SH14A-01
The Heliosheath: Temperature distributions and temporal changes
Voyager 2 has explored the heliosheath for more than a year. We now have enough data to look at statistical comparisons of the heliosheath with planetary magnetosheaths and compare the heliosheath data with that in the upstream solar wind. We find that the temperature distributions in the heliosheath are much broader than those in the solar wind. If the scatter in the temperatures results from differences in heating due to shock motion, then shock speeds of -100 to 100 km/s fit the observations. Fluctuations in the heliosheath are much larger than those seen in planetary magnetospheres, implying rapid temporal changes of the shock movement or structure. A significant number of spectra in the heliosheath have very cold temperatures suggesting this plasma was relatively unaffected by the shock. We show longer scale temporal events in the sheath which may be driven by solar wind changes. One unique 5-day event showed substantially higher density, temperature, and speeds than the ambient sheath plasma and we speculate on its origin.
SH14A-02
Observations of the heliosheath and solar wind termination shock by Voyager 2
This paper describes the principal features of the magnetic field strength variations B(t) and their relationships to the plasma and energetic particles observed prior to and after the crossing of the termination shock (TS) by Voyager 2 (V2). The solar wind (pre TS crossing) and heliosheath (post TS crossing) data extend from DOY 1 through 241, 2007 and from 2007 DOY 245 through 2008 DOY 80, respectively. In the solar wind, two merged interaction regions (MIRs) were observed in which the ratio beta of plasma pressure to magnetic pressure in the solar wind (excluding pickup protons) was relatively low, suggesting that they were formed from ejecta carrying strong magnetic fields away from the sun. Strong magnetic fields and low values of beta were also observed by V2 in the solar wind just prior to its crossing of the TS; it is possible that an MIR contributed to forming this structure, in addition to processes associated with the TS. The predicted correlation between peaks in the intensity of energetic particles in the solar wind when V2 crossed the heliospheric current sheet from positive to negative magnetic polarity in the solar wind was not observed. In the heliosheath, 40 days after crossing the TS, V2 observed a transition from small amplitude fluctuations in 24-hour averages B on a scale of days to large amplitude fluctuations of B on a scale of the order of ten days. A feature was observed in the heliosheath characterized by large enhancements of the density N and the proton temperature T, a small increase in speed V, and a depression in B producing an anticorrelation between the magnetic pressure and plasma pressure; this feature was not a MIR. The ratios of the average values of B, N, and 1/V in the heliosheath to the corresponding values of B, N, and 1/V in the solar wind are approximately the same (2.5, 2.0, and 2.6, respectively), consistent with a nearly perpendicular termination shock on average. The distributions of B and beta were approximately lognormal in both the solar wind and the heliosheath. The average of beta increased from 0.4 in the solar wind to 1.3 in the heliosheath. A unipolar region was observed for 73 days in heliosheath, as the heliospheric current sheet moved towards equatorial plane to latitudes lower than V2.
SH14A-03
Structure of the Heliospheric Termination Shock
Observations by the Voyager 2 spacecraft of the structure of the heliospheric termination shock revealed a quasi-perpendicular structure that possessed many of the characteristics that are familiar to us from perpendicular shocks observed in the inner heliosphere. Because the Voyager plasma instrument cannot measure the interstellar pickup ions directly, their behavior at the termination shock could not be gauged. Nonetheless, it had already been predicted by Zank et al. 1996 that the primary dissipation mechanism for the quasi-perpendicular termination shock would be the reflection of pickup ions rather than solar wind ions; this because of the shell-like structure of the pickup ion distribution function. We discuss the interaction of pickup ions through their trajectories within the structure of the shock observed by Voyager 2. A model for the shock structure is constructed on the basis of preferentially reflected pickup ions, and solutions are presented.
SH14A-04 INVITED
Sensing the Heliosheath from Inner Heliosphere via Energetic Neutral Atoms (ENA): a Review
Recently, L. Wang, R. P. Lin, D. E. Larson and J. G. Luhmann reported the detection of 4-20 keV energetic neutral atoms (ENA), most likely H atoms, coming from the frontal lobe of the heliosphere (Nature, Vol. 454, p. 81-83, 3 July 2008). The detection was performed by the suprathermal electron (STE) sensor on the STEREO A and B spacecraft from June to October 2007. The report showed the ENA flux peaking at about 5 and 20 degrees in ecliptic longitude, respectively, on either side of the Apex, the direction of Sun's motion relative to the local interstellar medium. Each peak has a full-width at half-maximum of about 20 degrees, with the one at the lower ecliptic longitude having a peak flux about three times that of the peak at the higher ecliptic longitude. The same report also derived the spectral shape of the shock-accelerated pick-up ions in the heliosheath to be two power-law spectra with a knee at about 11 keV. Assuming a healiosheath thickness of 40 AU at Voyager 2's crossing of the termination shock, the extrapolation of this proton spectrum into higher energy meets the extrapolation of the ion spectrum measured by Voyager 1 at about 25 keV. These results, especially the double peaking in the ENA flux, appear puzzling to some theorists and modelers of the heliosphere. This talk shall review the analysis of the STE/STEREO data, and go on to discuss the interpretation of the data and its consequences concerning the thickness and shape of the heliosheath as well as the variation of the availability of shock-accelerated ions in the frontal portion of the termination shock. We also compare this most recent ENA observation and implication on the heliosheath with the earlier observation of heliospheric neutral H atoms by SOHO in the energy interval 55-88 keV and the information on the heliosheath deduced from it. If we accept the analysis by Wang et al., then the STE/STEREO observation not only confirms that ion populations in the remotest regions of our heliosphere can be sensed near 1 AU via ENA, but also prepares the way for the next stage of observation by IBEX in an even lower energy interval, 0.01-6 keV. Observation of the same regions of the outer heliosphere in ENA by STE/STEREO and IBEX over the energy interval 0.01-20 keV would provide the much-welcomed information to guide theorists and modelers working on the termination shock and the heliosheath.
