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The meteor wind radar at Durham and the incoherent scatter radar at Millstone Hill, which are closely located at 43°N, 71°W, were used simultaneously during the first campaign of the Lower Thermosphere Coupling Study in September 1987 to map the structure of the semidiurnal tidal wind from 80 km to 135 km. The results from the two techniques are consistent and indicate overall continuity in the observed propagation of the semidiurnal tide in both amplitude and phase. Northward winds gradually increase from about 30 m/s at 80 km to 60 m/s above 110 km, while the eastward winds are near 20 m/s below 100 km and reach a maximum of 70 m/s at 115 km. Phase variations indicate a vertical wavelength of the propagating tidal mode of about 100 km at all altitudes for the eastward component, but the incoherent scatter results suggest a shorter wavelength for the northward component above 100 km. The results obtained during the campaign are similar to climatological averages obtained from nonsimultaneous data sets from the two radars. Comparison with numerical model results from Forbes and Vial (this issue) indicates that the model underestimates the wind amplitudes and reaches the maximum value about 10 km below the observed peak; the phase variations lag the observations by 2 hours but indicate an overall structure with altitude that is very similar to the data.

Ion-neutral collision frequencies determined from measurements obtained by the incoherent scatter radar located at Soendre Stroemfjord, Greenland, have been used to derive lower thermospheric neutral densities during the first Lower Thermosphere Coupling Study (LTCS 1), September 21-26, 1987. Periods of Joule and particle heating which might disturb the E region thermal equilibrium were systematically eliminated. The mean profile of neutral density for the period is in good agreement with the mass spectrometer incoherent scatter 1986 (MSIS-86) model between 92 and 104 km. A tendency to overestimate collision frequencies above 105 km may arise from range-smearing effects. The results of a tidal analysis performed on the neutral density between 92 and 109 km show that the amplitudes of the diurnal and semidiurnal components of the tides are approximately equivalent. The observations are generally in better agreement with the MSIS-86 predictions than with the thermosphere-ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval. The observed phase of the diurnal component is approximately constant with height above 98 km and is in close agreement with the MSIS-86 model phases; however, the TIGCM diurnal phases are shifted by 6-8 hours to later local times. The phase of the semidiurnal tide is inmore » good agreement with predictions of the MSIS-86 model and the TIGCM simulation of this interval, except near 98 km. The observed semidiurnal phase is also consistent with previous high-latitude results (Kirkwood, 1986). The relative amplitude of the observed semidiurnal oscillation is up to 15% larger than that previously observed at the European Incoherent Scatter facility but is consistent with the amplitudes presented in an earlier study of Millstone Hill measurements (Salah, 1974).« less

Theoretical predictions of global monthly and seasonally averaged semidiurnal temperatures and winds between 80 and 150 km are presented and compared with seasonal means from measurements at the Arecibo, Chatanika, Millstone Hill, and Sondrestrom incoherent scatter radars. The model values above 100 km are obtained by utilizing a set of tabulated ‘Hough mode extension’ functions to extrapolate the (2,2), (2,3), (2,4), and (2,5) propagating tidal modes at 100 km from a spectral model (not valid in the E region due to the way molecular dissipation is parameterized) to higher altitudes and by superimposing a component driven by in-situ excitation (important mainly above 140 km). The monthly simulations demonstrate the close coupling that exists between seasonal and latitudinal structures and argue for an observational program emphasizing adequate seasonal and latitudinal coverage. Despite constraints imposed by the distributions of existing data with respect to month and year, comparisons with the model reveal several trends that appear to be real: (1) the model amplitudes are usually 20–50% smaller than those observed; (2) vertical wavelengths are well-predicted by the model, but model phases generally lead the observations at middle and low latitudes by approximately 2–3 hours at all heights for both winds and temperatures; (3) semidiurnal temperatures at middle and low latitudes, in both the observations and model, indicate significant seasonal changes in the phase of the temperature oscillation, but not in the winds; and (4) the seasonal trends in the model and observations are consistent, with “summer leading winter” at Millstone Hill, while the reverse is true at Arecibo. The sources of the absolute amplitude and phase discrepancies in the model, which are probably connected with the adopted mean wind and thermal excitation specifications, need to be investigated. In addition, a concerted effort must be made within the Coupling Energetics and Dynamics of Atmospheric Regions (CEDAR) program to better delineate the seasonal-latitudinal thermal structure and dynamics of the E region, following the example of the extensive F region observational data base which has been accumulated over the last decade.

During the first Lower Thermosphere Coupling Study (LTCS), September 21-25 1987, data were recorded from the incoherent scatter radar sites at EISCAT, Millstone Hill, Sondrestrom, and Arecibo. These experimental facilities measured ionospheric parameters (Ne, Te, Ti, and plasma velocity) in the E and the F regions which have been used to determine the E region neutral wind and infer the neutral temperature in the height range 100-150 km. Propagating tides are clearly visible in some of the parameters, and the latitude structure and phase variations with height indicate the presence of at least the (2,2) and (2,4) global tidal Hough modes. The influence of geomagnetic forcing is also clearly present at high latitudes. The University College London-Sheffield University three-dimensional coupled thermosphere-ionosphere model has been used to simulate this period of observation, by imposing tidal forcing at the lower boundary and magnetospheric forcing at high latitudes, in an attempt to interpret and understand the experimental data. Model simulations are able to predict where the signature of a particular tidal mode is likely to be observed in the respective responses of the temperature and wind structure. The numerical simulations predict the range of observed tidal amplitudes at mid and high latitudes, providedmore » the tidal forcing functions imposed near the lower boundary of the model are larger (400 m geopotential height variation) than those inferred from linear tidal models.« less

The combined effect of the natural meteoroid flux and the man-made debris on future space operations is discussed. The radar and optical systems used to study orbital debris are examined with special attention given to the Haystack Debris Observation Project and the Haystack radar at Millstone Hill, which is being modified to enable it to observe the regions of interest to the Space Station. Consideration is also given to techniques designed to remove objects from space.

