ARTICLE IN PRESS

Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 www.elsevier.com/locate/jastp

Planetary wave signatures in the equatorial atmosphere– system, and mesosphere- E- and F-region coupling

M.A. Abdua,Ã, T.K. Ramkumarb, I.S. Batistaa, C.G.M. Bruma, H. Takahashia, B.W. Reinischc, J.H.A. Sobrala

aNational Institute for Space Research—INPE, Sao Jose dos Campos, 12245-970 SP, Brazil bNational MST Radar Facility, Gadanki, India cUniversity of Massachusetts, Lowell, USA

Available online 8 November 2005

Abstract

Upward transport of wave energy and momentum due to gravity, tidal and planetary waves from below and extra- tropics controls the phenomenology of the equatorial atmosphere–ionosphere system. An important aspect of this phenomenology is the development of small- and large-scale structures including thin layers in the mesosphere and E- region, and the formation of wide spectrum plasma structures of the equatorial F-region, widely known as equatorial spread F/plasma bubble irregularities (that are known to have significant impact on space application systems based on trans-ionospheric radio waves propagation). It seems that the effects of tidal and gravity waves at mesospheric and thermospheric heights and their control of ionospheric densities, electric fields and currents are relatively better known than are the effects originating from vertical coupling due to planetary waves. Results from airglow, radar and ionospheric sounding observations demonstrate the existence of significant planetary wave influence on plasma parameters at E- and F- region heights over equatorial latitudes. We present and discuss here some results showing planetary wave oscillations in concurrent mesospheric winds and equatorial intensity variations in the Indian sector as well as in the mesospheric airglow and F-layer height variation in Brazil. Also presented are evidences of planetary wave-scale oscillations in equatorial evening pre-reversal electric field (F-region vertical drift) and their effects on equatorial spread F / plasma bubble irregularity development and day-to-day variability. r 2005 Elsevier Ltd. All rights reserved.

Keywords: Planetary wave effects; Equatorial ionosphere; Pre-reversal vertical drift; ; MF radar winds; Equatorial spread F

1. Introduction

Vertical coupling in the atmosphere–ionosphere system through atmospheric waves (tidal gravity ÃCorresponding author. Fax: +55(12)3945-6990. E-mail addresses: [email protected] (M.A. Abdu), and planetary waves) that propagate to ionospheric [email protected] (T.K. Ramkumar), heights from the lower regions (tropo–stratosphere) [email protected] (B.W. Reinisch). of their generation, and the associated interactive

