Radio bursts from Saturn lightning

Jean-Mathias Griessmeier
LOFAR and the Transient Radio Sky, Amsterdam, December 2008

Transcript

1. Radio bursts from Saturn lightning ASTRON is part of the Netherlands Organisation for Scientific Research (NWO) griessmeier@astron.nl with: P. Zarka, A. Konovalenko, G. Fischer, B. Ryabov, D. Vavriv, V. Ryabov, H. Rucker, P. Ravier, M. Sidorchuk and the Radio-Exopla collaboration Jean-Mathias Grießmeier
2. 2 Saturn lightning Lightning atmospheric emission (lightning) magnetospheric emission
3. 3 Saturn lightning - - - - acceleration of charges ⇒ electromagnetic radiation Lightning
4. 4 Saturn lightning time the signal Intensity (12-28 MHz) Planetary lightning
5. 5 Saturn lightning ⇒ episodes repeat after one planetary rotation How do we know? ⇒ we even see the storms in IR (e.g. “Dragon storm”, 2004) Radiosource: lightning activity in corotating storm system Lightning as a radiosource
6. 6 Saturn lightning P=10.67 hr ⇒ episodes repeat after one planetary rotation How do we know? ⇒ we even see the storms in IR (e.g. “Dragon storm”, 2004) Radiosource: lightning activity in corotating storm system Lightning as a radiosource
7. 7 Saturn lightning P=10.67 hr ⇒ episodes repeat after one planetary rotation How do we know? ⇒ we even see the storms in IR (e.g. “Dragon storm”, 2004) Radiosource: lightning activity in corotating storm system Lightning as a radiosource
8. 8 Saturn lightning Why to study Saturn lightning? • existence of lightning • electrification processes • atmospheric dynamics and composition • geographical and seasonal variations • correlation with optical/IR observations (clouds) • comparison to Earth processes Radio search motivation
9. Radiobursts are: • faint signal (no ground detection until 2006) • transient signal (2 ms) • sporadic emission (low occurence rate: ~30d/year) • low frequency (20 kHz to 40 MHz) Observational challenges 9
10. 10 Saturn lightning • Voyager 1 & 2: 1980 & 1981 • Cassini: 2004-now Satellite observations: • UTR-2 & Nancay: 2006 • UTR-2: 2007 & 2008 Ground observations: Saturn lightning observations
11. 11 Saturn lightning • Voyager 1 & 2: 1980 & 1981 • Cassini: 2004-now Satellite observations: • UTR-2 & Nancay: 2006 • UTR-2: 2007 & 2008 Ground observations:  105 km instead of 1.5*109 km, i.e. signal 108 times stronger modern receivers, Cassini as trigger  Saturn lightning observations  large collecting area required
12. Radiobursts are: • faint signal (no ground detection until 2006) • transient signal (2 ms) • sporadic emission (low occurence rate: ~30d/year) • low frequency (20 kHz to 40 MHz) Observational challenges 12
13. τ ≈ 40-50 ms (Voyager, Cassini, t = 40 ms) τ ≈ 30 ms (UTR-2, t = 20 ms) Burst duration 13
14. τ ≈ 40-50 ms (Voyager, Cassini, t = 40 ms) τ ≈ 30 ms (UTR-2, t = 20 ms) τ ≈ 4.3 ms (UTR-2, t = 2 ms) Burst duration 14
15. 15 Saturn lightning dt=10 ms Not enough time resolution!
16. 16 Saturn lightning dt=10 ms dt=20 ms Not enough time resolution!
17. 17 Saturn lightning dt=10 ms dt=20 ms Not enough time resolution! τ=7.3ms τ=24.4ms
18. 18 Saturn lightning sampling time measured e-folding time e-folding time as a function of t
19. sampling time measured e-folding time unresolved res. e-folding time as a function of t  high time resolution required 19
20. Radiobursts are: • faint signal (no ground detection until 2006) • transient signal (2 ms) • sporadic emission (low occurence rate: ~30d/year) • low frequency (20 kHz to 40 MHz) Observational challenges 20
21. 21 trigger weak signal strong signal Cassini triggers ground-based observations Ground-based observation
22. 22 Ground-based observation UTR-2 Cassini  monitoring required • now:Cassini triggers ground-based observations • after Cassini (>2010)?
23. Radiobursts are: • faint signal (no ground detection until 2006) • transient signal (2 ms) • sporadic emission (low occurence rate: ~30d/year) • low frequency (20 kHz to 40 MHz) Observational challenges 23
24.  spectrum needed in wide band Spectrum 24 [Farrell et al, GRL, 2007] with the currently known spectrum, Saturn lightning could be • (A) 10000 times stronger than on Earth • (B) 10000 times weaker than on Earth
25. Spectrum 25 [Farrell et al, GRL, 2007] LOFAR with the currently known spectrum, Saturn lightning could be • (A) 10000 times stronger than on Earth • (B) 10000 times weaker than on Earth  spectrum needed in wide band
26. Radiobursts are: • faint signal (no ground detection until 2006) • transient signal (short duration) • sporadic emission (low occurence rate: ~30d/year) • low frequency (20 kHz to 40 MHz) Observational challenges 26
27. Radiobursts are: • faint signal (no ground detection until 2006) • transient signal (short duration) • sporadic emission (low occurence rate: ~30d/year) • low frequency (20 kHz to 40 MHz) Observational challenges 27 LOFAR has a high time resolution LOFAR has a large collecting area LOFAR can monitor LOFAR can observe up to 240 MHz
28. • Saturn: geographical variation seasonal variation correlation to optical surveys • Uranus: spectrum discharge timescale Observations: Predicted: • Neptune: follow-up tentative detections • Venus: follow-up tentative detections Tentative detections: • Mars: discharges in dust clouds? Observations with LOFAR 28
29. 29 Saturn lightning ? Lightning on other planets? ? observed to be observed (LOFAR)
30. The End