Temperature In Lower Thermosphere Research Articles

Climatology of the Nonmigrating Tides Based on Long-Term SABER/TIMED Measurements and Their Impact on the Longitudinal Structures Observed in the Ionosphere

This paper presents climatological features of the longitudinal structures WN4, WN3, and WN2 and their drivers observed in the lower thermospheric temperatures and in the ionospheric TEC. For this purpose, two long-term data sets are utilized: the satellite SABER/TIMED temperature measurements, and the global TEC maps generated with the NASA JPL for the interval of 2002–2022. As the main drivers of the longitudinal structures are mainly nonmigrating tides, this study first investigates the climatology of those nonmigrating tides, which are the main contributors of the considered longitudinal structures; these are nonmigrating diurnal DE3, DE2, and DW2, and semidiurnal SW4 and SE2 tides. The climatology of WN4, WN3, and WN2 structures in the lower thermosphere reveals that WN4 is the strongest one with a magnitude of ~20 K observed at 10° S in August, followed by WN2 with ~13.9 K at 10° S in February, and the weakest is WN3 with ~12.4 K observed over the equator in July. In the ionosphere, WN3 is the strongest structure with a magnitude of 5.9 TECU located at −30° modip latitude in October, followed by WN2 with 5.4 TECU at 30 modip in March, and the last is WN4 with 3.7 TECU at −30 modip in August. Both the climatology of the WSA and the features of its drivers are investigated as well.

Seasonal Variations in Ion Density, Ion Temperature, and Migrating Tides in the Topside Ionosphere Revealed by ICON/IVM

Based on the plasma parameters measured by the Ion Velocity Meter (IVM) instrument on the Ionospheric Connection Explorer (ICON) satellite from 2020 to 2021, we present an analysis of seasonal variations in ion density, ion temperature, and migrating tides in the low-latitude topside ionosphere. The interannual variations in total ion density and O+ density are directly impacted by solar radiation. However, the concentration of H+ is not highly related to solar activity. The measurements show that the hemispheric dividing latitude for the seasonal variation in Ti is at about 9°N. We suggest that the reason for the hemispheric dividing latitude being 9°N is because measurements at this geographical latitude represent the closest match to the geomagnetic equator. An anticorrelation in the seasonal variations between the total ion density (as well as the O+ density) and the ion temperature is observed at all observed latitudes while the correlations between H+ density and the ion temperature are positive in most of the latitudes except for serval degrees around 9°N. The latitudinal variations in the correlation coefficients lead us to suggest that thermal conduction is likely more important than ion-neutral collision in the ion energy budget at 600 km. Additionally, semiannual oscillations with peak amplitudes in winters and summers at the extra-equatorial latitudes are revealed in the observations of diurnal migrating tides in the topside ionosphere, which are different from the latitudinal and seasonal distributions of diurnal migrating tides captured in the lower thermospheric temperature and total electron content.

A Comparison of the CIR‐ and CME‐Induced Geomagnetic Activity Effects on Mesosphere and Lower Thermospheric Temperature

AbstractNeutral temperature responses in the mesosphere and lower thermosphere (MLT) to severe geomagnetic storms induced by coronal mass ejections (CMEs) are of growing interest to the space science research community. Recently, it was found that geomagnetic activities produced by the corotating interaction regions (CIRs) caused comparable effects on the Earth's upper atmosphere. In this work, we carried out a comparative study of the temperature responses in the MLT region to these two types of geomagnetic activities, using the temperature measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments onboard the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite. Our results demonstrate that CIR‐induced geomagnetic activity produced temperature variations in the MLT region and that this effect can penetrate downward to ∼100 km at high latitudes in both hemispheres. Temperature enhancements penetrated deeper during CME‐induced geomagnetic activities, but the heating effects lasted longer during CIR‐induced geomagnetic activities. There is a hemispherical asymmetry in the geomagnetical activity induced temperature changes in the MLT region. The temperature enhancements are stronger in the southern hemisphere than in the northern hemisphere during CME events.

