Lunar tide in the equatorial electrojet in relation to stratospheric warmings

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1 JOURAL O GEOPHYSICAL RESEARCH, VOL. 116,, doi:1.129/211ja1747, 211 Lunar tide in the equatorial electrojet in relation to stratospheric warmings R. J. Stening 1 Received 4 August 211; revised 27 September 211; accepted 28 September 211; published 1 December 211. [1] The relationship between sudden stratospheric warmings (SSWs) and large-amplitude lunar tides in the equatorial electrojet (EEJ) is studied. Analysis of ground magnetometer data shows that the lunar tide in the EEJ is maximum during the northern winter season except in the Pacific Ocean region. Since SSWs are also a northern winter phenomenon, it is suggested that the relation between the large lunar tide in the EEJ and the SSW may possibly be coincidental. The lunar tide in the geomagnetic variations at Huancayo is anomalously large compared with other EEJ stations. An examination of geomagnetic variations at EEJ stations during SSW events shows that afternoon counter-electrojets are frequently present at new moon and full moon, though the relationship is sometimes broken. The observation of large lunar EEJs when no SSW is present and of various different delay times suggests that other atmospheric processes are likely to be in play. Citation: Stening, R. J. (211), Lunar tide in the equatorial electrojet in relation to stratospheric warmings, J. Geophys. Res., 116,, doi:1.129/211ja Introduction [2] Recently several papers have suggested a relation between changes in the equatorial electrojet during stratospheric warmings in the north polar region [Chau et al., 29; Sridharan et al., 29; Goncharenko et al., 21a, 21b; Anderson and Araujo-Pradere, 21; Eccles et al., 211]. ejer et al. [21] presented evidence that the changes were in synchronism with the lunar tide. In this paper I will review some of the evidences for this and question whether there may be a causal link between a sudden stratospheric warming (SSW) and changes in the equatorial electrojet or whether these are just two phenomena that both happen to occur during the northern winter period. [3] The criteria for the existence of a major SSW are the reversal of the 1 hpa (3 km) zonal mean winds and of the temperature gradient poleward of 6 north latitude [Manney et al., 29]. A list of such events between 1957 and 22 was found at the Web site edu/ssws/index.php, though this Web site is no longer accessible at the time of writing. [4] The equatorial electrojet is a band of intense electric current flowing along the magnetic dip equator during daytime within the E region of the ionosphere. It is an outstanding feature found in the daily variations, measured from a local midnight baseline, of the northward (DX) or horizontal (DH) components of the magnetic field recorded by magnetometers on the ground in these equatorial regions. requently the electrojet current effect is isolated by subtracting 1 School of Physics, University of ew South Wales, Sydney, ew South Wales, Australia. Copyright 211 by the American Geophysical Union /11/211JA1747 the magnetic variation recorded at an observatory a short distance away from the dip equator from that recorded at the equatorial station. or example DH at Alibag (18.5, 72.9 E) is subtracted from DH at Trivandrum (8.5, 76.9 E) in India (geographic coordinates are given here). In this way ring current effects are mostly removed [see, e.g., Stolle et al., 28]. Because the E region electric conductivity becomes much smaller at nighttime, the DH at night is often taken as the base level and is subtracted. The normal daily magnetic variation then appears as a single maximum at, or a little before, local noon, with values near to zero at night. The abnormal changes to be discussed here are usually seen as excursions, during the morning or afternoon, below the nighttime level. These represent reversals in the direction of flow of the electrojet current and are often called counterelectrojets (CEJs). urthermore it has been established that there is a close relationship between the electrojet current magnitude and direction and the behavior of vertical plasma drift velocities in the region, as measured by incoherent scatter radars [Anderson et