Plasma Drift Velocity Research Articles

Comparative study of magnetic field effect on fast plasma flow of heavy and light species investigated by spectroscopic measurement

To understand the effects of magnetic fields on the propagating plasma flows of heavy and light ion species, a laboratory-scale experiment was conducted using a pulsed-power discharge. The plasma drift velocity and electron temperature were estimated by time-of-flight and line-pair methods, respectively, using spectroscopic measurements. Ion current waveforms were measured using an ion collector. When a magnetic field was applied, the plasma drift velocity decreased and the electron temperature increased in both heavy and light plasmas. The magnetic Reynolds number, pressure balance between the plasma and magnetic field, and ion current waveforms show that heavy plasma has a high possibility of deforming the magnetic field and generating accelerated ions through interaction with the magnetic field.

On the Role of Physical Processes in Controlling Equatorial Plasma Bubble Morphology

AbstractIn this study, we present the results of an analysis of the morphological features of Equatorial Plasma Bubbles (EPBs) over South America. In this context, we analyzed data from the Disturbance Ionosphere indeX (DIX) maps calculated using around 450 Global Navigation Satellite System (GNSS) stations. To mitigate the influence of magnetic disturbances on bubble development, only data from geomagnetically quiet days were utilized. This study covered the period from the post‐peak of solar cycle 24 (2015) to the pre‐peak of solar cycle 25 (2023), totaling 1321 nights with EPB occurrences, representing the largest data set of EPBs ever compiled for South America. Our analysis unveiled several key findings regarding EPBs and their behavior over the South American region. First, we observed that the amplitude of plasma depletions, as reflected in the DIX values, and the EPB latitudinal development follow an approximately 11‐year cycle driven by solar radiation levels. Furthermore, our analysis highlights the significant influence of factors such as the angle between the solar terminator and the magnetic meridian (T‐M angle), which varies inversely with the vertical plasma drift velocity during the pre‐reversal enhancement (PRE). Additionally, we discuss the longitudinal variations associated with magnetic declination, as well as the saturation behavior of EPB development with extreme solar flux.

Inter‐Hemispheric Energy Input Into the Ionosphere‐Thermosphere System During Magnetic Storms: What We Can and Cannot Learn From DMSP Observations

AbstractWe present a long‐term superposed epoch analysis and statistical study with 427 magnetic storms caused by coronal mass ejections and the subsequent magnetospheric energy input into the ionosphere‐thermosphere system. Electron and ion precipitating energy flux data along with plasma drift velocity and magnetic field data (used for Earth‐bound Poynting flux computations) are used in this study. By carefully exploring conjugated inter‐hemispheric energy inputs as a function of storm intensity and seasons, we find that near two‐decades of Defense Meteorological Satellite Program observations indicate that the northern hemisphere (NH) receives slightly more energy than the southern hemisphere (SH) (Poynting flux and integrated electron/ion precipitating energy flux), with the NH receiving more energy as the storm becomes more intense. However, for the Poynting flux case, we find that generally the SH receives more energy when the east‐west component of the interplanetary magnetic field (IMF) becomes highly positive (By ≥ +5 nT), whereas this pattern reverses for cases when IMF By is highly negative (By ≤ −5 nT). The inter‐hemispheric interpretation for electron and ion precipitation energy inputs is ambiguous due to a severe lack of conjugated observations around the midnight sector for both hemispheres.

Application of Wide‐Beam Transmission for Advanced Operations of SuperDARN Borealis Radars in Monostatic and Multistatic Modes

AbstractThe Super Dual Auroral Radar Network (SuperDARN) consists of more than 30 monostatic high‐frequency (HF, 8–20 MHz) radars to study dynamic processes in the ionosphere. SuperDARN provides maps of global‐scale ionospheric plasma drift circulation from the mid‐latitudes to the poles. The conventional SuperDARN radars consecutively scan through 16 beam directions with a lower limit of 1 minute to sample the entire field of view. In this work, we use the advanced capabilities of the recently developed Borealis digital SuperDARN radar system. Combining a wide transmission beam with multiple narrow reception beams allows us to sample all conventional beam directions simultaneously and to speed up scanning of the entire field‐of‐view by up to 16 times without noticeable deterioration of the data quality. The wide‐beam emission also enabled the implementation of multistatic operations, where ionospheric scatter signals from one radar are received by other radars with overlapping viewing areas. These novel operations required the development of a new model to determine the geographic location of the source of the multistatic radar echoes. Our preliminary studies showed that, in comparison with the conventional monostatic operations, the multistatic operations provide a significant increase in geographic coverage, in some cases nearly doubling it. The multistatic data also provide additional velocity vector components, increasing the likelihood of reconstructing full plasma drift velocity vectors. The developed operational modes can be readily implemented at other fully digital SuperDARN radars.

