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

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 ▿ · (Nv), 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.     

Comparison of total electron content calculated using the IRI with observations in China

Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the foF2 and M(3000)F₂ calculated using the Asia Oceania Region F₂-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile.     

Electron temperature and electron density in the F-region of the ionosphere. I. Observed relationship

Ionospheric data from three incoherent scatter stations over the height range 225–450 km were studied for all daylight hours over a wide range of solar conditions. The relationship between electron temperature T_e, electron density Nand solar flux at 10.7 cm wavelength S_10.7 was expressed as T_e = A−B·(N−5 × 10¹¹) + C·(S_10.7−750), where N is in units of m⁻³ and S_10.7 in kJy.This provided a very satisfactory expression for all data taken at Malvern and St. Santin between 0800 and 1600 LT. For data taken at Arecibo, however, the linearity broke down at low electron densities. The data from all three stations were therefore divided into two sets according to electron density and reexamined.ForN < 5 × 10¹¹ m⁻³ B increased steadily with height and decreased steadily with latitude.For N > 5 × 10¹¹ m⁻³ B did not appear to vary with height, with season or with latitude. C was approximately constant for all sets of data.The different mechanisms involved in the heat balance of the electron population are discussed and a qualitative explanation for the relationship is proposed.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

Studies of high latitude mesospheric turbulence by radar and rocket 1: energy deposition and wave structure

During early spring, 1985, the MAE-3 (Middle Atmospheric Electrodynamics) Program was conducted at Poker Flat Research Range, Alaska to study the origin of wintertime mesospheric echoes observed with the Poker Flat MST radar there, by probing the mesosphere with in situ rocket measurements when such echoes occurred. Pre-launch criteria required the appearance of echoes exhibiting some wave structure on the MST radar display; these could be met even under weak precipitation conditions with riometer absorption near or above 1.0 dB. Two morning rockets were launched under such conditions, the first (31.048) on 29 March 1985, at 1703 UT and the second (31.047) on 1 April 1985, at 1657 UT. Both payloads were deployed on a high altitude parachute near a 95 km apogee to provide a stable platform for data acquisition within the mesosphere (below 80 km). Each payload carried a solid state detector to measure energetic electrons between 0.1 and 1.0 MeV and an NaI crystal detector to measure x-rays from >5 to >80 keV. Payload 31.048 also carried a positive ion ‘turbulence’ probe which measured ion density changes (ΔN_i/N_i) during payload descent, whereas 31.047 carried a nose tip ‘turbulence’ probe designed to measure electron density changes (ΔN_e/N_e) during upleg ram conditions plus a Gerdien condenser for the measurement of bulk ion properties during downleg. The energy deposition curves for each event exhibited peak deposition rates between 75 and 80 km with a half width of 16–18 km, almost exclusively induced by precipitating relativistic electrons. They also showed a maximum bottomside gradient between 65 and 75 km. Radar echoes and atmospheric turbulence were observed in the same altitude domain, consistent with the anticipated need for adequate free thermal electron gradients to make such phenomena visible on the radar. The vertical wave structure from radar echoes was found to be consistent with that observed in horizontal wind and temperature profiles measured by Datasondes flown shortly after each large rocket. An analysis of the wave structure from radar data has shown that although large scale waves (λ_z ~ 7 km) were found to be present, a higher frequency shorter wavelength (∼ 1–3 km) component probably played a more significant role in modulating the signal-to-noise structure of the radar echoes.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

Polar thermospheric Joule heating,and redistribution of recombination energy in the upper mesosphere

Kellogg, W. W. (1961, J. Met. 18, 373) suggested that transport of atomic oxygen from the summer into the winter hemisphere and subsequent release of energy by three body recombination, O + O + N₂→O₂ + N₂ + E, may contribute significantly to the so-called mesopause temperature anomaly (increase in temperature from summer to winter). Earlier model calculations have shown that Kellogg's mechanism produces about a 10% increase in the temperature from summer to winter at 90 km. This process, however, is partly compensated by differential heating from absorption of UV radiation associated with dissociation of O₂. In the auroral region of the thermosphere, there is a steady (component of) energy dissipation by Joule heating (with a peak near 130 km) causing a redistribution and depletion of atomic oxygen due to wind-induced diffusion. With the removal of O. latent chemical energy normally released by three body recombination is also removed, and the result is that the temperature decreases by almost 2% near 90 km. Through dynamic feedback, this process reduces the depletion of atomic oxygen by about 25% and the temperature perturbation in the exosphere from 10% to 7% at polar latitudes. Under the influence of the internal dynamo interaction, the prevailing zonal circulation in the upper thermosphere (small in magnitude) changes direction when the redistribution of recombination energy is considered. The above described effects are very sensitive to the adopted rates of eddy diffusion. They are also strongly time dependent and are significantly reduced for disturbances associated with magnetic storms.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

