Mid-latitude Summer Nighttime Anomaly Research Articles

Ion Density Climatology Based on FPMU Measurements on Board the International Space Station

AbstractWe performed, for the first time, a season‐dependent geomagnetically quiet‐time climatology of mid‐ and low‐latitude ion densities during low and moderate solar flux conditions using Floating Potential Measurement Unit (FPMU) observations aboard the International Space Station (ISS) from 2008 to 2019. Our daytime observations indicate that the main characteristics of the equatorial ionization anomaly (EIA) at ∼400 km are consistent with those from other in situ and remote sensing probes. The FPMU daytime densities are also generally in good agreement with the corresponding results from the International Reference Ionosphere (IRI). However, the IRI does not reproduce the mid‐solar flux evening low‐latitude measured densities. In this period, the EIA exhibits strong longitude‐dependent crest‐to‐trough ratios and asymmetries due to the pre‐reversal enhancement (PRE) of the zonal electric field and thermospheric neutral winds. Our data also show strong structuring of the daytime and nighttime plasma densities. This includes a bulge, discussed for the first time here, in the EIA southern crest in the South Atlantic sector during the December solstice and equinox data, which we suggest being generated by the transport of plasma from the Weddell Sea anomaly (WSA). We also highlight and show the evolution of a midlatitude summer nighttime anomaly (MSNA) during the June solstice data in the North Atlantic sector. Our results give new insights into these two anomalies, where we show that they are stronger with increasing solar flux levels and that they last until the early morning. These latter results are not consistent with those from previous studies.

Analysis of temporal and spatial variation characteristics of midlatitude summer nighttime anomaly in low and middle solar activity period

The spatiotemporal variation characteristics of midlatitude summer nighttime anomaly (MSNA) are essential for ionospheric modeling in the MSNA region. Based on GIM TEC, COSMIC NmF2, COSMIC IEC (Integrate Electron Content), and GPS TEC, this paper analyzes the spatiotemporal variation characteristics of MSNA during the low and middle solar activity period. First, we use the fourth-order Fourier function to fit the TEC, NmF2, and IEC. Second, we calculate an MSNA index and give its spatiotemporal distribution during the low and middle solar activity period. Third, on the basis of GIM TEC and GPS TEC, we describe the spatiotemporal distribution of TEC peak moments from October to January (Oct-Jan) and May to August (May-Aug). Our detailed analysis shows the following: (1) MSNA in the Southern Hemisphere is most significant in Oct-Jan. Moreover, MSNA in the Northern Hemisphere is most significant in May-Aug. (2) MSNA mainly occurs in three regions: South America and Antarctica, Western Europe to eastern North America, and Northeast Asia to the Pacific. (3) When solar activity increases, the MSNA region in the Northern Hemisphere becomes smaller, and the MSNA index is unchanged. Moreover, the MSNA region in the Southern Hemisphere is unchanged, but the MSNA index increases. (4) From Oct-Jan, there are some apparent differences between the east and west sides of the MSNA region of the Antarctic Peninsula. The peak moments of TEC in the west region concentrate before midnight (20:00–22:00 LT), and the peak moments of TEC in the east region concentrate after midnight (00:00–08:00 LT). As the level of solar activity increases, the peak moments in the eastern region are significantly shifted to later local time. (5) As the level of solar activity increases, the incidence of MSNA at three GPS stations (OHI300ATA, YSSK00RUS, and BRST00FRA) shows a decreasing trend.

