Semimonthly oscillation observed in the start time of equatorial Spread-F

Using data from airglow an all sky imager and a coherent backscatter radar deployed at São João do Cariri (7.4S, 36.5W) and São Luís (2.6S, 44.2W), respectively, the start time of equatorial Spread-F were studied. Data from a period of over 10 years was investigated from 2000 to 2010. The semimonthly oscillations were clearly revealed in the start time of plasma bubbles from Oi6300 airglow images during three periods (September 2003, September-October 2005, November 2005 and 5 January 2008). Since the airglow measurements are not continuous in time, more than one cycle of oscillation in the start time of plasma bubbles cannot be observed from these data. Thus, coherent backscatter radar data appeared as an alternative to investigate the start time of the ionospheric irregularities. Semimonthly oscillation were observed in the start time of plumes (November 2005) and bottom type Spread-F (November 2008) with at least one complete cycle. Technical/climate issues did not allowed to observe the semimonthly oscillations simultaneously by the two instruments, but from September to December 1


Introduction
Equatorial plasma bubbles (EPBs) appear in the bottom side of the F-region in the equatorial ionosphere when there is an unstable F-layer.It generally occurs after the pre-reversal enhancement (PRE), after sunset.The pre-reversal enhancement consists of a rapid up shift of the F layer before the motion of the plasma be downward reverted.The main mechanism used to explain the development of the EPBs is the Rayleigh-Taylor (RT) instability.According to the theory, the RT growth rate is inversely proportional to the collision frequency between the neutral and ionic particles and it is proportional to the plasma density gradient.Thus, when the PRE is strong, it becomes more probable for EPBs occur.
Airglow measurements of the OI 630.0 nm (OI6300) have been recorded at São João do Cariri (7.4 o S, 36.5 o W) since September 2000.In this investigation data from September 2000 to December 2010 were used, which corresponds to the first generation of the all sky imager deployed in this observatory.
The all sky imager is composed by a fish eye lens, a telecentric set of lens, a filter wheel, a set of lens to reconstruct the image, a Charge Coupled Device (CCD) chip and a cooling system.This instrument has a field of view of 180 o of the sky.
Further details of this imager have been published elsewhere (e.g., Paulino et al., 2016).Airglow images of the OI6300 were taken by about 15 days around the New Moon with integration time of 90 s.Depending on the mode of operation, images can have 2-4 min of temporal resolution.The start and end times can be extracted directly from the image header after observing the appearance or disappearance of the structures.The start time was defined as the time when the plasma bubbles appeared in the images.It generally occurs in the Northwest part of the images.After that, the plasma bubbles start their development and dynamics.Figure 1 shows an example of the determination of the start time of EPBs on 27 January 2001.The supplementary short movie can help one to identify the time, in which the plasma bubbles start to extend to the southern part of the images.The altitude resolution is 2.5 km and noise band width of 120 kHz.These characteristics allow to observe irregularities of 5 m in the ionosphere.Further technical details of the São Luís' radar can be found in de Paula and Hysell (2004) and Rodrigues et al. (2008).
Start time of Plume and BTSF were defined in Cueva et al. (2013).Those parameters correspond to the exact time of appearance of plumes and BTSFs in the range time integration (RTI) maps.The temporal resolution of the start time of the spread-F calculated in the RTI maps were 12 min.Simulations have shown that the 16d planetary waves (PWs) have large amplitudes in the winter hemisphere at the lower levels of the atmosphere and high latitudes, but above the mesosphere there is a penetration of this wave to the summer hemisphere, which allows it to be observed in both hemispheres including in the equatorial region (Miyoshi, 1999).Forbes and Leveroni (1992) have pointed out that 16d oscillation in the E and F-region could be connected by the upward propagation of Rossby wave from the winter stratosphere.Although the 16d PW have a well defined seasonality in the lower atmosphere, according to the simulations, in the upper atmosphere the presence of this oscillation has been predicted to be more spread along the year (Miyoshi, 1999).It is also important to mention that the 16d oscillations were observed in the mesosphere and lower thermosphere from 85 to 100 km altitude in the equatorial region in the zonal wind during the period around the September equinox and solstices of 1994 (Luo et al., 2002), which coincides to the periods of observation of the present results.

