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A sequence of discrete solar wind structures within the sheath region of an interplanetary coronal mass ejection on November 6, 2015, caused a series of compressions and releases of the dayside magnetosphere. Each compression resulted in a brief adiabatic enhancement of ions (electrons) driving bursts of electromagnetic ion cyclotron (EMIC; whistler mode chorus) wave growth across the dayside magnetosphere. Fine‐structured rising tones were observed in the EMIC wave bursts, resulting in nonlinear scattering of relativistic electrons in the outer radiation belt. Multipoint observations allow us to study the spatial structure and evolution of these sheath structures as they propagate Earthward from L1 as well as the spatio‐temporal characteristics of the magnetospheric response. This event highlights the importance of fine‐scale solar wind structure, in particular within complex sheath regions, in driving dayside phenomena within the inner magnetosphere., Key Points A sequence of discrete structures within an interplanetary coronal mass ejection sheath are tracked via multipoint measurementsEach structure results in magnetospheric compression, adiabatic enhancement of ions and electrons, and cyclotron wave growthThis event highlights the importance of fine‐scale solar wind structure and sheath regions for driving dayside magnetospheric phenomena

We determine the solar wind structures (coronal mass ejection (CME)-related, corotating high-speed streams, and slow solar wind) driving geomagnetic storms of various strength over nearly three solar cycles (1972–2000). The most intense storms (defined by Kp) at both solar minimum and solar maximum are almost all (∼97%) generated by transient structures associated with CMEs. Weaker storms are preferentially associated with streams at solar minimum and with CMEs at solar maximum, reflecting the change in the structure of the solar wind between these phases of the solar cycle. Slow solar wind generates a small fraction of the weaker storms at solar minimum and maximum. We also determine the size distributions of Kp for each solar wind component.

The May, 1999 low density (< 1 cm -3 ) solar wind interval is one of a series of intervals of low density solar wind which have been detected since in-situ, near-Earth observations began. Examining the NSSDC OMNI database since 1965, covering solar Cycles 20-23, we show that such intervals, which are also periods of unusually low mass flux and low dynamic pressure, occur most frequently around sunspot maximum and are rarer at solar minimum. The occurrence rate of low-density plasma may be higher in weaker sunspot cycles (Cycle 20 and the current Cycle 23). Around two-thirds of periods with densities < 1 cm -3 are associated with transient solar wind structures, in particular with ejecta and post-shock flows. The majority of other events are associated with corotating streams. The May 1999 event is unusual because it is not associated with an ejecta or stream. A similar period was observed in July-August 1979.

StudiesusingSOHOspacecraftdatahavedemon- strated that frontside halo coronal mass ejections (CMEs) detected by the LASCO coronagraphs generally precede ge- omagnetic storms. Nonetheless, about three quarters of such CMEs do not result in even moderate geomagnetic activity. We study the relationship of all the ejecta (in- terplanetary CMEs) which passed Earth during 1996-1999 to coronagraph CMEs and geomagnetic activity. We reach the following conclusions: (1) Only about half of frontside halo CMEs encounter the Earth; (2) The geoeectiveness of ejecta depends strongly on the southward magneticeld strengthand, for thesame southwardeld, is irrespective of whetherornottheejectahasamagneticcloudstructure;(3) Transit speeds of ejecta to Earth are only loosely correlated with CME speeds, one influence being the prevailing solar windconditions betweentheSunandEarth; (4)Ejecta may be detected at Earth even when there is no preceding halo CME observed by LASCO. Such ejecta are not particularly geoeective.

We show that the variations of the interplanetary magnetic field strength (B) over a 22-year period are tracked by the inverted profile of the cosmic ray density measured by neutron monitors. We suggest that global changes in the Sun's magnetic field are more important for long-term modulation than magnetic field enhancements resulting from the merging of high-speed flows and coronal mass ejections in the outer heliosphere. The unexpectedly close relationship that we find between the “tilt angle” of the heliospheric current sheet and the cosmic ray density away from solar minimum for both polarity states of the solar magnetic field may be accounted for by the anticorrelation between the cosmic ray density and field strength variations.