A comparison of northern and southern hemisphere TEC storm behaviour

Changes in total electron content during magnetic storms are compared at stations with similar geographic and geomagnetic latitudes and eastward declinations in the northern and southern hemispheres.Mean patterns are obtained from 58 storms at ±35° and 28 storms at ± 20° latitude. The positive storm phase is generally larger (and earlier) in the southern hemisphere, while negative storm effects are larger in the north. These changes reduce the normal asymmetry in TEC between the two hemispheres. Composition changes calculated from the MSIS86 atmospheric model agree well with the maximum decreases in TEC in both seasons (when changes in the F-layer height are ignored). Recovery occurs with a time constant of about 35 h; this is 50% longer than in the MSIS86 model. There is a marked diurnal variation at 35°S, with a rapid overnight decay and enhanced values of TEC in the afternoon. This pattern is inverted (and weaker) at 35°N, where night-time decay is consistently slower than on undisturbed nights. These results require a diurnal change in composition of opposite sign in the two hemispheres, or enhanced westward winds at night changing to eastward near sunrise. There is some evidence for both these mechanisms. Following a night-time sudden commencement there is a large annual effect with daytime TEC increasing for storms near the June solstice and decreasing near December. Storms occurring between November and April tend to give large, irregular increases in TEC for several days, particularly at low latitudes. In summer and winter at both stations, the mean size of the negative phase does not increase for storms with K_p> 6. The size of the positive phase is proportional to the size of the change in a_p in winter, while in summer a positive phase is seen only for the larger storms.     

Subsidiary maxima in late evening NmF2 at a low latitude station at solar maximum

At solar maximum during the late evening hours (2100–2400 LT), N_mF2 at Tahiti frequently does not decrease monotonically but exhibits temporary subsidiary maxima. Thus, in 1980, of 66 evening periods for which good data were available, 20 showed monotonie decreases but the remainder exhibited such subsidiary maxima. In summer the subsidiary maxima correspond to hmF2 significantly lower than the monotonie decreases. This lower hmF2 during subsidiary maxima corresponds to a weakening or reversal of the equatorward neutral wind, accompanied by an increase in the flux from the equatorial fountain. In winter the subsidiary maxima are fully accounted for by increases in the flux from the fountain effect, probably due to increases in the strength of the equatorial electrojet.     

Variability of winds and temperatures in the lower thermosphere

High-resolution daytime incoherent scatter radar measurements of plasma temperatures and drifts in the ionospheric E-region above Millstone Hill (42.6°N, 71.5°W) have been used to derive horizontal neutral winds and temperatures in the lower thermosphere (105–130 km) during five multi-day campaigns in 1987–1991. The underlying semi-diurnal tidal component has been determined from the observations, with characteristic average amplitudes of 50 ± 15 m/s and 30 ± 10 K. Phase propagation with altitude follows the expected structure of semi-diurnal tidal modes, but reveals complex coupling of tidal modes, particularly above 115 km. Day-to-day variability in the winds and temperatures is large, and the deviations from the semi-diurnal harmonic can exceed 40 m/s and 50 K. No strong correlations have so far been found with geophysical parameters to explain the observed variability.     

Sodium density and atmospheric temperature in the mesopause region in polar summer

Sodium lidar measurements have been performed during three summer seasons at a polar latitude (69°N), yielding profiles of sodium number density and temperature of the 85–100 km altitude region. Density measurements were performed during the months of June–August; temperature measurements only were made in August. The sodium layer was found to be both significantly weaker and more variable in summer than in winter. Measurements in summer 1987 yielded an average maximum Na density of about 900 atoms per cm³. The average maximum Na density during summer 1986 and 1988 was near 2600 atoms per cm³. The observed Na column density in summer varied from 3.10⁸ to 3.10⁹ atoms per cm². Temperature measurements were performed in August of 1986 and 1987. Mesopause temperatures of less than 125 K were observed in early August, rising rapidly throughout the month. The mean altitude of the mesopause was found to be about 87 km.     

Antarctic polar cap total electron content observations from Casey Station

In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of − 80° A, and the ‘polar hole’, a region from −70 to − 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (K_p), IMF B_y direction, and universal time (UT) [sojkaet al. (1991) Adv. Space Res.11(10), 39].     