Modern digital ionosondes /1/ represent the most practical method for routine measurement of electron density profiles in the ionosphere. However, inversion of ionogram data to produce an N(h) profile suffers from the well known valley problem /2/. Regions of decreased ionization between the ionospheric layers do not reflect the radio wave and the resulting discontinuity leads to uncertainty in the true height above the valley region. Incoherent scatter radar is capable in principle of providing direct information on the electron density variation within the E-F valley. Colocation of a Digisonde 256 sounder with the Millstone Hill incoherent scatter radar affords the opportunity to carry out detailed comparisons.

The advent of digital vertical incidence ionosondes and automatic scaling has brought with it the capability of performing large-scale analyses of the detailed structure of the ionosphere. In this paper, we use about 6,000 Digisonde 256 ionograms recorded at Millstone Hill, Massachusetts during 1988 and scaled automatically by ARTIST, to investigate relationships which exist between various profile parameters and to investigate the accuracy of estimates of these parameters based on simple models of the ionosphere. In particular, we (1) confirm for the mid-latitude ionosphere the high level of accuracy of the Dudeney formula for hmF2, (2) illustrate the limited utility of Gulyaeva's value for the height at half the peak density, and (3) illustrate the relationship between the semi-thickness of the F2 layer and the value of hmF2.

[1] Sudden stratospheric warming (SSW) is a large-scale meteorological process in the winter hemisphere lasting several days or weeks. The Incoherent Scatter World Day campaign conducted on January 17–February 1, 2008 was arranged during a minor SSW event and focuses on studies of thermospheric and ionospheric response to stratospheric changes. We analyze ion temperature observations at 100–300 km height obtained by the Millstone Hill incoherent scatter radar (42.6°N, 288.5°E). Alternating regions of warming in the lower thermosphere and cooling above 150km altitude were observed by the radar. We use National Center for Environmental Prediction (NCEP) temperature data at 10hPa (∼30km) level and the F10.7 and Ap indices to identify any cause-effect relationship between observed variations in the temperature and stratospheric warming event. We conclude that the seasonal trend, solar flux and geomagnetic activity cannot account for the observed warming and cooling temperature variation and suggest that this variation is associated with stratospheric warming. This study demonstrates a link between the lower atmosphere and the ionosphere which has not been considered before and indicates that ionospheric variability as part of space weather should be considered in conjunction with stratospheric changes.

[1] A new technique for diagnostics of traveling ionospheric disturbances (TIDs) uses measurements of arrival angles and Doppler frequency shift variations of HF radio signals reflected from the ionosphere. This technique is a further improvement of a frequency-and-angular sounding method previously suggested. Here we solve the respective inverse problem of reconstructing TID parameters by using a physically realistic three-dimensional model of electron density waves propagating through an ionospheric layer. The developed diagnostic algorithms were tested through computer simulations demonstrating the high efficiency of the proposed method. We also reprocessed the data collected in an earlier experiment carried out at the Millstone Hill Observatory in March 2001 using the new algorithms. In that campaign, HF signal trajectory parameters were recorded using the Digisonde instrument as a receiver monitoring continuous transmission from the CHU station. Comparison of the TID parameters determined with the frequency-and-angular sounding method and simultaneous incoherent scatter radar observations showed good agreement between the two diagnostic techniques with the differences of no greater than 10%.

[1] This paper presents observations and numerical simulations of ionospheric and thermospheric disturbances associated with a moderate geomagnetic storm on 10 September 2005. During the event, the incoherent radars located in Millstone Hill and Arecibo observed a dayside positive storm phase as manifested by the enhanced electron density in the F region. The universal time (UT)-altitude profile of electron density measured by both radars displayed a structure that closely resembles the Greek letter Λ. A similar structure is also reproduced by the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIEGCM). This peculiar electron density distribution is found to be associated with vertical ion drift, which initially was upward and then became downward. Using realistic time-dependent ionospheric convection and auroral precipitation as inputs, the TIEGCM reveals that the primary cause of the positive storm response was the enhanced meridional neutral wind rather than the penetration magnetospheric electric field. This study reiterates the importance of neutral wind effects on ionospheric disturbance.

[1] HF radio propagation was observed regularly during the night at frequencies above the classical Maximum Usable Frequency (MUF) on the circuit from Fort Collins, Colorado, to Boston, Massachusetts, during 2003 (McNamara et al., 2006). This propagation was attributed to above-the-MUF propagation, in the sense of normal refraction by over-dense regions in the F2 layer of the ionosphere, following Wheeler (1966). Similar propagation was also observed on a regular basis on the Ottawa to Boston circuit, at an operating frequency of 7.335 MHz. We have reanalyzed the nighttime Ottawa to Boston propagation in conjunction with observations of foF2 recorded by the digisonde at Millstone Hill (near Boston). Normal propagation (via refraction) would be supported on this circuit only when foF2 exceeds the equivalent vertical frequency, which is ∼5.7 MHz. The actual propagation extends through the midnight to dawn period even when foF2 drops below 4 MHz, with signal powers decreasing from about −80 dBW to −110 dBW. We deduce that there are three possible modes of propagation that are, in order of appearance: (1) normal refraction when foF2 exceeds 5.7 MHz; (2) “sporadic” reflection from large scale irregularities (tens of kilometers) with plasma frequencies that exceed 5.7 MHz in a lower density background; and (3) a 2-hop ground side scatter mode with hop lengths greater than the 7.335 MHz skip distance. The second mechanism is the one discussed by Wheeler (1966).

[1] Increasing concentrations of greenhouse gases are expected on theoretical grounds to lead to a cooling of the upper atmosphere. Due to the close thermal coupling of the neutral and ionized components of the upper atmosphere the effects of this cooling are also expected to be seen in the ionospheric ion temperature. Long-term satellite observations of neutral density show a trend consistent with the expected cooling. Temperature data from the Millstone Hill incoherent scatter radar (46.2°N, 288.5°E) from 1978 to 2007 have been analyzed to provide a direct estimate of the temperature trend above the radar. The long-term trend in the directly measured ion temperature Ti at 375 km is found to be −4.7 K/year with a 95% confidence interval of −3.6 to −5.8 K. The estimated trend in the neutral temperature Tn is −5.0 K/year. These are significantly larger than predicted by theory.

