Multiple sounding rocket observations of charged dust in the polar winter mesosphere

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004ja010502, 2005 Multiple sounding rocket observations of charged dust in the polar winter mesosphere K. A. Lynch, 1 L. J. Gelinas, 2 M. C. Kelley, 2 R. L. Collins, 3 M. Widholm, 4 D. Rau, 4 E. MacDonald, 4 Y. Liu, 4 J. Ulwick, 5 and P. Mace 5 Received 26 March 2004; revised 15 September 2004; accepted 22 December 2004; published 4 March [1] We present data from a winter 2002 sounding rocket campaign for multiple observations of mesospheric charged dust. The campaign consisted of four identically instrumented payloads carrying detectors for charged mesospheric dust particles. The payloads reached an altitude of 100 km in the nighttime mesosphere and were flown from Poker Flat Research Range, Alaska, in conjunction with observations by the UAF sodium and iron resonance lidar system. Three of the four flights flew in sequence during the course of one night to study the temporal evolution of dust layers. Observations show good correlations between structure seen in the charged dust altitude profile, structure seen in the neutral metal layers observed by the lidars, and structure in the plasma density seen by the onboard Langmuir probes. The dust detector is sensitive to positively and negatively charged dust particles with ram energies of 1 to 11 ev for negatively and 3 11 ev for positively charged dust; that is, particles of approximately 5000 amu. The charged dust densities seen (estimated to be approximately 5 to 10 percent of the total dust density) are approximately 100 particles per cc, and the dust is negatively charged. Variations in the dust density of about 10% are seen in conjunction with structure in the plasma density and with the neutral metal layers. In this paper we present the details of the dust data and instrumentation; a companion paper explores the implications of the correlations seen between the dust and other mesospheric layers. Citation: Lynch, K. A., L. J. Gelinas, M. C. Kelley, R. L. Collins, M. Widholm, D. Rau, E. MacDonald, Y. Liu, J. Ulwick, and P. Mace (2005), Multiple sounding rocket observations of charged dust in the polar winter mesosphere, J. Geophys. Res., 110,, doi: /2004ja Science Goals and Mission Description 1.1. Science Background [2] About 40,000 metric tons of meteoric material enter the Earth s near environment every year [Love and Brownlee, 1993]. Some of this material vaporizes and recondenses into a layer of dust particles suspended in the Earth s mesosphere, near 90 km altitude. The mesospheric plasma density is high enough to cause charging of the suspended dust particles, through ion and electron attachment to the dust particles. The presence of this heavy, charged species may have consequences for mesospheric 1 Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire, USA. 2 School of Electrical Engineering, Cornell University, Ithaca, New York, USA. 3 Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA. 4 Space Science Center, University of New Hampshire, Durham, New Hampshire, USA. 5 Space Dynamics Laboratory, Utah State University, Logan, Utah, USA. Copyright 2005 by the American Geophysical Union /05/2004JA and atmospheric processes such as sudden atom layers (SAL), noctilucent clouds (NLC), and polar mesospheric summer echoes (PMSE); studies of the mesosphere may be useful for developing models of climate change [Thomas, 2003; von Zahn, 2003]. Models of this mesospheric dust population and its charging have existed for decades [Hunten et al., 1980; Havnes et al., 1987; Havnes et al., 1990; Goertz, 1989], but until recently no direct measurements had been made. Concerns about climate change and the need for quantitative modeling of the mesosphere have stimulated experimental research over the last decade to measure this dust population directly. [3] Recent measurements have included both polar latitude [Havnes et al., 1996; von Cossart et al., 1999] and midlatitude [Gelinas et al., 1998; Gelinas, 1999] sounding rocket and remote sensing studies. The polar measurements have concentrated on the connection to polar mesospheric summer echoes (PMSE), and the midlatitude measurement studied the relationship with sudden, or sporadic, metal atom layers (SAL) as measured by lidars and radars at Arecibo. In this paper we present the results of a follow-up experiment to the SAL measurements, using the lidars at Poker Flat Research Range in Alaska to monitor SAL activity before, during, and after four separate sounding rocket launches with identical in situ instrumentation. Three 1of10

