1. Plasma injection at the Earth and Saturn Abi Rymer (JHU-APL) Misha Sitnov (JHU-APL) Tom Hill (Rice University) Sasha Ukhorskhiy (JHU-APL) Barry Mauk (JHU-APL) Andrew Coates (MSSL-UCL) and Duane Pontius (Birmingham-Southern College) Polar Gateways Barrow, Alaska January, 2008

2. Abi Rymer Polar Gateways, 2008 Introduction  It is thought that small scale plasma injection might explain 80% of the mass, energy and momentum transport at the Earth, the small scale injections are commonly referred to as “bursty bulk flows” (BBFs)  Saturn’s magnetosphere has a large scale cold outflowing plasma component with small scale plasma injections superposed.  Our presentation will meander toward discussion of if BBFs at the Earth and plasma injection at Saturn are the same and how observations at Saturn might help to inform plasma processes at the Earth.

3. Abi Rymer Polar Gateways, 2008 Size:~7 x 4 m Weight at launch:5574 kg Number of instruments: Orbiter:12 Huygens6 Cost at launch: ~$3.5 billion The Cassini Spacecraft (Launched October 1997) Photo courtesy of JPL/ NASA CAPS

4. Abi Rymer Polar Gateways, 2008 Earth’s Magnetosphere

5. Abi Rymer Polar Gateways, 2008 Magnetosheath RERE 2010-10-20 Closest Approach UT -30-40-50 Plasmasphere Plasmasheet Tail lobe and MP crossings Earth Flyby August 18 1999 – introduction to the data #1 Photoelectrons Modulation due to the CAPS actuator

6. Abi Rymer Polar Gateways, 2008 photon, E h Photoelectrons with E electron > E  escape into space Photoelectrons with E electron < E  return to the spacecraft and can be measured in the low electron sensor Photoelectric effect gives Cassini a positive potential,  ‘real’ electrons are accelerated and are measured to have energy, E measured = E ’real’ electron + E  Photoelectron production

7. Abi Rymer Polar Gateways, 2008 …then 5 more years in space…

8. Abi Rymer Polar Gateways, 2008 Saturn’s Magnetosphere Magnetotail Magnetopause Cusps Plasma torus due to Titan Rings Cooler and less dense solar wind Aligned spin and dipole axes

9. Abi Rymer Polar Gateways, 2008 Saturn Arrival June 2004 – introduction to the data #2 Eclipse Dayside MSph Enter plasmasphere Dispersion features observed thoughout plasmasphere Radiation belts Evidence of plasmasheet dynamics Ions appear to be slightly faster than corotation velocity Two electron populations (both ~ Maxwellian)

10. Abi Rymer Polar Gateways, 2008 Birkeland current and aurora aka:  Field aligned current  Magnetic flux rope  Auroral electrojet Downward currents on morning side of the aurora Upward currents on the evening side Currents closes through the ionosphere In space current closes in the solar wind

11. Abi Rymer Polar Gateways, 2008 Birkeland currents were predicted by Kristian Birkeland based on three polar expeditions between 1896 and 1903. His 1908 book detailing their results and adventures has been made available online by the American Library. It is available at: www.archive.org/details/norwegianaurorap0 1chririch "The [first] expedition has not been described before, because it was such a sad adventure; but now that time has drawn a veil of melancholy oblivion over the misfortune that befell us, I will briefly relate some of our experiences." Birkeland, 1908

12. Abi Rymer Polar Gateways, 2008 “No one who has not tried it can imagine what it is to be out in such weather. Knudsen, for instance, once had one hand frost-bitten in the few minutes he was out to take a reading” Birkeland, 1908

13. Abi Rymer Polar Gateways, 2008 First direct measurements of Birkeland currents  The first direct measurements of Birkeland currents were made ~60 years after Birkelands predictions by an APL weather satellite (1968 3c).  The satellite used a bar magnet to maintain its course.  It was observed that the magnet began to oscillate at some locations.  These locations were eventually logged and collated.  It was recognised that the locations coincided with typical auroral locations – and so in situ measurements of the aurora by in-situ satellites began.

