Abstract.We study the momentum flux of the atmospheric motions in the height ranges between 6 and 22 km observed using the MU radar at Shigaraki in Japan during a 3 day period in January 1988. The data were divided by double Fourier transformation into data set of waves with downward-phase-velocity and data set of waves with upward-phase-velocity for independent momentum flux calculation. The result showed that both the 72 h averaged upward flux and downward flux of zonal momentum were negative at nearly each height, meaning that the upward flux was dominated by westward propagating waves while the downward flux was dominated by eastward propagating waves. The magnitude of the downward flux was approximately a factor of 1.5 larger than the upward flux for waves in the 2∼7 h and 7∼24 h period bands, and about equal to the upward flux in the 10-30 min and 30 min-2 h period bands. It is also observed that the vertical flux of zonal momentum tended to be small in each frequency band at the altitudes below the jet maximum (10∼12 km), and the flux increased toward more negative values to reach a negative maximum at some altitude well above the jet maximum. Daily averaged flux showed tremendous variation: The 1st 24 h (quiet day) was relatively quiet, and the fluxes of the 2nd and 3rd 24 h (active days) increased sharply. Moreover, the upward fluxes of zonal momentum below 17 km in the quiet day for each period band (10∼30 min, 30 min∼2 h, 2∼7 h, and 7∼24 h) were dominantly positive, while the corresponding downward fluxes were dominantly negative, meaning that the zonal momentum below 17 km in each period band under study were dominantly eastward (propagating along the mean wind). In the active days, both the upward fluxes and Correspondence to: F. S. Kuo (fskuo@msa.vnu.edu.tw) downward fluxes in each frequency band were dominantly negative throughout the whole altitude range 6.1-18.95 km.

Abstract. We study the horizontal structure of the atmospheric gravity waves (AGW) in the height ranges between 6 and 22 km observed using the MU radar at Shigaraki in Japan, during a 3 day period in January and a 4 day period in August 1988. The data were divided by double Fourier transformation into a data set of upward moving waves and a data set of downward moving waves for independent analysis. The phase and group velocity tracing technique was applied to measure the vertical group and phase velocity as well as the characteristic period of the gravity wave packet. Then the dispersion equation of the linear theory of AGW was solved to obtain its intrinsic wave period -horizontal wavelength and horizontal group velocity -and the vertical flux of horizontal momentum associated with each wave packet was estimated to help determine the direction of the characteristic horizontal wave vector. The results showed that the waves with periods in the range of 30 min∼6 h had horizontal scales ranging from 20 km to 1500 km, vertical scales from 4 km to 15 km, and horizontal phase velocities from 15 m/s to 60 m/s. The upward moving wave packets of wave period of 2 h∼6 h had horizontal group velocities mainly toward eastsouth-east and northeast in winter, and mainly in the section between the directions of west-north-west and north in summer.

