Universal Physical Processes in the Solar System II
Presiding: L Strachan, Harvard-Smithsonian Center for Astrophysics; E Moebius, Department of Physics and Institute for the Study of Earth, Oceans and Space, University of New Hampshire
SH54A-01 INVITED 15:30h
Shocks as a Universal Process
Shock waves are ubiquitous in low density flows in astrophysics. In most cases the shocks are collisionless, though in many cases the observable consequences of the shock occur far downstream after collisions have erased the signatures of collisionless processes. This talk will concentrate on a one aspect of shock wave physics addressed by observations within the heliosphere and magnetosphere, and by observations of shock waves in supernova remnants: How is the energy dissipated in a shock partitioned among thermal and non-thermal energies of different particle species and the magnetic field.
SH54A-02 15:45h
Energetic Particle Effects Associated With Reconnection in the Solar Wind
Reconnection is often invoked, for example in the contexts of the solar atmosphere and the geomagnetic tail, as a means of accelerating a small number of particles to energies well above 40 keV. The acceleration presumably results either from particle displacement relative to the electric field that must be present along the so-called reconnection line where reconnection occurs or from particle encounters with slow mode shocks within reconnection exhausts. We have recently obtained direct evidence for local, quasi-stationary, magnetic reconnection in the solar wind using plasma and magnetic field measurements obtained by the Advanced Composition Explorer, ACE. The prime evidence consists of intervals of accelerated or decelerated plasma flow observed within magnetic field reversal regions that we interpret as encounters with (generic) Petschek-type reconnection exhausts that are bounded by Alfven waves. We have made a preliminary examination of energetic particle observations obtained by ACE for the first 6 reconnection exhausts identified. We find evidence only for relatively minor modulations in the flux of greater than 40 keV electrons in association with these events (mostly small decreases in particle intensity that we suspect are associated with topological changes in the magnetic field), and no evidence for any substantial electron acceleration within the exhausts or the surrounding separatrix layers. This result contrasts strongly with interpretations given to energetic electron observations in the geomagnetic tail presumably associated with reconnection. On the other hand, we find some evidence for proton acceleration to energies greater than 50 keV in the heart of one of the exhausts and relatively small modulations in energetic proton intensity in two others (probably associated with changes in magnetic topology). We believe these results have implications for particle acceleration models invoking reconnection in other contexts.
SH54A-03 INVITED 16:00h
Mass Loading
Mass-loading occurs when magnetized plasma and neutrals coexist in space and have different bulk velocities. When the neutrals are ionized, they accelerate to the plasma speed and gain a thermal speed equal to the component of the speed difference perpendicular to the magnetic field. The energy for this acceleration may come from the bulk flow in the case of stellar winds or from the rotation of a magnetized body. We discuss several situations where mass-loading is important. In planetary magnetospheres, the transfer of energy from the planetary rotation to the plasma via the pickup process powers magnetospheric processes such as aurora and radiative emission. The solar wind is slowed significantly by pickup ions and shocks and other solar wind structures are mediated by these hot ions. In the astrophysical context, mass-loading affects shocked subsonic pulsar winds, may lead to shells of high-temperature material, and may through interaction with clumps of neutrals produce non-spherical structure. We compare and contrast mass-loading processes in these different settings.
SH54A-04 16:15h
Auroral Particle Acceleration by Kinetic Alfvén Waves
The auroral acceleration region is perhaps the most accessible and well-studied particle acceleration region in the solar system, and provides an example of particle acceleration in a strongly magnetized plasma that may give insight into other planetary magnetospheres, the solar corona and pulsar magnetospheres. It is now well accepted that in addition to quasi-static acceleration of auroral particles in parallel electric fields, some auroral particle signatures are clearly associated with field-aligned acceleration that can be attributed to kinetic Alfvén waves. Previous work has led to a non-local kinetic theory of Alfvén waves that takes into account the strong gradients in the Alfvén speed above the ionosphere on auroral field lines. However, this work has involved an equilibrium with no static parallel electric field. On the other hand, observations indicate that inverted-V signatures generally associated with static fields and field-aligned acceleration in Alfvén waves coexist on the same field lines. The kinetic theory of Alfvén waves in the auroral zone has been extended to include such static fields, which lead to new particular populations (reflected ionospheric electrons, trapped electrons) not present in the zero-field equilibrium.
