Close hyperlink's popup window

[mg]

The hyperlink code below causes a new page to popup above the main page,
where the code below runs.

<asp:HyperLink
id="HyperLink1"
runat="server"
NavigateUrl="http://www.microsoft.com"
Target="_new">
HyperLink
</asp:HyperLink>

How can I close the popup window using code that runs in the main page?

I use C#.