Observations and model calculations of the F 3 layer in the Southeast Asian equatorial ionosphere

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010ja016086, 2011 Observations and model calculations of the F 3 layer in the Southeast Asian equatorial ionosphere Jyunpei Uemoto, 1 Takashi Maruyama, 1 Takayuki Ono, 2 Susumu Saito, 3 Masahide Iizima, 4 and Atsushi Kumamoto 2 Received 3 September 2010; revised 30 October 2010; accepted 14 December 2010; published 9 March [1] To clarify the characteristics of the F 3 layer with a focus on magnetic latitude dependence and the relationship to the equatorial anomaly, we performed statistical analysis of F 3 layer occurrences using the ionosonde chain data in a magnetic meridional plane in Southeast Asia and performed model calculations. From comparison of the observational and model calculation results, it was found that the field aligned diffusion of plasma acts to make the F 3 layer prominent in the magnetic low latitude region while acting to decrease the peak density of the F 3 layer near the magnetic equator. The magnetic latitude dependence of the F 3 layer formation comes not only from the meridional neutral wind effect but also from the field aligned diffusion effect. The model calculations revealed that the F 3 peak corresponds to the electron density enhanced region associated with the equatorial anomaly. This relationship is consistent with the suggestion that the field aligned diffusion acts to make the F 3 layer prominent in the magnetic low latitude region since the fundamental factors for generation of the equatorial anomaly are also E B drift and field aligned downward diffusion. It is suggested that the local time and magnetic latitudinal variations of the F 3 layer result from those of the electron density enhanced region associated with the equatorial anomaly. Citation: Uemoto, J., T. Maruyama, T. Ono, S. Saito, M. Iizima, and A. Kumamoto (2011), Observations and model calculations of the F 3 layer in the Southeast Asian equatorial ionosphere, J. Geophys. Res., 116,, doi: /2010ja Introduction [2] The structure and dynamics of the daytime equatorial ionosphere have been extensively studied by various observational and theoretical methods since the equatorial anomaly was found [Namba and Maeda, 1939; Appleton, 1946]. The unique feature of the daytime F region equatorial ionosphere is characterized by the horizontal magnetic field line toward the northward and eastward electric field generated in the E region. The eastward electric field is mapped to the F region along the magnetic field line, and moves plasma upward by E B drift. Plasma uplifted by E B drift diffuses toward a higher latitude region along the magnetic field line, and two electron density enhanced regions appear in both the northern and southern magnetic low latitude regions. These regions are observed as the crests of the equatorial anomaly. Besides the E B drift and field aligned diffusion of plasma, the meridional neutral wind modulates the morphology of the equatorial anomaly 1 National Institute of Information and Communications Technology, Tokyo, Japan. 2 Graduate School of Science, Tohoku University, Sendai, Japan. 3 Electronic Navigation Research Institute, Tokyo, Japan. 4 Daijo Shukutoku Gakuen, Tokyo, Japan. Copyright 2011 by the American Geophysical Union /11/2010JA [e.g., Lyon and Thomas, 1963; Hanson and Moffett, 1966]. The crest of the equatorial anomaly is raised (declined) in the windward (leeward) hemisphere by the transequatorial meridional neutral wind through the ion drag effect. The dynamics of the daytime F region equatorial ionosphere are mainly controlled by the E B drift, field aligned diffusion and meridional neutral wind. [3] In the daytime F region equatorial ionosphere, it is well known that the F 2 layer often appears to divide into two layers in the ionogram. This split of the F 2 layer occurring in the equatorial ionosphere was identified as early as the 1940s [e.g., Bailey, 1948; Sen, 1949]. Later, the upper additional layer resulting from splitting of the F 2 layer was called the F 3 layer [Balan et al., 1997]. Recently, the F 3 layer has been extensively studied through observational and theoretical approaches, since Balan and Bailey [1995] modeled it by using the Sheffield University plasmasphere ionosphere model (SUPIM). It has been reported by analyzing ionogram data that occurrence of the F 3 layer depends on the season and on solar and magnetic activity [Ratcliffe, 1951; Jenkins et al., 1997; Balan et al., 1998, 2000, 2008; Hsiao et al., 2001; Batista et al., 2002; Rama Rao et al., 2005; Fagundes et al., 2007; Sreeja et al., 2009]. Balan et al. [1998] suggested a generation mechanism of the F 3 layer as follows: The combined effects of the E B drift and meridional neutral wind lift up the F 2 layer 1of14

2 Figure 1. Location of ionosonde stations in Chiang Mai, Thailand (CM); Chumphon, Thailand (CP); and Kototabang, Indonesia (KT). The magnetic equator is shown as a thin line. and form the F 3 layer, while the normal F 2 layer is formed by the usual photochemical and dynamical effects at a lower altitude. The peak density of the F 3 layer exceeds that of the F 2 layer at a lower altitude for a period of time. Subsequently, the peak density of the F 3 layer decreases as the local time progresses due to field aligned diffusion and chemical loss. The F 3 layer can be observed from ionosondes during the period when the peak density of the F 3 layer exceeds that of the F 2 layer. [4] The characteristics and dynamics of the F 3 layer have been greatly clarified by recent studies, but there are some remaining questions. One such question regards the magnetic latitude dependence of the F 3 layer occurrence. It was reported that in Fortaleza, Brazil (geographic latitude 4 S, geographic longitude 322 E, and magnetic latitude 4.5 S) the F 3 layer occurs frequently in the local summer and winter seasons and is less frequent in the equinox seasons [Balan et al., 1998; Batista et al., 2002]. On the other hand, Rama Rao et al. [2005] examined the variation of the occurrence probability of the F 3 layer in Waltair, India (17.7 N, 83.3 E, 8.2 N), and