the matter between stars , the interstellar medium , varies considerably from region to region in our galaxy .
the sun is inside a very large structure called the local bubble , a region of hot tenuous gas formed by supernova explosions tens of millions of years ago [ 13 ] .
adjacent to the local bubble is a similar but larger bubble , also formed from supernova explosions . inside the local bubble
are smaller , denser clouds which may have broken off from the bubble interaction region .
the h density of the local cloud is about 0.2  cm , the temperature is about 6000  k , and the cloud moves about 23  km / s with respect to the sun
. the magnetic field strength can not be directly measured , but based on models is 35  nt .
the sun is the source of the variable solar wind , with speeds measured near earth ranging from 250 to 2200  km / s , proton densities from 0.01 to > 100  cm , and an average magnetic field strength of 5  nt .
since the solar wind and local interstellar medium ( lism ) plasmas are both magnetized , they can not mix , so the lism flows around the heliosphere .
the boundary between the lism and solar wind is the heliopause ( hp ) , analogous to earth s magnetopause . since the solar wind is supersonic , a shock ( called the termination shock ) forms upstream of the hp . at the ts ,
the solar wind becomes subsonic and begins to turn toward the heliotail , the stretched - out downstream region analogous to earth s magnetotail .
if the lism were supersonic , a bow shock would form in the lism upstream of the hp , but recent data and analysis suggest that the lism flow is subsonic and thus the heliosphere does not have a bow shock .
voyagers 1 and 2 were launched in 1977 and are now exploring the interaction between the lism and the solar wind .
they both have crossed the ts and are in the region of shocked solar wind between the ts and the hp that is called the heliosheath . in late 2011 v1 was 119 au from the sun and v2 was 97 au , moving outward at 3.5 and 3.1 au / yr , respectively .
this paper reviews the observations made by these spacecraft as they enter unexplored regions of space .
the first observed influence of the lism on the solar wind was from the lism neutrals .
the neutrals are unaffected by the magnetic fields and flow into the heliosphere , where they are ionized in the solar wind and form hot , 1  kev , pickup ions .
these pickup ions dominate the thermal pressure outside about 30 au and play a major role in pressure balance structures outside this distance . accelerating the pickup ions to 1 
kev slows the solar wind ; this slowdown was first observed near 30 au and by 80 au the solar wind had slowed by about 20% .
some of the energy from the pickup ions is transferred to the thermal protons , causing the temperature of the solar wind to increase with distance .
about 2.5  years before each ts crossing , the voyagers detected a new energetic particle component with proton energies of tens of kev to tens of mev flowing along the magnetic field lines .
this new particle component , called termination shock particles , signified that the voyagers were entering a region analogous to earth s foreshock , with particles accelerated at the ts streaming into the heliosphere along the magnetic field . for these particles to be observed , the ts distance had to be further at the flanks than at the nose so that magnetic field lines at the voyagers would also pass through the ts .
the bluntness alone could not account for all the particle observations ; an additional asymmetry in the heliospheric boundaries due to the interstellar magnetic field was also required .
the realization that the supersonic solar wind must go through a termination shock to become subsonic was first reported by parker .
the location of this shock is determined by the hp location and the upstream plasma parameters .
the hp forms where the solar wind dynamic pressure is balanced by the total lism pressure ; the value of the lism pressure is not well determined the distance to the ts , and thus the scale size of the heliosphere , were determined when v1 crossed the ts at 94 au in 2004 .
it crossed the ts in 2007 at 84 au [ 1720 ] , 10 au closer than v1 .
calculations of the ts motion based on changes in the solar wind dynamic pressure suggested that ts motion was responsible for only 23 au of the distance change .
thus the heliosphere is asymmetric , with the ts closer in the v2 than v1 directions .
models of the interaction of the heliosphere with the lism show that an asymmetry occurs if the lism magnetic field is tilted from the lism flow direction and has a magnitude of > 3  nt .
if these conditions held , the magnetic field would drape around the heliosphere so that the magnetic field strength builds up outside the southern part of the heliosphere , and the increased magnetic pressure would push the boundaries of the southern heliosphere inward .
the ts crossing provided other surprises as well . the ts was a weak shock , with a compression ratio close to two . at voyager 2 ,
the speed decrease started about 80  days before the ts crossing as the speed went from 400 to 300  km / s in three discrete steps .
