Measurement of positively and negatively charged particles inside PMSEs during MIDAS

Transcription

1 1 Measurement of positively and negatively charged particles inside PMSEs during MIDAS SOLSTICE 2001 B. Smiley, S. Robertson, M. Horányi Physics Department, University of Colorado at Boulder, Colorado, 80309, USA T. Blix Forsvarets Forskningsinstitutt (FFI), 2027 Kjeller, Norway M. Rapp, R. Latteck Leibniz-Institut für Atmosphärenphysik (IAP), Kühlungsborn, Germany J. Gumbel Meteorology Department (MISU), Stockholm University, S Stockholm, Sweden

2 2 Abstract. A magnetically shielded, charge collecting rocket probe was used on two flights of the MIDAS (MIddle Atmosphere Dynamics and Structure) SOLSTICE (Studies of Layered STructures and ICE) 2001 rocket campaign over Andøya, Norway. The probe was a graphite collection surface with a permanent magnet underneath to deflect electrons. The first MIDAS was launched June 17, 2001 into a strong, multiply layered PMSE. The probe measured negative particles in an electron biteout inside the PMSE with a peak charge number density of charges per cubic centimeter. The second MIDAS was launched June 24, 2001 into another strong, multiply layered PMSE. The probe saw a band of positive particles centered in the lowest radar echo maximum, and a negative particle layer accompanied by a positive ion excess. The charge number densities for the positive and negative PMSE particles were several thousand charges per cubic centimeter. Unexpectedly, 2 km beneath the PMSE, the probe also found a very pronounced negative layer which was probably an NLC. Computer simulations of incoming, singly negatively charged ice aerosols were performed using a rarefied flow field representative of the MIDAS payload at zero angle of attack. Ice aerosols 1 nm in radius were diverted by the leading shock front, indicating the smallest detectable ice aerosol by this probe.

3 3 1. Introduction 1.1 Motivation Recent rocket, satellite, radar, and lidar studies of the polar summer mesosphere have investigated polar mesospheric summer echoes (PMSE) and noctilucent clouds (NLC, or more generally polar mesospheric clouds, PMC). PMSE s are thought to be composed of small, icy, charged aerosols ~10 nm in radius with charge number densities of several 1000 charges cm -3 [Havnes et al., 2001]. Repeated measurements by Havnes et al. [1996, 2001] showed positive aerosols in one PMSE and negative aerosols inside others. Recent rocket measurements of PMSEs during the DROPPS campaign measured both positively charged [Croskey et al., 2001] and negatively charged [Mitchell et al., 2001] aerosols in the same PMSE. Aerosol charging models [Rapp and Lübken, 2001] have shown that positive ion (electron) depletions or enhancements (depletions) can occur in aerosol populations. NLCs have frequently been seen with and beneath PMSEs [Wälchli et al., 1993, von Zahn et al., 1999; Stebel et al., 2000]. NLCs are made of ice particles [Hervig et al., 2001] that nucleate on either meteoritic dust or hydrated cluster ions [Thomas, 1991]. Multi color lidar studies [von Cossart et al., 1999; Alpers et al., 2000] have deduced properties of the detectable NLC aerosols assuming a lognormal size distribution: an average mode radius of ~50 nm and average number densities ~200 particles cm -3. Those lidar measurements also found that NLCs form at a recurrent average altitude of 82.5 km. Satellite measurements [Carbary et al., 2001, Stevens et al., 2001] have also seen that PMCs tend to form at approximately 82.5 km. The aerosols in a NLC region have been measured to have either sign of charge [Havnes et al., 1996], while aerosol charging models [Rapp and Lübken, 2001] predict negatively charged ice aerosols.

4 4 1.2 MIDAS SOLSTICE campaign description The MIDAS SOLSTICE rocket campaign was carried out at the Andøya Rocket Range (16E, 69N) in Norway during June The topics of investigation were the icy aerosols of NLCs, the meter scale plasma structures characteristic of PMSEs, and the neutral number density and temperature profiles. The MIDAS payload took simultaneous in situ measurements of positive ions, electrons, charged aerosols, and neutral density. The magnetically shielded probe [Horanyi et al., 2000] detected charged particles alongside the positive ion probe [Blix et al., 1990] and CONE (COmbined Neutral and Electron) instrument [Rapp et al., 2001]. Figure 1 shows the location of these probes on the MIDAS payload. There were two MIDAS launches into strong, multiply layered PMSEs: SO-MI-5 on June 17, 2001, SO-MI-11 on June 24, Cloudy weather was present prior to both launches, so simultaneous measurements from the ALOMAR lidar were unavailable. Thus the nearby ALWIN radar was the ground based monitor of the science conditions. 2. Experimental Methods 2.1. Probe description The magnetically shielded probe was a graphite surface that collected charge from impacting charged aerosols during flight. Beneath the graphite was a permanent magnet that completely deflected electrons and partially deflected positive ions. Since the graphite patch was at the same potential as the payload skin, the observed positive ion collection was likely assisted by the electric attraction of the payload potential, estimated to be 1.5 volts in the mesopause. The 6 cm by 2 cm graphite collection surface was located in a connecting ring 40 cm from the base of the rocket, see Figure 1. The permanent magnet had an effective dipole moment

