SH41B-01 INVITED 08:00h
Voyager in the Vicinity of the Termination Shock: Observations and Questions
For most of the last two and a half years, Voyager 1 has observed enhanced intensities of energetic ions streaming along the magnetic field, suggesting that the source is a durable rather than transient feature of this region of the heliosphere beyond 85 AU. If Voyager 1 is in the solar wind, the beaming requires a non-spherical termination shock, but is the asymmetry of the shock large enough and in the right location? The small average radial anisotropy requires a large radial gradient, but is that consistent with lack of energy dependent modulation at low energies? If Voyager 1 is in the heliosheath, the outward beaming direction and small radial anisotropy are consistent, but are strong, persistent beams expected in the turbulent heliosheath, and what is the mechanism for avoiding the expected compression of the magnetic field? Whether or not Voyager 1 crossed the shock, there appear to be two distinct sources of accelerated interstellar pickup ions. Why does the nearby source region connected directly to Voyager 1 accelerate He ions to less that ~10 MeV/nucleon, while the more distant source region accelerates He ions to much higher energies? These and other aspects of the observations are revealing the unexpected nature of the termination shock and particle acceleration.
SH41B-02 08:15h
Continuing the Search for the Heliosheath by Voyager 1
The Voyager 1 spacecraft, V1, has measured the Heliospheric Magnetic Field (HMF) since launch in 1977. This covers more than a full 22 years solar magnetic cycle. V1 data shows that the HMF, on average, is well described by Parker's archimedean spiral structure out to 95 AU and 35 degrees north heliographic latitude if due account is made for time variations of the source HMF field strength and solar wind velocity. The V1 magnetic field observations do not provide any evidence for entry into or exit from a subsonic SW region such as the heliosheath is expected to be nor any crossings of the heliosphere-heliosheath interface, the Termination Shock (TS), as has been reported at 85 AU by Krimigis et al [2003]. Merged Interaction Regions or transient flows are identified by increased fields and associated decreases in the flux of >70 MeV/nuc cosmic rays which are then followed by a recovery of the CR flux. This CR-B relationship has been identified and studied since 1982 when V1 was at 11 AU. A study of magnetic field fluctuations, up to 0.26 Hz, in 2001-2004, shows periods of increases of the daily mean standard deviation, which is a direct measure of the energy in the magnetic field variations. We are unable to identify any coherent correlation of these periods with the reported TS and heliosheath crossings.
SH41B-03 08:30h
Low-energy Ions and Electrons Observed near the Termination Shock by Voyager 1 in 2004 (93 AU)
From about 2002.6 to at least 2004.7, the Low Energy Charged Particle (LECP) instrument on Voyager 1 (85-93 AU, N34$^\circ$) has observed remarkable variations in low-energy ion (40 keV to 30 MeV) and electron (26 keV to 1.5 MeV) intensities, ion energy spectra, and ion angular distributions. Comparable phenomena are not observed at Voyager 2 (66-74 AU, S25$^\circ$). This report will focus on Voyager 1 LECP data acquired during the period 2004.1 to $\sim$2004.9. During 2004.1-2004.7 Voyager 1 observed a second major event (E2) associated with the termination shock (TS), the first major event (E1) having been observed during 2002.6-2003.1 [Krimigis et al., Nature, 426, 2003]. Voyager 1 also observed TS-associated energetic particle activity during the intervening period $\sim$2003.1-2004.1; but, intensities at ion energies $<$1 MeV were relatively low and sporadic. In February 2004 ion intensities at Voyager 1 increased to levels that continue to exceed those reached during E1. Ion intensities include superposed small-scale ($\sim$few hours) and quasi-recurrent ($\sim$13 days) variations, with the larger of these variations exhibiting little or no velocity dispersion over energies from 40 keV (speed $\sim$2 AU/day) to 30 MeV (speed $\sim$36 AU/day). Significant intensity increases of electrons with energies from at least 26 keV to 1.5 MeV (speed $\sim$160 AU/day) occurred during 2004.1-2004.2 and 2004.55 to at least 2004.7. As with the ions, these electron increases include smaller-scale, bursty ($\sim$few hours to $\sim$day) components, with the largest peaks time-coincident (non-dispersive) with those of the ions. Ion anisotropies during E2 are mainly unidirectional outward (away from the sun) along the near-azimuthal direction, very similar to those observed during E1. By contrast, electron intensities observed during the two large increases in 2004 (and also during E1) have nearly isotropic angular distributions. During E1, the energy spectrum of ions 40-4000 keV was well represented by a single power-law in energy with spectral index s$\sim$1.5. Thus far during E2 the spectral slope has shown more energy dependence, being notably harder at lower energy, with the index s$\sim$1 at 40 keV increasing with energy to s$\sim$1.5 at 4 MeV.
