SH43A-01 INVITED
The Outer Heliosphere: Recent Voyager Observations and Theoretical Predictions
The two Voyager spacecraft are exploring the outer heliosphere at a time of decreasing solar activity, decreasing tilt of the heliospheric current sheet, and decreasing solar modulation of galactic cosmic rays. Voyager 1 is observing the effects of these changes on the magnetic field and on cosmic ray intensities deep in the northern heliosheath at >102 AU, while Voyager 2 is making similar observations of the solar wind and energetic particles just upstream of the termination shock at >82 AU in the south. Insights gained from recent observations and theoretical models will be discussed.
SH43A-02 INVITED
Energetic Particles at Voyager 1 in the Heliosheath and Voyager 2 in the Termination Foreshock
As of 2007.16, Voyager 1 (V1) is in the heliosheath (HSH), having moved radially outward 8 AU since crossing the termination shock (TS) at 94 AU in late 2004 (V1 is at 102 AU, N34° lat., 173° long.). Voyager 2 (V2) is upstream of the TS in its foreshock (TFS) region (V2 is at 82 AU, S27° lat., 216° long.). We discuss variations of intensities and angular distributions of ions >40 keV and electrons >30 keV measured by the LECP instruments on V1 and V2. Measurements made in the TFS region are characterized by large intensity variations, by factors >10, that occur over a range of time scales, from a few hours to tens of days. TFS ion angular distributions often show unidirectional, or beamlike, anisotropies consistent with propagation along the solar wind magnetic field from the source region (i.e., from the TS and the HSH) to the spacecraft under relatively weak-scattering conditions. Notable differences between the V1 and V2 data in the TFS include (1) ion beaming directions that are oppositely directed at the two spacecraft, which may result from the different locations of the two spacecraft relative to an asymmetric TS, and (2) the TFS ion energy spectrum at V2 extends down only to about 0.2 MeV, while that at V1 extends down to at least 0.04 MeV. Data from V1 taken in the HSH are characterized by high intensities of low-energy ions that remain fairly steady, varying by factors <2, and by anisotropies produced mainly by convection at the HSH plasma flow velocity. V1 was crossed by the TS as the shock moved radially inward in late 2004. As a result, conditions in the HSH at V1 were highly disturbed until about mid-2005. Thereafter, the plasma flow velocity V in the HSH, estimated by analysis of low-energy ion angular distributions, has shown mean speed ~ 70 km/s and direction angle ~ -35° to the radial, i.e., on average VR ~ 58 km/s and VT ~ -40 km/s (in RTN coordinates). Also, during the past six months, the energy spectrum at V1 of ions 0.04 to ~1 MeV shows a persistent softening with energy, with the spectral index decreasing smoothly from 1.5 at 0.04 MeV to 1.7 at ~1 MeV.
SH43A-03 INVITED
The High-Latitude Heliosphere at Solar Minimum: Recent Results From Ulysses
Launched in October 1990, Ulysses is the first spacecraft ever to fly over the poles of the Sun. From this unique perspective, it has literally changed the way we view the Sun's environment, the heliosphere. The primary goal of the Ulysses mission is to explore the heliosphere in four dimensions: three spatial dimensions, and time. The scientific instruments on board Ulysses that help accomplish this goal measure the solar wind, the heliospheric magnetic field, natural radio emission and plasma waves, energetic particles and cosmic rays, interplanetary and interstellar dust, neutral interstellar helium atoms, and cosmic gamma-ray bursts. Ulysses first visited the Sun's poles in 1994 (in the south) and 1995 (north), as the solar activity approached a minimum. The measurements showed the heliosphere to be dominated by high-speed solar wind flowing from the Sun's polar caps, filling a large fraction of the heliosphere. Surprises included the presence of energetic particles at high latitudes, and the lack of an increase in cosmic radiation towards the poles. Following its path around the Sun, Ulysses arrived over the southern and northern poles for a second time in November 2000 and October 2001, this time near solar maximum. The environment over the poles was very different from that found by Ulysses during the first high- latitude passes. Nevertheless, equally compelling evidence was found for large-scale latitudinal transport of energetic charged particles. As Ulysses returns to the polar regions for the third time, again near solar minimum, a number of important questions remain: will the polarity reversal of the Sun's magnetic field that occurred in 2000-01 lead to the expected changes in the latitudinal gradients of positively and negatively charged particles? Will the north-south asymmetry found in 1994-95 be present, and if so, will the offset still be to the south of the solar equator? In this paper, we will review the recent observations from Ulysses in the light of some of these questions.