Analysis of the ionospheric variability based on wavelet decomposition

In this work, the ionospheric variability is analyzed by applying the wavelet decomposition technique to the noontime foF2, F10.7, interplanetary magnetic field (IMF) Bz, Ap, and lower thermospheric temperature at pressure of 10-4 hPa in 2002. Results show that the variance of periodic oscillations in the ionosphere is largest in the 2-4-day period and declines with the increase of the period. The maximum variance of the periodic oscillations in solar irradiation is in the 16-32-day period. For geomagnetic activities, most of the variance is about equally distributed on intervals of periods shorter than 32 days. Variance distributions of IMF Bz and lower thermospheric temperature are similar to those of the ionosphere. They show the maximum in the 2-4-day period and decline with the increase of the period. By analyzing the distributions of the variances, the potential connections between the ionosphere and the external sources are discussed.

A superposed epoch study of the effects of solar wind stream interface events on the upper mesospheric and lower thermospheric temperature

The response of mesosphere and lower thermosphere (MLT) temperature to energetic particle precipitation over the Earth’s polar regions is not uniform due to complex phenomena within the MLT environment. Nevertheless, the modification of MLT temperatures may require an event-based study to be better observed. This work examines the influence of precipitation, triggered by solar wind stream interfaces (SI) event from 2002 to 2007, on polar MLT temperature. We first test the relationship between the ionospheric absorption measured by the SANAE IV (South African National Antarctic Expedition IV) riometer and the layer of energetic particle precipitation from POES (Polar Orbiting Environmental Satellites). The combined particle measurements from POES 15, 16, 17 and 18 were obtained close in time to the pass of the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperature retrieval. Here, a superposed epoch technique is described and implemented to obtain average temperature profiles during SI-triggered particle precipitation. The superposed epoch average shows no significant temperature decrease below 100 km prior to the onset of SI-triggered precipitation, whereas a clear superposed average temperature decrease is observed at 95 km after the SI impact. A case study of SI event also yields similar observations. Results indicate that cooling effects due to the production of mesospheric odd hydrogen might be major contributors to temperature decrease under compressed solar wind stream.

Numerical investigation of the quasi 2 day wave in the mesosphere and lower thermosphere

The zonal wave number 3 planetary wave with about a 2 day period is a recurrent wave feature in the mesosphere and lower thermosphere (MLT). The quasi 2 day wave (QTDW) exhibits strong seasonal variability with peak amplitudes after summer solstice. In late January and early February, satellites also discovered two strong enhancements of the QTDW in meridional wind, one peak at summer midlatitudes near 90 km and the other in the tropical lower thermosphere. For the first time, this double‐peak characteristic of the QTDW meridional component is numerically investigated by the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) with the QTDW forcing prescribed at the lower model boundary and explained by the combined effect of baroclinic‐barotropic instability and Rossby normal mode. Baroclinic‐barotropic instability is capable of amplifying the QTDW, manifesting as Eliassen‐Palm (EP) flux divergence in the summer mesosphere. Without the direct contribution from baroclinic‐barotropic instability, the simulated QTDW response in a lower thermosphere temperature and horizontal wind resembles that of the (3, 0) Rossby‐gravity normal mode. In the summer middle atmosphere, the wave amplitude grows substantially, like an internal wave in the regions of large refractive index. As the wave amplitude growth ceases near the mesopause, where the zonal wind reverses direction, the QTDW reaches its maximum amplitude, displaying an enhanced meridional component in the tropical lower thermosphere. Several new aspects on the QTDWs in the MLT were also revealed. Compared with a prior model run, the propagation of the QTDW can also be prohibited by a self‐generated critical layer in a strong thermospheric easterly wind. In addition, a direct contribution from the migrating diurnal tide to the QTDW amplitude in the MLT is found. This is largely attributed to the change of the background zonal wind caused by the tide, thus leading to the increase of the QTDW refractive index in the summer middle atmosphere.

