Ion heating by broadband low‐frequency waves in the cusp/cleft

Ion conic distributions are often observed in the cusp/cleft region of the dayside magnetosphere. We show that these ions can be heated via the process of cyclotron resonance with broadband low‐frequency (near the ion gyrofrequency) waves. Data from two cusp/cleft crossings of the polar‐orbiting DE 1 satellite are studied in detail. There is very good agreement between the onset of low‐frequency waves and the onset of ion heating. Observed cool O+ distributions and observed wave intensities from one orbit are used as input to a Monte Carlo simulation. Given the assumptions underlying the simulation model, the resulting hot O+ distributions are in good agreement with the corresponding observed distributions. We explore this agreement using a further simplified analytic model which accounts for much of the agreement and many of the discrepancies. The mean ion energies of about 200 eV obtained from the simulation agree well with several minutes of observations, corresponding to a distance of nearly 1000 km along the satellite orbit. The O+ distribution functions from both simulation and observations show that heating near the equatorward edge of the cusp/cleft region is rather local, while ions observed well inside this region may be heated over altitudes of several thousand kilometers. This resonant heating by broadband low‐frequency waves is important for the outflow of ionospheric ions into the magnetosphere.