Mark Wilber

ABSTRACT Previous articles [1,2] have shown density holes are regions of density depletions below the solar wind level with scale length of an ion gyroradius. Density holes have steepened edges seen in both particles and fields. Here we present first observations of currents and wave polarizations associated with density holes. We show an example of current density determined from four point Cluster observations that has a value ~150 nA m-2. The waves are elliptically polarized and rotating in the sense of ions (left hand) in the plasma frame. The significance of these observations are still being studied. The waves appear to grow and steepen as the density holes are convected with the solar wind toward the Earth. The transient nature of density holes suggests that the temporal features could represent the different stages of nonlinear evolutionary processes that produce a shock-like structure.

The macroecological pattern known as Taylor's power law (TPL) represents the pervasive tendency of the variance in population density to increase as a power function of the mean. Despite empirical illustrations in systems ranging from viruses to vertebrates, the biological significance of this relationship continues to be debated. Here we combined collection of a unique dataset involving 11 987 amphibian hosts and 332 684 trematode parasites with experimental measurements of core epidemiological outcomes to explicitly test the contributions of hypothesized biological processes in driving aggregation. After using feasible set theory to account for mechanisms acting indirectly on aggregation and statistical constraints inherent to the data, we detected strongly consistent influences of host and parasite species identity over 7 years of sampling. Incorporation of field-based measurements of host body size, its variance and spatial heterogeneity in host density accounted for host id...

The terrestrial foreshock region plays an essential role in preprocessing the undisturbed solar wind en route to Earth's bow shock and magnetopause. Such preprocessing involves a rich array of plasma structures observed within the terrestrial ion foreshock, including short-duration large-amplitude magnetic structures (SLAMS), hot flow anomalies (HFAs), foreshock cavities and recently examined density holes. Much work has been done to characterize the macroscopic structure of these objects, as revealed in plasma and DC fields data, and to study higher frequency waves using wave spectra. Little has been done to date to examine high-frequency waveform data in the foreshock, and to place such measurements into context of the sub-structure of the features observed there. Here we present case studies of foreshock electrostatic waves observed by THEMIS, from a few tens to few thousand Hz. The THEMIS/EFI, FGM and SCM instruments provide long duration three-axis measurements of electric ...

Background/Question/Methods Species abundance distributions (SAD)s are a community level metric and their shape describes the commonness and rarity within a community. While the shape of the SAD is a result of biotic and abiotic processes, there is considerable debate whether a specific process will be reflected in the shape of the SAD. To this end, we examined how benthic macroinvertebrate community SADs responded to introduced trout in lakes in California's Sierra Nevada. In particular, we examined 42 paired trout and troutless streams, 10 paired trout and troutless lakes, and 379 and 361 independent trout and troutless lakes, respectively. For each lake and/or stream, we fit a flexible, neutral SAD (a discrete gamma distribution) and compared the shape parameters of this distribution between trout and troutless communities. Moreover, for the paired lakes and streams, we performed a strong test of SAD similarity in which we “trained” our SAD parameters on troutless communities...

Cold dense plasma sheet (CDPS) material is found along the flanks of the magnetopause during extended intervals of northward interplanetary magnetic field (IMF). The source for this population is the magnetosheath, and an un-resolved question is what mechanisms dominate in transporting, heating and accelerating it. Northward IMF is thought to be favorable for the Kelvin-Helmholtz instability (KHI), which can lead to turbulent mixing of plasma across the magnetopause. It is also thought favorable for high-latitude reconnection behind the cusps, which can permit direct injection and acceleration of particles. When newly-reconnected, cusp field lines on the day side contract due to magnetic tension, producing a characteristic velocity dispersion with faster particles arriving near Earth while field lines are still at higher latitudes (dE/dILAT~> 0). The low-altitude FAST spacecraft often observed overlapping energy dispersed ions (EDIs) during periods of northward IMF, leading to sp...

