Helios in Greek means the "sun". Hence Heliosphere means the "sphere of the sun". As the sun moves through the local interstellar medium, its supersonic solar wind carves out a cavity called the Heliosphere. The Heliosphere is a comet-like shaped bubble with a trailing tail filled with hydrogen and helium gases. The Heliosphere is extremely large, about 100 AU (Astronomical Units, one AU equals the distance from the earth to the sun) in the front and an extremely long tail in the rear.
The Solar Wind is a constant stream of charged particles in all directions emanating from the sun's extremely hot outer Corona atmosphere. The Corona is 5,800 K at the surface, but 20,000,000 K at the outermost hottest points! It is not clear at this time how to explain these extreme outer Corona temperatures.
The concept of a "flow of charged particles from the sun" surfaced in the scientific literature during the 1940's and 50's, but it was controversial. It was not until 1962 when the Mariner II spacecraft detected a continuous flowing particle wind that the issue was put to bed. The Mariner also discovered that the Solar Wind fluctuated in intensity in a 27 day cycle which was in concert with the rotation of the sun. The Solar Wind particles (ions of hydrogen and helium) can escape the sun's gravity because of their high energy.
For the first six billion miles from the sun, the Solar Wind travels about a million miles per hour. Even at a million miles per hour, it takes the Solar Wind about a year to reach the outer limits of the Heliosphere.
As it begins to come in contact with the Interstellar Wind (particles emanating from outer space), the Solar Wind slows down and finally comes to a stop. The point where the Solar Wind slows down to subsonic speed (less than the speed of sound) is called the Termination Shock, which is about 90 AU from the sun.
The edge of our solar system is called the Heliopause which is where the Solar Wind and the Interstellar Wind pressures balance. The area between the Termination Shock and the Heliopause is called the Heliosheath. In this region, the stream of charged particles, which have abruptly slowed down from supersonic speeds, become extremely turbulent.
The area where the Interstellar Wind coming from outer space meets the Solar Wind is called the Bow Shock. The Bow Shock is so named because it is similar to the water wave in front of a ship's bow, except in this case it is a gaseous wave. See the NASA Illustration Video of the Heliosphere. Top
No Heliosphere "Bow Shock"
Data (May, 2012) from NASA’s Interstellar Boundary Explorer (IBEX) showed that the Heliosphere moves through space too slowly to form a "bow shock". For over 20 years, researchers believed that the Heliosphere moved through the interstellar medium at a speed fast enough to form a bow shock. IBEX data indicated that the Heliosphere actually moves through interstellar space at about 52,000 miles per hour, roughly 7,000 miles per hour slower than previously thought. This is slow enough to create more of a "bow wave" than a shock.
IBEX data, as well as earlier Voyager observations, show that the magnetic field is stronger in the interstellar medium requiring even faster speeds to produce a bow shock. Combined, both factors now point to the conclusion that a bow shock is highly unlikely.
"The sonic boom made by a jet breaking the sound barrier is an example of a bow shock", says Dr. David McComas, principal investigator of the IBEX mission and assistant vice president of the Space Science and Engineering Division at Southwest Research Institute (SwRI). "While bow shocks certainly exist ahead of many other stars, we're finding that our Sun's interaction doesn't reach the critical threshold to form a shock. So a wave is a more accurate depiction of what's happening ahead of our Heliosphere, much like the wave made by the bow of a boat as it glides through the water." Top
Voyagers 1 and 2
In 1977 Voyagers 1 and 2, pictured at the top of this page's banner, were launched and headed out into space at 39,000 and 35,000 miles per hour respectively. After completing their planetary missions, they continued towards outer space in somewhat opposite directions, but still had enough power to continue to communicate back to earth. In December of 2004 Voyager I passed the Termination Shock at 94 AU (Astronomical Units) and Voyager 2 passed the Termination Shock in August of 2007 at 84 AU. The voyagers are proceeding through the Heliosheath and are getting close to the Heliopause, the edge of the Heliosphere, the outer boundary of our solar system.
In June of 2011 NASA surprised everyone when they announced that the Voyagers were encountering huge frothy magnetic bubbles at the Heliosphere boundary. See the diagram at the lower left. (The red and blue wavy lines represent the sun's magnetic waves.) This was totally unexpected. Some of the bubbles are 100 million miles wide. NASA explains: "Because the sun spins, its magnetic field becomes twisted and wrinkled, a bit like a ballerina's skirt. Far, far away from the sun, where the Voyagers
are now, the folds of the skirt bunch up. The crowded folds of the skirt reorganize themselves, sometimes explosively, into foamy magnetic bubbles. The actual bubbles appear to be self-contained and substantially disconnected from the broader solar magnetic field." See a 3 minute NASA video on Youtube regarding the Magnetic Bubbles.
The speed and magnetic strength of the Solar Wind varies in step with the Sunspot Cycles. During the sun's quiet periods, like the one we are just coming out of, the speed of the Solar Wind slowed considerably. The Ulysses spacecraft measured the decrease in speed to be as much as 20% during the last solar minimum compared to the previous solar minimum. These variations do affect our weather here on earth. The last "little ice age" corresponds in time to the "Maunder Minimum" period in the Sunspot Cycles. Most scientists believe that these variations also affect the size of the Heliosphere - i.e. the Heliosphere expands and contracts in synch with the Sunspot Cycles. However, at the moment this is just theory. Perhaps the Voyagers will shed some light on this phenomenon. For more information on Sunspot Cycles see the Sunspots Page. Top
Voyager 1 Cosmic Rays Increase
A sign of the Heliopause (edge of the Heliosphere) frontier’s approach is the number of cosmic rays hitting Voyager 1. Cosmic rays are high energy particles (protons and helium nuclei) accelerated to near-light speed by distant supernovas and black holes. The Heliosphere protects the solar system from these atomic bullets, deflecting and slowing them before they can reach the inner planets. As Voyager approaches the frontier, the number of cosmic ray hits has gone up. "From January 2009 to January 2012, there had been a gradual increase of about 25 percent in the amount of galactic cosmic rays Voyager was encountering," says Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena.
