Data from the IMAGE SI-13 instrument are used to study depletions in the OI 135.6 nm dayglow intensity due to two magnetic storms observed on 8 June and 15 July 2000. Results show that the SI-13 instrument is sensitive to changes in the 135.6 nm dayglow caused by depletion of the O/N2 column density ratio. It is found that depletion levels in the SI-13 images are weaker than those in the true O/N2 values or electron density parameters, such as NmF2 (peak F2 electron density) due to the N2 LBH contribution in the SI-13 intensities. A depletion of -20% (8 June 2000) and -50% (15 July 2000) in the SI-13 intensities corresponds to -50% and -90% reductions in NmF2, respectively. AURIC simulations indicate that -67% is the maximum depletion level that can be seen in the SI-13 intensities. The Millstone radar, digisonde and DMSP observations reveal that the electron density depletion extended from the low F-layer altitudes up to 840 km. Owing to the prevailing thermospheric circulation pattern, the depletions cover a wide area (at least 1/8 Earth surface) over the Northern Hemisphere for both of the cases. A deep depletion was always seen first in the morning side and then at later local times. The atmosphere took about 12 hours (8 June case) and more than 24 hours (15 July case) to recover. The results from the SI-13 images are in a good agreement with digisonde, radar, and DMSP observations. The depletion in the SI-13 intensity can be explained by the Joule and particle heating in the high-latitude regions. The heating has two effects: (1) The heated air is nitrogen-rich/oxygen-depleted and (2) a wind surge created by the heating in the night side moves the oxygen-deplete air upward and transports it to lower latitudes together with the neutral wind. Corotation with the Earth brings the oxygen-depleted air to the dayside.