Origins of the Ambient Solar Wind: Implications for Space Weather

The Sun's outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind. We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes such as turbulence, stream-stream interactions, and Coulomb collisions can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress -- in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue connected with a 2016 ISSI workshop on "The Scientific Foundations of Space Weather." 44 pages, 9 figure

Mechanisms underlying a thalamocortical transformation during active tactile sensation

During active somatosensation, neural signals expected from movement of the sensors are suppressed in the cortex, whereas information related to touch is enhanced. This tactile suppression underlies low-noise encoding of relevant tactile features and the brain’s ability to make fine tactile discriminations. Layer (L) 4 excitatory neurons in the barrel cortex, the major target of the somatosensory thalamus (VPM), respond to touch, but have low spike rates and low sensitivity to the movement of whiskers. Most neurons in VPM respond to touch and also show an increase in spike rate with whisker movement. Therefore, signals related to self-movement are suppressed in L4. Fast-spiking (FS) interneurons in L4 show similar dynamics to VPM neurons. Stimulation of halorhodopsin in FS interneurons causes a reduction in FS neuron activity and an increase in L4 excitatory neuron activity. This decrease of activity of L4 FS neurons contradicts the "paradoxical effect" predicted in networks stabilized by inhibition and in strongly-coupled networks. To explain these observations, we constructed a model of the L4 circuit, with connectivity constrained by in vitro measurements. The model explores the various synaptic conductance strengths for which L4 FS neurons actively suppress baseline and movement-related activity in layer 4 excitatory neurons. Feedforward inhibition, in concert with recurrent intracortical circuitry, produces tactile suppression. Synaptic delays in feedforward inhibition allow transmission of temporally brief volleys of activity associated with touch. Our model provides a mechanistic explanation of a behavior-related computation implemented by the thalamocortical circuit

Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Effects of temperature elevation on a field population of Acyrthosiphon svalbardicum (Hemiptera: Aphididae) on Spitsbergen

A manipulation experiment was carried out on a field population of the aphid Acyrthosiphon svalbardicum near Ny Ålesund, on the high arctic island of Spitsbergen, using cloches to raise temperature. An average rise in temperature of 2.8 deg. C over the summer season markedly advanced the phenology of both the host plant Dryas octopetala and the aphid. Advanced aphid phenology, with concomitant increases in reproductive output and survival, and successful completion of the life-cycle led to an eleven-fold increase in the number of overwintering eggs. Thermal budget requirements in day degrees above 0°C were calculated for key life-cycle stages of the aphid. Temperature data from Ny Ålesund over the past 23 years were used to calculate thermal budgets for the field site over the same period and these were compared with the requirements of the aphid. Each estimated thermal budget was then adjusted to simulate the effect of a +2, +4, and −2deg. C change in average temperature on aphid performance. This retrospective analysis (i) confirms that the life-cycle of A. svalbardicum is well suited to exploit higher summer temperatures, (ii) indicates that the annual success of local populations are sensitive to small changes in temperature and (iii) suggests that the aphid is living at the limits of its thermal range at Ny Ålesund based on its summer thermal budget requirements

Dispersive Nature of High Mach Number Collisionless Plasma Shocks: Poynting Flux of Oblique Whistler Waves

