Field Line Interhemispheric Plasma Research Articles

Coupled investigations of ionosphere variations over European and Japanese regions: observations, comparative analysis, and validation of models and facilities

This paper presents the results of a coordinated measurement campaign with ground based and satellite observations over European and Japanese regions during September 5–6, 2017. Two incoherent scatter radars, two satellite missions, International Reference Ionosphere (IRI-2016) empirical model, and Field Line Interhemispheric Plasma (FLIP) physical model were employed to examine the regular behavior of the F2-layer peak height and density and the topside ionosphere electron density, electron, and ion temperatures as well as traveling ionospheric disturbances (TIDs). The daily ionospheric variations over Kharkiv and Shigaraki exhibited similar behavior qualitatively and quantitatively. The results show that none of the empirical IRI-2016 models of F2-layer peak height, topside electron density, and temperature can be preferred for predicting the key qualitative features of variations in ionospheric plasma parameters over Kharkiv and Shigaraki. The likely reason is rapid day to day changes in solar activity and series of moderate enhancements of magnetic activity occurring in the observation period and preceding days. Compared with IRI-2016 model, the FLIP physical model was shown to provide the best agreement with the observations when constrained to follow the observed diurnal variations of F2-layer peak height both over Europe and Japan. This paper presents the first direct comparison of the mid-latitude electron density measured by the Swarm satellite with incoherent scatter radar data and it confirms the high quality of the space-borne data. For the first time, evidence of the possible need to increase the neutral hydrogen density in NRLMSISE-00 model by at least a factor of 2 was obtained for the Asian longitudinal sector. The TIDs, which have predominant periods of about 50 min over Europe and 80 min over Japan, were detected, likely caused by passage of the solar terminator. Such a difference in the periods could indicate regional features and is the topic for further research.

Coincident Observations by the Kharkiv IS Radar and Ionosonde, DMSP and Arase (ERG) Satellites, and FLIP Model Simulations: Implications for the NRLMSISE‐00 Hydrogen Density, Plasmasphere, and Ionosphere

AbstractThis paper reports the results of ionosphere and plasmasphere observations with the Kharkiv incoherent scatter radar and ionosonde, Defense Meteorological Satellite Program, and Arase (ERG) satellites and simulations with field line interhemispheric plasma model during the equinoxes and solstices of solar minimum 24. The results reveal the need to increase NRLMSISE‐00 thermospheric hydrogen density by a factor of ~2. For the first time, it is shown that the measured plasmaspheric density can be reproduced with doubled NRLMSISE‐00 hydrogen density only. A factor of ~2 decrease of plasmaspheric density in deep inner magnetosphere (L ≈ 2.1) caused by very weak magnetic disturbance (Dst > −22 nT) of 24 December 2017 was observed in the morning of 25 December 2017. During the next night, prominent effects of partially depleted flux tube were observed in the topside ionosphere (~50% reduced H+ ion density) and at the F2‐layer peak (~50% decreased electron density). The likely physical mechanisms are discussed.

The importance of neutral hydrogen for the maintenance of the midlatitude winter nighttime ionosphere: Evidence from IS observations at Kharkiv, Ukraine, and field line interhemispheric plasma model simulations

AbstractThis study investigates the causes of nighttime enhancements in ionospheric density that are observed in winter by the incoherent scatter radar at Kharkiv, Ukraine. Calculations with a comprehensive physical model reveal that large downward ion fluxes from the plasmasphere are the main cause of the enhancements. These large fluxes are enabled by large upward H+ fluxes into the plasmasphere from the conjugate summer hemisphere during the daytime. The nighttime downward H+ flux at Kharkiv is sensitive to the thermosphere model H density, which had to be increased by factors of 2 to 3 to obtain model‐data agreement for the topside H+ density. Other studies support the need for increasing the thermosphere model H density for all seasons at solar minimum. It was found that neutral winds are less effective than plasmaspheric fluxes for maintaining the nighttime ionosphere. This is partly because increased equatorward winds simultaneously oppose the downward H+ flux. The model calculations also reveal the need for a modest additional heat flow from the plasmasphere in the afternoon. This source could be the quiet time ring current.

