Taylor State Merging at SSX: Experiment and Simulation

We describe experiments and simulations of dynamical merging with two Taylor state plasmas in a Swarthmore Spheromak Experiment (SSX) device. Taylor states are formed by magnetized plasma guns at opposite ends of the device. We performed experiments with Taylor states of both senses of magnetic helicity (right-handed twist or left-handed twist). We present results of both counter-helicity merging (one side left-handed, the other right-handed) and co-helicity merging (both sides left-handed). Experiments show significant ion heating, consistent with magnetic reconnection. We suggest that the merged, warm state could be a suitable target for future magneto-inertial fusion experiments. Magnetohydrodynamic simulations of these experiments reveal the structure of the final relaxed, merged state.

Analysis and modelling of glacial climate transitions using simple dynamical systems

Glacial climate variability is studied integrating simple nonlinear stochastic dynamical systems with palaeoclimatic records. Different models representing different dynamical mechanisms and modelling approaches are contrasted; model comparison and selection is based on a likelihood function, an information criterion as well as various long-term summary statistics. A two-dimensional stochastic relaxation oscillator model with proxy temperature as the fast variable is formulated and the system parameters and noise levels estimated from Greenland ice-core data. The deterministic part of the model is found to be close to the Hopf bifurcation, where the fixed point becomes unstable and a limit cycle appears. The system is excitable; under stochastic forcing, it exhibits noisy large-amplitude oscillations capturing the basic statistical characteristics of the transitions between the cold and the warm state. No external forcing is needed in the model. The relaxation oscillator is much better supported by the data than noise-driven motion in a one-dimensional bistable potential. Two variants of a mixture of local linear stochastic models, each associated with an unobservable dynamical regime or cluster in state space, are also considered. Three regimes are identified, corresponding to the different phases of the relaxation oscillator: (i) lingering around the cold state, (ii) rapid shift towards the warm state, (iii) slow relaxation out of the warm state back to the cold state. The mixture models have a high likelihood and are able to capture the pronounced time-reversal asymmetry in the ice-core data as well as the distribution of waiting times between onsets of Dansgaard–Oeschger events.

A Continuum of Sudden Stratospheric Warmings

The k-means cluster technique is used to examine 43 yr of daily winter Northern Hemisphere (NH) polar stratospheric data from the 40-yr ECMWF Re-Analysis (ERA-40). The results show that the NH winter stratosphere exists in two natural well-separated states. In total, 10% of the analyzed days exhibit a warm disturbed state that is typical of sudden stratospheric warming events. The remaining 90% of the days are in a state typical of a colder undisturbed vortex. These states are determined objectively, with no preconceived notion of the groups. The two stratospheric states are described and compared with alternative indicators of the polar winter flow, such as the northern annular mode. It is shown that the zonally averaged zonal winds in the polar upper stratosphere at ∼7 hPa can best distinguish between the two states, using a threshold value of ∼4 m s−1, which is remarkably close to the standard WMO criterion for major warming events. The analysis also determines that there are no further divisions within the warm state, indicating that there is no well-designated threshold between major and minor warmings, nor between split and displaced vortex events. These different manifestations are simply members of a continuum of warming events.

A climatic thermostat making Earth habitable

The mean surface temperature on Earth and other planets with atmospheres is determined by the radiative balance between the non-reflected incoming solar radiation and the outgoing long-wave black-body radiation from the atmosphere. The surface temperature is higher than the black-body temperature due to the greenhouse warming. Balancing the ice-albedo cooling and the greenhouse warming gives rise to two stable climate states. A cold climate state with a completely ice-covered planet, called Snowball Earth, and a warm state similar to our present climate where greenhouse warming prevents the total glaciation. The warm state has dominated Earth in most of its geological history despite a 30% fainter young Sun. The warming could have been controlled by a greenhouse thermostat operating by the temperature control of the weathering process depleting CO2 from the atmosphere. This temperature control has permitted life to evolve as early as the end of the heavy bombardment 4 billion years ago.

Control of local heat gain by vasomotor response of the hand

Finger blood flow (BF) was measured by venous occlusion plethysmography using mercury-in-Silastic strain gauges during immersion of one hand in hot water (raised by steps of 2 degrees C every 10 min from 35 to 43 degrees C), the other being a control (kept immersed in water at 35 degrees C). The measurements were made in three different thermal states on separate days: 1) cool-25 degrees C, 40% rh, esophageal temperature (Tes) = 36.64 +/- 0.10 degrees C; 2) warm-35 degrees C, 40% rh, Tes = 36.71 +/- 0.11 degrees C; and 3) hot-35 degrees C, 80% rh with the legs immersed in water at 42 degrees C, Tes = 37.26 +/- 0.11 degrees C. When water temperature was raised at 42 degrees C, Tes = 37.26 +/- 0.11 When water temperature was raised to 39–41 degrees C in the warm state, finger BF in the hand heated locally (BFw) decreased. When water temperature was raised to 43 degrees C, however, BFw returned to the control value. In the hot state, Tes rose steadily, reaching 37.90 +/- 0.12 degrees C at the end of the 50-min sessions. BF in the control finger also increased gradually during the session. BFw showed a tendency to decrease when water temperature was raised to 39 degrees C, but the change was not greater than that observed in the warm state. In the cool state, no such reduction in BFw was observed when water temperature was raised to 39–41 degrees C. On the contrary, BFw increased at water temperatures of 41–43 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)