Signatures Of Coronal Mass Ejections Research Articles

Understanding the Effects of Spacecraft Trajectories through Solar Coronal Mass Ejection Flux Ropes Using 3DCOREweb

This study investigates the impact of spacecraft positioning and trajectory on in situ signatures of coronal mass ejections (CMEs). Employing the 3DCORE model, a 3D flux rope model that can generate in situ profiles for any given point in space and time, we conduct forward modeling to analyze such signatures for various latitudinal and longitudinal positions, with respect to the flux rope apex, at 0.8 au. Using this approach, we explore the appearance of the resulting in situ profiles for different flux rope types, with different handedness and inclination angles, for both high- and low-twist CMEs. Our findings reveal that CMEs exhibit distinct differences in signatures between apex hits and flank encounters, with the latter displaying elongated profiles with reduced rotation. Notably, constant, nonrotating in situ signatures are only observed for flank encounters of low-twist CMEs, suggesting the existence of untwisted magnetic field lines within CME legs. Additionally, our study confirms the unambiguous appearance of different flux rope types in in situ signatures in all of the cases, barring some indistinguishable cases, contributing to the broader understanding and interpretation of observational data. Given the model assumptions, this may refute trajectory effects as the cause for mismatching flux rope types as identified in solar signatures. While acknowledging limitations inherent in our model, such as the assumption of constant twist and a nondeformable torus-like shape, we still draw relevant conclusions within the context of the global magnetic field structures of CMEs and the potential for distinguishing flux rope types based on in situ observations.

Unveiling the journey of a highly inclined CME

Context. A fast (∼2000 km s−1) and wide (> 110°) coronal mass ejection (CME) erupted from the Sun on March 13, 2012. Its interplanetary counterpart was detected in situ two days later by STEREO-A and near-Earth spacecraft, such as ACE, Wind, and Cluster. We suggest that at 1 au the CME extended at least 110° in longitude, with Earth crossing its east flank and STEREO-A crossing its west flank. Despite their separation, measurements from both positions showed very similar in situ CME signatures. The solar source region where the CME erupted was surrounded by three coronal holes (CHs). Their locations with respect to the CME launch site were east (negative polarity), southwest (positive polarity) and west (positive polarity). The solar magnetic field polarity of the area covered by each CH matches that observed at 1 au in situ. Suprathermal electrons at each location showed mixed signatures with only some intervals presenting clear counterstreaming flows as the CME transits both locations. The strahl population coming from the shortest magnetic connection of the structure to the Sun showed more intensity. Aims. The aim of this work is to understand the propagation and evolution of the CME and its interaction with the surrounding CHs, to explain the similarities and differences between the observations at each spacecraft, and report what one of the most longitudinal expanded CME structures measured in situ would be. Methods. Known properties of the large-scale structures from a variety of catalogues and previous studies were used to have a better overview of this particular event. In addition, multipoint observations were used to reconstruct the 3D geometry of the CME and determine the context of the solar and heliospheric conditions before the CME eruption and during its propagation. The graduated cylindrical shell model (GCS) was used to reproduce the orientation, size and speed of the structure with a simple geometry. Also, the Drag-Based Model (DBM) was utilised to understand the conditions of the interplanetary medium better in terms of the drag undergone by the structure while propagating in different directions. Finally, a comparative analysis of the different regions of the structure through the different observatories was carried out in order to directly compare the in situ plasma and magnetic field properties at each location. Results. The study presents important findings regarding the in situ measured CME on March 15, 2012, detected at a longitudinal separation of 110° in the ecliptic plane despite its initial inclination being around 45° when erupted (March 13). This suggests that the CME may have deformed and/or rotated, allowing it to be observed near its legs with spacecraft at a separation angle greater than 100°. The CME structure interacted with high-speed streams generated by the surrounding CHs. The piled-up plasma in the sheath region exhibited an unexpected correlation in magnetic field strength despite the large separation in longitude. In situ observations reveal that at both locations there was a flank encounter – where the spacecraft crossed the first part of the CME – then encountered ambient solar wind, and finally passed near the legs of the structure. Conclusions. A scenario covering all evidence is proposed for both locations with a general view of the whole structure and solar wind conditions. Also, the study shows the necessity of having multipoint observations of large-scale structures in the heliosphere.

