Prominence height shows the proximity of an ejection

The onset of a coronal mass ejection (CME) is not preceded by any specific form of activity that could be recognized several days before the event. The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fills the whole volume of the dark cavity stretched in the corona along the photospheric polarity inversion line. Cold dense prominence matter accumulates in the lower parts of helical flux tubes, which serve as magnetic traps in the gravitation field. So, prominences and filaments are the best tracers of the flux ropes in the corona long before the beginning of eruption. A twisted flux rope is held by the tension of field lines of photospheric sources until parameters of the system reach critical values and catastrophe happens. The flux rope height above the photosphere is one of these parameters and it is revealed by the height of the filament. We had analyzed some 80 of filaments and found that eruptive prominences were near the limit of stability a few days before eruptions. We believe that a comparison of the real heights of prominences with the calculated critical heights could be helpful for revealing the filaments that are prone to erupt.