showed that the F 3 layer frequently occurs in the local summer season as well as equinox seasons and is less frequent in the local winter season. The difference of the F 3 layer morphology between Fortaleza and Waltair can come from the geographic longitudinal effect. Recently, many researchers [e.g., Sagawa et al., 2005; Immel et al., 2006] have reported that the equatorial ionosphere shows geographic longitudinal dependence. On the other hand, this difference can also come from the magnetic latitudinal difference of the F 3 layer. Lynn et al. [2000] and Uemoto et al. [2007] performed an event study and reported that the F 3 layer showed a different feature between near the magnetic equator and in the magnetic low latitude region. They suggested that the field aligned diffusion of plasma plays an important role in causing the magnetic latitudinal dependence of the F 3 layer. Batista et al. [2002] also showed that the F 3 layer occurrence depends on the magnetic latitude by analyzing a long period of data from 1975 to 2000, during which the magnetic inclination rapidly changed over Fortaleza. Balan et al. [1998] performed SUPIM model calculations and showed that the meridional neutral wind can modulate the latitudinal extent of the F 3 layer. [5] It is difficult to separate the meridional neutral wind and field aligned diffusion effects on the magnetic latitude dependence of the F 3 layer occurrence based on a single ionosonde observation. To examine these effects, we need a statistical investigation of the F 3 layer using ionogram data gathered from both the northern and southern low latitude regions and a near magnetic equator latitude in a magnetic meridional plane. [6] On the other hand, based on the mechanism suggested by Balan et al. [1998], a large E B drift is preferable for forming the F 3 layer. The equatorial anomaly is then expected to develop when the F 3 layer forms. It is valuable to investigate where the F 3 layer, which is seen in the vertical profile of electron density, locates in the two dimensional electron density distribution map showing the equatorial anomaly in the magnetic meridional plane. [7] In this paper, we present and discuss the occurrence features of the F 3 layer and the relationship to the equatorial anomaly. To clarify the occurrence features of the F 3 layer with a focus on magnetic latitude dependence, we performed a statistical analysis of the ionosonde chain data in the magnetic meridional plane of the Southeast Asian lowlatitude ionospheric network (SEALION). Furthermore, we performed model calculations using the SAMI2 code [Huba et al., 2000a, 2000b] to theoretically discuss the mechanism of occurrence features of the F 3 layer obtained through the statistical analysis, and to examine the relationship between the F 3 layer and the equatorial anomaly. 2. Observational Results [8] The ionosonde data (ionograms) analyzed in this paper were obtained in Chiang Mai, Thailand (CM) (geographic latitude 18.8 N, geographic longitude 98.9 E, magnetic latitude 13.2 N), Chumphon, Thailand (CP) (10.7 N, 99.4 E, 3.2 N), and Kototabang, Indonesia (KT) (0.2 S, E, 10.1 S). As shown in Figure 1, these three stations form a meridional chain including nearly magnetic conjugate points. The stations are a part of SEALION, which is a comprehensive network of ionospheric observations developed in Southeast Asia [Maruyama et al., 2007]. CP is regarded as a region near the magnetic equator, while CM and CP are regarded as being in the magnetic low latitude region in the Northern and Southern hemispheres, respectively. Ionograms with frequency and virtual height ranging from 2 MHz to 20 MHz (at KT) or 30 MHz (at CM and CP) and from 50 km to 950 km, respectively, have been obtained by using FM CW radar every 5 min. We examined ionogram data with a time resolution of 15 min throughout 1 day, since the characteristic time scale of the F 3 layer occurrence is about 1 h, as reported by Rama Rao et al. [2005]. To clarify both the seasonal and magnetic latitudinal dependences of the F 3 layer occurrence, two periods of the statistical analysis were selected to cover all months under the condition that the ionograms were successfully obtained at all three stations. The periods of the analysis 2of14

3 Figure 2. Sequence plots of the manually scaled O mode trace every 15 min during the local time period of LT on 31 March 2005 at (a) CM, (b) CP, and (c) KT and (d) local time variation of the F region critical frequency at CM (red), CP (black), and KT (blue). Red dots indicate the virtual height of the cusp between the F 2 and F 3 layers. It should be noted that the cusp (red dots) is indicated only when the F 3 layer was observed (red line) and that each O mode trace is compressed in the frequency range. Each trace is anchored at a frequency of 2 MHz to the local time when each trace is obtained. The frequency scale of O mode trace is linear, and the frequency scale at 0700 LT is shown under Figure 2a as an example. are from March to September 2005 and from October 2006 to February It should be noted that in September 2005, data on only 4 days were available at CM. Though Balan et al. [1998] and Rama Rao et al. [2005] reported that the F 3 layer occurrence depends on the solar activity, the two periods we analyzed do not degrade results from the point of view of solar activity, since they were during low solar activity. It should be also noted the days with the geomagnetic disturbance were included in the current statistical analysis. However, the number of days with the geomagnetic disturbance was much smaller than the nondisturbed days during the two periods. Then, this does not degrade statistical results significantly. [9] For depicting temporal variation of the virtual height of the F 3 layer, it is useful to display the O mode traces juxtaposed against variable local time. Figure 2 shows the sequential plots of the manually scaled O mode traces every 15 min on 31 March 2005 during the period from 0700 LT to 1900 LT at CM, CP, and KT, respectively (LT = UT + 7 h). It should be noted that each trace is anchored at the 3of14