5 5 of 9000 gauss cm 3 with an effective radial depth of 4.35 cm into the rocket, determined by a least squares fit of magnetic field data measured by a computer controlled apparatus. During both flights the MIDAS payload rotated at ~5 Hz. There was also precession, or coning motion, of the payload whose period was ten seconds. On downleg, this made the angle of attack smoothly oscillate between zero and 70 degrees, see figure 2b and 3b Probe operation During flight through the mesopause, the graphite patch collected a current on the order of nanoamps, sampled at 1085 Hz by a simple current to voltage amplifier. The dynamic range of the probe was + /-2 na with an accuracy of + /-6 pa. The probe measured the net current at a given altitude, meaning the net sum of all positive and negative collections. The net current can be divided into two main components. There was a positive background from the collection of positive ions embedded in the airflow across the patch, and an aerosol signal from the incoming heavy charge carriers. Impact ionization effects are neglected in this investigation because the probe had no constrictive geometry to collect charged particle fragments, a concern raised by Havnes et al. [1996] who used an electrically biased, blunt cup probe. The positive background took the form of multiple positive peaks with every rotation of the payload, see Figures 2a, 3a. These positive flow peaks formed as the probe turned through the different parts of the shock front. The amplitude of the flow peaks increased with altitude due to the increasing density of positive ions, saturating the input above 100 km. The coning motion shaped the flow peaks into an oscillatory envelope, reducing the peaks as the angle of attack passed through zero.

6 6. An additional aerosol current was collected when the graphite patch faced into the ram direction. This signal was either positive or negative, depending on the relative abundances of the aerosols at a given altitude. The aerosol current was in the 100 pa range and was superimposed on the larger flow modulation. Since the positive background was clearly proportional to the positive ion density, the positive ion probe signal was used to remove the positive ion contribution from the net positive aerosol probe data. 2.3 Extracting the aerosol residual from net positive probe data Data sorting technique The positive background at any instant was determined not only by the positive ion density but also by the flow field around the rocket. To freeze the flow dynamics, raw data points within + /-0.75 degrees of the ram direction were set aside, which selected one to two raw data points per rotation. This sorted data set only had variations due to the positive ions and aerosols. The ram direction was used because the most cross sectional area was presented to the aerosols at that moment. This careful sorting was accomplished with a velocity centered coordinate system, constructed with the rocket orientation data obtained from the onboard three axis magnetometer. For every data point, there existed a plane which held the rocket axis and the velocity vector, hereafter referred to as the velocity plane. The instantaneous rotation angle of the probe away from the velocity plane was calculated for all points. Small sketches of the velocity plane can be seen in figures 2a, 3a, and 6. The phase of the flow peaks with respect to the velocity plane was almost completely uniform during downleg, a fact that inspired the sorting technique. This regularity of the flow

7 7 peaks was not present during upleg, thus the downleg data was the only one used in this technique Removing the ion background For both MIDAS flights below 95 km, the ram direction data had a similar appearance to the positive ion data, one that was largely independent of the angle of attack, see Figures 4a and 5a. This suggests that positive ions were constantly forced into the graphite patch by the flow. Above 95 km, the ram data gradually diverged from the ion signal because the mean free path became large, reducing the forced ion collection. There was a correlation with the ion signal greater than 90% in the shaded regions of Figures 4a and 5a. This correlation would be expected if these regions were devoid of aerosols, or clean, since nothing but ions contributed to the signal. Applying this concept, a simple linear transform of the ion signal should reproduce clean ram data and indicate the ion contribution elsewhere. If the altitude dependent ion contribution to the ram data, in na, is called B(z), then B(z) = m(z) I(z) + b(z) (1) where z is the altitude in km, I(z) is the positive ion probe signal in ions m -3, m(z) is a scale factor in units of na m 3, and b(z) is an offset in units of na. Discrete values of m(z) and b(z) were readily found by performing least squares fits of I(z) to the ram data. The shaded areas in Figures 4a, 5a were considered clean and acceptable for determining the positive background because the correlation with the ion signal was greater than 90% and the absence of radar echoes. Since the sorted ram data had a reduced sampling period of 0.2 sec (a vertical resolution of ~150 meters), the ion profiles used with the least squares fits were smoothed to a matching time width. Linear interpolation was used to find m(z) and b(z)