Latitude Variation of the Post‐Sunset Plasma Density Enhancement During the Minor Geomagnetic Storm on 27 May 2021

AbstractIn this study, multiple instrumental observations including Global Navigation Satellite System total electron content (TEC), plasma drift velocity measured by Sanya (18.3°N, 109.6°E, dip latitude 12.6°N) Incoherent Scatter Radar (SYISR) and F2‐layer peak electron density (NmF2) and peak height (hmF2) from ionosonde and SYISR have been used to investigate ionospheric responses during a minor yet highly geo‐effective geomagnetic storm on 26–27 May 2021. Our findings revealed a significant time delay in the post‐sunset plasma density enhancement peak across different latitudes over East Asia, that is, the lower the geographic latitude, the earlier the peak appeared. The plasma density enhancement was accompanied by a decrease in hmF2 prior to NmF2 peak around sunset. The newly built SYISR measurements around sunset verified that the field‐aligned drift decreased the ionosphere with a notable time delay at latitude, beneficial to electron density enhancements at lower altitudes within the 16–30°N latitudinal band but a small TEC change. While at 30–50°N, it is possible that the competition between storm‐induced equatorward winds and downward field‐aligned drift depressed hmF2 decline and the buildup increased both NmF2 and TEC. The ICON observations suggested that the meridional wind during this minor storm event modulated the direction of plasma transport near sunset, playing a dominant role in post‐sunset plasma density enhancement from low to middle latitudes. These results provide fresh insight into the electrodynamic mechanisms of post‐sunset enhancements at middle and low latitudes over East Asia, and also enhance our understanding of the intricate behaviors within the ionosphere‐thermosphere system in response to a minor storm.

The Hemispheric Difference in Electric Potential and Electron Precipitation Observed by DMSP in the Auroral Zone

AbstractIn this case study, we focus on how hemispheric differences in the electric potential distribution in the auroral zones are related to hemispheric differences in the energetic particle precipitation that indicate the presence of field‐aligned potential drops. Utilizing data during the spring and fall equinox in 2013 and 2014 from the Defense Meteorological Satellite Program (DMSP) F16 and F17 satellites, we systematically examine simultaneous measurements of plasma drift velocity and particle precipitation in magnetically conjugate regions across the auroral zone in both hemispheres. This allows us to establish the degree to which the interhemispheric differences in ionospheric electric potential may be accounted for by field‐aligned potential drops. Thirty‐seven eligible cases examined show accelerated electron energy spectra at conjugate locations in the northern and/or southern hemispheres. Of these 37 cases, 23 show conjugate differences in electric potential that agree qualitatively with the observed electron acceleration and average energy of precipitating electrons in each hemisphere. This result suggests that the hemispheric differences in ionospheric electric potential in the auroral zone may be most frequently accounted for by field‐aligned potential drops along the magnetic flux tubes connecting conjugate points to the equatorial plane. The spatial and temporal variability of the potential distribution, electron precipitation, and magnetic field configuration could account for deviations from the normally observed situation in the remaining cases. More magnetically conjugate observations at various altitudes in both hemispheres are needed to investigate this hemispheric coupling in more detail.