4 Figure 3. Same as Figure 2 but on 4 March frequency of 2 MHz to the local time when each was obtained, and that the empty area of the traces indicates where the O mode trace could not be identified due to background noise. In this paper, the virtual height of the cusp between the F 2 and F 3 layers is utilized as a representative of the height variation of the F 3 layer. It should be noted that the virtual height of the cusp has no information about the peak height of the F 3 layer since its height corresponds to the lower boundary of the F 3 layer. [10] As seen in Figure 2, the F 3 layers showed similar behavior at CM and KT while the behavior of the F 3 layer at CP differed from the other two stations. The F 3 layer at CP moved rapidly upward, and was observed with an earlier onset time than those observed at the other two stations. On the other hand, at CM and KT, the F 3 layer remained near a certain altitude. Figure 2d shows the local time variation of the F region critical frequency of the O mode trace at each station. It should be noted that this is not plotted at the local time when it could not be identified due to difficulty in scaling. The F region critical frequency in the magnetic low latitude region (CM and KT) became higher than that near the magnetic equator (CP) around 1030 LT. This variation indicates that the plasma diffusion along the magnetic field lines started before 1030 LT, associated with the equatorial anomaly phenomenon. It should be noted that when the F 3 layer formed, the F region critical frequencies at CM and KT increased rather than decreased, while that at CP was almost constant. This feature of the F region critical frequency in the magnetic low latitude regions is inconsistent with the mechanism suggested by Balan et al. [1998]. Figure 3 is the same as Figure 2, but on 4 March On this date, the F 3 layer was observed 4of14

5 appear to differ from those at KT. To clarify the difference of the occurrence probability at CM and KT, the figure from CM was subtracted from that at KT (Figure 5d). The difference between CM and KT became conspicuous in the December solstice season. Figure 4. Monthly occurrence probabilities of the F 3 layer at CM (red), CP (black), and KT (blue). The analysis periods are from March to September 2005 and from October 2006 to February It should be noted that there were only 4 days of available data at CM in September Model Calculation Results [12] In section 2, it was found by analyzing the ionogram data that the F 3 layer at CP near the magnetic equator behaves in a different way than that at CM and KT in the only in the magnetic low latitude regions. The virtual height of the cusp was more or less constant at CM and KT. While it is difficult to precisely identify a tendency of the local time variation of the F region critical frequency at KT due to the lack of data, as shown in Figure 3d, it can be noticed that the frequency at CM did not decrease when the F 3 layer formed. [11] To deduce the seasonal variation of the F 3 layer occurrence, ionograms at CM, CP, and KT were statistically analyzed regarding the occurrence. To acquire the monthly occurrence probabilities, we counted the number of days in each month when at least one ionogram shows the F 3 layer. Figure 4 shows the seasonal dependence of the occurrence probabilities of the F 3 layer at CM, CP, and KT. At CM, the probability was lowest in the December solstice season. At CP, it was highest in the June solstice season. At KT, it was high in all seasons. The yearly averaged occurrence probabilities of the F 3 layer at CM, CP, and KT are 75%, 46%, and 97%, respectively. CP is somewhat less than CM and KT. This indicates that the occurrence probability of the F 3 layer strongly depends on the magnetic latitude. Moreover, the variation of occurrence probabilities at CM resembles that at Waltair (magnetic latitude 8.2 N) [Rama Rao et al., 2005], and that at CP resembles Fortaleza (magnetic latitude 4.5 S) [Balan et al., 1998]. These results suggest that the difference in seasonal dependence of the occurrence probability between Waltair and Fortaleza may not be mainly caused by the difference of the geographic longitude but that of the magnetic latitude. Figure 5 shows the local time and seasonal dependences of the occurrence probabilities of the F 3 layer for each station. Each bin is 1 h 1 month, and the occurrence probability is defined as the ratio of the number of ionograms showing the existence of the F 3 layer to that of ionograms available for analysis in each bin. It should be noted that the range of the color bar of CP differs from that of CM and KT. The F 3 layer occurred during the period from morning to presunset local time at CM and KT, while the occurrence of the F 3 layer at CP was localized in the morning of the local time period, except for in the June solstice season. In most of the days in the June solstice season when the F 3 layer observed in the presunset local time at CP, the F 3 layer was also observed in the morning local time. On the other hand, the local time and seasonal variations at CM also Figure 5. Local time and seasonal variations of the occurrence probabilities at (a) CM (red), (b) CP (gold), and (c) KT (blue) and (d) difference of the occurrence probabilities between KT and CM. It should be noted that the highest occurrence probability at CP is adjusted to 50% and that there were only 4 days of available data at CM in September of14

