This work presents the daytime behavior of the intermediate
layer (ILs) parameters (the virtual height – h'IL, and the top frequency – ftIL) over
the low-latitude region of Cachoeria Paulista (CP, 22.42∘ S;
45∘ W, I: -34.59∘) during the 2009 deep solar minimum.
Under such a unique condition, this research reveals the ILs' quiet state seasonal
behavior as well as its responses to moderate changes in the geomagnetic
activity. The main results show that even small variations of geomagnetic
activity (quantified by the planetary Kp index) are able to modify the
dynamics of the ILs parameters. For the first time, it was observed that during
the summer, the h'IL decreases rapidly with the increase of geomagnetic
activity, mainly in the early morning hours, while in the following hours, a
smoothed rise of the IL was found in all seasons analyzed. Regarding the
IL frequency, it was observed that after 12:00 LT, there is a tendency to
decrease with the increase of magnetic disturbances, this
characteristic being more intense after 16:00 LT for summer and winter. For the
equinox, such variation was detected, however with half of the amplitude of
the other seasons. In addition, the domain of the annual
periodicity of the ftIL stands out, while the h'IL presents a semiannual component under the
condition of geomagnetic quiet.
Introduction
The deep solar minimum of the solar cycle 23–24 provides an unprecedented
opportunity to understand the variability of Earth's ambient ionosphere
since 1947. During this period, an unusually inactive state of the Sun with
only relatively small sunspot-carrying active regions was observed. The
solar fluxes (UV, EUV, and X-rays) responsible for the heating of the upper
atmosphere and production of the ionosphere and the well-known 10.7 cm solar
radio flux (F10.7 cm) presented very low values when compared to the previous solar cycle of the modern era (see, for example, Tapping, 2013; Tapping and
Morgan, 2017; Balan et al., 2012; Kutiev et al., 2013). During this
period, a reduction of ionospheric temperatures and densities was detected
over several latitudes (Coley et al., 2010; Heelis et al., 2009; Yue et al.,
2010; Klenzing et al., 2011; Aponte et al., 2013). The thermospheric total
mass density from the prolonged minimum in solar activity between cycles 23
and 24 compared with that of the previous solar minima, presented a reduction of
about 10 %–30 % compared with the climatologically expected levels (Emmert
et al., 2010). Likewise, Heelis et al. (2009) and Aponte et al. (2013)
reported an unprecedented contraction of the topside ionosphere to altitudes
that had never been reported before.
Great efforts have been made to better understand the behavior of the
different ionospheric layers over the equatorial and low-latitude sectors
during this period. Under such conditions, it is expected that the effects
caused by geomagnetic activities are highlighted, since the variability of
radiation coming from the Sun in this case can be neglected. Liu et al. (2012), for example, discussed the impacts of the high-speed stream in the
equatorial ionization anomaly (EIA) development. They showed that the
inhibition in the EIA formation was probably due to a westward disturbance
dynamo electric field. Santos et al. (2016a) investigated the behavior of
the equatorial F region zonal and vertical plasma drifts over Jicamarca
during the weak geomagnetic storm of June 2008. Based on a realistic
low-latitude ionospheric model (SUPIM – Sheffield University
Plasmasphere-Ionosphere Model), they showed that the perfect
anticorrelation between the vertical and the zonal drifts close to the
evening prereversal enhancement of the zonal electric field was driven
mainly by a vertical Hall electric field induced by the primary zonal
electric field in the presence of an enhanced nighttime E region ionization
(see Abdu et al., 1998). Sreeja et al. (2011), in turn, showed that
the daytime E-region westward drift over Trivandrum (8.5∘ N,
77∘ E; dip latitude ∼0.5∘ N) presented a reduction
that was simultaneous with the disappearance of the equatorial sporadic E
layer (Esq) echoes in the ionograms. In this case, it was suggested that
an additional overshielding electric field (westward during the day and eastward during the night), superposed on the ionosphere during the storm main phase,
contributed to the observed reduction in the drift.
While the effects of the geomagnetic storms on the E, F, and sporadic-E (Es)
layers have been widely investigated, little information can be found about such
effects on those layers located in the ionospheric valley, especially during
the deep solar cycle minimum of 2008–2009. These layers, which are known as
“intermediate layers” (or just “ILs”) are defined as a region of enhanced
electron density located in the ionospheric valley that extends from the
peak altitude of the daytime E-region to the bottom side of the F-region.
Fujitaka and Tohmatsu (1973) reported that the solar semidiurnal
atmospheric tide can be the dominant cause of the intermediate layers at
night and that the vertical drift of the ionizations by the Sq electric
field seems to modify the altitude variation of the ILs during this time.
