Topside Electron Density Profiles Research Articles

Validation of the IRI-2020 topside ionosphere options through in-situ electron density observations by low-Earth-orbit satellites

The topside ionosphere extends from the F2-layer peak, where the electron density reaches its absolute maximum in the ionosphere, to the overlying plasmasphere and magnetosphere. In the topside ionosphere, the electron density decreases with height with a vertical variation rate strongly dependent on height itself. The last version of the International Reference Ionosphere (IRI) model, i.e., IRI-2020, describes this complex behavior through four topside options based on different sub-models (i.e., options) developed from the 1970s to the present. All these options have in common the F2-layer peak as an anchor point, while they differ in their topside electron density profile and/or plasma effective scale height formulations. In this work, we perform a validation of the accuracy of the four IRI-2020 topside options based on the comparison against in-situ electron density observations by Gravity Recovery and Climate Experiment (GRACE), Ionospheric Connection Explorer (ICON), and Defense Meteorological Satellite Program (DMSP) F15 low-Earth-orbit satellites. Datasets used in this study encompass observations recorded from 1999 to 2022, covering different diurnal, seasonal, and solar activity conditions, on a global basis and for the height range 400–850 km above the ground. The nearly two solar cycles dataset facilitated the evaluation of IRI-2020 topside options ability to reproduce the spatial and time variations of the topside ionosphere for different solar activity conditions. The weaknesses and strengths of each IRI-2020 topside option are highlighted and discussed, and suggestions on how to improve the modeling of the challenging topside ionosphere region within the IRI model are provided for future reference.

A Regional Model of Topside Ionospheric Effective Scale Heights Derived From Ionosonde and GNSS TEC

AbstractIonospheric scale height is critical for the structure of electron density profile in the ionosphere. However, mostly, the effective scale heights (fitted with mathematical functions) instead of topside ionospheric scale heights were used to reconstruct the topside profile. In this study, a new method was proposed to estimate topside ionospheric effective scale heights from ionosonde and Global Navigation Satellite System (GNSS) vertical total electron content (TEC). First, the bottomside electron density profile can be estimated from ionograms. Then, combined the parameters of the F2 layer and effective scale heights, topside electron density profile can be estimated by Epstein Layer. Furthermore, ionosonde TEC can be compared with GNSS vertical TEC by adjusting effective scale heights to obtain best‐fit ionosonde TEC and effective scale heights. In this study, ionosonde and GNSS TEC data recorded at Zhangye station (39.40°N, 100.13°E) were used to test the performance. Results show that diurnal and seasonal characteristics of effective scale heights are consistent with previous studies. Furthermore, the Empirical Orthogonal Function technique was used to build a regional and empirical model of effective scale heights. Results show that the accuracy (between ±2.5 TECU) of ionosonde TEC are about 91.26% by comparing with GNSS TEC. Considered that the traditional methods mostly underestimated TEC, it indicates that the accuracy of ionosonde TEC can be improved significantly by the new model of effective scale heights. Furthermore, results show that the topside electron densities estimated by the proposed method are comparable with in situ observations measured by Swarm B.

Characteristics of the Effective Scale Height in the Topside Ionosphere Extracted From Swarm A and Digisonde Observations: Preliminary Results

AbstractThe ionospheric scale height is a key parameter that defines the shape of the electron density profile. Given that its direct calculation requires knowledge of the physical and chemical state of the plasma, the concept of the effective scale height was proposed to facilitate its application. It is defined as the scale height that fits the electron density profiles with the α‐Chapman function. Its main characteristics, including the diurnal and altitudinal variation are still under investigation. This paper focuses on the specification of its key characteristics at middle latitudes based on coincident Digisonde and Swarm A satellite data. Based on the assumption that the topside electron density profile is approximated with the α‐Chapman model of variable scale height, the effective scale height at the Swarm A altitude is calculated and compared with the effective scale height at . A general conclusion is that the topside effective scale height at ∼450 km exhibits a diurnal variation with highest values in the early morning and evening sectors, while at it gets its maximum values at noon. At night, the effective scale height tends to increase with altitude, however in the daytime, its behavior is more complex. On a local scale, evidence for connection between spread F irregularities and high variability in the topside effective scale height is reported. These preliminary results indicate the potential to further exploit Swarm and Digisonde data for the development of a model for the effective scale height, able to support realistic electron density reconstruction models.

