The Outer Limits of the Heliosphere I
Presiding: M E Hill, Department of Physics, University of Maryland at College Park; M Opher, Jet Propulsion Laboratory, California Institute of Technology
SH22A-01 INVITED 10:30h
Voyager in the Vicinity of the Termination Shock
A new low energy component was observed beginning in mid-2002 when Voyager 1 was at 85 AU, suggesting that the spacecraft was in the vicinity of the termination shock. These termination shock particles (TSPs) are present with increased intensity as Voyager 1 approaches 95 AU, indicating this is a durable feature of this region of the heliosphere and not likely the result of occasional transient events. These observations pose a number of questions concerning the spectral differences of TSPs and anomalous cosmic rays, an apparent average anisotropy direction offset from the average magnetic field, the inferred solar wind velocity and lack of magnetic compression, and a TSP source that is radially inward of Voyager 1. These questions and further observations continue to stimulate new analysis and interpretations. This work was supported by NASA under contract NAS7-03001.
SH22A-02 INVITED 10:45h
Energetic Ions and Electrons Observed near the Termination Shock during 2004-05
The Voyager 1 (V1) spacecraft has observed large variations in ion (40 keV to 30 MeV) and electron (26 keV to 1.5 MeV) intensities, ion energy spectra, and ion angular distributions since at least mid-2002. Comparable phenomena have been not observed at Voyager 2 (66-74 AU, S25°), indicating that V1 (85-94 AU, N34°) has been measuring particles associated with the heliospheric termination shock (TS). We present observations from the Low Energy Charged Particle (LECP) instrument on V1, focusing on data acquired during 2004.1 to ~2005.4. V1 observed a second major episode of sustained increases of energetic particle intensities beginning ~2004.1 and lasting until ~2004.78 (the first major episode occurred during ~2002.6-2003.1). Enhanced ion intensities include superposed small-scale (~few hours) and quasi-recurrent (~13 days) variations, with the larger of these variations exhibiting little or no velocity dispersion over energies from 40 keV to 30 MeV. Ion anisotropies are mainly unidirectional outward (away from the Sun) along the near-azimuthal direction. Large intensity increases of electrons with energies from at least 26 keV to 1.5 MeV occurred during 2004.1-2004.2 and 2004.6-2004.8. As with the ions, these electron increases include small-scale, bursty (~few hours to ~day) components, with the largest peaks time-coincident (non-dispersive) with those of the ions; but, unlike the ions, the electrons have nearly isotropic angular distributions. Particle intensities decreased abruptly on ~2004.78, remained low for roughly 65 days, increased rapidly on ~2004.96, and have remained high until at least 2005.1. A remarkable feature of this latest increase is that the intensities of ions from at least 40 keV to ~350 keV recovered to values well above those reached prior to the late-2004 drop (e.g., factors ~2-3 higher). We will summarize these results and also report on revised estimates of convection speeds that are based upon reanalysis of angular distributions of ions 40 keV to 20 MeV.
SH22A-03 11:00h
Nonextensivity of the Large-scale Magnetic Field Strength Fluctuations in the Solar Wind from 7 to 87 AU
The solar wind is a driven nonlinear non-equilibrium system, with large jumps and fluctuations in the temporal profiles of the magnetic field strength B. Relatively large clusters of strong fields ("Merged interactions", MIRs) form beyond 1 AU become prominent at 15 to 30 AU, and they decay slowly between 40 AU and at least 90 AU. We analyze the probability distribution functions (PDFs) of fluctuations of the B observed by Voyager 1 between 7 and 87 AU on scales from 1 to 128 days during each of four years (1980, 1991, 2001, and 2002). The 32 PDFs of the increments of B for all of these data can be described by a single function, the Tsallis distribution of nonextensive statistical mechanics. The tails of the PDFs are described by the nonextensitivity parameter q, which is scale dependent. The Tsallis distribution has a finite variance when q < 5/3 and a divergent variance when q > 5/3. For the 1980 and 1991 data (near 8 and 45 AU respectively), q > 5/3 at all scales, owing to large tails of the PDFs caused by large fluctuations and jumps in B(t). For the 2001 and 2002 data (between 80 and 87 AU), q < 5/3, and q approaches 1 (the PDF tends to a Gaussian) at large scales. The standard deviation of the increments of B increases linearly and quadratically for the 1980 and 1991 data, respectively (q > 5/3) and it increases asymptotically to a limiting value for 2001 and 2002 data (q < 5/3). The transition from q > 5/3 at < 45 AU to q < 5/3 at > 80 AU suggests the possibility of a phase transition in the outer heliosphere.