Szuszczewicz et al. (1995) found that the ILs are observed throughout the
day and in all latitudes that covered the Northern and Southern Hemispheres.
Besides that, they also reported the formation of ILs at high altitudes
(>170 km) and a monotonic descent to lower altitudes at rates as
high as 8.5 km h-1. Rodger et al. (1981) noted that the ILs over South Georgia
(54∘ S, 37∘ W) are characterized by a prior downward movement of the F-layer,
followed by the formation of the intermediate layer and its subsequent drift
downwards to about 140 km. They also mentioned that initially this downward
movement of the ILs can be at the same rate as the F layer but decreases as
the ILs attains lower altitudes. Mridula and Pant (2021) studied the
behavior of ILs over the equatorial location of Thiruvananthapuram and noted
that the occurrence of ILs over this sector is higher in the summer and
winter solstice and lower in equinoxes. They also showed that the occurrence
of this layer is higher in the solar minimum than in the solar maximum
period. The possible influence of the gravity waves in determining these
characteristics is also discussed by the authors.
Recently, dos Santos et al. (2019) and Santos et al. (2020, 2021) studied the essential characteristics of the ILs over the Brazilian sector
during epochs of minimum and maximum solar activity. It was observed that
these layers are predominantly diurnal and present a typical downward
movement that can last from minutes to hours. Depending on the height at
which the ILs are formed, they can descend and merge with the normal ongoing
sporadic – E (Es) layers. The occurrence of ILs over Brazil is high and seems to be
dependent on the magnetic inclination angle and independent (or weakly
dependent) on the solar activity. Nocturnal ILs also were observed over Brazil,
but they are very unusual. Regarding the shape in which the ILs are seen in the
ionograms, it was verified that they presented a curved format similar to
the “h” type Es layer; however, ILs with a straight format and spreading
base appearance were also observed.
The studies conducted so far on the ILs over Brazil give us some indications
that the dynamics of these layers can be influenced by the atmospheric
tides, gravity waves, and electric fields (Nygrén et al., 1990;
Wilkinson et al., 1992). The day-to-day variability in the average ILs' descent
velocity also suggests the influence of a periodic perturbation with a
periodicity of some days. The velocity values found are compatible with
those of the semidiurnal and quarter-diurnal tides. However, the larger
descending rate (>10 km h-1) observed over the equatorial region
may reveal the additional influence of the gravity waves in IL's dynamics.
Additionally, Santos et al. (2021) reported interesting events in which the
ILs presented an upward movement at the same time in which the F layer rises due
to the evening prereversal enhancement of the zonal electric field. Such
a characteristic was observed in most of the cases during a period of high
solar activity, between October and April; however, a single case was also
observed in 2009. Another interesting characteristic observed is that
the ILs could suffer in some way from the influence of the prompt penetration
electric fields. Dos Santos et al. (2019), for example, showed a case in
which a daytime IL over the equatorial region of São Luis (2∘ S;
44∘ W) on 9 October 2009 presented a strong upward movement that
carried the IL to the base of the F2 layer in ∼ 1.5 h. This
anomalous rise was probably caused by the joint action of the atmospheric
gravity wave propagation and the dawn to dusk PPEF. Santos et al. (2021)
also reported the ascending ILs, but during sunset times. As was mentioned by
the authors, it is possible that the ILs in these cases were caused by the
action of the PRE and in some events by the additional contribution from the
prompt penetration electric fields. In all the studied events, the ILs were
located at altitudes higher than or equal to 175 km, except the event of
10 November 2003, when an Es layer located at about 120 km of altitude
presented an abrupt rise reaching 290 km of altitude in a time interval of
∼ 1.25 h. This rapid rise of the Es and IL layers was probably
caused by an eastward electric field of ∼ 0.6 mV m-1 arising
from the PRE and the PPEF (for more details, see Santos et al., 2021).
The focus of this paper is to investigate the geomagnetic activity effects
on the intermediate layers over the Brazilian low-latitude sector during the
deep solar minimum of 2009, regardless of the reasons why such storms were
generated. As was mentioned previously, this epoch is especially suited to
developing studies like the one proposed here due to the very low values and
little variation of the solar decimetric flux (10.7 cm). In this case, the
effects caused in the ILs by the variability of radiation coming from the Sun
can be neglected and only those caused by geomagnetic variations are considered.
The data and methodology used to investigate the possible influence of the
geomagnetic storms in the intermediate layers is given in Sect. 2. The
results are presented in Sect. 3, and finally, in Sect. 4, the discussion
and conclusions.
