Stream Interaction Regions Research Articles

Evidence for local particle acceleration in the first recurrent galactic cosmic ray depression observed by Solar Orbiter

Context.In mid-June 2020, the Solar Orbiter (SolO) mission reached its first perihelion at 0.51 au and started its cruise phase, with most of the in situ instruments operating continuously.Aims.We present the in situ particle measurements of the first proton event observed after the first perihelion obtained by the Energetic Particle Detector (EPD) suite on board SolO. The potential solar and interplanetary (IP) sources of these particles are investigated.Methods.Ion observations from ∼20 keV to ∼1 MeV are combined with available solar wind data from the Radio and Plasma Waves (RPW) instrument and magnetic field data from the magnetometer on board SolO to evaluate the energetic particle transport conditions and infer the possible acceleration mechanisms through which particles gain energy. We compare > 17–20 MeV ion count rate measurements for two solar rotations, along with the solar wind plasma data available from the Solar Wind Analyser (SWA) and RPW instruments, in order to infer the origin of the observed galactic cosmic ray (GCR) depressions.Results.The lack of an observed electron event and of velocity dispersion at various low-energy ion channels and the observed IP structure indicate a local IP source for the low-energy particles. From the analysis of the anisotropy of particle intensities, we conclude that the low-energy ions were most likely accelerated via a local second-orderFermiprocess. The observed GCR decrease on 19 June, together with the 51.8–1034.0 keV nuc−1ion enhancement, was due to a solar wind stream interaction region (SIR). The observation of a similar GCR decrease in the next solar rotation favours this interpretation and constitutes the first observation of a recurrent GCR decrease by SolO. The analysis of the recurrence times of this SIR suggests that it is the same SIR responsible for the4He events previously measured in April and May. Finally, we point out that an IP structure more complex than a common SIR cannot be discarded, mainly due to the lack of solar wind temperature measurements and the lack of a higher cadence of solar wind velocity observations.

Interplanetary Coronal Mass Ejections from MAVEN Orbital Observations at Mars

Abstract The measurements from the Mars Atmosphere and Volatile EvolutioN spacecraft, in orbit around Mars, are utilized to investigate interplanetary coronal mass ejections (ICMEs) near 1.52 au. We identify 24 ICMEs from 2014 December 6 to 2019 February 21. The ICME list is used to examine the statistical properties of ICMEs. On average, the magnetic field strength of 5.99 nT in ICMEs is higher than that of 5.38 nT for stream interaction regions (SIRs). The density of 5.27 cm−3 for ICMEs is quite comparable to that of 5.17 cm−3 for SIRs, the velocity of 394.7 km s−1 for ICMEs is slightly lower than that of 432.8 km s−1 for SIRs, and the corresponding dynamic pressure of 1.34 nPa for ICMEs is smaller than that of 1.50 nPa for SIRs. Using existing databases of ICMEs at 1 au for the same time period, we compare ICME average properties at 1.52 au with those at 1 au. The averages of the characteristic quantities decrease by a factor of 1.1–1.7 from 1 to 1.52 au. In addition, we analyze an unusual space weather event associated with the ICME on 2015 March 9–10, and propose that the extremely strong dynamic pressure with a maximum of ∼18 nPa on March 8 is caused by the combined effects of the enhanced density inside a heliospheric plasma sheet (HPS), the compression of the HPS by the forward shock, and the high velocity of the sheath ahead of the ICME.

A Generalized Magnetospheric Disturbance Index: Initial Application to Mars Using MAVEN Observations

AbstractSince MAVEN's arrival at Mars in 2014, in situ observations of the magnetic field have been made continually. During this time, the Martian magnetosphere has been disturbed by various solar transient events such as stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs). To understand the impact of transient events on magnetospheres, statistical studies with large sets of events are necessary. At Earth, quantitative disturbance metrics have been employed, primarily relying on ground‐based magnetometer observations, enabling statistical studies of transient events to various physical processes driven by solar wind‐planetary interactions. However, many of these indices require assumptions about the magnetic field background. Lacking an intrinsic global magnetic field, the Martian magnetosphere is a product of piled‐up interplanetary magnetic field resulting in a variable “nominal” state of the system. To estimate the level of the disturbance to the Martian system, we have developed a generalized Magnetospheric Disturbance Index (MDI). MDI normalizes each observation and quantifies the change in field strength and variability inside of the bow shock. As an initial application, we compare the effectiveness of more than 70 SIRs and ICMEs at disturbing the Martian system. We find that the strength of MDI is not dependent on the type of space weather event impacting Mars. Additionally, the duration of the disturbance is not correlated to the type of space weather event or strength of disturbance. The short duration of the disturbance may imply that the system reacts to enhanced dynamic pressure and quickly relaxes back to a nominal state.

