Motions of the equatorial ionization anomaly crests imaged by FORMOSAT-3/COSMIC

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L19101, doi: /2007gl030741, 2007 Motions of the equatorial ionization anomaly crests imaged by FORMOSAT-3/COSMIC C. H. Lin, 1 J. Y. Liu, 2 T. W. Fang, 2,3 P. Y. Chang, 2 H. F. Tsai, 4 C. H. Chen, 2 and C. C. Hsiao 1 Received 20 May 2007; revised 13 August 2007; accepted 24 August 2007; published 3 October [1] The equatorial ionization anomaly (EIA) structures and evolutions are imaged using radio occultation observation of the newly launched FORMOSAT-3/COSMIC (F3/C) satellite constellation. Three-dimensional ionospheric images provide unprecedented detail of the EIA structure globally. This paper presents images of the EIA structures during July August 2006 and discusses the development and subsidence of the EIA. Clear seasonal asymmetries in both ionospheric electron density and layer height are observed. Two-dimensional (cross section) maps at a meridian provide dynamic variations and motions of the northern and southern EIA crests. Results suggest that in addition to the asymmetric neutral composition effect, interactions between the summer-to-winter (transequatorial) neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the EIA crests as imaged by the F3/C. Citation: Lin, C. H., J. Y. Liu, T. W. Fang, P. Y. Chang, H. F. Tsai, C. H. Chen, and C. C. Hsiao (2007), Motions of the equatorial ionization anomaly crests imaged by FORMOSAT-3/ COSMIC, Geophys. Res. Lett., 34, L19101, doi: / 2007GL Introduction [2] Most of our knowledge of the ionosphere comes from remote sensing by radio waves. One of the earliest, and most used, ground-based radar devices is the ionosonde, which yields vertical electron density profiles up to but not above the altitude of the highest electron density [e.g., Davies, 1990; Hunsucker, 1991]. Currently, there are about 200s standard ionosondes routinely recording ionograms. Although, sophisticated incoherent scatter radars have the ability to make measurements from the ground to the topside ionosphere where is inaccessible by ionosondes, they are rather limited in number of about 10. Recently, the radio beacon, satellite-borne transmitter, of the global positioning system (GPS) has been used to derive the ionospheric total electron content (TEC). By applying an interpolation and/or a model smoothing on the derived data from 100 s 1000 s ground-based GPS receivers, a global 1 Science Research Division, National Space Organization, Hsinchu, 2 Institute of Space Science, National Central University, Jhongli, 3 Also at High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA. 4 Central Weather Bureau, Taipei, Copyright 2007 by the American Geophysical Union /07/2007GL ionospheric map can be constructed for studying the global horizontal structure of the ionospheric TEC [e.g., Mannucci et al., 1998; Komjathy et al., 2005; Mendillo and Klobuchar, 2006]. The significant shortcoming of existing groundbased observations is that they can not yield data over oceanic areas (Figure 1). This shortcoming could partially be redeemed by satellite observations, such as TOPEX/ Poseidon satellite [cf. Codrescu et al., 1999; Jee et al., 2004, and references therein] and TIMED-GUVI observations [cf. Kil et al., 2006, and references therein]. Meanwhile, to quickly yield a three-dimension (3-D) global coverage, worldwide vertical profiles simultaneously observed by multiple satellites are essential. [3] A powerful technique of using signals to derive the vertical profiles is known as the atmospheric/ionospheric radio occultation [Hajj and Romans, 1998; Schreiner et al., 1999; Hajj et al., 2000; Yunck, 2002]. The technique was first used by the Mariner missions in exploration of planetary atmosphere in 1960s [Fjeldbo and Eshleman, 1968; Fjeldbo et al., 1971]. The radio occultation technique was not applied to the Earth s atmosphere observation until an experiment satellite called GPS/MET in Using a GPS receiver on board a low-earth orbit (LEO) satellite to receive radio signal transmitted by GPS satellites at an altitude of 20,200 km, vertical distribution of the atmospheric/ionospheric parameters are derived. Following the successful GPS/MET experiment, similar satellite missions, such as CHAMP (Germany), SAC-C (Argentina), GRACE (2 satellites, US and Germany), and IOX (US), are carried out. Many studies have shown promised occultation results of ionospheric soundings in comparison with ground-based radar measurements since then [e.g., Hajj and Romans, 1998; Schreiner et al., 1999; Hernandez-Pajares et al., 2000; Hajj et al., 2000; Jakowski et al., 2002; Tsai and Tsai, 2004]. [4] However, the existing satellites performing radio occultation experiments are mainly solo-satellite missions that are not dedicated to rapidly monitor global space weather changes. To improve global space weather monitoring, six microsatellites termed Formosa Satellite 3 and Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC or F3/C in short) were launched into a circular low-earth orbit from Vandenberg Air Force Base, California, at 0140 UTC on 15 April Each microsatellite of the joint Taiwan-US satellite constellation mission has a GPS Occultation Experiment (GOX) payload to operate the ionospheric radio occultation (RO), a tiny ionospheric photometer (TIP) to observe the nighttime ionospheric airglow emission, and a tri-band beacon (TBB) to tomographically estimate fine structures of ionospheric electron density on the satellite- L of6

2 Figure 1. FOMOSAT-3/COSMIC 2500 observation points per day after the constellation reaches the mission orbit (green points); locations of 10 incoherent scattering radars that can observe both the top and bottom side of the ionosphere (red pentagrams); 44 available sounding radars, ionosondes ( that can observe from the bottom side to the peak altitude (opened magenta pentagrams); and about 1000s ground-based GPS receivers (blue triangles). to-receiver plane. The F3/C satellites were launched to the initial orbit at an altitude of 512 km, and 72 inclination angle [Cheng et al., 2006]. The six microsatellites are close to each other at the initial orbit. It will take about 16 months for constellation to reach the mission orbit at around 800 km altitude, 72 inclination angle, and 30 separation in longitude between each microsatellite. Currently (as of 5 February 2007), the GOXs on four initial- and two mission-orbit microsatellites globally observe about 2500 vertical electron density profiles per day. 2. Images of the Equatorial Anomaly Crests [5] In the past, 3-D ionospheric images are limited and only ionospheric tomographic image constructed by regional observation chain provides detailed observation of ionospheric structure [e.g., Andreeva et al., 2000; Yeh et al., 2001; Tsai et al., 2002]. With the worldwide dense occultation observations carried out by GOXs on board the F3/C, 3-D ionospheric images can be constructed routinely. It is the purpose of this paper to study the structures and motions of the equatorial ionization anomaly (EIA) with 3-D ionospheric images provided by the F3/C observation. The EIA is characterized by two enhanced plasma crests at low latitudes straddling the magnetic equator with the electron density depleted on the magnetic equator. It is the region that yields the greatest electron density in globe except in the auroral region during magnetic disturbance period. The EIA is produced by the equatorial plasma fountain, which lifts the plasma from magnetic equator to higher altitudes and then it diffuses down along magnetic field lines to higher latitudes creating two ionization crests on both sides of the magnetic equator [Namba and Maeda, 1939; Appleton, 1946; Duncan, 1960; Hanson and Moffett, 1966; Anderson, 1973; Balan and Bailey, 1995; Rishbeth, 2000]. Although the equatorial fountain is the major driver for producing EIA, field-aligned plasma transport produced by neutral winds and photochemical processes produced by neutral composition effects are also known to alter the EIA structure, especially during solstitial seasons. We present time-evolution images of the EIA during July August 2006 (close to June solstice) to study asymmetric seasonal effects to the EIA crest motions. [6] Although the constellation has not yet reached its final mission orbit, 2500 ionospheric soundings are performed daily providing measurements of global 3-D ionospheric electron density structure up to 500 km altitude. Monthly averaged global ionospheric soundings can be obtained by binning measurements from two months (e.g., July August 2006) of occultation observations in two-hour (or hourly) bins and taking median value of the soundings located in the same 2.5-by-2.5 degree (longitude-bylatitude) grid in every 5 km altitude range. Detailed description of the inversion technique applied to invert the F3/C occultation soundings to ionospheric electron density profiles are given by Schreiner et al. [1999] and Syndergaard et al. [2006]. Some initial validation works of the F3/C