1364-6826/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2005.03.019 ARTICLE IN PRESS 510 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 processes, controls the dynamics and phenomenol- 2. PW oscillations in the MLT E- and F-region ogy of the ‘quiet time’ ionosphere. Over the equatorial latitudes the unique feature of the Evidence of PW in the mesosphere has come from horizontal orientation of the ’s magnetic field extended observations by medium frequency lines leads to a condition in which the dynamo (MF) radars operated at different geographic electric fields generated by these waves play the locations (Harris, 1994; Haris and Vincent, 1993; leading role in associated ionospheric response Gurubaran et al., 2001). These waves are very large sequences extending to the upper layers of the horizontal scale atmospheric oscillations in neutral ionosphere. While the vertical coupling processes wind, density and pressure that propagate zonally driven by tidal forcing and the consequent dynamo and vertically from the troposphere–stratosphere electric fields are basically responsible for shaping regions of their generation to the MLT region and the major phenomenology of the equatorial region, perhaps beyond. Their oscillation periods have been the interactive processes involving planetary waves found to lie in the 2- to 20-day range and beyond (PW) and gravity waves are believed to play a (Forbes, 1996, Forbes et al., 1995), the main periods significant role in the day-to-day and short-term being near the normal Rossby modes, i.e., 2-, 5-, 10- variabilities widely observed in the important and 16-days, as well as at other (secondary) periods aspects of this phenomenology: equatorial electrojet resulting from the superposition of these periods. 2- (EEJ) current, F-layer plasma drifts and evening day waves in the MLT region is now well pre-reversal electric field (PRE) that controls the established (see, e.g., Pancheva et al., 2004), as are equatorial spread F (ESF) and related processes. the 5-, 10- and 16-day periods (Forbes et al., 1995). However, evidences on direct association between An important feature of the PW is that their the lower atmosphere wave forcing and the iono- occurrence is of episodic nature so that attention spheric response signatures continue to be sparse in needs to be focused on specific intervals. For this respect. Major sources of perturbations that can example, a study of the 2-day waves in mesospheric obscure a clear identification of the cause–effect zonal and meridional winds over Adelaide by Harris relationship in the vertical coupling processes, in (1994) showed their occurrences for 2–3 week this context, are known to be solar ionizing duration generally following mid-summer, and radiation flux variations and enhanced – Clark et al. (2002) presented mesospheric wind –ionosphere interaction during measurements showing occurrence of 7-day wave of events of disturbances when the zonal wave number S ¼ 1 that lasted more than penetrating/disturbance dynamo electric fields and 20–25 days during August–September 1993. PW of the disturbance wind system dominate the equator- quasi-2-day and 3- to 5-day periodicities in equator- ial region. Thus, only quiet time observational data ial mesospheric winds have been reported by sets will be useful in the investigations of the Gurubaran et al. (2001) and Vincent (1993), variability arising from the vertical coupling pro- respectively, and in mesospheric airglow intensity cesses. The entire vertical coupling process can be by Takahashi et al. (2002). Influence of PW on seen as a two-stage process: (a) dynamic processes ionospheric parameters had been indicated earlier by which the wave energy from the lower heights/ when Brown and Williams (1971) showed correla- regions (troposphere and stratosphere) propagates tion between stratospheric pressure variations and to higher levels in middle atmosphere and electron density. More recently, direct evidence of lower thermosphere (MLT) regions (including the PW role on mid-latitude sporadic E-layer genera- lower E-region heights); (b) the subsequent se- tion, based on f0Es data from an extended long- quences in which the electrodynamic processes take itudinal chain of stations was provided by over at height levels near and above the E-layer Haldoupis and Pancheva (2002). extending well into the F-region. In this paper, we Manifestation of the PW effects in the equatorial intend to present some new results, and briefly ionosphere came to be widely recognized after Chen evaluate some relevant results available recently on (1992) demonstrated 2-day oscillations in the these two aspects and discuss their possible con- intensity of the equatorial anomaly and Forbes sequences to our investigations towards a better and Leveroni (1992) demonstrated 16-day oscilla- understanding of the quiet time variability of the tions in the EEJ intensity and the F2-layer peak equatorial F-region plasma dynamics and ESF density. In view of the theoretical difficulty to irregularity processes. predict their upward propagation to ionospheric ARTICLE IN PRESS M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 511 heights (Hagan et al., 1993; Forbes et al., 1995) dip lat: 12.961). In the upper panel is shown the these results were attributed to the electrodynamic power spectrum of the zonal wind at 88 km, signature of the PW interaction with the dynamo concurrently measured by an MF radar at Tirun- region (lower E-region) of the ionosphere. The wave nelveli (located close to Trivandrum). We may note oscillations in the EEJ over Huancayo, Peru, in these spectra (which correspond to an observa- analyzed by Parish et al. (1994) for an interval in tional interval from 15 March to 26 April 1994) 1985 confirmed the similar periodicities, namely, 2-, prominent spectral peaks near the periodicities at 5-, 10- and 16-day, to those found earlier by Forbes 6.5-day and 14-day in both the EEJ strength and and Leveroni (1992) for an interval in 1979. Thus zonal wind. The prominent periodicities vary with the oscillation at some prominent PW periods in the the observational epoch as other such plots have EEJ appears to be reasonably well established. We demonstrated. These results might seem to indicate have attempted in this paper, as a first step, to verify that PW found at mesospheric heights could the possible association of such oscillations in the propagate up to the dynamo region where they EEJ strength with the oscillations in the MF radar modulate the ionospheric current system associated winds using their concurrent long duration mea- with EEJ at the PW time scales (more discussion on surements realized in the EEJ region over India. this aspect will follow later). Fig. 1 shows, as an example, the power spectrum Additional results based on concurrent data on obtained for the EEJ strength (lower panel), EEJ and MF radar winds for extended intervals are represented by DH, the difference of magnetic field presented in Fig. 2. Here, the contour plot shows the H-component variation over an EEJ station (Tri- time evolution of normalized power spectrum vandrum: 8.481N, 76.951E; dip lat: 0.281) and that determined by ‘‘Morlet wavelet’’ analysis for EEJ over an off-EEJ station (Alibag:18.681N, 72.861N; strength (upper panel) and zonal and meridional winds at 88 km (lower panel) for the first half of Power spectrum of winds & EEJ strength 1999 (left panels) and second half of 1993 (right [15 March to 26 April 1994] panels). The data availability dictated the selection 35 35 zonal wind at 88 km of these intervals. We may note in this figure that 30 30 during the late northern summer month of August 25 1993 the EEJ strength has significant spectral power

2 25 )

-1 for periodicities near 2-, 3-, 5- and 6.5-days. 20 20 Correspondingly, the zonal wind also presents 15 15 significant power at these periodicities, suggesting, thereby, that the mesospheric PW observed at 88 km Power (ms 10 10 might have propagated to dynamo region and/or 5 5 influenced the wind fields there, so that the EEJ strength is modulated at the PW time scales similar 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 to the example presented in Fig. 1. Also to note is the prominent 12–18 days oscillation in the EEJ 5 5 EEJ strength (ABG-TRD) strength from the last week of September till the end of October that is accompanied by a prominent 4 4 12–20-day oscillation in the zonal wind. We note )

2 further that the oscillation in zonal wind precedes 3 3 by 12 days that in the EEJ strength, which might 95 % Confidence level suggest that PW of 12–20 days period observed at 2 2

Power (nT 88 km could take some time (of the order of 12 days) 90 % to propagate to the dynamo region heights 1 1 (105 km) in the ionosphere. There are also frequent cases of oscillations present only in one 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 of the two parameters. Examples are the zonal wind Period of oscillation (days) periodicities of 5-day in July, 8–12-day in August Fig. 1. Power spectrum of the zonal winds at 88 km (top) and and 14–20-day in November–December, and EEJ strength (bottom) during the period of 15 March to 26 April meridional wind periodicities 14-day in August 1994. and 6–11-day in September. Conversely, the ARTICLE IN PRESS 512 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522

Normalaized Morlet wavelet power spectrum (1993 &1999) EEJS (nT) (top), Zon. (mid) & Mer. (bot.) wind (m/s) at 88 km 22 22 20 20 18 18 16 16 14 14 12 12 10 10 8 8 6 6 4 4 2 2