14. Abi Rymer Polar Gateways, 2008 Birkeland currents and plasma injection Plasma injection, aka: Bubble Transient fast flow Solitary electromagnetic pulse Bursty bulk flow Travelling compression region Flux transfer event Magnetic flux rope Rather than closing in the solar wind as with the auroral Birkeland currents, current associated with plasma injection closes in the plasma sheet. Footprint of plasma injection in the planetary ionosphere +++ - - - v bubble

15. Abi Rymer Polar Gateways, 2008 Pressure crisis Steady sunward convection consistent with the adiabatic condition PV 5/3 =constant is not possible in the tail-like magnetic configuration of the Earth. Yet overwhelming evidence exists that largescale sunward convection exists. (first recognised by Erickson and Wolf [1980] and referred to as a “mild dilemma” and “the pressure balance inconsistency” it has since been known as the “pressure crisis” or even “pressure catastrophe”

16. Abi Rymer Polar Gateways, 2008 Possible resolution: injection of plasma bubbles  First proposed by Pontius and Hill [1989] to explain Voyager observations at Jupiter.  Introduced as a mechanism applicable to the Earth by Pontius and Wolf [1990].  Observed to be a prolific feature of Saturn’s magnetosphere [e.g. Hill et al., 2005] after Pontius and Wolf, 1990 E0E0 Angelopoulos et al., 1992 and Baumjohann et al. 1990 showed that at the Earth the apparent steady sunward convection of the plasma sheet could, in reality, be a superposition of bursty high speed flows with intermittent intervals of near stagnant plasma and that small bubbles could accomplish earthward mass, energy and flux transport comparable with that expected from “stead state” convection.

17. Abi Rymer Polar Gateways, 2008 Small scale plasma injection is a vital aspect of large scale magnetospheric flow, but it is relatively difficult to observe at the Earth. The ratio of rotation speed to drift speed at Saturn make it an ideal place to observe plasma injection as explained by Tom Hill soon after Cassini arrival at Saturn: EarthJupiterSaturn Corotation velocity, kms -1 44135 Gradient drift velocity, kms -1 60.0451 e.g. 1 keV electron at L = 7 For a given energy E and a given L value. Plasma injection at the outer planets Plasma drift speed in a dipole field scales as:

18. Abi Rymer Polar Gateways, 2008 e-e- p+p+ magnetopause dusk SUN dawn Injection at midnight, t=0 (say) Saturn North Gradient and curvature drifts Dipole opposite to the Earth Corotating plasma Plasma circulation at Saturn Cassini B As drifted plasma corotates over Cassini, Cassini will measure first the hottest protons (which drift with corotation) then the coolest protons (which have drifted the least far) then the coolest electrons followed finally by the hottest electrons which have drifted the furthest in the direction opposite to corotation.

19. Abi Rymer Polar Gateways, 2008 Plasma corotation energies If we assume that charge exchange/photo-ionisation results in the production of one ion and one electron with zero energy each then they will experience the planetary field and accelerate to the local ion and electron speed respectively. Plasma corotation energies If we assume that charge exchange/photo-ionisation results in the production of one ion and one electron with zero energy each then they will experience the planetary field and accelerate to the local ion and electron speed respectively. Oxygen Proton Electron

20. Abi Rymer Polar Gateways, 2008 penetrating particles Lines of constant first adiabatic invariant,  Proton corotation energy Cassini electron observations at Saturn Rymer et al., 2007 We estimate it would take ~150 hours (15 Saturn rotations) for the electrons to equilibrate to the proton corotation energy. We therefore assume that the outflow of plasma is slow and that magnetic flux is returned via plasma injection - as proposed by Pontius and Hill [1989] for Jupiter.

21. Abi Rymer Polar Gateways, 2008 Electrons Ions Example of Electron and Ion spectra: 28 October 2004 Saturn’s magnetosphere is positively fizzing with plasma injection events…

22. Abi Rymer Polar Gateways, 2008 Hill et al., 2005

23. Abi Rymer Polar Gateways, 2008 Hill et al., 2005 The bubbles are not obviously organised by local time or planetary longitude

24. Abi Rymer Polar Gateways, 2008 e-e- Electron pitch angles – a powerful diagnostic of plasma production and transport.

25. Abi Rymer Polar Gateways, 2008 Inward transport of an isotropic distribution leads to a pancake distribution. Outward transport of an isotropic distribution goes field aligned Outward transport of a pancake distribution can go butterfly: depends on distance travelled and steepness of original distribution. Evolution of pitch angle distributions

26. Abi Rymer Polar Gateways, 2008 Observation of a young plasma bubble at Saturn Rymer et al. [2008]

27. Abi Rymer Polar Gateways, 2008 Burch et al., Nature 2007. Interpreted as being due to transport out to L=8 from Dione (L=6.3) Interpreted as being due to transport out to L=8 from Tethys (L=4.9) First butterfly electron observations in the warm electron component

28. Abi Rymer Polar Gateways, 2008 penetrating particles Cassini electron observations at Saturn Rymer et al., 2007 showed that the PSD at Dione and Tethys is insufficient for the butterfly PADs observed at ~8 Rs to originate there. Tethys Dione Electron PADs observed here Under outward conservative transport these electron PADs started here.

29. Abi Rymer Polar Gateways, 2008 Fit to butterfly pitch angle distribution for loss free transport from Enceladus L-shell Can vary the values of m and n in to optimise the fit.