Plasma flowing at super-Alfvénic speeds has been observed in Earth's magnetosheath by particle and field experiments crossing the Earth's bow shock. Examination of several hundred crossings on the dayside downstream of the bow shock shows magnetosheath plasma flows at super-Alfvénic speeds ~90% of the time. We also find significant heat flux in the magnetosheath for both super- and sub-Alfvénic flows. Field-aligned current (J||) of several 10-6 A/m^2 is observed across the transition region and smaller J|| persists in the magnetosheath. The Boltzmann H function and dH/dt show while dH/dt ~0 in the solar wind, a large negative dH/dt ``spike'' is observed across the ramp. These observations will be discussed and compared to predicted features of current shock models.

It has been reported that sometimes the plasma sheet in the magnetotail consists of cold, dense ions, especially during northward interplanetary magnetic field (IMF) periods. However, the mechanism explaining the formation of the cold, dense plasma sheet is not yet understood. In this study we will present multi-spacecraft observations showing transition of plasma sheet from cold, dense to hot, tenuous status in the near-Earth magnetotail. The transition tends to occur very rapidly, in less than 1 min. Before the transition, the ion density is typically ~1 cm-3, and the temperature ~1 keV. After the transition, the ion density drops to ~0.3 cm-3, while the temperature increases to ~2.5 keV. The transition boundary can be quite far from the flank magnetopause and low latitude boundary layer (LLBL). For example, transition can be observed in the vicinity of the central plasma sheet (|Bx| < 10 nT) at ~(-17.8, -7.06, 1.71 RE GSM). One interesting feature is that the transition is usu...

It has been reported that nonlinear steepening of waves and fluctuations frequently occurs in the solar wind upstream of Earth's bow shock. The observation of nonlinear steepening provides important clues how shocks form in collisionless space plasmas. Previously, using Cluster multi-spacecraft measurements we reported that a compressional pulse formed at the edge of a density hole can steepen and develop into a shock-like structure. In that study we found that ion thermalization is insignificant during the nonlinear steepening and suggested that electrons may play an important role in the steepening. In this study we will focus on the interaction between electrons and waves. Investigation of electron phase space distributions shows that electron distributions in the downstream region of the steepened shock-like edge still have similar shape with the upstream solar wind distribution instead of becoming flat-top as observed in the downstream region of Earth's bow shock. On th...

Short-term (~20~s), order of magnitude depletions in foreshock density and magnetic field intensity have been reported recently. These density holes typically exhibit a greatly reduced solar wind beam, and diffuse-like energetic ions. There is increased wave activity, including enhanced ion cyclotron waves and whistler mode waves in their interiors, along with strong electrostatic waves near density minima. These are often, but not exclusively, observed in the presence of large amplitude ULF waves, or at the interface between quasi-parallel and quasi-perpendicular foreshock regions resulting from abrupt interplanetary magnetic field (IMF) rotations. While they are commonly seen in regions containing large-amplitude magnetic pulsations, the relationship between density holes and those SLAMS has been uncertain. A recently identified statistical sample of density holes occurring without associated SLAMS provides insight into their character and development. Here we present a few repres...

Cluster's highly eccentric polar orbit at around 4 Re perigee permits sampling of the ring current population, and gives its latitudinal profile evolution. The CIS (Cluster Ion Spectrometry) experiment onboard Cluster is capable of obtaining full three-dimensional ion distributions (from about 5eV/q to 40 keV/q) with one spacecraft spin time resolution (4 sec) and with mass-per-charge composition determination. The ion distribution functions obtained in situ by CIS during numerous perigee passes reveal new and very interesting structures on the ion spectrograms: the presence, for all the major ion species, of different features in energy bands, all along the CLUSTER pass inside the diffuse auroral zone. The highly structured pitch-angle distributions, which are energy dependent, confirm the presence of distinct populations. Moreover, the four point measurements of the magnetic field, obtained by the FGM experiment, allow the determination of the ring current density profile alon...

