Unsolved Problems in Saturn's Magnetosphere

Unsolved Problems in Saturn’s Magnetosphere

Michelle F. Thomsen Planetary Science Instute

UPMP, Scarborough, 6-12 Sep 2015 If you’ve seen one magnetosphere,

you’ve seen one magnetosphere! Saturn’s magnetospheric features: • Radiaon belts • Plasmasphere • Plasma sheet • Magnetospheric tail • Aurorae • Bow shock • Magnetopause • Magnetospheric dynamics (convecon, reconnecon, wave-parcle interacons, etc.)

But each of these is significantly different from the corresponding features at Earth! Factors Affecng Planetary Magnetospheres

Saturn rel. to Earth • Size of the planet 10x • Strength of its magnetic ﬁeld 500x • Presence of satellites and rings Earth has none • plasma sources ✓ • plasma sinks ✓ • dynamic interactions ✓ • imbedded magnetospheres! ✓ • Rotational period 2.5x faster • Tilt of magnetic dipole rel. to rotational axis ~0 • Properties of external plasma (solar wind) n~0.02x; B~0.1x merged stream

regions; high MA Factors Aﬀecng Planetary Magnetospheres

Plasma sources, transport, sinks, global dynamics… 3.6x106 km Credit: NASA/JPL/Space Science Instute Credit: Univ. of Michigan

504 km Saturn’s Magnetospheric Imperave • Dominant source of plasma in Saturn’s magnetosphere is at low L (ionizaon of gas from Enceladus) • Recombinaon is too slow to maintain a steady state • All this plasma must ulmately be shed to the solar wind • How and where does that happen? Convecon at Saturn is dominated by corotaon

• Weaker interplanetary ﬁeld • Rapid rotaon

Therefore, no convecon plasmapause is expected.

Brice and Ioannidis, The magnetospheres of Jupiter and Earth, Icarus, 1970 Plasma Shedding at Saturn: A Two-Step Process

1. Centrifugally-driven interchange • Inner to middle magnetosphere

2. Tail reconnecon/Plasmoid release • Middle/outer magnetosphere Step One: Centrifugally-Driven Interchange Instability

Rayleigh-Taylor Instability Liu et al., 2010 High- Heavy fluid density plasma

Light fluid Low- density plasma Centrifugal force Gravity IonEnergy (eV)

UT r 3.9 4.7 5.5 6.3 7.0 7.7 8.3 Rs

Interchange- unstable when ∂η < 0 ∂L where η = ndV ∫V

Sittler et al., 2007 Southwood & Kivelson (1987) Step Two: Magnec Reconnecon of Loaded Flux Tubes Centrifugal force -> stretched ﬂux tubes -> B unable to conﬁne

Vasyliunas, V. M., Phys. Jov. Msp., 1983

Stretched flux tubes reconnect Step Two: Magnec Reconnecon of Loaded Flux Tubes Centrifugal force -> stretched flux tubes -> B unable to confine

“Vasyliunas-type reconnecon”

Dipolarizaon of returning closed ﬂux Deparng plasmoid Strong tailward ﬂows Dipolarizaon

View from above the dusk meridian View from above the noon meridian

X. Jia et al., JGR, 2012 PP Lobe 1# 2# Outer 3# Inner 25#Jun#2009# 2# 104# a# 103# )#

eV 2# # #( 10

Ei • Hot, low-density 101# accum 100# • Signiﬁcant energec O+ 4# CAPS 10 b# )# 103# ﬂuxes eV Log#counts/ #( 2#

Ee 10 • “Leading ﬁeld”=>evidence 101# 0# for supercorotaon 100# # )# '30 c# • nT '50# SKR enhancement and #( r B '70#