1 31 61 91 121 151 181 211 241 271 301 331 22 22 20 20 18 18 16 16 14 14 12 12 10 10 8 8 6 6 4 4 2 2 Fourier period of oscillation (days) 1 31 61 91 121 151 181 211 241 271 301 331 22 22 20 20 18 18 16 16 14 14 12 12 10 10 8 8 6 6 4 4 2 2

1 31 61 91 121 151 181 211 241 271 301 331 (1 Jan. to 20 Jun.1999) Day number (21 Jun. to 8 Dec. 1993)

Fig. 2. Time evolution of the power spectra determined by Morlet wavelet analysis method for the period of 1 January to 20 June 1999 (left panels) and 21 June to 8 December 1993 (right panels). The top, middle and bottom panels show, respectively, the results for EEJ strength, zonal and meridional winds at 88 km. The contours inside the thick dashed contour lines are above the 95% confidence level. The vertical axis denotes the Fourier periods of oscillation corresponding to the ‘‘Morlet wavelet’’ scales ranging from a fraction of a day to 22 days. The horizontal scale shows the observational period in terms of the day of the year. enhanced 8–11-day oscillation in EEJ is not in the first week of February and near 5–6-day in associated with corresponding mesospheric PW, April which are accompanied, respectively, by which might point to the possibility of in situ significant and less significant oscillations in zonal generation of these oscillations in dynamo region wind. Some times, dominant oscillations in meso- winds. As regards the results for the year 1999, we spheric winds are accompanied by less significant or may note that the EEJ strength presents pro- no oscillations in EEJ strength. For example, the nounced oscillations with periodicities near 2-day dominant 12-day oscillation in zonal wind in May ARTICLE IN PRESS M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 513 is accompanied by less significant 14-day oscilla- a clear sky nights and there are 10–14 nights of tion in EEJ, and the enhanced 14–20-day oscilla- observation surrounding each new period (see tions in zonal winds during February and March are also, Takahashi et al., 2005), whereas the h’F data not accompanied by any signatures in EEJ strength, has (usually) 15-min interval on a continuous basis. which might indicate that the waves might be For the present analysis we have used h’F values getting damped before penetrating to the dynamo averaged over 21–24 LT of each night. The result of region. It is interesting to note in the results of Fig. 2 Morlet wavelet power spectral analysis performed that there are more cases of correspondence of EEJ on this averaged h’F values are shown in the upper oscillations with the oscillation periods of zonal panel of Fig. 3, wherein shaded zones indicate the winds, than with those of meridional winds, which missing data gaps. Due to the relatively discrete might suggest that the EEJ processes are controlled nature of the airglow data its time series was more by zonal wind than by meridional winds. subjected to Lomb–Scargle (LS) spectral analysis Next we consider some evidences on associated for each monthly group, and the results are shown PW oscillations in the mesospheric airglow emission in the lower panels of Fig. 3 (diverging arrows and F-layer height in the equatorial region over joining the upper and lower panels indicate the Brazil. The upper mesosphere and lower thermo- intervals of the h’F data series for which the airglow sphere night airglow intensities in the OI5577 and was analyzed). A comparison between the two sets OH(6,2) bands monitored over Cariri (7.51S, of results brings out some cases of reasonably good 36.51W) and the virtual height of the F-layer coincidences in the oscillation periodicities. A 3–5- monitored by a Digisonde (Reinisch et al., 1989) day oscillation in the h’F seen during the interval over Sao Luis (2.61S, 44.21W) during the period 9–26 March can be easily identified with the airglow from March to November 1999 are used in this oscillation showing an 4-day period in both the analysis. It should be pointed out that the airglow OI5577 and OH rotational temperature. During the data time series has 10-min interval during 6–10 h of interval of 6–19 June a relatively weaker 2–3-day

21-LT 24-UT 1 151

2 100 4 50 8 Period (days) Period

16 0 Mar-99 May July Sept Nov

OI55 OI55 40 OI55 60 60 40

PSD 40 20 PSD 20 20 PSD 0 0 0 234567 234567 234567 40 OH-T OH-T 40 OH-T 20

PSD 20 PSD 20 PSD

0 0 0 234567 234567 234567 Period (days) Period (days) Period (days)

Fig. 3. Spectral distribution of the F-layer height oscillations obtained from Morlet wavelet analysis of h’F (F-layer base height) data over Sao Luis, Brazil, during the interval March–November 1999 (Top panel). The grey areas indicate data gaps. The h’F data represent averages of 21–24 LT values. Power spectral distribution of the airglow emission intensity at OI5577 (OI55) and OH rotational temperature (OH-T) are presented in the lower panels. ARTICLE IN PRESS 514 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522

