The occurrence climatology equatorial F-region irregularities in the COSMIC RO data

Transcription

1 The occurrence climatology equatorial F-region irregularities in the COSMIC RO data B. A. Carter 1, K. Zhang 1, R. Norman 1, V. V. Kumar 2, S. Kumar 3 and N. L. Yen 4 1 RMIT University, Australia, 2 School of Mathematical Sciences Monash University, Australia, 3 School of Engineering and Physics, The University of the South Pacific, Fiji, 4 National Space Organization (NSPO), Hsichu, Taiwan,

2 Outline Introduction: Equatorial F-region irregularities (EFIs), equatorial spread F (ESF) and equatorial plasma bubbles (EPBs) EFI characteristics within COSMIC RO scintillation data Ionospheric scintillation observations: Climatological distributions (i.e. seasonal/longitudinal variability) of EFI occurrence Role of solar and geomagnetic activity on COSMIC RO EFI detections Discussion and interpretation Summary and conclusions 2

3 Equatorial Plasma Bubbles Equatorial Spread F (ESF)/Equatorial F- region irregularities (EFI) Equatorial Plasma bubbles - Observations Huang et al., 2012 Generalised Rayleigh-Taylor instability: Courtesy of M. Francis, IPS (Gentile et al., 2006) Upward plasma drift - prereversal enhancement after sunset 3

4 Introduction to Radio Occultation Profile The Low Earth Orbit (LEO) satellites measure the GPS signals that are occulted by the Earth s atmosphere. These occulted signals are used to infer atmospheric properties such as wet temperature (troposphere) and electron density (ionosphere). Dataset: ~ 5 years ( ) of RO data collected by the COSMIC satellites. The s4max9s is used for each event instead of the s4max parameter to avoid the use of spurious S4 measurements. E region/ troposphere F region S4max9s 0.3 between km was classified as an F-region irregularity detection 4

5 Mag dip (deg) Mag dip (deg) Mag dip (deg) Magnetic dip-local time S4 distributions F-region irregularities Sporadic E The occurrences were calculated by counting the number of events were s4max9s 0.3 over all of the RO events between that took place within each 5 1 hr bin, for all intervals where 0 Kp < 3. Previous works have shown seasonal variations in the EPB/RSF characteristics for different longitude sectors 5

6 Additional data sorting Globe was separated into 4 longitude sectors 90 wide; African, Asian, Pacific and American sectors. Seasons were specified as the 3-month period surrounding the solstices and equinoxes for all 5 years considered; Mar and Sep equinoxes and Jun and Dec solstices

7 Seasonal and longitudinal dependence B Day Night Sorting the data according to season and longitude sector show strong differences in EFI occurrence. These trends match those found in previous studies of EPB occurrence; e.g. Burke et al. (2004) Occurrence of F-region irregularities is strongly controlled by the magnitude of the angle between the magnetic field and the direction of the day-night terminator (Tsunoda, 1985) 7

8 Seasonal and longitudinal dependence By decreasing the longitude bin sizes to 15, direct comparisons with previous works are possible COSMIC EFI trends are similar to those in previous works, with the exception of the equinox peaks at ~ 55 W The spatial occurrence trends show that EFI occurrence peaks are located within the South Atlantic Anomaly (SAA) region Previous studies have shown that the R-T growth rates are decreased near the SAA due to particle precipitation These results indicate that the SAA particle precipitation did not suppress EFI generation, in contrast to previous works 8

9 Dependence of EPB and ESF occurrence on solar and geomagnetic activities Huang et al. [2002] - DMSP 20% of EPBs observed when Kp 5 40% observed when ΔDst -5 nt Nishioka et al. [2008] Ground-based GPS Dependence of EFI (and EPB) occurrence on solar activity is clear. Geomagnetic activity is expected to increase DMSP EPB detection rates. 9

10 Solar activity effects on EFI generation Local time (LT) distribution: Increases in both LT range and EFI occurrence values with solar activity evident in most longitude/season segments, except for American sector. Magnetic dip angle (~ latitude) distribution: Increases in both dip angle range and EFI occurrence values with solar activity evident in most longitude/season segments, except for American sector. Effects of particle precipitation in the South Atlantic Anomaly region? 10

11 Geomagnetic activity effects on EFI generation Local time (LT) distribution: Effects of prompt penetration electric fields are evident in some plots, particularly in Asian and Pacific sectors during the Mar equinox months; EFI detection earlier during main phase than quiet and recovery phase periods. Gonzales and Echer [2005] Main Magnetic dip angle (~ latitude) distribution: ΔDst -5 nt/hr Dip angle distributions do not show any clear dependence on storm phase or activity level. Recovery ΔDst 5 nt/hr 11

12 Summary and conclusions 5 years of COSMIC RO ionospheric scintillation data were used to characterise the global EFI occurrence trends: The width of the scintillation band was found to be spread across ±40 in magnetic dip centered at the equator and extended from 19 LT until LT Seasonal and longitudinal climatology in the EFI occurrence largely agrees with that of EPBs as reported in previous works; i.e. high EFI occurrence at times and locations where magnetic field lines are close-to parallel to day-night terminator The EFI occurrence was higher than expected in the proximity of the SAA during the equinox months. It is suggested that particle precipitation has a significant effect on the generation of EFIs in the SAA region The dependence of EFI occurrence on the solar and geomagnetic activity was also investigated: Generally, the range of both the local time range and the magnetic latitude of the EFI band were found to increase with solar activity in line with expectations The only significant exception to this was the American longitude sector, that appeared to further show the effects of particle precipitation close to the SAA. Evidence was also uncovered of the effects of electric fields associated with geomagnetic storms on the EFI occurrence. Carter et al., (2013), On the occurrence of equatorial F-region irregularties during solar minimum using radio occultation measurements, J. Geophys. Res., 118, , doi: /jgra