Tidal waves in the polar lower thermosphere observed using the EISCAT long run data set obtained in September 2005

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009ja015237, 2010 Tidal waves in the polar lower thermosphere observed using the EISCAT long run data set obtained in September 2005 S. Nozawa, 1 Y. Ogawa, 2 S. Oyama, 1 H. Fujiwara, 3 T. Tsuda, 1 A. Brekke, 4 C. M. Hall, 4 Y. Murayama, 5 S. Kawamura, 6 H. Miyaoka, 2 and R. Fujii 1 Received 29 December 2009; revised 6 April 2010; accepted 19 April 2010; published 18 August [1] Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Tromsø (69.6 N, 19.2 E) over 23 days, from September 6 to 29, The derived semidiurnal amplitude exhibited day to day variations ( 5 30 ms 1 ) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17 22, Between September 6 and 16, the vertical wavelengths were estimated to be 58 km and 76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than 24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms 1 ) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5 6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere. Citation: Nozawa, S., et al. (2010), Tidal waves in the polar lower thermosphere observed using the EISCAT long run data set obtained in September 2005, J. Geophys. Res., 115,, doi: /2009ja Introduction [2] Atmospheric tides observed in the mesosphere and lower thermosphere exhibit substantial long and short term variability in amplitude and phase. Bernard [1981b] suggested that long period (longer than five days) variations are related to variations of the tidal excitation source or of propagation conditions in the middle atmosphere, whereas short term (less than a few days) fluctuations may be due to local perturbations. The difficulty in specifying the mechanisms producing short time variations is probably due to the existence of several such mechanisms acting together or separately. Pancheva [2001] summarized causes of tidal variability as follows: (1) changes in the zonal mean winds and temperature structure through which the tides propagate, 1 Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 2 National Institute of Polar Research, Tachikawa, Japan. 3 Faculty of Science, Tohoku University, Sendai, Japan. 4 Faculty of Science, University of Tromsø, Tromsø, Norway. 5 National Institute of Information and Communications Technology, Koganei, Japan. 6 Okinawa Subtropical Environment Remote Sensing Center, National Institute of Information and Communications Technology, Kunigami, Japan. Copyright 2010 by the American Geophysical Union /10/2009JA (2) changes in the tidal force, which include the global distribution of ozone, water vapor, the release of latent heat, and changes in the UV flux, (3) variations in eddy diffusivity and/ or dissipation, and (4) non linear interaction between the tides and the planetary waves. Teitelbaum and Vial [1991] indicated that nonlinear interactions between tides and planetary waves really occurred in the upper mesosphere and the lower thermosphere. The day to day variability is one of the characteristics in the lower thermosphere. [3] It is understood that the neutral atmosphere plays an important role in the magnetosphere ionosphere thermosphere coupling process. Our understanding of the wind dynamics in the lower thermosphere at high latitudes has been steadily advanced during the last three decades using model simulations [e.g., Kwak and Richmond, 2007; Kwak et al., 2007], satellite observations [e.g., Burrage et al., 1996; Palo et al., 2007], and Incoherent Scatter (IS) radars [e.g., Azeem and Johnson, 1997; Nozawa and Brekke, 1999a]. However, some of the details, such as the response to the magnetospheric disturbance, tidal variability, planetary wave activities, seasonal variations, solar cycle dependences, and so on, are not yet fully understood. To investigate tidal variability in the lower thermosphere, IS radar is one of the best tools. In contrast to MF and meteor radars, it is difficult for IS radars to run for longer time intervals (roughly, 1 week or longer) due to the high cost of their operation. For the last five years, 1of16

2 Figure 1. Temporal variations of the electron densities from 1830 UT on September 6 to 1500 UT on September 29, 2005 observed with the EISCAT UHF radar at Tromsø are illustrated between 90 and 600 km in altitude. however, operations of IS radars were conducted several times for longer intervals, and the observed data revealed daily variations of the ionosphere. From September 6 to 29, 2005, the EISCAT UHF radar (the so called KST radar) at Tromsø (69.6 N, 19.2 E) [Folkestad et al., 1983] operated continuously in Common Program two (CP2) mode [Collis, 1995], which enables us to derive wind velocity vectors in the lower thermosphere ( km). Although the KST radar had been in operation for 20 years by that time, this run was the first time that the EISCAT UHF radar operated continually over about a one month interval. [4] In this paper, we present characteristics of the lower thermospheric wind above Tromsø over 23 days from September 6 to 29, In section 2, a descriptive overview of the data sets is given. In section 3, the analysis method and observational results of the electric field and neutral wind are presented. In section 4, causes of the temporal variations of the diurnal and semidiurnal tides are discussed. In particular, special attention is paid to a mode change of the semidiurnal tide. In section 5, a summary and conclusions are given. 2. Data 2.1. EISCAT Operation [5] The EISCAT UHF radar experiment data obtained at Tromsø from 1830 UT on September 6 to 1500 UT on September 29, 2005 were analyzed. The operation was conducted almost continuously over about 23 days in a so called CP 2 mode (the beam swinging mode) [Collis, 1995] from which we can derive 3 D ion velocity vectors from 90 km up to 600 km. A data break occurred for a one day interval from 0700 UT on September 21 to 0700 UT on September 22, when the radar operated in another mode of the experiment in which we could not derive the wind velocity. In the CP2 mode at Tromsø, the line of sight of the combined transmitter and receiver antenna is pointed toward four consecutive positions, including one field aligned position, with a dwell time of 1 min in each position, making a six minute full cycle time for the antenna. We can thus derive 3 D ion velocity vectors every 6 min. The altitude resolution is 3km in the E region. [6] Figure 1 shows the temporal variation of the electron density observed by the EISCAT UHF radar over the experiment s period. Data coverage was good except for the one day interval starting at 0700 UT on September 21. Clear temporal variations of electron density can be found, such as high values, which are due to the solar ionization during the day time and are due to auroral particle precipitation at night. The enhancement of the electron density in the E region Figure 2. Temporal variations of (top) F10.7 and (bottom) Ap indices are presented for September 6 29, of16

