Persistent Longitudinal Features in the Low-Latitude Ionosphere H

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A06315, doi:10.1029/2012JA017570, 2012

Persistent longitudinal features in the low-latitude ionosphere H. Kil,1 W. K. Lee,2 Y.-S. Kwak,3 S.-J. Oh,4 L. J. Paxton,1 and Y. Zhang1 Received 26 January 2012; revised 28 March 2012; accepted 2 May 2012; published 23 June 2012.

[1] Various longitudinal wave patterns exist in the low-latitude ionosphere, but the investigation has been limited to the wave number (WN) 3 and 4 patterns. This study extends the investigation to the wave patterns that have not yet been explored. The persistent ionospheric wave patterns are investigated by using the measurements of the ion density during March 1999–June 2004 by the first Republic of China satellite. The investigation is performed with data sets in the magnetic south (20S–0) and magnetic north (0–20N). The dominant wave features in plasma density are the WN1, WN2, WN3, and WN4 patterns. Among them, the WN1 pattern in the magnetic south during the June solstice is the most pronounced feature. Except for this component, the WN3 pattern is the most persistent and pronounced feature. Fundamental difference exists between the WN1 and WN2 patterns and the WN3 and WN4 patterns. First, the wave phases in the south and north are the same in the WN3 and WN4 patterns, but they are opposite in the WN1 and WN2 patterns. Second, the amplitudes of the WN3 and WN4 patterns show similar annual variation in the south and north, but those of the WN1 and WN2 patterns show opposite annual trends in the opposite hemispheres. These observations indicate that the WN1 and WN2 patterns and the WN3 and WN4 patterns are created by different mechanisms. We discuss the creation of the WN1 and WN2 patterns in association with the geomagnetic field configuration. Citation: Kil, H., W. K. Lee, Y.-S. Kwak, S.-J. Oh, L. J. Paxton, and Y. Zhang (2012), Persistent longitudinal features in the low-latitude ionosphere, J. Geophys. Res., 117, A06315, doi:10.1029/2012JA017570.

1. Introduction These characteristics support that the creation of the WN4 pattern is associated with the diurnal eastward-propagating [2] When the plasma distribution in the low-latitude nonmigrating WN3 tide (DE3). DE3 winds modulate the F region is viewed at a fixed local time, different wave E-region dynamo electric field; the E-region dynamo elec- patterns appear in different seasons. The most striking wave tric field modulates the vertical plasma drift in the F region; patterns are the wave number 4 (WN4) and wave number and the longitudinal modulation of the vertical plasma drift 3 (WN3) patterns [Kil and Paxton, 2011, and references creates the WN4 pattern in the F-region plasma density therein]. The WN4 pattern has been investigated extensively [England et al., 2006; Immel et al., 2006; Jin et al., 2008; Kil by using various observations. The common characteristics et al., 2007]. This mechanism is supported by the observa- of the WN4 pattern are (1) the occurrence of the amplitude tions of a good correlation between the WN4 patterns in maximum during July–September and the amplitude mini- plasma density and the vertical plasma drift [Kil et al., 2007, mum during the December solstice and (2) the eastward 2008, 2011]. Model simulations further verified the linkage phase shift close to a rate of 90 per 24 h local time (LT) between DE3, vertical plasma drift, and WN4 pattern in [Fang et al., 2009; Kil et al., 2008, 2010; Lühr et al., 2008; plasma density [Fang et al., 2009; Hagan et al., 2007; Oh Ren et al., 2009; Scherliess et al., 2008; Wan et al., 2008]. et al., 2008; Ren et al., 2009, 2010]. [3] The WN3 pattern is also a persistent feature in the ionosphere. The WN3 pattern is apparent only during the months near the December solstice, although the WN3 pat- 1 Space Department, Johns Hopkins University Applied Physics tern is a pronounced feature in all months. This is because Laboratory, Laurel, Maryland, USA. 2COSMIC Project Office, University Corporation for Atmospheric the WN3 pattern is hidden under the superposition of the Research, Boulder, Colorado, USA. WN3 and WN4 patterns during the months when the WN4 3Space Science Division, Korea Astronomy and Space Science Institute, pattern is pronounced [Kil et al., 2011]. Similar to the cre- Daejeon, South Korea. 4 ation of the WN4 pattern, the creation of the WN3 pattern in Space Environment Laboratory, Seoul, South Korea. plasma density is explained by the longitudinal modulation Corresponding author: H. Kil, Space Department, Johns Hopkins of the E-region dynamo electric field [Kil et al., 2010]. The University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, diurnal eastward-propagating nonmigrating wave number MD 20723, USA. ([email protected]) 2 tide (DE2) is suggested as the source of the WN3 pattern in ©2012. American Geophysical Union. All Rights Reserved. the E-region dynamo electric field [England et al., 2009;

