On the Force-Freeness of the Photospheric Sunspot Magnetic Fields as Observed from Hinode (SOT/SP)

Abstract

A magnetic field is force-free if there is no interaction between the magnetic field and plasma in surrounding atmosphere i.e., electric currents are aligned with the magnetic field, giving rise to zero Lorentz force. Computation of various magnetic parameters such as magnetic energy, gradient of twist of sunspot fields and any kind of extrapolations, heavily hinge on the force-free approximation of the photospheric sunspot magnetic fields. Thus it is important to inspect the force-freeness of sunspot fields. The force-freeness of sunspot magnetic fields has been examined earlier by some researchers ending with incoherent results. Accurate photospheric vector field measurements with high spatial resolution are required to inspect the force-free nature of sunspots. We use several such vector magnetograms obtained from the Solar Optical Telescope/Spectro-Polarimeter aboard the Hinode. Both necessary and sufficient conditions for force-freeness are examined by checking global and local nature of magnetic forces over sunspots. We find that the sunspot magnetic fields are not much away from force-free configuration, although they are not completely force-free on the photosphere. The umbral and inner penumbral fields are more force-free than the middle and the outer penumbral fields. During their evolution, sunspot magnetic fields are found to maintain their proximity to force-free behaviour. Although a dependence of net Lorentz force components is seen on the evolutionary stages of the sunspots, we don't find a systematic relationship between the nature of sunspot fields and associated flare activity. Further, we examine whether the fields at photosphere follow linear or non-linear force free conditions. After examining this in various complex and simple sunspots we conclude that,in either case,the photospheric sunspot fields are closer to satisfy non linear force-free field approximation.