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China’s Mars exploration mission has stimulated tremendous interest in planetary science exploration recently. To propose potential scientific research projects, this study presents a concept simulation for the measurement of Martian atmospheric winds using the Doppler Michelson interferometry technique. The simulation is based on the satellite instrument initially designed for the Dynamic Atmosphere Mars Observer (DYNAMO) project to measure vertical profiles of winds from the 1.27 μm airglow observations in the Martian atmosphere. A comprehensive DYNAMO measurement simulation forward model based on an orbit submodel, an atmospheric background field submodel, and an instrument submodel is developed using the Michelson equation. The simulated interferogram signal over the field of view (FOV) calculated by the forward model is associated with the filter transmittance function, column emission rate of airglow, wind velocity, temperature, and the Michelson phase. The agreement between the derived atmospheric signals from the simulated interferogram without altitude inversion and the input parameters used to initiate the forward model confirms the validity of the forward model.

This paper presents a review of the progress in the understanding of an additional layer in the low latitude ionosphere, the F3 layer. A brief introduction to the daytime equatorial electrodynamics required for understanding the formation of F3 layer and formation of the usual ionospheric regions and layers are also included for continuity. Numerous papers on various aspects of F3 layer and topside ledge (F3 layer in the topside ionosphere) were published by different research groups using observations and modelling. The first time observations and important new aspects are highlighted and areas for further studies are suggested in this review. The important new aspects include an automated procedure for identifying F3 layer, global map of F3 layer and its longitudinal wave structure, solar activity variation of F3 layer and its long term trend, F3 layer in TEC, F3 layer at sunrise and sunset hours, simultaneous observations of F3 layer and upward ExB drift velocity, simultaneous observations of F3 layer in conjugate summer and winter locations, rapid ascend of F3 layer during geomagnetic storms as an indicator of eastward prompt penetration electric field and mechanism of F4 layer. Main points are summarized at the end.

Following a brief history and progress of ionospheric research, this paper presents a brief review of the recent developments in the understanding of two major phenomena in low and mid latitude ionosphere—the equatorial ionization anomaly (EIA) and involved equatorial plasma fountain (EPF) and ionospheric irregularities. Unlike the easy-to-understand misinterpretations, the EPF involves field perpendicularE×B plasma drift and field-aligned plasma diffusion acting together and plasma flowing in the direction of the resultant at all points along the field lines at all altitudes. The EIA is formed mainly from the removal of plasma from around the equator by the upward E×B drift creating the trough and consequently the crests with small accumulation of plasma at the crests when the crests are within ~±20° magnetic latitudes and no accumulation when they are beyond ~±25° magnetic latitudes. The strong EIA under magnetically active conditions arises from the simultaneous impulsive action of eastward prompt penetration electric field and equatorward neutral wind. Intense ionospheric irregularities develop in the post-sunset bottom-side equatorial ionosphere when it rises to high altitudes, and evolve nonlinearly into the topside. Pre-reversal enhancement (PRE) of the vertical upward E×B drift and its fluctuations amplified during PRE provide the driving force and seed, with neutral wind and gravity waves being the primary sources. At low solar activity especially in summer when fast varying PRE is absent, the slow varying gravity waves including large scale waves (LSW) seem to act as both driver and seed for weak irregularities. At mid latitudes, the irregularities are weak and associated with medium scale traveling ionospheric disturbances (MSTIDs). A low latitude minimum in the occurrence of the irregularities at March equinox predicted by theoretical models is identified. The minimum occurs on the poleward side of the EIA crest and shifts equatorward from ~25° magnetic latitudes at high solar activity to below 17° at low solar activity.

The 2015 St. Patrick's day geomagnetic storm with SYM-H value of -233 nT is an extreme space weather event in the current 24th solar cycle. In this work, we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and low latitudes in the Asian-Australian sector and the American sector during this super storm event. The results reveal that the disturbed electric fields, which comprise penetration electric fields (PEFs) and disturbance dynamo electric fields (DDEFs), play a decisive role in the ionospheric storm effects in low latitude and equatorial regions. PEFs occur on March 17 in both the American sector and the Asian-Australian sector. The effects of DDEFs are also remarkable in the two longitudinal sectors. Both the DDEFs and PEFs show the notable local time dependence, which causes the sector differences in the characteristics of the disturbed electric fields. This differences would further lead to the sector differences in the low-latitude ionospheric response during this storm. The negative storm effects caused by the long-duration DDEFs are intense over the Asian-Australian sector, while the repeated elevations of hmF2 and the EIA intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3-layer features are caught on March 17 in the American equatorial region, proving the effects of the multiple strong eastward PEFs.