We report on the finding of a 153-day periodicity in the magnetic field strength and solar wind speed measured at 1 AU during the years 1978–1982. The period and the occurence epoch are consistent with the “154-day” periodicity previously reported for events occurring at the Sun. In particular, the variations in the field strength and the occurence rate of energetic (tens of MeV) solar particle events are in phase. The similar periodicities in the interplanetary field and solar phenomena are consistent with a global phenomenon. Whereas this periodicity is quite strong for the magnetic field magnitude, there is only a weak periodicity for the individual field components. The field magnitude shows essentially no periodicity at this period during the previous and following solar maxima. The lack of persistence, and of significant harmonic components of the observed periodicity, does not support the proposal of a solar “clock” mechanism. The most significant variations in the complete near-earth magnetic field data base (1963–1997) with periods of less than 200 days occur at 166 and 146 days.

Evidence is presented that the enhanced geomagnetic activity, on April 10–11, 1997, was caused by one of two ejecta that left the Sun at ≈ 14 UT on April 7. This ejecta was not directly detected at the Earth. The evidence for this interpretation is based on WIND spacecraft observations in the solar wind (SW). It is consistent with: (i) measured velocities of the coronal mass ejections from the SOHO coronagraph; (ii) the initial propagation speed of the shock generated in this event, estimation from type II radio burst observations from the WAVES instrument on WIND, and (iii) the time profile of energetic ions observed by EPACT on WIND. This locally unobserved ejecta (moving at 600 to 700 kms−1) generated a fast shock which accelerated ions to several tens of MeV/amu. The inferred passage of the first ejecta close to Earth (on April 10 to 11) is based on the observation of an interplanetary shock (IS) ahead of a field and plasma compressional region where the draping of the SW flow and possibly the changes in the direction of the IMF are consistent with a location northward of a faster ejecta. This ejecta was responsible for disturbed SW conditions including approximately ten hours of southward orientation of the interplanetary magnetic field (IMF) and a ram pressure many times above normal. The slower moving ejecta was directed toward Earth and was observed with WIND from about 0550 until 1500 UT on April 11. It had a strong northward IMF and produced density enhancements which elevated the ram pressure to more than four times above normal.

[1] We present a statistical study of the sizes of interplanetary coronal mass ejections (ICMEs) and the preceding shock sheath regions in near-Earth space. The 46 events studied are a subset of the events responsible for intense (Dst ≤ −100 nT) geomagnetic storms in 1996–2005 in which only a single ICME was responsible for generating the storm. We find that the durations and radial sizes of these ICMEs range from 8.0 to 62.0 hr and 0.08 to 0.63 AU, respectively, with average values of 30.6 hr and 0.37 AU. The sheath durations and radial sizes range from 2.6 to 24.5 hr and 0.03 to 0.31 AU, with average values of 10.6 hr and 0.13 AU. On average, the ICME radial size is 2.8 times that of the sheath. In terms of their relative geoeffectiveness, ICMEs contribute on average about 71% of the total energy input (sheath + ICME) into the magnetosphere.

[11 ] While the classic picture of a geomagnetic storm is of a main phase eventually reaching maximum intensity, followed by a recovery, the profile can often be more complex. This has been recognized in past studies that have classified storms as having "one" or "two" "steps" during the main phase. However, the intense (Dst < -100 nT) storms studied during the LWS CDAW Workshop may be more complicated. We discuss the variety of interplanetary circumstances that gave rise to several storms of varying complexity.

[1] An intermittent quasi-periodicity of ∼150 days in various solar and interplanetary phenomena has been reported in earlier solar cycles. We suggest that variations in the occurrence of solar energetic particle events, interplanetary coronal mass ejections, and geomagnetic storm sudden commencements during solar cycle 23 show evidence of this quasi-periodicity, which is also present in the sunspot number, in particular in the northern solar hemisphere. It is not, however, prominent in the interplanetary magnetic field strength.

"Magnetic clouds" (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) characterized by enhanced magnetic fields with an organized rotation in direction, and low plasma beta. Though intensely studied, MCs only constitute a fraction of all the ICMEs that are detected in the solar wind. A comprehensive survey of ICMEs in the near- Earth solar wind during the ascending, maximum and early declining phases of solar cycle 23 in 1996 - 2003 shows that the MC fraction varies with the phase of the solar cycle, from approximately 100% (though with low statistics) at solar minimum to approximately 15% at solar maximum. A similar trend is evident in near-Earth observations during solar cycles 20 - 21, while Helios 1/2 spacecraft observations at 0.3 - 1.0 AU show a weaker trend and larger MC fraction.

