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Measurements of meteor smoke particles during the ECOMA-2006campaign: 1. Particle detection by active photoionization

Markus Rapp, Irina Strelnikova

Department of Radar Soundings and Sounding Rockets, Leibniz-Institute of Atmospheric Physics, University of Rostock, Schlosstr. 6, 18225 Kuhlungsborn, Germany

a r t i c l e i n f o

Article history:

Accepted 2 June 2008Available online 12 June 2008

Keywords:

Meteor smoke particles

In situ measurements

a b s t r a c t

We present a new design of an in situ detector for the study of meteor smoke particles (MSPs) in the

middle atmosphere. This detector combines a classical Faraday cup with a xenon-flashlamp for the

active photoionization/photodetachment of MSPs and the subsequent detection of corresponding

photoelectrons. This instrument was successfully launched in September 2006 from the Andya Rocket

Range in Northern Norway. A comparison of photocurrents measured during this rocket flight and

measurements performed in the laboratory proves that observed signatures are truly due to

photoelectrons. In addition, the observed altitude cut-off at 60 km (i.e., no signals were observed

below this altitude) is fully understood in terms of the mean free path of the photoelectrons in the

ambient atmosphere. This interpretation is also proven by a corresponding laboratory experiment.

Consideration of all conceivable species which can be ionized by the photons of the xenon-flashlamp

demonstrates that only MSPs can quantitatively explain the measured currents below an altitude of

90 km. Above this altitude, measured photocurrents are most likely due to photoionization of nitric

oxide. In conclusion, our results demonstrate that the active photoionization and subsequent detection

of photoelectrons provides a promising new tool for the study of MSPs in the middle atmosphere.

Importantly, this new technique does not rely on the a priori charge of the particles, neither is the

accessible particle size range severely limited by aerodynamical effects. Based on the analysis described

in this study, the geophysical interpretation of our measurements is presented in the companion paperby Strelnikova, I., et al. [2008. Measurements of meteor smoke particles during the ECOMA-2006

campaign: 2. results. Journal of Atmospheric and Solar-Terrestrial Physics, this issue, doi:10.1016/

j.jastp.2008.07.011].

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Meteor smoke particles (MSPs) are thought to be formed by

the recondensation of metal- and silicon-containing molecules

originating from the ablation of meteoroids in the 70120 km

altitude range (Plane, 2003). Starting with the 1960s, modelstudies suggested that MSPs should exist in the mesosphere/lower

thermosphere with number densities of up to 104 particles=cm3

and corresponding radii ranging from the sub-nanometer range to

a few nanometers (Rosinski and Snow, 1961; Hunten et al., 1980).

Despite these tiny dimensions, it has been suggested that MSPs

are involved in a variety of atmospheric processes such as the

nucleation of mesospheric ice clouds, heterogeneous chemistry,

and the formation of nitric acid trihydrate (NAT)-particles in polar

stratospheric clouds which are involved in ozone destruction in

the polar spring (e.g., Rapp and Thomas, 2006; Summers and

Siskind, 1999; Voigt et al., 2005). Despite obvious interest in MSPs,

measurements of such particles have proven difficult so that today

only a few measured altitude profiles are available from charged

particle measurements on sounding rockets (Schulte and Arnold,

1992; Gelinas et al., 1998; Horanyi et al., 2000; Croskey et al.,2001; Lynch et al., 2005; Rapp et al., 2005; Amyx et al., 2008 ) and

from incoherent scatter radar experiments (Rapp et al., 2007;

Strelnikova et al., 2007).

Most of these earlier in situ measurements used Faraday cup-

based instruments based on the original design of Havnes et al.

(1996) who was the first to detect charged (ice) particles in the

polar summer mesopause region. For this type of detector,

however, it has become clear in the meantime that it possesses

a severely limited detection efficiency for the smallest MSPs

(i.e., below$12 nm) and for which the smallest detectable radius

even varies with altitude as a consequence of aerodynamics

(Horanyi et al., 1999; Rapp et al., 2005; Hedin et al., 2007).

In addition, it has recently been speculated that such

ARTICLE IN PRESS

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jastp

Journal ofAtmospheric and Solar-Terrestrial Physics

1364-6826/$- see front matter& 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.jastp.2008.06.002

Corresponding author. Tel.: +49 3829368200; fax: +49 382936850.

E-mail address: [email protected] (M. Rapp).

Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 477485
http://dx.doi.org/10.1016/j.jastp.2008.07.011http://dx.doi.org/10.1016/j.jastp.2008.07.011http://www.sciencedirect.com/science/journal/atphttp://www.elsevier.com/locate/jastphttp://dx.doi.org/10.1016/j.jastp.2008.06.002mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.jastp.2008.06.002http://www.elsevier.com/locate/jastphttp://www.sciencedirect.com/science/journal/atphttp://dx.doi.org/10.1016/j.jastp.2008.07.011http://dx.doi.org/10.1016/j.jastp.2008.07.011

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measurements which rely on the impact of particles on the

detector electrode may be influenced by secondary charging

effects such as fragmentation and triboelectric charging (Barjatya

and Swenson, 2006; Havnes and Naesheim, 2007; Amyx et al.,

2008). Measurements with incoherent scatter radars, on the other

hand, have so far only proven to be feasible with the most

powerful incoherent scatter radar, i.e., the Arecibo radar, whereas

measurements with even the EISCAT UHF radar were onlyindicative of the existence of MSPs (Rapp et al., 2007). In addition,

even with the Arecibo radar, measurements could only be

performed at altitudes with sufficient D-region ionization, i.e., in

daytime and above $85km (Strelnikova et al., 2007). Hence, it is

obvious that new measurements which should cover a larger

altitude range and which should be able to detect MSPs of all sizes

are highly desirable.

In the current manuscript, we present the concept and first

results of a new rocket borne particle detector which uses active

photoionization to measure the concentration of MSPs and hence

largely avoids limitations from aerodynamics and secondary

charging effects and does not depend on the a priori charge of

the particles. In Section 2 we introduce the basic design of this

detector. Initial results from a rocket flight during the ECOMA-2006 campaign (ECOMA existence and charge state of meteor

smoke particles in the middle atmosphere) are presented in

Section 3. These flight data are then discussed in the scope

of laboratory measurements in Section 4.1 and with respect to

contamination from other ionizable species (like nitric oxide) in

Section 4.2. The geophysical results from the discussed rocket

flight are presented in the companion paper by Strelnikova et al.

(2008).

2. Instrument description

The particle detector applied for the current study is a

combination of a Faraday cup and a xenon-flashlamp for the

photoionization of particles. A photo of the instrument together

with a schematic of the detector is shown in Fig. 1.

The design of the Faraday cup itself is very close to the original

design ofHavnes et al. (1996) who were the first to detect charged

nanoparticles (in their case consisting of water ice) in the

mesosphere. The Faraday cup comprises a collector electrode

(held at payload potential by the negative feedback loop of the

electrometer) for the measurement of particles of either positive

or negative charge and two shielding grids (biased at 6:2 V

relative to payload potential) to shield the collector electrode from

ambient thermal electrons and ions. Note that unlike these

thermal electrons and ions the particles have a much larger mass

such that their kinetic energy (i.e., 12 mpv2r where mp is the particle

mass and vr is the rocket velocity) is large enough to penetrate thepotential barrier set by the shielding grids.

The novelty of this instrument, however, is the combination of

the Faraday cup design with a xenon-flashlamp. The flashlamp is a

commercially available lamp (Perkin Elmer FX1162, see http://

optoelectronics.perkinelmer.com for technical details) which we

operate at a repetition rate of 20 Hz with an energy per flash of

0.5 J. The broadband spectrum of each flash contains a sufficiently

large number of UV photons down to a minimum wavelength of

$110 nm corresponding to a maximum photon energy of 11.3 eV

(see Fig. 2). The UV photons may now create photoelectrons by

photoionization and/or photodetachment of an electron from a

neutral/negatively charged species with sufficiently low work

function (or electron affinity) which may in turn be detected at

the detector electrode as a short charge pulse. We note thatinstruments employing lamps to actively ionize atmospheric

constituents were applied earlier (e.g., Croskey et al., 2003, and

references therein).

In order to be able to record both the continuous charge signal

from the naturally charged MSPs which are large enough to

penetrate to the detector electrode and the very short charge

pulses excited by the UV flash, we have chosen to operate the

electrometer in the so-called integrate and dump mode. This has

the advantage that even the smallest charge signatures are not

missed provided the noise level of the electrometer is sufficiently

low. This means that the electrometer is in fact operated as an

integrator which has to be discharged shortly before a flash. The

charge pulse created by the flash is then integrated by an inverting

amplifier (i.e., a positive charge leads to a negative slope registered

by the integrator) and the resulting signal is digitized by a 16 bit

analog-to-digital converter. In addition to the charge pulse, the

amplifier also samples any existent DC-current due to naturally

charged particles of either polarity. In order to properly sample the

shape of the UV-created charge pulses on the one hand and tomeasure the rather small DC-currents due to naturally charged

particles on the other hand, we sample the output signal of the

integrator at two sampling frequencies: the charge pulsewhich

originates from an effective ionized volume between 2.5cm

(photoelectrons from closer distances are actually in the shadow

of the flashlight itself and cannot reach the electrode) and 70 cm

upstream of the detectorhas a typical duration ofo0:5ms and is

sampled with 100 kS/s (=kilosamples/second), stored in a memory,

and subsequently transmitted to ground by telemetry in non-real

time (note, however, that the start time of each such data dump is

exactly defined). DC-currents, however, are sampled with 1 kS/s

and transmitted to ground in real time (note that the first sample

of this signal also contains the charge pulse, however, at a much

poorer temporal resolution). If the integrator reaches its positive ornegative maximum value it is automatically discharged such that

