Structure of the Heliosheath and Heliopause
Abstract
We discuss the structure of the heliosheath (HS) and and heliopause (HP) when reconnection is taken place within the sector region. Observational constrains of reconnection within the sector are challenged by the resolution limitations of the magnetometer. However, indirect constraints such as the lack of conservation of magnetic flux in the heliosheath (Richardson et al. 2013) and the correlation of the variability of energetic particles with the sector region (Hill et al. 2013) indicate that reconnection might be taking place within the sector (Opher et al. 2011). The reconnected sector region in high beta plasma has a multitude of islands and is very similar to a crossing of a normal sector in terms of the overall configuration of the magnetic field and intensity. However, there is substantial reduction of magnetic tension. We show, that Rayleigh-Taylor (RT) instabilities can take place within the sector region where there is no magnetic tension to stabilize the interchange instability (Opher et al. 2013). The RT instability produces elongated flow structures that disturb the heliosheath flow pattern. This instability can explain the large differences between the flows at Voyager 1 and 2. V1 measurements indicate a constant decrease in the radial speed until a region with zero radial speeds while V2 radial speeds are constant. The structure of the HP has been explored with 2-D PIC simulations (Swisdak et al. 2013) to understand what underlies the complex particle and magnetic data seen by V1 in the latter half of 2012. We show using a global MHD model that because of draping the direction of the magnetic field in the interstellar medium (ISM) does not differ significantly from the azimuthal heliospheric field measured in the HS. Magnetic field profiles from cuts of the MHD simulation across the HP are used as input into the initial conditions of the PIC simulation. However, the HS in the PIC simulation is taken to have a sectored structure with a population of pickup ions.The sectored field reconnects first, forming magnetic islands with scales of the order of the sector spacing. These islands then begin reconnecting with the ISM across the HP, slowed by the higher density plasma in the ISM. The HP eventually develops a complex magnetic structure with nested magnetic islands where HS and ISM plasma has mixed. Multiple sharp jumps in the number density of the ISM plasma are seen in cuts across the HP which is revealed not as a single boundary but as a series of boundaries. The jumps occur at separatrices of magnetic islands that exhibit jumps in the population density but no jumps in the magnetic field direction. This important result is consistent with the striking absence of rotation of the magnetic field data seen during jumps in the ACR and GCR intensities seen by V1. Based on these simulation results and the Voyager magnetic and particle data we have constructed the possible magnetic structure of the HP boundary region, which includes a series of nested magnetic islands and separatrices, that produce a porous boundary. The jump in the magnetic field strength measured by Voyager on its approach to the HP very likely arises from the leakage of high pressure HS plasma across this porous boundary into the ISM where it is lost.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2013
- Bibcode:
- 2013AGUSMSH24A..06O
- Keywords:
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- 7835 SPACE PLASMA PHYSICS / Magnetic reconnection;
- 7827 SPACE PLASMA PHYSICS / Kinetic and MHD theory