On 11G user and system traces are on the same directory (%DIAGNOSTIC_DEST%\diag\rdbms\SID\trace on my machine) and automatic health monitoring looks like a bit more active than 10G. On 10G I wasn’t seeing automatic trace generation for user sessions, however on 126.96.36.199 Oracle nearly creates trace for every session and it realy drives me mad when I am working on 10046 traces. I checked unsupported parameters and I found _disable_health_check parameter. When you set this parameter to TRUE a Oracle stops generating trace files for every user session. Parameter needs restart of database.
It looks like there are 32 other diagnostic related parameters on 11G but _disable_health_check
was enough to solve my problem.
I can see this parameter is available on 10GR2 and 11GR1 but not available at least on 188.8.131.52
!!!!! As you see, it is unsupported dont use it on any production system without asking Oracle Support.