Equatorial electrojet plasma irregularities observed during late afternoon by the 30 MHz coherent scatter radar in São Luís, Brazil

Coherent scatter radar observations conducted right before twilight by the 30 MHz radar in São Luís (Brazil) during both quiet (December 10, 2001) and disturbed (December 12, 2001) conditions detected type I echoes on the shoulders of broad type II spectra (near zero Doppler velocity). Zonal electric fields inferred from the São Luís ionosonde have a larger magnitude during the period where these spectral observations were made than on December 18, 2001 when type I echoes were not observed during late afternoon. The IMF Bz and AE index indicate that the extended geomagnetic disturbance in the afternoon of December 12, 2001 has allowed the interaction between disturbance dynamo and prompt penetration electric fields. The latter electric field component could have exceeded the former to give rise to the observed electric fields, and make possible observation of type I echoes.

► We report 30 MHz radar observations of electrojet echoes measured before twilight in São Luís, Brazil. ► The observations were conducted during quiet and disturbed geomagnetic conditions. ► The radar detected type spectra on shoulders of broad type spectra (near zero Doppler velocity). ► Electric fields inferred from an ionosonde have a larger magnitude when the spectra were observed. ► Penetration electric field caused large electric field on the storm day giving rise to type echoes.