Intra-Annual Variation of Wave Number 4 Structure of Vertical E В B
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A05308, doi:10.1029/2009JA014060, 2009 Click Here for Full Article Intra-annual variation of wave number 4 structure of vertical E Â B drifts in the equatorial ionosphere seen from ROCSAT-1 Zhipeng Ren,1,2,3 Weixing Wan,1 Libo Liu,1 and Jiangang Xiong1 Received 9 January 2009; revised 21 March 2009; accepted 16 April 2009; published 23 May 2009. [1] This paper studies the intra-annual variation of the wave number 4 structure in the equatorial vertical E Â B drifts at high solar flux level based on of the ROCSAT-1 observations during the interval from 1999 to 2004. It is found that the daytime longitudinal structure is significant in northern summer and early autumn but weak in northern winter. This is consistent with the intra-annual variations of the tidal mode DE3. At night, the intra-annual variation of the wave number 4 structure shows some differences from that in daytime, which may relate to different dynamo mechanisms operating in different local time. We also found that the wave number 4 structure mainly shifts eastward during daytime in most of months, which is coincident with the eastward propagation of the DE3 tide. However, it is largely disturbed near sunrise and sunset. We attribute this disturbance as the jumping of the main dynamo region between E and F layers, as well as the different dynamo mechanisms in different dynamo regions. Citation: Ren, Z., W. Wan, L. Liu, and J. Xiong (2009), Intra-annual variation of wave number 4 structure of vertical E Â B drifts in the equatorial ionosphere seen from ROCSAT-1, J. Geophys. Res., 114, A05308, doi:10.1029/2009JA014060. 1. Introduction Heelis, 2007; Kil et al., 2007; Lu¨hr et al., 2007, 2008; Li et al., 2008; Liu et al., 2008; Oberheide and Forbes, 2008b; [2] The low-latitude ionosphere is a highly dynamical Ren et al., 2008]. environment that exhibits significant variations with alti- [3] The wave number 4 patterns in the low-latitude tude, latitude, longitude, local time, season, solar cycle, and ionosphere could not be explained by longitudinal varia- geomagnetic activity. The coupling processes between the tions of the geomagnetic fields, which were previously ionosphere and the atmosphere below play important roles considered as the main reasons responsible to the longitu- in these variations. Recently, using the nightglow images of dinal ionospheric structure. Instead, it was explained in OI 135.6 nm, Sagawa et al. [2005] and Henderson et al. terms of the coupling process between the ionosphere and [2005] individually observed the wave number 4 longitudinal the atmosphere below. Sagawa et al. [2005] postulated that structure in the equatorial ionization anomaly (EIA). These the E region nonmigrating semidiurnal tide [Hagan and wave number 4 patterns were also been noticed in total Forbes, 2003] should be the origin of the wave number 4 electron content (TEC) and electron density in the low- structure in F region. England et al. [2006a] further dem- latitude ionosphere [e.g., Lin et al., 2007a, 2007b; Liu and onstrated a clear correspondence between the nonmigrating Watanabe, 2008; Kil et al., 2008; Pedatella et al., 2008; Ren diurnal tides [Hagan and Forbes, 2002] and the morphol- et al., 2008; Scherliess et al., 2008; Wan et al., 2008; Liu et ogy of the post sunset EIA. Immel et al. [2006] pointed out al., 2009]. A number of researchers had studied the intra- that the peak locations of this longitudinal structure are annual variability of these wave number 4 patterns [e.g., Kil coincident with those of the diurnal tidal amplitude pre- et al., 2008; Pedatella et al.,2008;Ren et al., 2008; dicted by the Global Scale Wave Model (GSWM) [Hagan Scherliess et al., 2008; Wan et al., 2008; Liu et al., 2009]. and Forbes, 2002]. They further proposed this structure as Similar structure even occurred in other ionospheric and the effects of the eastward propagating zonal wave number thermospheric parameters, such as the equatorial electrojet 3 diurnal tide (DE3), which can achieve large amplitude in (EEJ), the vertical E Â B plasma drifts (V?), the plasma the ionospheric E region [Forbes et al., 2008; Oberheide bubble (PB) occurrence rates, the electron temperature, the and Forbes, 2008a]. vertical scale height (VSH), the nitric oxide density in the [4] DE3 is excited by latent heat release associated with low thermosphere and zonal neutral wind in the F region raindrop formation in the tropical troposphere [Hagan and [e.g., England et al., 2006b; Fejer et al., 2008; Hartman and Forbes, 2002; Oberheide and Forbes, 2008a]. It propagates vertically and is sometimes the single largest tidal compo- 1Beijing National Observatory of Space Environment, Institute of nent above the mesopause [Forbes et al., 2008; Oberheide Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. and Forbes, 2008a]. Because there are growing observa- 2Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, China. tional evidences that DE3 is the main source of the wave 3Graduate School of Chinese Academy of Sciences, Beijing, China. number 4 structure in the ionosphere, DE3 has attracted a considerable attention. Recently, the satellite observations Copyright 2009 by the American Geophysical Union. 