Upward Acceleration Research Articles

An analysis of the rebound of the body in backward human running

Step frequency and energy expenditure are greater in backward running than in forward running. The differences in the motion of the centre of mass of the body associated with these findings are not known. These differences were measured here on nine trained subjects during backward and forward running steps on a force platform at 3-17 km h(-1). In contrast to previous reports, we found that the maximal upward acceleration of the centre of mass and the aerial phase, averaged over the whole speed range, are greater in backward running than in forward running (15.7 versus 13.2 m s(-2), P=1.9×10(-6) and 0.098 versus 0.072 s, P=2.4×10(-5), respectively). Opposite to forward running, the impulse on the ground is directed more vertically during the push at the end of stance than during the brake at the beginning of stance. The higher step frequency in backward running is explained by a greater mass-specific vertical stiffness of the bouncing system (499 versus 352 s(-2), P=2.3×10(-11)) resulting in a shorter duration of the lower part of the vertical oscillation of the centre of mass when the force is greater than body weight, with a similar duration of the upper part when the force is lower than body weight. As in a catapult, muscle-tendon units are stretched more slowly during the brake at the beginning of stance and shorten more rapidly during the push at the end of stance. We suggest that the catapult-like mechanism of backward running, although requiring greater energy expenditure and not providing a smoother ride, may allow a safer stretch-shorten cycle of muscle-tendon units.

Upward acceleration of magnetospheric ions by oscillatory centrifugal force

The paper addresses the issue of upward acceleration of ions along geomagnetic field lines. It has been shown that ion acceleration by electric field oscillations (formerly known as magnetic moment “pumping” or MMP) may be treated as a centrifugal acceleration mechanism. More precisely, the case in point is oscillatory centrifugal acceleration; this brings up the question on comparing the MMP with the centrifugal acceleration caused by the quasi-static magnetospheric convection field. It has been found that at high geomagnetic latitudes, the oscillatory centrifugal force is weaker or stronger than the centrifugal force of magnetospheric convection if the ratio of the electric field oscillation amplitude to the mean field is correspondingly lower or higher than \(\sqrt 2 \). Analysis of data from measurements and calculations of magnetospheric electric fields suggests that, contrary to current opinion, the oscillatory centrifugal force may be comparable to the centrifugal force of magnetospheric convection and even exceed it when strong global Pc5 pulsations are excited in the magnetosphere.

Tidal variability of salinity and velocity fields related to intense point‐source submarine groundwater discharges into the coastal ocean

Velocity and hydrography measurements were used to determine the tidal variability and detailed structure of an intense (~ 43,200 m3 d−1) point‐source submarine groundwater discharge from a spring located in the coastal ocean of the Yucatán peninsula, Mexico. Measurements were obtained during a dry season with a combination of towed, profiling, and fixed instrumentation. The goal of these observations was to understand the effects of tides on the velocity and salinity structure of the water column that determine mixing and dispersion processes at spatial scales from meters to kilometers. Tidally averaged flows were characterized by an upward jet flanked by asymmetric downdrafts. The asymmetry was caused by a ~ 0.1 m s−1 ambient horizontal flow and by the ~ 40° angle (relative to the vertical) of discharge. The spatially averaged salinity distributions exhibited lowest values at low tides and highest values at high tides. This was attributed to tidal variations of hydrostatic pressure acting on the spring outflow, which allowed ∼1 m s−1 vertical flows at low tides and < 0.05 m s−1 vertical velocities at high tides. The vertical momentum balance consisted of upward accelerations produced by buoyancy and inertia from the spring and downward accelerations from gravity. On the basis of this balance and of energetic considerations, the vertical extent of the spring discharge likely depends on the density contrast between ambient and discharge waters (buoyancy forces), the upward speed of the spring (inertia forces), and the depth of the water column (hydrostatic pressure force), all of which vary with tides.

(A306) Community Resilience and the Christchurch Earthquake: Best Laid Plans or Practise Made Perfect?