Although the equatorial ionization anomaly (EIA) has been widely studied, it was seldom investigated from observations for the topside ionosphere. In this paper, the climatology characteristics of the latitudinal structure of topside ion density (Ni) were investigated in detail using ROCSAT-1 and DMSP Ni observations. EIA double-peak structure may exist at 600 km, depending on longitude, local time, season, and solar activity; while it is not a prevalent characteristic at 840 km even at solar maximum sunset. Local time evolution of the EIA at 600 km was presented. The double-peak structure begins to appear at noontime, being later than the appearance of the EIA in F2-peak region. The pronounced EIA induced by the strong prereversal enhancement at solar maximum begins to appear at 19:00 LT and can last to pre-midnight; EIA crest-to-trough ratio (CTR) is largest (lowest) at March equinox (June solstice) and reaches a maximum at 20:00 LT in all seasons. EIA structure shows evident longitudinal difference. Pronounced EIA exists around about 100°E at 13:00 LT at the two equinoxes and June solstice, while it exists at more extensive longitudes (about 90°E to 240°E) at December solstice. The trans-equator plasma transport induced by neutral winds can weaken the double-peak structure in the topside ionosphere. The longitudinal difference in the EIA structure at 600 km is related to the longitudinal variations of equatorial upward plasma drift and geomagnetic declination.

The study of ionospheric anomaly variation before large earthquakes has attracted the attention of geophysicists for many years. This is primarily because ionospheric anomaly variation has been considered to constitute one possible method to identify earthquake precursors. Since China covers several major earthquake zones, it is critical for Chinese scientists to study possible ionospheric earthquake precursors. In this report, we provide a brief summary of recent progress in the study of ionospheric earthquake precursors mainly from Chinese mainland researchers, as well as some other scientists. This report focuses on: (1) some case studies of ionospheric abnormal behaviors before some great earthquakes around the world (e.g., M7.9 Wenchuan Earthquake, M9.0 Tohoku-Oki Earthquake, M7.0 Haiti Earthquake); and (2) some statistical characteristics of ionospheric anomalies before earthquakes, including their temporal and spatial distributions, and the relationship with the forthcoming earthquake. The above-mentioned studies may provide some new and beneficial insights for the future study of ionospheric earthquake precursors.

In this paper ionosonde observations in the East Asia–Australia sector were collected to investigate dusk-to-nighttime enhancement of mid-latitude summer NmF2 (maximum electron density of the F2 layer) within the framework of NmF2 diurnal variation. NmF2 were normalized to two solar activity levels to investigate the dependence of the dusk-to-nighttime enhancement on solar activity. The dusk-to-nighttime enhancement of NmF2 is more evident at Northern Hemisphere stations than at Southern Hemisphere stations, with a remarkable latitudinal dependence. The dusk-to-nighttime enhancement shows both increasing and declining trends with solar activity increasing, which is somewhat different from previous conclusions. The difference in the dusk-to-nighttime enhancement between Southern Hemisphere and Northern Hemisphere stations is possibly related to the offset of the geomagnetic axis from the geographic axis. hmF2 (peak height of the F2 layer) diurnal variations show that daytime hmF2 begins to increase much earlier at low solar activity level than at high solar activity level at northern Akita and Wakkanai stations where the dusk-to-nighttime enhancement is more prominent at low solar activity level than at high solar activity level. That implies neutral wind phase is possibly also important for nighttime enhancement.