SH14A-05
STEREO Observations of Heliosheath ENAs in 2008
In 2007, the STE (SupraThermal Electron) sensors of the IMPACT instrument suite on the STEREO A and B spacecraft detected ~4-20 keV ENAs, most likely hydrogen, coming from the nose of heliosphere, with a wide (~60° in longitude), asymmetric double-peak source structure. These ENAs very likely originated from termination-shock-accelerated pickup ions in the heliosheath. Here we report on new STEREO A observations of heliosheath ENAs in 2008, a year later. We find generally the same ENA source structure - an asymmetric double peak straddling the nose of heliosphere, with the two peaks located at virtually same longitudes with comparable fluxes, on average. The observed ENA flux spectra also exhibit a similar double-power-law with a break at ~9-14 keV. Compared to 2007, the average ENA fluxes are almost the same in the direction of Voyage 1 (close to the major peak), while lower by a factor up to 2 in the direction of Voyager 2 (at the edge of ENA source). These observations indicate a rather steady ENA source in the heliosheath over a year, from 2007 to 2008. If the source is pickup ions accelerated at the termination shock, then their acceleration is quite stable. Furthermore, we will present STEREO B observations that are being taken in the next few months.
SH14A-06
IMAGE/LENA ISN Observations from 2003-2005: Implications for a Secondary Neutral Stream at 1 AU
The Imager for Magnetopause to Aurora: Global Exploration (IMAGE) spacecraft included a Low Energy Neutral Atom (LENA) imager that imaged neutrals in the energy range from about 10 eV to 300 eV. The imager used a tungsten conversion surface to convert low energy neutrals to negative ions. Higher energy neutrals were also detected by sputtering of negative ions (primarily water products) from the conversion surface. From December 2000 to about March 2001, a signal was observed in the LENA imager that was interpreted as a secondary stream of neutral atoms inside the heliosphere arriving from about 285 degrees ecliptic longitude. The primary neutral atom stream comes from ~254 degrees ecliptic longitude, or about 30 degrees away from this secondary stream. The interpretation of this secondary stream was based partly on the fact that the neutral atom flux peaked around 10 January 2001, which is about 30 days later than expected date for the peak flux for the primary neutral atom stream. To date, only the 2000-2001 and 2001- 2002 interstellar neutral atom observations have been analyzed. From 2002 to 2003, the spacecraft was in the ecliptic plane and background due to radiation belts was significant compared to the weak interstellar neutral signal. Here, fluxes from 2003-2004 and 2004-2005 are analyzed. The peak flux appears to be located near 10 January, in agreement with earlier observations. However, the final loss of the interstellar neutral signal from the LENA field of view in 2005 has important implications on the existence and properties of the secondary stream.
SH14A-07
Balancing Act: The Role of The Interstellar Magnetic Field and Neutral H in Voyager 1 and 2 Asymmetries
We present results from recently developed 5 fluid MHD model (4 neutral fluids and 1 ionized fluid). We present a benchmark comparison between our model and the kinetic Moscow model for the case of strong interstellar magnetic field, and no interplanetary field. The presence of neutral H, as pointed out by previous works, has the effect of diminishing the global heliospheric asymmetries. With a stronger interstellar field, however, the asymmetries are increased. Results of the 5-fluid MHD model have been employed as an starting point for a new kinetic-MHD model that combines the BATS-R-US MHD code with the 3D Monte- Carlo Moscow code. We present first results of this new coupled model. We discuss these results and compare with our previous work (Opher et al. 2006, 2007). Our goal is to constrain the orientation and intensity of the interstellar magnetic field that can satisfy the different constraints from the observed asymmetries (energetic particles streaming (east- west); the 10AU differences between V1 and V2 crossing; radio emission; and neutral H deflection).
SH14A-08
Modeling Heliospheric Interface: Observational and Theoretical Challenges
Observational data provided by Voyager 1 and Voyager 2 spacecraft ahead of the heliospheric termination shock (TS) and in the heliosheath require considerate reassessment of theoretical models of the solar wind (SW) interaction with the magnetized interstellar medium (LISM). Contemporary models, although sophisticated enough to take into account kinetic processes accompanying charge exchange between ions and atoms and address the coupling of the interstellar and interplanetary magnetic fields (ISMF and IMF) at the heliospheric interface, are still unable to analyze the effect of non-thermal pick-up ions (PUI's) in the heliosheath. The presence of PUI's undermines the assumption of a Maxwellian distribution of the SW ions. We discuss the ways to improve physical models in this respect. The TS asymmetry observed by Voyagers can be attributed to the combination of 3D, time- dependent behavior of the SW and by the action of the ISMF. It is clear, however, that the ISMF alone can account for the TS asymmetry of about 10 AU only if it is unexpectedly strong (greater than 4 microgauss). We analyze the consequences of such magnetic fields for the neutral hydrogen deflection in the inner heliosphere from its original direction in the unperturbed LISM. We also discuss the conditions for the 2-3 kHz radio emission, which is believed to be generated in the outer heliosheath beyond the heliopause, and analyze possible location of radio emission sources under the assumption of strong magnetic field. The quality of the physical model becomes crucial when we need to address modern observational and theoretical challenges. We compare the plasma, neutral particle, and magnetic field distributions obtained with our MHD-kinetic and 5-fluid models. The transport of neutral particles is treated kinetically in the former and by a multiple neutral-fluid approach in the latter. We also investigate the distribution of magnetic field in the inner heliosheath for large angles between the Sun's rotation and magnetic-dipole axes.