Title: A Porous, Layered Heliopause
Authors: M. Swisdak,1* J. F. Drake,1-2 and M. Opher3
Affiliations:
1Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742.
2 Department of Physics, the Institute for Physical Science and Technology and the Joint Space Science Institute, University of Maryland, College Park, MD 20742.
3Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215
*Correspondence to:
Abstract: The picture of the heliopause (HP) -- , which is the boundary between the domains of the sun and the local interstellar medium (LISM) -- , as a pristine closed interface fails to describe the recent Voyager 1Voyager 1 spacecraft data. Particle-in-cell simulations reveal that the sectored region of the heliosheath (HS) producesgenerates large-scale magnetic islands that reconnect with the interstellar magnetic field and mix the LISM and HS plasma. Cuts across the simulation data reveal multiple, anti-correlated jumps in the number density of LISM and HS particles at the magnetic separatrices of the islands as seen in the Voyager 1 data. Based on the Voyager 1 observations and simulation data a model is presented of the HP as a porous, multi-layered structure that is threaded by magnetic fields.
Main Text: The Voyager 1Voyager 1 (V1) and 2 spacecraft have been mapping the structure of the outer heliosphere on their trajectories out of the solar system. In 2005 Voyager 1 crossed the termination shock (TS) (1-3), where the supersonic solar wind becomes subsonic, and after this time has been traversing the inner HS. In the inner HS the magnetic field has remained dominantly in the azimuthal direction given by the Parker spiral magnetic field (Parker??). The outer boundary of the inner HS is the heliopause (HP), the magnetic boundary that separates the magnetic field and plasma from the sun from that of the LISM (4, 5). [you don’t want to mention that the HP was thought to be a tangential discontinuity-or not needed?!] . TThe location and structure of the HP is unknown. The expectation is that across the HP the magnetic field will rotate from azimuthal (east-west) in the HS to a direction with significant North-South and radial components (Opher076). In an ideal (non-dissipative) model of the heliosphere the local magnetic field is transverse to the boundary so the HP is a tangential discontinuity (4, 5). However, wWhether the HP is a smooth interface or breaks up due instabilities at the interface has been the subject of substantial discussion in the literature (7-11)10, , Baranov’92 or the most recent one Baranov ’99-I added these references at the end). The structure of the HP, and in particular whether the boundary is porous to some classes of particles, is of great importance because of its likely impact on the rate of transport of galactic cosmic rays from the LISM into the heliosphere.
[It also will impact the overall structure of the heliosphere due to pressure balance]
Starting on day 210 of 2012 the Voyager 1V1 spacecraft measured a series of dropouts in the intensity of energetic particles that are produced in the heliosphere, the Anomalous Cosmic Rays and the lower- energy Termination Shock Particles (TSPs)(121, 132 S SH11A-2194). Simultaneous with the dropouts in the heliospheric particles were abrupt increases in the Galactic Cosmic Ray (GCR) electrons and protons and increases in the intensity of the magnetic field (143 SH14B-01). Finally, on around day 238 the heliospheric-produced particles dropped and remained at noise levels and the GCR particles rose and remained constant. This behavior suggested that Voyager 1V1 might have crossed the HP into the LISM with the repeated dropouts and increases arising from the inward and outward motion of the HP resulting from the variability of the solar wind dynamic pressure. However, a key observation was that during these dropouts and increases the direction of the magnetic field remained dominantly azimuthal (143SH14B-01), consistent with the spacecraft remaining in the HS. While MHD models of the global structure of the heliosphere suggested that the draping of the interstellar magnetic field would reduce the rotation of the magnetic field across the HP at the location of Voyager 1V1 would be small (6??), the lack of any significant change in the magnetic field direction at the dropouts suggested that Voyager 1V1 remained within the magnetic domain of the HS during all of these events.