A new dynamic fluid‐kinetic model for plasma transport within the plasmasphere

AbstractA new dynamic fluid‐kinetic model is developed for investigating the plasma transport along a closed magnetic flux tube in the plasmasphere by coupling the field line interhemispheric plasma (FLIP) model with a generalized semikinetic (GSK) model. The coupling is achieved via an overlapped transition region (800 km–1100 km altitude) in each of the hemispheres. The flux tube is allowed to move both radially away from, toward, and azimuthally around the Earth. In addition to H+, ion species O+ and He+ are for the first time treated as simulation particles in a numerical model of the plasmasphere. The simulation particles are subjected to the field‐aligned electric field, magnetic mirror force, gravity, centrifugal force, and interspecies and intraspecies Coulomb collisions. The plasmaspheric refilling processes as an application of the model are studied. The simulation results show that the behaviors of O+ ions are substantially different from those of H+ and He+ ions.

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.

An assessment of the role of soft electron precipitation in global ion upwelling

AbstractThe role of electron precipitation in the auroral zone in driving thermal O+ upwelling is explored by comparison of observations and model results. Previous reports have shown how the ambiguities of such an assessment can be reduced when the problem is addressed in dynamic boundary‐related coordinates. Upwelling ion data from the Defense Meteorological Satellite Program (DMSP) satellites are compared using a modeling framework based on the Field Line Interhemispheric Plasma (FLIP) ionospheric model. We focus on geomagnetically quiet intervals, that is, nonstorm times with Dst > −50 nT. We find that low‐energy (<100 eV) electrons are a significant driver of O+ upwelling on the nightside, particularly in the 2100 magnetic local time sector. Our analysis suggests that DMSP electron observations and electron precipitation models derived from them significantly underestimate the actual flux of soft (i.e., <100 eV) electrons.

Reevaluation of thermosphere heating by auroral electrons

This paper presents a new calculation of neutral gas heating by precipitating auroral electrons. It is found that the heating rate of the neutral gas is significantly lower than previous determinations below 200km altitude. The neutral gas heating arises from the many exothermic chemical reactions that take place from the ions and excited species created by the energetic electrons. The calculations show that less than half the energy initially deposited ends up heating the neutral gases. The rest is radiated or lost in the dissociation of O2 because the O atoms do not recombine in the thermosphere. This paper also presents a new way of calculating the heating rate per ionization that can be used for efficient determination of the overall neutral gas heating for global thermosphere models. The heating rates are relatively insensitive to the neutral atmosphere when plotted against pressure rather than altitude coordinates. At high altitudes, the heating rates are sensitive to the thermal electron density and long-lived species. The calculations were performed with the Field Line Interhemispheric Plasma (FLIP) model using a 2-stream auroral electron precipitation model. The heating rate calculations in this paper differ from previous heating rate calculations in the treatment of backscattered electrons to produce better agreement with observed flux spectra. This paper shows that more realistic model auroral electron spectra can be obtained by reflecting the up going flux back to the ionosphere at the upper boundary of the model. In this case, the neutral gas heating rates are 20%–25% higher than when the backscattered flux escapes from the ionosphere.

Ion density calculator (IDC): A new efficient model of ionospheric ion densities

[1] We present a new computationally efficient and accurate model of ion concentrations in the bottomside ionosphere based on the photochemistry. There has long been a need for efficient and accurate specification of ionospheric molecular ion concentrations. Incoherent scatter radars need to specify the relative ion concentrations in order to accurately determine plasma temperatures. Full physical ionospheric models are available but too costly and cumbersome for many applications. The international reference ionosphere (IRI) model is an efficient empirical model that accurately specifies the electron density but the molecular ion concentrations are based on limited data sets. Our new ion density calculator (IDC) model uses chemical equilibrium to determine all ion concentrations except the O+ density, which cannot be derived from chemical equilibrium above ∼180 km due to the increasing importance of diffusion. The IDC model overcomes this problem by using an iterative technique to solve for the O+ density given the electron density that is provided by the radar or the IRI model and the fact that the total ion concentration must sum to the electron density. This quasi-chemical model produces very good agreement with satellite measured ion densities and significantly improves electron and ion temperatures from incoherent scatter radars. It also produces good agreement with the Field Line Interhemispheric Plasma (FLIP) physical ionosphere model, which solves the continuity, momentum, and thermal equations. Comparisons with the IRI model point out the shortcomings of the most recent version, IRI-2007 in representing molecular ion densities.