Data provided by the Cluster Ion Spectrometry (CIS) instruments are used to analyze Cluster crossings of the plasmasphere. The Cluster spacecraft orbit the Earth in a highly eccentric polar orbit at 4 Re perigee, and this permits them to sample the ring current, the radiation belts and the outer plasmasphere. CIS is capable of obtaining full three-dimensional ion distributions (about 0 to 40 keV/q) with a time resolution of one spacecraft spin (4 sec) and with mass-per-charge composition determination. In addition the CIS Retarding Potential Analyzer (RPA) allows more accurate measurements in the about 0 - 25 eV/q energy range, covering the plasmasphere energy domain. The low-energy ion distribution functions, obtained by CIS-RPA during the perigee passes, reveal new and interesting features, not reported by previous missions. The ion discrimination capability of CIS reveals how the density profile is different for each of the main ion species (H+, He+, O+): H+ and He+ present mostl...

The regions upstream of Earth's bow shock provide a natural laboratory for a variety of fundamental plasma processes, which include the interaction between particle populations that stream away from the shock with the solar wind and low frequency waves. Recent observations from Cluster and Double Star have exhibited novel short-duration (~4-20 s) density holes. These density holes are accompanied by a variety of plasma waves at ~ 0.01 Hz to several tens kHz. Preliminary analysis shows that the gross-scale structures appear fast magnetosonic-like, while in the interior ion cyclotron waves, whistler mode waves, and strong electrostatic waves are observed. A broad spectrum of electrostatic turbulence (a few kHz to ~50 kHz)is observed at the upstream edges of density holes.These waves are likely to play important roles in determining the holes' properties, including heating of the suprathermal particles seen. Some of the wave activity, especially at the upstream edges of density...

Different types of backstreaming ion distributions reported in the Earth s foreshock among them gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field have been frequently observed in association with large amplitude quasi-monochromatic right-hand mode low-frequency waves These waves reveal the existence of coherent wave-particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock It has been shown recently from a large data set including multi-spacecraft observations by Cluster that the gyrophase-bunched ion distributions are mainly produced by such a process from cyclotron-resonant field-aligned beams FABs observed just both at the edge of the gyrating ions region and the onset of the waves The parallel and perpendicular velocities of the gyrophase-bunched ion beams have been used to qualify a theoretical model of nonlinear pitch-angle trapping by the waves The d...

Larmor size transient structures with density depletions as large as 99% of ambient solar wind density levels occur commonly upstream of Earth's bow shock. Analysis of ~150 events show ``density holes'' have a mean duration of 17.9±10.4s but holes as short as 4s have been observed. The average fractional density depletion deltan/n inside the hole is ~0.68±0.14. The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions (ions and electrons) and waves show the steepened edge can behave like a shock, and measured local field geometries and Mach number support this view. The waves appear to steepen as the density holes are convected with the solar wind toward the Earth. Substantial heating of ions and electrons are observed inside density holes. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The mechanisms for produc...

Background/Question/Methods The ubiquity of disturbance in structuring ecological communities continues to motivate a search for generality in disturbance ecology. A better understanding of ecological perturbations and quantitative comparisons of their effects over multiple scales is required for both species-level and landscape-scale conservation efforts, however, few quantitative syntheses of cross-system comparisons of disturbance effects exist. Here we extend the Maximum Entropy Theory of Ecology (METE), an information entropy-based theory of macroecology, to disrupted and disturbed ecosystems. Despite explicitly not incorporating ecological interactions, METE has proven reliable for estimating the species-area relationship, endemics- area relationship, and species-abundance distribution in minimally disturbed, “steady state” ecosystems. As a first test of METE’s predictions for ecosystems with high levels of disturbance, we census plants in several ecosystems with known histori...

We report an observation by Cluster-CIS of an energetic (2--30~keV) upstream ion event presenting a clear double-peak spectrum. The lower-energy (E ˜~3.5~keV) peak is associated with an ion beam propagating along the magnetic field direction, while the higher-energy peak is associated with gyrating ions. Our analysis indicates that the gyrating ions had guiding centers on field lines downstream of the field-aligned component, but that both populations could be sampled simultaneously due to gyroradius effect. We find that downstream limit of the field-aligned beams is populated with protons having a speed 1.5 times the solar wind velocity, which is inconsistent with any known shock-related emission mechanisms. The spatial boundary separating the field-aligned beams from the gyrating ion guiding centers corresponds well with the ULF and intermediate ion foreshock boundary reported in previous studies. Like the field-aligned beams, the gyrating ions reported here have streaming speeds ...

