Search

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 are localized flows of enhanced dynamic pressure that are frequently observed downstream of the Earth's bow shock. They are significantly more likely to occur downstream of the quasi‐parallel shock than the quasi‐perpendicular shock. However, as the quasi‐perpendicular geometry is a more common configuration at the Earth's subsolar bow shock, quasi‐perpendicular jets comprise a significant fraction of the observed jets. We study the influence of solar wind conditions on jet formation by looking separately at jets during low and high interplanetary magnetic field (IMF) cone angles. According to our results, jet formation commences when Alfvén Mach number MA ≳ 5. We find that during low IMF cone angles (downstream of the quasi‐parallel shock) other solar wind parameters do not influence jet occurrence. However, during high IMF cone angles (downstream of the quasi‐perpendicular shock) jet occurrence is higher during low IMF magnitude, low density, high plasma beta (β), and high MA conditions. The distribution of quasi‐parallel (quasi‐perpendicular) jet sizes parallel to flow peaks at ∼0.3 RE (∼0.1 RE). Some quasi‐perpendicular jets formed during high β and MA are particularly small. We show two examples of high β and MA quasi‐perpendicular shock crossings. Jets were observed in the transition region, but not deeper in the magnetosheath. A more detailed look into one jet revealed signatures of gyrating ions, indicating that gyrobunched ions near the shock may produce jet‐like enhancements. Our results suggest that jets form as part of the quasi‐perpendicular shock dynamics amplified by high solar wind MA and β. Key Points: Jet formation is sensitive to SW parameters during high interplanetary magnetic field cone angles (quasi‐⊥), but not during low cone angles (quasi‐‖)Quasi‐‖ (quasi‐⊥) jets have an intrinsic size of ∼0.3 RE (∼0.1 RE) parallel to flowQuasi‐⊥ jet formation is related to shock dynamics amplified by higher β and MA [ABSTRACT FROM AUTHOR]

Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft have been sampling the subsolar magnetosheath since the first dayside science phase in 2008, and we finally have observations over a solar cycle. However, we show that the solar wind coverage during these magnetosheath intervals is not always consistent with the solar wind conditions throughout the same year. This has implications for studying phenomena whose occurrence depends strongly on solar wind parameters. We demonstrate this with magnetosheath jets—flows of enhanced earthward dynamic pressure in the magnetosheath. Jets emerge from the bow shock, and some of them can go on and collide into the magnetopause. Their occurrence is highly linked to solar wind conditions, particularly the orientation of the interplanetary magnetic field, as jets are mostly observed downstream of the quasi‐parallel shock. We study the yearly occurrence rates of jets recorded by THEMIS over solar cycle 24 (2008–2019) and find that they are biased due to differences in spacecraft orbits and uneven sampling of solar wind conditions during the different years. Thus, we instead use the THEMIS observations and their corresponding solar wind conditions to develop a model of how jet occurrence varies as a function of solar wind conditions. We then use OMNI data of the whole solar cycle to estimate the unbiased yearly jet occurrence rates. For comparison, we also estimate jet occurrence rates during solar cycle 23 (1996–2008). Our results suggest that there is no strong solar cycle dependency in jet formation. Key Points: Observed jet occurrence rates can be biased due to spacecraft orbits and uneven solar wind samplingWe created a statistical model of jet occurrence using interplanetary magnetic field cone angle, magnitude, solar wind speed, and densityThere is no strong solar cycle dependency in jet occurrence, but there may be a ∼10%–20% decrease around solar maximum [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 are localized high‐dynamic pressure pulses originating at Earth's bow shock and propagating earthward through the magnetosheath. Jets can influence magnetospheric dynamics upon impacting the magnetopause; however, many jets dissipate before reaching it. In this study we present a database of 13,096 jets observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft from 2008 to 2018, spanning a solar cycle. Each jet is associated with upstream solar wind conditions from OMNI. We statistically examine how solar wind conditions control the likelihood of jets forming at the shock, and the conditions favorable for jets to propagate through the magnetosheath and reach the magnetopause. We see that, for each solar wind quantity, these two effects are separate, but when combined, we find that jets are over 17 times more likely to reach and potentially impact the magnetopause when the interplanetary magnetic field (IMF) orientation is at a low cone angle, and approximately 8 times more likely during high speed solar wind. Low IMF magnitude, high Alfvén Mach number, and low density approximately double the number of jets at the magnetopause, while β and dynamic pressure display no net effect. Due to the strong dependence on wind speed, we infer that jet impact rates may be solar cycle dependent as well as vary during solar wind transients. This is an important step towards forecasting the magnetospheric effects of magnetosheath jets, as it allows for predictions of jet impact rates based on measurements of the upstream solar wind. Plain Language Summary: When the solar wind, a constant flow of plasma from the Sun, meets Earth's magnetic field, a shock wave forms in space. Like a rock in a stream, the plasma is diverted around the obstacle and a dense, turbulent layer—the magnetosheath—forms in front of it. We study instances of fast plasma jets bursting through the shock and traveling towards Earth. However, it appears that only a small proportion of jets hit the edge of our magnetic field—the magnetopause. To forecast their effects, we therefore need to know when jets will make it through. We use a database of 13,096 jets observed by spacecraft in the magnetosheath, alongside measurements of the solar wind, to determine when jets are most likely to hit the magnetopause. We find the highest probability is when the solar wind magnetic field is aligned with its flow direction and when it has a higher speed. We hope that, with this information, we may eventually be able to forecast space weather effects of jets based solely on measurements of the upstream solar wind. Key Points: Time History of Events and Macroscale Interactions during Substorms and OMNI data from 2008 to 2018 are used to constrain conditions for jet formation and propagation to the magnetopauseJets reach the magnetopause 17x more often during low IMF cone angles (<33°) and 8x more often during high solar wind speeds (>539 km/s)Low interplanetary magnetic field magnitude, high Alfvén Mach no., and low density double expected magnetopause impacts, dynamic pressure and β have no net effect [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]

