HF Radar Research Articles

On the vector plasma drift measurements over Equatorial Ionosphere using HF Doppler radar – A brief review

Background/ Objective: University of Kerala, Thiruvananthapuram ( 8.5◦N ,77◦E, dip 0.5◦N ) in India had a lively Space Physics research group since 1982, owing to proximity of the location to the geomagnetic dip equator. In 1982, a single frequency HF Doppler radar, operational at 5.5 MHz was installed in the University which provided many invaluable scientific results regarding the night time ionospheric F region. In 2003, it was upgraded to a new multifrequency HF Doppler Radar thereby enabling detailed examination of vertical plasma drift measurements at three different altitudes of F-region, in a nearsimultaneous manner. Methods: This study is basically a brief review of major results of F –region electrodynamics over Indian longitudinal sector obtained exclusively using HF Radar data. Findings: This review brings all the scientific results obtained using HF Radar about vertical drift studies over equatorial latitude and found that nearly about 40 research papers have been published in reputed national and international journals using the Radar data. Novelty: In a complementary point of view, this gives an opportunity to revisit and remember the important contributions of many of the earlier Indian scientists, who enriched ionospheric science using this unique facility. Keywords: HF Doppler Radar; Ionosphere; Vertical drift

Atmospheric resonances and their coupling to vibrations of the ground and waves in the ocean

Observations of the ionosphere with the airglow, GPS-TEC, and HF radar techniques reveal a resonant response of the middle and upper atmosphere to broad-band excitation by earthquakes, volcano eruptions, and convective storms. The resonances occur at such frequencies that an atmospheric wave, which is radiated at the ground level and is reflected from a turning point in the middle or upper atmosphere, upon return to the ground level satisfies boundary conditions on the ground. Using asymptotic and numerical models of atmospheric waves, this paper investigates atmospheric resonances and their excitation by seismic waves and infragravity waves in the ocean. It is found that “buoyancy” resonances with periods up to several hours arise in addition to “acoustic” resonances with periods of about 3–5 min. The acoustic and buoyancy resonances occur, respectively, on the acoustic and gravity branches of the dispersion curve of acoustic-gravity waves. Buoyancy of the atmosphere is important for the resonances of both kinds. Acoustic resonances are found to be sensitive to the temperature profile, especially around mesopause and tropopause, and are predicted to be a seasonal phenomenon in polar atmosphere. Unlike acoustic resonances, buoyancy resonances exhibit high sensitivity to the wind velocity profile and its variations. The resonances correspond to most efficient coupling between the atmosphere and its lower boundary and are promising for detection of such coupling.

Investigations into Synoptic Spatiotemporal Characteristics of Coastal Upper Ocean Circulation Using High Frequency Radar Data and Model Output

Numerical models and remote sensing observation systems such as radars are useful for providing information on surface flows for coastal areas. Evaluation of their performance and extracting synoptic characteristics are challenging and important tasks. This research aims to investigate synoptic characteristics of surface flow fields through undertaking a detailed analysis of model results and high frequency radar (HFR) data using self-organizing map (SOM) and empirical orthogonal function (EOF) analysis. A dataset of surface flow fields over thirteen days from these two sources was used. A SOM topology map of size 4 × 3 was developed to explore spatial patterns of surface flows. Additionally, comparisons of surface flow patterns between SOM and EOF analysis were carried out. Results illustrate that both SOM and EOF analysis methods are valuable tools for extracting characteristic surface current patterns. Comparisons indicated that the SOM technique displays synoptic characteristics of surface flow fields in a more detailed way than EOF analysis. Extracted synoptic surface current patterns are useful in a variety of applications, such as oil spill treatment and search and rescue. This research provides an approach to using powerful tools to diagnose ocean processes from different aspects. Moreover, it is of great significance to assess SOM as a potential forecasting tool for coastal surface currents.

