From Pascal.Demoulin@obspm.fr Tue May 27 17:50:29 2003 Date: Fri, 25 Apr 2003 17:40:57 +0200 From: Pascal Demoulin To: mitch Berger , Pablo Mininni , Richard Canfield , Alexander Nindos , BC Low , Jim Klimchuk , Marcelo Lopez-Fuentes , Sarah Gibson , sakurai@solar.mtk.nao.ac.jp, Terry Forbes , "Brian T. Welsch" , dana@mithra.physics.montana.edu, magara@mithra.physics.montana.edu, apevtsov@nso.edu, chae@cnu.ac.kr, haimin@sundog.caltech.edu, yjmoon@bbso.njit.edu, kusano@hiroshima-u.ac.jp Subject: 2 new papers on magnetic helicity [Part 1, Text/PLAIN 66 lines] [Unable to print this part] Dear colleagues, Magnetic helicity has became a hot subject in these past years. Here are two new contributions to the subject. Your comments are well come !! Hoping this will continue to be a fast developing subject in the coming years ! They are posted on the Max Millenium E-print archive http://solar.physics.montana.edu/cgi-bin/eprint/index.pl But you can also got them directly on my web page http://www.solaire.obspm.fr/demoulin/newest.html Magnetic energy and helicity fluxes at the photospheric level Demoulin, P., Berger, M.A. , Solar Physics in press The source of coronal magnetic energy and helicity lies below the surface of the sun, probably in the convective zone dynamo. Measurements of magnetic and velocity fields can capture the fluxes of both magnetic energy and helicity crossing the photosphere. We point out the ambiguities which can occur when observations are used to compute these fluxes. In particular, we show that these fluxes should be computed only from the horizontal motions deduced by tracking the photospheric cut of magnetic flux tubes. These horizontal motions include the effect of both the emergence and the shearing motions what ever the magnetic configuration complexity is. We finally analyze the observational difficulties involved in deriving such fluxes, in particular the limitations of the correlation tracking methods. Obervations of magnetic helicity (review) van Driel-Gesztelyi L., Demoulin P., Mandrini C.H, Adv. Space Research, in press. The first observational signature of magnetic helicity in the solar atmosphere (sunspot whirls) was discovered 77 years ago. Since then, the existence of a cycle-invariant hemispheric helicity pattern has been firmly established through current helicity and morphological studies. During the last years, attempts were made to estimate/measure magnetic helicity from solar and interplanetary observations. Magnetic helicity (unlike current helicity) is one of the few global quantities that is conserved even in resistive magnetohydrodynamics (MHD) on a timescale less than the global diffusion timescale, thus magnetic helicity studies make it possible to trace helicity as it emerges from the sub-photospheric layers to the corona and then is ejected via coronal mass ejections (CMEs) into the interplanetary space reaching the Earth in a magnetic cloud. We give an overview of observational studies on the relative importance of different sources of magnetic helicity, i.e. whether photospheric plasma motions (photospheric differential rotation and localized shearing motions) or the twist of the emerging flux tubes created under the photosphere (presumably by the radial shear in the differential rotation in the tachocline) is the dominant helicity source. We examine the sources of errors present in these early results and try to judge how realistic they are. My best wishes, Pascal *====================================================================* Pascal Demoulin Phone: 33 1 45 07 78 16 Observatoire de Paris Fax: 33 1 45 07 79 59 section Meudon, LESIA, Bat. 14 http://www.solaire.obspm.fr/demoulin/ F-92195 Meudon Principal Cedex Pascal.Demoulin@obspm.fr France *====================================================================*