8 8 between the clean areas since their values changed slowly. Figures 4b and 5b show the values of m(z) and b(z) normalized to the lowest value and the interpolation for all altitudes. Figures 4c and 5c are plots of the resulting background B(z) on top of the sorted ram data. 2.4 The particle charge number density The charge number density was deduced from the relation R = (q)(n)(a eff )(v), where R is the aerosol residual current in the hundreds of picoamps, q is the charge per particle, n is the aerosol number density, and A eff is the effective amount of area that sweeps up the aerosols. A eff could be less than the full geometrical area of 12 cm 2 but never more, since all conceivable corrections divert particles away from the patch. Since a smaller area results in a larger charge number density, the geometrical cross section was adopted to find the lower bound: (q)(n) R / [(A geometrical )(v)] (2) This (q)(n) is a sum over all the aerosols (of all charges) that were present during collection. 3. Results 3.1 Negative aerosol layer during SO-MI-11 The only net negative current event is shown in more detail in figure 6. This was interpreted as a negative aerosol layer because of the very low concentration of negative ions in the daytime mesosphere [Kopp et al., 1998, 2000]. This event was so brief in the sorted data it is more informative to look at the raw data. The negative current began at km, labeled event n0 in figure 6a, and went to negative saturation for three samples. Over the next two rotations the negative layer receded, exhibiting small scale density variations. The three largest sporadic

9 9 events were a wide negative peak at km, hereafter called event n1, a pair of positive spikes at km called p1, and single positive spike at km, dubbed p2. At 80.4 km the probe current became net positive again, but appeared depressed relative to the positive ion profile until 79 km, see figure 5c. The angle of attack was quite steep, starting at 0.2 degrees at n0 and only growing to 8.5 degrees by 80.4 km. The downleg positive ion data showed no change during this event, but this could be because of the geometry. The steep angle of attack kept the ion probe near the center of the rocket wake, making small ion enhancements or depletions hard to detect. During upleg of SO-MI-11, when the positive ion probe was at the leading end of the payload, there was a slight ion excess seen at p2, see figure 6. The rocket telemetry logs showed no electrical disturbances during passage through the negative layer, nor was there a change in the payload potential, so the negative layer data were considered genuine. 3.2 PMSE aerosols from SO-MI-11 There was evidence of both positive and negative aerosols in this PMSE, see figures 7a,b. The aerosol residual was extracted by subtracting B(z) from the ram direction data. The uncertainty of the derived charge number density was large because of the summed uncertainties of the positive ion signal (taken to be 25%), the raw probe current, m(z), and b(z). The electron probe did not function, but figure 7c shows the simultaneous positive ion density. Coincident with the lowest radar echo maximum at 84 km was a broad band of positive aerosols labeled as β. The positive aerosol density generally followed the outline of the radar echo, reaching a maximum of / charges cm -3. Below β the positive aerosol density reduced to zero along with the radar echo. At km was a negative layer that lasted for two

10 10 ram direction points, labeled α. Negative layer α peaked at charges + / cm -3 and was accompanied by an obvious enhancement in the positive ion signal, see figure 7c. Above β the aerosol density decreased, becoming net negative at km. The negative aerosols increased until event γ, above which the angle of attack fell below 20 degrees and the shock front began to divert aerosols, a consequence of the side mounting of the probe. The ion signal showed another small enhancement at γ, see figure 7c. The aerosols reemerged symmetrically, peaking at charges + / cm -3 at 89 km, labeled δ, approximately 2 km beneath the mesospheric temperature minimum, see figure 7a. 3.3 PMSE aerosols from SO-MI-5 On SO-MI-5 both the electron and positive ion probe returned data, see figure 8c. The probe showed negative aerosols punctuated by a positive peak of 400 charges + /- 600 cm -3 at 81.7 km, labeled ε, figure 8b. ε coincides with the lowest radar maximum similar to SO-MI-11, but the large uncertainty prevents confidence. The aerosol signal rose above the error bars in region ζ where there was a significant electron and ion depletion, assuming that the plasma densities above and below ζ were unperturbed. The probe saw negative aerosols in proportion to the electron biteout, even culminating in an identical maximum of charges + /- 700 cm -3 at 83.8 km, labeled η. Immediately above η the angle of attack went through zero, reemerging into more negative aerosols in θ. A maximum of charges + /- 800 cm -3 was reached at 90.8 km, location ι, approximately 1 km below the temperature minimum, see figure 8a. The ion density in θ appeared depleted, but notably the electron density either looked unperturbed or showed an excess.

