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The article analyzes the errors in estimating the parameters of the main ionospheric maximum, plasma frequency foF2 and its height hmF2, by automated systems for processing the vertical radio sounding data of the ionosphere at the IZMIRAN station in 2018, during a period of low solar activity. It considers the results of the two most commonly used programs that scale observatory registrations of the ionosphere state, ARTIST 5, part of the DPS-4 ionosonde (DPS–ARTIST complex), and Autoscala 5.1, adapted for the Parus-A ionosonde (Parus–Autoscala complex). An independent system for analyzing ionograms from the Parus-A ionosonde by an operator is used as an etalon in comparing the results. It is shown that the foF2 estimates by both automated systems are sufficiently reliable with a standard deviation of 0.1 MHz. The errors in hmF2 estimation differ significantly and are characterized by a general negative bias, depending on the year’s season, and are most significantly expressed in the local noontime. The statistical values of the hmF2 deviations, in the common case, are significantly larger for the DPS–ARTIST system and are principally close to the results of the comparison with the Millstone Hill incoherent scatter radar data.

A new method is proposed for improving the IRI2016 vertical electron density profile by ingesting the in situ Ne values from Swarm A, B and C. The method is validated by comparing the improved electron density profiles with measured full height profiles over three Incoherent Scatter Radar stations in the American sector (Jicamarca, Millstone Hill and Sondrestrom) during the period 5–11 September 2017 covering both quiet and storm days. We considered 90 epochs corresponding to times when one of the Swarm satellites passed near an Incoherent Scatter Radar at a time when the Incoherent Scatter Radar was measuring electron density profiles. The method results in improvements of the electron density profile, as measured in terms of the Root-Mean-Square-Error in 63% of the observed epochs. The fraction of improvements on quiet days is 63% of the observed epochs, while 37% of the improvements occurred on storm days. The change in Root-Mean-Square-Error of the IRI profile compared to the ISR profiles before and after the modification was not dependent on the distance between the observation location and the ISR, nor on the difference between the hmF2 predicted by the IRI and measured by the ISR.

Millstone Hill incoherent scatter radar (ISR) noontime $h_{\mathrm {m}}\text{F}_{2}$ observations in the (2000–2016) period have been used to test the thermospheric parameters from ionosonde observations (THERION) method. The input parameters apart from the standard indices of solar F10.7 and geomagnetic $A_{\mathrm {p}}$ activity include noontime $f_{o}\text{F}_{2}$ and plasma frequency at 180-km height read from the $f_{p}(h)$ height profiles scaled from the ionograms with the Autoscala program. The THERION method provides a self-consistent set of the main aeronomic parameters responsible for the mid-latitude daytime ionospheric F-region formation, $h_{\mathrm {m}}\text{F}_{2}$ in particular. Overall, 60 dates under various solar and geomagnetic activity levels have been used for the analysis. The retrieved $h_{\mathrm {m}}\text{F}_{2}$ values demonstrate a standard deviation of 10.6 km, and this is close to the expected inaccuracy of $h_{\mathrm {m}}\text{F}_{2}$ determination with the Millstone Hill ISR. The correlation coefficient between the retrieved and the observed $h_{\mathrm {m}}\text{F}_{2}$ is 0.937 ± 0.051, which is significant at the confidence level >99.9%. The undertaken analysis has confirmed the earlier obtained conclusion on the possibility to extract the main aeronomic parameters from the routine ionosonde observations with the THERION method.

Based on the results of simultaneous ionosonde observations during low solar and weak magnetic activities, a coupling was found between diurnal and quasi-periodic variations in ionospheric parameters over magnetically conjugated regions, where the Ukrainian Antarctic Station (UAS) and Millstone Hill Observatory are located. A significant impact of the summer hemisphere on the nighttime variations of the F2 layer critical frequency foF2 in the magnetically conjugated region in the winter hemisphere was found. The most characteristic manifestation of this impact is the control of foF2 variations over the UAS not by the local sunset (sunrise), but by the sunset (sunrise) over Millstone Hill. It was found that the sunset over Millstone Hill leads to an increase in foF2 over the UAS, while the sunrise leads to a decrease in foF2 with a subsequent sharp increase. Both phenomena are associated with changes in the photoelectron flux from the northern hemisphere, corresponding changes in the electron temperature in the ionosphere above the UAS and the effect of these changes on the compression or rarefaction of the ionospheric plasma and changes in the plasmaspheric fluxes of H + ions. It was shown that the transition from nighttime to daytime conditions over both observation points was characterized by a significant decrease in the F2 layer peak height, and the difference in the values of this ionospheric parameter over Millstone Hill and UAS at night is due to seasonal differences in the thermospheric circulation and the difference in the behavior of the ionospheric parameters in the Northern and Southern hemispheres. Manifestations of atmospheric gravity waves, caused by the passage of local sunrise terminators, as traveling ionospheric disturbances with periods of about 90 and 75 – 120 mins over Millstone Hill and UAS, respectively, were found. These waves were most likely generated in the region located between the ionospheric F1 and F2 layers, where the sharp gradients in the electron and ion densities occur during changes in the intensity of solar radiation. It is confirmed that wave disturbances in atmospheric and ionospheric parameters can be transferred between magnetically conjugated regions by slow magnetohydrodynamic waves generated both at the heights of the ionospheric dynamo region due to the modulation of atmospheric and ionospheric parameters by atmospheric waves and the occurrence of external currents, and at the top of the plasmaspheric tube, where sharp plasma compression and heating or rarefaction and cooling occur during the passage of the solar terminator. Keywords: the ionosphere, F2 region, ionosonde measurements, geomagnetic field tube, magnetoconjugate region coupling, atmospheric gravity waves, traveling ionospheric disturbances, generation of slow magnetohydrodynamic waves

In this report, we have evaluated the performance of International Reference Ionosphere (IRI-2016) model in predicting the diurnal variation of F-region peak electron density height (hmF2) at Millstone Hill (MH, 42.6°N, 288.5°E) during the solar cycle 24 for both high and low-solar activity years. The new methodology is adopted to derive hmF2 values from the Incoherent Scatter Radar (ISR) measurements. The hmF2 derived from ISR along with ionosonde measurements operating at MH has been used to evaluate the performance of IRI-2016 model. The IRI-2016 model has three options to predict hmF2, namely, BSE-1979, AMTB-2013, and SHU-2015. The observed hmF2 from ISR and ionosonde are compared with all the three options provided by IRI-2016 to predict hmF2. Results show that hmF2 from IRI-2016, ISR and ionosonde exhibit similar diurnal variation. The deviations between measurements and model predictions were found to increase during nighttime. The SHU-2015 model emerged out to be the best in predicting the hmF2 values over MH as compared to other two options. The BSE-1979 option also provides reasonably better prediction as compared to the relatively latest AMTB-2013 options. Based on this statistical analyses, we recommend SHU-2015 option of IRI-2016 hmF2 model over MH area.