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study, we processed the ROCSAT-1 data at the times of low and moderate levels of the Earth’s magnetic field dis- turbance (Kp index ≤ 3+). The persistency of wave patterns is investigated by using the data between 1200 and 1600 LT. This time interval was chosen because the longitudinal modulation in plasma density is most pronounced in the afternoon [e.g., Kil et al., 2008]. The wave components in plasma density were extracted by applying a Fourier analysis to the average density in the magnetic south (20S–0) and magnetic north (0–20N). The observation data are divided into six periods (January–February, March–April, May–June, July–August, September–October, and Novem- ber–December) of each year. Because the ROCSAT-1 data are available during March 1999–June 2004 (64 months), the wave components were extracted from 32 data sets.

3. Results and Discussion

Figure 1. The percentage wave amplitude distribution. The [7] To identify the primary longitudinal wave features in wave amplitudes are calculated by using the ROCSAT-1 ion the ionosphere, we have calculated the percentage wave density data between 1200 and 1600 LT in the magnetic lat- amplitudes using the data in three magnetic latitude intervals: itude interval 20 S–0 (red dots), 0 –20 N (blue dots), and 20S–0 (south), 0–20N (north), and 20S–20N (average) 20 S–20 N (black dots). The average amplitudes (solid dots) for the 32 data sets. The percentage wave amplitude is and standard deviations (vertical bars) are obtained from the obtained by applying a Fourier analysis to the density divided 32 period data sets described in the text. by the longitudinal average density. Throughout this study, we use the percentage wave components. The LT interval Pedatella et al., 2008], but Kil et al. [2010, 2011] argued was chosen between 1200 and 1600 LT. Figure 1 presents the that the DE2 characteristics do not appear in the WN3 pat- percentage amplitudes (solid dots) and standard deviations terns in the vertical plasma drift and plasma density. The (vertical bars). The amplitudes of the high wave number source of the WN3 pattern is still an open question. components (≥5) are small (<4%). Because those compo- [4] A significant portion of the longitudinal plasma dis- nents are considered minor features in the ionosphere, our tribution is described by the WN3 and WN4 patterns, but investigation of the persistency of the wave components is other wave features also exist in the ionosphere. Not much limited to the WN1–WN4 components. The WN1 compo- attention has been paid to the wave number 1 (WN1) and nent in the magnetic south has the largest amplitude. The wave number 2 (WN2) patterns. Those low wave number large standard deviation of this component is related to the patterns are not easily perceived when they co-exist with the large seasonal variation of this component (see Figure 3). WN3 and WN4 patterns. The characteristics and sources of Except for this WN1 component, the WN3 amplitude is the the WN1 and WN2 patterns have not yet been investigated. largest. [5] In this study, we determine which wave patterns are [8] In Figure 2, we examine the recurrence of the (a) WN1, persistent and dominant features in the ionosphere by (b) WN2, (c) WN3, and (d) WN4 components during the examining the measurements of the ion density during 32 periods. The wave components in the south and north March 1999–June 2004 by the first Republic of China sat- are shown by red and blue colors, respectively. In the plots, ellite (ROCSAT-1). A typical method to investigate the the observation month and year are not distinguished. The wave characteristics in the ionosphere is to extract the wave thick black curve in Figure 2a denotes the geographic lati- amplitude and phase by applying a Fourier analysis. How- tude of the magnetic equator. For the WN1 component, the ever, the wave components extracted by this method are WN1 amplitude in the south is much greater than that in the mathematical outputs. Only the wave components whose north, as is shown in Figure 1. In a comparison of the characteristics repeatedly occur would be considered per- southern WN1 component with the magnetic equator loca- sistent and physically meaningful features in the ionosphere. tion, we can identify the creation of the crest and trough of Because the characteristics of the WN3 and WN4 patterns the WN1 component in the longitude regions where the have been reported by many studies, the main interest of this magnetic equator is located in the geographic north and study is to understand the characteristics and source of the south, respectively. This tendency appears to be reversed in WN1 and WN2 patterns. the northern WN1 component. The opposite phases between the southern and northern components also appear in the WN2 component. In Figure 1, the WN2 amplitude obtained 2. Data Description from the average data in the magnetic south and north (black [6] ROCSAT-1 was launched on 27 January 1999 into a dots) is smaller than the amplitudes obtained from the data in circular orbit at an altitude of 600 km. Its orbital inclination each hemisphere (blue and red dots). This behavior is related was 35 and the orbital period was 97 min. The Ionospheric to the opposite phases of the WN2 components in the oppo- Plasma and Electrodynamics Instrument onboard ROCSAT-1 site hemispheres. The wave components in the opposite [Yeh et al., 1999] made the measurements of the ion density hemispheres do not show any notable difference in the WN3 and ion velocity from March 1999 to June 2004. For this and WN4 components. Among the four wave components,