11 11 4. Discussion 4.1 Computer modeling of incoming aerosol trajectories Computer simulations of incoming aerosols were performed to check which ice grains were capable of entering the aerosol probe. The flow field around the rocket was required for these simulations, previously described by Horanyi et al., [1999]. In short, the simulations integrate the equations of motion for nanometer-sized aerosols, taking into account viscous drag forces, magnetic forces, heating, and sublimation. At precisely zero degrees angle of attack, the flow around the MIDAS payload was azimuthally symmetric. This two dimensional fluid flow problem can be solved with Direct Simulation Monte Carlo (DSMC) methods [Gumbel, 2001]. Figure 9 shows the density field found from the DSMC simulation in multiples of the ambient mesospheric density. The simulation was performed at a neutral number density representative of 85 km in altitude. Note that a simplified outline of the rear mounted CONE instrument was used as the boundary condition rather than a blunt cylinder. The trajectories of spherical ice aerosols with radii of 100, 10, and 1 nm are shown in Figure 9. The grains were given a single negative charge. The shock completely diverted the 1 nm grains, but 10 nm and 100 nm grains pushed through the shock and hit the payload. The mass loss was completely negligible for 10 nm and 100 nm grains, while the most heavily sublimated 1 nm grain lost 40% of the starting mass. 100 nm grains passed directly over the graphite patch, but the v x B force was far too weak to turn them into the patch, even with an unreasonable charge of 300 electrons! These simulations reproduce the trajectory deflections that caused the aerosol signal to attenuate at zero angle of attack. They add confidence that the aerosol extraction process was accurate since angle of attack did not explicitly enter that calculation. While aerosols 1 nm are

12 12 highly influenced by individual molecular collisions and could conceivably bounce into the detector at zero angle of attack, the leading shock front sweeps out the approaching air volume. Thus the detectable ice grains for this aerosol probe are 1 nm. 4.2 Negative layer from SO-MI-11, the suspected NLC There were brilliant NLC displays seen throughout Europe on June 24 th, the night of SO- MI-11 (see the NLC reporting website htm.) In fact, one hour after the launch, observers in Oslo (60N, 11E, ~900 km southwest of the Andøya launch site) reported NLCs at a local elevation angle of 40 degrees which covered the sky from west to north. Those NLCs were assigned a brightness rating of 4 out of 5 and possessed every categorical form (diffuse, banded, wave-like, whirl-like) recorded by the site. Hence it is probable that NLCs were present above Andøya for SO-MI-11. The location of the negative layer was close to the observed locations of NLCs and PMCs. Measurements of PMC [Carbary et al., 2001] found an average height of /-1.3 km in the northern hemisphere. NLC lidar studies [von Cossart et al., 1999] also showed an average NLC altitude of 82.5 km, suggesting that NLCs and PMCs are identical phenomena simply viewed from different perspectives. The neutral density and temperature measurements from the CONE instrument can be used to find the water mixing ratio required for ice inside the negative layer. The ice saturation pressure from Stevens et al. [2001] shows that a water concentration of 16 (23) ppm was needed at 80.7 (80.4) km to maintain ice aerosols, a concentration seen (not seen) by [Summers et al., 2001] in the average 82 km - 84 km band. Thus, if the negative layer was an NLC, then it was at the lowest part of the local existence zone when observed. Stevens et al. [2001] has seen PMC as

13 13 low as 80 km, so the negative layer is plausibly close to all average altitudes. Wave motions like those seen by Gerrard et al. [1998] could also move the negative layer down from the average. The negative layer was 2 km beneath the bottom of the PMSE, similar to earlier observations [Walchli et al., 1993, von Zahn et al., 1999; Stebel et al., 2000] showing while NLC and PMSE can appear alone or together, NLCs are always present at the lower edges of the PMSEs or a few km lower. This has been partly attributed to a growth process where smaller PMSE aerosols at higher altitudes coagulate and descend, making larger NLC aerosols at lower altitudes. Another possibility was that the ambient ionization was only sufficient to create the PMSE above 83 km [Rapp et al., 2002], well above the negative layer. One striking feature of figure 6 was that negative charge was collected continuously through two rotations after n0. The steep angle of attack kept the flow (partially) azimuthally symmetric, sporadically distributing negative aerosols all the way around. The azimuthal symmetry was lost as the angle of attack grew and the omnidirectional negative signal ceased. Another important point is that n0 and n1 both occurred 45 degrees prior to the ram direction. The consistent forward facing angle suggests that they were larger aerosols that entered the detector with little deflection compared to the omnidirectional component. Figure 9 shows that the additional aerosols at n0 and n1 would need to be ~100 nm. Thus there was evidence of a particle size distribution in the negative layer: smaller aerosols swept in by the flow and additional, heavier ones that approached with straighter trajectories. The positive peaks p1 and p2 were most likely ion layers, given the serendipitous alignment of p2 and the ion excess seen on upleg. Since the horizontal distance between the upleg and downleg positions at this altitude was 20.7 km, the upleg ion data cannot be expected to match perfectly. Ion enhancements can exist in negative aerosol layers [Rapp and Lübken,

