Jiyao Xu

Seasonal and quasi-biennial variations in the migrating diurnal tide observed by Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED)

We present periodic variations of the migrating diurnal tide from Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics ( TIMED) temperature and wind data from 2002 to 2007 and meteor radar data at Maui (20.75 degrees N, 156.43 degrees W). There are strong quasi-biennial oscillation (QBO) signatures in the amplitude of the diurnal tidal temperature in the tropical region and in the wind near +/- 20 degrees. The magnitude of the QBO in the diurnal tidal amplitude reaches about 3 K in temperature and about 7 m/s ( Northern Hemisphere) and 9 m/s ( Southern Hemisphere) in meridional wind. The period of the diurnal tide QBO is around 24-25 months in the mesosphere but is quite variable with altitude in the stratosphere. Throughout the mesosphere, the amplitude of the diurnal tide reaches maximum during March/April of years when the QBO in lower stratospheric wind is in the eastward phase. Because the tide shows amplification only during a limited time of the year, there are not enough data yet to determine whether the tidal variation is truly biennial (24-month period) or is quasi-biennial. The semiannual (SAO) and annual oscillations (AO) in the diurnal tide support previous findings: tidal amplitude is largest around equinoxes ( SAO signal) and is larger during the vernal equinox ( AO signal). TIMED Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature and atmospheric pressure data are used to calculate the balance wind and the tides in horizontal wind. The comparison between the calculations and the wind observed by TIMED Doppler Interferometer (TIDI) and meteor radar indicates qualitative agreement, but there are some differences as well.

Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations

[1] Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/ Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) temperature observations are used to study the global structure and variability of the mesopause altitude and temperature. There are two distinctly different mesopause altitude levels: the higher level at 95–100 km and the lower level below 86 km. The mesopause of the middle- and high-latitude regions is at the lower altitude in the summer hemisphere for about 120 days aroundsummer solstice and is at the higher altitude during other seasons. At the equator the mesopause is at the higher altitude for all seasons. In addition to the seasonal variation in middle and high latitudes, the mesopause altitude and temperature undergomodulationbydiurnalandsemidiurnaltidesatalllatitudes.Themesopauseisabout 1 km higher at most latitudes and 6–9 K warmer at middle to high latitudes around December solstice than it is around June solstice. These can also be interpreted as hemispheric asymmetry between mesopause altitude and temperature at solstice. Possible causes of the asymmetry as related to solar forcing and gravity wave forcing are discussed.

Global structure and long‐term variations of zonal mean temperature observed by TIMED/SABER

Received 14 February 2007; revised 9 August 2007; accepted 17 September 2007; published 25 December 2007. [1] In this paper, we present a method of extracting zonal mean temperature and tides from TIMED/SABER satellite and discuss the features of the zonal mean temperature. The global temperature structure is presented, and the mean variations at each latitude and altitude are decomposed into semiannual (SAO), annual (AO), and quasi-biennial (QBO) components. The SAO is strong in the tropical upper stratosphere, mesosphere, and lower thermosphere. The SAO phase (measured by the time of the maximum) is at the equinox at 85 km and at solstice at 75 km. The amplitude is large compared to the annual mean temperature structure, which leads to a mesospheric inversion layer (MIL) in the zonal mean temperature around the equator at equinox. The AO is most evident at middle latitudes and displays a clear hemispheric asymmetry at solstices. The QBO in temperature is strongest in the tropical lower stratosphere; its period there is 26.6 months. There are also weak QBO signals near the mesopause and throughout the middle atmosphere at midlatitudes. The analysis of longer-term variations of the zonal mean temperature, probably affected by the solar cycle but also containing any other trends, indicates that in most regions, the zonal mean temperature decreases during the period of 5 years and is positively correlated with the solar radiation. These results use version 1.06 of the SABER temperature data, which have some known biases in the vicinity of the mesopause.

Global distribution and interannual variations of mesospheric and lower thermospheric neutral wind diurnal tide: 2. Nonmigrating tide

On the basis of the TIDI mesospheric and lower thermospheric neutral wind observations from 2002 (March) to 2007 (June), we analyze the interannual variations of nonmigrating diurnal tides from eastward zonal wave number 3 (E3) to westward zonal wave number 3 (W3). We focus on possible QBO-related variations in these nonmigrating diurnal tide components. We found: (1) a strong reverse QBO effect on the W2 meridional diurnal tide in the September equinox and the December solstice, which suggests a W2 source of nonlinear interaction between planetary wave 1 and the migrating diurnal tide; (2) the QBO effect on the peak height during the June solstice on the E3 zonal diurnal tide; (3) several nonmigrating tide components (E3, E2, E1, W3 meridional, and W3 zonal) with similar eastward phase QBO enhancement during the March equinox to the migrating diurnal tide, although to a lesser degree from 2002 to 2005; (4) the QBO effects, in some cases, during 2006 and 2007 are either less or opposite those observed between 2002 and 2005.