Climatology of TEC Longitudinal Difference in Middle Latitudes of East Asia

In this paper, a statistical analysis of the diurnal, seasonal and solar cycle variation in the TEC longitudinal difference in midlatitudes of East Asia is presented using CODE GIMs data in 2015–2019. Moreover, the empirical neutral wind model HWM-14 and geomagnetic field model IGRF-2020 were employed to analyze the influence of geomagnetic configuration-neutral wind mechanism on the TEC longitudinal difference, and the F2 layer peak electron density (NmF2) data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) were also used to study the role of local electron density in the TEC longitudinal difference. For the high solar activity year, the results show that east-west TEC longitudinal difference index Re/w is negative in the noon and positive at evening-night. Moreover, the longitudinal difference of daytime TEC is most evident in summer, less in autumn and least in spring and winter, while the nighttime difference is most obvious in equinox, followed by summer and winter during nighttime. The model simulation shows that the TEC longitude difference around noon is mainly caused by the zonal wind-declination mechanism, and a 4-h time delay seems to be an optimal result for the vertical drift velocity to cause the longitudinal TEC difference during pre-noon hours. At night, the uplifting electron flux, which is a product of local electron density and vertical drift velocity, shows a good correlation with Re/w, indicating that the local electron density is also an important factor affecting the TEC longitudinal difference during the nighttime. Moreover, there was about a 3-h time delay between the TEC longitudinal variations and the uplifting electron flux at night. For the low solar activity years, the western TEC is greater than eastern TEC during most of the year except in the summer nighttime. The TEC diurnal variation in the east and west suggested that the nighttime Re/w should be related to other physical process, such as the midlatitude summer nighttime anomaly (MSNA) in the east and the ionospheric nighttime enhancement (INE). The current study provides evidence for the longitudinal difference of NmF2 in East Asian midlatitudes and geomagnetic configuration-neutral wind mechanism proposed in previous studies and finds some new features which need further studying to improve our current understanding of ionospheric longitudinal difference in the low solar activity years. The results provide new insight into TEC longitudinal variations at midlatitudes, and they can contribute to understanding the ionosphere-thermosphere coupling system.

Extension of the Electron Density Enhancement From Middle to High Latitudes Observed by Swarm‐A in Summer of the Southern Hemisphere

AbstractThe major causal mechanism for the eastward contraction of the electron density (Ne) enhancement associated with the southern Midlatitude Summer Nighttime Anomaly (MSNA) was attributed to the thermospheric neutral wind at the middle geomagnetic latitude in the fixed local time coordinate. In the study, the Ne and horizontal cross‐track ion drifts observed by the Swarm‐A satellite at 462 km altitude show that the horizontal drifts can drive the Ne enhancement of the southern MSNA in the local summer of the Southern Hemisphere. Both the eastward and westward ion drifts transport the enhancement from the middle to the high geomagnetic latitudes, which results in the enhancement evolving at all the longitudes in the whole day.

Equinoctial asymmetry during solar minima at low to mid latitude

Equinoctial asymmetry (EA) of F2-layer critical frequency (foF2) during minimum years of solar cycle 22–23 (1996) and 23–24 (2008) has been studied for low and mid latitude stations ranges from 20°N - 55° N. Year 2008 provided an opportunity to study this phenomenon in extreme low EUV conditions. Pronounced/reasonable increase in EA strength at low/mid latitudes from 1996 to 2008 indicates role of EUV in EA. In addition to an increase in strength of EA from 1996 to 2008, variation in its strength and sign (positive if Mar Equinox foF2means > Sep Equinox foF2means and vice versa) with latitude and longitude are also observed during both years. Similar to previous studies, EA is observed in daytime during both years. However at low latitude stations, two peaks of foF2 are also observed at night: the first peak in summer and the second peak in the beginning of the September equinox which shows superposition of semi-annual and annual components. Hourly analysis showed asymmetry (Sep Equinox foF2means > Mar Equinox foF2means) from sunset to mid-night at low mid to mid latitude. It appears that the Mid latitude Summer Nighttime Anomaly (MSNA) is extending to September equinox with decreasing magnitudes from Summer to Sep equinox. IRI-16 showed EA during daytime only with over- and under- estimation as compared to observed data. Serious discrepancy between observed and IRI-2016 model values especially during deep minimum suggests modification of model for extreme low EUV condition as well as incorporation of parameters related to global wind pattern in the model.

