Theory The Heliosphere

The heliosphere is a giant, 6.2 billion miles-wide bubble around our Sun and filled with the million-mph moving solar wind, ionized gas plasma, and magnetic fields. When these particles reach the edges of the heliosphere, their motion becomes more complicated. Outside the heliosphere lies the interstellar medium, with plasma that has different speed, density, and temperature than solar wind plasma, as well as neutral gases. These materials interact at the heliosphere’s edge to create a region known as the inner heliosheath, bounded on the inside by the termination shock, which is more than twice as far from us as the orbit of Pluto, and on the outside by the heliopause, the boundary between the solar wind and the comparatively dense interstellar medium. Out at the boundary of our solar system, pressure runs high due to plasma, magnetic fields and particles like ions, cosmic rays and electrons exerting on one another. The heliosphere, as it is magnetically tied to the Sun, and rotates together with it over a period of about 27 days hence its density at a given place in the solar system varies too accordingly. The heliosphere's size and shape is function of the balance between the outward push of the solar wind and the inward one from the interstellar wind, or the gas in the Local Interstellar Cloud. The heliosphere is situated near the inside edge of the Local Interstellar Cloud and the two move past each other at a velocity of 50,000 miles per hour creating a wind of neutral interstellar atoms. From Earth's perspective, the interstellar wind flows in from a point just above the constellation Scorpius. The direction from which the interstellar flow looks like coming has changed some 4 to 9 degrees since the 1970s as the Sun being close to the edge of Local Interstellar Cloud, that area might experience turbulence. A fluorescence occurs when the extreme ultraviolet radiation coming from the Sun scatters off the interstellar helium wind which managed to our star, and neutral helium atoms get caught by the gravity, forming a focusing cone. Studies in 2013 found that the magnetic field in the local interstellar medium near the heliosphere is 3.7–5.5 mG as it is tilted about 20–30° from the interstellar medium flow direction -which is resulting from the peculiar motion of the Sun in the Galaxy as it is at a angle of about 30° from the Galactic plane. The magnetic field of the interstellar medium thus likely is turbulent or has a distortion in the solar vicinity. Cosmic rays are as much as four times more abundant in interstellar space than in the vicinity of Earth at the magnetic field of the local interstellar medium is wrapped around the heliosphere. Low-frequency radio waves exist in the heliosphere

->The Heliosphere, Not Comet-Shaped?
New data from NASA’s Cassini mission -- data of which already had been analysed since 2009 -- combined with measurements from the two Voyager spacecraft and NASA’s Interstellar Boundary Explorer, or IBEX, suggests that the heliosphere is a rounded system, calling into question the view of the heliosphere trailing behind the Sun in a long cometary shape. That latter view was due to that most other stars' heliospheres observed features such a shape. The hot population of the solar wind's charged ions interaction with the interstellar medium is significantly controlled by the interstellar magnetic field. The round shape would be due to a interstellar magnetic field much stronger than what was anticipated in the past and compacting the trail, combined with the fact that the ratio between particle pressure and magnetic pressure inside the heliosheath is high. A simulation of the heliosphere further by early 2015 showed the heliosphere might actually be dominated by two giant jets of material shooting backwards over the Sun's north and south poles. If true, those views would bring to a much smaller heliosphere of some 23 billions miles across only

The heliosphere may be divided into different zones and layers:

• the termination shock. The termination shock is the zone where the solar wind speed falls under sound's speed, from about 700,000-1.5 million mph to less than 100,000-250,000 mph (435,000-933,000 km/h to less than 62,000-156,000 km/h). At this distance from the Sun, the solar wind has become a mere thin plasma, but its speed remains very high. Due to fundamentals of supersonic fluid flow, the solar wind's speed abruptly falls when it reaches the interstellar medium (which itself is very thin and exerts a small pressure). The conversion of the kinetic energy yields heat and the temperature raises to more than a million degrees. Particles are accelerated. The termination shock is 90 AU from the Sun in direction of the leading edge, and about 170 AU in direction of the trailing edge
• the heliosheath (or heliotrail). After having crossed the termination shock, the solar wind further slows and turns into the ambient flow in the interstellar medium: it is this region which forms the comet-tail core of the heliosphere. This turbulent area is located approximately at 8.7 or 11 billion miles (14 or 18 billion km) from the Sun. Magnetic bubbles there might be the result that the Sun's magnetic field extends all the way to that border and that, because the Sun spins, its magnetic field becomes twisted and wrinkled as the folds eventually bunch up. When a magnetic field gets severely folded like this, lines of magnetic force criss-cross, and reconnect into such magnetic bubbles
• the heliopause. The heliopause is the outer border of the heliosheath. The heliosphere meets the interstellar medium. This frontier is about 110 AU from the Sun towards the leading edge, and about 260 AU crosswise
• the bow shock. The bow shock is the point, downstream, where the Sun's medium is plowing into the interstellar medium. The bow shock acts as a shield relative to the latter and deflects interstellar particles all along the bubble of the heliosphere. Bow shock is at 230 AU from the Sun, towards the leading edge. The bow shock should have the shape of a bow-shaped wall of hydrogen as the interaction between the neutral hydrogen being part of the interstellar medium and the plasma of the Sun medium, produces more neutral hydrogen, hence the wall. Neutral hydrogen is leaking inside the heliosphere, it be neutral at the origin, or ionized. When ionized it eventually turn neutral, via charge exchange. At the fore of the heliosphere, on the other hand, some atoms are heatep up and lose their electrical charge, thus no longer hindered by magnetic fields, flow back toward the bow shock. Neutral, interstellar ions are encountering the heliosphere at speeds varying from 100,000 mph (160,000 km/h) to more than 2.4 million mph (3.8 million km/h)! A study by February 2016 have shown that the IBEX ribbon, which lies at the interstellar boundary at the very edge of the heliopshere, in which more particles are streaming in through a long, skinny swath in the sky than anywhere else, are actually solar material reflected back like neutral atoms at us after they interacted with the interstellar magnetic field (ISMF) at the edges of Sun’s magnetic boundaries, after a complex series of charge exchanges lasting between 3 to 6 years on average. That precisely determines the strength and direction of the magnetic field outside our heliosphere. Latest studies in 2012 are thinking that our heliosphere just isn't moving fast enough to create a bow shock in the tenuous and highly magnetized region in our local part of the Galaxy. The speed of the heliosphere with respect to the local cloud of material is only 52,000 miles per hour, instead of the previously believed 59,000 which translates to a quarter less pressure exerted on the boundaries of the heliosphere and a much weaker interaction. Recent studies are showing that it does not indeed exist a bow shock, but a bow wave instead at the forefront of the heliosphere, there where our protective bubble and interstellar material are siding each other. That creates strong magnetic fields and the lack of a bow shock would be explained through density, the pressure of the magnetic fields and the relatively slow speed of the heliosphere add up to indicate that there is no bow shock at the front of the heliosphere but a bow wave instead
• astronomers thought that there might be a transition layer at the ultimate frontier between the outer layer of the heliosphere and interstellar space, which they called a 'stagnation region'. That was indeed observed by the Voyager 1 craft which reached such that region by late 2011. That is a area where, after the solar wind stopped, the solar magnetic field piled up because inward pressure from interstellar space is compacting it, interstellar high-energy electrons are increasing in number an density

->IBEX Finds That the Solar Wind is Mostly Encountering the Interstellar Space Along a Ribbon at The Termination Shock Only!
The IBEX mission, which launched since 2008 as a science orbiter about Earth to study the first, closest to the Sun, boundary of the heliosphere at 10 billion miles (16 billion km) from the Earth, has discovered in 2009, that the solar ions mostly are interacting with the interstellar medium along a relatively small ribbon on the termination shock only! The termination is the first chaotic frontier of where the solar medium -the solar wind mostly- is encountering the first elements coming from the interstellar space, the neutral atoms of hydrogen that is. Where the astronomers expected to see a gradual variation between both media, IBEX has seen that their are interacting where the interstellar -or galactic- magnetic field is most parallel to the surface of the termination shock! Albeit one knows that the charged ions of the solar wind may turn neutral when a electron of a interstellar neutral atoms jumps unto those, the fundamental aspect of the interaction -hence, likely, the ribbon aspect- is still ill-known. Personally, the site 'Amateur Astronomy' thinks that it might that it's the interstellar magnetic field -or galactic- which is pervading the heliosphere down to the termination shock, where it interacts with the solar magnetic field, through some sorts of mechanisms of reconnexion, determining places of where the atoms of the solar wind and the neutral interstellar hydrogen come to transition