Dataset
In this paper, the ionospheric sounding data collected by the Digisonde
operated over the low-latitude site, Cachoeira Paulista (CP,
22.42∘ S; 45∘ W, I: -34.59∘), during the deep
solar minimum of 2009 are used to verify the possible dependence of the ILs on
geomagnetic activity. The ionospheric survey made by Digisonde is
based on the reflection of the electromagnetic signal transmitted vertically
to the ionosphere with a peak power of the order of 10 kW (for the case of
Digisonde DGS256, that is the model used to collect data for 2009 over CP)
at frequencies ranging from 0.5 to 30 MHz. The vertical radio sounding makes
use of the fact that radio waves are reflected in the ionosphere at the
height where the local cutoff frequency equals the frequency of the radio
wave. The ionospheric information is recorded in the form of ionograms that
display the virtual height of the returned echoes versus their frequency,
generally registered at 10 and or 15 min intervals. The Digisonde data used
in this work were preprocessed through ARTIST software (Automatic Real
Time Ionogram Scaler with True Height) and also manually postprocessed
using the SAO-explorer software following the same criteria described by dos
Santos et al. (2019). For more details about Digisonde, see, for example,
Reinisch (1986) and Reinisch et al. (2009). The ILs' virtual height (h'IL) and top
frequency (ftIL) are analyzed as a function of the Kp index. All the
ILs observed were included in the analysis, regardless of whether they present a descending or
an ascending movement.
Before going into details on the topic that this work proposes, we will first
give an overview of the behavior of ILs on the sector of CP. Figure 1 shows
the variability of the parameter of frequency and height of the ILs (panels a
and b), their distribution with the local time (panel c), as well as their
rate of occurrence for different seasons of the year. The red, blue, and gray
colors are used to represent the summer (December solstice), winter (June
solstice), and equinoxes, respectively. It can be observed that in general
the ILs attain higher frequencies (>6 MHz) after 11:00 LT (panel a)
(except in some cases) and present a high variability in height during the
entire period analyzed (panel b). The downward movement of the ILs is an
important characteristic that also can be observed in panel (b). As indicated
in panel (c), the occurrence of the ILs during the day is not continuous, which
means that the ILs can appear and disappear many times during the day or simply
not occur. Additionally, panel (c) also shows that in some periods (especially
in winter, with some exceptions), there is a tendency that the ILs be formed a
little later (after 08:00–09:00 LT). Regarding panel (d), it can be seen that the
occurrence of the ILs in the low-latitude sector of CP increases significantly in the
first hours of the day, attaining its maximum at ∼ 14:00 LT in
summer, ∼ 12:00 LT in winter, and ∼ 10:30 LT in the
equinoxes. In general, the probability of occurrence decays drastically as
the nighttime period approaches (note that the seasons were equally divided,
i.e., 121 d around the solstices and 61 d around the equinoxes).
Behavior of the frequency (ftIL) and height (h'IL) parameters of the
intermediate layers over Cachoeira Paulista during 2009 (a and b,
respectively) as functions of local time; distribution of the ILs occurrence
with the local time as a function of day of the year for 2009 (c), and
the seasonal occurrence probability of the ILs. In each panel, red, blue, and
gray are used to represent summer, winter, and equinoxes, respectively (d).
Figure 2 summarizes the geophysical condition of the data distribution
according to the solar and geomagnetic activities based on the F10.7P index
and Kpav index, respectively. The F10.7P (gray line in the top left panel)
is a combination of the daily decimetric solar flux index (F10.7) and one
more term (F10.7A), which corresponds to the average of the 81 previous
days, thus F10.7P = (F10.7A + F10.7) /2 (given in solar flux units (SFU);
1 SFU = 10-22 W m−2 Hz−1). F10.7P was chosen because several
authors have shown that the ionospheric parameters are better described by
this index (Brum et al., 2011, 2012; Goncharenko et al., 2013 and
references therein). In fact, Brum et al. (2011) and Brum et al. (2012) have
shown that the best description of the UV-EUV (based on UV-EUV irradiance
data from Pioneer Venus Orbiter (10–150 nm) and by the Solar EUV Monitor on
board the Solar Heliospheric Observatory (26–34 and 0.1–50 nm bands))
is given by F10.7P when compared with F10.7. In addition, their works have
shown that the UV-EUV emissions tend to increase with F10.7P until a certain
threshold (around 175 SFU). However, for low solar activity, the UV-EUV
variations with the F10.7P can be well represented by a linear function, and
this feature is very important for the methodology employed in this work, as
seen below. For more details about F10.7 index, see Tapping (2013) and
Tapping and Morgan (2017). The Kpav (grey line in left bottom panel)
is the average of the 3 h data current Kp value (Kp(ref)) and the
previous 3 and 6 h, that is, Kpav= (Kp(ref)+ Kp(ref-3)+ Kp(ref-6))/3,
which gives the standard behavior
of the geomagnetic activity and avoids sharp gradients in the temporal edges
of this index (every 3 h). Then, in the case of Kpav, different
values can be defined per day, since the ILs can occur in different intervals
of the day.
