Magnetic field and unstable accretion during AM Herculis low states

A study of AM Her low states in September 1990 and 1991 and June-July 1997 is reported from a coordinated campaign with observations obtained at the H aute-Provence observatory, at the 6-m telescope of the Special Astrophysica l Observatory and at the 2.6m and 1.25m telescopes of the Crimean observato ry. Spectra obtained at different dates when the source was in low states a t a comparable V magnitude, show the presence of strong Zeeman absorption f eatures and marked changes in emission lines with a day-to-day reappearance of the Hell (4686 Angstrom) emission lines in 1991. Despite this variabili ty, the magnetic held inferred from the fitting of the absorption spectrum with Zeeman hydrogen splitting, is remarkably constant with a best value of (12.5+/-0.5)MG. Detailed analysis of the UBVRI light curves shows the pres ence of repetitive moderate amplitude (similar to 0.3-0.5 mag) flares predo minantly red in colour. These flares are attributed to small accretion even ts and are compared to the large (similar to 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We suggest that the general flaring activity ob served during the low states is generated by accretion events. The differen t characteristics of the flares (colour and polarization) are the results o f different shock geometries depending on the net mass accretion flux.