Measured and modeled ionospheric densities, temperatures, and winds during the international polar year

This paper examines the ability of ionospheric models to reproduce measured electron density, winds, and temperatures during the International Polar Year (IPY) in 2007. The models include the field line interhemispheric plasma (FLIP) model, the international reference ionosphere (IRI) model, and the empirical horizontal neutral wind models (HWM) (HWM93, HWM07). For Poker Flat, Alaska, there is exceptionally good agreement between the FLIP model and measured electron density, winds, and temperatures in equinox and winter. This research shows an interesting post sunset peak in Te from late fall through early spring that is reproduced by the FLIP model. In June and July the FLIP model underestimates the measured peak electron density by a factor of 2. Although both the data and model show evidence of an F1 peak near 150 km in summer, the model F1 peak electron density tends to be larger than the F2 peak electron density and that is not seen in the data. The summer discrepancy is most likely due to incorrect atomic to molecular neutral density ratios. The FLIP model reproduces the Millstone Hill data well throughout 2007. The IRI model agrees well with the electron density data during the day but overestimates the peak electron density and the height of the peak at night. The equivalent winds from the FLIP model and the winds from the HWM93 model agree well with the measured winds. The HWM07 winds are different from the earlier HWM93 winds at Poker Flat and do not agree as well with the data.

Temporal and spectral variations of the photoelectron flux and solar irradiance during an X class solar flare

Photoelectrons are the main energy source of airglow used to diagnose the state of the ionosphere‐thermosphere system. Because of measurement uncertainties and substantial gaps in the historical record, parameterized models of the EUV irradiance and photoelectron flux are generally used to estimate airglow intensities. This paper compares observed and modeled photoelectron spectra from an X3 class flare that occurred on July 15, 2002. The photoelectron data were obtained from the FAST satellite. Model photoelectron spectra were obtained from the Field Line Inter‐hemispheric Plasma (FLIP) model using 10 s cadence solar spectra at 1 nm resolution from the Flare Irradiance Spectral Model (FISM). The observed and modeled spectra agree well temporally and spectrally within the uncertainties of the models and data. Systematic differences found between observed and modeled photoelectron spectra suggest that the solar irradiance from FISM could be improved at wavelengths shortward of 17 nm.

Photoelectron impact excitation of OI 8446 Å emission observed from Arecibo Observatory

Photoelectron (PE) impact on thermospheric oxygen atoms is a major source of OI 8446 Å emission excitation at midlatitudes. However, historical discrepancies between observed twilight emission brightnesses and PE model predictions have not only prompted speculation regarding secondary sources of excitation but also precluded the use of observed brightness as a much‐needed diagnostic of thermospheric O density. In an effort to improve understanding of the physical mechanisms responsible for its excitation, this paper presents new photometric measurements of twilight OI 8446 Å emission brightness, together with calculations using the Field Line Interhemispheric Plasma (FLIP) PE model, acquired from Arecibo Observatory under geomagnetically quiet conditions during three winter campaigns from 1999–2002. Winter is a particularly favorable season for twilight 8446 Å observation in the northern hemisphere since significant PE production, and thus 8446 Å excitation, persists for several hours in the fully illuminated, geomagnetic conjugate hemisphere in the absence of any local thermospheric illumination. The winter 8446 Å brightness data are fully consistent with a dominant PE impact excitation source. However, the new data also confirm the Lancaster et al. (2000) report of excess early morning brightness at Arecibo with respect to PE models which use a tilted‐dipole approximation to the geomagnetic field. By using the International Geomagnetic Reference Field (IGRF) geomagnetic field model to refine Arecibo's conjugate point location specified in the FLIP PE model, we demonstrate significantly improved agreement between the modeled and observed brightness decay profiles during both morning and evening twilight intervals.

Photoelectron flux variations observed from the FAST satellite

This paper examines high resolution (Δ E/ E = 0.15) photoelectron energy spectra from 10 eV to 1 keV, created by solar irradiances between 1.2 and 120 nm. The observations were made from the FAST satellite at ∼3000 km, equatorward of the auroral oval for the July–August, 2002 solar rotation. These data are compared with the solar irradiance observed by the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and fluxes calculated using the Field Line Interhemispheric Plasma (FLIP) code. The 41 eV photoelectron flux, which corresponds to solar EUV fluxes near 20 nm, shows a clear solar rotation variation in very good agreement with the EUV flux measurements. This offers the possibility that the 41 eV photoelectron flux could be used as a check on measured solar EUV fluxes near 20 nm. Because of unexpected noise, the solar rotation signal is not evident in the integral photoelectron flux between 156 and 1000 eV corresponding to EUV wavelengths between 0.1 and 7 nm measured by the SEE instrument. Examination of daily averaged photoelectron fluxes at energies between 25 and 500 eV show significant changes in the photoelectron spectra in response X and M class flares. The intensity of photoelectrons produced in this energy region is primarily due to two very narrow EUV wavelength regions at 2.3 and 3 nm driving Auger photoionization in O at 500 eV and N 2 at ∼360 eV. Comparison of calculated and daily averaged electron fluxes shows that the HEUVAC model solar spectrum used in the FLIP code does not reproduce the observed variations in photoelectron intensity. In principle, the 21 discrete photoelectron energy channels could be used to improve the reliability of the solar EUV fluxes at 2.3 and 3 nm inferred from broad band observations. In practice, orbital biases in the way the data were accumulated and/or noise signals arising from natural and anthropogenic longitudinally restricted sources of ionization complicate the application of this technique.

