Reconciling the electron counterstreaming and dropout occurrence rates with the heliospheric flux budget

Counterstreaming electrons (CSEs) are treated as signatures of closed magnetic flux, i.e., loops connected to the Sun at both ends. However, CSEs at 1 AU likely fade as the apex of a closed loop passes beyond some distance R, owing to scattering of the sunward beam along its continually increasing path length. The remaining antisunward beam at 1 AU would then give a false signature of open flux. Subsequent opening of a loop at the Sun by interchange reconnection with an open field line would produce an electron dropout (ED) at 1 AU, as if two open field lines were reconnecting to completely disconnect from the Sun. Thus EDs can be signatures of interchange reconnection as well as the commonly attributed disconnection. We incorporate CSE fadeout into a model that matches time-varying closed flux from interplanetary coronal mass ejections (ICMEs) to the solar cycle variation in heliospheric flux. Using the observed occurrence rate of CSEs at solar maximum, the model estimates R ∼ 8–10 AU. Hence we demonstrate that EDs should be much rarer than CSEs at 1 AU, as EDs can only be detected when the juncture points of reconnected field lines lie sunward of the detector, whereas CSEs continue to be detected in the legs of all loops that have expanded beyond the detector, out to R. We also demonstrate that if closed flux added to the heliosphere by ICMEs is instead balanced by disconnection elsewhere, then ED occurrence at 1 AU would still be rare, contrary to earlier expectations.

Role of coronal mass ejections in the heliospheric Hale cycle

The 11-year solar cycle variation in the heliospheric magnetic field strength can be explained by the temporary buildup of closed flux released by coronal mass ejections (CMEs). If this explanation is correct, and the total open magnetic flux is conserved, then the interplanetary-CME closed flux must eventually open via reconnection with open flux close to the Sun. In this case each CME will move the reconnected open flux by at least the CME footpoint separation distance. Since the polarity of CME footpoints tends to follow a pattern similar to the Hale cycle of sunspot polarity, repeated CME eruption and subsequent reconnection will naturally result in latitudinal transport of open solar flux. We demonstrate how this process can reverse the coronal and heliospheric fields, and we calculate that the amount of flux involved is sufficient to accomplish the reversal within the 11 years of the solar cycle.

Intermittent release of transients in the slow solar wind: 1. Remote sensing observations

The Heliospheric Imager (HI) instruments on board the STEREO spacecraft are used to analyze the solar wind during August and September 2007. We show how HI can be used to image the streamer belt and, in particular, the variability of the slow solar wind which originates inside and in the vicinity of the streamer belt. Intermittent mass flows are observed in HI difference images, streaming out along the extension of helmet streamers. These flows can appear very differently in images: plasma distributed on twisted flux ropes, V‐shaped structures, or “blobs.” The variety of these transient features may highlight the richness of phenomena that could occur near helmet streamers: emergence of flux ropes, reconnection of magnetic field lines at the tip of helmet streamers, or disconnection of open magnetic field lines. The plasma released with these transient events forms part of the solar wind in the higher corona; HI observations show that these transients are frequently entrained by corotating interaction regions (CIRs), leading to the formation of larger, brighter plasma structures in HI images. This entrainment is used to estimate the trajectory of these plasma ejecta. In doing so, we demonstrate that successive transients can be entrained by the same CIR in the high corona if they emanate from the same corotating source. Some parts of the streamers are more effective sources of transients than others. Surprisingly, evidence is given for the outflow of a recurring twisted magnetic structure, suggesting that the emergence of flux ropes can be recurrent.

Cusp observations during a sequence of fast IMF B-Z reversals

In recent years, a large number of papers have reported the response of the cusp to solar wind variations under conditions of northward or southward Interplanetary Magnetic Field (IMF) Z-component (BZ). These studies have shown the importance of both temporal and spatial factors in determining the extent and morphology of the cusp and the changes in its location, connected to variations in the reconnection geometry. Here we present a comparative study of the cusp, focusing on an interval characterised by a series of rapid reversals in the BZ-dominated IMF, based on observations from space-borne and ground-based instrumentation. During this interval, from 08:00 to 12:00 UT on 12 February 2003, the IMF BZ component underwent four reversals, remaining for around 30 min in each orientation. The Cluster spacecraft were, at the time, on an outbound trajectory through the Northern Hemisphere magnetosphere, whilst the mainland VHF and Svalbard (ESR) radars of the EISCAT facility were operating in support of the Cluster mission. Both Cluster and the EISCAT were, on occasion during the interval, observing the cusp region. The series of IMF reversal resulted in a sequence of poleward and equatorward motions of the cusp; consequently Cluster crossed the high altitude cusp twice before finally exiting the dayside magnetopause, both times under conditions of northward IMF BZ. The first magnetospheric cusp encounter, by all four Cluster spacecraft, showed reverse ion dispersion typical of lobe reconnection; subsequently, Cluster spacecraft 1 and 3 (only) crossed the cusp for a second time. We suggest that, during this second cusp crossing, these two spacecraft were likely to have been on newly closed field lines, which were first reconnected (opened) at low latitudes and later reconnected again (re-closed) poleward of the northern cusp.

