F region vertical drifts (Vz) are the result of the
interaction between ionospheric plasma with the zonal electric field and
the Earth's magnetic field. Abrupt variations in Vz are strongly
associated with the occurrence of plasma irregularities (spread F) during
the nighttime periods. These irregularities are manifestations of space
weather in the ionosphere's environment without necessarily requiring a solar
burst. In this context, the Brazilian Space Weather Study and Monitoring
Program (Embrace) of the National Institute for Space Research (INPE) has
been developing different indexes to analyze these ionospheric
irregularities in the Brazilian sector. Therefore, the main purpose of this
work is to produce a new ionospheric scale based on the analysis of the
ionospheric plasma drift velocity, named AV. It is based on the maximum
value of Vz (Vzp), which in turn is calculated through its
relationship with the virtual height parameter, h′F, measured by the Digisonde
Portable Sounder (DPS-4D) installed in São Luís (2∘ S,
44∘ W; dip: -2.3∘). This index quantifies the time
relationship between the Vz peak and the irregularity observed in the
ionograms. Thus, in this study, we analyzed 7 years of data, between 2009
and 2015, divided by season in order to construct a standardized scale. The
results show there is a delay of at least 15 min between the Vzp
observation and the irregularity occurrence. Finally, we believe that this
proposed index allows for evaluating the impacts of ionospheric phenomena in the
space weather environment.
Introduction
Ionospheric irregularities (spread F) occur in the F region, which are
characterized by regions of signal scattering in ionosondes. In general, the
spread F may be associated with plasma bubbles characterized by regions
where the plasma density is reduced. Also, these irregularities usually
develop after sunset (Abdu et al., 1983; Abdu, 2001). The plasma bubbles are
generated through the nonlinear evolution of Rayleigh–Taylor instability
in equatorial regions (Bittencourt and Abdu, 1981; Abdu et al., 2006, 2009a; Huang
et al., 2002).
One of the most useful parameters to analyze these irregularities is the
vertical drift velocity (Vz), which is a response of the zonal electric
field in the F region, and it is controlled by the interaction between the
E and F layers, which is positive (upward) during the day. In the nighttime
period, the Vz becomes negative due to the inversion of neutral wind
(Abdu et al., 2006). Soon before the inversion, an increase in plasma drift
occurs, lifting the equatorial F layer and controlling the generation of
plasma bubbles (Fejer, 1991; Fejer et al., 2008; Huang et al., 2002). This
phenomenon is named prereversal enhancement (PRE) of the vertical plasma
drift, and it gives rise to a maximum in the vertical velocity drift (Vzp)
around 18:00–19:00 LT (Heelis et al., 1974; Farley et al., 1986).
It is well established that the PRE presents a great variability in relation
to seasonality, solar cycle, and magnetic activity (Fejer, 1991; Abdu et al.,
1995; Fejer et al., 2008). In the Brazilian sector, ionospheric
irregularities occur frequently in summer due to the relatively small angle
between the solar terminator and the magnetic meridian (Batista et al.,
1986). Therefore, there is an almost instantaneous decoupling between the E
and F regions. Thus, the polarization electric fields of the F region
associated with the PRE peak have higher amplitudes, and the Vz reaches
higher velocities.
Regarding the solar flux, some works have pointed out a direct correlation
between the Vz and the F10.7 radio index (Fejer et al., 1979; Fejer, 1991; Batista et al., 1996). In fact, the number of free electrons in the
ionosphere will increase with the intensification of solar flux causing intense
electric fields. Consequently, larger amplitudes of the Vz parameter
are observed.
Although several studies report the occurrence of plasma irregularities in the
F region increases with the plasma Vz, there is still no climatological
study that determines the relationship of solar–terrestrial conditions applied to
the ionosphere in such a way as to construct an ionospheric index. Therefore, there are no
indexes showing the relationship to the irregularities in ionospheric plasma as spread F (plasma bubbles). Jakowski et al. (2012) suggested a disturbance ionosphere
index (DIX), which describes the perturbation degree of the ionosphere
using global navigation satellite systems (GNSS) data. Recently,
Nishioka et al. (2017) reported on plasma irregularity occurrences using an
index based on total electron content (TEC) measurements. However, an
index which correlates to the Vz with irregularity/plasma bubble
occurrences was not found in the literature.
