Time Variations of the Nonpotential and Volume-threading Magnetic Helicities

Relative magnetic helicity is a gauge-invariant quantity suitable for the study of the magnetic helicity content of heliospheric plasmas. Relative magnetic helicity can be decomposed uniquely into two gauge-invariant quantities, the magnetic helicity of the nonpotential component of the field and a complementary volume-threading helicity. Recent analysis of numerical experiments simulating the generation of solar eruptions have shown that the ratio of the nonpotential helicity to the total relative helicity is a clear marker of the eruptivity of the magnetic system, and that the high value of that quantity could be a sufficient condition for the onset of the instability generating the eruptions. The present study introduces the first analytical examination of the time variations of these nonpotential and volume-threading helicities. The validity of the analytical formulae derived are confirmed with analysis of 3D magnetohydrodynamics (MHD) simulations of solar coronal dynamics. Both the analytical investigation and the numerical application show that, unlike magnetic helicity, the nonpotential and the volume-threading helicities are not conserved quantities, even in the ideal MHD regime. A term corresponding to the transformation between the nonpotential and volume-threading helicities frequently dominates their dynamics. This finding has an important consequence for their estimation in the solar corona: unlike with relative helicity, their volume coronal evolution cannot be ascertained by the flux of these quantities through the volume's boundaries. Only techniques extrapolating the 3D coronal field will enable both the proper study of the nonpotential and volume-threading helicities and the observational analysis of helicity-based solar-eruptivity proxies.