regions passing over FZ and CZ) can be observed propagating westward. These depletions propagated over FZ and CZ at 23:12 LT, in agreement with the spread-F traces seen in the ionograms from FZ.
Sequence of OI 630 nm images showing the time evolution of depletions on 25–26 July 2011 between 23:12 and 01:26 LT at Cajazeiras, Brazil. The images are projected onto geographic coordinates over the Brazil map. In the plot, FZ is Fortaleza, SL is São Luís, and CZ is Cajazeiras. Arrows indicate the propagation direction of the depletions (dark regions passing over FZ and CZ).
Sky maps registered over FZ from 00:12 to 00:42 LT on 26 July 2011, showing the echo locations and Doppler frequencies (color coded) for F-region echoes from digisondes. Doppler velocities are positive (negative) for irregularities arriving (leaving) the station.
Automatic drift-mode routines were used to obtain information about the
location of echo sources in the F layer associated with plasma
irregularities. These routines provide information about the distance of the
reflected echoes using measurements of the radar ranges to the vertical and
oblique echoes as well as their directions, as described by Reinisch et al. (2004). The distribution of the echoes can be displayed in sky maps, as shown
in Fig. 4. Sky maps between 00:12 and 00:42 LT were constructed using data
from FZ during the spread-F event studied; reflected echoes appear and
are distributed in a west–east elongated pattern covering a total horizontal
distance of 1200 km (from west to east). It may be noted that, in general,
negative Doppler velocity (yellow) of the echoes dominates the western
azimuth, while the eastern azimuth is dominated by positive Doppler velocity
(blue), a characteristic indicative of an overall westward
motion of the irregularity structures. Additional directional information is
obtained from the temporal evolution of each spread-F echo in plots of the
horizontal distance of the echoes (horizontal axis) as a function of time
(vertical axis), presented as directograms. A directogram for the night of
25–26 July 2011 constructed using data from FZ is shown in Fig. 5. Each
horizontal line of the directogram corresponds to a single ionogram. The
spread echoes are distributed to the east and west of the station mainly from
23:00 to
Directogram for Fortaleza on 26 July showing the location and the
horizontal distances of the irregularities detected by digisonde and seen in
the ionograms as spread F. At left is the F-region height (km), where
The unusual spread-F echoes were observed at both equatorial sites, SL and
FZ, with a zonal separation of
Besides the capabilities of the digisonde to sound and detect the occurrence of plasma irregularities seen in the ionograms as spread-F echoes, the F-region height variations, and their vertical drifts, there is a method that uses the true heights to obtain information about gravity wave oscillations at specific plasma frequencies. The true heights are extracted from virtual heights by an inversion algorithm used in the SAO Explore software. This method was described in detail by Abdu et al. (2009) in a comprehensive study about the influence of gravity waves on the equatorial spread F. In their work, the same both locations were analyzed: the off-Equator station FZ and the equatorial station SL. Because both stations are more separated in longitude than in latitude, it was assumed that GW oscillations observed in the bottom-side F layer in FZ could have the same features at SL, considering a few differences attributed to the magnetic field inclination at each one. In this work, we also took advantage of the simultaneous digisonde sounding at these stations in order to verify the possible influence of GWs as a precursor to instability growth, which leads to the late development of the spread F studied.
Figure 7 presents the oscillations in F-layer true height at fixed frequencies (1.5–5.0 MHz) at both stations, SL and FZ. It is possible to observe oscillations prior to the development of spread F, especially in FZ, with periods around 1 h, which will be discussed later in Sect. 4.3.4.
Oscillations in the real height of the F layer at fixed frequencies
(1.5 to 5.0 MHz) before the spread F in São Luís
Figure 8 shows the measured thermospheric zonal (top panel) and meridional
(bottom panel) wind on 25–26 July, taken from the FPI installed in CZ, the
same location where the airglow images were obtained. The shaded region
encloses the standard deviation of the monthly average; the green lines are
the average winds on 25–26 July (
Measured zonal and meridional winds in CZ, Brazil, on 25–26 July 2011. The shaded region is the monthly average with standard deviation; the green lines are the mean winds on 25–26 July (mean of 2 d), and the red line is for 25–26 July.