[1] An incoherent scatter radar experiment at Millstone Hill covering 30 consecutive days in September 2005 has enabled this study of day-to-day ionospheric variability. This was a period of low solar activity with few magnetically disturbed periods. Our discussion focuses on ionospheric variability during quiet magnetic activity in the 100–500 km height range, with emphasis on its height variation at noon. (1) Very large midday variability is present for the ion temperature Ti near 120 km, which is verified by two other 30-d experiments at Millstone Hill. This is not apparently associated with solar flux and magnetic activity. The percentage variability in the midday electron density Ne changes with height, being smaller between 150 and 250 km and larger in the topside. (2) With increasing solar flux, Ne decreases between 170 km and the F2 peak and increases elsewhere, being essentially unchanged near the F2 peak. With increasing magnetic activity, Ne decreases between 160 and 325 km. Ti increases with solar flux and magnetic activity, in particular in the F2 region. (3) There is a time lag of ionospheric responses, varying with height, to changes in solar-geophysical conditions: In the E region, the lag is almost zero; above the F2 peak, both Ne and Ti respond to F10.7 with a 2–3-d delay. The delay in response to 3-hourly ap index changes for Ne above the F2 peak can be 9–12 h and between 160 km and the F2 peak can be 0–3 h. The time delay for Ti is 6–9 h. (4) We estimate that the majority of the topside variability in Ti and Ne can be explained in terms of solar flux F107 and magnetic activity ap effects. Near the F2 peak, Ne variability seems to be complicated, and nearly one half (10%) of it cannot be ascribed directly to F10.7 and ap effects.

[1] A direct comparison between simulation results from the Global Ionosphere Thermosphere Model (GITM) and measurements from the Millstone Hill incoherent scatter radar (ISR) during the month of September 2005 is presented. Electron density, electron temperature, and ion temperature results are compared at two altitudes where ISR data is the most abundant. The model results are produced, first using GITM running in one dimension, which allows comparison at the Millstone Hill location throughout the entire month. The model results have errors ranging from 20% to 50% over the course of the month. In addition, the F2 peak electron density (NmF2) and height of the peak (HmF2) are compared for the month. On average the model indicates higher peak electron densities as well as a higher HmF2. During the time period from 9 September through 13 September, the trends in the data are different than the trends in the model results. These differences are due to active solar and geomagnetic conditions during this time period. Three-dimensional (3-D) GITM results are presented during these active conditions, and it is found that the 3-D model results replicate the trends in the data more closely. GITM is able to capture the positive storm phase that occurred late on 10 September but has the most difficulty capturing the density depletion on 11 and 12 September that is seen in the data. This is probably a result of the use of statistical high-latitude and solar drivers that are not as accurate during storm time.

The Cluster spacecraft and ground-based Digisonde network observed on November 7, 2004 a strong interplanetary shock interaction with Earth's magnetosphere which initiated a strong magnetic storm with Dst = −373 nT. When the interplanetary shock encountered the Earth system, the Cluster fleet was traveling in the inner magnetosphere region (L shell = 4.2) at almost exactly the Cluster's perigee (around 0900 MLT). This event offers an excellent opportunity to study the geospace response to a powerful interplanetary shock. The angle between the sun-Earth line and the normal direction of the shock front is only 3.0 degree indicating that the shock hit the geospace at ∼12 LT initially. It is found that energetic particle fluxes are strongly enhanced and the shock related ionospheric phenomena have obvious longitudinal and latitudinal distribution. The interplanetary shock has a significant influence on the dayside mid-high latitude stations, e.g., Millstone Hill, Wallops Island, etc. whereas the stations in t...

The upper thermosphere and F -region ionosphere system at 43°N is modelled for equinox and moderate solar conditions via a series of iterative calculations employing a thermospheric wind model and a one-dimensional ionospheric model which are mutually coupled. Several feedback loops within the system involving F 2 -layer peak height, F 2 -layer peak density, zonal wind, meridional wind, and Coriolis force are investigated to better understand the interactive aspect of ionosphere-thermosphere coupling. The interplay of primary importance involves the night-time ascent/descent of the F -layer due to equatorward/poleward neutral winds, the resulting changes in ion drag presented to the meridional and zonal wind fields, and the Coriolis force modification of the ion drag coupling. Wind shear and plasma profile shape are not significantly coupled. For magnetically undisturbed conditions, self-consistent treatment of these effects modifies a non-interactive “control” calculation by 20–35 m s −1 in the wind field. During geomagnetically disturbed periods interactive processes play a more crucial role in determining thermospheric and ionospheric storm responses. Our calculations reveal wind enhancements of up to 100 m s −1 associated with the lifting and negative-phase depletion of the F -region for prolonged magnetic disturbance conditions, the former mechanism accounting for a major portion of the effect.

A sounding rocket experiment was conducted to release molecules of H2O and CO2 into the daytime ionosphere within the simultaneous fields of view of the Millstone Hill radar and the Dynamics Explorer UV imager. Preliminary results confirm the creation of chemically-induced F-region plasma depletions and reduced UV intensities from the modified region.

In a series of experiments at Millstone Hill the total electron content along the path to different GPS satellites has been compared using simultaneous integrated phase and incoherent scatter measurements. An analysis of these data allows verification of our technique for determining the ionospheric and tropospheric path delay from the integrated phase data. It also provides the opportunity for evaluation of the IRI model in an operational sense.

The present status of understanding and modeling of the variation of electron and ion temperatures with solar activity is reviewed. All atmospheric and ionospheric densities and temperatures are positively correlated with solar activity except the electron temperature, which exhibits a rather complex variation pattern during a solar cycle. The ion temperature at low altitudes closely follows the variation patterns of the neutral temperature. The electron temperature at high altitudes increases with increasing solar activity; the increase is larger during day than during night and exhibits a latitudinal variation. At low altitudes during daytime, tha amplitude of the seasonal variation of the electron temperature increases toward the solar maximum. At 400 km during daytime, the summer electron temperatures for Millstone Hill increase slightly toward higher solar activities, whereas the winter temperatures decrease distinctly. For Arecibo, an opposite trend is noticeable.

