PUBLICATIONS. Journal of Geophysical Research: Space Physics

Transcription

1 PUBLICATIONS RESEARCH ARTICLE Key Points: An alternative possibility to equatorial plasma bubble forecasting through mathematical modeling and Digisonde data Advances in space weather research Advances in equatorial F-region collisional interchange instability Correspondence to: J. Sousasantos, Citation: Sousasantos, J., E. A. Kherani, and J. H. A. Sobral (2017), An alternative possibility to equatorial plasma bubble forecasting through mathematical modeling and Digisonde data, J. Geophys. Res. Space Physics, 122, , doi: / 2016JA Received 27 JUL 2016 Accepted 9 DEC 2016 Accepted article online 16 DEC 2016 Published online 2 FEB American Geophysical Union. All Rights Reserved. An alternative possibility to equatorial plasma bubble forecasting through mathematical modeling and Digisonde data J. Sousasantos 1, E. A. Kherani 1, and J. H. A. Sobral 1 1 National Institute for Space Research, São José dos Campos, Brazil Abstract Equatorial plasma bubbles (EPBs), or large-scale plasma depleted regions, are one of the subjects of great interest in space weather research since such phenomena have been extensively reported to cause strong degrading effects on transionospheric radio propagation at low latitudes, especially over the Brazilian region, where satellite communication interruptions by the EPBs have been, frequently, registered. One of the most difficult tasks for this field of scientific research is the forecasting of such plasma-depleted structures. This forecasting capability would be of significant help for users of positioning/navigation systems operating in the low-latitude/equatorial region all over the world. Recently, some efforts have been made trying to assess and improve the capability of predicting the EPB events. The purpose of this paper is to present an alternative approach to EPB prediction by means of the use of mathematical numerical simulation associated with ionospheric vertical drift, obtained through Digisonde data, focusing on telling beforehand whether ionospheric plasma instability processes will evolve or not into EPB structures. Modulations in the ionospheric vertical motion induced by gravity waves prior to the prereversal enhancement occurrence were used as input in the numerical model. A comparison between the numerical results and the observed EPB phenomena through CCD all-sky image data reveals a considerable coherence and supports the hypothesis of a capability of short-term forecasting. 1. Introduction In the last decades, several advances in the scientific knowledge about ionospheric plasma instabilities have been achieved, due to improvements in numerical simulations and also to the implementation of large new networks of instruments, markedly the Global Navigation Satellite Systems network which is capable of providing data with very refined temporal resolution. In the nightside equatorial ionosphere, electrodynamical processes may lead to the development of the collisional interchange instability (CII) [Zargham and Seyler, 1987] which, under appropriate conditions, is capable of triggering large regions of plasma depletions known as equatorial plasma bubbles (EPBs). The conditions for the CII to evolve into EPBs depend, in general, on steep gradients in the F-region density, on the presence of a considerable prereversal vertical drift (PRVD) and on the geomagnetic activity. Since the beginning of mathematical modeling studies concerning plasma instabilities and EPBs in the ionosphere, a large number of studies were presented over the past years such as those of Scannapieco and Ossakow [1976], Ott [1978], Ossakow et al. [1979], Zalesak [1979], Zalesak and Ossakow [1980], Zargham and Seyler [1987], Raghavarao et al. [1992], Huang et al. [1993], Keskinen et al. [2003], Kherani et al. [2005], Huba and Joyce [2007, 2010], Sousasantos et al. [2013], Kherani and Patra [2015], and so forth. The amount of information provided by these mathematical approaches increased significantly, although the predictability of the EPBs still continues to be a demand for this field of space physics research. Anderson et al. [2004] discussed the forecasting of scintillation activity based on a statistical analysis of Digisonde information about the prereversal vertical drift (PRVD). These scintillation events are claimed to be related to the F-region bottomside irregularities in the plasma distribution, which may be an evidence of the presence of spread F and, possibly, of the EPB development. In their analysis, a proposition of a threshold velocity of 20 m/s for the PRVD as a necessary condition for the existence of critical levels of scintillation activity, namely, the S4 index 0.5 was established. Also, Retterer et al. [2005] presented one more approach to the scintillation forecasting problem by using vertical plasma drift data from Jicamarca incoherent radar, in association with numerical simulation of the F-region plasma density. Their report, in a similar way to that of Anderson et al. [2004], addresses the role of the PRVD on the scintillation/epb forecasting. Another work on this subject SOUSASANTOS ET AL. EPB FORECAST 2079

