Ring Rain and Other Drivers Luke Moore, Marina Galand, Arv Kliore, Andy Nagy, James O Donoghue

Transcription

1 Ring Rain and Other Drivers Luke Moore, Marina Galand, Arv Kliore, Andy Nagy, James O Donoghue

2 Outline Introduction to Saturn s ionosphere Basic properties and theory Observations: what do we know? Radio occultations: Saturn Electrostatic Discharges: Ring rain: N e (h) N MAX (SLT) H 3 (12 SLT) Theory/Models: what do we think we know? Comparisons with observations Summary Remaining uncertainties, future observations

3 Atmospheric Layers T exo h MAX Upper atmosphere (aeronomy) - Key transition region between lower atmosphere and magnetosphere - Energy and momentum sources: EUV/FUV solar radiation Energetic particles Forcing from below (e.g., gravity waves) N MAX Lower atmosphere (meteorology) Wikipedia

4 Vertical Temperature Profile Reference altitude (0 km) = 1 bar level Moses et al (2000a)

5 Latitudinal Temperature Behavior UV: from solar and stellar occultations, represent T exo IR: from H 3 emissions, represent effective column temperature Moore et al (2014)

6 Thermosphere of Saturn Reference altitude (0 km) = 1 bar level Heterosphere Molecular diffusion Convective mixing Homopause / Turbopause Homosphere Moses and Bass (2000)

7 Initial Theory: hν H 2 H e - He CH 4 H H He CH X H 3 H He CH X 1 Photoionization (solar EUV photons) 2 Charge-exchange and recombination HH 2 HH 2 HH 3 HH FAST H 2 HH 3 ee HH 2 OO HH OH 2H FAST H 2 ~90% e - HH ee HH SLOW

8 Observations

9 Radio Occultations Time delay and bending angle (α) provide electron density vs. altitude (4) Electron density vs. radius (1) Frequency residual vs. time (2) Bending angle α vs. time (3) Refractivity vs. radius nn ee ee 2 μμ ee 1 = υυ ee = 8ππ 2 mm ee εε oo ff 2 Withers et al (2014)

10 Summary of Outer Planet Radio Occultations Pioneer 10 Pioneer 11 Voyager 1 Voyager 2 Galileo Cassini 3 Mar Apr Sep Aug Oct Oct 1997 x 2 x 2 x 2 x 2 x 5 * x 59 ** = 13 x 2 x 2 x 2 = 65 x 2 = 2 x 2 = 2 ingress (N) and egress (X) orbiters * analyzed; ** taken to-date

11 Pre-Cassini Saturn Radio Occultations Pioneer Voyager Lindal et al (1985) Kliore et al (1980) Narrow low-altitude layers of N e N MAX ~ 10 4 cm -3 h MAX ~ km

12 Cassini Equatorial Radio Occultations DAWN DUSK Nagy et al (2006)

13 Cassini Equatorial Dawn/Dusk Asymmetry DAWN N MAX < DUSK N MAX DAWN h MAX > DUSK h MAX Cassini equatorial radio occultation averages DAWN DUSK Kliore et al (2009)

14 Cassini Latitudinal Trend in N e 39 profiles high-latitude (>60 o ) averages Low-latitude (<20 o ) mid-latitude (>20 o,<60 o ) Kliore et al (2009)

15 Cassini Latitudinal Trend in N e Minimum N MAX at equator; N MAX increases with latitude Moore et al (2010)

16 Saturn Electrostatic Discharges (SEDs) Broadband, short-lived, impulsive radio emission, ~10 hr periodicity Initially thought to originate in Saturn s rings, later shown to be associated with powerful lightning storms in Saturn s lower atmosphere Detected by Voyager and Cassini (~9 SED storms to-date, each lasting weeks-months) Observed low-frequency cutoff can be used to derive N MAX (SLT) Specific latitudes (primarily -35 o ), single storm locations Powerful lightning also observed at Jupiter, but no JEDs Perhaps due to attenuation of radio waves by Jupiter s ionosphere

17 N MAX (SLT) from SEDs LT of storm from images, angle of incidence α calculated from storm location and Cassini position f = cutoff f pe,max cos( α) N = e f 2 pe,max / 81 Fischer et al (2011)