Interplanetary coronal mass ejections (ICMEs) are a significant feature of the heliospheric environment and the primary cause of adverse space weather at the Earth. ICME propagation and the evolution of ICME magnetic field structure during propagation are still not fully understood. We analyze the magnetic field structures of 18 ICME magnetic flux ropes observed by radially aligned spacecraft in the inner heliosphere. Similarity in the underlying flux rope structures is determined through the application of a simple technique that maps the magnetic field profile from one spacecraft to the other. In many cases, the flux ropes show very strong underlying similarities at the different spacecraft. The mapping technique reveals similarities that are not readily apparent in the unmapped data and is a useful tool when determining whether magnetic field time series observed at different spacecraft are associated with the same ICME. Lundquist fitting has been applied to the flux ropes, and the rope orientations have been determined; macroscale differences in the profiles at the aligned spacecraft may be ascribed to differences in flux rope orientation. Assuming that the same region of the ICME was observed by the aligned spacecraft in each case, the fitting indicates some weak tendency for the rope axes to reduce in inclination relative to the solar equatorial plane and to align with the solar east‐west direction with heliocentric distance. Plain Language Summary: Coronal mass ejections (CMEs) are large eruptions of magnetic field and plasma from the Sun. When they arrive at the Earth, these eruptions can cause significant damage to ground and orbital infrastructure; forecasting this "space weather" impact of CMEs at the Earth remains a difficult task. The impact of individual CMEs is largely dependent on their magnetic field configurations, and an important aspect of space weather forecasting is understanding how CME field configuration changes with distance from the Sun. We have analyzed the signatures of 18 CMEs observed by pairs of lined‐up spacecraft and show that their basic magnetic field structures often display significant self‐similarities, that is, they do not often show significant reordering of field features with heliocentric distance. This similarity points to the general usefulness of placing spacecraft between the Sun and Earth to act as early‐warning space weather monitors. CME signatures observed at such monitors would likely be similar to the signatures subsequently arriving at the Earth and could be used to produce space weather forecasts with longer lead times. Key Points: Eighteen interplanetary flux ropes observed by radially aligned spacecraft in the inner heliosphere have been examinedMany of the flux ropes showed significant self‐similarities in magnetic field structure at the aligned spacecraftMacroscale differences in the magnetic field profiles are consistent with the flux ropes displaying different axis orientations [ABSTRACT FROM AUTHOR]