Multifrequency measurements of HF Doppler velocity in the auroral E region

[1] HF measurements in Prince George, British Columbia (Canada), at five radar frequencies between 9.3 and 15.7 MHz are considered to study the Doppler velocity of E region coherent echoes. One event showing a regular variation of velocity with radar frequency, slant range, and azimuth of observations is analyzed in detail. For this event, plasma drifts were in access of 700 m s−1, but the observed velocities were below 250 m s−1 since measurements were performed at large flow angles (L shell angles 45°< ϕ< 100°). We show that measured Doppler velocity depends on irregularity scale but only within the Farley-Buneman (F-B) instability cone (45°< ϕ< 75°). We demonstrate that maximum velocities measured at the highest radar frequency are ∼1.3 times larger than those at the lowest frequency. We also show that for observations inside the instability cone, the velocity magnitude strongly decreases with aspect angle and the rate of the decrease is scale sensitive. The effect can be described by the fluid theory formula if the nominal electron collision frequencies are replaced by anomalous collision frequencies that are ∼5 times larger. However, for observations outside the F-B instability cone (75°< ϕ< 100°), the Doppler velocity does not show significant variation with aspect angle. For these directions, velocity change with flow angle was insignificant, very similar at all radar frequencies, and not consistent with the expected “cosine” law. The implications of the measurements on the theory of electrojet instabilities and the processes of coherent echo formation are discussed.

[1]  R. Greenwald,et al.  Obliquely propagating ion acoustic waves in the auroral E region: Further evidence of irregularity production by field‐aligned electron streaming , 1990 .

[2]  J. St.‐Maurice,et al.  A turbulent theoretical framework for the study of current‐driven E region irregularities at high latitudes: Basic derivation and application to gradient‐free situations , 1993 .

[3]  Mark Lester,et al.  Simultaneous observations at different altitudes of ionospheric backscatter in the eastward electrojet , 1997 .

[4]  D. Moorcroft,et al.  HF detection of slow long-lived E region plasma structures , 2000 .

[5]  G. Sofko,et al.  On the origin of narrow non‐ion‐acoustic coherent radar spectra in the high‐latitude E region , 1994 .

[6]  E. Nielsen.  Aspect angle dependence of mean Doppler velocities of 1‐m auroral plasma waves , 1986 .

[7]  V. Virvilis,et al.  Simultaneous observations of E region coherent radar echoes at 2-m and 6-m radio wavelengths at , 1997 .

[8]  S. Wing,et al.  A new magnetic coordinate system for conjugate studies at high latitudes , 1989 .

[9]  P. E. Johnston,et al.  Auroral radar observations at Siple Station, Antarctica , 1982 .

[10]  J. Delloue,et al.  Midlatitude E region coherent backscatter observed simultaneously at two HF radar frequencies , 1996 .

[11]  B. Fejer,et al.  Correction [to “Ionospheric irregularities”] , 1981 .

[12]  D. André,et al.  Observations of 50- and 12-MHz auroral coherent echoes at the Antarctic Syowa station , 2001 .

[13]  N. Nishitani,et al.  Implications of statistics of near-range Doppler velocity observed with the Syowa East HF radar , 2001 .

[14]  C. Senior,et al.  Statistical study of high-latitude E-region Doppler spectra obtained with the SHERPA HF radar , 1991 .

[15]  G. Hussey,et al.  Wavelength dependence of Doppler spectrum broadening in midlatitude E region coherent , 1998 .

[16]  J. M. Ruohoniemi,et al.  Large-scale imaging of high-latitude convection with Super Dual Auroral Radar Network HF radar observations , 1998 .

[17]  G. Sofko,et al.  Aspect angle dependence of the radar aurora Doppler velocity , 1994 .

[18]  Y. Feldstein,et al.  Electric potential patterns in the northern and southern polar regions parameterized by the interplanetary magnetic field , 1994 .

[19]  D. T. Farley,et al.  Radar studies of the equatorial electrojet at three frequencies , 1971 .

[20]  John D. Sahr,et al.  Auroral electrojet plasma irregularity theory and experiment: A critical review of present understanding and future directions , 1996 .

[21]  Dieter Bilitza,et al.  International reference ionosphere—Status 1995/96 , 1997 .

[22]  C. Hanuise,et al.  Ionospheric refraction effects in slant range profiles of auroral HF coherent echoes , 1994 .

[23]  Mark Lester,et al.  A classification of spectral populations observed in HF radar backscatter from the E region auroral electrojets , 2001 .

[24]  T. Pulkkinen,et al.  CUTLASS HF radar observations of high-velocity E-region echoes , 2001 .

[25]  Mark Lester,et al.  On the altitude dependence of the spectral characteristics of decametre-wavelength E region backscatter and the relationship with optical auroral forms , 2001 .

[26]  A. Koustov,et al.  On the power‐velocity relationship for 12‐ and 50‐MHz auroral coherent echoes , 2001 .

[27]  M. Lester,et al.  Spectral and flow angle characteristics of backscatter from decametre irregularities in the auroral electrojets , 1999 .

[28]  I. McCrea,et al.  Doppler spectrum statistics obtained from three different-frequency radar auroral experiments , 1995 .

[29]  K. Schlegel,et al.  Coherent backscatter from ionospheric E-region plasma irregularities , 1996 .

[30]  T. B. Jones,et al.  DARN/SuperDARN , 1995 .