)# expansion to lower d# 40# nT

# #(

38 θ B MAG 36# frequencies 4# )# 0# nT # #(

φ '4

B '8# e# CHEMS H+ CHEMS H+ 55# 5 25# 2 4

'1# 4 )# 10 f# Freshly injected plasma H+ ) 3 eV 3 Energy (kev) keV Energy (kev) 1 # 1 2 4# 2 EH#( 10 sr

units 1 / ( cm^2 ster keV sec ) from tail reconnecon of 1 # units 1 / ( cm^2 ster keV sec ) Time (UTC) 06:00:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00:00 #s# 1 1 2009/176 CHEMS W+ 2009/176 2 CHEMS W+ Time (UTC) Dipole06:00:00 L (RS) 10.67 07:009.83 9.17 08:008.69 09:008.35 8.13 10:008.03 8.0311:00 8.12 12:00 13:00 14:00:00 Radial Dist2009/176 (RS) 7.22 7.24 7.29 7.36 7.46 7.59 7.73 7.9 8.08 2009/176 DipoleSZS L (RS) Local Time10.67 (hrs) 7.62 9.837.96 8.26 9.17 8.54 8.698.8 9.04 8.359.26 9.478.13 9.66 8.03 8.03 8.12 Radial Dist (RS) 7.22 7.24 7.29 7.36 7.46 7.59 7.73 3 # 7.9 8.08 3

)# 5# 2 SZS Local Time (hrs) 1027.62 g# 7.96 8.26 8.54 8.8 9.04 9.26 3 9.47 previously-closed ﬁeld 9.66 eV

CHEMS 2 2

Energy (kev) O+ Energy (kev) EO#( log#ﬂux(cm 11# units 1 / ( cm^2 ster keV sec ) 1 lines.

Time (UTC) 06:00:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00:00 units 1 / ( cm^2 ster keV sec ) 2009/176 2009/176 Dipole L (RS) 10.67 9.83 9.17 8.69 8.35 8.13 8.03 8.03 8.12 Time (UTC) Radial Dist06:00:00 (RS) 7.22 07:007.24 7.29 08:007.36 09:007.46 7.59 10:007.73 11:007.9 8.08 12:00 13:00 14:00:00 SZS Local Time2009/176 7#(hrs) 7.62 7.96 8.26 8.54 8.8 9.04 9.26 9.47 9.66 2009/176 Dipole L (RS) 1010.67 9.83 9.17 8.69 8.35 8.13 8.03 8.03 8.12 Radial Dist (RS) 7.22 7.24 7.29 7.36 7.46 7.59 7.73 # # 7.9 8.08 SZS Local Time (hrs) 7.62 7.96 8.26 8.54 8.8 9.04 9.26 30 9.47 9.66⇒ “Vasyliunas cycle”

105# bkg 20# #(Hz)# h# 103# RPWS

Freq #

dB#above# 10 101# 0# UT# 0600# 0700# 0800# 0900# 1000# 1100# 1200# L# 10.67# 9.83# 9.17# 8.69# 8.35# 8.13# 8.03# LT# 7.62# 7.96# 8.26# 8.54# 8.80# 9.04# 9.26# Thomsen et al., 2015 Lat# '34.6# '30.9# '27.0# '23.0# '19.0# '15.0# '11.2# 1# 2# 3# 25#Jun#2009# 2# 104# a# 103# )#

eV 2# # #( 10 Ei 101# accum 100# 104# b# )# 103# eV Log#counts/ #( 2#

Ee 10 101# 0# 100# # )# '30 c#

nT '50# #( r B '70# d# 40# )# nT

# #(

38 θ 36# B 4# )# 0# nT # #(

φ '4

B '8# e# CHEMS H+ CHEMS H+ 55# 5 25# 2 4

'1# 4 )# 10 f# )

3 eV 3 Energy (kev) # keV Energy (kev) 1# 2# 3# 1 25#Jun#2009 # 1 2 4# 2 EH#( 10 sr # units 1 / ( cm^2 ster keV sec ) 1 2#