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Figure 2. Persistency test of the (a) WN1, (b) WN2, (c) WN3, and (d) WN4 components. The wave components in the magnetic south (red) and north (blue) are obtained from the 32 data sets. The geographic latitude of the magnetic equator is shown with a thick black curve in Figure 2a. the WN3 component shows the most consistent phase during result provides insight into the source of the ionospheric the 32 periods. The WN4 component also shows a consistent wave patterns. The creation of the opposite wave patterns phase during the periods when the WN4 amplitude is large. in the opposite hemispheres is not explained by the effect During the periods when the WN4 amplitude is small, how- of the vertical plasma drift (or dynamo electric fields). ever, the phase of the WN4 component is not consistent. The Thus WN1 and WN2 patterns are created by a mechanism WN1 and WN2 components show opposite phases in the that is different from the creation mechanism (i.e., dynamo opposite hemispheres, but those components are consistent in electric fields) of the WN3 and WN4 patterns. each hemisphere. [10] The annual and yearly variations of the percentage [9] The fundamental difference between the WN3 and wave amplitudes are shown in Figure 3. The results obtained WN4 components and the WN1 and WN2 components is with the data sets in the north and south are presented in the phase of the southern and northern wave components. Figures 3a–3d and Figures 3e–3h, respectively. Note the use They are in-phase in the WN3 and WN4 components and of different amplitude scale in the WN1 component. In gen- opposite phase in the WN1 and WN2 components. This eral, repeatable annual patterns exist in all wave components.

Figure 3. The annual and yearly variations of the percentage wave amplitude. (a–d) Wave components in the magnetic north. (e–h) Wave components in the magnetic south.

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m). Here K is a variable expressed by the optical depth, scale height, and height, and m is the solar zenith angle. The variation of the photoionization rate is greater for large solar zenith angles than for small solar zenith angles because of the cos m term. Therefore, the magnitude of the solar zenith angle as well as its longitudinal difference causes the seasonal and hemispheric difference of the WN1 component amplitude. Neutral winds and neutral composition at the same magnetic latitude vary with longitude in association with the geomagnetic field configuration. These factors would also cause a large-scale longitudinal asymmetry in plasma density. [12] The WN1 feature is associated with a longitudinally asymmetric plasma distribution. Although the clarification Figure 4. Comparison of the longitudinal plasma distribu- of the source of the large-scale longitudinal asymmetry in tion and the magnetic equator location. The red curves show plasma density requires further investigation by model simulations, creation of the WN1 feature in the ionosphere the average plasma density (Ni) normalized by the longitudi- might be predictable by the reasons described above. How- nal mean density (No) in the magnetic south (20 S–0 ) during June–July of each year. The magnetic equator location is ever, the source of the WN2 feature is clueless. The WN2 denoted with a black dashed curve. component and the location of the geomagnetic equator do not seem to show any correlation. The observation of the opposite phases between the southern and northern WN2 The absence of an increasing or decreasing trend of the wave components rules out the possibility that the tidal modula- amplitude as a function of year may indicate that the wave tion of the E-region dynamo electric fields is the source. We amplitude does not depend on the solar cycle. The WN1 deduce the possible connection of the WN1 and WN2 component in the south shows a clear annual variation; the components from their similar behavior: the opposite phases amplitude maximum occurs during the June solstice and of the longitudinal wave patterns and the opposite annual the amplitude minimum occurs during the December sol- patterns of the wave amplitudes in the opposite hemispheres. stice. This annual trend appears to be reversed in the north. [13] To test the connection between the WN1 and WN2 The annual variation of the WN2 amplitude is small. A minor components, we examine the wave components embedded tendency that we can identify in the WN2 component is on a schematic model ionosphere that resembles the plasma the greater amplitude during the spring equinox than during distribution shown in Figure 4 (ignoring the high-frequency the fall equinox in the south and the reversal of this ten- features). The model ionosphere in Figure 5a is produced dency in the north. For the WN3 component, the amplitude with a half sine curve to represent the low plasma density in during solstices is greater than that during equinoxes. The the longitude 180E–360. The wave components extracted amplitude of the WN4 component shows a clear annual variation; the maximum amplitude occurs during July–August and the minimum amplitude occurs near the December solstice. The correlated annual variation of the wave amplitudes in the southern and northern data in the WN3 and WN4 components is contrasted to their anti-correlated variation in the WN1 and WN2 components. [11] The WN1 component in the magnetic south during the June solstice is the most pronounced feature in the ionosphere. We interpret this phenomenon in association with the geomagnetic field configuration. The red curves in Figure 4 show the average plasma density in the magnetic south (20S–0) acquired during June–July. The plasma density of each year is normalized by the longitudinal average density. The geographic latitude of the magnetic equator is shown with a black dashed curve. Ignoring the high-frequency wave features, the longitudinal variation of the plasma density shows a good correlation with the location of the magnetic equator. Creation of the large WN1 amplitude during the June solstice is related to the low plasma density in the longitude near 300E, where the magnetic equator is located farthest distance from the geographic equator. The solar zenith angle at the magnetic equator is the largest near 300E during the June solstice. Thus the low photoionization rate near 300E during the June solstice is presumably the primary cause of the WN1 Figure 5. (a) Schematic model ionosphere. (b) Wave com- feature. Photoionization rate is proportional exp (ÀK / cos ponents extracted from the schematic model ionosphere.

