The Outer Limits of the Heliosphere II Posters
Presiding: J D Richardson, Massachusetts Institute of Technology; E Moebius, Department of Physics and Institute for the Study of Earth, Oceans and Space, University of New Hampshire
SH23A-01 1330h
New Geoeffective Parameters of Very Fast Halo Coronal Mass
We have examined the physical characteristics of very fast coronal mass ejections (CMEs) and their geoeffective parameters. For this we consider SOHO/LASCO CMEs whose speeds are larger than 1300 km s-1. By examining all SOHO/EIT and SOHO/LASCO images of the CMEs, we selected 38 front-side very fast CMEs and then examined their associations with solar activity such as X-ray flares and Type II bursts. As a result, we found that among these frontside fast CMEs, 25 are halo (or full halo) CMEs with span of 360 degrees; 12 are partial halo CMEs with span greater than 130 degrees; only 1 is broadside CME with span of 53 degrees. There are 13 events that are shock deflected CMEs: 6 are full halo CMEs and 7 are partial halo CMEs. It is found that about 60 % (23/38) CMEs were ejected from the western hemisphere. We also note that these very fast CMEs have very high associations with other solar activities: all the CMEs are associated with X-ray flares (X-12, M-23, C-3), and about 80 % of the CMEs (33/38) were accompanied by Type II bursts. For the examination of CME geoeffectiveness, we select 12 halo CMEs whose longitudes are less than 40 degrees, which are thought to be the most plausible candidates of geoeffective CMEs. Then we examine the relationship between their CME physical parameters (mass, column density, location of an associated flare, and direction) and the Dst index. Especially, a CME direction parameter, which is defined as the maximum ratio of its shorter front from solar disk center and its longer one, is proposed as a new geoeffective parameter. Its major advantage is that it can be directly estimated from coronagraph observation. It is found that while the location of the associated flare has a poor relationship with the Dst index, the new direction parameter has a relatively good relationship. In addition, the column density of a CME also has a comparable good correlation with the Dst index. Noting that the CME column density is strongly affected by the direction of a CME, our results imply that the CME direction seems to be the most important parameter that controls the geoeffectiveness of very fast halo CMEs.
SH23A-02 1330h
Characteristics of the Solar Wind Termination Shock From Voyager 1 Observations
For much of the time since mid-2002, the Voyager 1 spacecraft has apparently been on interplanetary magnetic field lines connected to a source of particles at the solar wind termination shock. The source point is back along the field lines towards the Sun, as suggested by the streaming anisotropies. The energy spectra often resemble the energy spectra expected for anomalous cosmic rays (ACRs) at the shock, with one important difference. Although the spectra often have a power-law dependence at low energies and a roll off to a much steeper power-law spectrum at higher energies, as expected for the ACR spectrum at the shock, the energy of this roll off is much lower than observed in the ACR spectrum at the same time. Hence, the ACRs must be originating from a different part of the shock. The observed energy spectrum sometimes appears modulated at low energies, as if there is turbulence along the field line between Voyager 1 and the shock. At other times, it appears that there is little scattering in the intervening space. We will examine the observed energy spectra to infer key characteristics of the shock region sampled by Voyager 1, such as the shock strength, the perpendicular mean free path, and the rigidity dependence of the mean free path. This work was supported by NASA under contract NAS7-03001.
SH23A-03 1330h
Three Possible Energetic Particle Sources at the Termination Shock
As we approach mid-2005, we mark the end of three years during which Voyager 1 (V1) witnessed extraordinary particle events as it traveled through a region, beyond 85 AU from the Sun, which is strongly influenced by the solar wind termination shock (TS). V1 has traveled 10 AU since entering this region and appears to be near the TS throughout. This suggests either that there is a chance relationship between the speed of V1 and that of the expanding TS or that this TS-dominated region is large, but variable. We report on up to three populations of energetic particles observed in this region, with sources at the termination shock. (1) Above 40 MeV is the classical anomalous cosmic ray (ACR) component, accelerated across the global TS. The ACR intensities are modulated at lower energies, forming a spectral peak---an unmistakable feature of transport---but exhibit only small variations during this period. (2) Particles from ~50 keV -- 40 MeV have intense, falling shock-accelerated spectra that vary greatly throughout this period and show no evidence of transport down to ~50 keV. Therefore, it appears that V1 is intermittently magnetically connected to a nearby region of the TS, which is the source of these particles. (3) There is evidence of a third particle population, which may be due to particles accelerated at a nearby deformation of the shock with a local radius of curvature that is much smaller than that of the global TS. This population's source appears not to be magnetically connected and is distant enough for transport effects to be obvious, but the small radius of curvature results in a spectral peak at ~5 MeV, well below that of classical ACRs.