->The Heliosphere in a Weakening Episode?
Latest data by the joint NASA-ESA mission Ulysses, in September 2008, are showing that the activity of the solar wind was at is half-a-century lowest in 2007, leading to a weak heliosphere -that protective bubble- for the solar system. The solar wind data were found to be lesser by 20 percent than its values during the last solar cycle. That might mean space travels a little riskier, and an increase in the intensity of cosmic rays. The upper atmosphere could cool too, meaning less drag on satellites hence more debris left (and thus more danger too for the astronauts). Those same scientists which made that discovery are saying that the state of the heliosphere might not impact the phenomenon of the global warming, which is, according to them, a different phenomenon as other studies, like for example, the famed 'Eddy's diagram' is showing that a lesser importance of the heliosphere leads to colder episodes of the Earth's global climate -albeit, that, of course, is true when any change in the heliosphere is a long-running one

When fast-moving protons in the solar wind reach the edge of the heliosphere, they sometimes grab electrons from the slower-moving interstellar atoms around them. Such a charge exchange creates electrically neutral hydrogen (ENA) atoms that are no longer controlled by magnetic fields. Suddenly, they're free to go wherever they want — and because they're still moving fast, they quickly zip away from the interstellar boundary in all directions. Those heliospheric interactions vary with time

Zeta Ophiuchi, a young, six times hotter, eight times wider, 20 times more massive, and about 80,000 times as bright than our Sun, which is moving at a amazing 54,000 mph in the interstellar medium is yielding a spectacular bow shock, far more obvious than the one of our own heliosphere (the bow shock further has been imaged in the infrared with NASA’s Spitzer Space Telescope). The bow shock is seen here at about half a light-year away from the star. picture courtesy NASA/JPL-Caltech