Thermal structure and CO distribution for the Venus mesosphere/lower thermosphere: 2001–2009 inferior conjunction sub-millimeter CO absorption line observations

Sub-millimeter 12CO (346 GHz) and 13CO (330 GHz) line absorptions, formed in the mesosphere and lower thermosphere of Venus (70–120 km), have been mapped across the nightside Venus disk during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as well as Doppler wind fields (described in the companion paper, Clancy et al., 2012). Temporal sampling over the hourly, daily, weekly and interannual timescales was obtained over 2001–2009. On timescales inferred as several weeks, we observe changes between very distinctive CO and temperature nightside distributions. Retrieved nightside CO, temperature distributions for January 2006 and August 2007 observations display strong local time, latitudinal gradients consistent with early morning (2–3 am), low-to-mid latitude (0–40NS) peaks of 100–200% in CO and 20–30 K in temperature. The temperature increases are most pronounced above 100 km altitudes, whereas CO variations extend from 105 km (top altitude of retrieval) down to below 80 km in the mesosphere. In contrast, the 2004 and 2009 periods of observation display modest temperature (5–10 K) and CO (30–60%) increases, that are centered on antisolar (midnight) local times and equatorial latitudes. Doppler wind derived global (zonal and should be SSAS) circulations from the same data do not exhibit variations correlated with these CO, temperature short-term variations. However, large-scale residual wind fields not fit by the zonal, SSAS circulations are observed in concert with the strong temperature, CO gradients observed in 2006 and 2007 (Clancy et al., 2010). These short term variations in nightside CO, temperature distributions may also be related to observed nightside variations in O 2 airglow (Hueso, H., Sánchez-Lavega, A., Piccioni, G., Drossart, P., Gérard, J.C., Khatuntsev, I., Zasova, L., Migliorini, A. [2008]. J. Geophys. Res. 113, E00B02. doi:10.1029/2008JE003081) and upper mesospheric SO and SO 2 layers (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60). The retrieved temperature profiles also exhibit 20 K long-term (2001–2009) variations in nightside (whole disk) average mesospheric (80–95 km) temperatures, similar to 1982–1991 variations identified in previous millimeter CO line observations (Clancy et al., 1991). Global average diurnal variations in lower thermospheric temperatures and mesospheric CO abundances decreased by a factor-of-two between 2000–2002 versus 2007–2009 periods of combined dayside and nightside observations. The infrequency and still limited temporal extent of the observations make it difficult to assign specific timescales to such longer term variations, which may be associated with longer term variations observed for cloud top SO 2 (Esposito, L.W., Bertaux, J.-L., Krasnopolsky, V., Moroz, V.I., Zasova, L.V. [1997]. Chemistry of lower atmosphere and clouds. In: Bougher, S.W., Hunten, D.M., Phillips, R.J. (Eds.), VENUS II, 1362pp) and mesospheric water vapor (Sandor, B.J., Clancy, R.T. [2005]. Icarus 177, 129–143) abundances.

Kelvin waves in stratosphere, mesosphere and lower thermosphere temperatures as observed by TIMED/SABER during 2002–2006

Abstract Temperature measurements from the SABER instrument on the TIMED spacecraft are used to elucidate the properties of Kelvin waves and other equatorial oscillations over the altitude range 20–120 km during 2002–2006. The dominant Kelvin waves transition from long periods (52–10 days) and short wavelengths (9–13 km) in the stratosphere, to shorter periods (2–3 days) and longer wavelengths (35–45 km) in the 80–120 km height region. Ultra-Fast Kelvin Waves (UFKW) with periods of 2.5–4.5 days intermittently exist at amplitudes of order 3–10 K between 80–120 km during all months of the year, with variability at periods typically in the 20–60 day range. An Intra-seasonal oscillation (ISO) of zonal mean temperatures also exists with periods 20–60 days that may be driven by Eliassen-Palm Flux Divergences (EPFD) due, at least in part, to UFKW and migrating diurnal tides.

