Artificial and Natural Disturbances in the Equatorial Ionosphere: Results from the MOSC Experiment

Transcription

1 Artificial and Natural Disturbances in the Equatorial Ionosphere: Results from the MOSC Experiment and the C/NOFS satellite mission Dev Joshi (1) 1 Institute for Scientific Research, Boston College, MA USA 1

2 Presentation Overview 1. Metal Oxide Samarium Cloud (MOSC) NASA-launched sounding rocket released samarium vapor to create artificial plasma cloud in the valley region (low electron density) of the ionosphere, altering the HF propagation environment. 2. Electron Density Irregularity Observations from the Joint AF- NASA C/NOFS Satellite Characterize the spatial extent of lowlatitude irregularities as a function of solar flux. 2

3 Ionosphere 101 Formed by solar EUV/UV radiation Reflects, refracts, diffracts & scatters radio waves, depending on frequency, density, and gradients Refractive Index n 2 = 1 f p f 2 f p 9 N e Subject to Rayleigh- Taylor instability during day to night transition; other instabilities in regions of large gradients, strong flows TURBULENT PLASMA BUBBLES Leads to highly variable reflection/refraction = SCINTILLATION PRN 7 Scintillated GPS Signal 3

4 Equatorial Dynamics Presence of anomaly crests strengthens off-equator scintillations State of anomaly formation is indicative of equatorial dynamics Anomaly crests are areas of maximum F-region ionization density off equator Daytime eastward electric field (E) drives plasma up (E B) Plasma moves toward crests (g, P ) (View looking east)

5 MOSC Overview First experiment to comprehensively diagnose artificial plasma cloud generated by release of samarium metal vapor carried out by the AFRL with support from the NASA sounding rocket program in May 2013 Understand the dynamics, evolution and chemistry of Sm atoms in the upper atmosphere of the earth Investigate the interaction of artifical ionization and the background plasma Measure the effects on high-frequency (HF) radio wave propagation Payload for each rocket included - Two canisters of samarium ( 5 kg yield) - Dual Frequency RF Beacon (NRL CERTO) Ground Diagnostics from 5 sites included: Incoherent Scatter Radar, GPS/VHF Scintillation Rxs, All-Sky Cameras, Optical Spectrograph, Ionosondes, Beacon Rx, HF Rx/Tx 5

6 Motivation The use of high frequency radio wave systems for communications and remote sensing has grown rapidly over the past decade. The performance of these systems relies on understanding the propagation paths of the HF energy. The ionosphere frequently represents the limiting factor in our knowledge of the propagation modes. The MOSC experiment injected an artificial plasma cloud into the upper atmosphere to test our ability to understand and model HF propagation effects. 6

7 THE RELEASE : MOVIE 7

8 Samarium Cloud 8

9 Two Releases : 01 May and 09 May 2013 Ionosphere during first release was disturbed, rising rapidly and Spread F formed within minutes after release Ionosphere during second release is quiescent 9

10 Transmitter/Receiver Geometry Tx Rongelap N E Wotho MOSC Release Location & Likiep-Wotho Mid-Point Rx Likiep Tx ALTAIR Rx Kwajalein Rongelap-Wotho link geometry is predominantly N-S and great-circle path is far from release region Likiep-Wotho path is nearly E-W and release point lies nearly on midpoint of the link should be ideal for observing SmO+ layer 10

11 09 May 2013 Pre-Release Sweep Wotho Receiver Rongelap TX Likiep TX F region F region second hop F region Ground Wave Ground Wave Sweeps from 2-30 MHz were completed every five (5) minutes Slightly higher peak frequencies on Wotho Likiep path relative to Rongelap- Wotho links 11

12 09 May st Post-Release Sweep Wotho Receiver Rongelap TX Likiep TX F-region secondary echo F-region secondary echo MOSC Layer MOSC Layer On the Likiep -Wotho geometry the layer extends up to 10 MHz peak frequency There is also a prominent secondary F region echo 12

13 MOSC : Modeling Steps I. Assimilate the model ionospheric profile with the observed ALTAIR radar profile II. Insert the model cloud in the assimilated model ionosphere III. Apply ray-tracing to understand the effects of the model cloud on the propagation modes of HF propagation 13