The behaviour of the ionospheric F2-layer during geomagnetic storm is studied using the critical frequency parameter foF2. The magnetic storms of 2nd October 2013 and 22nd June 2015 are considered. The variations of foF2 at Millstone Hill (42.6° N, 288.5° E), Ramey (18.5° N, 292.9° E), Boa Vista (2.8° N, 299.3° E) and Port Stanley (51.6° S, 302.1° E) along the longitude of about 300° E are analysed. The main observations of this study are (1) strong and short term positive storm effects are observed at the beginning of recovery phase for the moderate storm at all the stations and during the main phase for the intense storm at Millstone Hill and Port Stanley, (2) long term negative phase is observed at the northern hemispheric stations for the intense storm and (3) the southern winter hemisphere station Port Stanley recorded positive storm effects during the main and recovery phases of the intense storm on 22-06-2015. The observed deviations in foF2 are discussed in terms of Prompt Penetration Electric Fields, Disturbance Dynamo Electric Fields, Disturbance neutral winds and Travelling Atmospheric Disturbance.

This study presents a suggestion for improvement of the ion temperature (Ti) model in the International Reference Ionosphere (IRI). We have re-examined ion temperature data (primarily available from NASA’s Space Physics Data Facility (SPDF)from older satellites and combined them with newly available data from the Defense Meteorological Satellite Program (DMSP), the Communication Navigation Outage Forecasting System (C/NOFS), and from the recently launched Ionospheric Connection Explorer (ICON). We have compiled these data into a unified database comprising in total Ti data from 18 satellites. By comparisons with long term records of ion temperature from the three incoherent scatter radars (ISRs) (Jicamarca, Arecibo, and Millstone Hill), it was found that an intercalibration is needed to achieve consistency with the ISR data and among individual satellite data sets. This database with thus corrected data has been used for the development of a new global empirical model of Ti with inclusion of solar activity variation. This solar activity dependence is represented by an additive correction term to the Ti global pattern. Due to the limited data coverage at altitudes above 1000 km, the altitude range described by the model ranges from 350 km to 850 km covering only the region where generally Ti is higher than the neutral temperature (Tn) and lower than the electron temperature (Te). This approach is consistent with the current description of Ti in the IRI model. However, instead of one anchor point at 430 km altitude as in the current IRI, our approach includes anchor points at 350, 430, 600, and 850 km. At altitudes above 850 km Ti is merged using a gradient derived from the model at 600 and 850 km, with the electron temperature described by the IRI-2016/TBT-2012 option. Comparisons with the ISR data (Jicamarca, Arecibo, Millstone Hill, and Kharkiv) for high and low solar activity and equinox show that the proposed Ti model captures local time variation of Ti at different altitudes and latitudes better than the current IRI-2016 Ti model.

The ionosphere plays an important role in satellite navigation, radio communication, and space weather prediction. However, it is still a challenging mission to develop a model with high predictability that captures the horizontal-vertical features of ionospheric electrodynamics. In this study, multiple observations during 2005–2019 from space-borne global navigation satellite system (GNSS) radio occultation (RO) systems (COSMIC and FY-3C) and the Digisonde Global Ionosphere Radio Observatory are utilized to develop a completely global ionospheric three-dimensional electron density model based on an artificial neural network, namely ANN-TDD. The correlation coefficients of the predicted profiles all exceed 0.96 for the training, validation and test datasets, and the minimum root-mean-square error of the predicted residuals is 7.8 × 104 el/cm3. Under quiet space weather, the predicted accuracy of the ANN-TDD is 30%–60% higher than the IRI-2016 at the Millstone Hill and Jicamarca incoherent scatter radars. However, the ANN-TDD is less capable of predicting ionospheric dynamic evolution under severe geomagnetic storms compared to the IRI-2016 with the STORM option activated. Additionally, the ANN-TDD successfully reproduces the large-scale horizontal-vertical ionospheric electrodynamic features, including seasonal variation and hemispheric asymmetries. These features agree well with the structure revealed by the RO profiles derived from the FORMOSAT/COSMIC-2 mission. Furthermore, the ANN-TDD successfully captures the prominent regional ionospheric patterns, including the equatorial ionization anomaly, Weddell Sea anomaly and mid-latitude summer nighttime anomaly. The new model is expected to play an important role in the application of GNSS navigation and in the explanation of the physical mechanisms involved.

This work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019...

Published by AGU. Copyright (2020) American Geophysical Union. Longley, W. J., Erickson, P. J., Vierinen, J., Oppenheim, M. M., Lind, F. D., Dimant, Y. S. (2020), Millstone Hill ISR Measurements of Small Aspect Angle Spectra, Journal of Geophysical Research: Space Physics, volume 125, issue 6. https://doi.org/10.1029/2019JA027708 The Millstone Hill incoherent scatter (IS) radar is used to measure spectra close to perpendicular to the Earth's magnetic field, and the data are fit to three different forward models to estimate ionospheric temperatures. IS spectra measured close to perpendicular to the magnetic field are heavily influenced by Coulomb collisions, and the temperature estimates are sensitive to the collision operator used in the forward model. The standard theoretical model for IS radar spectra treats Coulomb collisions as a velocity independent Brownian motion process. This gives estimates of Te/Ti< 1 when fitting the measured spectra for aspect angles up to 3.6°, which is a physically unrealistic result. The numerical forward model from Milla and Kudeki (2011, https://doi.org/10.1109/TGRS.2010.2057253) incorporates single‐particle simulations of velocity‐dependent Coulomb collisions into a linear framework, and when applied to the Millstone data, it predicts the same Te/Ti ratios as the Brownian theory. The new approach is a nonlinear particle‐in‐cell (PIC) code that includes velocity‐dependent Coulomb collisions which produce significantly more collisional and nonlinear Landau damping of the measured ion‐acoustic wave than the other forward models. When applied to the radar data, the increased damping in the PIC simulations will result in more physically realistic estimates of Te/Ti. This new approach has the greatest impact for the largest measured ionospheric densities and the lowest radar frequencies. The new approach should enable IS radars to obtain accurate measurements of plasma temperatures at times and locations where they currently cannot.

