Have at look at this page on SDL Live Content, which explains various types of scenarios for connecting as different users:
As per the docs, instead of impersonation you may want to establish your Core Service connection as follows using NetworkCredential:
using (ChannelFactory<ISessionAwareCoreService> factory =
new ChannelFactory<ISessionAwareCoreService>("netTcp_2011"))
{
NetworkCredential networkCredential =
new NetworkCredential("username", "password", "domain");
factory.Credentials.Windows.ClientCredential = networkCredential;
ISessionAwareCoreService client = factory.CreateChannel();
Console.WriteLine(client.GetCurrentUser().Title);
}