al., 24; Stolle et al., 28]. [5] The possible association between CEJs and SSWs was earlier discussed by Stening [1977] where a very crude statistical relationship was established. Later Stening et al. [1996] performed a more detailed investigation. That paper suggested the possibility that CEJs might be due to a lunar atmospheric semidiurnal tide since the vertical propagation of such a tide may be enhanced during the atmospheric conditions associated with an SSW. In fact 15 out of the 23 events mentioned there occur near to full moon or new moon, as do the events found by ejer et al. [21]. Whereas SSWs primarily occur in northern winter and are evidenced by winds measured at 3 km altitude, the winds examined by Stening et al. [1996] came from Saskatoon (52, 17 W) at a much greater height, 99 km. Also about half of the events 1of7

2 STEIG: LUAR EECTS I THE EQUATORIAL ELECTROJET Table 1. Details of Observatories Used in Analysis of Lunar Geomagnetic Tides Observatory Latitude Longitude Years Analyzed umber of Days Analyzed Error (nt) Davao Trivandrum Addis Ababa Huancayo Koror are not in northern winter. Indeed one event showing CEJs accompanied by a zonal wind reversal at Saskatoon occurs in September. [6] ejer et al. [21] have examined electrojet behavior during the January 23 SSW event. They first show large perturbations in the vertical plasma drifts at Jicamarca with large downward drifts in the afternoon accompanied by increased upward drifts in the morning, thus giving a semidiurnal character to the variation. urthermore the time of maximum downward perturbation becomes later on succeeding days, indicating a possible lunar influence. Similar changes were observed in the ground-based geomagnetic variations DH which occurred earlier and stronger at American longitudes and later and weaker in the Pacific Ocean region. [7] Anderson and Araujo-Pradere [21] examined region drifts, derived from ground-based magnetometers in Peru and in the Philippines during 23 and 24 SSWs. Sometimes the morning increase and afternoon decrease coincided with a new moon or full moon in Peru, but the similar changes in the Philippines were either 3 days later, and so not coincident with the expected lunar phase, or hardly visible at all. The large drift differences reported by Chau et al. [29] in January 28 occur near to full moon. [8] The effects of the changes in the equatorial electrodynamics are also found in the equatorial ionospheric anomaly. Goncharenko et al., 21a report a large semidiurnal effect in the Jicamarca drifts on 27 January 29. This is 1 day after new moon and 3 days after the SSW criterion is fulfilled. The morning increase and afternoon decrease is also found in the GPS total electron content (TEC) at the crests of the equatorial anomaly. That the ionospheric effect occurs some days after the SSW has significance for ionospheric forecasting. Goncharenko et al., 21b present further cases where changes in the TEC have a semidiurnal character. Sometimes these changes maximize near a new moon or full moon, sometimes not. Chau et al. [21] report changes in electron density and in TEC at Arecibo (18.3, 66.7 W) associated with SSWs. Probably the concurrent large changes in the equatorial electric fields move the crests of the equatorial anomaly poleward at these times. Pedatella and orbes [21] have taken Global Ionospheric Maps (GIMs) of GPS TEC and analyzed the data into migrating and nonmigrating tides. Of particular interest is the SW2 (migrating semidiurnal) tide which they show has a 14 day periodicity in its amplitude at the anomaly crests: this is suggestive of a lunar effect which persists from January to March 29. Other relations between planetary waves and nonmigrating tides are found. In particular they suggest that similar oscillations of the amplitude of the planetary wave 1 and of the nonmigrating semidiurnal tide zonal wave number 1 indicate that planetary waves are driving changes in global tides through nonlinear interactions. [9] It is important to note that no major warmings occurred during the nine winters between and [Manney et al., 25]. 