The goal of this study is to present a new model for global topside electron temperature ( urn:x-wiley:jgra:media:jgra55532:jgra55532-math-0001) using a deep neural network (DNN) that is trained using measurements from incoherent scatter radars (ISRs). This study is also an investigation into whether this model can be used to generate the electron temperature in the topside ionosphere using GNSS ionospheric radio occultation (GNSS‐IRO) data as the input. ISR is one of the most reliable and long‐term sources to measure topside ionospheric electron density and plasma temperature information simultaneously. However, a drawback of ISR databases is the relatively poor spatial coverage due to the low number of ISR stations around the world. In contrast, GNSS‐IRO can be used to measure the global distributed electron density, but urn:x-wiley:jgra:media:jgra55532:jgra55532-math-0002 information is not directly detected. The relationship between the electron density and the electron temperature has been investigated by many researchers, but these studies have not explicitly considered the parameters that are known to influence the electron temperature, such as solar and geomagnetic activity level, and the features of electron density profile ( urn:x-wiley:jgra:media:jgra55532:jgra55532-math-0003, urn:x-wiley:jgra:media:jgra55532:jgra55532-math-0004, and scale height). This study uses a DNN technique to create a new global topside electron temperature model from three submodels that have been trained using data from three ISR stations: Arecibo (low latitude), Millstone Hill (midlatitude), and Poker Flat (high latitude). This global model is trained using electron density profile information (e.g., vertical scale height [VSH], urn:x-wiley:jgra:media:jgra55532:jgra55532-math-00052, and urn:x-wiley:jgra:media:jgra55532:jgra55532-math-00062) and solar and geomagnetic activity ( urn:x-wiley:jgra:media:jgra55532:jgra55532-math-0007 and urn:x-wiley:jgra:media:jgra55532:jgra55532-mat

We present the results of comparative study of traveling ionospheric disturbances (TIDs) obtained at middle latitudes of different longitudinal sectors during two coordinated observational campaigns. The joint measurements were conducted near the vernal equinox and summer solstice in 2016 using Kharkiv (49.6 N, 36.3 E) and Millstone Hill (42.6 N, 288.5 E) incoherent scatter radars. The same methods and software were used for analysis of both data sets to ensure consistency. We found that TIDs with periods of 40–80 min are observed during all measurements and concentrated predominantly near the sunrise and sunset terminators over both sites. There is no obvious relationship between the observed wave processes and variations in the auroral electrojet. Absolute and relative amplitudes, time of appearance, durations and phase differences of TIDs show strong height and seasonal variability. Relative amplitudes are substantially greater over Millstone Hill, whereas higher absolute amplitudes are observed over Kharkiv. During the summer solstice, the overall wave activity is smaller than during vernal equinox. Additional joint observations are needed to identify the seasonal and longitudinal dependences of TID characteristics.

Formation mechanism of the spring–autumn asymmetry of the F2-layer peak electron number density of the midlatitudinal ionosphere, NmF2, under daytime quiet geomagnetic conditions at low solar activity are studied. We used the ionospheric parameters measured by the ionosonde and incoherent scatter radar at Millstone Hill on March 3, 2007, March 29, 2007, September 12, 2007, and September 18, 1984. The altitudinal profiles of the electron density and temperature were calculated for the studied conditions using a one-dimensional, nonstationary, ionosphere–plasmasphere theoretical model for middle geomagnetic latitudes. The study has shown that there are two main factors contributing to the formation of the observed spring–autumn asymmetry of NmF2: first, the spring–autumn variations of the plasma drift along the geomagnetic field due to the corresponding variations in the components of the neutral wind velocity, and, second, the difference between the composition of the neutral atmosphere under the spring and autumn conditions at the same values of the universal time and the ionospheric F2-layer peak altitude. The seasonal variations of the rate of O+(4S) ion production, which are associated with chemical reactions with the participation of the electronically excited ions of atomic oxygen, does not significantly affect the studied NmF2 asymmetry. The difference in the degree of influence of O+(4S) ion reactions with vibrationally excited N2 and O2 on NmF2 under spring and autumn conditions does not significantly change the spring–autumn asymmetry of NmF2.

Ionospheric ionosonde and radar observations and theoretical calculations of the F2-layer peak altitude, hmF2, and number density, NmF2, over Millstone Hill during winter, spring, summer, and autumn geomagnetically quiet time periods at low solar activity are used to study the causes of the observed daytime NmF2 semi-annual anomaly. It follows from the model simulations that this anomalous phenomenon arises in the ionosphere mainly as a result of seasonal variations of the following atmospheric parameters: (1) the plasma drift along geomagnetic field lines due to corresponding changes in neutral wind components, (2) temperature and number densities of the neutral atmosphere, and (3) an optical thickness of the atmosphere caused by the dependence of the solar zenith angle on the day of the year for the same solar local time. Seasonal variations of the production rate unexcited O+ ions due to chemical reactions involving electronically excited O+ ions contribute to the formation of the NmF2 semi-annual anomaly during the predominant part of the existence time of this anomalous phenomenon. However, these seasonal variations are not significant, and this mechanism should be considered only as an additional source of the NmF2 semi-annual anomaly during its time of existence. The reactions of unexcited O+ ions with vibrationally excited N2 and O2 cause only weak changes of NmF2 and these changes are close in magnitude at a given solar local time during the winter, spring, summer, and autumn daytime conditions under consideration. Ignoring these reactions cannot produce a significant impact on the formation of the NmF2 semi-annual anomaly.

The global statistical median behavior of the electron temperature (Te) in the topside ionosphere was investigated through in-situ data collected by Langmuir Probes on-board the European Space Agency Swarm satellites constellation from the beginning of 2014 to the end of 2020. This is the first time that such an analysis, based on such a large time window, has been carried out globally, encompassing more than half a solar cycle, from the activity peak of 2014 to the minimum of 2020. The results show that Swarm data can help in understanding the main features of Te in the topside ionosphere in a way never achieved before. Te data measured by Swarm satellites were also compared to data modeled by the empirical climatological International Reference Ionosphere (IRI) model and data measured by Jicamarca (12.0°S, 76.8°W), Arecibo (18.2°N, 66.4°W), and Millstone Hill (42.6°N, 71.5°W) Incoherent Scatter Radars (ISRs). Moreover, the correction of Swarm Te data recently proposed by Lomidze was applied and evaluated. These analyses were performed for two main reasons: (1) to understand how the IRI model deviates from the measurements; and (2) to test the reliability of the Swarm dataset as a new possible dataset to be included in the underlying empirical dataset layer of the IRI model. The results show that the application of the Lomidze correction improved the agreement with ISR data above all at mid latitudes and during daytime, and it was effective in reducing the mismatch between Swarm and IRI Te values. This suggests that future developments of the IRI Te model should include the Swarm dataset with the Lomidze correction. However, the existence of a quasi-linear relation between measured and modeled Te values was well verified only below about 2200 K, while for higher values it was completely lost. This is an important result that IRI Te model developers should properly consider when using the Swarm dataset.