On February 22, 2011 at 12:51 pm an earthquake measuring 6.3 on the Richter scale struck the city of Christchurch, population 376,700 in the South Island of New Zealand. This followed a 7.3 magnitude earthquake in September 2010, but the shallowness (5km) and proximity of the February earthquake to the central city, resulted in far more devastation, with Modified Mercalli scores reaching ten in some areas and upward ground acceleration exceeding 2.4G. The application of the Coordinated Incident Management System (CIMS) routinely used by New Zealand Civil Defence agencies was swift, innovative and efficient, facilitating rapid deployment of local and international emergency teams and response resources. The effectiveness of this response was partially attributed to lessons learnt from the September earthquake which, with hindsight, was a practise for the more serious February event. The community response was equally remarkable, with standard approaches to measuring preparedness and resilience suggesting that community resilience in Canterbury was high. A number of initiatives by the New Zealand Ministry of Civil Defence and Emergency Management may have fostered some of this resilience,particularly community- based resilience-building projects initiated by the Regional Emergency Management Office on 2009 and 2010, supported by the Ministry of Civil Defence and Emergency Management. In addition, website education resources and media promotion (“Get Ready Get Thru”) and a travelling exhibition called “The Pandemic Roadshow” had been particularly well received and remembered by Canterbury residents. However, two key events provided an impetus for the Canterbury community to burnish its resilience. First, the Swine flu (AH1N1) pandemic in 2009 resulted in a greater awareness of public health in emergencies along with a doubling of neighbourhood support groups. Secondly, the September 2010 earthquake resulted in the establishment of the student army of volunteers and improvement of public information management. This presentation will describe the markers of community resilience following the Christchurch earthquake and discuss how such resilience can be fostered in communities where emergency preparedness is not recognised as a priority.

Wirewalker Dynamics

AbstractA wirewalker exploits the difference in vertical motion between a wire attached to a surface buoy and the water at the depth of a profiling body to provide the power to execute deep profiles: when the wire’s relative motion is upward, the profiler lets go; when it is downward, the profiler clamps on, and the weight attached at depth pulls the wire down, dragging the profiler downward against its buoyancy. The difference between the upward wire and profiler motion has to exceed the buoyancy-driven upward acceleration of the profiler body for this to work. Because the relative motion of the wire and water decreases as the surface is approached, the profiler might get stuck near the surface, especially when it is calm. However, two things mitigate this: 1) the system has a damped resonant response (~1.3 Hz), which induces relative motion between the buoy and water even at the surface; and 2) for waves too gentle to directly exceed the required acceleration, drag on the profiler can pull the clamped-together system down sufficiently that the buoy and wire without the profiler attached can suddenly release and bob upward faster than the profiler. For system parameters as estimated here, the latter requires submersion of less than 0.005 m below its equilibrium depth. Several such “bounces” can occur over a portion of the wave phase. These two effects explain why, in practice, the profiler does not stay long near the surface (although it does proceed downward a bit more slowly there).

Toward understanding the early stages of an impulsively accelerated coronal mass ejection

The expanding magnetic flux in coronal mass ejections (CMEs) often forms a cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of an impulsively accelerated CME on 25 March 2008, which displays a well-defined extreme ultraviolet (EUV) and white-light cavity of nearly circular shape already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion r(t) of the cavity are obtained in the whole height range of the main acceleration. We interpret them as the axis height and as a quantity proportional to the minor radius of a flux rope, respectively. The three-dimensional expansion of the CME exhibits two phases in the course of its main upward acceleration. From the first h and r data points, taken shortly after the onset of the main acceleration, the erupting flux shows an overexpansion compared to its rise, as expressed by the decrease of the aspect ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident with the impulsive rise of the acceleration and is followed by a phase of very gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~ 1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by flux conservation around a rising flux rope of decreasing axial current and by the addition of flux to a growing, or even newly forming,flux rope by magnetic reconnection. Further analysis will be required to decide which of these contributions is dominant. The data also suggest that the horizontal component of the impulsive cavity expansion (parallel to the solar surface) triggers the associated EUV wave, which subsequently detaches from the CME volume.

Muscle contributions to propulsion and support during running

Muscles actuate running by developing forces that propel the body forward while supporting the body’s weight. To understand how muscles contribute to propulsion (i.e., forward acceleration of the mass center) and support (i.e., upward acceleration of the mass center) during running we developed a three-dimensional muscle-actuated simulation of the running gait cycle. The simulation is driven by 92 musculotendon actuators of the lower extremities and torso and includes the dynamics of arm motion. We analyzed the simulation to determine how each muscle contributed to the acceleration of the body mass center. During the early part of the stance phase, the quadriceps muscle group was the largest contributor to braking (i.e., backward acceleration of the mass center) and support. During the second half of the stance phase, the soleus and gastrocnemius muscles were the greatest contributors to propulsion and support. The arms did not contribute substantially to either propulsion or support, generating less than 1% of the peak mass center acceleration. However, the arms effectively counterbalanced the vertical angular momentum of the lower extremities. Our analysis reveals that the quadriceps and plantarflexors are the major contributors to acceleration of the body mass center during running.