Comparative Analysis of Summer Upwelling and Downwelling Events in NW Spain: A Model-Observations Approach

Upwelling and downwelling processes play a critical role in the connectivity between offshore waters and coastal ecosystems, having relevant implications in terms of intense biogeochemical activity and global fisheries production. A variety of in situ and remote-sensing networks were used in concert with the Iberia–Biscay–Ireland (IBI) circulation forecast system, in order to investigate two persistent upwelling and downwelling events that occurred in the Northwestern (NW) Iberian coastal system during summer 2014. Special emphasis was placed on quality-controlled surface currents provided by a high-frequency radar (HFR), since this land-based technology can effectively monitor the upper layer flow over broad coastal areas in near-real time. The low-frequency spatiotemporal response of the ocean was explored in terms of wind-induced currents’ structures and immediacy of reaction. Mean kinetic energy, divergence and vorticity maps were also calculated for upwelling and downwelling favorable events, in order to verify HFR and IBI capabilities, to accurately resolve the prevailing surface circulation features, such as the locus of a persistent upwelling maximum in the vicinity of Cape Finisterre. This integrated approach proved to be well-founded to efficiently portray the three-dimensional characteristics of the NW Iberian coastal upwelling system regardless of few shortcomings detected in IBI performance, such as the misrepresentation of the most energetic surface dynamics or the overestimation of the cooling and warming associated with upwelling and downwelling conditions, respectively. Finally, the variability of the NW Iberian upwelling system was characterized by means of the development of a novel ocean-based coastal upwelling index (UI), constructed from HFR-derived hourly surface current observations (UIHFR). The proposed UIHFR was validated against two traditional UIs for 2014, to assess its credibility. Results suggest that UIHFR was able to adequately categorize and characterize a wealth of summer upwelling and downwelling events of diverse length and strength, paving the way for future investigations of the subsequent biophysical implications.

Validation and Evaluation of a Ship Echo-Based Array Phase Manifold Calibration Method for HF Surface Wave Radar DOA Estimation and Current Measurement

Shore-based phased-array HF radars have been widely used for remotely sensing ocean surface current, wave, and wind around the world. However, phase uncertainties, especially phase distortions, in receiving elements significantly degrade the performance of beam forming and direction-of-arrival (DOA) estimation for phased-array HF radar. To address this problem, the conventional array signal model is modified by adding a direction-based phase error matrix. Subsequently, an array phase manifold calibration method using antenna responses of incoming ship echoes is proposed. Later, an assessment on the proposed array calibration method is made based on the DOA estimations and current measurements that are obtained from the datasets that were collected with a multi-frequency HF (MHF) radar. MHF radar-estimated DOAs using three calibration strategies are compared with the ship directions that are provided by an Automatic Identification System (AIS). Additionally, comparisons between the MHF radar-derived currents while using three calibration strategies and Acoustic Doppler Current Profilers (ADCP)-measured currents are made. The results indicate that the proposed array calibration method is effective in DOA estimation and current measurement for phased-array HF radars, especially in the phase distortion situation.

Ensemble 4DVAR (En4DVar) data assimilation in a coastal ocean circulation model. Part II: Implementation offshore Oregon–Washington, USA

The ensemble four-dimensional variational (En4DVar) data assimilation (DA) system introduced in Part I (Pasmans and Kurapov, 2019) is tested in the coastal waters offshore Oregon and Washington, U.S. West coast, during the spring and summer of 2011. The background error covariance B is derived from the forecast ensemble. Satellite sea-surface temperature (SST), sea-surface height (SSH), and daily-averaged radial surface currents from high-frequency radars (HFRs) are assimilated. The performance of the En4DVar system is compared with a “traditional” 4DVAR system using a static B. It is found that the presence of the Columbia River plume has a profound impact on the ensemble B. Near the plume front the SST–SSS covariance can be up to a factor 20 larger in magnitude than in the static B. This introduces large spatial and temporal variability in the ensemble B. The En4DVar system is more successful than the 4DVAR with the static B preserving the temperature–salinity properties when compared to glider data. The En4DVar system also produces more accurate forecasts and analyses for temperature in the subsurface below 30 m at a buoy location on the continental shelf. In comparisons with other surface and subsurface observations En4DVar shows consistent, albeit not significant, improvement over traditional 4DVAR. Large surface temperature–salinity covariances in combination with the episodic occurrence of large-scale errors in the SST observations lead to erroneous freshening in the centre of the model domain. Adding constraints on the surface salinity corrections based on the prior model reduces this effect.

