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
A key goal for space weather studies is to define severe and extreme conditions that might plausibly afflict human technology. On 23 July 2012, solar active region 1520 (approx. 141 deg W ...heliographic longitude) gave rise to a powerful coronal mass ejection (CME) with an initial speed that was determined to be 2500+/-500 km/s. The eruption was directed away from Earth toward 125 deg W longitude. STEREO-A sensors detected the CME arrival only about 19 h later and made in situ measurements of the solar wind and interplanetary magnetic field. In this paper, we address the question of what would have happened if this powerful interplanetary event had been Earthward directed. Using a well-proven geomagnetic storm forecast model, we find that the 23-24 July event would certainly have produced a geomagnetic storm that was comparable to the largest events of the twentieth century (Dst approx. - 500 nT). Using plausible assumptions about seasonal and time-of-day orientation of the Earth's magnetic dipole, the most extreme modeled value of storm-time disturbance would have been Dst= - 1182 nT. This is considerably larger than estimates for the famous Carrington storm of 1859. This finding has far reaching implications because it demonstrates that extreme space weather conditions such as those during March of 1989 or September of 1859 can happen even during a modest solar activity cycle such as the one presently underway. We argue that this extreme event should immediately be employed by the space weather community to model severe space weather effects on technological systems such as the electric power grid.
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.
Abstract
In situ measurements from spacecraft typically provide a time series at a single location through coronal mass ejections (CMEs), and they have been one of the main methods to investigate ...CMEs. The CME properties derived from these in situ measurements are affected by temporal changes that occur as the CME passes over the spacecraft, such as radial expansion and aging, as well as spatial variations within a CME. This study uses multispacecraft measurements of the same CME at close separations to investigate both the spatial variability (how different a CME profile is when probed by two spacecraft close to each other) and the so-called aging effect (the effect of the time evolution on in situ properties). We compile a database of 19 events from the past 4 decades measured by two spacecraft with a radial separation of <0.2 au and an angular separation of <10°. We find that the average magnetic field strength measured by the two spacecraft differs by 18% of the typical average value, which highlights nonnegligible spatial or temporal variations. For one particular event, measurements taken by the two spacecraft allow us to quantify and significantly reduce the aging effect to estimate the asymmetry of the magnetic field strength profile. This study reveals that single-spacecraft time series near 1 au can be strongly affected by aging and that correcting for self-similar expansion does not capture the whole aging effect.
We have conducted a survey of 575 slow-to-fast stream interaction regions (SIRs) using
Solar Terrestrial Relations Observatory
(STEREO) A and B data, analyzing their properties while extending a ...Level-3 data product through 2016. Among 518 pristine SIRs, 54% are associated with heliospheric current sheet (HCS) crossings, and 34% are without any HCS crossing. The other 12% of the SIRs often occur in association with magnetic sectors shorter than three days. The SIRs with HCS crossings have slightly slower speeds but higher maximum number densities, magnetic-field strengths, dynamic pressures, and total pressures than the SIRs without an HCS. The iron charge state is higher throughout the SIRs with an HCS than the SIRs without an HCS, by about
1
/
3
charge unit. In contrast with the comparable phases of Solar Cycle 23, slightly more SIRs and higher recurrence rates are observed in the years 2009 – 2016 of Cycle 24, with a lower HCS association rate, possibly attributed to persistent equatorial coronal holes and more pseudo-streamers in this recent cycle. The solar-wind speed, peak magnetic field, and peak pressures of SIRs are all lower in this cycle, but the weakening is less than for the comparable background solar-wind parameters. Before STEREO-B lost contact in October 2014, 151 SIR pairs were observed by the twin spacecraft. Of the dual observations, the maximum speed is the best correlated of the plasma parameters. We have obtained a sample of plasma-parameter differences analogous to those that would be observed by a mission at Lagrange points 4 or 5. By studying several cases with large discrepancies between the dual observations, we investigate the effects of HCS relative location, tilt of stream interface, and small transients on the SIR properties. To resolve the physical reasons for the variability of SIR structures, mesoscale multi-point observations and time-dependent solar-wind modeling are ultimately required.
Abstract
On 2020 April 19–20, a solar ejection was seen by spacecraft in a radial alignment that included Solar Orbiter and Wind. The ejection contained a magnetic flux rope where magnetic field and ...plasma parameters were well correlated between spacecraft. This structure is called an “unperturbed magnetic flux rope” (UMFR). Ahead of the UMFR is a portion of the ejection (not sheath) that is referred to as “upstream” (US). We focus on the US and inquire why the correlation is so much weaker there. Specifically, we analyze data collected by Solar Orbiter at 0.81 au and Wind at L1. We show that a plausible cause for the lack of coherence in the US is a combination of front erosion and internal reconnection occurring there. Front erosion is inferred from an analysis of azimuthal magnetic flux balance in the UMFR. In the present case, we contend that the US, rather than the UMFR, is the source of the eroded field lines. The presence of erosion is supported further by a direct comparison of the magnetic field data at both spacecraft that shows, in particular, a massive shrinkage of the front portion of the US. Internal reconnection is also happening at thin current sheets inside the US. Strong nonradial flows are reconfiguring the structure. As a result of these reconnection processes, a whole section of the US is disrupted and field lines move down the flanks of the ejection and out of view of Wind.
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.
The giant, superfast, interplanetary coronal mass ejection, detected by STEREO A on 2012 July 23, well away from Earth, appears to have reached 1 AU with an unusual set of leading bow waves ...resembling in some ways a subsonic interaction, possibly due to the high pressures present in the very energetic particles produced in this event. Eventually, a front of record high-speed flow reached STEREO. The unusual behavior of this event is illustrated using the magnetic field, plasma, and energetic ion observations obtained by STEREO. Had the Earth been at the location of STEREO, the large southward-oriented magnetic field component in the event, combined with its high speed, would have produced a record storm.
The Interstellar Mapping and Acceleration Probe (IMAP) is a revolutionary mission that simultaneously investigates two of the most important overarching issues in Heliophysics today: the acceleration ...of energetic particles and interaction of the solar wind with the local interstellar medium. While seemingly disparate, these are intimately coupled because particles accelerated in the inner heliosphere play critical roles in the outer heliospheric interaction. Selected by NASA in 2018, IMAP is planned to launch in 2024. The IMAP spacecraft is a simple sun-pointed spinner in orbit about the Sun-Earth L1 point. IMAP's ten instruments provide a complete and synergistic set of observations to simultaneously dissect the particle injection and acceleration processes at 1 AU while remotely probing the global heliospheric interaction and its response to particle populations generated by these processes. In situ at 1 AU, IMAP provides detailed observations of solar wind electrons and ions; suprathermal, pickup, and energetic ions; and the interplanetary magnetic field. For the outer heliosphere interaction, IMAP provides advanced global observations of the remote plasma and energetic ions over a broad energy range via energetic neutral atom imaging, and precise observations of interstellar neutral atoms penetrating the heliosphere. Complementary observations of interstellar dust and the ultraviolet glow of interstellar neutrals further deepen the physical understanding from IMAP. IMAP also continuously broadcasts vital real-time space weather observations. Finally, IMAP engages the broader Heliophysics community through a variety of innovative opportunities. This papersummarizes the IMAP mission at the start of Phase A development.
PDF