Thus, in this study we present the newly developed ionospheric index, AV,
based on the Vz parameter. This index quantifies the time relationship
between the Vz peak and the irregularity observed in the ionograms.
The time relationship between these parameters (Vzp and irregularity
observations) is at least 15 min for values of Vzp less than 60 m s-1.
Finally, this study demonstrates that the AV index can be used for space
weather forecast, and it will help in the evaluation of phenomena
impacts in the ionosphere.
Dataset
We used ionospheric data in São Luís in the Brazilian sector
(2∘31′ S, 44∘16′ W; dip: -2.3∘). The data were
acquired by a Digisonde Portable Sounder (DPS4), an ionospheric radar that
operates in variable high frequency (HF). The data are composed of the
signal reflected by the ionospheric layers, in which they are registered in
ionograms, graphs of frequency versus virtual height (h′F). Therefore, it is
possible to calculate the electron density profile and parameters of the
different regions in the ionosphere (Reinisch, 1986, 2009).
The Vz parameter is a representation of the vertical drift velocity of
the post-sunset F region (Bittencourt and Abdu, 1981; Abdu et al., 1983, 2006). It is important to mention that the heights below 300 km
were not considered in this study since the ionosphere plasma is subject to recombination effects. We have calculated the Vz using its relation
with h′F determined as Vz=Δh′FΔt. The h′F parameter is
collected in 15 min intervals on a continuous basis. We collected
h′F values from 18:00 to 21:00 UTC (from 21:00 to 24:00 LT) each night to obtain the
Vz parameter for the present analysis.
The higher vertical drift velocity each day, or Vzp, is considered
to represent the PRE. As the purpose of this analysis is to quantify the
time relationship between the Vzp and the ionospheric irregularity occurrences,
we firstly analyzed data from 2 years representing high (2001) and low
(descending phase; 2015) solar flux. It is important to mention that
the criterion was to choose 2 whole years in different phases of the solar
cycle that had a complete and revised amount of data to avoid any error in
the results. We used this analysis to construct the AV index scale. In the
following step, we performed a climatological study considering the data
acquired from the years 2009 to 2014 in order to validate the AV
scale. Lastly, the dataset was separated in seasons: equinoxes (March, April,
and May; September and October), summer (November, December, and January),
and winter (June, July, and August), since the irregularities are observed
every night between November and January. In other words, since plasma
bubbles occur between September and March, we eliminate the 2 months
before and after this period in this analysis. The same criterion was used in the winter
season.
Results and discussionAV index scale
The purpose of this analysis is to quantify the interrelationship between the
Vzp and the ionospheric irregularity occurrences that may be associated
with plasma bubbles (spread F). Based on previous studies about this
correlation (Batista et al., 1986, 1996; Abdu et al., 2009a),
it can be assumed that the lowest and highest variations in the Vzp
amplitude are approximately 20 and 70 m s-1, respectively. Therefore, we
analyzed the irregularity occurrence after the highest Vzp value, and
we proposed a scale to quantify this relationship.
The new ionospheric scale AV is divided into five levels as shown in Table 1. The colors indicate the level of disturbance in increasing order of
magnitude. The AV1 and AV2 (blue/green) imply typical conditions,
when no irregularities in the ionospheric plasma were observed. From
the AV3 (yellow) and above it is possible to observe the spread in the
F region detected by the Digisonde. The AV4 and AV5 indexes
are represented by orange and red, respectively, meaning extreme
conditions, in which the existence of plasma bubbles is more probable. The
color selection was used based on the Embrace index development program (http://www2.inpe.br/climaespacial/portal/the-embrace-program/, last access: 6 September 2019).
The new ionospheric plasma index, AV, divided into five levels. The colors indicate the level of disturbance. Vzp is given in meters per second.
It is known from previous studies that irregularities in the F region can
be observed when Vzp amplitudes are higher than 30 m s-1 (Abdu et al.,
1985; Fejer et al., 1999). In the Brazilian region, Abdu et al. (1983) and
Abdu et al. (2009a) found that the Vzp should be around 30–50 m s-1 to
observe the irregularity/spread F. Other authors found a threshold of 40 m s-1
in different locations (Basu et al., 1996; Whalen, 2003). Furthermore, the
irregularity occurrence probability becomes higher when the Vzp is
greater than 40 m s-1 (Huang et al., 2015). In fact, we do not observe in our
analysis an expressive irregularity occurrence for the Vz less than 40 m s-1.
