Fireflies for my burning earth I watched this blender guru tutorial on Making Fire. Around 34:00 he starts explaining how to use a particle system to create "fireflies". The result is something like this



Now I have this idea to create a "burning earth". For this I have a UV sphere inside the earth as an emitter of fire and smoke. And the smoke domain is placed like this.



This results in an image like this.



So far so good. But when I add a particle system to my "emitter sphere" the whole sphere is emitting fireflies and I want only fireflies around my fire. I thought to solve that by creating a circle under the fire and add a particle system to that circle. But how to get this working like that the circle emits fireflies that are driven upwards by the smoke domain ?? 

I guess (with hindsight) this question is a possible duplicate for this question How can I control where smoke and fire are emitted from? but I think because my question is specific for the Particle System it adds new knowledge to Blender SE
You should either define a vertex group on your sphere and change the particle settings (also for your fire and smoke) to limit emission to that vertex group or, seeing that you seem to be using this sphere for nothing else, simply delete the parts of the mesh that should not emit particles or smoke.

Quick explanation of vertex group approach:


Select your mesh, 
go into Edit mode, 
then select the vertices/faces that you want to emit particles/smoke, 
then go to the object data tab and create a new vertex group by clicking the "+"-button in that area. You should give the  group a meaningful name. 
Then click the "Assign"-button beneath the list of vertex groups while your new group is selected and the selected vertices will be assigned to that group with a weight of 1.0. 
Next, go to the particle settings under Vertex Groups and select your new group for the Density setting.
For smoke, look at the Smoke Flow Advanced section of the emitter object's smoke settings. You can select the vertex group there.