14 MOSC Launch 2: May 9, 2013 Modeling the Cloud BEFORE AFTER The ALTAIR cloud profile is used to model the cloud in MATLAB with latitude/longitude increment at degree and height increment at 1.55 km. The central pixel corresponds to 7.44 MHz 14

15 3D Ray Trace Analysis Rongelap To Wotho Rongelap Wotho MOSC release point HF signals received well off the great circle path to the receiver MOSC Wotho Rongelap The artificial cloud bends the HF energy through large angles Excellent agreement between model and observations 15

16 3D Ray Trace Analysis Likiep To Wotho Wotho MOSC release point Likiep Likiep MOSC Wotho Great circle path to the receiver passes through the MOSC volume Multiple paths between ionosphere, cloud and receiver expected 16

17 MOSC Summary Ray tracing confirms sounder observations to high degree of fidelity and identifies the modes of HF propagation The change in ambient natural propagation environment due to small artificial modification can be successfully modeled Samarium cloud behaved like a divergent lens resulting in HF voids or shadow zones Wotho Rongelap MOSC release point Publication : Joshi, D., K. M. Groves, W. McNeil, C. Carrano, R. G. Caton, R. T. Parris, T. R. Pederson, P. S. Cannon, M. Angling, and N. Jackson-Booth (2017), HF propagation results from the Metal Oxide Space Cloud (MOSC) experiment, Radio Sci., 52, doi: /2016rs

18 Motivation: Equatorial Irregularities Our goal is to understand the spatial distribution of equatorial irregularities as a function of solar flux. These irregularities are associated with intense radio wave scintillations that impact space-based navigation, communications and remote sensing systems. While models of occurrence frequency exist (e.g., WBMOD), the extent of the regions affected is poorly understood and the validity of physics-based models of the underlying instability is unknown. 18

19 GPS Position Error Scintillation can cause rapid fluctuations in GPS position fix; Typical night from recent field experiments 19

20 Why do disturbances form? Unique Equatorial Magnetic Field Geometry Equatorial scintillation occurs because plasma disturbances readily form with horizontal magnetic field Plasma moves easily along field lines, which act as conductors Horizontal field lines support plasma against gravity unstable configuration E-region shorts out electrodynamic instability during the day F Region Magnetic (Dip) Equator Magnetic Field Lines E Region Earth Unstable Plasma Daytime Shorting 20

21 Spread F Echoes from the radio sounders are spread due to the disturbances in the ionosphere hence the name spread F Ionosonde : Reflectometry diagnostic instrument 21

22 Methodology The assumed mapping of large-scale electric fields along magnetic field lines suggests that the meridional extent of spread F structures can be estimated from the altitude distribution of equatorial bubbles. To test the consistency of these findings we compare results from C/NOFS satellite in situ observations with ground-based scintillation measurements. Height determines horizontal extent Irregularities map poleward along B as instability expands vertically 22

23 DMSP Observations Peak occurrence rates 50 % observed during active periods ( ) in the American-African longitude sectors at the DMSP altitude of ~830 km* The solar minimum climatology, in contrast to that of the solar maxima, show very low EPB rates *See, e.g., Gentile et al., 2006;

24 C/NOFS Payload Description GPS Occultation Receiver C/NOFS Occultation Receiver for Ionospheric Sensing and Specification (CORISS) Developed by Aerospace (P. Straus PI) Measures: Remote sensing of LOS TEC & Scintillations Electric Field Instrument Vector Electric Field Instrument Developed by NASA/GSFC (R. Pfaff PI) Measures: Vector AC and DC electric fields RF Beacon Coherent EM Radio Tomography (CERTO) Developed by NRL (P. Bernhardt PI) Measures: Remote sensing of RF scintillations and LOS TEC RAM Plasma Sensors Planar Langmuir Probe (PLP) Developed by AFRL/RV (J. Ballenthin PI) Measures: Ion Density, Ion Density Variations, Electron Temperature Ion Velocity Meter (IVM) Developed by UT Dallas (R. Heelis PI) Measures: Vector Ion Velocity, Ion Density, Ion Temperature Neutral Wind Meter (NWM) Developed by UT Dallas with NASA support (R. Heelis PI) Measures: Vector Neutral Wind Velocity Orbit well-suited to assess altitude characteristics: 13 inclination; elliptical 400 x 850 km altitude