Results and Discussion
Lunar semidiurnal tides have been pointed out as important factor to the appearance and the start time of EPBs.The main reason for the influence of the Lunar tides in the EPB variability is the capability of the lunar tides propagate upward to high levels of the atmosphere and consequently it can affect the pre-reversal enhancement (PRE) amplitude and time (Stening and Fejer, 2001).Another factor to be considered is the moon phase (New Moon) that coincides to zero position of the oscillation for all observed cases, including the case studies observed from the coherent backscatter radar that will be shown ahead.The real mechanism that allows the lunar tides to act in the PRE is not well defined, but some works have pointed out as either the direct propagation to the bottom side of ionospheric F region (e.g., Evans, 1978;Forbes, 1982) or coupling of the E region dynamo to the F region (e.g., Immel et al., 2009;Eccles et al., 2011).
In order to corroborate the present results, data from the backscatter radar deployed in São Luís have been used to investigate the start time of the bottom type spread-F (BTSF) and plumes.The main goal of these analysis is trying to observe more than one cycle of the semimonthly oscillation in the start time of spread-F, since the radar was operating continuously and does not depend on tropospheric weather conditions.Figures 3 and 4 show that the spread-F structures can be controlled by the semimonthly oscillation.However, the strong day-to-day variability of the spread-F does not allow to observe this signature always.Another difficulty in the radar data analysis was the algorithm does not give an exact start time of the oscillation, i.e., there was a temporal resolution of 15 min in The present results indicate that one semimonthly dynamical structure can control either the start time or the amplitude of the PRE that can consequently produce Spread-F.These results must contribute to understanding the day-to-day variability of equatorial spread-F.However, the results shows that besides the semimonthly oscillations, other phenomena are important to the day-to-day variability occurrence of EPBs since this oscillations is not dominant in the whole period of observation.
Regarding to the agents that are causing this oscillation, further investigation are necessary and they are out of the scope of this work.Lunar semidiurnal tides, which have semimonthly period of oscillation have been pointed out as a likely agent to produce this kind of oscillation in the start time of Spread-F.Besides, we have discussed the importance of 16d PWs that must be further investigated before being neglected.
2008.The VHF coherent radar of São Luís operates at 30 MHz with a power peak of 4 kW.It has antenna half-power-fullbeam-width of 10 o and inter-pulse-period of 9.34 ms.The coverage in altitude of 87.5 to 1267 km and velocity of ± 250 ms −1 .

Figure 2
Figure 2(a) shows the evolution of start time of the EPBs observed in September 2003 over São João do Cariri.The solid linerepresents the best fit for a periodicity of 14.5 days, the stars correspond to the exact time in which the plasma bubble appeared in the OI6300 images and the filled circle shows the New Moon time.In this case, one can see a good agreements of the fit line with the observation during a half cycle of the oscillation.The amplitude of this oscillation was calculated from the fitting as ∼52 min, i.e., there was a difference of ∼52 min in the start time of EPBs along the observed nights.

Figure 2
Figure 2(b) shows the best fit 14.5 days oscillation in the start time of EPBs observed from the later September to early October 2005.For the whole period of airglow observation, it was the best case study observed because it covers a full circle of the oscillation.There was an amplitude of ∼37 min and the position of the New Moon was observed on 03 October 205.The predominance of this oscillation persists up to November 2005 as shown in Figure 2(c) with higher amplitude ∼70 min.Similar results to September 2003 and November 2005 were found in January 2008 as one can see in Figure 2(d), inclusive the position of the New Moon in the cycle.The estimated amplitude was ∼45 min.

Figure 2 .
Figure 2. Start time of plasma bubble (stars) as function of time.Solid line represents the best fit to a sinusoidal oscillation with period of 14.5 days.The respective amplitudes are shown on the middle top of the panels.Panel (a) shows the results for September 2003.Panel (b) shows the results for September -October 2005.Panel (c) shows the results for October-November 2005.Panel (d) shows the results for January 2008.Filled circles indicate the New Moons.

Figure 3
Figure3shows a complete cycle of 14.5 days in the start time of plume observed on November 2005, which coincide with the same period of observation of EPBs in the airglow images.For this case, an amplitude of ∼ 1 hour was observed.

Figure 3 .
Figure 3. Same as Figure 2, but for start time of plumes.Open circles indicate the Full Moon.

Figure 4
Figure 4 shows the start time evolution of the bottom type spread-F in November 2008, one can observe two complete cycles of the start time of the BTSF fitting a semi-month oscillation with an amplitude of ∼ 12 min.

Figure 4 .
Figure 4. Same as Figure 3, but for start time of bottom type spread-F.

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. Discuss., https://doi.org/10.5194/angeo-2019-62Manuscript under review for journal Ann.Geophys.Discussion started: 2 May 2019 c Author(s) 2019.CC BY 4.0 License.Using almost one solar cycle of data from OI630 airglow images and range time integration maps from a backscatter radar in the equatorial region over Brazil, semimonthly oscillations in the start time of spread-F (EPBs, BTSF and Plumes) were observed and the results are summarized as follow: -Four periods of airglow observation showed amplitudes higher than 36 min in the start time of EPBs for 14.5 days oscillation, three periods of observations (September 2003, October 2005 and January 2008) revealed good fit for half cycle and the another case (September 2005) showed and complete cycle; -Two complete cycles of 14.5 days with amplitude of ∼ 12 min were observed in the bottom type spread-F in November 2008; Plumes observed in the RTI data showed a 14.5 cycle oscillation with amplitude of 1 hour in the start time of plumes during November 2005; Ann. Geophys.Discuss., https://doi.org/10.5194/angeo-2019-62Manuscript under review for journal Ann.Geophys.Discussion started: 2 May 2019 c Author(s) 2019.CC BY 4.0 License.Ann.Geophys.Discuss., https://doi.org/10.5194/angeo-2019-62Manuscript under review for journal Ann.Geophys.Discussion started: 2 May 2019 c Author(s) 2019.CC BY 4.0 License.