Validation and Improvement of NeQuick Topside Ionospheric Formulation Using COSMIC/FORMOSAT‐3 Data

AbstractWe examine systematic differences between topside electron density measurements and different topside model formulations including α‐Chapman, NeQuick, and an improved NeQuick (NeQuick‐corr) topside formulation, recently proposed. The global topside electron density data set used, was extracted from Radio Occultation (RO) topside electron density profiles on board Low‐Earth Orbit satellites from the COSMIC/FORMOSAT‐3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite) mission. By using RO topside electron density measurements collocated with digisonde stations, we ensure that our investigation is based on two independent data sets (RO and digisondes). A subset of these profiles, with matched (within 5%) peak RO‐digisonde characteristics (foF2 and hmF2) is also exploited. This subset is exploited to extend the investigation on the basis of a higher quality validation data set. The comparison demonstrates that α‐Chapman and NeQuick‐corr underestimate, whereas NeQuick overestimates COSMIC topside electron density observations. The main outcome of this study is the significant NeQuick topside representation improvement that can be achieved near the F region peak, if the key parameter g, which controls the change of scale height with respect to altitude, is optimized to a value of 0.15 (compared to a currently adopted value of 0.125). The NeQuick‐corr topside formulation using the optimized g value of 0.15 outperforms all other topside formulations.

A Novel Method for Deriving the Vary‐Chap Scale Height Profile in the Topside Ionosphere

AbstractThe modeling of topside ionospheric electron density using height‐varying scale height methods have received increasing attention. The modified Chapman function with a variable scale height, known as Vary‐Chap function, can well represent the topside electron density profiles, and is considered as a promising direction to improve the International Reference Ionosphere model. In order to facilitate modeling, a new formula with four unknown coefficients is proposed to fit the Vary‐Chap scale height, and these unknowns can be determined by the least squares technique. The approximated electron density profile is reconstructed by using the fitted scale height, and its average percentage error relative to the measured electron density profile is quantitatively analyzed by using the Alouette/ISIS topside sounder data sets and COSMIC radio occultation databases, and the accuracies of five other approaches (i.e., Nava et al., 2008, https://doi.org/10.1016/j.jastp.2008.01.015; Nusmei et al., 2012, https://doi.org/10.1029/2012RS004989; Porl et al., 2019, https://doi.org/10.1007/s10712-019-09521-3; Reinisch et al., 2007, https://doi.org/10.1016/j.asr.2006.05.032; Wu et al., 2020, https://doi.org/10.1029/2019JA027637) to approximate the topside electron density profiles are also compared. From the perspective of average percentage error, our formula performs well to reconstruct the topside electron density profiles, especially at the altitudes between hmF2 and hmF2 + 1,200 km.