Simulating plasmaspheric field‐aligned density profiles measured with IMAGE/RPI: Effects of plasmasphere refilling and ion heating

The radio plasma imager (RPI) on board the IMAGE satellite measures electron density profiles along magnetic lines through radio sounding techniques. We simulate three density profiles measured by the RPI on 8 June 2001 using the field line interhemispheric plasma (FLIP) model and compare the modeled density profiles with observed profiles. While careful consideration of depletion of flux tubes followed by refilling over several days allows fair agreement between simulated and measured densities around the equator, the simulated densities in the off‐equator regions remain significantly lower than those measured for all simulated flux tubes. It is shown that direct heating of ions in the plasmasphere during the plasmasphere refilling, in addition to heating through Coulomb collisions with hotter plasmaspheric electrons, can improve the agreement in the off‐equator regions. The enhanced ion pressure in the high altitude part of magnetic field line due to the ion heating impedes the ionospheric inflow from both hemispheres toward the equator, giving rise to decreased transport of ionospheric ions into the equatorial region and thus increased density gradient along the magnetic field line.

Ion and neutral density variations during ionospheric storms in September 1974: Comparison of measurement and models

This paper examines the ability of the field line interhemispheric plasma (FLIP) physical ionospheric model to reproduce the O+, NO+, O2+, and N2+ ion densities, and nitric oxide density in the ionosphere and thermosphere over Australia in September 1974. The study includes ionospheric electron density data from Australian ionosonde stations as well as ion and neutral density composition data from elliptical orbits of the Atmosphere Explorer‐C (AE‐C) satellite. The mass spectrometer and incoherent scatter (MSIS) neutral atmosphere model accurately reproduces the O and N2 densities during quiet periods in September 1974, and this enables the FLIP model to also accurately reproduce the measured ion and NO densities as well as ion and electron temperatures during quiet periods. However, the MSIS model produces poor results during the disturbed period of 15 and 16 September because it underestimates the increase in neutral temperature by several hundred degrees and the depletion of the O column density by a factor of 2. When the measured neutral densities are used in the FLIP model instead of MSIS densities, there is fair agreement between the measured and modeled O+, O2+, and NO+ densities above 200 km on 15 September, but the FLIP model overestimates the O2+ density below 200 km. The FLIP model is also in fair agreement with the NmF2 from the ionosonde and reproduces the measured O+ density on 16 September but severely overestimates the satellite measured O2+ and NO+ densities at all altitudes. As found in previous studies, the FLIP model was able to accurately reproduce a positive ionospheric storm that occurred during this period. An important result to emerge from this study is the observation that even during these severely disturbed periods the altitude distributions of the measured O and N2 densities can be reasonably well represented by hydrostatic equilibrium profiles. This suggests that the basic philosophy of the MSIS model remains intact and that it should be possible to improve the MSIS model storm behavior just by improving the specification of the neutral boundary densities and the exospheric temperature. The assumption of hydrostatic equilibrium also simplifies the task of deducing neutral composition from remote sensing.

An investigation of ionospheric responses, and disturbance thermospheric winds, during magnetic storms over South American sector

