The Relationship Between Cusp Region Ion Outflows and East‐West Magnetic Field Fluctuations at 4,000‐km Altitude

A number of interdependent conditions and processes contribute to ionospheric‐origin energetic ( ∼ 10 eV to several keV) ion outflows. Due to these interdependences and the associated observational challenges, energetic ion outflows remain a poorly understood facet of atmosphere‐ionosphere‐magnetosphere coupling. Here we demonstrate the relationship between east‐west magnetic field fluctuations ( Δ B EW ) and energetic outflows in the magnetosphere‐ionosphere transition region. We use dayside cusp region FAST satellite observations made near apogee ( ∼ 4,180‐km altitude) near fall equinox and solstices in both hemispheres to derive statistical relationships between ion upflow and Δ B EWspectral power as a function of spacecraft frame frequency bands between 0 and 4 Hz. Identification of ionospheric‐origin energetic ion upflows is automated, and the spectral powerP EW in each frequency band is obtained via integration of Δ B EWpower spectral density. Derived relationships are of the formJ ‖ , i = J 0 , iP EW γfor upward ion fluxJ ‖ , iat 130‐km altitude, withJ 0 , ithe mapped upward ion flux for a nominal spectral powerP EW = 1  nT2 . The highest correlation coefficients are obtained for spacecraft frame frequencies ∼ 0.1–0.5 Hz. Summer solstice and fall equinox observations yield power law indices γ ≃ 0.9–1.3 and correlation coefficients r ≥ 0 . 92 , while winter solstice observations yield γ ≃ 0.4–0.8 with r ≳ 0 . 8 . Mass spectrometer observations reveal that the oxygen/hydrogen ion composition ratio near summer solstice is much greater than the corresponding ratio near winter. These results reinforce the importance of ion composition in outflow models. If observed Δ B EWperturbations result from Doppler‐shifted wave structures with near‐zero frequencies, we show that spacecraft frame frequencies ∼ 0.1–0.5 Hz correspond to perpendicular spatial scales of several to tens of kilometers.