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The DEMETER satellite was launched on June 29, 2004 into a polar, quasi-helio-synchronous circular orbit at 710 km altitude in the ~10:30-22:30 LT plane, operating nearly continuously at invariant latitudes of less than ~ 60°. The payload includes an ion analyzer and a Langmuir probe to study the thermal ionospheric plasma and a wave package that measures 3 components of electric fields in the frequency range from DC to 3 MHz and 3 components of magnetic fields from 10 Hz to 20 kHz. Although the main objective of the DEMETER mission is to search for ionospheric effects of seismic activity, the instruments gather data that are also highly relevant to many areas of ionospheric and space weather research. After a brief overview of the DEMETER satellite and its instruments, we report observations of equatorial plasma depletions that were gathered in the nighttime equatorial ionosphere on two satellite passes during the initial phase of a major magnetic storm on November 10, 2004. Deep plasma depletions of nearly 3 orders of magnitude were observed on both passes with an extension of more than 2000 km along the satellite orbit. Either the satellite travelled within an elongated, depleted "spread-F" flux-tube over nearly 20° latitude, or the F-peak had risen well above DEMETER's trajectory at 710 km. Within the region of very low density plasma, the most striking observations are the presence of intense plasma waves at the lower hybrid (LH) frequency near 1.2 kHz as well as ELF emissions near 280 Hz that is well below the hydrogen cyclotron frequency. Emissions near the LH frequency are observed to be triggered by strong upward-propagating whistlers. Enhanced electrostatic turbulence, up to 200 Hz above the LH frequency, lasts for ~ 20 seconds and is followed by a nearly pure sine wave reminiscent of lower hybrid auroral solitons with amplitudes of a few mV/m. The ELF emissions are characterized by a stable, narrow frequency spectrum with a superimposed, irregular ULF amplitude modulation. Detected only inside the deep depletions with a very sharp drop at the edges, such emissions appear as electromagnetic waves generated and efficiently trapped within the plasma depletion. The ion measurements also provide a new view of equatorial plasma depletions. Fe ions were detected with small but measurable densities in the undisturbed plasma outside of the depletion, whereas the presence of NO ions were detected inside the depletion with variable densities that are, on average, a few percent of the O densities detected by the same instrument. This composition is similar to that at altitudes of about 200 km where instabilities may initially lift the low density plasma up, into the upper F-region. Furthermore, inside the depletion, a number of ion energy spectra obtained by the retarding potential analyzer show the existence of suprathermal ions with temperatures ranging from a few eV to ~ 20 eV and densities between 0.1% to 1% of the major ions. Heating of thermal ions by the LH waves may be one mechanism responsible for the observed suprathermal energy distribution.