The long-duration positive storm (LPS) in the equatorial regions is relatively poorly understood. In this report, we conducted a statistical analysis of the LPS effects in the equatorial ionosphere over Jicamarca (12.0°S, 283.2°E) in 1998–2010. There are 250 geomagnetic storms (minimum Dst

In this paper, we use the C/NOFS and ROCSAT-1 satellites observations to analyze the storm time evolution of the disturbance plasma drifts in a 24-hour local time scale during three magnetic storms driven by long-lasting southward IMF Bz. The disturbance plasma drifts during the three storms present some common features in the periods dominated by the disturbance dynamo. The newly formed disturbance plasma drifts are upward and westward at night, and downward and eastward during daytime. Further, the disturbance plasma drifts are gradually evolved to present significant local time shifts. The westward disturbance plasma drifts gradually migrate from nightside to dayside. Meanwhile, the dayside downward disturbance plasma drifts become enhanced and shift to later local time. The local time shifts in disturbance plasma drifts are suggested to be mainly attributed to the evolution of the disturbance winds. The strong disturbance winds arisen around midnight can constantly corotate to later local time. At dayside the westward and equatorward disturbance winds can drive the F region dynamo to produce the poleward and westward polarization electric fields (or the westward and downward disturbance drifts). The present results indicate that the disturbance winds corotated to later local time can affect the local time features of the disturbance dynamo electric field.

Chapman scale height is a valuable key parameter measuring the shape of the profile of plasma density in the F2 layer ionosphere. Currently, the data of Chapman scale height are routinely derived from ionogram observations at many ionosonde stations in terms of the SAO explorer software. In this report, we collected the in situ observations of plasma density at altitudes around 600 km from the ROCSAT-1 satellite and of simultaneous F peak parameters from an ionosonde operated at Wuhan (30.6°N, 114.4°E), a low-latitude station in central China, to estimate the topside plasma density profiles by using the Chapman α function and further retrieve Chapman scale height. Evident solar cycle, seasonal variation, and local time variation are presented in the retrieved Chapman scale height over Wuhan. The climatological features of the derived Chapman scale height are significantly different from those from the ground-based ionograms. Such significant discrepancy suggests that further improvements are required in the present extrapolating topside electron density profiles from ionosonde observations. Furthermore, the attempt to constructing plasma density profiles through combining ionosonde and satellite in situ observations provides a new way to reanalyze observations from different sources and normalize plasma density recorded at varying altitudes to specified altitudes, which is critical and more convenient for ionospheric climatology studies.

The GPS total electron content (TEC) data were used to investigate the ionospheric abnormal behaviors prior to the 2011 Tohoku-Oki earthquake. The observations show a significant enhancement in TEC on March 8, 2011, and during the same period the solar activity also has a large increase. Both an empirical model and a theoretical model were used to check whether the TEC anomalies were entirely contributed by the increase in solar radiation. The comparison between the observations and the simulation results shows that only the solar radiation enhancement was not enough to produce the observed significant TEC enhancement. It means that some additional mechanisms, such as the per-earthquake ionospheric disturbance or the geomagnetic activities, may play a significant role in the significant TEC enhancement on March 8. The temporal–spatial distribution of the extreme TEC enhancement within 30 days before the earthquake was particularly pursued. It is found that the extreme enhancement was persistently located in the region adjacent to the epicenter and the magnetic conjugate point for a long time of 16 h. In addition, a geomagnetic disturbance with Kp = 4 occurred on March 7. Therefore, the significant TEC enhancement on March 8 might be related to the M9.0 Tohoku-Oki earthquake and the geomagnetic disturbance.

On the basis of previous work, we develop a middle and low latitude theoretical ionospheric model in this paper, named Theoretical Ionospheric Model of the Earth in the Institute of Geology and Geophysics, Chinese Academy of Sciences (TIME-IGGCAS). TIME-IGGCAS solves the equations of mass continuity, motion and energy of electron and ions self-consistently and uses an eccentric dipole field approximation to the Earth’s magnetic field. We combine the Eulerian and Lagrangian approaches in the model and take account of the plasma E×B drift velocity. Calculation results reveal that the model is steady and credible and can reproduce most large-scale features of ionosphere. By using TIME-IGGCAS, we carried out an observation system data assimilation experiment. Assimilation results show that the E×B drift velocity can be accurately estimated by ingesting the observed foF2 and hmF2 into the model applying nonlinear least-square fit method. We suggest that this work is of great significance in the development of ionospheric data assimilation model to give better nowcast and forecast of ionosphere.

