Ground-based Fabry-Perot Interferometer Research Articles

Inversion of Wind and Temperature from Low SNR FPI Interferograms

The temperature and wind in the middle and upper atmosphere can be obtained by recording the Doppler shift and broadening of the airglow emission, which is reflected by the interference ring from a ground-based Fabry–Perot interferometer (FPI) system. FPI observations are highly susceptible to weather and the external environment, which seriously affect the signal-to-noise ratio (SNR) of FPI interferograms. An SNR can significantly increase errors in determining the center of the interferogram, leading to inaccurate wind and temperature inversions. The calculation shows that the wind inversion from the interferogram decreases and the temperature increases for larger central errors. In this paper, we propose the maximum standard deviation method (MSDM) with high accuracy and robustness to determine the interference ring center. The performance of the MSDM is better achieved by using more than 100 1D interferogram bins to determine the center of interferograms. The robustness of the MSDM is investigated by computing numerous simulated interferograms with white Gaussian noise and Poisson noise, and compared with the two algorithms of binarization and peak fitting, which are usually used to invert wind and temperature from the interference ring of FPI. The results show that MSDM has higher accuracy and robustness than the other two algorithms. We also simulate the distortion interferogram when the FPI may be illuminated by inhomogeneous background light, which can introduce additional errors in wind and temperature, and the MSDM still performs better. Finally, we invert the wind and temperature from the real airglow interferogram by the Kelan (38.7°N, 111.6°E) FPI, which shows that both the wind and temperature inverted by MSDM better agree well with the FPI product than the other two algorithms. Therefore, the MSDM helps to improve the accuracy and stability to invert the wind and temperature.

Validation of ICON-MIGHTI Thermospheric Wind Observations: 1. Nighttime Red-Line Ground-Based Fabry-Perot Interferometers.

Observations of the nighttime thermospheric wind from two ground-based Fabry-Perot Interferometers are compared to the level 2.1 and 2.2 data products from the Michelson Interferometer Global High-resolution Thermospheric Imaging (MIGHTI) onboard National Aeronautics and Space Administration's Ionospheric Connection Explorer to assess and validate the methodology used to generate measurements of neutral thermospheric winds observed by MIGHTI. We find generally good agreement between observations approximately coincident in space and time with mean differences less than 11 m/s in magnitude and standard deviations of about 20-35 m/s. These results indicate that the independent calculations of the zero-wind reference used by the different instruments do not contain strong systematic or physical biases, even though the observations were acquired during solar minimum conditions when the measured airglow intensity is weak. We argue that the slight differences in the estimated wind quantities between the two instrument types can be attributed to gradients in the airglow and thermospheric wind fields and the differing viewing geometries used by the instruments.

Comparing high-latitude thermospheric winds from Fabry–Perot interferometer (FPI) and challenging mini-satellite payload (CHAMP) accelerometer measurements

Abstract. It is generally assumed that horizontal wind velocities are independent of height above the F1 region (> 300 km) due to the large molecular viscosity of the upper thermosphere. This assumption is used to compare two completely different methods of thermospheric neutral wind observation, using two distinct locations in the high-latitude Northern Hemisphere. The measurements are from ground-based Fabry–Perot interferometers (FPI) and from in situ accelerometer measurements onboard the challenging mini-satellite payload (CHAMP) satellite, which was in a near-polar orbit. The University College London (UCL) Kiruna Esrange Optical Platform Site (KEOPS) FPI is located in the vicinity of the auroral oval at the ESRANGE site near Kiruna, Sweden (67.8∘ N, 20.4∘ E). The UCL Longyearbyen FPI is a polar cap site, located at the Kjell Henriksen Observatory on Svalbard (78.1∘ N, 16.0∘ E). The comparison is carried out in a statistical sense, comparing a longer time series obtained during night-time hours in the winter months (DOY 300–65) with overflights of the CHAMP satellite between 2001 and 2007 over the observational sites, within ±2∘ latitude (±230 km horizontal range). The FPI is assumed to measure the line-of-sight winds at a height of ∼240 km, i.e. the peak emission height of the atomic oxygen 630.0 nm emission. The cross-track winds are derived from state-of-the-art precision accelerometer measurements at altitudes between ∼450 km (in 2001) and ∼350 km (in 2007), i.e. 100–200 km above the FPI wind observations. We show that CHAMP wind values at high latitudes are typically 1.5 to 2 times larger than FPI winds. In addition to testing the consistency of the different measurement approaches, the study aims to clarify the effects of viscosity on the height dependence of thermospheric winds.