Preliminary Results of the Three-Dimensional Plasma Drift Velocity at East Asian Low-Latitudes Observed by the Sanya Incoherent Scattering Radar (SYISR)

As the first advanced modular phase array incoherent scatter radar (ISR) established in the Eastern Hemisphere at low latitudes, Sanya ISR (SYISR) can measure the line-of-sight (LOS) velocity of ion drift in multiple directions, potentially yielding the spatial distribution of ionospheric plasma drift. Three beam-scanning modes are designed for plasma drift detection: meridian, zonal and cross-shaped (both meridian and zonal) plane, which will provide the distribution of plasma drift in latitude/longitude as well as altitude. The altitude profile of plasma drift and the first presented distribution of low latitude plasma drift in the meridian plane for March to May 2021 are inversed through LOS velocity using cross-shaped and meridian beam-scanning modes, respectively. A statistical correlation coefficient between the vpn and crest-to-trough ratio (CTR) of equatorial ionization anomaly (EIA) TEC and a case study of magnetic storm response in plasma drift show that the inversed plasma drift can be a good indicator in response to the changes in atmospheric tide and solar wind at different time scales and explain the corresponding ionospheric electron density variations at low and equatorial latitudes. This study proves that the SYISR-measured plasma drift is reliable and will play an important role in the atmosphere-ionosphere-magnetospheric coupling study in the East Asian region.

Ionosphere response to geospace storm on 25 September 2016 over Kharkiv (Ukraine)

Instrumental observations of variations in the level of the geomagnetic field and ionospheric parameters were conducted during a weak geospace storm on September 25, 2016. It was found that a moderate magnetic storm was accompanied by variations in the level of the H component of the geomagnetic field up to 20–30 nT, as well as variations in the level of fluctuations of the horizontal components of the geomagnetic field up to ±(2–3) nT. It was revealed that the ionospheric storm was three-phase, mostly moderate. The decrease in the electron density reached a factor of 1.5, and its increase reached a factor of 2.2. The increase in the electron temperature during the negative phase of ionospheric storm did not exceed 200–300 K, while its increase during the positive phase of ionospheric storm reached 400–500 K. The ion temperature variations were within ±100 K. During the negative phases of the ionospheric storm, individual deviations in the vertical plasma drift velocity from those on a geomagnetically quiet day were observed both in the direction of decreasing the downward drift velocity (increasing the upward drift velocity at high altitudes) and vice versa. Significant (17–53 m/s) deviations in velocity variations towards positive values (decrease of downward and increase of upward drift velocity), accompanied by the rise of the F2 layer, after significant changing in the zonal component of the magnetospheric electric field (from –2 to 3.5 mV/m), contributed to the increase in the electron density and to the appearance of the positive phase of the ionospheric storm. During the positive phase, the increase in the velocity of downward plasma drift reached 9–20 m/s at altitudes above 450 km. The negative phases of the ionospheric storm were formed due to an increase in the electron and ion temperatures and a decrease in the O/N2 ratio during the storm. The generation of ionospheric storm positive phase at mid-latitudes apparently was caused by several mechanisms at once: a penetrating electric field, a meridional wind directed toward the equator, and downward plasma flows in the protonosphere. The phases of the ionospheric storm were accompanied by significant variations in the dynamic and thermal processes in the ionosphere.

Equilibrium Studies of the magnetic surfaces function  of Tokamak system with poloidal divertor

An analytical solution for the magnetic field B produced by current filaments and the related magneticsurface function are given. The use of as a coordinate in this system is explored, and its hyperbolicform is given. The simple character of the plasma drift velocity for this coordinate is derived.

Investigating the Role of Gravity Waves on Equatorial Ionospheric Irregularities Using TIMED/SABER and C/NOFS Satellite Observations

In this paper, for the first time, simultaneous atmospheric temperature perturbation profiles obtained from the TIMED/SABER satellite and equatorial ion density and vertical plasma drift velocity observations with and without ESF activity obtained from the C/NOFS satellite are used to investigate the effect of gravity waves (GW) on ESF. The horizontal and vertical wavelengths of ionospheric oscillations and GWs are estimated by applying wavelet analysis techniques. In addition, vertically propagating GWs that dissipate energy in the ionosphere-thermosphere system are investigated using the spectral analysis technique. We find that the vertical wavelength of GW, corresponding to dominant wavelet power, ranges from 12 to 31 km regardless of the conditions of the ionosphere; however, GWs with vertical wavelengths between about 1 to 13 km are found every day, saturated between 90 and 110 km at different longitudinal sectors. Filtering out vertical wavelengths above 13 km from temperature perturbations, ranges of zonal wavelengths of GW (i.e., from about 290 to 950 km) are found corresponding to irregular and non-irregular ionosphere. Similarly, corresponding to dominant oscillations, the zonal wavelength of ion density perturbations is found within 16 to 1520 km. Moreover, we find an excellent agreement among the median zonal wavelengths of GW for the cases of irregular and non-irregular ionosphere and ion density perturbations that are 518, 495, and 491 km, respectively. The results imply that seed perturbations due to GW with a vertical wavelength from about 1 to 13 km evolve to ion density irregularity and may be amplified due to post-sunset vertical upward drift velocity.