2 was done by Retterer [2005] using unidimensional ionospheric data provided by the C/NOFS satellite with a low-inclination orbit and a network of receivers associated with numerical models. Kelley and Retterer [2008] proposed the use of the solar wind data combined with physics-based assimilative numerical model in the attempt of predicting ionospheric responses to a magnetic storm. Additionally, Redmon et al. [2010] showed some information about plasma depletions forecasting, through h 0 F ionospheric parameter (virtual height of the bottomside F region) examination in the Peruvian and the Kwajalein Atoll longitudinal sectors, suggesting that a high altitude of the F region must be attained approximately at 19:30 LT in order to trigger those instabilities. Carter et al. [2014a] proposed an approach to forecast the GPS scintillation believed to be related to the EPB phenomena. Their approach was based on the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM), driven by the solar radiation flux (F 10.7 ) and the geomagnetic Kp index. Carter et al. [2014b] applied geomagnetic activity predictions from solar wind data, TIEGCM and WideBand MODel-Ionospheric Scintillation Model models to forecast the occurrence and suppression of EPBs in the African and Asian longitude sectors, with some appreciable results. In all these previous studies, some points such as the day-to-day-variability of the ionospheric ambient make difficult a reliable forecasting of the EPB events. This work presents an alternative approach to handle the issue of the EPB forecasting, based on the activities of the gravity waves (GWs) identified in the vertical drift calculated through Digisonde data, specifically from the ionospheric true height, obtained by using different probing frequencies, namely, 5 MHz, 6 MHz, and 7 MHz and the h m F 2 (peak height F 2 layer) parameter together with a mathematical bidimensional model of the CII instability. The results present a very good agreement between the simulation results and the ionospheric observations, suggesting that the method presented here may be a promising tool in the search of the capability of prediction of such plasma irregularities. 2. Methodology Two important parameters, which decide the evolution of the EPB, are the PRVD and the seeding nature of the GWs. Both parameters may be deduced from the Digisonde data [Abdu et al., 2009a]. The primary decisive parameter is the PRVD, while for a given favorable PRVD, EPB may or may not grow, depending on the nature of seeding [Kherani et al., 2009]. Both these parameters vary on a day-to-day basis, turning forecasting of the EPB into a challenging task. We propose the following hybrid approach of observation and simulation for the forecasting: for a seasonally averaged PRVD which is a well-known and well-documented parameter [Fejer et al., 2008], the EPB evolution is examined from the simulation of the CII, with the day-to-day varying seeding nature of the GWs as an input parameter, as deduced from the Digisonde. The seasonally averaged PRVD itself may be derived from the Digisonde data and will be considered as the representative PRVD for every day in a given season, therefore leaving the GW amplitude as the only parameter to be deduced. The amplitude of the GWs may be deduced between 1 h to 30 min prior to the occurrence of the peak of the PRVD. Both of them are then fed into the simulation to monitor the eventual evolution of the EPB. Such numerical experiments are carried out for several days where the occurrence/nonoccurrence of the EPB is known from the observations. The simulation results are then compared with the observations to evaluate the efficiency of the forecasting method proposed here. In this study the attention was focused in geomagnetic calm periods (Kp index always less than 4). Furthermore, the period presented here corresponds to an equinox with very quiet solar conditions, namely, September and October, This choice allows the handling of moderate PRVD magnitudes, minimizing this way, the hypothesis of the EPB seeding through longitudinal PRVD gradient [Sousasantos et al., 2013]. Moreover, the geomagnetic quiet condition will minimize external influences on the ionospheric frame under analysis Estimation of Seasonally Averaged Vertical Drift (V D ) The vertical drift (V D ) was calculated through the Digisonde true height and the h m F 2 parameter data from a Brazilian low-latitude station, namely, São Luís (geographic coordinates: S, W; dip angle: 2.7 ) for a set of days in which the EPB activity varied. The h m F 2 data follow directly from the ionogram analysis. Recorded ionograms are manually edited with SAO-explorer software, assuring the reliability of the data, being then converted to true height electron density profiles by the NHPC program [Huang and Reinisch, 1996]. The SAO-explorer software was developed by the Center of Atmospheric Research of the SOUSASANTOS ET AL. EPB FORECAST 2080

