In situ measurements of plasma irregularity growth in the cusp ionosphere

The Investigation of Cusp Irregularities (ICI‐2) sounding rocket was launched on 5 December 2008 from Ny‐Ålesund, Svalbard. The high‐resolution rocket data are combined with data from an all‐sky camera, the EISCAT Svalbard Radar, and the SuperDARN Hankasalmi radar. These data sets are used to characterize the spatial structure of F region irregularities in the dayside cusp region. We use the data set to test two key mechanisms for irregularity growth; the Kelvin‐Helmholtz (KH) and gradient drift (GD) instabilities. Except for a promising interval of 4–6 km irregularities, the KH growth rate was found to be too slow to explain the observed plasma irregularities. The time history of the plasma gives further support that structured particle precipitation could be an important source of kilometer‐ to hectometer‐scale “seed” irregularities, which are then efficiently broken down into decameter‐scale irregularities by the GD mechanism.