We present the results of a global MHD simulations of the heliosphere pared with a local PIC simulation of the HP that suggest that due to reconnection a complex nested set of magnetic islands develop at the boundary. Tongues of LISM plasma penetrate into the magnetic domain of the HS along field lines that connect the domain of the LISM with that of the HS. These tongues correspond to local depletions of the HS plasma and local enhancements in the local magnetic pressure. A model of the magnetic structure of the HP at the location of Voyager 1V1 is constructed that produces particle and magnetic signatures consistent with the Voyager 1V1 observations.
We first explore the large-scale structure of the heliosphere to establish the local conditions at the HP using a global MHD simulation model (Prested12,154) that includes neutral and ionized components (see Supplementary material). The MHD simulation did not include the sector zone (where the solar spiral magnetic field periodically reverses polarity as result of the tilt between the solar magnetic and rotation axes) since this leads to field reversals that can’t be numerically resolved artificial dissipation of the HS magnetic field upstream of the HP (15-6-176). The magnetic field strength B from the global MHD simulation reveals the solar wind compression at the termination shock, the downstream HS and the HP (Fig. 1). Profiles (solid curves in Fig. S1) along the Voyager 1V1 trajectory of the density of the pick-up (npui), and thermal (nth) ions, and the azimuthal (BT) and normal (BN) magnetic fields near the HP are used as input for the PIC simulations. In the LISM the reduction of BN (Fig. S1C) is small at the latitude of V1. The BT component of B flips direction across the HP but remains the dominant component on both sides of the boundary (Fig. S1D). below the nominal value based on BISM is a consequence of draping as the LISM flow piles up against and then flows around the heliosphere [but Jim, Marc at r=300AU in the MHD BN=-0.001; BT=-0.17 and BR=-0.35]. Because Voyager 1V1 has continued to measure sector boundaries in the HS during 2012 and was therefore in the sector zone, the direction of BT in the HS in the MHD model is irrelevant since a “correct” model should include the reversals associated with the sectors that fill the HS upstream of the HP (16-17). ((the sentence you cut clarified this point: The sector region is carried by the flows and fills the HS upstream of the HP (15, 16)-therefore V1 should be within the sector)). On the other hand, the strength of the field in the HS and the strength and orientation of the field in the LISM should be correct. Therefore Voyager 1V1 should not measure a large rotation of B across the HP.
The initial profiles for the magnetic field density and temperature for the 2-D PIC simulations of the HP (dotted lines in Fig. S1) were constructed with input from the profiles from the MHD model. The simulations are evolved in time with no initially imposed magnetic perturbations. Because of the lower density in the HS, which leads to a locally higher Alfvén speed, magnetic reconnection first starts in the sectored HS (movie S1). Small magnetic islands grow on individual current layers in the HS and merge to become larger islands until they are comparable in size to the sector spacing (Figs. S2A, S3A, 2A) (187). A chain of small islands growsew at the HP (movie S1, Fig. S2A). These islands merged to form larger islands and arewere then compressed by islands in the HS pushing against the HP (movie S1, Figs. S3A, 2A). By late time the HS magnetic field has reconnected with that of the LISM, forming a complex, nested chain of islands (Fig. 2A) at the HP with sizes scales comparable to the original sector spacing.
In our PIC model we are able to independently track all particles and we therefore can explore the mixing of the LISM and HS particles as reconnection at the HP takes place. Overall, reconnection at the HP produces a highly structured distribution in the density of LISM (nLISM) and HS (nHS) plasma with the density of each species undergoing sharp jumps across the separatrices that bound the outflows of plasma ejected from reconnection sites (Figs. S2, S3, 2A-C). The particles initially in the LISM continue to dominate the number density of the un-reconnected field in the LISM, have mixed with HS particles in the nested islands formed as a result of reconnection between the HS and LISM fields, and are largely excluded from islands that resulted from reconnection of the HS sectored field (Fig. 2B). The particles initially in the HS dominate in islands resulting from reconnection of the sectored field, are mixed with LISM particles in the HP islands and are nearly excluded from regions of the LISM with un-reconnected fields (Fig. 2C).
