Ground Echoes Observed by the Meteor Radar and High‐Speed Auroral Observations in the Substorm Growth Phase. Issue 11 (8th November 2019)

• Record Type: Journal Article
• Authors: Kozlovsky, A.; Shalimov, S.; Oyama, S.; Hosokawa, K.; Lester, M.; Ogawa, Y.; Hall, C.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 11(2019)
• Page Start: 9278
• Page End: 9292
• Publication Date: 2019-11-08
• Subjects: Meteor radar -- Substorm growth phase -- Ionosonde -- Pulsating aurora -- Radar ground scatter; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA026829
• Languages: English
• ISSNs: 2169-9380

Abstract

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⋅10 12 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, f r, 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 ≈ f r 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. Plain Language Summary: It is known that due to the ionospheric recombination time of the order of tens of seconds, fast few‐hertz variations of auroral precipitation cannot produce noticeable enough variations in the ionospheric plasma density to be detected by radars. Nevertheless, such high‐frequency variations were detected in the ground scatter of the meteor radar operated at a frequency of 37 MHz. It was possible due to the enhanced ionospheric electron density at about 110 km altitude associated with the auroral arc in the substorm growth phase. We have shown that if the electron density exceeds some critical value, the radar waves can be bended toward the ground, reflected from the ground and returned to the radar. Otherwise, the radio waves pass through the ionosphere and never come back. Thus, even small changes of the E layer electron density (less than 1%) may cause noticeable modulation of the radar return. This makes possible radar monitoring of the fast variations of auroral precipitation. Key Points: The growth phase auroral arc was associated with the enhanced E layer electron density corresponding to about 13‐MHz plasma frequency Enhanced E layer ionization caused bending and ground scatter of the 36.9‐MHz radio waves transmitted at low elevation Amplitude of the radar ground scatter was modulated at a frequency of a few hertz, and such modulation was found in auroral luminosity