On the role of tidal winds in the descending of the high type of sporadic layer (Esh)

As the prevailing tidal winds in the E region are generated by heating mechanisms, the dynamics of Es layers impacted by solar tides is a relevant theme in the space weather studies. This paper aims to identify the tidal wind component involved in the mechanism of formation and descending of the high type of sporadic layer (Es_h). The Es_h layers observed at altitudes between around 120 and 150 km in the Brazilian low latitude stations of Jataí and São José dos Campos during the months of April, June, September and December of 2016 are used in this analysis. The height variability and altitude descent of the Es_h layers are analyzed from the h′Es parameter obtained by ionosonde data. In this study, the observational data are compared with the simulations generated by an extended version of the Ionospheric E-Region Model (MIRE). At higher altitudes in the E region, the results show that the prevailing tidal pattern and wind direction controlling the Es_h layer formation and descent are different depending on month: (a) in April and June the zonal wind component and the associated semidiurnal tidal oscillations prevail, with some differences in terms of time of occurrence and descending speeds, and (b) in September and December the diurnal tidal periodicities become dominant, and both the meridional and zonal wind components seem to control the descending of the Es_h layers. Since the role of the tidal periodicities and wind directions changed depending on the month, the results suggest a possible seasonal tidal wind pattern, which is not well understood from the present study but requires further investigation. Other relevant aspects of the observations and the modeling are highlighted and discussed.     

Analysis of global TEC prediction performance of IRI-PLAS model

The International Reference Ionosphere (IRI) empirical model provides valuable data for many fields including space and navigation applications. Since the IRI model gives the ionospheric parameters in the altitude range from 50?km to 2000?km, researchers focused on the IRI-PLAS model which is the plasmasphere extension of the IRI model. In this study, Total Electron Content (TEC) prediction performance of the IRI-PLAS model was examined at a global scale using the location of globally distributed 9 IGS stations. Besides the long term (01.01.2015–31.12.2015) behavior of the model, TEC predictions during the equinox and solstice days of 2014–2017 were also tested. IRI-PLAS-TEC values were examined in comparison with GPS-TEC data. Hourly interval of yearly profile exhibits that when the geomagnetic and solar active days are ignored, differences between IRI-PLAS-TEC and GPS-TEC are rather small (～2–3 TECU) at stations in the northern hemisphere, generally ～4–5 TECU level at the southern hemisphere stations and reaching above 10 TECU for few hours. While the IRI-PLAS-TEC generally overestimates the GPS-TEC at southern hemisphere stations during quiet days, the model-derived TEC underestimates GPS-TEC during solar active days. IRI-PLAS-TEC and GPS-TEC values exhibit similar trend for the equinoxes 21 March and 23 September which refer equivalent conditions.     

电离层水平电场与风场的耦合模拟研究

设计了一个将电离层水平电场与风场耦合的模拟方案,研究了电流函数和风场在耦合前后的变化与差异. 研究发现,水平电场与风场相互反馈后,风场的变化比电流函数小. 经向风在白天有较明显的差异,夜晚的差异比白天小,主要出现在中高纬地区,并随高度的增加而增大,300km左右达到最大值,其后几乎保持不变. 纬向风有与经向风相似的变化,但纬向风耦合前后的差异比经向风小. 电流函数在耦合后有较大改变,两个涡旋强度都有较强增加,并且北半球的增强大于南半球,而夜晚差异较小. 结果表明,在研究的高度范围内,风场对电场的控制作用大于电场对风场的影响.      

Height and time variability of planetary wave activity

The height–season and year-to-year regularities of parameters of first and second spatial harmonics determine the structure of the stratosphere and mesosphere circulation and its variability. In the period 1992–2002 at heights 0–55 km, the amplitudes and phases of the first and second spatial harmonics in the field of temperature, geopotential height, zonal and meridional wind were calculated by the method of harmonic decomposition. Dispersion (standard or mean square deviation) of their day-to-day and year-to-year variations was calculated by their wavelength constants. Height and season patterns of variability have been estimated. The difference in height–longitude variability for wave numbers m = 1 and 2 has been discovered. At the same time, the intensity of wave disturbances for m = 1 is less than for m = 2 excluding the polar areas, where a significant variability appears at the heights 0–55 km. There is also a tendency for the intensity of year-to-year variations to decrease in comparison with day-to-day variations. In cold and warm periods the amplitude of perturbation waves with m = 2 both for day-to-day and year-to-year variations is greater than for waves with m = 1. Transient height areas in the interval of 20–30 km are more distinct for day-to-day variations of polar area.     

