[1] We simulate the three-dimensional structure of the heliosphere during solar activity minimum by specifying boundary conditions at the coronal base. We compare the output of the model with Ulysses observations obtained during the spacecraft's first fast latitude transition in 1994–1995. The polytropic MHD equations are solved for a steady coronal outflow that includes the addition of Alfven wave momentum and energy in the WKB approximation. A solution for the outflow in a tilted dipole magnetic field in the inner computational region (1–20 R⊙) is combined with a three-dimensional solution in the outer region which extends to 10 AU. The inner region solution is essentially the same as in the work of Usmanov et al. [2000] but has been obtained for slightly different boundary conditions using a different numerical algorithm. The dipole orientation is chosen to match the one inferred from photospheric magnetic field observations at the Wilcox Solar Observatory. The steady solution in the outer region is constructed using a marching-along-radius method and models both solar rotation and interaction regions. The bimodality of solar wind with a rapid change in flow parameters with latitude and the observed extent of the slower wind belt are reproduced well. We compare our simulation also with the results of Bruno et al. [1986] and empirical models of coronal density. We show that the simulation results are in good agreement with the empirical model of Wang and Sheeley [1990].

[1] The solar wind experiment on the Advanced Composition Explorer (ACE) has detected at least 328 impulsive solar electron bursts that exhibited energy and angle dispersion at onset at energies below 1.4 keV in the interval from 1 January 1998 through 30 September 2003. Discontinuous, dispersionless modulations in burst intensity and pitch angle occurred in a subset of these events. These dispersionless modulations were similar to discontinuous modulations in the solar wind electron strahl intensity often observed at these same energies at other times. During the bursts themselves, dispersionless modulations in burst intensity commonly, but not always, were associated with discontinuous changes in the underlying strahl intensity. At times the burst intensity modulations were correlated with the strahl modulations and at times they were anticorrelated with the strahl modulations. We find that the variety of types of dispersionless burst/strahl modulations observed within low-energy solar electron bursts can largely be explained in terms of a simple model that assumes spatially limited burst source regions, a strahl intensity that varies with coronal magnetic connection point, and magnetic field line foot point motions in the solar atmosphere during the 2–4 days it takes solar wind plasma and the embedded heliospheric magnetic field to travel from the Sun to 1 AU.

[1] The photospheric sources of solar wind observed by the Ulysses and ACE spacecraft from 1998 to early 2001 are determined through a two-step mapping process. Solar wind speed measured at the spacecraft is used in a ballistic model to determine a foot point on a source surface at a solar distance of 2.5 solar radii. A potential-field source-surface model is then used to trace the field and flow from the source surface to the photosphere. Comparison of the polarity of the measured interplanetary field with the polarity of the photospheric source region shows good agreement for spacecraft latitudes equatorward of 60°. At higher southern latitudes, the mapping predicts that Ulysses should have observed only outward directed magnetic fields, whereas both polarities were observed. A detailed analysis is performed on four of the solar rotations for which the mapped and observed polarities were in generally good agreement. For those rotations, the solar wind mapped to both coronal holes and active regions. These findings for a period of high solar activity differ from the findings of a similar study of the solar wind in 1994–1995 when solar activity was very low. At solar minimum the fastest wind mapped to the interior of large polar coronal holes while slower wind mapped to the boundaries of those holes or to smaller low-latitude coronal holes. For the data examined in the present study, neither spacecraft detected wind from the small polar coronal holes when they existed and the speed was never as high as that observed by Ulysses at solar minimum. The principal difference between the solar wind from coronal holes and from active regions is that the O7+/O6+ ion ratio is lower for the coronal hole flow, but not as low as in the polar coronal hole flow at solar minimum. Furthermore, the active-region flows appear to be organized into several substreams unlike the more monolithic structure of flows from coronal holes. The boundaries between plasma flows from neighboring sources are marked by large magnetic holes, plasma sheets, and low entropy, independent of whether the sources have the same or opposite magnetic polarities. The evolution of solar wind properties between 1 AU and the 1.6–5.4 AU solar distance of Ulysses is also briefly discussed.

