A comparative investigation is made, for the first time, of direct (ion-composition measurements) and indirect (from topside ion density profiles) methods to determine the O+/light-ion transition height over equatorial latitudes. The transition height determined from the ion-composition follows similar latitudinal and local time variations as the underlying Equatorial Ionization Anomaly (EIA) in the F-region ionosphere. The north-south hemispheric asymmetries in the transition height from ion composition are consistent with the summer-to-winter interhemispheric neutral wind patterns. On the other hand, the transition height derived from ion density profiles is chiefly influenced by changes in the scale height (shape) due to vertical E × B drift at the equator and steep vertical ion density gradients in topside ionosphere. As a result, the transition height derived from topside profiles are systematically lower and exhibit inconsistent latitudinal and local time variations when compared to direct ion-composition measurements at equatorial latitudes.
 Topside ionosphere is mainly composed of electrons and O+, H+ and He+ ions. The dominant ions around the F2-layer peak are O+. As the altitude increases, the dominant ion gradually changes from O+ to light-ions such as H+ and He+. The height where the number density of O+ and light-ions (H+ + He+) are become equal is defined as the ionospheric upper transition height which is the boundary dividing the O+ dominated topside ionosphere and the light ions dominated plasmasphere. The transition height is very important parameter in many empirical models to describe the vertical distribution of ion density and composition [Bilitza et al., 2006, and references therein]. Since the transition height is the base of the plasmasphere, the accuracy of transition height plays a crucial role in empirical models to reconstruct the topside profiles that smoothly connects with plasmaspheric profiles measured from topside [Reinisch et al., 2007].
 The transition height can be accurately determined from direct observations of the in-situ ion composition made from satellites [González et al., 1992]. However, the large diurnal variation of the transition height makes it inaccessible, at some local times, to satellites that have limited orbital altitude range. On the other hand, the transition height can also be derived indirectly from the topside ion concentration profiles and several studies have reported the variability of transition height from topside sounder data. In general, there are two approaches to determine the transition height from topside profiles. First, theoretical ion density profiles described by analytical functions are fit to measured topside profiles by iteratively changing the topside parameters such as temperature, temperature gradient and transition height until a best fit is obtained [Titheridge, 1976; Webb et al., 2006]. Secondly, the transition height is derived from shape of the topside profile. Assuming a constant plasma scale height and temperature, the O+ density profile is determined from the scale height just above the F2-layer peak. The transition height is then obtained at an altitude where the O+ density is equal to half of the ion density [Kutiev et al., 1994; Marinov et al., 2004; Kutiev and Marinov, 2007]. Rigorous validations of this second method with direct ion composition measurements, particularly in the equatorial latitudes, have never been reported before, perhaps, primarily due to a lack of simultaneous observations of ion composition and topside ion density profiles.
 Recently, Heelis et al.  investigated the variability of O+/H+ transition height over equatorial latitudes using ion composition measurements from the Coupled Ion Neutral Dynamics Investigation (CINDI)-Retarding Potential Analyzer (RPA) on board the Communication/Navigation Outage Forecast System (C/NOFS). They have reported that the O+/H+ transition height during the extreme solar minimum of 2008 is within the C/NOFS altitudinal range at all local times. The low inclination (13°) and elliptic orbit (apogee = 850 km and perigee = 400 km) of C/NOFS provides an unprecedented coverage of ion composition measurements in the equatorial latitudes. Therefore, the combination of ion composition measurements from C/NOFS and topside ion density profiles from Formosat-3/COSMIC (F3/C) Radio Occultation (RO) measurements provide us a unique opportunity to compare the transition heights derived from topside profiles with those derived from direct ion-composition measurements. The F3/C RO profiles in F-region and above are found to be in good agreement with the ground based observations [Lei et al., 2007].
 The O+, H+ and He+ ion concentration data from CINDI-RPA and the topside profiles from F3/C during the extreme solar minimum period from Sep 2008 to Aug 2009 are considered. Figure 1a shows the O+ and light-ion (H+ + He+) concentrations measured by CINDI-RPA. The transition heights are determined as the altitude at which the O+ and light-ion concentrations becomes equal as indicated by red solid circles. Figure 1b shows a typical example of an ion density profile from F3/C. The scale height of the O+ density profile is determined by least squares fit using α-Chapman function (equation (1)) to the portion of the profile where the vertical ion density gradient does not exceed its lowest by 30% (just above the F2-layer peak as represented by red solid circles) [Marinov et al., 2004; Kutiev et al., 2006]
where z = and HT is the scale height.
 Considering this scale height is independent of height, the O+ density profile (green dotted line) is reconstructed using equation (1) to higher altitudes and the transition height is marked at an altitude where the O+ density becomes half of the total ion density [n(O+) = Ni/2] as marked by pink horizontal line. For convenience, the transition heights derived from composition and profile data are further denoted as THC and THP, respectively.
