Q:

What are SEP events in this article?

I am studying this paper; at page 5 it says:

The occurrence of SEP events with fluences larger than the saturation
  limit was estimated to be less tha n one per year for the period the
  LPF spacecraft remained into orbit around L1

but what are SEP events?

A:

A SEP is a Solar Energetic Particle: basically a cosmic ray, but specifically from the sun. These particles come in bursts, called SEP events.
Some relevant links:
http://lasp.colorado.edu/home/maven/science/instrument-package/sep/
http://lasp.colorado.edu/home/maven/files/2017/01/The-MAVEN-Solar-Energetic-Particle-Investigation.pdf
https://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=2013-063A-05

