Accretion-Ejection Instability and a "Magnetic Flood" scenario for GRS 1915+105

We present an instability, occurring in the inner region of magnetized accretion disks, which seems to be a good candidate to explain the low-frequency QPO observed in many X-ray binaries. We then briefly show how, in the remarkable case of the microquasar GRS 1915+105, identifying this QPO with our instability leads to a scenario for the $\sim$ 30 mn cycles of this source. In this scenario the cycles are controlled by the build-up of magnetic flux in the disk.