A topside investigation over a mid-latitude digisonde station in Cyprus

We examine the systematic differences between topside electron density measurements recorded by different techniques over the low-middle latitude operating European station in Nicosia, Cyprus (geographical coordinates: 35.14oN, 33.2oE), (magnetic coordinates 31.86oN, 111.83 oE). These techniques include space-based in-situ data by Langmuir probes on board.European Space Agency (ESA) Swarm satellites, radio occultation measurements on board low Earth orbit (LEO) satellites from the COSMIC/FORMOSAT-3 mission and ground-based extrapolated topside electron density profiles from manually scaled ionograms. The measurements are also compared with International Reference Ionosphere Model (IRI-2016) topside estimations and IRI-corrected NeQuick topside formulation (method proposed by Pezzopane and Pignalberi (2019)). The comparison of Swarm and COSMIC observations with digisonde and IRI estimations verifies that in the majority of cases digisonde underestimates while IRI overestimates Swarm observations but in general, IRI provides a better topside representation than the digisonde. For COSMIC and digisonde profiles matched at the F layer peak the digisonde systematically underestimates topside COSMIC electron density values and the relative difference between COSMIC and digisonde increases with altitude (above hmF2), while IRI overestimates the topside COSMIC electron density but after a certain altitude (~150 km above hmF2) this overestimation starts to decrease with altitude. The IRI-corrected NeQuick underestimates the majority of topside COSMIC electron density profiles and relative difference is lower up to approximately 100 km (above the hmF2) and then it increases. The overall performance of IRI-corrected NeQuick improves with respect to IRI and digisonde.

On the link between the topside ionospheric effective scale height and the plasma ambipolar diffusion, theory and preliminary results

Over the years, an amount of models relying on effective parameters were implemented in the challenging issue of the topside ionosphere description. These models are based on different analytical functions, but all of them depend on a parameter called effective scale height, that is deduced from topside electron density measurements. As their names state, they are effective in reproducing the topside electron density profile only when applied to the analytical function used to derive them. Then, in principle, they do not have any physical meaning. It is the goal of this paper to mathematically link the effective scale height modeled through the Epstein layer to the vertical scale height theoretically deduced from the plasma ambipolar diffusion theory. Firstly, effective and theoretical scale heights are linked through a mathematical relation by showing that they tend to each other in the topside ionosphere. Secondly, their connection is preliminarily demonstrated by calculating effective scale height values from the entire COSMIC/FORMOSAT-3 radio occultation dataset. Thirdly, a possible connection between the vertical gradient of the topside scale height (as obtained by COSMIC/FORMOSAT-3 satellites) and the electron temperature (as obtained by ESA Swarm B satellite) is studied by highlighting corresponding similarities in the diurnal, seasonal, solar activity, and latitudinal variability.

New Vary‐Chap Scale Height Profile Retrieved From COSMIC Radio Occultation Data

AbstractThe Global Navigation Satellite System radio occultation technique provides materials for the validation of a mathematical description of the topside ionosphere up to the low Earth orbit (LEO) satellite altitude. An attempt to represent the topside electron density profile is using α‐Chapman function with a continuously varying scale height. In this study, the Vary‐Chap scale height profiles are obtained based on Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron density profiles from 1 January 2008 to 31 December 2013 and fitted by a shape function composed of two weighted patterns representing the ion and electron contributions of lower and higher altitudes. The associated fitting parameters a and b are analyzed to reveal their temporal and spatial features and variations along with enhancement of solar activity. The prominent dependence on latitudes, longitudes, the local time, the season, and the solar cycle shown in the parameters inspires future modeling of the Vary‐Chap scale height in constructing empirical topside ionospheric models.

Latitudinal Variation of α‐Chapman Scale Height During Quiet and Disturbed Geomagnetic Conditions

AbstractScale height is an important ionospheric parameter which determines the shape of the topside electron density profile. In this paper, the latitudinal variation of α‐Chapman scale height (Hm) is investigated using data obtained from four ground‐based ionosondes located in the African‐European longitudes. A Hm pattern reveals a marked dependence on seasons, latitudes, and magnetic activity. The daytime Hm peak magnitude decreases with increasing latitudes. Daytime Hm is higher relative to that of nighttime at low‐ and mid‐latitudes irrespective of the season. However, at high latitude, the opposite is observed in the solstice seasons and comparable magnitudes during equinoxes. Seasonally, Hm magnitude maximizes/minimizes in summer/winter at all latitudes considered – revealing hemispheric difference emanating from the opposite occurrence of winter and summer. Further, the difference in the magnitude of the summer‐winter Hm is most pronounced at high latitudes at all hours. However, at low latitude, magnitude of the difference between the nighttime summer‐winter values is insignificant. Hm also exhibits weak equinoctial asymmetry that reveals hemispheric difference. A strong positive linear relationship exists between Hm and a bottomside thickness parameter (B0) across all seasons and latitudes. However, Hm versus the total electron content, F2‐layer critical frequency (foF2), and peak height (hmF2) generally revealed a poor relationship and does not show any significant seasonal and latitudinal trends. The excellent Hm‐B0 relationship (r = 0.85–0.97) suggests a convenient method to develop an empirical model of Hm from a more easy‐to‐obtain B0 data regardless of the latitude.