We present an unusual event of PMI and spread-F depletions over the equatorial site in Brazil that exhibits singular features. This is the first report of such a distinct pattern of spread F for the Brazilian equatorial region, though it was observed earlier at the low-latitude station CP (Brazil) for the solar minimum 2008–2009 by Candido et al. (2011). By distinct we mean that it occurs in postmidnight hours propagating westward, which is not usually observed during solar minimum unless there is a previous eastward EPB structure propagation, as mentioned by Paulino et al. (2010). A careful analysis of equatorial ionograms and other plots from digisonde soundings suggests modifications in the ionospheric plasma density structuring, such as those associated with plasma density depletions, which are responsible for a variety of spread-F-layer patterns.
Airglow images show an apparent southwestward propagation of depletions on
this night, which differs from the typical propagation direction of
post-sunset EPBs. However, this atypical propagation can be a characteristic
of postmidnight depletions and needs further investigation with a long-term
airglow database. The depletions also propagated over CZ (350 km south of
FZ) with mean westward velocities
Moreover, Sobral et al. (2011) interpreted westward-traveling plasma
bubbles (WTPBs) observed in the same region as associated with westward
zonal thermospheric winds (simulated results). On the other hand, Fisher et
al. (2015) presented a climatological study of quiet-time thermospheric
winds and temperatures with measurements of the OI 630.0 nm airglow emission
spectral line shape over the same region. They noticed that during low solar
activity (F10.7
As mentioned before, spread-F echoes in ionograms generally appear first at the low-frequency end, as satellite traces, evolving into spread-F echoes extended in frequency and range. These characteristics were not seen in the present study. In this work, the reflected echoes observed in the ionograms first came from oblique directions and at heights that could possibly be considered higher than those observed overhead. The spread echoes appear at the higher-frequency edge of the F layer, with a top frequency higher than the layer critical frequency. Subsequently, the low-frequency edge of the cusp merges with the main trace, while the baseline of the spread-F traces gradually decreases in height. Anomalous traces in F-layer ionograms, such as cusps or “spurs”, were described in earlier studies as associated with traveling disturbances in the ionosphere. Munro and Heisler (1956) and Heisler (1958) have observed the occurrence of anomalous traces in ionograms and attributed them to manifestations of TIDs. As is well known, TIDs can be described as frontal gravity waves propagating horizontally in the ionosphere, causing increases and decreases in ionization, i.e., horizontal gradients in ionization. According to Munro and Heisler (1956), changes in ionization would be responsible for the anomalous traces in the F-layer ionogram. Similar occurrences were reported by Ratcliffe (1956) for ionograms from Huancayo, Peru. Calvert and Cohen (1961) have pointed out that some spread-F traces observed over Huancayo presented characteristics similar to frequency spread F from “temperate” latitudes, which is mainly associated with TIDs. Also, they studied distinct configurations of spread F with echoes coming from oblique directions, similar to what is presented in this work. The oblique echoes observed in ionograms alone could not provide their zonal direction (from east or west). However, additional directional information provided from the drift-mode sounding of the digisonde (DPS-4) and their appearance first in the ionograms over FZ, followed by their occurrence over SL (a western site in relation to FZ), suggested that they propagated westward. Late to pre-dawn spread F was also reported by MacDougall et al. (1998) for solstices in the Brazilian sector. However, they considered the occurrence of late-time spread F during the December solstice at Fortaleza as patches of ionization, which cause spread echoes at the high-frequency end or the frequency spread F. They also concluded that the echoes did not come from overhead structures but the east or west directions.
As is well known, poor alignment between the sunset terminator and the magnetic field lines during the June solstice in Brazil is responsible for the low occurrence rate of post-sunset spread F and EPBs, since the vertical plasma drifts are very weak. However, an occurrence peak of late-night spread-F and/or plasma irregularities is observed in the June solstice, especially around midnight and postmidnight. For this, it is necessary to have an F-layer uplift, which creates favorable conditions for the development of the RT instability. These conditions are not entirely understood, and they have been discussed by several authors (McDougall et al., 1998; Nicolls et al., 2006; Abdu et al., 2009; Nishioka et al., 2012; Yokoyama et al., 2011; Ajith et al., 2016).