The Millstone Hill incoherent scatter radar operated during 21–25 September 1987 as part of the Lower Thermosphere Coupling Study (LTCS). During the daytime, local E and F-region correlation functions were obtained using a combination of multipulse and lag profile techniques, while during the night, spectral techniques were employed to probe the local and high-latitude F-region. Emphasis has been placed so far on the analysis of temperatures in the lower thermosphere over Millstone Hill which reveal an increase of about 60°K in the mean daytime temperature on the geomagnetically disturbed day, 25 September, relative to the previous quieter days. This is considered to be due to the transport of Joule heat from higher latitudes. Temperature oscillations of 40°K with a 12-hour period were observed on 23 and 25 September exhibiting downward phase propagation consistent with the characteristics of upward propagating semidiurnal tides from the lower atmosphere. Although large variability is noted from day to day, there is a general consistency with tidal theory predictions for those days when tidal oscillations are apparent.

[1] In this paper, electron densities during 25-28 September 2000 observed by the Millstone Hill incoherent scatter radar (ISR) are assimilated into a one-dimensional midlatitude ionospheric theoretical model by using an ensemble Kalman filter (EnKF) technique. It is found that (1) the derived vertical correlation coefficients of electron density show obvious altitude dependence. These variations are consistent with those from ISR observations. (2) The EnKF technique has a better performance than the 3DVAR technique especially in the data-gap regions, which indicates that the EnKF technique can extend the influences of observations from data-rich regions to data-gap regions more effectively. (3) Both the altitude and local time variations of the root mean square error (RMSE) of electron densities for the ensemble spread and ensemble mean from observation behave similarly. It is shown that the spread of the ensemble members can represent the deviations of ensemble mean from observations. (4) To achieve a better prediction performance, the external driving forces should also be adjusted simultaneously to the real weather conditions. For example, the performance of prediction can be improved by adjusting neutral meridional wind using equivalent wind method. (5) In the EnKF, there are often erroneous correlations over large distance because of the sampling error. This problem may be avoided by using a relative larger ensemble size.

Neutral oxygen (O I) is a dominant species between about 250 and 500 km in the thermosphere. A complete thermospheric model requires measurements of the species density ([O]) to incorporate into forward models. One way to measure [O] is to detect Bowen fluorescence at triplet 844.6 nm. Bowen fluorescence is generated when thermospheric oxygen absorbs Solar Lyman-β and de-excites through a path eventually leading to 844.6 nm emission. This emission must be distinguished from the brighter 844.6 nm emission caused by photoelectron (PE) impact, which can be done by measuring the intensity ratio between two branches of the 844.6 triplet. An instrument to measure Mid-latitude Bowen fluorescence has been installed at Millstone Hill Observatory. The instrument is a Spatial Heterodyne Spectrometer (SHS), a novel type of Fourier transform spectrometer. This SHS was first used to observe Na Fraunhofer lines from dayglow at 589 nm, and is to be turned to the 844.6 region. With simple readjustments, it can also be reset for other visible and near IR wavelengths.

[1] In this paper, ionospheric electron temperature (Te) data for more than two solar cycles measured by the incoherent scatter radars (ISR) at Millstone Hill (42.6°N, 71.5°W) and Arecibo (18.3°N, 66.7°W) are compared with the theoretical Te calculated from the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR-TIEGCM) to investigate the temporal variations of Te. The comparisons are made for both low and high solar activity conditions and for three seasons: equinox, summer, and winter. The observations show that the diurnal variation of Te is characterized by morning and evening peaks at Arecibo and by a morning peak at Millstone Hill. The occurrence and strength of the peaks at Arecibo are significantly different from those at Millstone Hill. Daytime Te tends to increase with solar activity at both stations below ∼300 km. Te above 300 km generally decreases with solar activity; however, it increases with solar activity in equinox and summer at Arecibo, whereas it does so only in summer at Millstone Hill. The TIEGCM model can reproduce these variations. However, the modeled evening peak is weaker than that from observations at Arecibo. The simulations show that the daytime bulge of Te tends to occur at low latitudes and high solar activity, as seen in the observations, and the significant morning peak at low solar activity over Arecibo is associated with the equatorial anomaly. Moreover, an interesting feature predicted by the model is that the midday Te at the F2 peak height increases with solar activity when F10.7 values are less than about 100 × 10−22 W m−2 Hz−1 or larger than 190 × 10−22 W m−2 Hz−1 at Millstone Hill; so does at Arecibo when F10.7 values are larger than 100 × 10−22 W m−2 Hz−1. As a result, a positive correlation between daytime Te and Ne occurs under these conditions.

[1] We analyze ground-based Fabry-Perot interferometer observations of upper thermospheric (∼250 km) horizontal neutral winds derived from Doppler shifts in the 630.0 nm (red line) nightglow. The winds were measured over the following locations: South Pole (90°S), Halley (76°S, 27°W), Arequipa (17°S, 72°W), Arecibo (18°N, 67°W), Millstone Hill (43°N, 72°W), Sondre Stromfjord (67°N, 51°W), and Thule (77°N, 68°W). We derive climatological quiet time (Kp 150). The seasonal dependence of the winds is generally annual, but there are isolated cases in which a semiannual variation is observed. Within the austral winter, winds measured from the South Pole show a substantial intraseasonal variation only along longitudes directed toward the magnetic pole. IMF effects are described in a companion paper.

[1] Recent observations with the new mid-latitude SuperDARN HF radar located at Wallops Island, Virginia have identified a class of ionospheric irregularities that is prevalent in the nightside sub-auroral ionosphere under low-to-moderate Kp conditions. These irregularities can be observed for many hours and generally exhibit very low Doppler velocities. A recent collaborative experiment using the Wallops radar and the Millstone Hill incoherent scatter radar has determined that these irregularities are located at the ionospheric footprint of the plasmapause and in a region of opposed electron density and electron temperature gradients. We conclude that the irregularities are produced by the temperature gradient instability (TGI) or by turbulent cascade from primary irregularity structures produced from this instability. This is the first experimental confirmation that the TGI is effective in producing decameter-scale ionospheric irregularities.

