A statistical investigation of the slowly varying component of the 9.1-cm solar radio emission is based upon the Stanford radioheliograms covering the years 1962-66. On the average the peak value of the brightness temperature Tb is proportional to the area covered by the corresponding spot group. However, in individual cases the observed Tb is definitely lower or higher than is to be expected from the size of the spot group. We introduce the concept 'microwave importance'/in of the spot group, which is the Tb to be expected from the Zi.irich class and spot number; and the concept 'relative brightness' Br, which is the ratio of the observed Tb to Ira. This leads to the distinction of 'faint', 'normal' and 'bright' sources with Br ~< 0.8, 0.8 < Br < 1.2 and Br >~ 1.2 respectively. Br is correlated with the maximum magnetic-field strength H observed in the spot group and with the flux-density spectrum of the source. The yearly average of Br and the average flux-density spectrum vary with the phase of the solar cycle.An analysis of the results is based upon the electron-density distribution in the condensation, which was visible at the solar limb during the eclipse of February 5, 1962, and on an adopted temperature distribution with a central value of 4 • 106 K. The computed Tb, including gyro-resonance absorption, agrees with the value derived from the microwave importance of the spot group and Br = 0.5, which shows that in the current gyro-resonance models the electron density is underestimated. The variation of Tb with the size of the spot group can be explained by varying the dimensions of the condensation in area and in height, if the central density and temperature remain constant. The statistical relationships between B~ and H and between B~ and the flux-density spectrum yield a model for the differences between faint and bright sources: Br increases with the contribution from the gyro-resonance absorption and with the central electron density.
