The IceTop array should trigger at near the 90% level for air showers above 500 TeV, which will require stations of roughly 7 m2 area located near the top of each string. The tanks should have a dynamic range of ∼ 104 to cover the PeV–EeV energy range without saturation over the full lateral extent of the showers. This will likely require using separate low gain and high gain modules within each tank.
To reduce the background rate each station should consist of two tanks, each roughly 3.5 m2 in area. A local coincidence should be implemented between neighboring tanks to eliminate single muon events as well as reduce the likelihood of triggering on random electron and photon hits not associated with larger air showers. The separation of these individual tanks should be large enough to keep the number of local triggers to a level that can be handled by the DOM communications, perhaps several meters.
A denser subarray within IceTop should consist of smaller tanks on the order of 1 m2 area separated by 10 m so that they will have a much lower threshold near 10 TeV to trigger on showers most likely to contain only one high energy muon. This subarray could be composed of the present SPASE array with an embedded standard IceTop station. The subarray should record only a fraction of the data that triggers depending on what is needed for calibration and most acceptable for the DAQ.
In addition, it is possible that we will decide to record as much of this low energy data as DOM communications will allow, storing several days of data in a disk cache at the Pole. This cache should be accessed when interesting IceCube events come in over that same time, when all low energy surface array data from a wide time window on each side of the interesting events should be separated out and stored for later investigations.
