Ground echoes observed by the meteor radar and high-speed auroral observations in the substorm growth phase

Abstract Multi‐instrument observations by a meteor radar (MR), auroral cameras, ionosondes, and ground magnetometers were made in Northern Europe at auroral latitudes (between 64° and 72° corrected geomagnetic latitude) at 22–24 magnetic local time in the substorm growth phase. The southward drifting growth phase auroral arc was associated with enhanced electron density up to 2⋅1012 m‐3 (corresponding to a plasma frequency, foEs of about 13 MHz) at about 110‐km altitude. Such an enhanced E layer electron density caused bending toward the ground of the MR radio waves transmitted at a frequency, fr, of 36.9 MHz and at low elevation (el. < 25°), such that the radar received ground echoes characterized by a near‐zero Doppler shift. The amplitude of the echoes was modulated at a frequency of a few hertz, and a similar modulation was found in the auroral luminosity at 427.8 nm near the location of the bending of MR radio waves. The modulation was due to irregular (random) fluctuations of auroral precipitation. Although such a few‐hertz variation of the auroral precipitation cannot produce more than 1% modulation of the ionospheric electron density, even such a small modulation can lead to 50% modulation of the MR ground scatter provided foEs ≈ fr sin (el.). The ionosonde and MR data provide evidence that this condition was satisfied in the present case. Due to a high‐frequency (>2 Hz) amplitude modulation of the ground scatter, the MR erroneously accepts such signals as echoes from meteor trails