Cuts through the simulation data along the radial direction reveal that the increases and decreases in the number densities of the LISM and HS plasma are typically anti-correlated (Fig. 2E). Moving from a pure HS magnetic island into an island or outflow jet where LISM and HS plasma has mixed reduces nHS and increases nLISM. On a cut from the HS to the LISM the first drops in nHS take place just downstream of magnetic separatrices where HS particles have an open path to the LISM along open field lines (sΔR/di ~13 in Fig. 2D I am still confused with Δ instead of just R/d_i. What is Δ here?maybe you change the figure – the axis I have is R/d_i
). Such mixing behavior has been documented in satellite measurements at the Earth’s magnetopause (18Gosling??) and is one of the striking observations on Voyager 1V1’s approach to the HP – the variation of the flux of ACRs and TSPs are anti-correlated with galactic electrons and GCRs (121-132). The cuts through the simulation data further reveal that when crossing a magnetic separatrix the sharp increases and decreases of the number density of the LISM and HS plasma do not correspond to a directional change in the magnetic field (Fig. 2D). The absence of a direction change in B in the simulations at locations with strong variation of the particle density is consistent with one of the most significant of the recent Voyager 1V1 observations (143). In the simulation the change in field direction to that of the LISM occurs across the midplanes of the chain of magnetic islands at the HP (ΔR/di s ~31 in Fig. 2D same comments as above, why not just R/d_i?
).
Finally, the cuts through the simulation data reveal that local decreases in the HS density correspond to increases in the local magnetic field (Figs. 2D-E). The total pressure across the HP is balanced. While the dominant pressure in the HS is from the plasma (heated pickup particles heated at the termination shock (2019)), the dominant pressure in the LISM is magnetic. Thus, when reconnection opens a path for HS plasma to escape into the LISM and mix with the lower- pressure LISM plasma, there is nothing to balance the total pressure and, as a result, the region undergoes a local compression to increase the magnetic field amplitude. This behavior is primarily seen at separatrix crossings remote from where reconnection locally reduces the magnetic field strength (Fig. 2D). In the Voyager 1V1 data the magnetic field strength is also observed to increase where the local fluxes of HS plasma decrease (121-143).
Thus, based on our simulations we suggest that the Voyager 1V1 observations of simultaneous drops (increases) in HS (LISM) particle fluxes take place at a series of separatrix crossings associated with a nested set of magnetic islands that formed at the HP (Fig. 3). At such crossings the magnetic field direction will not change significantly while particle fluxes can change sharply, as seen in the satellite data. Three active reconnection sites at the HP and associated separatrices with two nested islands are sufficient to explain the sequence of Voyager events. On day 168 the spacecraft crossed a broad current layer, on day 190 the flux of HS electrons dropped, on day 208 a narrow current layer was crossed and on days 210, 226 and 238 were three successive drops (increases) in the HS (LISM) particle fluxes occurred . The day 190 drop in the HS electrons suggests that the magnetic field after this time was no longer laminar so that these electrons could leak into the LISM. In the Voyager observations the highest energy ACRs suffered the largest drops and the size of the drop increased in each successive event until the third event when the HS particles dropped to noise levels. Such behavior is consistent with our schematic. Islands and x-lines flowing away from an active x-line (right-most x-line in Fig. 3) correspond to reconnection sites that developed earlier in time. Thus, the day 210 drop in HS particles occurred on field lines that had just reconnected and formed an open path to the LISM. The drop in intensity was therefore modest. The intensity rose as the spacecraft approached field lines that were close to the separatrix of the right-most HP magnetic island. This is because the effective length of a field line increases dramatically when it wraps around an island and passes close to the x-line where the field line moves mostly in the out-of-plane direction. HS particles in the separatrix region therefore must move farther to escape into the LISM so their intensity recovers. The drop in HS particles on day 226 was greater than on day 210 because reconnection with the LISM field had occurred earlier in time. The final drop of the HS particle fluxes on day 238 occurred downstream of the oldest x-line. Essentially all of the HS particles had drained into the LISM on these field lines.