Electric field control of E region coherent echoes: Evidence from radar observations at the South Pole

Abstract

Characteristics and formation mechanisms of E region plasma irregularities at high latitudes are investigated using observations with the newly deployed Super Dual Auroral Radar Network (SuperDARN) radar at the South Pole Antarctic station (SPS) near a magnetic latitude (MLAT) of 75°S. It is shown that E region echo occurrence at SPS exhibits a diurnal variation that is significantly different from those at auroral and polar cap latitudes. Moreover, analysis of major spectral populations also showed a distinct and previously unreported diurnal pattern. The plasma drift velocity estimates are derived at E region ranges of SPS, leveraging the SPS radar's position well within the MLAT region where SuperDARN convection estimates are well constrained by the data. It is shown that E region irregularity occurrence increases when the convection direction is within the SPS field of view and/or when the plasma drift component is comparable with the nominal ion‐acoustic speed Cs of 350 m/s. This is the expected behavior for irregularities generated directly by the modified two‐stream plasma instability (MTSI). On the other hand, irregularity velocity dependence on convection velocity showed an unexpected saturation at velocity values smaller than nominal Cs. It is demonstrated that the convection velocity at which irregularity velocity starts to differ from the convection component and to approach a maximum value is dependent on the magnetic aspect angle. Moreover, the maximum velocity value itself also depends on the aspect angle. The observed behavior is discussed in context of recent models that involve evolving aspect angles as a key characteristic of MTSI saturation.

Asymmetry in the Farley‐Buneman dispersion relation caused by parallel electric fields

“…Coherent radars within the Super Dual Auroral Radar Network (SuperDARN) measure irregularity phase velocity very accurately (with uncertainty of less than about 1%). Many of the high southern latitude SuperDARN radars including radars at the South Pole, McMurdo, Dome Concordia, and Syowa Antarctic stations have aspect angles of different polarity within their E region fields of view (FoVs) [ Makarevich et al , ; Forsythe and Makarevich , ]. The phase velocity along the same radar beam can be examined as a function of the aspect angle [ Ogawa et al , ; Nielsen , ; Kustov et al , ; Makarevich et al , , ].…”

“…The gyrofrequencies for ions and electrons (negative in this notation convention) were taken to be constant with altitude and equal to 150 s −1 and −10 7 s −1 , respectively. The geographic coordinates of 80°E, 60°S and the time of 14:00 magnetic local time (MLT) on 15 February 2013 were selected to represent summer daytime conditions at high southern latitudes which were recently demonstrated to have a significant presence of echoes of likely FBI origin [ Makarevich et al , ; Forsythe and Makarevich , ].…”

Global view of the E region irregularity and convection velocities in the high‐latitude Southern Hemisphere

“…Ground scatter was excluded using the standard SuperDARN criteria of low velocity and low spectral width ( m/s, m/s) and further cleaned, by excluding isolated interference and other spurious echoes with low signal‐to‐noise ratio (SNR) P < 3 dB, large spectral width W > 500 m/s, and very large Doppler velocity V > 2000 m/s. This approach was adapted from the previous SPS study by Makarevich et al [].…”

“…The E region velocity and convection component distributions are analyzed first using a similar approach as that in Makarevich et al [] but for the entire 11 month data set and for both SPS and ZHO. Figure shows the point occurrence of (a) E region velocities, (b) convection component deduced from the map potential technique, and (c) F region l‐o‐s velocities for SPS for February 2013.…”

“…Recent deployment of the SuperDARN South Pole radar that samples locations deep within the typical convection pattern and at small flow angles has enabled further studies of the FBI echoes and their control by the convection component. Using this radar, Makarevich et al [] demonstrated that during 1 month of February 2013, the occurrence of the FBI echoes with negative velocities was greater than that of the positive FBI echoes and that this occurrence was strongly controlled by the convection component. Following the same idea, one expects that a radar with the E region FoV at the same MLAT, but with an opposite orientation, would observe similar numbers of the FBI echoes but with predominantly positive velocities.…”