Hi Steve,
you can try changing the log level of the authentication log by running the command logconfig on the CLI. Select EDIT and the authentication log file. The log level will be most likely 3 Information. You can try chaning it to 2 Warning and see if that helps.
Log level:
1. Critical
2. Warning
3. Information
4. Debug
5. Trace
Otherwise you will need to filter it out once downloaded from the appliance.
Regards,
Enrico