  Previous studies have identified two emission modes in PSR B1859+07: a normal
mode that has three prominent components in the average profile, with the
trailing one being the brightest, and an anomalous mode (i.e. the A mode) where
emissions seem to be shifted to an earlier phase. Within the normal mode,
further analysis has revealed the presence of two sub-modes, i.e. the cW mode
and cB mode, where the central component can appear either weak or bright. As
for the anomalous mode, a new bright component emerges in the advanced phase
while the bright trailing component in the normal mode disappears. New
observations of PSR B1859+07 by using the Five-hundred-meter Aperture Spherical
radio Telescope (FAST) have revealed the existence of a previously unknown
emission mode, dubbed as the Af mode. In this mode, all emission components
seen in the normal and anomalous modes are detected. Notably, the mean
polarization profiles of both the A and Af modes exhibit an orthogonal
polarization angle jump in the bright leading component. The polarization
angles for the central component in the original normal mode follow two
distinct orthogonal polarization modes in the A and Af modes respectively. The
polarization angles for the trailing component show almost the same but a small
systematic shift in the A and Af modes, roughly following the values for the cW
and cB modes. Those polarization features of this newly detected emission mode
imply that the anomalous mode A of PSR B1859+07 is not a result of ``phase
shift" or ``swooshes" of normal components, but simply a result of the varying
intensities of different profile components. Additionally, subpulse drifting
has been detected in the leading component of the Af mode.