It is an interesting topic how the ionosphere varies when solar extreme ultraviolet (EUV) irradiance decreases far below normal levels. When extrapolating the total electron content (TEC)-EUV relation, significantly negative TECs at the zero solar EUV point are obtained, which indicates that TEC-EUV variation under extremely low solar EUV (ELSE) conditions does not follow the TEC-EUV trend during normal solar cycles. We suggest that there are four types of nonlinear TEC-EUV variations over the whole EUV range from zero to the solar maximum level. The features of the ionosphere under ELSE conditions were investigated using the TEC extrapolated with cubic TEC-EUV fitting. With the constraint of zero TEC at zero EUV, the cubic fitting takes not only observations but also the trend of the ionosphere (only an extremely weak ionosphere can exist when EUV vanishes) into account. The climatology features of TEC under ELSE conditions may differ from those during normal solar cycles at nighttime. Ionospheric dynamic processes are supposed to still significantly affect the ionosphere under ELSE conditions and induce this difference. With solar EUV decreasing, global electron content (GEC) should vary largely in accordance with the GEC-EUV trend during normal solar cycles, and the seasonal fluctuation of GEC declines, owing to the contraction of the ionosphere.

[1] In the present work, using the three-dimensional theoretical ionospheric model of the Earth in Institute of Geology and Geophysics, Chinese Academy of Sciences (TIME3D-IGGCAS), we simulate the temporal-spatial distribution of the midlatitude summer nighttime anomaly (MSNA) and the influences of thermospheric meridional and zonal winds on the formation of MSNA. MSNA mainly appears in three distinct regions, the East Asian region, the northern Atlantic-Europe region and the South Pacific region. MSNA in the South Pacific region is obviously stronger than that in the other two regions. In these three regions, MSNA not only occurs in local summer, but also often occurs in Equinox. Through lifting up or lowering the F region ionization, thermospheric winds mainly drive the formation of MSNA. In the East Asian region and the northern Atlantic-Europe region, MSNA are mainly driven by the thermospheric meridional wind, and the influence of the thermospheric zonal wind can be neglected. However, although MSNA in the South Pacific region (or Weddell Sea Anomaly) are mainly driven by the thermospheric meridional wind, the thermospheric zonal wind also plays an important role in the formation of MSNA.

The F-3 layer is a common feature within +/- 10 degrees of the magnetic equatorial ionosphere in the daytime. According to Balan et al. (1998) the F-3 layer occurs mainly during the morning-noon period due to the combined effect of the upward E x B drift and the neutral wind that provides upward plasma drifts at and above the F-2 layer. The F-3 layer occurrence rate is higher in summer and decreases with increasing solar activity. In this study, the characteristic of the sunset F-3 layer is first investigated using a solar cycle of ionosonde data (1995-2010) from the magnetic equatorial station at Jicamarca, and compared with the features derived from the four subtropical stations at Sao Luis, Fortaleza, Kwajalein, and Vanimo. Evidence shows that the local time distribution of the occurrence of the F-3 layer can extend to the postsunset time (1800-2100 local time). The sunset F-3 layer has a strong seasonal dependence occurring mainly during the summertime. Unlike the daytime F-3 layer, the occurrence of the sunset F-3 layer clearly increases and the virtual height of the bottom side of the F-3 layer statistically increases from 620 to 1000 km with increasing solar activity. In addition, the occurrence of the sunset F-3 layer at the other stations is much less than that at Jicamarca. These features of the dependence on the season, solar activity, and latitude are clearly related to the geomagnetic control of the evening prereversal enhancement of the equatorial zonal electric field and geomagnetic configuration.