Comparison of Retrieval Methods for Neutral Wind Based on Airglow Measurements by a Ground-based Fabry-Perot Interferometer

利用半径法、完整傅里叶级数描述法和非线性回归法分别对安装在河北省境内国家天文台兴隆园区（40.2°N，117.4°E）的Fabry-Perot干涉仪（FPI）获得的地基中高层大气气辉观测数据进行风速反演.反演结果表明，半径法、完整傅里叶级数描述法和非线性回归法均能很好地反演风速，在误差范围内反演结果存在很好的一致性.相对于半径法和完整傅里叶级数描述法，非线性回归方法更完整地利用了观测数据信息，反演结果可信度较高.

Retrieval of middle and upper atmospheric wind based on non-full circular fringe recorded by Fabry-Perot Inteferometer

Fabry-Perot Interferometer (FPI) has been used widely for wind measurements of the middle and upper atmosphere. To date, most of FPIs have been based on full-closed circular fringe, which needs 15–25 min to obtain a group of wind velocity (zonal and meridional). However, it is hard to improve the temporal resolution because full-closed circular fringe in several directions cannot be easily imaged onto the same Charge-Coupled Device (CCD) with enough airglow intensity. In this paper, a data processing method is proposed for non-full circular fringe of FPI, which can support CCD with enough area of observations in several directions simultaneously. The method is focused on the center determination of non-full fringe. It includes radial cross-section, peak coordinate determination, and center calculation. Based on the calculated center, the fringe is annular summed. Then its radius is determined subsequently using Gaussian fitting. Finally, the wind is retrieved from the fringe radius. For validation, fringes from two ground-based FPIs were used, which are deployed in Kelan (38.71°N, 111.58°E) and Xinglong (40.40°N, 117.59°E) in China. The results retrieved from non-full fringes of FPIs were compared with that from full-closed circular fringe. The averaged wind deviation between them demonstrates reasonable difference with 5.38 m s−1 for 892.0 nm airglow emission, 5.81 m s−1 for 630.0 nm emission, and 3.03 m s−1 for 557.7 nm emission. Besides, wind results of Xinglong FPI are compared roughly with measurements of meteor radar which is deployed in Ming Tombs of Beijing (40.3°N, 116.2°E). Good agreement demonstrates that this method is robust enough for FPI wind retrieval of mesosphere and thermosphere.

A Tiny Fabry-Perot Interferometer with Postpositional Filter for Measurement of the Thermospheric Wind

A tiny and low-cost ground-based Fabry-Perot interferometer (FPI) was designed using a filter behind etalon and Galilean telescope system for the thermospheric wind observation with OI 630.0 nm nightglow emissions (~250 km). Based on the instrument, experiments were carried out at Langfang (39.40° N, 116.65° E) site for a rough comparison and Kelan (38.71° N, 111.58° E) site for a detailed validation. Wind results of Langfang experiment are well consistent with measurements of two other FPIs deployed at Xinglong (40.40° N, 117.59° E) and Kelan which are retrieved by the American National Center for Atmospheric Research (A-NCAR). In Kelan experiment, the averaged wind deviation between our FPI and A-NCAR FPI is 11.8 m/s. The averaged deviation of wind measurement error between them is 2.9 m/s. The comparisons suggest good agreement. Then, the analysis of influencing factors was made. The center determination offset has an exponential relation with wind deviation, while the radius calculation offset is linear with wind deviation.

Thermosphere variation at different altitudes over the northern polar cap during magnetic storms

In this study, we report observations and simulation results of heated neutrals at various altitudes inside the polar cap during two magnetic storms in January 2005. The Poynting flux measurements from the Defense Meteorological Satellite Program (DMSP) satellites show enhanced energy input in the polar cap during the storm main phase, which is underestimated in the TIE-GCM simulation. Neutral temperature measurements at 250km from the ground-based Fabry-Perot Interferometer (FPI) at Resolute Bay are presented, along with the neutral density observations at 360km and 470km from Challenging Minisatellite Payload (CHAMP) and the Gravity Recovery and Climate Experiment (GRACE) satellites, respectively. These data have been analyzed to demonstrate the altitudinal dependence of neutral response to the storm energy input. By comparing the TIE-GCM simulation results and the observations, we demonstrate that Poynting fluxes as well as the thermosphere response were underestimated in the model. The simulated neutral temperature at Resolute Bay increases by approximately 260° and 280°K for the two events, respectively, much lower than the observed temperature enhancements of 750° and 900°K. Neutral density enhancements with more than 30% increase over the background density were also observed at polar latitudes, with no clear distinction between the auroral zone and polar cap. All measurements indicate enhancements at high latitudes poleward of 80° magnetic latitude (MLAT) implying that substantial heating can occur within the polar cap during storms.