SH41B-04 08:45h
Highest Intensity of Low-Energy Particles Observed in Outer Heliosphere as Voyager 1 Approaches 94 AU
Beginning in mid-2002, the 2-3 MeV H intensity measured by the Cosmic Ray experiment on Voyager 1 (V1) (at 85.3 AU and 33.8$^{\circ}$N) increased by a factor of $\sim$100 for $\sim$7 months, subsided for approximately 6 months, and then increased again to the highest intensities yet seen in the outer heliosphere, higher than have been recorded since solar energetic particles were observed at $\sim$15 AU in 1983. This type of activity was not observed at Voyager 2, some 18 AU closer to the Sun than is V1. The longevity of the enhanced intensities at V1 suggests that this is a durable feature of the outer heliosphere and not a transient phenomenon. The particles are thought to be coming from the termination shock of the solar wind. The anisotropy of the 3.3-7.8 MeV H particles is primarily outward from the Sun along the nominal interplanetary magnetic field line. Presumably, the field line along which the particles are streaming towards V1 intersects with the termination shock back along the direction to the Sun. We will report the latest observations of the energy spectra and anisotropies of these particles as Voyager 1 approaches 94 AU. This work was supported by NASA under contract NAS7-03001.
SH41B-05 09:00h
Voyager-1 Observations of MeV Ions and Electrons in the Vicinity of the Heliospheric Termination Shock
Beginning in 2002.5 at a heliospheric distance of some 85AU, the CRS (Cosmic Ray Subsystem) experiment on Voyager-1 observed two large increases in MeV ions and electrons accompanied by more modest increases in Galactic and Anomaloous cosmic rays (GCR and ACRs). The first event persisted for some 6.5 months and was terminated by the passage of a large outward moving interplanetary disturbance. The second event, which began in 2003.62, is still in progress more than a year later. We have interpreted these events as the expected energetic particle precursors as Voyager-1 approaches the heliospheric termination shock and we have termed them TSP (Termination Shock Particle) increases. There are small, but significant, differences between TSP 1 and 2 both in the time history of the ions and electrons and in the short term (13 - 52 day) variations. In addition the GCR and ACR flux levels are somewhat smaller for the 2nd event. These TSP events have not yet been observed at Voyager-2, which is at a heliocentric distance some 18AU less than Voyager-1. However, at Voyager-2 there is a remarkable series of solar/interplanetary energetic particle events associated with specific episodes of solar activity. The effects of these interplanetary transients is clearly evident in the time history of the Voyager-1 TSPs, thus providing another means of probing this new region of space.
SH41B-06 09:15h
Uncertainties of Solar Wind Speed Determination Using Voyager Energetic Particle Anisotropy Measurements at 85 AU
Recent measurements from the LECP experiments on Voyager 1 show that low-energy particles have a very anisotropic pitch-angle distribution in the event of late 2002. In fact, the anisotropy is often so large that it almost appears as a beam of particles streaming along magnetic field lines. Given the large field-aligned pitch angle anisotropy, the Compton-Getting effect needs to be considered very carefully because an uncertainty of magnetic field direction can seriously affect the analysis. In this paper, we will present a calculation of Compton-Getting anisotropy in the presence of field-aligned beams using Lorentz transform of energetic particles with a power law energy spectrum. Our calculation shows that the anisotropy measurements from Voyager 1 can qualitatively fit either a model having a moderate solar wind speed with a Parker spiral magnetic field or a model with an approximately zero solar wind speed in a non-Parker field. The solar wind speed determination crucially depends on three assumptions (1) magnetic field direction, (2) cross-field diffusion anisotropy and (3) background from response to isotropic cosmic rays in the low-energy channels of the instrument. We will discuss how large uncertainties these assumptions may bring to the determination of solar wind speed.
SH41B-07 INVITED 09:30h
Interaction of the solar wind and the interstellar medium: global structure
The global structure of the heliosphere is determined by a combination of the interstellar plasma pressure, the movement of the Sun through the ISM, the interstellar magnetic field, and the interstellar neutrals, and of course, the supersonic solar wind itself. It can be affected by variability in the solar wind (solar cycle dependent) and by variability in the interstellar environment. The past 5 years has seen the development of sophisticated 2D steady-state models of the heliosphere that incorporate much of the basic physics thought to determine structure. More recent developments have begun to focus on the inclusion of time dependence, fully 3D structure (which implies the inclusion of the interplanetary and interstellar magnetic fields), and more sophisticated physical processes such as galactic and anomalous cosmic rays, turbulence excited by pickup ions, and so forth. We will review the modeling efforts of the past 5 years, relate this to observational tests, both current and proposed, and discuss briefly new directions and trends.
SH41B-08 09:45h
A Three-Dimensional MHD Model of the Solar Wind with Pickup Protons
We present initial results from a three-dimensional steady-state solar wind model that accounts for the effects of pickup protons in the distant heliosphere. Inside 1~AU, we employ the model developed earlier [{\it Usmanov and Goldstein}, 2003] that consists of two regions: region~I (1--20 $R_\odot$) where a steady-state solution is obtained using time-relaxation, and a region~II (20 $R_\odot$ -- 1~AU) where the solution is constructed by integrating the governing MHD equations along radius. In the present work, we modified that model to include a region~III that extends beyond 1 AU where we consider a population of interstellar pickup protons and its interaction with the solar wind protons. Following {\it Isenberg} [1986] and {\it Whang} [1998], we take into account both ionization and charge exchange for the production of pickup protons and write down the energy equations for the pickup and solar wind protons, independently. For the interstellar neutral hydrogen, we presently use a zero order approximation that assumes that the neutral hydrogen is homogeneously distributed throughout the heliosphere. We compute the global structure of the solar wind from the coronal base to the outer heliosphere and present a study of the effects of the pickup protons on solar wind structure.