The occurrence number in hours of the Kpav level during 2009 is
presented in the right bottom panel of Fig. 2 (red bars). It is observed
that all of the data were acquired during very low to normal geomagnetic
activity (Kpav≤3+ or 3.3) according to the Wrenn et al. (1987) classification. Such a distribution is very similar to that found by
Terra et al. (2020) when the authors analyzed the MSTID events for the
period starting in the middle of 2018 to the end of 2019 (also low solar
activity). Note that the occurrence of various levels of magnetic activity
is well distributed throughout the year (left bottom panel of Fig. 2), and
this behavior is the optimum condition for the kind of analysis of this
work, as will be seen in this report. Tsurutani et al. (2011) have studied
part of the period in analysis and showed that the causes of the low
geomagnetic activity during the end of cycle #23 can be related to the
solar midlatitude small coronal holes, low IMF Bz variances, low solar wind
speeds, and low solar magnetic fields. Regarding the solar activity, the
period that encompasses our dataset is the end of solar cycle #23 and the
beginning of solar cycle #24. A growth of activity and fluctuations of
F10.7P along the year is observed, varying from 66.5 to 78.1 SFU
(average of 70.1 SFU, top right panel), and an uneven distribution of F10.7P
(left upper panel) may be noted. Schrijver et al. (2011) showed that in
agreement with the yearly averaged sunspot number, only 5 of 28 cycles since
1700 had a minimum lower than in early 2009. From mid-2008 until September 2009,
the fraction of spot-free days fluctuated around 82 %, which was unprecedented in
the age of modern instrumentation. Using Johann Heinrich Müller's
sunspot observations from 1709 (Fig. 5 of Hayakawa et al., 2021a) and the
sunspot catalog published by the Kislovodsk Mountain Astronomical Station of
the Central Astronomical Observatory at Pulkovo for the recent solar cycles
(1996–2019), Carrasco et al. (2021) showed that one of the most active
years in the Maunder Minimum (1709), was still less active than most years
in the Dalton Minimum and also less active than those of the most recent
solar minima. Additionally, they mentioned that only the solar activity
levels in 2008, 2009, and 2019 were similar to or lower than (as in the case
of 2008) the most probable active day fraction value for 1709 (for more
details, see Fig. 2 of Carrasco et al., 2021). This reinforces how special
the period chosen here is to analyze the possible dependence of ILs on
geomagnetic activity. For more details about the Maunder Minimum, see Usoskin et
al. (2015, 2021), Carrasco et al. (2021), and Hayakawa et al. (2021a, b).
Variability of the solar and geomagnetic activity quantified by
the F10.7P (a, b) and the Kpav indices (c, d). (a) and (c) show the geophysical conditions as a function of the day of the year, while (b) and (d) show their corresponding number of occurrences (in hours) under different geophysical conditions. The dots of (a) represent the 41 d' averages and the standard deviation of F10.7P, while the dots of (c) represent the same range of days of (a) and (b) and its respective standard deviation but for Kpav≤2.3 (geomagnetic condition used to construct the quiet time condition of h'IL and ftIL) (Note: (a) and (c) show the geophysical conditions as a function of the day of the year (gray line)). The red continuous lines are the reconstruction of these variabilities using fast Fourier transform (FFT). The given occurrence in the right column of panels is the number of hours for a given interval of Kpav (0.125) and F10.7P (0.5 SFU).
From the h'IL and ftIL data, an empirical climatological model was developed that
accounted for the dependencies of these parameters on time and season, under
low solar and geomagnetic activities. Determining the variability of ILs
parameters in function of time and season make the isolation of
any changes related to geomagnetic activity possible. The first step in our
methodology was to extract the seasonal quiet time behavior of the h'IL and
ftIL parameters. To this end, the weighted arithmetic mean defined
as x(trefd) represented as (Eq. 1) was employed
x‾(tref,dref)=∑dref-20dref+20xtref,ddref-d∑dref-20dref+20dref-d,
where x denotes h'IL or ftIL values under the geomagnetic activity
condition below Kpav≤2.3 for the time reference tref and the
selected day of the year (d= DOY). The average value of height and frequency of
the ILs was calculated considering 20 d adjacent to the dref and 30 min around the tref.