http://space.umd.edu/voyager
SH23A-04 1330h
Turbulent Heating of the Distant Solar Wind by Interstellar Pickup Protons with a Variable Solar Wind Speed
We have been investigating the turbulent heating of the equatorial solar wind, as modeled by a phenomenological description of the turbulent evolution under the action of spherical expansion, shear driving in the inner heliosphere, and interstellar pickup proton driving in the outer heliosphere. This model is able to reproduce the radial trend of solar wind temperature observed at the Voyager 2 spacecraft out to nearly 80 AU. However, a detailed comparison with the observed solar-rotation-averaged temperature, which shows considerable spatial/temporal variation, is less than satisfactory. In particular, the solar wind temperature beyond 55 AU, measured during the most recent solar maximum period, is found to be consistently lower than predicted by our model and the variations of the observed and model temperatures are not well matched in phase. These differences may indicate the need for modifications in the turbulent evolution model, which presently contains a number of simplifications. However, the present model does not include several important physical effects, which should be treated before considering the effect of more complicated turbulence models. In particular, the well-defined slowdown of the solar wind due to the momentum loading by the pickup protons has not been incorporated. Neither has the dependence of the charge-exchange cross-section on solar wind flux been taken into account. Both of these effects are expected to reduce the predicted model temperature in the distant solar wind. We will extend the turbulent evolution model to allow these effects in the theory, and determine whether they result in significant improvement in the comparison with the Voyager observations.
SH23A-05 1330h
Deflection of the Interstellar Neutral Hydrogen Flow Across the Heliospheric Interface: an Interstellar Magnetic Compass
Analyses of SOHO-SWAN observations show that the interstellar neutral H flow direction differs by about 4 degrees from the neutral He flow direction recently derived with an unprecedented accuracy using combined data sets (Mobius et al, 2004). The most likely explanation is a distortion of the heliospheric interface under the action of an inclined interstellar magnetic field, with imprints of the distorsion on the neutral H flow due to charge-transfer reactions between H atoms and ions. The direction of the ambient interstellar magnetic field and the heliospheric shape can be derived from the observed deviation. Implications for Voyager trajectories are discussed.
SH23A-06 1330h
Anomalous Cosmic Ray Oxygen Intensity Gradients Between 1 AU and Voyager During the Cycle 23 Solar Maximum
The Solar Isotope Spectrometer (SIS) on NASA's Advanced Composition Explorer (ACE) has measured the composition of energetic nuclei at 1 AU over an energy interval of ~10 to 100 MeV/nucleon for more than 7 years. During solar quiet times at solar minimum, the particle fluxes at the low energy end of this interval are dominated by anomalous cosmic rays (ACRs). After being suppressed by a factor of ~100 or more from 2001 through 2003 during solar maximum, ACR intensities are once again recovering at 1 AU. However, their intensities are lower relative to higher rigidity galactic cosmic rays than they were during entry into solar maximum in 1998-2000. We review ACE/SIS measurements of the ACR composition and spectra obtained during solar minimum and of the variation of the ACR O intensity at 1 AU during the solar cycle. Using the 1 AU ACR oxygen intensity along with that measured by the Voyager 1 and 2 spacecraft in the outer heliosphere, we obtain the radial ACR intensity gradient during solar maximum and compare it with the galactic cosmic ray gradient during that time. This work was supported by NASA at Caltech (under grants NAG5-6912 and NAS7-03001), JPL, and GSFC.
SH23A-07 1330h
Effects of a Tilted Heliospheric Current Sheet in the Heliosheath
Effects of a Tilted Heliospheric Current Sheet in the Heliosheath Recent observations indicate that Voyager 1, now beyond 90 AU, is in a region unlike any encountered in it's 26 years of exploration. There is currently a controversy as to whether Voyager 1 has already crossed the Termination Shock, the first boundary of the Heliosphere (Krimigis et al. 2003; McDonald et al. 2003, Burlaga et al. 2003). An important aspect of this controversy is our poor understanding of this region. The region between the Termination Shock and the Heliopause, the Helisheath, is one of the most unknown regions theoretically. In the Heliosheath magnetic effects are crucial, as the solar magnetic field is compressed at the Termination Shock by the slowing flow. Therefore, to accurately model the heliosheath the inclusion of the solar magnetic field is crucial. Recently, our simulations showed that the Heliosheath presents remarkable dynamics, with turbulent flows and a presence of a jet flow at the current sheet that is unstable due to magnetohydrodynamic instabilities (Opher et al. 2003; 2004). We showed that to capture these phenomena, spatial numerical resolution is a crucial ingredient, therefore requiring the use of an adaptive mesh refinement (AMR). These previous works assumed that the solar rotation and the magnetic axis were aligned. Here we present including, for the first time, the tilt of the heliocurrent sheet using a 3D MHD AMR simulation with BATS-R-US code. We discuss the effects on the global structure of the Heliosheath, the flows, turbulence and magnetic field structure. We access the consequences for the observations measured by Voyager 1 since mid-2002. This intensive computational run was done at the supercomputer Columbia at NASA/AMES
http://butch.engin.umich.edu/~merav