Performance evaluation of IRI-2016 with GPS-derived TEC at the meridian of 110oE in China of 2014

The latest version of the International Reference Ionosphere (IRI-2016) is evaluated for ionospheric total electron content (TEC) over China during a high solar activity phase of 2014. TEC is derived from observations by the GNSS receiver network of Crustal Movement Observation Network of China (CMONOC). IRI-2016 uses the NeQuick option for the topside electron density profile. The comparisons shows that IRI-2016 underestimates GPS-TEC slightly in spring and winter, but overestimates slightly in summer and autumn at the mid-latitudes of 35oN-45oN. The deviation of TEC estimation is about 5 TECU at 35oN-45oN. As a whole, IRI-2016 underestimates GPS-TEC at the lower latitudes of 15oN-35oN, and the deviations of TEC estimation are about 4 TECU, 10 TECU, 26 TECU and 33 TECU in summer, autumn, winter and spring, respectively. The largest difference between IRI-TEC and GPS-TEC is about 50 TECU at the northern crest of equatorial ionization anomaly (EIA) in spring. There is an obvious winter anomaly for GPS-TEC, while it is not predicted by IRI-2016. Due to the influence of nighttime enhancement of ionosphere, the extreme latitudinal gradient of GPS-TEC appears after sunset in spring and winter, while it is not shown on that of IRI-TEC. Because of the equatorial fountain effect, the maximum time rate of GPS-TEC appears at the beginning of EIA (∼10:00–11:00 LT) in equinoxes, while it is not shown on that of IRI-TEC. The most rapid decrease of IRI-TEC appears after sunset (∼19:00 LT) in spring and winter, while GPS-TEC quickly decreases at ∼22:00 LT.

New Aspects of the Ionospheric Behavior Over Millstone Hill During the 30‐Day Incoherent Scatter Radar Experiment in October 2002

AbstractThe geomagnetic storm‐driven ionospheric changes and the involved processes are interesting and challenging topics in understanding and predicting the ionosphere. In this study we investigate the response of the ionosphere to geomagnetic disturbances during the 30‐day incoherent scatter radar measurements conducted at Millstone Hill (42.6°N, 71.5°W) from 4 October to 4 November 2002. During geomagnetically disturbed periods, while the peak electron density of the F2 layer (NmF2) and total electron content deviate remarkably from the quiet time ones in a similar way, the incoherent scatter radar measurements reveal that the changes in electron density are frequently different between low and high altitudes. The electron density is significantly depleted at low altitudes; however, at topside it either changes slightly or sometime is enhanced. The enhanced vertical scale height around 600 km under geomagnetically active conditions implies that the topside electron density profiles become much steeper. The increase in the peak height of F2 layer (hmF2) indicates the upward motions under the action of the storm‐driven dynamic processes. Further, sometimes strong differences are shown in total electron content between Millstone Hill and longitude 100°W. The competing contributions from dynamic processes and disturbance composition to the storm‐time ionospheric changes over Millstone Hill are indicated in the different responses in electron density at the bottomside and topside of the ionosphere.

