Tidal Perturbations Research Articles

On the stability of hypothetical satellites coorbital to Mimas or Enceladus

In the present work, we study the stability of hypothetical satellites that are coorbital with Enceladus and Mimas. We performed numerical simulations of 50 particles around the triangular Lagrangian equilibrium points of Enceladus and Mimas taking into account the perturbation of Mimas, Enceladus, Tethys, Dione, Titan and the oblateness of Saturn. All particles remain on tadpole orbits after 10 000 yr of integration. Since in the past the orbit of Enceladus and Mimas expanded due to the tidal perturbation, we also simulated the system with Enceladus and Mimas at several different values of semimajor axes. The results show that in general the particles remain on tadpole orbits. The exceptions occur when Enceladus is at semimajor axes that correspond to 6:7, 5:6 and 4:5 resonances with Mimas. Therefore, if Enceladus and Mimas had satellites librating around their Lagrangian triangular points in the past, they would have been removed if Enceladus crossed one of these first-order resonances with Mimas.

On the use of Ajisai and Jason-1 satellites for tests of general relativity

In this paper we analyze in detail some aspects of the proposed use of Ajisai and Jason-1, together with the LAGEOS satellites, to measure the general relativistic Lense–Thirring effect in the gravitational field of the Earth. A linear combination of the nodes of such satellites is the proposed observable. The systematic error due to the mismodelling in the uncancelled even zonal harmonics would be ∼1% according to the latest present-day CHAMP/GRACE-based Earth gravity models. In regard to the non-gravitational perturbations especially affecting Jason-1, only relatively high-frequency harmonic perturbations should occur: neither semisecular nor secular bias of non-gravitational origin should affect the proposed combination: their maximum impact is evaluated to ∼4% over 2 years. Our estimation of the root-sum-square total error is about 4–5% over at least 3 years of data analysis required to average out the uncancelled tidal perturbations.

An assessment of the measurement of the Lense–Thirring effect in the Earth gravity field, in reply to: “On the measurement of the Lense–Thirring effect using the nodes of the LAGEOS satellites, in reply to “On the reliability of the so far performed tests for measuring the Lense–Thirring effect with the LAGEOS satellites” by L. Iorio,” by I. Ciufolini and E.

In this paper we reply to recent claims by Ciufolini and Pavlis about certain aspects of the measurement of the general relativistic Lense–Thirring effect in the gravitational field of the Earth. (I) The proposal by such authors of using the existing satellites endowed with some active mechanism of compensation of the non-gravitational perturbations as an alternative strategy to improve the currently ongoing Lense–Thirring tests is unfeasible because of the impact of the uncancelled even zonal harmonics of the geopotential and of some time-dependent tidal perturbations. (II) It is shown that their criticisms about the possibility of using the existing altimeter Jason-1 and laser-ranged Ajisai satellites are groundless. (III) Ciufolini and Pavlis also claimed that we would have explicitly proposed to use the mean anomaly of the LAGEOS satellites in order to improve the accuracy of the Lense–Thirring tests. We prove that it is false. In regard to the mean anomaly of the LAGEOS satellites, Ciufolini himself did use such an orbital element in some previously published tests. About the latest test performed with the LAGEOS satellites, (IV) we discuss the cross-coupling between the inclination errors and the first even zonal harmonic as another possible source of systematic error affecting it with an additional 9% bias. (V) Finally, we stress the weak points of the claims about the origin of the two-nodes LAGEOS–LAGEOS II combination used in that test.

Estimation of lunar and solar tides in the Earth’s thermosphere from density measurements by the CACTUS microaccelerometer

The densities measured by the CACTUS microaccelerometer at altitudes from 270 to 600 km are used to analyze the effect of tidal perturbations in the Earth’s thermosphere caused by the gravitational attraction of the Moon and the Sun. These tidal perturbations are considered a priori small and are not taken into account in modern atmospheric density models. The residuals between the densities measured by the CACTUS microaccelerometer and calculated by models are analyzed, and the density variations correlating with variations of the zenith angles from the Moon to the center of the Earth to the satellite and from the Sun to the center of the Earth to the satellite are found at altitudes from 270 to 600 km. The amplitude of the perturbations revealed in the study grows with height. The phase of the tidal perturbations also varies with height. The amplitude of the density variations is about 30% at 270–320 km and increases to 80% at 520–570 km. The results agree with a priori theoretical estimates obtained for tidal motion of gaseous matter with a variable density.