2 Figure 1. The DustOrions dust detector. of the launches occurred in quick succession over the course of one SAL event, and the fourth was on a separate night. All launches were in darkness and without auroral activity Mission Plan [4] In this DustOrions sounding rocket mission and its corresponding SAL midlatitude mission, we addressed the following scientific objectives: (1) determine the temporal evolution of the charged dust layer over the timescales of sudden atom layers (twilight, night, and after SAL activity); (2) determine the electrodynamic relationships between the dust layer and other components of the mesosphere including sporadic E layers, sudden atom layers, wave electric fields, and variations in the plasma density; and (3) characterize the morphology of the Earth s mesospheric dust layer, as functions of latitude, local time, and layering activities. In this paper we concentrate on the charged dust measurements themselves; a companion paper [Gelinas et al., 2005] details their relationships with both the remote sensing measurements and the plasma density measurements. [5] The four DustOrion flights were planned to investigate the relationship between charged dust layer parameters and sporadic atom layers (SALs). One flight was planned for a night with no SAL activity, and the other three were to be flown before, during, and after a SAL event. Monitoring of the neutral metal layer activity was provided by the University of Alaska Fairbanks Geophysical Institute sodium and iron lidars [Collins and Smith, 2004; Hou, 2002]. As part of this mission, nightly lidar data were recorded for 14 nights. The March 2002 launch window was chosen as a balance between darkness (required for lidar operation) and the springtime increase in SAL activity. A further requirement for the launches was a total lack of auroral activity, as electron populations with energies comparable to 2of10

3 Figure 2. Accepted negative dust mass ranges versus altitude for the four flights. Differences between the flights stem from varying motor performances. The upper curves correspond to particles with 1 ev of ram energy, while the lower curves correspond to 11 ev. the dust ram energy would be indistinguishable from the charged dust population. The auroral activity (or lack thereof) was monitored by the Poker Flat Research Range all-sky camera and meridian scan photometer equipment Instrumentation [6] Figure 1 shows the Dust Detector, which is a Faraday cup. The detector is designed to collect charged particles rammed into its aperture by the rocket velocity and is aligned with its long axis parallel to the spin axis of the rocket. The energy of the particles collected is assumed to be 0.5 m dust v 2 eff, where v eff is the component of the ram velocity into the aperture. For a 2400 amu particle at 800 m/s, this energy is approximately 8 ev. A negatively biased screen near the entrance repels thermal electrons, and the positive voltage of the anode repels thermal ions. In between, a modulation screen switching between positive and negative 11 V at 1 KHz modulates the charged dust flux to the anode, allowing a synchronous detection scheme to amplify significantly the signal-to-noise ratio [Horowitz and Hill, 1989]. The detector is sensitive to both positively and negatively charged dust; current from negative dust fluxes reaches the anode when the modulation screen is at +11 V and from positive dust fluxes when it is at 11 V. [7] Currents reaching the anode are monitored by two separate preamplifier circuits. The low-frequency DC-coupled LF channel averages the total net current to the anode over several modulation cycles. The highfrequency AC-coupled AB channel is phased to the modulation cycle. For this AB channel, current A is monitored when the modulation gate is at +11 V, current B is monitored when the modulation gate is at 11 V, and the AC-coupling circuit offsets these signals to maintain the sum of A + B at a constant level with a time constant of several seconds. The difference current AB = A B then provides a high-gain, low-noise measure of the current collected from particles within the ±11 ev bandwidth of the modulation gate. Particles with energy greater than 11 ev, being unmodulated by the gate, are collected during both cycles of the modulation. Thus their contribution is monitored by the LF channel but should be cancelled out in the AB difference channel. The LF channel has a preamplifier gain of 2 mv/pa, and the AB channel has a gain of 430 mv/pa. [8] A few hypothetical examples are illustrative for understanding the AB and LF channels. (1) If the dust is all of negative polarity and if its mass is such that its ram energy is less than 11 ev, the LF current will be half the AB current, since the LF current is accumulated during both phases of the modulation cycle. (2) If the dust is all of positive polarity, the LF current will change sign but the AB current will remain negative; note that the AB current is derived from the difference A B, with floating offsets. Thus the AB current does not contain information about the sign of the dust polarity; this information must be obtained from the DC-coupled LF current. (3) If the dust population consists of equal