3 Figure 3. Temporal variations of the electric field from 1830 UT on September 6 to 1500 UT on September 29, 2005 are shown for (top) the meridional component and (bottom) the zonal component. One hour running averaged values are presented. due to auroral particles occurred almost every day, which rendered good quality data in the E region. At Tromsø, local time (LT) is 1 hr ahead of Universal Time (UT) (i.e., LT = UT + 1hr), and magnetic local time (MLT) is 2.5 hrs ahead of UT (i.e., MLT = UT + 2.5hrs). In the present study a right handed geographic coordinate system is used, with positive values in the northward (x), eastward (y), and downward (z) directions. [7] Figure 2 shows temporal (i.e. daily) variations of the solar 10.7 cm flux indices (F 10.7 ) and Ap indices (the arithmetic mean of the 3 hour ap index) from September 6 to 29, The F 10.7 index is a proxy for extreme ultraviolet (EUV) radiative flux from the Sun. The F 10.7 index varied smoothly with time, showing that the solar activity was relatively high (F 10.7 > 100 ) between September 9 and 18, 2005, and went lower for the rest of the experiment s interval. Geomagnetic activity was high (Ap > 16: Kp = >3) for 8 days from September 9 to 16, The Sun was active until September Derivation of the Electric Field [8] Assuming that the F region plasma drifts perpendicularly to the geomagnetic field B solely due to the electric field E, this field can be obtained by E ¼ v ð F BÞ ð1þ where v F is the ion velocity vector in the F region (data at 276 km are used in this study). For B we use the International Geomagnetic Reference Field (IGRF) model [IAGA Division I Working Group 1, 1987]. [9] Figure 3 shows temporal variations of the northward (top panel) and eastward (bottom panel) electric field over 23 days from 1830 UT on September 6 to 1500 UT on September 29, In Figure 3, one hour running averaged values are presented. The meridional electric field varied from 40 mv m 1 to 60 mv m 1, and it exhibited clear diurnal variations almost over the entire observational interval. The zonal electric field was smaller than that of the 3of16

4 Figure 4. Spectra of normalized amplitudes of the electric field derived from the data set with a 22.5 day length starting at 1830 UT on September 6 are shown for (top) meridional and (bottom) zonal components. Horizontal broken lines denote 99% significance levels. Vertical broken lines denote locations with periods of 8 hrs, 12 hrs, 16 hrs, 24 hrs, 2 days, and 6 days. meridional component and generally varied from 10 mv m 1 to 10 mv m 1, except for 3 days from September 11 to 13, when it ranged from 20 to 20 mv m 1. [10] Figure 4 presents spectra of the normalized amplitude of the meridional (top panel) and zonal (bottom panel) components of the electric field. The analysis was made using the Lomb Scargle method, which is based on least squares frequency analysis of unequally spaced data [cf. Hocke, 1998] with a data window length of 22.5 days, starting at 1830 UT on September 6, The significance level (s) for a specific value of probability p is obtained from s ¼ ln 1 ð1 pþ ð1=nþ ð2þ where N is the number of data points in the time series [Nozawa et al., 2003]. Thus, significance levels with 99% and 50% are calculated from equation (1) with p = 0.01 and 0.5, respectively. Data with periods shorter than 6 days are presented. The 24hr variation is the strongest, and the 12hr variation is also seen for both horizontal components. The peak with the 24 hr period of the meridional component (corresponding amplitude is 15 mv/m) is the most significant, and it could affect the diurnal tidal variation of the neutral winds through the ion drag force (e.g., Lorenz force) [cf. Tsuda et al., 2007]. We will discuss the ion drag effect in Section Neutral Winds in the Lower Thermosphere [11] The derivation of the neutral wind velocity in the lower thermosphere ( km) is founded on knowledge of how it is related to the ion velocity and the electric field in that region. This idea was first introduced by Brekke et al. [1973], thoroughly evaluated by Comfort et al. [1976] and later revised by Rino et al. [1977], and it has since been used by a string of authors [see Nozawa and Brekke, 1999a, and references therein]. According to Rino et al. [1977] the steady 4of16