Ionospheric variations over Chinese EIA region using foF2 and comparison with IRI-2016 model

In the present work, we have analyzed data of critical frequency of the F2 region (foF2) for the period, 2008–2013 over low latitude Chinese station Guangzhou (Geog. Lat. 23.10°N, Geog. Long. 113.40°E, dip, Lat. 13.49°N) and results thereof have been compared with IRI-2016 model. foF2 data set of the present study encompasses period of unusual and extended solar minimum, i.e., the years 2008–2009 and rising phase of solar cycle 24. IRI data have been obtained by choosing topside electron density profile IRI-NeQuick for two F peak models, CCIR and URSI. It is found that the general trend of variation in foF2 closely follows the trend of the solar flux during the period of study. A linear regression analysis gave a correlation coefficient of 0.98 which shows strong dependence of foF2 variation over solar flux variation. Semi-annual and annual oscillations are clearly brought out in the foF2 data using the Lomb-Scargle periodogram. A presence of semiannual and winter anomaly in observed as well as modeled foF2 at Guangzhou have found to be consistent throughout the period 2008–2013 irrespective of the phases of the solar activity. Our results also show the stronger presence of winter anomaly during the years of higher solar flux and it has been confirmed by normalizing the difference of winter to summer foF2 values for each year. Comparative results of ionosonde observation and IRI-2016 model show a significant discrepancy with regard to values of foF2 in different seasons and local time variations. IRI 2016 model underestimates the foF2 values in winter and equinoxes and overestimates foF2 values in summer. IRI modeled foF2 values using CCIR and URSI F peak models were found greater during forenoon hours and smaller during afternoon hours than the observed foF2 values throughout the period 2008–2013.

Topside Electron Density Representations for Middle and High Latitudes: A Topside Parameterization for E‐CHAIM Based On the NeQuick

AbstractIn this study, we present a topside model representation to be used by the Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM). In the process of this, we also present a comprehensive evaluation of the NeQuick's, and by extension the International Reference Ionosphere's, topside electron density model for middle and high latitudes in the Northern Hemisphere. Using data gathered from all available incoherent scatter radars, topside sounders, and Global Navigation Satellite System Radio Occultation satellites, we show that the current NeQuick parameterization suboptimally represents the shape of the topside electron density profile at these latitudes and performs poorly in the representation of seasonal and solar cycle variations of the topside scale thickness. Despite this, the simple, one variable, NeQuick model is a powerful tool for modeling the topside ionosphere. By refitting the parameters that define the maximum topside scale thickness and the rate of increase of the scale height within the NeQuick topside model function, r and g, respectively, and refitting the model's parameterization of the scale height at the F region peak, H0, we find considerable improvement in the NeQuick's ability to represent the topside shape and behavior. Building on these results, we present a new topside model extension of the E‐CHAIM based on the revised NeQuick function. Overall, root‐mean‐square errors in topside electron density are improved over the traditional International Reference Ionosphere/NeQuick topside by 31% for a new NeQuick parameterization and by 36% for a newly proposed topside for E‐CHAIM.

Examining the use of the NeQuick bottomside and topside parameterizations at high latitudes

An examination of the high latitude performance of the bottomside and topside F-layer parameterizations of the NeQuick electron density model is presented using measurements from high latitude ionosonde and Incoherent Scatter Radar (ISR) facilities.For the bottomside, we present a comparison between modeled and measured B2Bot thickness parameter. In this comparison, it is seen that the use of the NeQuick parameterization at high latitudes results in significantly underestimated bottomside thicknesses, regularly exceeding 50%. We show that these errors can be attributed to two main issues in the NeQuick parameterization:(1) through the relationship relating foF2 and M3000F2 to the maximum derivative of F2 electron density, which is used to derive the bottomside thickness, and (2) through a fundamental inability of a constant thickness parameter, semi-Epstein shape function to fit the curvature of the high latitude F-region electron density profile.For the topside, a comparison is undertaken between the NeQuick topside thickness parameterization, using measured and CCIR-modeled ionospheric parameters, and that derived from fitting the NeQuick topside function to Incoherent Scatter Radar-measured topside electron density profiles. Through this comparison, we show that using CCIR-derived foF2 and M3000F2, used in both the NeQuick and IRI, results in significantly underestimated topside thickness during summer periods, overestimated thickness during winter periods, and an overall tendency to underestimate diurnal, seasonal, and solar cycle variability. These issues see no improvement through the use of measured foF2 and M(3000)F2 values. Such measured parameters result in a tendency for the parametrization to produce a declining trend in topside thickness with increasing solar activity, to produce damped seasonal variations, and to produce significantly overestimated topside thickness during winter periods.

