  We compare the size distributions of self-organized criticality (SOC) systems
in the solar photosphere and the transition region, using magnetogram data from
Helioseismic and Magnetic Imager (HMI) and Interface Region Imaging
Spectrograph (IRIS)} data. For each dataset we fit a combination of a Gaussian
and a power law size distribution function, which yields information on four
different physical processes: (i) Gaussian random noise in IRIS data; (ii)
spicular events in the plages of the transition region (described by power law
size distribution in IRIS data); (iii) salt-and-pepper small-scale magnetic
structures (described by the random noise in HMI magnetograms); and (iv)
magnetic reconnection processes in flares and nanoflares (described by power
law size distributions in HMI data). We find a high correlation (CCC=0.90)
between IRIS and HMI data. Datasets with magnetic flux balance are generally
found to match the SOC-predicted power law slope a_F=1.80 (for mean fluxes F),
but exceptions occur due to arbitrary choices of the HMI field-of-view. The
matching cases confirm the universality of SOC-inferred flux size
distributions, and agree with the results of Parnell et al.~(2009), a_F=1.85
+/- 0.14.