SH43A-04
Effects of Upstream Turbulent Fluctuations on the Termination Shock and Heliosheath
The crossing of the solar-wind termination shock by the Voyager 1 spacecraft and the subsequent observations in the heliosheath have presented new challenges to our theoretical understanding of the physics of space plasmas and their interactions with energetic particles. Observations suggest strongly that the shock was moving rapidly inward at some 60-100 km/sec at the time of the termination shock crossing and that this motion continued for some time after the shock crossing. The termination-shock motions and flow patterns are likely to be significantly non-steady, and to vary with both time and space as a consequence of large-scale, pre-existing upstream turbulent fluctuations being convected across the shock. The shock motions, turbulence and non-radial flow downstream of the shock have important conseqences for the heliosheath magnetic field, and for the acceleration and transport of energetic particles. The consequences of these effects of large-scale upstream turbulence for the plasma, magnetic field and energetic particles in the inner heliosheath will be discussed.
SH43A-05
The Effect of Unsteady Solar Wind on the Termination Shock Asymmetries
Possible asymmetries of the solar wind (SW) termination shock due to the action of the interstellar magnetic field (ISMF) are summarized using numerical simulations based on the Riverside model of the heliosphere. The model is based on a three-dimensional, multi-fluid, MHD-neutral approach that takes into account the important consequences of charge exchange between neutral and charged particles in partially ionized plasma. Special attention is paid to the numerically obtained asymmetry in the Voyager 1 and Voyager 2 trajectory directions, which can be compared with the spacecraft measurements. Possible effects of the solar cycle and CME/GMIR propagation on the simulated asymmetry is investigated. The solar cycle model includes the variations in the latitudinal extent of the slow SW and the angle between the Sun's magnetic and rotation axes. Solar cycle related polarity reversals of the dipolar magnetic field of the Sun are also taken into account. The interplay is analyzed between an ISMF-induced asymmetry and unsteady phenomena. It is shown that a GMIR interaction with the TS initially drives it further from the Sun. On reaching its maximum distance from the Sun, the TS location begins to recede, overshooting its initial location. The relaxation of the termination shock to its initial location can last for more than 2 years.
SH43A-06
Propagation Delays of Features in the Energetic Particle Intensity vs. Time Profiles Between Voyager 2 and Voyager 1 in the Outer Heliosphere
We examine the Voyager 1 (V1) and Voyager 2 (V2) intensity vs. time profiles in several energy bands of low-energy particle populations during the period 2005 through early 2007. There appear to be a set of propagating structures with ~1 year duration that appear first at V2 and then ~115 days later at V1. During this period V1 is continually in the heliosheath whereas V2 is upstream of the solar wind termination shock. We will discuss these observations in the context of the source and transport of termination shock particles and anomalous cosmic rays. This work was supported by NASA under contract NAS7-03001.
SH43A-07
The Orientation of the Local Interstellar Magnetic Field and Induced Asymmetries on the Heliosphere
We combine radio emission and energetic particle streaming measurements with extensive 3D MHD computer simulations of magnetic field draping over the heliopause to obtain information on the inclination angle of the local interstellar magnetic field. The orientation of the local interstellar magnetic field introduces asymmetries in the heliosphere that affect the location of radio emission and the streaming direction of ions from the termination shock of the solar wind. In this talk we discuss the orientation of the plane of the local interstellar field and the global asymmetries induced in the heliosphere and in the heliosheath.