I'd like to take Lio's numbers and spin them a different way. His research found:
[The sun] has a density of 150 g/cm³ (150 times the density of liquid water) at the center, and a temperature of 15 million degrees Celsius. The core is made of hot, dense gas in the plasmic state (ions and electrons), at a pressure estimated at 265 billion bar (3.84 trillion psi or 26.5 petapascals (PPa)) at the center. Due to fusion, the composition of the solar plasma drops from 68-70% hydrogen by mass at the outer core, to 33% hydrogen at the core/Sun center.
Two things are going to come through this portal:
EM radiation is going to consist of photons passing through the portal while it is open, while hot matter is matter which will pass through first and then transfer its heat to the environment once it's on this side.
The key question is going to be how much energy gets to the Earth, so we can focus entirely on that.
One thing which is not explicitly specified in the question is how the expansion of the matter occurs. As the high pressure plasma from the sun spews through the stargate, it is going to pick up velocity and it is going to drop in pressure. The balance of these will affect the flux of matter. If the stargate acts like a giant nozzle, then all of the pressure will be converted to forward velocity, and the exit of the stargate will not result in a sudden decrease in pressure. If this is the case, we will see the entire raw temperature of the sun on both sides of the gate. On the other hand, if it acts more like a hole in a pressure cylinder, there will be less time to accelerate the matter. Less matter will pour through per second, and the matter that does pour through will undergo rapid expansion and cooling. In this case, the temperature differential across the gate will be larger, and infra-red radiation passing through will matter.
Fortunately, there is a reasonable guess as to what will happen: choked flow. Under choked flow conditions, the exit velocity of the material is the speed of sound at the outside pressure. This is a natural limit if you aren't paying attention to what you're doing with a nozzle, so its reasonable to guess that our wizard, failing to specify the conditions of the gate, will run into this limit. This limit is quite slow compared to the speeds that could be achieved with a better nozzle shape, but it does mean that calculating the EM radiation will be difficult because there will be a sudden drop in pressure. I'll ignore EM radiation from here on out (the matter itself is damaging enough).
The speed of sound is roughly 340m/s. Your 20ft diameter gate has an area of about 30m^2. This means we will have a volumetric flow of plasma around 10,200 m^3/s. At the specified density, that's 1530000000kg/s of matter streaming through the gate!
That's a lot of matter. This is going to get hot quite quickly!. The center of the sun is mostly helium. To keep the numbers simple I'll just pretend it's all helium. Helium has a heat capacity of 5.19 kJ/kg-K. Since the sun is roughly 15 million degrees K hotter than the earth, each kilogram of helium will carry 77,400,000 kJ of energy. Multiplying this by our mass flow rate we get a total energy flux coming through the gate of: 118,422,000,000,000,000 kJ/s. That's 118 EJ/s, or 118PJ/ms, if I do some conversions.
I do those conversions to make it easier to reference Order of Magnitude (Energy), one of the more fun pages on Wikipedia. They list:
- 210PJ - Energy released in the detonation of Tsar Bomba, the largest nuclear weapon ever detonated.
So right off the bat, we can see the thresholds. In the first millisecond or two that the gate is left open, the damage will be as mighty as the strongest nuclear bomb ever tested. Your 10 mile damage marker would have been more in line with what Fat Man did to Nagasaki in WWII. Fat Man was a mere 21kT, cmpared to Tsar Bomba's 50Mt, so your 10 mile line is broken in the first microseconds that the gate is left open!
As for ecosystems, it's not clear how resilient ecosystems are. However, it is worth pointing out that in the first hundred seconds, this gate would pump out more energy than strikes the earth from the sun in a single day. I would not want this gate open for any measurable periods of time!