Output data formats

Three kinds of data

Vlasiator produces three kinds of output files during a simulation run, the contents of which vary based on simulation parameters:

logfile.txt, the simulation run log. This is a timestamped ascii file providing basic diagnostic output of the run, including memory usage, time steps etc.
diagnostic.txt. The contents of this file can be configured by the diagnostic = options in the run config file. In general, this ascii file will contain one line per simulation timestep, with the columns determined by the selected data reducers. These include, for example, simple scalar values like overall plasma mass, number of velocity space blocks in the simulation, charge balance, divergence of magnetic field etc.
VLSV files are the main output data products. These files come in multiple varieties:

Restart files. These contain the whole simulation state, including the full phase space density, all relevant electromagnetic fields and metadata. Simulations can be restarted from them (hence the name), but they tend to be very heavy, easily multiple terabytes in size for production runs.
Bulk files. In these, reduced spatial simulation data is written for further scientific analysis. Usually, this includes moments of the distribution functions and electromagnetic fields, but can also contain much more complex data reducer operators, as listed below. It is also possible to configure a subset of the velocity distribution functions to be written for further analysis.