On the other side of the boundary of our heliosphere, electrically charged particles from the galactic wind blow as they rebound off the heliosheath, never to enter the solar system. The galactic wind has a speed of 52,000 mph (83,600 km/h) and it affects the shape of the heliosphere. It is yielded from the material and magnetic fields in our Milky Way Galaxy. Interstellar space, generally, the region between stars is filled with a thin soup of charged particles, also known as a plasma. When our Sun is bursting, it sends a shock wave outward. Incoming helium from the same source, on a other hand, has a higher speed of 59,000 mph (94,900 km/h) and streams from another direction. Such data also provide a glimpse about the so-called 'Local Interstellar Cloud,' a 30 light years across interstellar cloud into which our solar system currently resides. The solar system with its heliosphere moved into our local cloud at some point during the last 45,000 years as we might be at the boundary rather than the center of it and transitioning into a new region of space sometime in the next hundred to few thousand years. Interstellar neutral oxygen flux, as far as it is concerned, is able to cross the heliosheath barrier and to travel inside the solar system down to the Sun, during 30 years, where it slingshots. That internal flow has a composition different from the outside, with more oxygen in our solar system than there is in the nearby interstellar material. That suggests that either the Sun formed in a different part of the Galaxy or that oxygen, outside the solar sytem, lies trapped in interstellar dust or ice grains and is unable to move freely in space. In the galactic medium, for every 20 neon atoms in the galactic wind, there are 74 oxygen atoms as 111 oxygen atoms in the solar system. That might have also life-originating implications. Some late observations by the SOHO solar satellite have seen that the flow of the neutral hydrogen atoms has been seen lately coming from a slightly different direction than other neutral matter like helium. This might point to a specific orientation of the local interstellar magnetic field which would not be aligned nor perpendicular to the direction of motion of the solar system. Further, this would mean that the shape of the heliosphere is not perfectly symmetric. This last point has been confirmed, on the other hand, by the two Voyager probes -Voyager 1 and Voyager 2- which are heading to the outside of the heliosphre's bubble in two different directions. They are reporting a large North-South asymmetry in the shape of the heliosphere, which could be due to an interstellar magnetic field pressing inwards on the southern hemisphere of it. Voyager 1 reached by 2004 the first frontier of the heliosphere, at 8.4 billion miles (90 AU; 5.2 billion km), or the termination shock. Voyager 1 too found that a invisible shock was extant at the location where the particles of the solar wind are hitting the neutral interstellar gas! Voyager 1 then has reached by late 2010 a point at the edge of our solar system where there decidedly is no more outward motion of solar wind, at some 10.8 billion miles (17.4 billion kilometers) from the Sun, which marks a major milestone into the heliosheath, a turbulent outer shell where solar wind is heating up. The decrease of solar wind motion is steady. Crossing into interstellar space further will mean a sudden drop in the density of hot particles and an increase in the density of cold particles. Researchers currently estimate Voyager 1 will take 10 to 20 years more to reach the next layer, the heliopause and, then beyond the bow shock, the craft will drift forever in the interstellar space. It should reach a star of the constellation of the Giraffe, the AC+79388 red dwarf, in about 40,000 years. Voyager 1 and 2, missions to the gas giants, were turned since into exploratory ones into the heliosphere. Voyager 1 is traveling at a speed of about 38,000 mph (61,100 km/h) and Voyager 2 by 35,000 mph (56,300 km/h) and both in the direction of the front side of the heliosphere, there where, with the bow shock, crafts have a lesser distance to journey to cross into interstellar space. Voyager 2 has reached a position 8.8 billion miles (14.2 billion km) from the Sun and is bound to pass the same steps that its sibling in the heliosphere by the coming years. Voyager 2 should reach Sirius by some 296,000 years from now. The Pioneer 10 and 11, former missions in 1972 and 1973 to Jupiter and Saturn also are pioneering in the region albeit they do not send data anymore, with Pioneer 11 heading too to the front side of the heliosphere as Pioneer 10 is leaving the solar system in the opposite direction. Of note, at last, is that the Oort Cloud is the final frontier to the solar system, beyond the heliosphere

Interstellar Boundary Explorer, or IBEX, spacecraft, reveals that conditions at the edge of our solar system may be much more dynamic than previously thought. The local interstellar flow, generally, is significantly hotter than believed previously. A new set of "all-sky" maps of our solar system's interaction with the galaxy are revealing changing conditions in the region that separates the nearest reaches of our galaxy, called the local interstellar medium, from our heliosphere. Since 2009 already, first map produced by IBEX had revealed an unpredicted bright ribbon of energetic neutral atoms emanating toward the Sun from the edge of the solar system. The interstellar boundary region is creating particles referred to as energetic neutral atoms as charged particles from the Sun, called the solar wind, flow outward far beyond the orbits of the planets and collide with material between stars. These collisions cause energetic neutral atoms to travel inward toward the sun from interstellar space at velocities ranging from 100,000 mph to more than 2.4 million mph (160 000 à 3,9 millions de km/h). The area at the edge of our solar system shields us from most of the dangerous galactic cosmic radiation that would otherwise enter from interstellar space. Recentest observations have shown that the interaction of our Sun and the Galaxy is amazingly dynamic, and variations taking place on remarkably short timescales. Observations from NASA's both Voyager spacecraft suggest the edge of our solar system may not be smooth, but filled with a turbulent sea of magnetic bubbles. Using a new computer model to analyze Voyager data, scientists found the Sun's distant magnetic field is made up of bubbles approximately 100 million miles wide. By 2012, the Voyager 1 entered a region which constitutes the limit between our solar system and the interstellar medium, where the solar wind is settling and even runs in the reverse direction, the magnetic field of the Sun is packing, very energetic solar particles are running away in the interstellar space, or the number of cosmic rays hitting the craf is increasing. Voyager 1 entered the interstellar space in 2012, leaving the planets and the solar wind behind. The magnetic interstellar field has been slowly turning ever since as that could be a effect of the nearby boundary of the solar wind. A more pristine region of the interstellar space could be reached by about 2025. The particle density since border crossing is observed 40 times greater than inside the solar system, which could also change. The so-called "IBEX ribbon" might be produced by a flow of neutral hydrogen atoms from the solar wind that is re-ionized in nearby interstellar space and then picking up electrons to become neutral again. For a better apprehension of our solar system boundary, NASA is combining data from several spacecraft like Ulysses, Voyager, IBEX and others. The complex appearance of supernovae remnants, for example, hints to that interstellar medium probably has a knotty structure. A 'tsunami wave' occurs outside the heliosphere, when the Sun emits a coronal mass ejection, generating a wave of pressure. When the wave runs into the interstellar plasma, a shock wave results that perturbs the plasma

