We report on the in‐flight performance of the Solar Wind Ion Analyzer (SWIA) and observations of the Mars‐solar wind interaction made during the Mars Atmosphere and Volatile EvolutioN (MAVEN) prime ...mission and a portion of its extended mission, covering 0.85 Martian years. We describe the data products returned by SWIA and discuss the proper handling of measurements made with different mechanical attenuator states and telemetry modes, and the effects of penetrating and scattered backgrounds, limited phase space coverage, and multi‐ion populations on SWIA observations. SWIA directly measures solar wind protons and alpha particles upstream from Mars. SWIA also provides proxy measurements of solar wind and neutral densities based on products of charge exchange between the solar wind and the hydrogen corona. Together, upstream and proxy observations provide a complete record of the solar wind experienced by Mars, enabling organization of the structure, dynamics, and ion escape from the magnetosphere. We observe an interaction that varies with season and solar wind conditions. Solar wind dynamic pressure, Mach number, and extreme ultraviolet flux all affect the bow shock location. We confirm the occurrence of order‐of‐magnitude seasonal variations of the hydrogen corona. We find that solar wind Alfvén waves, which provide an additional energy input to Mars, vary over the mission. At most times, only weak mass loading occurs upstream from the bow shock. However, during periods with near‐radial interplanetary magnetic fields, structures consistent with Short Large Amplitude Magnetic Structures and their wakes form upstream, dramatically reconfiguring the Martian bow shock and magnetosphere.
Key Points
SWIA provides direct and proxy measurements of solar wind input to Mars
The Mars‐solar wind interaction varies with solar wind conditions and season
Mars only perturbs the upstream medium weakly, except during radial IMF
We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both ...outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).
Key Points
MAVEN ion measurements are mapped to a spherical surface around Mars
Planetary ion fluxes are organized in four spatial regions on the shell
Heavy ion escape rates exceed 2 × 1024 s−1 for energies >25 eV
Several recent studies suggest that magnetic reconnection is able to erode substantial amounts of the outer magnetic flux of interplanetary magnetic clouds (MCs) as they propagate in the heliosphere. ...We quantify and provide a broader context to this process, starting from 263 tabulated interplanetary coronal mass ejections, including MCs, observed over a time period covering 17 years and at a distance of 1 AU from the Sun with Wind (1995–2008) and the two STEREO (2009–2012) spacecraft. Based on several quality factors, including careful determination of the MC boundaries and main magnetic flux rope axes, an analysis of the azimuthal flux imbalance expected from erosion by magnetic reconnection was performed on a subset of 50 MCs. The results suggest that MCs may be eroded at the front or at rear and in similar proportions, with a significant average erosion of about 40% of the total azimuthal magnetic flux. We also searched for in situ signatures of magnetic reconnection causing erosion at the front and rear boundaries of these MCs. Nearly ~30% of the selected MC boundaries show reconnection signatures. Given that observations were acquired only at 1 AU and that MCs are large‐scale structures, this finding is also consistent with the idea that erosion is a common process. Finally, we studied potential correlations between the amount of eroded azimuthal magnetic flux and various parameters such as local magnetic shear, Alfvén speed, and leading and trailing ambient solar wind speeds. However, no significant correlations were found, suggesting that the locally observed parameters at 1 AU are not likely to be representative of the conditions that prevailed during the erosion which occurred during propagation from the Sun to 1 AU. Future heliospheric missions, and in particular Solar Orbiter or Solar Probe Plus, will be fully geared to answer such questions.
Key Points
MCs are frequently eroded at the front or at the rear in similar proportion
Nearly 30% of selected MC boundaries show reconnection signatures
The amount of eroded MCs and solar wind parameters do not seem to be correlated
During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we ...quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19–20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large‐scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12–0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo‐effectiveness.