Global equivalent slab thickness model of the Earth’s ionosphere

The shape of the vertical electron density profile is a result of production, loss and transportation of plasma in the Earth’s ionosphere. Therefore, the equivalent slab thickness of the ionosphere that characterizes the width of vertical electron density profiles is an important parameter for a better understanding of ionospheric processes under regular as well as under perturbed conditions. The equivalent slab thickness is defined by the ratio of the vertical total electron content over the peak electron density and is therefore easy to compute by utilizing powerful data sources nowadays available thanks to ground and space based GNSS techniques. Here we use peak electron density data from three low earth orbiting (LEO) satellite missions, namely CHAMP, GRACE and FORMOSAT-3/COSMIC, as well as total electron content data obtained from numerous GNSS ground stations. For the first time, we present a global model of the equivalent slab thickness (Neustrelitz equivalent Slab Thickness Model – NSTM). The model approach is similar to a family of former model approaches successfully applied for total electron content (TEC), peak electron density NmF2 and corresponding height hmF2 at DLR. The model description focuses on an overall view of the behaviour of the equivalent slab thickness as a function of local time, season, geographic/geomagnetic location and solar activity on a global scale. In conclusion, the model agrees quite well with the overall observation data within a RMS range of 70 km. There is generally a good correlation with solar heat input that varies with local time, season and level of solar activity. However, under non-equilibrium conditions, plasma transport processes dominate the behaviour of the equivalent slab thickness. It is assumed that night-time plasmasphere–ionosphere coupling causes enhanced equivalent slab thickness values like the pre-sunrise enhancement. The overall fit provides consistent results with the mid-latitude bulge (MLB) of the equivalent slab thickness, described for the first time in this paper. Furthermore, the model recreates quite well ionospheric anomalies such as the Night-time Winter Anomaly (NWA) which is closely related to the Mid-latitude Summer Night-time Anomaly (MSNA) like the Weddell Sea Anomaly (WSA) and Okhotsk Sea Anomaly (OSA). Further model improvements can be achieved by using an extended model approach and considering the particular geomagnetic field structure.

Comparison of Electron Density and Temperature From the CSES Satellite With Other Space‐Borne and Ground‐Based Observations

AbstractIn this paper we provide a comprehensive comparison of in situ electron density (Ne) and temperature (Te) measured by Langmuir probe (LAP) on board the China Seismo‐Electromagnetic Satellite (CSES), with nearly simultaneous measurements from the Swarm B satellite, incoherent scatter radar (ISR) at Millstone Hill, as well as predictions from empirical models including the incoherent scatter radar ionospheric model (ISRIM) for Millstone Hill and International Reference Ionosphere model (IRI‐2016). Results reveal that the global distributions and their relative variations of Ne/Te from CSES and Swarm are quite consistent during conjunction periods of the two satellites, although the absolute values of Swarm Ne are proportionally larger than that of CSES. The large‐scale ionospheric structures, such as the equatorial ionization anomaly (EIA), the longitudinal wave number (WN3/4), the Weddell Sea anomaly (WSA), the northern midlatitude summer nighttime anomaly (MSNA), and the midlatitude ionospheric trough (MIT), are well represented by the CSES measurements. For the temporal variation over Millstone Hill station, CSES Ne at nightside shows some different characteristics from the predictions of IRI and ISRIM, possibly due to the influences of MIT and midlatitude arc (MLA) that are often observed at latitudes of Millstone Hill. Our results suggest that the CSES in situ plasma parameters are reliable with a high scientific potential for investigation of geophysics and space physics.

Nighttime Ionospheric TEC Study Over Latin America During Moderate and High Solar Activity

