Large scale electrodynamic and plasma density variations in the low latitude ionosphere have recently been associated with sudden stratospheric warming (SSW) events. We present average patterns of largely enhanced lunar semidiurnal equatorial vertical plasma drift perturbations during arctic winter low and high solar flux SSW events. These perturbations play a dominant role in the electrodynamic response of the low latitude ionosphere to SSWs. Our models indicate that the amplitudes of the enhanced lunar semidiurnal drifts are strongly local time and solar flux dependent, with largest values during early morning low solar flux SSW periods. These results suggest that ionospheric conductance strongly modulate low latitude ionospheric changes during SSWs. They also indicate that lunar semidiurnal effects need to be taken into account by global ionospheric models for their improved forecasting of the low latitude ionospheric response to SSW events, especially for low solar flux conditions.
 Middle and low latitude upper atmospheric and ionospheric perturbations during sudden stratospheric SSWs have been the subject of intensive recent studies. SSWs are strong large scale metereological events in the winter middle atmosphere with large rapid temperature, wind and circulation changes [e.g., Labitzke, 1981; Andrews et al., 1987]. These phenomena, driven by the growth of upward propagating quasi-stationary planetary waves and their interaction with stratospheric mean circulation [Matsuno, 1971], strongly affect the vertical coupling from the stratosphere to the thermosphere [e.g., Liu and Roble, 2002].
 Numerous studies reported large perturbations in the lower atmosphere and ionosphere at low latitudes during SSWs. These include multi-day reversals from eastward to westward in the daytime equatorial electrojet (driven by the zonal electric field) and winds in the 90–105 km height range, as well as sudden stratospheric cooling [e.g., Stening et al., 1996; Sridharan et al., 2009; Vineeth et al., 2009]. Chau et al.  discussed large semidiurnal perturbations (upward in the morning and downward in the afternoon) on the F region vertical plasma drifts over the Jicamarca Radio Observatory (11.9°S, 76.8°W; dip latitude 1°N) during a minor SSW event in January 2008. Fejer et al.  showed that these perturbations are preceded by large dayside downward drifts and westward equatorial electrojet currents, and suggested that they are due to enhanced lunar semi-diurnal lunar tidal wave effects. CHAMP satellite and ground-based magnetic field measurements showed that SSW associated equatorial electrodynamic perturbations have significant longitudinal dependence [e.g., Fejer et al., 2010]. Large variations in low latitude total electron content (TEC) and peak electron densities consistent with enhanced semidiurnal equatorial plasma drifts were also observed during these events [e.g., Goncharenko et al., 2010a, 2010b; Pedatella and Forbes, 2010a; Chau et al., 2010; Yue et al., 2010]. Liu et al.  showed that the relationship between the equatorial and low latitude TEC response in the Asian sector during the 2009 SSW was in good agreement with the latitudinal structure of the lunar semi-diurnal tides.
 Low latitude ionospheric changes during SSW events are indicative of strong global lower and upper atmosphere coupling. Sridharan et al.  suggested that the semi-diurnal modulation of the vertical plasma drifts is due to the nonlinear interaction of migrating tides and planetary waves. Modeling studies by Liu et al.  suggested that this interaction causes global enhancements of migrating and non-migrating tides. Simulation studies of the January 2009 SSW using the whole atmosphere model (WAM) suggest large increases in the amplitudes of the ter-diurnal tide in the dynamo region at the expense of semidiurnal tides [Fuller-Rowell et al., 2011]. This simulation reproduced fairly well some of the general characteristics of the drifts reported by Chau et al. . A detailed review of the low latitude ionospheric response to these warmings was recently presented by Chau et al. .
 In this study, we use vertical plasma drift measurements over Jicamarca to determine their solar flux dependent response to arctic winter SSWs. Our results show very strongly enhanced lunar semidiurnal vertical plasma drift amplitudes during early morning low solar flux warmings. They also clearly indicate that these tidal effects need to be included in global ionospheric models for more accurate representation of the low latitude ionospheric weather.
 We used daytime vertical plasma drifts measured by the incoherent scatter and JULIA radar systems at the Jicamarca Radio Observatory. The incoherent scatter radar drifts correspond to average values in the height range from about 300–400 km. The accuracy is typically 1–2 m/s and the time resolution is about 5 min [e.g., Fejer, 1997]. The JULIA drifts are derived from Doppler measurements of 150-km radar echoes generally between about 0800 and 1600 LT with a 5 min resolution. They closely follow, but are not identical to the F region vertical drifts due to the zonal electric field altitudinal variation. We also used, to smaller extent, vertical drifts derived from Jicamarca and Piura (5.2°S, 80.6°W; 6.9° magnetic) magnetic field data [e.g., Anderson et al., 2002]. These derived drifts are generally in good agreement with the radar drifts, but are less accurate in the early morning and late afternoon sectors. Over Jicamarca, an upward drift velocity of 40 m/s corresponds to an eastward electric field of about 1mV/m.