6 empirical model proposed by Scherliess and Fejer [1999]. The spatial variation of the E B drift in the present calculations is given by V EB ð; LÞ ¼ V model L 2 cos 3 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ 3 sin 2 ð 150 kmþ; ð1þ V EB ð; LÞ ¼ V model L 2 cos 3 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ 3 sin 2 " # 150 h 2 exp ð< 150 kmþ; ð2þ 20 Figure 6. (a) Local time variations of the E B drift during March equinox, June solstice, and December solstice and (b) daily local time variations of the meridional neutral wind throughout 2005 at an altitude of 400 km over the magnetic equator. magnetic low latitude region. The F 3 layer moves rapidly upward near the magnetic equator, while it stays almost at a certain altitude in the magnetic low latitude region. The yearly averaged occurrence probability near the magnetic equator was lower than in the low latitude regions. The occurrence is confined to the morning local time period near the magnetic equator, except for in the June solstice season, while it spreads from the morning to presunset local time in the low latitude region. These observational results strongly indicate that the temporal variations and occurrence features of the F 3 layer depend on the magnetic latitude. To explain the magnetic latitude dependence and the relationship between the F 3 layer and the equatorial anomaly, we performed model calculations using the SAMI2 code. [13] The SAMI2 code was the first comprehensive lowlatitude to midlatitude ionosphere model to be developed at the U.S. Naval Research Laboratory [Huba et al., 2000a, 2000b]. In the SAMI2 model, the dynamic plasma and chemical evolution of seven ion species (H +,He +,N +,O +, N 2 +,NO +, and O 2 + ) are addressed. The ion continuity and momentum equations are solved for all seven species, and thermal balance equations are solved for three ion species (H +,He +, and O + ) as well as electrons in an offset, tilted dipole system. The empirical codes of NRLMSISE 00 [Picone et al., 2002] and HWM93 [Hedin et al., 1996] are used to specify the neutral atmosphere. The E B drift in the magnetic meridional plane is calculated from the where V model, L, l, and h in the equations above are the value of E B drift calculated from the empirical model, L value, magnetic latitude and height above the sea level, respectively. Based on equation (1), the E B drift becomes larger when increasing the altitude with the L 2 dependence at the magnetic equator. On the other hand, the strength of the E B drift decreases exponentially below the threshold altitude of 150 km as modeled in equation (2). The other input parameters such as photoionization rates, chemical reaction rates, recombination loss rates, collision frequencies are described in detail by Huba et al. [2000b]. In the model calculations described below, 384 magnetic field lines and 202 points along each magnetic field line are displaced between the altitude range of 85 km to 6,000 km at the geographic longitude of CP. The magnetic equator in the current model calculation is at the approximate geographic latitude of 9 N. It should be noted that the dipole field used in the model calculation is shifted slightly northward compared with the geomagnetic field calculated from the IGRF model. The geographic latitudes of CM, CP, and KT in the SAMI2 model, at which each dip angle coincides with that of the IGRF model at an altitude of approximately 400 km, are 20.7 N, 11.9 N, and 0.0, respectively. [14] We performed two sets of model calculations. The first was to examine field aligned diffusion effects on the magnetic latitude dependence of the F 3 layer (model 1). This model calculation was done for the March 2005 equinox without the meridional neutral wind. The second was to examine the seasonal dependence of the latitudinal extent of the F 3 layer (model 2). This model calculation was performed with the meridional neutral wind for the 21st day of each month in Model 1 [15] The local time variation of the E B drift over the magnetic equator at the altitude of approximately 400 km for the March equinox, as well as for the June and December solstices, is shown in Figure 6a. Examples of the calculated N(h) profiles at the latitudes corresponding to CM, CP, and KT and the O mode traces that were calculated from the N(h) profiles at 1030 LT are shown in Figure 7. It should be noted that the shape of O mode traces showing the existence of the F 3 layer at CM and KT resembles that in actual ionograms. Since the existence of the F 3 layer can be much more clearly seen in the simulated O mode traces than in the N(h) profiles as shown in Figure 7, the F 3 layer was identified by using the simulated O mode 6of14

7 Figure 7. (a) N(h) profiles and (b) O mode traces at 1030 LT over CM (dash dotted line), CP (solid line), and KT (broken line). Arrows shown in Figure 7b indicate the existence of the cusp between the F 2 and F 3 layers. traces in the following analyses and discussion. As shown in Figure 7b, the F 3 layers appeared at the latitudes of CM and KT. However, the F 3 layer did not appear at the latitude of CP at any local time. To depict the magnetic latitude dependence of the F 3 layer occurrences in more detail, the sequence plots of the simulated O mode traces with 30 min intervals and the local time variations of the F region critical frequency are plotted every 3 within the geographic latitudinal range of 9 S to 27 N (Figure 8). Each trace is anchored at the frequency of 2 MHz to the local time it was obtained. The hatched area indicates local time when the F 3 layer formed. The calculated results are roughly symmetrical to the magnetic equator, which is because the calculation was done for the March equinox without the meridional neutral wind. The features of the simulated F 3 layer shown in Figure 8 are summarized as follows: (1) The F 3 layers formed in the geographic latitude range from 3 S to 6 N and from 15 N to 24 N. The F 3 layer formed only in the magnetic lowlatitude regions but not near and over the magnetic equator. (2) The F 3 layer started to form at an earlier local time nearer the magnetic equator. (3) The virtual height of the cusp is more or less constant in the magnetic low latitude region. (4) The F region critical frequency clearly increased as the local time progressed, even when the F 3 layer formed, especially in the magnetic low latitude region. All of these features of the F 3 layer are consistent with the observational results mentioned in section Model 2 [16] To examine the seasonal dependence of the latitudinal extent of the F 3 layer, we performed another model calculation with the meridional neutral wind (see Figure 6b) and E B drift on the 21st day of each month in Figure 9 shows the simulated latitudinal extent of the occurrence of the F 3 layer. The F 3 layer formed in the southern magnetic low latitude region when the strong meridional neutral wind blew northward (January, February, October, November, and