[1] The angle (θBn) between the normal to an interplanetary shock front and the upstream magnetic field direction, though often thought of as a property “of the shock,” is also determined by the configuration of the magnetic field immediately upstream of the shock. We investigate the interplanetary circumstances of 105 near-Earth quasi-perpendicular shocks during 1996–2005 identified by θBn ≥ 80° and/or by evidence of shock drift particle acceleration. Around 87% of these shocks were driven by interplanetary coronal mass ejections (ICMEs); the remainder were probably the forward shocks of corotating interaction regions. For around half of the shocks, the upstream field was approximately perpendicular to the radial direction, either east-west or west-east or highly inclined to the ecliptic. Such field directions will give quasi-perpendicular configurations for radially propagating shocks. Around 30% of the shocks were propagating through, or closely followed, ICMEs at the time of observation. Another quarter were propagating through the heliospheric plasma sheet (HPS), and a further quarter occurred in slow solar wind that did not have characteristics of the HPS. Around 11% were observed in high-speed streams, and 7% in the sheaths following other shocks. The fraction of shocks found in high-speed streams is around a third of that expected based on the fraction of the time when such streams were observed at Earth. Quasi-perpendicular shocks are found traveling through ICMEs around 2–3 times more frequently than expected. In addition, shocks propagating through ICMEs are more likely to have larger values of θBn than shocks outside ICMEs.

[1] Statistical analyses of Polar plasma wave data are performed to determine the occurrence frequency, intensity, and Poynting direction of ∼360 Hz to ∼1.8 kHz extremely low-frequency (ELF) electromagnetic waves (chorus, magnetosonic mode, and hiss) in the dayside sector of the magnetosphere. The study is limited to an L* range of 2 to 9 and a magnetic local time (MLT) range of 0900 to 1500, a region infrequently covered in past statistical surveys. The study was performed on 1996–1997 data, an interval near solar minimum. It is determined that in the outer region of the magnetosphere, from L* = 6 to 9, the ∼360 to ∼800 Hz waves at Polar altitudes are typically characterized by downward (toward Earth) propagation. The downgoing waves have been previously identified as chorus in Tsurutani et al. (2011). The downgoing chorus have intensities of ∼10−2 nT2, are right-hand circularly polarized and are propagating close to parallel to the ambient magnetic field B0. The high rate of occurrence of these downward propagating waves narrows to a smaller region of L* = 6 to 7 for ∼1.2 kHz waves. In the inner region of the magnetosphere, L* = 3 to 6, the ∼360 to 800 Hz waves are characteristically oblique (to B0) and upwards propagating, away from the Earth. The upcoming waves are most likely plasmaspheric hiss and low altitude magnetospherically reflected waves. These waves are an order of magnitude less intense and less coherent than the downward propagating chorus waves. At low frequencies, ∼360 Hz, there is a region near L* = 4 to 5, where obliquely propagating waves are detected. These are most probably a mixture of obliquely propagating plasmaspheric hiss and magnetosonic waves. A detailed (case study) examination of upward propagating waves is made for one Polar pass to add context to the statistical results. The upward propagating waves are quasicoherent and slightly elliptically polarized at Polar altitudes. From this and the statistical results, we ascribe to the scenario that ∼360 to 800 Hz chorus enters the plasmasphere at low altitude entry points and propagates through the plasmasphere as semicoherent hiss, in basic agreement with the Bortnik et al. (2008, 2009a) hypothesis for the origin of some plasmaspheric hiss. However, for ∼1.2 and 1.8 kHz waves inside the nominal location of the plasmasphere, downward propagating waves have higher intensities than upward propagating or oblique waves, perhaps indicating effects associated with different source locations, different entry points and different reflection regions and/or damping/amplification. At ∼800 Hz and ∼1.2 kHz upward propagating waves with weak intensities are common in the range L* = 8–9. These may be chorus waves magnetospherically reflected to larger L*. For frequencies above ∼1.5 kHz, most wave events are low intensity and upward propagating. It is possible that sferics and power line harmonics are contributors to these signals.

