Manifestation of the stellar wind cycle in infrared images: Example of the heliosphere

TL;DR

The paper investigates how the solar wind cycle, via changes in the interplanetary magnetic field (IMF), alters the mid-infrared appearance of the heliosphere through interstellar dust distribution. A Monte Carlo dust model coupled to a global heliosphere MHD-kinetic framework is used to generate synthetic mid-infrared maps at 50 microns, accounting for heliosphere boundaries and a non-stationary IMF. Results show that very small dust grains remain outside the heliosphere, medium-sized grains form IMF-driven density waves in the tail, and large grains build a behind-the-Sun bulge, producing observable brightness patterns whose spacing scales with the solar cycle and the Sun's motion through the ISM. The work demonstrates that infrared astrosphere imaging can serve as a diagnostic of stellar activity cycles in Sun-like stars.

Abstract

The regions in which stellar winds interact with the interstellar medium, also known as astrospheres, can be observed in detail through the thermal emission of the interstellar dust particles, resided in plasma. Interstellar dust is also directly observed in the vicinity of the Sun with dust detectors onboard spacecraft, and it is known to be affected by the interplanetary magnetic field. The main goal of this work is to show how the change in the interplanetary magnetic field with the solar cycle affects the infrared picture of the heliosphere. To compose a synthetic intensity map of the interstellar dust thermal emission, we used the Monte Carlo method to calculate the distribution of the dust particles inside the heliosphere. We considered the effects of the heliosphere boundaries and the non-stationary current sheet. The change in the Parker magnetic field caused by the solar activity cycle leads distinguishable features in the mid-infrared emission maps of the heliosphere. The distribution of the interstellar dust in the vicinity of the Sun we calculated suggests that small particles linger outside of the heliosphere, and medium-size particles are mostly affected by the changing interplanetary magnetic field, which leads to number density waves in the tail region of the heliosphere. Finally, large particles form a bulge behind the Sun.

Figures (4)

• Figure 1: Coordinate system used for calculations. The Sun is at the origin, $V_{ISM}$ is the relative velocity of the interstellar dust and plasma, $B_{ISM}$ is the interstellar magnetic field, and $\theta$ is an angle between the direction towards the observer and the velocity of the ISM.
• Figure 2: Resulting number density for the interstellar dust particles with sizes of 1000 nm (a), 300 nm (b), and 10 nm (c) in the BV plane. The Sun is at the origin. The solid white lines depict the position of the HP (outer parabolic line) and the TS (inner circular line).
• Figure 3: Synthetic intensity maps of the thermal emission of interstellar dust (at wavelength of $50 \mu m$) in the vicinity of the Sun. The angle between the line of sight and the relative velocity vector of the Sun is $90^\circ$ (left), $60^\circ$ (middle), and $30^\circ$ (right). $x_{LOS}$ and $y_{LOS}$ stand for the local coordinates in the projection plane.
• Figure 4: Spectral emission distribution of interstellar dust particles inside the heliosphere for three different size distributions of particles. Classical MRN (green) Mathis1977, a truncated power law (blue) Mathis1996, and the most recent (orange) Hensley2023. The dashed line indicates the interplanetary dust SED from Poppe2019.