18 N MAX (SLT) from SEDs Strong diurnal variation in N MAX N MAX (noon) ~ 10 5 cm -3 N MAX (midnight) ~ cm -3 Moore et al (2012) log N e = A - B cos(lt - φ)

19 Protonated Molecular Hydrogen: H 3 First astronomical spectroscopic detection in the universe at Jupiter Auroral IR measurements with CFHT (Drossart et al., 1989) Bright emission lines in K-band (2-2.5 mm) and L-band (3-4 mm) atmospheric windows Strong methane absorption in the L-band Therefore, at the giant planets (where H 3 is above the homopause), H 3 appears as bright emission above a dark background Highly temperature dependent Can be used to derive ion temperatures, densities, and velocities Important as a coolant, e.g.: Efficient thermostat at Jupiter Hot exoplanets with dissociated H 2 lose a key cooling mechanism Connerney and Satoh (2000)

20 First Low-Latitude Measurements of H 3 H 3 frequently used as a diagnostic of outer planet ionospheres (Jupiter, Saturn, Uranus), BUT: H 3 only detected in Saturn s auroral regions until 17 April 2011 Keck NIRSPEC observations Ring reflection methane slit Wavelength (micron) O Donoghue et al (2013)

21 Latitudinal Variations in H 3 Emission local extrema mirrored at magnetically conjugate latitudes, and also map to structures in the rings Ring Radius (R S ) I H3 (nw m -2 ) O Donoghue et al (2013) NORTH Latitude (deg.) SOUTH

22 Latitudinal Variations in H 3 Emission local extrema mirrored at magnetically conjugate latitudes, and also map to structures in the rings I H3 (nw m -2 ) II HH3 = ff NN HH3, TT HH3 NORTH Latitude (deg.) SOUTH

23 Summary of Observational Constraints Radio occultations Unusual vertical structure: Average peak values: Dawn/dusk asymmetry: Latitudinal variation: Narrow low-altitude layers of N e N MAX ~ 10 4 cm -3 h MAX ~ km DAWN N MAX < DUSK N MAX DAWN h MAX > DUSK h MAX Minimum N MAX at equator; N MAX increases with latitude Saturn Electrostatic Discharges (SEDs) Strong diurnal variation: Noon and midnight values: Ring Rain Latitudinal structure in H 3 : 1-2 order of magnitude variation in N MAX N MAX (noon) ~ 10 5 cm -3 N MAX (midnight) ~ cm -3 Non-solar structure in H 3 emission; coupling to rings Mid- and low-latitude temperatures and densities?

24 Theory

25 Overview of Saturn s Main Ionosphere hν H 2 H e - He CH 4 H H He CH X H 3 H He CH X 1 Photoionization (solar EUV photons) 2 Charge-exchange and recombination HH 2 HH 2 HH 3 HH FAST H 2 HH 3 ee HH 2 OO HH OH 2H FAST H 2 ~90% e - HH ee HH SLOW

26 Hydrocarbon photochemistry Simplified Schematic of Hydrocarbon Photochemistry at Saturn Moses and Bass (2000) 300 additional reactions!

27 Hydrocarbon/metallic ion ledge And many more Kim and Fox (1994) Meteoroid ablation deposition leads to Mg/Mg, Fe/Fe, Si/Si, O/O, S/S, C/C, etc. Kim and Fox (2001)

28 Predicted Ionospheric Densities H 3 N e z (km) HeH Observed N MAX values n (cm -3 ) McElroy (1973) N MAX = ~10 5 cm -3

29 Radio Occultation Constraints: N MAX ~ 10 4 cm -3 h MAX ~ km Altitude (km) Data Model N MAX h MAX Water H H 2 O H 2 O H H 2 O HH 2 H 3 O H H 3 O HH e 2 OO HH OOOO 2HH N e (cm -3 ) Vibrationally Excited H 2 H H 2 (νν 4) H 2 H H 2 H 2 H 3 H H 3HH 3 e HH 2 HH hν H 2 H e - He CH 4 H 2 H He CH X H 3 H He CH X