Nocturnal temperature structure in the mesopause region over the Arecibo Observatory (18.35°N, 66.75°W): Seasonal variations

We present the mean seasonal climatology of the nocturnal temperature structure in the mesopause region (80–105 km) above the Arecibo Observatory, Puerto Rico (18.35°N, 66.75°W) from 106 nights of potassium Doppler lidar observations between December 2003 and September 2006. This first complete range‐resolved mesopause climatology for a tropical latitude exhibits several unique features. Compared to higher latitude sites, mesospheric temperature inversion layers in the nocturnal means are much weaker at Arecibo. Seasonally large inversions occur in summer but are almost non‐existent during the rest of the year. The Arecibo climatology shows a three‐level mesopause: a high altitude in summer (∼100 km), a medium altitude in late autumn and winter (∼96 km), and a low altitude in early spring (∼91 km). The mesopause is cold in the solstices (∼171 K in summer, ∼176 K in winter) and warm around equinoxes, particularly late autumn when it is near 195 K, while the spring mesopause temperature is close to 185 K. The lower thermosphere around 100 km at Arecibo shows a decreasing temperature from spring to summer when it reaches its coldest temperature, which is contrary to the increasing temperature observed at all midlatitude locations. Semiannual variations in the seasonal temperature have amplitudes as large as the annual variations through most of the MLT altitude range at Arecibo. These observed seasonal variations appear to be associated with the semi‐annual oscillation, a predominantly tropical phenomenon. This report provides one of the very few observations of the semi‐annual oscillation in lower thermosphere temperature.

Ground-based mesospheric temperatures at mid-latitude derived from O 2 and OH airglow SATI data: Comparison with SABER measurements

Rotational temperatures obtained from the O 2 Atmospheric (0–1) nightglow band and from the OH (6–2) band, with a Spectral Airglow Temperature Imager (SATI) instrument at Sierra Nevada Observatory ( 37 . 06 ∘ N , 3 . 38 ∘ W ) are presented. A revision of the temperatures obtained from the Q branch of the (6–2) Meinel band has been undertaken. First, new experimental Einstein coefficients for these lines have been introduced and the temperatures derived from the Q lines (1, 2 and 3) of the (6–2) OH Meinel band have been compared to those deduced from the P lines (2 and 4) of the same band of spectra taken by a spectrograph at Boston University. The new set of SATI data has been used to analyse the seasonal behaviour of the mesospheric and lower thermospheric temperatures. Atmospheric temperatures deduced from SATI and from satellite observations with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the TIMED satellite, have also been compared. SABER temperatures at 95 km are slightly warmer (about 2.5 K) than SATI temperatures while at 87 km they are slightly colder (about 5.7 K). Also, similar patterns of seasonal and day to day variations are found in the temperatures retrieved from both instruments at the latitude of 37 ∘ N .

Observations on Stratospheric-Mesospheric-Thermospheric temperatures using Indian MST radar and co-located LIDAR during Leonid Meteor Shower (LMS)

Abstract. The temporal and height statistics of the occurrence of meteor trails during the Leonid meteor shower revealed the capability of the Indian MST radar to record large numbers of meteor trails. The distribution of radio meteor trails due to a Leonid meteor shower in space and time provided a unique opportunity to construct the height profiles of lower thermospheric temperatures and winds, with good time and height resolution. There was a four-fold increase in the meteor trails observed during the LMS compared to a typical non-shower day. The temperatures were found to be in excellent continuity with the temperature profiles below the radio meteor region derived from the co-located Nd-Yag LIDAR and the maximum height of the temperature profile was extended from the LIDAR to ~110 km. There are, how-ever, some significant differences between the observed profiles and the CIRA-86 model profiles. The first results on the meteor statistics and neutral temperature are presented and discussed below. Key words. Atmospheric composition and structure (pres-sure, density, and temperature) History of geophysics (at-mospheric sciences) Meteorology and atmospheric dynamics (middle atmosphere dynamics)