International audienceWhistler wave trains are observed in the foot region of high Mach number quasiperpendicular shocks. The waves are oblique with respect to the ambient magnetic field as well as the shock normal. The Poynting flux of the waves is directed upstream in the shock normal frame starting from the ramp of the shock. This suggests that the waves are an integral part of the shock structure with the dispersive shock as the source of the waves. These observations lead to the conclusion that the shock ramp structure of supercritical high Mach number shocks is formed as a balance of dispersion and nonlinearity. Shock plasma waves are ubiquitous in our Universe. They play an important role in redistributing kinetic energy in supersonic flow into plasma thermal energy and energetic particles. In particular, Earth's bow shock defines the boundary between the supersonic solar wind plasma and the subsonic region of the near-Earth space environment. Despite the absence of collisions, low Mach number collisionless shocks are treated as steady state fast magne-tosonic nonlinear waves or discontinuities in a dissipative MHD approximation. This allows one to determine the asymptotic state of the plasma and magnetic field across a shock, by using the Rankine-Hugoniot conservation laws. Any deviation from MHD such as two-fluid or kinetic descriptions results in the appearance of dispersive effects. When the Mach number of the shock increases past a critical Mach number, M crit , inferred in the frame of a MHD description, neither resistive nor viscous effects can provide sufficient dissipation to sustain a stationary shock transition [1]. For these so-called supercritical shocks the major dissipation mechanism is related to reflected ions [2–4] that require a kinetic description. It is well known that a subcritical shock has a nonlinear whistler wave train upstream of its front [5,6]. The major transition of such a dispersive shock, the ramp, behaves as the largest peak of the whistler precursor wave package [7–10]. The presence of whistler or fast magnetosonic precursor wave trains in supercritical shocks as well was experimentally established in [11–13]. These whistler waves have rather large amplitudes, and their role in energy transformation and redistribution between different particle populations and in the formation of the shock front structure is still an open question. Often the precursor waves are almost phase standing in the shock frame. However, their group velocity can still be greater than zero in the shock reference frame, which would allow energy flow in the form of Poynting flux to be emitted towards the upstream of the shock transition. In this Letter we address this problem and present the first direct measurement of the Poynting flux of the upstream whistler waves. It has been suggested that the shock front structure of quasiperpendicular supercritical shocks is formed similarly to that of subcritical shocks [14]. The observed dynamic features of shocks have also been studied extensively using computer particle-in-cell (PIC) or hybrid simulations, often with focus on the precursor wave activity and reflected ions [15–17]. From a kinetic viewpoint, however, it may be argued that the shock-reflected ions change the physical picture and that the principal scales, temporal and spatial, could be determined by the characteristics of the reflected ion population [18]. Upstream waves can then be generated due to counterstreaming ions and electrons in the shock front region, forming unstable particle distributions with respect to some wave modes [16,17,19]. While this is probably the case for some higher frequency waves, our analysis below leads to the conclusion that the source of the upstream low frequency whistler waves is related to the presence of the nonlinear ramp transition, emitting smaller scale dispersive waves towards the upstream flow. The existence of phase-standing upstream whistler waves depends on the value of the upstream flow speed Mach number relative to the phase velocity. If the Mach number of the shock does not exceed the whistler critical Mach number M w ¼ V w;max =V A ¼ 1=2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi m i =m e p cos Bn , the highest possible phase velocity, then phase-standing (linear) whistler wave trains can exist upstream of the shock [9,14]. In the above equation V A is the Alfvén speed and Bn is the angle between the upstream magnetic field and the normal to the shock. Below we establish the energy source of the waves by calculating the Poynting flux of the waves in the normal incidence frame (NIF) of the shock, using multisatellit

Extreme adaptive life-cycle in a high arctic aphid, Acyrthosiphon svalbardicum

1 The year-round biology of a high arctic aphid is described for the first time. 2 The life-cycle is shown to be genetically determined, and thus markedly different to temperate species where the observed polymorphism is governed primarily by external environmental cues. 3 The fundatrix, which emerges from the overwintering egg, gives birth directly to sexual morphs, a phenomenon previously undescribed in the Aphidinae. This process is essentially prevented in temperate aphids by an endogenous mechanism, the interval timer. 4 In addition to the sexual morphs, the fundatrix produces a small number of parthenogenetic individuals (viviparae) that give rise to a third generation. This last generation consists exclusively of oviparae and males that would increase the number of overwintering eggs provided there is sufficient thermal budget for them to mature and oviposit before conditions become adverse. 5 The position of particular morphs in the birth sequences of the second and third generations maximize the chances of survival in harsh conditions, whilst enhancing the likelihood that individuals from the third generation will add to the number of overwintering eggs. 6 Guaranteed egg production combined with an in-built flexibility to produce an extra generation in particularly favourable seasons, confer adaptations to the high arctic environment, and ideally suit this aphid to exploit elevated temperatures in an era of climate change

Identification of three previously unknown morphs of Acyrthosiphon svalbardicum Heikinheimo (Hemiptera: Aphididae) on Spitsbergen

Identification of three previously unknown morphs of Acyrthosiphon svalbardicum Heikinheimo (Hemiptera: Aphididae) on Spitsbergen