Therefore, the threshold was selected as 40 m s-1 here to validate the
proposed AV index. In other words, values smaller than 40 m s-1 mean that
the irregularity will not appear (AV1) or will rarely appear (AV2)
since the interest of this study is related to general cases.
Since the Vzp intensification may indicate the occurrence of plasma
irregularities in ionograms, mainly plasma bubbles, we performed a
statistical analysis to find a correlation between the instant in which the
Vzp increases and the time that the irregularity is identified. An
example is shown in Table 2 for 11 January 2001. It is observed that
the Vzp reaches 43 m s-1 at 21:45 UTC, corresponding to the AV3 index.
The irregularity can be observed in the ionogram 45 min after the
Vzp peak (22:30 UTC), as shown in Fig. 1. The red arrows indicate the
spread in the F region.
Example of the relationship between the irregularity occurrence and the
Vzp parameter.
DateVzpIndexTimeTime ofscalespread-Foccurrence1 Nov 200143 m s-1AV321:45 UTC22:30 UTC
Sequences of ionograms showing the spread F over São
Luís on 11 January 2001 (red arrows).
AV index validation
Table 3 presents statistics of the data used in the analysis for 2001
and 2015, considering the total number of observations, number of days in
which the irregularities were observed, and the number of days that
were classified as the AV3, AV4, or AV5.
Statistics of the data used in this study for 2001 and 2015.
YearSeasonNumber ofNumber ofIndexNumber of daysobservationsirregularitiesper index2001Summer8266AV336AV425AV55Equinoxes16763AV341AV421AV51Winter8461AV310AV40AV502015Summer8461AV329AV423AV59Equinoxes15358AV335AV421AV52Winter7023AV323AV40AV50
Figure 2 shows the statistical analysis between the Vzp values
considering only the indexes AV3, AV4, and AV5 and the time
that the irregularity starts to appear in the ionogram. Firstly, the
analysis takes into account the data gathered in the summer of 2001 and 2015
over São Luís. The interval between the Vzp peak and the
observation of the spread F was discretized into five intervals for the sake of
the analysis: 0, 15, 30, 45, and ≥60 min. This quantity is called the
Δtvi hereafter. Notice that this granularity of 150 min is a
limitation of the Digisonde sampling time, and all the occurrences in which
the Δtvi is higher than 60 min were placed in the last
interval.
The time relationship between the Vzp parameter, considering
the AV3, AV4, and AV5 indexes, and the irregularity observations
in ionograms in the summer of 2001 and 2015 over São Luís.
Comparing the results for the AV3 index, it is observed that the Δtvi is at least 15 min in both solar cycles. In 2015, no data with
the Δtvi equal to 45 min were observed. There was also a
significant change comparing both scenarios with respect to the occurrences
with the Δtvi equal to or greater than 60 min. In 2001, only
16 % of the cases fell into this interval, whereas in 2015 it was 50 %. The
high occurrence in the descending phase of the solar cycle could be related
to the post-midnight occurrence of the irregularities and, generally, they are
observed in low solar activity (Otsuka, 2018).
Regarding the AV4 index, 10 % of the cases had a Δtvi
equal to 0 min during the maximum solar flux (2001), meaning that the
spread F occurred at the same time of the Vzp. On the other hand, in
2015 no data fell into this interval. However, notice that there is a high
probability that the Δtvi is equal to or higher than 15 min.
When the index reached the AV5 corresponding to extreme cases, we
observe a high occurrence of events in which the Δtvi equals 0 min.
In fact, notice that almost 40 % of that data in 2001 and 70 % of the
data in 2015 comprise this interval.
It can be inferred from our results that the probability density function of the
Δtvi for the AV3 and AV4 approaches a uniform
distribution with its lower bound at 15 min, except for 2001 and the AV4.
Regarding the AV5 in 2001, the distribution seems uniform with its lower bound
at 0 min and its upper bound at 30 min. For the same index, the
distribution appears to be exponential in 2015. Furthermore, it can be
concluded that there is a high probability of observing a Δtvi of less than 15 min, given that the index for the AV3 or AV4 is negligible.