Solar activity and QBO influence on the temperature regime of the subauroral middle atmosphere

The relationship between the temperature of the stratosphere at ∼27 km , the mesopause at ∼87 km and lower thermosphere at ∼97 km and the flux density of solar 10.7 cm radio emission (the F 10.7-index) depending on the QBO phase of the stratospheric equatorial zonal wind is analyzed using night sky temperature data. These data have been obtained from the Doppler broadening of the 557.7 nm [OI] line contour using the Fabry–Perot interferometer in Maimaga (geogr. ϕ=63°N; λ=129.7°E) for the period of 1979–1991 and from the distribution of the rotational structure intensities of the OH(6,1) hydroxyl band measured in Yakutsk ( ϕ=62°N; λ=129.7°E) for 1965–1970. It is shown that for the west QBO phase in winter their is a positive correlation between temperature variations in the stratosphere and the F 10.7-index, with a negative correlation at the mesopause and in the lower thermosphere. In the east QBO phase an anticorrelation between the stratosphere and lower thermosphere temperatures is observed, with a weak positive correlation at the mesopause.

Comparison of mesospheric and lower thermospheric residual wind with High Resolution Doppler Imager, medium frequency, and meteor radar winds

The objective of this study is to compare observed mean meridional winds with those deduced from theory. The diabatic circulation is computed from High Resolution Dopper Imager (HRDI) mesospheric and lower thermospheric temperatures during January and July conditions. The meridional wind component is compared with HRDI Eulerian mean meridional winds near 95 km and with seasonal averages of meridional winds at a number of radar medium‐frequency (MF) and meteor wind (MW) sites. The diabatic wind is directed from the summer toward the winter hemisphere. Peak values exceed 20 m s−1 and are observed at 105 km near 20° in the summer hemisphere. A secondary maximum of about 10 m s−1 is observed in the wintertime lower mesosphere during the July case. The diabatic wind is qualitatively consistent with HRDI 95‐km mean meridional winds at latitudes equatorward of 50°. Time‐averaged summertime radar winds are consistent with HRDI and diabatic winds between 50°S and 20°N. At winter midlatitudes, MF radar winds are directed oppositely to the diabatic wind, while one available MW measurement is directed with the diabatic wind. The zonal acceleration implied by the diabatic wind is about 150–200 m s−1 d−1 in the midlatitude summer lower thermosphere.

Long-term changes of the mesosphere and lower thermosphere temperature and composition

A brief review of the recent publications on long-term trends of atmospheric and ionospheric parameters in the mesosphere and lower thermosphere is presented. The rocket measurements of the ion composition in the ionospheric E region are analyzed. A systematic decrease of the ratio of two principal molecular ions NO + and O 2 + from the sixties to the end of the eighties is detected. It is shown that the derived trends of the ion composition may be important in linking the temperature trends around the mesopause and the electron concentration trends in the E region.

Retrieval of thermospheric atomic oxygen, nitrogen and temperature from the 732 NM emission measured by the ISO on ATLAS 1

The Imaging Spectrometric Observatory (ISO) was a part of the ATLAS 1 Mission flown on the shuttle Atlantis from March 24 to April 2, 1992. During limb scanning operations, the ISO measured the O+(2P) ion emission at 732 nm. We have used a numerical inversion technique to retrieve thermospheric atomic oxygen, molecular nitrogen and temperature profiles. These preliminary results indicate a lower thermospheric temperature cooler than that predicted by MSIS for the solar conditions during the mission. Although the densities agree at low altitudes, the reduced scale height produces O and N2 densities 25% lower than the MSIS at 300 km.