0148-0227/09/2009JA014060$09.00 A05308 1of7 A05308 REN ET AL.: BRIEF REPORT A05308 reveal the climatology of nonmigrating tides including DE3. number 4 structure in V? using the vertical plasma drifts Forbes et al. [2003a, 2003b] and Oberheide et al. [2006] velocity observed by ROCSAT-1. found that the zonal wind of DE3 is stronger and occurs primarily during northern summer and autumn, while the 2. Measurements and Data Processing meridional component is relatively weaker and occurs primarily during the northern winter. Oberheide and Forbes [7] ROCSAT-1 has a circular orbit at an altitude of about [2008a, 2008b] found that the seasonal change of the 600 km with an inclination of 35°. The period of an orbit is temperature fluctuation of DE3 is very similar to that of about 97 min, and consecutive orbits are separated in the zonal wind component, and pointed out that the intra- longitude by about 24.5°. The 24-h local times are covered annual DE3 variation should be interpreted in terms of in the latitude À35°35° every 25 days. This satellite latitudinally symmetric versus antisymmetric modes carries Ionospheric Plasma and Electrodynamics Instrument (Hough modes). Recent studies have shown that similar (IPEI), which is designed to take in situ measurements of intra-annual variations are observed in the wave number 4 ion density, temperature, composition, and drift velocity, structure in the ionization parameters and the DE3 tide. For over a large dynamic range with high accuracy, to monitor example, Wan et al. [2008] found that the seasonal variation the behavior of plasma. The cross-track plasma drift veloc- of the wave number 4 patterns of TEC in the low latitude is ity is measured with the onboard ion drift meter (IDM). consistent with that of the zonal wind of the nonmigrating ROCSAT-1 was launched on 27 January 1999, and the tide mode DE3. Kil et al. [2008] compared the amplitude of equatorial F region plasma drifts were continuously mea- the DE3 tide at 110 km altitude with the power of the wave sured by IPEI from mid-March 1999 to early June 2004. number 4 component in the density structure and found that The accuracy of the cross-track plasma drift velocity meas- they show similar intra-annual variations. urements from this IDM depends on the total ion density [5] Immel et al. [2006] first proposed a production and the proportion of oxygen ions. The cross-track plasma mechanism of the ionospheric wave number 4. First, the drift velocity can be determined accurately (error <10%) 3 À3 DE3 tide, when propagates into the ionospheric E region, when the total ion density is greater than 10 cm and the can affect the dynamos and lead to modulations on the percentage of the oxygen ion exceeded 85%. Because of ionospheric zonal electric fields and V?. Then the modulat- the near horizontal magnetic field near the dip equator, the ed plasma drifts further induce the motion and redistribution equatorial F region vertical plasma drifts correspond basi- of the ionospheric plasma in F region and finally produce cally to the electrodynamic drift driven by the zonal iono- the observed longitudinal structure [see also Kil et al., 2007; spheric electric fields. Here, we will use the observations of England et al., 2008; Oh et al., 2008]. This mechanism has vertical E Â B plasma drifts (V?) near the dip equator from been demonstrated by simulations with the TIME-GCM 1999 to 2004. model [Hagan et al., 2007]. The simulation by Jin et al. [8] Only measurements of V? within 5° of the dip [2008] further indicates that the wave number 4 structure in equator have been used, since they have higher statistical zonal electric field is directly from charge separation significance. Observations taken during periods of high induced by the Hall current, which is driven by the zonal magnetic activity marked by the Kp > 3 were additionally wind of DE3. At the same time, observations also show that removed from the data set in order to confine our attention the wave number 4 structures exist indeed in parameters of to quiet time variations. As Fejer et al. [2008] had done, we the ionospheric electric fields (EEJ and V ) [e.g., England only using V? corresponding to total ion density larger than ? 5 À3 et al., 2006b, Hartman and Heelis, 2007; Kil et al., 2007; 10 cm between 0800 LT and 0200 LT, and larger than 4 À3 Fejer et al., 2008].