[1] The shape of the topside electron density (Ne) profile is studied using incoherent scatter radar (ISR) measurements. On the basis of more than 90,000 and 84,000 Ne profiles measured over Arecibo (18.4°N, 66.7°W, dip 49.7°) and Millstone Hill (42.6°N, 71.5°W, dip 71.6°), respectively, shape factors have been derived by fitting the ISR observed profile using a Chapman-type layer with a height-independent scale height. The results reveal that the shape factor shows a general departure from the typically used values of 0.5 (Chapman α layer) except during the night and also undergoes appreciable variation with local time, season, solar activity, and latitude. Over Arecibo, the averaged shape factor is characterized by a daytime maximum of ∼0.55–0.75, a nearly constant nighttime value close to 0.5, and a marked morning decline; over Millstone Hill, the shape factor shows pronounced seasonal variations, and the diurnal variation in summer is opposite to that in other seasons. Over both stations, the shape factor exhibits a high correlation with the F2 layer peak electron density (NmF2), and it has strong solar cycle dependence during the late morning hours. It indicates that the temperature structure of the topside ionosphere can explain much of the variation of the shape factor when the plasma density is low, especially during nighttime. During daytime hours, the topside shape factor is thought to be associated with ion-neutral drag during periods of large plasma density.

International audience; Empirical ionospheric local models have been developed from long-term data sets of seven incoherent scatter radars spanning invariant latitudes from 25 to 75 in American, European and Asian longitudes at Svalbard, Tromsø, Sondrestrom, Millstone Hill, St. Santin, Arecibo and Shigaraki. These models, as important complements to global models, represent electron density, ion and electron temperatures, and ion drifts in the E and F regions, giving a comprehensive quantitative description of ionospheric properties. A case study of annual ionospheric variations in electron density and ion temperature is presented based on some of these models. Clear latitudinal, longitudinal, and altitude dependency of annual and semiannual components are found.

[1] We present a global view of large-scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low-latitude, auroral, and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high-latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high-latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m 3 ) and low electron and ion temperatures (2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high-latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was 50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m 2 s 1 ) were observed in the topside ionosphere

[1] Measurements from the incoherent scatter radar (ISR) and ionosonde over Millstone Hill (42.6 D N, 288.5 D E) are analyzed to explore ionospheric temporal variations. The F-2 layer peak density NmF2, peak height h(m)F(2), and scale height H are derived from a Chapman a layer fitting to observed ISR electron density profiles. Diurnal, seasonal, and solar activity variations of the ionospheric characteristics are presented. Our study on the solar activity dependence of NmF2, h(m)F(2), and H indicates that the peak parameters (NmF2 and h(m)F(2)) of the F-2 layer increase with daily F-10.7 index and saturate ( or increase with a much lower rate) for very high F-10.7; however, they show almost linear dependence with the solar proxy index F-10.7p = (F-10.7 + F-10.7A)/2, where F-10.7A is the 81-day running mean of daily F-10.7. This suggests that the overall effect of solar EUV and neutral atmosphere changes on the solar activity variation of ionospheric ionization is linear with F-10.7p. The rate of change in the ionospheric characteristics with solar activity exhibits a seasonal and local time variation. Over Millstone Hill, NmF2 in summer is characterized by the evening peak in its diurnal variation, and NmF2 exhibits winter anomaly under low and high solar activity levels. The temporal variations of the topside effective scale height H-0 can be explained in terms of those in the slab thickness. The IRI model overestimates the N-e effective topside scale height over Millstone Hill; therefore our analysis for the effective topside scale height from the Millstone Hill measurements might help to improve the IRI topside profiles at middle latitudes.

[1] It is well known that the interplanetary electric field can penetrate to the low-latitude ionosphere. It is generally believed that the penetration of electric fields can last only for ∼30 min because of the shielding effect in the ring current. In this paper we present the observations of the dayside ionospheric electric field enhancements at middle and low latitudes in association with reorientations of the interplanetary magnetic field (IMF). In six cases, the eastward electric field in the dayside equatorial ionosphere, measured by the Jicamarca incoherent scatter radar, was enhanced for 2–3 hours after the IMF turned southward and remained continuously southward. In one case the eastward electric field in the dayside midlatitude ionosphere, measured by the Millstone Hill incoherent scatter radar, was continuously enhanced for ∼10 hours during southward IMF. Since Millstone Hill is close to the equatorward boundary of the auroral zone during magnetic storms, the penetration electric field there may be different from that at the equatorial ionosphere. The most striking feature of the measurements is that the enhancements of the ionospheric electric field can last for many hours without significant decay. The electric field enhancements in the middle- and low-latitude ionosphere are closely related to magnetic activity and occur during the main phase of magnetic storms. The observations show that the interplanetary electric field can continuously penetrate to the low-latitude ionosphere without shielding for many hours as long as the strengthening of the magnetic activity is going on under storm conditions.

