Foreshock Ion Motion Across Discontinuities: Formation of Foreshock Transients

Abstract

In the ion foreshock, hot flow anomalies (HFAs) and foreshock bubbles (FBs) are two types of foreshock transients that have the strongest fluctuations, which can disturb the magnetosphere‐ionosphere system and increase shock acceleration efficiency. They form due to interaction between the foreshock ions and solar wind discontinuities: the direction of the foreshock ion‐driven current and whether it decreases or increases the magnetic field strength behind the discontinuity determine whether the transient's formation can be promoted or suppressed. Thus, to predict the HFA and FB formation and forecast their space weather effects, it is necessary to predict the foreshock ion‐driven current direction. In this study, we derive analytical equations of foreshock ion velocities within discontinuities to estimate foreshock ion‐driven current direction, which provides a quantitative criterion of HFA and FB formation. To validate the criterion, we use Acceleration Reconnection Turbulence & Electrodynamics of Moon's Interaction with the Sun to observe pristine solar wind discontinuities and calculate discontinuity parameters. We use Magnetospheric Multiscale to observe the foreshock ion motion around the discontinuities and show that the data support our model. This study is another step toward a predictive model of HFA and FB formation so that we can forecast their space weather effects at Earth using solar wind observations at lunar orbit or L1.

THEMIS Observations of Magnetosheath‐Origin Foreshock Ions

“…The case studies and statistical studies such as by Liu et al. (2024) support the latter possibility. Global hybrid simulations and multi‐point observations could be conducted to further investigate the reason for the anisotropy in the future.…”

“…Likely related to the ion gyroradii, the leaked ions were more aligned with the IMF than with the magnetosheath field line in two examples (Figures S3 and S4 in Supporting Information ), suggesting a more adiabatic process (the IMF strength was ∼8 and 5 nT in these two cases, respectively). Statistical study in the accompanying paper (Liu et al., 2024) shows that most of events have field‐aligned heat flux toward the upstream direction and ∼56% of them are more than 5 × 10 10 eV/cm 2 /s. The possible causes of this anisotropy are discussed in Section 4.…”

Analytical Model of Foreshock Ion Interaction With a Discontinuity: A Statistical Study

“…In the accompanying paper (Liu, Angelopoulos, et al., 2023), we have derived that $$$j(x)\sim q{n}_{f}\left(\frac{1}{\pi {v}_{th\perp }^{2}}\right)\,\int \nolimits_{0}^{\infty }\,\mathrm{exp}\left(-\frac{{\left({V}_{\perp }\right)}^{2}}{{v}_{th\perp }^{2}}\right)\left(-{V}_{\perp }{\increment}\mathrm{sin}\,{\varphi }_{c}\,\mathrm{cos}\,\alpha (x)+\left({V}_{{\Vert} 0}-C\right)\mathrm{sin}\,\alpha (x){\increment}{\varphi }_{c}\right){V}_{\perp }d{V}_{\perp }$$$ assuming that foreshock ions follow Maxwellian distributions, where $${n}_{f}$$ is foreshock ion density, $${\varphi }_{c}$$ is the initial gyrophase of foreshock ions that can reach position x within a discontinuity along the normal direction, $$\alpha $$ is the shear angle of discontinuity, parameter $$C={{\Omega }}_{i}\frac{dx}{d\alpha }$$, and for easy application in observations, we simplify that the foreshock ions approximately follow Maxwellian distributions. When $$j(x)< 0$$, the field strength decreases at the discontinuity, which favors further growth of the transient structure.…”

“…In this study, we take yet another step in the quantitative description of foreshock transients and their effects by combining the foreshock ion motion model (Liu, Angelopoulos, et al., 2023) and the expansion speed model (Liu, Vu, et al., 2023) to quantitatively describe the expansion speed of the transient due to the foreshock ion‐discontinuity interaction. This model describes the early interaction between the foreshock ions and a discontinuity, which generates small perturbations that may or may not evolve into an FB, an HFA, or a structure that belongs to neither.…”

“…When foreshock ions encounter a solar wind discontinuity, as their gyroradii are comparable to the discontinuity thickness, they perform partial gyration. In the accompanying paper (Liu, Angelopoulos, et al., 2023), we derived the analytical equations of foreshock ion motion across a discontinuity. Because electrons are magnetized, the motion difference between the foreshock ions and electrons leads to a current.…”