JICAMARCA - March 2000 oscillation in h’F is associated with strong oscilla- 80 tion of the same period in the airglow emission. In 60 the interval 3–14 September, when the airglow 40 emission presents strong oscillation of 4–5-day ) period the h’F shows a tendency for 4–8-day -1 20 oscillation which gets intensified, staying strong 0 -20 during the later half of September. These results do Vz (m.s suggest associated oscillations in the equatorial -40 mesosphere and F-region on PW scales. -60 15 - min Vz values -80 40 2.1. PW scale modulations in the evening zonal 0 electric field and spread F -40 Dst (nT) -80 0 5 10 15 20 25 30 It is well known that equatorial F-layer height Day of the month undergoes a large increase at sunset due to the zonal electric field enhancement, PRE (Woodman, 1970) Fig. 4. F-region vertical drift calculated as dhF/dt (hF being the that develops under the action of the F-layer wind true height of a specific plasma frequency of the F-layer) using the true heights at 4 and 5 MHz plasma frequencies obtained from dynamo that intensifies at this time. The height of a digisonde data over Jicamarca. The average of velocities obtained specific plasma frequency can reach values from the two frequencies is plotted in the figure. The lower panel 4300 km and, therefore, its rate of change with shows the hourly Dst index variation (downloaded from the time is a good/reliable indicator of the vertical Kyoto WDC for geomagnetic data). plasma drift of the F-layer as discussed in detail by Bittencourt and Abdu (1981) and Abdu et al. (1981). Thus the nighttime F-layer height (at a attained slightly higher values only on 1 and 31 given plasma frequency) is an approximate repre- March, the corresponding SKp (diurnal sum of the sentation of the time-integrated vertical drift of the 3-h values) being 28 and 29+, respectively. Thus it post-sunset F-region (subject to the recombination appears evident that the large amplitude oscillations effects that set in for heights o300 km). We have in the Vzp are unlikely to have been caused by any calculated the vertical drift velocity (eastward disturbance electric field. Consideration of Vzp electric field) over Jicamarca using the F-layer variations during some specific intervals such as height information available from digisonde data. 2–6, 17–21 and 22–29 March, when the Dst The result for the entire month of March 2000 remained stable and very low, testifies to the above (using the almost uninterrupted half-hourly data) is affirmation. Thus we may attribute these oscilla- presented in Fig. 4 (the day numbers on the x-axis tions in the Vzp amplitude to PW-induced effects in mark the end of the day in UT, ‘0’ being the the ionosphere similar to the oscillations in the h’F beginning of the day ‘1’). The large increase in the presented in Fig. 3. vertical drift velocity that falls nearly on the day The Vzp values of March 2000 (from Fig. 4) are markings, represents the evening vertical drift plotted in the top panel of Fig. 5. These were velocity enhancement (PRE) that maximizes near subjected to Morlet wavelet analysis and the result 19 LT (note that the local time (LT) over Jicamarca is shown in the lower panel of this figure. The cone is 5 h behind the UT). We note that the vertical drift of influence due to edge effect in the data series velocity peak, Vzp, has relatively large values analyzed is indicated by the thick (parabolic) curves. typical of the seasonal/equinoctial maxima of a Significant amplitude of 3- and 7-day waves is solar maximum epoch for this location (Fejer et al., present during the second week of March and the 1991). Large variation in the Vzp amplitude is wave periods evolve to 2–3-day, 4–5-day and 7–8- observed during this period, the lowest and highest day towards the third week of the month. These values attained being 20 and 70 m/s, respectively. periodicities are in good agreement with the widely Oscillations of a few days periodicity can be clearly observed PW periods discussed before (see, e.g. identified in them. The magnetic activity level, in Forbes, 1996; Forbes and Leveroni 1992; Guru- terms of the hourly Dst values plotted in the bottom baran et al., 2001; Haldoupis et al., 2004; Pancheva panel, indicates generally quiet conditions for this et al., 2003, Takahashi et al., 2002). Thus we note period, as confirmed also by the Kp indices that that the PW disturbances are capable of modulating ARTICLE IN PRESS M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 515

JICAMARCA 1.6 0.075 0.3 2 0.45 0.75 0.9 1.2 1.051.351.5 1.20.9 0.075 0.75 4 0.075 0.8 0.3 0.15 6 0.75 0.075 0.4 POWER Period (days) 0.9 1.05 1.35 8 0.3 1.2 10 0.45 0.60.750.9 0 Morlet mother function 80 ) -1 60

40

Vzp (m.s 20 1 4 7 10 13 16 19 22 25 28 31 MARCH 2000

Fig. 5. Vzp values (the peak vertical drift velocity) time series for March 2000 over Jicamarca taken from Fig. 4 (upper panel), and the Vzp oscillation periods obtained by Morlet wavelet spectral analysis (lower panel).

JICAMARCA ESF MAR 2000 33 5.5 30

UT 27 4.5 24 3.5

33 2.5 30 1.5

UT 27 0.5 24 80 ) -1 60 40

Vzp (m.s 20 12 3 4 5 61 64 67 70 73 76 79 82 85 88 91 DAY OF THE YEAR

Fig. 6. Equatorial spread F intensity from ionograms quantified (as a rough indicator) in terms of the range of the echo spreading (one unit representing 100 km of range spreading) is shown in the top and middle panels (white areas mark missing data). The top panel shows the echo spreading intensity at the higher frequency range of the F-layer trace and in the middle panel is plotted similar parameter for the lower frequency range. The bottom panel shows the corresponding Vzp variations. The data set is for March 2000 over Jicamarca.

