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  • 1
    Online Resource
    Online Resource
    F region ion temperature enhancements resulting from Joule heating
    American Geophysical Union (AGU) ; 1983
    In:  Journal of Geophysical Research: Space Physics Vol. 88, No. A5 ( 1983-05), p. 4114-4118
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 88, No. A5 ( 1983-05), p. 4114-4118
    Abstract: F region ion temperature measurements were made by the Chatanika and Millstone Hill incoherent scatter radars as part of the Magnetosphere‐Ionosphere‐Thermosphere Radar Studies program of coordinated high‐latitude observations. At both radars, periods of enhanced ion temperature associated with Joule heating events were detected. A regular feature of the observations was the existence of larger and longer lasting temperature enhancements in the morning sector as contrasted with the evening sector during periods of comparable electric field magnitudes. Because the ion temperature increases in proportion to the square of the vector difference between the ion and neutral velocities, the morning/evening temperature enhancement asymmetry implies a morning/evening neutral wind asymmetry. The neutral wind in the evening must be more closely aligned to the ion flow vector. This might arise as a consequence of the higher plasma density in the evening sector, enabling the ions to set the neutral air in motion. Comparison of the simultaneous plasma density and ion velocity measurements with the ion temperature data supports the foregoing explanation for the observed greater morning sector temperature enhancements.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA088iA05p04114
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1983
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  • 2
    Online Resource
    Online Resource
    Lower thermospheric structure from Millstone Hill Incoherent Scatter Radar Measurements: 1. Daily mean temperature
    American Geophysical Union (AGU) ; 1983
    In:  Journal of Geophysical Research: Space Physics Vol. 88, No. A9 ( 1983-09), p. 7201-7209
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 88, No. A9 ( 1983-09), p. 7201-7209
    Abstract: Daytime temperature determinations from 151 days of incoherent scatter radar measurements at Millstone Hill (42.6°N) from 1970 to 1975 were analyzed to characterize the mean daily temperature in the lower thermosphere (105–125 km). An analytical model fitted to the measured temperatures contained terms to specify the dependencies of the mean daily temperature on day of year (annual and semiannual terms), solar cycle, solar rotation, and geomagnetic activity. The model representation, whose coefficients are tabulated, showed that seasonal, solar cycle, and geomagnetic effects were all of comparable magnitude and could each produce a variation of 5–10% in the mean temperature. The solar rotation effects were found to be small. The annual term and the smaller semiannual term combined to produce a summer or later summer temperature maximum. Geomagnetic activity effects were determined on the basis of a delayed Kp index, and a propagation delay of 3.7 hours was obtained from the model fit to the lower thermospheric temperature measurements. An increase in either mean solar flux or Kp produced an increase in the mean daily temperature, the magnitude of the increase generally being larger at the highest altitudes. Comparison of the model results with a model based on temperatures measured at Saint Santin (44°N) showed similar seasonal variations but a larger solar cycle dependence at Millstone Hill. The strong Kp dependence found at Millstone Hill is attributed to the relatively high geomagnetic latitude of this station.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA088iA09p07201
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1983
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  • 3
    Online Resource
    Online Resource
    Incoherent scatter observations of an artificially modified ionosphere
    American Geophysical Union (AGU) ; 1984
    In:  Journal of Geophysical Research: Space Physics Vol. 89, No. A1 ( 1984-01), p. 203-215
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 89, No. A1 ( 1984-01), p. 203-215
    Abstract: The launch of NASA's HEAO‐C satellite by an Atlas/Centaur rocket on September 20, 1979, provided the first “experiment of opportunity” to test incoherent scatter radar techniques for the diagnostic study of a chemically‐induced modification of the ionosphere. The cause of the disturbance was the rocket's exhaust cloud of H 2 and H 2 O molecules that cause a rapid recombination of the F region plasma at heights above 250 km. The launch from the Kennedy Space Center (28°N, 81°W) was monitored by the incoherent scatter radar at Millstone Hill (42.6°N, 71.5°W) using low elevation angle observations over ranges exceeding 2000 km. The experiment employed various pulse lengths, integration times and scanning modes to explore optimization methods for an upcoming series of Space Shuttle induced ionospheric hole experiments. The results from the HEAO‐hole campaign include the first unambiguous observations of a gas expansion “snowplow effect,” the derivation of local and height‐integrated plasma recombination rates, and the full spatial, temporal, and dynamical morphologies of a large‐scale ionospheric hole. The campaign succeeded in demonstrating that incoherent scatter radars can play a primary role in space plasma physics “active experiments” during the next decade.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA089iA01p00203
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1984
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  • 4
    Online Resource
    Online Resource