A new method of measuring Forbush decreases

Context. Forbush decreases (FDs) are short-term depressions in the galactic cosmic ray (GCR) flux and one of the common signatures of coronal mass ejections (CMEs) in the heliosphere. They often show a two-step profile, the second one associated with the CME’s magnetic structure. This second step can be described by the recently developed analytical FD model for flux ropes (FRs) – ForbMod. Aims. The aim of this study is to utilise ForbMod to develop a best-fit procedure to be applied on FR-related FDs as a convenient measurement tool. Our motivation is to develop a best-fit procedure that can be applied to a data series from an arbitrary detector. Thus, the basic procedure would facilitate measurement estimation of the magnitude of the FR-related FD, with the possibility of being adapted for the energy response of a specific detector for a more advanced analysis. Methods. The non-linear fitting was performed by calculating all possible ForbMod curves constrained within the FR borders to the designated dataset and minimising the mean square error (MSE). In order to evaluate the performance of the ForbMod best-fit procedure, we used synthetic measurements produced by calculating the theoretical ForbMod curve for a specific example CME and then applying various effects to the data to mimic the imperfection of the real measurements. We also tested the ForbMod best-fit function on the real data, measured by detector F of the SOHO/EPHIN instrument on a sample containing 30 events, all of which have a distinct FD corresponding to the magnetic obstacle. The extraction of FD profiles (from the onset to the end) was performed manually by an observer, whereby we applied two different versions of border selection and assigned a quality index to each event. Results. We do not find notable differences between events marked by a different quality index. For events with a selection of two different borders, we find that the best fit applied on extended interplanetary coronal mass ejection (ICME) structure borders results in a slightly larger MSE and differences compared to the traditional method due to a larger scatter of the data points. We find that the best-fit results can visually be categorised into six different FD profile types. Although some profiles do not show a visually pleasing FD, the ForbMod best-fit function still manages to find a solution with a relatively small MSE. Conclusions. Overall, we find that the ForbMod best-fit procedure performs similar to the traditional algorithm-based observational method, but with slightly smaller values for the FD amplitude, as it’s taking into account the noise in the data. Furthermore, we find that the best-fit procedure has an advantage compared to the traditional method as it can estimate the FD amplitude even when there is a data gap at the onset of the FD.

3D MHD Time-dependent Charge State Ionization and Recombination Modeling of the Bastille Day Coronal Mass Ejection

Heavy ion signatures of coronal mass ejections (CMEs) indicate that rapid and strong heating takes place during the eruption and early stages of propagation. However, the nature of the heating that produces the highly ionized charge states often observed in situ is not fully constrained. An MHD simulation of the Bastille Day CME serves as a test bed to examine the origin and conditions of the formation of heavy ions evolving within the CME in connection with those observed during its passage at L1. In particular, we investigate the bimodal nature of the Fe charge state distribution, which is a quintessential heavy ion signature of CME substructure, as well as the source of the highly ionized plasma. We find that the main heating experienced by the tracked plasma structures linked to the ion signatures examined is due to field-aligned thermal conduction via shocked plasma at the CME front. Moreover, the bimodal Fe distributions can be generated through significant heating and rapid cooling of prominence material. However, although significant heating was achieved, the highest ionization stages of Fe ions observed in situ were not reproduced. In addition, the carbon and oxygen charge state distributions were not well replicated owing to anomalous heavy ion dropouts observed throughout the ejecta. Overall, the results indicate that additional ionization is needed to match observation. An important driver of ionization could come from suprathermal electrons, such as those produced via Fermi acceleration during reconnection, suggesting that the process is critical to the development and extended heating of extreme CME eruptions, like the Bastille Day CME.

Parker Solar Probe Encounters the Leg of a Coronal Mass Ejection at 14 Solar Radii

We use Parker Solar Probe (PSP) observations to report the first direct measurements of the particle and field environments while crossing the leg of a coronal mass ejection (CME) very close to the Sun (∼14 Rs). An analysis that combines imaging from 1 au and PSP with a CME model, predicts an encounter time and duration that correspond to an unusual, complete dropout in low-energy solar energetic ions from H–Fe, observed by the Integrated Science Investigation of the Sun (IS⊙IS). The surrounding regions are populated with low-intensity protons and heavy ions from 10s to 100 keV, typical of some quiet times close in to the Sun. In contrast, the magnetic field and solar wind plasma show no similarly abrupt changes at the boundaries of the dropout. Together, the IS⊙IS energetic particle observations, combined with remote sensing of the CME and a dearth of other “typical” CME signatures, indicate that this CME leg is significantly different from the magnetic and plasma structure normally assumed for CMEs near the Sun and observed in interplanetary CMEs farther out in the solar wind. The dropout in low-energy energetic ions may be due to the cooling of suprathermal ions at the base of the CME leg flux tube, owing to the rapid outward expansion during the release of the CME.

