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
In 2010 May 23-24, Solar Dynamics Observatory (SDO) observed the launch of two successive coronal mass ejections (CMEs), which were subsequently tracked by the SECCHI suite on board STEREO. Using the ...COR2 coronagraphs and the heliospheric imagers (HIs), the initial direction of both CMEs is determined to be slightly west of the Sun-Earth line. We derive the CME kinematics, including the evolution of the CME expansion until 0.4 AU. We find that, during the interaction, the second CME decelerates from a speed above 500 km s super(-1) to 380 km s super(-1), the speed of the leading edge of the first CME. STEREO observes a complex structure composed of two different bright tracks in HI2-A but only one bright track in HI2-B. In situ measurements from Wind show an "isolated" interplanetary CME, with the geometry of a flux rope preceded by a shock. Measurements in the sheath are consistent with draping around the transient. By combining remote-sensing and in situ measurements, we determine that this event shows a clear instance of deflection of two CMEs after their collision, and we estimate the deflection of the first CME to be about 10degrees toward the Sun-Earth line. The arrival time, arrival speed, and radius at Earth of the first CME are best predicted from remote-sensing observations taken before the collision of the CMEs. Due to the over-expansion of the CME after the collision, there are few, if any, signs of interaction in in situ measurements. This study illustrates that complex interactions during the Sun-to-Earth propagation may not be revealed by in situ measurements alone.
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
Kelvin-Helmholtz Instability is ubiquitous at Earth's magnetopause and plays an important role in plasma entry into the magnetosphere during northward interplanetary magnetic fields. Here, using one ...solar cycle of data from NASA THEMIS (Time History of Events and Macro scale Interactions during Substorms) and MMS (Magnetospheric Multiscale) missions, we found that KHI occurrence rates show seasonal and diurnal variations with the rate being high near the equinoxes and low near the solstices. The instability depends directly on the Earth's dipole tilt angle. The tilt toward or away from the Sun explains most of the seasonal and diurnal variations, while the tilt in the plane perpendicular to the Earth-Sun line explains the difference between the equinoxes. The results reveal the critical role of dipole tilt in modulating KHI across the magnetopause as a function of time, highlighting the importance of Sun-Earth geometry for solar wind-magnetosphere interaction and for space weather.
Magnetic reconnection is an energy conversion process that occurs in many astrophysical contexts including Earth's magnetosphere, where the process can be investigated in situ by spacecraft. On 11 ...July 2017, the four Magnetospheric Multiscale spacecraft encountered a reconnection site in Earth's magnetotail, where reconnection involves symmetric inflow conditions. The electron-scale plasma measurements revealed (i) super-Alfvénic electron jets reaching 15,000 kilometers per second; (ii) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures in the velocity distributions; and (iii) the spatial dimensions of the electron diffusion region with an aspect ratio of 0.1 to 0.2, consistent with fast reconnection. The well-structured multiple layers of electron populations indicate that the dominant electron dynamics are mostly laminar, despite the presence of turbulence near the reconnection site.
A significant portion of transients measured by spacecraft at 1 AU does not show the well‐defined properties of magnetic clouds (MCs). Here we propose a new class of complex, non‐MC ejecta resulting ...from the interaction of two coronal mass ejections (CMEs) with different orientations, which differ from previously studied multiple‐MC events. At 1 AU, they are associated with a smooth rotation of the magnetic field vector over an extended duration and do not exhibit clear signs of interaction. We determine the characteristics of such events based on a numerical simulation and identify and analyze a potential case in the long‐duration transient event measured in situ on 19–22 March 2001. Such events may result in intense, long‐duration geomagnetic storms, with a sequence sawtooth events, and may sometimes be misidentified as isolated CMEs.
Key Points
Long‐duration isolated‐like ejecta at 1 AU may result from multiple CMEs
Such events may drive the magnetosphere for long periods with sawtooth events
In‐situ measurements may be misleading and propagation must be accounted for
Using Van Allen Probe observations of the inner magnetosphere during geomagnetic storms driven by interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs), we ...characterize the impact of these drivers on the storm‐time ring current development. Using 25 ICME‐ and 35 CIR‐driven storms, we have determined the ring current pressure development during the prestorm, main, early‐recovery, and late‐recovery storm phases, as a function of magnetic local time, L shell and ion species (H+, He+, and O+) over the 100‐ to 600‐keV energy range. Consistent with previous results, we find that during the storm main phase, most of the ring current pressure in the inner magnetosphere is contributed by particles on open drift paths drifting duskward leading to a strong partial ring current. The largest difference between the ICME and CIR ring current responses during the storm main and early‐recovery phases is the difference in the response of the <~55‐keV O+ to these drivers. While the H+ pressure response shows similar source and convection patterns for ICME and CIR storms, the O+ pressure response is significantly stronger for ICME storms. The ICME O+ pressure increases more strongly than H+ with decreasing L and peaks at lower L shells than H+.