From the quiet time variability of the h'IL and ftIL obtained by weighted arithmetic
mean process described above, a simple model was built using finite Fourier
series reconstruction following the procedure by Souza et al. (2010) and
Brum et al. (2011), given by
xVt,d=A0t+2∑m=14[Am(t)cos2πmf1d+Bm(t)sin2πmf1d],
where xVt,d is the reconstructed variable as a
function of time in LT (t), and DOY (d) (xV stands for h'IL or ftIL), f1 is the
fundamental frequency of the parameter to be reconstructed (1/365),
A0t is the annual average of such a parameter for a
given (t), and finally, Am(t) and Bm(t) are the mth
Fourier coefficients also as a function of time. The terms A0t, Am(t) and Bm(t) were incorporated into the model
using polynomial fittings in function of time (LT), as shown in Fig. 3,
for the harmonics m=1 (1 year), m=2 (∼ 6 months), m=3
(∼ 4 months), and m=4 (∼ 3 months). The upper
left (h'IL) and right (ftIL) panels show the time dependence of A0t open circles for Fourier coefficients. Similarly, the values of
Am(t) and Bm(t) are presented in the lower panels by the blue
and red circles, respectively. In all the panels of this figure, the best
polynomial fitting is represented by the continuous lines following the same
color scale described above.
Based on the model output described above, Fig. 4 shows the behavior of
the h'IL and ftIL during the year from 06:00 to 18:00 LT (top and bottom panel,
respectively). The right panels show the dispersion diagram between the
model and its respective weighted arithmetic mean (under Kpav≤2.3) obtained by Eq. (1), wherein it is possible to see the good
correlation between the observation and the modeled data. The left panels
show the dominance of the semiannual and annual variation of the ILs' virtual
height and top frequency, respectively. It is interesting to observe that
the upper intermediate layers (>160 km) are formed as winter
approaches in the Southern Hemisphere between ∼ 06:00 and 11:00 LT, with a maximum in April–May (DOY 92-153) before the local noon. A second
maximum is observed from the beginning of November to the middle of January (DOY 304-15), however, in a more restricted range of time (prior ∼ 09:00 LT). After 12:00 LT, the ILs are generally located at altitudes at or
below 150 km. In addition, it is observed that the evolution of the ILs to
altitudes below 120 km was more evident between the months of April and May
(DOY 92-153) at the end of the day. The bottom left panel shows an annual
variation of the top frequencies, with a maximum at about 12:00–13:00 LT
from November to February (DOY 304-62). It can be observed that the upper
ILs present lower frequencies when compared to the layers located near 150 km.
As the ILs descend, they can reach the E region and merge with the existing
sporadic-Es layer increasing, in this way, the top frequency of the layer due
to the presence of the metallic ions.
Dependence of the h'IL / ftIL's FFT coefficients as a function of LT (left or right, respectively). The circles are the values obtained by the FFT decomposition, while the continuous lines are the best polynomial
approximation. The colors blue
and red are used to represent the coefficients values of
Am(t) and
Bm(t), respectively.
Contour plot of the annual variation of the modeled virtual height
(h'IL, a) and top frequency (ftIL, c) of the
intermediate layers over Cachoeira Paulista. The dispersion diagrams on the
right-hand side show the correlation between the weighted arithmetic mean
and model results. In (b) and (d), the correlation coefficient
(R) and root mean square error (RSME) values are also provided.
Figure 5 exemplifies how the dependence of the different parameters of the
intermediate layers in respect to geomagnetic activity was investigated in
this work using ΔKpav. The ΔKpav is the mean of
the respective Kpav (gray line in the left bottom panel of Fig. 2,
Kpav= (Kp(ref)+ Kp(ref-3)+ Kp(ref-6))/3) minus
the average of any value below Kpav≤2.3 in a range of
± 20 d (this is the geomagnetic condition that the model was
developed for, the red line shown in the left bottom panel of Fig. 2).