FARSIDE: A Low Radio Frequency Interferometric Array on the Lunar Farside

FARSIDE (Farside Array for Radio Science Investigations of the Dark ages and Exoplanets) is a Probe-class concept to place a low radio frequency interferometric array on the farside of the Moon. A NASA-funded design study, focused on the instrument, a deployment rover, the lander and base station, delivered an architecture broadly consistent with the requirements for a Probe mission. This notional architecture consists of 128 dual polarization antennas deployed across a 10 km area by a rover, and tethered to a base station for central processing, power and data transmission to the Lunar Gateway. FARSIDE would provide the capability to image the entire sky each minute in 1400 channels spanning frequencies from 100 kHz to 40 MHz, extending down two orders of magnitude below bands accessible to ground-based radio astronomy. The lunar farside can simultaneously provide isolation from terrestrial radio frequency interference, auroral kilometric radiation, and plasma noise from the solar wind. This would enable near-continuous monitoring of the nearest stellar systems in the search for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres for the nearest candidate habitable exoplanets. Simultaneously, FARSIDE would be used to characterize similar activity in our own solar system, from the Sun to the outer planets, including the hypothetical Planet Nine. Through precision calibration via an orbiting beacon, and exquisite foreground characterization, FARSIDE would also measure the Dark Ages global 21-cm signal at redshifts z=50-100. The unique observational window offered by FARSIDE would enable an abundance of additional science ranging from sounding of the lunar subsurface to characterization of the interstellar medium in the solar system neighborhood.

Transformative science from the lunar farside: observations of the dark ages and exoplanetary systems at low radio frequencies.

The farside of the Moon is a pristine, quiet platform to conduct low radio frequency observations of the early Universe's Dark Ages, as well as space weather and magnetospheres associated with habitable exoplanets. In this paper, the astrophysics associated with NASA-funded concept studies will be described including a lunar-orbiting spacecraft, DAPPER, that will measure the 21 cm global spectrum at redshifts ≈40-80, and an array of low frequency dipoles on the lunar farside surface, FARSIDE, that would detect exoplanet magnetic fields. DAPPER observations (17-38 MHz), using a single cross-dipole antenna, will determine the amplitude of the 21 cm spectrum to the level required to distinguish the standard ΛCDM cosmological model from those produced by exotic physics such as nongravitational dark matter interactions. FARSIDE has a notional architecture consisting of 128 dipole antennas deployed across a 10 km area by a rover. FARSIDE would image the entire sky each minute in 1400 channels over 0.1-40 MHz. This would enable monitoring of the nearest stellar systems for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres of the nearest candidate habitable exoplanets. In addition, FARSIDE would provide a pathfinder for power spectrum measurements of the Dark Ages. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.

EUV Wave Detection and Characterization Using Deep Learning

Coronal Mass Ejections (CMEs) are the most violent solar bursts. They cause severe disturbances in the solar–terrestrial space and affect human activities in many aspects, especially causing damage to high-tech infrastructure. It usually takes few hours for a CME to arrive at the Earth after eruption. Therefore, many efforts have been devoted to CME arrival time prediction, so that we have enough time to take action before a CME arrives at the Earth. For predicting CME arrival time, it is vital to detect the CME origin, arrival and departure speed in a coronagraph. It has been widely accepted that Extreme Ultraviolet (EUV) waves are associated with CMEs, so EUV waves are the signatures of CMEs as CMEs originate and traverse the solar disk, specifically for front-side CMEs. In this paper, two deep neural networks are developed to first detect EUV waves and then outline their wavefronts, giving early signatures of CMEs. Usually, CMEs are recorded by coronagraphs as they transit the corona, so our proposed method can obtain a certain time ahead compared with conventional CME forecasting. In addition, the parameters for describing EUV waves can be more easily deduced, benefiting the subsequent statistical analysis of CMEs. The experimental results demonstrate the effectiveness of the proposed model for detecting EUV waves and generating their outlines.

