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Context: There is now growing evidence that some brown dwarfs (BDs) have verystrong magnetic fields, and yet their surface temperatures are so low that thecoupling is expected to be small between the matter and the magnetic field inthe atmosphere. In the deeper layers, however, the coupling is expected to bemuch stronger. Aims: This raises the question of whether the magnetic fieldstill leads to the formation of structures in the photosphere. Methods: Wecarried out a spectroscopic monitoring campaign of two ultracool dwarfs thathave strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110.LP944-20 was observed simultaneously in the optical and in the near infraredregime, 2MASSW J0036159+182110 only in the infrared. Results: Both dwarfsturned out to be remarkably constant. In the case of LP944-20, theTeff-variations are <50K, and the rms-variations in the equivalent widths ofHalpha small. We also find that the equivalent widths of photospheric lines areremarkably constant. We did not find any significant variations in the case of2MASSW J0036159+182110 either. Thus the most important result is that nosignificant variability was found at the time of our observations. Whencomparing our spectra with spectra taken over the past 11 years, we recognizesignificant changes during this time. Conclusions: We interpret these resultsas evidence that the photosphere of these objects are remarkably homogeneous,with only little structure in them, and despite the strong magnetic fields.Thus, unlike active stars, there are no prominent spots on these objects.

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