the last step coincided with a sharp gradient in the energetic particle pressure , with the inward pressure gradient force large enough to produce the observed slowdown . at the v2 ts ( v1 does not have a working plasma instrument ) , the speed decreased from 300 to 150  km / s , the density and magnetic field increased by a factor of 2 , and the ion temperature increased by a factor of 30 . a major surprise ( but see zank et al . ) was that the heating of the thermal ions was much less than the decrease in the flow energy .
about 15% went to heating the energetic ( tens of kev ) ions , but the majority seems to have gone into heating the pickup ions , which are not directly observed .
the ts was the source of the low - energy particles observed in the foreshock ; the intensities of these particles peaked at the ts .
however , the anomalous cosmic ray ( acr ) intensities did not peak at the ts as expected , at least not where crossed by v1 and v2 .
acrs are singly ionized particle with 10100  mev / nuc ; they were observed first near earth and their origin was thought to be pickup ions formed from lism neutrals which were then accelerated at the ts .
the acr intensity did not increase at the ts ; no evidence an acr source at the ts was observed at either of the voyager crossing locations .
the heliosheath was thought to be , in analogy with planetary magnetosheaths , a highly turbulent region and this expectation has been correct [ 2325 ] . figs .
1 and 2 show the daily average plasma parameters obtained by fitting the observed spectra to convected , isotropic proton distributions .
the broad envelope of the data and the 25-day running averages that are superposed show consistent trends . however , the individual sets of spectra very greatly on time scales of tens of minutes .
the magnetic field also varies by factors of 23 over similar time scales , confirming the very dynamic and turbulent nature of this region .
as v2 moves deeper into the heliosheath , these fluctuations decrease slowly in magnitude but remain significant . by the end of 2011 , v2 was 14 au past the ts crossing distance of 84 au .
models suggest that the ts has moved inward 8 au since the ts crossing due the very low solar wind dynamic pressure during the recent solar minimum .
the expectation was that the plasma speed would decrease across the heliosphere and the flow direction would turn tailward .
1 shows that , contrary to these expectations , the average speed at v2 has remained roughly constant at 150  km / s for over 4  years , with a brief dip in speed at 2009.7 followed by a recovery in 2010.5 .
these observations of steady speeds are not predicted by models   and are not understood .
although the speed is not slowing , the direction of the flow at v2 is turning as expected .
1 shows that the flow in the rt plane ( the rtn coordinate system has r radially outward , t parallel to the solar equatorial plane and positive in the direction of solar rotation , and n completes a right - handed system ) is about 20 after the ts crossing and increases to about 45 at the end of 2011 .
the flow in the rn plane was toward the south as expected , starting at about 10 after the ts , then oscillating for about a year before it started a monotonic increase to 25 at the end of 2011 .
the initial deflections at the ts must be due to the ts being at an angle to the radial flow . as discussed above
, the ts is blunt near the nose , less curved than a circle , so the flow at the ts is deflected away from the nose of the heliosphere . as the plasma moves across the heliosheath it continues to turn away from the nose , as expected .
the rt angle plot shows a cutoff at about 50. this cutoff is an instrumental effect ; when the flow direction is at too large an angle to the instrument look direction the plasma is not detected . in this case
the distributions of the plasma properties in the heliosheath are well represented by gaussian distributions .
the observed distributions of the plasma parameters are fit to gaussians to find the average properties and standard deviations . for the rt angle , which is cut off at about 50
, we can fit the distribution below 50 with a gaussian and determine the average flow angle .
the flow angles determined from these fits are shown by the dashed line in fig .
1 , which shows the flow is 56 from radial in the t direction in 2011 .
note that the rt angles ( and thus speed ) are greater than the rn angles throughout the heliosheath .
more of the plasma goes around the sides than over the top of the heliosheath , at least in the southern hemisphere where v2 is located , which suggests that the heliosheath is compressed at the southern pole .
