The Sun's Heat Increases As It Travels Further Away

Earth's Magnetic Fields Deflect the Solar Wind

Everyone knows that you can melt lead on Mercury if you leave some on the driveway, because Mercury is relatively close to the Sun (to which I will refer as Sol, for obvious reasons). The best you can do with Sol's heat on Earth, which is much further away than Mercury, is to fry some eggs during a local weather report during the peak of summer. But Sol's heat does not necessarily dissipate over distance. In fact, it sometimes increases, even without an additional heat source.

Mercury, as close as 46 million km from Sol, has an average temperature of 167° C (332° F), although the side directly facing Sol can reach up to 427° C (800° F). Lead, by the way, melts at about 328° C (622° F). Despite Mercury's proximity to Sol, it's not the hottest planet in the Solar System.

Venus is. About 108 million km from Sol, its thick atmosphere traps Sol's heat near the surface, and the planet's average temperature is 462° C (864° F). Yes, that also melts lead -- which makes one wonder what the surface consists of if everything is melting? I'm not going to be answering that here, sorry.

So, the first thing we are learning here is that distance is not always the measure of how hot Sol's heat is. Sometimes the heat increases or decreases, based on how thick an atmosphere is present. Sometimes, in the case of Earth, the heat is deflected by magnetic bands. However, what about the heat of Sol as it travels through Space, irrespective of heating up planets or atmospheres? Ok, let's talk about the Corona and the Solar Wind.

Corona

Sol's surface temperature is about 5,500° C (10,000° F), which would actually turn lead -- and the driveway it's on -- into a gas. So, it's not exactly a "surface" as we know the word. However, as you move above Sol's surface, and past its outer atmosphere, the temperature actually rises, and its Corona is much hotter: As high as 2 million° C (3.5 million° F). Sol's Corona is a plasma field that extends over a million km above the surface. So, as you travel further from the surface of Sol, the heat of your surroundings actually increases by over 15,000%. How can this be, since the Corona is actually further away from Sol's heat source deep within its confines? Well, no one is really sure how this happens. However, there is a theory that interaction among Sol's magnetic fields causes the heat increase, but the process is not yet fully understood.   

Solar Wind

The Corona is the source of something else: the Solar Wind (SW). The SW is a plasma, 99% of which consists of Hydrogen and Helium ions (protons), and electrons. One question you probably have is this: Since Sol has enormous gravity, how can particles escape from Sol into Space? Excellent question; I'm glad you thought of it. In order to escape Sol's gravitational field, objects must exceed Solar Escape Velocity (SEV), which is about 618 km/s, or Mach 1,800 for you jet travel buffs. Sol's thermal energy and magnetic fields hurl particles into the corona. The particles that reach SEV escape Sol's gravitational field and travel outward into the Solar System in the form of the SW. You've seen evidence of the SW when you observe the Aurora Australis and Aurora Borealis (the SW particles are interacting with Earth's own magnetic fields) or the tails of comets. In fact, if a comet is traveling away from Sol, the tail could be ahead of the comet if the SW is traveling faster than the rocky/icy chunk.

The Solar Wind travels through the Solar System at various speeds, between 200 km/s and 800 km/s -- which could be like circling the Earth in less than an hour. By the time the SW hits the Earth, it is traveling about 450 km/s, and its temperature has dropped to "only" 150,000° C (270,000° F). By the time the SW has reached the edge of the Solar System, beyond Neptune and Pluto, its temperature is a more balmy 9,700° C (17,500° F). However, then something astounding happens: The temperature of the SW significantly increases!

Termination Shock

Pluto's furthest point from Sol is about 5.9 billion km. At a distance approximately 12-14 billion km from Sol, the Solar Wind hits a wall and strange things happen. Ok, it's not a "wall" as we know it, but it has the same braking effect. The SW abruptly slows down to under 100 km/s, and its temperature rises 10-fold to nearly 100,000° C (180,000° F). This event is called Termination Shock. What's going on? How did the SW know that the Solar System ended?

Interstellar Medium

Space, by its name alone, leads one to think of it as empty. But it is not. Our atmosphere at sea level on Earth consists of about 10 trillion million molecules/cm³. The SW, by contrast has perhaps 10 particles within the same area. That is relatively quite empty, but still not empty enough. In Interstellar Space (the areas between solar systems), there can be as many as 1 million particles/cm³. This bunch of material is call the Interstellar Medium (ISM). It surrounds our Solar System, and exerts pressure upon it. The SW exerts pressure from Sol. The Termination Shock is the point where the inward pressure of the ISM equals the outward pressure of the SW. At the Termination Shock point, the SW becomes greatly compressed at a very fast rate. This combination creates more friction between particles, thereby causing thermal heating, thus increasing the temperature of the SW. Voilà! Eventually, the SW grinds to a complete halt at the Heliopause, about 18 billion km from Sol, and that is considered the final boundary of our Solar System. 

Fun Fact: Because our Solar System itself is traveling through the ISM at almost 84,000 kmh (52,000 mph), the Heliopause is much farther away in the opposite direction of movement. In other words, our Solar System is shaped more like a comet with a following tail than like a round globe.

The Solar Wind Experiences Termination Shock