[1] Solar wind fast streams emanating from solar coronal holes cause recurrent, moderate intensity geomagnetic activity at Earth. Intense magnetic field regions called Corotating Interaction Regions or CIRs are created by the interaction of fast streams with upstream slow streams. Because of the highly oscillatory nature of the GSM magnetic field z component within CIRs, the resultant magnetic storms are typically only weak to moderate in intensity. CIR-generated magnetic storm main phases of intensity Dst < −100 nT (major storms) are rare. The elongated storm “recovery” phases which are characterized by continuous AE activity that can last for up to 27 days (a solar rotation) are caused by nonlinear Alfven waves within the high streams proper. Magnetic reconnection associated with the southward (GSM) components of the Alfven waves is the solar wind energy transfer mechanism. The acceleration of relativistic electrons occurs during these magnetic storm “recovery” phases. The magnetic reconnection associated with the Alfven waves cause continuous, shallow injections of plasma sheet plasma into the magnetosphere. The asymmetric plasma is unstable to wave (chorus and other modes) growth, a feature central to many theories of electron acceleration. It is noted that the continuous AE activity is not a series of substorm expansion phases. Arguments are also presented why these AE activity intervals are not convection bays. The auroras during these continuous AE activity intervals are less intense than substorm auroras and are global (both dayside and nightside) in nature. Owing to the continuous nature of this activity, it is possible that there is greater average energy input into the magnetosphere/ionosphere system during far declining phases of the solar cycle compared with those during solar maximum. The discontinuities and magnetic decreases (MDs) associated with interplanetary Alfven waves may be important for geomagnetic activity. In conclusion, it will be shown that geomagnetic storms associated with high-speed streams/CIRs will have the same initial, main, and “recovery” phases as those associated with ICME-related magnetic storms but that the interplanetary causes are considerably different.

[1] Cluster and ACE magnetic field measurements from the 13 February 2001 are presented. On this day the four Cluster spacecraft made two crossings of the heliospheric current sheet (HCS) while in the solar wind and observed a large-scale rotation of the magnetic field in the magnetosheath. These features were also observed by ACE upstream of the Earth at the L1 point. Minimum variance analysis (MVA) is used to show the similarity of the HCS orientation at ACE and Cluster, and Cluster multispacecraft analysis of the timing, orientation, and changes across the reversal is used to show that these results are consistent with crossings of the HCS. The structure of the currents in the HCS is also investigated using the Cluster curlometer technique. The field rotation observed by Cluster in the magnetosheath and ACE in the solar wind is identified as a magnetic flux rope, and MVA shows that although the structure and ordering of the flux rope are unchanged by the bow shock, the orientation of the axis changes. This is most likely caused by the rotation of the interplanetary magnetic field (IMF) as it is processed by the bow shock, augmented by draping effects deeper in the magnetosheath.

While Pioneer 11 and Galileo are two deep space missions that experienced radio communication disruptions due to space weather, the success of a mission like Solar Probe, whose goal is to fly by the Sun within a few solar radii of its surface, may depend critically on space weather. It is therefore crucial to thoroughly understand how space weather affects radio communications and to identify ways to predict it. In this study, we explain how enhanced small‐scale electron density variations in the solar corona represent adverse space weather to radio communications because they give rise to enhancements in radio propagation phenomena such as spectral broadening and fluctuations in intensity and Doppler of the radio signal known as scintillation. The inability of narrowband radio receivers to track the broadened and rapidly fluctuating radio signal leads to disruption in communication and loss of telemetry. Recent advances have been made in understanding enhanced small‐scale density variations and their relationship to the Sun, and we show how this makes it possible to use daily observations of unprecedented solar missions like the Solar and Heliospheric Observatory (SOHO) or Solar Terrestrial Relations Observatory (STEREO) to diagnose, monitor, and forecast adverse space weather for deep space communications.

We present a four-category classification algorithm for the solar wind, based on Gaussian Process. The four categories are the ones previously adopted in Xu & Borovsky [2015]: ejecta, coronal hole origin plasma, streamer belt origin plasma, and sector reversal origin plasma. The algorithm is trained and tested on a labeled portion of the OMNI dataset. It uses seven inputs: the solar wind speed $V_{sw}$, the temperature standard deviation $\sigma_T$, the sunspot number $R$, the $f_{10.7}$ index, the Alfven speed $v_A$, the proton specific entropy $S_p$ and the proton temperature $T_p$ compared to a velocity-dependent expected temperature. The output of the Gaussian Process classifier is a four element vector containing the probabilities that an event (one reading from the hourly-averaged OMNI database) belongs to each category. The probabilistic nature of the prediction allows for a more informative and flexible interpretation of the results, for instance being able to classify events as 'undecided'. The new method has a median accuracy larger than $90\%$ for all categories, even using a small set of data for training. The Receiver Operating Characteristic curve and the reliability diagram also demonstrate the excellent quality of this new method. Finally, we use the algorithm to classify a large portion of the OMNI dataset, and we present for the first time transition probabilities between different solar wind categories. Such probabilities represent the 'climatological' statistics that determine the solar wind baseline.