For the first time thermospheric parameters (neutral composition, exospheric temperature and vertical plasma drift related to thermospheric winds) have been inferred for ionospheric G-conditions observed with Millstone Hill ISR on 11–13 September 2005; 13 June 2005, and 15 July 2012. The earlier developed method to extract a consistent set of thermospheric parameters from ionospheric observations has been revised to solve the problem in question. In particular CHAMP/STAR and GOCE neutral gas density observations were included into the retrieval process. It was found that G-condition days were distinguished by enhanced exospheric temperature and decreased by ~2 times of the column atomic oxygen abundance in a comparison to quiet reference days, the molecular nitrogen column abundance being practically unchanged. The inferred upward plasma drift corresponds to strong ~90 m/s equatorward thermospheric wind presumably related to strong auroral heating on G-condition days.

The low latitude ionospheric data observed by digisonde at Hainan station (19.5°N, 109.1°E) in a whole solar activity cycle period from 2002 to 2012 within Ap

The F-region peak electron densities NmF2 measured during daytime quiet geomagnetic conditions at low solar activity on January 22, 2008, April 8, 1997, July 12, 1986, and October 26, 1995, are compared. Ionospheric parameters are measured by the ionosonde and incoherent scatter radar at Millstone Hill and calculated with the use of a 1D nonstationary ionosphere–plasmasphere model of number densities and temperatures of electrons and ions at middle geomagnetic latitudes. The formation of the semiannual anomaly of the midlatitudinal NmF2 under daytime quiet geomagnetic conditions at low solar activity is studied. The study shows that the semiannual NmF2 anomaly occurs due to the total impact of three main causes: seasonal variations in the velocity of plasma drift along the geomagnetic field due to the corresponding variations in the components of the neutral wind velocity; seasonal variations in the composition and temperature of the neutral atmosphere; and the dependence of the solar zenith angle on a number of the day in the year at the same solar local time.

We use incoherent scatter radar observations at Millstone Hill (MHO) and Arecibo (AO) and topside ionosphere in-situ DMSP observations during the great geomagnetic storm on 17-18 March 2015 to conduct a focused study on ion-neutral coupling and storm-time ionosphere and thermosphere dynamics. Some of these observations were made around the time of large ionospheric drifts within a Sub-Auroral Polarization Stream (SAPS). During the storm main phase, we identify multiple disturbance characteristics in the North American late afternoon and dusk sector. (1) Strong sub-auroral westward drifts occurred between 20-24 UT near MHO, accompanied by a storm enhanced density plume passage over MHO in the afternoon with a poleward/upward ion drift. The strongly westward flow reached 2000 m/s speed near the poleward plume edge. (2) Prompt penetration electric field signatures, appearing as poleward/upward ion drifts on the dayside over both MHO and AO, were consistent with DMSP vertical drift data, and contributed to plume development. (3) Meridional wind equatorward surges occurred during daytime hours at MHO, followed by 2-3 hr period oscillations at both MHO and AO. The zonal electric field at AO was strongly correlated with the wind oscillation. (4) Large ion temperature enhancements as well as 50+ m/s upward ion drifts throughout the E and F regions were observed during the SAPS period. These were presumably caused by strong frictional heating due to large plasma drifts. The heating effects appeared to drive significant atmospheric upwelling, and corresponding ion upflow was also observed briefly. This study highlights some of the important effects of fast plasma transport as well as other disturbance dynamics on ion-neutral coupling during a single intensification period within a great geomagnetic storm.

The changes in the ionosphere during geomagnetic disturbances is one of the prominent Space Weather effects on the near-Earth environment. The character of these changes can differ significantly at different regions on the Earth. We studied ionospheric response to five geomagnetic storms of March 2012, using data of Total Electron Content (TEC) and F2-layer critical frequency (foF2) along the meridian of 70&deg, W in the Northern Hemisphere. There are few ionosondes along this longitudinal sector: in Thule, Sondrestrom, Millstone Hill and Puerto Rico. The lacking foF2 values between the ionosondes were determined by using the experimental latitudinal dependences of the equivalent ionospheric slab thickness and TEC values. During geomagnetic storms, the following features were characteristic: (a) two-hours (or longer in one case) delay of the ionospheric response to disturbances, (b) the more prominent mid-latitude trough and (c) the sharper border of the EIA northern crest. During four storms of 7&ndash, 17 March, the general tendency was the transition from negative disturbances at high latitudes to intense positive disturbances at low latitudes. During the fifth storm, the negative ionospheric disturbance controlled by O/N2 change was masked by the overall prolonged electron density increase during 21&ndash, 31 March. The multiple correlation analysis revealed the latitudinal dependence of dominant Space Weather parameters&rsquo, impacts on foF2.

Although the horizontal density structures of the polar ionosphere have been extensively studied mostly using the F-region peak density or total electron content, there are relatively few studies on the vertical density structures. In this review, we present the climatology of the polar ionospheric density not only in the F-region but also in the E-region and topside ionosphere, in comparison with the mid-latitude ionosphere, using long-term incoherent scatter radar (ISR) observations at Millstone Hill, Tromso, and Svalbard. The ISR data during the period of 1995–2015 are analyzed to study on the variations with local time, season, and solar/geomagnetic activity. The diurnal variations of the F-region density are much smaller in the polar region than in the mid-latitude, particularly in summer. At Svalbard, there is a characteristic double-peak structure in the diurnal variation of the polar ionosphere in winter only for high solar activity. The diurnal variation of hmF2 decreases with increasing latitude and eventually disappears at Svalbard for low solar activity but the hmF2 and its diurnal variations in the polar ionosphere are remarkably enhanced for high solar activity. The distinctive irregularity in the mid-latitude F1-layer nearly disappears in the polar region, especially at Svalbard. The anomalous seasonal variations of the F-region density are less evident in the polar ionosphere especially for low solar activity and for high magnetic activity conditions. The polar E-region density shows characteristic nighttime peaks induced by auroral precipitation but it does not necessarily increase with solar activity. The topside ionospheric density variations are much stronger in the polar region for high solar activity. Finally, it is found that the polar ionospheric density profiles more strongly respond to increasing solar activity as well as the magnetic activity compared with the mid-latitude ionosphere.

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.

