LF/MF Whistler mode dispersive signals observed with rocket‐borne instruments in the auroral downward current region

The SIERRA sounding rocket launched 14 January 2002, northward from Poker Flat, Alaska, to an altitude of 735 km in active aurora. On the downleg, in the altitude range 500–700 km, the onboard high‐frequency wave experiment detected structured whistler mode signals in the frequency range 100–1000 kHz, lasting nearly 200 s during which the rocket moved approximately 200 km downrange. The most prominent feature consisted of narrowband signals with frequency descending from 500 kHz to 250 kHz in about 0.5 s, with bandwidths ∼10 kHz, with frequency spacings ∼25 kHz, and with amplitudes in the range 10–400 μV/m. The structured whistler mode signals coincide with a region of Alfvénically accelerated electrons of the type often associated with the downward current region, poleward of the inverted‐V electron structures usually associated with the upward current region. However, on short timescales there was no clear correlation between the whistler mode signals and the locally detected electron flux or density. Ray‐tracing calculations indicate that concentrated sources of waves originating on the whistler mode resonance cone and moving upward through a model ionosphere, with speeds of 1000–5000 km/s, to rocket altitudes, exhibit the magnitude of dispersion of the observed structured whistler mode features. We evaluate the strengths and weaknesses of the hypothesis that these concentrated sources could be associated with phase space electron holes shedding electrostatic whistlers as they move upward along the field line at altitudes much higher than the rocket in the downward current region.