2. Lunar Geomagnetic Tide in the Equatorial Region [1] irst the behavior of the lunar tides in the horizontal geomagnetic variation DH will be examined. Hourly values of DH are subjected to a ourier decomposition as outlined in the work of Stening and Winch [1979] and described in more detail in the work of Winch and Cunningham [1972]. Twenty ourier components are isolated of the form L sin(nt 2v +(k 2)h + l) where L is the amplitude, t is the local time, n = 1 to 4, n is the lunar age, k = to 4 and l is a phase. The data were downloaded from the Edinburgh World Data Center for Geomagnetism ( ac.uk/catalog/master.html). The five international disturbed days of each month were excluded from the analysis. Details of the data used are given in Table 1 and the monthly variations of the amplitude of the lunar tide at local noon are plotted in igure 1. The size of the error bars, representing one standard deviation, are also given in Table 1. Except at Davao, a clear maximum amplitude is seen during December January. In these months the variations at Huancayo in South America are often twice as large as elsewhere. It is well known that the equatorial electrojet at Huancayo is expected to be larger owing to the increased Cowling conductivity associated with the weaker main magnetic field in the South American anomaly. However, this does not fully explain the large lunar amplitude at Huancayo. If the solar variation of DH is extracted from the same data, the familiar semiannual variation with season is found with maxima near the equinoxes. If the amplitudes of the solar component at local noon at Huancayo are compared to those at Addis Ababa, the ratio is 1.1. The same ratio for the lunar components is 2., so there is some other mechanism enhancing the lunar amplitude at Huancayo. [11] One hesitates to emphasize too much the different seasonal variation at Davao since fewer data are available. Yet a similar seasonal variation is obtained at Koror, also in the Western Pacific region, though even fewer data are used. [12] Some estimate of the variation of the amplitude of the lunar tide in DH at Huancayo and Trivandrum/Tirunelveli was obtained by using the Winch and Cunningham [1972] analysis method on smaller data sets. Each data set comprised hourly values from March in one year to the end of ebruary in the following year, thus including one northern winter in each analysis. This calculation provides amplitudes of the lunar semidiurnal tide for each local time and each month. The amplitude is characteristically largest near noon, as shown in igure 2. The noontime amplitudes for December, January and ebruary are averaged and plotted in 2of7

3 STEIG: LUAR EECTS I THE EQUATORIAL ELECTROJET igure 1. Variation of the amplitude of the lunar tide in DH with the month of the year at Davao (DAV), Trivandrum (TRD), Addis Ababa (AAE), and Huancayo (HUA). igure 3. The indicated year is the later one, so December 2 to ebruary 21 is plotted at 21. Large year-to-year changes can be seen. Sometimes these changes are in a similar sense in Peru and India, sometimes not. [13] igure 3 shows that the lunar tide in DH in South America was indeed outstandingly large during the winter with an average value of 39 nt at noon. The point to be made here is that much of the day-to-day variation occurs in the middle of the day, where it is less obvious as there are hardly any CEJs near noon while these can be easily seen in the afternoon and morning. In order to show this some of the raw data have been plotted, where, for each individual data set, the mean of all values for a UT day is subtracted from each hourly value. The lunar periodicity can be seen in igure 4 where four cycles of the lunar semimonthly tide are clearly visible. In igure 4 the DH values at uquene (5.5, E) are subtracted from those at Huancayo (12. S, E). [14] The phase of the lunar tide at Huancayo in January 23, t, (lunar hour of maximum) is 8.5 h, as derived during the ourier analysis process. ew moon is on 2 January when the lunar age n =. The local time when the lunar tide is maximum is t = t + n = 8.5 h, with a corresponding minimum about 6 h later at 14.5 h and so we see an afternoon minimum Hours LT igure 2. Amplitude of the lunar tide in DH at Addis Ababa in December as it varies with local time, using hourly data from 1985 to 1989, with the five monthly disturbed days omitted. 3of7

