Evidence for long period Alfvén waves in the inner solar system

Magnetic field and plasma data of Helios 1 and 2 between 0.3 and 1 AU have been used to investigate the Alfvénic character of the solar wind fluctuations with periods above 1 hour. Clear evidence for the existence of very long period Alfvén waves, up to ∼15 hours (in the spacecraft frame) close to the sun, has been obtained. The observations at different heliocentric distances suggest that the longest wavelengths are removed from the Alfvénic regime leaving the sun.