Key Points
Demonstrate the occurrence of magnetic cloud erosion during propagation
Investigate all expected signatures of this mechanism
Highlight the implications in terms of impact in the Heliosphere and at Earth
The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has been continuously observing the variability of solar soft X‐rays and EUV irradiance, monitoring the upstream solar wind and ...interplanetary magnetic field conditions and measuring the fluxes of solar energetic ions and electrons since its arrival to Mars. In this paper, we provide a comprehensive overview of the space weather events observed during the first ∼1.9 years of the science mission, which includes the description of the solar and heliospheric sources of the space weather activity. To illustrate the variety of upstream conditions observed, we characterize a subset of the event periods by describing the Sun‐to‐Mars details using observations from the MAVEN solar Extreme Ultraviolet Monitor, solar energetic particle (SEP) instrument, Solar Wind Ion Analyzer, and Magnetometer together with solar observations using near‐Earth assets and numerical solar wind simulation results from the Wang‐Sheeley‐Arge‐Enlil model for some global context of the event periods. The subset of events includes an extensive period of intense SEP electron particle fluxes triggered by a series of solar flares and coronal mass ejection (CME) activity in December 2014, the impact by a succession of interplanetary CMEs and their associated SEPs in March 2015, and the passage of a strong corotating interaction region (CIR) and arrival of the CIR shock‐accelerated energetic particles in June 2015. However, in the context of the weaker heliospheric conditions observed throughout solar cycle 24, these events were moderate in comparison to the stronger storms observed previously at Mars.
Key Points
We present a comprehensive overview of the first 1.9 years of MAVEN space weather conditions measured upstream at Mars
We characterize a subset of Mars‐impacting events due to an extensive period of SEP electrons, a succession of ICMEs, and a strong CIR
We discuss the space weather implications of the weaker solar cycle 24 heliospheric conditions on the events observed by MAVEN
We present observations by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission of a substantial plume‐like distribution of escaping ions from the Martian atmosphere, organized by the upstream ...solar wind convection electric field. From a case study of MAVEN particle‐and‐field data during one spacecraft orbit, we identified three escaping planetary ion populations: plume fluxes mainly along the upstream electric field over the north pole region of the Mars‐Sun‐Electric field (MSE) coordinate system, antisunward ion fluxes in the tail region, and much weaker upstream pickup ion fluxes. A statistical study of O+ fluxes using 3 month MAVEN data shows that the plume is a constant structure with strong fluxes widely distributed in the MSE northern hemisphere, which constitutes an important planetary ion escape channel. The escape rate through the plume is estimated to be ~30% of the tailward escape and ~23% of the total escape for > 25 eV O+ ions.
Key Points
Three escaping planetary ion populations near Mars are identified in MAVEN observations
MAVEN observed a substantial plume with strong escaping ion fluxes over the MSE north pole region
The plume contributes ~23% to the total >25 eV O+ escape of ~2 × 1024 s−1
We utilize suprathermal ion and magnetic field measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, organized by the upstream magnetic field, to investigate the morphology ...and variability of flows, fields, and forces in the Mars‐solar wind interaction. We employ a combination of case studies and statistical investigations to characterize the interaction in both quasi‐parallel and quasi‐perpendicular regions and under high and low solar wind Mach number conditions. For the first time, we include a detailed investigation of suprathermal ion temperature and anisotropy. We find that the observed magnetic fields and suprathermal ion moments in the magnetosheath, bow shock, and upstream regions have observable asymmetries controlled by the interplanetary magnetic field, with particularly large asymmetries found in the ion parallel temperature and anisotropy. The greatest temperature anisotropies occur in quasi‐perpendicular regions of the magnetosheath and under low Mach number conditions. These results have implications for the growth and evolution of wave‐particle instabilities and their role in energy transport and dissipation. We utilize the measured parameters to estimate the average ion pressure gradient, J × B, and v × B macroscopic force terms. The pressure gradient force maintains nearly cylindrical symmetry, while the J × B force has larger asymmetries and varies in magnitude in comparison to the pressure gradient force. The v × B force felt by newly produced planetary ions exceeds the other forces in magnitude in the magnetosheath and upstream regions for all solar wind conditions.
Key Points
MAVEN measures the global distribution of suprathermal ions and magnetic fields around Mars, from which we can derive macroscopic forces
The flows, fields, and forces in the Mars‐solar wind interaction vary with both upstream magnetic field orientation and Mach number
Ion temperature and temperature anisotropy vary spatially and with solar wind parameters, with implications for plasma instabilities
Plain Language Summary
The solar wind that flows out from the Sun and pervades our solar system is largely deflected around Mars by its interaction with the upper atmosphere. However, this interaction also transfers energy to planetary ions, giving some of them sufficient velocity to escape from Mars. Therefore, the Mars‐solar wind interaction has implications for the long‐term evolution of the Martian atmosphere and its habitability. In this work, we study the structure and variability of the interaction and the macroscopic forces responsible for decelerating and deflecting the solar wind around Mars as well as those that accelerate planetary ions. We also investigate the asymmetries in this interaction and how they change in response to variations in the incoming solar wind flow and the magnetic field carried with the flow.