Note that the usage of the residuals minimizes the background quiet time
behavior variation along the time (LT and season), enhancing in this way the
detection of the real contribution or not of the geomagnetic activity on the
ILs parameters. The upper panel of Fig. 5 shows the whole dataset sorted
from the lowest to the highest ΔKpav values and divided into
eight sections with the same percentage of samples for each range of ΔKpav (12.5 %, represented by the black vertical lines) for the
summer (December solstice) at 17:30 LT ± 30 min. Specifically, for
this example, the selected range represents 178 data points, i.e., each
12.5 % displays the behavior of ∼ 22 individual data
samples. This panel also displays the respective F10.7P values (red line)
and its respective average and standard deviation (blue open circles) for
the same sorted 12.5 % occurrence range of ΔKpav. Note that
the F10.7P mean variation for each range does not vary much, which leads us
to emphasize that the following variations of ILs are due to geomagnetic
activity. The bottom panels show the h'IL and ftIL responses to the geomagnetic
activity by the residual average obtained by the difference of the data and
the model output presented in Fig. 4 in function of ΔKpav.
The open circles represent the average values of the height and frequency
residuals (Δ h'IL and Δ ftIL, respectively) for the eight different
levels of ΔKpav and their respective standard deviations
(vertical and horizontal lines). The linear fitting is indicated by the blue
lines. The slope (SLP) of the dependence of h'IL and ftIL with respect to the
geomagnetic activity variation (km ΔKp-1 and MHz ΔKp-1) and the correlation factor (R) are also shown. In this example,
it can be clearly observed that as the geomagnetic activity increases, the
height of the intermediate layers also increases. The opposite occurs with
the frequency when an increase of ΔKpav causes a decrease
in this parameter.
Responses of the intermediate layer to the geomagnetic activity
for summer at 17:30 LT ± 30 min. (a) shows the ΔKpav data organized from the lowest
to the highest values and divided into eight sections with the same
percentage of samples. In addition, the values of F10.7P with respect to
ΔKpav and the average of the F10.7P (blue open circles) for
each section are also presented. (b) and (c) show the linear
regression fitting over the height and frequency residual variability
relative to the average ΔKpav values.
The same methodology explained in the case of Fig. 5 was applied to all
the data between 06:00 and 18:00 LT for each season. Figure 6
shows the dependence of ILs on the geomagnetic activity in terms of height (the first
two columns from left to right) and frequency (two columns on the left) for
the different seasons of the year (the data were grouped in seasons as shown
in panel c of Fig. 1). The correlation coefficient (R) of both parameters
is also shown at the right column of each block of panels. The variations of
the geomagnetic activity presented in this figure were 2.13 ± 0.28
(summer), 2.18 ± 0.24 (equinoxes), and 1.72 ± 0.21 (winter) in Kp
index. It is observed that the higher variability in the height of ILs with
geomagnetic activity occurs during the summer period. In this case, the IL was
located lower than the expected position with the increase of the
geomagnetic activity in the beginning of the day. In the following hours,
this condition decreases until a moment in which the opposite behavior
occurs, that is, a small rise of the IL begins to be observed with the
increase of the ΔKpav. Although some fluctuation in the R-value
can be observed (mainly during the equinox and winter), there is a tendency
for the height of the ILs' to increase with the ΔKpav variation in all
seasons after ∼ 12:00 LT, as can be seen by the positive
values of km ΔKpav-1. Regarding the behavior of the
frequency, it is noticed that in the summer, the ftIL parameter presented a
tendency to increase with the geomagnetic activity in the beginning of the
day (∼ 0.1 MHz ΔKpav-1); however, from
∼ 12:00 LT on there is a significant decrease in the ftIL with
ΔKpav (mainly after 16:00 LT), as can be confirmed by the
negative values of the coefficient correlation (R). During the equinox, the
general tendency is that the ftIL decreases during the day, and during winter,
and little or no response of the top frequency to ΔKpav variability
can be observed prior 16:00 LT, and there is a sharp decrease after the period referred to
similar to what is observed during the summer.
Geomagnetic activity effects on h'IL and ftIL parameters for different
seasons. The first two panels (from left to right) show the linear
regression of the h'IL as a function of the ΔKpav index over the
different times of the day and the correlation coefficient R. The two right
panels indicate the same but for the ftIL parameter.
Discussion and conclusions
It is well known that geomagnetic activity can drastically modify the
low-latitude ionospheric dynamics. During the last solar minimum, a unique
opportunity was available to investigate such dynamics, since the effects of
the solar activity, which dominates the temporal variability of ionospheric
properties, could practically be disregarded due to very low solar radiation
variation. Using Digisonde data from a Brazilian low-latitude station,
Cachoeira Paulista, we studied the impacts of the geomagnetic activity in
the height and top frequency of the intermediate layers during the deep
solar minimum of 2009.