A spectral slope versus perihelion distance correlation for planet-crossing asteroids

Abstract The spectral slope of sunlight reflected by an asteroid surface may change with time as a result of several physical processes acting on the surface layer known as ‘space weathering’. The effects of space weathering are important for asteroid studies because they may help us to explain the paucity of spectroscopic analogues of ordinary chondrites (OCs) among the main belt asteroids. In this Letter we report a new result that may be used to understand better how asteroid surfaces evolve with time. Using a large data set of spectroscopic observations of planet-crossing asteroids (near-Earth objects and Mars-crossers), we have found a statistically significant correlation between their spectral slope and the perihelion distance. We have also determined that the Q- and Sq-type planet-crossing asteroids are more abundant (relative to more space-weathered S-type asteroids) at small perihelion distances that characterize dynamically evolved orbits. We take these results as evidence for some (so far neglected) process that modifies surfaces of planet-crossing asteroids. As originally proposed by Nesvorný et al., the close encounters of these objects with the terrestrial planets may produce tidal perturbations to their surfaces that may partially, or even completely, remove the old and weathered layers and expose non-weathered OC-like material. Our findings may provide new evidence for this process because standard models for the dynamical evolution of asteroids show that planet-crossing asteroids with small perihelion distances have undergone more frequent planetary encounters than those residing in more distant orbits. Therefore, if planetary encounters are important, the distribution of Q-type asteroids in the planet-crossing space should show a correlation with the perihelion distance, exactly as we have found here.

The effect of a dense atmosphere on the tidally induced potential of Titan

The effect of the dense atmosphere of Titan on the tidal variations of the external gravitational potential of degree two is quantified. The atmospheric tides perturb the external gravitational potential of Titan in two ways. First, the atmosphere itself contributes directly to the external gravitational potential with a period of 15.945 days. Second, the variable loading of the atmosphere induces mass redistribution within Titan, which also changes the external gravitational potential. It is shown that the relative atmospheric contributions to the tides are most likely less than 2% and vanish almost completely for the most plausible models with a subsurface ocean. This suggest that atmospheric tidal perturbations will contribute only negligibly to Cassini measurements of Titan's gravitational field so that the tidal Love numbers derived from these observations can be directly interpreted in terms of the satellite's interior.

Seasonal variation of diurnal perturbations in mesopause region temperature, zonal, and meridional winds above Fort Collins, Colorado (40.6°N, 105°W)

On the basis of lidar observations from May 2002 through April 2003, covering both day and night, we performed a harmonic analysis to extract the diurnal perturbations in mesopause region temperature, zonal and meridional winds over Fort Collins, Colorado (40.6°N, 105°W), binned every 2 months. The results were compared to predictions of the 2000 and 2002 versions of Global‐Scale Wave Model (GSWM00 and GSWM02). The diurnal tidal period oscillations showed a mixture of propagating and evanescent (trapped) modes, but the propagating modes dominated for most of the year. The agreement in temperature diurnal phases between observation and GSWM prediction is marginal. On the other hand, other than July‐August meridional winds, the observed diurnal phases in both wind components are in good agreement with GSWM predictions for most of the altitude range reported. The diurnal amplitude predictions of GSWM00 were reasonably close to lidar observations, while other than January‐February, the GSWM02 amplitude prediction overestimated the observations, typically by a factor of two. We also conducted comparisons on tidal perturbations in zonal wind between radar campaigns and our lidar observations. The lidar data agreed reasonably well with the MF radar data from 2000 to 2001 at nearby Platteville, Colorado (40.2°N, 104.7°W), but showed considerable differences with the data from other midlatitude stations from 1992 to 1993. The dominance of the evanescent mode in the temperature diurnal tidal oscillation during the early winter (November and December), which reached a peak value at midnight, was interesting and anomalous. By invoking the more recent data (November and December in 2003), as well as the diurnal temperature observations from December 1998, we report that the evanescent (trapped) diurnal tidal perturbations were robust and persisted from one year to the next.