Transpolar voltage and polar cap flux during the substorm cycle and steady convection events

Transpolar voltages observed during traversals of the polar cap by the Defense Meteorological Satellite Program (DMSP) F-13 spacecraft during 2001 are analyzed using the expanding-contracting polar cap model of ionospheric convection. Each of the 10,216 passes is classified by its substorm phase or as a steady convection event (SCE) by inspection of the AE indices. For all phases, we detect a contribution to the transpolar voltage by reconnection in both the dayside magnetopause and in the crosstail current sheet. Detection of the IMF influence is 97% certain during quiet intervals and >99% certain during substorm/SCE growth phases but falls to 75% in substorm expansion phases: It is only 27% during SCEs. Detection of the influence of the nightside voltage is only 19% certain during growth phases, rising during expansion phases to a peak of 96% in recovery phases: During SCEs, it is >99%. The voltage during SCEs is dominated by the nightside, not the dayside, reconnection. On average, substorm expansion phases halt the growth phase rise in polar cap flux rather than reversing it. The main destruction of the excess open flux takes place during the 6- to 10-hour interval after the recovery phase (as seen in AE) and at a rate which is relatively independent of polar cap flux because the NENL has by then retreated to the far tail. The best estimate of the voltage associated with viscous-like transfer of closed field lines into the tail is around 10 kV.

Helices, chirality and interpenetration: the versatility and remarkable interconversion of silver-copper cyanide frameworks

The structural transformations between cesium silver-copper cyanides under modest conditions, both in solution and in the solid state, are described. Three new cesium silver(I) copper(I) cyanides with three-dimensional (3-D) framework structures were prepared as single crystals from a one-pot reaction initially heated under hydrothermal conditions. The first product to appear, Cs3Ag2Cu3(CN)(8) (I), when left in contact with the supernatant produced CsAgCu(CN)(3) (II) and CsAgCu(CN)(3)center dot 1/3H(2)O (III) over a few months via a series of thermodynamically controlled cascade reactions. Crystals of the hydrate (III) can be dehydrated to polycrystalline CsAgCu(CN)(3) (II) on heating at 100 degrees C in a remarkable solid-state transformation involving substantial breaking and reconnection of metal-cyanide linkages. Astonishingly, the conversion between the two known polymorphs of CsAg2Cu(CN)(4), which also involves a major change in connectivity and topology, occurs at 180 degrees C as a single-crystal to single-crystal transformation. Structural features of note in these materials include the presence of helical copper-cyanide chains in (I) and (II), which in the latter compound produce a chiral material. In (II) and (III), the silver-copper cyanide networks are both self- and interpenetrating, features also seen in the known polymorphs of CsAg2Cu(CN)(4).

Suprathermal electron flux peaks at stream interfaces: Signature of solar wind dynamics or tracer for open magnetic flux transport on the Sun?

The high variability of the intensity of suprathermal electron flux in the solar wind is usually ascribed to the high variability of sources on the Sun. Here we demonstrate that a substantial amount of the variability arises from peaks in stream interaction regions, where fast wind runs into slow wind and creates a pressure ridge at the interface. Superposed epoch analysis centered on stream interfaces in 26 interaction regions previously identified in Wind data reveal a twofold increase in 250 eV flux (integrated over pitch angle). Whether the peaks result from the compression there or are solar signatures of the coronal hole boundary, to which interfaces may map, is an open question. Suggestive of the latter, some cases show a displacement between the electron and magnetic field peaks at the interface. Since solar information is transmitted to 1 AU much more quickly by suprathermal electrons compared to convected plasma signatures, the displacement may imply a shift in the coronal hole boundary through transport of open magnetic flux via interchange reconnection. If so, however, the fact that displacements occur in both directions and that the electron and field peaks in the superposed epoch analysis are nearly coincident indicate that any systematic transport expected from differential solar rotation is overwhelmed by a random pattern, possibly owing to transport across a ragged coronal hole boundary.

Pre-CME onset fuses: do the STEREO heliospheric imagers hold the clues to the CME onset process?