We investigate the role of coronal mass ejections (CMEs) in the global coronal magnetic field reconfiguration, a debate that has lasted for about two decades. Key evidence of the coronal field restructuring during the 2007 December 31 CME is provided by combining imaging observations from widely separated spacecraft with the potential-field source-surface (PFSS) model, thanks to the extraordinarily quiet Sun at the present solar minimum. The helmet streamer, previously disrupted by the CME, re-forms but is displaced southward permanently; the preexisting heliospheric plasma sheet (HPS) is also disrupted as evidenced by the concave-outward shape of the CME. The south polar coronal hole shrinks considerably. Plasma blobs moving outward along the newly formed HPS suggest the occurrence of magnetic reconnection between the fields blown open by the CME and the ambient adjacent open fields. A streamer-like structure is also observed in the wake of the CME and interpreted as a plasma sheet where the thin post-CME current sheet is embedded. These results are important for understanding the coronal field evolution over a solar cycle as well as the complete picture of CME initiation and propagation.

Understanding the transport of solar energetic particles (SEPs) from acceleration sites at the Sun into interplanetary space and to the Earth is an important question for forecasting space weather. The interplanetary magnetic field (IMF), with two distinct polarities and a complex structure, governs energetic particle transport and drifts. We analyze for the first time the effect of a wavy heliospheric current sheet (HCS) on the propagation of SEPs. We inject protons close to the Sun and propagate them by integrating fully 3D trajectories within the inner heliosphere in the presence of weak scattering. We model the HCS position using fits based on neutral lines of magnetic field source surface maps (SSMs). We map 1 au proton crossings, which show efficient transport in longitude via HCS, depending on the location of the injection region with respect to the HCS. For HCS tilt angles around 30°–40°, we find significant qualitative differences between A+ and A− configurations of the IMF, with stronger fluences along the HCS in the former case but with a distribution of particles across a wider range of longitudes and latitudes in the latter. We show how a wavy current sheet leads to longitudinally periodic enhancements in particle fluence. We show that for an A+ IMF configuration, a wavy HCS allows for more proton deceleration than a flat HCS. We find that A− IMF configurations result in larger average fluences than A+ IMF configurations, due to a radial drift component at the current sheet.

This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high-intensity, long-duration, continuous auroral electrojet activity) and the so-called Russell-McPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood. (2) A serious controversy exists as to whether the successive occurrence of intense substorms plays a direct role in the energization of ring current particles or whether the enhanced electric field associated withmore » southward IMF enhances the effect of substorm expansions. While most of the {ital Dst} variance during magnetic storms can be solely reproduced by changes in the large-scale electric field in the solar wind and the residuals are uncorrelated with substorms, recent satellite observations of the ring current constituents during the main phase of magnetic storms show the importance of ionospheric ions. This implies that ionospheric ions, which are associated with the frequent occurrence of intense substorms, are accelerated upward along magnetic field lines, contributing to the energy density of the storm-time ring current. An apparently new controversy regarding the relative importance of the two processes is thus created. It is important to identify the role of substorm occurrence in the large-scale enhancement of magnetospheric convection driven by solar wind electric fields. (3) Numerical schemes for predicting geomagnetic activity indices on the basis of solar/solar wind/interplanetary magnetic field parameters continue to be upgraded, ensuring reliable techniques for forecasting magnetic storms under real-time conditions. There is a need to evaluate the prediction capability of geomagnetic indices on the basis of physical processes that occur during storm time substorms. (4) It is crucial to differentiate between storms and nonstorm time substorms in terms of energy transfer/conversion processes, i.e., mechanical energy from the solar wind, electromagnetic energy in the magnetotail, and again, mechanical energy of particles in the plasma sheet, ring current, and aurora. To help answer the question of the role of substorms in energizing ring current particles, it is crucial to find efficient magnetospheric processes that heat ions up to some minimal energies so that they can have an effect on the strength of the storm time ring current. (5) The question of whether the {ital Dst} index is an accurate and effective measure of the storm time ring-current is also controversial. In particular, it is demonstrated that the dipolarization effect associated with substorm expansion acts to reduce the {ital Dst} magnitude, even though the ring current may still be growing. {copyright} 1998 American Geophysical Union« less

