A corridor to the Sun for select nanodust particles Print E-mail
Wednesday, 03 October 2018 11:09

The smallest dust grains in the circumsolar dust cloud are the nanodust particles, i.e., the dust grains with the sizes of a few to a few tens millionth parts of millimeter. They are so small they include just several dozen thousands of atoms.

As all dust grains in the Solar Systems, they are electrically charged, and the large charge to mass ratio they have makes the Lorentz force from the magnetic field in the solar wind similar in strength to the solar gravity force, or even larger. Therefore, the motion of nanodust grains significantly differs from the motions of typical dust grains, which resemble the motions of asteroids.

Nanodust is predominantly produced by collisional fragmentation of larger dust particles. The initial velocities of the nanodust particles produced in this mechanism are close to the orbital velocities of the parent dust grains. The newly-created dust grains quickly become electrically charged and consequently are picked up by the solar magnetic field, frozen in the solar wind, and accelerated to velocities comparable to that of solar wind and run away from the Sun. However, some of nanodust particles created sufficiently close to the Sun become “trapped” in bound orbits around the Sun due to an interplay between the solar gravity and magnetic forces. Therefore, it is likely that a population of trapped nanodust particles is present in the vicinity of the Sun.

Another hypothetic source of nanodust are comets, and in particular the sungrazing comets, i.e., the comets with perihelia deep inside the Mercury orbit and aphelia somewhere between the orbits of Mars and Jupiter. The initial velocities of nanodust particles released by these comets shortly before perihelion are much larger than these of collisional nanodust particles. If the release of a nanodust particle occurs inside the Mercury orbit, its speed may be comparable to that of the solar wind, oppositely directed. Therefore it is expected that the orbital dynamics of these nanodust particles is different to that of the collisional nanodust grains. This topic was investigated by A. Czechowski from CBK PAN and I. Mann from the Arctic University of Norway in Tromosoe, Norway, who created a model of motion of these particles.

Based on numerical simulations of the forces acting on nanodust particles, Czechowski and Mann concluded that unlike the “regular” nanodust particles, the particles from sungrazing comets cannot be trapped close to the Sun, even those released very close to the Sun. Therefore, sungrazing comets are unlikely to be an additional source of the trapped nanodust population. However, the two researchers identified an interesting phenomenon that they called a “corridor to the Sun”.

Some of nanodust particles released in the inbound leg of the comet orbit can enter peculiar trajectories leading them deep into the solar corona, i.e., to the upper hot part of the solar atmosphere visible from Earth during solar eclipses. Due to electromagnetic forces, these particles approach the Sun to a distance much closer than the perihelion of the parent comet, which may lead to destruction of nanodust by sublimation or collisions with the ions. Most nanodust particles are, however, picked up by the solar wind and ultimately escape away from the Sun.

Results of this study were presented in a paper published in Astronomy & Astrophysics, available at https://www.aanda.org/articles/aa/abs/2018/09/aa32922-18/aa32922-18.html.

Andrzej Czechowski

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