Qihou Zhou

The micrometeoroid mass flux into the upper atmosphere : Arecibo results and a comparison with prior estimates

Radar micrometeor observations at Arecibo Observatory have enabled direct estimates of the meteoroid mass flux into the upper atmosphere. We report mass flux determinations from November 1997/1998 observations that are based on the observed number of meteor events per day in the 300-m diameter Arecibo beam and on particle mass determinations from that fraction of all particles for which deceleration is measured. The average mass of the Arecibo micrometeoroids that manifest observable deceleration is ∼0.32/0.76 µgm/particle with a resultant annual whole-Earth mass flux of 1.6 × 106/2.7 × 106 kg/yr over the ∼10−5−10² µgm mass range for 1997/1998, respectively. The annual whole-earth mass flux per decade of particle mass is calculated and compared with that of Ceplecha et al. [1998] (3.7 × 106 kg/yr) and with that derived by Love and Brownlee [1993] (LB) from small particle impact craters on the orbital Long Duration Exposure Facility (LDEF). We also give the LDEF results as significantly modified using the Arecibo-determined average particle velocity of ∼50 km/sec—much larger than the effective value of 12 km/sec used by LB. This modification results in a net LDEF mass flux of 1.8×106 kg/yr—7% of the value we determined from reanalysis of the LB data using their original 12 km/sec mean impact speed. These results may provoke some debate.

First observation of meteoritic charged dust in the tropical mesosphere

We discuss a recent sounding rocket experiment which found charged dust in the Earths tropical mesosphere. The dust detector was designed to measure small (5000–10000 amu) charged dust particles, most likely of meteoric origin. A 5 km thick layer of positively charged dust was found at an altitude of 90 km, in the vicinity of an observed sporadic sodium layer and sporadic E layer. The observed dust was positively charged in the bulk of the dust layer, but was negatively charged near the bottom.

A study of the role of ion-molecule chemistry in the formation of sporadic sodium layers

Over two campaigns in 1998 and 1999, multiple sporadic sodium events were observed by the Arecibo Observatory sodium density lidar while simultaneously monitoring the plasma density using the incoherent scatter radar. In this paper, we test the theoretical explanation proposed by Cox and Plane (1998) where Na + in a plasma layer is neutralized via an ion–molecule mechanism to form a sporadic sodium layer. A particular challenge is to interpret observations made in a Eulerian frame of observation where the spatial and temporal characteristics of events cannot easily be separated. The reaction scheme in the original mechanism is modi=ed to include the reactions NaO + +N2 → Na + ·N2 +O and NaO + +O2 → Na + +O3, following the results of theoretical quantum calculations. Six unique case studies of sporadic sodium layers are presented here, and excellent agreement between simulation and observations was obtained for =ve of them. c � 2002 Published by Elsevier Science Ltd.

Observations of ion layer motions during the AIDA campaign

Abstract The AIDA-89 campaign has yielded the most comprehensive set of low-latitude incoherent scatter radar power profiles and derived electron concentration results ever made. These results have been used to study the time-height trajectories of 80–150 km ion layers and serve to gauge both the periodicity and variability of ion layer structure throughout the campaign. Features of the AIDA ion layer trajectories point to a dynamics ‘zoo’ of processes ranging from multiday-period waves, tides and acoustic-gravity waves (AGWs) to geomagnetic storm effects and evidence of coupled neutral sodium and ion layer/plasma processes. The semidiurnal and diurnal tides are evidenced in the almost always present layers, the Tidal Ion Layers (TILs), which are identified by their regular and periodic trajectories that also display regions of variable mixing or confluence of the various tides. The TILs are contrasted with the truly sporadic layers that include sporadic E and sporadic intermediate layers. The sporadic layers may be formed due to interaction of the tidal wind system with AGWs. The formation process may involve horizontal as well as vertical ion convergence mechanisms and/or various non-linear effects. Limits to the study derive from volume undersampling due to use of the single radar beam.

An analysis of tidal and planetary waves in the neutral winds and temperature observed at low-latitudeEregion heights

We present an extensive analysis of tidal and planetary waves in the altitude range of 94 to 144 km for the January 20-30, 1993, period using the temperature and winds measured by the Arecibo incoherent scatter radar (ISR). This is the first time that simultaneous observational results for the 6-8, 12, 24 hour tides and a quasi 2-day planetary wave at E region heights have been reported at tropical latitudes. In order to derive the major oscillations from the mainly daytime data, we fill in the nighttime periods with assumed data values and large error bars when valid measurements are not obtainable and then fit the data using the measured and assumed errors as weight. Simulated results show that such a method is demonstratively better than leaving the nighttime as a gap. The salient features of the tidal analysis results include the following: (1) Despite their large day-to-day variabilities in amplitude, the 6-8 hour oscillations are shown to be upward propagating tides. (2) The vertical wavelength of the semidiurnal tide in the zonal wind lengthens from 25 to 110 km for the altitude range from 94 to 135 km while the vertical wavelength of the meridional wind remains constant. The semidiurnal component of the meridional wind experiences little dissipation above 106 km, which is indicative of the turbopause height. (3) The diurnal tide, which has often been assumed to be negligible in earlier observations, can be quite substantial above 110 km. (4) A 2-day planetary wave dominates the diurnal and semidiurnal tides in the meridional wind between 97 and 108 km. Continuous ISR operation during the January 20-30, 1993, campaign also provides an opportunity to examine the tidal variability both in phase and amplitude.