Study of ionospheric topside variations based on NeQuick topside formulation and comparisons with the IRI-2012 model at equatorial latitude station, Chumphon, Thailand

It is well-known that the equatorial anomaly at equatorial and low latitudes is caused by the fountain effect, therefore, the modeling of topside electron density profiles (EDPs) and the knowledge of electron density distribution in the region are particular challenging. Chumphon station, Thailand, is located within this region. However, at this station, since only an FMCW ionosonde system is installed, the topside EDP cannot be automatically obtained. Therefore, in this work, the topside EDPs are derived using the NeQuick topside formulation of the NeQuick 2 model (namely, NeQuick-derived topside EDPs) and then compared with those of three options of the IRI-2012 model including NeQuick, IRI01-corr, and IRI2001 options. The results show that, at Chumphon station, the NeQuick-derived topside EDPs are generally closer to the topside EDPs of IRI01-corr option in winter season and the topside EDPs of NeQuick option in equinox and summer seasons. When analyzing the topside TEC values obtained from each profile and the IGS TEC, it is found that the topside TEC values integrated from NeQuick-derived topside EDPs and predicted by both IRI01-corr and IRI-2001 options of the IRI-2012 model are sometimes higher than the IGS TEC values observed at Chumphon. In addition, we study the diurnal variations of the scale height computed from NeQuick topside formulation (Hsc), hmF2 and foF2 parameters. The correlation of Hsc with the parameters hmF2 and the bottomside thickness parameter of NeQuick model (B2bot). The computed Hsc values during daytime are lower than those during nighttime and they show non-linear correlations with hmF2 and B2bot. The main cause of these discrepancies are possibly due to the B2bot expressions used to compute the Hsc.

Software for scaling and analysis of vertical incidence ionograms-ionoScaler

A software tool named ionoScaler (ionogram Scaler) was developed to carry out manual and automatic scaling of vertical incidence ionograms. Additionally, ionosonde Total Electron Content (ITEC) can be estimated by this tool through reconstructing the bottomside and topside electron density profiles of ionograms. Ionograms recorded at Puer (22.7°N, 101.05°E) and Wuhan (30.5°N, 114.3°E) were adopted to test features of the ionoScaler. Results show that the software tool performed well for scaling of ionograms and estimation of ITEC. However, bad quality ionograms would reduce the accuracy of automatic scaled (autoscaled) values. The ionoScaler presented in this paper will greatly expedite the interpretation of ionograms, and it is also an alternative method for estimation of TEC values. Moreover, this tool will be useful for research in the ionosphere and space weather.

Assessment of IRI and IRI‐Plas models over the African equatorial and low‐latitude region