The glow of the night ionosphere due to energetic ion beams

This paper discusses photometric measurements made of the ionospheric excitation of the line $\text{λ = 5577}\text{A}\text{?}$ at the time of electron beam injection from a rocket into the Earth's ionosphere. The gradual increase of the glow intensity per impulse occurs due to accumulation of the energy of excited states of N₂(A³Σ⁺_u) and O(′S) during their lifetimes. The large disturbed zone in the near-rocket environment (size >500 m) is connected via the interaction of ions accelerated in the rocket potential field with ionospheric components. The glow intensity modulation is observed at a height of ～98 km during the electron beam injection simultaneously with the ignition of the beam-plasma discharge (BPD). The intensity minima are explained by a decrease of the energy of accelerated ions due to effective neutralization of the rocket body by the BPD plasma. The height profile of the glow intensity revealed two maxima at heights of ～103 km and ～115 km. The second maximum (at ～115 km) indicates that, at these heights, both collision and collision-free mechanisms of accelerated ion energy transport to ionospheric components exist.     

Ionospheric responses to the 21 August 2017 great American solar eclipse – A multi-instrument study

Herein, we report on the ionospheric responses to a total solar eclipse that occurred on 21 August 2017 over the US region. Ground-based GPS total electron content (TEC) data along with ground-based measurements (Millstone Hill Observatory (MHO) and digital ionosondes) and space-based measurements (COSMIC radio occultation (RO) technique) allowed us to identify eclipse-associated ionospheric responses. TEC data at ~20°, ~30°, and ~40°N latitudes from the west to east longitudes show not only considerable depression but also wave-like characteristics in TEC both in the path of totality and away from it, exclusively on the day of eclipse. Interestingly, the observed depressions are associated with lesser (higher) magnitudes at stations over which the solar obscuration percentage was meager (significant), a clear indication of bow-wave-like features. The MHO observes a 30% reduction in F₂-layer electron densities between 180 and 220 km on eclipse day. Ionosonde-scaled parameters over Boulder (40.4°N, 100°E) and Austin (30.4°N, 94.4°E) show a significant decrease in critical frequencies while an altitude elevation is seen in the virtual heights of the F-layer only during the eclipse day and that decreases are associated with wave-like signatures, which could be attributed to eclipse-generated waves. The estimated vertical electron density profile from the COSMIC RO-based technique shows a maximum depletion of 40%. Relatively intense and moderate depths of TEC depression, considerable reductions in the F₂-layer electron densities measured by the MHO and COSMIC RO-measured densities at the F₂-layer peak, and elevations in virtual heights and reduction in the critical frequencies measured by ionosondes during the eclipse day could be due to the eclipse-induced dynamical effects such as gravity waves (GWs) and their associated electro-dynamical effects (modification of ionospheric electric fields due to GWs).     

Mean solar quiet daily variations in the earth’s magnetic field along East African longitudes

Solar quiet daily (Sq) variation in the earth’s magnetic field along the East African meridian was studied using data of the H, D and Z components recorded with Magnetic Data Acquisition System of SERC. One year data recorded at ten African geomagnetic observatories was used in the analysis of worldwide solar quiet daily variation (Wsq). The study revealed that the focus of Sq (H) in the southern hemisphere lies at the boundary of low and middle latitude region. Noon-time enhancement of Sq (H) was generally noticed at all stations along the meridian, though it is latitudinal dependent in terms of magnitude as it reduces with distance from dip equator. In addition, night-time variations also occur in small magnitude along African meridian in Sq (H) and Sq (Z) which could be attributed to non-ionospheric sources. Semi-diurnal variation was noticed in Sq (D) at all stations except in AAB that is under the influence of electrojet current. Dusk sector calm condition of Sq (D) current was notice in some stations and the same condition was also noticed at dawn sector in some other stations. The usual sunrise maximum and sunset minimum for D component at stations north of dip equator as well as sunrise minimum and sunset maximum was found to increase with distance away from dip equator. Day-time perturbation of Sq current was noticed to be more pronounced in all the three field elements. Mass plots of annual mean hourly value show contrasting phase pattern about the focus in H element and the results of the variations at each region with the associated standard error. It was concluded from the result of correlation coefficients computed that different currents system flowing in opposite directions could be responsible for contrasting patterns.     