A dipolarization event was observed by the Magnetospheric Multiscale (MMS) spacecraft at L=3.8 and 19.8 magnetic local time (MLT) starting at ∼ 23:42:36 UT on 7 October 2015. The magnetic and electric fields showed initially coherent variations between the spacecraft. The sunward convection turned tailward after the dipolarization. The observation is interpreted in terms of the pressure balance or the momentum equation. This was followed by a region traversed where the fields were irregular. The scale length was of the order of the ion gyroradius, suggesting the kinetic nature of the fluctuations. Combination of the multi-instrument, multi-spacecraft data reveals a more detailed picture of the dipolarization event in the inner magnetosphere. Conjunction ionosphere-plasmasphere observations from DMSP, two-dimensional GPS TEC, the Millstone Hill mid-latitude incoherent scatter radar, and AMPERE measurements imply that MMS observations are located on the poleward edge of the ionospheric trough where Region 2 field aligned currents flow.

It is now recognized that Earth's upper atmosphere is experiencing a long-term cooling over the past several solar cycles. The potential impact of the cooling on societal activities is significant, but a fundamental scientific question exists regarding the drivers of the cooling. New observations and analyses provide crucial advances in our knowledge of these important processes. We investigate ionospheric ion temperature climatology and long-term trends using up-to-date large and consistent ground based datasets as measured by multiple incoherent scatter radars (ISRs). The very comprehensive view provided by these unique observations of the upper atmospheric thermal status allows us to address drivers of strong cooling previously observed by ISRs. We use observations from two high latitude sites at Sondrestrom (Invariant latitude 73.2°N) from 1990-2015, and Chatanika/Poker Flat (Invariant latitude 65.9°N) over the span of 1976-2015 (with a gap from 1983-2006). Results are compared to conditions at the mid-latitude Millstone Hill site (Invariant latitude 52.8°N) from 1968-2015. The aggregate radar observations have very comparable and consistent altitude dependence of long-term trends. In particular, the lower F region (< 275 km) exhibits dayside cooling trends that are significantly higher (-3 to -1K/year at 250 km) than anticipated from model predictions given the anthropogenic increase of greenhouse gases. Above 275 km, cooling trends continue to increase in magnitude but values are strongly dependent on magnetic latitude, suggesting the presence of significant downward influences from non-neutral atmospheric processes.

We present detailed observations of dynamic, fine-scale inner magnetosphere-ionosphere coupling at ∼3.9 RE in the Region 2 Birkeland field-aligned current (FAC). We find that observed electrodynamic spatial/temporal scales are primarily characteristic of magnetically mapped ionospheric structure. On 15 September 2015, conjugate Magnetospheric Multiscale (MMS) spacecraft and Millstone Hill radar observations show plasmasphere boundary region subauroral polarization stream (SAPS) electric fields at L = 4.0–4.2 near 21 MLT. MMS observations reveal high-altitude ∼1 mV/m fine-scale radial and azimuthal electric field perturbations over ≤0.15 L with high spatial coherence over ≥2–3 min and show outward motion within a broader FAC of ∼0.12 μA/m2. Our analysis shows that MMS electric field fluctuations are most likely reflective of SAPS ionospheric structure at scales of ∼22 km and with ionospheric closure of small-scale filamentary FAC perturbations. The results highlight the ionosphere's importance in regulating fine-scale magnetosphere-ionosphere structure.

Recent studies reveal that midlatitude ionospheric irregularities are less understood due to lack of models and observations that can explain the characteristics of the observed wave structures. In this paper, the cascading processes of both the temperature gradient instability (TGI) and the gradient drift instability (GDI) are investigated as the cause of these irregularities. Based on observations obtained during a coordinated experiment between the Millstone Hill incoherent scatter radar and the Blackstone Super Dual Auroral Radar Network radar, a time series for the growth rate of both TGI and GDI is calculated for observations in the subauroral ionosphere under both quiet and disturbed geomagnetic conditions. Recorded GPS scintillation data are analyzed to monitor the amplitude scintillations and to obtain the spectral characteristics of irregularities producing ionospheric scintillations. Spatial power spectra of the density fluctuations associated with the TGI from nonlinear plasma simulations are compared with both the GPS scintillation spectral characteristics and previous in situ satellite spectral measurements. The spectral comparisons suggest that initially, TGI or/and GDI irregularities are generated at large-scale size (kilometer scale), and the dissipation of the energy associated with these irregularities occurs by generating smaller and smaller (decameter scale) irregularities. The alignment between experimental, theoretical, and computational results of this study suggests that in spite of expectations from linear growth rate calculations, cascading processes involving TGI and GDI are likely responsible for the midlatitude ionospheric irregularities associated with GPS scintillations during disturbed times.

Since 1987, an all-sky airglow imaging system has operated from a site at the Millstone Hill/Haystack Observatory in Westford, MA. During the ~2.5 solar cycles from 1987 to 2014, many studies using all-sky images, in conjunction with incoherent scatter radar and satellite data, described subauroral, ionospheric disturbances observed during individual geomagnetic storms. The most prominent storm time optical feature from a subauroral site is a stable auroral red (SAR) arc. The standard use of a SAR arc's position is to locate the ionospheric footprint of the narrow plasmapause-ring current interaction region where heat conduction from the inner magnetosphere excites emission within the F layer trough. When mapped from an emission altitude of 400 km to the geomagnetic equatorial plane, SAR arcs from Millstone Hill give the location of the plasmapause at radial distances between 2 to 4.5 Earth radii. A total of 314 SAR arcs have been observed during the 27 years of imaging at Millstone Hill. We find no single morphology for all SAR arcs, but rather patterns that occasionally depart from stability in space and time. We have classified these into five categories: longevity, multiplicity, zonal structure, latitudinal inhomogeneity, and tilt with respect to geomagnetic coordinates. In each case, the implications for the inner magnetosphere sources that drive SAR arcs are explored. While individual SAR arc variability characteristics have been noted in previous studies, here we describe for the first time all five types from the same site—an aspect not yet addressed in either magnetosphere or ionosphere modeling studies.

Ionospheric F2 region peak densities (NmF2) are expected to have a positive correlation with total electron content (TEC), and electron densities usually show an anticorrelation with electron temperatures near the ionospheric F2 peak. However, during the 17 March 2015 great storm, the observed TEC, NmF2, and electron temperatures of the storm-enhanced density (SED) over Millstone Hill (42.6°N, 71.5°W, 72° dip angle) show a quiet different picture. Compared with the quiet time ionosphere, TEC, the F2 region electron density peak height (hmF2), and electron temperatures above ~220 km increased, but NmF2 decreased significantly within the SED. This SED occurred where there was a negative ionospheric storm effect near the F2 peak and below it, but a positive storm effect in the topside ionosphere. Thus, this SED event was a SED in TEC but not in NmF2. The very low ionospheric densities below the F2 peak resulted in a much reduced downward heat conduction for the electrons, trapping the heat in the topside in the presence of heat source above. This, in turn, increased the topside scale height so that even though electron densities at the F2 peak were depleted, TEC increased in the SED. The depletion in NmF2 was probably caused by an increase in the density of the molecular neutrals, resulting in enhanced recombination. In addition, the storm time topside ionospheric electron density profiles were much closer to diffusive equilibrium than the nonstorm time profiles, indicating less daytime plasma flow between the ionosphere and the plasmasphere.