 We studied Jicamarca vertical drift perturbations during 9 SSW events from November 2000 through February 2010. These residual drifts were obtained by subtracting from each measurement the corresponding quiet-time climatological average drift. We used the Scherliess and Fejer  model for the incoherent scatter measurements and bi-monthly quiet-time JULIA drift models for the lower altitude data. In this study, we will compare the 08–16 LT perturbation drifts during SSWs with JULIA lunar semidiurnal drift models, and in the 06–08 and 16–18 LT sectors, where the JULIA data are more sparse, with incoherent scatter radar drift models. These semidiurnal drift models were obtained from bi-hourly and bi-monthly average drifts binned as a function of the day after new or full Moon. The standard deviations vary from about 2 m/s in the early morning and late afternoon periods to about 1 m/s near noon. These model drifts and their standard deviations are in good agreement with those from the Stening and Fejer  lunar semidiurnal model.
 Low latitude ionospheric disturbances during the very strong 2009 SSW were discussed in several papers [e.g., Fejer et al., 2010; Goncharenko et al., 2010a, 2010b; Chau et al., 2010; Pedatella and Forbes; 2010a; Yue et al., 2010; Liu et al., 2011, Rodrigues et al., 2011]. Figure 1 shows in the top panel bi-hourly averaged daytime drift perturbations and the climatological lunar semi-diurnal drifts. These perturbation drifts were obtained from incoherent scatter radar and JULIA data between January 17–February 3, and from magnetometer data during the other days The bottom panels show the zonal temperatures at 90° N, mean zonal winds over 60°N, and the 30-year mean temperatures and zonal winds at 10 hPa (about 32 km), obtained from the National Center for Environmental Prediction (NCEP). In this period, the F10.7 index was about 70, and geomagnetic activity was low, except for occasional minor disturbances.
Figure 1 shows strong quasi-two day activity during this entire period, which is typical of SSW periods [e.g., Fejer et al., 2010]. Figure 1 also shows large daytime perturbation drifts after the rapid increase in the arctic stratospheric temperature and decrease in the high latitude eastward wind at about 19 January. The drift perturbations systematically shifted to later local times closely following the climatological lunar semi-diurnal drifts, but had much larger magnitudes between about 19 January and 4 February. In this period, the perturbation drift patterns closely resemble those reported previously [e.g., Fejer et al. 2010; Chau et al., 2010]. Later, the drifts generally decreased and did not follow as closely the lunar pattern.
 The bi-hourly averaged perturbation drifts, climatological lunar semi-diurnal drifts and the high latitude temperatures and mean zonal winds during January 18–February 14, 2010 are presented in Figure 2. These drifts were measured by the incoherent scatter radar from January 18th through the early morning of February 4th, and later by the JULIA system. The average F10.7 index was about 80, and geomagnetic conditions were mostly quiet, except for a few disturbances. In this case again, large quasi-two day drift perturbations occurred over most of the observational period and the multi-day perturbation pattern followed closely the variation of the climatological lunar semi-diurnal drifts after the SSW onset. In the 06–10 LT sector the perturbation drifts were significantly down shifted relative to the climatological values suggesting the occurrence of an additional local time dependent perturbation process. The amplitudes of these semi-diurnal fluctuations reached about 10 m/s up to about February 9, and significantly decreased later.
 We used measurements during the 2006, 2008, 2009, and 2010 SSW events to determine the average effects of these warmings on low solar flux lunar semidiurnal drifts. First, we minimized the short-term variability (e.g., quasi-two day wave) by carrying out 3-point running averages of the bi-hourly perturbation drifts. Then, we selected the perturbation drifts during the first 15 days after SSW onset (when the temperatures systematically increased above and the mean eastward winds decreased below their mean values), since they usually decreased after about 10–12 days. In addition to the 2009 and 2010 data presented earlier, we used the radar perturbation drifts during 19 January–1 February 2006, and 22–27 January 2008. These data were binned as a function of the number of days after the new or full moon. For the 2009 SSW period, for example, the data from January 19 (see Figure 2) were placed in the 8th bi-hourly bins. Finally, we did 3-point running averages on the data in these bins.