the intensity of the evening equatorial F-region should be mentioned here that we have used as a vertical drift velocity (zonal electric field). rough indicator of the ESF intensity the spread The evening F-layer uplift due to the PRE, of range of the ESF echoes in the ionograms, one unit which Vzp is the best indicator, is known to be the of intensity being represented by 100 km of range main cause for the generation of the post-sunset spreading around the F-layer trace. The middle ESF/plasma bubble irregularities (see e.g., Abdu panel shows the spread F intensity at the lower et al., 1983, Fejer et al., 1999). We have plotted in frequency limit of the ionogram which indicates the the top two panels of Fig. 6 the ESF intensity as a first onset of the bottomside spread F. The onset of function of UT and day of the year for the same SF at the higher frequency range that follows in interval as in Fig. 5 (March 2000). The correspond- sequence (with some time delay, usually of ing Vzp values are plotted in the bottom panel. It 15–30 min) marks the evolution towards topside ARTICLE IN PRESS 516 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 bubble formations when the intensity (spread range) 91–103 km region. Several mechanisms have been generally increases, as can be verified from the plots proposed to explain the PW period oscillations in the top panel. We note that there is significant observed in the dynamo region (100–170 km) and control of the SF intensity by the Vzp amplitude, at higher levels of the ionosphere. They include, as higher intensity of the SF, in general, corresponding explained by Forbes (1996), (a) stratosphere/meso- to larger amplitude of the Vzp. For example, during sphere PW modulation of the accessibility of gravity two of the lowest values of Vzp identified by the waves to the upper levels, involving mechanisms vertical lines 2 and 5, corresponding to VzpE20 m/ that may lead to secondary source of PW excitation s, the SF was nearly totally absent, and during three at higher levels and (b) PW modulation of the of the higher Vzp values identified by the vertical upward propagating that participate in the lines 1, 3 and 4, which corresponded to dynamo action to generate electric fields at higher VzpE65–70 m/s, the SF was clearly more intense. levels. The latter mechanism seems to be especially Of course a closer examination of this figure would promising and more easily verifiable, in view of the show some important exceptions to a direct fact that the forcing by upward propagating diurnal relationship between the two parameters, of which and semidiurnal tides is basically responsible for the we will be discussing later. Independent of the phenomenology of the quiet time ionosphere, detailed nature of the relationship between Vzp and especially over equatorial latitudes. In fact, evidence ESF intensity we may affirm that PW scale on the influence of PW modulated atmospheric tides oscillations are clearly present in the evening PRE of diurnal and semi-diurnal periodicities on mid- (vertical drift) which in turn modifies/controls the latitude sporadic E-layer formation has been formation and intensity of the ESF. provided recently by Pancheva et al. (2003). Our results in Figs. 1 and 2 showing cases of associated 3. Discussion planetary scale oscillations in DH and mesospheric winds (especially zonal winds) can be examined in We have presented observational results in the light of this discussion. The DH represents the evidence of vertical coupling processes involving diurnal range of the magnetic field H-component PW scale oscillations in the mesosphere E- and F- variation due to the EEJ, which is a direct measure regions of the equatorial atmosphere–ionosphere of the product of dynamo electric field intensity and system. The results call our attention to two electron density/conductivity at E-layer heights. important aspects of the coupling processes under- Assuming that the PW oscillations in the E-layer lying these results: (1) Mesosphere E-region dy- electron density are small (there is no independent namic coupling (MEDC) through upward evidence of such oscillations so far) we attribute the propagation of PW and (2) E- and F-region periodicities observed in DH to the corresponding electrodynamic coupling (EFEC) that produces oscillations in the dynamo electric field, which is manifestations at PW oscillation periods. produced dominantly by a diurnal tidal wind Both these aspects pose an important question as dynamo. In view of the observation by Pancheva to the accessibility of the PW to the ionospheric et al. (2003) that mid-latitude Es-layer formation is dynamo region and to higher levels. In the case of affected indirectly by PW through their modulation the MEDC, the penetration of the PW into the E- of the diurnal and semidiurnal tides, we may expect region could result in dynamo action by the that the equatorial DH oscillations at PW periods associated winds, leading to the generation of an may arise in the dynamo region also through the electric field responsible for the effects observed in action of PW modulated tidal winds. This point the E-regions, such as the PW scale oscillations in needs to be checked, however, from further analysis DH observed in Figs. 1 and 2. However, the existing of the data. In any case, a more direct role of PW in model calculations do not seem to support direct the dynamo region cannot be ruled out either, penetration of PW into the E-region capable of without examining additional concurrent data of inducing wind amplitudes required for a viable winds in the equatorial dynamo region. dynamo mechanism (Forbes, 1996; Hagan et al., As regards the aspect of the EFEC mentioned 1993; Forbes et al., 1995). Further, recent analysis above, the PW scale oscillations in the F-region results by Pancheva et al. (2003) on MF radar winds parameters that result from the EFEC processes seem to show a certain amplitude attenuation in have important implications to our understanding meridional wind with height increase in the of the causes of the widely observed day-to-day ARTICLE IN PRESS M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 517 variability in the equatorial F-region evening directly induced by the PW. There is a possibility of vertical drift (PRE) and the related spread F PW modulation of the solar diurnal thermal development processes. This problem is complex that is mainly responsible for the winds at higher enough that a detailed discussion is not attempted levels of the thermosphere, which is difficult to here. For a brief discussion we may simplify this verify however. A more likely source of these problem into the two following aspects: (a) PW oscillations may be sought in the possible depen- scale oscillations in the evening PRE/vertical drift dence of the E-layer conductivity longitude/LT and (b) ESF variability arising from the oscillations gradient on the lower thermosphic zonal wind as in the PRE as one of the causes. explained below. Fig. 7 taken from Abdu et al. Regarding the aspect (a), it is known that the (2003) shows the E-layer integrated Pedersen con- equatorial evening electric field enhancement arises ductivity as a function of time calculated for diurnal from the F-layer wind dynamo, which involves tidal zonal winds of different amplitudes ranging electrical and electrodynamical coupling of the E- from 0 to 100 m/s. The time t ¼ 0 is adjusted to and F-regions of the equatorial ionosphere (Rish- coincide with 18 LT, close to local sunset (for details beth, 1971; Heelis et al., 1974). Briefly, the thermo- of the calculation see Abdu et al., 2003). We note spheric zonal wind (eastward in the evening hours) that the LT gradient (which is equivalent to in the presence of the strong longitudinal/LT longitudinal gradient), positive towards dayside, gradient in the E-layer conductivity (caused by the increases significantly with increase of the zonal conductivity decay towards night side of the wind amplitude in the E-region (90–140 km). On the terminator) produces by dynamo action vertical basis of this dependence we may expect that the (downward) electric field increasing towards the PRE amplitude which depends upon the long- night side, as can be verified with the help of the itudinal gradient in the E-layer integrated conduc- following equation: tivity (as a dominant controlling factor) could hiX .X X undergo fluctuations due to corresponding oscilla- E ¼ U B þ , (1) tions in the amplitude of the E-layer zonal wind of z y 0 F F E diurnal or semi-diurnal tidal mode. The amplitude where Uy is thermospheric zonal windP and BP0 is the of the tidal wind itself could be modulated by PW Earth magnetic field intensity, and F and E are through nonlinear interaction, as mentioned before. the integrated conductivities, respectively, of the E- Here again we cannot rule out the possibility of a and F-regions (Abdu et al., 2003).P Due to the faster direct role of the PW in causing the zonal wind decayP of the E-layer conductivity, E, as compared fluctuation of the required magnitude at E-layer to F, in the post-sunset hours, Ez tends to increase heights. towards the nightside. The application of a curl-free The aspect (b) concerns the day-to-day variability condition to such an electric field could lead to the in the ESF occurrence and intensity due to enhanced zonal (eastward followed by westward) oscillations in the PRE amplitudes as one of its electric field as proposed originally by Rishbeth causes. It has been known from previous studies (1971) and modeled by Eccles (1998) (for other different approaches used in modeling this problem integrated Pedersen conductivity see e.g., Farley et al., 1986; Batista et al., 1986; 1.0 Haerendel et al., 1992; Crain et al., 1993, Fesen et 0.8 al., 2000). Thus it appears evident that the magnitudes of the zonal wind as well as that of 0.6 0 m/s the longitudinal conductivity gradient across the 0.4 100 terminator could control the intensity of the PRE. 50 Therefore, the PW oscillations observed in the PRE 0.2

