Thermospheric Neutral Temperatures Research Articles

Reply to comment by Jan Lastovička on “Long‐term trends in thermospheric neutral temperature and density above Millstone Hill”

Reply to comment by Jan Lastovička on “Long‐term trends in thermospheric neutral temperature and density above Millstone Hill”

Comment on “Long‐term trends in thermospheric neutral temperatures and density above Millstone Hill” by W. L. Oliver et al.

Comment on “Long‐term trends in thermospheric neutral temperatures and density above Millstone Hill” by W. L. Oliver et al.

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

AbstractIncoherent 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.

Near-continual ground-based nighttime observations of thermospheric neutral winds and temperatures over equatorial Brazil from 2009 to 2012

We present measurements of thermospheric neutral winds and temperatures measured by two Fabry–Perot interferometers (FPIs) operating in northeastern Brazil, in the equatorial region, over the period of September 2009 through June 2012. The FPIs observe the Doppler shift and broadening of the nighttime 630.0-nm emission originating at altitudes of approximately 240km. The temperatures indicate a strong dependence on solar flux conditions, as captured by the F10.7 index, which ranged from ∼72 to 175 solar flux units over the duration of the study. The signature of the midnight temperature maximum is seen in the data, recurring each year during the local summer months (October through February). Throughout the year, the thermospheric zonal winds show an initial increase in eastward flow after sunset followed by a reduction in the midnight and early morning hours. This reduction in the zonal flow is most rapid in the local summer months. The thermospheric meridional winds show the expected signature of trans-equatorial flow from the summer to winter hemisphere. Superposed on this during the local summer months is a time of equatorward flow during the mid-evening period, 22–23LT, which is likely a manifestation of the semi-diurnal thermal tidal wave moving upward into the lower thermosphere region from below. The thermospheric neutral winds do not show as much of a dependence on the solar flux as the thermospheric neutral temperatures do, at least for the range of fluxes covered during the study period.

Longitudinal variability of thermospheric temperatures from WINDII O(1S) dayglow

A topic of great current interest in atmospheric dynamics is the developing recognition of the influence of the diurnal eastward‐propagating non‐migrating tide of wave number 3 (DE3) on the thermosphere. Longitudinal variations of the equatorialFregion ionospheric electron density show a zonal wave number 4 (wave‐4, WN4) pattern, expected for satellite observations at a fixed local time with respect to a rotating Earth. WINDII observations of airglow O(1S) volume emission rate (VER), excited by photoelectron impact on atomic oxygen at 250 km have previously been interpreted as neutral density observations; they have shown the wave 4 to be a common density perturbation in the equatorial thermosphere. The analysis has been extended further by examining thermospheric neutral temperatures from September 1992 to April 1993 at 245 km height derived from the vertical gradient of the observed daytime O(1S) VERs at 235–255 km yielding the atomic oxygen density scale heights centered at this altitude. The thermospheric temperatures show a distinct and persistent wave‐4 signature at 20°S–40°S with a pronounced increase during March equinox.

Variations in foF2 at the end of the 1990s and the beginning of the 2000s according to the SPIDR system data

Variations in the critical frequency of the F2 layer at 22 ionospheric stations within the period 1990–2010 according to the SPIDR system data are considered. A confirmation of the negative trends in foF2 for seven stations considered by one of the authors earlier on the basis of median data is obtained. It is found that both negative (a decrease in foF2 with time) and positive (growth in foF2 with time) trends of the critical frequency are observed. During the later part of the considered period (after 1997), negative trends dominate. This fact manifests itself, apparently, in an increase in the role of the decrease in the thermospheric neutral temperature in the formation of foF2 trends.

Periodic spacing between consecutive equatorial plasma bubbles

We analyze three‐years of data collected by a field‐aligned airglow imaging system located at the Cerro Tololo Inter‐American Observatory near La Serena, Chile to determine the occurrence of equatorial plasma bubbles (EPBs). On 317 of the 552 predominately clear nights of observations, structure indicative of EPBs is present. On 123 of these nights, multiple EPBs with periodic spacings were recorded with 88 nights showing 3 or more consecutive bubbles. We suggest that the periodic spacing of EPBs could be related to the properties of an underlying seed mechanism, namely gravity waves (GWs). The distribution of spacings compares favorably to the spectrum of GW induced traveling ionospheric disturbances (TIDs) measured by Vadas and Crowley (2010) from a similar geographic latitude in the northern hemisphere. Furthermore, the distribution of spacings decreases from 2006 through 2009, tracking the corresponding decrease in the thermospheric neutral temperature, Tn. As Tn decreases, GWs with larger horizontal wavelengths have smaller initial amplitudes and cannot propagate as easily to EPB seeding altitudes. Thus, our observations are consistent with GW theory.