The lack of reliable global E-region electron density or conductivity measurements has hampered a detailed and quantitative understanding of the E-region electrodynamic processes. The characterization of the global E-region ionospheric irregularities as a function of local time, longitude, and season remains a great challenge. In this study, we evaluate the electron densities retrieved by Wu (2018) in the D and E regions using ground-based radar data and empirical models. Unlike in the conventional top-down retrieval methodology, the new approach avoids the Abel weighting function that carries substantial sensitivity to the electron density residuals from the F-region even after they have been corrected. The new technique uses a bottom-up approach in which the F-region contributions in the excess phase are more effectively removed compared to the Abel inversion, and hence more accurate electron density retrievals in D and E regions are determined. The results are compared and evaluated with two low and mid-latitude incoherent scatter radar observations - the Arecibo Observatory’s 430 MHz radar, located in Puerto Rico (18 ∘ N, 67 ∘ W) and the Millstone Hill Observatory’s 440 MHz radar, located in Massachusetts (42 ∘ N, 72 ∘ W). In addition, the results are also compared with two models - the International Reference Ionosphere model (IRI-2016) and the Faraday International Reference Ionosphere model (FIRI-2018) at these two locations. The comparison of electron density height profiles from GPS-RO retrievals at the two locations show good agreement with radar measurements, as well as the FIRI-2018 model predictions, especially at ∼ 85 - 100 km. The seasonal and diurnal climatology comparisons of GPS-RO retrieved electron density show good agreement with radar observations at both locations, and also reflect the influence of the solar cycle on E-region electron density. It is noticed that the GPS-RO background electron density at Millstone Hill increases by ∼ 20% at around 105 km from solar minimum to the maximum during cycle 24 which is in agreement with radar observations and FIRI-2018 model predictions.

The total electron content TEC finds wide application in various scientific and technological areas. One of applications is the use of TEC to obtain critical frequencies foF2 of the ionosphere. The most studied is the situation in middle latitudes. High and low latitudes are considered as problematic zones. In the present work, the situation in high and low latitudes is investigated, including at comparison with results in middle latitudes. Possibility of the TEC use to obtain foF2 is connected with their high correlation however in details this correlation was not investigated. In the present work, on an example of three stations of the American zone (Thule, Millstone Hill, Puerto Rico), latitudinal section of the northern hemisphere on a meridian 75°W it is shown, that: 1) correlation coefficients p(TEC, foF2) between foF2 and TEC lay in a range 0.7-1.0 in a seasonal course and 0.6-0.85 in a daily course with the least values for the station Thule. 2. The most clear seasonal dependence is related to that in the winter (January, December) correlation is worse, in an equinox (March, September) is better. 3. There is a tendency to higher correlation and smaller scatter of values in years of high solar activity, than in low activity. 4. Correlation coefficients ρ(δTEC, δfoF2) of deviations between δTEC and δfoF2 lay in the same range, as coefficients for magnitudes, and not strongly worsen during the disturbed periods, the least values here are at the station Puerto Rico. Estimation of a dependence ρ(δTEC, δfoF2) from indexes Kp and Dst and its approximations by a polynomial with degree n has shown, that: 1) degree of polynomial approximation and the best index of correlation depend on latitude (for high-latitude stations the greatest correlation is observed with index Kp, for middle - and low-latitude stations correlation with Dst can prevail). 2. Degree and the best index of correlation depend on a season (the greatest correlation is observed in winter months for high-latitude stations and in the summer for low-latitude stations). 3. Degree n=3 is insufficient for approximation, often degree of reliability R^2 for n=4-6 exceeds reliability for n=3 twice. 4. Reliability degree can exceed the level 0.5 testifying strong dependence and possibility of the forecast of parameters. As a whole, differences between high and low latitudes consist in a difference of coefficients of correlation, similarities - in possibility of the TEC use to obtain foF2 practically at all latitudes, i.e., details of behavior of TEC can differ, but the main result is identical: the use of TEC allows to improve conformity of the calculated values of foF2 to experimental data in 1.5-2.5 times. Residual deviations do not exceed 10 %.

Thermospheric neutral winds at Kiruna, Sweden (67.4°N, 20.4°E) are compared using both direct optical Fabry-Perot Interferometer (FPI) measurements and those derived from European incoherent scatter radar (EISCAT) measurements. This combination of experimental data sets, both covering well over a solar cycle of data, allows for a unique comparison of the thermospheric meridional component of the neutral wind as observed by different experimental techniques. Uniquely in this study the EISCAT measurements are used to provide winds for comparison using two separate techniques: the most popular method based on the work of Salah and Holt (1974) and the Meridional Wind Model (MWM) (Miller et al., 1997) application of servo theory. The balance of forces at this location that produces the observed diurnal pattern are investigated using output from the Coupled Thermosphere and Ionosphere (CTIM) numerical model. Along with detailed comparisons from short periods the climatological behaviour of the winds have been investigated for seasonal and solar cycle dependence using the experimental techniques. While there are features which are consistent between the 3 techniques, such as the evidence of the equinoctial asymmetry, there are also significant differences between the techniques both in terms of trends and absolute values. It is clear from this and previous studies that the high-latitude representation of the thermospheric neutral winds from the empirical Horizontal Wind Model (HWM), though improved from earlier versions, lacks accuracy in many conditions. The relative merits of each technique are discussed and while none of the techniques provides the perfect data set to address model performance at high-latitude, one or more needs to be included in future HWM reformulations. Key words. Meteorology and atmospheric dynamics (thermospheric dynamics), Ionosphere (ionosphere-atmosphere interactions, auroral ionosphere)

A multi-day (6–8 May 2013), apparently medium-scale traveling ionospheric disturbance (MSTID) event with an ∼1 hour period was recorded using both the vertical-looking and steerable Millstone Hill incoherent scatter radar (ISR) systems during quiet geomagnetic activity. The dense network of Global Positioning System (GPS) total electron content (TEC) receivers has allowed the horizontal imaging of this event across a far larger scale than is possible with the ISR(s) alone. This MSTID event was identified in the GPS-TEC dataset on the mesoscale centered at Haystack. However, GPS-TEC imaging also revealed that this event was coherent across the United States. The GPS-TEC imaging also revealed that similar events occurred globally. We conclude that this MSTID event was actually a coherent ionospheric pulsing structure (CIPS) that, when viewed locally, masqueraded as an MSTID event. The GPS-TEC images reveal that, on a global scale, the CIPS were stationary in longitude, but moved southward in the Northern Hemisphere—imaging of the southern hemisphere was not possible. The CIPS originated in high latitudes and at one instance were coherent across 23,000 km. The large-scale coherence, ubiquity, and source location of the CIPS points to an unidentified auroral zone source with sufficient energy to continually force the observed MSTID-like structures.