Additional radiative cooling of the upper mesosphere and lower thermosphere caused by tidal perturbations in temperature

The effect of tidal perturbations in temperature on the rate of cooling the upper mesosphere and lower thermosphere in the CO2 15-µm band is revised. It is shown that the effect of solar semidiurnal tide on radiative cooling can be neglected. However, the solar diurnal tide leads to a marked increase in the rate of radiative cooling (up to a relative increase by about 0.1 to 0.2) near the equator at altitudes below 100 km. It is possible that this additional cooling is responsible for the air descent observed in the near-equatorial thermosphere at altitudes above 105 km.

The Dynamical Evolution of the Short‐Period Extrasolar Planet around υ Andromedae in the Pre–Main‐Sequence Stage

We study the dynamical evolution of short-period planets in the multiple planetary system during the epoch of protostellar disk depletion. Through a detailed case analysis for the triple-planet system around υ Andromedae, we identify the necessary condition for the survival of its short-period planet b. In this study, we calculate the planets' orbit evolution including effects of the post-Newtonian potential of the host star, the potential of an evolving disk, and the potential due to the flattening of the star produced by the stellar rotation and the tide from the planet. At its present-day semimajor axis of 0.059 AU, the negligible eccentricity of planet b is most likely damped by the dissipation of the tidal perturbation induced by the star on the planet. But this process also leads to the heating of the planets' interior and probably the inflation of their sizes. If planet b once had an eccentricity greater than 0.1 at its present location, the tidal dissipation process would induce it to overflow its Roche lobe, lose its mass, and undergo an orbital expansion. Using this constraint, we reconstruct the eccentricity evolution of planet b. We find that even if planet b arrives at its present location on a nearly circular orbit, its eccentricity could have been excited by the sweeping secular resonances of two outer planets, c and d, during the disk depletion. In addition to the contribution from the disk potential, the resonant condition is also modified by the precession of planet b due to the post-Newtonian correction and rotational distortion of υ And's gravitational potential. Today, the former effect offsets the secular resonance, whereas the latter effect is weak. But during the depletion of the disk, precession due to the relativistic correction is outpaced by that due to the disk potential. The passage of the sweeping secular resonance near the surface of υ And cannot be avoided unless it had a sufficiently fast spin to provide a flattened shape with a finite quadrupole moment, which dominates the precession of the planet's orbit. Finally, we show that the survival of planet b requires its eccentricity to be low at all times, which would be possible only if the spin period of υ And was shorter than 2 days during the depletion of the disk.

Temperature tides and waves near the mesopause from lidar observations at two latitudes

Daytime and nighttime temperature measurements covering several days enable the detection of tidal and wave perturbations in the mesopause region. This study presents two potassium lidar observations from 28°N and 54°N which cover more than 100 hours each. The temperature variations during this time show significant wave features, and numerical analyses allow to separate those of tidal and nontidal origin. The tidal activity under winter conditions is found to be dominated by upward propagating semidiurnal tides with amplitudes of about 10 K. Whereas a diurnal tide was also seen at 54°N, it was not detectable at 28°N. The data are discussed with respect to various available data sets and models. Comparisons with current numerical models show that especially the semidiurnal tide is clearly underestimated at both latitudes. The tidal activity exhibits a strong variability. This indicates nonlinear interactions with planetary waves, which have also been observed.

