Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session S3: The Heliospheric Termination Shock |
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Sponsoring Units: DAP Chair: Martin Lee, University of New Hampshire Room: Hyatt Regency Dallas Landmark C |
Monday, April 24, 2006 3:30PM - 4:06PM |
S3.00001: Voyager at the Heliosphere's Termination Shock: Energetic Particle Observations Invited Speaker: The Voyager 1 spacecraft encountered the termination shock of the solar wind in mid-December of 2004 and entered the heliosheath. Voyager 1 was then 94.0 AU from the Sun and at N34$^{o}$ heliographic latitude. Since that time until at least day 10 of 2006 (97.8 AU), the spacecraft has remained in the heliosheath. We will describe intensities and angular distributions of ions $>$40 keV and electrons $>$26 keV associated with the termination foreshock region, termination shock, and heliosheath. These measurements are from the Low Energy Charged Particle instruments on Voyagers 1 and 2. Notable features observed thus far in the heliosheath particle data from Voyager 1 include: (1) high, relatively smooth intensities devoid of the large, quasi-recurrent fluctuations that characterized intensities in the foreshock region; (2) ion energy spectra from 40 keV to several MeV that are well fit by a power-law in energy with spectral slope $\approx $-1.5; (3) large reductions in the amplitudes and occurrence rates of anti-sunward, near-azimuthal beaming anisotropies that were routinely seen in the foreshock ion data; and, (4) radial plasma flow speeds, estimated from analysis of low-energy ion angular distributions, that remain less than $\sim $100 km/s. We are fortunate to be currently receiving heliosheath data from Voyager 1 and solar wind data from Voyager 2, which is now in termination foreshock region. Voyager 2 began measuring low intensities of termination shock precursor protons in late 2004 (75 AU, S26$^{o})$. By mid-2005 (77 AU), the intensity of 3-17 MeV protons measured at Voyager 2 reached levels comparable to those observed at Voyager 1 during the latter half of 2002 (85-87 AU). Ion data at Voyager 2 show quasi-recurrent intensity variations and near-azimuthal beaming anisotropies that are directed mainly sunward, opposite to the mainly anti-sunward beaming anisotropies measured in the termination foreshock by Voyager 1. [Preview Abstract] |
Monday, April 24, 2006 4:06PM - 4:42PM |
S3.00002: Imaging the Global Interaction between the Heliosphere and the Interstellar Medium with IBEX Invited Speaker: The solar wind carves out a large void - the heliosphere - into the surrounding interstellar plasma. Because the sun moves relative to the local interstellar cloud at $\approx $26 km/s an interstellar wind of neutral gas blows through our solar system, and in spite of the large size of the helio-sphere we can probe this flow for He and O near Earth. At a distance of about 100 AU the solar wind slows down to subsonic speed through interaction with the interstellar medium, thus form-ing the termination shock. Here ions are accelerated efficiently, preferably those created from inflowing interstellar gas and then picked up by the solar wind. Through charge exchange with interstellar gas atoms these accelerated ions form energetic neutral atoms (ENA), which - if re-leased towards the Earth -- will arrive on a straight trajectory, carrying with them the information about the energy spectra and spatial distribution of the energetic ions at the termination shock. Two highly sensitive ENA cameras on the Interstellar Boundary Explorer (IBEX) will capture the interstellar O flow and take ENA images of the termination shock over the energy range 10 -- 6000 eV for the first time. These observations will provide insight into the interaction of the he-liosphere with the interstellar medium in the mutual boundary region as well as into the nature of the termination shock and its global structure. [Preview Abstract] |
Monday, April 24, 2006 4:42PM - 5:18PM |
S3.00003: The Mysteries Associated with the Acceleration of Energetic Particles at the Termination Shock of the Solar Wind Invited Speaker: The Voyager 1 spacecraft has crossed the Termination Shock of the solar wind, a long-awaited goal in the exploration of the outer heliosphere. There were great expectations for the Termination Shock as the site for the acceleration of Anomalous Cosmic Rays (ACRs), presumably by diffusive shock acceleration. ACRs result from interstellar neutral gas that is ionized and accelerated in the solar wind, by over four orders of magnitude in energy. In fact, the intensity of the ACRs did not peak at the shock crossing seen by Voyager. Low-energy energetic ions ($<$3 MeV/nucleon) are abruptly accelerated at the Termination Shock, but these particles have the unusual feature that their spectral shape is constant, independent of the change in flow speed at the shock, unlike the predictions of diffusive shock acceleration. Observational constraints on theories for particle acceleration at the Termination Shock will be reviewed and a possible theory for the acceleration of the low-energy energetic ions will be discussed. [Preview Abstract] |
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