ULF fluctuations observed along the SEGMA array during very low solar wind density conditions

• Ground occurrence of ULF waves in correspondence to very low solar wind densities.
• The solar wind density is important in the process of upstream wave generation.
• ULF waves are detected in the magnetosheath and correspondently on the ground.

We examined several events of geomagnetic field fluctuations related to upstream waves, focusing the attention on the role of the solar wind density in the control of the wave activity. Recently, it was suggested that, when the solar wind density is lower than 2 cm−3, ULF wave activity on the ground dramatically drops. Based on geomagnetic field measurements conducted at low latitude along the SEGMA array, our analysis shows that, for extremely low solar wind densities (below 1 cm−3), the wave activity on the ground disappears, regardless of cone angle conditions favourable to the transmission of upstream waves through the subsolar magnetopause and independently on the solar wind speed; conversely, for solar wind densities in the range ∼1–2 cm−3, ULF fluctuations can be still observed, during favourable cone angle conditions, if the solar wind speed is higher than ∼600 km/s. Such results are confirmed by the analysis of simultaneous measurements on the ground and from the Cluster spacecraft, when moving in the upstream region and in the magnetosheath, during one of the examined events. We suggest that, for extremely low SW densities, the wave generation mechanism is not operating efficiently, since very few ions are reflected by a very weak bow shock.