2 shows the radial speed vr , the density n and the temperature t in the heliosheath .
although the speed has remained roughly constant as shown above , vr decreased from 130 to 100  km / s as the flow turned tailward . after the ts , the density initially averaged about twice the 0.001  cm value in the solar wind but had large , factor of 34 , fluctuations . by the end of 2008
the cause of the density decrease is likely partially the reduced solar wind flux coming from the sun in the recent solar minimum   and partially a heliolatitude effect . at solar
minimum the solar wind flux decreases with heliolatitude , so v2 at 30 s should observe less flux than observed near earth at low - latitudes .
however , a problem with this hypothesis is that these lower fluxes are associated with higher flow speeds , which are not observed in the heliosheath .
the decrease in fluctuations may result from the very quiet solar wind conditions in this solar minimum combined with v2 moving further from the ts .
the density increased by a factor of two during a 6  month period in 2011 , perhaps because of a diminishment of the heliolatitudinal flux gradient as solar minimum ends .
the average density at the end of 2011 is similar to that observed just after the ts crossing , but the fluctuations in daily averages are much smaller .
the temperature decreased from 150,000  k after the ts to about 40,000  k in 2011 .
perhaps it reflects cooler solar wind encountering the ts or less heating at the ts due to differences in the upstream flow parameters .
the temperature increased slightly in 2011 in concert with the increase in density , but the reason is unclear .
although the plasma instrument on v1 does not work , the speeds in the r and t direction can be calculated from the low energy charged particle ( lecp ) instrument observations of tens of kev ion intensities using the compton - getting effect .
the speed after the ts was below 100  km / sand monotonically decreased from 70  km / s in mid-2007 to 0  km / s in early 2011   and has since become negative .
the t component of the speed averaged 40  km / s until mid-2010 , when it started to decrease .
.   suggest that the decrease of the speed to near zero signifies that v1 has entered a boundary region in front of the hp in which flow is parallel to the hp .
the v1 spacecraft was recently reprogrammed to do roll maneuvers so that vn could also be determined , and vn is also small < 20  km / s .
thus the v1 has entered a region with nearly stagnant flow which was not predicted .
models show that such a region could be part of the global spatial character of the heliosphere   or a time dependent feature near the boundary of the fast and slow solar wind regimes near solar minimum .
since the observed speeds are very low , comparable to the 23  km / s expected in the lism , one might wonder if v1 has already crossed the hp . the magnetic field increased by about a factor of 2 in the stagnation region but that the direction has not changed .
the field is still consistent with the parker spiral direction ; this direction is expected to change in the lism , so v1 likely has not crossed the hp .
the increase in magnetic field magnitude is consistent with predictions that the field will be compressed as it pushes up against the hp boundary .
the most probably explanation for these data is that v1 has entered a boundary layer near the hp but has not yet crossed the hp .
the acr intensity has increased slowly as the voyager spacecraft move deeper into the heliosheath . at v1 ,
the spectra are almost power laws , indicating that v1 is near the source region .
one is that they are accelerated on the flanks of the heliosphere where the particles can interact with the ts longer , then move along the magnetic field lines to the voyager spacecraft .
another hypothesis is that acrs are accelerated by second order fermi acceleration by magnetic islands or ridges near the hp .
a third is that reconnection occurs as the current sheets are compressed near the hp , leading to particle acceleration .
the voyagers may be able to differentiate between these possibilities as they approach and cross the hp .
the voyager spacecraft celebrate their 35th year in space in august 2012 and continue exploring new regions of space .
they should continue to return data until 2025 , when we expect they will be well into the interstellar medium .
this paper describes some of the new discoveries and new mysteries resulting from recent observations .
some of the more intriguing puzzles are the source of the acrs , the very different speed profiles observed in the v1 and v2 directions , and the formation of a boundary layer in front of the hp .
 this paper provides a brief review and update on the voyager observations of the interaction of the heliosphere with the interstellar medium . 
 voyager has found many surprises : ( 1 ) a new energetic particle component which is accelerated at the termination shock ( ts ) and leaks into the outer heliosphere forming a foreshock region ; ( 2 ) a termination shock which is modulated by energetic particles and which transfers most of the solar wind flow energy to the pickup ions ( not the thermal ions ) ; ( 3 ) the heliosphere is asymmetric ; ( 4 ) the ts does not accelerate anomalous cosmic rays at the voyager locations ; and ( 5 ) the plasma flow in the voyagers 1 ( v1 ) and 2 ( v2 ) directions are very different . at v1 
 the flow was small after the ts and has recently slowed to near zero , whereas at v2 the speed has remained constant while the flow direction has turned tailward . 
 v1 may have entered an extended boundary region in front of the heliopause ( hp ) in 2010 in which the plasma flow speeds are near zero . 
Introduction
Pre-termination shock
Termination shock
Heliosheath
Summary