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In recent years, it has become increasingly clear that space weather disturbances can be triggered by transient upstream mesoscale structures (TUMS), independently of the occurrence of large-scale solar wind (SW) structures, such as interplanetary coronal mass ejections and stream interaction regions. Different types of magnetospheric pulsations, transient perturbations of the geomagnetic field and auroral structures are often observed during times when SW monitors indicate quiet conditions, and have been found to be associated to TUMS. In this mini-review we describe the space weather phenomena that have been associated with four of the largest-scale and the most energetic TUMS, namely, hot flow anomalies, foreshock bubbles, travelling foreshocks and foreshock compressional boundaries. The space weather phenomena associated with TUMS tend to be more localized and less intense compared to geomagnetic storms. However, the quiet time space weather may occur more often since, especially during solar minima, quiet SW periods prevail over the perturbed times. [ABSTRACT FROM AUTHOR]

Magnetosheath jets constitute a significant coupling effect between the solar wind (SW) and the magnetosphere of the Earth. In order to investigate the effects and forecasting of these jets, we present the first‐ever statistical study of the jet production during large‐scale SW structures like coronal mass ejections (CMEs), stream interaction regions (SIRs) and high speed streams (HSSs). Magnetosheath data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft between January 2008 and December 2020 serve as measurement source for jet detection. Two different jet definitions were used to rule out statistical biases induced by our jet detection method. For the CME and SIR + HSS lists, we used lists provided by literature and expanded on incomplete lists using OMNI data to cover the time range of May 1996 to December 2020. We find that the number and total time of observed jets decrease when CME‐sheaths hit the Earth. The number of jets is lower throughout the passing of the CME‐magnetic ejecta (ME) and recovers quickly afterward. On the other hand, the number of jets increases during SIR and HSS phases. We discuss a few possibilities to explain these statistical results. Key Points: Occurrence rate of magnetosheath jets is found to vary due to the arriving CMEs and SIRsFewer jets are found when magnetic ejecta regions of CMEs hit the Earth, more jets are found when SIRs and high speed streams hit the EarthThe jet duration does not appear to vary much during individual SW structures [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]