AbstractA reliable ionospheric specification by empirical models is important to mitigate the effects of the ionosphere on the operations of satellite‐based positioning and navigation systems. This study evaluates the capability of the International Reference Ionosphere (IRI) and IRI extended to the plasmasphere (IRI‐Plas) models in predicting the total electron content (TEC) over stations located in the southern hemisphere of the African equatorial and low‐latitude region. TEC derived from Global Positioning System (GPS) measurements were compared with TEC predicted by both the IRI‐Plas 2015 model and the three topside options of the IRI 2012 model (i.e., NeQuick (NeQ), IRI 2001 corrected factor (IRI‐01 Corr), and the IRI 2001(IRI‐01)). Generally, the diurnal and the seasonal structures of modeled TEC follow quite well with the observed TEC in all the stations, although with some upward and downward offsets observed during the daytime and nighttime. The prediction errors of both models exhibit latitudinal variation and these showed seasonal trends. The values generally decrease with increase in latitude. The TEC data‐model divergence of both models is most significant at stations in the equatorial region during the daytime and nighttime. Conversely, both models demonstrate most pronounced convergence during the nighttime at stations outside the equatorial region. The IRI‐Plas model, in general, performed better in months and seasons when the three options of the IRI model underestimate TEC. Factors such as the height limitation of the IRI model, the inaccurate predictions of the bottomside and topside electron density profiles were used to explain the data‐model discrepancies.

Evaluation and correction of the IRI2016 topside ionospheric electron density model

The international reference ionosphere (IRI) is the internationally recommended empirical model. The IRI2016 is now the latest version, and it includes three options for the prediction of topside electron density profiles: IRI2001, a correction of IRI2001 (IRI2001corr), and NeQuick model. In the paper, we use the Arecibo, Jicamarca, and Millstone Hill incoherent scatter radar observed topside electron density data ranging from year 2001 to 2014 to assess the prediction capabilities of these three model options. The results show that the NeQuick model outputs perform best at these areas from the point view of relative difference distribution, followed by the IRI2001corr model, and the IRI2001 option is worst. To further improve the performance of NeQuick and IRI2001corr options, the correction factors are introduced in their model formulation and are determined by the least squares estimation technique. Compared with the original models, the corrected models perform in average better, especially at low and medium solar activities.

Evidence of scale height variations in the Martian ionosphere over the solar cycle

AbstractSolar cycle variations in solar radiation create density changes in any planetary ionosphere, which are well established in the Earth's case. At Mars, however, the ionospheric response to such changes is not well understood. We show the solar cycle impact on the topside ionosphere of Mars, using data from the Mars Advance Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express. Topside ionospheric variability during the solar cycle is analyzed through neutral scale height behavior. For moderate and high solar activity phases, the topside electron density profile is reproduced with an altitude‐variable scale height. However, for the period of extremely low solar activity in 2008 and 2009, the topside was smaller in density than in the other phases of the solar cycle, and there is evidence that it could be reproduced with either a constant scale height or a height‐variable scale height with lower electron density. Moreover, the ionosphere during this time did not show any apparent dependence on the EUV flux. This singular behavior during low solar activity may respond to the presence of an induced magnetic field which can penetrate to lower ionospheric altitudes than in other phases of the solar cycle due to the reduced thermal pressure. Numerical simulations of possible scenarios for two different solar cycle phases indicate that this hypothesis is consistent with the observations.

New method for deriving the topside ionospheric Vary‐Chap scale height

AbstractAn α‐Chapman function with a continuously varying scale height H(h) can be used to describe the topside ionosphere electron density profile that seamlessly connects the ionosphere with the plasmasphere. Huang and Reinisch () have derived the analytic expression of the scale height from the Vary‐Chap function, which can be applied to the topside F2 region. However, the H(h) is still difficult to obtain because H(h) is a function of the unknown value H(h) at hmF2. In this paper, the maximum entropy method is introduced to solve this ill‐posed problem, and the Vary‐Chap scale height for each electron density profile is derived. Then the average percentage errors that defined as the differences between the measured electron density profiles and the Vary‐Chap profiles obtained from the Vary‐Chap scale heights are calculated with the Alouette/ISIS satellites topside sounder database. The results reveal that the average percentage errors are generally less than 5% and the Vary‐Chap profiles can well represent the topside electron density profiles. What is more, the local time, seasonal, solar cycle, and latitudinal variations of the Vary‐Chap scale height are preliminarily analyzed.