AB STRACTThe wind field ef fect on the phase ve loc i ties of 3-to 10-me ter Farley-Buneman two-stream waves in the equa to rial E re gion ion o sphere at al ti tudes in the range of 95 -110 km is stud ied by nu mer i cal sim u la tion. The be hav ior of this two-stream wave in the uni form wind field U n in a plane per pen dic u lar to the Earth's mag netic field is sim u lated with a two-di men sional two-fluid code in which elec tron in er tia is ne glected while ion in er tia is re tained. It is con firmed that, the thresh old con di tion for the ap pear ance of two-stream waves is V C U D th s nand the phase ve loc ity of the two-stream wave at the thresh old con di tion is V p » C s + U n cos q, where q is the el e va tion an gle of the wave prop a ga tion in a lim ited range andThe first for mula in di cates that the wind field par al lel (anti-par al lel) to the elec tron drift ve loc ity will raise (lower) the thresh old drift ve loc ity by the amount of the wind speed. This means that par al lel wind is a sta ble fac tor, while anti-par al lel wind is an un sta ble fac tor of two-stream waves. This may ex plain why high speed (larger than acous tic speed) two-stream waves were rarely ob served, since larger thresh old drift ve loc ity de mands larger po lar iza tion elec tric field. The re sult of the sim u la tions at the sat u ra tion stage show that when V D was only slightly larger than V D th , the hor i zon tal phase ve loc ity of the two-stream wave would grad u ally down-shift to the thresh old phase ve loc ity C s + U n . The physical implications of which are discussed. . A three-dimen sional nu mer i cal model cal cu la tion (Hysell et al. 2002) in di cated that large hor i zon tal neu tral winds have strong effects on the equa to rial elec tro-jet and low lat i tude ion ospheric cur rent sys tem. These re ports sug gest that rou tine neu tral wind mea sure ments are re quired to ad vance the research of the ef fects of neu tral wind on the equa to rial electro-jet, and a proper for mula of wind ef fects is needed. Balsley et al. (1976) at tempted to es ti mate wind ve loc ities from mea sur ing the phase ve loc i ties of type 1/type 2 waves in the elec tro-jet. They as sumed in their mea sure ments that the phase ve loc ity of type 1 waves (two stream waves) in the frame of ref er ence of ions was the ion acous tic ve loc ity; hence, the Dopp ler ve loc ity ob served by ra dar on the ground would be (1) where r V p is the wave phase ve loc ity; r U n is the neu tral wind ve loc ity; and r k is the ra dar wave vec tor. Broche et al. (1978) also de rived the phase ve loc i ties of type 1 and type 2 ir reg u lar i ties to il lus trate the role of neu tral winds, and obtained the same Eq. (1) at the thresh old con di tion. Hanuise and Cro chet (1981) ap plied Eq. (1) to re duce the phase veloc i ties of 5 -50-m wave length type 1 waves from their radar ob ser va tion data, and found that the phase ve loc ity of short scale waves (l @ 5 m) was near the nom i nal ion acoustic ve loc ity (around 3...

Abstract. The wave packets of atmospheric gravity waves were numerically generated, with a given characteristic wave period, horizontal wave length and projection mean wind along the horizontal wave vector. Their projection phase and group velocities along the oblique radar beam (v pr and v gr ), with different zenith angle θ and azimuth angle φ, were analyzed by the method of phase-and group-velocity tracing. The results were consistent with the theoretical calculations derived by the dispersion relation, reconfirming the accuracy of the method of analysis. The RTI plot of the numerical wave packets were similar to the striation patterns of the QP echoes from the FAI irregularity region. We propose that the striation range rate of the QP echo is equal to the radial phase velocity v pr , and the slope of the energy line across the neighboring striations is equal to the radial group velocity v gr of the wave packet; the horizontal distance between two neighboring striations is equal to the characteristic wave period τ . Then, one can inversely calculate all the properties of the gravity wave responsible for the appearance of the QP echoes. We found that the possibility of some QP echoes being generated by the gravity waves originated from lower altitudes cannot be ruled out.

AB STRACTIn this pa per, a two-di men sional nu mer i cal study of a plasma den sity gra di ent ef fect on Farley-Buneman waves (FB waves) is per formed via a two-fluid code in which the elec tron in er tia is ne glected while the ion in er tia is re tained. We fo cused the sim u la tions on the in ter ac tion be tween a sin gle wave mode and the back ground E re gion where the ver ti cal den sity gra di ent pro file and the weaker than FB thresh old am bi ent elec tric field were con sid ered. From 2D den sity con tour maps, it was found that the FB wave grows in the re gion ofÑN e is the elec tron den sity gra di ent and v E is the elec tric field), the ini tial growth rate was in rea son able agree ment with the pre dic tion of the com bined lin ear the ory of Farley-Buneman and gra di ent drift in sta bil i ties, and the prop a ga tion speed was mod u lated by the gra di ent strength. Ac cord ing to the phase ve loc ity eval u ated by the Fou rier anal y sis and peak to peak es ti ma tion method, the den sity gra di ents were found to have an ef fect of low er ing the phase ve loc ity at sat u ra tion, which is smaller than ion-acous tic speed for large scale waves, and the re sults dem on strated that the re duc tion of phase ve loc ity by a den sity gra di ent ef fect was larger for a lon ger wave length wave than shorter wave length curve. It was also found that the plau si ble den sity gra di ent ef fects seem to be re lated to the thick ness of the den sity gra di ent re gion and ver ti cal elec tric field where the FB wave was trav el ing. The thicker un sta ble layer would cause a greater phase ve loc ity re duc tion than the thin ner un sta ble layer might cause, and the large driving electric field would reduce the wavelength dependence of density gradient effect on the saturation phase velocity.