ARTICLE IN PRESS

Fig. 1. Photo and schematic of the ECOMA-particle detector. See text for details.

M. Rapp, I. Strelnikova / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 477485478
http://optoelectronics.perkinelmer.com/http://optoelectronics.perkinelmer.com/http://optoelectronics.perkinelmer.com/http://optoelectronics.perkinelmer.com/

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this amplifier setup provides the capability to continuously

measure a broad current range, i.e., from the sub-picoamp to the

0:5mA range. Finally, after 50 ms (i.e., shortly before the next flash)the next discharge of the integrator is initiated and the whole

sequence starts from the beginning.

As applied to the measurement of MSPs, the conceptual idea

behind our particle detector may be summarized as follows: In

Fig. 3 we have sketched the particle size distribution of MSPs as

predicted by microphysical models (Hunten et al., 1980; Megner

et al., 2006). In this size distribution, particles with radii larger

than a minimum radius (here indicated by the vertical blue line)

may be directly detected by particle impact and subsequent

charge deposition at the collector electrode of the Faraday cup.

Particles with radii smaller than this minimum radius may not be

detected because of aerodynamic effects on their trajectories

(e.g., Rapp et al., 2005; Hedin et al., 2007). All of the particles,

however, may be photoionized and corresponding photoelectrons

may be detected as a charge pulse. Hence, the particle detector

provides information on the number density of charged particles

which are larger than a certain threshold size and on the total

number density of all MSPs.

Note that first results of conventional Faraday cup measure-

ments with this instrument from a rocket flight in October 2004were already presented in Rapp et al. (2005). At that time,

however, the photoelectron measurements with the detector

were impaired by electromagnetic interference from the flash-

electronics and could not be used. In the present manuscript, we

will focus on new measurements obtained in September 2006

during which photoelectron measurements yielded high quality

data. In the following section we will present actual data from this

rocket flight with which we will demonstrate the feasibility of the

detector concept described above.

3. Atmospheric measurements

In September 2006, the ECOMA-2006 campaign was conductedfrom the North-Norwegian Andya Rocket Range 69N. ECOMA

( existence and charge state of MSPs in the middle atmosphere)

is an international research program led by the Leibniz Institute of

Atmospheric Physics in Germany and the Norwegian Defence

Research Establishment in Norway (with additional contributions

from Austria, Sweden, and the USA) dedicated to the study of

MSPs and their atmospheric environment. Besides the particle

detector described in the preceding section, the payload carried

instruments to characterize the D-region plasma, a particle

sampler for the in-flight collection of MSPs, and instruments

to measure neutral air density, temperature, and turbulence.

The atmospheric background state was monitored by means of

the EISCAT UHF and VHF radars, the ALOMAR RMR lidar, and the

ALOMAR sodium lidar. An overview of all measurements obtained

during the campaign along with a more detailed description of all

involved instruments is presented in the companion paper by

Strelnikova et al. (2008). In this manuscript, however, our focus is

on the flight performance of the ECOMA particle detector.

The ECOMA-01 payload was launched on September 8, 2006 at

22:17:00 UT and reached an apogee of 130.6km. During the entire

rocket flight the particle detector worked nominally and provided

data in both data channels. In order to demonstrate the quality of

these measurements we present an arbitrarily chosen raw data

sample from about 83:95 0:05 km on the upleg part of the rocketflight. In Fig. 4, the black symbols and lines show the charge signal

from the integrator circuit in digital units (du). The red symbols

and lines are corresponding currents derived by taking the time

derivative of the integrator output and taking into account that

1 du corresponds to a charge of 0.18 fC. Blue vertical lines indicate

the times when the integrator was discharged after which a flash

was triggered. The upper panel shows results from the 1 kHz data

channel which primarily records charge signatures and currents

due to the impact of particles on the collector electrode. In the

lower panel, corresponding signals from the fast (100 kHz) data

channel are shown which clearly show the pulse due to

photoelectrons with an amplitude of $ 4:5n A on top of a

smooth DC-background, in this case of about 50 pA. Note that

the photoelectron current due to the flash is certainly alsorecorded in the slow data channel, however, due to its poorer

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Fig. 2. Spectrum of the xenon-flashlamp in the UV wavelength range measured with a plane grating vacuum spectrometer. Note that the cut-off at 110nm is due to the

transmission properties of the window of the flashlamp.