5 Figure 5. Spectra of normalized amplitudes of the wind velocities derived from the data set with 22.5 day length starting at 1830 UT on September 6 are shown for (left) meridional and (right) zonal components for 5 heights at 96, 102, 109, 115, and 120 km. Horizontal broken lines denote 99% significance levels. Vertical broken lines denote locations with periods of 6 hrs, 8 hrs, 12 hrs, 24 hrs, 2 days, and 6 days. state ion mobility equation can be solved for the neutral wind velocity vector u in the following way: u ¼ v W i B in ðe þ v BÞ ð3þ where n in is ion neutral collision frequency, W i (= eb/m i )is the gyrofrequency of the ions, e is the elementary charge (= C), m i (=30.5 amu) is the mean ion mass, and B is the magnitude of B. The model neutral atmosphere used for calculating n in is MSIS90 [Hedin, 1991], and the formula for the ion neutral collision frequency is that given by Schunk and Walker [1973]. With these models W i is found to equal n in at 120 km altitude under all conditions. There is not yet consensus for the choice of the model for the ion neutral collision frequency, and therefore it is possible that it can introduce uncertainties to the derived wind velocities at the uppermost heights [e.g., Johnson and Virdi, 1991; Williams and Virdi, 1989]. The electric field is projected onto heights between 96 and 120 km where the neutral wind is calculated. Due to the high inclination of the geomagnetic field at Tromsø and because the potential along field lines is approximately constant, this projection involves no systematic uncertainties in the magnitude and direction of the electric field of significance for this study. Small scale electric field components that vary in time at a rate comparable to that of the observation cycle, or that vary in space on a scale comparable to that of the volume of the multipoint observations, will cause errors in the determination of both E and v, but these are of a random nature and should not contribute significantly to the estimation of the slowly varying winds studied here. 3. Results 3.1. Temporal Variation of Neutral Wind in the Lower Thermosphere [12] The neutral wind velocity was derived every 6 min at 9 heights between 96 and 120 km from 1830 UT on September 6 to 1500 UT on September 29. Data coverage was good except for the one day data gap occurring from 0700 UT on September 21 to 0700 UT on September 22. By utilizing the wind data, we have derived quasi two day wave (Q2DW) as well as tidal components using the Lomb Scargle periodogram method. Figure 5 illustrates spectra of the normalized amplitude of the wind velocity for 5 heights of 96, 102, 109, 115, and 120 km. Here we use the data sets with a length of 22.5 days starting at 1830 UT on 5of16

6 Figure 6. Altitude profiles of the (top) amplitudes and (bottom) corresponding phases (the local time of maximum) (left) of a 6 day wave for the meridional and (right) of a 4.7 day wave for the zonal components are shown. Filled circles denote data above 99% significance levels. September 6. Thus, determined periodic components shown in Figure 5 can be thought to exist over 22.5 days or can be thought of as the average of the corresponding components. At and below 115 km, the semidiurnal component is the strongest in the meridional and zonal components, while the diurnal component becomes the strongest at 120 km. The Q2DW component is not obviously seen in this case except above 115 km, where the Q2DW amplitude is slightly above or close to the 99% significance level. [13] It should be noticed that in Figure 5, clear peaks, which are above 99% significance level, with a period of 6 days for the meridional component and 4.7 days for the zonal component are identified between 96 and 120 km and between 102 km and 120 km, respectively, suggesting the existence of a planetary wave. Here we describe its features briefly. Figure 6 shows altitude profiles of the 6 day wave for the meridional component and of the 4.7 day wave for the zonal component from 96 to 120 km. Solid circles denote data values which are above 99 % significance level. The amplitudes of both the components tend to increase as altitude increases. The amplitudes are 10 ms 1 below 102 km and become 28 ms 1 at around km. Their phases (the local time of maximum) tend to shift toward earlier hours as altitude increases. The vertical wavelengths are estimated to be 41 km (±5 km) and 63 km (±3 km) for the meridional and zonal components, respectively, using the least squares method. We are not sure about the reasons of the difference of the periods between the meridional and zonal components. One possible interpretation is that not only one wave existed, but also at least two waves existed in the lower thermosphere in September [14] To investigate day to day variations of Q2DW and diurnal and semidiurnal tides, the analysis was performed with the following data window lengths for each component: a 2 day interval for the semidiurnal tide, a 4 day interval for the diurnal tide, and a 8 day interval for the Q2DW. Each data window starts at 1900 UT, and the start date of the data window is shifted by 1 day from September 6 toward the end 6of16