Simulating observable comets

This is the second in a series of papers presenting an attempt to reproduce the mechanisms acting currently on the Oort cloud of comets (Oort 1950, Bull. Astron. Inst. Nether., 11, 91) and producing the observed sample of long-period comets. We combine the effect of the close, recent stellar passage with the continuous action of the Galactic tidal perturbation, and concentrate on the dominant term of this effect, namely the tidal force induced by the galactic disk matter. The main results presented in the previous paper of this series are fully confirmed within a much more realistic model. The results we obtained is that the observable subpopulation of the Oort cometary cloud remained the same in number, even after the close stellar passage. The main output of such a passage is a short time variation in the observable influx of comets and strong asymmetries present in their perihelion direction distribution.

Driving Spiral Arms in the Circumstellar Disks of HD 100546 and HD 141569A

With two-dimensional hydrodynamic simulations of disks perturbed externally by stars, brown dwarfs, or planets we investigate possible scenarios that can account for the spiral structure in circumstellar disks. We consider two scenarios: spiral structure driven by an external bound planet or low-mass star, and that excited by a previous stellar close encounter or flyby. We find that both scenarios produce a morphology similar to that observed in the outer disks of HD 141569A and HD 100546: moderately open two-armed outer spiral structure. Our simulations exhibit some trends. While bound prograde objects effectively truncate a disk, a close encounter by a star instead pulls out spiral arms, spreading out the disk. Following a flyby, the morphology of the spiral structure can be used to limit the mass of the perturbing object and the time since the encounter occurred. Eccentric bound planetary perturbers tend to clear gas away from the planet more efficiently, resulting in a reduction in the excited spiral amplitude. Bound perturbers in thicker disks excite more open but lower amplitude spiral structure. When the bound object has higher mass (stellar), the disk is truncated by the Roche lobe of the planet at periapse, and each time the companion approaches periapse, spiral arms may be pulled out from the disk. Thinner disks tend to exhibit more steeply truncated disk edges. We find that the outer two-armed spiral structure at radii greater than 300 AU observed in the disk of HD 141569A is qualitatively reproduced with tidal perturbations from its companion binary HD 141569B and HD 141569C on a prograde orbit near periapse. Our simulation accounts for the outer spiral arms, but is less successful than the secular model of Augereau and Papaloizou at matching the lopsidedness or asymmetry of the disk edge at 300 AU. Our simulations suggest that the disk has been previously truncated by the tidal force from the binary and has aspect ratio or thickness h/r 0.1. The simulated disk also exhibits an enhanced density at the disk edge. We find that a bound object (stellar or planetary) is unlikely to explain the spiral structure in the disk of HD 100546. A coeval planet or brown dwarf in the disk of sufficient mass to account for the amplitude of the spiral structure would be detectable in NICMOS and Space Telescope Imaging Spectrograph images; however, existing images reveal no such object. A previous encounter could explain the observed structure, provided that the disk is thin, the mass of the perturbing star is greater than ~0.1 M?, and the encounter occurred less than a few thousand years ago. This suggests that the object responsible for causing the spiral structure is currently within a few arcminutes of the star. However, the USNO-B proper motion survey reveals no candidate object associated with HD 100546 or any moving object that could have recently encountered HD 100546. Furthermore, the probability that a field star encountered HD 100546 in the past few thousand years is very low.

The impact of the new Earth gravity models on the measurement of the Lense–Thirring effect with a new satellite