AbstractThe present work is a comprehensive study of the ionospheric vertical total electron content (vTEC) variations during the nighttime, based on data collected by ground‐based Global Navigation Satellite System (GNSS) receivers over the Latin American region. We provide a qualitative and quantitative analysis of the ionospheric vTEC trend at 21:00, 00:00, and 03:00 local time (LT), during geomagnetically undisturbed days of 2011 (ascending phase) and 2014 (maximum phase), which encompassed (a) the response to the solar flux variation, (b) the seasonal trend in different latitudes and longitudes, and (c) the interhemispheric asymmetry. One significant result of this study is the development of TEC maps for the Latin American region, which are used for the monitoring and forecasting of the ionosphere for space weather purposes. The nighttime vTEC variations showed a strong latitudinal dependence, especially in the Northern Hemisphere. For 2011, the semiannual anomaly was similar to that observed in daytime; however, in 2014, the receivers at midlatitude presented asymmetric behavior. Similarly, the nighttime winter anomaly (NWA) was very weak in both years. The Equatorial Ionospheric Anomaly (EIA) signature was absent from June to August, a period in which the hemispheric disparity in the vTEC values became more evident, suggesting a feeble interhemispheric circulation. The Midlatitude Summer Nighttime Anomaly (MSNA) was also identified in the Southern Hemisphere, during January and February of 2011 (moderate solar activity). Model approximations suggest that the equatorward winds and the EIA were involved in the formation of the MSNA.

A study on Nighttime Winter Anomaly (NWA) and other related Mid-latitude Summer Nighttime Anomaly (MSNA) in the light of International Reference Ionosphere (IRI) – Model

The ionospheric Nighttime Winter Anomaly (NWA) is a feature observed in the Northern Hemisphere at the American and in the Southern Hemisphere at the Asian longitude sector under low solar activity conditions. Jakowski et al. (2015) analyzed ground-based GPS derived TEC and peak electron density data from radio occultation measurements on Formosat-3/COSMIC satellites and confirmed the persistence of the phenomenon. Further, they assumed that Mid-latitude Summer Nighttime Anomaly (MSNA) and related special anomalies such as the Weddell Sea Anomaly (WSA) and the Okhotsk Sea Anomaly (OSA) are closely related to the NWA via enhanced wind-induced uplifting of the ionosphere. The aim of this paper is to study the factors causing these anomalies and also to investigate if these anomalies are re-produced by IRI. The results show that IRI model does include the NWA effect, though at a different longitude and could be improved for better predictions. The IRI-2016 model does show WSA in TEC but not in NmF2. Further, the IRI-2016 model could clearly predict the OSA both in NmF2 and TEC.

Variations of the main nighttime ionospheric density anomalies observed by DEMETER during the descending phase of solar cycle 23

This paper is related to analysis of the Weddell Sea Anomaly (WSA) and the Mid-latitude Summer Nighttime Anomaly (MSNA) observed by the low altitude satellite DEMETER during nighttime between 2004 and 2010. This time interval corresponds to the decrease of the solar cycle 23 which was unusually long. It appears that, if these two anomalies have a peak in local summer (December in the Southern hemisphere for the WSA, June in the Northern hemisphere for the MSNA), the anomalies are also observed during the months around December and June with a decreased intensity. But at the end of the solar cycle 23 the summer peaks dramatically decrease and even relatively more quickly than the solar index F10.7. This phenomenon is much more significant for the WSA. It is shown that the mechanism producing the two anomalies (thermospheric neutral winds and magnetic declination effects) is strengthened by the solar ionization which is active during the night above the WSA and the MSNA areas. But at solar minimum, this mechanism is weakened. These results are valid at the satellite altitude (660 km) and may vary at lower altitudes.

Regional TEC model under quiet geomagnetic conditions and low-to-moderate solar activity based on CODE GIMs

Global empirical total electron content (TEC) models based on TEC maps effectively describe the average behavior of the ionosphere. However, the accuracy of these global models for a certain region may not be ideal. Due to the number and distribution of the International GNSS Service (IGS) stations, the accuracy of TEC maps is geographically different. The modeling database derived from the global TEC maps with different accuracy is likely one of the main reasons that limits the accuracy of the new models. Moreover, many anomalies in the ionosphere are geographic or geomagnetic dependent, and as such the accuracy of global models can deteriorate if these anomalies are not fully incorporated into the modeling approach. For regional models built in small areas, these influences on modeling are immensely weakened. Thus, the regional TEC models may better reflect the temporal and spatial variations of TEC. In our previous work (Feng et al., 2016), a regional TEC model TECM-NEC is proposed for northeast China. However, this model is only directed against the typical region of Mid-latitude Summer Nighttime Anomaly (MSNA) occurrence, which is meaningless in other regions without MSNA. Following the technique of TECM-NEC model, this study proposes another regional empirical TEC model for other regions in mid-latitudes. Taking a small area BeiJing–TianJin–Tangshan (JJT) region (37.5°–42.5° N, 115°–120° E) in China as an example, a regional empirical TEC model (TECM–JJT) is proposed using the TEC grid data from January 1, 1999 to June 30, 2015 provided by the Center for Orbit Determination in Europe (CODE) under quiet geomagnetic conditions. The TECM–JJT model fits the input CODE TEC data with a bias of 0.11TECU and a root mean square error of 3.26TECU. Result shows that the regional model TECM–JJT is consistent with CODE TEC data and GPS–TEC data.