SH22A-04 11:15h
Large-Scale Structure of the Heliosphere: the Impact of Recent Observations
Recent observations from the Voyager 1 spacecraft as it approaches the heliospheric termination shock, and other data, have led to a more detailed confrontation between theory and observations of the interaction of the solar wind with the interstellar medium. The issues arising in this confrontation have provided new and important constraints on the structure of the heliosphere and on the basic physics of charged-particle acceleration at collisionless shocks propagating in a turbulent astrophysical plasma. Of particular relevance is the energetic particle streaming anisotropy, which has a sensitive dependence on the large and small scale structure of the termination shock and any lateral asymmetry. Constraints imposed by the observed anisotropies will be discussed. Factors affecting the large-scale structure and lateral asymmetry of the heliosphere, such as the very local interstellar magnetic field will also be discussed.
SH22A-05 11:30h
Recent Voyager Observations of Radio Emissions and Plasma Waves in the Outer Heliosphere
During the past year or so, two new types of observations have been obtained from the plasma wave instrument on the Voyager spacecraft. First, starting on February 12-13, 2004, again on August 29 to September 1, 2004, and several times since then, the plasma wave instrument on Voyager 1, which is now at about 95 AU (Astronomical Units), began detecting sporadic narrowband emissions at frequencies of a few hundred Hz. These emissions usually occur in only a single frequency channel, typically 178 or 311 Hz, and in one case 562 Hz, and have field strengths ranging up to about 1 microvolt per meter. The simple and most straightforward interpretation of these emissions is that they are electron plasma oscillations of the type that have been predicted to occur in the region upstream of the termination shock (upstream electron plasma oscillations are a common feature of planetary bow shocks). The frequencies are consistent with the electron plasma frequency computed from a 1/R2 extrapolation to 95 AU of the long term average plasma density measured by the plasma instrument on Voyager 2. If this interpretation is correct, these observations indicate that Voyager 1 is still upstream of the termination shock, but is now close enough for low energy electrons from the shock to occasionally reach the spacecraft, thereby exciting the electron plasma oscillations. Second, starting in early September 2004, another new heliospheric radio emission event was detected. This event is characterized by an emission frequency that gradually drifted upward from about 2.4 to 3.0 kHz over a period of several months, very similar to several earlier events of this type. Radio emission events of this type are thought to be produced when the shock or shocks associated with an intense period of solar activity interact with the heliosphere. The causative solar event is not yet certain, but one possibility is that it is related to the intense series of solar flares that occurred in late October and early November 2003.
SH22A-06 INVITED 11:45h
Diagnostics of the Local Interstellar Medium in the Inner Heliosphere - A Coordinated Approach
The neutral component of the local interstellar medium (LISM) flows through the inner heliosphere due to the relative motion of the Sun and the surrounding medium. Diagnosing the recognizable flow distribution and flow pattern using UV backscattering, pickup ion, and direct neutral gas observations for several species allows us to unravel the conditions in the LISM and filtering processes that occur in the heliospheric boundary region. The combination of several solar, heliospheric, and astrophysics spacecraft, serving as a "great heliospheric observatory" are providing a synoptic view of the heliosphere-LISM interaction. A recent coordinated analysis effort of several simultaneous observations hosted by the International Space Science Institute (ISSI) has produced a consolidated set of the physical parameters for He. Direct neutral gas observations provide the kinetic parameters and pickup ions return the density with the least uncertainties. Starting with the reasonable assumption that all species in the undisturbed LISM have the same physical parameters as He, which reaches the inner heliosphere without significant modification by the heliospheric interface, a similar effort has started for interstellar H, whose characteristics is substantially altered. Making use of pickup ion, solar wind slowdown, and UV scattering observations at various distances from the Sun and a full chain of modeling from the pristine LISM to the inner heliosphere, the interstellar H density, element abundances, and ionization fractions, as well as the effects of the heliospheric interface on flow vector and temperature can be deduced. As LISM oxygen is similarly affected by the heliospheric interface as H the expected addition of O observations with IBEX in the near future will provide the crucial neutral gas distributions for a filtered species of the LISM.