This paper presents the results of a study of the ionospheric and thermospheric response to magnetic storm disturbances for four stations covering a wide range of latitude in the South American region. The study is based on the F layer critical frequency (foF2) and the peak height (hmF2) from Fortaleza (FZ), which is equatorial, Cachoeira Paulista (CP), which is low latitude, Concepción (CON), a midlatitude site, and King George Island (KGI), a middle to high latitude site. Equivalent neutral winds are extracted from the measured hmF2 by employing the Field Line Interhemispheric Plasma (FLIP) model. The Equatorial Ionization Anomaly (EIA) can undergo enhancement due to magnetospheric disturbance electric field that penetrates to low latitudes during the growth phase of a storm/substorm, whereas EIA inhibition occurs more often under disturbance dynamo (DD) electric field. Season‐dependent meridional/transequatorial winds can significantly alter the EIA response to disturbance electric fields. Large amplitudes of DD electric field is observed during evening and morning hours in equinoctial, but not in winter, months. Over the Brazilian low latitude of large westward magnetic declination angle, large equatorward meridional wind surges seem to be helped by the presence of disturbance zonal (westward) winds. The low latitude disturbance can, in some cases, be traced to specific disturbance fronts originating promptly from auroral activity enhancements. This study confirms earlier studies that show that the quiet time meridional wind increases with increase of latitude and that over midlatitude it shows decreases with increase of solar flux due to the effect of the ion drag force. This study further shows that the disturbance meridional wind and the intensity of DD electric field increase with increasing intensity of high latitude energy input. The negative ionospheric storm phase over middle latitude, observed in equinox and not in winter, is also intensified with the increasing intensity of high latitude energy input.

Seasonal and solar cycle variations of the ionospheric peak electron density: Comparison of measurement and models

This paper examines the ability of empirical and physical models to reproduce the peak electron density of the midlatitude ionospheric F2 region (NmF2) from 1976 to 1980. The data from all midlatitude stations show a tendency toward a semiannual variation in noon NmF2 with peaks at the equinoxes for all levels of solar activity. The Southern Hemisphere semiannual variation is more pronounced than in the Northern Hemisphere primarily because the winter density is relatively low in the Southern Hemisphere. At most locations the equinox density peaks are approximately equal. However, the September peak is much weaker than the March peak at most Australian stations. This leads to a distinct longitudinal variation between the Australian and South American sectors. On the other hand, there is remarkably little longitudinal variation in the Northern Hemisphere. We present calculations from the field line interhemispheric plasma (FLIP) model from 1976 to 1980 at six representative midlatitude stations around the globe. The FLIP model reproduces the average seasonal and solar cyclical behavior of the measured NmF2 remarkably well most of the time. The greatest differences of 50% occur at the March equinox in the South American region and at the September equinox in the Australian region during September solstice solar maximum. The international reference ionosphere (IRI) model reproduces the average NmF2 even better than the FLIP model but, unlike the FLIP model, it has little day‐to‐day variation. A factor of 2 increase in the solar EUV ion production rate and in the atomic to molecular density ratio at the F2 region peak height (hmF2) produces a factor of 4 increase in NmF2 over a solar cycle. Most of this increase takes place before the average solar activity index (F10.7) reaches 175. At solar maximum in 1979 and 1980, there is little relationship between daily F10.7 and NmF2. Changes in the atomic to molecular density ratio at hmF2 are primarily responsible for the semiannual variation in the FLIP model NmF2. The inclusion of vibrationally excited N2 in the FLIP model improves the relative seasonal and solar cycle NmF2 variations in the FLIP model, but it causes the overall NmF2 to be too low at most stations.

A dynamic diffusive equilibrium model of the ion densities along plasmaspheric magnetic flux tubes

A new approach, based in part on diffusive equilibrium, has been developed to model the dynamic H + content of plasmaspheric magnetic flux tubes. Called the Global Plasmasphere Ionosphere Density (GPID) model, its aim is to reproduce the main dynamic features of the more complete Field Line Interhemispheric Plasma (FLIP) model, with a simpler formulation that allows for more rapid computation of the ions temporal variation in a magnetic flux tube. By simultaneously modelling several thousand flux tubes, a global representation of the plasmasphere can be obtained in a realistic time frame. A global model allows the plasmaspheric contribution to the Total Electron Content (TEC), measured from Global Position System (GPS) satellite radio signals, to be determined and removed, hence permitting the underlying ionosphere to be observed. This paper presents the approach used to model a single magnetic flux tube. GPID assumes an ionosphere/plasmasphere composition of O + and H +, and includes chemical processes and simple diffusive transport. In almost all situations, GPID reproduces the predictions of the FLIP model. Comparisons between GPID and the refilling fluxes obtained from whistler observations generally show good agreement, both in diurnal variations and during prolonged refilling of empty flux tubes. Comparisons to seasonal variations during solar maximum at L=2.5 show good agreement with observations, but at solar minimum there is variable agreement.

