Particle energization at Saturn C. Paranicas 1, E. Roussos 2, P. Kollmann 2, N. Krupp 2, J. F. Carbary 1, D. G. Mitchell 1, S. M. Krimigis 1, B. H. Mauk 1, and G. Clark 3 (1) Johns Hopkins University/APL (2) Max Planck Inst. für Sonnensystemforschung (3) SouthWest Research Institute Magnetospheres of the Outer Planets Boston, Massachusetts July 12, 2011

Overview It is hard to energize charged particles from the cold plasma energies into the MeV One mechanism commonly considered is inward radial transport and Fermi/betatron acceleration At Jupiter, whistler waves were proposed to raise particle energies (e.g., Horne et al. 2008) At Saturn, energization by inward transport alone is complicated by the dust/cold neutral torus Energetic ions are lost from the system as ENAs and carry away the energy; electrons tend to be cooled and scattered So why does Saturn have radiation belts?

Protons The main source of protons in the MeV energy range at Saturn is likely to be crand (cosmic ray albedo neutron decay), e.g., Cooper 1983 This is consistent with features such as the shape of the radiation belts (intensity by radius) near the inner moons and a separate peak in the proton energy spectrum Also some trapped fluxes correlate well with solar cycle phase – during solar max, GCR’s have a harder time penetrating the heliosphere (e.g., Lario and Pick 2008), crand’s source is weaker, and fewer MeV protons are observed in the belts Part of the distribution of ions by energy is heavily influenced by the neutral gas torus. In these cases, such as protons below ~100 keV, the losses significantly dominate sources and transport. It is also therefore less likely these are the seed population for MeV protons

Correlation between trapped proton belts and solar cycle This plot from Roussos et al. (2011) suggests that the ability of GCR’s to reach Saturn’s distance is correlated with the trapped belt protons. Alternatively, but less likely, is that during active periods on the sun and high solar wind speeds, the magnetosphere is so heavily perturbed that it is harder for high fluxes to be sustained. Sunspot number peaked ≈2001 and decreased to a min ≈2009.

These are six-year average proton energy spectra (Paranicas et al. 2011, PSS) The source of this part of the energy spectrum is harder to understand These keV protons at Tethys, Dione, and Rhea, are essentially absent inward of Tethys, except within injections The crand peak near Mimas’s orbit

Even though we do not know the source of MeV protons, this figure from Bishop (1996), showing the cross- section of stripping rising above the CE one near 50 keV, suggests that however these protons are created, their fluxes can be maintained longer in the neutral torus than lower energy protons

Summary energetic p + energization Except inside the D Ring, we don’t observe high fluxes of keV protons inward of Tethys – so no need to identify the source The protons inward of the D Ring have likely undergone a CE/stripping process (Krimigis et al. 2005) Above about 1 MeV, a crand source fits with solar cycle variation Between MeV, we don’t know the source, and CE and other processes may play a role (see poster at this meeting by Roussos/Kollmann)

Electron distributions Average fluxes of electrons at Saturn have different distributions by energy (next group of slides) High energy electrons are trapped in radiation belts, approximately inward of Tethys’s orbit Lower energy (tens to hundreds of keV) electrons have their highest fluxes near the edge of the dense neutral gas torus Plasma electrons have asymmetries in summary data How do these different energy distributions communicate with each other?

This is a mission averaged plot of 0.79 – 4.75 MeV electron intensities versus dipole L and local time from the MIMI/LEMMS sensor. This figure illustrates the confinement of the ~ MeV electron belt with peak fluxes inward of about Tethys’s orbital distance (Paranicas et al. 2011).

Many year averages of electron intensity (see, Paranicas et al – top panel; Carbary et al – lower 2 panels). See Thomsen et al. (this meeting) for the radial behavior of the most intense flux versus local time. Average Electron Fluxes Top: keV Middle: keV Bottom: keV

DeJong et al. (2011) found that there is a day/night asymmetry even in the plasma electrons with particles reaching lower L shells near midnight

Sources at each energy Do the electrons get energized from the plasma range through the keV into the MeV or are there other processes to consider? Kollmann et al. (2011), for example, argued that some hundreds of keV electrons would result from the crand process Our modeling of the injection process shows that large-scale distributions that cover a wide range of L shells do follow an energization process (see below), perhaps between the tens of keV to ~MeV range Missing pieces in the electron energization story: –Transport into the belt region for ~MeV energies –Energization from plasma to ~keV, is this understood?

Fitting the electron data with a model that conserves mu and J

In our model, a single electron injection (all L shells populated simultaneously but with a mu-J cutoff in energy) was launched about an hour before local midnight on day at 07:12 UTC inward to a distance of L=5.8. This figure shows how such an injection, about 2+ days old, would appear in a time-energy spectrogram using the actual Cassini orbit (see Paranicas et al. 2010).

Superposition of previous single injection and LEMMS data obtained on days to 118

Day 2010/289 electron data. These injections to high energy appear to involve populations that span a large range of radial distances. These tens of keV injections at low energy do not usually involve a wide range of L shells

As above, some older injections track/define the upper edge of the trapped particle population Changes in pitch angle occur here

Injections come from different distances, so it is not likely the aggregate will follow a single invariant curve. Curves of constant  (upper) and mu/J (lower) are drawn, mu/J seems to track the “edge” of the trapped distribution.  ~p 2 (L/B eq ) 2/3  /J

Injections In our injection modeling, we have found that we can reproduce features in the higher energy portion of the spectrogram using a simple model of a rapid inward injection and subsequent drift Our current modeling assumes the first two adiabatic invariants of motion are conserved and this seems to explain the observed behavior at the high energies Problems with “injecting” the radiation belts: injections to high energies seem to cutoff at around 4-5 R S, but we observe the ~MeV type belts inward of about 4.5 R S (note that we see some injections at lower energies -- tens of keV to about 100 keV -- even inward to 3 R S, so this helps) At the other end of the energy range, how do we create a reservoir of hot particles that can be then further accelerated into the MeV range?

From MIMI/CHEMS/Telescope-1 only. Here it does not appear that a separate flux tube is displacing the background particles (during this time, T-1 is detecting particles with pitch angles of about 45 o ). Remnants of a previous proton injection that are dispersed

Some discussion points Where there are energetic protons observed at Saturn, we have some understanding of their source in the MeV range, less so at 0.1 – 1 MeV in the inner magnetosphere For electrons up to about 1 MeV, can see energization with injections but harder to explain spatial distribution of belts without further interchange How is the seed population of the electrons that get injected up to 1 MeV created?

Extras

This is a representation electron energy spectrogram detected around periapsis, with more dispersed (older) injections at the highest energies and recent injections at the lowest energies (see, Paranicas et al. 2010)

Another piece of evidence is that electrons outside about 10 RS have field- aligned PADs, suggesting they are accelerated in the polar region (Carbary et al. 2011). If these particles are transported inward and conserve invariants, they are likely to become more pancake in the inner regions.