Rapid Buildup of a Magnetic Flux Rope during a Confined X2.2 Class Flare in NOAA AR 12673

Abstract Magnetic flux ropes (MFRs) are believed to be the core structure in solar eruptions; nevertheless, their formation remains intensely debated. Here we report a rapid buildup process of an MFR system during a confined X2.2 class flare occurred on 2017 September 6 in NOAA active region (AR) 12673, three hours after which the structure erupted to a major coronal mass ejection (CME) accompanied by an X9.3 class flare. For the X2.2 flare, we do not find extreme ultraviolet dimmings, separation of its flare ribbons, or clear CME signatures, suggesting a confined flare. For the X9.3 flare, large-scale dimmings, separation of flare ribbons, and a CME show it to be eruptive. By performing a time sequence of nonlinear force-free fields extrapolations we find the following. Until the eruptive flare, an MFR system was located in the AR. During the confined flare, the axial flux and the lower bound of the magnetic helicity for the MFR system were dramatically enhanced by about 86% and 260%, respectively, although the mean twist number was almost unchanged. During the eruptive flare, the three parameters were all significantly reduced. The results evidence the buildup and release of the MFR system during the confined and the eruptive flare, respectively. The former may be achieved by flare reconnection. We also calculate the pre-flare distributions of the decay index above the main polarity inversion line and find no significant difference. It indicates that the buildup of the magnetic flux and helicity of the MFR system may play a role in facilitating its final eruption.

The Association of Solar Flares with Coronal Mass Ejections During the Extended Solar Minimum

We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007-2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the {\it Solar Terrestrial Relations Observatory} (STEREO). Although all of the fast (v $>$ 900 km s$^{-1}$) {\it and} wide ($\theta >$ 100$\arcdeg$) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. $\gtrsim$1 \times 10^{-6}$ W m$^{-2}$ or C1) are not associated with CMEs. Intense flares tend to occur in active regions with strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales comparable to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91%, 74%, 57%, and 74%, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs.

THE ROLE OF STREAMERS IN THE DEFLECTION OF CORONAL MASS EJECTIONS: COMPARISON BETWEENSTEREOTHREE-DIMENSIONAL RECONSTRUCTIONS AND NUMERICAL SIMULATIONS

On 2009 September 21, a filament eruption and the associated coronal mass ejection (CME) were observed by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. The CME originated from the southern hemisphere and showed a deflection of about 15 ◦ toward the heliospheric current sheet (HCS) during the propagation in the COR1 field of view. The CME source region was near the central meridian, but no on-disk CME signatures could be seen from the Earth. The aim of this paper is to provide a physical explanation for the strong deflection of the CME observed on 2009 September 21. The two-sided view of the STEREO spacecraft allows us to reconstruct the three-dimensional travel path of the CME and the evolution of the CME source region. The observations are combined with a magnetohydrodynamic simulation, starting from a magneticfield configuration closely resembling the extrapolated potential field for that date. By applying localized shearing motions, a CME is initiated in the simulation, showing a similar non-radial evolution, structure, and velocity as the observed event. The CME gets deflected toward the current sheet of the larger northern helmet streamer due to an imbalance in the magnetic pressure and tension forces and finally gets into the streamer. This study shows that during solar minima, even CMEs originating from high latitude can be easily deflected toward the HCS, eventually resulting in geoeffective events. How rapidly they undergo this latitudinal migration depends on the strength of both the large-scale coronal magnetic field and the magnetic flux of the erupting filament.