SH54A-05 INVITED 16:30h
Comparative Planetary Aurora: Earth, Jupiter and Saturn
Observations of Jupiter and Saturn's auroral emissions, their morphology, and time variations are now sufficiently advanced to begin some detailed comparisons with the Earth's aurora. Specifically, campaigns of Hubble Space Telescope (HST) UV imaging conducted as the Cassini spacecraft approached each planet have now been carried out, permitting direct comparisons with the local solar wind properties. While Jupiter has been much more thoroughly studied by HST than Saturn, the number of HST images of Saturn's aurora has multiplied by several times with the approach to and arrival of Cassini at Saturn. Where one used to think of the Earth's aurora as driven by the solar wind, Jupiter's aurora as driven by internal sources of plasma and rapid rotation, and Saturn as being intermediate between the two, it is now possible to determine directly how Saturn's aurora compare with the other planets. It has been found that Saturn's aurora exhibit characteristics reminiscent of both the Earth's and Jupiter's, and also show features that have not been observed at the other planets. The observations of Jupiter and Saturn will be reviewed, with a summary of current thinking on the interpretation of the various emission processes, what they tell us about each planet's magnetospheric processes, and how these relate to the Earth.
SH54A-06 16:45h
Suggested Evidence for Coherent Synchrotron Microwaves by Microbunching Instability in a Solar Burst
The presence of solar flare submm-wave spectral emission component peaking in the THz range simultaneous to the well-known microwaves component, has been recently reported. It was particularly well observed in the November 4, 2003 large flare by the Solar Submillimeter-wave Telescope, El Leoncito, Argentina Andes, bringing serious difficulties for interpretation using existing models. The largest measured fluxes were of about 12,000 and 21,000 solar flux units (sfu) at 212 and 405 GHz, respectively. The source remained remarkably stable in space (within 15 arcseconds). Burst size smaller than about 10 arcseconds was evaluated, which imply in brightness temperatures larger than about 9-2 106 K, at 200 and 400 GHz, indicating the emission from an optically thick non-thermal source. Intense microwaves were observed by the Owens Valley Solar Array, CA, USA, in the range 2-20 GHz, exhibiting spectral maxima in the range 15-19 GHz for different time structures. Peak flux of about 45,000 sfu determined at about 18 GHz was considerably larger than the THz component extrapolated down to 18 GHz (being of the order of 1,000 sfu). The fluxes became equal for the microwave and THz components at about 80-100 GHz. Spectral peaks at different frequencies may arise from different populations electrons accelerated, although there are no evidence to support this assumption. However the simultaneous presence of a double spectral component, at microwaves and at submillimeter waves, added to characteristics of the bursting features at both ranges, have striking similarities to coherent synchrotron radiation (CSR) produced at microwaves by high energy electron beams ( E > 100 MeV) in accelerators undergoing microbunching instabilities. This phenomenon is well known, although the super-radiant cm-mm wavelength CSR bursting mechanism is not fully understood. The THz solar flare component might due to incoherent synchrotron emission, produced by bunches of electrons, represented by the impulsive burst time structures. CSR is produced when the electron beam is submitted to a density modulation on the scale of the order or smaller than the emitting wavelength, originating microbunches, emitting power proportional to the incoherent component power squared. In laboratory accelerators this is accomplished submitting the electron beam to repetitive magnetic undulations, modifying the bunch shape, causing the microbunching instability. Models predict the creation of microbunching by the interaction of electron beams with its own synchrotron radiation, which amplifies itself via coherent emission. In the fine magnetic structures of solar active regions it is not difficult to conceive the presence of magnetic chicanes traversed by the accelerated bunches of electrons. Only a small fraction of the total number of particles accelerated undergoing microbunch instability would be needed to produce a substantial broadband coherent emission enhancement in the cm-mm wavelength range. The solar burst CSR reduces for smaller wavelengths, where the condition for instability vanishes. Rapid periodic bursts, with amplitude and frequency of occurrence increasing with the number of electrons in the beam (for larger intensities) are observed in accelerators and in solar bursts. They might be signatures of microbunching instabilities enhanced by wave-particle interaction. This suggested interpretation of solar flare emission require the complete knowledge of the spectrum into the THz range, which usually is not known for most of bursts.