M. Rapp, I. Strelnikova / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 477485 479

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time resolution, its peak value is not captured (though the

recorded charge signal is certainly the same).From the raw data of the type described above, we have now

determined the background DC-currents from the slow data

channel (where the first sample after each flash which contains

the additional charge due to photoelectrons has been omitted)

and the maximum current from each photoelectron pulse due to

the xenon-flash from the fast data channel. Corresponding

altitude profiles are presented in Fig. 5 for both the upleg and

downleg part of the rocket flight. The background measurements

of the particle charge (left panel in Fig. 5) reveal a prominent

negative current layer in the altitude range between 80 and 90 km

indicative of negatively charged particles. This layer is qualita-

tively very similar to the previous measurements by Faraday cup

instruments reported, e.g., by Lynch et al. (2005) and Rapp et al.

(2005). A striking difference to the Rapp et al. (2005) results is,however, the opposite polarity of the particle charge. As discussed

ARTICLE IN PRESS

Fig. 3. Sketch of the particle size distribution of meteor smoke particles as predicted by microphysical models ( Hunten et al., 1980; Megner et al., 2006). The blue vertical

line indicates that only a part of the size distribution may be detected by direct particle impact in a particle detector due to aerodynamical effects on the trajectories ofsmall particles (e.g., Rapp et al., 2005; Hedin et al., 2007). All of the particles, however, may be photoionized and corresponding photoelectrons may be detected as a charge

pulse.

Fig. 4. Raw data samples for the slow (i.e., 1 kHz, upper panel) and fast data

channel (i.e., 100kHz, lower panel) from an altitude of $83:95km on the upleg

part of the rocket trajectory. The blacksymbols and lines show the charge signal

from the integrator circuit in digital units (du). The red symbols and lines are

corresponding currents derived by taking the time derivative of the integrator

output. Blue vertical lines indicate the times when the integrator is discharged

after which the flash is triggered. Note that a positive slope in the integrator output

indicates a negative current, in this case of about 50 pA. Note further that the

photoelectron current due to the flash is also recorded in the slow data channel,however, due to its poorer time resolution, its peak value is not captured.

Fig. 5. Overview of current measurements from the ECOMA flight on September 8,

2006. Left panel: current measurements due to direct particle impacts on the

electrode recorded as a DC-current in the slow data channel. Right panel:

corresponding peak photoelectron currents recorded in the fast data channel.

Black lines are for upleg measurements, red lines are for downleg.


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in detail in Strelnikova et al. (2008), the major difference between

these two flights is the much larger background ionization and a

different electrode material in 2006 which might have caused

potential differences in triboelectric charging.

Above the negative layer, the current turns positive and even

shows a very prominent positive peak at $93km which is the

altitude where the EISCAT radars observed a sporadic E-layer

(see Strelnikova et al., 2008, for details). It is interesting to notethat Horanyi et al. (2000) observed a similar morphology with a

broad negative layer below a narrow positive layer which they

interpreted as a sudden E-layer related to the deposition of

meteoric metal ions. In our case, however, the positive peak is also

seen on the downleg of the rocket flight when the instrument is

not facing the ram (and no particles can enter the detector

volume). Hence, we conclude that these currents must be due to

contamination by positive ions. These measurements will be

further discussedfor example with respect to the observed

particle charge and triboelectric effectsin the companion

paper by Strelnikova et al. (2008). Here, we now focus

on the measurements due to the flash of the detector as shown

in the right panel of Fig. 5. On upleg, large currents of up to

10 nA were recorded above an altitude of$80 km. This currentdecays to about 2 nA at apogee but never returns to zero. On

downleg, the measured current shows a similar behavior with

slightly enhanced values from apogee down to 80 km. The largest

difference, however, is that on downleg the (negative) current

further increases with decreasing altitude until it reaches a

maximum value of 15nA at $75km and disappears at about

60 km, i.e., about 20 km lower than on upleg.

In the next section we will critically discuss the following

questions:

(1) Is it possible to prove that the recorded currents in the fast

data channel (called photo-currents hereafter) are indeed due

to photoelectrons?

(2) Is it possible to understand the differences of the recorded

photo-currents on upleg and downleg?

(3) And finally: Is it possible to show that the observed photo-

currents originate from MSPs particles and not from other

ionizable species such as nitric oxide, excited oxygen, or metal

atoms?

