BARREL Observations of Electron Precipitation Driven by Solar Wind Periodic Density Structures

The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission was designed to study the loss of radiation belt electrons to the upper atmosphere. Measurements of losses are in the form of bremsstrahlung X-rays generated from the collision of electrons with the Earth’s atmosphere. Analysis of data from balloon pairs showed that electron precipitation events manifest coherence on long time scales, on the order of ≈60 minutes, and large-scale size, encompassing mostly the near noon local times. These events are frequently unrelated to internal magnetospheric activity, suggesting that the loss mechanism is related to the changes in solar wind driving involving modulation of the magnetospheric cavity size (forced breathing) and/or triggering of Ultra Low Frequency (ULF) waves. In fact, these magnetospheric field fluctuations can determine electron loss directly, via loss cone modulation, or indirectly, triggering the growth of hiss waves which in turn scatter electrons, causing an increase in the precipitation loss. Among the possible solar wind driving processes, we focused on the magnetospheric “forced breathing” due to solar wind periodic density structures, which are frequently remnant of solar corona processes. We performed a spectral analysis on time series of X-ray counts, from both the first and second campaign of the BARREL mission, and solar wind density measurements from the Wind spacecraft. Considering the solar wind transit time from Wind to the magnetosphere, we selected the time intervals in which the same periodicity occurred in the solar wind and BARREL mission observations. Events that occurred in proximity of other sources of X-rays (e.g., flares, solar energetic particles, and galactic cosmic rays) are removed from our analysis. Here, we show preliminary results of our investigation discussing the general properties of these events in terms of duration, location, and observed periodicity.