Observations of the lower thermospheric neutral temperature and density in the DELTA campaign

Abstract The rotational temperature and number density of molecular nitrogen (N2) in the lower thermosphere were measured by the N2 temperature instrument onboard the S-310-35 sounding rocket, which was launched from Andøya at 0:33 UT on 13 December 2004, during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. The rotational temperature measured at altitudes between 95 and 140 km, which is expected to be equal to neutral temperature, is much higher than neutral temperature from the Mass Spectrometer Incoherent Scatter (MSIS) model. Neutral temperatures in the lower thermosphere were observed using the auroral green line at 557.7 nm by two Fabry-Perot Interferometers (FPIs) at Skibotn and the Kiruna Esrange Optical Platform System site. The neutral temperatures derived from the look directions closest to the rocket correspond to the rotational temperature measured at an altitude of 120 km. In addition, a combination of the all-sky camera images at 557.7 nm observed at two stations, Kilpisjärvi and Muonio, suggests that the effective altitude of the auroral arcs at the time of the launch is about 120 km. The FPI temperature observations are consistent with the in situ rocket observations rather than the MSIS model.

Middle ultraviolet remote sensing of the equatorial thermosphere during a geomagnetic storm

Abstract. We present measurements of the equatorial middle ultraviolet airglow taken during the week of 21-28 May 2000, including a strong geomagnetic storm on 24 May. Limb spectra were taken by the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) experiment on the Advanced Research and Global Observation Satellite (ARGOS). OII lines near 247.0nm and NII lines near 214.3nm provide a measure of thermospheric O and N2 changes, respectively. Our results show increases in airglow brightness of both emissions during the storm that we interpret as increases in the concentrations of both species, but the ratio of intensities that correspond to O/N2 is lower than expected during and after the storm. We also invert the emission profiles using discrete inverse theory, to obtain a measure of thermospheric neutral temperatures during this time. Although temperatures increase after the main phase of the storm, the increases do not correlate with the observed features in the intensity ratio. These data could be explained by vertical winds and tides modifying the equatorial neutral composition during storms, in addition to common heating responses.

Novel Fabry-Perot interferometer measurements of F-region ion temperature

Novel optical measurements of F-region ion temperatures have been made in conjunction with thermospheric neutral temperatures. A ground-based Fabry-Perot interferometer has been used to observe high-latitude F-region ion temperatures using the O+(2P) auroral emission at 732 nm, and upper-thermospheric neutral temperatures using the O(1D) airglow emission at 630 nm. Dual wavelength measurements were made during February 2001 and clearly show that the ion temperature is equal to or greater than the neutral temperature, as expected. EISCAT radar measurements show that there is good agreement between the incoherent scatter and optical Doppler broadening methods of inferring ion temperature.

A comparison of thermospheric winds and temperatures from Fabry-Perot interferometer and EISCAT radar measurements with models

Abstract During the nights of 8–9 and 9–10 February 1997, Fabry-Perot interferometers were operated from the EISCAT radar site at Ramfjord (69.59° N, 19.23° E) and Skibotn (69.35° N, 20.36° E). From Ramfjord, horizontal neutral winds were measured in the lower and upper thermosphere using the auronal/airglow emissions at 557.7 and 630 nm, respectively. From Skibotn, thermospheric neutral temperatures were measured using the same wavelengths. The EISCAT radar measured ion temperatures up the local magnetic field line in the height range 90 – 580 km during the first night. Neutral winds are compared to the HWM-90 and CTIP-200 models with poor agreement. Neutral temperatures are compared to the MSISE-90 and CTIP-200 models as well as EISCAT ion temperatures with good agreement.