Figure 2 shows the comparison of transition heights derived from C/NOFS ion composition measurements (THC) with those from F3/C ion density profiles (THP) during the noon (11 to 14 LT) hours. For better coverage, the data of Sep–Oct 2008 and Mar–Apr 2009 are together shown as equinox (Figures 2a and 2d), Nov 2008–Feb 2009 as December (Figures 2b and 2e) and May–Aug 2009 as June (Figures 2c and 2f) solstices. At the outset, it can be observed that the values of THP differ significantly from THC with THP being lower than THC by about 100 km during all the seasons. Most importantly, the latitudinal variation of THC (Figures 2a–2c) exhibits a similar variation to that of the well known EIA, with THC being lower along the geomagnetic equator (trough) and increases with increasing latitudes (crests). Further, the THC is higher and the maximum height is closer to the equator in the summer hemisphere than the winter hemisphere (Figures 2b and 2c). On the other hand, the latitudinal variation of THP is quite different, with THP being a maximum over equator and decreases with increasing latitude (Figures 2d–2f). However, the values of THP appear to be slightly higher in the northern summer hemisphere (Figure 2f).
Figure 3 shows the longitudinally averaged THC and THP as a function of local time and geomagnetic latitude. Again it is clearly seen that the THP values during the day are lower than THC by about 100 km and slightly higher than THC during the night. Also clear are the similarities between the latitudinal variation of THC and the EIA with lower THC values over the equator and increases with increasing latitudes. THC values are higher and maxima are closer to the geomagnetic equator in the summer hemisphere than the winter hemisphere (Figures 3b and 3c), consistent with Figures 2b and 2c.
 The zonal mean values of THP exhibit much less seasonal variation than do THC within ±13° geomagnetic latitudes (Figure 3). Further, the most significant difference between the local time variation of THC and THP is that, the values of THC exhibit their lowest values during the post-midnight to pre-dawn hours (02 to 04 LT). They increase immediately after sunrise and decrease after sunset. As expected, the increase in THC at sunrise occurs earlier and the decrease after sunset occurs later in the summer hemisphere than the winter hemisphere (Figures 3b and 3c) due to seasonal difference in the sunrise/sunset times. On the other hand, the values of THP are found to decrease after the sunrise and reach a minimum around 08 LT. The local time variation of THP exhibits a secondary maximum around the post-sunset hours of equinoxes and December solstices.
 Since the O+ ions are dominant in F-region ionosphere, the transition height where the O+ density becomes 50% of the total ion concentration depends primarily on the O+ densities lower in the F-region ionosphere where the production and loss of ionization take place. During day time, the E × B drift at the equator vertically transports the F-region plasma (primarily electrons and O+ ions) to higher altitudes, which then diffuse along the magnetic field lines under gravitational and pressure gradient forces forming the two crests of ionization on either side of the equator, and a trough over the equator. These transport processes produce similar latitudinal variation in THC with lower values over the equator and higher values at latitudes away from the equator during the equinoxes (Figures 2a and 3a). However during the solstices, the equatorward wind in the summer hemisphere moves the F-region plasma upward along the field lines closer to the equator and to higher altitudes where the recombination rate is small. On the other hand, the poleward wind in the winter hemisphere pushes the plasma away from the equator and to lower altitudes. Hence, stronger/weaker EIA crests are formed at higher/lower altitudes and closer/further from the equator in the summer/winter hemispheres [Lin et al., 2007]. The north–south asymmetry of THC during solstices (Figures 2b–2c and 3b–3c) is also a manifestation of the underlying hemispheric asymmetry of the EIA in O+ densities due to inter-hemispheric neutral winds.
Figure 4 shows the longitudinally averaged O+ concentrations (percentage) measured from CINDI-RPA as function of geomagnetic latitude and altitude during the noon (11 to 14 LT) and night (00 to 03 LT) hours. The thin black lines in Figure 4 represent the O+ concentration level corresponding to 50% of total ion density, indicating the location of the transition height. During the equinoxes when the inter-hemispheric wind is insignificant, the O+ concentrations and the transition heights are lower at the equator and increase around ±10–13° geomagnetic latitudes during the noon time (Figure 4a). Whereas during the solstices it can be clearly observed that the O+ concentration, and the transition height, is higher in the respective summer hemispheres than in the winter hemispheres (Figures 4b and 4c). During the night time, in the absence of ion production and with the subsidence of EIA, both the O+ concentration as well as the transition height is much lower and decreases with latitudes away from the equator. However, the inter-hemispheric wind still operates producing a noticeable asymmetry in the transition height (Figures 4e and 4f).