The Relationship Between Solar Wind Dynamic Pressure Pulses and Solar Wind Turbulence

Solar wind dynamic pressure pulses (DPPs) are small-scale plasma structures with abrupt and large-amplitude plasma dynamic pressure changes on timescales of seconds to several minutes. Overwhelming majority of DPP events (around 79.13%) reside in large-scale solar wind transients, i.e., coronal mass ejections, stream interaction regions, and complex ejecta. In this study, the intermittency, which is a typical feature of solar wind turbulence, is determined and compared during the time intervals in the undisturbed solar wind and in large-scale solar wind transients with clustered DPP events, respectively, as well as in the undisturbed solar wind without DPPs. The probability distribution functions (PDFs) of the fluctuations of proton density increments normalized to the standard deviation at different time lags in the three types of distinct regions are calculated. The PDFs in the undisturbed solar wind without DPPs are near-Gaussian distributions. However, the PDFs in the solar wind with clustered DPPs are obviously non-Gaussian distributions, and the intermittency is much stronger in the large-scale solar wind transients than that in the undisturbed solar wind. The major components of the DPPs are tangential discontinuities (TDs) and rotational discontinuities (RDs), which are suggested to be formed by compressive magnetohydrodynamic (MHD) turbulence. There are far more TD-type DPPs than RD-type DPPs both in the undisturbed solar wind and large-scale solar wind transients. The results imply that the formation of solar wind DPPs could be associated with solar wind turbulence, and much stronger intermittency may be responsible for the high occurrence rate of DPPs in the large-scale solar wind transients.

Field‐Aligned and Ionospheric Currents by AMPERE and SuperMAG During HSS/SIR‐Driven Storms

AbstractThis study considers 28 geomagnetic storms with Dst nT driven by high‐speed streams (HSSs) and associated stream interaction regions (SIRs) during 2010–2017. Their impact on ionospheric horizontal and field‐aligned currents (FACs) have been investigated using superposed epoch analysis of SuperMAG and AMPERE data, respectively. The zero epoch () was set to the onset of the storm main phase. Storms begin in the SIR with enhanced solar wind density and compressed southward oriented magnetic field. The integrated FAC and equivalent currents maximize 40 and 58 min after , respectively, followed by a small peak in the middle of the main phase ( + 4 hr), and a slightly larger peak just before the Dst minimum ( + 5.3 hr). The currents are strongly driven by the solar wind, and the correlation between the Akasofu and integrated FAC is 0.90. The number of substorm onsets maximizes near . The storms were also separated into two groups based on the solar wind dynamic pressure pdyn in the vicinity of the SIR. High pdyn storms reach solar wind velocity maxima earlier and have shorter lead times from the HSS arrival to storm onset compared with low pdyn events. The high pdyn events also have sudden storm commencements, stronger solar wind driving and ionospheric response at , and are primarily responsible for the first peak in the currents after . After days, the currents and number of substorm onsets become higher for low compared with high pdyn events, which may be related to higher solar wind speed.

Electric field measurements at the plasma frequency around comet 67P by RPC-MIP on board Rosetta

Context. The Mutual Impedance Probe (RPC-MIP) carried by the Rosetta spacecraft monitored both the plasma density and the electric field in the close environment of comet 67P/Churyumov–Gerasimenko (67P), as the instrument was operating alternatively in two main modes: active and passive. The active mode is used primarily to perform plasma density measurements, while the passive mode enables the instrument to work as a wave analyzer. Aims. We are reporting electric field emissions at the plasma frequency near comet 67P observed by RPC-MIP passive mode. The electric field emissions are related to Langmuir waves within the cometary ionized environment. In addition, this study gives feedback on the density measurement capability of RPC-MIP in the presence of cold electrons. Methods. We studied the occurrence rate of the electric field emissions as well as their dependence on solar wind structures like stream interaction regions (SIRs) and coronal mass ejections (CMEs). Results. We are showing that strong electric field emissions at the plasma frequency near 67P were present sporadically throughout the period when Rosetta was escorting the comet, without being continuous, as the occurrence rate is reported to be of about 1% of all the measured RPC-MIP passive spectra showing strong electric field emissions. The Langmuir wave activity monitored by RPC-MIP showed measurable enhancements during SIR or CME interactions and near perihelion. Conclusions. According to our results, Langmuir waves are a common feature at 67P during the passage of SIRs. Comparing the plasma frequency given by the RPC-MIP passive mode during Langmuir wave periods with the RPC-MIP active mode observations, we conclude that the measurement accuracy of RPC-MIP depends on the operational submode when the cold electron component dominates the electron density.