3 Figure 1. Standard vertical drift assumed as background profile input in the mathematical simulation for geomagnetic quiet, low solar flux equinox conditions. University of Lowell Massachusetts, and it is available on The EPBs have been reported for the first time over the brazilian region by Sobral et al. [1980a, 1980b]. The calculation of the vertical drift is given by V D ¼ dðhmf2þ dt. Based on the day-to-day vertical drift for h m F 2, an average vertical drift was calculated representing the standard vertical drift for equinox, low solar flux period with geomagnetically calm conditions, following V D ¼ no days i¼1 ðv D Þ ðno daysþ (no days = 22 days). These results are in complete agreement with those presented in Fejer et al. [1991] for similar conditions (Figure 1 in their paper). The standard vertical drift profile is shown in Figure 1. The maximum value which the PRVD reaches is around 30 m/s, and no fluctuations are present on the vertical motion in the neighborhood of the PRVD starting time. This profile was taken as our background profile for simulation purposes, and it will represent the initial conditions of the ionospheric vertical drift in all the mathematical simulations (actually, the model presented in Fejer et al. [1991] can be used with identical results). The h m F 2 parameter was selected mainly because it is based on the true height being a more reliable data. Furthermore, h m F 2 is always at higher altitudes ( 300 km), where recombination processes do not interfere significantly in the vertical drift calculation, so the oscillations may be extracted in a more precise way. Since the solar and geomagnetic conditions were approximately the same for the entire data set, and once gravity waves are known modulators of the electrodynamics of the ionosphere, it was assumed that, presumably, the deviation of the daily measured dh m F 2 /dt, i.e., daily-standard is caused due to the gravity wave action Estimation of Gravity Wave Amplitude (ΔV D ) In Figure 2, a more general V D = dh/dt is shown where h is the true height at frequencies 5, 6, and 7 MHz and f o F 2 (the critical frequency of the F 2 layer). V D represents the vertical drift of the ionosphere mainly arising from the east-west electric field. This electric field is generated by wind dynamo mechanism in which the global-scale tidal wind and small-scale gravity wave wind contribute. The phase propagation was used as a confirmation of the gravity wave presence, in order to assure the validity of the analysis. In case of no phase propagation it was assumed that no gravity waves are acting on the F-region altitudes [Abdu et al., 2009a]. This phase propagation may be easily identified in the figure and is indicated with black dotted lines. The phase propagation as shown in Figure 2 ensures the presence of GW; however, the amplitude of GW cannot be determined from V D. In order to extract the amplitude of GW, a band-pass filter between 20 min and 3 h is generally applied to V D [Abdu et al., 2009a]. This filtered V D is denoted by ΔV D and is referred as the drift SOUSASANTOS ET AL. EPB FORECAST 2081

4 Figure 2. Vertical drift calculated based on true height (5 MHz, 6 MHz, and 7 MHz) and h m F 2. The black dotted lines represent the phase propagation of gravity waves in the ionospheric F region in hours just before the PRVD initiation. disturbance. The amplitude of gravity wave may be considered approximately equal to ΔV D prior to the occurrence of the PRVD and the EPB [Abdu et al., 2009a]. However, in the present study, since the seasonally averaged V D will be used in the simulation instead of filtering V D, ΔV D is estimated by subtracting V D from the seasonally averagedv D as shown in Figure 1, i.e., ΔV D = V D V D. It should be pointed out that ΔV D is estimated for f o F 2 since V D in Figure 1 is estimated for f o F 2 due to the already exposed reasons and only until the PRVD onset. In essence, the presence of GW is confirmed from V D by plotting it at more than one frequency, as shown in Figure 2, while the amplitude of GW, i.e., ΔV D, is estimated through the frequency f o F 2.In Figure 3, ΔV D is shown for equinoctial season five selected days (11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October 2009, dd/mm/yyyy format) representing the general results of our data set. Also, the EPB activity as observed from the CCD all-sky imager was analyzed. The main purpose is to analyze these five different curves within a time interval focused in hours just before the PRVD Figure 3. Amplitude of ΔV D for five different days in the data set, namely, 11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October The oscillatory amplitude is noticeably different in each day. The magnitudes of ΔV D for 11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October 2009 were 3, 6, 15, 17, and 9, respectively. SOUSASANTOS ET AL. EPB FORECAST 2082

