Conjugate observation of electron microburst groups by Bremsstrahlung X ray and riometer techniques

The first evidence is reported of simultaneous conjugate electron microburst group precipitation. Groups of bremsstrahlung X ray microbursts (E >25 keV) were observed during a substorm recovery phase by a balloon‐borne scintillation counter over Roberval, Quebec, Canada. The microburst groups were accompanied one‐to‐one by time‐delayed and broadened pulses of ionospheric absorption measured by a high sensitivity 30‐MHz riometer at Siple Station, Antarctica (L ≈ 4.1). For the interval of highest correlation, the absolute lag between the two data sets was 4±1 s, to the limit of the relative timing accuracy. Approximately 2 s of the observed lag had been introduced by a low‐pass filter in the riometer data acquisition unit. The remainder (2 s) was due to the ionospheric recombination process, which evidently had a response time (τ∼5 s) during this event much shorter than that ordinarily associated with the D region of the ionosphere. Model calculations of the ionospheric response to time‐varying precipitation, derived from the profile of the measured X ray flux, provide a consistent picture of simultaneous microburst group precipitation at conjugate points, absolute absorption and the electron spectrum derived from X rays, the degree of variation in absorption and X ray fluxes, and the characteristic ionospheric time constant at the altitude of maximum energy deposition.