Modeling of coronal mass ejections that caused particularly large geomagnetic storms using ENLIL heliosphere cone model

In our previous paper we reported the results of modeling of 14 selected well‐observed strong halo coronal mass ejection (CME) events using the WSA‐ENLIL cone model combination. Cone model input parameters were obtained from white light coronagraph images of the CME events using the analytical method developed by Xie et al. (2004). This work verified that coronagraph input gives reasonably good results for the CME arrival time prediction. In contrast to Taktakishvili et al. (2009), where we started the analysis by looking for clear CME signatures in the data and then proceeded to model the interplanetary consequences at 1 AU, in the present paper we start by generating a list of observed geomagnetic storm events and then work our way back to remote solar observations and carry out the corresponding CME modeling. The approach used in this study is addressing space weather forecasting and operational needs. We analyzed 36 particularly strong geomagnetic storms, then tried to associate them with particular CMEs using SOHO/LASCO catalogue, and finally modeled these CMEs using WSA‐ENLIL cone model. Recently, Pulkkinen et al. (2010) developed a novel method for automatic determination of cone model parameters. We employed both analytical and automatic methods to determine cone model input parameters. We examined the CME arrival times and magnitude of impact at 1 AU for both techniques. The results of the simulations are compared with the ACE satellite observations. This comparison demonstrated that WSA‐ENLIL model combination with coronagraph input gives reasonably good results for the CME arrival times for this set of “geoeffective” CME events as well.

Simultaneous interplanetary scintillation and Heliospheric Imager observations of a coronal mass ejection

We describe simultaneous Interplanetary Scintillation (IPS) and STEREO Heliospheric Imager (HI) observations of a coronal mass ejection (CME) on 16 May 2007. Strong CME signatures were present throughout the IPS observation. The IPS raypath lay within the field-of-view of HI-1 on STEREO-A and comparison of the observations shows that the IPS measurements came from a region within a faint CME front observed by HI-1A. This front may represent the merging of two converging CMEs. Plane-of- sky velocity estimates based on time-height plots of the two converging CME structures were 325 kmsﰀ1 and 550 kmsﰀ1 for the leading and trailing fronts respectively. The plane- of-sky velocities determined from IPS ranged from 420 ± 10 kmsﰀ1 to 520 ± 20 kmsﰀ1. IPS results reveal the presence of micro-structure within the CME front which may represent interaction between the two separate CME events. This is the first time that it has been possible to interpret IPS observations of small-scale structure within an interplanetary CME in terms of the global structure of the event.

Solar Sources of Geoeffective CMEs: a SOHO/EIT View

Observations of the low solar corona, in particular in the EUV, are an eectiv e means of identifying the solar sources of coronal mass ejections (CMEs). SOHO/EIT, with its continuous 24 hours per day coverage, is well suited to perform this task. Source regions and start times of frontside full and partial halo CMEs (that may be geoeectiv e) can thus be determined. The most frequent EUV signatures of CMEs are coronal dimmings. EIT waves, eruptive lamen ts and post-eruption arcades are also reliable signatures. Frontside halo CMEs with source regions close to the solar disc center have the strongest chance to hit the Earth. The inspection of the EIT data together with photospheric magnetograms may give an idea about the ejected interplanetary ux rope magnetic eld and, in particular, about the presence or absence of southward (geoeectiv e) eld. If a source region is situated close to the solar limb, the corresponding CME also may be geoeectiv e, as the CME-driven shocks have large angular extent. In this case the storm can be produced by the sheath plasma behind the shock, provided it contains strong enough southward interplanetary magnetic eld. Some implications for the operational space weather forecast are discussed. EIT and LASCO are capable to identify the solar sources of the most of geomagnetic storms. In some cases, however, the identication is uncertain, so the observations by the future STEREO mission will be needed for the investigation of similar events.

Launch of solar coronal mass ejections and submillimeter pulse bursts

The rapid solar spikes (100–500 ms) recently discovered at submillimeter waves bring new possibilities to investigate energetic processes near the solar surface that might have an important role in the launch and propelling of ionized mass away from the Sun. We present a study on the association between the launch time of coronal mass ejections (CMEs) observed by the LASCO instruments on the SOHO spacecraft and the onset of the new kind of rapid solar spikes (100–500 ms) observed at submillimetric waves (212 and 405 GHz) by the new Solar Submm‐wave Telescope (SST). We investigated six submm‐wave events, all found associated to CMEs. Seven related CME were identified. Five of them were associated with flares with large GOES class soft X‐rays, presenting distinct time histories and associations at other energy ranges, and two of them were related to flares behind the solar limb, with simultaneous related activity observed in the visible solar disk. Ultraviolet images from EIT on SOHO show some kind of small or large‐scale magnetic activity or brightening for all events. The extrapolation of apparent CME positions to the solar surface show that they occurred nearly coincident in time with the onset of submm‐wave pulses for all six events. These results suggest that pulse bursts might be representative of an important early signature of CMEs, especially for events beginning near the center of the solar disk, sometimes identified as “halo” CMEs. They lead to several challenging questions relative to the physical nature of the pulses and its association to the launch and acceleration of coronal mass ejections. Although these evidences may favor multiple rapid energy releases at the origin near the solar surface, they require further research in order to better understand both diagnostics and model descriptions.