Figure 3 shows the average bi-hourly perturbation drifts as a function of the number of days after new or full Moon and the climatological semi-diurnal drifts. The average F10.7 index was about 80 and the mean temperature increase was about 25 K. Figure 4 shows that, except for usually small phase shifts, the perturbation drifts during warmings closely follow the lunar semi-diurnal drifts, but with significantly larger amplitudes especially in the morning and late afternoon. The standard deviations are largest in the 06-08 LT sector, mostly due to the 2010 early morning downshifted perturbation drifts (see Figure 2), and the very large upward (about 40 m/s) drifts near full Moon on 24–26 January 2008. Since the Jicamarca mid-morning and mid-afternoon low solar flux average drifts are about 16 and −1 m/s, respectively, SSW effects strongly modulate these drifts and, therefore, also the daytime low latitude TEC. Our results (not shown) indicate that these drift perturbations extend into nighttime with significant amplitudes.
 We studied the drift lunar semi-diurnal perturbations during 5 high solar flux (average F10.7 of about 170) SSW events using the methodology described above. These SSWs had onsets on 7 December 2000, 30 January 2001, 22 December 2001, 27 December 2002, and 16 December 2003. During these events, there was a larger percentage of magnetometer derived drifts, and few measurements from 16–18 LT. Figure 4 shows the average lunar perturbation drifts during high flux SSWs have well defined semidiurnal patterns with amplitudes comparable to the November-February climatological drifts, but large phase shifts and standard deviations. In this case, the average high latitude temperature enhancement was about 20 K. The perturbation drifts during the December 2002–January 2003 (F10.7 of about 140) SSW studied by Fejer et al.  had significantly enhanced amplitudes and followed fairly closely the average SSW drifts shown in Figure 3, but that was not the case for the drifts during other high flux events.
3. Discussion and Conclusions
 We have seen that significantly enhanced lunar semidiurnal tidal wave effects are likely the dominant source of equatorial vertical plasma drift perturbations during low solar flux SSW events. The enhancements on the semidiurnal drifts start at about the SSW onset and generally last for about two weeks. They have largest amplitudes in the morning and smallest mid-afternoon and, except for small phase shifts, follow closely the low solar flux non-SSW lunar semi-diurnal drifts. The peak morning upward and afternoon downward drifts occur close to new or full Moon, as reported by Fejer et al. . The lunar semidiurnal average drifts during high solar flux SSWs have smaller amplitudes and larger phase shifts than the low flux SSW drifts. Since equatorial vertical drifts are smaller during low solar flux periods, lunar tidal effects should modulate low latitude TEC more strongly during low flux SSW events. This is consistent with the larger variability of the equatorial plasma drifts during low solar flux periods [Fejer and Scherliess, 2001]. As pointed out by Chau et al. , large semidiurnal vertical drift perturbations have been observed only during SSW events. Our data also showed that during these events, quasi-two day wave activity [e.g., Pancheva et al., 2006] is an additional important source of quiet-time low latitude electrodynamic variability.
 Our results are consistent with simulation studies presented by Stening et al.  that showed significant effects on lunar tidal amplitudes and phases when SSW effects were included.
 Although lunar semi-diurnal tidal effects seem to play dominant roles on the low latitude electrodynamic perturbations during warmings, other processes resulting from the nonlinear interaction between migrating tides and planetary waves could also be responsible for significant semi-diurnal [e.g., Sridharan et al., 2009; Liu et al., 2010; Pedatella and Forbes, 2010a], and enhanced 8-hour ter-diurnal tidal electrodynamic perturbations [e.g., Fuller-Rowell et al., 2011]. This was probably the case in the early morning period during the 2010 SSW.
 Recent studies showed significant lunar tidal effects also on global TEC [Pedatella and Forbes, 2010b], equatorial vertical plasma drifts and F region plasma densities [Eccles et al., 2011], and equatorial spread F [Aveiro and Hysell, 2010]. Our data and these studies indicate that lunar tidal effects, which are not taken into account in most current global electrodynamic models, play important roles on low latitude ionospheric weather, especially during low solar flux SSW conditions. These updated models could provide important information on the processes responsible for these enhanced tidal effects.
 This work was supported by the NASA through grant NNX09AN55G. The Jicamarca Radio Observatory is a facility of the Instituto Geofisico del Peru, Ministry of Education, and is operated with support from the NSF cooperative agreement AGS-0905448 through Cornell University.
 The Editor thanks Andrei Mikhailov and an anonymous reviewer for their assistance in evaluating this paper.