Intermediate Layers responses to Geomagnetic Activity During the 2009 Deep Solar Minimum Over the Brazilian Low Latitude Sector

. This work presents the daytime behaviour of the Intermediate Layers ( ILs) parameters (the 10 virtual height - h’IL , and the top frequency - ftIL ) over the low latitude region of Cachoeria Paulista (CP, 22.42° S; 45° W, I: -34.4°) for the deepest solar minimum of the last 100 years. In such a unique condition, this research reveals the ILs ' quiet state seasonal behaviour 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 15 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 analysed. Regarding to the IL frequency, it was observed that after 12:00 LT, there is a tendency to decrease with the increase of the magnetic disturbances, being this characteristic more intense after 16:00 LT for summer and winter. For the equinox, such variation was detected, however 20 with half of the amplitude of the other seasons. In addition, it stands out the domain of the annual periodicity of the ftIL while the h’IL presents a semiannual component under the geomagnetic quiet condition.


Introduction
The deep solar minimum of the solar cycle 23/24 provides an unprecedented opportunity to understand the variability of Earth's ambient ionosphere.During this period, an unusually inactive state 2 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 (F10.7cm)presented very low values when compared to the previous solar cycle of the modern era (see for example Balan et al., 2012 andKutiev et al., 2013).During this period, reduction of ionospheric temperatures and densities were 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 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 never reported before.
Great efforts have been made to better understand the behaviour of the different ionospheric layers over the equatorial and low latitudes sectors during this period.Under such condition, 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 highspeed 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. (2016) investigated the behaviour 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 their 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/eastward during the day/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 are 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.As the behaviour of the ILs is little discussed in the literature, and in order for the reader to become familiar with the subject that this paper is going to deal with, we will first present a brief summary of the main characteristics of the ILs over the equatorial and low latitudes sectors over Brazil, and then we will be able to understand how the magnetic activity can affect these layers.
Recently, dos Santos et al. (2019) and Santos et al. (2020;2021) have 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 ILs' occurrence 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 the ILs with a straight format and spreading base appearance also were 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) 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 4 prereversal enhancement of the zonal electric field.Such characteristic was observed in most of the cases during a period of high solar activity, between October and April months, however a single case also was observed in 2009.Another interesting characteristic observed is that the ILs could suffer in some way 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 o S; 44 o W) on October 9, 2009, presented a strong upward movement that carried the IL to the base of the F2 layer in ~ 1.5 hours.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, however, during sunset times.As mentioned by the authors, it is possible that the ILs in these cases had been 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 November 10, 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 hours.This rapid rise of the Es/IL layers probably was caused by an eastward electric field of ~ 0.6 mV/m 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 mentioned previously, this epoch is especially suited to develop studies like the one proposed here due to the very low values and little variation of the solar decimetric flux (10.7cm).In this case, the effects caused in the ILs by the variability of radiation coming from the Sun can be neglected, and consider only those caused by geomagnetic variations.The data and methodology used to investigate the possible influence of the geomagnetic storms in the intermediate layers is given in Section 2. The results are presented in Section 3 and finally, in Section 4, the discussion and conclusions.