On Turbulent Features of E × B Plasma Motion in the Auroral Topside Ionosphere: Some Results from CSES-01 Satellite

The recent Chinese Seismo-Electromagnetic Satellite (CSES-01) provides a good opportunity to investigate some features of plasma properties and its motion in the topside ionosphere. Using simultaneous measurements from the electric field detector and the magnetometers onboard CSES-01, we investigate some properties of the plasma E × B drift velocity for a case study during a crossing of the Southern auroral region in the topside ionosphere. In detail, we analyze the spectral and scaling features of the plasma drift velocity and provide evidence of the turbulent character of the E × B drift. Our results provide an evidence of the occurrence of 2D E × B intermittent convective turbulence for the plasma motion in the topside ionospheric F2 auroral region at scales from tens of meters to tens of kilometers. The intermittent character of the observed turbulence suggests that the macro-scale intermittent structure is isomorphic with a quasi-1D fractal structure, as happens, for example, in the case of a filamentary or thin-tube-like structure. Furthermore, in the analyzed range of scales we found that both magnetohydrodynamic and kinetic processes may affect the plasma dynamics at spatial scales below 2 km. The results are discussed and compared with previous results reported in the literature.

Nighttime morphology of vertical plasma drifts over Vietnam during different seasons and phases of sunspot cycles

This study is the first to provide the nighttime morphology of vertical plasma drifts over Vietnam at the equatorial trough and the northern tropical crest of ionization anomaly in the Asian sector. We use the h’F data in Vietnam from Bac Lieu observatory (9.28°N, 105.73°E, dip: 1.73°N) during the 2006–2019 period and Phu Thuy observatory (21.03°N, 105.96°E, dip: 14.49°N) for the 1964–2011 period, to calculate the vertical plasma drift velocity (Vd) and analyze the annual, seasonal, sunspot cycle, and sunspot cycle (SC) phase evolutions. For Bac Lieu, the PRE and minimum reversal peaks evolve according to the sunspot cycles. The seasonal average of the drift pattern shows that the PRE peak appears earlier one hour in summer (1800 LT) than in equinoxes and winter (1900 LT) and the minimum reversal peak occurred about 2100–2200 LT, except in summer, in which this peak identifies around 0100 LT. This drift pattern exhibits a semiannual asymmetry during PRE/minimum reversal periods with peaks in March and September/October. The evening-time pre-reversal enhancement (PRE)at Bac Lieu is dependent on the phase of the sunspot cycle. For Phu Thuy, the mean annual peak magnitudes of Vd during the PRE and minimum reversal periods exhibit a poor correlation with the sunspot number (Rz). For the SC seasonal variation, the lowest PRE peak magnitude in autumn appeared one hour later compared with winter, one hour earlier than summer, and the same time as spring, while the minimum reversal peak is larger in summer for SCs 20–23 and in autumn for SCs 20, 23, and 24 at around 0200–0300 LT than in another season at about 2000, 2200, and 2300 LT. The morphology of the seasonal average variations of Vd is rather similar for all SCs except SC 24 at Phu Thuy. The PRE peak during the high solar activity years (2000 LT) appears earlier about 4 h than that during the low solar years (at 2300 LT/0000 LT). The monthly average of plasma drift also exhibits semiannual asymmetry during PRE events for cycles 22, 24, and minimum reversal period for cycle 24 with peaks in March and September. In addition, it shows an annual variation for the remaining cycles during PRE/minimum reversal periods with peaks in the summer months. The pre-sunrise enhancement characteristic (0500 LT) was observed during all seasons and phases (ascending, descending, minimum) at Bac Lieu, summer for all SCs, equinoxes for SC 24, the ascending and minimum phases for the SC 24, and the descending phase for the SC 23 at Phu Thuy.