Coronal mass ejections: Initiation and detection

Coronal mass ejections (CMEs) are large-scale magnetic structures expelled from the Sun due to MHD processes involving interaction between plasma and magnetic field in closed field regions. I provide a summary of the observational signatures and current models on CME initiation. I also discuss the multiwavelength signatures of CMEs, which have helped us obtain a global picture of the CME phenomenon in the inner heliosphere.

Solar Radio Astronomy at Low Frequencies

This review is concerned to study of sun at frequencies lower than 1.4 GHz. Emphasis is made on results which illustrate the topics in which GMRT could play a major role. Coordinated studies including spectral and imaging radio observations are important for research in solar physics. Joint observations between the Giant Meter Radio Telescope (GMRT) with radio instruments located in the same longitude range are encouraged. This review inludes three distinct topics: Electron beams and radio observations- Radio signatures of Coronal Mass Ejections- Radio signatures of coronal and interplanetary shocks.

Relation between Flare-associated X-Ray Ejections and Coronal Mass Ejections.

In an attempt to identify the direct signatures of coronal mass ejections (CMEs) in soft X-ray wavelengths, we have searched for plasma ejections in Yohkoh soft X-ray telescope (SXT) images in a total of 17 limb flares, and compared the results with the Solar and Heliospheric Observatory LASCO data. A general correlation exists between the presence/absence of the X-ray ejection and the CME. Although the height versus time relation often indicates (under the assumption of constant speed) that the CME onset coincides with the X-ray ejection, the latter probably does not represent the CME front, because the CME speed must result from acceleration, which would put the estimated onset at an earlier time. In some cases, the estimated CME onset time comes well before the impulsive phase of the associated flare. Although the role of the flare-associated plasma ejection in a CME is still unclear, we propose that its occurrence depends on the presence of open field lines, which can be due to a preceding CME. Lastly, we present a rare example of SXT observations of what appeared to be the three-part structure of a CME, which was seen a few minutes before a major flare started.

SOHO/EIT observations of an Earth‐directed coronal mass ejection on May 12, 1997

An earth‐directed coronal mass ejection (CME) was observed on May 12, 1997 by the SOHO Extreme ultraviolet Imaging Telescope (EIT). The CME, originating north of the central solar meridian, was later observed by the Large Angle Spectrometric Coronagraph (LASCO) as a “halo” CME: a bright expanding ring centered about the occulting disk. Beginning at about 04:35 UT, EIT recorded several CME signatures, including dimming regions close to the eruption, post‐eruption arcade formation, and a bright wavefront propagating quasi‐radially from the source region. Each of these phenomena appear to be associated with the same eruption, and the onset time of these features corresponds with the estimated onset time observed in LASCO. We discuss the correspondence of these features as observed by EIT with the structure of the CME in the LASCO data.

The topology of intrasector reversals of the interplanetary magnetic field

A technique has been developed recently to determine the polarities of interplanetary magnetic fields relative to their origins at the Sun by comparing energetic electron flow directions with local magnetic field directions. Here we use heat flux electrons from the Los Alamos National Laboratory (LANL) plasma detector on the ISEE 3 spacecraft to determine the field polarities. We examine periods within well‐defined magnetic sectors when the field directions appear to be reversed from the normal spiral direction of the sector. About half of these intrasector field reversals (IFRs) are cases in which the polarities match those of the surrounding sectors, indicating that those fields have been folded back toward the Sun. The more interesting cases are those with polarity reversals. We find no clear cases of isolated reverse polarity fields, which suggests that islands of reverse polarity in the solar source dipole field probably do not exist. The IFRs with polarity reversals are strongly associated with periods of bidirectional electron flows, suggesting that those fields occur only in conjunction with closed fields. We propose that both those IFRs and the bidirectional flows are signatures of coronal mass ejections (CMEs). In that case, many interplanetary CMEs are larger and more complex than previously thought, consisting of both open and closed field components.