parts positively and negatively charged particles, the LF current will be zero and the AB current will be double that of the first case. Cases of mixed polarity dust are somewhat ambiguous in the detector response, and in the data analysis below we assume a single polarity (negative) consistent with the LF current. (4) If the current to the anode includes a contribution from particles with energy greater than 11 ev, this contribution should be monitored by the LF current but rejected by the AB difference current. However, a fraction of this higherenergy flux, particularly from particles with energies just above 11 ev, is partially modulated by the gate. This changes the offset to the AC-coupled AB current and appears as a contribution to the AB current despite the differencing. We examine the separation of these signals in the data analysis below. [9] The nominal mass acceptance bandwidth of the AB current is a function of the payload velocity, since the 11 ev particle energy modulation limit in the detector frame is derived from the ram motion into the detector. Figure 2 shows the accepted mass range as a function of altitude for the four DustOrion flights. [10] In addition to the Dust Detector, each payload carried a three-axis fluxgate magnetometer, three electron density probes, a small two-probe electric field sensor, and a GPS receiver. Table 1 lists the launch times and apogees of the four flights. All instrumentation other than the electric field sensor functioned nominally, with the exception of the GPS receiver on flight Magnetometer and GPS data are used in the interpretation of the 3of10

4 Table 1. Flight Information for the Four DustOrion Flights Flight Time, UT Time, LT Apogee, km 7 March March dust data given here. The density probe data are presented and discussed in the companion paper [Gelinas et al., 2005]. 2. Flight and Ground Data Summaries [11] Figures 3 and 4 give an overview of the mission data. Figure 3 is a two-dimensional false color plot of the iron concentration profiles derived from the resonance lidar measurements. The lidar data in Figure 3 show the behavior of the neutral iron layer over the course of the three-flight night. Note the sporadic event beginning near 0100 LT and 105 km altitude. Over the course of the night, this layer diffuses downward and joins the main layer, with some additional structuring later in the night. Figure 4 shows altitude profiles of the charged dust data, detailed below, and the bottom panels show lidar altitude profiles during each flight Dust Detector Signal Processing [12] The upper panels of Figure 4 show the LF (blue) and AB (black) currents as functions of altitude for each flight. Since the LF current is negative in all cases, we assume that all the current to the anode is negative, as discussed above, in order to proceed. Properly modulated fluxes of particles within the 11 ev passband should always contribute to negative AB currents; the high- and low-altitude regions of Figure 3. Iron lidar profiles for the night of the three DustOrion flights. The figure is two-dimensional false color plot of the iron concentration profiles derived from the resonance lidar measurements. The color indicates neutral iron concentration [/cc] as a function of time (x-axis) and altitude (y-axis), as in the single-time line plots in the bottom panels of Figure 4. Each profile represents 100 s of measurements. The maximum concentrations (yellow) are about atoms/cc. The iron concentrations are derived from the raw lidar data using standard inversion methods [e.g., Tilgner and von Zahn, 1988]. Note the sporadic event beginning near 0100 LT, 105 km. 4of10

5 Figure 4. Altitude profiles from the four flights. (top) Anode currents as a function of altitude; AB (black), LF (blue), and j bg (red). (middle) Negatively charged dust density from 1 11 ev energy range dust channel (black) and bg channel (blue). (bottom) The 1 11 ev negatively charged dust density times 10 (black), and neutral sodium (turquoise) and iron (red) densities from the lidars. The iron (red) density is divided by two here to fit better on the plot; the real iron density is twice what is shown. positive polarity AB current are caused by offsets from fluxes outside the 11 ev passband as follows. [13] Assume there is a background flux due to something outside the 11 ev energy band such as electron precipitation or heavy dust. High-energy electron precipitation or very heavy dust, significantly above 11 ev, would be relatively unaffected by the screen modulation and cancelled out in the AB AC-coupled signal. Fluxes near the 11 ev limit, 5of10