radio occultation observations have been carried out by Schreiner et al. [2007] and Lei et al. [2007]. Schreiner et al. [2007] compared occultation soundings of the two nearby F3/C microsatellites when the constellation is still clustered together. They found that the root mean square difference of electron density in the ionosphere between 150 and 500 km altitude for collocated occultations is about 10 3 cm 3. Lei et al. [2007] also shows good agreements between radar observations and F3/C radio occultation results during June August 2006 which covers the time period of observations presented in this paper. A correlation coefficient of 0.85 is obtained from their study by comparing 276 collocated radar and radio occultation observations during July In this study, observations during magnetically disturbed periods are excluded in the data bins. Figure 2 shows the constructed peak altitude (or hmf 2 ), peak density (or NmF 2 ), and total electron content (TEC) map (integrated from km altitude) at around 12:00 LT. The peak altitude in the northern/summer hemisphere is generally higher than that in the southern/winter hemisphere (Figure 2a). The peak density, on the other hand, shows larger magnitude in the southern/winter hemisphere. Both peak altitude and density reveal clear longitudinal variability. 2of6

3 Figure 2. Ionospheric maps in (a) peak altitude (hmf 2 ), (b) peak density (NmF 2 ), and (c) total electron content (TEC) integrated between km altitude range at global constant local time at 1200 LT. These longitudinal variations may be due to differences in magnetic declination, E B drift, and neutral winds in different longitudes. The TEC map, however, does not show clear seasonal asymmetry, except in between E longitude regions. The deviations between the NmF 2 and TEC distributions provides important information to ionospheric researchers, since the global TEC distribution is widely available and used for studying the ionospheric seasonal effects [e.g., Tsai et al., 2001; Codrescu et al., 1999; Jee et al., 2004] while the NmF 2 observations are rather limited. [7] To have a better understanding on the ionospheric dynamics in producing the EIA structure and the seasonal asymmetry in the northern and southern hemispheres as shown in Figure 2, a sequence of images of electron density on the 90 E meridian plane, where prominent north south asymmetry in EIA peak density is seen, are constructed to depict the time evolution of EIA development and subsidence (Figure 3). The increasing ionization appears in the northern/summer hemisphere at around 05:30 06:30 LT, possibly due to earlier sun-rise time in the northern hemisphere and north south asymmetry of the photo-ionization process. The EIA crest in the southern hemisphere is intensified after 07:00 LT, and becomes stronger than the northern EIA crest between 08:30 10:30 LT. The stronger southern EIA crest may result from both the summer-towinter hemisphere plasma transport and the asymmetric neutral composition distribution. After 10:30 LT, the electron density starts to pile up at the magnetic equator and the northern hemisphere forming discernible northern EIA crest at 12:00 LT. The northern EIA crest becomes stronger than the southern crest at 15:00 LT. Later on, after 17:00 LT, the southern EIA crest starts to diminish, possibly due to reduction of the photo-ionization effect and weakening of the equatorial plasma fountain while the northern EIA crest subsides with descending peak altitude after 19:00 LT. These time dependent meridian images of the ionospheric density provide unprecedented details of the EIA structure, which also illustrate formation of the latitudinal asymmetry. 3. Discussion and Summary [8] The hemispheric asymmetry of the EIA shown in Figure 2 is consistent with GPS TEC observation at the West Pacific region by Tsai et al. [2001]. Their observations show greater TEC values in the southern/winter hemisphere during July and August 1997 (also at low solar activity period). Their results also indicated that the EIA crest in the winter hemisphere formed earlier than that in the summer hemisphere. This result is consistent with time-sequence images shown in the Figure 3. Meanwhile, Figure 3 shows that the southern EIA crest starts to decay after 14:00 LT, 3of6