Data set
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: -34.4°), during the deep solar minimum of 2009 are used to verify the possible dependence of the ILs on the geomagnetic activity.The ionospheric survey made by the Digisonde is based on the reflection of the electromagnetic signal transmitted vertically to the ionosphere with a peak power of the order of 10kW (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 cut-off 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 pre-processed through the ARTIST software (Automatic Real Time Ionogram Scaler with True Height) and also manually post-processed using the SAO-explorer software following the same criteria described by Dos Santos et al. (2019).For more details about Digisonde, see for example Reisnish (1986) and Reisnish et al. (2009).The ILs' virtual height (h'IL) and top frequency (ftIL) are analysed as a function of the Kp index.All the observed ILs were included in the analysis, regardless if they present a descending or an ascending movement.
Before going into detail on the topic that this work proposes, we will first give an overview of the behaviour 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 grey 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 all the analysed period (panel b).The downward movement of the ILs is one of the important characteristic that also can be observed in panel b.As indicated in panel c, the occurrence of the ILs along 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 08-09 LT).Regarding panel d, it can be seen that the ILs occurrence in the low latitude sector 6 6 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, ~10:30 LT in the equinoxes.In general, the probability of occurrence decays drastically as the night-time period approaches (Note: the seasons were equally divided, i.e., 121 days around the solstices and 61 days around the equinoxes).(given in Solar Flux Units (SFU); 1 SFU = 10 -22 W/(m 2 Hz)).F10.7P was chosen because several authors have shown that the ionospheric parameters are better described by this index (Brum et al., 2011 and2012;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 nm 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)  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 Figure 2) and this behaviour 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 SFU 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  and  () 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 colour scale described above.
11 Based on the model output above described, Figure 4 shows the behaviour of the h'IL and ftIL  The same methodology explained in the case of Figure 5 was applied to all the data between 06:00 and 18:00 LT for each season.Figure 6 shows the ILs' dependence on the geomagnetic activity in terms of height (first two columns from left to right) and frequency (two columns of the left) for the different seasons of the year (the data were grouped in seasons as shown in panel c of Figure 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 ILs' height 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 an opposite behaviour 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 that the ILs' height increase with the ΔKpav variation in all seasons after ~12:00 LT, as can be seen by the positive values of km.∆  −1 .
Regarding the behaviour of the frequency, it is noticed that in the summer, the ftIL parameter presented a tendency of increase with the geomagnetic activity in the beginning of the day (~ 0.1 MHz.∆  −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 decrease along the day, and during the winter, little or no response of the top frequency to ΔKpav variability can be observed prior 16:00 LT and sharp decrease after the referred period similar to what is observed during the summer.The results summarized in Figure 6 revealed, for the first time, that the most expressive response of the ILs over the low latitude region of Brazil to the geomagnetic activity occurred during the early morning hours (~06:00 -08:00 LT) of the summer when the ILs presented a significant variation of their altitude with the increase of the ΔKpav (as indicated by the negative values of km.ΔKpav -1 in h'IL panels).
One of the hypotheses to explain such variation in the h'IL parameter is that this behaviour can be related to dusk-to-dawn directed PPEF (see for example Tsurutani et al., 2008).Such electric fields have westward polarity during 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 disturbance electric field can affect considerably the vertical displacement of the intermediate layers.
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/west during the daytime/nighttime hours.Therefore, it is possible that in the first 2-3 hours 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 Figure 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 also was accompanied by a decrease in their top frequency.Wakai (1967) reported that the height of the intermediate layer over Boulder (40 o N; 105 o W) also can be influenced by the magnetic disturbances, however their observation was made during the night-time.
At the same time in which a decrease in the ILs height is observed, an increase in the ftIL parameter occurs 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 18 decrease with the increase of ΔKpav.Note that the after 16:00 LT, this decreased 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/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 comfortable below the IL's minimum height used in this work (Brum et al., 2006;Brum et al., 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 o S; 34 o 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 shown by Santos et al. (2020), the ILs' occurrence 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 referred two periods of geomagnetic activity are totally different from each other.However, their development/dynamics over Brazilian sector can be affected by disturbed electric fields, as shown by the results here presented and others previous publications (dos Santos et al. 2019, andSantos et al. 2021).
Although the impacts of the geomagnetic activity on different layers of the ionosphere have been extensively studied, 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 2.During the summer, the height of the ILs tend to be lower with the increase of the magnetic activity in the first hours of the day.This characteristic was probably caused by a dusk to dawn electric field; 3. During daytime, the smoothed rise of the h'IL can be related to the regular day-to-day undisturbed zonal electric field of the ionosphere; 20 4. In respect to the top frequency dependence with geomagnetic activity, before 12:00 LT it was observed positive or null variation in all seasons.After midday, there is a tendency that the ftIL's decrease with the magnetic disturbances, being this characteristic more intense after 16:00 LT for the summer and winter, and; 5. The domain of a semi-annual and an annual component variation was observed in parameters of height and top frequency of the ILs, respectively, for geomagnetic very quiet time condition.

Figure 1 :
Figure 1: Behaviour of the frequency (ftIL) and height (h'IL) parameters of the intermediate layers over Cachoeira Paulista during 2009 (panels a and b, respectively) in function of local time; distribution of the ILs occurrence with the local time in function of day of the year for 2009 (panel c), and; the seasonally occurrence probability of the ILs.In each panel, red, blue and grey are used to represent Summer, Winter and Equinoxes, respectively (panel d).

Figure 2
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 (grey line in 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 and Tapping and Morgan (2017).The Kpav (grey line in left bottom panel) is the average of the 3 hours data current Kp value (Kp(ref)) and the previous 3 and 6 hours, that is, Kpav = (Kp(ref) + Kp(ref-3) + Kp(ref-6))/3, which gives the standard behaviour of the geomagnetic activity and avoid sharp gradients in the temporal edges of this index (every 3 hours).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 on the right bottom panel of Figure 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 early 2009.From mid-2008 until 2009/09, the fraction of spot-free days fluctuated around 82%, unprecedented in the age of modern instrumentation.Using Johann Heinrich Müller's sunspot observations from 1709,Carrasco et al. (2021) concluded 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 cycles.As commented by the authors, the solar activity level in 2009 was similar to that in 1709 according to its most probable value.All the characteristics mentioned above reinforce the importance of the period chosen here to analyse the possible dependence of ILs on geomagnetic activity since 2009 can be, in some way, comparable to the Maunder Minimum epoch of greatly suppressed solar activity and considered as the weakest cycle of the past 100+ years(Zharkova, 2020).

Figure 2 :
Figure 2: Variability of the solar and geomagnetic activity quantified by the F10.7P and the Kpav indices (upper and bottom block of panels, respectively) for the studied period.Left panels show the geophysical conditions as a function

Figure 3 :
Figure 3: Dependence of the h'IL/ftIL's FFT coefficients as a function of LT (left/right, respectively).The circles are the values obtained by the FFT decomposition, while the continuous lines are the best polynomial approximation (more information in the manuscript body).The colours blue and red are used to represent the coefficients values of  () and  () , respectively.
230during 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 equation 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 semi-annual 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 southern hemisphere between ~ 06:00 LT and 11:00 LT, with a maximum in April/May (DOY 92-153) before the local noon.A second maximum is observed from the begging of November to 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 to 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.

Figure 4 : 13 Figure 5 Figure 5 :
Figure 4: Contour plot of the annual variation of the modeled virtual height (h'IL, left top panel) and top frequency (ftIL, left bottom panel) 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 each panel on the right, the Correlation Coefficient (R) and Root Mean Square Error (RSME) values are also provided.

16 16Figure 6 :
Figure 6: 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.
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 follow: 1.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;