14 ], although a localized shift in aerosol size or ion recombination coefficient would be needed to confine the ion excess to a narrow band. It is also possible that the same mesospheric winds that collect water vapor across large distances for the growth of NLC aerosols [Thomas, 1991] could also gather ions into a layer by windshear. The charge number density of the negative layer as seen in the ram data is in figure 7b. Ignoring the single positive data point that came from p1, the layer had a peak negative charge number density of /-1100 charges cm -3. If an average charge of 3 electrons per NLC aerosol is assumed, an estimate for 50 nm aerosols in NLC conditions taken from [Rapp and Lübken, 2001], this makes an aerosol number density of 870 cm -3. The next lowest negative peak in the ram data was charges cm -3, making an aerosol number density of 570 cm -3. On the average, lidar derived NLC aerosol number densities are somewhat lower. A range of particles cm -3 was found by Alpers et al. [2000], and von Cossart et al. [1999] reported an average of 82 + /-52 particles cm -3, with one NLC event above 1000 particles cm -3. If the negative layer is assumed to be an average NLC, then the top of the negative layer had a little more negative charge than estimated by applying average aerosol charges to average lidar number densities. Since lidar backscatter rapidly diminishes as aerosols become smaller than 20 nm in radius, the upper parts of the negative layer could have contained aerosols smaller than the detection threshold. If the negative layer is assumed to be a strong NLC, (a reasonable assumption based on the observations) then the upper number density is comparable to the most intense NLC event from von Cossart et al. [1999]. Overall, the tapering of the aerosol density within the NLC with altitude was consistent with the top of the negative layer containing more numerous, smaller aerosols which coagulated during descent into fewer, larger aerosols.

15 PMSE aerosols, SO-MI-11 The positive aerosol layer at β is not an unprecedented measurement. Croskey et al. [2001] saw evidence of two groups of positive aerosols, corresponding to sizes of 1 nm and 10 nm, within a PMSE during the DROPPS campaign of The number density of the 10 nm positive aerosols was seen to approach that of the 1 nm positive aerosols precisely within the radar echo peak, becoming nearly 3000 charges cm -3. Assuming a similar situation existed during SO-MI-11, positive 10 nm aerosols are likely candidates for the positive peak. Using figure 9 as an estimate, 10 nm aerosols are clearly heavy enough to penetrate the shock front but 1 nm aerosols are going to be deflected. Layer β was 1700 charges cm -3, lower than the DROPPS result if singly positively charged aerosols are assumed. Negative aerosols, like those seen by Mitchell et al. [2001] in the same PMSE from DROPPS, could have existed alongside the positives and reduced the intensity of the aerosol signal. Thus, even if 1000 cm -3 singly charged negative aerosols were in the mix, making a true positive aerosol density of 2670 cm -3, this is still below the maximum value seen by Croskey et al [2001]. The negative layer α is reminiscent of the dust measurements of Havnes et al. [2001] because α occurred exactly in a small inflection in the radar profile with a charge number density in the thousands of charges cm -3. Because there was an ion enhancement, the charging model of Rapp and Lübken [2001] can be checked to see which aerosol sizes theoretically produce ion excesses. Without electron data the relative electron depletion is unknown, but that can be left as a free parameter. If the relative ion excess is taken to be approximately 25%, then figure 1 of [Rapp and Lübken, 2001] shows that thousands of singly negatively charged aerosols sized 2 to 5 nm in radius are eligible candidates. Thus, the charging model does not explicitly forbid ion

16 16 enhancements for the aerosol sizes that are detectable by the magnetically shielded probe. These arguments would apply to the ion excess seen at γ as well. The altitude of δ suggests that the temperature minimum is a source of aerosols. The net current nature of the probe revealed the most numerous aerosols at different heights, showing mostly positive (negative) aerosols that were correlated with the radar echo (temperature profile) at the low (high) regions of the mesopause. 4.3 PMSE aerosols from SO-MI-5 The correlation of the negative aerosol residual and the electron reduction in ζ was encouraging. At 86.1 km, the aerosol charge number density was -700 cm -3, while the electron density appeared to have a deficit of roughly 2000 cm -3 (a relative depletion near 30%). Assuming only negative aerosols were present, this implies just under 3 electrons per negative aerosol. Rapp and Lübken [2001] showed that smaller, more numerous PMSE aerosols capture one electron on the average. So, if it were assumed that 2000 singly negatively charged aerosols cm -3 captured the electrons, 1300 positive aerosols cm -3 would be needed to explain the net density of 700, which is reasonable given the results of Croskey et al. [2001] and the previous results from SO-MI-11. At event η the negative aerosols peaked at cm -3. The relative electron reduction was nearly 50% with roughly 4500 cm -3 electrons absent. The same estimate of 3 electrons per aerosol results if only negative aerosols are assumed. Singly negatively charged aerosols could have existed but would need an accompanying population of 3000 positive aerosols cm -3, still within the bounds of Croskey et al. [2001]. The charging model of Rapp and Lübken [2001] can be applied to region ζ to check for obvious contradictions. Assuming the relative electron depletion was somewhere between 25%