6 however, are partially modulated. This portion of the background flux causes some unwanted modulation that shows up as modulated difference current in AB. Then AB ¼ j j dust jþj bg * bgmod; where j dust is the true current to the anode from properly modulated particle fluxes in the 11 ev range, j bg is the current reaching the anode from fluxes outside the 11 ev range, and bgmod is the fraction of j bg that shows up as modulated current. The LF current, which averages the net current to the anode over several modulation cycles, averages both current contributors together LF ¼ j dust =2 þ j bg : [14] Remember that j dust is accumulated only half the time in the LF monitoring, as it does not reach the anode during the half cycle when the modulation gate is closed to its polarity. [15] An algebraic combination of the above allows us to change from the measured AB and LF currents to the more useful j dust and j bg currents just defined. Under the assumption made above that the j dust has a negative polarity, we have jj dust j = j dust, so we can write j dust ¼ AB j bg * bgmod j bg ¼ LF j dust =2 j bg ¼ ð2 * LF ABÞ= ð2 bgmodþ: [16] The bgmod variable is empirically determined in order to make the j dust return to zero at apogee, where no dust fluxes should be rammed into the aperture since the look direction is normal to the ram. The value bgmod = 0.35 satisfies this criterion, meaning that 35% of the j bg current shows up as an oppositely signed offset to the AB current. The red curves in the top panels of Figure 4 show the j bg current. The j dust current is twice the difference between LF (blue) and j bg (red); it goes to zero at apogee as desired. [17] This simple j bg calculation assumes that all the modulated background current comes from the precipitating electron flux with energies greater than 11 ev or from heavy dust. It also assumes that the bgmod value of 0.35 is correct in all regions. There are potential problems with this calculation. At high altitudes, electrons with energy below 11 ev can still enter the detector despite ram considerations, so the assumption that the j dust should go to zero at apogee is an approximation. At low altitudes, this background calculation may be incorrect because the electron flux does not reach there and instead there is a significant population of heavy dust particles with masses just outside the acceptance band. These heavy dust particles contribute a partially modulated background signal to AB, possibly in a different manner than the electrons above. Still, this simple model provides what appears to be a self-consistent interpretation of the currents reaching the anode, as we shall see in the discussions below. [18] A modeling effort is under way to model the mass response of the LF and AB channels more rigorously using laboratory calibrations of the detector response. In this simulation, the modulation of high-energy particle currents is caused by slightly different acceptances in the detector for these particles during the different phases of the detector modulation. Initial results of this study agree well with the j dust and j bg calculated here using the simple background model. However, the more rigorous calculation allows a more explicit understanding of the LF channel response and the relative fluxes of different types of contamination : heavy dust and electron precipitation Dust Data [19] Given the above interpretation of the currents j dust and j bg, we can convert the currents to the anode into density of particulates by assuming that all the particle velocity is caused by the rocket motion density ½=cc Š ¼ current½paš= ðe * v eff * areaþ; where e is the unit charge in pc, v eff is the component of ram velocity into the detector, and area is the size of the detector aperture. The angle between v eff and v ram is less than 20 degrees through the dust layer and then moves rapidly to 90 degrees near apogee, where the rocket velocity is nearly perpendicular to the spin-axis-aligned detector aperture look direction. Since the payload cone angle is seen by the magnetometer to be less than 2 degrees and the aperture has a 20 degree acceptance, coning effects on the ram velocity vector are not considered. [20] The middle panels of Figure 4 show dust (black), the density/cc of particles corresponding to the j dust current, and bg (blue), the density/cc of particles corresponding to the j bg current. Thus dust represents the charged dust density of particles with ram energy within the modulation bandwidth and within the mass acceptance range plotted in Figure 2. This measurement is the primary goal of the Dust Detector. The bottom panels of Figure 4 repeat the dust trace in the context of the lidar profiles at the time of each flight. These Na and Fe lidar profiles represent 15 min of data that have been filtered at 4 km. [21] The dust density as thus calculated reflects the collection of negative charged dust with ram energies between 1 and 11 ev; with a corresponding dust mass acceptance at each altitude as shown in Figure 2. The bg density reflects negative current reaching the anode of the detector from particles outside this energy bandwidth, including, in particular, electrons and heavy negatively charged dust. Differences