 The transition height (THP) derived from the ion density profiles primarily depends on the scale height (shape) of the topside profile [Marinov et al., 2004; Kutiev and Marinov, 2007]. The constant scale height assumed in this method implies a constant plasma temperature in the topside ionosphere. However, during the daytime and in a region where the thermal conductivity between electrons and H+ is higher, the plasma temperature increases with height and the assumption of constant plasma temperature is violated. Therefore, the O+ density profile reconstructed with the assumption of constant scale height decreases more rapidly with height resulting in THP values that are too low as illustrated in Figures 2 and 3. Further, in the equatorial latitudes, the scale height of the topside profile is greatly controlled by the vertical E × B drift at the equator [Liu et al., 2008; Tulasi Ram et al., 2009]. During noon time when the E × B drift is at its maximum, the vertically transported plasma causes a decrease in the ion density at F-region peak altitudes and an enhancement in the topside ionosphere. This is manifest as a less steepen topside density profiles with a large effective scale height (thickness) over the equator than the off-equatorial latitudes. Since, the transition height derived from the topside profiles are proportional to the vertical scale height [Kutiev and Marinov, 2007], the large effective scale height results in higher values of THP over the equator. While the values are larger than those derived at latitudes away from the equator, as may be seen from Figures 2d–2f and 3d–3f, they remain significantly lower than the real transition height from ion composition measurements.
 Soon after sunrise, the ion density in the bottom side ionosphere increase rapidly because of photoionization of large quantities of ionizable gases. However, the topside ionospheric density is mainly determined by diffusion and charge exchange chemistry, hence, the ion density increase after the sunrise is relatively slower. Thus, the difference between the plasma densities at F-region peak altitudes and the topside ionosphere increases after sunrise which results in vertically steep negative ion density gradients and smaller scale heights around 07–08 LT [Liu et al., 2008; Tulasi Ram et al., 2009]. Therefore, the low THP values after sunrise and a minimum observed at 08 LT (Figures 3d–3f) is also due to the changes in the scale height owing to vertically steep negative ion density gradients. The secondary maximum in the THP values observed around 18–19 hrs LT (Figures 3d and 3e) can be attributed as due to the post-sunset enhancement in the vertical E × B drift and resultant increase in the scale height.
 The assumption of spherical symmetry involved in the Abel inversion of F3/C RO can induce significant error in the bottom side profiles at EIA latitudes. However, the error analyses of Abel retrieval illustrate that the error in the topside profile data (above F2-peak) is only ∼10% [Liu et al., 2010; Yue et al., 2010]. Since only the topside profile data from F3/C RO is considered, this small error from Abel inversion will not greatly affect the derivation of THP. A simple test analysis (not presented here) indicates that the deviation of 10–50% of electron density at F2-peak will only result in an error of ∼6–16 km for THP. On the other hand, the values of THP are lower than THC by ∼100 km during noon time. Thus, the large difference in the magnitude can be attributed as primarily due to the assumption of constant scale height (temperature) involved in the derivation of THP and contributions due to small errors in Abel inversion of RO profiles is meager. Further, the latitudinal and local time variations of THP deviate quite significantly from those of THC which can be explained well by the changes in the scale height (shape) brought about by the vertical E × B drift and steep vertical gradients after sun rise as discussed previously.
 The transition height derived from the topside ion concentration profiles (THP) are systematically compared, for the first time, with those determined from direct in-situ ion composition measurements (THC) at equatorial latitudes. The values of THP are lower than THC by about 100 km during the day time and slightly higher during the night. The local time and latitudinal variations of THC are similar to that of EIA and consistent with the underlying O+ concentrations in F-region ionosphere. During solstices, the THC values are higher and peaks in the THC are closer to the equator in the respective summer hemispheres than in the winter hemispheres. This behavior is consistent with the prevailing inter-hemispheric wind patterns during the solstices.
 On the other hand, THP is derived by characterizing the vertical ion density profiles and assuming a constant scale height that is independent of altitude. In the equatorial region, THP significantly departs from the traditional description of ion transition heights (THC) that are produced in the presence of neutral winds and E × B drifts. Furthermore, in the presence of significant H+ concentrations during the daytime, the plasma scale height will be a strong function of altitude. These conditions produce transition heights derived from the topside profiles (THP) that are generally lower and show inconsistent variations in latitude and local time compared with THC at equatorial latitudes. The present comparison is made under very low solar activity conditions. However, the strength of vertical E × B drift increases during high solar activity that could increase the discrepancy between the latitudinal and local time variations of THC and THP. Also, the increase of plasma temperature and scale height with solar activity could increase both THC and THP, therefore, needs a further investigation for a quantitative assessment of difference between the two methods.
 S. Tulasi Ram is supported by post-doctoral fellowship from ASIAA, Academia Sinica. The authors acknowledge NSPO, UCAR-CDAAC for providing the F3/C ion profile data, and CINDI-RPA data are supported by NASA grant NAS5-01068.