Correlation of LOFAR and Cassini observations of Saturn's lightning

Julien Girard
LOFAR Transients Key Project Meeting, Meudon, December 2011

Transcript

1. Correlation of LOFAR and Cassini observations of Saturn's lightning J. Girard, P. Zarka and the (e)PWG team TKP Meeting - Meudon - 14/16 December 2011 LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, Meudon
2. Outline Saturn electrostatic discharges Cassini and LOFAR datasets plans for correlation
3. Saturn Electrostatic Discharges (SED) First discovered by Voyager Observed by Cassini correlated with optical observations (Warwick et al., 1981) F T • Subject to propagation effects in the kronian ionosphere • Storm rotating with period ~10.7 h • Wideband unpolarized bursts • Bursts duration: few ms to 100s ms • Spectrum: up to ≥40 MHz (F max Cassini = 16 MHz)
4. A B (Farrell et al., 2007) Why LOFAR is interesting for SEDs LOFAR LBA ? LOFAR in the LBA range (up to >40 MHz) allow to distinguish A/B. Cassini SED detection around 10 MHz
5. Cassini swept-frequency analyser RPWS (Radio & Plasma Wave Science) instrument f t events seen as narrow-band bursts ➔ List of detected events with 10 s resolution ➔ Time resolution increased by knowing the in-sweep durations between channels and integration time ➔ higher time resolution ~10 ms Cassini Data 1 sweep (16 MHz) ~10s Broadband SED events Intercepted chunks (credits: Zarka Cecconi)
6. LOFAR observation 2011/07/08 Cassini Data
7. SED Detectability with LOFAR (Zarka et al. 2004) Unobservable from ground Ionospheric cut-off SEDs @ Earth ~ 10-1000 Jy SED A1 Station ~ 48λ2/3 (van Haarlem, 2001) (Zarka, 2004) Nstations f b ȷ
   σsky 1 station 30 MHz 195 kHz 82µs 104 Jy 10 stations incoh sum 30 MHz 6 MHz 20ms 37 Jy 5 stations coh sum 20 MHz 67 MHz 195 kHz 9.1ms 233 Jy 126 Jy
8. 2011/07/08 : LOFAR observation 13:00 - 19:00 UT Storm visibility and timing Saturn Earth ΔT Sat-Saturn ΔT Sat-Saturn ~ 5 s @ 22 Saturn radii ΔT Sat-S/C ~ 80 min @ ~10 AU Coherent sum of 5 superterp stations ON & OFF Beam (122 SB each) T= 6H dt=1.3 ms df=195/16 kHz F= [ 20, 67 ] MHz
9. ON OFF 20 MHz 70 MHz 13:00 17:00 2011-07-08 : first 4 hours LOFAR Data After 1 cleaning step
10. 20 70 MHz 0 1600 sec 13h 13h27 2011-07-08 : first 1600 sec, beam ON Saturn After 2nd cleaning step
11. 20 70 MHz 0 1600 sec 2011-07-08 : first 1600 sec, beam OFF Saturn 13h 13h27 After 2nd cleaning step
12. Average spectrum On Off (MHz) 42.0-47.8 50.6-52.5 55.6-58.4 61.1-63.1 64.2-66.2 Selection of 5 clean bands
13. Excess in ON between 13h-14h (LOFAR Time) Cleaned Data !Ntrue /5min ΔN true = N ON -N OFF Integration in clean SBs and spike detection
14. Comparing with Cassini data 14:00 18:00
15. SED detection in LOFAR Data • Remaining low level RFI seen both in ON & OFF beam • First step of cleaning in t-f plane and rebinned (dt=9.1ms, df=195kHz) ON OFF - 1 0 1000
16. SED detection in LOFAR Data • RFI detection (>3σ in 201 px window) in OFF integrated data ➔ mask of bad pixels applied in ON integrated data ➔ interpolating bad values with nearest good data • Remaining low level RFI seen both in ON & OFF beam • First step of cleaning in t-f plane and rebinned (dt=9.1ms, df=195kHz)
17. ON OFF 0 1000 sec
18. SED detection in LOFAR Data • RFI detection (>3σ in 201 px window) in OFF integrated data ➔ mask of bad pixels applied in ON integrated data ➔ interpolating bad values with nearest good data • (On-going) detection of new spikes • Remaining low level RFI seen both in ON & OFF beam t 1 0 dt = 9.1 ms • First step of cleaning in t-f plane and rebinned (dt=9.1ms, df=195kHz)
19. From list of detected events Data bin # of event Date (sec of day) Int time (msec) Freq (kHz) Flux (W.m-2) ... 152 122.28421 17.3280 1018.75 6.50709e-20 ... 157 122.46661 17.3280 1081.25 8.41127e-20 ... ... ... ... ... ... ... t 1 0 build synthetic time series at LOFAR data resolution dt = 9.1 ms Cassini Data
20. Correlation plans t dt = 9.1 ms t • prior time shift to correct light travel delays • but HUGE numbers of SED in Cassini Data Cassini LOFAR • Correlation in ~1 min windows ~ 21000 events (0-30 dB) (dB above near bins)
21. ➔ selecting threshold 10%,20%,30%... of most powerful SEDs ➔ study how correlation get better/worse with threshold ~ 700 events (20-30 dB) ~ 21000 events (0-30 dB) (dB above near bins)
22. Work plan • Currently no storm on Saturn ➔ no possible observation yet • Some observations were taken under ‘extreme’ conditions (daytime, low elevation target, storm visibility) 4/2011 7/2011 10/2011 12/2011 NOW 08/07/11
23. • Currently no storm on Saturn ➔ no possible observation yet • With clearly identified and matched events (LOFAR/Cassini): ➔ statistics on burst durations • Some observations were taken under ‘extreme’ conditions (daytime, low elevation target, storm visibility) Work plan D0~ 50 ms (as seen by Voyager I & II & Cassini) LOFAR data of 8 apr 2011
24. Work plan • Currently no storm on Saturn ➔ no possible observation yet • With clearly identified and matched events (LOFAR/Cassini): ➔ statistics on burst durations ➔ mean power spectrum • Some observations were taken under ‘extreme’ conditions (daytime, low elevation target, storm visibility) f -0.7 Spectrum of 1 event of 7/8/11 f-Ћ with Ћ=1-2 in >40 MHz
25. Work plan • Currently no storm on Saturn ➔ no possible observation yet • With clearly identified and matched events (LOFAR/Cassini): ➔ statistics on burst durations ➔ mean power spectrum ➔ SED energy -> A/B scenario • Some observations were taken under ‘extreme’ conditions (daytime, low elevation target, storm visibility) • When storm activity resumes, do joint observations with TBB people during the storm maximum. • Check with TBB people for existing TBB data taken during the same window. (Manual beamforming of antenna signals on Saturn (ask Sander) during storm events) ➔ with high time resolution data (δt~ns) ➔ fine time structure (strokes).