I have some actual weather data that contains global solar radiation values. I have noticed that it has positive and negative values at night time, please see below. Should it be zero during night time or I am missing something?

P.S: The unit is wrong for global solar radiation.

I have some actual weather data that contains global solar radiation values. I have noticed that it has positive and negative values at night time, please see below. Should it be zero during night time or I am missing something?

P.S: The unit is wrong for global solar radiation.