Large‐scale solar wind fluctuations in the inner heliosphere at low solar activity

In this study we investigate the properties of solar wind fluctuations with time scales from 1 hour to 3 days in the inner heliosphere (0.3 to 1.0 AU) close to the minimum of solar cycle 21. Hourly averages of plasma and magnetic field data from Helios spacecraft have been used. Two different ranges in time scale have been considered separately and it has been found that the features of the fluctuations depend dramatically upon the scale. At scales from 1 to 12 hours, Alfvénic correlation is dominant. Alfvénic fluctuations are observed in all the regions investigated and in any solar wind condition. The highest degree of Alfvénicity is found in fast streams near the Sun. No strong evidence of pressure balance structures, as found by other studies in the outer heliosphere, has been obtained. At scales from 0.5 to 3 days the amount of Alfvénic fluctuations is greatly reduced. A positive correlation between fluctuations in thermal and magnetic pressures is progressively built up inside 0.7‐0.8 AU by the interaction between flows with different speed.