Asymmetrical structure in the thermosphere during magnetic stroms as deduced from the CACTUS accelerometer data

During the last solar activity minimum, a great deal of very precise total density data was obtained in the equatorial regions from the CACTUS accelerometer experiment. Due to the eccentricity of the orbit, it is also possible to determine a density scale height by considering that the density profiles between the perigee (270 km) and 400 km are quasi-vertical. Densities and density scale heights are analysed during magnetic storms and their variations are compared with their behaviour during quiet periods. For densities as well as for scale heights, an asymmetrical structure in latitude and longitude is exhibited with respect to the magnetic equator. Their values are relatively higher in the northern hemisphere than in the southern one. The hypothesis (previously suggested) of a greater energy input in the southern hemisphere inducing asymmetrical winds, explains the results well.     

The solar wind control of the equatorial ionosphere dynamics during geomagnetic storms

The interplanetary magnetic field, geomagnetic variations, virtual ionosphere height h′F, and the critical frequency foF2 data during the geomagnetic storms are studied to demonstrate relationships between these phenomena. We study 5-min ionospheric variations using the first Western Pacific Ionosphere Campaign (1998–1999) observations, 5-min interplanetary magnetic field (IMF) and 5-min auroral electrojets data during a moderate geomagnetic storm. These data allowed us to demonstrate that the auroral and the equatorial ionospheric phenomena are developed practically simultaneously. Hourly average of the ionospheric foF2 and h′F variations at near equatorial stations during a similar storm show the same behavior. We suppose this is due to interaction between electric fields of the auroral and the equatorial ionosphere during geomagnetic storms. It is shown that the low-latitude ionosphere dynamics during these moderate storms was defined by the southward direction of the B_z-component of the interplanetary magnetic field. A southward IMF produces the Region I and Region II field-aligned currents (FAC) and polar electrojet current systems. We assume that the short-term ionospheric variations during geomagnetic storms can be explained mainly by the electric field of the FAC. The electric fields of the field-aligned currents can penetrate throughout the mid-latitude ionosphere to the equator and may serve as a coupling agent between the auroral and the equatorial ionosphere.     

由电离层电位所建立的大气电状态

本文在已知电离层电位分布下, 解析地计算了大气电位, 电场和电流强度的全球分布.结果表明, 在大气导电率随高度呈指数增加的情况下, 100km高度上的电离层电位, 几乎无衰减地扫到25km以下.大气电场较强的区域主要在20km以下的低层大气区, 其垂直分量比水平分量大4个数量级.而中高层大气电场较弱, 且两分量量级相当.本文还提出了一种考虑地面形状对大气电场影响的解析方法.      

Response of low latitude D-region ionosphere to the total solar eclipse of 22 July 2009 deduced from ELF/VLF analysis

Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (Geog. lat., 25.27° N; Geog. long., 82.98° E; Geomag. lat. = 14° 55’ N) was investigated using ELF/VLF radio signal. Tweeks, a naturally occurring VLF signal and radio signals from various VLF navigational transmitters are first time used simultaneously to study the effect of total solar eclipse (TSE). Tweeks occurrence is a nighttime phenomena but the obscuration of solar disc during TSE in early morning leads to tweek occurrence. The changes in D-region ionospheric VLF reflection heights (h) and electron density (n_e: 22.6–24.6 cm⁻³) during eclipse have been estimated from tweek analysis. The reflection height increased from ∼89 km from the first occurrence of tweek to about ∼93 km at the totality and then decreased to ∼88 km at the end of the eclipse, suggesting significant increase in tweek reflection height of about 5.5 km during the eclipse. The reflection heights at the time of totality during TSE are found to be less by 2–3 km as compared to the usual nighttime tweek reflection heights. This is due to partial nighttime condition created by TSE. A significant increase of 3 dB in the strength of the amplitude of VLF signal of 22.2 kHz transmitted from JJI-Japan is observed around the time of the total solar eclipse (TSE) as compared to a normal day. The modeled electron density height profile of the lower ionosphere depicts linear variation in the electron density with respect to solar radiation as observed by tweek analysis also. These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days.     