Effect of repetitive irradiation on ion saturation current of laser ablation plasma using 1 kHz repetitive laser

We demonstrated 100 shots of burst laser irradiation with an Al target using a 1 kHz repetitive pulse laser. By using the laser, we generated ablation plasmas with a repetition rate of 1 kHz. The time-evolution of ion saturation current density in the generated ablation plasma was measured using the orifice probe. The result shows that the waveforms on 100 spikes were observed. The ion saturation current density decreased rapidly from the first to the third shot and reached a minimum value at the fourth shot. After the fourth shot, it reached 1 μA/mm2 at the 53rd shot and settled at 0.5 μA/mm2. We extracted the time-evolution of ion saturation current densities of the first, 10th, 50th and 100th shots. The drift velocities were estimated by Time of Flight from the time-evolution of current. The result shows that the drift velocity of the ablation plasma increases with the increase in the number of shots.

Characteristics of F-region velocity vortex during sunrise: Observations from the Mid-Latitude Station Nicosia

This manuscript presents the dawn characteristics of zonal plasma drift and the resultant plasma drift velocity vector (the resultant vector of vertical and zonal plasma velocity components) over the mid latitude station Nicosia, Cyprus (35°N, 33°E, and Dip angle, I = 51.7°), analyzed for a duration of one year from 2013 October to 2014 September. A westward enhancement in zonal drift is observed during sunrise and is termed as Sunrise Westward Enhancement (SWE), similar to Sunrise Downward Enhancement (SDE) in the vertical drift. The enhancement in westward drift during sunrise is evident in all seasons and is more prominent in Summer. The novelty of the present study is that the simultaneous observations of zonal and vertical drift components are used to obtain vector plasma drift, done for the first time for a mid-latitude station during morning hours. Pre-sunrise resultant plasma drift vector shows a counter clockwise circulation in all seasons.

Current Understanding of the Equatorial E x B Drift velocities in the African Sector: A Short Review

A short review of the pattern and morphology of the equatorial plasma drift velocities, particularly during the evening-time Pre-reversal enhancement (PRE) period in the African region had been presented. The seasonal PRE peak values across some locations in the West-African region were considered and compared with other sectors of the world. While most plasma drift observations in the African region were calculated from ionosonde measurements, the observations from other sectors involved direct measurement from satellite and the Incoherent Scatter Radar (ISR) observations. The importance of the PRE in ionospheric electrodynamics was highlighted, the better in the use of either the virtual or real heights of the F-layer in inferring vertical drift velocities were enumerated, revealing the strengths and weakness of each method. The general observations revealed that PRE peak magnitude is commonly weaker in the African region in comparison with the American/Peruvian and Indian sectors, seasonal and solar activity dependent, and could be higher during either magnetic quiet or disturbed activity than when both magnetic activity conditions are combined. The first work to present a regional PRE model around the African equatorial ionization anomaly region (Adebesin et al model) was mentioned. The relevance of the E × B drift in quantifying the daytime equatorial electrojet (EEJ) current was also discussed.

On the Dependence of Amplitude and Phase Scintillation Indices on Magnetic Field Aligned Angle: A Statistical Investigation at High Latitudes

This letter presents for the first time the results of a statistical study that had looked into the dependence of scintillation on the propagation geometry of GPS signals at high latitudes by combining the geometrical parameters, namely elevation and azimuth, along with the magnetic field vector into a single variable called the magnetic field aligned angle (MFAA). An increase in the value of scintillation indices when MFAA approaches zero is observed. This indicates the presence of field-aligned irregularities and the fact that MFAA is sensitive to irregularities whose size extends to Fresnel scales as well. Contrary to previous experimental and modeling studies which have shown higher variations of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{\phi }$ </tex-math></inline-formula> index as compared to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{4}$ </tex-math></inline-formula> index with the propagation geometry of GPS signals, our results suggest that these higher fluctuations in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{\phi }$ </tex-math></inline-formula> were refractive in origin and were the result of improper detrending of the phase at high latitudes where the use of constant cutoff of 0.1 Hz is not accurate due to higher ionospheric plasma drift velocities at these latitudes.