4. Discussion

4.1. Laboratory experiments

In order to answer questions 1 and 2 raised in Section 3 we

devised a laboratory experiment as sketched in Fig. 6. The ECOMA

particle detector was mounted inside a large vacuum chamber

which was evacuated to a pressure below 104 mbar. Opposite thedetector we mounted a conical target coated with an aluminum

surface shaped such that flash photons emitted by the xenon-

flashlamp of the detector were not reflected back towards the

collector electrode. In consequence, currents due to unwanted

photoemission from the collector electrode could be avoided. UV

photons from the xenon-flash, however, were sufficiently ener-

getic (maximum energy 11.3 eV, see Section 2) to create photo-

electrons at the surface of the aluminum target (workfunction

4.2eV), part of which were then flying towards the collector

electrode. A typical current pulse recorded during such measure-

ments along with a current pulse recorded during the rocket flight

in September 2006 is presented in Fig. 7. Obviously, both pulses,

i.e., from the laboratory experiment and from the rocket flight,

have exactly the same shape (note that the coincidence inamplitude is certainly by chance and is not meant to imply that

the two pulses have exactly the same origin) which indicates that

the pulses recorded during the rocket flight are indeed photo-

electrons emitted by an ionizable species (see Section 4.2 below

for arguments that these species must have been MSPs in most of

the altitude range where photo-currents were observed). Note

further that we were able to verify that the signals recorded in the

laboratory were due to photoelectrons by varying a retarding

potential between the target and the collector electrode.

As expected for photoelectrons, the signal dropped to zero when

the retarding potential reached a value of $ 6 V, i.e., at the

expected kinetic energy of the photoelectrons ( flash energyminus work function). Note further that we also studied the effect

of the potentials of the shielding grids on the collection of the

photoelectrons. This showed that the potentials of the two grids

only had a negligible effect on the collection of the photoelectrons

indicating that the rejection of electrons in the eV energy range by

this grid type is poor. Note, however, that another experiment in

which we tested the rejection of thermal electrons (energy

o0:1 eV) in a plasma chamber revealed that the rejection

efficiency of these was close to 100%.

Finally, we used the laboratory setup described above to study

the pressure dependence of the recorded photocurrents. I.e., we

varied the stationary pressure in the vacuum chamber between

104 and 1 mbar and measured the maximum current due to

photoelectrons emitted from the target. The results from thisexperiment are shown in the upper panel of Fig. 8. This indicates

ARTICLE IN PRESS

Fig. 6. Setup of laboratory measurements to identify the origin (and other

properties) of the recorded currents due to the xenon-flash. The ECOMA-particle

detector is brought into a vacuum chamber and flashes at a conical target coated

with aluminum. The electrode voltage with respect to the vacuum chamber was

varied between 8 V in order to determine the kinetic energy of the recorded

photoelectrons (retarding field method).

Fig. 7. Comparison of recorded current pulses due to photoelectrons produced bythe Xe-flash from the actual rocket flight (red curve) and laboratory measurements

(black curve).


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that for pressures below $102 mbar, all photoelectrons can reach

the collector electrode and are not affected by collisions with gas

molecules. At higher pressures, however, the current due to

photoelectrons drops until at $0:1 mbar no more photoelectrons

can be detected. The reason for this behavior can be understood

by considering the lower panel in Fig. 8. This figure shows

calculations of the mean free path of an electron in air as a

function of pressure. A pressure of 102 mbar corresponds to a

mean free path of $5cm, which is about the dimension of our

particle detector. For lower pressures, the mean free path of a

photoelectron is hence sufficiently large such that it can be

detected in our instrument without undergoing collisions with

gas molecules. For higher pressures, however, collisions with gas

molecules lead to the loss of a part of the photoelectrons until at a

pressure of 0.1 mbar the mean free path has dropped by about a

factor of 10 and no more photoelectrons can reach the collector

electrode.

We now come back to the results from our rocket flight,

showing that on upleg the minimum altitude for photoelectrondetection was 80 km, whereas it decreased to 60 km on downleg.