By late 2018, both Voyager probes had reached out of the heliopshere, crossing the heliopause, the Voyager 1 by 2012 and the Voyager 2 by late 2018. The heliopause is where the warm and tenuous solar wind meets the denser and colder interstellar medium. Voyager 2 carries a working instrument that will provide first-of-its-kind observations of the nature of this gateway into interstellar space as date will take 16.5 hours to travel to reach Earth. A both probes are exiting the heliosphere on the bow shock's side, they won't reach the Oort Cloud's inner border in about 300 years and possibly will take 30,000 years to fly beyond it

->Steps of Voyager 1 Outside the Heliosphere
One of both missions to gas giants in the solar system, Voyager 1 launched in September 1977 and was drawn above ecliptic's plane after it reached the Saturnian system, and bound to exit the solar system sphere of influence. Since then, the Voyager 1 spacecraft closed to the end of the heliosphere

• by Dec. 2004 Voyager 1 crossed the termination shock
• since then, the craft has been exploring the heliosphere's outer layer, the heliosheath where the stream of charged particles from the Sun -the solar wind- abruptly slows down from supersonic speeds and becomes turbulent. Voyager 1's environment was consistent for about five and a half years
• after that, the spacecraft then detected that the outward speed of the solar wind slowed to zero
• by 2012, the Voyager 1 spacecraft entered a new region which likely is the final area to cross before reaching interstellar space. Such a region is likely a magnetic highway for charged particles because our Sun's magnetic field lines connect there to interstellar magnetic field lines. This connection allows lower-energy charged particles that originate from inside our heliosphere to zoom out and higher-energy particles from outside to stream in. Before entering this region, the charged particles bounced around in all directions, as if trapped on local roads inside the heliosphere. The Voyager team infers this region is still inside our solar bubble because the direction of the magnetic field lines has not changed. The direction is predicted to change when Voyager breaks through to interstellar space. It's likely the Voyager is there just a few months to a couple years away from exiting the heliosphere. The magnetic field of that area is about 10 times more intense than before the termination shock
• each star is generating its own plasma environment as the interstellar medium is bathed by particles released from the explosion of supernovas millions of years ago and denser that the solar particles density by the end of the heliosphere. Only theoretical models until now are allowing to figure what the transition between our heliosphere and the interstellar medium is. Both medium, on a other hand, are carrying their own magnetic field lines, the one of the Sun, the one of interstellar magnetic field respectively. At the border between our heliosphere and the interstellar medium, we should expect a drop of the charged particles originating at the Sun and a increase of galactic cosmic rays. A first such discrepancy occurred in May 2012 and July 2012 (when the levels turned back to usual after five days). By August 2012, Voyager 1 then reached a area -likely a intermediate one ahead of the heliopause- where solar particles dropped by a large factor but analysis of the magnetic field showed that the spacecraft still was in the one of the Sun, in terms of direction of the magnetic lines. Solar outburts, further, eventually reached until the frontier of the solar domain after a year or so, where they plow into the interstellar plasma and cause it to oscillate. By April 2013, Voyager 1 was seen to have reached that area as the craft was able to pick up local plasma oscillations as the density of interstellar plasma was measured too with a density 40 times that observed earlier in the heliosheath. It is likely that the heliosphere's frontier is large, or variable or both as dense interstellar plasma might make inroads into with its typical largest density. The last sign that Voyager 1 would lie inside the interstellar medium only will be that it will be hit by particles uniformly from all directions. Voyager 1 has enough electrical power to keep operating the fields and particles science instruments through at least 2020 but at that point, mission managers will have to start turning off these instruments one by one to conserve power, with the last one turning off around 2025. The craft will then journey silent during 40,000 years until it comes near to star AC +79 3888. Beyond the question of the interstellar space -or the one of a star's realm of plasma influence- scientists also define a line from the concept of gravity: the realm of the Sun ends after the Oort Cloud, where the gravity of other stars begins to dominate. Voyager 1 now is still 300 years from the inner edge of the Oort Cloud as it will possibly fly 30,000 years until the outer edge. Both Voyagers currently lie within the Local Interstellar Medium, a bubble of material that encompasses the Solar System as Voyager 2 will exit it in several thousand years and then enter another cloud beyond. It’s unclear when Voyager 1 will break through the bubble

