Polar Gateways Barrow, Alaska January, 2008

1. Plasma injection at the Earth and SaturnAbi 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. 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. Introduction Polar Gateways, 2008

3. The Cassini Spacecraft (Launched October 1997) Size: ~7 x 4 m Weight at launch: 5574 kg Number of instruments: Orbiter: 12 Huygens 6 Cost at launch: ~$3.5 billion CAPS Photo courtesy of JPL/ NASA Polar Gateways, 2008

4. Earth’s Magnetosphere Polar Gateways, 2008

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

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

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

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

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

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

11. 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/norwegianaurorap01chririch "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 Polar Gateways, 2008

12. “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 Polar Gateways, 2008

13. 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. First direct measurements of Birkeland currents Polar Gateways, 2008

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

15. Pressure crisis Steady sunward convection consistent with the adiabatic condition PV5/3=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” Polar Gateways, 2008

16. 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] Possible resolution: injection of plasma bubbles E0 after Pontius and Wolf, 1990 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. Polar Gateways, 2008

17. Plasma injection at the outer planets 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: For a given energy E and a given L value. Plasma drift speed in a dipole field scales as: e.g. 1 keV electron at L = 7 Polar Gateways, 2008

18. Plasma circulation at Saturn Dipole opposite to the Earth Gradient and curvature drifts SUN Corotating plasma North dawn p+ B Injection at midnight, t=0 (say) Saturn Saturn Cassini e- magnetopause dusk 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. Polar Gateways, 2008

19. Plasma corotation energies Plasma corotation energies Oxygen Proton Electron 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. 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. Polar Gateways, 2008

20. penetrating particles Cassini electron observations at Saturn Lines of constant first adiabatic invariant,  Proton corotation energy 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. Polar Gateways, 2008

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

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

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

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

25. Evolution of pitch angle distributions 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. Polar Gateways, 2008

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

27. First butterfly electron observations in the warm electron component 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) Burch et al., Nature 2007. Polar Gateways, 2008

28. penetrating particles Cassini electron observations at Saturn Dione Tethys Electron PADs observed here Under outward conservative transport these electron PADs started here. 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. Polar Gateways, 2008

29. 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. Polar Gateways, 2008

30. 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. Polar Gateways, 2008

31. Observation of a young plasma bubble at Saturn 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. Cold plasma formed from Saturn’s icy moons, rings and neutral cloud Rymer et al. [2008] Polar Gateways, 2008

32. Speed of injection/BBF at the Earth, Jupiter and Saturn • Saturn estimate 1: • Age ~ 16 minutes • Distance travelled ~ 4 Rs • Speed ~ 260 kms-1 Saturn estimate 2: Ukhorskiy et al., [2007] estimate a maximum floating speed of the bubble ~ 200 kms-1 where: B0=0.21 G, L=7, ly=3.410-2 Rs and Te=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] Polar Gateways, 2008

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

34. The bubble moves due to a relative build up of charge causing planetward ExB drift. 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, Jװ. Current closure through the ionosphere leads to collapse of the bubble. planet + - Density depleted bubble Ukhorskiy et al., AGU 2007 Polar Gateways, 2008

35. 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. Polar Gateways, 2008

36. 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? Polar Gateways, 2008

37. Summary, musings and future work Midnight 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. Dusk Dawn 10 10 5 5 Gas Cloud Noon ~ 24 Rs Polar Gateways, 2008

38. END Polar Gateways, 2008

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

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

41. Pressure crisis EARTH SATURN 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 PV-5/3 Polar Gateways, 2008

42. Plasma injection Polar Gateways, 2008