Evolution of Interplanetary Coronal Mass Ejection Complexity: A Numerical Study through a Swarm of Simulated Spacecraft

Abstract In-situ measurements carried out by spacecraft in radial alignment are critical to advance our knowledge on the evolutionary behavior of coronal mass ejections (CMEs) and their magnetic structures during propagation through interplanetary space. Yet, the scarcity of radially aligned CME crossings restricts investigations on the evolution of CME magnetic structures to a few case studies, preventing a comprehensive understanding of CME complexity changes during propagation. In this Letter, we perform numerical simulations of CMEs interacting with different solar wind streams using the linear force-free spheromak CME model incorporated into the EUropean Heliospheric FORecasting Information Asset model. The novelty of our approach lies in the investigation of the evolution of CME complexity using a swarm of radially aligned, simulated spacecraft. Our scope is to determine under which conditions, and to what extent, CMEs exhibit variations of their magnetic structure and complexity during propagation, as measured by spacecraft that are radially aligned. Results indicate that the interaction with large-scale solar wind structures, and particularly with stream interaction regions, doubles the probability to detect an increase of the CME magnetic complexity between two spacecraft in radial alignment, compared to cases without such interactions. This work represents the first attempt to quantify the probability of detecting complexity changes in CME magnetic structures by spacecraft in radial alignment using numerical simulations, and it provides support to the interpretation of multi-point CME observations involving past, current (such as Parker Solar Probe and Solar Orbiter), and future missions.

The Effect of Stream Interaction Regions on ICME Structures Observed in Longitudinal Conjunction

Abstract We study two interplanetary coronal mass ejections (ICMEs) observed at Mercury and at 1 au by spacecraft in longitudinal conjunction, investigating the question: what causes the drastic alterations observed in some ICMEs during propagation, while other ICMEs remain relatively unchanged? Of the two ICMEs, the first one propagated relatively self-similarly, while the second one underwent significant changes in its properties. We focus on the presence or absence of large-scale corotating structures in the ICME propagation space between Mercury and 1 au, which have been shown to influence the orientation of ICME magnetic structures and the properties of ICME sheaths. We determine the flux rope orientation at the two locations using force-free flux rope fits as well as the classification by Nieves-Chinchilla et al. We also use measurements of plasma properties at 1 au, the size evolution of the sheaths and magnetic ejecta with heliocentric distance, and identification of structures in the propagation space based on in situ data, remote-sensing observations, and simulations of the steady-state solar wind to complement our analysis. Results indicate that the changes observed in one ICME were likely caused by a stream interaction region, while the ICME exhibiting little change did not interact with any transients between Mercury and 1 au. This work provides an example of how interactions with corotating structures in the solar wind can induce fundamental changes in ICMEs. Our findings can help lay the foundation for improved predictions of ICME properties at 1 au.

A new view of energetic particles from stream interaction regions observed by Parker Solar Probe

Early observations from the first orbit of Parker Solar Probe (PSP) show recurrent stream interaction regions that form close to the Sun. Energetic particle enhancements were observed on the 320th–326th day of the year 2018, which corresponds to ~1–7 days after the passage of the stream interface between faster and slower solar wind. Energetic particles stream into the inner heliosphere to the PSP spacecraft near 0.33 au (71 solar radii) where they are measured by the Integrated Science Investigation of the Sun (IS⊙IS). The large 6-day time interval over which energetic particles are observed after the stream passage provides a unique perspective on the development of stream interactions within the heliosphere. The long duration of energetic particle enhancements suggests that particles stream in through the inner heliosphere more directly along magnetic field lines that form a sub-Parker spiral structure due to magnetic footpoint motion at the Sun and shearing of the magnetic field in the rarefaction region behind the stream interface. The strong build-up of energetic particle fluxes in the first 3 days after the passage of the stream interface indicates that suprathermal populations are enhanced near the interaction region through compression or other acceleration processes in addition to being diffusively accelerated. The early increases in energetic particle fluxes (in the first 3 days) in the formation of these events allows for the characterization of the acceleration associated with these suprathermal seed populations. Thus, we show that the time history of energetic particle fluxes observed by IS⊙IS provides a new view of particle acceleration at stream interaction regions throughout the inner heliosphere.