The Impact of Assimilating COSMIC‐2 Observations of Electron Density in WACCMX

AbstractThe present study investigates the impact of assimilating electron density profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2) mission in a whole atmosphere data assimilation system. The observations are assimilated into the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCMX) using the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Assimilation of the COSMIC‐2 electron density profiles during the evaluation time period of 25–30 April 2020 leads to improvement in both the 1 hr forecast and analysis electron densities in WACCMX + DART. Compared to a control experiment that does not assimilate COSMIC‐2 observations, the assimilation of the COSMIC‐2 electron density profiles reduces the 1 hr forecast root mean square error (RMSE) and bias with respect to COSMIC‐2 observations at 300 km by 6.76% and 24.91%, respectively. Assimilation of the COSMIC‐2 electron density profiles does not significantly influence the RMSE and bias with respect to ground‐based Global Navigation Satellite System vertical total electron content observations. The equatorial vertical plasma drift velocity in WACCMX + DART is changed by ±5–10 ms−1 due to the assimilation of the COSMIC‐2 electron density profiles, indicating that the model representation of the electrodynamics of the low latitude ionosphere are significantly impacted by the assimilation of COSMIC‐2 observations.

Development of a set of movable electrostatic probes to characterize the plasma in the ITER neutral beam negative-ion source prototype

We present the development and first use of a set of movable electrostatic probes on the full-scale ITER heating neutral beam prototype negative-ion source SPIDER. The probes access the ion source plasma from the multi-aperture accelerator aiming at the study of the plasma formation and expansion from the RF drivers through the transverse magnetic filter. The magnetic filter separates the relatively high electron-temperature region for plasma formation and hydrogen dissociation form the negative-ion extraction region in which low electron-temperature is required to avoid negative-ion destruction, but also causes non-uniformities and drifts in the large plasma discharge which will be studied by this setup. The set of electrostatic probes encompasses eight RF-compensated Langmuir probes, one double probe, one Mach probe for the assessment of plasma drift velocities, and two gridded retarding field energy analyzers to measure the positive-ion energy distribution function. Electric aspects as well as mechanical constraints given by the large in-vacuum movable structure, and thermal requirements of these relatively heat-flux components made the design challenging. Prototyping and commissioning of the measurement system is discussed offering examples of measured characteristics with the various probes.

Variations in Ionospheric Parameters during the Formation of a Polarization Jet

The calculation results and experimental data on the formation and development of a polarization jet in subauroral latitudes are compared. It is shown that the alternation of vertical and horizontal components of the ionospheric plasma–drift velocity leads to different variations in the main parameters of the F2 layer. An increase in the critical frequency of the F2 layer is revealed in certain cases before the formation of a polarization jet. This behavior of the F2 layer may be caused by a decrease in the rate of the recombination of charged particles due to an increase in the altitude of the layer maximum, which results in particle accumulation at these altitudes. An increase in the critical frequency of the F2 layer can be an additional indicator of the development of a polarization jet over an observation station.

Diurnal and Longitudinal Variations in the Equatorial Anomaly for Winter Solstice According to Interkosmos-19 Satellite Data

The equatorial anomaly dynamics in the winter solstice is studied based on data from topside sounding of the ionosphere from the Interkosmos-19 satellite for high solar activity. This is the final study on the construction of the pattern of equatorial anomaly variations under conditions of high solar activity. Changes in the equatorial anomaly structure with local time and longitude are considered in detail. It is shown that the anomaly begins to form at ~0800 LT during the creation of the winter northern crest, but the well-developed equatorial anomaly is formed only by 1000–1100 LT. The daytime maximum of the equatorial anomaly development is reached at 1200 LT. The foF2 value above the equator and the degree of the equatorial anomaly development (equatorial anomaly intensity, EAI) at 1200 LT vary with longitude according to changes in the vertical plasma drift velocity W. Three harmonics are observed in the longitudinal variations in W, foF2, and EAI at this time. The EAI increases up to the diurnal maximum 1.5−2.0 h after the evening W burst. Longitudinal variations in foF2 at 2000 LT are also associated with the relevant variations in W, in which two harmonics are revealed. The degree of equatorial anomaly development drops after the evening peak but equatorial anomaly is still well-developed at midnight and is completely absent at 0500–0700 LT. The EAI at different moments of local time differs rather strongly for different longitudinal sectors. The average position of northern crest is ~25° of the geomagnetic inclination I, and the southern crest is at ~–30° I. During the day, the winter crest is higher than the summer crest; by night-time, it is quite the opposite. Thus, a well-expressed equatorial anomaly is observed from 1000–1100 to 0000–0200 LT during the winter solstice.