4 STEIG: LUAR EECTS I THE EQUATORIAL ELECTROJET igure 3. Amplitude of local noon lunar tide in DH during northern winter at Huancayo (HUA) and Trivandrum (TRD)/Tirunelveli (TIR). (reversed electrojet) on this day. This occurs again around full moon on 18 January. [15] On 9 January a morning minimum is seen. On this day the lunar age n is 78 and so the local time of maximum is /15 or about 14 h with a corresponding minimum about 8 h as observed. Also, 9 January marks the overall maximum amplitude followed by the next maximum on 23 January, 14 days (half a lunation) later. [16] There is a short-lived SSW starting about 1 January 22 [Manney et al., 25, igure 8]. The lunar tide in DHis less pronounced in January 22 (igure 5) than in January 23 (igure 4), though again four cycles of the lunar semimonthly tide may be noticed. The calculated amplitude is also smaller than for 23 (see igure 3). The maximum amplitude on 5 January 22 coincides with a predicted minimum of the lunar tide at 8 h LT, using the calculated lunar hour of maximum of 9 h. In fact the minimum occurs at 6 h, possibly owing to a lesser contribution from the ever-present solar tidal influences at that time. Two days later, on 7 January, the observed minimum has moved to 8 h LT. The next major maximum after 5 January is on 2 ebruary, 28 days later. [17] By contrast the variations of DH are presented for the winter in igure 6. Again a clear lunar periodicity can be seen with maxima separated by 13 to 15 days, but there was no SSW during this winter, as shown in the work of Manney et al., 25, igure 8]. urthermore the amplitude of the lunar tide in DH (igure 3) is not significantly different from that in years when there was an SSW. 3. Indications of SSW Effects at Equatorial Stations [18] The times of occurrence of SSWs were obtained from the above-mentioned Web site and the presence of counterelectrojets was determined by visually inspecting the daily variations of DH at the equatorial stations Addis Ababa, Huancayo and Trivandrum (or Tirunelveli). It was hoped that Hours igure 4. Huancayo DH minus uquene DH for January and ebruary 23. The plot starts at : UT on 1 January 23. The SSW temperature peaks on 31 December 22. Times of new moon () and full moon () are indicated. 4of7

5 STEIG: LUAR EECTS I THE EQUATORIAL ELECTROJET Hours igure 5. Huancayo minus uquene DH for January ebruary 22. The plot starts at : UT on 1 January 22.The SSW temperature peaks on 3 December 21. this procedure might give some insight into how much the CEJs depended on an SSW and whether the CEJs fitted a lunar tidal source. The results are given in Table 2. Whereas a fixed date is given for each SSW, representing the time of maximum warming, the warming period may extend over two weeks or more, giving, at some time during that interval, atmospheric conditions favorable for the propagation of lunar tides and so generating an afternoon CEJ around the time of new moon or full moon. As a result there are very few cases in Table 2 where there is no relation between the warming and CEJ occurrence. The days of occurrence of afternoon CEJs are fairly tightly controlled by the lunar phase, but there a few examples where the phase is wrong and a solar semidiurnal tide might be invoked to explain these. It is also apparent that the CEJs do not always appear simultaneously at all three longitude sectors, being sometimes seen only at one or two of the electrojet stations. 4. Discussions [19] One of the features emphasized by ejer et al. [21] is that the supposed SSW effect on the equatorial electrojet occurs earlier in America than in the Pacific Ocean sector. This property is also mentioned by Anderson and Araujo- Pradere [21], who suggest that the effect is 3 days later in the Pacific. It has been shown above that a negative departure in DH in afternoon hours due to a lunar tide should occur near to the time of new moon or full moon. Yet some examples presented by Anderson and Araujo-Pradere, for 8 1 January 23, occur close to the first quarter lunar age. This leads one to speculate whether some tide other than lunar may be responsible here. During this particular time slot (8 1 January) there is also a lessening in the stratospheric warming in between two larger warming maxima [ejer et al., 21, igure 4] while the amplitude of the effect in the Pacific (Philippines), namely a morning maximum followed by an afternoon minimum, is clearly the largest that these authors present. [2] Modeling studies such as that of Stening et al. [1997] show that changes in the background atmospheric conditions associated with an SSW do produce significant global changes in the amplitude and phase of the lunar atmospheric tide at an altitude of 9 km. urthermore analysis of winds at 9 km at Saskatoon (52, 17 W) has shown year-to-year Hours igure 6. Huancayo minus uquene DH for December 1997 to January The plot starts on 1 December Day numbers for the various maximums are indicated at the top. 5of7

