Scott Robert Knappmiller

Rocket-borne in situ measurements of meteor smoke: Charging properties and implications for seasonal variation

Rocket-borne observations of meteoric smoke particles (MSPs) are presented from three campaigns at polar latitudes (69 degrees N) in September 2006, and in the summers of 2007 and 2008. MSPs are detected using a novel technique based on photoelectron emission from the particles after stimulation by UV photons emitted by a xenon flashlamp. Resulting photoelectron currents are shown to be proportional to particle volume density. September results match model predictions qualitatively at altitudes from 65 to 85 km while measurements at higher altitudes are contaminated by photoelectrons from NO and O-2((1)Delta(g)). Contamination below this altitude can be excluded based on concurrent satellite observations. The observations show a large variability from flight to flight. Part of this variability can be attributed to differences in the charging of MSPs during day and night. Finally we find that MSP volume density in summer can exceed that during September. Analyzing model simulations of the global transport and microphysics of these particles, we show that our observations are in agreement with the model predictions, even though number densities of particles with radii >1 nm, which have long been thought to be suitable condensation nuclei for mesospheric ice particles, show the opposite behavior. It is shown that this discrepancy is caused by the fact that even larger particles (similar to 3 nm) dominate the volume density and that transport affects these different particle sizes in different ways. These results reinforce previous model findings according to which seasonal MSP variability is mainly driven by the global circulation and corresponding transport.

A rocket-borne mass analyzer for charged aerosol particles in the mesosphere

An electrostatic mass spectrometer for nanometer-sized charged aerosol particles in the mesosphere has been developed and tested. The analyzer is mounted on the forward end of a rocket and has a slit opening for admitting a continuous sample of air that is exhausted through ports at the sides. Within the instrument housing are two sets of four collection plates that are biased with positive and negative voltages for the collection of negative and positive aerosol particles, respectively. Each collection plate spans about an order of magnitude in mass which corresponds to a factor of 2 in radius. The number density of the charge is calculated from the current collected by the plates. The mean free path for molecular collisions in the mesosphere is comparable to the size of the instrument opening; thus, the analyzer performance is modeled by a Monte Carlo computer code that finds the aerosol particles trajectories within the instrument including both the electrostatic force and the forces from collisions of the aerosol particles with air molecules. Mass sensitivity curves obtained using the computer models are near to those obtained in the laboratory using an ion source. The first two flights of the instrument returned data showing the charge number densities of both positive and negative aerosol particles in four mass ranges.

Energy balance and plasma potential in low-density hot-filament discharges

Electron energy balance is shown to play an important role in determining the plasma potential in low-density hot-filament discharges. The confined electrons that are lost to the walls are those with energy just above the plasma potential, thus the electron energy loss rate is the product of the electron loss rate and the height of the potential barrier. The sources of the electron energy are the energy at creation plus the energy gained from equilibration with energetic, unconfined electrons. An experiment in a soup-pot plasma device demonstrates that the plasma potential has values that satisfy the energy balance equation. The ion loss rate affects the electron loss rate through the quasi-neutrality condition, thus collisions of ions play a role in determining the plasma potential by reducing the particle loss rates

Microphysical Properties of Mesospheric Aerosols: An Overview of In Situ-Results from the ECOMA Project

Six sounding rockets were launched within the ECOMA (=“Existence and Charge state Of Meteoric smoke particles in the middle Atmosphere”) project to study the characteristics of meteoric smoke particles (MSPs) and mesospheric ice particles, as well as their possible microphysical relation. The launches were conducted during three campaigns from the Andoya Rocket Range (69°N, 16°E), one in September 2006, and the other two in the summers of 2007 and 2008. This chapter provides an overview of these observations and presents the corresponding geophysical results with special emphasis on our understanding of the micropyhsics of mesospheric ice particles. Most notably, we are able to confirm the existence of MSPs at all altitudes between 60 and 85 km in September, and a seasonal variation that is consistent with previous model studies in which MSP-variability is mainly driven by the global circulation. Together with these model studies as well as recent satellite observations of MSPs our results hence cast some doubt on a standard assumption of state-of-the-art microphysical models of mesospheric ice clouds, namely that ice nucleation mainly occurs heterogeneously on MSPs.

Comparison of two microwave and two probe methods for measuring plasma density

Four types of electron density measurements are compared in the same device: cylindrical probe, disk probe, microwave cavity, and microwave hairpin. The measurements are made in hot-filament discharges in a soup-pot type of plasma device with and without multidipolar surface magnetic fields with densities up to 1times109 cm -3 and electron temperatures of 0.1-1.3 eV. The cylindrical probe and hairpin give densities that are in close agreement for all conditions. The density from the cylindrical probe, hairpin, and cavity measurements are in good agreement (les12% difference) in plasma without magnetic containment. With magnetic containment, the different methods give greater differences in the densities (les30%), perhaps as a result of the higher fraction of energetic electrons. For all conditions, the densities from the disk probe are lower than the densities from the other methods

Analysis of the electron and ion fluxes to the wall of a hot-filament discharge device

The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density (0.2–4.5)×109cm−3, electron temperature 0.14–0.21eV, and pressure 0.3–12mTorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10–65eV range as being mostly secondaries from ionization and those in the 0–10eV range as being mostly secondaries f...

Origin of higher temperatures in multidipolar plasma devices

Hot-filament discharge devices with multidipolar surface magnetic fields have densities and temperatures higher than in these devices without multidipolar fields. Probe data show a much higher density of secondary electrons from the wall with multidipolar fields that is best explained by the wall secondaries being confined by the magnetic mirror effect. A relatively simple mathematical model for energy balance shows that the heating of the bulk plasma electrons by collisions with the greater number of secondaries from the wall accounts quantitatively for the increased temperature.

Dependence of Langmuir probe data on distance from the axis of a collisionless plasma

A cylindrical Langmuir probe in a low-density, collisionless plasma (density ~108 cm−3, electron temperature 0.2 eV) has been scanned radially through the presheath region to determine the effect of distance from the axis on the current-voltage characteristic. In the ion part of the probe characteristic, the collected ion current decreases with distance from the axis as a consequence of ion acceleration by the presheath. The part of the ion current from charge-exchange collisions remains relatively constant. In the electron part of the probe characteristic, the collected current decreases with distance from the axis, consistent with the existence of a small potential barrier from the presheath between the axis and the probe. The electron temperature from the slope of the probe characteristic is nearly constant across the presheath region. The plasma potential from the Langmuir probe characteristic is also nearly constant, indicating that the probe analysis finds the plasma potential on the axis, even when...

Analysis of the Electron and Ion Fluxes to the Wall of a Hot-Filament Discharge Device

Summary form only given. The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary-electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density 0.2-4.5times109 cm-3, electron temperature 0.14-0.21 eV, pressure 0.3-12 mtorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10-65 eV range as being mostly secondaries from ionization and those in the 0-10 eV range as being mostly secondaries from the wall. Ions are collected at the most negative grid bias voltages. The measured ion currents are within about 40% of values calculated from a model with charge exchange collisions of ions, and at the highest pressure differ by about a factor of three from the collisionless value, indicating that the Bohm ion current should be corrected for collisions when the charge exchange mean free path is less than about 0.2 of the plasma radius.