(Figs. 4 and 5) could be caused by such oscillations S conductivity, Integrated 20 m/s 0.0 either in the intensity of the thermospheric zonal 0246 wind or in the degree of the conductivity long- time, hours itudinal gradient. From the discussion on the Fig. 7. One-dimensional simulation results of height integrated vertical penetration of the PW presented above it E-layer Pedersen conductivity in unit of Siemens (S) as a function looks less likely that significant oscillations in the of time for zonal (westward) wind amplitudes 0, 20, 50 and neutral winds at thermospheric height could be 100 ms1. ARTICLE IN PRESS 518 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 that ESF can develop and increase in intensity for has not been adequately established/investigated so Vzp amplitude of 20 m/s and higher (see for far (see e.g. Mendillo et al., 2001; Abdu, 2001). example, Abdu et al., 1983; Fejer et al., 1999). This Another source of variability for the field line relationship is evident also in the plots of these two integrated conductivity seems to reside in sporadic parameters shown in Fig. 6. The ESF development E-layers when present at the extremities of the flux and intensity depend also on other conditions tubes that participate in the ESF development besides a direct dependence on the PRE amplitude. processes which include also the development of To understand its variability we need to examine the the PRE. Model calculations have shown that linear growth rate, g, of the ESF/plasma bubble increase of field line integrated conductivities of instability by the generalized Rayliegh–Taylor sufficient magnitude to affect the instability growth mechanism that uses the background ionospheric rate can occur due to the presence of Es-layers of parameters expressed in terms of their values significant intensity (Stephan et al., 2002). However, integrated along the magnetic flux tube in which observational evidence on the effect of Es-layers on the instability begins to develop at the bottom-side ESF development so far available lead to diverging gradient region of a rapidly rising evening F-layer. conclusions. For example, while an isolated case of Based on flux tube integrated quantities, as pro- ESF inhibition in the presence of sporadic E-layer posed by Haerendel (1973, unpublished report) over the equatorial station Jicamarca has been (also, Haerendel et al., 1992), a generalized form reported by Stephan et al. (2002) an exactly of the linear growth rate derived by Sultan et al. opposite effect from the statistical data set of ESF (1996) is given by enhancement in the presence of sporadic E-layers over an equatorial anomaly (low latitude) station, SF E g 1 Chung-Li, has been shown by Bowman and g ¼ P U P þ b . (2) FT E F FT FT Mortimer (2003). Such apparently ambiguous/con- SP þ SP B neff LFT tradictory manifestations of the Es-layer–ESF E;F Here, SP is the field-line integrated conductivities relationship can be understood in the light of a P for the E- and F-region segments of a field line; U FT direct relationship between the PRE development is the conductivity weighted flux tube integrated and the evening Es-layer formation conditions that vertical wind; b is the recombination loss rate; neff is has been recently demonstrated by Abdu et al. the effective ion-neutral collision frequency. The (2003). It was shown based on analysis of extensive subscript FT in Eq. (2) stands for flux tube data sets that Es-layer formation in the evening integrated quantity. The influence of the different hours over a sub-equatorial station (such as terms in the ESF variability has been discussed Fortaleza, Brazil) can be disrupted during the briefly in Abdu (2001). We note from Eq. (2), development of the PRE of significant amplitude. especially, that larger flux tube integrated conduc- An example of this result (taken from Abdu et al., tivity (the denominator of the conductivity term) 2003) is presented in Fig. 8 in which the right panel reduces the instability growth rate. Even if gFT is shows the interruption of an ongoing Es-layer at the sufficiently positive for the initiation of the in- time of the maximum F-layer vertical drift (zonal stability the ensuing nonlinear growth of the electric field) near sunset, the Es-layer formation instability into developed topside bubble structures resuming after 2–3 h. When the vertical drift is can be retarded or even totally inhibited depending relatively less intense such disruption of the Es- upon the magnitude of the flux tube integrated layers does not occur, as can be seen in the results conductivity. It has been shown by Maruyama plotted in the left panel. This was explained (by (1988) based on model calculations that trans- Abdu et al., 2003) in terms of the effect of a vertical equatorial winds (that blow from summer to winter electric field in causing vertical plasma transport at hemisphere) can cause significant increase of the E-layer heights that can act, depending upon the E- flux tube integrated conductivity. Its effect will be to field direction, to retard or enhance the vertical ion reduce instability growth rate as well as to inhibit convergence driven by a wind/wind shear mechan- the polarization electric field responsible for the ism that is basically responsible for the Es-layer nonlinear development of the plasma bubbles. Thus formation. An upward-directed vertical electric field possible day-to-day variability in the intensity of can disrupt the vertical ion convergence while a trans-equatorial winds could be a significant source downward electric field can enhance it (see Abdu of ESF day-to-day variability although this effect et al., 2003, for further details). The vertical ARTICLE IN PRESS M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 519