17 17 and 50%, and taking the relative ion depletion to be 25%, then figure 6 of [Rapp and Lübken, 2001] reveals that aerosols 5 to 20 nm in radius are eligible, which are all detectable aerosol sizes. Similar to SO-MI-11, the residual showed more negative aerosols reaching a maximum at ι, 1 km below the temperature minimum. Positive ions appear depleted in θ, indicating aerosols of some kind, but CONE showed either an unperturbed electron profile or a small enhancement. The transition from the good agreement in ζ to the disagreement in θ is puzzling. One explanation for the disagreement is that the high electron density (>10,000 cm -3 ) overcame the magnetic shielding. This explanation is unlikely because the peak at ι would not have formed since electron density continued to rise. Another explanation is that the negative aerosols in θ were small and mobile enough ( 1 nm) to be detected by the CONE instrument as if they were electrons. However, this is even more unlikely, since the attractive +6 volt potential of the CONE instrument would not create an effective cross section for negative aerosols as large as the one for electrons. Also note that the shock front would easily deflect such small aerosols. Perhaps a size distribution of aerosols could explain the disagreement of the aerosol probe and CONE. Suppose that aerosols in θ had a low work function because they contained sodium, like those discussed in [Rapp and Lübken, 1999]. Constructing a simple example, assume a double distribution of 2 nm and 10 nm grains where the 2 nm grains outnumber the 10 nm grains by a few thousand cm -3. Figure 2 of [Rapp and Lübken, 1999] shows that 2 nm sodium grains will tend to be singly negatively charged, while the 10 nm grains would have one positive charge. The smaller aerosols would only scavenge a few thousand electrons cm -3 from the preexisting distribution because the large aerosols emitted electrons, offsetting the depletion. The electron profile would not appear depleted because a few thousand electrons cm -3 was only 10%

18 18 of the total seen by CONE at ι. When the net aerosol current is measured, the smaller negatively charged grains outnumber the larger positive ones and create a net negative aerosol residual. If small negative aerosols were the source of the aerosol / CONE disagreement, and the mesospheric temperature minimum was the source of them, then the disagreement should be greatest near the minimum and decrease with altitude, since aerosols grow by coagulation as they descend. This is exactly what was seen inside θ, a negative aerosol density that peaked near the temperature minimum and decreased with altitude, thus this simple example is self-consistent. 5. Conclusions During SO-MI-11, the magnetically shielded probe detected a pronounced negative charge layer at the probable location of an NLC. This layer had a charge number density of charges cm -3 that was large but consistent with lidar derived number densities multiplied by model aerosol charges. The probe detected positive and negative particles in the PMSEs at charge number densities which were mutually consistent with Havnes et al. [2001], Croskey et al. [2001], and Mitchell et al. [2001]. The lower negative aerosol collection was simultaneous with a positive ion excess, another consistency with aerosol charging models. During SO-MI-5, the probe saw negative aerosols in an electron biteout, as expected from charging models and earlier results from Havnes et al. [2001] and Mitchell et al. [2001]. More negative aerosols were seen in regions where the electron density did not look depleted, but this could be explained by a net negative aerosol distribution. This magnetically shielded probe offers a technologically distinct strategy for measuring PMSE aerosols, possessing an open geometry very different from the electrically biased blunt cup probes. Improvements to the magnetically shielded probe to reduce positive ion collection will include stronger magnetic shielding and a small positive bias

19 19 (~1 V) for the graphite patch, making an instrument which only sees the heaviest charge carriers in a mixed plasma / aerosol environment. Acknowledgments. The authors thank DLR for their willingness to include the magnetically shielded probe in the MIDAS payload on short notice. B.S., S.R., and M.H. supported by NASA. References Alpers, M., Gerding, M., Höffner, J., von Zahn, U., NLC particle properties from a five color lidar observation at 54 degrees, J. Geophys. Res., 105, , Blix, T.A., Thrane, E.V., Andreassen, Ø., In situ measurements of the Fine Scale Structure and Turbulence in the mesosphere and lower thermosphere by means of electrostatic positive ion probes, J. Geophys. Res., 95(D5), , Carbary, J.F., Morrison, D., Romick, G.J., Hemispheric comparison of PMC altitudes, Geophys. Res. Lett., 28, , Croskey, C., Mitchell, J., Friedrich, M., Torkar, K., Hoppe, U., Goldberg, R., Electrical structure of PMSE and NLC regions during the DROPPS program, Geophys. Res. Lett., 28, 8, , von Cossart, G., Fiedler, J., von Zahn, U., Size distributions of NLC particles as determined from 3 color observations of NLC by ground based lidar, Geophys. Res. Lett., 26, , Gerrard, A., Kane, T., Thayer, J., Noctilucent clouds and wave dynamics: observations at Sondrestrom, Greenland, Geophys. Res. Lett., 25, , Gumbel, J., Aerodynamic influences on atmospheric in situ measurements from sounding rockets, J. Geophys. Res., 106, , 2001.