Simulation results from the NRL ionospheric model SAMI2 indicate that the changes in the F-region over Millstone Hill during the geomagnetic storm beginning on 3 April 2004 are primarily due to the influence of a long-lasting eastward electric field, as was previously suggested by {\it Huang et al.} [2005]. A simulation of the storm day agrees well with the observational data and shows that, compared with the ionosphere of the previous quiet day, the peak electron density in the F-region (NmF2) increased by a factor of $\approx 2$, the altitude of the peak density (hmF2) rose by $\approx 80$ km, and the F-region electron temperature decreased by $\approx 1000$ K. Further simulations in which either the neutral atmosphere and winds or the electric field were replaced by their quiet day counterparts clearly suggest that the electric field played the dominant, although not exclusive, role in producing these effects., Comment: Submitted for publication in GRL

[1] A long-duration incoherent scatter radar (ISR) experiment was conducted at Millstone Hill and Svalbard from October 4–November 4, 2002. Along with the simultaneous GUVI/TIMED neutral composition measurements, this 30-day run enabled us to study a number of thermosphere-ionosphere-magnetosphere phenomena. This paper focuses on the day-to-day variability and quasiperiodic oscillation of the ionosphere. The day-to-day variability under quiet magnetic conditions in electron density Ne, ion temperature Ti and electron temperature Te, respectively, changed with local time and height, with the largest variability in Ne and the smallest in Ti. Midnight through dawn was the period of largest variability. Quasiperiodic Ne oscillations were present with periods >1 day. Some of these oscillations were correlated with changes in the neutral composition originating from geomagnetic activity, which altered the global atmospheric circulation as a result of high latitude heating processes as indicated in Svalbard ion temperature enhancements. However, the wave-type oscillation of Ne exhibits a downward phase progression which persists up to 600 km and prevails until a large storm appears to impose an upward phase progression.

[1] High spectral resolution measurements of OI 6300 A emissions during daytime on 30 October 2003 from Boston revealed prolonged auroral activity from 1415–1900 LT. The solar zenith angles varied from 70°–115° during that interval. Intense enhancement in brightness of 38 Kilo Rayleighs were observed at 1509 LT, which were six times larger than the normal daytime emission rates for that day. During this event the solar zenith angle was 74° and the solar background continuum was 4 × 106 Rayleighs A−1. Such large enhancement in brightness were due to the daytime aurora generated in response to an X-class Coronal Mass Ejection on the Sun. The measured daytime airglow emissions show good agreement, both, in the magnitudes and in the temporal variability with the emissions modeled by using as inputs the Ne, Te and Ti profiles that were measured by the Millstone Hill Incoherent Scatter Radar (ISR) located approximately 60 km away from the spectrograph.

[1] Two ionospheric modification experiments were carried out over the incoherent scatter radar (ISR) located at Millstone Hill, Massachusetts. These experiments are part of the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) program at the Naval Research Laboratory. The experiments use 10-s burns of the dual orbital maneuver subsystem (OMS) engines to produce the injection of high-speed molecules in the ionosphere near 380 km altitude. Charge exchange between the high-speed exhaust molecules and the ambient oxygen ions yields molecular ion beams that disturb the natural state of the ionosphere. Radar scatter provides measurements of the ion velocity distributions and plasma turbulence that result from the ion beam interactions. Ground-based observations with the University of Massachusetts Digisonde record the ionospheric density depressions resulting from recombination of the molecular ions with electrons. Prompt signatures of nonequilibrium ion distributions in the OMS engine plume are seen in the data taken during the SIMPLEX III and IV experiments for the space shuttle flights STS-108 and STS-110, respectively. The SIMPLEX III observations are much weaker than those during SIMPLEX IV. These differences are primarily attributed to the changes in the viewing directions for the radar beam. During SIMPLEX IV, the radar is looking more downstream from the exhaust injection and the stimulation of plasma turbulence is seen with the ISR for over 30 s at distances up to 200 km from the burn altitude along the radar beam. Strong backscatter in the radar spectra is attributed to ion acoustic waves driven by the pickup ion beams. Both experiments provide large-scale cavities detected by the Digisonde for up to 20 min after the engine burn. These cavities are the result of ion-electron recombination of the pickup ions.

[1] During geomagnetic storms, the ionospheric F region electron density may be greatly increased or decreased, which are termed positive storms or negative storms. It is generally accepted that negative storm phases are caused by neutral composition changes. In contrast, different mechanisms have been proposed to explain the generation of positive storm phases. In this paper, we present observations of a strong positive storm at middle latitudes. The Millstone Hill incoherent scatter radar detected significant increases of the midlatitude ionospheric F region electron density during a magnetic storm on 3 April 2004. The positive phase of the ionospheric storm started to occur in the morning sector (0912 LT at Millstone Hill) and lasted for more than 10 hours. Compared with the quiet-time ionosphere, the daytime F peak altitude over Millstone Hill during the storm was ∼80 km higher, the F region electron density was increased by a factor of 2–4, and the F region electron temperature was decreased by ∼1000 K (or ∼40%). The radar also measured an enhanced eastward electric field and a slightly more equatorward neutral wind. Global GPS measurements show that total electron content (TEC) was increased at middle latitudes and decreased at lower latitudes, and the large TEC enhancements occurred primarily in the Atlantic sector. We suggest that electric fields may play an important role in the generation of the observed positive storm phase. The eastward electric field will cause increases in the midlatitude ionospheric electron density by moving the plasma particles upward and decreases in the equatorial ionospheric electron density by strengthening the fountain effect. The daytime poleward wind was reduced or even reversed during the storm, which may also contribute to the occurrence of the positive storm.

[1] Coherent backscatter observations with the Millstone Hill UHF radar (MHR) are used to investigate spatial/temporal variations in the ionospheric sub-auroral polarization stream (SAPS) electric field. For the 440 MHz MHR, coherent amplitude is on average linearly proportional to electric field strength. The use of both main-beam and sidelobe returns and the great sensitivity of the MHR system permits observations spanning 3° of the SAPS region with 1-sec temporal and 10-km spatial resolution. For a moderately disturbed event on May 25, 2000, the SAPS channel moved steadily equatorward. Large-scale (30 mV/m peak to peak) wave-like oscillations in the electric field magnitude (200s–300s periodicity) were seen to propagate across the SAPS channel throughout the hour-long event. It is suggested that such localized electric field intensifications, which exhibit many of the characteristics of the narrow SAID features described in the literature, arise as wavelike perturbations within the SAPS channel.