between the two channels are informative. First, the high-altitude increase in bg density is presumed to be from ionospheric electron populations with temperatures above 11 ev; the sweeping increase with altitude is an artifact of the ram velocity correction (inappropriate for electrons with thermal velocity much greater than the rocket motion) as the ram vector becomes perpendicular to the detector look direction near apogee. Since these electrons reach the anode during both halves of the synchronous-detection cycle they are cancelled out in the dust channel. The low-altitude bg density is presumed to be from heavy dust. The exact altitude of the transition from electron to heavy dust contributions to bg is unknown. [22] Second, the difference between the two channels in the lower-altitude region where the dust signal is nonzero is 6of10

7 Figure 5. The dust layer falls with time together with the neutral layers. Profiles are from the last two flights, 050 (black) and 051 (blue). Here, as in Figure 4, the dust density is multiplied by 10, and the iron signal is divided by 2. an indication of the abundance of heavier dust particles; that is, dust particles with a mass larger than the mass acceptance given in Figure 2. [23] Third, the difference between the lower-altitude edge of the bg density and the sharp lower cutoff of the dust density is thought to come from two sources. First, presumably the heaviest dust particles are more abundant at the bottom of the layer, and these particles are beyond the range of the dust channel. Thus the bg density is a better indicator of the lower edge of the charged dust layer, but it does not supply knowledge of the dust mass range. Second, the ACcoupled AB current measurement is sensitive to time of flight through the detector to the anode. Collisional drag from pressure buildup within the rammed detector cup may slow the dust particles sufficiently to change their flight time and misalign the synchronous detection scheme. The fact that all four flights have inflections in the AB current within 0.5 km of 77 km altitude suggests that this drag from neutral atmospheric buildup within the detector is the cause of this sharp cutoff. Thus the dust and bg measurements are suspect below 77 km altitude. 3. Data Analysis 3.1. Charged Dust Density Versus Time [24] Figure 3 shows the overall profile of the neutral iron layer over the course of the three-flight night. From 0215 to 0445 LT, the time spanning the flights, the top of the layer diffuses downward in time. Similar motion can be seen in the dust layer; Figure 5 shows the falling motion of the neutral iron layer between the times of the last two flights, together with the motion of the top of the dust layer at the same times. The one kilometer drop in altitude is approximately the same for both species. [25] From the first to the second flight that night (see Figure 4), the depression of the top edge of the dust layer is accompanied by an increase in the average dust density, such that the total dust layer content remains roughly constant but is compressed in altitude. There is a small loss of total content in the last flight seen in the dust density, indicating either a loss mechanism such as recombination, a mass redistribution, or a movement of the payload out of the dust layer region. [26] On longer timescales, we can compare the dust layer seen on 7 March (left column of Figure 4) to those of 15 March (remaining columns of Figure 4). The first flight had a considerably stronger dust density peak than the later flights. However this difference in intensity is localized to the lower edge of the layer; the topsides of the layers, though they move in altitude, stay near a constant density of /cc. The lower edge of the layer seems more volatile but this may be an instrumental effect (see section 3.4) as the dust signal appears to have a sharp mass cutoff in the middle of the dust mass spectrum at these altitudes Charged Dust Density Versus Lidar Fe/Na Ratio Profiles [27] In general the charged dust layer appears to coexist with the neutral iron layer and sits underneath the neutral sodium layer, as can be seen in the altitude profiles of Figure 4. Figure 6 illustrates this relationship in a different way, showing the dust density as a function of the Fe density for all four flights. The log of the charged dust density (in units of 1/cc) increases linearly with the neutral Fe density. The dust density is well-correlated with the neutral Fe density, for a range of altitudes (82 to 95 km) over both nights studied Charged Dust Density: Polar Versus Midlatitude Study [28] Measurements from the SAL mission [Gelinas et al., 1998] were made at midlatitudes, near twilight. In that mission, some of the dust population was seen to be positively charged. This is not yet understood but is presumed to be related to the twilight conditions; surface chemistry effects on the surface of the dust particles could lengthen substantially the recombination times for electrons attached to the dust, to times long enough to include the sunlit time just before the twilight observation [Gelinas et al., 1998]. The high-latitude DustOrion observations represent the steady state density of the charged dust layer, with conditions unchanged for many hours before the observation; the midlatitude observations were made within an hour of twilight and thus may not represent equilibrium charging states, especially if recombination times are longer than gas phase chemistry would predict. 7of10