Disk-Shaped Random Scatterers With Application to Model-Based PolSAR Decomposition

Polarimetric SAR (PolSAR) imagery offers an enhanced capability to reveal the salient scattering properties of scene content. PolSAR-based target decomposition has been widely used to show different apparent scattering mechanisms for various target classes, empowering a direct yet powerful technique for SAR imagery analysis. Among those common targets, modeling the random volume scattering from vegetation is one of the most important. Generally, one models vegetation as a cloud of randomly oriented thin cylinders, mainly intended for twigs and branches. At high radar frequencies, PolSAR imagery shows a strong response from leaves in the vegetation canopy. In this letter, we derive the polarimetric scattering theory for general random volume scatterers, including both thin cylinders and thin disks as limiting cases for leaf response. Adding the proposed random thin disk model explains the observed scattering difference between deciduous forest and coniferous forest, which we then incorporate into a new model-based PolSAR target decomposition scheme.

Assessment of HF Radar in Mapping Surface Currents under Different Sea States

AbstractTwo Ocean State Monitoring and Analyzing Radar (OSMAR071) (7.8 MHz) high-frequency (HF) radars and four moored ADCPs were operated concurrently in the southwestern Taiwan Strait during January–March 2013. Qualitative and quantitative comparisons of surface currents were conducted between the HF radars and the ADCPs. Except for a location probably affected by shallow water and sand waves on the Taiwan Banks, the HF-radar-derived radial currents (radials) showed good agreement with the ADCP measured results (correlation coefficient: 0.89–0.98; rms difference: 0.07–0.13 m s−1). To provide further insight into the geophysical processes involved, the performance of the HF-radar-derived radials was further evaluated under different sea states (sea states: 2–6). It was found that both the data returns of the radar-derived radials and the differences between the radar-derived radials and the ADCP-derived radials varied with sea state. The HF radar performed best at sea state 4 in terms of data returns. The spatial coverage increased rapidly as the waves increased from sea state 2 to 4. However, it decreased slowly from sea state 4 to 6. Second, the radial differences were relatively high under lower sea states (2 and 3) at the location where the best agreement was obtained between the radar and ADCP radials, whereas the differences increased as the sea states increased at the other three locations. The differences between the radials measured by the HF radars and the ADCPs could be attributed to wave-induced Stokes drift and spatial sampling differences.

The Evolution of Complex Es Observed by Multi Instruments Over Low‐Latitude China

AbstractA complex daytime sporadic E (Es) case with extremely high critical frequency (foEs) was observed over the low latitude of China on 19 May 2018. Simultaneous observational results from two very high frequency (VHF) radars, two ionosondes, and multiple Global Navigation Satellite System total electron content and scintillation receivers are analyzed to investigate the evolution of the complex Es occurrence, which consisted of a relatively weak ambient Es layer (foEs < 8 MHz) and band‐like strong Es structures (foEs > 17 MHz) drifting from higher latitude. The strong Es structures elongated more than 500 km in the northwest‐southeast direction, drifted southwestward at a speed of ~65 m/s. VHF radar backscatter echoes were generated when the strong Es structures passed the radar field of view, with different echo patterns due to different radar and antenna configurations. No VHF radar backscatter echo was associated with the ambient Es layer. The mechanisms responsible for the formations of the ambient Es layer and band‐like strong Es structures are addressed and discussed.

Bistatic Observations With SuperDARN HF Radars: First Results

AbstractSuper Dual Auroral Radar Network (SuperDARN) radars operate in a coordinated but monostatic configuration whereby high‐frequency (HF) signals scattered from ionospheric density irregularities or from the surface of the Earth return to the transmitting radar where Doppler parameters are then acquired. A bistatic arrangement has been developed for SuperDARN radars in which HF signals transmitted from one radar are received and analyzed by another radar that is separated by a large distance (>1,000 km). This new capability was developed and tested on radars located in Oregon and Kansas. Numerous 3‐day bistatic campaigns have been conducted over a period extending from September 2019 through March 2020. During these campaigns three distinct bistatic propagation modes have been identified including a direct mode in which signals are transmitted and received through the radar side lobes. Direct mode signals propagate along the great‐circle arc connecting the two bistatic radar sites, reflecting from the ionosphere at both E region and F region altitudes. Two additional modes are observed in which HF signals transmitted from one radar scatter from either ionospheric density irregularities or from the surface of the Earth before propagating to the bistatic receiving radar. Ray tracing simulations performed for examples of each mode show good agreement with observations and confirm our understanding of these three bistatic propagation modes. Bistatic campaigns continue to be scheduled in order to improve technical aspects of this new capability, to further explore the physical processes involved in the propagation and scattering of HF bistatic signals and to expand the coverage of ionospheric effects that is possible with SuperDARN.