4 Figure 3. Time evolution of the ionospheric plasma density structure at the mid- and low-latitude regions at around 90 E (right) meridian sector (±15 longitude intervals). The plasma density structure plot at each time interval is constructed by taking the median of the FORMOSAT-3/COSMIC observations within the two-hour interval during July August, Time marked in each plot is the central hour of the two-hour interval. which is again consistent with Tsai et al. [2001]. From their GPS-TEC analysis in 1997, the southern EIA crest shows its maximum at 12:00 LT and 13:00 LT in July and August, respectively. [9] The two basic processes that affect the EIA formation significantly are the strength of the equatorial plasma fountain and thermospheric neutral winds [Balan and Bailey, 1995; Balan et al., 1997; Su et al., 1997; Rishbeth, 2000; Abdu, 2001; Lin et al., 2005]. Figure 3 shows that more ionization starts to form in the sunlit/summer hemisphere at 06:30 LT and may be transported to the winter hemisphere by summer-to-winter meridional wind later on, forming the EIA crest in south earlier than north (08:30 10:30 LT). The electron density starts to pile up at around magnetic equator and northern hemisphere after 10:30 LT, which suggests a growing strength of the equatorial fountain effect. According to Scherliess and Fejer [1999], the equatorial upward E B drift reaches its maximum value at around 10:00 11:00 LT and more pronounced plasma fountain effect occurs after that. The fountain effect may interact with the transequatorial neutral wind effect in the following ways. In the southern/winter hemisphere, a downward diffusion produced by the fountain effect has the same direction as that produced by the summer-to-winter wind, thus both effects accelerate the development process of the southern EIA crest. In the northern/summer hemisphere, the summer-towinter wind brings the plasma upward/equatorward while the fountain effect tends to diffuse the plasma to downward/ poleward direction. If the neutral wind effect prevails, the electron density accumulates at location closer to the magnetic equator or to the southern/winter hemisphere. On the other hand, if the fountain effect is more dominant, the northern EIA crest forms in poleward location with more plasma accumulated there. Images presented in Figure 3 may result from competing interactions between the transequatorial neutral wind effect and the equatorial plasma fountain effect. The possible scenario described here is consistent with the hypothesis described by Kil et al. [2006] in explaining the seasonal asymmetry observed by multiple satellites with transequatorial neutral winds obtained from HWM 93 model [Hedin et al., 1991]. Effects of the transequatorial neutral winds to plasma transport in the EIA region during solstitial seasons are modeled by National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamic General Circulation Model (NCAR-TIEGCM) and Sheffield University Plasmasphere 4of6

5 Ionosphere Model (SUPIM) showing qualitative agreements with electron density structures presented herein [Lin, 2005; Fang et al., 2006]. It is worthwhile to note that the asymmetric neutral composition effect, upwelling in the summer hemisphere and the downwelling in the winter hemisphere [e.g., Rishbeth et al., 2004], may also contribute to the observed structures presented here. We focus our discussions on competitions between the neutral wind and the fountain effect here, since the two effects are more dominant from theoretical examination of Lin [2005] which indicates good characteristic agreement with density structure shown in Figure 3. [10] Seasonal asymmetry of the north south EIA crests and their diurnal variations are imaged by the F3/C radio occultation observation. We summarize our observation features as follows. [11] (1) Stronger electron density of EIA crest appeared in the southern/winter hemisphere, while higher peak altitude is seen in the northern/summer hemisphere at 12:00 LT (Figure 2). [12] (2) The southern/winter EIA crest formed earlier than the northern/summer crest, which is possibly due to summer-to-winter neutral wind effect. The northern/summer crest starts to develop after the fountain effect becomes dominant (Figure 3). [13] (3) The northern EIA crest forms at 12:00 LT and becomes stronger than the southern crest after 15:00 LT, while the southern EIA crest becomes diminished at 17:00 LT. The entire EIA structure becomes indiscernible after 21:00 LT (Figure 3). [14] In conclusion, FORMOSAT-3/COSMIC provides three-dimensional images of electron density for globally studying structures and dynamics in the ionosphere. [15] Acknowledgments. The authors would like to thank Arthur D. 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