"Recently, we have seen a very rapid escalation in that part of the energy spectrum. Beginning in May, 2012, the cosmic ray hits have increased five percent in a week and nine percent in a month" said Stone. The sharp increase means that Voyager 1 might be on the verge of a breakthrough 11 billion miles from Earth. (The signal from Voyager 1 takes approximately 17 hours to travel to Earth.) As Voyager 1 actually exits the Heliosphere, researchers expect energetic particles from the sun to become scarce. Top
Voyager 1 Enters A Magnetic Highway
In December, 2012 NASA's Voyager 1 spacecraft entered a new region at the frontier of our solar system that scientists think is the final area the spacecraft has to cross before reaching interstellar space. Much to their surprise, scientists did not previously know this region was there. Astro-physicists refer to this new region as a magnetic highway for charged particles because our sun's magnetic field lines are connected to the interstellar magnetic field lines. See the illustration to the left. This magnetic region is unlike any Voyager has been in before. It is about 10 times more intense than inside the termination shock.
This magnetic pathway allows lower-energy charged particles that originate inside the Heliosphere to move out and allows higher-energy particles from outside to enter. (Before Voyager entered this region, the charged particles bounced around in all directions, as if trapped inside a dome.) Top
Voyager 1 Is Out Of The Solar System
On September 12, 2013 NASA officially announced that Voyager 1 was the first human-made object to leave the Solar System. In retrospect, they have estimated the exact date that it left was August 25, 2012. An eruption on the sun in March 2012 sent waves of solar material out into space. When this ejection reached Voyager 1 13 months later in April 2013, it set the local interstellar plasma vibrating. Here is the definitive quote from NASA:
"Voyager 1 does not have a working plasma sensor, so scientists needed a different way to measure the spacecraft's plasma environment to make a definitive determination of its location. A coronal mass ejection, or a massive burst of solar wind and magnetic fields, that erupted from the sun in March 2012 provided scientists the data they needed. When this unexpected gift from the sun eventually arrived at Voyager 1's location 13 months later, in April 2013, the plasma around the spacecraft began to vibrate like a violin string.
On April 9, Voyager 1's plasma wave instrument detected the movement. The pitch of the oscillations helped scientists determine the density of the plasma. The particular oscillations meant the spacecraft was bathed in plasma more than 40 times denser than what they had encountered in the outer layer of the Heliosphere. Density of this sort is to be expected in interstellar space."
NASA considers the above "proof" that Voyager 1 is in interstellar space. The Heliopause (the edge of the Heliosphere where the sun's influence ends) now appears to be a rather sharp line, not an extended region as previously thought. Top
What's Next For Voyager 1?
As for the future of the Voyagers spacecraft, they each lose about 4 watts of power a year and by 2020, the team will have to turn off some instruments to conserve power. By 2025, there will probably not be enough power for any of the instruments to run, but there will be enough power to “ping” the spacecraft and have it answer. By that time, they should be very far out of the solar system. Voyager 1 is on a course that should eventually take it within 1.6 light years of a star called AC+79 3888, which lies in the constellation of Camelopardalis. It’s due to arrive there in about 40,000 years, long after the spacecraft has lost its ability to talk to anyone on earth. Top
Star Mira - Example Of An "Astrosphere"
NASA has observed a real comet-like sphere similar to our Heliosphere attached to the star Mira (pronounced My-rah) 350 light years away in the constellation Cetus. These spheres are now called "astrospheres". Mira has a comet-like shield and glowing tail similar to our sun as shown in the picture to the left. The tail contains carbon, oxygen and other matter capable of eventually forming new planets. The Bow Shock can be clearly seen in front of the star. Mira is a highly evolved star traveling at 290,000 miles an hour. It is a " red giant" - bloated as if our sun stretched all the way to Mars.
These are signs of a star approaching its end of life. As the hydrogen gas in its surrounding sphere cools and loses energy, it gives off ultraviolet light which the NASA space telescope GALEX (Galaxy Evolution Explorer) was able to photograph in 2006. Mira's comet-like tail stretches an amazing 13 light years across the sky. (For comparison sake, the nearest star to our sun is Proxima Centauri 4 light years away.) See the Mira Video.
To the left is an artist's conception (from the Adler Planetarium in Chicago) of what our sun and Heliosphere might look like sailing through space. Since the sun is not dying and throwing off lots of materials, the tail should be much shorter than Mira's. Top
Cosmic Ray Shield
The chart on the left, also from the Adler Planetarium in Chicago, depicts the flow of very "high energy cosmic rays" (greater than 100 million electron volts, 100 MeV) that flow from outer space. Cosmic rays consist of 90% hydrogen protons, 9% alpha particles (two protons, two neutrons) and 1% other. The very red area represents 100% of the flow as it hits the Bow Shock, the outermost region of our Heliosphere. About 20% of the cosmic rays do not make it to the Heliopause. However, from the Heliopause to the Termination Shock, about 60% of the rays are neutralized by our sun's Solar Wind. Once inside the Termination Shock, another 10% are absorbed by the Solar Wind. Finally, about 10% of the cosmic rays make it into our Solar System, depicted by the color yellow in the chart. The rays that make it through the above are almost all blocked by the earth's magnetic field and the ozone layer of the atmosphere. This drop off process is depicted on the chart by the thick black line and the scale to the left. The Heliosphere provides a vital shield for life here on earth. Without it, we would be exposed to these deadly cosmic rays from the Interstellar Wind.