8 [29] The charged dust layers seen in the DustOrions mission under study here were substantially lower in altitude and stronger in intensity than the SAL midlatitude observation. In the SAL measurement, both the dust layer at km and the sodium layer at 94 km and above were about 10 km higher in altitude than these high-latitude measurements. The high-latitude observations are also an order of magnitude stronger in density but this may be a reflection of the nonequilibrium charge state of the midlatitude observations. Whether the observed increase is an increase in the neutral dust source or an increase in the charging percentage of the neutral dust is an open question. Figure 6. The dust density is correlated with the density of the Fe neutral metal atoms. The scatterplot combines data points from all four flights, from 82 km to 95 km altitude Comparison of Dust and bg Densities [30] The difference between the 1 11 ev dust densities and the >11 ev bg densities can be interpreted as a crude mass analyzer, during the altitude range where these densities and their ratio are changing rapidly with altitude. The left panel of Figure 7 repeats the Figure 2 information for a limited altitude range, showing the maximum mass accepted by the dust channel as a function of altitude for each rocket (varying motor performances mean that the ram velocities are different for each flight). Data are shown from altitudes above where the AB data are suspect (near 77 km) until the altitude where the bg signal becomes contaminated with electron precipitation (about km). The right panel of Figure 7 shows the ratio of dust to dust + bg, that is, the fraction of dust which is within the 1 11 ev mass acceptance bandwidth at each altitude. The ratio is small at low altitudes, where heavy dust dominates the layer. With Figure 7. (left) The mass acceptance limit of the dust channel as a function of altitude for each flight; flight 49 (red), flight 52 (black), flight 50 (blue), flight 51 (turquoise). (right) The ratio of dust/(dust + bg) as a function of altitude, with same color scheme. 8of10

9 increasing altitude, smaller dust particles are seen and the ratio increases. The ratio would approach 1 if there were no bg, i.e., if all the charged dust were in the 1 11 ev band and there were no electron precipitation. However, each curve asymptotes near values of 0.5 (flights 52, 50, and 51) or 0.7 (flight 49) when electron contamination of the bg density dominates the measurement. [31] The altitude dependence of the ratio dust/(dust + bg) is determined both by the altitude-dependent mass cutoff of the dust channel and by the mass distribution of dust in the layer. Both effects can be seen in the right panel of Figure 7. At 79 km altitude, where the mass cutoff is 6900 amu for flight 52 (black), 6250 for flight 50 (blue), and slightly less for flight 51 (turquoise), the observed ratios are 30% (flight 52), 25% (flight 50), and 20% (flight 51). Thus as the dust bandwidth decreases, the percentage of dust seen in the dust channel decreases accordingly. We could interpret this to mean that at 79 km altitude, 30% of the dust has mass below 6900 amu, 25% below 6250 amu, and 20% below 6200 amu. It is also possible that the dust layer has shifted in altitude during the time from one flight to the next. However, the decreasing ratio is consistent with the decreasing bandwidth. For the single-flight night, however, it seems clear that the dust altitude profile is different, as at 79 km, the cutoff is near 7200 amu, but the ratio is only 25%. [32] While the altitude-dependent cutoff has some effect on the ratio, the dominant effect seems to be the altitude dependence of dust masses in the layer. A plot of dust/(dust + bg) as a function of mass limit for the four flights (not shown) does not show a consistent pattern, as it would if the dust layer had a uniform distribution of masses with altitude. If we instead view the right panel of Figure 7 with the assumption of a roughly constant average mass cutoff of amu, we can consider the altitude dependence of the dust mass distribution. For altitudes below 78 km, all the dust is too heavy for the dust channel, that is, greater than 6500 amu. At 79 km, 25% of the dust is less than 6500 amu. At 80 km (using the less-contaminated flight 49 data), 40% and at 81 km, almost 