Solar cycle variations of the thermospheric meridional wind over Japan derived from measurements of hmf

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A10, PAGES 22,427-22,431, OCTOBER 1, 1999 Solar cycle variations of the thermospheric meridional wind over Japan derived from measurements of hmf Seiji Igi Communications Research Laboratory, Koganei, Tokyo, Japan William L. Oliver Center for Space Physics, Boston University, Boston, Massachusetts Department of Electrical, Computer, and Systems Engineering, Boston University, Boston, Massachusetts Tadahiko Ogawa Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan Abstract. This paper describes an analysis of the meridional equivalent neutral wind for geomagnetically quiet conditions as determined from data collected at the Kokubunji ionosonde station (35.7N, 139.5E), Japan over one solar cycle ( ). The wind is derived from the altitude of peak F 2 layer electron density using the Field Line Interhemispheric Plasma (FLIP) model. For low and moderate solar activity the wind is poleward in the daytime and equatorward in the nighttime. For high solar activity the wind is weak and almost always poleward throughout the day. In winter the local time of peak poleward velocity occurs in the afternoon for low solar activity but in the night for high solar activity. In summer the peak poleward wind occurs in the morning for all levels of solar activity. The diurnal amplitude decreases with increasing solar activity. It also reaches its maximum around the solstices and its minimum around the equinoxes. The mean wind is larger at solar maximum than solar minimum, except for the year The daily-mean wind is smallest in summer and largest in winter. Qualitatively, the meridional wind at Asian midlatitudes has characteristicsimilar to those seen in other sectors (such as Millstone Hill, Boulder, Wallops Island, King George Island, and Saint-Santin), but the details in behavior are different and warrant further investigation. 1. Introduction Thermospheric neutral winds are strongly dependent on solar activity. Buonsanto [1990, 1991] derived meridional winds from h mf2, the altitude of peak ionospheric electron density in the F2 region, over a full solar cycle at two mid-latitude stations, Boulder (40.0N, 254.7E) and Wallops Island (37.8N, 284.5E). The results of these studies showed that the diurnal amplitude of the wind is considerably larger at solar minimum than at solar maximum. Hagan [1993] reported from incoherent scatte radar (ISR) measurements over Millstone Hill that the nighttime wind was significantly weaker at solar maximum than at solar minimum, but, in contrast, the daytime wind was stronger at solar maximum than at solar minimum. The ISR measurements also indicate increasing diurnal amplitudes with decreasing solar activity. The Coordinated Analysis of the Thermosphere (CAT) program was carried out to understand the seasonal and solar cycle variability of thermospheri circulation [Hedin et al., 1994]. Wind data from ISR, Fabry-Perot interferometer, and hmf 2 measurements all showed decreasing diurnal amplitudes with increasing solar activity during all seasons, except for Saint-Santin (47.4N, 2.3E) data, which showed a slight increase with increasing solar activity for summer conditions. The HWM90 empirical model amplitudes increase slightly with solar activity during all seasons. Hedin et al. Copyright 1999 by the American Geophysical Union. Paper number 1999JA /99/1999JA [1994] concluded that there are significant differences between the trends and between the absolute values for the various data sets and models that need further investigation. Recently, Arriagada et al. [1997] reported the solar activity variations of meridional winds derived from hmf2 measurements over King George Island, Antarctica. They found that the solar cycle dependence in the Southern Hemisphere differed from that reported previously for the Northern Hemisphere. Oliver et al. [1990] first reported the behavior of the winds above Japan between 1987 and 1988 (low, ascending solar activity) from ISR measurements made with the Middle and Upper atmosphere (MU) radar at Shigaraki (38.9N, 136.1E), Japan. Igi et al. [1995] calculated winds for these same periods from hmf 2 measurements at Kokubunji (35.7N, 139.5E), Japan. The agreement between these two sets of measurements served to validate the h mf 2 method. The purpose of this paper is to investigate the diurnal, seasonal, and solar cycle variations of the neutral wind over Kokubunji, which is supposed to be representative of midlatitudes in the Asian sector, over one solar cycle, and to note how these may differ from behavior seen in other sectors. 2. Method A wind, in the magnetic meridian, is derived from h mf 2 by determining what wind is required by the Field Line Interhemispheric Plasma (FLIP) model to reproduce that value of h, F 2. The value of hmf2 is controlled by four factors: (1) 22,427