FORTALEZA, BRAZIL (2001)

600 2001-Mar.6,10, 2001-Nov.2,4,6,8, 12,18,20,21,22, 9,10,12,14,18,20, 23,24,25,28,29, 23,25,27,29, 500 31,Apr.3,4 Dez.3,4,7,8,12

400

h'F (5MHz) (km) 300

200 -1 -1 ) 40 Vzp=37m.s Vzp=62m.s -1

0 Vz (m.s -40 12 8 4 ftEs (MHz) 0 200

160

120 h'Es (km) 80 9 12151821 0369121518210369 Local Time

Fig. 8. F-layer height versus Es-layer relationship using digisonde data over Fortaleza for March–April and November–December 2001. Left panels: the case of post-sunset Es-layer non-disruption; and right panels: the case of Es-layer disruption. (Note the large difference between the values of Vzp for the two cases). structure of the evening vertical electric field over observations during December 1988 of simulta- the equator, which is associated with the PRE, as neous Es-layer variations over Fortaleza and modeled by Haerendel et al. (1992) consists of an Cachoeira Paulista, together with the F-layer height upward directed field in the height region and the associated vertical drift variations over the 90–300 km which reverses to a downward directed former station. It is clear that Es-layer disruption field above 300 km (see Haerendel et al., 1992, for over Fortaleza is accompanied by its formation over further details). When this electric field is field-line Cachoeira Paulista when the PRE development was mapped to off-equatorial latitudes we have an at its peak over the former station, which is in upward-directed electric field over Fortaleza complete agreement with the vertical electric field (3.91S, 38.451W, dip angle: 91) and a downward- control of the Es-layer just explained above. We directed electric field at a location farther away in have verified that ESF was present over Fortaleza latitude, such as Cachoeira Paulista (22.61S, 3151E; on all these nights as expected for Vzp amplitudes dip angle:281). As explained by Abdu et al. (2003) 420 m/s, in agreement with the results of Fig. 6 as this situation leads to disruption of Es-layer well. Thus we note that the nature of the association formation over Fortaleza while uninterrupted, or between the ESF and Es-layer occurrences to be even enhanced, Es-layer formation over Cachoeira expected in observational data would depend upon Paulista during the PRE development. Results in the latitude region for which the data analysis is support of this explanation are presented in Fig. 9, performed. Further, it appears that a possible effect which shows, as an example, a few days of on ESF development and intensity due to an ARTICLE IN PRESS 520 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522

FORTALEZA-December 1988 495 h´F 420

345

270 h´F (km)

195

120 )

