Corrected photometric uncertainty for the J-band magnitude in the 2MASS survey. If rd_flg=2, this is the measurement error from the profile-fitting procedure, corrected to be consistent with observed repeatability statistics. If rd_flg=1 or 4, this is the RMS of the brightness measured in aperture photometry on the individual frames. If rd_flg=3, the uncertainty is derived from the residuals to the 1-d radial profile fit (averaging over azimuth) to the unsaturated wings of the bright source. If J_MAG_SIG > 8.0, it is a flag value indicating that a meaningful uncertainty could not be determined for the source. This column is null if the magnitude is a 95% confidence upper limit (rd_flg=0 or 6) or if the source is not measurable (rd_flg=9).