Line‐by‐line radiative excitation model for the non‐equilibrium atmosphere: Application to CO2 15‐μm emission

We describe a new line‐by‐line (LBL) algorithm for radiative excitation in infrared bands in a non local thermodynamic equilibrium (non‐LTE) planetary atmosphere. As a specific application, we present a predictive model for the terrestrial CO2 15‐μm emission that incorporates this generic algorithm, and validate the model by comparing its results with emission spectra obtained in a limb‐scanning rocket experiment. The radiative‐excitation algorithm has certain unique features. Being a completely monochromatic calculation, it includes the detailed layer‐by‐layer variation of the shapes of the individual lines in its evaluation of atmospheric transmittivity; and, being an iterative algorithm, it avoids the need to construct and invert large matrices, so that a fine layering scheme can be implemented. It also includes a simple correction procedure to minimize the most serious error due to having discrete layers. These features contribute to accurate radiative transfer results and reliable atmospheric cooling rates. For altitudes above 40 km, we present results of model calculations of CO2(v2) vibrational temperatures, 15‐μm limb spectral radiances, and cooling rates, for the main band as well as for weaker hot and isotopic bands. We calculate the excitation and deexcitation rates due to different processes, including the radiative pumping due to individual atmospheric layers. We compare the predicted limb radiance with earthlimb spectral scans obtained in the SPIRE rocket experiment over Poker Flat, Alaska, and get excellent agreement as a function of both wavelength and tangent height. This constitutes the first validation of a long‐wavelength CO2 non‐LTE emission model using an actual atmospheric data set, and it verifies the existence of certain aeronomic features that have only been predicted by models. It also constrains the previously unknown value of the very important rate constant for deactivation of the CO2 bending mode by atomic oxygen to the range of 5–6 × 10−12 cm3/(mol s) at mesospheric and lower thermospheric temperatures. We discuss the significance of this large value for the terrestrial and Venusian thermospheres. We also discuss the convergence rate of the iterative scheme, the model's sensitivity to the background atmosphere, the importance of the lower boundary surface contribution, and the effects of the choice of the layer thickness and the neglect of line overlap.

Winter-season mesopause and lower thermosphere temperatures in the northern polar region

Mesopause/lower thermosphere nocturnal temperatures have been deduced from spectrophotometric observations of the night airglow OH emissions at Longyearbyen, Svalbard (78°N), during four winter seasons (1980–1985). A monthly average temperature maximum of 223 K is found to occur in January with monthly averages of 206, 212,212 and 198 K respectively for November, December, February and March. A relatively low temperature in late December followed by a very warm mesopause in early January seems to be consistent for all four seasons and might be associated with changes in transmission of gravity waves to the upper mesosphere in connection with stratospheric and lower mesospheric circulation changes.

Geomagnetic activity effects on semidiurnal winds in the lower thermosphere

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.

Lower thermospheric structure from Millstone Hill Incoherent Scatter Radar Measurements: 1. Daily mean temperature

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.

Lower-thermosphere temperature determined from the line profiles of the O I 17,924-K (5577 Å) emission in the night sky, 2, Interaction with the lower atmosphere during stratospheric warmings

Interaction between the lower thermosphere, at 40°N, and the stratosphere has been found during stratospheric warming events. This interaction shows a temperature decrease of the lower thermosphere of about 10°K (with a 95% confidence interval from 4.2°K to 17°K). Cooling already occurs by the time of peak stratospheric warming; however, maximum cooling appears nearly one lunation later. The measurements show the average magnitude of the cooling effect to be independent of solar activity, magnetic activity, and other long-term secular variations to which the temperatures in the lower thermosphere are sensitive and to be independent of a 34° longitude separation in observing sites. The results indicate that the magnitude of the cooling effect may be dependent on the time of occurrence of the stratospheric warming during the winter; however, there are not enough measurements to test this contention fully.