The results summarized in Fig. 6 revealed, for the first time, that the
most expressive response of the ILs over the low-latitude region of Brazil to
geomagnetic activity occurred during the early morning hours
(∼ 06:00–08:00 LT) of the summer when the ILs presented a
significant variation of altitude with the increase of the ΔKpav (as indicated by the negative values of the km.ΔKpav-1 in h'IL panels). One of the hypotheses to explain such
variation in the h'IL parameter is that this behavior can be related to
dusk-to-dawn directed PPEF (see, for example, Tsurutani et al., 2008). Such
electric fields have westward polarity during the daytime and, therefore, it may
be one of the factors responsible for the occurrence of lower h'IL at this time.
As pointed out by Santos et al. (2021), depending on the height at which the
ILs are located, the electric field disturbance can affect the
vertical displacement of the intermediate layers considerably.
Another interesting point that needs to be considered is that the movement
of the ILs can also be influenced by the regular undisturbed day-to-day
variations in the zonal electric field of the ionosphere that is directed to
east during the daytime and west during the nighttime hours. Therefore, it is possible that
in the first 2–3 h of our analysis period, the lowering of the h'IL with the
increase of ΔKpav (negative values of the rate km ΔKpav-1) could be a result of a competition between the eastward
zonal electric field created by the E-region dynamo and the disturbance
westward electric field arising from the overshielding process. In the following hours, an
opposite situation was observed, that is, a small rise of the ILs occurred in
all seasonal periods analyzed, as denoted by the positive values of the rate
km ΔKpav-1 in the first set of panels on the left
in Fig. 6. As mentioned by dos Santos et al. (2019), the rise of an IL
during daytime can also be a result of the joint action of the of the
eastward PPEF (undershielding) and gravity wave propagation. In the case studied by dos
Santos et al. (2019), the rise of the IL was also accompanied by a decrease in
their top frequency. Wakai (1967) reported that the height of the
intermediate layer over Boulder (40∘ N; 105∘ W) can also be
influenced by the magnetic disturbances; however, their observation was made
during the nighttime.
At the same time in which a decrease in the ILs height was observed, an increase
in the ftIL parameter occurred during the summer between 08:00 and 12:00 LT
attaining a maximum of 0.2 MHz ΔKpav-1 at 09:00 LT. It
is interesting to observe that before 12:00 LT, for example, the rate
variation of the ftIL was positive, indicating that when they are located in
lower altitudes, the ILs' top frequency increased in relation to its quiet time
values. This increase in the frequency is expected since as the layer
descends, it can suffer an additional increase of ionization arising from
the metallic ions that contribute to the ion density in the ongoing
sporadic-E (Es) layers. As the ILs presented a rise after 13:00 LT, the tendency
was that the ftIL decrease with the increase of ΔKpav. Note that
the after 16:00 LT, this decrease is more accentuated during the summer and
winter. Analyzing the incoherent scatter data from the mid-latitude region
of Arecibo, Raizada et al. (2017) showed that the integrated electron
content (E-region total electron content – ErTEC) between 80 and 150 km
altitude regions presented a maximum variability throughout the night due to
geomagnetic activity for both low and high solar activity during equinox
periods. Besides that, the authors also verified that the integrated
electron content during geomagnetically disturbed or normal conditions and high
solar flux periods displays positive changes during summer and equinox,
while it is negative in winter. Wakai (1967) reported a study about the
maximum electron concentration of the nighttime E layer, the valley above it,
and the appearance of the intermediate layer from analysis of the
low-frequency ionogram obtained at Boulder on three nights of quiet,
moderate, and severe geomagnetic activity. They observed an increase of the
ionization in the nighttime valley at times of increased magnetic activity
and the appearance of an intermediate layer in ∼ 150–160 km
during periods of moderated activity. Eventually, the IL can be impacted by
the energetic particle precipitation (EPP) (see, for example, Santos et al.,
2016a, b), mainly during the occurrence of intense geomagnetic storms.
Furthermore, as the present study refers to a period in which the
geomagnetic storms were considerably weaker. That said, we believe that if
ILs were impacted in any way by the EPP, it would not be relevant to our
investigation at this moment. In addition, theoretical simulation of
ion-pair production by EPP over Cachoeira Paulista have shown that the peak
production of electrons is comfortably below the IL's minimum height used in
this work (Brum et al., 2006; Brum, 2021).