4# units 1 / ( cm^2 ster keV sec ) Time (UTC) 06:00:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00:00 #s# 1 1 10 2 2009/176 CHEMS W+ 2009/176 CHEMS W+ Dipole06:00:00 L (RS) 10.67a# 07:009.83 9.17 08:008.69 09:008.35 8.13 10:008.03 8.0311:00 8.12 12:00 13:00 14:00:00 Time (UTC) Radial Dist2009/176 (RS) 7.22 7.24 7.29 7.36 7.46 7.59 7.73 7.9 8.08 2009/176 DipoleSZS L (RS) Local Time10.67 (hrs) 7.62 9.837.96 8.26 9.17 8.54 8.698.8 9.04 8.359.26 9.478.13 9.66 8.03 8.03 8.12 Radial Dist (RS) 7.223# 7.24 7.29 7.36 7.46 7.59 7.73 3 # 7.9 8.08 3 SZS Local Time (hrs))# 1027.625# 2 7.96 8.26 8.54 8.8 9.04 9.26 9.47 9.66 )# 10 g# 3 eV eV 2# 2

# 2 #( 10 Energy (kev) Energy (kev) Ei EO#( 1# log#ﬂux(cm 11# 10 units 1 / ( cm^2 ster keV sec ) 1

Time (UTC) 06:00:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00:00 units 1 / ( cm^2 ster keV sec )

2009/176 2009/176 accum Dipole L (RS) 10.67 9.83 9.17 8.69 8.35 8.13 8.03 8.03 8.12 Time (UTC) Radial Dist06:00:00 7#(RS)0# 7.22 07:007.24 7.29 08:007.36 09:007.46 7.59 10:007.73 11:007.9 8.08 12:00 13:00 14:00:00 SZS Local Time2009/17610 (hrs) 7.62 7.96 8.26 8.54 8.8 9.04 9.26 9.47 9.66 2009/176 Dipole L (RS) 1010.674# 9.83 9.17 8.69 8.35 8.13 8.03 8.03 8.12 Radial Dist (RS) 107.22 7.24 7.29 7.36 7.46 7.59 7.73 # # 7.9 8.08 SZS Local Time (hrs) 7.62 b# 7.96 8.26 8.54 8.8 9.04 9.26 30 9.47 9.66 bkg

)# 5# 10103# eV 20# Log#counts/ #(Hz)# #( 3#2# h#

Ee 1010

Freq #

dB#above# 10 101# 101# 0# 100# 0# # UT#)# '300600#c# 0700# 0800# 0900# 1000# 1100# 1200#

L#nT '5010.67## 9.83# 9.17# 8.69# 8.35# 8.13# 8.03# #( PP r

LT#B 7.62# 7.96# 8.26# 8.54# 8.80# 9.04# 9.26# '70# L~9

Lat# '34.6# '30.9# '27.0# '23.0# '19.0# '15.0# '11.2# )# d# 40# Tail reconnecon of loaded ﬂux tubes nT

# #(

38 θ 36# B • sheds inner magnetospheric mass 4# )# 0# nT # #(

• creates a sharp boundary between the dense inner-φ '4 B '8# e# CHEMS H+ CHEMS H+ 55# 5 magnetospheric plasma that has escaped reconnecon, 25# 2 4

'1# 4 )# 10 f# )

3 eV 3 Energy (kev) keV and the injected hot tenuous plasma of the reconnected Energy (kev) 1 # 1 2 4# 2 EH#( 10 sr units 1 / ( cm^2 ster keV sec ) 1 # units 1 / ( cm^2 ster keV sec ) Time (UTC) 06:00:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00:00 #s# 1 1 2009/176 CHEMS W+ 2009/176 2 ﬂux region: Plasmapause CHEMS W+ Time (UTC) Dipole06:00:00 L (RS) 10.67 07:009.83 9.17 08:008.69 09:008.35 8.13 10:008.03 8.0311:00 8.12 12:00 13:00 14:00:00 Radial Dist2009/176 (RS) 7.22 7.24 7.29 7.36 7.46 7.59 7.73 7.9 8.08 2009/176 DipoleSZS L (RS) Local Time10.67 (hrs) 7.62 9.837.96 8.26 9.17 8.54 8.698.8 9.04 8.359.26 9.478.13 9.66 8.03 8.03 8.12 Radial Dist (RS) 7.22 7.24 7.29 7.36 7.46 7.59 7.73 3 # 7.9 8.08 3