5 starting in order to concentrate the attention in the presence of gravity wave velocities with considerable amplitudes, constituting potential aspirants to CII triggers and eventually leading to the subsequent EPB development. The amplitudes of the velocities were extracted by using critical point analysis (inflexion points) of ΔV D, and it was assumed that the maximum difference between such values represents the amplitude of vertical GW velocity since these velocities are the manifestation of a vertical component beyond the background usual behavior. Digisonde data are generated at each 10 min and then transmitted in near real time to the databases of the server where the data can be instantaneously accessed and the calculations may be performed within few minutes. From Figure 3, the amplitude of gravity wave velocities clearly varied widely in the day-to-day dynamics, in such a way that the magnitudes of ΔV D for 11 October 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October 2009 were 3, 6, 15, 17, and 9 m/s, respectively. Based on this variable feature, this information was included in our mathematical model; hence, the value of gravity wave vertical velocity amplitude was set as the seeder for the instability in the numerical approach in each day of the data set. Gravity waves are known to be an efficient seeder of F-region instabilities, leading to the development of the EPBs. Evidences for such assertive were found with both observations [Abdu et al., 2009a] and numerical simulations [Huang and Kelley, 1996; Kherani et al., 2009]. This proposition is used to establish the relation between the presence of gravity waves, their characteristics, and the development of the EPBs in the numerical model for each day, since the main threshold condition for the CII development, i.e., the magnitude of the PRVD >22 m/s [Farley et al., 1970; Abdu et al., 1983; Fejer et al., 1999; Abdu et al., 2009b], was present during the entire data set, a given GW could, hypothetically, find the proper conditions to seed the CII. The results are compared with data from the post-prvd Digisonde information and the CCD all-sky images (6300 Å). The CCD all-sky images were located at São João do Cariri (geographic coordinates: S, W; dip angle: 11 ). Since São João do Cariri is located eastward from São Luís, in case of occurrence of an EPB event the structure would pass through São João do Cariri imager some few hours after reaching higher altitudes. 3. Simulation Model The mathematical model corresponds to a 2-D grid (altitude-longitude) perpendicular to the geomagnetic field numerical representation. The following set of equations [Kherani et al., 2004] were adopted: n t þ nu e ¼ βn αn 2 (1) J ¼ e n u i u e ¼ 0 (2) where, u i;e ¼ κ i;e 1 1 þ κ 2 v i;e b _ þ i;e 1 þ κ 2 v i;e (3) i;e v i;e ¼ C s 2 i;e logðnþþ g h þ b i;e E þ W i B 0 þ W (4) ν i;en ν i;en Equations (1) and (2) stand for the ion continuity and the divergence free current equations. Here subscripts i and e refer to ions and electrons, respectively. A macroscopic charge neutrality condition was assumed (n i = n e = n) as ensured by equation (2). u i;e are the ion and electron velocities, respectively, obtained through neglecting the inertial dynamics compared to the collisional dynamics in the corresponding momentum equations. β corresponds to the loss of electrons through charge exchange process, and α corresponds to the recombination process losses. Furthermore, κ i,e represent the ratios of gyrofrequencies (Ω i,en ) to collision frequencies (ν i,en ) of a given species. The mobilities are represented by b i;e ¼ m e i;e ; m i,e are the mass of corresponding species. E is the electric field in the neutral wind (W ) frame, and B 0 is the magnetic field at the _ equator, b being the unit vector on its direction. Moreover, C si,e are the thermal velocities of ions and electrons, respectively, although in our study diffusion effects were neglected, since large-scale phenomena were the main interest. SOUSASANTOS ET AL. EPB FORECAST 2083

6 Assuming an electrostatic perturbation (δ E ¼ Φ) of the electric field, in such a way that E ¼ E 0 Φ, (E 0 being the undisturbed total electric field in the frame of neutral wind) and substituting into equation (2) the perturbed potential for F region as described by the equation (5) may be obtained. σp Φ ¼ B 0Δ u 0 σ P (5) where Δ u 0 ¼ E B þ g ν b _ B 2 This coupled closed system of equations is then solved through finite difference methods, applying Crank- Nicolson implicit scheme and Successive-Over-Relaxation algorithm evolving with time. The simulation domain occupies a 2-D Cartesian plane corresponding to an equatorial plane, perpendicular to the Earth s magnetic field which consists of longitude (x) and altitude (y). The domain covers km in altitude and km in longitude with equal grid resolutions, Δx = Δy = 5 km. At t = 0, a sinusoidal wind perturbation (W) with wavelength of 200 km is given along the longitude. The amplitude of this perturbation varies from a day-to-day basis and is derived from the Digisonde between 1 h and 30 min prior to the occurrence of the peak of the PRVD, and the model can produce the results within 5 min. V D as shown in Figure 1 and ΔV D as shown in Figure 3 were inserted as inputs in the simulation model of the CII. Both V D and ΔV D entered in the simulation model through the velocity as given by the equation (4) and these contributions may be written in the following form: S V ¼ μ P E 0 þ E W þ δ E þ μ H E 0 þ E W þ δ E b _ (6) where μ P and μ H are the Pedersen and the Hall mobilities, respectively. Also, _ _ _ _ E 0 ¼ μ H V D x þ μp V D y and W E ¼ μ H ΔV D x þ μp ΔV D y, in such a way that E 0 and E W are the ambient and GW-associated electric fields, respectively; moreover, V D and ΔV D are considered to be strictly in the vertical direction. 