Implications of meteor observations by the MU Radar

We report high resolution meteor echo observations using the Kyoto University Middle and Upper (MU) Atmosphere 46.5 MHz Radar. When the MU radar was pointed perpendicular to the geomagnetic field lines (B), numerous long-lived range spread trail echoes were observed which were largely absent when the beam was pointed in the vertical and parallel-to-B directions. This shows that this type of trail echo is largely due to scattering structures aligned along B. Additionally, nearly all the head echoes displaying an along-the-beam velocity component were followed by range spread echoes in the perpendicular-to-B pointing geometry. This demonstrates that meteoric field aligned irregularity is present in essentially all meteors up to the detection limit of the MU radar. Practically all the spectra are limited within a bandwidth corresponding to a Doppler shift of 320 m/s, suggesting that the two stream instability is absent most of the time. Meteoric field aligned structures can be a potential error source for aeronomical applications if they are not appropriately considered.

Meteor observations by the Arecibo 430 MHz incoherent scatter radar. II. Results from time-resolved observations

Abstract The paper discusses an incoherent scatter radar (ISR) power profile interference detection and meteor filtering technique and presents the meteor interference statistics derived from the Arecibo 430 MHz ISR. The statistics obtained from the Arecibo ISR are comparable with the meteor statistics obtained by other radar and optical techniques. Using the hourly rate of meteor flux, the average visual magnitude of the meteors i s estimated to be as faint as + 14, probably, the faintest ever reported. For low echo meteors, the mean ablation height exhibits a logarithmic relationship with the returned power below 105 km. However, once the echo power exceeds a certain threshold, the mean height remains constant at 107 km for all meteors irrespective of their returned power. The unique aspect of the meteor trails reported here is that they are observed in the radial direction of the radar beam. The lack of aspect sensitivity and the high meteor rate detected by the Arecibo 430 MHz system seem to suggest that the scattering mechanism in the UHF range of frequencies may be different from the mechanism operating at VHF. Some future prospects of exploiting; meteor trails to study the neutral wind and other aspects of the meteor region are also proposed.

Simultaneous atomic and ion layer enhancements observed in the mesopause region over Arecibo during the Coqui II Sounding Rocket Campaign

The NASA Coqui II sounding rocket campaign in Puerto Rico provided the opportunity to obtain a large number of lidar and incoherent scatter radar observations of atomic sodium and ion layers in the upper mesosphere and lower thermosphere. Sodium layer enhancements, coupled with ion layers, were frequently observed in the range of 90–105 km altitude. We found that above 97 km all of the enhanced Na layers were observed to have an associated ion layer, and below that altitude some Na enhancements could occur in their absence. Finally, we show one extraordinary case of a sporadic Na layer that grew to near its peak concentration before the associated ion layer appeared at its altitude.

Simultaneous meteor echo observations by large‐aperture VHF and UHF radars

We report simultaneous meteor echo observations using the Arecibo 430-MHz and 46.8-MHz radars. Using identical data-taking and meteor selection criteria, 1868 and 367 meteors were found in the 430-MHz and 46.8-MHz beams, respectively, while 145 were found in both beams during the 7 hours of observation. Of the 367 VHF echoes, there were only 10 trail echoes, while the rest were head echoes, which was quite contrary to expectation. The smaller number of meteors detected by the VHF system and its wider beam width show that UHF meteors are far smaller than the VHF meteors. We estimate that VHF head echoes have a typical effective scattering cross section of the order of 10−3 m2, while the accompanying UHF echoes have an effective scattering cross section of the order of 10−6 m2. The paucity of VHF trail echoes observed leads us to suggest that the ratio of head echo power to the trail echo power increases with decreasing meteor size. When a meteor is too small, a radar can observe the head echo but not the trail echo. Of the 145 meteors observed by both radars, the powers received by the two systems were not correlated. Although antenna beam pattern contributes to the lack of correlation, it is also possible that UHF and VHF echoes may be enhanced by different scattering mechanisms.

A proposed temperature dependent mechanism for the formation of sporadic sodium layers

Abstract We examine the influence of temperature fluctuations on the formation of sporadic sodium layers (SSLs) with particular emphasis on AIDA (Arecibo Initiative in the Dynamics of the Atmosphere) results. We present evidence suggesting that sodium abundance is very sensitive to the temperature. A 10 K increase in mesopause temperature may double the sodium concentration. Thus the sodium profile may change significantly if appropriate thermal fluctuations due to tides and/or acoustic-gravity waves (AGWs) occur. Gravity wave theory predicts that the ion convergence node, without other influences, coincides with a temperature maximum for a westward propagating wave. In this case, the ion layer coincides with the temperature maximum which results in a higher sodium concentration at or near the ion layer height. This proposed temperature dependency can, for the tidal wind field, account for the observed correlation between sodium and ion column abundances and is supported by the average O 2 (0–1) rotational temperature determinations made at Arecibo. Specifically, we propose that the formation of SSLs is due to the temperature fluctuations induced by AGWs, or other wave processes, in conjunction with a background tidal wind system. Additionally, we argue that when an AGW propagates westward, the SSL coincides with an existing tidal ion layer or with a true sporadic- E layer which forms in the net wave field convergence zone. We also note that roughly the same processes may apply to the production of intense sporadic- E layers.