[1] A successful ionospheric electron density model assimilates relevant data for determination of its driving parameters. We process about two hours worth of two-frequency GPS ground-receiver data from five satellites in order to separate the least squares solution of up to four driving parameters of the electron density profile (EDP) model from the solution of four relative (between satellites) differential hardware biases. The EDP model we use is part of the Ionosphere and Troposphere Raytrace Model (ITRAY), which is our upgrade of the Raytrace/Ionospheric Conductivity and Electron Density model (ICED)-Bent-Gallagher (RIBG) model. We use the Westford GPS receiver in Massachusetts to update the EDP model, which we then use to predict EDP distributions at nearby Millstone Hill for comparison with ground truth incoherent-scatter radar and Digisonde data. We find that a simple procedure often works well when the model time dependence is correct, but we find that forcing the ionospheric model to more closely fit GPS data actually degrades ionospheric specification in the F layer region. We suggest a data diversification remedy.

[1] For the large geomagnetic disturbance on April 11, 2001, we combine data from the Millstone Hill radar, a network of GPS TEC receivers, and the DMSP F-12 and IMAGE satellites, to estimate the sunward flux of thermal plasma associated with erosion of the outer plasmasphere/ionosphere in the dusk sector. Direct radar observations of the E×B advection of SED plasma determine a flux of >10E26 ions/s to the noontime cusp at F-region heights. DMSP in situ observations provide a similar estimate. Assuming a dipolar magnetic field, we project our low-altitude observations into the outer plasmasphere, obtaining a total sunward flux of >10E27 ions/s. High-altitude IMAGE EUV observations of the plasmasphere drainage plume provides an estimate of 1.5 × 10E27 ions/s for the sunward flux. Such rates of sunward plasma transport are sufficient to deplete a 1-Re shell of the outer plasmasphere in ∼1 hour.

Ionospheric plasma temperature variations have recently been studied based on incoherent scatter radar (ISR) observations at a lower midlatitude site, Shigaraki, in East Asia [Otsuka et al., 1998] and Millstone Hill, a typical subauroral midlatitude site in North America [Zhang and Holt, 2004]. The French Saint Santin ISR, with a geographic latitude slightly higher but an apex latitude 14° lower than Millstone, collected bistatic and quadristatic measurements for over two solar cycles beginning in September 1965. A database of these data, containing observations between 1966 and 1987, has been used in this study in order to establish the midlatitude ionospheric climatology, in particular that of the upper atmosphere thermal status, as well as empirical models for space weather applications. This paper presents, in comparison with the Millstone Hill results, variations of ion and electron temperatures (Ti and Te) with solar activity, season, time of the day, and altitude. The F2 region Te at St. Santin is found to be lower than at Millstone between March and July, when the St. Santin electron density Ne is relatively higher. The midday Te below 300 km increases with F10.7, as at Millstone Hill. Above 300 km it tends to decrease with F10.7 at St. Santin, while it increases in summer at Millstone Hill. Ti between 250 and 350 km peaks midway between spring and summer. We have also created St. Santin ionospheric models for Ne, Te, and Ti using a bin-fit technique similar to that used for the Millstone Hill models. Comparisons with corresponding IRI predications indicate good agreement in Ti at high solar activity, and above the F2 peak, Te from the IRI tends to be higher than both the St. Santin and Millstone Hill models.

[1] High-resolution observations of the dynamics of the ionospheric E region (100–150 km) at midlatitudes have been made with the incoherent scatter radar at Millstone Hill (42.6°N, 71.5°W) during several intense geomagnetic storms (Kp ∼ 8–9). These included the storms of 25 September 1998, 15 July 2000, and 31 March 2001. The observations reveal electric fields of up to 100 mV/m during these events and indicate E region zonal plasma drifts of 300–1000 m/s, westward in the evening sector and eastward in the morning sector, with smaller meridional drifts. The tidal pattern of neutral winds in the lower thermosphere is heavily disrupted. The zonal component of the neutral wind follows convection-driven ion flow, while the meridional component shows a more complex response to geomagnetic forcing.

[1] A single digisonde, used as a receiver for ionospheric high frequency signals from broadcast stations, was able to determine the wave characteristics of traveling ionospheric disturbances (TIDs) using frequency shift and arrival angle measurements. During a measurement campaign, aimed at recovering large-scale wave-like processes in the upper atmosphere, in March 2001 at the MIT Haystack Observatory (Millstone Hill, MA), a Digisonde Portable Sounder (DPS) made simultaneous observations with the incoherent scatter radar (ISR). The DPS was upgraded to include the frequency and angular sounding (FAS) technique previously developed at the Institute of Radio Astronomy in Kharkov, Ukraine, for TID diagnostics. The DPS with four spaced receive antennas received the radio signals at 3.33 MHz and 7.335 MHz from Radio CHU of the Canadian Time Service (Ottawa, Ontario). The FAS technique recovered the basic parameters of TIDs, i.e., amplitude, speed, and the direction of the propagation vector by modeling the ionosphere as a perfectly reflecting surface. For three days during the campaign the Millstone Hill ISR monitored the ionospheric midpoint using a triangulation mode to identify the direction of motion and speed of the wave-like ionospheric disturbances. Comparison of the results from the two diagnostic techniques showed good agreement. The wave directions were within 10 to 15 degrees, and speed and wavelength were within 10 to 12%.

[1] Quasi-periodic variations in ionospheric F-region parameters associated with interplanetary magnetic field fluctuations have been reported by Huang et al. [2002]. Two mechanisms that could generate these ionospheric fluctuations are a time-varying electric field or neutral wind. We present simulation results of the response of the mid-latitude F-region ionosphere to these drivers using the NRL ionosphere model SAMI2. We obtain good agreement with Millstone Hill density and velocity observations for a zonal F-region electric field E ≃ ±2.4 mV/m. We also obtain good agreement with density observations if the neutral wind fluctuates ∼±40 m/s along the geomagnetic field. However, this neutral wind variation corresponds to ∼±140 m/s in the meridional component of the horizontal neutral wind, and the fluctuating ion velocity is ∼30% larger than those observed. This suggests that a fluctuating penetration electric field is probably responsible for the Millstone Hill observations.