7 Figure 7. Altitude profiles of the amplitudes of the northward component of the Q2DW (45.2 hrs) are shown every 2 days from September 10 to 24, Filled larger diamonds, filled smaller diamonds, and open diamonds denote data above 99% significance levels, above 50 % (and below 99 %) significance levels, and below 50 % significance levels, respectively. of the experiment. The date of the data is defined to be the center of the data interval. For example, the September 8 data of the diurnal tide are derived from the wind data over a fourday interval from 1900 UT on September 6 to 1900 UT on September Quasi Two Day Wave [15] We investigated temporal variations of the quasi two day wave (Q2DW) activity with an 8 day data window in the lower thermosphere. Here we assumed the period of the Q2DW is between 54.9 hrs and 45.2 hrs (i.e., peaks are at 54.9 hrs, 51.2 hrs, 48 hrs, and 45.2 hrs), conforming to the definition used by Nozawa et al. [2003]. We have found that the 45.2 hr component was the strongest among the four components over the entire observational interval. Figure 7 illustrates altitude profiles of the amplitudes of the northward component of the Q2DW (45.2 hrs) shown every 2 days from September 10 to 24, At and above 111 km, we can identify the Q2DW activity during September and September when the amplitude was above 99 % significance level. The amplitude reached 30 ms 1 at the upper heights, and the amplitude maximized at 120 km on September 12, 2005 over the observational interval with value of 45 ms 1. On the other hand, the activity was low or below noise level between 96 and 109 km over the observational interval. This feature is similar to that found by the Upper Atmosphere Research Satellite (UARS) for the Southern Hemisphere [cf. Lieberman, 1999]. They indicated additional amplitude maxima between 110 and 120 km and above 135 km, separated by a local minimum between 120 and 125 km [Ward et al., 1996]. In the Southern Hemisphere, the wave period is close to 48 ± 3 hrs. Therefore, our result is consistent with the UARS observations and has proved the existence of the Q2DW in the lower thermosphere on some occasions in September at high latitudes in the northern hemisphere Day to Day Variability of the Diurnal Tide [16] Figure 8a illustrates together 19 altitude profiles of diurnal tidal amplitudes (top 2 panels) and phases (the local time of maximum) (bottom 2 panels) for meridional (left panels) and zonal (right panels) components. To clarify differences of the temporal variation between 3 intervals, the following three symbols are used depending on the date of the data: filled circles, open circles, and filled triangles denote data values between September 8 and 12, September 13 and 20, and September 21 and 26, respectively. From Figure 8a, it is identified that altitude profiles of the meridional amplitude differ from each other among the three time intervals. The amplitudes between September 13 and 20 were much smaller than those obtained for the other intervals above 110 km, and the altitude profiles differ from those for the 7of16

8 Figure 8a. Altitude profiles (19 sets) of the (top) amplitudes and (bottom) corresponding phases (the local time of maximum) of the diurnal tides for (left) meridional and (right) zonal components are shown. Symbols denote data obtained for different intervals. Filled circles (with solid lines), open circles (with dotted lines), and filled triangles (with solid lines) denote data for September 8 12, September 13 20, and September 21 26, respectively. other intervals. The amplitudes for September did not show an increase with height and were less than 40 ms 1 over the height region, while the amplitudes for the other two intervals increased with height and reached ms 1 at the upper heights. In the lower thermosphere at high latitudes, the meridional component of the diurnal tide is thought to be in situ generated mode [cf. Nozawa and Brekke, 1999a], and the phase is usually almost constant with the height for the two time intervals of September 8 12 and September For September 13 and 20, the phase of the meridional diurnal component tends to shift toward an earlier time at and above 105 km, as if the diurnal tide were in upward migration mode. Concerning the zonal component, there seems to be no clear differences between the three time intervals. [17] The differences of the meridional components among the three intervals are clearly identified in Figure 8b where the amplitude (left panels) and phase (right panels) of the meridional components are illustrated as a function of the day for three heights of 118, 111, and 105 km. Vertical bars associated with each data value denote errors (one sigma). At 105 km, the amplitude is in the range of 10 to 30 m s 1, and amplitude values between September 21 and 26 are smaller than those for the other intervals. The phase is fairly constant with time over the period at 105 km, but there is a small phase shift (1 2 hrs) found for September At the upper heights, the difference of the amplitude and the phase becomes more obvious between the 3 time intervals. At 111 km and 118 km, the amplitudes for the interval of September are smaller than (less than half of) those for the other intervals. This is a more salient feature than the day to day variation. At 111 km and 118 km, the phase is fairly constant with time for September 8 12 and September 21 26, while a 8of16

