Its original purpose of HAARP was to ionize the atmosphere. You would then be able to bounce radio signals off the conductive layer over the horizon for long range communications. This study was then made a bit irrelevant once satellites became the primary method of communication. Then they switched to studying the effects the sun has on radio communication and the atmosphere. As the sun also Ionizes the atmosphere quite drastically, so much so you can actually see the atmosphere ionize (we call it the Aurora Borealis). Problem is the aurora dosn't tend to happen where you want it to and in controlled amounts. So instead they use HAARP to ionize the atmosphere. However the amount it can is only a fraction of what the sun is able to, so you cant actually see the auroras it creates.
Since HAARP is broadcasting on frequencies similar to what the sun has be putting out for millions of years but at drastically lower amounts, pretty safe to assume HAARP isn't dangerous.