Dynamics Explorer 2 observations of equatorial thermospheric winds and temperatures: Local time and longitudinal dependences

We have analyzed a large database of Dynamics Explorer 2 (DE 2) measurements of equatorial (<±9° latitude) thermospheric neutral temperatures and horizontal wind components in the altitude range from 200 to 400 km for quiet and active geomagnetic conditions (Kp < 3 and Kp > 3, respectively). Local time variations and longitudinal dependences on the duskside, dayside, and nightside have been separately examined. The dayside hourly averaged neutral zonal winds are relatively insensitive to variations in geomagnetic activity and appear to show an inverse dependence (lower zonal wind speed for higher Kp) in the midnight‐dawn local time sector. The duskside neutral temperatures show a region of enhancement (by ∼60 K) over the South Atlantic magnetic anomaly. The quiet time zonal winds are weaker than the corresponding active time zonal winds on the dayside near the South Atlantic magnetic anomaly. Enhanced ionization in the region, which increases the ion drag on the neutral gas, is suggested as the causal mechanism for these effects. The DE 2 averaged measurements have also been compared with MSIS‐86/HWM90 and VSH models. The data and models are in good general agreement, though, the MSIS‐86/HWM90 model predicts stronger dayside zonal winds than are observed under active conditions and the VSH model appears to underestimate the magnitude of the dayside zonal winds for both high and low Kp cases.

Thermospheric temperatures and meridional winds measured at Albany, New York during geomagnetically disturbed periods

High quality optical data, obtained by using a high-resolution Fabry-Perot interferometer, were used to derive the thermospheric neutral temperatures and meridional wind velocities at a mid-latitude station, Albany (42.7°N, 73.8°W), New York, during the period 1978–1982. A total of 24 geomagnetically active nights, when the K p index was more than 3, were selected during the observation period. The neutral temperatures varied between about 900 and 1600 K, with a mean value of 1276 ± 54 K. Thermospheric temperatures were also estimated for these selected nights, using the 83 and 86 versions of the MSIS thermospheric model to compare with the experimental values. The experimentally determined temperature variations at the F-region heights were found to be in qualitative accord with the predicted variations based on the thermospheric models. However, the MSIS-83 model gave temperatures smaller than the measured values by about 250 K, while the difference was less than 200 K with the MSIS-86 estimates. In summary, the results of this study show a fair agreement between the observed temperature variations and the estimated variations using the MSIS models for geomagnetically active periods, and there is an increasing need to resolve the quantitative differences. The meridional wind velocities are generally equatorward and they increase from about 125 m s −1 shortly after sunset, to a maximum value of around 200 m s −1 before local midnight, and then decrease to less than about 100 m s −1 toward the early morning hours and become poleward by morning twilight. The mean meridional wind is essentially equatorward in agreement with the thermospheric circulation theory and also in general accord with other mid-latitude optical wind observations.

Geomagnetic quiet-time behavior of the thermospheric meridional winds and temperature at Albany, New York

High resolution Fabry-Perot interferometer data were used to deduce the thermospheric temperatures and meridional winds at a mid-latitude station, Albany (42.7°N, 73.8°W), New York, during 6 geomagnetically quiet nights in the year 1981. The meridional wind velocities are generally equatorward and they increase from less than 100ms −1 shortly after sunset, to a maximum value of around 170ms −1 before local midnight, and then decrease to small values toward early morning hours and become poleward by morning twilight. The neutral temperature varied between 890 and 1450 K and a detailed comparison was made with the predictions from the empirical models of MSIS-83 and MSIS-86. The observed variations in the thermospheric neutral temperatures are in qualitative accord with the computed structure based on the MSIS models. In general, the MSIS-86 model gave temperatures smaller than the observed values by nearly 42 K compared to the 106 K difference obtained between the MSIS-83 model values and the experimental temperatures. In summary, the results of this study show a good agreement between the observed temperature structure and the MSIS model values for quite-time geomagnetic conditions. The mean meridional wind is essentially equatorward in agreement with the thermospheric circulation theory and is also in general accord with other mid-latitude optical wind observations.

Non-thermal distribution of O( 1D) atoms in the night-time thermosphere

The 6300 Å O( 1D- 3P) emission has been used for many years to remotely monitor the thermospheric temperature from the Doppler width of its line profile. The O( 1D) atoms in the night-time thermosphere are initially produced by the dissociative recombination of O 2 + ions with kinetic energy much greater than the thermal energy of the ambient neutrals. The validity of the technique to monitor neutral ambient temperature by measuring O( 1D) 6300 Å emission depends on the degree of thermalization of the O( 1D) atoms. The object of this study is to calculate the velocity distribution of the O( 1D) atoms and to examine the effect of non-thermal distribution on the night-time thermospheric neutral temperature determined.

