Dust mobilization in the presence of magnetic fields

We present a study of surface dust mobilization due to photoelectric charging in the presence of a magnetic field. Dust mobilization is observed to be inhibited in certain regions and is correlated with the orientation of the magnetic field. The recent patched charge model, which describes a mechanism for dust charging and mobilization, is extended to explain the effects of magnetic fields seen in our laboratory results. We propose that ambient electrons collected in photoemitting areas precipitate changes in the emission and reabsorption of photoelectrons inside microcavities between dust grains. This affects the charging, repulsion, and subsequent mobilization of the dust grains surrounding the microcavities. The magnetic field controls the movement of ambient electrons across the dusty surface, resulting in active and inactive regions of dust mobilization. Computer simulations show that regions of ambient electron accumulation as imposed by the magnetic field match the areas of high dust activity.

Figure 1

Schematic of the experimental setup. A circular bed 4 cm in diameter and 1-mm deep of Martian simulant dust ($<38$ microns in diameter) is placed on an insulating plate in a vacuum chamber. A permanent magnet is placed underneath the dust bed. The dust is irradiated and charged by UV light (172 nm or 7.2 eV).

Figure 2

Dust activity profiles. Dust activity levels extracted from recorded images when the dipole magnet is a) perpendicular and (b) parallel to the dust surface. The center of the magnet are 7.5 and 10.5 mm away from the dust surface, respectively.

Figure 3

Extended PCM. (a) Collection of ambient electrons on a photoemitting dust grain within the microcavity is added to the original PCM [25]. (b) The potential barrier created between grains that emit and collect photoelectrons across the microcavity. The collection of ambient electrons causes a downward shift in the potential barrier, indicating more photoemission and, subsequently, increased accumulation of negative charges on the surrounding grains. Repulsive forces between the negatively charged grains are, therefore, increased.

Figure 4

Measured and simulated magnetic fields. Magnetic field lines measured from the permanent magnet (blue) and from the theoretical dipole (orange). At the origin, which corresponds to the center of the surface of the permanent magnet, the magnetic field strengths of both are 1.2 kG.

Figure 5

Simulated electron landing patterns. Electron landing patterns generated by the simulations when the magnetic dipole moment is (a) perpendicular and (b) parallel (along the $Y$ axis) to the surface. The dipoles are placed right below the origin. The green circles indicate the area of the dust bed in the experiment.