9 Figure 8b. Temporal variation (or day to day variation) of the (left) diurnal amplitude and (right) phase as a function of the day from September 8 to 26 for 3 heights at (top) 118 km, (middle) 111 km, and (bottom) 105 km. Symbols denote different intervals. Filled circles, open circles, and filled triangles denote data for September 8 12, September 13 20, and September 21 26, respectively. large change with time as well as altitude is found during September In particular the deviation is the largest for September 15, 16, and 17. [18] To summarize the features of the diurnal tide found here, a day to day variation of the amplitude is seen, but the phase is fairly stable with time and height in general. The more significant temporal variations of the meridional component are that 1) the amplitudes were significantly smaller for September than those for the other 2 intervals at and above 111 km and 2) the phases for September varied with height at and above 105 km, while the phases for the other two intervals were almost constant with height. Causes of the difference will be discussed in Section Day to Day Variability of the Semidiurnal Tide [19] Figure 9a shows 18 altitude profiles of the semidiurnal amplitudes and phases (the local time of maximum) from September 7 to 27 except for 3 days from September 20 to 22. Each semidiurnal tidal value is derived by the Lomb Scargle method with a 2 day length of data starting at 1900 UT. From the altitude variation of the phase (i.e., vertical wavelength), the data sets are divided into 3 time intervals. In Figure 9a, three symbols are used for differentiating time intervals. A filled circle, an open circle, and a filled triangle denote data values obtained between September 7 and 16 (1st interval), between September 17 and 23 (2nd interval), and between September 24 and 27 (3rd interval), respectively. Due to the data gap for the one day interval starting at 0700 UT on September 21, the data sets are not shown for 3 days for the September 20, 21, and 22. Amplitudes over the height region were generally between 5 and 80 ms 1 for meridional and zonal components regardless of the intervals. Contrary to variations of the diurnal amplitudes, the variations of the semidiurnal amplitudes with altitude as well as with time did not exhibit any salient differences among the three intervals. [20] The altitude profiles of the phase for the meridional and zonal components did not change significantly with the day over the 1st interval. This is also true over the 3rd interval. For the 2nd interval, however, the altitude profile varied significantly with time. Altitude profiles of the phase for the meridional and zonal components exhibited a change of the vertical wavelength during the 2nd interval. The vertical wavelengths of the semidiurnal tide between September 7 and 16 were estimated to be km using the least squares method, while they became shorter and are km between September 24 and 27. These results imply that a mode change of the semidiurnal tide occurred in the lower thermosphere during September 17 23, [21] To investigate the temporal variation of the semidiurnal tidal amplitudes and phase in more detail, Figures 9b and 9c 9of16

10 Figure 9a. Altitude profiles (18 sets) of the (top) amplitudes and (bottom) corresponding phases (local time of maximum) of the semidiurnal tide for the (left) meridional and (right) zonal components are shown. Symbols denote data obtained for different intervals. Filled circles, open circles, and filled triangles denote data for September 7 16, September 17 23, and September 24 27, respectively. show the temporal variations of the semidiurnal amplitudes and phases as functions of time (day), respectively. Associated vertical bars denote estimated error values. Figure 9b shows the temporal variations of the amplitude for 5 heights for both horizontal components. The amplitude generally ranged from 5 to 80 ms 1 between 96 and 120 km, and it indeed varied day by day. It should be pointed out that although the amplitude varied with time (like day to day variability), it appears to vary relatively smoothly, as if it was modulated by a planetary wave. It would be related to the planetary wave shown in Figure 6. Figure 9c shows the temporal variation of the phase for 5 heights for both horizontal components. The variation with time (day) was small over the 1st time interval and over the 3rd time interval, and it was less than 2 hrs. From Figure 9c, the mode change is clearly identified, occurring from September 18 to 23. For example, for the meridional component at 102 km, the difference of the phases between the 1st and 3rd intervals is 6 hrs. 4. Discussion 4.1. Effect of the Ion Drag on the Diurnal Tide [22] As shown in Figure 8a, the altitude profile of the meridional diurnal tide between September 13 and 20 is different from that of the other intervals. The difference becomes obvious at and above 111 km, where the ion drag can play an important role in the wind dynamics [cf. Tsuda et al., 2007; Nozawa et al., 2005]. We analyzed the electric field data and derived a 24 hr variation component for four intervals, such as all (all: September 6 29), the 1st interval (P1: September 6 12), the 2nd interval (P2: September 13 20), and the 3rd interval (P3: September 22 29), which we summarized in Table 1. It is a bit of a surprise that there 10 of 16