A New Ground-based Fabry-Perot Interferometer for Measurement of the Thermospheric Wind

地表海拔大约250\,km高度处大气非常稀薄,目前被动光学观测是该层风场探测最有效手段.Fabry-Perot干涉仪(FPI)由于具有较高能量利用率及光谱分辨率等特点,是该层大气最有效的地基风场探测仪器之一.基于采用光路缩束系统及滤光片后置(标准具之后)方法研制的小型化FPI,2014年分别在河北廊坊(39.40°N,116.65°E)和山西岢岚(38.71°N,111.58°E)进行了地基观测试验,将观测数据与反演算法相结合得到高层大气风场数据,并将数据结果与美国大气研究中心两台FPI的风场数据进行了比较研究,在气辉整体辐射较弱的情况下得到岢岚站的风速平均反演偏差为11.8 m·s^-1.

Mesospheric and thermospheric observations of the January 2010 stratospheric warming event

We use two ground based Fabry–Perot interferometers (FPI) at Boulder (40°N, 105°W), Resolute (75°N, 95°W), and a meteor radar at Bear Island (75°N, 19°E) to examine the mesospheric and thermospheric winds during a sudden stratospheric warming (SSW) event in January 2010. The two high latitude wind instruments allow us, for the first time, to infer the zonal wavenumber of the semidiurnal tide on a daily basis during an SSW event. The ground based FPIs recorded enhanced lower thermosphere semidiurnal tide before and during the SSW. The two high latitude stations did not show much westward propagating semidiurnal tide with zonal wavenumber one (SW1) as predicted by model simulations. Several indications of westward propagating zonal wavenumber three semidiurnal tide (SW3) require further verification. In addition, the TIMED TIDI lower thermospheric winds at 95km also appear to show sign of SW3. High latitude terdiurnal and quaddiurnal oscillations were observed. Enhancements in the mid-latitude thermospheric semidiurnal and terdiurnal tides during the SSW were observed, which may be related to the gravity wave filtering by solar tides in the stratosphere and mesosphere. Unusually large thermospheric winds were observed before the SSW at Boulder, its connection to the planetary wave activity related to the SSW is still to be determined.

Wind Retrieval and Error Analysis of Ground-based Fabry-Perot Interferometer for the Middle and Upper Atmosphere

基于子午工程地基法布里-帕罗干涉仪（Fabry-Perot Interferometer，FPI）的气辉观测数据，结合地基独特的观测模式（天顶角为0° 的天顶方向和天顶角为45°的东西北南四个方向）对地基中高层大气风速进行反演，包括数据预处理、干涉环圆心确定、干涉环半径计算和风速反演. 将2010年5月6-13日8天十个环（十个干涉环同时参与反演）的反演结果与地基FPI风速实测数据进行比较，得到557.7nm，630.0nm，892.0nm三种谱线气辉的反演平均偏差分别为2.7m·s^-1，5.5m·s^-1，7.7m·s^-1. 此外，基于反演算法对上述反演精度影响因素进行了分析. 研究发现，气辉辐射强度对风速的反演精度影响较大，气辉辐射越强，外环的半径计算精度越高，可参与的反演环数越多，则最终的风速反演精度越高. 而圆心偏差± 2pixel（五个环）和± 1pixel（十个环）及焦距变化（±10mm）对风速反演精度的影响相对较小，但当超出这一偏差范围，风速反演偏差会迅速增大.