A single‐station empirical model for TEC over the Antarctic Peninsula using GPS‐TEC data

AbstractCompared with regional or global total electron content (TEC) empirical models, single‐station TEC empirical models may exhibit higher accuracy in describing TEC spatial and temporal variations for a single station. In this paper, a new single‐station empirical total electron content (TEC) model, called SSM‐month, for the O'Higgins Station in the Antarctic Peninsula is proposed by using Global Positioning System (GPS)‐TEC data from 01 January 2004 to 30 June 2015. The diurnal variation of TEC in the O'Higgins Station may have changing features in different months, sometimes even in opposite forms, because of ionospheric phenomena, such as the Mid‐latitude Summer Nighttime Anomaly (MSNA). To avoid the influence of different diurnal variations, the concept of monthly modeling is proposed in this study. The SSM‐month model, which is established by month (including 12 submodels that correspond to the 12 months), can effectively describe the diurnal variation of TEC in different months. Each submodel of the SSM‐month model exhibits good agreement with GPS‐TEC input data. Overall, the SSM‐month model fits the input data with a bias of 0.03 TECU (total electron content unit, 1 TECU = 1016 el m−2) and a standard deviation of 2.78 TECU. This model, which benefits from the modeling method, can effectively describe the MSNA phenomenon without implementing any modeling correction. TEC data derived from Center for Orbit Determination in Europe global ionosphere maps (CODE GIMs), International Reference Ionosphere 2012 (IRI2012), and NeQuick are compared with the SSM‐month model in the years of 2001 and 2015–2016. Result shows that the SSM‐month model exhibits good consistency with CODE GIMs, which is better than that of IRI2012 and NeQuick, in the O'Higgins Station on the test days.

Variation of the topside ionosphere during the last solar minimum period studied with multisatellite measurements of electron density and temperature

AbstractUsing the ionospheric measurements of CHAMP, DEMETER, and DMSP F15, the seasonal and spatial variations of the topside ionosphere during the last solar minimum period were investigated and compared with ionospheric models. In all the satellite measurements, equatorial ionization anomaly (EIA) shows clearly longitudinal asymmetry with wave number −3 or −4 patterns. Anomalous increases of Ne in the nighttime surpassing daytime Ne, known as the Weddell Sea anomaly (WSA) or midlatitude summer nighttime anomaly (MSNA), were also observed in the global Ne distribution with differences in detailed geometry of the geomagnetic field according to the altitude. In the nighttime ionosphere, the reduced Te in the equatorial region at the DMSP altitude, identified as the equatorial plasma temperature anomaly (EPTA), was ascribed to the leftover of the prereversal enhancement of the upward plasma drift. Though the EIA, WSA, MSNA, and EPTA are all associated with the upward plasma movement, the difference in the thermal evolution is ascribable to the geometry of drift in which the plasma moves across the geomagnetic field line for the EIA and the EPTA, while along the field line for the WSA and the MSNA.