December), and formed in the northern magnetic low latitude region when the strong meridional neutral wind blew southward (May, June, and July). This variation of the latitudinal extent in the months in which the strong transequatorial neutral wind blows supports the meridional neutral wind effect proposed by Balan et al. [1998]. They showed a SUPIM model calculation in which the F 3 layer has a tendency to form in the windward hemisphere. When the transequatorial meridional neutral wind was not as strong as the other months in Southeast Asia (March, April, August, and September), the F 3 layer formed in both the northern and southern magnetic lowlatitude regions. On the other hand, the seasonal dependence of the F 3 layer occurrence near the magnetic equator has a different tendency from that in the magnetic low latitude region. The F 3 layer formed near the magnetic equator in both hemispheres around the June solstice season, and the F 3 layer did not form over the magnetic equator throughout a year. 4. Discussion 4.1. Field Aligned Diffusion Effect [17] The important observational results in section 2 with respect to the role of the field aligned diffusion of plasma are that the F 3 layer showed a similar nature in both the magnetic low latitude regions, and the F region critical frequency did not decrease. When we consider the effect of the E B drift, the upward drift velocity becomes largest over the magnetic equator, as expressed in equation (1). Thus the F 3 layer is expected to form at the magnetic equator based on the mechanism proposed by Balan et al. [1998]. On the other hand, when we consider the neutral wind effect the F 3 layer is expected to form in the windward region, since the downward motion of plasma along the magnetic field line is reduced by the meridional neutral wind in the windward region. Combining both the effects of the E B drift and the meridional neutral wind, the F 3 layer cannot appear in both the Northern and Southern hemispheres without formation of the F 3 layer near the magnetic equator. Thus it is difficult to explain the latitudinal extent of the F 3 layer observed on 4 March 2005 by considering only the E B drift and meridional neutral wind effects. The observation result on the similar behavior of the F 3 layer in both the magnetic low latitude regions indicates that the field aligned diffusion plays an important role in forming the F 3 layer in the magnetic low latitude region, because transportation of plasma by field aligned diffusion from the 7of14

8 Figure 8. (a m) Sequence plots of the O mode traces (solid curves) and F region critical frequency (blue dots) every 30 min during the local time period of LT in the geographic latitude range from 27 N to 9 S every 3. The magnetic equator is approximately 9 N (Figure 8g). The red curves indicate the traces showing the existence of the F 3 layer. The red dots indicate the cusp between the F 2 and F 3 layers. The hatched area in each panel indicates the local time when the F 3 layer formed. Note that each trace is anchored at a frequency of 2 MHz to the local time when each trace is obtained. The frequency scale of O mode trace is linear, and the frequency scale at 0700 LT is shown under Figure 8a as an example. 8of14

9 Figure 8. (continued) lower latitude region into the higher latitude region is symmetrical in both hemispheres. [18] The model calculation results in section 3 support this indication. The model 1 results show that the F 3 layer formed in both the magnetic low latitude regions, and the F region critical frequency increased even when the F 3 layer formed (Figure 8). Moreover, in model 2, the F 3 layer formed in both the northern and southern magnetic low latitude regions in the months when the transequatorial meridional neutral wind is not so strong (Figure 9). Figure 10a shows the N(h) profile and simulated O mode trace at 21 N at 1030 LT, and Figure 10b shows the altitude profiles of the terms that appear in the continuity equation and contribute to the electron density change. As shown in Figure 8c, this local time is when the F 3 layer is being prominent. It should be noted that these profiles 9of14

10 Figure 9. Seasonal variation of the latitudinal extent of the F 3 layer. Hatched bins indicate the latitude regions where the F 3 layer formed. for the electron density shown in Figure 10b were estimated by performing summation of all the simulated ion species. The F 2 and F 3 layers were defined from the O mode traces, as mentioned earlier. In the F 3 layer, the field aligned diffusion term, shown by the blue line, is the dominant contribution to the density increase. In other words, plasma is transported into the F 3 layer by the field aligned diffusion from higher altitude regions closer to the magnetic equator. This is the evidence that the field aligned diffusion acts to form the F 3 layer in the magnetic low latitude region Relationship to Equatorial Anomaly [19] The driving forces of the F 3 layer are the E B drift, meridional neutral wind, and the field aligned diffusion of plasma. The diffusion acts to make the F 3 layer inconspicuous near the magnetic equator, as suggested by Balan et al. [1998], and to make it prominent in the magnetic lowlatitude region, as concluded in section 4.1. Therefore, it can be pointed out that the same driving factors act on the F 3 layer and the equatorial anomaly. In this context, it is natural to consider that the F 3 layer would have some relationships with the equatorial anomaly. [20] Figure 11 is a result of model 1 showing the electron density distribution at 0854, 1030, and 1200 LT. The peak altitudes of the F 2 and F 3 layers are superimposed as light blue and red squares, respectively. As shown in Figure 11, it was found that the F 3 peaks correspond to the electron density enhanced region associated with the equatorial anomaly (crest of the equatorial anomaly), and that the F 3 layer moves to a higher latitude region together with the density enhanced region as local time progresses. This relationship is consistent with our conclusion that the fieldaligned diffusion acts to make the F 3 layer prominent in the magnetic low latitude region, because the density enhanced region of the equatorial anomaly also becomes prominent Figure 10. (a) N(h) and simulated O mode trace and (b) rate of change in electron density at 1030 LT at 21 N. The F 2 and F 3 layers are defined by using the O mode trace shown in Figure 10a. In Figure 10b, the total change of the electron density, changes due to photochemical reactions, divergence of the field aligned diffusion, and divergence of the transverse motion are represented by dash dotted, dash double dotted, solid, and dotted curves, respectively. 10 of 14