Properties of Interplanetary Fast Shocks Close to the Martian Environment

Abstract Magnetic field and plasma measurements from the Mars Atmosphere and Volatile EvolutioN mission in orbit around Mars are analyzed for interplanetary fast shock properties and drivers from 2014 October to 2018 November. We identified 52 fast shocks, including 39 fast forward (FF) shocks and 13 fast reverse (FR) shocks. Most (79%) of the FF shocks are driven by stream interaction regions (SIRs) with only a few cases being driven by interplanetary coronal mass ejections, and all of the FR shocks are driven by SIRs. A total of 92% of the identified shocks are quasi-perpendicular shocks. On average, the shock strengths of SIR-driven forward and reverse shocks are comparable, and they are greater than that of ICME-driven forward shocks. The shock strengths show no systematic dependence on the shock locations relative to the Martian bow shock. We find no evidence that the shock shapes are affected by Mars and its bow shock as an obstacle in the propagating medium. The results can help us understand the nature of interplanetary shocks propagating in different environments.

Time evolution of stream interaction region energetic particle spectra in the inner heliosphere

We analyze an energetic proton event associated with a stream interaction region (SIR) that was observed at Parker Solar Probe on day 320 of 2018 when the spacecraft was just 0.34 AU from the Sun. Using the Integrated Science Investigation of the Sun instrument suite, we perform a spectral analysis of the event and show how the observed spectra evolve over the course of the event. We find that the spectra from the first day of the event are much more consistent with local acceleration at a weak compression, while spectra from later on are more typical of SIR-related events in which particles accelerated at distant shocks dominate. After the first day, the spectra remain approximately constant, which indicates that the modulation of energetic particles during transit from the presumed source region is weaker than previously thought. We argue that these observations can be explained by a sub-Parker spiral magnetic field structure connecting the spacecraft to a source region in the SIR that is relatively close to the Sun. We further propose that acceleration at weak, pre-shock compressions likely plays an important role in observations of SIR-related events in the inner heliosphere and that future modelling of such events should consider acceleration all along the compression region, not just at the distant shock region.

Geosynchronous Magnetopause Crossings and Their Relationships With Magnetic Storms and Substorms

AbstractThe paper investigates the strengthening of magnetospheric activity related to geosynchronous magnetopause crossings (GMCs). We make a list of GMC events using the empirical magnetopause model (Lin et al., 2010, https://doi.org/10.1029/2009ja014235) and hourly averaged OMNI data and find which solar wind and magnetospheric conditions accompany and follow the GMCs. The GMCs are mostly caused by the impact of interplanetary coronal mass ejections (ICMEs) and/or interplanetary shocks often with a strong increase in the density and a moderate increase in velocity. The average solar wind density during the first GMC hour is higher than 20 cm−3 in 70% cases, while the velocity is higher than 500 km/s in 56% cases. The hourly interplanetary magnetic field (IMF) BZ is negative in 87% cases. The average over all events SMU (SML), Kp, and PC indices reach maxima (minima) in 1 h after the GMC beginning, while the delay of the minimum of the Dst index is usually 3–8 h. These average time delays do not depend on the strength of the storms and substorms. The SML (Dst) minimum is less than −500 nT (−30 nT) in the next 24 h in 95% (99%) cases, that is, the GMC events are mostly followed by magnetic storms and substorms. We compare solar wind and magnetospheric conditions for GMCs connected with ICMEs and stream interaction regions (SIRs). Our study confirms that the ICME‐related events are characterized by stronger ring current and auroral activity than the SIR‐related events. The difference might be explained by the different behavior of the solar wind velocity.