Using TIMED/SABER nightglow observations to investigate hydroxyl emission mechanisms in the mesopause region

Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations of vertical profiles of the OH nightglow emission rates, temperature, and ozone are used along with a theoretical model of the OH nightglow to distinguish the dominant mechanism for the nightglow. From the comparison between the model fit and the observations we conclude that the chemical reaction O-3 + H -> OH(v OH(v = 4. The analysis also determines the best fits for quenching of OH(v) by O-2 and O. The results show that the quenching rate of OH(v) by O-2 is smaller and that the removal by O is larger than currently used for the analysis of SABER data. The rate constant for OH(v) quenching by O-2 decreases with temperature in the mesopause region. The vertical profiles of atomic oxygen and hydrogen retrieved using both 2.0 and 1.6 mu m channels of Meinel band emission of the OH nightglow and the new quenching rates are slightly smaller than the profiles retrieved using only the 2.0 mu m channel and the quenching rate coefficients currently used for the analysis of SABER data. The fits of the model to the observations were also used to evaluate two other assumptions. The assumption of sudden death quenching of OH by O-2 and N-2 (i.e., quenching to the ground state rather than to intermediate vibrational levels) leads to poorer agreement with the SABER observations. The question of whether the reaction with or quenching by atomic oxygen depends on the OH vibrational level could not be resolved; assumptions of vibrational level dependence and independence both gave good fits to the observed emissions.

Nonlinear interactions between gravity waves with different wavelengths and diurnal tide

By using a compressible nonlinear two-dimensional gravity wave model, we simulate the nonlinear interactions between gravity waves (GWs) with three different vertical wavelengths and a meridional component of the diurnal tidal wind and temperature (30 degrees N, September) calculated from GSWM-00. We also compare the results with the simulation of GWs propagation in a background with zero wind. Consistent with the dispersion relation, the numerical experiments show that tidal wind reduces the vertical wavelengths of the GWs when it is in the same direction as the wave propagation, and thus increases the perturbative shear and the likelihood of instability and wave breaking, especially for waves with shorter vertical wavelengths. The breaking of GWs below the critical level can increase the amplitude of diurnal tidal wind due to the momentum deposition. Because the GW penetration height increases with vertical wavelength, the amplitude of diurnal tidal wave at higher altitudes is more likely to be affected by GWs with large vertical wavelengths. Therefore gravity wave breaking not only accelerates the mean winds, but also increases the amplitudes of the diurnal tide at various altitudes.

The features and a possible mechanism of semiannual variation in the peak electron density of the low latitude F2 layer

Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974–1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer (NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a “double-humped structure”, which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2.

Seasonal and QBO variations in the OH nightglow emission observed by TIMED/SABER

[1] Using TIMED/SABER observations, we present global distribution of the semiannual oscillation (SAO), annual oscillation (AO), and quasi‐biennial oscillation (QBO) in the OH nightglow peak emission rate and height as well as the intensity. The latitudinal variations of the SAO, AO, and QBO in the peak emission rate are similar to those in the intensity. For the peak emission rate and the intensity, the SAO and QBO amplitudes have three peaks (one at the equator and others at about 35°S and 35°N). The AO amplitude peaks at about 20°S and 20°N, respectively. The SAO phase is delayed poleward from the equinoxes at the equator to the solstices at 50°S/N; in addition, the phases of the AO are delayed poleward from 30°S. For the peak height, the SAO and QBO amplitudes have three peaks (around the equator, 40°S, and 40°N). Its AO amplitudes at 50°S and 50°N are larger than those at other latitudes; the phase of the SAO shifts from the solstice at the equator to near the equinoxes at 50°S/N. The airglow QBO is stronger in tropics than midlatitude and is likely the real QBO oscillation at the equator. In addition, the emission in the Southern Hemisphere is weaker than that in the Northern Hemisphere. The SAO and QBO are hemispherically symmetrical, and the AO is hemispherically antisymmetrical at some latitudes. The peak emission rate and peak height SAOs are generally in antiphase. The peak emission rate and intensity SAOs are generally in phase.

Perturbations of the sodium layer: controlled by chemistry or dynamics?

The chemical lifetime of the mesospheric sodium layer is calculated from the eigenvalues and eigenvectors of the sodium chemical system. This method determines a lifetime that is an excellent estimate of the relaxation time of the sodium chemical system. The lifetime is more than a day in the vicinity of the mesospheric sodium layer and is much longer than the traditionally defined chemical lifetime for sodium of a few minutes. This verifies the assumption often made that transport determines the sodium perturbations in the presence of gravity waves and other rapid air motions. At the bottom of the sodium layer, below about 85 km, photochemistry is the dominant process.

Comparison between the temperature measurements by TIMED/SABER and lidar in the midlatitude

Comparisons of monthly mean nighttime temperature profiles observed by the sodium lidar at Colorado State University and TIMED/SABER overpasses are made. In the altitude range from 85 km to about 100 km, the two observations are in very good agreement. Though within each others error bars, important differences occur below 85 km in the entire year and above 100 km in the summer season. Possible reasons for these difference are high photon noise below 85 km in lidar observations and less than accurate assumptions in the concentration of important chemical species like oxygen (and its quenching rate) in the SABER retrieval above 100 km. However, the two techniques both show the two-level mesopause thermal structure, with the times of change from one level to the other in excellent agreement. Comparison indicates that the high-level (winter) mesopause altitudes are also in excellent agreement between the two observations, though some difference (2-3 km) may exist in the low-level (summer) mesopause altitudes between ground- based and satellite-borne data. In addition, the difference in local time dependency between lidar and SABER is investigated; this difference makes the comparison of mesopause altitudes difficult.