Seasonal variations of day-time ionisation flows inferred from a comparison of calculated and observed NmF2

This paper reports on a comparison of calculated and observed monthly mean day-time ionospheric F2-peak density (NmF2) at a chain of stations from Japan to Australia for both solar minimum (1976) and solar maximum (1980). Nm values are calculated using the MSIS model for the observed peak heights (hmF2) and a simplified version of the continuity equation for day-time equilibrium conditions. The observed NmF2 values are always higher than the calculated ones in winter. This implies that a substantial downward flow of ionisation from above into the winter ionosphere is induced by the strongly poleward winter neutral wind which drives the ionisation down the field lines, lowering the peak height hmF2. In summer, winds are smaller, and the fluxes are more upward in comparison to winter. The seasonal variation of the ionisation fluxes and neutral winds are estimated for solar minimum, and compared with results of detailed calculations.     

Short term variabilities of ionospheric electron content (IEC) and peak electron density (NP) during solar cycles 20 and 21 for a low latitude station

Hourly values of IEC and of f₀F₂ (critical frequency) for a low latitude station, Hawaii (21.2°N, 157.7°W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimum years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum for this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC_max and NP_max) are not closely associated with the variations in F_10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NP_max, are somewhat reduced during the solar maximum (when the variability in F_10.7 solar flux is much higher compared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC_max and NP_max during both solar phases of the two solar cycles compared to the corresponding F_10.7 solar flux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

Inter-annual variability of tides in the mesosphere and lower thermosphere

The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (35°S), Christchurch (44°S) and Saskatoon (52°N) and the meteor wind radar at Durham (43°N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidal amplitudes and solar activity. In contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the semi-diurnal tide, but only during the southern summer and simultaneous northern winter.     

The vertical distribution of CH4, N2O,CFC-12 and CFC-11 in the middle atmosphere at mid-latitudes

CH₄, N₂O, CFC-12 and CFC-11 data analysed from samples collected by means of cryogenic samplers from five balloon flights conducted during the 1982–1984 period, are presented. The vertical distributions of these gases show structures with dominant scale size of 16 ±2.5 km. The amplitude of the perturbation is larger for the species having steeper gradient. A comparison of the present results with the results of earlier measurements representing the period of 1977–1979 do not show any appreciable change in the mixing ratios of CH₄ and N₂O. However, an average tropospheric increase of (6.5±1)% per year is found for CFC-12 and CFC-11. This annual change in the mixing ratios of CFC-12 and CFC-11 is found to increase with height suggesting a time lag of about 6 years between the troposphere and the altitude region around 25 km. The average mixing ratios of CH₄ and N₂O are compared with the available satellite/spacelab data. This data set shows a satisfactory agreement with the present in situ measurements for the height region above 30 km.     

Observed and calculated F2 peak heights and derived meridional winds at mid-latitudes over a full solar cycle

The peak height of the F2 layer, hmF2, has been calculated using the ‘servo’ model of Rishbeth et al. [(1978), J. atmos. terr. Phys. 40, 767], combined with the hedin et al. [(1988), J. geophys. Res. 93, 9959] neutral wind model. The results are compared with observed values at noon and midnight derived from ionosonde measurements at two mid-latitude stations, Boulder and Wallops Island, over a full solar cycle. The reduced height of the F2 layer, zmF2, is also computed for the same period using the observed hmF2 values and the MSIS-86 model. Day-night, seasonal, and solar cycle variations in zmF2 are attributed to neutral composition changes and winds. Anomalously low values of hmF2 and zmF2 during summer both at solar minimum and during the solar cycle maximum in magnetic activity may be associated with increases in the molecular to atomic ion concentration ratio. Under these circumstances the F2 peak may lie significantly below the O⁺ peak height calculated by the servo model. Neutral meridional winds at Wallops Island are derived from the servo model using the observed hmF2 values and the calculated O⁺ ‘balance height’. It is shown that if the anomalously low hmF2 values are used, unrealistically large poleward winds are derived, which are inconsistent with both theory and observations made using other techniques. For most conditions the F2 peak is clearly an O⁺ peak, and daily mean winds at hmF2 derived from the servo model are consistent with the hedin et al. (1988) wind model. Unexpectedly, the results do not show an abrupt transition in the thermospheric circulation at the equinoxes. Diurnal curves of the servo model winds reveal a larger day-night difference at solar minimum than at solar maximum.