Here's my aggregated understanding of panic changes. There is still a good bit of uncertainty on some of the more rare cases. Please edit this post if you have more detailed or more correct information! Also feel free to fix my crappy formatting.
- Successfully assault a UFO landing site or crash site
- -2 panic in the nation the UFO landed/crashed in
- Complete a council mission (VIP escort, bomb defuse, etc)
- -1 panic in all nations in all continents?
- This seems to vary occasionally and may even be random. If it were random I would guess it's a panic reduction between 0 and 2 for each nation
- Complete a major combat mission objective (i.e. Assault the Alien Base)
- -2 panic in all nations in all continents
- This may be different for other mission objectives
- Build a satellite over a nation
- -2 panic in the associated nation (-3 or even -4 if the nation specifically requested a satellite)
- Successfully stop a terror attack
- -2 panic in all nations on the continent the terror attack occurred in
Mixed Panic Response
- Successfully respond to an abduction (the 3-choice scenario that happens frequently)
- -2 panic in the nation you responded to
- NO panic change in other nations in the continent you responded to
- +1 panic in every nation in the continents which had an abduction you did not respond to
- In some cases, I have seen +2 panic in the specific nations that I didn't respond to instead of just +1. I think this may depend on either a) the intensity of the attack or b) the panic level before the abduction took place.
- Fail to stop a terror attack
- +1 (or 2?) panic in all nations on the continent which was attacked
- The attacked nation leaves the XCOM funding council instantly
- Fail any other type of mission
- I haven't actually had this happen yet, but at the least it has no benefit and probably increases panic further than just ignoring the mission would have.
I believe there are slight variations in a lot of these, and panic may even go up somewhat randomly in individual nations which weren't attacked. My justification for this is that I often have individual nations which haven't been targeted by specific activity being at a higher panic level than the other nations on their continent.
Many of the events above aren't directly under the player's control. The things that the player can affect are:
- When and in what nations you build satellites.
- It's actually "wasteful" on potential panic reduction to put up a satellite in a nation with less than 3 panic. Consider this carefully.
- You may be better off waiting until near the end of the month to put up satellites. Unless you're finishing coverage on a continent, you don't get any immediate benefit, and unless you fail a mission a nation won't leave the council (they leave if the report comes in and they're at 5 panic). A nation that's at panic level 3+ might end up getting attacked, and you can reduce its panic level by responding to it. If you had already put the satellite down you've lost that opportunity.
- You have to aggressively go after engineers and satellite uplinks to have enough satellites to keep everyone calm. Putting your HQ in Africa for the 30% funding bonus helps in this.
- Funding from an affluent nation like the United States can provide as much as $120 MORE than the poorer nations, which is the cost of two satellites, so keep it in mind.
- When you do major combat missions like the alien base assault
- Seems advisable to save these for when you can really get a lot of use out of the panic reduction.
- Obviously you have to have gone through the main mission objectives to get to this point at all, which can be challenging as you're focusing on satellites.
- Which nation you respond to when abductions happen
- Can be a tough choice. Engineers are really important. Money is too. And panic rises in the two continents you don't respond to. Scientists and new soldiers are almost certainly bad choices in the first few months of the game.