HttpContext.Current.User.Identity.IsAuthenticated

Hi, 

i have two web applications in my asp.net solution. One of them is portal and other one is forum.. When user login from portal,

then click to forum link; i want to authenticate user in forum application too.. I m using asp.net forms authentication in my applications

but when i login from portal, and then go to forum "HttpContext.Current.User.Identity.IsAuthenticated" value is false..

How can i set it true or how can i set user logged in in other web application?

thanks in advance..
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