On the causes of the annual variation in the plasmaspheric electron density

This paper examines the relative importance of ionospheric and thermospheric densities and temperatures in producing the annual variation in the plasmaspheric electron density ( N eq). In the Eastern American sector, whistler observations show a factor of 2–3 variation in N eq at low solar activity with a minimum in June and a maximum in December. The field line interhemispheric plasma (FLIP) model also shows an annual N eq variation in the American sector at solar minimum, but the model densities are about 30% higher. By using the ability of the FLIP model to be constrained by the measured h mF 2, N mF 2, and T e, we are able to show that, contrary to previous studies, it is the plasmaspheric thermal structure, and not ionospheric density that plays the key role in producing the annual variation in the model N eq at solar minimum. We find that the model annual N eq variations in the American and Australian sectors are out of phase by six months as would be expected from the opposite tilt of the Earth's magnetic field. This study also shows that the plasmaspheric electron density is anticorrelated with magnetic and solar activity due to changes in neutral hydrogen density. Thermospheric neutral winds appear to play only a small role in the annual N eq variations because longitudinal variations in the measured h mF 2 are small. However, magnetic storm induced variability in ionospheric density through changes in thermospheric winds and neutral densities do contribute to the day-to-day variability of N eq. At solar maximum in November 1989 the model densities are generally in good agreement with both whistler and satellite observation in both the American and New Zealand sectors. However, the standard model is unable to reproduce the relatively small annual variation in equatorial electron density that was observed from Dunedin, New Zealand at solar maximum in 1990. We show that the low measured densities at solar maximum may be explained by magnetic storm induced depletion of the plasmasphere by convection electric fields.

On the relative importance of convection and temperature to the behavior of the ionosphere in North America during January 6–12, 1997

This paper is primarily concerned with the causes of the large density and temperature enhancements that are often observed during magnetically quiet periods on winter nights at mid‐latitudes in the North American sector. Measurements from a network of Digisondes and an incoherent scatter radar are compared with the field line interhemispheric plasma (FLIP) model for January 6–12, 1997, in order to examine the temporal evolution and geographical extent of the enhancements in eastern North America. Postsunset measurements at Millstone Hill show high electron temperatures accompanied by rapid density decay until midnight followed by a rapid temperature decay accompanied by a pronounced density enhancement in the early morning hours. The FLIP model reproduces the nighttime density enhancement well, provided the model is constrained to follow the topside electron temperature and also that the overlying plasmaspheric flux tube is full. The dramatic reduction in plasmaspheric heat flux near midnight results in a sharp decrease in ionospheric temperature, inducing a large downward flow of plasmaspheric ions which creates the nighttime enhancement in ionospheric density. We find that the nighttime plasmaspheric heat flux variation drives the nighttime ionospheric density variation, which is opposite to conclusions in previously published work. Although the plasmaspheric heat flux variation can explain the ionospheric density variation, the reasons for this heat flux variation are not understood. Convection of plasma from higher magnetic latitudes is now included in the FLIP model but is not needed to produce the observed nighttime density maximum. We have found that the fraction of light ions in the topside ionosphere at 500 km altitude in the model is very close to that obtained from chemical equilibrium and agrees well with the measured fraction. The model generally reproduces the daytime electron density very well at all stations except Bermuda, where the difference is as much as 50%.

Solar cycle variations of the thermospheric meridional wind over Japan derived from measurements of hmF2

This paper describes an analysis of the meridional equivalent neutral wind for geomagnetically quiet conditions as determined from data collected at the Kokubunji ionosonde station (35.7N, 139.5E), Japan over one solar cycle (1981–1991). The wind is derived from the altitude of peak F2 layer electron density using the Field Line Interhemispheric Plasma (FLIP) model. For low and moderate solar activity the wind is poleward in the daytime and equatorward in the nighttime. For high solar activity the wind is weak and almost always poleward throughout the day. In winter the local time of peak poleward velocity occurs in the afternoon for low solar activity but in the night for high solar activity. In summer the peak poleward wind occurs in the morning for all levels of solar activity. The diurnal amplitude decreases with increasing solar activity. It also reaches its maximum around the solstices and its minimum around the equinoxes. The mean wind is larger at solar maximum than solar minimum, except for the year 1984. The daily‐mean wind is smallest in summer and largest in winter. Qualitatively, the meridional wind at Asian midlatitudes has characteristics similar to those seen in other sectors (such as Millstone Hill, Boulder, Wallops Island, King George Island, and Saint‐Santin), but the details in behavior are different and warrant further investigation.