In this paper we investigate the opportunities offered by the new Earth gravity models from the dedicated CHAMP and, especially, GRACE missions to the project of measuring the general relativistic Lense–Thirring effect with a new Earth’s artificial satellite. It turns out that it would be possible to abandon the stringent, and expensive, requirements on the orbital geometry of the originally prosed LARES mission (same semimajor axis a = 12,270 km of the existing LAGEOS and inclination i = 70°) by inserting the new spacecraft in a relatively low, and cheaper, orbit ( a = 7500–8000 km, i ∼ 70°) and suitably combining its node Ω with those of LAGEOS and LAGEOS II in order to cancel out the first two even zonal harmonic coefficients of the multipolar expansion of the terrestrial gravitational potential J 2, J 4 along with their temporal variations J ˙ 2 , J ˙ 4 . The total systematic error due to the mismodelling in the remaining even zonal harmonics would amount to ∼1% and would be insensitive to departures of the inclination from the originally proposed value of many degrees. No semisecular long-period perturbations would be introduced because the period of the node, which is also the period of the solar K 1 tidal perturbation, would amount to ∼10 2 days. Since the coefficient of the node of the new satellite would be smaller than 0.1 for such low altitudes, the impact of the non-gravitational perturbations of it on the proposed combination would be negligible. Then, a particular financial and technological effort for suitably building the satellite in order to minimize the non-conservative accelerations would be unnecessary.

On the impossibility of using the longitude of the ascending node of GP-B for measuring the Lense?Thirring effect

The possibility of analyzing the node Ω of the GP-B satellite in order to measure also the Lense–Thirring effect on its orbit is examined. This feature is induced by the general relativistic gravitomagnetic component of the Earth gravitational field. The GP-B mission has been launched in April 2004 and is aimed mainly to the measurement of the gravitomagnetic precession of four gyroscopes carried onboard at a claimed accuracy of 1%. of better. The aliasing effect of the solid Earth and ocean components of the solar K1 tidal perturbations would make the measurement of the Lense–Thirring effect on the orbit unfeasible. Indeed, the science period of the GP-B mission amounts to almost one year. During this time span the Lense–Thirring shift on the GP-B node would be 164 milliarcseconds (mas), while the tidal perturbations on its node would have a period of the order of 103 years and amplitudes of the order of 105 mas.

Geophysical studies with laser-beam detectors of gravitational waves**In the very last days of preparation of this paper for publication we have tragically lost our young talented colleague and friend—Andrej Serdobolski.

The existing high technology laser-beam detectors of gravitational waves may find very useful applications in an unexpected area—geophysics. To make possible the detection of weak gravitational waves in the region of high frequencies of astrophysical interest, ∼30–103 Hz, control systems of laser interferometers must permanently monitor, record and compensate much larger external interventions that take place in the region of low frequencies of geophysical interest, ∼10−5–3 × 10−3 Hz. Such phenomena as tidal perturbations of land and gravity, normal mode oscillations of the Earth, oscillations of the inner core of the Earth, etc will inevitably affect the performance of the interferometers and, therefore, information about them will be stored in the data of control systems. We specifically identify the low-frequency information contained in distances between the interferometer mirrors (deformation of the Earth) and angles between the mirrors' suspensions (deviations of local gravity vectors and plumb lines). We show that access to angular information may require some modest amendments to the optical scheme of the interferometers, and we suggest ways of doing that. The detailed evaluation of environmental and instrumental noises indicates that they will not prevent, even if only marginally, the detection of interesting geophysical phenomena. Gravitational-wave instruments seem to be capable of reaching, as a by-product of their continuous operation, very ambitious geophysical goals, such as observation of the Earth's inner core oscillations.

Tidal perturbations and variability in the mesopause region over Fort Collins, CO (41N, 105W): Continuous multi‐day temperature and wind lidar observations

An unusually long data set was acquired at the sodium lidar facility at Colorado State University (41N, 105W), between Sep 18 and Oct 01, 2003, including a 9‐day continuous observation. This time is long enough to average out the perturbations of gravity waves and short‐period planetary waves. As such, it can be used to define tidal‐period perturbations in temperature and horizontal wind. Assuming the sodium mixing ratio is a constant of motion, the observed tidal‐period oscillation in sodium density follows that of vertical wind. Thus, the data set defines tidal‐period perturbations of temperature and wind vector. The observed amplitudes and phases were compared to Global Scale Wave Model predictions (both GSWM00 and GSWM02). We found excellent agreement in diurnal phases and reasonable agreement in semidiurnal phases. However, GSWM02 overestimates diurnal amplitudes and both model versions underestimate observed semidiurnal amplitudes. Since the data period is long enough for the study of planetary waves and of tidal variability, we perform spectral analysis of the data, revealing a strong quasi 3‐day wave in meridional wind, a 14 hour perturbation in zonal wind, and both 14‐hour and 10‐hour periods in meridional wind, likely the result of nonlinear interactions. The observed semidiurnal amplitudes are much larger than the corresponding diurnal amplitudes above 85 km, and over a few days the diurnal and semidiurnal amplitudes vary by factors of 2–3. Causes for the observed tidal variability in terms of planetary wave modulation and tide‐gravity wave interaction are explored qualitatively.