-1 40 Vz 0 -40 Vz (m.s 20 ftEs 15

10

ftEs (MHz) 5

0 180 h´Es 160 140 120 h´Es (km) 100 80 10 ftEs 8 6

ftEs (MHz) 4 2 180 h´Es 160 140 120 h´Es (km) 100 80 9 12151821 0369 LOCAL TIME

0 Fig. 9. F-layer height h’F, vertical drift velocity Vz, and Es-layer parameters, f tEs and h Es, over Fortaleza, plotted in the top four panels and the Es-layer parameters over Cachoeira Paulista in the bottom two panels. enhanced field line-integrated conductivity arising Es-layer, to be associated with ESF occurrence/ from the mere presence of Es-layers is a challenging intensity, be located in common areas in the question warranting experimental/observational conjugate E-layers at the feet of a potentially verification. In any attempt to verify this point it unstable flux tube. Further, the direct association is necessary, as a minimum requirement, that the (in an interactive manner) that exists between the ARTICLE IN PRESS M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522 521

PRE and the Es-layer, as shown by Abdu et al. widely observed day-to-day variability in the ESF (2003), needs to be isolated in order to establish any phenomenon. possible effect on the ESF arising from the mere presence of the Es-layers. These considerations Acknowledgements show that while the PW scale oscillations in the PRE amplitude is an important factor contributing The authors wish to acknowledge the supports to the day-to-day variability in the ESF, other from the Fundac-a˜o de Amparo a Pesquisa do factors such as the meridional/ trans-equatorial Estado de Sa˜o Paulo- FAPESP through the projects winds, field line integrated conductivities, etc., 1999/00437-0, 04/07695-5 and 05/52654-8, Conselho mentioned before, as well as the Es-layer connec- Nacional de Desenvolvimento Cientifico e Tecno- tions, also contribute to its variability in ways logico- CNPq through Grants no 502804/2004-1. difficult to evaluate from limited data bases. We thank the Jicamarca Radio Observatory for the use of their Digisonde data that we have retrieved 4. Conclusions from the UML DIDBase. BWR was supported by AF Grant F19628-02-C-0092. We thank Maria The following conclusions may be noted from the Goreti dos Santos Aquino for assistance in proces- present study: PW scale oscillations of different sing the digisonde data and in preparing some of the periodicities with episodic nature occur simulta- figures. neously in the mesospheric winds and EEJ intensity. It is not clear from the present analysis if the EEJ effects are due to direct penetration of the PW up to References the dynamo region or indirectly by a mechanism of tidal wave modulation by PW interaction at lower Abdu, M.A., 2001. Outstanding problems in the equatorial height as found to be operative in the case of mid- ionosphere-thermosphere electrodynamics relevant to spread F. Journal of Atmospheric and Terrestrial Physics 63, latitude Es-layer responses. Nighttime F-layer 869–884. height oscillations at PW periodicities are found to Abdu, M.A., Bittencourt, J.A., Batista, I.S., 1981. Magnetic be associated with such oscillations in the meso- declination control of the equatorial F region dynamo field spheric airglow intensity and OH rotational tem- development and spread-F. Journal of Geophysical Research perature. In this case the F-region oscillations 86, 11443–11446. Abdu, M.A., Medeiros, R.T., Bittencourt, J.A., Batista, I.S., represent the time-integrated effect of the evening 1983. Vertical ionization drift velocities and range type spread vertical drift (zonal electric field) due to F-region F in the evening equatorial ionosphere. Journal of Geophy- dynamo. For the first time we have provided sical Research 88, 399–402. evidence for oscillations at PW periods in the Abdu, M.A., MacDougall, J.W., Batista, I.S., Sobral, J.H.A., equatorial evening zonal electric field believed to Jayachandran, P.T., 2003. Equatorial evening pre reversal electric field enhancement and sporadic E layer disruption: a result from the combined action of thermospheric manifestation of E and F region coupling. Journal of zonal wind and the evening longitudinal gradient in Geophysical Research 108 (A6), 1254. the E-layer Pedersen conductivity. Considerations Batista, IS., Abdu, M.A., Bittencourt, J.A., 1986. Equatorial F on the relevant electrodynamic processes suggest region vertical plasma drift: seasonal and longitudinal that PW oscillations in zonal winds at E-layer asymmetries in the American sector. Journal of Geophysical Research 91, 12055–12064. height, rather than such oscillations in the zonal Bittencourt, J.A., Abdu, M.A., 1981. A theoretical comparison winds at F-region height, could be a more likely between apparent and real vertical ionization drift velocities cause of the observed PW scale oscillations in the in the equatorial F region. Journal of Geophysical Research evening zonal electric field /F-region vertical plasma 86, 2451–2455. drift. A direct consequence of the PW scale Bowman, G.G., Mortimer, I.K., 2003. Spread–F/sporadic E coupling at Chung-Li, especially for postsunset periods of oscillations in the evening electric field is its role in sunspot maximum years. Journal of Geophysical Research the quiet time day-to-day variability of the ESF/ 108 (A4), 1148. plasma bubble occurrence and intensity. However, Brown, G.M., Williams, D.C., 1971. Journal of Atmospheric and other factors, such as the relationship between the Terrestrial Physics 33, 1321. PRE field and sporadic E-layer formation in the Chen, P.R., 1992. Two-day oscillations of the equatorial ionization anomaly. Journal of Geophysical Research 979 evening hours, to mention as an example, should be (A5), 6343–6357. considered in any investigations aiming for a better Clark, R.R., Burrage, M.D., Franke, S.J., Manson, A.H., Meek, understanding and evaluation of the causes of the C.E., Mitchell, N.J., Muller, H.G., 2002. Observations of ARTICLE IN PRESS 522 M.A. Abdu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 509–522

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