4of6 A06315 KIL ET AL.: IONOSPHERIC LONGITUDE FEATURES A06315 from the model ionosphere are shown in Figure 5b. As WN4 components, but those show opposite trends in the expected, the WN1 component has the largest amplitude. WN1 and WN2 components. These results indicate that The WN2 amplitude is the second largest and is about the WN1 and WN2 components are created by a mechanism 40% of the WN1 amplitude. The WN4 amplitude is about that is different from the creation mechanism of the WN3 10% of the WN1 amplitude. The WN3 amplitude is zero. and WN4 components. In other words, the WN1 and WN2 The WN1 and WN2 components obtained from the simple components are not created by the modulation of the model ionosphere are similar to those obtained from the E-region dynamo electric field. The creation of the WN1 magnetic south data in Figures 2a and 2b (red curves). component might be explained by the longitudinal variation This comparison provides insight into the source of the of the solar zenith angle at the magnetic equatorial region, WN2 component. The large-scale longitudinal asymmetry in neutral winds, and neutral composition in association with plasma density might be the source of the WN2 component the magnetic field configuration. The large-scale longitudinal as well as the WN1 component. In the southern hemisphere asymmetry also produces the WN2 component. It would be during the June solstice, the WN2 component may be con- adequate to understand the WN1 and WN2 components as sidered a subordinate feature of the WN1 component. How- longitudinal asymmetry features rather than as wave features. ever, the WN2 amplitude is not proportional to the WN1 amplitude. In Figure 2, the WN2 amplitude in the north is [17] Acknowledgments. H. Kil acknowledges support from the NASA Grant NNX12AD17G. W. K. Lee acknowledges support by the Post comparable to that in the south, although the WN1 compo- Doctoral Research Program at the University of Science and Technology in nent in the north is much smaller than that in the south. In this South Korea. case, the WN2 component is not a subordinate feature of the [18] Robert Lysak thanks the reviewers for their assistance in evaluat- WN1 component. The relative amplitude of the WN1 and ing this paper. WN2 component depends on the morphology of the large- scale longitudinal asymmetry. The reduction of the longitude References range of the density drop in the schematic model ionosphere England, S. L., S. Maus, T. J. Immel, and S. B. Mende (2006), Longitudi- causes the increase of the WN2 amplitude. The seasonal and nal variation of the E-region electric fields caused by atmospheric tides, hemispheric variations of the WN1 and WN2 amplitudes Geophys. Res. Lett., 33, L21105, doi:10.1029/2006GL027465. shown in Figure 3 further demonstrate that the WN2 ampli- England, S. L., X. Zhang, T. J. Immel, J. M. Forbes, and R. DeMajistre (2009), The effect of non-migrating tides on the morphology of the equa- tude is not proportional to the WN1 amplitude. We note that torial ionospheric anomaly: Seasonal variability, Earth Planets Space, 61, our result partially explains the creation of the WN2 com- 493–503. ponent in association with the WN1 component. The longi- Fang, T.-W., H. Kil, G. Millward, A. D. Richmond, J. Y. Liu, and S.-J. Oh (2009), Causal link of the wave-4 structures in plasma density and verti- tudinal and hemispheric asymmetries of the plasma density cal plasma drift in the low-latitude ionosphere, J. Geophys. Res., 114, induced by other factors such as the magnetic declination A10315, doi:10.1029/2009JA014460. would also affect the WN2 amplitude. Hagan, M. E., A. Maute, R. G. Roble, A. D. Richmond, T. J. Immel, and S. L. England (2007), Connections between deep tropical clouds and [14] The WN4 amplitude is nonzero in Figure 5b, the Earth’s ionosphere, Geophys. Res. Lett., 34, L20109, doi:10.1029/ although the schematic