The physical mechanism for energy transfer from the solar wind to the magnetosphere is magnetic reconnection between the interplanetary field and the earth's field. During and a few years after solar maximum, the dominant interplanetary phenomena causing intense magnetic storms (D ST < -100 nT) are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two interplanetary regions are important for intense southward IMFs: the sheath region just behind the forward shock, and the ejecta material itself. Whereas the initial phase of a storm is caused by the increase in plasma ram pressure associated with the increase in density and speed at and behind the shock (accompanied by a sudden impulse [SI] at Earth), the storm main phase is due to southward IMFs. If the fields are southward in both of the sheath and solar ejecta, two-step main phase storms can result. The storm recovery phase begins when the IMF turns less southward, with delays of ∼1-2 hours. The recovery phase has a decay time of ∼10 hours and is physically due to a combination of several different energetic particle loss processes (Coulomb collisions, charge exchange and wave-particle interactions). During solar minimum, high speed streams from coronal holes dominate the interplanetary medium activity. The high-density, low-speed streams associated with the heliospheric current sheet (HCS) plasma sheet impinging upon the Earth's magnetosphere cause positive D ST values (storm initial phases if followed by main phases). In the absence of shocks, SIs are infrequent during this phase of the solar cycle. High-field regions called Corotating Interaction Regions (CIRs) are created by the fast stream (emanating from the coronal hole) interaction with the HCS plasma sheet. However, because the B z component is typically highly fluctuating within the CIRs, the main phases of the resultant magnetic storms typically have highly irregular profiles and are weaker. Storm recovery phases during this phase of the solar cycle are also quite different in that they can last from many days to weeks. The southward magnetic field (B s ) component of Alfven waves in the high speed stream proper cause intermittent reconnection, intermittent substorm activity, and sporadic injections of plasma sheet energy into the outer portion of the ring current, prolonging its final decay to quiet day values. This continuous auroral activity is called High Intensity Long Duration Continuous AE Activity (HILDCAAs).

The interaction of an interplanetary shock with the heliospheric plasma sheet (HPS) is simulated using the 2 12D MHD model in the meridional plane. The shock structure is generated by a velocity pulse and consists of a strong broad forward shock (FS) with a concave shape and a much weaker and narrower reverse shock (RS) with a convex shape. The flat equatorial HPS, with its larger mass density and slower flow velocity, modifies this shock structure. A dimple is formed at the FS and a reverse dimple is formed at the RS. A large distortion of the heliospheric current sheet (HCS) occurs when the velocity pulse is introduced outside the HPS. The interaction of a shock with the HPS leads to very large southward values of the interplanetary magnetic field due to an effect of shock compression, field-line draping, and deflection of the HCS.

The heliospheric current sheet (HCS) is the boundary between open oppositely directed magnetic field lines which commonly originate as the outward extension of the solar magnetic dipole. The dipole tilt, the rotation of the Sun, and the outward propagation of the solar wind cause peaks and valleys in the current sheet which spiral outward. The HCS extends throughout the heliosphere to the greatest distances reached by Pioneer and Voyager. It serves as a magnetic equator, and solar wind parameters including speed, temperature, density, and composition vary with distance from the HCS. Extrapolated back to the Sun, especially near solar minimum, the HCS corresponds to the low-latitude streamer belt. Both features are closely related to a neutral line obtained by extrapolating photospheric magnetic fields to a source surface at several solar radii. The current sheet and sector structure persist throughout the solar cycle including solar maximum. At 1 AU the width of the HCS is approximately 10,000 km while a surrounding plasma sheet is thicker by a factor of ∼30. The field inside the HCS does not simply decrease to a null and then reappear with the opposite sense. Instead, the field rotates at nearly constant magnitude from one polarity to the other. In spite of theoretical expectations that fields on opposite sides of the HCS will merge or reconnect, there is little evidence that such is occurring. Many scientific questions remain unanswered. What are the global properties of the HCS near solar maximum, and how faithfully are they reproduced by source surface models? Are multiple HCS crossings caused by waves on the current sheet or by multiple current sheets? What is the effect of coronal mass ejections on the HCS and vice versa?

