J. M. Holt

Multiradar observations of the polar tongue of ionization

[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

Optimal analysis of incoherent scatter radar data

A lag-profile data collection mode has recently become operational at Millstone Hill. A new data analysis technique has been developed in which one or more matrices of ion-line lagged products are analyzed simultaneously, along with any available a priori information on ionospheric and system parameters such as ionosonde and plasma-line estimates of f0F2. The analysis yields spline function estimates of height-varying ionospheric parameter profiles. The effect of the full two-dimensional (range-lag) radar ambiguity function is included in this analysis. Millstone Hill single-pulse lag profile data are presented and analyzed and the new technique is shown to produce ionospheric parameter profile estimates with significantly better height resolution than would be possible if the lag profile matrix were first divided into ACFs by application of a summation rule.

Millstone Hill measurements on 26 February 1979 during the solar eclipse and formation of a midday F-region trough☆

On 26 February 1979, the Millstone Hill incoherent scatter radar was operated to observe the F-region over Canada along the path of the total eclipse of the sun. A continuous scanning mode was used with the radar elevation fixed at 4° and the azimuth swept continuously from 350° to 298°. Each scan required 20 min to complete and useful results were obtained at ranges up to 2992 km. The path of totality crossed the center of the region swept by the radar beam. The F-region electron density, ion temperature, electron temperature and ion line-of-sight drift were measured. Electric field components have been extracted from the radar line-of-sight component of the ion drift by assuming that the electric field may be represented by a quasi-static two-dimensional potential with the potential assumed constant along geomagnetic field lines. Electron density decreases were observed in association with the eclipse. At 300 km an ∼50% decrease occurred near the region of totality, but the variations were small above 450 km. However, apparently unrelated to the eclipse, very large electric fields developed in the region under view. Somewhat later a well defined trough formed north of Λ = 70°, whose equatorward edge is extremely sharp and appears to be approaching the radar with time. Large values of the electron and ion temperatures were observed in the trough and the trough formed where the drift was westwards and exceeded 1000 m s−1. It is suggested that the observed through may be due to an increase in the rate of the charge transfer reaction O+ + N2 → NO+ + N in the presence of a very large electric field impressed from the magnetosphere in association with a large magnetic disturbance.

Estimation of the O+, O collision frequency from coincident radar and Fabry‐Perot observations at Millstone Hill

The formula for the O+, O momentum transfer collision frequency has been uncertain due to a discrepancy between results of theoretical calculations and some of the joint radar/optical studies. The former suggest a multiplicative factor equal to 1.2–1.3 times the formula derived by Dalgarno [1964] and Banks [1966], while the latter suggest a multiplicative factor F = 1.7 [Salah, 1993]. We present results of a new analysis of data from 30 nights of coincident incoherent scatter radar (ISR) and Fabry-Perot interferometer (FPI) experiments conducted at Millstone Hill between 1988 and 1992. The O+, O collision frequency is estimated from FPI measurements of the horizontal neutral wind in the magnetic meridian, ISR measurements of the ion drift velocity parallel to the Earths magnetic field and other data at the calculated height of peak 630 nm emission, and the mass spectrometer and incoherent scatter 86 model. A complete error analysis is carried out for each derived value of F. This allows us to carry out Monte Carlo simulations which confirm that random errors lead to an increase in the mean value of F and which provide us with an unbiased result, F = 1.15 ± 0.2. However, this result was obtained from an analysis which neglected vertical neutral winds, about which we have little information. The most likely effect of these winds would be an increase in the value of F, so that our best estimate from this study is F = 1.4 ± 0.3, which is consistent with theoretical calculations.

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

[1] This paper examines the ability of ionospheric models to reproduce measured electron density, winds, and temperatures during the International Polar Year (IPY) in 2007. The models include the field line interhemispheric plasma (FLIP) model, the international reference ionosphere (IRI) model, and the empirical horizontal neutral wind models (HWM) (HWM93, HWM07). For Poker Flat, Alaska, there is exceptionally good agreement between the FLIP model and measured electron density, winds, and temperatures in equinox and winter. This research shows an interesting post sunset peak in Te from late fall through early spring that is reproduced by the FLIP model. In June and July the FLIP model underestimates the measured peak electron density by a factor of 2. Although both the data and model show evidence of an F 1 peak near 150 km in summer, the model F 1 peak electron density tends to be larger than the F 2 peak electron density and that is not seen in the data. The summer discrepancy is most likely due to incorrect atomic to molecular neutral density ratios. The FLIP model reproduces the Millstone Hill data well throughout 2007. The IRI model agrees well with the electron density data during the day but overestimates the peak electron density and the height of the peak at night. The equivalent winds from the FLIP model and the winds from the HWM93 model agree well with the measured winds. The HWM07 winds are different from the earlier HWM93 winds at Poker Flat and do not agree as well with the data.

Long‐term trends in thermospheric neutral temperature and density above Millstone Hill

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.

Measurements of gradients in ionospheric parameters with a new nine-position experiment at Millstone Hill

Abstract A new nine-position experiment is now routinely carried out with the Millstone Hill incoherent scatter radars which allows estimation of spatial gradients in the measured ionospheric scalar parameters N e , T e , and T i , and in the components of the ion velocity vector v i . Use of this technique results in improved estimates of basic and derived parameters from incoherent scatter data at times of significant gradients. We detail the data analysis method and present the first results from this new experiment. The gradients in N e and in the components of vi are used to compute the motion term in the ionospheric F region continuity equation ▿ · ( N v), which is then combined with ∂N / ∂t to estimate the O + recombination rate β at night. Meridional neutral winds U mer are computed from the field-aligned ion velocity v ∥ and a calculation of the O + diffusion velocity v d , and it is found that horizontal gradients in the ion velocity field at times significantly affect the calculation of the neutral winds.