It is well established that the spread-F occurrence depends on the season
and epoch in the solar cycle (Abdu et al., 1983, 1995; Abdu,
2001). Abdu et al. (1985) showed that the drift velocities were small during low-sunspot years, which weakens the development of irregularities. Huang et
al. (2002) observed that the maximum irregularity rates were significantly
higher during the maximum phase of the solar cycle. This behavior happens
because the thermospheric winds and longitudinal gradients in conjugate-E
layer conductivity are more effective. Those are the key parameters that
control the evening F region dynamo electric field. Therefore, during
high solar activity, there are significant variations in thermospheric winds
and in longitudinal conductivity gradients of the evening conjugate E
layers, which lead to higher values of the Vzp and, consequently, a
favorable environment for irregularity formation (Abdu et al., 1983).
In our analysis, we observed a significantly high number of irregularities
in 2015. We believe that this is caused by the descending phase of the
solar cycle. However, although the irregularities reached the AV5 level
in 2015, its duration in ionograms was lower than in 2001. This can be
seen in Fig. 3, where we show the duration of spread F in ionograms for the
AV3 level in 2001 and 2015. The duration of the irregularities was divided
into five intervals: less than 6, between 6 and 7, between 7 and
8, between 8 and 9, and more than 9 h (t<6, 6<t<7, 7<t<8, 8<t<9, and ≥9 h). The difference between the years is very
clear, and during the high solar cycle the duration of the irregularities is more than
9 h for most of the events. On the other hand, in 2015 the spread F lasted
less than 6 h in almost 70 % of cases, revealing a solar flux influence
and agreeing with the previous study (Abdu et al., 1985; Huang et al.,
2002).
Occurrence of spread F in ionograms for the AV3 index in 2001
and 2015 over São Luís to exemplify the differences between the
solar flux levels.
In the early hours, the irregularities were controlled by the polarization
electric field, but after 6 h the ionospheric plasma dynamics
control the bubble (Huang et al., 2011). Barros et al. (2018) show an
important role of the zonal wind plays in the evolution of the plasma bubbles in
the TEC data between 2012 and 2016. They did not discuss the differences in
the solar cycle phases. However, they showed a good agreement between the
zonal drift velocities and the thermospheric winds regarding the occurrence of plasma bubbles. Therefore, we believe that the thermospheric winds in 2015 were
lower than in 2001, interfering with the growth rate of Rayleigh–Taylor
instability (discussed below). This fact will be evaluated in greater detail
in future work.
As mentioned before, the spread F is directly related to the plasma bubbles.
These plasma bubbles are formed due to the Rayleigh–Taylor gravitational
instability process, which is operational on the steep upward gradient in the
nighttime bottom side of the F region at the magnetic equator (Abdu et al.,
2006). This Rayleigh–Taylor mechanism is related to the linear growth rate
(γ) given by Haerendel et al. (1992), Sultan (1996), and Abdu et al. (2009a, 2006).
γ=∑FP∑EP+∑FPEB-UP+gν1L-β,
where the ∑P is the Pedersen conductivity in the field line
integrated for E and F, UP is the Pedersen
conductivity-weighted neutral wind perpendicular to the magnetic field in
the magnetic meridian plane, E and B are the intensities of the ambient zonal electric field and
the magnetic field, g is the gravitational acceleration, ν is the collision frequency, L is the McIlwain parameter, and β is
the recombination loss rate. All the terms in this equation were discussed
in Abdu (2001). However, we highlight here that the angle formed between the
solar terminator and the magnetic meridian is relatively small in summer,
and consequently there is an almost instantaneous decoupling between the E
and F regions (Batista et al., 1986). Thus, the polarization electric fields
of the F region associated with the Vz peak have higher
amplitudes, favoring instability growth. Thus, the irregularity is more
favored to occur in summer than winter (Tsunoda, 1985; Barros et al., 2018).
In order to investigate the seasonal behavior, we analyzed the AV index for
the equinoxes and the winter of 2001 and 2015 over São Luís. The results
are presented in Figs. 4 and 5 for the equinoxes and winter, respectively.