Unipolar streamers (also known as pseudo-streamers) are coronal structures that, at least in coronagraph images, and when viewed at the correct orientation, are often indistinguishable from dipolar ...(or “standard”) streamers. When interpreted with the aid of a coronal magnetic field model, however, they are shown to consist of a pair of loop arcades. Whereas dipolar streamers separate coronal holes of the opposite polarity and whose cusp is the origin of the heliospheric current sheet, unipolar streamers separate coronal holes of the same polarity and are therefore not associated with a current sheet. In this study, we investigate the interplanetary signatures of unipolar streamers. Using a global MHD model of the solar corona driven by the observed photospheric magnetic field for Carrington rotation 2060, we map the ACE trajectory back to the Sun. The results suggest that ACE fortuitously traversed through a large and well-defined unipolar streamer. We also compare heliospheric model results at 1 AU with ACE
in-situ
measurements for Carrington rotation 2060. The results strongly suggest that the solar wind associated with unipolar streamers is slow. We also compare predictions using the original Wang–Sheeley (WS) empirically determined inverse relationship between solar wind speed and expansion factor. Because of the very low expansion factors associated with unipolar streamers, the WS model predicts high speeds, in disagreement with the observations. We discuss the implications of these results in terms of theories for the origin of the slow solar wind. Specifically, premises relying on the expansion factor of coronal flux tubes to modulate the properties of the plasma (and speed, in particular) must address the issue that while the coronal expansion factors are significantly different at dipolar and unipolar streamers, the properties of the measured solar wind are, at least qualitatively, very similar.
We present the extension of the magnetic breakout model for CME initiation to a fully three-dimensional, spherical geometry. Given the increased complexity of the dynamic magnetic field interactions ...in three dimensions, we first present a summary of the well known axisymmetric breakout scenario in terms of the topological evolution associated with the various phases of the eruptive process. In this context, we discuss the analogous topological evolution during the magnetic breakout CME initiation process in the simplest three-dimensional multipolar system. We show that an extended bipolar active region embedded in an oppositely directed background dipole field has all the necessary topological features required for magnetic breakout, i.e., a fan separatrix surface between the two distinct flux systems, a pair of spine field lines, and a true three-dimensional coronal null point at their intersection. We then present the results of a numerical MHD simulation of this three-dimensional system where boundary shearing flows introduce free magnetic energy, eventually leading to a fast magnetic breakout CME. The eruptive flare reconnection facilitates the rapid conversion of this stored free magnetic energy into kinetic energy and the associated acceleration causes the erupting field and plasma structure to reach an asymptotic eruption velocity of image1100 km s super(-1) over an image15 minute time period. The simulation results are discussed using the topological insight developed to interpret the various phases of the eruption and the complex, dynamic, and interacting magnetic field structures.
We have conducted a survey of 341 interplanetary coronal mass ejections (ICMEs) using STEREO A/B data, analyzing their properties while extending a Level 3 product through 2016. Among the 192 ICMEs ...with distinguishable sheath region and magnetic obstacle, the magnetic field maxima in the two regions are comparable, and the dynamic pressure peaks mostly in the sheath. The north/south direction of the magnetic field does not present any clear relationship between the sheath region and the magnetic obstacle. About 71% of ICMEs are expanding at 1 au, and their expansion speed varies roughly linearly with their maximum speed except for ICMEs faster than 700 km s−1. The total pressure generally peaks near the middle of the well-defined magnetic cloud (MC) passage, while it often declines along with the non-MC ICME passage, consistent with our previous interpretation concerning the effects of sampling geometry on what is observed. The hourly average iron charge state reaches above 12+ ∼31% of the time for MCs, ∼16% of the time for non-MC ICMEs, and ∼1% of the time for non-ICME solar wind. In four ICMEs abrupt deviations of the magnetic field from the nominal field rotations occur in the magnetic obstacles, coincident with a brief drop or increase in field strength-features could be related to the interaction with dust. In comparison with the similar phases of solar cycle 23, the STEREO ICMEs in this cycle occur less often and are generally weaker and slower, although their field and pressure compressions weaken less than the background solar wind.
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