Comparison of topside ionosphere scale height modeled by the Topside Sounder Model and incoherent scatter radar ionospheric model

The topside ionosphere scale height extracted from two empirical models are compared in the paper. The Topside Sounder Model (TSM) provides directly the scale height (H_T), while the incoherent scatter radar ionospheric model (ISRIM) provides electron density profiles and its scale height (H_R) is determined by the lowest gradient in the topside part of the profile. H_T and H_R are presented for 7 ISR locations along with their dependences on season, local time, solar flux F10.7, and geomagnetic index ap. Comparison reveals that H_T values are systematically lower than respective H_R values as the average offset for all 7 stations is 55 km. For the midlatitude stations Arecibo, Shigaraki, and Millstone Hill this difference is reduced to 43 km. The range of variations of H_R is much larger than that of H_T, as the H_T range overlaps the lower part of the H_R range. Dependences on ap, DoY and LT are much stronger in the ISRIM than in TSM. This results in much larger values of H_R at higher ap. Diurnal amplitude of H_R is much larger than that of H_T, with large maximum of H_R at night. The present comparison yields the conclusion that the ISR measurements provide steeper topside Ne profiles than that provided by the topside sounders.     

电离层电场半年变化的模拟研究

利用一个中低纬电离层电场理论模式，模拟太阳活动低年、地磁活动平静情况下，中低纬地区电离层电场全年的变化情况．结果显示，单独计算南、北半球(去耦合)得到电离层电场具有明显的周年变化特征，且两个半球电场的相位相差半年左右．而同时计算南、北半球(计及耦合)时，电场则是以半年变化为主，且这种半年变化的幅度和相位随地方时和地磁纬度有变化．提出一个南、北半球耦合电路的简单物理模型给予解释．电路模型初步计算发现，即使两个半球电离层电场分别具有周年变化，只要它们变化的幅度相当，相位相差半年左右，由于跨越南北半球磁力线的耦合效果，耦合的电离层电场会产生明显的半年变化分量．由于缺少连续的电离层电场观测资料，将模拟结果与Richmond基于非相干散射雷达数据建立的经验模式(ISR Model)相比较，结果符合较好．     

Large-scale traveling ionospheric disturbances of auroral origin according to the data of the GPS network and ionosondes

The intensity of large-scale traveling ionospheric disturbances (LS TIDs), registered using measurements of total electron content (TEC) during the magnetic storms on October 29–31, 2003, and on November 7–11, 2004, had been compared with that of local electron density disturbances. The data of TEC measurements at ground-based GPS receivers located near the ionospheric stations and the corresponding values of the critical frequency of the ionospheric F region f_oF2 were used for this purpose. The variations of TEC and f_oF2 were similar for all events mentioned above. The previous assumption that the ionospheric region with vertical extension from 150 to 200 km located near the F-layer maximum mainly contributes to the TEC variations was confirmed for the cases when the electron density disturbance at the F region maximum was not more than 50%. However, this region probably becomes vertically more extended when the electron density disturbance in the ionospheric F region is about 85%.     

Hemisphere-coupled modeling of nighttime medium-scale traveling ionospheric disturbances

Nighttime medium-scale traveling ionospheric disturbances (MSTIDs), which have tilted frontal structures in the midlatitude ionosphere, are investigated by the midlatitude ionosphere electrodynamics coupling (MIECO) model in this study. It has been proposed that the electrodynamic coupling between the E and F regions plays an important role in generating MSTIDs within a few hours. An intriguing aspect of MSTIDs is that they were simultaneously observed at magnetic conjugate locations in the Northern and Southern Hemispheres. In order to study the hemisphere-coupled electrodynamics, the MIECO model has been upgraded to consist of two simulation domains for both hemispheres in which the electrostatic potential is solved by considering electrodynamics in both hemispheres. The simultaneous occurrence of MSTIDs at the magnetic conjugate stations has clearly been reproduced when the F-region neutral wind satisfies the unstable condition in both hemispheres and a sporadic-E layer is given only at the Northern (summer) Hemisphere. Even if the unstable condition is satisfied in the summer hemisphere, an unfavorable F-region neutral wind in the winter hemisphere largely suppresses the growth of MSTIDs in both hemispheres.     