Key Points
During the storm main and early‐recovery phases, most of the ring current pressure is contributed by particles on open drift paths
The largest difference between ICME and CIR responses in the main and early recovery phase is the <~55‐keV O+ pressure contribution
During the storm main and early‐recovery phases, the plasma beta increases to values greater than ~1 for L > 4 for ICMEs and L > 4.5 for CIRs
We identify all fast‐mode forward shocks, whose sheath regions resulted in a moderate (56 cases) or intense (38 cases) geomagnetic storm during 18.5 years from January 1997 to June 2015. We study ...their main properties, interplanetary causes, and geoeffects. We find that half (49/94) such shocks are associated with interacting coronal mass ejections (CMEs), as they are either shocks propagating into a preceding CME (35 cases) or a shock propagating into the sheath region of a preceding shock (14 cases). About half (22/45) of the shocks driven by isolated transients and which have geoeffective sheaths compress preexisting southward Bz. Most of the remaining sheaths appear to have planar structures with southward magnetic fields, including some with planar structures consistent with field line draping ahead of the magnetic ejecta. A typical (median) geoeffective shock‐sheath structure drives a geomagnetic storm with peak Dst of −88 nT pushes the subsolar magnetopause location to 6.3 RE, i.e., below geosynchronous orbit and is associated with substorms with a peak AL index of −1350 nT. There are some important differences between sheaths associated with CME‐CME interaction (stronger storms) and those associated with isolated CMEs (stronger compression of the magnetosphere). We detail six case studies of different types of geoeffective shock‐sheaths, as well as two events for which there was no geomagnetic storm but other magnetospheric effects. Finally, we discuss our results in terms of space weather forecasting and potential effects on Earth's radiation belts.
Key Points
Forty‐nine shocks/sheaths in a previous transient and 45 isolated shocks drove moderate or intense geomagnetic storms from 1997 to 2015
Fifty percent of shocks in transients and 13% of shocks in normal solar wind had a sheath region that drove at least a moderate storm
A median geoeffective shock/sheath pushed the magnetopause to 6.3 RE, had strong substorms (AL = ‐1350 nT) and moderate storm (Dst = ‐88 nT)
A method is described to model the magnetic field in the vicinity of three‐dimensional constellations of satellites (at least four) using field and plasma current measurements. This quadratic model ...matches the measured values of the magnetic field and its curl (current) at each spacecraft, with ∇ • B zero everywhere, and thus extends the linear curlometer method to second order. Near the spacecraft, it predicts the topology of magnetic structures, such as reconnecting regions or flux ropes, and allows a tracking of the motion of these structures relative to the spacecraft constellation. Comparisons to particle‐in‐cell simulations estimate the model accuracy. Reconstruction of two electron diffusion regions definitively confirms the expected field line structure. The model can be applied to other small‐scale phenomena (e.g., bow shocks) and can also be modified to reconstruct the electric field, allowing tracing of particle trajectories.
Key Points
Three‐dimensional model of magnetic field is constructed using magnetic field and current data
The constructions are able to visualize the local magnetic topology around spacecraft
Motion of magnetic structures can be derived
On 5 April 2010 an interplanetary (IP) shock was detected by the Wind spacecraft ahead of Earth, followed by a fast (average speed 650 km/s) IP coronal mass ejection (ICME). During the subsequent ...moderate geomagnetic storm (minimum Dst = −72 nT, maximum Kp = 8−), communication with the Galaxy 15 satellite was lost. We link images from STEREO/ SECCHI to the near–Earth in situ observations and show that the ICME did not decelerate much between Sun and Earth. The ICME flank was responsible for a long storm growth phase. This type of glancing collision was for the first time directly observed with the STEREO Heliospheric Imagers. The magnetic cloud (MC) inside the ICME cannot be modeled with approaches assuming an invariant direction. These observations confirm the hypotheses that parts of ICMEs classified as (1) long–duration MCs or (2) magnetic–cloud–like (MCL) structures can be a consequence of a spacecraft trajectory through the ICME flank.
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