30 Radio Occultation Constraints: N MAX ~ 10 4 cm -3 h MAX ~ km Forced vertical drift ( h MAX ) Enhanced H 2 (ν 4) population( N MAX ) Topside H 2 O influx ( N MAX ) A A z (km) C B z (km) C B Majeed and McConnell (1991) n (cm -3 ) A observations B nominal model C model fit n (cm -3 )

31 Radio Occultation Constraints: DAWN N MAX < DUSK N MAX DAWN h MAX > DUSK h MAX Cassini equatorial radio occultation averages DAWN DUSK Ionospheric model simulation Moore et al (2004) H H3 Ne DAWN DUSK DAWN DUSK Kliore et al (2009) Ionospheric model simulation At Saturn s equator: N MAX ~ 10 3 cm -3 h MAX ~ km H H H H3 Galand et al (2009)

32 Radio Occultation Constraints: Narrow low-altitude layers of N e Structure driven by vertical wind shear interactions with magnetic field. Such shears could result from gravity wave breaking. Other sources remain possible, such as meteoric layers.

33 Radio Occultation Constraints: Topside H 2 O influx (derived) Minimum N MAX at equator; N MAX increases with latitude Model solstice DUSK Model solstice DAWN Cassini DUSK * Cassini DAWN Model equinox DUSK Model equinox DAWN Mueller-Wodarg et al (2012)

34 SED constraints: 1-2 order of magnitude variation in N MAX Significant ionization enhancements required to match dawn-noon rise Drastic losses required to match nighttime decline Moore et al (2012) Non-photochemical solution? Low altitude ion layers? Voyager result (A) Cassini results (dotted and dashed) Various attempted model fits (B-E) Various attempted model fits (grey) Best model fit (solid lines) Majeed and McConnell (1996) Moore et al (2012)

35 Ring rain constraints: Non-solar structure in H 3 emission; coupling to rings NN HH3 = ff II HH3, TT HH3 TT HH3 = ff φφ pppp UV IR

36 Ring rain constraints: Non-solar structure in H 3 emission; coupling to rings Global water influx at Saturn Family of solutions matching ring rain H 3 column density at Water influx mapped to ring plane H2(ν 4) population Moore et al (2014)

37 Ring rain: where is it? 2011 H Ring sunlight reflection Ring sunlight reflection Hydrocarbon reflection of sunlight See poster by James O Donoghue

38 Summary of Model Data Comparisons Radio occultations Unusual vertical structure: Average peak values: Dawn/dusk asymmetry: Latitudinal variation: Saturn Electrostatic Discharges (SEDs) Strong diurnal variation: Noon and midnight values: Ring Rain Latitudinal structure in H 3 : Narrow low-altitude layers of N e Gravity waves. (Meteors?) N MAX ~ 10 4 cm -3 DAWN N MAX < DUSK N MAX h MAX ~ km DAWN h MAX > DUSK h MAX Water influx and/or H 2 (ν 4) enhancements. Minimum N MAX at equator; N MAX increases with latitude Latitudinal variation in water influx. 1-2 order of magnitude variation in N MAX N MAX (noon) ~ 10 5 cm -3 N MAX (midnight) ~ cm -3 Require extreme ionization enhancement process. Low altitude layers? Non-solar structure in H 3 emission; coupling to rings Latitudinal variation in water influx and/or heating?

39 A look towards the future Radio occultations Further mid- and high-latitude occultations to help solidify the trend in N MAX there Proximal orbits Ion densities? Electron densities? SEDs? (attenuation varies with frequency, so Cassini SED measurements close to Saturn may alter derived N MAX trend) Ring Rain observations Self-consistent H 3 temperatures and densities (H densities) Water measurements Help reinforce influxes derived from Ionospheric model-data comparisons

40 x

41 Representative Ionospheric Structure Basic Ionospheric structure Basic

42 Thermal Profile Comparisons Thermosphere: - Positive temperature gradient - Collective (fluid) behavior Exosphere: - Constant temperature ( exospheric temperature ) - Infrequent collisions kinetic particle behavior and escape I. Müller-Wodarg