4. Results and Discussion In the current paper, 5 days from the data set are presented representing the general results of our analysis, strictly speaking, 11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October Data from 22 October 2009 were omitted here because there was no CCD all sky images before 22h30LT; however, its results follow narrowly those ones for 11October 2009, whether through observations (Digisonde and later CCD-All sky images) or via numerical modeling results. On each day, the simulation begins at the time of maximum ΔV D showed in Figure 3. One can notice that ΔV D shows periodic nature, which, in fact, may be translated in terms of the spatial variation in the east-west direction with a given wavelength. Due to the similar periodic nature for every day in the data set, a 200 km horizontal wavelength for every day in the data set was assumed. Markedly, a strong dependence of the EPB development on the gravity wave characteristic was found. In all the treatises about the EPB and instabilities in equatorial the F-region ionosphere, the central role of the PRVD is well discussed; however, despite the fixed profile for the PRVD in the simulations for all the days in our data set, the EPB activity was found to be largely variable, and always following the gravity wave trend. By this hypothesis, it may be argued that knowing the gravity wave activity in hours before the beginning of the PRVD, the possibility of forecasting whether the EPB development will occur may be achieved with at least min of antecedence through numerical modeling and Digisonde data. Since the time evolution of the PRVD may be hypothetically assumed, based on decades of previous extensive studies about its behavior and dependence on solar activity, seasonality, and geomagnetic conditions, such restriction may be removed from our analysis. This approach provides an emancipation of the short-term forecasting, given the independence between the results and the day-to-day PRVD information. Since the inputs of the numerical model are the standard vertical drift (V D ; showed in Figure 1) and the ΔV D calculated in each day when events of gravity wave phase propagation were verified in the true-height Digisonde data, specifically, in hours surrounding the PRVD beginning, thus, the numerical procedure may be done before the PRVD event in each day. SOUSASANTOS ET AL. EPB FORECAST 2084

7 Figure 4. (top left) The evolution of maximum velocities (upward) inside the depletions for each day. (top middle, top right, bottom left, bottom middle, and bottom right) The evolution of EPB in the numerical model for 11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October 2009, respectively for a given instant in time in which EPBs are expected to be already well developed. Figure 4 shows the model results for each day (11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October 2009) under different ΔV D conditions. In Figure 4 (top left), the temporal evolution of the depletion maximum drift velocity is plotted. These velocities indicate the movement inside the depletion rising and reaching higher geomagnetic field lines, where after some time this depletion may be mapped through the geomagnetic field lines to off-equator latitudes and are in agreement with observations [e.g., Abdu et al., 1983; Dabas and Reddy, 1990] and numerical previous studies [e.g., Ossakow and Chaturvedi, 1978]. The black (18 October 2009, ΔV D = 17), blue (13 October 2009, ΔV D = 15), and magenta (19 October 2009, ΔV D = 9) curves reach 280 m/s within a period less than 2500 s, constituting ideal conditions in which instabilities driven by gravity waves can evolve into EPB structures, capable to reach higher altitudes. On the other hand, the green curve corresponding to the results for 11 October 2009 ( ΔV D = 6) presents a slower increase in its velocity demanding a larger time to reach the multiexponential growth and subsequently the development of the EPB structure. A noticeable more circumspect behavior may be verified in the red curve, representing the results for 11 September 2009 ( ΔV D = 3) in such a way that the velocity was not capable to reach 280 m/s within our simulation temporal window, in an opposite way to what was verified on the other days in Figure 4, thus, corresponding to a non-epb event. These trends follow narrowly the ΔV D tendency, exactly as our hypothesis had foreseen. It must be emphasized that the black curve (18 October 2009) behaves in a more accelerated manner; the blue curve (13 October 2009) presents a bit slower growth in its velocity, being followed by the magenta curve (19 October 2009), and then the green curve (11 October 2009); and finally, the red curve (11 September 2009) which is not capable to reach the velocity achieved in the other simulation cases, and these are exactly the same trends which were found in ΔV D values. Figure 4 (top middle, top right, bottom left, bottom middle, and bottom right) present the evolution of the EPB in our mathematical model for 11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October 2009, respectively, for a given instant in time when the EPBs are expected to be already well developed. The color bar indicates the electron density in m 3. The drop-shaped region in the middle of each panel corresponds to the EPB structure, rising toward higher altitudes. The initial background density SOUSASANTOS ET AL. EPB FORECAST 2085