6 STEIG: LUAR EECTS I THE EQUATORIAL ELECTROJET Table 2. Equatorial Electrojet Responses to SSWs a SSW Date Was CEJ Detected? Comments 15 Jan 196? magnetic activity 28 Jan 1963 yes coincident M; CEJs at AAE and TRD 16 Dec 1965 no CEJs at M/M at HUA but not at SSW time 23 eb 1966? magnetic disturbance 7 Jan 1968? CEJs occur before SSW near M 28 ov 1968 no no relation 1 Jan 197? disturbed at SSW; CEJs later at M at AAE 18 Jan 1971? CEJs before SSW just after M 31 Jan 1973 yes CEJ on 2 and 4 eb 1973 at AAE at M 9 Jan 1977 yes CEJs at AAE, TRD, and HUA 22 eb 1979? disturbed 29 eb 198? coincident M; disturbed; nothing clear 4 Mar 1981 yes coincident M, but morning CEJ at TRD 4 Dec 1981 no noon deion at TRD, wrong lunar phase 24 eb 1984? uncertain 1 Jan 1985 yes afternoon CEJs frequent the whole month 23 Jan 1987? CEJs near M 6 days after SSW at TRD and AAE 7 Dec 1987 yes coincident with M 14 Mar 1988 no CEJs at SSW but wrong lunar phase 21 eb 1989 yes near M; CEJ at TRD and AAE but not HUA 15 Dec 1998 yes CEJs at HUA; TRD at M during warming 26 eb 1999 yes CEJ at TRD only 2 Mar 2 no M and SSW coincide but no CEJ 11 eb 21? small CEJ at TIR 3 Dec 21 yes coincident M, CEJs TRD, and AAE but not HUA 17 eb 22 yes morning CEJ, expected lunar phase at SSW 15 Jan 23 yes CEJs near noon at M during warming 18 eb 23? unclear 2 Jan 24? possible CEJs at M during warming; disturbed 23 Jan 28 yes coincident M 24 Jan 29 yes coincident M a Using available data from Addis Ababa (AAE), Trivandrum (TRD), Huancayo (HUA), and Tirunelveli (TIR) to search for counter-electrojets (CEJs), noting coincidences with new moon (M) or full moon (M). changes in the amplitude and phase of the lunar atmospheric tide [Stening et al., 1994], though these changes could not be directly related to SSWs. [21] uller-rowell et al. [21] were able to generate an SSW within their Whole Atmosphere Model and found significant changes in the propagation of various atmospheric tides. In particular the terdiurnal tide correlated best with the SSW with a 3 day time delay. [22] Doubtless changes in the background atmosphere during SSWs will also affect solar tides, particularly the solar semidiurnal tide. In this respect, Pedatella and orbes [21] discuss how the planetary wave activity associated with an SSW, particularly PW1 with zonal wave number 1, may have a nonlinear interaction with the migrating (solar) semidiurnal tide to yield the nonmigrating semidiurnal tide with zonal wave number 1, evidence of which they find in total electron content data during the 29 SSW event. This is in addition to the approximate 14 day periodicity seen in their data, as mentioned above, indicating a possible lunar rather than solar semidiurnal tidal influence. [23] Eccles et al. [211] have presented evidence of lunar tides near the magnetic equator in Peru, both in the magnetic variations at Huancayo and in the region, driven by the fountain effect. They also run a model to yield lunar tides in DH and in f o 2, though their adopted amplitude of lunar tidal winds in the model (65 m/s) is much higher than is observed or is found in other models such as the Global Scale Wave Model (GSWM) of Hagan et al. [1995] in which the lunar tidal winds do not exceed 2 m/s. The GSWM was used by Stening et al. [22] to model the global lunar geomagnetic variations and this yielded amplitudes reasonably close to those derived from observations. 