In relation to the equinoxes (Fig. 4), the Vzp did not reach the AV5 in
both years. For the AV4, only a few cases were observed in the high solar
cycle. Among those, 10 % had a Δtvi equal to 0 min, 34 % had
a Δtvi equal to 15 min, 15 % had a Δtvi equal to 30 min, 10 % had a Δtvi equal to 45 min, and 31 % had
a Δtvi equal to or greater than 60 min. Regarding the AV3,
the irregularities were observed between 15 and 45 min after the Vzp
peak in both years. No significant values in which the Δtvi was equal to 0
min or greater than 60 min were found.
The time relationship between the Vzp parameter, considering the AV3
and AV4 indexes, and the irregularity observations in ionograms in the
equinoxes of 2001 and 2015 over São Luís. The Vzp did not
reach the AV5 scale.
The time relationship between the Vzp parameter, considering
the AV3 index, and the irregularity observations in ionograms in the winter
of 2001 and 2015 over São Luís. The Vzp did not reach the
AV4 and AV5 scales.
As seen in Fig. 5, the index did not reach the AV4 or the AV5 in winter.
During the high solar cycle, 10 % of the cases had a Δtvi equal to 0 min, 10 % had a Δtvi equal to 15 min, 10 % had a Δtvi equal to 30 min, and almost 20 % had a Δtvi equal to 45 min. Thus, most of the irregularity observations occurred after 1 h
from the Vzp peak. In general, the irregularities observed in the
ionograms had a Δtvi equal to or greater than 60 min in 2015.
Therefore, we observed only a few cases of spread F in both seasons,
agreeing with previous studies that irregularities related to plasma bubbles
are more frequent in summer (Barros et al., 2018). Thus, we considered the
analysis of the summer as enough to validate the proposed AV scale. However,
it is important to mention that the index is valid for the other stations of
the year too, as shown in the examples above. The climatological results are
presented in the following section.
Climatological study of the AV index and spread F in summer
Figure 6 shows the relationship between Vzp intensification, the AV index, and
the time that the irregularity starts to appear in the ionogram. This study
considered the data obtained from São Luís station from the
summer of 2009 to 2014. The years 2009 and 2010 occurred during the minimum
solar cycle 24, 2011 and 2012 occurred during the ascending phase of the
same solar cycle, and 2013 and 2014 occurred during its maximum. In this figure, we also
show the quantity of available observations and the number of days that we
used in the analysis (below the year). Also, we show the quantity for each
scale in the AV index used in this analysis.
It is possible to observe from the results that there is no regular pattern
between the highest Vzp value and the irregularity observations.
However, it is important to mention that the probability to observe a Δtvi equal to or higher than 15 min is still very high in all
phases of the solar cycle.
After the AV3 and AV4 results, we did not observe significant
values with a Δtvi equal to 0 min. Most of the events found
that the Δtvi lies between 30 min and greater than 60 min, showing the same pattern of the results presented in the previous
section. Additionally, a high number of observations with a Δtvi
greater than 60 min were found, mainly in 2009, 2010, and 2014. As
mentioned before, this percentage can be associated with the irregularities
that occurred in the post-midnight hours, which is a common observation in
low solar activity. Otsuka (2018) affirms that during solar minimum
conditions, post-midnight irregularities may occur mostly in association
with plasma bubbles initiated around midnight. In addition,
post-midnight plasma bubbles could be caused by atmospheric gravity wave
seeding of Rayleigh–Taylor instability and/or an increase in the growth rate of Rayleigh–Taylor instability due to F-layer uplift.
The time relationship between the Vzp parameter, considering the
AV3, AV4, and AV5 indexes, and the irregularity observations
in ionograms in the summer of 2009 until 2014 over São Luís.
We notice that when the index reached the AV5, there is a high occurrence
of events with a Δtvi equal to 0 min. This behavior is clearly
observed during the ascending and maximum phases of the solar cycle,
which accounts for almost 70 % of the occurrences in this interval. During the
minimum solar cycle (2009 and 2010), this probability for such occurrences reduces to less
than 60 %. This fact can serve as evidence that the zonal drift reversal time
and the weak zonal neutral wind magnitude can cause a delay in the
irregularity occurrence during the solar minimum activity phase (Abdu et
al., 1985). Despite all these promising results, we shall recall that we
are working with a 7-year interval. Thus, any definitive conclusion on the
solar cycle dependence must be accompanied by a more comprehensive study that confirms our results.