On equilibrium tides in fully convective planets and stars

We consider the tidal interaction of a binary consisting of a fully convective primary star and a relatively compact mass. Using a normal-mode decomposition we calculate the evolution of the primary angular velocity and orbit for arbitrary eccentricity. The dissipation acting on the tidal perturbation is assumed to result from the action of convective turbulence, the effects of which are assumed to act through an effective viscosity. A novel feature of the work presented here is that, in order to take account of the fact that there is a relaxation time tc (the turnover time of convective eddies) associated with the process, this is allowed to act non-locally in time, producing a dependence of the dissipation on tidal forcing frequency. Results are expressed in terms of the Fourier coefficients of the tidal potential, assumed periodic in time. We find useful analytical approximations for these valid for sufficiently large values of eccentricity e > 0.2. We show that in the framework of the equilibrium tide approximation, when the dissipative response is frequency-independent, our results are equivalent to those obtained under the often used assumption of a constant time-lag between tidal response and forcing. We go on to consider the case when the frequency dependence of the dissipative response is ∝ 1/[1 + (ωm,ktc)p], where ωm,k is the apparent frequency (pattern speed ×m) associated with a particular harmonic of the tidal forcing as viewed in the frame corotating with the primary. We concentrate on the case ωm,ktc≫ 1, which is thought to be appropriate to many astrophysical applications. We study numerically and analytically the orbital evolution of the dynamical system corresponding to different values of the parameter p. We present results from which the time to circularize from large eccentricity can be found. We find that when p 1 differs drastically. In that case the system evolves through a sequence of spin–orbit corotation resonances with Ωr/Ωorb=n/2, where Ωr and Ωorb are the rotation and orbital frequencies and n is an integer. When p= 2 we find an analytic expression for the evolution of semimajor axis with time for arbitrary eccentricity assuming that the moment of inertia of the primary is small. We confirm the recent finding of Ivanov & Papaloizou, on the basis of an impulsive treatment of orbits of high eccentricity, that equilibrium tides associated with the fundamental mode of pulsation and dissipative processes estimated using the usual mixing-length theory of convection seem to be too weak to account for the orbital circularization from large eccentricities of extrasolar planets. Generalizations and limitations of our formalism are discussed.

Statistical Tests of Load-Unload Response Ratio Signals by Lattice Solid Model: Implication to Tidal Triggering and Earthquake Prediction

Statistical tests of Load-Unload Response Ratio (LURR) signals are carried in order to verify statistical robustness of the previous studies using the Lattice Solid Model (MORA et al., 2002b). In each case 24 groups of samples with the same macroscopic parameters (tidal perturbation amplitude A, period T and tectonic loading rate k) but different particle arrangements are employed. Results of uni-axial compression experiments show that before the normalized time of catastrophic failure, the ensemble average LURR value rises significantly, in agreement with the observations of high LURR prior to the large earthquakes. In shearing tests, two parameters are found to control the correlation between earthquake occurrence and tidal stress. One is, A/(kT) controlling the phase shift between the peak seismicity rate and the peak amplitude of the perturbation stress. With an increase of this parameter, the phase shift is found to decrease. Another parameter, AT/k, controls the height of the probability density function (Pdf) of modeled seismicity. As this parameter increases, the Pdf becomes sharper and narrower, indicating a strong triggering. Statistical studies of LURR signals in shearing tests also suggest that except in strong triggering cases, where LURR cannot be calculated due to poor data in unloading cycles, the larger events are more likely to occur in higher LURR periods than the smaller ones, supporting the LURR hypothesis.