During high solar activity, the longitudinal variation of the declination angle is predominant on the F-layer vertical drift and the occurrence of plasma irregularities, while it is not essential during solar minimum. During low solar activity and/or a solar minimum, in the absence of geomagnetic disturbances, the seeding processes related to gravity waves seem to be more critical, especially when the pre-reversal enhancement (PRE) amplitude is small or absent (Balachandran et al., 1992; Abdu et al., 2009). In this way, we should address the conditions that precede the occurrence of the postmidnight irregularities observed in this work. It is noted that spread-F traces associated with plasma irregularities were detected first at oblique directions at least 500 km east or west from the station, as seen in the directograms in Fig. 5, which we can consider to be ionospheric conditions favorable in a wide longitudinal range.
Nicolls et al. (2006) discussed the nocturnal F-layer uplifts associated
with the secondary maximum of the spread-F occurrence rate in low solar
activity. As is well understood, the nocturnal westward electric field is
responsible for the downward movement of the F layer. During solar minimum,
these electric fields can be easily reversed by a weak geomagnetic
disturbance. However, in the absence of the geomagnetic disturbance, which
is the case studied in this work, other sources should be considered.
Analyzing F-layer uplifts for different conditions of solar activity,
Nicolls et al. (2006) verified that during downward F-layer movement
(decreasing westward electric field), even a small contribution of a
meridional equatorward wind (
Moreover, it was discussed that neutral winds could not uplift the
equatorial plasma directly, but they are sources of meridional advection
(movement) for plasma driven by a latitudinal gradient in electron density and
responsible for F-layer uplifts. They concluded that the uplifts could be due
to the decreasing, not to the reversal, of the westward zonal electric field
associated with departures in the wind system related to the midnight
temperature maximum (MTM), recombination processes, and the plasma flux. In
this way, we analyze the zonal and meridional neutral wind variation in
Fig. 8 in order to verify that there are suitable conditions for F-layer
uplift. As is observed in Fig. 8 (top panel), the zonal wind is
Nishioka et al. (2012) discussed the causes of the postmidnight uplifts that
occurred during winter in Chumphon, Thailand (low latitude), and the
postmidnight field-aligned irregularities (FAIs) in Kototaband, Indonesia
(equatorial region). As is well known, the zonal electric field is
westward during the night, as the vertical drift ExB is downward. This
condition leads to a negative RT instability growth rate. In this way, it is
crucial to address the importance of the term
F-layer plasma density profile for 25–26 July derived from digisonde data and SAO Explorer data for several hours (LT).
The role of the Es layer has been considered as a possible cause for late-time RT instability development. The low-latitude Es layer can provide a sufficient polarization electric field that maps to the equatorial F-layer bottom side, causing F-layer uplift, as pointed out by Yizengaw et al. (2013). They interpreted the occurrence of late plasma irregularities and EPBs over the African coast during the same period of this work, the June solstice 2011, and discussed the fact that during quiet geomagnetic nights, there were favorable conditions for the action of polarization electric fields associated with low-latitude Es-layer instability, which mapped to the equatorial F layer along the geomagnetic field lines, seeding RT instability and irregularities. In fact, in this work, we can observe the occurrence of the Es layer at both quasi-equatorial stations FZ and SL at around 00:00 and 02:50 LT, respectively. However, the influence of Es layers on late-time F-layer uplift in this work is not clear since they occur at the same location as the spread F. Their influence on the postmidnight spread F during solar minimum is worthy of investigation in further work.