Novel Fabry-Perot interferometer measurements of F-region ion temperature

Novel optical measurements of F-region ion temperatures have been made in conjunction with thermospheric neutral temperatures. A ground-based Fabry-Perot interferometer has been used to observe high-latitude F-region ion temperatures using the O+(2P) auroral emission at 732 nm, and upper-thermospheric neutral temperatures using the O(1D) airglow emission at 630 nm. Dual wavelength measurements were made during February 2001 and clearly show that the ion temperature is equal to or greater than the neutral temperature, as expected. EISCAT radar measurements show that there is good agreement between the incoherent scatter and optical Doppler broadening methods of inferring ion temperature.

Polar cap mesosphere wind observations: comparisons of simultaneous measurements with a Fabry–Perot interferometer and a field-widened Michelson interferometer

Polar cap mesospheric winds observed with a Fabry-Perot interferometer with a circle-to-line interferometer optical (FPI/CLIO) system have been compared with measurements from a field-widened Michelson interferometer optimized for E-region winds (ERWIN). Both instruments observed the Meinel OH emission emanating from the mesopause region (approximately 86 km) at Resolute Bay, Canada (74.9 degrees N, 94.9 degrees W). This is the first time, to our knowledge, that winds measured simultaneously from a ground-based Fabry-Perot interferometer and a ground-based Michelson interferometer have been compared at the same location. The FPI/CLIO and ERWIN instruments both have a capability for high temporal resolution (less than 10 min for a full scan in the four cardinal directions and the zenith). Statistical comparisons of hourly mean winds for both instruments by scatterplots show excellent agreement, indicating that the two optical techniques provide equivalent observations of mesopause winds. Small deviations in the measured wind can be ascribed to the different zenith angles used by the two instruments. The combined measurements illustrate the dominance of the 12-h wave in the mesopause winds at Resolute Bay, with additional evidence for strong gravity wave activity with much shorter periods (tens of minutes). Future operations of the two instruments will focus on observation of complementary emissions, providing a unique passive optical capability for the determination of neutral winds in the geomagnetic polar cap at various altitudes near the mesopause.

Comparisons between Canadian prairie MF radars, FPI (green and OH lines) and UARS HRDI systems

Abstract. Detailed comparisons have been completed between the MF radars (MFR) in the Canadian prairies and three other systems: two ground-based Fabry-Perot interferometers (FPI) and the UARS high resolution Doppler imager (HRDI) system. The radars were at Sylvan Lake (52°N, 114°W), Robsart (49°N, 109°W) and the main continuing facility is at Saskatoon (52°N, 107°W). Statistical comparisons of hourly mean winds (1988-1992) for the Saskatoon MFR and FPI (557.7 nm green line) using scatter plots, wind speed-ratios, and direction-difference histograms show excellent agreement for Saskatoon. No serious biases in speeds or directions occur at the height of best agreement, 98 km. If anything, the MFR speeds appear bigger. The same applies to the Sylvan Lake MFR and Calgary FPI, where the best height is 88 km. In both cases these are close to the preferred heights for the emission layers. Differences between measurements seen on individual days are likely related to the influence of gravity waves (GW) upon the optical and radar systems, each of which have inherent spatial averaging (350, 50 km respectively), as well as the spatial difference between the nominal measurement locations. For HRDI, similar statistical comparisons are made, using single-overpass satellite winds and hourly means (to improve data quality) from MFR. Heights of best agreement, based upon direction-difference histograms, are shown; there is a tendency, beginning near 87 km, for these MFR heights to be 2 or 3 km greater than the HRDI heights. Speeds at these heights are typically larger for the satellite (MFR/HRDI = 0.7–0.8). Reasons for the differences are investigated. It is shown that the estimated errors and short-term (90 min) differences are larger for HRDI than for the MFR, indicating more noise or GW contamination. This leads to modest but significant differences in median speed-ratio (MFR/HRDI < 1). Also, comparison of the two systems is made under conditions when they agree best and when they show large disagreement. For the latter cases both systems show higher relative errors, and the HRDI vectors are frequently small. It is suggested that spatial or temporal GW wind fluctuations are the likely cause of the larger HRDI-MFR disagreement when wind speeds are small. No satisfactory explanation exists for the overall discrepancy in speeds between the MFR and HRDI.