)# 5# 2 SZS Local Time (hrs) 1027.62 g# 7.96 8.26 8.54 8.8 9.04 9.26 3 9.47 9.66 eV 2 2 Energy (kev) Energy (kev) EO#( => Unstable to centrifugal interchange log#ﬂux(cm 11# units 1 / ( cm^2 ster keV sec ) 1

105# bkg 20# #(Hz)# h# 103#

Freq #

dB#above# 10 101# 0# UT# 0600# 0700# 0800# 0900# 1000# 1100# 1200# L# 10.67# 9.83# 9.17# 8.69# 8.35# 8.13# 8.03# LT# 7.62# 7.96# 8.26# 8.54# 8.80# 9.04# 9.26# Lat# '34.6# '30.9# '27.0# '23.0# '19.0# '15.0# '11.2# Major Open Quesons • How important is the solar wind? – Magnetopause reconnecon is inhibited but does occur => flux return is another imperave, which can’t be accomplished with Vasyliunas reconnecon. – Auroral signature of tail reconnecon is enhanced by Pdyn increases. – Recent evidence for sustained lobe reconnecon (Dungey cycle) during high Pdyn intervals => Does Saturn have a recurrent high-speed-stream-driven storm cycle? – Does it just affect the outermost L shells or is the influence more pervasive? Major Open Quesons • Are smaller-scale loss processes (“dribble”) important? – Observed plasmoid loss rate inadequate to shed the full load produced by Enceladus? (But see recent work by Cowley …) – Duskside “planetary wind” – Pre-dawn tail flow beyond ~15 Rs dominantly toward magnetopause Major Open Quesons • How does interchange really work? – Is it strongly driven by tail dynamics? – Does it enable tail dynamics? – Does impoundment happen? – Is the inflow in the form of channels? Bubbles? Highly structured? – How much energy is carried in? How much material is carried out? – Is there nonadiabac heang? Major Open Quesons • What is the nature of the auroral/subauroral magnetosphere? – Interchange remnants + heated plasma from tail reconnecon? – Local heang? Field-aligned acceleraon? Major Open Quesons • How variable is the inner magnetosphere? – Radiaon belts? – Plasma source? – Hot populaon? – Any relaon to solar wind variability? Major Open Quesons • What causes the dawnward convecon component in the inner magnetosphere? – Too large, wrong direcon for solar-wind imposed convecon as at Earth – Related to ongoing tail dynamics? – Related to magnetospheric size (wave propagaon speeds)? – Does it show up in global MHD simulaons? Major Open Quesons • What causes the ubiquitous periodicies at ~planetary rotaon period?

Saturn Kilometric Radiation (SKR)

Energetic electrons (20-900 keV)

Magnetic Field

(Carbary et al., 2015, Saturn in the 21st Century, in review. Thanks to Tom Hill) !

The SKR modulation period differs between N and S hemispheres:

Total N + S SKR

LH SKR South

RH SKR North

! 2004 05 06 07 08 09 10 11 12 13 (Lamy, 2011 Plan. Radio Emissions VII; Carbary et al., 2015 Saturn in the 21st Century, in review) T. Hill MOP Conference, Atlanta, GA, USA, 1 June 2015 24 Major Open Quesons • What causes the ubiquitous periodicies at ~planetary rotaon period?

– Why does Saturn have any spin modulaon? – Why are there two disnct periods (N, S)? – Why do Saturn’s periods (both N and S) exhibit large (% level) me variaons on seasonal me scales? Summary Saturn’s magnetosphere is a fascinang place, with many familiar yet not-so-familiar structures and processes. Cassini observaons have led to a credible framework for understanding it, but there are MANY open quesons to answer before we achieve anything near the level of understanding we have of the Earth’s magnetosphere.

Comparave magnetospheric studies enable us to expand our understanding and appreciaon of the many wonderful ways the same physical processes can combine with diﬀerent relave importance to produce unique systems.