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A particle with a permanent electric or magnetic dipole moment
would experience an orienting torque in the terrestrial electric
or magnetic field that would, as with gravito- photophoresis,
produce a time-averaged force parallel to the orienting field.
One might, by analogy, call the resulting forces electro- or magnetophotophoretic....
Magnetic or electrostatic torques can greatly exceed gravitational
torques for small aerosols allowing particles that were engineered
to exploit electro- or magnetophotophoresis to exhibit
dynamical properties not found in natural aerosols. For example,
electro- or magnetophotopheresis could be used to enable a
particle to levitate in the lower atmosphere and could be used
to levitate particles smaller than 0.1 μm.
Engineered particles need not be spherical. The most massefficient
geometry for a scattering is a thin disk with radius larger
than the wavelength of light. Such a design would produce minimal
forward scattering when the disk’s radius is substantially larger
than the wavelength of light, eliminating the diffuse radiation
problem encountered with sulfate aerosols. The use of electric or
magnetic materials allows disks to be oriented horizontally, and
the use of material with contrasting accommodation coefficients
allows for levitation.
An Idealized Example. As a specific example, consider a thin disk
with radius ∼5 μm and thickness 50 nm composed of three layers:
5 nm aluminum oxide, 30 nm of metallic aluminum, and finally 15 nm of barium titanate (Fig. 1C). The thickness of the Al layer
is chosen so that it has high solar-band reflectivity and is nearly
transparent to outgoing thermal infrared so as to produce a large
mass-specific negative radiative forcing (cooling) (9). The Al2O3
layer serves to protect the Al layer from oxidization. The thickness
of the BaTiO3 is chosen so that the electrostatic torque from
the atmospheric electric field is sufficient to orient the disk horizontally
against torques arising from reasonable asymmetries in
thickness or α across the disk (24). Assuming a relatively small,
and therefore conservative, 15% difference in α between the
two materials (23), the photophoretic force on the disk would
exceed 2 times its weight under diurnally averaged illumination
at altitudes in the middle stratosphere or mesosphere assuming it
absorbed only 10% of the solar flux (Fig. 2 and SI Text).