Comparisons between Canadian prairie MF radars, FPI (green and OH lines) and UARS HRDI systems

Detailed comparisons have been completed between the MF radars (MFR) in the Canadian prairies and three other systems: two ground-based Fabry-Perot interferometers (FPI) and the UARS high resolution Doppler imager (HRDI) system. The radars were at Sylvan Lake (52°N, 114°W), Robsart (49°N, 109°W) and the main continuing facility is at Saskatoon (52°N, 107°W). Statistical comparisons of hourly mean winds (1988-1992) for the Saskatoon MFR and FPI (557.7 nm green line) using scatter plots, wind speed-ratios, and direction-difference histograms show excellent agreement for Saskatoon. No serious biases in speeds or directions occur at the height of best agreement, 98 km. If anything, the MFR speeds appear bigger. The same applies to the Sylvan Lake MFR and Calgary FPI, where the best height is 88 km. In both cases these are close to the preferred heights for the emission layers. Differences between measurements seen on individual days are likely related to the influence of gravity waves (GW) upon the optical and radar systems, each of which have inherent spatial averaging (350, 50 km respectively), as well as the spatial difference between the nominal measurement locations. For HRDI, similar statistical comparisons are made, using single-overpass satellite winds and hourly means (to improve data quality) from MFR. Heights of best agreement, based upon direction-difference histograms, are shown; there is a tendency, beginning near 87 km, for these MFR heights to be 2 or 3 km greater than the HRDI heights. Speeds at these heights are typically larger for the satellite (MFR/HRDI = 0.7–0.8). Reasons for the differences are investigated. It is shown that the estimated errors and short-term (90 min) differences are larger for HRDI than for the MFR, indicating more noise or GW contamination. This leads to modest but significant differences in median speed-ratio (MFR/HRDI < 1). Also, comparison of the two systems is made under conditions when they agree best and when they show large disagreement. For the latter cases both systems show higher relative errors, and the HRDI vectors are frequently small. It is suggested that spatial or temporal GW wind fluctuations are the likely cause of the larger HRDI-MFR disagreement when wind speeds are small. No satisfactory explanation exists for the overall discrepancy in speeds between the MFR and HRDI.

Thermospheric dynamics in the southern polar region

The dynamical behaviour of the upper thermosphere in the southern polar region has been investigated for a range of solar and geomagnetic activity and as a function of season, using a series of simulations by a coupled thermosphere - ionosphere model. The predicted behaviour in the southern winter period can be compared with recent ground-based Fabry-Perot interferometer observations of the OI 1D emission from the Amundsen-Scott station at the South Pole.

How close are ground-based Fabry-Perot thermospheric wind and temperature measurements to exospheric values? A simulation study

Ground-based Fabry-Perot interferometers (FPIs) have been used extensively to determine thermodynamical and hydrodynamical properties of the upper atmosphere by measuring emission line profiles from the O( 1D) nightglow at high spectral resolution. The resulting thermospheric winds and temperatures are normally referred to the altitude of the peak of the O( 1D) emission layer, near 250 km, and the effects of wind shear and temperature gradations along the line of sight are neglected. In order to quantify the significance of these effects, and to obtain a better understanding of the “effective” (volume-emission-rate-weighted) altitude of the FPI measurement, a computer simulation of the measurement technique has been performed, incorporating realistic profiles of winds, temperatures and O( 1D) volume emission rates along the instrumental line-of-sight. The atmospheric profiles used for the simulations are derived from a recently-developed thermospheric computer subroutine based on the predictions of the NCAR thermospheric general circulation model. The simulation code is used to calculate synthetic FPI spectrograms for different viewing geometries and FPI station locations, and the spectrograms are analyzed using standard data reduction techniques to derive fitted winds and temperatures. These are then compared with the atmospheric profiles used as input to the simulation code to determine the effective altitude of each simulated measurement and to study the differences between recovered winds and temperatures and the corresponding exospheric values. A first investigation using the simulation code for geophysical conditions corresponding to December solstice at solar maximum has indicated that FPI-derived Doppler temperatures may be lower than exospheric temperatures by ∼10% in the winter hemisphere and ∼15% in the summer hemisphere. Furthermore, FPI measurements of neutral winds, particularly at high latitudes, can differ appreciably from exospheric values due to the weighting of the FPI measurement to altitudes near ∼250 km.

