Intensity of low-latitude nighttime F-region ionospheric density irre- gularities observed by ROCSAT and ground-based GPS receivers in solar maximum Yang-Yi Sun a , Jann-Yenq Liu a,b,c,n , Chi-Kuang Chao a , Chia-Hung Chen d a Institute of Space Science, National Central University, Chung-Li, Taiwan b Center for Space and Remote Sensing Research, National Central University, Chung-Li, Taiwan c National Space Program Origination, Hsin-Chu, Taiwan d Department of Earth Science, National Cheng Kung University, Tainan, Taiwan article info Article history: Received 13 August 2014 Received in revised form 30 December 2014 Accepted 31 December 2014 Available online 1 January 2015 Keywords: Irregularity intensity ROCSAT ion density GPS phase fluctuation Hilbert–Huang transform abstract This study examines the global correlation between the instantaneous total amplitude of ion density fluctuations observed by ROCSAT and the phase fluctuation of the total electron content (TEC) recorded by worldwide ground-based Global Positioning System (GPS) receivers during the high solar activity period of March 1999–December 2002 for Kp o3. The Hilbert–Huang transform (HHT) is applied to compute the instantaneous total amplitude of ROCSAT ion densities. The event-based and climatological comparisons of the total amplitude and occurrence probability of irregularities observed by ROCSAT show that the total amplitude can reveal both the occurrence probability of irregularities and the as- sociated intensity. The noise level of the total amplitude is about 10 3.5 (near 3000) ions/cm 3 . The high correlation (correlation coefficient ¼0.81) between the GPS TEC phase fluctuation index F P and in- stantaneous total amplitude of ROCSAT electron densities suggests that the total amplitude can be used to globally monitor the intensity of irregularities at equatorial and within the latitude belt of 715°. The relationship between the ionospheric background ionization and the irregularity intensity is further investigated. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction One of the advantages of using satellite probing is to yield a global coverage. The morphology of the seasonal and geographical distributions of the occurrence probability of low-latitude F-region irregularities has been comprehensively observed by AE-E (At- mosphere Explorer-E) (Kil and Heelis, 1998; McClure et al., 1998; Hei et al., 2005), DMSP (Defense Meteorological Satellite Program) (Burke et al., 2004; Gentile et al., 2006), ROCSAT (Republic of China Satellite 1) (Su et al., 2006, 2008, 2010; Kil et al., 2009), CHAMP (Challenging Minisatellite Payload) (Stolle et al., 2006), and FOR- MOSAT-3 (Formosa Satellite Mission 3)/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) (Uma et al., 2012). The magnitude of the vertical ion drift velocity of the evening prereversal enhancement (PRE) plays an important role in the development of equatorial irregularity in postsunset hours (e.g., Farley et al., 1970; Ossakow et al., 1979; Rastogi, 1980; Abdu et al., 1983; Sultan, 1996; Fejer et al., 1999; Anderson et al., 2004; Li et al., 2008; Su et al., 2008; Kil et al., 2009). The good agreement between the seasonal/longitudinal distributions of the vertical drift and the occurrence probability of irregularities in the equa- torial ionosphere has been reported by means of ROCSAT (e.g., Su et al., 2008; Kil et al., 2009). Su et al. (2008) indicated that the global distribution of the vertical drift velocity and the occurrence probability of irregularity are controlled by the global variation of the postsunset ionospheric condition, which is related to the magnetic declination effect and the seasonal variation of the io- nospheric density level at the dip equator located with respect to the geographic equator. Kil et al. (2009) suggested that the gen- eration of equatorial plasma bubbles (EPBs) is not guaranteed by the occurrence of an intense PRE. Other mechanisms, in addition to the PRE, should be considered as an explanation for the oc- currence of EPBs in the topside ionosphere. On the other hand, due to the increase of ground-based GPS (Global Positioning System) receivers in the recent decade, the derived GPS total electron content (TEC, 1 TEC unit (TECU) ¼ 1  10 16 el/m 2 ) has been widely applied to study iono- spheric density irregularities globally (e.g., Pi et al., 1997; Nishioka et al., 2008; Li et al., 2010). The changes in the differential carrier Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics http://dx.doi.org/10.1016/j.jastp.2014.12.013 1364-6826/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author at: Institute of Space Science, National Central University, Chung-Li, Taiwan. E-mail address: jyliu@jupiter.ss.ncu.edu.tw (J.-Y. Liu). Journal of Atmospheric and Solar-Terrestrial Physics 123 (2015) 92–101