Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields

rapid rotator of their sample exhibits no emission in Hα. In a subsequent study, Mohanty & Basri (2003) find a drastic drop in activity and a sharp brake in the rotation-activity connection. The Hα emission levels in very late type dwarfs are much lower than in earlier types and often undetectable, in spite of very rapid rotation. The photometric variability of L-type BDs has henceforth been interpreted in terms of clouds and weather, as on planets (Morales-Calderón et al. 2006 and the reference therein). At first glance, magnetic fields and spots seemed to be unimportant for the structure of the atmospheres of old BDs.

The detection of X-ray emission and large flares indicating the presence of a corona and of strong magnetic fields in at least some BDs changed the picture dramatically (Liebert et al. 2003;Burgasser & Putman 2005;Preibisch & Zinnecker 2002;Preibisch et al. 2005;Ozawa et al. 2005;Fleming et al. 2003). The question thus arises whether BDs are like M-dwarfs in this respect. For both active and inactive stars, there is a correlation between the X-ray and the radio emission of the corona, which works for over 10 orders of magnitude in activity level. It was thus very surprising when Berger et al. (2005) discovered that LP944-20 is 4 to 5 orders of magnitude too bright in the radio regime. The same phenomena has also been observed for a few other BDs. The coronae of these objects thus must be quite different from those of normal stars! Observation at 8.46 GHz with the VLA of the old BD 2MASSW J0036159+182110 (from now on called 2M0036+1821) imply a magnetic field strength of ∼ 175 G at about two radii above the surface of the object (Berger 2006). The field strength at the surface must be ≥ 1kG. Thus, it is now clear that at least these old BDs have strong magnetic fields indeed, but are these BDs just like active stars?

It is possible that not only do the coronae differ from those of stars but also the topology of the magnetic field itself. As shown by Dobler et al. (2006), fields of fully convective objects (like BDs) are expected not to be concentrated in small spots but to be distributed on a global scale. Chabrier & Küker (2006) find that the field for fully convective objects should be generated by an α 2 dynamo: the fields on a large scale, and are non-axis symmetric. In this respect it is interesting to note that Zeeman-Doppler imaging observation of a fully convective, rapidly rotating 0.28 M ⊙ -star shows a strong, large-scale, but axisymetric field (Donati et al. 2006). This shows that more observations and theoretical work are needed to understand the fields of fully convective objects. That brown dwarfs are rapid rotators (Zapatero Osorio et al. 2006) must, however, be related to the absence of any winds that are similar to the solar-wind that could brake these objects. Possibly the absence of such winds is related to the topology of the magnetic field, rather than to its strength. A solar magnetic field topology is only expected for very old, very massive BDs, which have conductive cores. The other difference for the solar-like stars is the low temperature of the atmosphere, resulting in a low degree of ionization in the atmosphere, which in turn lead to a very low degree of coupling between the magnetic field and the atmosphere. The coupling between the gas and the magnetic field is usually described in terms of the Reynolds number R m = lυ/η (where l is a length scale, υ a velocity scale, and η the magnetic diffusivity (Priest 1982).

Following a suggestion by Meyer & Meyer-Hofmeister (1999), Mohanty et al. (2002) studied the conductivity of the atmospheres of late M and L dwarfs. They find that the atmospheres have very high electrical resistivities because they are predominantly neutral. For example, ionization fraction at τ J-band = 1 is only between 10 -5.5 and

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