20 20 Havnes, O., Aslaksen, T., Brattli, A., Charged dust in the Earth s middle atmosphere, Physica Scripta, T89, , Havnes, O., Troim, J., Blix, T., Mortensen, W., Naesheim, L., Thrane, E., Tonnesen, T., First detection of charged dust in the Earth s mesosphere, J. Geophys. Res., 101, , Hervig, M., Thompson, R., McHugh, M., Gordley, L., Russell, J., Summers, M., First confirmation that water ice is the primary component of polar mesospheric clouds, Geophys. Res. Lett., 28, , Horanyi, M., Gumbel, J., Witt, G., Robertson, S., Simulation of rocket borne particle measurements in the mesosphere, Geophys. Res. Lett., 26, , Horanyi, M., Robertson, S., Smiley, B., Gumbel, J., Witt, G., Walsh, B., Rocket borne mesospheric measurement of heavy (m>>10 amu) charge carriers, Geophys. Res. Lett., 27, , Kopp, E., A Global Model of Positive and Negative Ions in the Lower Ionosphere, Adv. Space. Res., 25, , Kopp, E., Fritzenwallner, J., Model calculations of the negative ion chemistry in the mesosphere with special emphasis on the chlorine species and the formation of cluster ions, Adv. Space. Res., 21, , Mitchell, J.D., Croskey, C.L., Goldberg, R.A., Evidence for charged aerosols and associated meter scale structure in identified PMSE/NLC regions, Geophys. Res. Lett., 28, , Rapp, M., Lübken, F.J., Modelling of positively charged aerosols in the polar summer mesopause region, Earth Planets Space, 51, , 1999.

21 21 Rapp, M., Gumbel, J., Lübken, F., Absolute density measurements in the middle atmosphere, Ann. Geophys., 19, , Rapp, M., Lübken, F.J., Modelling of particle charging in the polar summer mesosphere: part 1 general results, Journal of Atmospheric and Solar Terrestrial Physics, 63, , Rapp, M., Gumbel, J., Lübken, F., Latteck, R., D-region electron number density limits for the existence of PMSE, accepted by J. Geophys. Res., Stebel, K., Barabash, V., Kirkwood, S., Polar mesospheric summer echoes and noctilucent clouds: simultaneous and common volume observations by radar, lidar, and CCD camera, Geophys. Res. Lett., 27, , Stevens, M., Conway, R., Englert, C., Summers, M., Grossmann, K., Gusev, O., PMCs and the water frost point in the Artic summer mesosphere, Geophys. Res. Lett., 28, , Summers, M., Conway, R., Englert, C., Siskind, D., Stevens, M., Discovery of a water vapor layer in the arctic summer mesosphere: implications for polar mesospheric clouds, Geophys. Res. Lett., 28, , Thomas, G., Mesospheric clouds and the physics of the mesopause region, Reviews of Geophysics, 29, , Walchli, U., Stegman, J., Witt, G., Cho, J.Y.N., Miller, C.A., Kelley, M.C., Swartz, W.E., First height comparison of NLC and simultaneous PMSE, Geophys. Res. Lett., 20, , von Zahn, U., Bremer, J., Simultaneous and common volume observations of noctilucent clouds and polar summer mesospheric echoes, Geophys. Res. Lett., 26, , 1999.

22 22 Figure captions Figure 1. Schematic of the MIDAS payload used in the SOLSTICE 2001 campaign. The downleg velocity was ~720 meters/sec in the mesopause. Figure 2. (a) Raw aerosol data from the magnetically shielded probe during downleg of SO-MI- 05. The small sketch shows the angle of attack and velocity plane. (b) Angle of attack during downleg of SO-MI-05. Figure 3. (a) Raw aerosol data from downleg of SO-MI-11. The collected current became net negative at 80.7 km when the payload went through a suspected NLC. (b) Angle of attack during downleg of SO-MI-11. Figure 4. (a) The sorted ram direction data from the magnetically shielded probe. Also shown is the positive ion profile smoothed to a time period that matched the sorted ram data. (b) Normalized plots of m(z) and b(z) used for aerosol extraction on SO-MI-05. m 0 was m 3 na, b 0 was na. (c) Sketch of the ion background B(z) on top of the ram direction data. Figure 5. (a) Sorted ram direction data and smoothed positive ion profile for flight 2. (b) Normalized plots of m(z) and b(z) used in the aerosol extraction for SO-MI-11. m 0 was m 3 na, b 0 was na. (c) Sketch of the ion background B(z) on top of the ram direction data. Figure 6. Detail of the negative layer encountered on SO-MI-11 along with the upleg positive ion data. Note how a small ion enhancement coincides with p2. The Os are when the probe was