The Ulysses spacecraft is now well into its third polar orbit around the Sun. Using stackplot displays, we summarize the wind speeds and interplanetary sector polarities recorded by Ulysses since its launch in 1990 and relate the observed patterns to the global evolution of open magnetic regions (coronal holes) over the solar cycle. We verify that the wind speeds are inversely correlated with the rate of flux-tube divergence in the corona, as derived from a current-free extrapolation of the measured photospheric field. We identify the source of each of the long-lived, high-speed streams encountered by Ulysses and discuss their formation, evolution, and rotational properties.

Solar energetic particles (SEPs), a major component of space weather, propagate through the interplanetary medium strongly guided by the interplanetary magnetic field (IMF). In this work, we analyze the implications that a flat Heliospheric Current Sheet (HCS) has on proton propagation from SEP release sites to the Earth. We simulate proton propagation by integrating fully 3D trajectories near an analytically defined flat current sheet, collecting comprehensive statistics into histograms, fluence maps, and virtual observer time profiles within an energy range of 1–800 MeV. We show that protons experience significant current sheet drift to distant longitudes, causing time profiles to exhibit multiple components, which are a potential source of confusing interpretations of observations. We find that variation of the current sheet thickness within a realistic parameter range has little effect on particle propagation. We show that the IMF configuration strongly affects the deceleration of protons. We show that in our model, the presence of a flat equatorial HCS in the inner heliosphere limits the crossing of protons into the opposite hemisphere.

One of the goals of machine learning is to eliminate tedious and arduous repetitive work. The manual and semi-automatic classification of millions of hours of solar wind data from multiple missions can be replaced by automatic algorithms that can discover, in mountains of multi-dimensional data, the real differences in the solar wind properties. In this paper we present how unsupervised clustering techniques can be used to segregate different types of solar wind. We propose the use of advanced data reduction methods to pre-process the data, and we introduce the use of Self-Organizing Maps to visualize and interpret 14 years of ACE data. Finally, we show how these techniques can potentially be used to uncover hidden information, and how they compare with previous empirical categorizations.

Occurrences of solar wind plasma with abnormally low proton temperatures (Tp) have long been associated with the interplanetary manifestations (which we term “ejecta”) of coronal mass ejections (CMEs). We survey the National Space Science Data Center Omni solar wind database for 1965–1991, and data from the Helios 1 and 2 spacecraft for more limited periods, to identify plasma in which Tp is less than the temperature expected (Tex) from the well-established correlation between the solar wind speed and Tp for normal solar wind expansion. The occurrence rate of low-temperature plasma (specifically with Tp/Tex ≤ 0.5) is shown to be correlated with solar activity levels. Individual low-temperature regions have durations from 1 to ∼80 hours with a mean of ∼10 hours. Around one third are encounters with the heliospheric plasma sheet (HPS). These events are observed most frequently during the increasing phase of solar activity when the HPS lies closer to the ecliptic. The abnormally low temperatures may be intrinsic to the HPS or may provide support for the proposal that the coronal streamer belt underlying the HPS is a frequent source of ejecta. The remaining events have an occurrence rate which shows a particularly clear correlation with solar activity levels and with the CME rate at the Sun (when CME observations are available), consistent with an association with ejecta. These events also show greater than chance associations with other ejecta signatures. We suggest that the Omni plasma data can provide information on the rate of ejecta passing the Earth, and hence give an indication of the CME rate, for a period commencing before spacecraft coronagraph CME observations became available in the early 1970s. Our findings suggest that Tp depressions may provide a more comprehensive indication of the presence of ejecta than other ejecta signatures, such as bidirectional solar wind electron heat fluxes and energetic ion flows, which alone do not identify all ejecta.

Faraday rotation observations are unique amongst radio occultation measurements in that they respond to magnetic field in addition to electron density, making it possible to probe the coronal magnetic field. Signatures interpreted as transients in Faraday rotation measurements made by Pioneer 6 in 1968 in the heliocentric distance range of 6–11 Ro are shown to be caused by coronal streamer stalks of angular size 1–2° observed in white‐light measurements rather than the magnetic bottles interpreted earlier. This identification makes available the first magnetic field observations of coronal streamers. The detection of the reversal of magnetic field polarity high in the corona provides observational evidence confirming what has previously only been inferred from modeling — that streamers observed in white‐light measurements are the manifestation of the heliospheric current sheet. These results reinforce the apparent connection between the coronal streamer stalk observed in white‐light coronagraphs and the heliospheric plasma sheet identified by in situ fields and particles measurements beyond 0.3 AU.