A living catalog of stream interaction regions in the Parker Solar Probe era

Stream interaction regions (SIRs) and corotating interaction regions (CIRs) are important phenomena in heliospheric physics. These large-scale structures vary temporally and spatially, both in latitude and with radial distance. The additions of Parker Solar Probe (PSP) and Solar Orbiter have allowed for investigations into the radial evolution of these structures over a wide range of heliocentric distances for the first time since the Helios era. To better enable investigations of SIRs and CIRs within the inner heliosphere, we have developed a living catalog of SIR and CIR observations by Parker Solar Probe with corresponding observations by STEREO-A as well as ACE and Wind at 1 au. The methodology used for the identification of events and the generation of this catalog, the initial catalog of PSP observations spanning orbits one through five along with corresponding 1 au observations, and information on accessing the living catalog for future studies is described. This list of SIR and CIR events from PSP and corresponding observations from other heliophysics missions will enable case studies utilizing unique orbital arrangements, as well as aid in future statistical studies to further understand the properties and evolution of these structures.

Using a Heliospheric Upwinding eXtrapolation Technique to Magnetically Connect Different Regions of the Heliosphere

Understanding how coronal structure propagates and evolves from the Sun and into the heliosphere has been thoroughly explored using sophisticated MHD models. From these, we have a reasonably good working understanding of the dynamical processes that shape the formation and evolution of stream interaction regions and rarefactions, including their locations, orientations, and structure. However, given the technical expertize required to produce, maintain, and run global MHD models, their use has been relatively restricted. In this study, we refine a simple Heliospheric eXtrapolation Technique (HUX) to include not only forward mapping from the Sun to 1 AU (or elsewhere), but backward mapping toward the Sun. We demonstrate that this technique can provide substantially more accurate mappings than the standard, and often applied “ballistic” approximation. We also use machine learning (ML) methods to explore whether the HUX approximation to the momentum equation can be refined without loss of simplicity, finding that it likely provides the optimum balance. We suggest that HUX can be used, in conjunction with coronal models (PFSS or MHD) to more accurately connect measurements made at 1 AU, Stereo-A, Parker Solar Probe, and Solar Orbiter with their solar sources. In particular, the HUX technique: 1) provides a substantial improvement over the “ballistic” approximation for connecting to the source longitude of streams; 2) is almost as accurate, but considerably easier to implement than MHD models; and 3) can be applied as a general tool to magnetically connect different regions of the inner heliosphere together, as well as providing a simple 3-D reconstruction.

Properties of stream interaction regions at Earth and Mars during the declining phase of SC 24

Aims.We inspect the evolution of stream interaction regions (SIRs) from Earth to Mars, covering the distance range 1–1.5 AU, over the declining phase of solar cycle 24 (2014–2018). So far, studies only analyzed SIRs measured at Earth and Mars at different times. We compare existing catalogs for both heliospheric distances and arrive at a clean dataset for the identical time range. This allows a well-sampled statistical analysis and for the opposition phases of the planets an in-depth analysis of SIRs as they evolve with distance.Methods.We use in situ solar wind data from OMNI and the Mars Atmosphere and Volatile EvolutioN spacecraft as well as remote sensing data from Solar Dynamics Observatory. A superposed epoch analysis is performed for bulk speed, proton density, temperature, magnetic field magnitude and total perpendicular pressure. Additionally, a study of events during the two opposition phases of Earth and Mars in the years 2016 and 2018 is conducted. SIR related coronal holes with their area as well as their latitudinal and longitudinal extent are extracted and correlated to the maximum bulk speed and duration of the corresponding high speed solar wind streams following the stream interaction regions.Results.We find that while the entire solar wind high speed stream shows no expansion as it evolves from Earth to Mars, the crest of the high speed stream profile broadens by about 17%, and the magnetic field and total pressure by about 45% around the stream interface. The difference between the maximum and minimum values in the normalized superposed profiles increases slightly or stagnates from 1–1.5 AU for all parameters, except for the temperature. A sharp drop at zero epoch time is observed in the superposed profiles for the magnetic field strength at both heliospheric distances. The two opposition phases reveal similar correlations of in situ data with coronal hole parameters for both planets. Maximum solar wind speed has a stronger dependence on the latitudinal extent of the respective coronal hole than on its longitudinal extent. We arrive at an occurrence rate of fast forward shocks three times higher at Mars than at Earth.