The effects of the magnetic storms on the intermediate layer were studied
also by Rodger et al. (1981) using ionosonde data from South Georgia
(54∘ S; 34∘ W). They showed that the rate of the downward movement
and the final height of the nocturnal intermediate layer are independent of
the season or magnetic activity. Additionally, they observed that the
probability of formation of an IL when the minimum virtual height of the F2
layer is above 220 km is very low, but it can increase during magnetically
disturbed periods. As was shown by Santos et al. (2020a), the occurrence of ILs over
Cachoeira Paulista (22.42∘ S; 45∘ W) was very high both
in 2009 (a low solar activity year and the same period of this report) and
2003 (a high solar activity year). These results show that, in general, the
ILs occurrence resulted to be independent of the magnetic disturbances, since
the two periods of geomagnetic activity referred to are totally different from
each other. However, their development and dynamics over the Brazilian sector can be
affected by disturbed electric fields, as shown by the results
presented here and previous other publications (dos Santos et al., 2019; Santos et al., 2021).
Although the impacts of the geomagnetic activity on different layers of the
ionosphere have been studied extensively, there is a lack of information
about what happens in the ionospheric valley region during such conditions,
mainly over the low and equatorial latitudes. Using the low-power VHF radar
data over the equatorial site of Jicamarca, Chau and Kudeki (2006) showed
that the 150 km echoes were not affected by the electric field reversal
caused by a magnetic disturbance (Kp = 5). As mentioned by them, a
statistical study on the ILs occurrence based on the magnetic activity index Kp
did not identify any correlation between magnetic activity and the 150 km
echoes. On the other hand, our results show that a small variation in
ΔKpav index (∼ 2.0) can affect the ILs, especially
in the morning period of the summer and late afternoon of all season over
the low-latitude sector over Brazil. Although the techniques used by us are
different from those used by Chau and Kudeki (2006), the contrasting result
reveals that the ionospheric valley is a complex region, and additional
studies need to be performed to understand the physical mechanism governing
the generation of the intermediate layers during the occurrence of magnetic
disturbances. It is important to emphasize that for the first time, it was
shown that a small variation in ΔKpav index (by ∼ 2.0) was able to impact the dynamics of the intermediate layer over the low-latitude region during the period of deep solar minimum. The main results of
this work are summarized as follows:
A small variation in the geomagnetic activity during low solar activity can
affect both the parameter of height and frequency of the ILs over the low
latitude Brazilian sector, and such responses are dependent on local time and
season.
During the summer, the height of the ILs tends to be lower with the increase of magnetic activity in the first hours of the day. This characteristic was
probably caused by a dusk to dawn electric field.
During the daytime, the smoothed rise of the h'IL can be related to the regular
day-to-day undisturbed zonal electric field of the ionosphere.
With respect to the top frequency dependence with geomagnetic activity, before
12:00 LT, positive or null variation in all seasons was observed. After
midday, there is a tendency that the ftILs decrease with the magnetic
disturbances, this characteristic being more intense after 16:00 LT for the
summer and winter.
The domain of a semiannual and an annual component variation was observed
in parameters of height and top frequency of the ILs, respectively, for
very quiet time, geomagnetic conditions.
Data availability
The Digisonde data can be downloaded in Zenodo (identified as CAJ2M 2009 in 10.5281/zenodo.3967542, Santos et al., 2020b).
Author contributions
AMS and CGMB processed the data, performed the analysis and wrote the paper. ISB, MAA, JHAS, JRS contributed in the interpretation of the data.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “From the Sun to the Earth's magnetosphere–ionosphere–thermosphere”. It is a result of the VIII Brazilian Symposium on Space Geophysics and Aeronomy & VIII Symposium on Physics and Astronomy, Brazil, March 2021.
Acknowledgements
The Kp index was obtained from the World Data Center for Geomagnetism, Kyoto
(http://wdc.kugi.kyoto-u.ac.jp/index.html, last access: 8 November 2019) and Solar Radio Flux (F10.7 cm) from the National Oceanic and Atmospheric Administration (NOOA). The Arecibo Observatory is operated by the University of Central Florida under a cooperative agreement with the
National Science Foundation (AST-1744119) and in alliance with Yang
Enterprises and Ana G. Méndez-Universidad Metropolitana.
Financial support
Ângela M. Santos is grateful for the financial support
from FAPESP (process number: 2015/25357-4) and CNPq (grant no. 165743/2020-4). Inez S. Batista is grateful for CNPq grant numbers 306844/2019-2 and 405555/2018-0. One of us (José H. A. Sobral) had Grant number 303383/2019-4 from the Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq). Jonas R. Souza would like to thank the CNPq (grant no. 307181/2018-9) for the research productivity sponsorship and the INCT GNSS-NavAer supported by CNPq (grant no. 465648/2014-2), FAPESP (grant no. 2017/50115-0) and CAPES (grant no. 88887.137186/2017-00).
Review statement
This paper was edited by Luis Vieira and reviewed by two anonymous referees.
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