We included an analysis of the average Δtvi considering all the
years for the AV3 (yellow line), AV4 (orange line), and AV5 (red
line) in Fig. 7. It is possible to observe that the mean Δtvi is greater than 60 min for the AV3 and AV4. In severe events,
i.e., AV5, we had the mean Δtvi equal to 0 min. Thus, we can
infer that, with considerable probability (around 50 %), regarding the AV3 and AV4, the elapsed time between the
Vzp peak and the irregularity occurrence is greater than 60 min.
The average time relationship between the Vzp parameter, considering the
AV3 (yellow), AV4 (orange), and AV5 (red) indexes, and the
irregularity observations in ionograms in summer from 2009 until 2014 over
São Luís.
It is well known that the EB term in Eq. (1) arises from the
evening vertical drift enhancement, also known as PRE (Abdu et al., 2009b).
Therefore, when the base of the F layer lies above 300 km,
EB is approximately equal to the Vzp (Bittencourt and Abdu,
1981; Batista et al., 1986). Furthermore, the term gν can be
enhanced due to a large Vzp, leading to an enhancement of the linear
growth rate (γ). In other words, there is a clear compromise between
the Vzp and the linear growth rate of the plasma bubble. Moreover,
several authors have reported the necessity to have a wave-like perturbation
to trigger Rayleigh–Taylor instability (Abdu et al., 2009b; Takahashi et
al., 2018; Tsunoda, 2006).
Using data from ground-based experiments conducted during the 2005 SpreadFEx
campaign in Brazil, Abdu et al. (2009c) have shown a few case studies where it
is possible to identify a relationship between the Vzp and plasma bubble
occurrence. The authors observed that the spread F occurs 30 min after
the Vzp reached values of around 40 m s-1. They concluded that the
relationship between the Vzp and plasma bubble occurrence is a significant
factor in understanding the development of these irregularities. Moreover, as
mentioned before, several authors have reported the necessity to have a
wave-like perturbation trigger Rayleigh–Taylor instability, which is
responsible for plasma irregularity formation in the F region (Abdu et
al., 2009c; Tsunoda, 2006). Narayanan et al. (2014) studied the connection
between the equatorial spread F and satellite traces over an Indian
equatorial station, Tirunelveli (8.7∘ N, 77.8∘ E; dip:
1.1∘ N). They used ionosonde observations with 5 min intervals
from March 2008 to February 2009 during the extended solar minimum
conditions. Among their results, they found an average spread-F onset delay
of about 30 min compared to observations made with the satellite traces. Therefore, they
believed that the satellite trace remarks may be used as a precursor to
irregularity occurrences. Additionally, they recommended validating their
analysis with the Vzp parameter, but they did not perform this analysis.
In a recent study, Sousasantos et al. (2019) developed a three-dimensional
numerical model to analyze plasma bubble structure. An input parameter is
the PRE (ranging between 20 and 60 m s-1) obtained using the SAMI2 model (Huba
et al., 2000), which is a necessary condition in
addition to all mechanisms in forming plasma bubbles. Thus, the authors
showed that the plasma bubble structure is generated approximately 20–30 min after the PRE enters in the model. They attributed this delay to the
parallel conductivity that may reduce the growth rate on the equatorial
region. In turn, this implies longer times are required to reach the nonlinear
stage.
As presented here, the relationship between the Vzp and irregularity onset
time is an open important scientific issue. Based on the previous studies,
both wave-like perturbations or parallel conductivity can modify the
outset of the irregularity, being the responsible for the delay. In order to
determine the key factor, a more comprehensive and specific study shall be
designed.
Additionally, the ionospheric indexes found show a relationship between TEC or
satellite measurements and plasma irregularity occurrence (Huang et
al., 2015; Nishioka et al., 2017). On the other hand, there is no
ionospheric index in the available literature that found a relationship between the drift
velocity and irregularity/plasma bubble occurrences. Furthermore, this
study confirms that this proposed index can be used to warn users about irregularity occurrences, since it was shown that regarding the AV3 and
AV4 there is at least 15 min between observation of the Vzp peak and the occurrence of the irregularity.