Estimating the Polytropic Indices of Plasmas with Partial Temperature Tensor Measurements: Application to Solar Wind Protons at ~1 au

We examine the relationships between temperature tensor elements and their connection to the polytropic equation, which describes the relationship between the plasma scalar temperature and density. We investigate the possibility to determine the plasma polytropic index by fitting the fluctuations of temperature either perpendicular or parallel to the magnetic field. Such an application is particularly useful when the full temperature tensor is not available from the observations. We use solar wind proton observations at ~1 au to calculate the correlations between the temperature tensor elements and the scalar temperature. Our analysis also derives the polytropic equation in selected streamlines of solar wind plasma proton observations that exhibit temperature anisotropies related to stream-interaction regions. We compare the polytropic indices derived by fitting fluctuations of the scalar, perpendicular, and parallel temperatures, respectively. We show that the use of the parallel or perpendicular temperature, instead of the scalar temperature, still accurately derives the true, average polytropic index value, but only for a certain level of temperature anisotropy variability within the analyzed streamlines. The use of the perpendicular temperature leads to more accurate calculations, because its correlation with the scalar temperature is less affected by the anisotropy fluctuations.

Venus and solar storms: Solar Energetic Particles, Stream Interaction Regions and Coronal Mass Ejections

Venus and solar storms: Solar Energetic Particles, Stream Interaction Regions and Coronal Mass Ejections

Associations between Space Weather Events and the Incidence of Acute Myocardial Infarction and Deaths from Ischemic Heart Disease

The effects of charged solar particles hitting the Earth’s magnetosphere are often harmful and can be dangerous to the human organism. The aim of this study was to analyze the associations of geomagnetic storms (GSs) and other space weather events (solar proton events (SPEs), solar flares (SFs), high-speed solar wind (HSSW), interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs)) with morbidity from acute myocardial infarction (AMI) and mortality from ischemic heart diseases (IHDs) during the period 2000–2015 in Kaunas (Lithuania). In 2000–2015, 12,330 AMI events (men/women n = 6942/5388) and 3742 deaths from IHD (men/women n = 2480/1262) were registered. The results showed that a higher risk of AMI and deaths from IHD were related to the period of 3 days before GS—a day after GS, and a stronger effect was observed during the spring–autumn period. The strongest effect of HSSW was observed on the day of the event. We found significant associations between the risk of AMI and death from IHD and the occurrence of SFs during GSs. We also found a statistically significant increase in rate ratios (RRs) for all AMIs and deaths from IHD between the second and fourth days of the period of ICMEs.

A Self-consistent Simulation of Proton Acceleration and Transport Near a High-speed Solar Wind Stream

Abstract Solar wind stream interaction regions (SIRs) are often characterized by energetic ion enhancements. The mechanisms accelerating these particles, as well as the locations where the acceleration occurs, remain debated. Here, we report the findings of a simulation of a SIR event observed by Parker Solar Probe at ∼0.56 au and the Solar Terrestrial Relations Observatory-Ahead at ∼0.95 au in 2019 September when both spacecraft were approximately radially aligned with the Sun. The simulation reproduces the solar wind configuration and the energetic particle enhancements observed by both spacecraft. Our results show that the energetic particles are produced at the compression waves associated with the SIR and that the suprathermal tail of the solar wind is a good candidate to provide the seed population for particle acceleration. The simulation confirms that the acceleration process does not require shock waves and can already commence within Earth’s orbit, with an energy dependence on the precise location where particles are accelerated. The three-dimensional configuration of the solar wind streams strongly modulates the energetic particle distributions, illustrating the necessity of advanced models to understand these particle events.

Interaction of Space Weather Phenomena with Mars Plasma Environment During Solar Minimum 23/24

AbstractWe study the interaction of three solar wind (SW) structures, two stream interaction regions, and one interplanetary coronal mass ejection, with Mars’ plasma environment during November 20–27, 2007. This period corresponds to the solar minimum between the solar cycles 23 and 24 which was characterized by very low values of the SW density and dynamic pressure and low interplanetary magnetic field magnitude. During that time the Mars Express orbit was in the terminator plane, while the Earth, Sun, and Mars were almost aligned, so we use the Advance Composition Explorer and Solar Terrestrial Relations Observatory probes as SW monitors in order to identify and characterize the structures that later hit Mars. We find that the passage of these structures caused strong variations in the bow shock location (between 2.2 and 3.0 RM), compression of the magnetospheric cavity (up to 45%), and an increased transterminator flow below 2 RM (by a factor of ≤ 8). This study shows that during times of low solar activity, modest space weather phenomena may cause large variations of plasma flow at Mars.