On the occurrence of the equatorial F-region irregularities during solar minimum using radio occultation measurements

Transcription

1 On the occurrence of the equatorial F-region irregularities during solar minimum using radio occultation measurements B. A. Carter1, K. Zhang1, R. Norman1, V. V. Kumar2 and S. Kumar3 RMIT University, Australia School of Mathematical Sciences Monash University, Australia School of Engineering and Physics The University of the South Pacific, Fiji 3

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 An investigation of the role of atmospheric gravity wave seeding using lightning location data Discussion and interpretation Summary and conclusions 2

3 Equatorial Plasma Bubbles Equatorial Spread F (ESF)/Equatorial F- Equatorial Plasma bubbles - Observations region irregularities (EFI) Generalised Rayleigh-Taylor instability: (Gentile et al., 2006) Upward plasma drift - prereversal enhancement after sunset Simulation (Huang et al., 2012) 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. F region E region/ troposphere S4max9s 0.3 between km was classified as an F-region irregularity detection 4

5 Seasonal and longitudinal dependence low Kp B Day Night Sorting the data according to season and longitude sector show strong differences in F-region irregularity 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 5

6 Inter-tropical convergence zone (ITCZ) The ITCZ is the location where the trade winds from the northern and southern hemispheres. It is characterised by clouds, rainfall and occasional thunderstorms. 6

7 Longitudinal and seasonal dependence of EPB and ESF occurrence α Tsunoda [2010] B Day Night Burke et al. [2004] The longitudinal and seasonal variation of ESF and EPB occurrence is mostly related to the longitudinal conductivity gradient magnitude, which is maximised when both E-region footprints enter darkness simultaneously. PESF=PS PI, where PS is the seeding probability and Pi is the instability probability. Difference between June and December was attributed to difference in PS 7

8 Tropospheric seeding WWLLN data The World Wide Lightning Location Network (WWLLN) is made up of more than 50 instruments across the planet (red stars) that detect VLF waves from lightning strikes. The location of each stroke is determined using at least 5 sensors, limiting the dataset to strong strikes only; 15-30% of detections at any one location are detected by 5 or more stations in total. Recent research has determined the detection efficiency to be approx. 30% globally. All 2007 data were available for this study 8

9 Seasonal and longitudinal dependence Tropospheric seeding Is the WWLLN dataset a good proxy for determining the location of the ITCZ? A brief comparison shows that it is! 9

10 Seasonal and longitudinal dependence Tropospheric seeding If tropospheric seeding is important for EFI generation, then one would expect differences between the strike rates for EFI versus no EFI observations. -85 Ngeo Nmag TRO-TSTRIKE < 5 hrs km RO plane Strikes Coincident strikes 95 (Criteria from modelling by Vadas [2007]) The coincident strike characteristics are generally very similar for both EFI and no EFI observations, which suggests that tropospheric seeding is not a dominant factor in EFI generation. 10

11 Tropospheric seeding ECMWF Preliminary results ω500 at 12 UT The difference in the coincident ω500 from the background values is not dependent on whether EFIs are observed, on average. This result further supports the conclusions drawn from the WWLLN analysis. (a) FMA (b) ASO (c) MJJ (d) NDJ ω500 represents the vertical velocity of air packets at 500 hpa. Positive (negative) values represent downward (upward) velocities. 11

12 Summary and conclusions The Radio Occultation technique is a very useful new technique for studying and investigating EFIs associated with EPBs: The longitudinal/seasonal (s/l) EFI occurrence variability largely matches that found in previous works that employed different measurement techniques. The COSMIC RO dataset was employed to test the hypothesis that tropospheric atmospheric gravity waves strongly control the s/l EFI occurrence trends: It was found that the number of coincident lightning strikes did not change significantly between RO events where EFIs were detected versus RO events for which they were not. This result was further supported by a similar test using the Ω500 data from the ECMWF. It is suggested that the longitudinal variability in another important parameter in the generation of EPBs, such as the upward plasma drift, is the cause of the unexplained EFI occurrence characteristics. 12