Understanding the onset of coronal mass ejections (CMEs) is surely one of the holy grails of solar physics today. Inspection of data from the Heliospheric Imagers (HI), which are part of the SECCHI instrument suite aboard the two NASA STEREO spacecraft, appears to have revealed pre-eruption signatures which may provide valuable evidence for identifying the CME onset mechanism. Specifically, an examination of the HI images has revealed narrow rays comprised of a series of outward-propagating plasma blobs apparently forming near the edge of the streamer belt prior to many CME eruptions. In this pilot study, we inspect a limited dataset to explore the significance of this phenomenon, which we have termed a pre-CME ‘fuse’. Although, the enhanced expulsion of blobs may be consistent with an increase in the release of outward-propagating blobs from the streamers themselves, it could also be interpreted as evidence for interchange reconnection in the period leading to a CME onset. Indeed, it is argued that the latter could even have implications for the end-of-life of CMEs. Thus, the presence of these pre-CME fuses provides evidence that the CME onset mechanism is either related to streamer reconnection processes or the reconnection between closed field lines in the streamer belt and adjacent, open field lines. We investigate the nature of these fuses, including their timing and location with respect to CME launch sites, as well as their speed and topology.

How is open solar magnetic flux lost over the solar cycle?

The Sun's open magnetic field, magnetic flux dragged out into the heliosphere by the solar wind, varies by approximately a factor of 2 over the solar cycle. We consider the evolution of open solar flux in terms of a source and loss term. Open solar flux creation is likely to proceed at a rate dependent on the rate of photospheric flux emergence, which can be roughly parameterized by sunspot number or coronal mass ejection rate, when available. The open solar flux loss term is more difficult to relate to an observable parameter. The supersonic nature of the solar wind means open solar flux can only be removed by near-Sun magnetic reconnection between open solar magnetic field lines, be they open or closed heliospheric field lines. In this study we reconstruct open solar flux over the last three solar cycles and demonstrate that the loss term may be related to the degree to which the heliospheric current sheet (HCS) is warped, i.e., locally tilted from the solar rotation direction. This can account for both the large dip in open solar flux at the time of sunspot maximum as well as the asymmetry in open solar flux during the rising and declining phases of the solar cycle. The observed cycle-to-cycle variability is also well matched. Following Sheeley et al. (2001), we attribute modulation of open solar flux by the degree of warp of the HCS to the rate at which opposite polarity open solar flux is brought together by differential rotation.

On the importance of interplanetary magnetic field (By)on polar cap patch formation

A number of poleward moving events were observed between 1130 and 1300 UT on 11 February 2004, during periods of southward interplanetary magnetic field (IMF), while the steerable antenna of the European Incoherent Scatter (EISCAT) Svalbard radar (ESR)and the Tromsø VHF radar pointed nearly northward at low elevation. In this interval, simultaneous SuperDARN CUTLASS Finland radar measurements showed poleward moving radar aurora forms (PMRAFs) which appeared very similar to the density enhancements observed by the ESR northward pointing antenna. These events appeared quasiperiodically with a period of about 10 min. Comparing the observations from the above three radars, it is inferred that there is an almost one‐to‐one correspondence between the poleward moving plasma concentration enhancements (PMPCEs) observed by the ESR and the VHF radar and the PMRAFs measured by the CUTLASS Finland radar. These observations are consistent with the interpretation that the polar cap patch material was generated by photoionization at subauroral latitudes and that the plasma was structured by bursts of magnetopause reconnection giving access to the polar cap. There is clear evidence that plasma structuring into patches was dependent on the variability in IMF |By|. The duration of these events implies that the average evolution time of the newly opened flux tubes from the subauroral region to the polar cap was about 33 min.

Direct observations of the evolution of polar cap ionization patches

Patches of ionization are common in the polar ionosphere where their motion and associated density gradients give variable disturbances to High Frequency (HF) radio communications, over-the-horizon radar location errors, and disruption and errors to satellite navigation and communication. Their formation and evolution are poorly understood, particularly under disturbed space weather conditions. We report direct observations of the full evolution of patches during a geomagnetic storm, including formation, polar cap entry, transpolar evolution, polar cap exit, and sunward return flow. Our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a “tongue” of ionization (TOI).

Solar origin of heliospheric magnetic field inversions: evidence for coronal loop opening within pseudostreamers

The orientation of the heliospheric magnetic field (HMF) in near‒Earth space is generally a good indicator of the polarity of HMF foot points at the photosphere. There are times, however, when the HMF folds back on itself (is inverted), as indicated by suprathermal electrons locally moving sunward, even though they must ultimately be carrying the heat flux away from the Sun. Analysis of the near‒Earth solar wind during the period 1998–2011 reveals that inverted HMF is present approximately 5.5% of the time and is generally associated with slow, dense solar wind and relatively weak HMF intensity. Inverted HMF is mapped to the coronal source surface, where a new method is used to estimate coronal structure from the potential‒field source‒surface model. We find a strong association with bipolar streamers containing the heliospheric current sheet, as expected, but also with unipolar or pseudostreamers, which contain no current sheet. Because large‒scale inverted HMF is a widely accepted signature of interchange reconnection at the Sun, this finding provides strong evidence for models of the slow solar wind which involve coronal loop opening by reconnection within pseudostreamer belts as well as the bipolar streamer belt. Occurrence rates of bipolar‒ and pseudostreamers suggest that they are equally likely to result in inverted HMF and, therefore, presumably undergo interchange reconnection at approximately the same rate. Given the different magnetic topologies involved, this suggests the rate of reconnection is set externally, possibly by the differential rotation rate which governs the circulation of open solar flux.