MSTIDs have been reported at Brazilian low latitudes using airglow and
ionosonde (Candido et al., 2008, 2011; Pimenta et al., 2008). They appear as
large-scale dark bands aligned from northeast to southwest, propagating
northwestward, mainly during low solar activity, and are associated with
electrodynamic forces at midlatitudes (Perkins instability); they can also be associated with the
propagation of gravity waves at ionospheric heights at low latitudes or
in the equatorial region. If they propagate at equatorial ionospheric heights, they
can be seen as oscillations in the F-layer bottom side and can trigger
RT instability and plasma bubbles. In this work, the plasma irregularities
seen by the ionosonde are preceded by small oscillations in the F-layer
bottom (
Finally, we should address the fact that, as shown in Figs. 2 and 7, late height rise
(in both
It is plausible to consider the depletions observed in this work to be associated with atypical EPBs triggered by GWs–MSTIDs at locations to the east of FZ and SL or with F-layer uplifts caused by departures from the wind system simultaneously with a weakening of the westward zonal electric field (not shown here) during low solar activity. We should note that the observational techniques used in this work are complementary and validate each other to identify “anomalous” spread-F patterns associated with plasma irregularities and depletions, and they can also help us understand the ionosphere during low solar activity. The drift mode is advantageous and suitable for tracking plasma irregularities and their evolution in the absence of other techniques.
In this paper, we have presented and discussed an unusual spread-F pattern
associated with unusual depletions on the OI 630 nm airglow emission
observed during geomagnetically quiet conditions during the June solstice of
2011 over the equatorial region in Brazil. We summarize our findings as follows.
The unusual spread-F pattern studied in this work presents a distinct
feature from those usually observed at post-sunset hours, with spread F
appearing first at the higher-frequency edge of the F-layer trace to
further develop into a mixed (frequency and range) spread F. The spread F and depletions occurred during low-plasma-density conditions,
geomagnetically quiet nights, and low solar activity; they propagated
westward. For the studied case, there is no evidence of previous depletions
propagating eastward. The processes to generate spread F at equatorial latitudes during quiet
times seem to be associated with later-time F-layer uplifts, possibly caused
by departures in the neutral wind system. In turn, departures in the
neutral wind system may be caused by increased auroral activity, which in
this present study may be associated with the occurrence of a short-duration
event of high-speed stream (this possible influence is the subject of an
ongoing study). Moreover, departures of the wind system associated with a
weakening of the westward electric field, or with the propagation of GWs at
ionospheric heights, favor the development of late-time RT instability.
Further studies including simulations are in progress. The spread-F event discussed here presents characteristics similar to
those of earlier cases reported for the low latitudes in CP (around the
south crest of the EIA) during the June solstice of solar minimum 2008–2009 by
Candido et al. (2011), which were interpreted as the signature of the propagation of
midlatitude MSTIDs in the ionograms. The instrumental approach in this work seems to be suitable for further
ionospheric studies, modeling, and forecasting during low solar activity.
The processed data used in this work can be requested from the author, Claudia M. N. Candido, by
email: claudia.candido@inpe.br. The authors thank the EMBRACE/INPE
program for the digisonde raw data, which can be downloaded from
CMNC wrote the paper and plotted the graphics of the ionospheric parameters. FBG contributed with part of the graphics and revised the paper. JS, ISB, EC, MAA, NB, ZL, and CW read and made suggestions regarding the paper. JM and NC provided the airglow figures and Fabry–Pérot data and plots, as well as reading the paper and suggesting corrections. All the authors read, commented, and made suggestions regarding the work and agreed with the content and submission of this paper.
The authors declare that they have no conflict of interest.
Claudia M. N. Candido thanks the China–Brazil Joint Laboratory for Space Weather, CBJLSW, for the postdoctoral fellowship program. The authors thank the EMBRACE/INPE/MCTIC program for providing ionospheric data for this work. We are grateful to the Universidade Federal de Campina Grande and Ricardo A. Buriti for support regarding the imaging systems installed at Cajazeiras. Narayan Chapagain thanks the NASA Living with a Star Heliophysics Postdoctoral Fellowship Program, administered by the University Corporation for Atmospheric Research (UCAR). Work at the University of Illinois at Urbana–Champaign was performed in collaboration with John Meriwether at Clemson University.
This research has been supported by the China–Brazil Joint Laboratory for Space Weather (CBJLSW) and a Brazilian funding agency (CNPq, grant no. 64537/2015-5). Jiankui Shi was supported by the National Natural Science Foundation of China (NSSC, grant nos. 41474137 and 1674145). Work at the University of Illinois at Urbana–Champaign was supported by National Science Foundation CEDAR (grant no. AGS 09-40253).
This paper was edited by Steve Milan and reviewed by two anonymous referees.