11 Figure 11. Contour plot of the electron density with the height of the F 2 and F 3 peaks at (a) 0854, (b) 1030, and (c) 1200 LT. Light blue and red squares indicate the heights of the F 2 and F 3 peaks, respectively. The blue curves are the magnetic field lines. via the combined effect of the E B drift and field aligned diffusion. This relationship is also compatible with the meridional neutral wind effect on the F 3 layer suggested by Balan et al. [1998]. The density enhanced region becomes higher (lower) in altitude and lower (higher) in density in the windward (leeward) region. Thus, the F 3 layer is naturally expected to appear in the windward region with a small electron density difference from the F 2 layer, as reported by Balan et al. [1998] and Batista et al. [2002] because the F 3 layer should be located at a higher altitude region sufficiently separated from the F 2 layer at a lower altitude generated through photochemical reactions (see Figure 10) Local Time Variation [21] It was clarified by the current model calculations that the F 3 layer peaks correspond to the electron density enhanced region associated with the equatorial anomaly. Ignoring the meridional neutral wind effect, the densityenhanced region associated with the equatorial anomaly moves symmetrically to higher latitude regions and rises to higher altitudes as the local time progresses and then stagnates. This temporal evolution of the density enhanced region is consistent with that of the F 3 layer (see Figure 8). [22] This temporal variation is also consistent with the ionosonde data analysis described in section 2. The temporal variation of the observed F 3 layer was similar in the northern and southern magnetic low latitude regions (CM and KT), while the F 3 layer near the magnetic equator (CP) shows a different temporal variation (Figure 2). The virtual height of the cusp between the F 2 and the F 3 layer was almost constant at CM and KT, while the F 3 layer moved rapidly upward and its duration time was shorter at CP. The occurrence of the F 3 layer at CP was also localized in the morning period, except for in the June solstice season, while it was extended from the morning to presunset local time at CM and KT (see Figure 5). Thus, it is suggested that the temporal variation of the F 3 layer observed at certain latitude reflects that of the electron density enhanced region associated with the equatorial anomaly Magnetic Latitude Dependence and Seasonal Variation [23] The magnetic latitude dependence and seasonal variation of the occurrence probability of the F 3 layer obtained from the ionogram data analysis are summarized as follows: At CM, the occurrence probability became lowest in the December solstice season. At CP, it was highest in the June solstice season. At KT, it was high in all seasons. The yearly probabilities at CM, CP, and KT are 75%, 46%, and 97%, respectively. The yearly averaged occurrence probability at CP is somewhat less than at CM and KT. The occurrence of the F 3 layer at CP was localized in the morning period, except for in the June solstice season, while the local time period at CM and KT extended from the morning to presunset local time. In the December solstice season, there was conspicuous difference of probability between CM and KT. [24] Based on the SAMI2 model calculation results, the appearance of the F 3 layer can be divided into three regions: in the magnetic low latitude region, near the magnetic equator, and over the magnetic equator (see Figure 9). Thus we discuss the occurrence of the F 3 layer in the three regions, respectively Magnetic Low Latitude Region [25] In the magnetic low latitude region, the F 3 layer formed in the Northern Hemisphere when the meridional neutral wind blew from the Northern to Southern hemispheres (May, June, and July), while it formed in the Southern Hemisphere when the meridional neutral wind blows from the Southern to Northern hemispheres (January, February, October, November, and December) (see Figure 9). This tendency is consistent with the meridional neutral wind effect on the F 3 layer suggested by Balan et al. [1998] as well as that on the equatorial anomaly. Since the equatorial anomaly is modulated by the meridional neutral wind, it is then reasonable that the F 3 layer which corresponds to the electron density enhanced region associated with the equatorial anomaly is also modulated by the meridional neutral wind. The density enhanced region becomes higher in altitude and lower in density in 11 of 14

12 Figure 12. (a) O mode traces simulated without the meridional neutral wind for the December solstice during the morning period using the normal E B drift and (b) the local time shifted one at 12 N. The reversal time of the E B drift is shifted to coincide with that in the June solstice. The arrow in Figure 12b indicates the existence of the cusp between the F 2 and F 3 layers. the windward region. Thus the F 3 layer is expected to appear at a higher altitude in the windward region with a small electron density difference compared with the F 2 layer. This expectation is consistent with the statistical results in Fortaleza reported by Balan et al. [1998] and Batista et al. [2002]. The conspicuous difference of probability between CM and KT in the December solstice season can be explained by the fact that the transequatorial meridional neutral wind in Southeast Asia blows stronger in the December solstice season than the June one. This tendency may be caused by the fact that the magnetic equator is shifted northward from the geographic equator in Southeast Asia by approximately 10. In the other