Knowledge of the thermospheric neutral wind and its horizontal components is critical for an improved understanding of F region dynamics and morphology. However, to date their reliable estimation remains a challenge because of difficulties in both measurement and modeling. We present a new method to estimate the climatology of the zonal and meridional components of thermospheric neutral wind at low and middle latitudes using a Kalman filter technique. First, the climatology of the magnetic meridional wind is obtained by assimilating seasonal maps of F region ionosphere peak parameters (NmF2 and hmF2), obtained from Constellation Observing System for Meteorology, Ionosphere, and Climate radio occultation data, into the Global Assimilation of Ionospheric Measurements Full Physics (GAIM-FP) model. GAIM-FP provides the 3-D electron density throughout the ionosphere, together with the magnetic meridional wind. Next, the global zonal and meridional wind components are estimated using a newly developed Thermospheric Wind Assimilation Model (TWAM). TWAM combines magnetic meridional wind data obtained from GAIM-FP with a physics-based 3-D thermospheric neutral wind model using an implicit Kalman filter technique. Ionospheric drag and ion diffusion velocities, needed for the wind calculation, are also taken from GAIM-FP. The obtained wind velocities are in close agreement with measurements made by interferometers and with wind values from the Horizontal Wind Model 93 (HWM93) over Millstone Hill, Arecibo, and Arequipa during December and June solstices, and March equinox. In addition, it is shown that compared to HWM93 the winds from TWAM significantly improve the accuracy of the Ionosphere/Plasmasphere Model in reproducing the observed electron density variation over the Weddell Sea Anomaly.

An 11year (1989-1995 and 2010-2013) nighttime thermospheric wind (at similar to 250km) data set from Fabry-Perot interferometer at Millstone Hill (42.6 degrees N, 71.5 degrees W) is used to investigate multiday oscillations (6-30days) in the thermospheric nighttime winds. Dependencies of multiday oscillations on solar radiation (F-10.7), solar wind speed (SW), and geomagnetic activity (Kp) are determined. We found that (1) there exist prominent quasi 27day oscillations in the thermospheric zonal (meridional) wind, which appears to be more correlated to the same periodic oscillations in Kp and SW than in F-10.7 during the solar maximum (solar declining phase and minimum); (2) quasi 13.5day oscillations in the zonal wind occur during the solar maximum and increasing phases and are also more correlated to the same periodic oscillations in Kp and SW; and (3) in general, the correlations of the multiday oscillations in the thermospheric winds are more correlated with Kp and SW than F-10.7. Our analyses also show that the zonal wind is more sensitive to SW than the meridional wind. We further find that (4) for high F-10.7, the premidnight zonal wind is more westward toward increasing SW; for low F-10.7, the postmidnight zonal wind is also more westward toward increasing SW; and (5) meridional winds tend to be more southward with increasing SW for medium and high F-10.7.

We use a continuous 30 day incoherent scatter radar experiment at Millstone Hill in October 2002 to examine day-to-day thermospheric variability in exospheric temperature Tex. Solar flux and magnetic activity influences as the main driving factors for day-to-day variability are investigated quantitatively. Solar ultraviolet flux levels are based on the TIMED/SEE space weather product, allowing for analysis of ultraviolet flux-Tex correlation. Tex is most sensitive to solar EUV flux with approximately a 2 day delay at wavelengths of 27–34 nm (including 30.4 nm). In particularly, a 20–60 h time delay occurs in Tex response to EUV flux at 27–34 nm band, with shorter delays in the morning and longer delays in the afternoon and at night. The 1 ∼ 2 day delayed Tex response to solar ultraviolet flux and associated thermospheric solar preconditioning (“memory”) are most significant in the daily mean for the 27–34 nm band, in the diurnal and semidiurnal amplitudes for the soft X-ray flux at 0.1–7 nm, and in the diurnal amplitude for longer wavelengths. An empirical model driven only by EUV flux at 27–34 nm from 2 days in advance reproduces 90% of the observed variability in the Tex daily mean. With a 2 day time delay, solar X-ray flux at 0.1–7 nm is correlated positively with Tex diurnal amplitude and negatively with Tex semidiurnal amplitude. Finally, magnetic activity control, as represented by the Dst index, is weaker during the day and stronger at night and is important for the semidiurnal amplitude but not important for the daily mean.

Incoherent scatter radar measurements of ionospheric temperature and density collected above Millstone Hill over the years 1976–2013 are analyzed to show the long-term trends in noontime neutral temperature and neutral O density over the height region 120–500 km. Exospheric temperature cooled by 69.3 ± 6.4 K over the period, an order of magnitude greater than that expected from greenhouse gas action. The O density dropped 0.081 ± 5.6% at 400 km altitude but rose by 36.9 ± 5.0% at 120 km over this period. This trend in density at 400 km agrees with that determined from satellite drag. The increase in density at 120 km counteracts the thermal contraction of the thermosphere expected to be associated with the cooling, resulting in only a small density response at 400 km. The long-term O density increase at 120 km may be caused by a long-term descent of the turbopause height of 4.2 km. Such a descent has been documented by a series of rocket mass spectrometer measurements.

Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry-Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all-sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low-energy O+ ions from the magnetosphere. The presence of low-energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low-energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

The diurnal variation of the mid-latitude upper thermosphere zonal winds during equinoxes has been studied using data recently generated from CHAMP measurements from 2002 to 2004 using an iterative algorithm. The wind data was separated into two geomagnetic activity levels, representing high geomagnetic activity level (Ap > 8) and low geomagnetic activity level (Ap ⩽ 8). The data were further separated into two solar flux levels; with F10.7 > 140 for high and F10.7 ⩽ 140 for low. Geomagnetic activity is a correlator just as significant as solar activity. The response of mid-latitude thermospheric zonal winds to increases in geomagnetic disturbances and solar flux is evident. With increase in geomagnetic activity, midday to midnight winds are generally less eastward and generally more westward after the about midnight transitions. The results show that east west transitions generally occurred about midnight hours for all the situations analyzed. The west to east transition occurs from 1400–1500 MLT. Enhanced westward averaged zonal wind speeds going above 150 ms −1 are observed in the north hemisphere mid-latitude about sunrise hours (∼0700–1100 MLT). Nighttime winds in the north hemisphere are in good agreement with previous single station ground observations over Millstone Hill. Improved ground observations and multi satellite observations from space will greatly improve temporal coverage of the Earth’s thermosphere.