model ionosphere does not have the 2007GL030142. WN4 feature. This result indicates that some wave compo- Immel, T. J., E. Sagawa, S. L. England, S. B. Henderson, M. E. Hagan, S. B. nents with small amplitude in observation data are not real Mende, H. U. Frey, C. M. Swenson, and L. J. Paxton (2006), Control of equatorial ionospheric morphology by atmospheric tides, Geophys. Res. features. During the December solstice, the WN4 component Lett., 33, L15108, doi:10.1029/2006GL026161. in plasma density has a small amplitude and the phase of the Jin, H., Y. Miyoshi, H. Fujiwara, and H. Shinagawa (2008), Electrodynam- WN4 component is variable year to year. The eastward phase ics of the formation of ionospheric wave number 4 longitudinal structure, J. Geophys. Res., 113, A09307, doi:10.1029/2008JA013301. shift at a rate of 90 /24 h LT did not appear in the WN4 Kil, H., and L. J. Paxton (2011), Causal link of longitudinal plasma den- component during the December solstice [Fang et al., 2009; sity structure to vertical plasma drift and atmospheric tides—A review, Kil et al., 2011; Wan et al., 2008]. Thus the WN4 compo- in Aeronomy of the Earth’s Atmosphere and Ionosphere, IAGA Spec. nents identified during the December solstice may not be the Sopron Book Ser., vol. 2, edited by M. A. Abdu and D. Pancheva, pp. 349–361, Springer, New York, doi:10.1007/978-94-007-0326-1_26. physically meaningful features created by the effect of DE3. Kil, H., S.-J. Oh, M. C. Kelley, L. J. Paxton, S. L. England, E. Talaat, K.-W. Min, and S.-Y. Su (2007), Longitudinal structure of the vertical E Â B drift and ion density seen from ROCSAT-1, Geophys. Res. Lett., 34, 4. Conclusions L14110, doi:10.1029/2007GL030018. Kil, H., E. R. Talaat, S.-J. Oh, L. J. Paxton, S. L. England, and S.-J. Su [15] We have investigated the persistency of the longitudi- (2008), Wave structures of the plasma density and vertical E Â B drift nal wave features in the low-latitude ionosphere by using the in low-latitude F region, J. Geophys. Res., 113, A09312, doi:10.1029/ plasma density data during March 1999–June 2004 provided 2008JA013106. Kil, H., L. J. Paxton, W. K. Lee, Z. Ren, S.-J. Oh, and Y. S. Kwak (2010), Is by ROCSAT-1. Our results show that the longitudinal plasma DE2 the source of the ionospheric wave number 3 longitudinal structure?, distribution in the low-latitude F region can be described pri- J. Geophys. Res., 115, A11319, doi:10.1029/2010JA015979. marily by the wave number 1–4 patterns. Among them, the Kil, H., Y.-S. Kwak, S.-J. Oh, E. R. Talaat, L. J. Paxton, and Y. Zhang (2011), The source of the longitudinal asymmetry in the ionospheric tidal WN3 pattern shows the most persistent behavior with signif- structure, J. Geophys. Res., 116, A09328, doi:10.1029/2011JA016781. icant amplitude in all months. Lühr, H., M. Rother, K. Häusler, P. Alken, and S. Maus (2008), The influ- [16] Fundamental difference exists between the WN1 and ence of nonmigrating tides on the longitudinal variation of the equatorial electrojet, J. Geophys. Res., 113, A08313, doi:10.1029/2008JA013064. WN2 components and the WN3 and WN4 components. The Oh, S.-J., H. Kil, W.-T. Kim, L. J. Paxton, and Y. H. Kim (2008), The wave components in the magnetic southern and northern role of the vertical E Â B drift for the formation of the longitudinal hemispheres are in-phase in the WN3 and WN4 compo- plasma density structure in the low-latitude F region, Ann. Geophys., nents, but they are opposite phases in the WN1 and WN2 26, 2061–2067, doi:10.5194/angeo-26-2061-2008. Pedatella, N. M., J. M. Forbes, and J. Oberheide (2008), Intra-annual vari- components. The annual patterns of the wave amplitude in ability of the low- latitude ionosphere due to nonmigrating tides, Geophys. the magnetic south and north are similar in the WN3 and Res. Lett., 35, L18104, doi:10.1029/2008GL035332.

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