8 Figure 5. CCD all-sky data for some days in the data set. (top left; 11 September 2009) The absence of EPB structures. (top middle; 11 October 2009) The presence of a small structure may be verified at the top of the image. (top right; 13 October 2009) A well-developed EPB structure with branches spreading along field lines. (bottom left; 18 October 2009) A much more intense EPB structure may be noticed. (bottom right; 19 October 2009) Again, a considerable EPB structure may be seen. profile is also presented in the left side of each of these plots, and its maximum is m 3. A standard initial background density profile was used, since a later complete density profile from Digisonde, close to the PRVD onset, would decrease significantly our forecasting power. Furthermore, such profile does not vary widely under similar solar and geomagnetic conditions, and the eventual small fluctuations would not imply in any significant difference in the simulation results. The results presented in Figure 4 (top left) are considerably elucidated by these complementary panels. The characteristics of EPB evolution for each one of these days follow narrowly the trend discussed just above despite the presence of the same PRVD condition, namely, the standard assumed. A wider investigation demonstrates the same results for the whole data set with negligible dissimilitudes. The EPB structure on 13 October 2009 is well developed before 4400 s and reaches above 450 km of altitude. Similarly, in 18 October 2009 the EPB is already present, but at the same instant the structure reaches higher altitudes (>550 km) and may be mapped toward off-equator latitudes. In 19 October 2009 the EPB structure is also present, but showing a subtle development and lying between 400 and 450 km at the same instant, in a very similar way to the presented on 13 October The panel showing the result for 11 October 2009 presents an instability growing at a much slower speed, which was not able to overpass the topside of the F region and may represent a weak EPB structure, increasing with time under some specific conditions. On the other hand, when verifying results for 11 September 2009, one can notice the inefficacy of the EPB structure in attaining apex altitudes, i.e., the instability was not capable to give rise to an EPB structure which reaches higher altitudes, perhaps, only corresponding to a weak spread-f event on the F-region bottomside. For further validation of our proposition the numerical simulation was compared to the CCD all-sky data (6300 Å). Figure 5 shows the images from the CCD all-sky located at São João do Cariri, eastward from São Luis station (from where the Digisonde data were taken), between 20:44 and 20:47 LT (23:44 and 23:47 UT) SOUSASANTOS ET AL. EPB FORECAST 2086

9 corresponding approximately to min after the PRVD peak at São Luis latitudinal sector, when the EPBs generated are already expected to be registered in the CCD all-sky images. In Figure 5(top left) (11 September 2009) the absence of the EPB structures is clear, and even in later hours no EPB event was found. In Figure 5 (top middle) (11 October 2009) a small structure may be noticed at the top of the image (the darker region), indicating a slower evolution and later mapping through field lines. Figure 5 (top right) (13 October 2009) shows a well-formed EPB structure with branches spreading along the field lines. Figure 5 (bottom left) (18 October 2009) presents a huge structure with extensive regions of depletion and branches propagating along the field lines over the entire image. Again, in Figure 5 (bottom right) (19 October 2009) one can notice a well-developed structure spreading along the geomagnetic field lines analogously to the results for 13 October The behavior registered on the images matches tightly to the results found in our simulations. Even though the time presented in Figure 5 may be a bit longer than the simulation results, one must notice that some structures are already present in prior hours; also, one more realistic 3-D model would also present a considerable slower evolution due to the parallel current, absent in our 2-D model. In the presence of such parallel current, inexorably, the accumulation of charges driving the instability growth will decrease. This current offers an alternative path with much larger conductivity in the direction of the geomagnetic field lines. Acknowledgments The authors would like to thank CAPES and CNPQ (under process: /2015-1) for financial support to carry out this work. E.A.K. wishes to acknowledge FAPESP-2011/ funding. Also, we acknowledge the DAE/INPE for kindly providing the Digisonde and 6300 Å all-sky imager data. We wish to thank also the reviewers for their valuable comments and contribution to improve this work. The authors wish also to thank the Editor for his useful comments and guidance. The Digisonde data used in this study may be available by contacting the Responsible Coordinators in DAE/INPE (Inez S. Batista, inez. batista@inpe.br). 