The Research Activities of the Dusty Plasma Group

Rocket Measurements of Aerosol Particles in The Polar Mesosphere

Noctilucent Clouds as Dusty Plasmas

NocCloud.gif

Click here for the NLC web page

Motivation and background:
Noctilucent clouds (NLC) form in the arctic mesosphere (~80-110 km altitude) in the summer. As the name implies, these clouds are seen at night. Shortly after sunset, the sky is black because the lower atmosphere is in the earth's shadow but the upper atmosphere is still illuminated. Thus these clouds are often seen against a starry background. They occur in the summer at about 85 km altitude because the rising arctic air is strongly cooled by expansion, and is in fact colder than in the winter (~120 K rather than ~240 K). The cloud particles are ice. This is inferred from their forming in conditions where the water vapor would be supersaturated.
The altitude of these clouds puts them in the ionosphere where there are free electrons created by solar wind particles, and during the daytime, UV irradiation. The electrons can attach to the ice particles thus creating a "dusty" plasma.
There is also thought to be meteor dust in the ionosphere. These particles can also become charged and may act as the nucleation centers for the NLC particles. Meteoric dust particles in the mesosphere have never been detected in situ and are only known from models.

What we are doing:
We have developed plasma probes that are carried by rockets into the mesosphere for the purpose of finding charged aerosol particles. These could be either meteoric dust or NLC particles. The probe is simply a patch of graphite (about 2 cm x 6 cm) mounted flush with the side of the payload. The velocity component of the rocket transverse to the payload axis carries the particles into the patch. A circuit measures the current I to the patch. We can then find the density of particles using I = nevA where physics students will recognize the variables. The plasma has an equal number of plus and minus charges so you might think we could collect no net charge. However, we mount a simple permanent magnet behind the patch and push away the unattached electrons, which have a small Larmor radius and we collect the charge residing on dust particles. The Larmor radii of the dust particles is very large so the dust particles are not deflected. The atomic and molecular ions are deflected but not enough to prevent some detection of ions. Also, the collisions of these ions with the flowing neutral gas tends to overcome the magnetic force. So our probe doesn't separate ions from dust particles as cleanly as we would like. So we are trying to find the right amount of positive bias voltage to put on the patch to help get rid of the pesky ions.

Computer modeling:
The flow around the rocket carries with it the aerosol particles and may cause them to evaprorate as well as cause them to miss being collected. We have quantified these effects using computer simulations.
“Simulation of rocket-borne particle measurements in the mesosphere,” M. Horányi, J. Gumbel, G. Witt and S. Robertson, Geophysical Research Letters 26, 1537-1540 (1999).

Launches:

Our first launch was from White Sands, N.M. which does not have NLC. However, ablated meteoric material forms ions layers called sporadic E.
“Rocket-borne mesospheric measurement of heavy charge carriers,” M. Horanyi, S. Robertson, B. Smiley, J. Gumbel, G. Witt, and B. Walch, Geophysical Research Letters 27, 3825-3828 (2000).

We had two launches in 2001 as part of the MIDAS/SOLSTICE campaign. This will appear in
"Measurement of positively and negatively charged particles inside PMSEs during MIDAS/SOLSTICE 2002," B. Smiley, S. Robertson, M. Horanyi, T. Blix, M. Rapp, R. Latteck and J. Gumbel, to appear in Journal of Geophysical Research - Atmospheres, topical collection. PDF file.

We had three more launched in June 2002 during the MIDAS/MacWave campaign. These launches were from the Norwegian rocket range on the island of Andoya, off the Norwegian coast at about 69 N latitude (above the Arctic Circle). The launches were made into NLC conditions. Charged aerosols were seen at the NLC altitudes.

 

The Andoya launch facility is the building at the base of the radio tower.
The occasion for the bonfire is the summer solstice, June 23, 2002.

Byron helps attach the payload to the rocket motor.

 

Left: The assembled payload. The probe is just above the bottom row of bolts.
Right: The launch on an Orion rocket motor, July 1, 2002.

RETURN TO TOP

Dust Charging on Surfaces

Motivation: Floating dust particles often get there by lifting off a surface. What is the charge on the dust particle while it is on the surface? It may be affected by UV and plasma above the surface. But before we can understand that, we need to know what the charge will be without plasma or UV.

What did we do?

We sat dust particles on a surface having a small hole in the middle and agitated the surface with an electromagnet (see figure below). The particles fell through the hole into a Faraday cup where we measured the charge. This was done in vacuum with both conducting and nonconducting particles. The surfaces were various metals and glass.

1) For clean metals on clean metals, there is a charge due to the difference in work function. Then the dust charge is Q = CV (you've seen that before) where V is the work function difference in eVs and C is the capacitance of a spherical particle resting on a plane, but not quite touching (see the paper for details). This C is proportional to the radius experimentally and in theory.

2) Many metals are oxidized and the effective work function is about 5.5 eV, so use this value for the metal if it is oxidized. We found that V, W, and Hf have the work function for oxide, not for the metal. The metals Co, Ni, and Pt showed the textbook work functions (no oxide).