2 22,428 IGI ET AL.: BRIEF REPORT loo 60. E,, 'o 40 ß,o I 989(SSN=157.6), 1 Mar ß (SSN=13.4), -----* 1 983(SSN=66.6) E.c_. : 20 o o -20 ";s. -"". Jun. ''... ß. -8O Local Time (hots) loo DE c. 20 o O Figure 1. Diurnal variation of the meridional wind (positive equatorward) for March, June, September, and December. The bold solid line with open square shows the variation in 1989 (solar maximum; SSN = 157.6), the dashed line with solid circles shows the variation in 1986 (solar minimum; SSN = 13.4), and the thin solid line with black asteriskshows the variation in 1983 (solar descending year; SSN = 66.6). magnetic meridional neutral winds, (2) electric fields, (3) diffusion, and (4) production and recombination. A poleward neutral wind lowers the layer while an equatorward wind raises the layer. An eastward electric field raises the layer, while a westward electric field lowers the layer. Miller et al. [1986] show that the neutral wind U can be estimated as U = (hmf2- ho)/, (1) where h0 is the height of the F 2 layer peak when U - 0 and a is a factor computable using an ionospheric model, such as the FLIP model, giving the sensitivity of the layer height to the wind. This formula attributes all layer motion to the wind, but in reality the layer motion contains contributions from both the magnetic south wind component Ux and the magnetic east electric field component Ey: S = Sx + Ey/Bj, (2) where B z is the vertical component of the magnetic field. Thus the wind U should be called an equivalent meridional neutral wind [Richards, 1991]. We do not compensate for the effect of the electric field in this work since Miller et al. [1987] indicate that its effect is not significant for quiet or moderate magnetic conditions at the magnetic latitude of Kokubunji. For our study we estimate h mf 2 from Kokubunji ionograms by using the empirical relationship proposed by Bilitza et al. [1979] between h mf 2 and the parameters M(3000)F2, fof2, and fo E readable from the ionograms. 3. Description of Data Set The meridional neutral wind in the thermosphere over Kokubunji were calculated for each month of the solar cycle from 1981 to 1991 by using the method described in section 2. Minimum solar activity was attained in 1986 and maximum activity in For each month, hourly median values of M(3000)F 2, fof2, and fo E were used. We used median values to eliminate undue effects from geomagnetically disturbed pe- riods. Therefore it is assumed that the effect of electric fields is minimal. The average Ap index for each month from 1981 to 1991 was less than eight except for September 1982, when it was nine, and for March 1989, when it was twelve. As a result, the winds calculated in this paper are the equivalent meridional neutral winds for quiet conditions for the midlatitude Asian sector. 4. Results and Discussion 4.1. Diurnal Variations Figure 1 shows the diurnal variation of the wind in March, June, September, and December for the years 1983, 1986, and

3 ß IGI ET AL.: BRIEF REPORT 22, The year 1989 represents high solar activity (sunspot number SSN = 157.6), 1986 represents low activity (SSN = 13.4), and 1983 represents moderate activity (SSN = 66.6). The uncertainties in these median winds are no more than 4 m/s. The uncertainty assigned to any median calculation is the difference between the 25th and 75th percentile values divided by the square root of the number of values in the collection of data, a measure analogous to the probable error of the mean in the computation of an average. For each month for low and moderate solar activity, the wind is poleward in the daytime and equatorward in the nighttime. This result is qualitatively consistent with the expected upper thermospheric behavior of winds blowing from the dayside pressure maximum to the nightside pressure minimum. For high solar activity, (1) the wind in June is poleward in the daytime and equatorward in the nighttime with small diurnal variation, (2) the wind in December is poleward at all local times (LT) except at 0100, 0200 and 0400 LT, and (3) the wind in March and September is always poleward. The difference between the diurnal maximum and diurnal Santin. Figure 1 indicates that the diurnal variations in March and September are similar, but the local times of the peak wind is different. For all levels of solar activity the poleward wind reaches its peak speed at 2100 LT in March but at LT in September. Figure 2 shows the diurnal variation of the wind for each month in 1987 and The local times of peak poleward velocity are connected by broken (1987) and solid (1989) lines. We showed the wind in 1987 (SSN = 29.4) instead of 1986 for low solar activity, because we cannot estimate the wind of whole month due to instability of calculation. For low solar activity the local time of peak poleward velocity changes from afternoon (1600 LT) to morning (0500 LT) between March and April, and from morning (1000 LT) to afternoon (10 LT) between September and October. For high solar activity this time changes from night (2100 LT) to morning (0900 LT) between March and July, and from morning (0900 LT) to night (1700 LT) between September and October. Generally speaking, during the winter the local time maximum occurs in the afternoon for low solar activity and in the night for high solar activity. During the summer the maximum occurs in the morning for all levels of solar activity. The local time of peak poleward velocity is