Hα intensity oscillations in solar flares

Based on the analysis of high spatial resolution H-alpha filtergrams, we report here the detection of H-alpha intensity oscillations in two extended flares of 15 November 1989 and 20 April 1991. The relative intensities measured with time at 18 different flare and chromospheric locations were analyzed to obtain the oscillation modes, The analysis shows prominent 5 and 3 min modes in flares in addition to their existence in the chromosphere. However, there exists a frequency difference between the flare and chromospheric modes. This frequency deviation of about 300 microhertz is proposed as an influence of higher magnetic field, location of measurements (height) in the chromosphere and high temperature in the flare. We extended our study to a recently observed solar flare on 12 May 1997, and a large sunspot active region on 14 March 1998 with a CCD camera to achieve high time resolution (2 - 10s) in contrast to earlier flares. The relative intensity is measured with the motion of 6 unambiguously identified flare kernels and 15 steady points inside the flare. Our analysis not only confirms the presence of 5 and 3 min oscillation modes but surprisingly shows the prominent modes near 7, 8.3 and 10 mHz in the flare. The similar analysis of filtergrams of 14 March 1998 further confirms the chromosphere oscillates predominantly with the 3 min mode while the plage shows shorter periods of oscillations observed in the same area under study. This indicates that high spatial and time resolution observations are extremely important. Further, the presence of short period oscillations in the hares indicates that the loops and hence flux tubes are oscillating in the active region very fast. We ask whether short period oscillations in flare kernels (foot points of flux tubes) may provide continuously enough energy in the fluxtubes from beneath the convection zone so as to accumulate at the top of the loop in the corona. When the situation of saturation takes place, then a minor instability or a disturbance may cause imbalance between magnetic field and plasma forces in the corona and hence the flare takes place. In this kind of scenario energy for the flare is pumped from beneath in pockets in a magnetic helical form by several oscillation modes, most prominently, perhaps, by > 8.3 mHz modes.