Interstellar Dust Transport Through the Heliosphere Including the Sector Region

Abstract

Interstellar dust has been detected in situ flowing through the heliosphere. Understanding the implications of this dust for the nature of interstellar dust in the very local interstellar medium requires modeling the transport of the grains as they interact with the solar wind magnetic field. The magnetic field in the sector region (SR) that contains the heliospheric current sheet is substantially different from that in the monopolar solar wind. The rapid polarity flips that occur in the SR can present an effectively very low averaged field strength to grains that have gyroradii of tens of au. We present new calculations of dust transport through the heliosphere using a model that includes the SR. We show that the SR can act as a window allowing even relatively small grains to penetrate deep into the heliosphere. The presence of the SR reduces the variation in dust density with the solar cycle (as compared to models without it), with very little concentration or dilution of the dust for grains larger than $\sim 0.1$ $μ$m for most of the solar cycle (except for a focusing overall polarity of the field at solar minimum.) While the lack of time dependence of the magnetic field during transport of grains through the heliosphere is a limitation of the model, the relative lack of variation as a function of the point in the solar cycle of the grain density in the inner heliosphere suggests that our results will not deviate dramatically from a model that fully incorporates time dependence.

Figures (7)

• Figure 1: Time variation of the sector boundary latitude ($\alpha$) over a full 22-year solar cycle. We consider the magnitude of the latitude ($\theta$) for a given solar wind parcel at the inner boundary of 1 au with respect to the sector boundary latitude. $|\theta| < \alpha$ indicates the plasma is within the sector region, and $|\theta| > \alpha$ indicates the plasma is within a unipolar region. The time on the $x$-axis is model time and does not correspond to actual real world times.
• Figure 2: The sector region in a meridional plane that includes the upwind direction. Yellow regions contain the current sheet, dark blue/purple regions are inside the heliopause but do not include the current sheet and greenish regions are outside the heliopause. The times shown in the panels are model times. The solar cycle phases that correspond to the panels (l-to-r) are: transition from solar max to min, solar min, transition from solar min to max, solar max. The thickness of the sector region clearly varies strongly in time. The sector region gets advected to high and low ecliptic latitudes in the heliosheath.
• Figure 3: Magnetic field, $B_y$ (in $\mu$G), for the time of transitioning from solar min to solar max (model time 2204) in the meridional plane that includes the upwind direction. The overall polarity imposed is focusing (magnetic north in the ecliptic north). The azimuthal field in this plot is $-B_y$.
• Figure 4: Comparison of dust density relative to interstellar for 0.1 $\mu$m grains and for different assumptions about the magnetic field. The images are for a slice perpendicular to the ecliptic at a distance 20 au upstream of the Sun. As indicated by the labels, the heliosphere model for the calculation either included the sector region or did not and the overall polarity is either focusing or defocusing. Clearly there is much less of a difference between the focusing/defocusing cases when the sector region is included.
• Figure 5: Grain density relative to that in the undisturbed ISM in the ecliptic plane. The field here is for solar max conditions (model time 2207). The top row is for overall focusing polarity and the bottom row is for defocusing polarity. The grain sizes are as indicated in the labels. The white lines indicate the location of the termination shock (to the right) and heliopause (to the left).