Magnetic Clouds along the solar cycle: expansion and magnetic helicity

DASSO, S.; DEMOULIN, P.; GULISANO A.M.

Comparative Magnetic Minima: characterizing quiet times in the Sun and stars

Cambridge University Press

Año: 2012; p. 139 - 148

Magnetic clouds (MCs) are objects of extreme importance in the heliosphere. Theyhave a major role on releasing magnetic helicity from the Sun (with crucial consequences onthe solar dynamo), they are the hugest transient object in the interplanetary medium, and themain actors for the Sun-Earth coupling. The comparison between models and observations isbeginning to clarify several open questions on MCs, such as their internal magnetic configurationand their interaction with the ambient solar wind. Due to the decay of the solar wind pressurewith the distance to the Sun, MCs are typically in expansion. However, their detailed andlocal expansion properties depend on their environment plasma properties. On the other hand,while it is well known that the solar cycle determines several properties of the heliosphere, theeffects of the cycle on MC properties are not so well understood. In this work we review twomajor properties of MCs: (i) their expansion, and (ii) the magnetic flux and helicity that theytransport through the interplanetary medium. We find that the amount of magnetic flux andhelicity released via MCs during the last solar minimum (years 2007-2009) was significantly lowerthan in the previous one (years 1995-1997). Moreover, both MC size and mean velocity are inphase with the solar cycle while the expansion rate is weakly variable and has no relationshipwith the cycle.