A new regional total electron content empirical model in northeast China

Using total electron content (TEC) data over one and a half solar cycles (1999–2015) provided by the Center for Orbit Determination in Europe (CODE), this paper proposes a new empirical TEC model for northeast China (40–50N, 120–130E). The model, called TECM-NEC, involves the multiplication of four separable components, including diurnal variation, seasonal variation, geomagnetic field dependency, and solar dependency. Diurnal variation is composed of three parts: the typical daily variation of TEC; corrections of Mid-latitude Summer Nighttime Anomaly (MSNA) that depend on geographic location, season, and local time; and corrections of day-to-night ratio under different seasons and solar activities. Four sub-harmonics of the year with annual, semiannual, four-, and three-month periods are used to describe seasonal variations. For geomagnetic variation, geomagnetic latitude is based on the latest International Geomagnetic Reference Field (IGRF12) model. Compared with similar empirical models, the solar proxy index F10.7P = (F10.7 + F10.7A)/2, where F10.7A is the 81-day running mean of daily F10.7, is chosen as having linear relationship with TEC for the model. This model has 43 coefficients, which are determined by nonlinear least squares fitting (NLSF) technique. The TECM-NEC model fits with the TEC/CODE input data with a bias of 0.03TECU and a RMS deviation of 2.76TECU. The proposed TECM-NEC model can reproduce the MSNA and nighttime TEC enhancements phenomenon over northeast China.

Three-dimensional electron density along the WSA and MSNA latitudes probed by FORMOSAT-3/COSMIC

In this paper, we employ electron density profiles derived by the GPS radio occultation experiment aboard the FORMOSAT-3/COSMIC (F3/C) satellites to examine the electron density on geographic latitudes of 40° to 80° in the Southern hemisphere and 30° to 60° in the Northern hemisphere at various global fixed local times from February 2009 to January 2010. The results reveal that an eastward shift of a single-peak plasma density feature occurs along the Weddell Sea Anomaly (WSA) latitudes, while a double-peak plasma density feature appears along the northern Mid-latitude Summer Nighttime Anomaly (MSNA) latitudes. A cross-comparison between three-dimensional F3/C electron density and HWM93 simulation confirms that the magnetic meridional effect and vertical effect caused by neutral winds exhibit the eastward shifts. Furthermore, we find that the eastward shift of the peaks when viewed as a function of local time suggests that they could be interpreted as being comprised of different tidal components with distinct zonal phase velocities in local time.

Field‐aligned neutral wind bias correction scheme for global ionospheric modeling at midlatitudes by assimilating FORMOSAT‐3/COSMIC hmF2 data under geomagnetically quiet conditions

AbstractThis study demonstrates the usage of a data assimilation procedure, which ingests the FORMOSAT‐3/COSMIC (F3/C) hmF2 observations to correct the model wind biases to enhance the capability of the new global Ionosphere Plasmasphere Electrodynamics (IPE) model under geomagnetically quiet conditions. The IPE model is built upon the field line interhemispheric plasma model with a realistic geomagnetic field model and empirical model drivers. The hmF2 observed by the F3/C radio occultation technique is utilized to adjust global thermospheric field‐aligned neutral winds (i.e., a component of the thermospheric neutral wind parallel to the magnetic field) at midlatitudes according to a linear relationship between time differentials of the field‐aligned wind and hmF2. The adjusted winds are further applied to drive the IPE model. The comparison of the modeled electron density with the observations of F3/C and ground‐based GPS receivers at the 2012 March equinox suggests that the modeled electron density can be significantly improved in the midlatitude regions of the Southern Hemisphere, if the wind correction scheme is applied. Moreover, the F3/C observation, the IPE model, and the wind bias correction scheme are applied to study the 2012 Southern Hemisphere Midlatitude Summer Nighttime Anomaly (southern MSNA)/Weddell Sea Anomaly (WSA) event at December solstice for examining the role of the neutral winds in controlling the longitudinal variation of the southern MSNA/WSA behavior. With the help of the wind bias correction scheme, the IPE model better tracks the F3/C‐observed eastward movement of the southern MSNA/WSA feature. The apparent eastward movement of the southern MSNA/WSA features in the local time coordinate is primarily caused by the longitudinal variation in the declination angle of the geomagnetic field that controls the field‐aligned projection of both geographic meridional and zonal components of the neutral wind. Both the IPE simulations and the F3/C observations show the significant longitudinal variation in the speed of the eastward movement of the southern MSNA/WSA.

