What is the spatial distribution of magnetic helicity injected in a solar active region?
Etienne Pariat Submitted: 2006-03-06 02:38
Magnetic helicity is suspected to play a key role in solar phenomena
such as flares and coronal mass ejections. Several investigations have
recently computed the photospheric flux of magnetic helicity in
active regions. The derived spatial maps of the helicity flux density,
called GA, have an intrinsic mixed-sign patchy distribution.
Pariat et al. (2005) recently showed that GA is only a proxy of
the helicity flux density, which tends to create spurious polarities.
They proposed a better proxy, G heta. We investigate here the
implications of this new approach on observed active regions.
The magnetic data are from MDI/SoHO instrument and the photospheric
velocities are computed by local correlation tracking.
Maps and temporal evolution of GA and G heta are compared using
the same data set for 5 active regions.
Unlike the usual GA maps, most of our G heta maps show
almost unipolar spatial structures because the nondominant helicity
are significantly suppressed.
In a few cases, the G heta maps still contain spurious
bipolar signals. With further modelling we infer that the real
helicity flux density is again unipolar.
On time-scales larger that their transient temporal variations, the
time evolution of the total helicity fluxes derived from GA and
G heta show small differences. However, unlike GA,
with G heta the time
evolution of the total flux is determined primarily by the
predominant-signed flux while the nondominant-signed flux is roughly
stable and probably mostly due to noise.
Our results strongly support the
conclusion that the spatial distribution of helicity injected into
active regions is much more coherent than previously thought: on the
active region scale the sign of the injected helicity is
predominantly uniform. These results have implications for the
generation of the magnetic field (dynamo) and for the physics of both flares and coronal mass ejections.
Authors: Pariat E., Nindos A., Démoulin P., Berger M.A.
Publication Status: A&A accepted
Last Modified: 2006-03-06 02:38