Near-Earth initiation of a terrestrial substorm

Despite the characterization of the auroral substorm more than 40 years ago, controversy still surrounds the processes triggering substorm onset initiation. That stretching of the Earth's magnetotail following the addition of new nightside magnetic flux from dayside reconnection powers the substorm is well understood; the trigger for explosive energy release at substorm expansion phase onset is not. Using ground-based data sets with unprecedented combined spatial and temporal coverage, we report the discovery of new localized and contemporaneous magnetic wave and small azimuthal scale auroral signature of substorm onset. These local auroral arc undulations and magnetic field signatures rapidly evolve on second time scales for several minutes in advance of the release of the auroral surge. We also present evidence from a conjugate geosynchronous satellite of the concurrent magnetic onset in space as the onset of magnetic pulsations in the ionosphere, to within technique error. Throughout this time period, the more poleward arcs that correspond to the auroral oval which maps to the central plasma sheet remain undisturbed. There is good evidence that flows from the midtail crossing the plasma sheet can generate north-south auroral structures, yet no such auroral forms are seen in this event. Our observations present a severe challenge to the standard hypothesis that magnetic reconnection in stretched magnetotail fields triggers onset, indicating substorm expansion phase initiation occurs on field lines that are close to the Earth, as bounded by observations at geosynchronous orbit and in the conjugate ionosphere.

Polar cap patch segmentation of the tongue of ionization in the morning convection cell

Two types of poleward moving plasma concentration enhancements (PMPCEs) were observed during a sequence of pulsed reconnection events, both in the morning convection cell: Type L (low density) was associated with a cusp flow channel and seems likely to have been produced by ionization associated with particle precipitation, while Type H (high density) appeared to originate from the segmentation of the tongue of ionization by the processes which produced the Type L events. As a result, the Type L and Type H PMPCEs were interspersed, producing a complex density structure which underlines the importance of cusp flow channels as a mechanism for segmenting and structuring electron density in the cusp and shows the necessity of differentiating between at least two classes of electron density patches.

Comparative Cluster/Double Star observations of the high and low latitude dayside magnetopause

The launch of the Double Star mission has provided the opportunity to monitor events at distinct locations on the dayside magnetopause, in coordination with the quartet of Cluster spacecraft. We present results of two such coordinated studies. In the first, 6 April 2004, both Cluster and the Double Star TC-1 spacecraft were on outbound transits through the dawn-side magnetosphere. Cluster observed northward moving FTEs with +/- polarity, whereas TC-1 saw -/+ polarity FTEs. The strength, motion and occurrence of the FTE signatures changes somewhat according to changes in IMF clock angle. These observations are consistent with ongoing reconnection on the dayside magnetopause, resulting in a series of flux transfer events (FTEs) seen both at Cluster and TC-1. The observed polarity and motion of each FTE signature advocates the existence of an active reconnection region consistently located between the positions of Cluster and TC-1, lying north and south of the reconnection line, respectively. This scenario is supported by the application of a model, designed to track flux tube motion, to conditions appropriate for the prevailing interplanetary conditions. The results from the model confirm the observational evidence that the low-latitude FTE dynamics is sensitive to changes in convected upstream conditions. In particular, changing the interplanetary magnetic field (IMF) clock angle in the model predicts that TC-1 should miss the resulting FTEs more often than Cluster, as is observed. For the second conjunction, on the 4 Jan 2005, the Cluster and TC-1 spacecraft all exited the dusk-side magnetosphere almost simultaneously, with TC-1 lying almost equatorial and Cluster at northern latitudes at about 4 RE from TC-1. The spacecraft traverse the magnetopause during a strong reversal in the IMF from northward to southward and a number of magnetosheath FTE signatures are subsequently observed. One coordinated FTE, studied in detail by Pu et al, [this issue], carries and inflowing energetic electron population and shows a motion and orientation which is similar at all spacecraft and consistent with the predictions of the model for the flux tube dynamics, given a near sub-solar reconnection line. This event can be interpreted either as the passage of two parallel flux tubes arising from adjacent x-line positions, or as a crossing of a single flux tube at different positions.