The geomagnetic Sqvariation—annual, semi-annual and solar cycle variations and ring current effects

The difference between H at two magnetic observatories having the same longitude, but latitudes equatorwards and polewards of the latitude of the S q focus, yields a measure of S q intensity largely free from disturbance. Data from all days may be used, which results in improved time resolution of the variations of S q . As well as an annual variation in S q with a maximum in local summer, there is a substantial semi-annual variation with equinoxial maxima. It is proposed that the annual variation is due mainly to the annual variation in conductivity of the E-region and that the semi-annual variation arises from the semi-annual variation in the thermospheric neutral temperature. The solar cycle variation is also delineated. There is a non- S q annual variation in the midnight values of H that appears to exist also at other hours of the night and probably throughout the 24 h. It is consistent with an annual north-south motion of the ring current. The implication of this for the S q baseline problem is discussed.

Global model of longitude/UT variations in thermospheric composition and temperature based on mass spectrometer data

Measurements of N2, O, He, and Ar densities from neutral gas mass spectrometers on four satellites (Ogo 6, Aeros A, AE‐C, and AE‐D) and inferred O2 and H densities from an ion mass spectrometer on AE‐C have been combined to produce a model of longitude/universal time (UT) variations in thermospheric neutral temperature and composition. The longitude/UT model is an extension of the mass spectrometer‐incoherent scatter thermospheric model and uses spherical harmonic terms dependent on geographic latitude, longitude, and universal time. The terms which depend only on longitude indicate a temperature enhancement of about 30°K somewhat equatorward of the magnetic poles. The temperature varies also in universal time, the greatest enhancement being about 30°K near 2130 UT in the northern hemisphere and about 70°K near 0930 UT in the southern hemisphere. The extrapolated 120‐km variations are generally in phase for Ar and O2 and out of phase for He, O, and H. Changes during magnetic storms are relatively small. The combined longitude and UT variations reflect the influence of the earth's magnetic field but indicate that the variations may not be simply represented in magnetic coordinates. The longitude/UT model helps reduce differences in the local time variation between the incoherent scatter stations.

A global thermospheric model based on mass spectrometer and incoherent scatter data MSIS, 1. N2density and temperature

Measurements of neutral nitrogen density from mass spectrometers on five satellites (AE-B, Ogo 6, San Marco 3, Aeros A, and AE-C) and neutral temperatures inferred from incoherent scatter measurements at four ground stations are combined to produce a model of thermospheric neutral temperatures and nitrogen densities similar to the Ogo 6 empirical model (Hedin et al., 1974). This global model is designated MSIS (mass spectrometer and incoherent scatter). The global average temperature, the annual temperature variation, lower bound density, and lower bound temperature are discussed. The data set covers the time period from the end of 1965 to mid-1975 and also a wide range of solar activities. Diurnal and semidiurnal variations in lower bound density and temperature are considered, as is magnetic activity.

Comparison of the correlation of incoherent scatter and ionosonde measurements of temperature with calcium plage and 2800-megahertz intensities

The variation in the diurnal thermospheric neutral temperatures has been studied. Minimum and maximum temperatures were calculated from 20 sets of diurnal incoherent scatter measurements taken at Arecibo, Puerto Rico, which corresponded to a range of 2800-MHz intensity of 120 to 167×10−22watt m−2 sec−1. Minimum temperatures were also derived from 850 reduced ionosonde records obtained from Ramey, Puerto Rico, for which the 2800-MHz intensity varied between 74 and 252×10−22 watt m−2 sec−1. Both sets of measurements were seasonally distributed. Various combinations of the monthly mean and daily values of the 2800-MHz and Ca II solar indices were fitted to the data. A formula incorporating the difference between the daily and monthly average Ca II index and the monthly average 2800-MHz index provided a better fit to all the data than did formulas incorporating only the Ca II index or the 2800-MHz index alone. The phase and amplitude of the semiannual component and the term dependenton Kp were found to remain almost the same for the maximum and minimum temperature. The term dependent on the 27-day component of the solar activity was found to be about 4 times as large for the diurnal maximum as for the minimum.