Variations of thermospheric winds in northern scandinavia between 1980 and 1986: A study of geomagnetic activity effects during the last solar cycle

Since late 1980, a ground-based Fabry-Perot interferometer has been in operation near Kiruna, Sweden (20.4°E, 67.8°N). This instrument has recorded thermospheric neutral winds under a wide range of geophysical conditions which have occurred during six observing winters. These data cover the period from the time of maximum geomagnetic activity during the last solar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sunspot minimum. A statistical analysis of the data from about 400 nights of observation provides a graphic description of the generally rapid time-dependence of thermospheric winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances. The data also provide an excellent data base describing the auroral oval. The data describe ion convection patterns as they respond to variable geomagnetic activity, and also the mean distribution of O I 630 nm emission as a function of local time, latitude and geomagnetic activity. These results can be used to examine the geomagnetic input parameters to a global thermospheric model (for example the semi-empirical global models of magnetospheric convection) as required to bring the simulations of thermospheric circulation into overall improved agreement with the observations.

Observations of the variations of thermospheric winds in Northern Scandinavia between 1980 and 1986: A study of geomagnetic activity effects during the last solar cycle

Since late 1980, a ground-based Fabry-Perot interferometer has been in operation near Kiruna, Sweden (20.4° E, 67.8° N). This instrument has recorded upper thermospheric neutral winds under a wide range of geophysical conditions which have occurred during six observing winters. These data cover the period from the time of maximum geomagnetic activity during the last solar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sunspot minimum. A statistical analysis of the data from about 400 nights of observation provides a graphic description of the generally rapid time-dependence of upper thermospheric winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances. The data also provide an excellent data base describing the auroral oval. The data describe ion convection patterns as they respond to variable geomagnetic activity, and also the mean distribution of OI 630 nm emission as a function of local time, latitude and geomagnetic activity. These results can be used to examine the geomagnetic input parameters to a global thermospheric model (for example the semi-empirical global models of magnetospheric convection) as required to bring the simulations of thermospheric circulation into overall improved agreement with the observations.

Comparisons of theoretical models and observations of the thermosphere and ionosphere during extremely disturbed geomagnetic conditions during the last solar cycle

During the last solar cycle, a series of major geomagnetic disturbances, at times of moderate to high solar activity, were well observed by a combination of ground-based and space-borne instruments capable of observing both thermospheric and ionospheric structure, dynamics and their responses to geomagnetic inputs. A wide range of information is now available describing the major geomagnetic driving processes, and their magnitudes and large-scale variations during major disturbances, as a result of concerted experimental programmes of the past 6 to 8 years. Using these data, some of the largest geomagnetic disturbances recorded during the past 30 years have been simulated. Observations from Dynamics Explorer and ESRO IV, in addition to those from ground-based Fabry-Perot interferometers, provide detailed thermospheric data against which these simulated disturbances can be compared. These are presented as several case studies. The simulations provide a considerable insight into the behaviour of the thermosphere during extreme events, in addition to a valuable body of data for examination of extreme conditions of the thermospheric and ionospheric environment for spacecraft operations.

A theoretical and empirical study of the response of the high latitude thermosphere to the sense of the “ Y” component of the interplanetary magnetic field

The strength and direction of the Interplanetary Magnetic Field (IMF) controls the transfer of solar wind momentum and energy to the high latitude thermosphere in a direct fashion. The sense of “ Y” component of the IMF ( BY) creates a significant asymmetry of the magnetospheric convection pattern as mapped onto the high latitude thermosphere and ionosphere. The resulting response of the polar thermospheric winds during periods when BY is either positive or negative is quite distinct, with pronounced changes in the relative strength of thermospheric winds in the dusk-dawn parts of the polar cap and in the dawn part of the auroral oval. In a study of four periods when there was a clear signature of BY, observed by the ISEE-3 satellite, with observations of polar winds and electric fields from the Dynamics Explorer-2 satellite and with wind observations by a ground-based Fabry-Perot interferometer located in Kiruna, Northern Sweden, it is possible to explain features of the high latitude thermospheric circulation using three dimensional global models including BY dependent, asymmetric, polar convection fields. Ground-based Fabry-Perot interferometers often observe anomalously low zonal wind velocities in the (Northern) dawn auroral oval during periods of extremely high geomagnetic activity when BY is positive. Conversely, for BY negative, there is an early transition from westward to southward and eastward winds in the evening auroral oval (excluding the effects of auroral substorms), and extremely large eastward (sunward) winds may be driven in the auroral oval after magnetic midnight. These observations are matched by the observation of strong anti-sunward polar-cap wind jets from the DE-2 satellite, on the dusk side with BY negative, and on the dawn side with BY positive.