Subject: Why does the air temperature increase with altitude in the stratosphere?

I understand from 
Subject: Re: where atmosphere layers got their names
what the layers of the atmoshpere are and that some layers have decreasing 
temperature with increasing altitude.  

It would seem to me that air temperature increases with altitude in the 
Thermoshpere because of the Sun's particles hitting the earth's 
atmosphere's particles.  These collisions increase the movement of the 
particles, thereby increasing the temperature.  Inform me if I am wrong, 
please.

But what is it about the stratosphere that causes the air temperature to 
increase with altitude?

Nice site, by the way.  I saw it in Popular Science.  

Thank you.

Re: Why does the air temperature increase with altitude in the stratosphere?