25 PLP Observations from C/NOFS Ion density C/NOFS sampled all altitudes between km every orbit Sat trajectory Irregularities Sat altitude FFT of 1s density B

26 Irregularity Detection We define an irregularity parameter, σ as follows: σ ( % ) = 100 x i= logn i log N 0i 2 log N 0i The above equation represents the standard deviation of ion density variations in logarithmic scale divided by the mean of ion density in logarithmic scale. 11 i=1 1 2

27 C/NOFS Observations : Consistent with other studies, the occurrence probability is generally high in equinoctial months and low around June solstice, maximizes in the longitude sector from 280 E to 30 E Peak occurrence rates above 700 km are 50%, similar to that observed with DMSP in the previous solar cycle

28 C/NOFS Observations : The plots show less activity at higher apex altitudes during low solar activity years

29 Solar Cycle DMSP C/NOFS The C/NOFS satellite flew in the weakest solar cycle. 29

30 Ground based Observations Mod SSN High SSN Ground-based VHF measurements show that scintillation occurrence at Ascension Island (-17 Mlat) reached % during the peak seasons between Assuming bubble height determines meridional extent, structures must rise to over 1000 km to reach Ascension, but only about 400 km to reach Cape Verde

31 Ground based Comparison Approach To compare the ground scintillation and space-based irregularity observations we map the C/NOFS in situ observations into the magnetic field geometry at Ascension Island and Cape Verde We expect to see in situ irregularities whenever scintillation is observed on the ground and the satellite is sampling below the apex altitude of the F- region field lines above the site Data are plotted as functions of physical and apex altitude and geographic and geomagnetic latitude 31

32 Flux Tube Mapping: Ascension Island Color Key S4 > 0.6 YES YES NO NO σ > 1% YES NO YES NO ASI CVD Example shows the distribution of points from 2011 days

33 Flux Tube Mapping: Cape Verde Island Color Key S4 > 0.6 YES YES NO NO σ > 1% YES NO YES NO ASI CVD Example shows the distribution of points from 2011 days

34 F10.7 The plot shows values of F10.7 flux over a variety of averaging periods Solar fluxes <~ 80 and >~ 110 are well represented statistically, but the transition between 80 to 110 was rapid and has few samples Sampling density is certainly not uniform Transition 34

35 Distribution of Apex Altitude Sampling Solar Minimum Solar Maximum Distribution of apex altitude bins sampled during the respective measurement windows; similar but not identical. Sampling window includes active months only (Jan - April and Sep Dec). Apex altitudes sampled are predominantly below 1000 km, but the full range of expected bubble altitudes are covered (~2000 km max) 35

36 Irregularity Occurrences for Solar Min/Max Solar Minimum Solar Maximum The altitude distributions of all sampled bins with σ > 0.5 Number of irregularities detected at solar maximum is about 2.3x greater than solar minimum The number detected above 700 km is more than 4x greater 36

37 Occurrence Percentage (Detections normalized by total samples) The height distribution of irregularities decreases ~twice as rapidly at solar minimum relative to maximum. Confirms that low occurrence rates reported from DMSP during solar minimum result from altitude bias; results not representative of actual occurrence. 37

38 Modeling: PBMOD F10.7 = 60 F10.7 = 120 F10.7 = 180 F10.7 = 240 PBMOD indicates that the bubble height increases with increasing solar flux

39 Future Work Extend analysis to all longitude sectors Apply physics based modeling (e.g., PBMOD installed at CCMC) to improve physical understanding and improve model Develop algorithm to predict irregularity altitude as a function of location and solar flux 39

40 Conclusions The C/NOFS bubble occurrence statistics are consistent with previous work related to this topic (Huang, et al. 2012, Gentile et al., 2006) During solar minimum C/NOFS shows that the occurrence of irregularities decreases by 30% - 40%, but the altitude distribution decreases rapidly by a factor of ~2 above 700 km. The flux-tube paradigm for the growth of equatorial electron density irregularities seems consistent with our results. More analysis with PBMOD will be performed to determine if a physicsbased model can reproduce the observed dependence of bubble height on solar flux. The results confirm that the area routinely affected by equatorial bubbles increases significantly ( > 4x ) during solar maximum relative to solar minimum.