    Seasonal variations of lower thermospheric winds from the Millstone Hill Incoherent Scatter Radar
    American Geophysical Union (AGU) ; 1983
    In:  Journal of Geophysical Research: Space Physics Vol. 88, No. A11 ( 1983-11), p. 9227-9241
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 88, No. A11 ( 1983-11), p. 9227-9241
    Abstract: From July 1976 to November 1977 regular incoherent scatter observations of E and F region ion‐drifts were conducted at Millstone Hill (42.6°N) with the steerable ( L band) and fixed (UHF) radars. Data were collected on a total of 46 days and analyzed for ionospheric electric fields and daytime neutral winds in the lower thermosphere at 105, 115, 125, and 135 km. The wind measurements characteristically exhibited the dominance of the semidiurnal oscillation with both wind components, showing a 12‐hour variation and an eastward wind maximum 3 hours ahead of a southward wind maximum. A model fit to the full set of wind measurements also allowed the smaller annual mean diurnal winds to be determined. The winds measured on 38 geomagnetically quiet days were grouped by day‐number to determine the seasonal dependence of the semidiurnal tide as well as the prevailing wind. The semidiurnal winds exhibited the following general characteristics. They typically grew in amplitude reaching a maximum in the vicinity of 125 km with larger values at the equinoxes than the solstices. The wind field at 105 km showed a significant polarization, with southward wind amplitudes larger than eastward amplitudes. Seasonal variations in tidal phase were generally less than one hour. The amplitude and phase of the measured diurnal wind were in reasonable agreement with theoretical expectations for the in situ tidal component, and there was no indication of the presence of an upward propagating diurnal component. On the other hand, the measured semidiurnal winds were consistent with being the result of the upward propagation of tidal energy from below 100 km rather than in situ tidal excitation. Attempts to match the measured vertical wind structure with that expected for the different semidiurnal Hough mode extensions in the lower thermosphere lead to no definite mode identification. The measured vertical wavelengths (70–100 km) and the altitude of maximum amplitude were in best agreement with the S 2,2 mode, while the temperature/wind ratio suggested the S 2,4 A mode. There is overall agreement of the Millstone Hill southward wind measurements with results from the French incoherent scatter radar at Saint Santin, but differences are noted as to the seasonal variation, the altitude of maximum wind, and the vertical wavelength. Some of these differences might be explained by a solar cycle dependence of tidal dissipation in the lower thermosphere.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA088iA11p09227
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1983
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  • 5
    Online Resource
    Online Resource
    Geomagnetic activity effects on semidiurnal winds in the lower thermosphere
    American Geophysical Union (AGU) ; 1983
    In:  Journal of Geophysical Research: Space Physics Vol. 88, No. A11 ( 1983-11), p. 9243-9248
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 88, No. A11 ( 1983-11), p. 9243-9248
    Abstract: Over the period from July 1976 to November 1977 regular incoherent scatter observations of E and F region ion‐drifts were conducted at Millstone Hill (42.6°N) by using the steerable ( L band) and fixed (UHF) radars. Data were collected on a total of 46 days and analyzed to give the ionospheric electric field and daytime neutral winds in the lower thermosphere at 105, 115, 125, and 135 km. The wind measurements characteristically exhibited strong 12‐hour oscillations with the eastward wind component reaching a maximum 3 hours ahead of the southward wind component. These results imply that the neutral winds are being dominated by semidiurnal tides. The geomagnetic activity dependence of the semidiurnal winds was determined by dividing the observations into quiet and disturbed periods on the basis of the Kp index. During disturbed conditions the winds were found to be significantly altered from the quiet time behavior, with the semidiurnal amplitudes reduced by 20 to 50% at the lowest altitudes and the altitude of maximum wind increased to be above 125 km. The vertical wavelength also appeared to decrease under disturbed conditions. Some of these effects might be explained by geomagnetic‐activity induced changes in lower thermospheric temperature and density, which alter the dissipation of the upward propagating tidal component. Changes in the prevailing winds also appear to be associated with Kp increases, implying a modification of the general thermospheric circulation by high‐latitude heating. The incremental winds are small (∼5m/s) and eastward/equatorward at 105 km, becoming stronger ( ∼25m/s) and westward/equatorward at higher altitudes.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA088iA11p09243
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1983
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  • 6
    Online Resource
    Online Resource
    Lower thermospheric structure from Millstone Hill Incoherent Scatter Radar Measurements: 2. Semidiurnal temperature component
    American Geophysical Union (AGU) ; 1983
    In:  Journal of Geophysical Research: Space Physics Vol. 88, No. A9 ( 1983-09), p. 7211-7224
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 88, No. A9 ( 1983-09), p. 7211-7224