[1] We present periodic (∼2 hour) magnetospheric and ionospheric oscillations observed with multiple space-based and ground-based instruments during the severe magnetic storm of 25 September 1998. Periodic particle injections were observed at geosynchronous orbit. Ground magnetometers registered very large, as high as 3000 nT, magnetic deviations in the nightside auroral zone. The Millstone Hill incoherent scatter radar at magnetic latitude 55° measured periodic enhancements of ionospheric eastward ion velocity from midnight to ∼1000 MLT. The periodic variations in the magnetometers and Millstone Hill radar were well correlated with the magnetospheric particle injections. The observations are consistent with the occurrence of periodic magnetospheric substorms during a magnetic storm. During each substorm cycle the ionospheric auroral zone expanded to the latitude of the Millstone Hill radar, so the radar was periodically within the auroral zone and detected the auroral plasma return flows. After substorm activity stopped, the Millstone Hill radar measured near-noon in-phase periodic oscillations of the ionospheric F-region electron density and temperature, which may be due to particle precipitation from the ring current and/or plasmasphere into the midlatitude ionosphere. We suggest that magnetospheric substorms during magnetic storms have a cycle time of ∼2 hours. The magnetosphere can sustain similar oscillations for several cycles even though the substorm activity has stopped. All the observed periodic variations in the geosynchronous particle injections, in the auroral magnetic deviations, and in the ionospheric ion velocity are related to the periodic magnetospheric substorms and oscillations.

Neutral winds in the lower thermosphere at altitudes of 94-130 km were measured in 1987-2000 by the ground-based incoherent scatter radar (ISR) at Millstone Hill (42.6°N, 288.5°E) and in 1992-1997 by the space-based Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The data from the ISR and WINDII have been separately grouped into four seasons. The daytime wind measurements from both instruments are found to be in good agreement, with similar structures and similar magnitudes. Winds from both instruments show an annual variation with a winter minimum. Winds in spring and summer are generally about a factor of two larger than those in fall and winter. The semidiurnal winds are dominant, but diurnal tides, mainly the in situ thermospheric diurnal tide, are comparable. The tidal characteristics and the background winds (the diurnal mean) derived from WINDII data show that the amplitude of the semidiurnal tide is generally 10 ms -1 larger than that of the in situ thermospheric diurnal tide, and the zonal and meridional background winds are less than 40 ms -1 and 15 ms -1 in all seasons, respectively. The TIME-GCM simulations of wind fields at 42.5°N for the March equinox and the December solstice are in reasonable agreement with the observations, but the comparisons for the June solstice are less satisfactory. More research needs to be done to understand the cause of disagreement at the June solstice and the September equinox and to properly model the tidal effects in the lower thermosphere during those periods.

Storm effects at F1-layer heights (160–200 km) were analyzed for the first time using Millstone Hill (mid-latitudes) and EISCAT (auroral zone) incoherent scatter (IS) observations. The morphological study has shown both increases (positive effect) and decreases (negative effect) in electron concentration. Negative storm effects prevail for all seasons and show a larger magnitude than positive ones, the magnitude of the effect normally increasing with height. At Millstone Hill the summer storm effects are small compared to other seasons, but they are well detectable. At EISCAT this summer decrease takes place only with respect to the autumnal period and the autumn/spring asymmetry in the storm effects is well pronounced. Direct and significant correlation exists between deviations in electron concentration at the F1-layer heights and in the F2-layer maximum. Unlike the F2-layer the F1-region demonstrates a relatively small reaction to geomagnetic disturbances despite large perturbations in thermospheric parameters. Aeronomic parameters extracted from IS observations are used to explain the revealed morphology. A competition between atomic and molecular ion contributions to Ne variations was found to be the main physical mechanism controlling the F1-layer storm effect. The revealed morphology is shown to be related with neutral composition (O, O2, N2) seasonal and storm-time variations. The present day understanding of the F1-region formation mechanisms is sufficient to explain the observed storm effects.Key words. Atmospheric composition and structure (thermosphere-composition and chemistry); ionosphere (ion chemistry and composition; ionospheric disturbances)

[1] We make direct comparisons between GPS maps of total electron content (TEC) over the North American continent, Millstone Hill radar observations of storm enhanced density, and low and high-altitude satellite measurements of the perturbation of the outer plasmasphere during the March 31, 2001 geomagnetic storm. We find that storm enhanced density (SED) and plumes of greatly-elevated TEC are associated with the erosion of the outer plasmasphere by strong sub-auroral polarization electric fields. The SED/TEC plumes identified at low altitude map closely onto the magnetospheric determination of the boundaries of the plasmapause and plasmaspheric tail determined by EUV imaging from the IMAGE spacecraft. Characteristics of the SED/TEC plumes/tails for the March 31, 2001 event are: TEC ∼ 100 TECu; F-region sunward velocity ∼1000 m/s; sunward flux ∼5*1024 ions s−1; total transport to dayside magnetopause/merging region (3-hr event) ∼5*1028 ions.

[1] We use 630.0 nm nightglow Fabry-Perot measurements over Millstone Hill from 1989–1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late-night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is low, the winter and equinox average meridional winds change from equatorward to poleward at ∼2200 LT, and the summer winds are equatorward throughout the night. The high solar flux meridional winds are poleward, with magnitudes increasing from dawn to dusk at all seasons. These disturbance winds patterns are in poor agreement with results from the empirical horizontal wind model, HWM-93. The zonal and meridional disturbance winds show very large variations relative to their average values. We have also studied the time-dependent response of the midlatitude thermospheric winds to enhanced magnetic activity. The early night westward winds build up to large amplitudes (about twice their climatological values) in ∼6 hours; the late-night eastward winds are smaller and reach their peak values ∼3 hours after the increase in magnetic activity. The storm time dependence of the meridional winds is considerably more complex than that of the zonal winds, and it varies with season and solar flux. Following enhanced magnetic activity, equatorward winds are observed at all local times and seasons, but the increase of their amplitudes with storm time is fastest in the late local time sector. Near midnight, and when the solar flux is low, the meridional winds reverse from equatorward to poleward ∼6–10 hours after the increase in magnetic activity. This reversal is fastest (slowest) during December (June) solstice. At later local times, and for high solar flux conditions, the variation of the meridional disturbance winds is season independent. The observed storm time dependence partly explains the large variability of the disturbance winds.