Validation of CODAR 25MHz High-Frequency Radar derived Waves in Donghea, Korea

In order to examine the possibility of wave observation of CODAR HF radar, which is mostly used in Korea for surface ocean current observation, CODAR HF radar was installed on the coast to observe real-time waves. Unlike other HF radars, CODAR HF radars are easy to install because they are integrated with transmission and reception. and It is suitable for the domestic coastal environment where the coastline was developed. The CODAR HF radar used in this study transmits and receives waves using a center frequency of 25.4 MHz and a bandwidth of 150 kHz to observe the ocean wave. From the center of the observation point, a circular area of 140 degrees excluding land is observed, and ocean wave data is observed every 10 minutes from the total observation point to the shells 1 to 31 through the spatial average in units of 1 km from 31 km. According to the specifications of the 25Mhz CODAR HF radar, it is said that it is possible to observe a wave height of 4m, but the observation shows that it can be observed up to 6m.The observed CODAR HF radar wave data was compared and validated with the wave data observed by three wave measurement equipments (WAVE rider, AWAC, KMA wave buoy). In the case of the result of comparison of significant wave height and wave period in each installed shell region 1) WAVE rider was Corr. 0.76, 0.87 and RMSE 0.57 m, 0.65 s 2) AWAC was Corr. 0.92, 0.91 and RMSE 0.46 m, 0.45 s 3) KMA wave buoy was Corr. 0.80, 0.89 RMSE 0.65 m, 0.55 s, and all three types of underwater wave observation equipment confirmed high similarity to the wave result observed in HF radar. However, since the underwater wave observation equipment was located only at the 2 and 3 shells (2 to 3 km from the HF radar), additional verification is necessary through the installation of the underwater wave observation equipment at a remote shell location. Also, it is necessary to technically solve that the wave data acquisition rate of HF radar is lower than that of the radial vector. However, HF radar s wide-area wave observation data is expected to compensate for the disadvantages of underwater wave observation data (fixed point observation, difficulty in management and installation, and high maintenance cost). The wave observation data of HF radar could contribute greatly to ocean wave research.

HF Radar Measurements of Surface Waves in the Gulf of Naples (Southeastern Tyrrhenian Sea): Comparison With Hindcast Results at Different Scales

HF radar systems wave measurements are evaluated against numerical simulations in the Gulf of Naples (Southeastern Tyrrhenian Sea). Wave measurements are obtained from three CODAR SeaSonde HF radars installed along the coast of the Gulf of Naples. The numerical models employed are WavewatchIII, implemented on a regional scale with a resolution of about 10 km in longitude and latitude in the whole Mediterranean Sea, and SWAN, implemented with a 200 m resolution in the area of interest. Numerical simulations are also validated against experimental data acquired by a buoy installed offshore the Gulf of Naples. The agreement between HF radar measurements and model hindcasts is evaluated through the estimate of statistical error indices for the main wave characteristics (significant wave height, mean period and mean direction). The consistency between wave parameters retrieved by HF radars and hindcasted by the models opens the way to future integration of the two systems as well as to the utilization of HF radar wave parameters that could be envisaged for data assimilation in wave models.

Wind Speed Extraction Based on High Frequency Radar Retrieved Wind-Driven Current

High frequency (HF) radar often indirectly inverts wind field from wave height field, so the accuracy, range, and spatial resolution of the estimated wind field are usually limited by the wave field estimation performance. However, the sea surface current including the wind-driven current can often be accurately measured by radar. Thus, based on the wind-driven current estimated by HF radar, a new wind speed inversion algorithm is proposed in this letter. First, the relationship between wind speed and wind-driven current speed is established according to the ocean dynamics theory, but it contains several uncertain parameters. To avoid solving these parameters, an artificial neural network is used to train an accurate model between wind-driven current speed and wind speed. Final, the wind-driven current estimated by radar is substituted into the model to extract wind speed. A field experiment shows that the average correlation coefficient between the radar-estimated wind speed and the reference wind speed is 0.86, and the average root mean square error is 2.38 m/s. In addition, the proposed algorithm has larger measurement range and better spatial resolution than traditional algorithms.

