This map of May 20, 2012 covers from 23:05z to 23:30z.
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This map shows the evolution, size and MUF of the sporadic-E cloud that caused the opening on 144 MHz. and it is an estimation based on the information from DX-Spots collected by DXMAPS. I don't claim it to be 100% accurate but just an estimation that could help understand what happened that day.
Also please notice that sometimes you will see MUF rise very quickly. This is because of the almost complete lack of data for frequencies between 51 and 144 MHz. that leads to shown MUF rising almost 90 MHz in one minute. You should understand that this is not the usual behaviour of MUF, but the result of that lack of data.