Taking into account the laboratory measurements shown in Fig. 8,

this can be explained as follows: On upleg, the instrument is

facing the ram such that the pressure inside the instrument is

enhanced because of the aerodynamical compression of the

ambient air. Monte Carlo simulations for a cup-like detector

geometry as well as calculations using the RayleighPitot formula

imply that for a rocket flight with an apogee of 130km the

pressure should increase by about a factor of 13 at an altitude of

80km (see Fig. 7 in Rapp et al., 2001). Using the densities and

temperatures for the beginning of September from the climatol-

ogy of Lubken (1999) this implies that at 80 km, the pressure

should have been 13 102 mbar 0:13 mbar. Comparing this

pressure to the pressure for which our laboratory measurementsshow that the photoelectron-current should have dropped to zero,

we find that laboratory and flight results are in excellent

agreement. Note that we may ignore the effect of the rather weak

compression in the first few centimeters upstream of the detector

on the creation of photoelectrons because the first 2.5 cm of the

ionization volume are actually in the shadow of the flashlight such

that photoelectrons from this volume can never reach the detector

electrode (see Hedin et al., 2007 and our discussion of the

effective ionization volume above). As for the downleg measure-ments, the Lubken (1999)-climatology predicts a pressure of

0.24mbar at 60 km altitude. During this part of the rocket flight,

however, the particle detector is in the wake, for which Monte

Carlo simulations by Gumbel (2001) predict that the pressure

should be reduced by about a factor of 2. Hence, on downleg,

the current due to photoelectrons vanishes at a pressure of about

0:5 0:24mbar 0:12 mbar, which is again almost exactly the

pressure predicted by our laboratory experiments. Hence, the

difference in the photoelectron measurements between upleg

and downleg can be completely understood on the basis of the

pressure dependence of the mean free path of the photoelectrons

in the environment of the particle detector and its modification by

the aerodynamics of the rocket flight. Interestingly, this analysis

shows that the altitude range over which our method is capable ofdetecting signatures from ionizable species like meteor smoke

particles is enhanced by$20 km on the downleg part of the rocket

flight when the detector is in the wake.

4.2. Ionization of other species than MSPs

After we have demonstrated that the photocurrents recorded

in our fast data channel are indeed due to photoelectrons excited

by the photons of the xenon-flash, we now turn to the question if

these photoelectrons truly originate from MSPs or from some

other species.

In order to answer this question, a survey of middle atmo-

spheric species and corresponding threshold energies for either

photoionization of a neutral molecule or an atom (i.e., theionization potential), or for the photodetachment of an electron

from a negative ion (i.e., the electron affinity) reveals a list of

candidates which we have summarized in Table 1. These

candidates are nitric oxide (NO), excited molecular oxygen

O21Dg, the metal atoms Fe and Na, and the negative ions NO

3 ,

CO3 , and O2 . Note that also other metal atoms like K, Ca, Mg, and

Si have sufficiently low ionization potentials, however, for the

calculations below we restrict ourselves to the case of Fe and Na

which are by far the most abundant metal atoms in altitude range

of interest (e.g., Plane, 2003).

In order to judge whether the recorded photocurrents could

be due to these species, we have tried to estimate the

expected currents based on altitude profiles of their concentra-

tions (see Fig. 9) and their properties with respect to photo-ionization/photodetachment (see Table 1). NO number densities

have been taken from the HALOE-climatology of Siskind et al.

(1998), O21Dg number densities from the rocket measurements

by Gumbel et al. (1998), metal atom profiles are from Plane

(2003), and number densities of negative ions are taken from the

model study of Thomas and Bowman (1985). As for the meteor

smoke particles, hypothetical profiles of number densities of MSPs

of different sizes have been taken from the model study by

Megner et al. (2006). References for corresponding photoioniza-

tion cross sections and threshold energies are provided in Table 1.

The photoionization/photodetachment cross sections of MSPs

with radius rp at photon wavelength l are estimated using Mie-

theory, i.e.,

sMSPphotorp;l pr2p Qabsrp;l; nl; kl Y (1)

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Fig. 8. Upper panel: pressure dependence of the photoelectron current recorded in

the laboratory (black symbols with blue error bars). The red lines are drawn to

guide the eye. Lower panel: results of the calculation of the mean free path of a

photoelectron in air as a function of air pressure.


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where Qabs is the Mie absorption efficiency which we calculated

using the publicly available Mie-code from the text book byBohren and Huffman (1983), n and k are the real and imaginary

parts of the refractive index of the MSP material, and Y is the

quantum yield for photoemission/photodetachment. Guided by

the experimental finding that the yields of nanoparticles may

reach very large values which are up to three orders of magnitude

larger than the yields for the corresponding bulk material

(e.g., Schmidt-Ott et al., 1980; Muller et al., 1991) and in order

to derive upper estimates of the currents that we may expect, we

have chosen a value of Y 1:0 for our calculations. Finally,

following Plane (2003) who argues that MSPs should be some sort

of iron or silicon oxides, we consider refractive indices for Fe2O3and SiO, respectively (see http://www.astro.uni-jena.de/Labora-

tory/OCDB/oxsul.htm and http://luxpop.com/RefractiveIndexList.