->Voyager 2 And Other Data see the Heliosphere Assymetric
Voyager 2, the twin mission to Voyager 1, launched in August 1977, and his trajectory bent out from the ecliptic after it had reached Neptune, has in turn reached the 'heliosheath', the beginning of the frontier of the 'heliosphere', this bubble equivalent for the Sun to the Earth's magnetosphere and protecting the whole solar system from the interstellar medium, by the end of the year 2007. With the second of the Voyager craft reaching there, the scientists, further, got the evidence that the heliosphere is not perfectly round, with its southern face closer to the Sun by one billion miles (1.6 billion km). The 'heliosheath' is the region where the solar wind ceases and leaves room to the interstellar medium. Such a assymetrical shape of the heliosphere likely is caused by the force and direction of magnetic fields ramming into the heliosphere from outside

With our Sun passing through the Local Interstellar Cloud, at last, into the apparent direction of constellation Scorpius, the Scorpion, it sweeps up neutral interstellar helium into a cone of helium behind it. The Local Interstellar Cloud (LIC) is a haze of hydrogen and helium approximately 30 light-years across and blocked by our heliosphere. A thin stream of gas however makes it past and take its form of a interstellar wind inside the solar system. That wind shifted by 6 degree over 40 years. A fog of low-energy X-rays or the 'soft X-ray diffuse background,' is observed over the entire sky as much of this glow stems from a region of million-degree interstellar plasma known as the local hot bubble, or LHB, a bubble of hot gas extending out a few hundred light-years from the solar system in all directions. Interstellar gas in the region of our Sun is unusually sparse meaning that our solar system is moving through a region that may have been blasted clear by one or more supernova explosions during the past 20 million years. Our Sun is also producting low-energy, 'soft X-rays' through interactions between the solar wind and neutral atoms in comets, the outer atmospheres of planets, and even interstellar gas, a process called 'solar wind charge exchange,' which can occur anywhere neutral atoms interact with solar wind ions. The solar system is currently passing through a small cloud of cold interstellar gas as it moves through the Milky Way Galaxy. The cloud’s neutral hydrogen and helium atoms stream through the planetary system at about 56,000 mph (90,000 km/h). While hydrogen atoms quickly ionize and respond to numerous forces, the helium atoms travel paths are largely governed by the Sun's gravity. This creates a "helium focusing cone" downstream from the Sun that crosses Earth's orbit and is located high in the sky near midnight in early December. A solar wind charge exchange occurs too, which is caused by a atom of interstellar helium colliding with a solar wind ion, losing one of its electrons to the other particle. As it settles into a lower-energy state, the electron emits a soft X-ray. 40 percent of the soft X-ray background eventually originates within the solar system