3) For nonconductors, the charge builds up on patches as the particles jostles around on the surface. So the charge depends on the history of contacts. As we agitate the surface, the charge on the grains that fall is larger. Nonconductors can be assigned an effective work function.

4) For conducting grains, if there is an electric field above the surface there is an induced charge proportional to both E and grain the dust area in projection. For nonconducting grains, the effect is more complicated.

4) We did this again for the simulated lunar dust and simulated Martian dusts.

References:
"Charging of dust particles on surfaces," Z. Sternovsky, M. Horanyi and S. Robertson, Journal of Vacuum Science and Technology A 19, 2533-41, Sept. 2001.(MS Word file) or
"Contact charging of Lunar and Martian dust simulants," Zoltan Sternovsky, Amanda Sickafoose, Joshua Colwell, Scott Robertson and Mihaly Horanyi, accepted July 2002 for the Journal of Geophysical Research. (PDF file)

RETURN TO TOP

Fundamental Charging Processes of Dust in a Plasma

Motivation: To understand the processes by which dust charges in plasma.

Abstract: Experiments are carried out in a double plasma device that creates a low density plasma with energetic electrons. Dust particles are dropped through the plasma and their charge is measured after they exit. Materials investigated include spherical glass microballoons and powders of graphite, copper, and silicon. The particles are sieved to obtain samples with a narrow range of size. The data show that the particles charge to a negative potential determine by the fast electron energy unless this energy is sufficient to release a comparable current of secondary electrons.

Photograph of the the Double Plasma Machine used for charging measurements.
The dust is dropped into the machine by a dropper at the top and the charge is measured by a Faraday cup below the plasma.

Photomicrographs of a) glass microballoons and b) Minnesota Lunar Simulant (MLS-1), a simulated lunar soil.
All samples were sieved to select a 53-63 micron grain size. The frames are approximately 200 microns square. 

References for dust charging in plasma:
"Measurement of the Charge on Individual Dust Grains in a Plasma," B. Walch, M. Horányi, and S. Robertson, IEEE Trans. Plasma Sci. 22, 97 (1994).
"Experimental studies of charged dust particles," S. Robertson, Phys. Plasmas 2, 2200-2206, 1995.
"Charging of dust grains in plasma with energetic electrons," B. Walch, M. Horányi and S. Robertson, Phys. Rev. Lett. 75, 838-841 (1995).
"Laboratory Measurements of Dust Charging in Plasmas," Mihaly Horanyi, Scott Robertson, and Bob Walch, in The Physics of Dusty Plasmas, P. K. Shukla, D. A. Mendis, V. W. Chow, eds. (World Scientific, Singapore, 1996).

RETURN TO TOP

Undergraduate Research in Dust Dynamics

Motivation: Orbital motion is of fundamental importance in mechanics, yet there are few means of examining this motion in the lab other than by simulation. These experiments have been conducted as a part of our Research Experiences for Undergraduates Program.

Abstract: We have developed several devices which capture negatively charged dust particles in orbit about a positively charged electrode. In the first device the particle orbited a wire (the Kingdon trap used in atomic physics) and in the second device the particle orbits a sphere. Students may observe and videotape orbital motion, precession, and the effects of perturbations. Particles gradually spiral into the center conductor due to drag from residual gas molecules in the vacuum chamber. Some particles have orbited for six hours or about a half million orbits.

The photos below are from the charged dust orrery. An orrery is a device for illustrating planetary motion and is often a small mechanical model of the solar system.

These are negative video photorgraphs of charged dust particles (hollow glass microballoons) orbiting a 12 mm diameter sphere in vacuum. The sphere is at +8 kV potential and the charge on the particle is about a half million electrons. 

Chance Mahr, sitting at the orrery experiment.

To see a QuickTime movie of dust particles orbiting a sphere: Click on dust orbiting movie .

Papers authored by undergraduates:

"An Electrostatic Orrery for Celestial Mechanics, " T. Biewer, D. Alexander, S. Robertson, and B. Walch, Am J. Phys. 62, 821 (1994).
"A spherical electrostatic orrery," C. Smetana, D. Alexander, S. Robertson, K. Vilkaitis and B. Walch, Am. J. Phys. 64, 1356 (1996).
"Confinement Time of Dust Particles in a Kingdon Trap," C. Mahr, R. Walch, and S. Robertson, Bull. Am. Phys. Soc. 41, 1451(1996).

Other papers:
"Collective behavior of a non-neutral plasma in a Kingdon trap," S. Robertson and D. Alexander, Phys. Plasmas 2, 3, (1995).
"Trapping of dust particles in a Kingdon trap, " S. Robertson, in Non-neutral Plasma Physics, edited by J. Fajans and D. Dubin, AIP Conference Proceedings 331, (American Institute of Physics, New York, 1995) pp. 171-183.

