In crude terms, a flag is a message sent by an instrument to another instrument, which enables the latter one to respond by operating in a more efficient manner. The implication is that the data/command loop for responses via the ground would be far too long to be of use for the flagged event.
The coronal extreme ultraviolet and ultraviolet instrumentation on-board SOHO will have limited fields of view and limited telemetry streams. For this reason, one has to examine ways of increasing the efficiency of the available system. Short time scale events may best be detected by one SOHO instrument which may relay information to the others to generate operating mode changes or more precise pointing.
With SOHO we are considering a range of flags which will enable an efficient programme for observing a range of features. The inputs to this study from the various experiment teams have resulted in the list of flags given in Tables 2.2 and 2.3. For each entry we list the event-type, the time-scale needed for a response, the instrument which generates the flag, and the instruments which may wish to respond to such a flag. Since the features being studied are not necessarily as easy to identify as a flare, we also define the nature of the activity which can be monitored in order to generate the flag.
Table 2.2: Transient events to be studied by the use of flags
Table 2.3: Flag generating and receiving instruments on SOHO
Timing (for flag to be read by receiving instrument): within 16 seconds of being detected by generator.The rules and conventions adopted in the generation and reception of flags are described in Annex C.
Form of flag words: 2 x 10 bit words (X, Y location on Sun)
1 x 4 bit word (Identifier)
Notes: all flags sent to all receiving instruments
one flag generating instrument enabled at a time
planned from EOF