60% of the dust is in the dust bandwidth. While this interpretation is limited by the assumption of an average mass cutoff, it does show roughly the altitude dependence of the dust mass distribution. It could of course also be the result of the 15 20% increase in the dust bandwidth. This cannot be determined by these data. However, it seems reasonable that heavier dust would reside near the bottom of the layer. [33] A final note concerning instrument response in the different channels at different altitudes involves the charging rate for dust particles of different masses. The expected charging ratio (charged dust particles: all dust particles) increases with dust radius and mass. The expected charging fraction for 1 nm dust particles is only about 6 7% while that for higher masses can be twice that, as the charging fraction increases linearly with the dust particle radius, like the capacitance of a sphere [Havnes et al., 1987; Gelinas et al., 2005]. In our measurement of charged dust densities, there is an implicit assumption that we can extract from this measurement an estimate of the total (neutral and charged) dust density. However, the altitude profile of total dust density will not be linearly proportional to the charged dust density profile, since the charging fraction is a function of particle size. The charged dust detector is more sensitive to large dust particles, with their higher charging fraction. 4. Summary and Future Work [34] The results of the four DustOrion flights show a clear consistent signature of a ,000 amu negatively charged dust layer, located at km, with a charged density of a few hundred per cc. There is a strong correlation between the dust layer and its dynamics and the variability of other mesospheric layers (electron density, neutral Na, neutral Fe), both in altitude and in temporal evolution. These auroral latitude measurements are distinct from observations made at lower latitudes, being lower in altitude and made in equilibrium charging conditions. [35] Two remaining Dust Detector payloads will be flown, one at midnight and one in twilight, to examine the difference in charge state brought about by sunlight. A modeling effort of collisional drag inside the detector has been started to evaluate the low-altitude sharp cutoff of the AB signal. Finally, a new version of the detector is in the planning stages that will include a mass-spectrum separation. [36] Acknowledgments. The authors thank NASA Wallops Flight Facility, the NSROC payload team, and the Poker Flat Research Range personnel; UAF/GI and their enthusiastic science support during the launch campaign; University of Colorado at Boulder for flow modeling of the instrument response; University of Massachusetts at Lowell for Lowell Digisonde Sheep Creek Radar realtime data support; and NASA grant NAG The lidar observations at Poker Flat Research Range were funded with support from NASA and the NSF CEDAR program under contracts NRA MITM-049 and NSF-ATM , respectively. [37] Arthur Richmond thanks Mihaly Horanyi and another reviewer for their assistance in evaluating this paper. References Collins, R. L., and R. W. Smith (2004), Evidence of damping and overturning of gravity waves in the Arctic mesosphere: Na lidar and OH temperature observations, J. Atmos. Sol. Terr. 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10 Hou, T. (2002), Development of high spectral resolution Boltzmann lidar, M.S. thesis, Univ. of Alaska Fairbanks, Fairbanks, Alaska. Hunten, D. M., R. P. Turco, and O. B. Toon (1980), Smoke and dust particles of meteoric origin in the mesosphere and stratosphere, J. Atmos. Sci., 37, Love, S. G., and D. E. Brownlee (1993), A direct measurement of the terrestrial mass accretion rate of cosmic dust, Science, 262, 550. Thomas, G. E. (2003), Comment on Are noctilucent clouds truly a miner s canary for global change?, Eos Trans. AGU, 84(36), 352. Tilgner, C., and U. von Zahn (1988), Average properties of the sodium density distribution as observed at 69 degrees N latitude in winter, J. Geophys. Res., 93, von Cossart, G., J. Fiedler, and U. von Zahn (1999), Size distributions of NLC particles as determined from 3-color observations of NLC by ground-based lidar, Geophys. Res. Lett., 26, von Zahn, U. (2003), Are noctilucent clouds truly a miner s canary for global change?, Eos Trans. AGU, 84(28), 261. R. L. Collins, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA. (rlc@gi.alaska.edu) L. J. Gelinas and M. C. Kelley, School of Electrical Engineering, Cornell University, Ithaca, NY, USA. (lynett@ee.cornell.edu) Y. Liu, E. MacDonald, D. Rau, and M. Widholm, Space Science Center, University of New Hampshire, Durham, NH 03824, USA. (mark.widholm@unh.edu) K. A. Lynch, Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA. (kristina.lynch@ dartmouth.edu) P. Mace and J. Ulwick, Space Dynamics Laboratory, Utah State University, Logan, UT, USA. (petemace@cc.usu.edu) 10 of 10