11 Figure 9b. Temporal variations (or day to day variation) of the semidiurnal amplitude as a function of the day from September 7 to 27 for 5 heights at 120, 115, 109, 102, and 96 km are shown for (left) meridional and (right) zonal components. Vertical bars associated with each symbol denote error bars. Symbols denote different intervals. Filled circles, open circles, and filled triangles denote data for September 7 16, September 17 23, and September 24 27, respectively. are no significant differences of the amplitude between the P1 and P2 intervals and that there are no significant differences of the phase between the three intervals (i.e., P1, P2, P3). The meridional amplitude for P1 and P2 is 18 mv m 1, while it is 9 mvm 1 for P3. The corresponding phase (the local time of maximum) is almost the same for P1 and P2, and there is a small phase shift of 1.5 hrs later for P3. [23] Furthermore, we have derived ion drag acceleration values using the EISCAT data and model values [cf. Nozawa et al., 2005]. The ion drag force on the neutrals, which equals r n n ni (v u), where r n is the neutral mass density and n ni is the neutral ion collision frequency (=(r i /r n )n in, where r i is ion mass density), is equal to the Lorentz force on the ions. Figure 10 compares altitude profiles of the meridional component of the ion drag for three intervals (P1, P2, P3). First of all, the phase difference between the three intervals is small, less than 2 hrs. The amplitude is a few times larger for P1 than it is for P3, and this would be a reason for the difference of the meridional diurnal amplitude at and above 115 km between the two intervals. However, the amplitude for P1 is only slightly larger than that for P2, and the phase difference is only 1 hour more or less. These results indicate that the electric field cannot make any significant difference in the diurnal tide through the ion drag force between P1 and P2. [24] It is reported that the Sun had been active between September 5 and 20, For example, a Solar proton event (SPE) took place at 0215 UT on September 8 and maximized at 0425 on September 11. In the mesosphere, however, no significant difference of the diurnal amplitude is found between the three intervals from the Tromsø MF radar data. The study by Tsuda et al. [2007] suggested that the contribution due to the pressure gradient force produced by the Joule heating is important to the diurnal tidal amplitude in the lower thermosphere at high latitudes. Therefore, we could conclude that one of the possible causes would be the pressure gradient force due to the Joule heating, although we do not have any direct evidence Mode Change of the Semidiurnal Tide [25] According to Riggin et al. [2003], several factors combine to produce the variability of the semidiurnal tide observed at high latitudes. Potential sources of the semidiurnal tide variability can be summarized as: (i) forcing in the troposphere and stratosphere, (ii) mode coupling between migrating modes, (iii) non migrating modes, (iv) wave wave 11 of 16

12 Figure 9c. Same as Figure 9b but for phase (local time of maximum). interactions including gravity waves and planetary waves, and (v) changes in the background mean state (horizontal winds and temperatures). During September 19 23, the vertical wavelength of the meridional and zonal components in the lower thermosphere changed from km to km, as shown in Figure 9a. Here we focus on the mode change and present data obtained with the MF radar at Tromsø as well. To investigate differences of the semidiurnal tide between two intervals, semidiurnal tidal amplitudes and phases were derived with a data window length of 10 days (starting at 1900 UT on September 6: named ) and 6 days (starting at 0700 UT on September 23: named ). [26] Figure 11 shows altitude profiles for the 2 intervals between 70 and 120 km. Circles (with dashed lines) and triangles (with solid lines) denote data values for the earlier interval (050911) and for the latter interval (050926), respectively. EISCAT data are presented between 96 and 120 km, while Tromsø MF radar data were presented between 70 and 91 km. The vertical wavelengths in the mesosphere and the lower thermosphere are listed in Table 2. For , the altitude profile of the semidiurnal amplitude exhibited two clear peaks, where the amplitude maximized in the mesosphere and then in the lower thermosphere. In general, the amplitude tends to increase with height from 5 ms 1 at 70 km to 50 ms 1 at a peak height. The peak of the amplitude at the lower height is located at 79 km in both horizontal components with values of 20 ms 1, and the other peak is found at 115 km for the meridional component and at 109 km for the zonal component with values of 56 m s 1 and 44 m s 1, respectively. From the altitude profile of the phase, there seems to be a mode change occurring around km in altitude, and the vertical wavelength became longer in the lower thermosphere than that in the mesosphere. The vertical wavelengths are estimated to be 25 km (±1 km) and 58 km Table 1. Amplitudes and Phases of the 24h Variation Component of the Electric Field Amp. (mv/m) Phase a (hrs) NS EW NS EW Data Interval All UT on Sept 6 to 0630 UT on Sept 29 P UT on Sept 6 to 2348 UT on Sept 12 P UT on Sept 13 to 2400 UT on Sept 20 P UT on Sept 22 to 1000 UT on Sept 29 a Phase is local time of maximum (hrs). 12 of 16