5. Summary and Conclusions This work presents an alternative approach to the EPB forecasting based on the simulation of the CII with true height inputs derived from the Digisonde data. A solar minimum, geomagnetic calm, equinox period was selected to avoid external influences in the results, strictly speaking, September and October From the Digisonde true height data for 5, 6, and 7 MHz and h m F 2 (peak height F 2 layer) the vertical drifts were extracted; furthermore, a standard profile for the vertical drift was constructed based on the average covering the data set (strictly with h m F 2 information on each day). Subsequently, the verification of the presence/absence of the propagation of gravity wave phase in the vertical drift calculated following Abdu et al. [2009a] was done. In every case, when the presence of such GWs was perceived, the attention was focused on their oscillatory amplitude, i.e., the gravity wave vertical velocity amplitude. Afterward, the value of the amplitude of the gravity wave vertical velocity was subtracted from the standard vertical velocity amplitude in the time interval window nearby the PRVD onset (18:00 21:00 UT). Such value was defined as ΔV D, representing the particular characteristic from each day. The standard vertical drift was then inserted in our mathematical model of the CII instability as the ionospheric background profile. Also, the ΔV D values were inserted as an input in our simulations, similarly to the procedure adopted by Kherani et al. [2009]. Finally, the model results were compared to the images registered by the CCD all-sky in the red line emission (6300 Å). Five days are presented, representing the general results found in the whole data set, namely, 11 September 2009, 11 October 2009, 13 October 2009, 18 October 2009, and 19 October Even though a very similar behavior may be expected in the numerical simulations for these days, since one of the major rulers of such phenomena, the PRVD, is fixed (the standard profile) the results present a considerable variation. The CCD data strongly corroborate with our modeling results, and this trend was verified every day inside the data set, with few imperceptible discrepancies. This is the first step toward a more complete investigation of the viability of such hypothesis, but the results seem to be promising and may offer an alternative way to forecast the EPB events with at least min of antecedence. A more detailed analysis may help further validation of the method/approach presented here, for a variety of climatological conditions including different solar cycle phases. References Abdu, M. A., R. T. de Medeiros, J. H. A. Sobral, and J. A. Bittencourt (1983), Spread F plasma bubble vertical rise velocities determined from spaced ionosonde observations, J. Geophys. Res., 88, , doi: /ja088ia11p Abdu, M. A., E. A. Kherani, I. S. Batista, E. R. de Paula, D. C. Fritts, and J. H. A. Sobral (2009a), Gravity wave initiation of equatorial spread F/plasma bubble irregularities based on observational data from the SpreadFEx campaign, Ann. Geophys., 27, , doi: / angeo Abdu, M. A., I. S. Batista, B. W. Reinisch, J. R. de Souza, J. H. A. Sobral, T. R. Pedersen, A. F. Medeiros, N. J. Schuch, E. R. de Paula, and K. M. Groves (2009b), Conjugate Point Equatorial Experiment (COPEX) campaign in Brazil: Electrodynamics highlights on spread F development conditions and day-to-day variability, J. Geophys. Res., 114, A044308, doi: /2008ja Anderson, D. N., B. W. Reinisch, C. Valladares, J. Chau, and O. Veliz (2004), Forecasting the occurrence of ionospheric scintillation activity in the equatorial ionosphere on a day-to-day basis, J. Atmos. Sol.-Terr. Phys., 66, , doi: /j.jastp Carter, B. A., et al. (2014a), Geomagnetic control of equatorial plasma bubble activity modeled by the TIEGCM with Kp, Geophys. Res. Lett., 41, , doi: /2014gl SOUSASANTOS ET AL. EPB FORECAST 2087