Finally, as a perspective for future work, this index will be useful to
study the seasonality, solar cycle, and onset time of plasma
irregularities. Notice that the temporal error is around 15 min, since
the ionograms have this range of time. However, we believe that the error is
not significant in terms of spread-F occurrence. Therefore, the AV index
is suitable to be incorporated into the products offered by the Embrace
program, and it will help in the evaluation of phenomena impacts in the
space weather environment.
Conclusions
In this study, we develop an ionospheric index, AV, based on the Vz
parameter in the F region. The index quantifies the time relationship between the
Vzp and the ionospheric irregularity occurrences that may be associated
with plasma bubbles, Δtvi. We analyzed data from 2 years, 2001 and 2015, representing different solar flux in order to construct the AV index scale.
After, we performed a climatological study from 2009 to 2014, in order to
validate the AV scale.
In general, the results show that a Δtvi is at least 15 min
in both solar cycles for the AV3 and AV4 indexes. However, when the
index reached the AV5, in which case it is considered an extreme event, we
observed a rate of events with a Δtvi equal to 0 min
(60 % of the cases).
Additionally, we observed a significantly high number of irregularities in
2015 (61 cases in summer, 58 during the equinoxes, and 23 in winter). We attributed
this fact to the descending phase of the solar cycle. However, although
the irregularities reached the AV5 in 2015, its duration in ionograms was
lower than in 2001. We believe that the thermospheric winds are the
main agent responsible for this behavior, since they interfere in the growth
rate of Rayleigh–Taylor instability. This fact will be evaluated in greater
detail in future work.
We performed a climatological study during the summer since this season is
more significant regarding spread-F occurrences. Thus, we considered the data
obtained from the São Luís station from 2009 to 2014, almost covering
the ascending solar cycle. We show an irregular pattern between the highest
Vzp value and irregularity observations. However, it is important
to mention that the probability to observe a Δtvi equal to or
higher than 15 min is still very high in all phases of the solar cycle.
In fact, we can infer that, with very high probability under the AV3 and AV4, the elapsed time
between the Vzp peak and the irregularity occurrence is greater than 60 min.
Finally, as stated in previous studies the wave-like perturbations which trigger Rayleigh–Taylor instability or parallel conductivity can modify the outset
of the irregularity, and this might be responsible for the delay between the
Vzp and irregularity occurrence. However, more studies are needed to
understand this relationship. In fact, we did not find any study about the
ionospheric index that showed the relationship between the drift velocity and
irregularity/plasma bubble occurrences that is shown in this work. Thus, the AV
index is suitable to be incorporated into the products offered by the
Embrace program, and it will help in the evaluation of phenomena impacts
in the space weather environment.
Data availability
The data used in this study are available by contacting the Responsible Coordinator at the DAE/INPE (Inez S. Batista, email: inez.batista@inpe.br).
Author contributions
LCAR, CMD, GP, DB, and CF designed the study, analyzed and interpreted the data, and wrote the paper. JM and RP helped to improve the paper. All authors read and approved the paper
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “7th Brazilian meeting on space geophysics and aeronomy”. It is a result of the Brazilian meeting on Space Geophysics and Aeronomy, Santa Maria/RS, Brazil, 5–9 November 2018.
Acknowledgements
Laysa Cristina Araujo Resende would like to thank the National Space Science Center
(NSSC), the Chinese Academy of Sciences (CAS) for supporting her postdoctoral research,
and the CNPq/MCTIC (grant no. 169404/2017-0). Clezio Marcos Denardini thanks the CNPq/MCTI (no. grant 03121/2014-9). Giorgio Arlan Silva Picanço would like to thank the CNPq for the
financial support received during his M.Sc. (grant no. 132252/2017-18). Juliano Moro
would like to thank the National Space Science Center (NSSC), the Chinese
Academy of Sciences (CAS) for supporting his postdoctoral research, and the
CNPq/MCTIC (grant no. 429517/2018-01). Diego Barros thanks the CNPq for his fellowship (grant no.
301211/2019-1). Cosme Alexandre Oliveira Barros Figueiredo thanks the FAPESP for his postdoctoral
fellowship (grant no. 2018/09066-8). Régia Pereira Silva thanks the CNPq for their support (grant no. 300329/2019-9). The authors thank DAE/INPE for kindly providing the
ionospheric data.
Review statement
This paper was edited by Igo Paulino and reviewed by Ricardo Cueva and one anonymous referee.
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