months, the F 3 layer formed in both the northern and southern low latitude regions. This results from the positive fieldaligned diffusion effect on the F 3 layer in the magnetic low latitude region. The seasonal variation of the occurrence probability at CM and KT deduced from the current statistical analysis is basically consistent with the model calculation results, except for the high occurrence probability at KT in the June solstice season. This exception is further discussed in section Region Near the Magnetic Equator [26] Near the magnetic equator, the F 3 layer formed in the June solstice season not only in the northern hemisphere but also in the southern hemisphere in the current model calculation (Figure 9). We suggest that this occurrence feature is mainly due to a later reversal time of the E B drift from nighttime downward to daytime upward in the June solstice season as compared with the other seasons in Southeast Asia (see Figure 6). Figure 12 demonstrates the reversal time effect of the E B drift on the F 3 layer. Figure 12a is a calculation result at 12 N for the December solstice condition with the normal E B drift, while Figure 12b is the same as Figure 12a but calculated with the local time shifted E B drift. These calculations were performed without the meridional neutral wind. The reversal time was adjusted to that in the June solstice season. It should be noted that the F 3 layer did not form near the magnetic equator in either of the hemispheres for the December solstice in model 2 (see Figure 9). As shown in Figure 12, the F 3 layer only forms when calculated with the local time shifted E B drift, though it is very weak. As shown in Figure 12b, the F 2 layer becomes higher density due to the long duration of density inflation before being uplifted by the E B drift. This long duration of density inflation would cause the large difference in peak density between the F 2 and F 3 layers. In addition to this mechanism, this occurrence feature could partially come from the transequatorial meridional neutral wind during the morning local time period in the June solstice season being relatively weak compared to that in the other seasons in Southeast Asia, as shown in Figure 6b. The combined effect of the later reversal time of the E B drift and the relative weak meridional neutral wind during the morning local time period could make the F 3 layer distinct near the magnetic equator in the June solstice season. This consideration is consistent with the seasonal variation of the occurrence probability at CP deduced from the current statistical analysis. Though the occurrence probability at KT in the June solstice season was found to become high, as shown in Figures 4 and 5, this tendency could also be explained by this combined effect; KT is located near the boundary latitude between the magnetic low latitude region and the region near the magnetic equator. [27] On the other hand, SEALION observations demonstrate that the F 3 layer occurrence probability at CP is relatively high in the presunset local time period only in the June solstice season. In most of the days in the June solstice season when the F 3 layer observed in the presunset local time at CP, the F 3 layer was also observed in the morning local time. In other words, the reoccurrence of the F 3 layer reported by Balan et al. [2000] was common in the June solstice season at CP. This feature should be examined in more detail in a future study Region Over the Magnetic Equator [28] Over the magnetic equator, it was found by the current model calculation that the F 3 layer did not form in all the seasons. This could stem from the field aligned diffusion over the magnetic equator and the reversal of the E B drift starting too early to form the F 3 layer. To form the layer, a density enhanced region must be generated at a high altitude sufficiently separated from the F 2 layer, and the 12 of 14

13 peak density of the F 3 layer must be sufficiently higher than that of the F 2 layer. The start time of the field aligned diffusion and the uplifted amount of plasma decide whether or not the F 3 layer forms. Under the consideration described above, a large E B drift with a late reversal time, which can be during a magnetically disturbed period, may form the F 3 layer in the vicinity of the magnetic equator. Detailed statistical analysis of the F 3 layer occurrence over the magnetic equator is needed. 5. Conclusion [29] To clarify the characteristics of the F 3 layer with a focus on the magnetic latitude dependence and the relationship to the equatorial anomaly, we performed statistical analysis of ionosonde chain data in the magnetic meridional plane of SEALION, and performed model calculation using the SAMI2 code. [30] By analyzing the ionogram data at CM, CP, and KT during the periods from March to September 2005 and from October 2006 to February 2007, we confirmed that the occurrence features of the F 3 layer depend on the magnetic latitude in the magnetic meridional plane. At CP, near the magnetic equator, the F 3 layer moves upward rapidly with an early onset time, while it stays roughly at a certain altitude at CM and KT with a late onset time in the magnetic low latitude regions. The F region critical frequency did not decrease at CM and KT even when the F 3 layer appeared. The occurrence probability of the F 3 layer was highest in the June solstice season at CP. The probability was lowest in the December solstice season at CM, and it was high in all seasons at KT. The yearly averaged occurrence probabilities of the F 3 layer at CM, CP, and KT are 75%, 46%, and 97%, respectively. The yearly averaged occurrence probability at CP is somewhat less than that at CM and KT. The occurrence concentrates in the morning local time period at CP, except for in the June solstice season, while it spread from the morning to presunset local time at CM and KT. [31] From comparing the observational and model calculation results, we conclude that the field aligned diffusion acts to decrease the peak density of the F 3 layer near the magnetic equator, as suggested by Balan et al. [1998], while it acts to make the F 3 layer prominent in the magnetic