Reconstruction of topside density profile by using the topside sounder model profiler and digisonde data

To improve the accuracy of the real time topside electron density profiles given by the Digisonde software a new model-assisted technique is used. This technique uses the Topside Sounder Model (TSM), which provides the plasma scale height (H_s), O⁺–H⁺ transition height (H_T), and their ratio Rt = H_s/H_T, derived from topside sounder data of Alouette and ISIS satellites. The Topside Sounder Model Profiler (TSMP) incorporates TSM and uses the model quantities as anchor points in construction of topside density (Ne) profiles. For any particular location, TSMP calculates topside Ne profiles by specifying the values of foF2 and hmF2. In the present version, TSMP takes the F2 peak characteristics – foF2, hmF2, and the scale height at hmF2 – from the Digisonde measurements. The paper shows results for the Digisonde stations Athens and Juliusruh. It is found that the topside scale height used in Digisonde reconstruction is less than that extracted from topside sounder profiles. Rough comparison of their bulk distributions showed that they differ by an average factor of 1.25 for locations of Athens and Juliusruh. When the Digisonde scale heights are adjusted by this factor, the reconstructed topside profiles are close to those provided by TSM. Compared with CHAMP reconstruction profiles in two cases, TSMP/Digisonde profiles show lower density between 400 and 2000 km.     

Acceleration of charged particles by fluctuating and steady electric fields in a X-type magnetic field

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies (ω in dimensioless units) in the range 0.5 ? ω ? 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ? ω ? 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.     

Equatorial M(3000)F2 estimation of F2-layer models peak heights during low solar activity

M(3000)F2 estimation of h_mF2 based on four different formulated models viz: (1) Shimazaki (1955) (2) Bradley and Dudeney (1973), (3) Dudeney (1974) and (4) Bilitza et al. (1979) at an equatorial station in West Africa during low solar activity period (1995) are used to validate its conformity with observed and International Reference Ionosphere (IRI) model. Local time analyses of data from fifteen (15) selected days during the January and July solstices and April and October equinoxes are used. The results obtained show that the M(3000)F2 estimation of h_mF2 from the ionosonde-measured values using the Ionospheric Prediction Service (IPS-42) sounder compared to the observed values which were deduced using an algorithm from scaled virtual heights of quiet day ionograms are highly correlated with Bilitza model. International Reference Ionosphere (IRI 2007) model for the equatorial region also agrees with the formulation developed by Bilitza et al. (1979) for the four different seasons of the year. h_mF2 is highest (425 km) in summer (June solstice) season and lowest (386 km) in autumn (September equinox) season with daytimes peaks occurring at 1100–1200 LT during the solstices and at 1000 LT during the equinoxes respectively. Also, the post-sunset peaks are highest (362 km) at the spring (March equinox) and lowest (308 km) at the summer (June solstice) both occurring between 1800 and 2000 LT.     

A statistical study of the mid-latitude NmF2 winter anomaly

Hourly values of the F2-layer peak density, NmF2, measured by 62 ionosonde stations from 1957 to 2008 at middle geomagnetic latitudes of the northern and southern hemispheres are used in a statistical study of the F2-region winter anomaly. We analyze a maximum value, R, of the winter/summer geomagnetically quiet daytime NmF2 ratio over each ionosonde for approximately the same winter and summer solar activity conditions. The conditional probability of an occurrence of R in an interval of R, the most frequent value of R, the mean expected value of R, and the conditional probability to observe the F2-region winter anomaly are calculated and studied for low, moderate, and high solar activity conditions. It is found that northern hemisphere stations show significant cross-correlations of winter anomaly statistics with geomagnetic latitude for moderate and high solar activities.