[1] The longest total solar eclipse of this century occurred in East and South Asia on 22 July 2009. The eclipse was accompanied with a medium magnetic storm, whose main phase onset occurred ∼27 min after the passage of the Moon's umbral shadow. Using TEC data from 60 GPS stations, we construct differential TEC maps to investigate the ionosphere response to the solar eclipse in central China in the range of 26°N–36°N, 108°E–118°E (i.e., the magnetic latitude 15°N–25°N). During the eclipse's totality, a “shadow” in the ionosphere shown as TEC depletion area was formed ∼100 km south of the Moon's umbral path with a maximum decrease of 5 TECU. The TEC depletion area moved eastward, following the movement of the totality area with a time lag of ∼10 min. Enhancements of TEC due to the storm are observed after the main phase onset. The relative drop of TEC due to the solar eclipse is evidently larger at lower latitudes than that at higher ones and around noontime than that in the morning. By modeling work, we find that the latitudinal dependence of the TEC response may result from latitudinal variation of magnetic inclination, which influences the diffusion of ionization among different layers. Besides, the local time dependence of TEC response is closely related to the local time variation of background atmosphere density, which affects the electron loss efficiency in the ionosphere.

[1] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.

National Natural Science Foundation of China[40725014, 40674090, 40636032]; National Important Basic Research Project[2006CB806306]

[1] We statistically analyze the ionospheric scale heights in the lower topside ionosphere based on the electron density (Ne) and temperature profiles observed from the incoherent scatter radar (ISR) at Arecibo (293.2E, 18.3N), Puerto Rico. In this study, a database containing the Arecibo ISR observations from 1966 to 2002 has been used in order to investigate the diurnal and seasonal variations and solar activity dependences of the vertical scale height (VSH), which is deduced from the electron concentration profiles

The ionospheric mid-latitude trough minimum position as a function of geomagnetic storm time is identified and investigated statistically in terms of the category and the intensity of storms. The data of ion density derived from DMSP and DEMETER satellites were used to extract the trough position. The variations of mid-latitude trough in 41 moderate magnetic storms and 88 intense magnetic storms in the 23rd solar cycle were studied. The results show that the trough moves toward the equator as Dst index decreases and toward the pole as Dst index increases. Compared with the ICME, MC and CIR storms, in sheath storms the trough shifts to lower latitude at the end of the main phase, although the average storm intensity is weak. During the storm recovery phase, the rapid recovery of the trough position can be seen at the start of the recovery phase for moderate CIR storms. We also calculated the correlation between the minimum latitude of the trough position and the storm magnitude as well as other related main phase parameters during all storms. We found that the minimum latitude of the trough position exhibits a strong correlation with the storm magnitude during magnetic storms. However, the correlation coefficients between the trough position and other related main phase parameters are very low.

In this paper, the responses of the ionosphere to the solar cycle and solar rotation variations of extreme ultraviolet (EUV) irradiance are comparatively investigated using daily mean global electron content (GEC) and 0.1–50 nm EUV daily flux. GEC is well correlated with EUV on both the solar cycle and solar rotation timescales; however, the responses of GEC to the solar cycle and solar rotation variations of EUV are significantly different in terms of the following two aspects: (1) There is a significant time lag between the solar rotation variations of GEC and EUV; the lag is dominated by a 1-day lag and generally presents a decrease trend with solar activity decreasing. For the solar cycle variations of GEC and EUV, however, there are no evident time lags. (2) The GEC versus EUV slopes are different for the solar cycle and solar rotation variations of GEC and EUV; the solar cycle GEC versus EUV slope is higher than the solar rotation GEC versus EUV slope, and this difference occurs in different seasons and latitudinal bands. The results present an aspect of the difference between ionospheric climatology and weather.

This study statistically investigated the seasonal variation, magnetic local time (MLT) variation, geomagnetic activity dependence, and solar activity dependence of the mid-latitude trough using GPS total electron content (TEC) data from 2000 to 2014. The daily median Kp index was used to characterize the daily geomagnetic activity level. The results showed that the trough minimum position depended primarily on the geomagnetic activity, MLT, and the season. The trough depth depended primarily on the solar flux index (F107) and, to a lesser degree, on MLT. The trough depth increased as F107 increased and as the incidence angle of solar flux decreased. The trough equatorward half-width decreased as the geomagnetic activity increased. These variations in the GPS-TEC trough minimum position were compared with the variations in the TEC trough derived from the International Reference Ionosphere (IRI)-2007 model. The GPS-TEC trough minimum position changed little with respect to F107, whereas the IRI-TEC trough minimum position showed a strong F107 dependence.