pre-reversal enhancement of the evening vertical plasma drifts On the occurrence of equatorial F-region irregularities and the magnitude of the pre-reversal enhancement of the evening vertical plasma drifts J. M. Smith1, F. S. Rodrigues1 and M. A. Milla2 UT Dallas, William B. Hanson Center for Space Sciences, Richardson, TX 2. Jicamarca Radio Observatory, Lima, Peru Abstract: We conducted a comprehensive analysis of the climatological and day-to-day variability of the vertical plasma drifts and equatorial spread F (ESF) measurements made by the Jicamarca incoherent scatter radar (ISR) between 1996 and 2013. We also analyzed the relationship between the average vertical drifts and ESF, as well as the daily evening drifts and ESF. Due, at least in part, to the high sensitivity of the Jicamarca ISR, some interesting features were identified during our analysis. 4. RESULTS AND DISCUSSION CLIMATOLOGY OF ESF We computed the occurrence rate of coherent scatter echoes based on our procedure for ESF irregularity detection (given in panel c in Figure 1). DAY-TO-DAY RELATIONSHIP BETWEEN ESF AND PRE Sao Luis a. b. 1. OBJECTIVES AND SIGNIFICANCE The extent to which the vertical drifts control ESF, particularly on a short-term (day-to-day) basis is still an open question. Several studies have indicated that the magnitude of the pre-reversal enhancement (PRE) of the evening drifts is the main parameter controlling the development of ESF [Fejer et al., 1999; Basu et al., 1996; Abdu, 2001; Anderson et al., 2004]. It has even been suggested that the PRE of significant magnitude is a necessary condition for ESF development [e.g. Fejer et al., 1999; Basu et al., 1996; Abdu, 2001]. Other studies have emphasized the importance of the time history of the evening drifts [Stolle et al., 2008]. The present study was motivated by the necessity of a comprehensive and quantitative analysis of the relationship between the vertical drifts and ESF. The goals of this study are: To provide updated climatological results of ESF and the vertical plasma drifts based on ISR measurements [Kudeki et al., 1999] made over the past two solar cycles To investigate the relationship between the evening drifts (i.e. PRE) and ESF on a day-to-day basis Figure 4a: The PRE peak versus ESF severity. Figure 4b: The Mean PRE versus ESF severity. For reference, the dashed line indicates 10% ESF severity, where ESF severity is on a scale of 0 (no coherent echoes detected for all times/heights) to 1 (coherent echoes detected for all times/heights). The red markers in Dec. Sol. indicate significant ESF events that followed negative PRE peaks on Jan. 24 and 25, 2008. Figure 2: Occurrence rate of quiet-time F-region echoes as a function of local time and height. Each panel represents a season and solar flux condition. The number within square brackets indicates the number of the different days when the measurements were made. The black solid lines are iso-contours of the 0.4 (40%) occurrence rates and are shown for reference. ASSESSMENT Despite different data sets from previous solar cycles, our results in Figures 4a,b tend to agree with the hypothesis that a threshold PRE peak is necessary for ESF development, and that this threshold changes with season and solar flux [e.g. Fejer et al., 1999]. ASSESSMENT Results in Figure 2 can be classified as follows: Pre-midnight cases: Highest rates during Dec. Sol. and lowest in Jun. Sol. Height at which irregularities are first observed increases with solar flux The results agree with previous experimental results made by the coherent scatter radar at Jicamarca [Chapagain et al., 2009]. Post-midnight cases: Highest rates during Dec. Sol. and low solar flux conditions Previous studies using different types of observational techniques also found a higher occurrence rate of post-midnight ESF during low solar flux years [Huang et al., 2010]. 5. SUMMARY In this analysis, we performed a comprehensive analysis of F-region plasma drifts and the occurrence of ESF. MAIN FINDINGS: Higher than expected occurrence of post-midnight F-region irregularities during Dec. Sol. and low solar flux conditions. (See Figure 2). This is due, mostly, to the high sensitivity of the radar. High/low ESF occurrence rates tends to follow enhancements/depressions in the vertical drifts. (See Figures 2,3a). On a daily basis, significant ESF events tend to follow PRE peaks reaching a threshold value. Additionally, the use of the mean PRE metric appears to provide a better correlation. (See Figures 4a,b). Threshold PRE peak values seem to change with season and increase with solar flux activity, but the limited number of measurements during moderate and high solar flux conditions does not allow for definitive conclusions to be made. We also note exceptions in Dec. Sol., low solar flux conditions, where slightly negative evening drifts produced significant ESF. These events correspond to days during the 2008 Jan. sudden stratospheric warming event. 2. METHODOLOGY To address our goals, we have processed ~20 years of ISR measurements given as a function of local time and height. We only used geomagnetically quiet data (Kp at the time of measurement and three previous values not exceeding 4). For goal 1: We derived a technique to identify ESF based on the signal-to-noise (SNR) ratio and compute mean F-region drift curves. ESF: Detection when coherent SNR exceeded a threshold [dB] within specified time/range bin Mean drifts: Used good quality drift measurements between 200-400 km For goal 2: We proposed the following metric for ESF severity and two metrics for evening drifts: ESF severity: Area of coherent echoes detected between 1900-2400 LT/200-800 km Evening drift intensity (PRE peak): Magnitude of drifts between 1700-2000 LT Evening drift average (Mean PRE): Average of drifts between 1700-2000 LT CLIMATOLOGY OF VERTICAL DRIFTS In an attempt to better understand the climatological occurrence of coherent echoes (i.e. ESF irregularities), we determined the averages of the drift curves. a. b. 3. MEASUREMENTS AND PROCESSING REFERENCES -Abdu [2001] J. Atmos. Sol.-Terr. Phys., 63, 869-884. -Anderson et al. [2004] J. Atmos. Sol.-Terr. Phys., 66, 1567-1572. -Basu et al. [1996] J. Geophys. Res., 101, 26,795-26,809. -Chapagain et al. [2009] J. Geophys. Res., 114, A06316. -Fejer et al. [1999] J. Geophys. Res., 104, 19,859-19,,869. -Huang et al. [2010] J. Geophys. Res., 115, A07316. -Kudeki et al. [1999] J. Geophys. Res., 104, 28,145-28,162. -Stolle et al. [2008] Ann. Geophys., 26, 3979-3988. Figure 3a: Climatological quiet-time curves of mean F-region drifts as a function of local time. Figure 3b: Daily curves of quiet-time mean F-region drifts as a function of local time. ASSESSMENT The drifts are, in general, positive (upward) during the day and negative (downward) at night. It is also possible to distinguish the enhancement of the vertical drifts around sunset hours prior to its reversal. We also find that there is a strong correlation between average drift curves (Figure 3a) and the occurrence of ESF (Figures 2). Higher occurrence rates are associated with the enhancement of the evening drifts. ACKNOWLEDGEMENTS We would like to thank the Jicamarca Radio Observatory for providing the measurements. Work at UT Dallas was supported by NSF. 2015 CEDAR Workshop, Seattle, WA Figure 1: Example of measurements and analysis. (a) SNR of coherently scattered echoes, (b) ISR vertical drifts, (c) time versus height map of coherent scatter echoes/irregularities (red/blue indicates the occurrence/non-occurrence of irregularities, respectively), (d) mean F-region vertical drifts from 200-400 km. CONTACT INFORMATION Jessica Smith jessica.m.smith@utdallas.edu