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Solar energetic particles (SEPs) are considered a serious radiation threat to space technologies and humans in space. SEPs are accelerated to high energies by solar explosive phenomena such as solar flares and in particular by shocks driven by coronal mass ejections (CMEs). We aim to better understand the effects of magnetic field gradient-induced adiabatic focusing on the coronal acceleration of SEPs and to test whether free-escape boundaries produce the same effects as focusing. We present results from a one-dimensional oblique shock model with a mean free path similar to Bell’s (1978) theory using Monte Carlo simulations. We show that the momentum spectrum at a shock and far upstream will attain a steady state in a model with adiabatic focusing, whereas it does not in a non-focusing model. However, the effects of focusing can be mimicked in a non-focused simulation by introducing a free-escape boundary ahead of the shock close to the position where the particles will escape from the shock by focusing in a focused transport simulation. This provides a promising avenue for constructing computationally efficient codes that can model the particle emission from shocks.

The shocked solar wind flows around the Earth’s magnetosphere in the magnetosheath downstream of the Earth’s bow shock. Within this region, faster flows of plasma, called magnetosheath jets, are frequently observed. These jets have been shown to sometimes exhibit supermagnetosonic speeds relative to the magnetosheath flow and to develop bow waves or shocks of their own. Such jet-driven bow waves have been observed to accelerate ions and electrons. We model electron acceleration by magnetosheath jet-driven bow waves using test-particle Monte Carlo simulations. Our simulations suggest that the energy increase of electrons with energies of a few hundred eV to 10 keV can be explained by a collapsing magnetic trap forming between the bow wave and the magnetopause with shock drift acceleration at the moving bow wave. Our simulations allow us to estimate the efficiency of acceleration as a function of different jet and magnetosheath parameters. Electron acceleration by jet-driven bow waves can increase the total acceleration in the parent shock environment, most likely also at shocks other than the Earth’s bow shock. [ABSTRACT FROM AUTHOR]

Magnetosheath jets travel from the bow shock toward the magnetopause, and some of them eventually impact it. Jet impacts have recently been linked to triggering magnetopause reconnection in case studies by Hietala et al. (2018, https://doi.org/10.1002/2017gl076525) and Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357). In this study, we focus on the enhancing or suppressing effect jets could have on reconnection by locally altering the magnetic shear via their own magnetic fields. Using observations from the years 2008–2011 made by the Time History of Events and Macroscale Interactions during Substorms spacecraft and solar wind OMNI data, we statistically study for the first time BZ within jets in the Geocentric Solar Magnetospheric coordinates. We find that BZ opposite to the prevailing interplanetary magnetic field (IMF) BZ is roughly as common in jets as in the non‐jet magnetosheath near the magnetopause, but these observations are distributed differently. 60–70% of jet intervals contain bursts of opposite polarity BZ in comparison to around 40% of similar non‐jet intervals. The median duration of such a burst in jets is 10 s and strength is ±10nT. We also investigate the prevalence of the type of strong BZ≤−24nT pulses that Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357) linked to a substorm onset. In our data set, such pulses were observed in around 13% of jets. Our statistical results indicate that jets may have the potential to affect local magnetopause reconnection via their magnetic fields. Future studies are needed to determine whether such effects can be observed. Plain Language Summary: Fast earthward flows called jets are often observed within the magnetosheath. They form at the Earth's bow shock, where the solar wind is slowed down before diverting around the magnetosphere. These jets may hit the boundary of the magnetosphere, the magnetopause, with high dynamic pressure. Such impacts have been observed to trigger magnetic reconnection, in which the interplanetary magnetic field (IMF) of the solar wind connects with the Earth's magnetic field and solar wind can enter the magnetosphere. Magnetopause reconnection usually occurs when the IMF points southward, opposite to the Earth's northward field. However, jets have been proposed to trigger reconnection also during northward IMF. We study the magnetic field within jets using data from the Time History of Events and Macroscale Interactions during Substorms spacecraft and solar wind OMNI data from 2008–2011. We find that it is statistically more likely for a magnetosheath interval, associated with jet conditions, to exhibit short pulses of magnetic field directed opposite to the upstream IMF, than for a non‐jet interval of the same duration. Therefore, jets may have the potential to affect local reconnection at the magnetopause. Our results motivate future studies that investigate these possible jet‐related effects on reconnection. Key Points: A statistical study of BZ in jets is conducted using Time History of Events and Macroscale Interactions during Substorms 2008–2011 data from the subsolar magnetosheath60‐70% of jet intervals contain a pulse of BZ opposite to the interplanetary magnetic field BZ while majority of non‐jet intervals do notThe median duration of the longest opposite BZ burst in jets is ∼10s and strength is ∼±10nT [ABSTRACT FROM AUTHOR]