lowlatitude region. The magnetic latitude dependence of the F 3 layer comes not only from the meridional neutral wind effect suggested by Balan et al. [1998] but also the fieldaligned diffusion effect. [32] On the other hand, we found by the present model calculation that the F 3 peak corresponds to the electron density enhanced region associated with the equatorial anomaly. This relationship is consistent with the suggestion that the field aligned diffusion acts to form the F 3 layer in the magnetic low latitude region since the fundamental factors for generation of the equatorial anomaly are also E B drift and field aligned diffusion. This relationship does not contradict the suggestion by Balan et al. [1998] that the F 3 layer is likely to form in the windward region, since the density enhanced region associated with the equatorial anomaly is located at a high altitude in the windward region from the meridional neutral wind. Moreover, the local time and magnetic latitudinal variations of the F 3 layer can be regarded as those of the density enhanced region; the electron density enhanced region associated with the equatorial anomaly moved to a higher latitude region and rose to a higher altitude as the local time progresses, and then stagnated. This temporal variation of the density enhanced region is consistent with that of the F 3 layer. [33] The dynamics of the F 3 layer in the magnetic meridional plane are suggested as follows: The electron density enhanced region (F 2 layer) is horizontally located on a low altitude region at an early morning local time. After the zonal electric field is reversed from westward to eastward, the density enhanced region begins to be uplifted by the E B drift. The density enhanced region reaches faster a higher altitude region nearer the magnetic equator due to the inclination of the magnetic field and the magnetic latitudinal dependence of the E B drift. Near the magnetic equator, the density enhanced region diffuses to a higherlatitude region with moving upward from the E B drift and starts to form the crest of the equatorial anomaly. The occurrence of the F 3 layer near the magnetic equator depends on the reversal time of the E B drift. When the reversal time is too early, the F 2 layer becomes merely a broad structure in altitude without formation of the F 3 layer. On the other hand, if the reversal time is sufficiently late, the F 3 layer can form when the density enhanced region associated with the equatorial anomaly reaches the altitude separated sufficiently from the F 2 layer formed by the photochemical and dynamical processes. Since the densityenhanced region associated with the equatorial anomaly continues to move to a higher altitude and a higher latitude region due to the E B drift and field aligned diffusion with decreasing of the peak density of the F 3 layer near the magnetic equator, the F 3 layer is observed as moving upward, as reported by Balan et al. [1998]. Subsequently, the density enhanced region continues to move to a higher altitude and a higher latitude region due to the E B drift and field aligned diffusion. The latitudinal location of the F 3 layer is shifted to a higher latitude region. As local time progresses, the density enhanced region moves horizontally rather than vertically, due to the local time variation of the E B drift and field aligned diffusion. In the magnetic low latitude region, the peak density of the F 3 layer increases due to the positive field aligned diffusion effect. During this local time period, the cusp between the F 2 and F 3 layers on the ionogram is observed at the same altitude or moving slightly downward. Finally, the density enhanced region stagnates at a higher latitude region. When the density enhanced region approaches the altitude of the F 2 layer existing below, the F 3 layer becomes unobservable. The meridional neutral wind modulates the density enhanced region associated with the equatorial anomaly to become higher (lower) altitude in the windward (leeward) due to the ion drag effect. Therefore the F 3 layer is preferably formed in the windward magnetic low latitude region, as suggested by Balan et al. [1998]. The location and duration time of the F 3 layer are controlled by the balance between the E B drift, meridional neutral wind, and field aligned diffusion. [34] Acknowledgments. The ionosonde stations in Chiang Mai, Chumphon, and Kototabang are operated under agreements between NICT, Japan, and Chiang Mai University, Thailand, and between King Mongkut s Institute of Technology Ladkrabang (KMITL), Thailand, and the Indonesian National Institute of Aeronautics and Space (LAPAN), Indonesia. 13 of 14

14 The operation at Kototabang is also supported by the Equatorial Atmosphere Radar project of Kyoto University, Japan. This work uses the SAMI2 ionosphere model written and developed by the Naval Research Laboratory. This research was partially supported by a grant in aid for JSPS fellows, Ministry of Education, Culture, Sports, Science and Technology, Japan. Activities of two authors (T. Ono and A. Kumamoto) are supported by the Global COE Program Global Education and Research Center for Earth and Planetary Dynamics at Tohoku University. [35] Robert Lysak thanks C. C. Hsiao and Paulo Fagundes for their assistance in evaluating this paper. References Appleton, E. V. (1946), Two anomalies in the ionosphere, Nature, 157, 691. Bailey, D. K. (1948), The geomagnetic nature of the F 2 layer longitude effect, J. Geophys. Res., 53, 35 39, doi: /te053i001p Balan, N., and G. J. Bailey (1995), Equatorial plasma fountain and its effects: Possibility of an additional layer, J. Geophys. 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Lett., 34, L02110, doi: /2006gl M. Iizima, Daijo Shukutoku Gakuen, Maeno cho, Itabashi ku, Tokyo , Japan. A. Kumamoto and T. Ono, Graduate School of Science, Tohoku University, 6 3 Aoba, Aramaki, Aoba ku, Sendai, Miyagi , Japan. T. Maruyama and J. Uemoto, National Institute of Information and Communications Technology, Nukui Kitamachi, Koganei, Tokyo , Japan. (juemoto@nict.go.jp) S. Saito, Electronic Navigation Research Institute, Jindaiji Higashi , Chofu, Tokyo , Japan. 14 of 14