5. Conclusions [24] The results of ejer et al. [21] show a clear association of an SSW with an enhanced lunar tide in the equatorial electrojet and modeling studies suggest that changes in background winds accompanying an SSW will alter the atmospheric tides, both lunar and solar, which ultimately drive the currents in the EEJ. However, large-amplitude lunar tides in the EEJ and SSWs both occur most frequently in the northern winter and so reported correlations may be coincidental. Large-amplitude lunar tides in the EEJ, some clearly seen in the raw data, have been shown when there is no SSW. [25] Acknowledgments. Geomagnetic variation data were obtained from the World Data Centre for Geomagnetism (Edinburgh) Web site. [26] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. References Anderson, D., and E. A. Araujo-Pradere (21), Sudden stratospheric warming event signatures in daytime E B drift velocities in the Peruvian and Philippine longitude sectors for January 23 and 24, J. Geophys. Res., 115, AG5, doi:1.129/21ja Anderson, D., A. Anghel, J. Chau, and O. Veliz (24), Daytime vertical E B drift velocities inferred from ground-based magnetometer observations at low latitudes, Space Weather, 2, S111, doi:1.129/ 24SW95. Chau, J. L., B. G. ejer, and L. P. Goncharenko (29), Quiet variability of equatorial E B drifts during a sudden stratospheric warming event, Geophys. Res. Lett., 36, L511, doi:1.129/28gl Chau, J. L.,. A. Aponte, E. Cabassa, M. P. Sulzer, L. P. Goncharenko, and S. A. Gonzalez (21), Quiet time ionospheric variability over Arecibo during sudden stratospheric warming events, J. Geophys. Res., 115, AG6, doi:1.129/21ja Eccles, V., D. D. Rice, J. J. Sojka, C. E. Valladares, T. Bullett, and J. G. Chau (211), Lunar atmospheric tidal effects in the plasma drifts observed by the Low-Latitude Ionospheric Sensor etwork, J. Geophys. Res., 116, A739, doi:1.129/21ja ejer, B. G., M. E. Olson, J. L. Chau, C. Stolle, H. Lühr, L. P. Goncharenko, K. Yumoto, and T. agatsuma (21), Lunar-dependent equatorial ionospheric electrodynamic effects during sudden stratospheric warmings, J. Geophys. Res., 115, AG3, doi:1.129/21ja uller-rowell, T.,. Wu, R. Akmaev, T.-W. ang, and E. 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7 STEIG: LUAR EECTS I THE EQUATORIAL ELECTROJET Arctic stratosphere since the late 199s, J. Geophys. Res., 11, D417, doi:1.129/24jd5367. Manney, G. L., M. J. Schwartz, K. Krüger, M. L. Santee, S. Pawson, J.. Lee, W. H. Daffer, R. A. uller, and. J. Livesey (29), Aura Microwave Limb Sounder observations of dynamics and transport during the record-breaking 29 Arctic stratospheric major warming, Geophys. Res. Lett., 36, L12815, doi:1.129/29gl Pedatella,. M., and J. M. orbes (21), Evidence for stratosphere sudden warming-ionosphere coupling due to vertically propagating tides, Geophys. Res. Lett., 37, L1114, doi:1.129/21gl4356. Sridharan, S., S. Sathishkumar, and S. Gurubaran (29), Variabilities of mesospheric tides and equatorial electrojet strength during major stratospheric warming events, Ann. Geophys., 27, , doi:1.5194/ angeo Stening, R. J. (1977), Electron density changes associated with the equatorial electrojet, J. Atmos. Terr. Phys., 39, , doi:1.116/ (77)919-x. Stening, R. J., and D. E. Winch (1979), Seasonal changes in the global lunar geomagnetic variation, J. Atmos. Terr. Phys., 41, , doi:1.116/ (79) Stening, R. J., A. H. Manson, C. E. Meek, and R. A. Vincent (1994), Lunar tidal winds at Adelaide and Saskatoon at 8 to 1 km heights, , J. Geophys. Res., 99, 13,273 13,28. Stening, R. J., C. E. Meek, and A. H. Manson (1996), Upper atmosphere wind systems during reverse equatorial electrojet events, Geophys. Res. Lett., 23, , doi:1.129/96gl2611. Stening, R. J., J. M. orbes, M. E. Hagan, and A. D. Richmond (1997), Experiments with a lunar atmospheric model, J. Geophys. Res., 12, 13,465 13,471, doi:1.129/97jd778. Stening, R. J., C. Carmody, and J. Du (22), Simulating the lunar geomagnetic variations, J. Geophys. Res., 17(A7), 1125, doi:1.129/21ja24. Stolle, C., C. Manoj, H. Lühr, S. Maus, and P. Alken (28), Estimating the daytime equatorial ionization anomaly strength from electric field proxies, J. Geophys. Res., 113, A931, doi:1.129/27ja Winch, D. E., and R. A. Cunningham (1972), Lunar magnetic tides at Watheroo: Seasonal, elliptic, evectional, variational and nodal components, J. Geomagn. Geoelectr., 24, , doi:1.5636/jgg R. J. Stening, School of Physics, University of ew South Wales, Sydney, SW 252, Australia. (r.stening@unsw.edu.au) 7of7