How active can the Sun be?

The last half of solar cycle 23 (SC23) and SC24 quantified by the MgII index (left) and a "photograph" of the Sun at the MgII emission line (right).

Activity on the Sun has a direct influence on the space environment near the Earth. We have been actively measuring solar activity throughout the space age, i.e. starting in the late 1970s. Some indicators of solar activity, such as sunspots, have been measured for hundreds of years. Those records show long stretches of very low activity, and some periods of high activity. The natural question is: “What are the limits of solar activity?” Using observations from the Interface Region Imaging Spectrograph (IRIS), we can assemble a hyperspectral image of the Sun.

Every pixel in the image has a spectrum, and can be correlated to magnetic field information from the Solar Dynamics Observatory (SDO). What would a theoretical Sun composed of all quiet pixels do to the Earth environment? Perhaps more importantly, what is the maximum activity level of the Sun, and how would it impact space weather?