23 23 facing exactly in the ram direction, as shown by the sketch in the lower right. The same ram points are also present in Figure 7b. Figure 7. (a) Thermal structure and radar profile during SO-MI-11. (b) The derived aerosol charge number density, showing areas of both positive and negative aerosols. The zeroes along the central axis show when angle of attack was zero. The positive spike at 80.7 comes from the ram direction point in p1, figure 6, and is thought to be ions. (c) Downleg positive ion profile during SO-MI-11. Note the ion enhancements at α and γ coincide with negative aerosols, consistent with aerosol charging models. Figure 8. (a) Thermal structure and radar profile during SO-MI-5. (b) The derived aerosol charge number density. The zeroes along the central axis show when angle of attack was zero. The negative aerosol density followed the electron biteout closely in ζ, but not θ where there was either no biteout or a small enhancement. (c) Downleg positive ion and electron profile during SO-MI-05. Figure 9. (a) The density of the supersonic, rarefied flow past the MIDAS payload base at exactly zero degrees angle of attack. The unperturbed flow speed was 700 m/sec. The compression is shown in multiples of the ambient number density of cm -3. (b) The trajectories of 1 nm spherical ice grains with 1 charge. 1 nm grains are completely deflected, missing the payload entirely. (c) Same but for 10 nm grains with 1 charge. The grains easily push through the shock. (d) Same but for 100 nm grains with 1 charge, which are too heavy to be turned into the patch by the v x B force.

24 24 Figure 1 Figure 1. Schematic of the MIDAS payload used in the SOLSTICE 2001 campaign. The downleg velocity was ~720 meters/sec in the mesopause.

25 25 Figure 2 Figure 2. (a) Raw aerosol data from the magnetically shielded probe during downleg of SO-MI- 05. The small sketch shows the angle of attack and velocity plane. (b) Angle of attack during downleg of SO-MI-05.

26 26 Figure 3 Figure 3. (a) Raw aerosol data from downleg of SO-MI-11. The collected current became net negative at 80.7 km when the payload went through a suspected NLC. (b) Angle of attack during downleg of SO-MI-11.

27 27 Figure 4 Figure 4. (a) The sorted ram direction data from the magnetically shielded probe. Also shown is the positive ion profile smoothed to a time period that matched the sorted ram data. (b) Normalized plots of m(z) and b(z) used for aerosol extraction on SO-MI-05. m 0 was m 3 na, b 0 was na. (c) Sketch of the ion background B(z) on top of the ram direction data.

28 28 Figure 5 Figure 5. (a) Sorted ram direction data and smoothed positive ion profile for flight 2. (b) Normalized plots of m(z) and b(z) used in the aerosol extraction for SO-MI-11. m 0 was m 3 na, b 0 was na. (c) Sketch of the ion background B(z) on top of the ram direction data.

29 29 Figure 6 Figure 6. Detail of the negative layer encountered on SO-MI-11 along with the upleg positive ion data. Note how a small ion enhancement coincides with p2. The Os are when the probe was facing exactly in the ram direction, as shown by the sketch in the lower right. The same ram points are also present in Figure 7b.

30 30 Figure 7 Figure 7. (a) Thermal structure and radar profile during SO-MI-11. (b) The derived aerosol charge number density, showing areas of both positive and negative aerosols. The zeroes along the central axis show when angle of attack was zero. The positive spike at 80.7 comes from the ram direction point in p1, figure 6, and is thought to be ions. (c) Downleg positive ion profile during SO-MI-11. Note the ion enhancements at α and γ coincide with negative aerosols, consistent with aerosol charging models.

31 31 Figure 8 Figure 8. (a) Thermal structure and radar profile during SO-MI-5. (b) The derived aerosol charge number density. The zeroes along the central axis show when angle of attack was zero. The negative aerosol density followed the electron biteout closely in ζ, but not θ where there was either no biteout or a small enhancement. (c) Downleg positive ion and electron profile during SO-MI-05.

32 32 Figure 9 Figure 9. (a) The density of the supersonic, rarefied flow past the MIDAS payload base at exactly zero degrees angle of attack. The unperturbed flow speed was 700 m/sec. The compression is shown in multiples of the ambient number density of cm -3. (b) The trajectories of 1 nm spherical ice grains with 1 charge. 1 nm grains are completely deflected, missing the payload entirely. (c) Same but for 10 nm grains with 1 charge. The grains easily push through the shock. (d) Same but for 100 nm grains with 1 charge, which are too heavy to be turned into the patch by the v x B force.