Modeling Ocean Currents Through Complex Random Fields Indexed in Time

Surface ocean currents are often of interest in environmental monitoring. These vectorial data can be reasonably treated as a finite realization of a complex-valued random field, where the decomposition in modulus (current speed) and direction (current direction) of the current field is natural. Moreover, when observations are also available for different time points (other than at several locations), it is useful to evaluate the evolution of their complex correlation over time (rather than in space) and the corresponding modeling which is required for estimation purposes. This paper illustrates a first approach where the temporal profile of surface ocean currents is considered. After introducing the fundamental aspects of the complex formalism of a random field indexed in time, a new class of models suitable for including the temporal component is proposed and applied to describe the time-varying complex covariance function of current data. The analysis concerns ocean current observations, taken hourly on 30 April 2016 through high frequency radar systems at some stations located in the Northeastern Caribbean Sea. The selected complex covariance model indexed in time is used for estimation purposes and its reliability is confirmed by a numerical analysis.

An extended radar relative calibration adjustment (eRCA) technique for higher-frequency radars and range–height indicator (RHI) scans

Abstract. This study extends the relative calibration adjustment technique for calibration of weather radars to higher-frequency radars as well as range–height indicator (RHI) scans. The calibration of weather radars represents one of the most dominant sources of error for their use in a variety of fields including quantitative precipitation estimation and model comparisons. While most weather radars are routinely calibrated, the frequency of calibration is often less than required, resulting in miscalibrated time periods. While full absolute calibration techniques often require the radar to be taken offline for a period of time, there have been online calibration techniques discussed in the literature. The relative calibration adjustment (RCA) technique uses the statistics of the ground clutter surrounding the radar as a monitoring source for the stability of calibration but has only been demonstrated to work at S- and C-band for plan-position indicator (PPI) scans at a constant elevation. In this work the RCA technique is modified to work with higher-frequency radars, including Ka-band cloud radars. At higher frequencies the properties of clutter can be much more variable. This work introduces an extended clutter selection procedure that incorporates the temporal stability of clutter and helps to improve the operational stability of RCA for relatively higher-frequency radars. The technique is also extended to utilize range–height scans from radars where the elevation is varied rather than the azimuth. These types of scans are often utilized with research radars to examine the vertical structure of clouds. The newly extended technique (eRCA) is applied to four Department of Energy Atmospheric Radiation Measurement (DOE ARM) weather radars ranging in frequency from C- to Ka-band. Cross comparisons of three co-located radars with frequencies C, X, and Ka at the ARM Cloud, Aerosol, and Complex Terrain Interactions (CACTI) site show that the technique can determine changes in calibration. Using an X-band radar at the ARM Eastern North Atlantic (ENA) site, we show how the technique can be modified to be more resilient to clutter fields that show increased variability, in this case due to sea clutter. The results show that this technique is promising for a posteriori data calibration and monitoring.

Surface Circulation off the Andaman Islands from HF Radar Observations

Surface Circulation off the Andaman Islands from HF Radar Observations

Submesoscale Eddies Observation Using High-Frequency Radars: A Case Study in the Northern South China Sea

Eddies are of great concern to ship operations and have an important influence on heat transfer in the sea. In this article, a highly concurrent and accurate method incorporating the winding angles and density-based clustering (WA-DBC) algorithms is proposed for automatic eddy detection from surface current maps generated by high-frequency radars (HFRs). As a case study, eddies and their characteristics, such as centers, radii, and lifespans, were extracted using both the WA-DBC and vector geometry algorithms from the HFR surface current maps of the north of Qiongzhou Strait, South China Sea. The eddy trajectories obtained show that most eddies in this region during the experiment period had a radius of 3-30 km and negative correlation existed between the radius and the life span (i.e., eddies with a smaller radius survived a longer lifespan). Furthermore, most of the eddies appeared in the central part of the study area and lasted for 40-100 min. This study shows that HFR is useful for detecting submesocale eddies for research works and applications in marine observation.