htm, for corresponding internet data bases).Assuming that only single photoelectrons are emitted, that the

photoemission/photodetachment processes are independent, and

finally that the maximum charge is collected by the electrode

during the duration of the first sampling interval after the flash,

we estimate the corresponding maximum currents as

IMSPmax

Z1rmin

ZveDt2:5 cm

Zhc=Wp110 nm

dNpdrp

dF

dl sMSPphotorp;l P drp dl dl

e

Dt(2)

where rmin 0:2 nm is the minimum assumed size of MSPs

following the original proposal of Hunten et al. (1980) and taking

into account the argument that immediate re-condensation in ameteor trail is highly unlikely (Plane, 2000). Note that the exact

value of rmin is not critical for the value of Imax because these

smallest particles contribute only weakly to the overall current

since sMSPphoto / r3p. Furthermore, ve is the velocity of a photoelec-

tron, Dt 10ms is the sampling interval in the fast data channel(and hence also the charge integration time of our integrator

circuit), h is Plancks constant, cis the speed of light, and Wp is the

threshold energy for photoionization/photodetachment of a

particle, i.e., the workfunction or electron affinity of thecorresponding material. dF=dl is the number of photons per

wavelength interval emitted in one flash and l is the distance from

the particle detector. P S=4pl2 is the probability that the

photoelectron is emitted towards the detector electrode with area

S. dNp=drp is the number density of MSPs per size interval drp, and

dl and dl are the length and wavelength elements over which the

integrations above are carried out. Finally, e is the charge of

an electron. Note that the integration over the wavelengthl starts at 110 nm because of the transmission properties of the

MgF2-window of the Xe-flashlamp.

Likewise, photocurrents due to photoionization/photodetach-

ment of the species in Table 1 can be calculated as

Imax ZveDt

2:5 cm

Zhc=Wi110 nm

dFdl sil P ni dl dl

! eDt

(3)

% Fitot si ni

ZveDt2:5 cm

P dl

!e

Dt(4)

where ni is the number density of species i, sil is the cross

section for photoionization/photodetachment of species i at

wavelength l, and Wi is the corresponding threshold energy

(i.e., workfunction or electron affinity).

Note that for approximation (4) we have assumed thatRdF=dlsildl may be approximated as F

itot si where F

itot is

the total number of photons emitted in one flash with an energy

larger than Wi and si is the average cross section over the

corresponding wavelength interval.

Altitude profiles of expected maximum currents based onEqs. (2) and 4 and the altitude profiles shown in Fig. 9 assuming

either Fe2O3 or SiO particles are presented in Fig. 10. This figure

clearly shows that photoionization/photodetachment of MSPs

quantitatively explains the observed photocurrents. In contrast,

however, potential contributions to the detected photocurrents

from the photoionization of metal atoms or from the photo-

detachment of negative ions reach maximum currents ofo1 pA

and can hence be safely ignored. Even in the case of O21Dg,

which is by far the most abundant of the considered species, we

see that expected currents reach a maximum of$100 pA. This is

still a factor of 10100 less than the currents observed during our

rocket flight (gray shaded area). Likewise, currents due to NO can

only dominate the measurements above an altitude of$90 km. In

the companion paper by Strelnikova et al. (2008) we will comeback to this point and try to estimate the actual NO density above

ARTICLE IN PRESS

Table 1

Electron production by photoionization/photodetachment

Species Threshold energy (eV) Threshold wavelength (nm) Cross section cm2 Reference

NO 9.25 134.0 1 1018 Watanabe et al. (1953)

O21Dg 11.1 111.8 2 10

18 Clark and Wayne (1970)

Na 5.14 241.0 1 1019 Bautista et al. (1998)

Fe 7.87 157.5 2 1018 Kelly and Ron (1971)NO3 3.90 317.9 1 10

19 Smith et al. (1979)

CO3 2.90 427.6 1 1018 Cosby et al. (1976)

O2 0.43 2883.6 1 1018 Cosby et al. (1976)

Fig. 9. Altitude profiles of the concentration of constituents in the mesosphere/

lower thermosphere which could potentially be photoionized, or from which an

electron could be photodetached. Concentrations of sodium and iron atoms (black

lines) are taken from Plane (2003), of nitric oxide (blue line) from Siskind et al.