O+ field-aligned ambipolar diffusive velocities V-d and fluxes Phi(d) in the topside ionosphere have been calculated from the observed profiles of electron density, ion, and electron temperatures during a 30 day incoherent scatter radar experiment conducted at Millstone Hill (288.5 degrees E, 42.6 degrees N) from 4 October to 4 November 2002. Two geomagnetic storms took place during this period. During the negative phases (depleted electron densities) of these two storms, the magnitudes of the daytime upward V-d and Phi(d) were less than their averaged quiet time values. Whereas at nighttime, the downward V-d and Phi(d) were sometimes larger than the averaged quiet time values. The variations in diffusive velocity and flux during the storm main and recovery phases were caused by changes in the ionospheric scale height or the shapes of ionospheric density profiles. The negative storm effect further reduced daytime diffusive flux. During these two storms, positive ionosphere phases (enhanced electron densities) were also observed. The diffusive velocity was much smaller during the period of positive storm effect, which led to a smaller diffusive flux than the quiet time one, although electron density was higher. It appears that storm time variations in diffusive velocity were more the results of storm time changes in the plasma vertical profile, rather than the cause of these plasma density changes.

Three-year (2010-2013) observations of thermospheric winds (at similar to 250 km) by Fabry-Perot interferometers at Xinglong (XL, 40.2 degrees N, 117.4 degrees E) and Millstone Hill (MH, 42.6 degrees N, 71.5 degrees W) are used to study the climatology of atmospheric planetary wave-type oscillations (PWTOs) with periods of 4-19 days. We find that (1) these PWTOs occur more frequently in the months from May to October. They are consistent with the summertime preference of middle-latitude ionospheric electron density oscillations noted in other studies. (2) The month-to-month variations in PWTOs show phase changes between MH and XL, switching from antiphase to in phase when PWTO periods vary from short to long. (3) Typical PWTOs show annual and semiannual variations. The relative intensity of annual over semiannual components for PWTOs is different between XL and MH. (4) Magnetic storms and substorms have little influences on the annual and semiannual variations of the typical PWTO amplitudes. (5) Meridional wind PWTOs with typical periodicity bands around 5, 8, and 16 days appear to be correlated to both solar wind speed and K-p oscillations, suggesting a possible influence of the solar wind corotating interaction regions on neutral wind dynamics.

The first midlatitude conjugate thermospheric wind observations in the American sector showed various degrees of conjugacy between Palmer (64°S, 64°W, magnetic latitude (MLAT) 50°S) and Millstone Hill (42.82°N, 71.5°W, MLAT 53°N) under three different geomagnetic conditions (recovery after a substorm, moderately active, and quiet). The agreement with the National Center for Atmospheric Research's Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations also varies with the geomagnetic activity level. During substorm recovery, the observations at Palmer (PA) and Millstone Hill (MH) both showed strong westward zonal winds, which the standard TIEGCM greatly underestimated. Inadequate ion convection pattern size and lack of effect from Subauroral Polarization Streams (SAPS) may be the cause of the large discrepancy. The TIEGCM with a SAPS model produced stronger westward zonal winds near PA but did not change the zonal wind near MH. The empirical SAPS model needs further refinements. In general, there is better conjugacy with moderate geomagnetic activity levels. The TIEGCM also agrees better with the observations. Under geomagnetically quiet conditions, the meridional winds appear to be less conjugate. The agreement between the observations and model is reasonable. Optical conjugate observations are severely limited by the seasons and weather conditions in the two hemispheres. Yet they are necessary to understanding the thermospheric dynamics in the subauroral region and its relationship with geomagnetic activity levels. The comparisons with TIEGCM are necessary for future model improvements.

Solar flares are explosive events on the Sun that release energetic particles, X-rays, EUV, and radio emissions that have an almost immediate impact on Earth's ionosphere-thermosphere (IT) system and/or on operational systems that are affected by IT conditions. To assess such impacts, it is a key that we know how the ionosphere is modified. An objective of this paper is to evaluate how digisondes might serve in this role. Toward this end we utilize data from the Millstone Hill digisonde to reveal the height versus time bottomside F region responses to three X-class flares (X28, X8.3, and X1.7) at a middle latitude site. In terms of percent increase with respect to a preflare hourly mean, the long-lived (> 15–30 min) responses to these flares maximize between about 150 and 250 km and measurably last ~0.75–1.5 h after flare maximum. The relative magnitudes of these responses are complicated by flare position on the solar disk, which determines how much of the EUV solar emissions are attenuated by the solar atmosphere. At Millstone Hill there was little measurable response to these flares near the F2 layer peak; however, at the magnetic equator location of Jicamarca, the F2 peak electron density increased by ~15–40%. Herein, all of these flare response characteristics are interpreted in terms of available modeling results. We propose that such digisonde data, in combination with first-principles models and high-resolution measurements of solar EUV flux emissions (e.g., from Solar Dynamics Observatory/EUV Variability Experiment), can lead us to a deeper understanding of the ionospheric photochemistry and dynamics that underlies a predictive capability.

The seasonal variation of F region neutral wind from the midlatitude conjugate Fabry-Perot interferometer observations has been studied. The meridional wind at Palmer station (64°S,64°W) has a significant local time dependence with strong equatorward wind at midnight and polarward wind at dawn and dusk. The zonal wind switches from eastward to westward in the early morning section. From the June solstice (austral winter) to equinox, the maximum meridional wind increases from 90 m/s to 130 m/s, and the zonal wind switches direction at an earlier local time. The neutral winds from Palmer have been compared with those from the geomagnetic conjugate location, Millstone Hill (MH). At equinox, the local time variation of neutral wind shows a very good conjugacy between these two locations. But at June solstice, the similarity in the zonal wind becomes less clear. This seasonal dependence can be attributed to the seasonal variation of solar and geomagnetic forcings. The annual variation of daily average neutral wind from Palmer and MH has also been compared. The meridional wind shows a clear offset of season, and the magnitude at Palmer is averagely 40 m/s more equatorward than that at MH. The zonal wind is dominantly westward at Palmer and eastward at MH. The annual variation of neutral wind, especially the zonal component, is much less symmetric between the two sites than the local time variation. The empirical horizontal wind model shows a good agreement with the observations in both local time and annual variations.