Thermospheric tidal effects on the ionospheric midlatitude summer nighttime anomaly using SAMI3 and TIEGCM

This paper is the first study to employ a three‐dimensional physics‐based ionosphere model, SAMI3, coupled with the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) and Global Scale Wave Model to simulate the mesospheric and lower thermospheric tidal effects on the development of midlatitude summer nighttime anomaly (MSNA). Using this coupled model, the diurnal variation of MSNA electron densities at 300 km altitude is simulated on both June solstice (day of year (DOY) 167) and December solstice (DOY 350) in 2007. Results show successful reproduction of the southern hemisphere MSNA structure including the eastward drift feature of the southern MSNA, which is not reproduced by the default SAMI3 runs using the neutral winds provided by the empirical Horizontal Wind Model 93 neutral wind model. A linear least squares algorithm for extracting tidal components is utilized to examine the major tidal component affecting the variation of southern MSNA. Results show that the standing diurnal oscillation component dominates the vertical neutral wind manifesting as a diurnal eastward wave‐1 drift of the southern MSNA in the local time frame. We also find that the stationary planetary wave‐1 component of vertical neutral wind can cause diurnal variation of the summer nighttime electron density enhancement around the midlatitude ionosphere.

Long‐term variations of the nighttime electron density enhancement during the ionospheric midlatitude summer

This study, for the first time, presented the long‐term variations of Midlatitude Summer Nighttime Anomaly (MSNA) in the two hemispheres by using 66 ground‐based ionosonde observations from 1957 to 2010. MSNA is characterized by the feature of higher nighttime electron density than daytime density in the midlatitude region during local summer months. Observations from 66 ionosonde stations were used to calculate the MSNA index which is defined by the difference between nighttime and noontime NmF2 values. The MSNA occurrence is determined by positive value of the MSNA index. The global distribution map of the MSNA index shows that there are three regions of intense MSNA. Three ionosonde stations in each of active MSNA regions were chosen to study the long‐term variation of MSNA covering longer than one solar cycle. One station in the southern hemisphere is AIJ6N (Argentine IS; 65.2°S, 64.3°W geographic) and two stations in the northern hemisphere are LN047 (Lannion; 48.8°N, −3.4°E geographic) and MG560 (Magadan; 60.0°N, 151.0°E geographic). Results show that there is a clear solar activity negative dependence of the MSNA index, high MSNA in the low solar activity condition and low MSNA in the high solar activity condition. The seasonal and solar activity variations of the MSNA index are explained by the combined effects of the vertical plasma drift induced by the neutral wind and photoionization during the nighttime.

Simulated midlatitude summer nighttime anomaly in realistic geomagnetic fields

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 O I 135.6 nm airglow observations of the midlatitude summer nighttime anomaly by TIMED/GUVI

[1] This study presents the O I 135.6 nm airglow observation of the middle-latitude electron density enhancement during local summer nighttime by Global Ultraviolet Imager (GUVI) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. The nighttime density enhancement at magnetic middle latitudes in the Southern Hemisphere, known as the Weddell Sea Anomaly (WSA), had been studied by using multipoint observations, and a similar anomalous nighttime enhancement was also found in the Northern Hemisphere recently. The resemblance of both anomalies at magnetic middle latitudes in the Northern and Southern Hemispheres suggests that they should be categorized as the midlatitude summer nighttime anomaly (MSNA). To further explore the three-dimensional structure of the MSNA and its day-to-day variation, the two-dimensional global radiance maps and the vertical electron density profiles derived from disk and limb scans of the TIMED/GUVI 135.6 nm airglow observations are utilized in this study. These global observations show that the northern MSNA mainly occurs in Asia, Europe, and the North Atlantic Ocean regions, while the southern MSNA occurs in the South America-Antarctica region, near the WSA region. The GUVI day-to-day observations in 2006 further illustrate that the southern MSNA appears nightly in January-February and November-December, while the northern MSNA appears in 36 out of 41 total observation nights in May-June.