(1998) for 70N and equinox conditions, O2(1Dg) number densities (yellow line)

are from Gumbel et al. (1998), number densities of negative ions (red lines) from

Thomas and Bowman (1985), and of meteor smoke particles (green lines) from

Megner et al. (2006).

http://www.astro.uni-jena.de/Laboratory/OCDB/oxsul.htmhttp://www.astro.uni-jena.de/Laboratory/OCDB/oxsul.htmhttp://luxpop.com/RefractiveIndexList.htmhttp://luxpop.com/RefractiveIndexList.htmhttp://luxpop.com/RefractiveIndexList.htmhttp://luxpop.com/RefractiveIndexList.htmhttp://www.astro.uni-jena.de/Laboratory/OCDB/oxsul.htmhttp://www.astro.uni-jena.de/Laboratory/OCDB/oxsul.htm

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90 km on the basis of our measured photocurrents. Below 90 km,

however, measured photocurrents can only be explained byphotoionization/photodetachment of MSPs. Looking at Fig. 10 we

also find it noteworthy to point out that differences between

currents owing to photoionization of neutral particles and

photodetachment of electrons from negatively charged particles

are minor. The reason for this lies in the fact that the absorption

cross sections vary as $r3p=l, i.e., the cross sections at the shortest

relevant wavelengths are considerably larger than the ones at

longer wavelengths. Hence, it does not matter too much if the

integration over wavelength starts at corresponding energies

of 2 or 5.5eV.

In conclusion of this section, we may state that all available

arguments do support our initial assumption, i.e., that the

recorded photocurrents are due to photoelectrons from MSPs, at

least at altitudes below $90km.

5. Conclusions

In the current study we have introduced a new detector design

for the measurement of MSPs in the middle atmosphere. The new

detector combines the classical Faraday cup-design of Havnes et

al. (1996) with a xenon-flashlamp for the active photoionization/

photodetachment of MSPs. Accordingly, our new detector has two

measurement channels, one for the detection of a priori charged

MSPs which can penetrate into the Faraday cup and deposit their

charge signature on the electrode, and one for the detection of

very short photoelectron pulses which are actively created by

xenon-flash photons. The first successful launch of this detectortook place in September 2006 from the Andya Rocket Range and

yielded high quality data in both data channels. The direct Faraday

cup measurements showed signatures of negatively charged MSPs

in the limited altitude range between 80 and 90 km in qualitative

agreement with earlier measurements with this technique. In our

companion paper by Strelnikova et al. (2008) we show quantita-

tively that this limited altitude range is most probably a mere

consequence of aerodynamical effects and does not reflect a

layering process in the atmosphere. In agreement with this

conclusion, measured photocurrents were detected in a much

broader altitude range between 60 and 110 km. We have then

devised a laboratory experiment which verifies that measured

charge pulses truly originate from photoelectrons. With the same

laboratory setup we could further demonstrate that the observeddisappearance of photocurrents at 60 km on the descent of the

rocket flight is a direct consequence of the mean free path of

the photoelectrons in the ambient atmosphere. Interestingly, this

effect makes our measurement more sensitive if the instrument is

in the wake of the rocket than when it is in the ram. Finally, we

investigated the question whether the observed photocurrents

originated from MSPs or whether they could be due to other

ionizable species like NO, excited molecular oxygen, metal atoms,

or negative ions. This analysis shows that all these alternativespecies fall short to explain the observed photocurrents by several

orders of magnitude below an altitude of 90km. Below an altitude

of 90km, we succeeded to explain the measured currents

quantitatively by assuming that they originated from the photo-

ionization/photodetachment of MSPs where we assumed that the

particles consisted of iron or silicon oxides and had a photo-

ionization/photodetachment yield Y 1. Above 90 km, measured

photocurrents are most likely due to NO and can hence be used to

derive NO concentrations. This is discussed in detail in the

companion paper by Strelnikova et al. (2008).

In summary, our results demonstrate that the active photo-

ionization and subsequent detection of photoelectrons provides a

promising new tool for the study of meteor smoke particles in the

middle atmosphere. Importantly, this new technique does not relyon the a priori charge of the particles, neither is the accessible

particle size range severely limited by aerodynamical effects.

In the companion paper by Strelnikova et al. (2008) we apply the

expressions for the photocurrents presented in this study to

derive quantitative information on MSP properties which were so

far unaccessible by any other technique.

Acknowledgments

We are indebted to F.-J. Lubken for his support of the ECOMA

project, to J. Gumbel and M. Friedrich for valuable comments to

the manuscript, to von Hoerner System GmbH in Schwetzingen,

Germany, for their excellent contribution to the development of

the ECOMA flight electronics, to H.-J. Heckl for building thehardware of the ECOMA-detector, and to Misha Khaplanov,

Department of Meteorology of Stockholm University, for his help

with measuring the spectrum of the xenon-flashlamp. We also

appreciate the excellent support by the mobile rocket base of the

German Space Center. This work was supported by the German

Space Agency (DLR) under Grant 50 OE 0301 (Project ECOMA).

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Fig. 10. Expected maximum currents to be detected due to photoelectrons from

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