Velocity of electrojet irregularities, ion-acoustic speed and ExB plasma drift

1. Velocity of electrojet irregularities, ion-acoustic speed and ExB plasma drift (EISCAT supported study) A.V. Koustov, U of Saskatchewan, Saskatoon, CANADA M.V. Uspensky, Fin Meteorological Inst, Helsinki, Finland S. Nozawa, STElab, Nagoya University, Japan The velocity of electrojet irregularities at large flow angles has been traditionally related to the ExB component of the electron drift. Recently it was hypothesized that it is the ion-acoustic speed (projected onto a radar beam) that determines the irregularity velocity. In this presentation, joint observations between EISCAT and STARE VHF radars are considerate to explore both assumptions.

2. Motivation: Theory of the electrojet irregularities 2 Bahcivan et al. (2005) hypothesized that Cs saturated irregularities exist only along the flow while at all other directions, the velocity is Cs cosq. We term this hypothesis the “Cs model”. It has been believed, for a long time, that the velocity of electrojet irregularities is “saturated” at the ion-acoustic speed Cs within the FB instability cone ( 600 in azimuth) and the velocity is the ExB electron flow component along a certain direction for irregularities propagating outside the instability cone

3. 3 EISCAT/STARE/CUTLASS/ experimentation EISCAT facility is conveniently located for irregularity studies. VHF STARE and HF CUTLASS radar data can be tested against the theory. In this project, we consider EISCAT E field data and STARE velocities

4. 4 EISCAT and STARE data Overall data set: ExB, Cs and q angles. Total number of points is 2085 for the Norway and 1379 for the Finland radars. The observed VHF velocity does not match neither the Cs, not ExB component, most of the time

5. 5 Testing the Cs model of Bahcivan et al. (2005) A comparison of the predicted and measured velocity for the Norway and Finland STARE radars over the EISCAT spot for the entire data set. For predictions, the irregularity phase velocity was assumed to be the line-of-sight component of the ion-acoustic speed at 111 km. Panels (b) and (d) present the flow angles of observations for the Norway and Finland radars, respectively. The correlation coefficients and the slopes of the best linear fit line are given in the top left part of plots (a) and (c). The model predictions are reasonable for flow angles of < 500 and the agreement deteriorates at larger ExB.

6. Testing the hypothesis that the azimuth should be counted not from the ExB direction 6 The correlation coefficient and the slope of the best linear fit line to observations for various shifts in the direction of the maximum irregularity velocity. Irregularity velocity variation with the flow angle according to Bahcivan et al. (2005) was assumed. Positive shift corresponds to the CW direction from ExB.

7. 7 Testing the empirical model by Nielsen (2002) A comparison of the predicted and measured velocity for the Norway and Finland STARE observations over the EISCAT spot for all events. For predictions, the irregularity phase velocity was assumed to follow the empirical formula of Nielsen et al. (2002). Top panel (a) illustrates the expected velocity decrease with the flow angle according to the empirical formula

8. 8 All three cases show close results. Why? A comparison of the measured ion-acoustic speed at 111 km and the predicted one from the empirical equation by Nielsen and Schlegel (1985), solid dots. The velocity dependence used in the model by Uspensky et al. (2004) with b=0.588 is shown by the solid line.

9. 9 Testing the linear theory combined with altitude integration effect (Uspensky et al., 2004) Irregularity velocity predictions based on the assumption that the irregularity phase velocity is the line-of-sight component of the electron drift, panels (a) and (b), and on the assumption that the observed velocity is the l-o-s component scaled down owing to the aspect angle effect and echo reception from various heights (Uspensky et al., 2004), panels (c) and (d)

10. 10 Effect of Te, and b increase with ExB Measured Te against ExB magnitude and expected variation of b for the Finland radar Norway and Finland STARE velocities as a function of the EISCAT l-o-s ExB drift, blue points at all panels. Red points at the top panels are the linear theory predictions at 110 km for fixed electron collision frequency. Red points at the bottom panels are the linear theory predictions with the temperature-dependent electron collision frequency. Black dashed line is the line of ideal velocity agreement.

11. 11 Conclusions, I • 1. The Bahcivan et al.’s (2005) hypothesis gives the correlation coefficient between measurements and predictions of ~0.4 and the slope of the linear fit line of ~0.4. The overall dominating tendency for the underestimation of the irregularity velocity is especially pronounced at large drifts of > 1000 m/s. • 2. The velocity estimates based on the Bahcivan et al. (2005) hypothesis are of better quality if the considered flow angles are less than 600. The estimates deteriorate if the actual height of echoes is assumed to be less than 111 km. • 3. The irregularity phase velocity estimates based on the assumption that it is the line-of-sight component of the electron drift disagree severely with the STARE data. • 4. For Nielsen et al.’s (2002) empirical equation describing the irregularity velocity variation with the flow angle, agreement between the predictions based solely on EISCAT data and STARE measurements is comparable in quality with Bahcivan et al. (2005). • 5. The observed STARE velocity is best predicted by assuming that the irregularity phase velocity is the line-of-sight component of the electron drift scaled down, due to the off-orthogonality of irregularity propagation (non-zero effective aspect angle of observations) and echo collection from various heights, by a factor of ~0.5-0.6, as suggested by Uspensky et al. (2004). • 6. The match between the EISCAT/STARE data and the Uspensky et al.’s (2004) model improves if one assumes that the velocity of ~1 m electrojet irregularities at large flow angles can be described by the fluid linear theory in which the effective aspect angles are assumed to be ~0.90 and ~10 for the Finland and Norway radars, respectively, and the collision frequencies are temperature-dependent.

12. 12 Conclusions, II 5. The observed STARE velocity is best predicted by assuming that the irregularity phase velocity is the line-of-sight component of the electron drift scaled down, due to the off-orthogonality of irregularity propagation (non-zero effective aspect angle of observations) and echo collection from various heights, by a factor of~0.5-0.6, as suggested by Uspensky et al. (2004). 6. The match between the EISCAT/STARE data and the Uspensky et al.’s (2004) model improves if one assumes that the velocity of ~1 m electrojet irregularities at large flow angles can be described by the fluid linear theory in which the effective aspect angles are assumed to be ~0.90 and ~10 for the Finland and Norway radars, respectively, and the collision frequencies are temperature-dependent. 7. Further experimental tests of theoretical predictions with respect to the electrojet irregularity velocity variation with the flow angle have to concentrate on observations at drifts of >1000 m/s and flow angles of more that 600. For these conditions, predictions of various theories divert the most. Non-availability of EISCAT data is the major obstacle in pursuing this comparison. Bahcivan, H., Hysell, D.L., Larsen, M.F., and Pfaff, R.F.: The 30-MHz imaging radar observations of auroral irregularities during the JOULE campaign, J. Geophys. Res., 110, A05307, doi:10.1029/2004/JA010975, 2005. Nielsen, E., del Pozo, C. F., and Williams, P. J. S.: VHF coherent radar signals from the E region ionosphere and the relationship to electron drift velocity and ion-acoustic velocity, J. Geophys. Res., 107, 10.1029/2001JA900111, 2002. Uspensky, M., Koustov, A., Janhunen, P., Nielsen, E., Kauristie, K., Amm, O., Pellinen, R., Opgenoorth, H., and Pirjola, R.: STARE velocities: 2. Evening westward electron flow, Ann. Geophys., 22, 1077-1091, 2004. Uspensky, M.V., A.V. Koustov, and S. Nozawa, 2006. STARE velocities at large flow angles: Is it related to the ion-acoustic speed? Annales Geophysicae, 24, 873-885