qualitatively consistent with the result at Boulder of Buonsanto [1991], that is, the time of maximum northward wind is later at solar maximum than at solar mini- mum, and in winter than the other seasons. However, the phases are different between Boulder and Kokubunji. Although the phase is observed to be latest in winter at solar maximum at both stations, this phase at Boulder is 1625 LT and that at Kokubunji is around 2000 LT. The difference of the local times of peak poleward wind between the vernal and autumnal equinoxes in Figure 1 reflects the delicate transition timing from winter to summer and from summer to winter. Low so lar act i vi ty (1987) H i gh so l ar act i vi ty (1989) JAN : FEB MAR..'" ß.' JUL ''...,,'"'***'ø**". i..' ' AUG i ß.:.,.... SEP ",',.,.,,j ;-'": minimum wind velocities (hereafter called the diurnal amplitude) has a seasonal dependence. For all years the diurnal dhn amplitude in December is larger than that in June. The diurnal amplitude in both March and September is smaller than that in June and December. We quantitatively discuss the seasonal dependence of the diurnal amplitude in section 4.2. Figure 2. Diurnal variation of the meridional wind (positive The wind difference between the vernal and autumnal equi- equatorward) for each month in 1987 (dashed line: low solar noxes has not been studied adequately to date. Duboin and activity) and 1989 (solid line: high solar activity). The bold solid Lafeuille [1992] report that there are no appreciable differand bold dashed lines indicate the local time of peak poleward wind. ences between the vernal and autumnal winds above Saint 4.2. Seasonal Variation Plate la shows the seasonal variation of the diurnal amplitude for different levels of solar activity. The error bars indicate the standard deviation of the data. The diurnal amplitude is lowest for high solar activity and highest for low solar activity throughout the year. This solar activity dependence of the diurnal amplitude is consistent with the results by Buonsanto [1990, 1991] and Duboin and Lafeuille [1992]. The amplitudes have similar seasonal trends at low and moderate solar activity, with maximum amplitudes in January and July and minimum amplitudes in April and October (the minimum in October for low solar activity is not clear owing to a lack of data). For low solar activity the maximum values are 165 m/s in January and 169 m/s in July, and the minimum values are 109 m/s in April and 105 m/s in September. The corresponding values for moderate activity are 136 m/s in January, 109 m/s in July, 84 m/s in April, and 63 m/s in October. The diurnal amplitude for high solar activity does not have a large seasonal variation, although the amplitude in winter is larger than that in the other seasons. To summarize the seasonal behavior of the diurnal amplitude, we can say that for any time of year the lower the solar activity, the larger the diurnal amplitude. The diurnal amplitude for low and moderate solar activity has maxima -1 month after the solstices, and minima -1 month after the equinoxes. Although the seasonal variation at high activity is small, the maximum occurs at winter solstice and the minimum at vernal NOV equinox. The larger ion drag must play an important role in restraining the amplitude at high solar activity [Hagan and Oliver, 1985]. Plate lb shows the seasonal variation of the daily mean wind. When averaged over the year the daily mean wind is most '. '

4 ... 22,430 IGI ET AL.: BRIEF REPORT / '' '"...,.!.....,1,,,2' (s/m),pn!ldme lemn!0 (s/m) pufa ueem,!seo NSS! (s/m) epm,!lcluje lumn!(] (s/w) pu!t ueew Xl!e o

5 IGI ET AL.: BRIEF REPORT 22,431 equatorward for high solar activity and most poleward for low solar activity. Additionally, the daily mean wind is most strongly poleward in June or July and most strongly equatorward in October or November (we cannot determine the definite month for low activity due to a lack of autumn data). Buonsanto [1991] reports that the seasonal-average mean poleward wind, averaged over all levels of solar activity, is 11 m/s in summer, -4 m/s in fall, -18 m/s in winter, and 6 m/s in spring. These winds are comparable with the seasonal average mean for moderate solar activity at Kokubunji. However, the mean wind at Millstone Hill has different features, that is, the wind at moderate solar activity is always equatorward [Hedin et al., 1994]. activity and increases with decreasing solar activity. It is largest around the solstices and smallest around the equinoxes. 