Interference Patterns Observed in the Indian Coastal Ocean Radar Network

Suseentharan, V.; Arunraj, K.S.; Jena, B.K.; Ramana Murthy, M.V., and Tushar K., 2020. Interference patterns observed in the Indian coastal ocean radar network. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 63-66. Coconut Creek (Florida), ISSN 0749-0208.Indian Coastal Ocean Radar Network (ICORN) runs 10 shore- based High-Frequency Radar (HFR) systems for measuring 2D surface currents and waves, which cover four coastal states and the Andaman Islands. These coastal remote sensing systems are subjected to various interferences. This paper discusses the periodic interference patterns observed in the network and the possible causes for these patterns. The radial data file size from the HFR sites is used as a proxy to identify these patterns. Similarly, vertical Total Electron Content (TEC) is used to identify the influence of Ionosphere on HFR data. The Fast Fourier Transform (FFT) on these data sets showed a strong correlation between HFR and TEC data at diurnal and semi-diurnal frequencies as well as at 8, 6 and 4.8 hours periods. However; low pass filter HFR data does not show any seasonal or intra-seasonal pattern, except at the Andaman Islands, where it showed fortnightly and monthly patterns.

Sub-Mesoscale Circulation Features along the Andhra Pradesh Coast, Bay of Bengal: Observations from HF Radars

Mandal, S.; Pramanik, S.; Sil, S.; Arunraj, K.S., and Jena, B.K., 2020. Sub-mesoscale circulation features along the Andhra Pradesh coast, Bay of Bengal: Observations from HF radars. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 132-138. Coconut Creek (Florida), ISSN 0749-0208.This work presents the coastal circulation variability along the Andhra coast in western Bay of Bengal (BoB) using hourly high frequency radar (HFR, 4.4 MHz) derived ocean surface currents during 2015. The comparison of daily averaged HFR surface currents with satellite derived currents shows high correlations (0.86, 0.70) and low errors (0.28 m s-1, 0.25 m s-1) for the zonal and meridional components. The HFRs captured and quantified the dominant circulation features associated with the reversal of boundary currents. The northward (southward) currents are observed from mid-March till May end (October to November with highest current speed of ∼1.58 (1.24) m s-1 along the Andhra coast (between 15-17°N). In addition, the sub-mesoscale features associated with dominant circulation features are observed, like, the propagation of an anticyclonic eddy (81.80°E, 15.25°N) during March and cyclonic eddy (82.25°E, 15.40°N) during November. The mean vorticity pattern from the HFR currents shows the dipole nature during April - May to indicate the near coast upwelling signatures which are also supported by chlorophyll observations from satellite. The tidal driven currents, extracted from the HFRs, show that the semi-diurnal tidal constituent, M2 dominates during both the seasons with less intensity in March than in November. The distributions of the M2 tidal ellipses match well with the broad-scale current pattern in the offshore region. They follow the bathymetry in the nearshore region to indicate that it is driven by tides resulting in the generation of the shallow water tidal constituents (M3, M4, MS4 and M8) with significant amount of amplitudes.

On the Origin of Far‐Aspect Angle Irregularity Regions Seen by HF Radars at 100‐km Altitude

AbstractA special summer midday population of echoes has been reported by high‐frequency (HF) radars at ranges less than 300 km. These echoes come from a narrow altitude region near 100 km. Their spectral width indicates that they decay at a rate controlled by ambipolar diffusion. We label the echo regions as “far‐aspect angle irregularity regions” by contrast with “high‐aspect angle irregularity regions” (HAIR) documented in the past. Both types are from structures that decay after an initial growth phase. We show that each type decays at a rate controlled by the altitude where it is produced. The large aspect angles and decaying process come from a monotonic aspect angle increase due to a weak altitude dependence of the “eigenfrequency.” HAIR echoes are produced by the decay of otherwise standard E‐region irregularities. However, HAIR echoes are usually not far‐aspect angle irregularity regions because the latter are the result of a Gradient Drift Instability in the bottomside of the E‐region where the vertical density gradients can sometimes be so strong that a less than 10 mV/m northward electric field is able to destabilize 10‐m structures in the afternoon sector. Owing to smaller dip angles, the Gradient Drift Instability requirement is smaller at midlatitudes although it still needs a combined electron drift and eastward neutral wind contribution of the order of 100 m/s. With northward electric fields of 2 to 3 mV/m, the minimum midlatitude eastward wind requirement is of the order of 50 m/s.