RETURN TO TOP

Dust Charging in Sheaths

Motivation:Dust grains suspended above the lunar surface have been observed on multiple occasions. For example, a horizon glow tens of centimeters above the surface of the Moon was detected by Surveyor 5, 6, and 7 (Figure 1) and more recently by the Clementine spacecraft. At spacecraft sunrise, Apollo astronauts observed high altitude streaks due to light scattered off particles extending from the lunar surface to above the spacecraft. Evidence for horizontal dust transport on the surface of the Moon at terminator crossings was also detected by the Lunar Ejecta and Meteorite Experiment (LEAM) deployed by Apollo 17. Dust dynamics such as these are thought to be the result of the interaction between charged dust particles and a photoelectron layer above the surface.

Understanding the dynamics of dust near surfaces in space is important for many reasons. In the solar system, dust is continually released by the interplanetary micrometeoroid flux, ring particle collisions, and asteroid or meteor collisions. Therefore, many small, airless bodies in the solar system are coated with a dusty regolith. In addition to the observed lunar dust dynamics, dust levitation and transport may occur on Mars, Mercury, planetary satellites, planetary ring particles, asteroids, comets, and even planetesimals. Furthermore, the depth and distribution of the dusty regolith on planetary bodies affects infrared observations. Most importantly, any human or spacecraft activity on planetary bodies is affected by dust. Spacecraft will release dust upon impact with the surface, which can interfere with the operation or sensitivity of instruments. Surface activity can agitate dust and inject particles into the photoelectron layer, possibly causing contamination of instruments. Hence, investigation of the charging and dynamics of dust near planetary surfaces is a necessary component of future manned and unmanned exploration of the solar system.

 

 Figure 1. Surveyor image of the western lunar horizon shortly after sunset. The white arrow is pointing at a layer of dust levitated ~ 1 meter above the surface.

Background on photoelectron sheaths and dust charging: Objects in space charge to a floating potential determined by the balance between charging currents in the local plasma environment. Charge transfer continues until the sum of the charging currents is zero and the object has reached an equilibrium value. Surfaces in space that are exposed to high-energy photons from the sun emit electrons due to the photoelectric effect. These emitted electrons form a photoelectron layer, or sheath, near the surface. Typical sheath dimensions are tens of centimeters to one meter at distances of one AU. Dust grains on a planetary surface can also become charged due to photoemission. Particles that are ejected from the surface by external disturbances, such as micrometeorite impacts or human/spacecraft activity, can become charged by photoemission and electron collection when immersed in the photoelectron sheath. Then, particles can be levitated in the sheath when the Lorentz force balances gravity and can be transported vertically and horizontally above the surface by electric fields in the sheath.

In addition to particles becoming charged due to photoemission or collection of electrons from a sheath, dust can also be triboelectrically charged. Triboelectric charging refers to the charging of grains via contact. This charge may accumulate and can result in potential differences of kilovolts. Triboelectric charging is rarely discussed for grains at rest but is frequently invoked as a mechanism for charging of windblown grains. On Earth, strong electric fields in dust devils and lightning discharges in volcanic plumes are thought to result from triboelectric charging. This process may play an important role in electrical discharges from dust storms on Mars.

Experiments: We have produced and characterized photoelectron sheaths above metal plates of various compositions. For a typical photoelectron emission of 20 mA, the photoelectron density immediately above the surface is 4 x 104 cm-3. This density is doubled when the photoemitter is held at floating potential and emitted electrons return to the surface. A typical energy distribution for this apparatus peaks at ~ 1.1 eV with a full width of ~ 2 eV. We have also done studies on the electrostatic charging of dust grains. Conducting particles (zinc, copper, graphite) typically charge a few volts positive when exposed to UV light and a few volts negative when they pass through a photoelectron sheath.Nonconducting particles (SiC, glass, lunar regolith simulant, martian regolith simulant) have a triboelectric charge distribution of up to ± 15 V, roughly centered on zero.These grains are weak photoemitters but attain a negative floating potential when exposed to a photoelectron sheath.Our results are consistent with charging theory and indicate that triboelectric charging may be the dominant charging process for planetary regolith analogs (see references).

Next, we plan to study the conditions under which surface dust can be mobilized by electrostatic forces and the dynamics of that mobilized dust. The primary objectives of this continuing research are:

  1. to create a simulated surface environment in which to study levitation and transport of dust in the photoelectron layer;
  2. to investigate dust dynamics as a function of dust composition, dust size distribution, illumination, topography, and ambient thermal plasma properties;
  3. to study the effects of external disturbances (simulating ejecta from micrometeorite impacts or human/spacecraft activity) on dust dynamics.

Figure 2. Vacuum chamber in which dust levitation experiments are performed.

References:

"Experimental investigations on photoelectric and triboelectric charging of dust," A.A. Sickafoose, J.E. Colwell, M. Horányi, and S. Robertson, J. Geophys. Res. 106, 8343(2001).

"Photoelectric charging of dust particles in vacuum," A.A. Sickafoose, J.E. Colwell, M. Horányi, and S. Robertson, Phys. Rev. Lett., 84, 6034-3037, (2000).

RETURN TO TOP