13 Figure 10. Altitude profiles of the (left) amplitudes and (right) corresponding phases (the local time of maximum) of the 24 hr component of the meridional ion drag acceleration for three intervals, containing September 6 12 (circles), September (triangles), and September (diamonds). Filled symbols denote data values above 99% significance level. (±4 km) for the meridional component and 29 km (±3 km) and 76 km (±6 km) for the zonal component in the mesosphere and the lower thermosphere, respectively. Here a one sigma value given by the least squares method is used as an error value for the vertical wavelength. [27] For the latter interval (050926), features are different than those for the earlier interval (050911). The meridional amplitude exhibited several peaks, while the zonal amplitude maximized at km. There is a tendency for the vertical wavelengths in the lower thermosphere to become shorter than those in the mesosphere for the meridional and zonal components. The vertical wavelengths are estimated to be 65 ± 6 km ( 50 ± 4 km) and 21 ± 2 km ( 24 ± 3 km) in the mesosphere and in the lower thermosphere, respectively, for the meridional (zonal) component. According to the phase variation with altitude shown in Figure 11, there would be a mode change occurring around 105 km in the lower thermosphere for the zonal component, and thus estimation of the vertical wavelength in the lower thermosphere is less accurate in this case. [28] The meridional amplitude for is smaller than that of over the height region, except at 85 km. In particular, the amplitude for is almost double that of above 100 km. The zonal amplitude does not have such a tendency, and the amplitude is comparable between the two intervals. It should be noticed that although the vertical wavelength for is shorter than that for in the lower thermosphere, in the mesosphere the feature is opposite, i.e. it is shorter for Shorter wavelength (< 30 km) implies several modes coexisting [cf. Bernard, 1981a]. Riggin et al. [2003] pointed out that the mesospheric semidiurnal tide exhibited a seasonal dependence with horizontal wind amplitudes maximizing around the autumn equinox at 86 km at latitudes around 70. This was interpreted to mean that the several modes became in phase and the amplitude maximized in the upper mesosphere. This idea seems to be reasonable for our results over the height region between 70 and 120 km, although the amplitude maximized at 79 km in the mesosphere for We presented the fact that the vertical wavelengths of the semidiurnal tides differed from each other between the two intervals in September 2005 and also the fact that they varied with altitude between 70 and 120 km. These results suggest that several modes existed in the mesosphere and lower thermosphere and that the tidal amplitude, which we observed, was dependent on that interrelation. [29] With estimated vertical wavelengths of 58 and 76 km in the lower thermosphere for , it would be associated in classical tidal theory with the migrating (2,4) mode [e.g., Virdi et al., 1986; Forbes, 1995]. The amplitude and phase variations of the semidiurnal tide with height for the earlier interval have the properties of an upwardpropagating tide that grows in amplitude up to around 115 km (meridional component) and 109 km (zonal component) in altitude and then dissipates at higher altitudes. However, in the mesosphere, the vertical wavelength is shorter ( 25 and 29 km), and there must be several modes coexisting. On the other hand, the amplitude of the meridional component for does not show such a behavior, but it does not have a clear maximum in the lower thermosphere. The short (<25 km) vertical wavelengths of the both horizontal components in the lower thermosphere for suggest that the mode is not a simple classical tidal mode. In the mesosphere, however, the vertical wavelength would be interpreted to be in (2,4) mode. [30] There would be contributions due the non migrating tide to some extent. Non migrating tides have several sources. They can be excited by a large scale latent heat release, nonlinear interaction between global scale waves, and interaction between gravity waves and tides [cf. Hagan and Forbes, 2002; Miyahara and Miyoshi, 1997]. Model predictions by Miyahara and Miyoshi [1997] demonstrated that the westward propagating s = 1 non migrating tide around km has a large amplitude (5 ms 1 through 15 ms 1 ) in the polar region. To investigate the existence of the non migrating semidiurnal tides in the mesosphere during the observational interval here, we further analyzed MF radar data obtained at Poker Flat (65.2 N, W) in the same manner and derived semidiurnal amplitudes and phases for the two intervals as well. Comparison of the semidiurnal tide between Tromsø and Poker Flat for the two intervals is summarized below. (1) The amplitude is generally larger (smaller) at Tromsø than it is at Poker Flat for the (050926) interval. (2) The phase difference between the two sites is 13 of 16

14 Figure 11. Altitude profiles of the (top) amplitudes and (bottom) corresponding phases (local time of maximum) of the semidiurnal tides for (left) meridional and (right) zonal components for two intervals are shown between 70 and 120 km. Circles (with dashed lines) denote results from the 10 day data window starting at 1900 UT on September 6, while triangles (with solid lines) denote results from the 6 day data window starting at 0700 UT on September 23. Larger filled symbols denote data above 99% significance level, smaller filled symbols denote data between 50% and 99% significance levels, and open symbols denote data below 50% significance level. EISCAT data are shown at and above 96 km, while Tromsø MF radar data are shown at and below 91 km. small (less than 2 hrs). (3) For , the vertical wavelength of the zonal component at Poker flat is longer than that at Tromsø, while for , the vertical wavelength is longer at Tromsø than it is at Poker Flat. These results suggest that during the observational intervals, the migrating tide as well as non migrating tide existed in the mesosphere. However, since the difference of the phases between the two sites was less than 2 hrs, mainly the migrating tide was Table 2. Vertical Wavelengths of the Semidiurnal Tide for Two Time Intervals a Name Interval Mesosphere Lower Thermosphere NS EW NS EW September ± 1 km 29 ± 3 km 58 ± 4 km 76 ± 6 km September ± 6 km 50 ± 4 km 21 ± 2 km 24 ± 3 km a Here a one sigma value given by the least squares method is used as an error value for the vertical wavelength. 14 of 16