The comparison of particle spectrograms from different altitudes in the cusp is important to our understanding of magnetospheric entry processes. In practice, however, this remains a difficult task. Low altitude measurements detect a smaller cusp that is crossed relatively quickly, while at mid altitudes the extent of the cusp is larger and the traversals much slower, blurring the distinction between temporal and spatial features and complicating conjunction studies with low altitude data. The Cluster multi-satellite mission allows the opportunity to better understand the features at mid altitudes, facilitating direct comparisons with concomitant low altitude observations by one or more of the DMSP spacecraft. Orbit and particle data from both missions have been searched over a nine-month period for near-simultaneous cusp region crossings. Model magnetic field line footprints at 100 km altitude give an indication of the quality of the magnetic conjunctions in space and time. Example...

ABSTRACT AimThe recently proposed maximum entropy theory of ecology predicts that all nested species-area relationships (SARs) will collapse to a single, universal curve that exhibits a decreasing log-log slope with increasing scale, suggesting that the power law form of the SAR is invalid at any scale. In this analysis we test the generality of this scale collapse behaviour and determine the scale at which approximate power law behaviour of the SAR is predicted to occur.LocationGlobal.Methods We use common SAR models to recursively upscale SARs to (1) look for scale collapse of SARs built from different community assumptions and (2) identify the scale at which approximate power law SARs are recovered. We explore four SAR models in which species abundance distributions and the spatial aggregation of species vary within and across scales according to observed patterns.ResultsWe show that scale collapse is a property of nested SARs, not of maximum entropy theory, and that many types of SARs can exhibit scale-collapsed curves that follow the general pattern of empirical communities. Moreover, we use the scale-collapsed curves to show that power law behaviour is not predicted to occur at the scale of most SAR studies and that variation in species spatial aggregation or the scaling of species abundance distributions are needed to produce power law SARs at realistic scales.Main conclusionsOur findings show that power law behaviour of nested SARs is rare and that communities displaying approximate power law SARs may exhibit predictable characteristics within and across scale, such as interspecific variation in aggregation. Our findings regarding the general rarity of power law behaviour suggests that careful consideration of the curvature of a SAR and the scale of an ecological community are necessary before using power law SARs to predict biodiversity across scales.

The polar cap region in the 30 to 60 minute period prior to the onset of the auroral substorm has been examined using global images from the Polar Ultraviolet Imager (UVI) to look for observational evidence of processes related to the substorm growth phase. In particular, the area of the polar cap has been measured to determine changes in its size in relation to the orientation of the interplanetary magnetic field (IMF). It was found that the size of the polar cap region increases during the growth phase even if the IMF has no southward component. Three phenomena have been observed to produce the increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense auroral precipitation in the polar cap region. The first phenomenon has been considered to be a result of the growth of the tail lobe magnetic field and the second is related to the thinning of the plasma sheet. Both of these have been suppo...

Global images of the polar cap region during the substorm growth phase by the Polar Ultraviolet Imager reveals evidence of the processes which are not completely explained by current models. In particular, it was found that size of the polar cap region increases during the growth phase even if the interplanetary magnetic field has no southward component. Three phenomena were observed to produce an increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense aurora[ precipitation in the polar region. Correlation of image intensities with in situ particle measurements from the FAST satellite are being conducted to study the three growth phase phenomena; and to help identify the source regions of the particles, the mechanisms involved in producing the auroral structures and what may be reducing the polar cap precipitation during the substorm growth phase.

... A variety of smaller, transient, phenomena can also be found in the upstream region (see Wilber et al., 2008, for an overview). ... Density holes ( [Parks et al., 2006] and [Wilber et al., 2008] ) are sub-minute duration events that share some features with HFAs. ...

During this part of the pass the CODIF ion spectrometer on spacecrafts 1 and 3 were in the retarding potential analyzer mode, which captures particles in the energy range 1.7­25 eV. Within the cusp, the EFW electric field instrument recorded a very low spacecraft potential ˜2 eV, indicating that the measured energies were close to the actual values. Inside the