The comment of Gopalswamy et al. (thereafter GMY) relates to a letter discussing coronal mass ejections (CMEs), interplanetary ejecta and geomagnetic storms. GMY contend that Cane et al. incorrectly identified ejecta (interplanetary CMEs) and hypothesize that this is because Cane et al. fail to understand how to separate ejecta from "shock sheaths" when interpreting solar wind and energetic particle data sets. They (GMY) are wrong be cause the relevant section of the paper was concerned with the propagation time to 1 AU of any potentially geoeffective structures caused by CMEs, i.e. upstream compression regions with or without shocks, or ejecta. In other words, the travel times used by Cane et al. were purposefully and deliberately distinct from ejecta travel times (except for those slow ejecta, approx. 30% of their events, which generated no upstream features), and no error in identification was involved. The confusion of GMY stems from the description did not characterize the observations sufficiently clearly.

Recent studies have proposed that the presence or absence of an interaction with a preceding coronal mass ejection (CME) or other coronal structure within approximately 50R(sub s), of the Sun discriminates large, fast CMEs associated with major solar energetic particle (SEP) events from those that are not. We conclude that there is no compelling evidence that, if such interactions take place, they play an important role in SEP acceleration. Reasons include: The reported statistical results are consistent with a chance association between interacting CMEs and SEP events; Energetic SEPs are detected at Earth typically before or around the time when the primary CME enters the LASCO C2 field of view - interactions higher in the corona cannot play a role in acceleration of these particles; For approximately 60% of major SEP events in 1997-2001, the preceding CME fades into the background corona or is relatively narrow (less than 40 deg), suggesting any interaction will be weak; Radio signatures attributed to CME interaction occur after SEP acceleration has commenced.

We use observations from the Goddard particle detectors on IMP 8 to investigate near-Earth interplanetary disturbances and the associated solar events in January–May 1997. We infer the presence of high-speed streams and ejecta (material in the solar wind associated with coronal mass ejections (CMEs) at the Sun) using depressions in the counting rate of >60 MeV/amu particles measured by the anti-coincidence guard. In addition to a sequence of high-speed streams, the guard detected four ejecta. A fifth ejecta (January 10–11), evident in solar wind observations, did not produce a particle decrease. During the study period there were five particle enhancements, indicating the occurrence of energetic CMEs at the Sun. All the particle enhancements were associated with halo CMEs observed by the SOHO LASCO coronagraphs. Five of the total of nine halo CMEs in this period impacted the Earth. None of the other ∼120 CMEs detected by LASCO did so. Only the four CMEs associated with traditional flare signatures were also associated with >10 MeV particle increases.

Measurements made by the Energetic Particle Anisotropy Spectrometer (EPAS) on the International Cometary Explorer (ICE) spacecraft show that relatively abrupt changes occur in energetic heavy ion properties near the periphery of the mass-loaded flow region surrounding comet P/Giacobini-Zinner. Corresponding features are observed both inbound and outbound from the comet. As the distance to the comet decreases, a change in the ion distribution takes place over approximately 5 minutes, indicative of flow deflection away from the comet-sun axis. Shortly afterwards, the ion angular distribution broadens over a further 6 minutes, indicative of a slowing of the ion bulk flow by about 100 km s−1. Taken together with magnetic field and plasma data, these observations are suggestive of a shock wave standing in the flow.

Using energetic ion data obtained by the EPAS experiment on the ICE spacecraft we investigate the three dimensional ion bulk flow in the mass loading region around the comet P/Giacobini-Zinner. We find relatively abrupt changes in flow speed (∼ 100 km s−1) at the boundaries between the pickup and mass loading regions. Flow deflections away from the comet tail of ∼15° are observed at the boundaries with peaks upto ∼30° inside the mass loading region. Ion energies of ∼300 keV are found inside the mass loading region which suggest the need to consider other ion acceleration mechanisms in addition to solar wind pickup.