4. The daily mean wind has seasonal and solar cycle dependencies. It is larger at solar maximum than solar minimum (except in 1984). Also, it is most strongly poleward in summer and most strongly equatorward in winter. Although the meridional wind at this midlatitude Asian location has characteristics similar to the winds seen in other sectors (such as Millstone Hill, Boulder, Wallops Island, King George Island, and Saint-Santin), the details of the behavior are different. More theoretical work is needed to explain the longitudinal difference Solar Cycle Dependence Acknowledgments. We thank P. G. Richards for providing us with Plate 2a shows the solar cycle dependence of the diurnal the FLIP code. The ionospheric data were provided by the World Data amplitude for the four seasons. The monthly mean sunspot Center C2 for the ionosphere, Communications Research Laboratory, numbers are shown in Plate 2c. The diurnal amplitude de- Tokyo. Hiroshi Matsumoto thanks S. Maeda and P. G. Richards for their creases with increasing solar activity and increases with deassistance in evaluating this paper. creasing solar activity. This basic behavior is consistent with the results of Buonsanto [1991] andarriagada et al. [1997], but the details are different. In contrast to the result of Buonsanto References [1991] at Boulder, showing that the solar cycle variation of the Arriagada, M. A., A. J. Foppiano, and M. J. Buonsanto, Solar activity diurnal amplitude for summer conditions is smaller than that variations of meridional winds over King George Island, Antarctica, for other seasons, our result shows that the solar cycle variation. J. Atmos. Terr. Phys., 59, , Bilitza, D., N.M. Sheikh, and R. Eyfrig, A global model for the height of the diurnal amplitude for summer conditions is larger than of the F2 peak using M3000 values from the CCIR numerical map, that for other seasons. Furthermore, the diurnal amplitude in Telecommun. J, 46, , winter is larger than that in any other season throughout the Buonsanto, M. J., Observed and calculated F2 peak heights and deperiod except for 1982 and The diurnal am- rived meridional winds at mid-latitudes over a full solar cycle, J. Atmos. Terr. Phys., 52, , plitude for low solar activity is larger than that reported by Buonsanto, M. J., Neutral winds in the thermosphere at mid-latitudes Buonsanto [1991] and Hedin et al. [1994]. Although Plate 2a over a full solar cycle: A tidal decomposition, J. Geophys. Res., 96, shows that the maximum diurnal amplitude is m/s, , depending on season, the corresponding amplitudes reported Duboin, M.-L., and M. Lafeuille, Thermospheric dynamics above by Buonsanto and Hedin et al. are in the range m/s. The Saint-Santin: Statistical study of the data set, J. Geophys. Res., 97, , diurnal amplitudes in winter at King George Island have no Hagan, M. E., Quiet time upper thermospheric winds over Millstone solar cycle dependence [Arriagada, 1997], while those at Boul- Hill between 1984 and 1990, J. Geophys. Res., 98, , der and Kokubunji have a clear solar cycle dependence for any Hagan, M. E., and W. L. Oliver, Solar cycle variability of exospheric seasonal condition. The minimum amplitude occurred in 1989, temperature at Millstone Hill between 1970 and 19, J. Geophys. Res., 90, , which corresponds to solar maximum, and its value was in the Hedin, A. E., M. J. Buonsanto, M. Codrescu, M.-L. Duboin, C. G. range m/s in spring, summer, and autumn but 61 m/s in Fesen, M. E. Hagan, K. L. Miller, and D. P. Sipler, Solar activity winter. The maximum amplitude occurred during 1984 to 1986, variations in mid-latitude thermospheric meridional winds, J. Geodepending on the season. phys. Res., 99, , Plate 2b shows the solar cycle dependence of the daily-mean Igi, S., T. Ogawa, W. L. Oliver, and S. Fukao, Thermospheric winds over Japan: Comparison of ionosonde and radar measurements, J. wind for the four seasons. The daily mean is largest in winter Geophys. Res., 100, , and smallest in summer. The fact that the daily mean wind is Miller, K. L., D. G. Torr, and P. G. Richards, Meridional winds in the larger at solar maximum than solar minimum (except for the thermosphere derived from measurement of F2 layer height, J. year 1984) disagrees with the report by Buonsanto [1991] describ- Geophys. Res., 91, , Miller, K. L., J. E. Salah, and D. G. Torr, The effect of electric fields ing no obvious solar cycle variation in the daily mean wind. on measurements of meridional neutral winds in the thermosphere, Ann. Geophys., Ser..4, 91, , Oliver, W. L., S. Fukao, T. Takami, T. Yamamoto, T. Tsuda, T. 5. Summary Nakamura, and S. Kato, Thermospheric meridional winds measured by the middle and upper atmosphere radar, J. Geophys. Res., 95, , Richards, P. G., An improved algorithm for determining neutral winds from the height of the F2 peak electron density, J. Geophys. Res., 96, , The analysis of the meridional winds derived from h mf 2 measurements at Kokubunji over one solar cycle shows the following: 1. For low and moderate solar activity the wind is poleward in the daytime and equatorward in the nighttime. For high solar activity the wind is weak and almost always poleward throughout the day. 2. The local time of peak poleward wind changes dramatically after the equinoxes. Namely, during the winter the peak poleward wind occurs in the afternoon for low solar activity but in the night for high solar activity, while during the summer the maximum occurs in the morning for all levels of solar activity. 3. The diurnal amplitude decreases with increasing solar S. Igi, Communications Research Laboratory, Koganei, Tokyo , Japan. (igi@crl.go.jp) T. Ogawa, Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, , Japan. W. L. Oliver, Department of Electrical, Computer, and Systems Engineering, Boston University, Boston, MA (Received January 29, 1999; revised May 7, 1999; accepted May 24, 1999.)