15 dominant, but the amplitude was modulated by contributions of the non migrating tide. [31] Furthermore, one of the possible causes of the difference of the semidiurnal tide is the change of the solar irradiance. As shown in Figure 2, the F 10.7 index was larger for the earlier interval by 30% than it was for the latter interval. Since the F 10.7 index is a proxy for EUV radiative flux from the Sun, this change might be a cause of the difference. Indeed, the solar flare (X class) occurred on September 7, 8, 9, and 13. Based on 56 days of EISCAT data sets, Nozawa and Brekke [1999b] showed that the semidiurnal tidal amplitude and phase depends on the solar activity for the equinox season, where the amplitude was larger for the high solar activity period than it was for the low solar activity period and where the altitude profiles of the phase differed from each other. The result is consistent with our results here, so the change of the solar irradiance would be one of the causes. Another possible cause is the ion drag effect. The ion drag acceleration had a 12 hr variation component, but it was very small at and below 109 km. At and above 111 km, the 12 hr variation component of the ion drag acceleration increases with altitude and reaches to be 0.05 ms 2 at 120 km. The acceleration values are smaller than those of the 24 hr variation. There is no significant difference found between the two intervals in amplitude, and there is a only small phase shift (< 1 hrs). Therefore, the ion drag effect cannot be the major cause of the difference of the semidiurnal tide between the two intervals. [32] To summarize, the mode change occurred in the lower thermosphere in September 2005, and the vertical wavelengths differed from each other in the mesosphere and the lower thermosphere for each interval and also between the two intervals. The major cause would be the existence of several modes, including a non migrating mode. The change of the solar irradiance would be one candidate, but the ion drag is not the major cause. 5. Summary and Conclusions [33] We have investigated characteristics of the lower thermospheric wind for September 2005 using data obtained by the EISCAT UHF radar at Tromsø (69.6 N, 19.2 E) over 23 days from September 6 to 29, It is found that the diurnal and semidiurnal amplitude varied with the day, while variations of a relatively longer temporal interval were more significant. For the diurnal tide, the amplitudes above 109 km were much smaller for September than those for the earlier and latter intervals. Corresponding phases also differed from each other. We evaluated the electric field and ion drag variations to elucidate causes of the difference for three intervals (September 6 12, September 13 20, and September 22 29). We have found that (1) the phase difference of the 24 hr component of the ion drag force was small between the three intervals and (2) the difference of the 24hr amplitude of the ion drag between September 6 12 and September was small. These results indicated that the ion drag force (i.e., electric field) could not cause such a difference. In the mesosphere, no significant difference of the diurnal amplitude is found between the three intervals from the Tromsø MF radar data. Therefore the pressure gradient force due to the Joule heating is probably a major force causing observed difference of the diurnal tide. [34] For the semidiurnal tide, in the lower thermosphere, the wavelengths for September 6 16 are 58 km and 76 km for the meridional and zonal components, respectively, while they are less than 24 km for September 23 29, indicating that a mode change occurred in the middle of September By using wind data from the MF radar co located at Tromsø, we identified that the major semidiurnal mode in the mesosphere is different than that in the lower thermosphere and that the wavelength was shorter for September 6 16 than it was for September in the mesosphere. Furthermore, a comparison of the semidiurnal tide between Tromsø and Poker Flat suggests contributions due to the non migrating tide, although the migrating tide was mainly dominant in September These results suggest that several modes (including non migrating tide) of the semidiurnal tide coexisted in September 2005 and that the major mode in the mesosphere as well as the lower thermosphere changed in a week. [35] The amplitude of Quasi 2 day wave was small (below 50% significance level) and less than 10 ms 1 at and below 109 km over the observational interval. However, Quasi 2 day wave activity was found for September and at and above 111 km. The amplitudes during the intervals reached 30 ms 1 at the upper heights and maximized at 120 km on September 12 with value of 45 ms 1. Between September 6 and 29, planetary waves with a period of 6 days and 4.7 days were found for the meridional and zonal components, respectively. The amplitude reached 28 ms 1 at the upper heights ( 115 km). [36] Acknowledgments. S.N. thanks K. Hocke for letting us use his Lomb Scargle periodogram method routines and thanks fruitful discussions made at International Space Science Institute (ISSI) meetings (International Team 152). We are indebted to the director and staff of EISCAT for operating the facility and supplying the data. EISCAT is an international association supported by research organizations in China (CRIRP), Finland (SA), Germany (DFG), Japan (NIPR and STEL), Norway (NFR), Sweden (VR), and the United Kingdom (PPARC). This research has been partly supportedbyagrant in Aid for Scientific Research B ( , ) and Research C ( ), and Special Funds for Education and Research (Energy Transport Processes in Geospace) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. This research was also partially supported by the Grant in Aid for Nagoya University Global COE Program, Quest for Fundamental Principles in the Universe: from Particles to the Solar System and the Cosmos, from the Ministry of Education, Culture, Sports, Science and Technology of Japan. [37] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. References Azeem, S. M. I., and R. M. Johnson (1997), Lower thermospheric neutral winds at Søndre Strømfjord: A seasonal analysis, J. Geophys. 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Kawamura, Okinawa Subtropical Environment Remote Sensing Center, National Institute of Information and Communications Technology, 4484 Aza Onna, Onna, Kunigami, Okinawa , Japan. (s kawamura@nict. go.jp) H. Miyaoka and Y. Ogawa, National Institute of Polar Research, 10 3, Midoricho, Tachikawa, Tokyo , Japan. (miyaoka@nipr.ac.jp; yogawa@nipr.ac.jp) Y. Murayama, National Institute of Information and Communications Technology, Nukui Kitamachi, Koganei, Tokyo , Japan. (murayama@nict.go.jp) 16 of 16