Upper panel: electron column density fluctuations ΔN_e from each single station at a solar offset of 4.9 R_s on 2017 July 29 and 9.9 R_s on 2017 August 3. Middle panel: the corresponding differential electron column density ﹩{\rm{\Delta }}{N}_{e}^{\mathrm{AB}}﹩ in the spatial scale of the baseline length. Lower panel: power-density spectra of ΔN_e and ﹩{\rm{\Delta }}{N}_{e}^{\mathrm{AB}}﹩. Comparing with ΔN_e, the power density of ﹩{\rm{\Delta }}{N}_{e}^{\mathrm{AB}}﹩ decreases significantly when ν < 0.01 Hz.