10 Carter, B. A., et al. (2014b), Using solar wind data to predict daily GPS scintillation occurrence in the African and Asian low-latitude regions, Geophys. Res. Lett., 41, , doi: /2014gl Dabas, R. S., and B. M. Reddy (1990), Equatorial plasma bubble rise velocities in the Indian sector determined from multistation scintillation observations, Radio Sci., 25, , doi: /rs025i002p Farley, D. T., B. B. Balsley, and R. F. Woodman (1970), Equatorial spread F: Implications of VHF radar observations, J. Geophys. Res., 75(34), , doi: /ja075i034p Fejer, B. G., E. R. de Paula, S. A. González, and R. F. Woodman (1991), Average vertical and zonal F region plasma drifts over Jicamarca, J. Geophys. Res., 96, 13,901 13,906, doi: /91ja Fejer, B. G., L. Scherliess, and E. R. de Paula (1999), Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F, J. Geophys. Res., 104, 19,859 19,870, doi: /1999ja Fejer, B. G., J. W. Jensen, and S.-Y. Su (2008), Quiet time equatorial F region vertical plasma drift model derived from ROCSAT-1 observations, J. Geophys. Res., 113, A05304, doi: /2007ja Huang, C. S., and M. C. Kelley (1996), Nonlinear evolution of equatorial spread F: 2. Gravity wave seeding of Rayleigh-Taylor instability, J. Geophys. Res., 101(A1), , doi: /95ja Huang, C. S., M. C. Kelley, and D. L. Hysell (1993), Nonlinear Rayleigh Taylor instabilities, atmospheric gravity waves, and equatorial spread-f, J. Geophys. Res., 98, 15,631 15,642, doi: /93ja Huang, X., and B. W. Reinisch (1996), Vertical electron density profiles from the Digisonde network, Adv. Space Res., 18(6), , doi: / (95) Huba, J. D., and G. Joyce (2007), Equatorial spread F modeling: Multiple bifurcated structures, secondary instabilities, large density biteouts, and supersonic flows, Geophys. Res. Lett., 34, L07105, doi: /2006gl Huba, J. D., and G. Joyce (2010), Global modeling of equatorial plasma bubbles, Geophys. Res. Lett., 37, L17104, doi: /2010gl Kelley, M. C., and J. Retterer (2008), First successful prediction of a convective equatorial ionospheric storm using solar wind parameters, Space Weather, 6, S08003, doi: /2007sw Keskinen, M. J., S. L. Ossakow, and B. G. Fejer (2003), Three-dimensional nonlinear evolution of equatorial ionospheric spread-f bubbles, Geophys. Res. Lett., 30(16), 1855, doi: /2003gl Kherani, E. A., and A. K. Patra (2015), Fringe field dynamics over equatorial and low-latitude ionosphere: A three-dimensional perspective, J. Geophys. Res. Space Physics, 120, , doi: /2015ja Kherani, E. A., E. R. de Paula, and F. C. P. Bertoni (2004), Effects of the fringe field of Rayleigh-Taylor instability in the equatorial E and valley regions, J. Geophys. Res., 109, A12310, doi: /2003ja Kherani, E. A., M. Mascarenhas, J. H. A. Sobral, E. R. de Paula, and F. C. Bertoni (2005), A three dimension simulation model of collisional interchange instability, Space Sci. Rev., 121, , doi: /s x. Kherani, E. A., M. A. Abdu, E. R. de Paula, D. C. Fritts, J. H. A. Sobral, and F. C. de Meneses Jr. (2009), The impact of gravity waves rising from convection in the lower atmosphere on the generation and nonlinear evolution of equatorial bubble, Ann. Geophys., 27, , doi: /angeo Ossakow, S. L., and P. K. Chaturvedi (1978), Morphological studies of rising spread F bubbles, J. Geophys. Res., 83(A5), , doi: / JA083iA05p Ossakow, S. L., S. T. Zalesak, B. E. McDonald, and P. K. Chaturvedi (1979), Nonlinear equatorial spread F: Dependence on altitude of the F peak and bottomside background electron density gradient scale length, J. Geophys. Res., 84, 17 29, doi: /ja084ia01p Ott, E. (1978), Theory of Rayleigh-Taylor bubbles in the equatorial ionosphere, J. Geophys. Res., 83(A5), , doi: / JA083iA05p Raghavarao, R., R. Sekar, and R. Suhasini (1992), Nonlinear numerical simulation of equatorial spread F Effects of winds and electric fields, Adv. Space Res., 12, , doi: / (92) Redmon, R. J., D. Anderson, R. Caton, and T. Bullett (2010), A Forecasting Ionospheric Real-time Scintillation Tool (FIRST), Space Weather, 8, S12003, doi: /2010sw Retterer, J. M. (2005), Physics-based forecasts of equatorial radio scintillation for the Communication and Navigation Outage Forecasting System (C/NOFS), Space Weather, 3, S12C03, doi: /2005sw Retterer, J. M., D. T. Decker, W. S. Borer, R. E. Daniell Jr., and B. G. Fejer (2005), Assimilative modeling of the equatorial ionosphere for scintillation forecasting: Modeling with vertical drifts, J. Geophys. Res., 110, A11307, doi: /2002ja Scannapieco, A. J., and S. L. Ossakow (1976), Nonlinear equatorial spread F, Geophys. Res. Lett., 3, , doi: /gl003i008p Sobral, J. H. A., M. A. Abdu, I. S. Batista, and C. J. Zamlutti (1980a), Association between plasma bubble irregularities and airglow disturbances over Brazilian low latitudes. Geophys. Res. Lett., 7(11), , doi: /gl007i011p Sobral, J. H. A., M. A. Abdu, and I. S. Batista (1980b), Airglow studies on ionosphere dynamics over low latitude in Brazil, Ann. Geophys., 36(2), Sousasantos, J., E. A. Kherani, and J. H. A. Sobral (2013), A numerical simulation study of the collisional-interchange instability seeded by the pre-reversal vertical drift, J. Geophys. Res. Space Physics, 118, , doi: /2013ja Zalesak, S. T. (1979), Fully multidimensional flux-corrected transport algorithms for fluids, J. Comput. Phys., 31, , doi: / (79) Zalesak, S. T., and S. L. Ossakow (1980), Nonlinear equatorial spread F: Spatially large bubbles resulting from large horizontal scale initial perturbations, J. Geophys. Res., 85, , doi: /ja085ia05p Zargham, S., and C. E. Seyler (1987), Collisional interchange instability: 1. Numerical simulations of intermediate-scale irregularities, J. Geophys. Res., 92, 10,073 10,088, doi: /ja092ia09p SOUSASANTOS ET AL. EPB FORECAST 2088