    Abstract: Daytime temperature determinations from 151 days of incoherent scatter radar measurements at Millstone Hill (42°N) from 1970 to 1975 were analyzed to characterize the semidiurnal temperature oscillation in the lower thermosphere (105–125 km). An analytical model fitted to the measured temperatures contained terms to specify the dependencies of the semidiurnal term on day of year (annual and semiannual terms), solar activity, and geomagnetic activity. The model representation, whose coefficients are tabulated, showed that seasonal effects were larger than effects associated with the other parameters. The annual mean semidiurnal oscillation had a maximum amplitude of 28 K at 115 km and a vertical wavelength of 43 km. Variations associated with season were large, for example, ±17 K in amplitude and ± 1.2 hours in phase at 115 km when referred to the annual mean semidiurnal vector. Generally, the altitude of maximum was lowest at the solstices, and the longest vertical wavelength occurred in winter. Semidiurnal temperature measurements from Saint Santin showed good agreement with the Millstone Hill model results in winter, but some significant amplitude and phase differences were apparent in other seasons. Theoretical predictions indicated that the observed semidiurnal oscillation at Millstone Hill is primarily the upward propagating tide rather than an in situ tide. Comparisons with thermospheric Hough mode extensions indicated a temperature structure best matched by the S 2,4 mode, whereas previously reported wind measurements were found to be best matched by the S 2,2 mode. Because of their different vertical structures it was postulated that it might be possible to reproduce both the observed temperature and wind measurements by a suitable synthesis of the S 2,2 and S 2,4 modes. The observed solar flux effects could be reasonably attributed to changes in tidal dissipation in the lower temperature produced by changes in mean density and temperature with solar cycle.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA088iA09p07211
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1983
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  • 7
    Online Resource
    Online Resource
    A model representation of the ionospheric electric field over Millstone Hill (Λ=56°)
    American Geophysical Union (AGU) ; 1981
    In:  Journal of Geophysical Research: Space Physics Vol. 86, No. A7 ( 1981-07), p. 5801-5808
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 86, No. A7 ( 1981-07), p. 5801-5808
    Abstract: Incoherent scatter observations of E and F region drifts over Millstone Hill (42.6°N) were made routinely from May 1976 to November 1977 at monthly intervals. Some 667 hours of data gathered over this 19‐month period were analyzed to yield the F region polarization electric field in the magnetic south and magnetic east directions. The data have been used to construct an analytical model for these two field components as functions of (1) local time, (2) day of year, and (3) magnetic activity (represented by the Kp index). Models were constructed both with and without the constraint that the daily average of the E‐W field be zero. The model employs 35 coefficients for each electric field component which were found by a least mean squares fit to the data and are tabulated.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA086iA07p05801
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1981
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  • 8
    Online Resource
    Online Resource
    Seasonal and magnetic activity variations of ionospheric electric fields over Millstone Hill
    American Geophysical Union (AGU) ; 1981
    In:  Journal of Geophysical Research: Space Physics Vol. 86, No. A1 ( 1981-01), p. 103-118
    Details
    In: Journal of Geophysical Research: Space Physics, American Geophysical Union (AGU), Vol. 86, No. A1 ( 1981-01), p. 103-118
    Abstract: Incoherent scatter observations of E and F region drifts over Millstone Hill (42.6°N) were made routinely from May 1976 to November 1977 at monthly intervals. Some 667 hours of data gathered over this 19‐month period have been analyzed to yield the ionospheric polarization electric field in the magnetic south and magnetic east directions for three seasons during quiet magnetic conditions, as well as the annual average during magnetically disturbed conditions (Kp ⩾3 o ). The results for the quiet days are compared with earlier results obtained at Millstone Hill [Kirchhoff and Carpenter, 1976] and with those reported from St. Santin (44.6°N) by Blanc and Amayenc [1979] . We find that there is best agreement between Millstone and St. Santin for the east‐west component of the field which exhibits a roughly 12‐hour period variation during the daytime in all seasons. The north‐south field exhibits a predominantly diurnal variation that, in winter and equinox, is almost identically opposite at the two stations. This component shows substantial changes in summer at both stations and, at Millstone, appears to exhibit a significant (∼1mV/m) average northward component not seen at St. Santin. We attribute the differences in the quiet time behavior observed at Millstone and St. Santin to the very large difference in the latitude of their conjugate points where, presumably, the tides (and hence the dynamo effects) are dissimilar. There are marked differences in the electric field patterns between disturbed and quiet times which mainly are evident in the nighttime period. Commencing after noon, there is a gradual enhancement of both the northward and eastward fields on disturbed days. The eastward field reverses near midnight and becomes westward until dawn, while the other component remains northward for most of the night. These changes are thought to result from the penetration of auroral electric fields inside the plasmasphere.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JA086iA01p00103
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1981
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