Perihelion evolution of observed new comets implies the dominance of the galactic tide in making Oort cloud comets discernable

For an Oort cloud comet to be seen as a new comet, its perihelion must be moved from a point exterior to the loss cylinder boundary to a point interior to observable limits in a single orbit. The galactic tide can do this continuously, in a non-impulsive manner. Near-parabolic comets, with specific angular momentum H∝ q , will most easily be made observable. Therefore, to reduce the perihelion distance H must decrease. Since weakly perturbed comets are, in general, more numerous than strongly perturbed comets, we can anticipate that new comets made observable by a weak tidal torque will more likely be first observed when their slowly changing perihelion distances are approaching their minimum osculating values under the action of the tide, rather than receding from their minimum values. That is, defining Δ H tide as the vector change due to the galactic tidal torque during the prior orbit, and H obs as the observed vector, the sign S≡Sign( H obs·Δ H tide) will more likely be −1 than +1 if a weak galactic tidal perturbation indeed dominates in making comets observable. Using comet data of the highest quality class (1A) for new comets ( a>10,000 AU), we find that 49 comets have S=−1 and 22 have S=+1. The binomial probability that as many or more would exhibit this characteristic if in fact S=∓1 were equally likely is only 0.0009. This characteristic also persists in other long-period comet populations, lending support to the notion that they are dominated by comets recently arrived from the outer Oort cloud. The preponderance of S=−1 also correlates with weakly perturbed (i.e., smaller semimajor axis) new comets in a statistically significant manner. This is strong evidence that the data are of sufficiently high quality and sufficiently free of observational selection effects to detect this unique imprint of the tide.

Study of the tidal variations in mesospheric temperature at low and mid latitudes from WINDII and potassium lidar observations

Abstract. Zonal mean daytime temperatures from the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) and nightly temperatures from a potassium (K) lidar are employed in the study of the tidal variations in mesospheric temperature at low and mid latitudes in the Northern Hemisphere. The analysis is applied to observations at 89km height for winter solstice, December to February (DJF), at 55° N, and for May and November at 28° N. The WINDII results are based on observations from 1991 to 1997. The K-lidar observations for DJF at Kühlungsborn (54° N) were from 1996–1999, while those for May and November at Tenerife 28° N were from 1999. To avoid possible effects from year-to-year variability in the temperatures observed, as well as differences due to instrument calibration and observation periods, the mean temperature field is removed from the respective data sets, assuming that only tidal and planetary scale perturbations remain in the temperature residuals. The latter are then binned in 0.5h periods and the individual data sets are fitted in a least-mean square sense to 12-h and 8-h harmonics, to infer semidiurnal and terdiurnal tidal parameters. Both the K-lidar and WINDII independently observed a strong semidiurnal tide in November, with amplitudes of 13K and 7.4K, respectively. Good agreement was also found in the tidal parameters derived from the two data sets for DJF and May. It was recognized that insufficient local time coverage of the two separate data sets could lead to an overestimation of the semidiurnal tidal amplitude. A combined ground-based/satellite data set with full diurnal local time coverage was created which was fitted to 24h+12h+8h harmonics and a novel method applied to account for possible differences between the daytime and nighttime means. The results still yielded a strong semidiurnal tide in November at 28° N with an amplitude of 8.8K which is twice the SD amplitude in May and DJF. The diurnal tidal parameters were practically the same at 28° N and 55° N, in November and DJF, respectively, with an amplitude of 6.5K and peaking at ~9h. The diurnal and semidiurnal amplitudes in May were about the same, 4K, and 4.6K, while the terdiurnal tide had the same amplitudes and phases in May and November at 28° N. Good agreement is found with other experimental data while models tend to underestimate the amplitudes. Key words. Atmospheric composition and structure (pressure, density and temperature) – Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)