30. Abi Rymer Polar Gateways, 2008 Plasma production injection and drift and circulation at Saturn. Rymer et al., 2008 Rymer et al., 2008 proposed an alternative explanation wherein the butterlfy PADs evolve from magnetospheric circulation.

31. Abi Rymer Polar Gateways, 2008 Observation of a young plasma bubble at Saturn Rymer et al. [2008] Cold plasma formed from Saturn’s icy moons, rings and neutral cloud The pitch angle distribution of the injected plasma is consistent with injection from L~11 The drift indicates that the injection is ~16 minutes old.

32. Abi Rymer Polar Gateways, 2008 Speed of injection/BBF at the Earth, Jupiter and Saturn Saturn estimate 1: Age ~ 16 minutes Distance travelled ~ 4 R s  Speed ~ 260 kms -1 Saturn estimate 2: Ukhorskiy et al., [2007] estimate a maximum floating speed of the bubble ~ 200 kms -1 where: B 0 =0.21 G, L=7, l y =3.4  10-2 R s and T e =1keV Jupiter estimate: Thorne et al., [1997] estimate a bubble observed near the Io torus had a speed of ~100 kms -1 Earth measurements: Earthward flows 200-600 km/s [e.g. Zesta et al., 2004]

33. Abi Rymer Polar Gateways, 2008 Sergeev et al., 2004 Link between plasma bubble and the ionosphere At the Earth it is believed that the plasma bubbles are elongated structures with footprints that map to the auroral zone.

34. Abi Rymer Polar Gateways, 2008 The bubble moves due to a relative build up of charge causing planetward ExB drift. + - Ukhorskiy et al., AGU 2007 planet Density depleted bubble Consider a bubble depleted in plasma compared to its surroundings. Protons drift onto one side of the bubble and electrons drift onto the other side. This creates an electric field, E, across the bubble and the bubble ExB drifts planetward. The electric field across the bubble is strong enough to generate field-aligned Birkeland currents, J װ. Current closure through the ionosphere leads to collapse of the bubble.

35. Abi Rymer Polar Gateways, 2008 What we know about bubbles They are generally 1-2 planetary radii in azimuthal extent They propagate quickly (a few hundred km/s) They contain reduced density compared to surroundings Contained plasma is hotter than the surroundings (especially at the outer planets) Contained magnetic field is more dipolar. It is estimated that bubbles could be responsible for as much as 80% of the mass, energy and momentum transport in Earth’s plasma sheet. The fast drift speed and slow corotation speed at the Earth make it difficult to unambiguously link the ion and electron drift from a single injection. At the outer planets, especially at Saturn, the fundamental plasma timescales and abundance of plasma injection make the outer planets an ideal laboratory for studying this phenomenon.

36. Abi Rymer Polar Gateways, 2008 Summary and musings We are increasingly confident that plasma injection at Saturn and BBFs or bubbles at the Earth are related phenomena. Both return dipole field to the inner magnetosphere and apparently play an important role in largescale plasma convection. There exist some key differences and mysteries which should be resolved. Injections at Saturn happen closer (5-12 Rs) to the planet than those at the Earth – they therefore map to lower latitudes in the ionosphere – how does the conductivity of the ionosphere affect the progress of the bubble? At Saturn we observe the pressure inside the injection to be reduced, models usually presume the BBFs at the Earth to be pressure depletions. What is the origin of the plasma depleted region?

37. Abi Rymer Polar Gateways, 2008 Summary, musings and future work Dawn Dusk Midnight Noon 5 510 Gas Cloud ~ 24 Rs Saturn’s magnetosphere is positively fizzing with plasma injections. It seems likely that these injections play a significant part in plasma transport at Saturn. Very high energy injections observed by MIMI are apparently superposed on this fizzy regime – the role of the very high energy injections is as yet poorly understood.

38. Abi Rymer Polar Gateways, 2008 END

39. Abi Rymer Polar Gateways, 2008 Transient Petschek-type reconnection [Semenov et al., 2005] Secondary islands and/or BBF flux ropes [Slavin et al., 2003] M. Shay simulations in [Ohtani et al., 2004] Geotail fast flow statistics [Ohtani et al., 2004] Earthward (818 events) Tailward (290 events) Earthward-moving flux ropes Consistent with original BBF observations [Angelopoulos et al., 1992] and distinct from reconnection, the plasma sheet retains its integrity

40. Abi Rymer Polar Gateways, 2008 Roussos et al., 2007

41. Abi Rymer Polar Gateways, 2008 Pressure crisis EARTH SATURN P  V -5/3 P  V -2/3 Black diamonds: Saturn pressure derived from thermal and energetic particle measurements Solid black line: Saturn pressure derived from wave measurements of electron density and energetic particle measurements

42. Abi Rymer Polar Gateways, 2008 Plasma injection