The Slapdown Phase in High-acceleration Records of Large Earthquakes

The 2008 Iwate-Miyagi Nairiku earthquake (M_w 6.9, M_(jma) 7.2) produced strong shaking throughout northern Honshu, Japan, with severe damage to buildings and extensive landslides. The shallow event occurred in southwestern Iwate Prefecture (39.03°N, 140.88°E, depth 8 km) on 13 June 2008 at 23:43:45 GMT (Japan Meteorological Agency 2008). This earthquake produced relatively high-frequency ground motions, which resulted in large values of peak ground acceleration (PGA). The surface accelerometer of the station IWTH25 of KiK-net, located 3 km southwest of the epicenter, produced one of the largest strong-motion values of PGA (4,278 cm/s^2 for the vector sum of the three components) ever recorded (http://www.kik.bosai.go.jp/kik/index_en.shtml). The new accelerometers installed in KiK-net last year have a recording range up to 4,000 cm/s^2, which made it possible to record such large ground motions near the source (http://www.kik.bosai.go.jp/kik/index_en.shtml). The sampling rate of the record of IWTH25 is 100 Hz (http://www.kik.bosai.go.jp/kik/index_en.shtml). The surface acceleration record at station IWTH25 shows an asymmetric amplification in the vertical components (Aoi et al. 2008). The upward vertical acceleration is much larger than the downward direction, although in the borehole record at a depth of 260 m at the same site, the upward and downward accelerations have symmetric amplitudes (Figure 1). On the other hand, the horizontal components do not show this asymmetric effect. This difference between the surface and borehole recordings for the vertical component implies a strong nonlinear amplification. In this paper, we will analyze these records and propose a mechanism to produce the large vertical accelerations. The predominance of large upward acceleration spikes is not unique to the Iwate-Miyagi Nairiku earthquake, so our proposed mechanism may be applicable to a number of large vertical acceleration records.

Muscle Contributions to Propulsion and Support During Running

Understanding how muscles propel our body forward and support our body weight during running is challenging because important variables, such as muscle forces, are generally not measurable. Muscle-driven simulations of walking have been analyzed to gain insight into muscle actions; however, simulating running presents new challenges due to high accelerations and forces. PURPOSE: To create a 3-D, muscle-driven simulation of running and quantify how muscles contribute to propulsion and support (i.e., the forward and upward accelerations of the body mass center). METHODS: Motion and ground reaction force data were measured on an adult male subject running at 3.9 m/s on a treadmill. OpenSim, open-source dynamic simulation software, was used to determine the muscle activations and forces needed to drive a 31 degree-of-freedom musculoskeletal model to track the experimentally measured motion. Contributions of individual muscles to propulsion and support were examined by independently perturbing each muscle's force during the simulated movement to calculate its contribution to the acceleration of the subject's mass center. This analysis focused on the propulsive phase of stance, the time period when a foot is on the ground and the body mass center is accelerating forward. RESULTS: Gastrocnemius, soleus, and hamstrings contributed most to propulsion, while the quadriceps resisted forward propulsion. Soleus, gastrocnemius, and quadriceps contributed the most to vertical support, while hamstrings acted to pull the body mass center downward. CONCLUSIONS: The ankle plantarflexors are a key muscle group for both propulsion and support of the body mass center during the propulsive phase of running. Gastrocnemius contributes more to propulsion and soleus more to support, which is consistent with their actions during walking. We have used the first 3-D, muscle-driven simulation of a running gait cycle to answer the fundamental question of which muscles propel the body forward and support body weight during running. Supported by NSF and Stanford Fellowships, and NIH Grant GM63495.

Implications of Thrown-Out Boulders for Earthquake Shaking

In the source areas of some large shallow earthquakes, we have found many dislodged boulders struck by severe ground shaking. Some boulders were located at quite distances from the former sockets, which remained undisturbed with surrounding clear edges. This fact indicates the possibility that vertically upward seismic acceleration exceeded the earth's gravity (1g). This phenomenon of upthrown boulders is investigated herein by examining the effects of waves, which emanate deep in the ground due to an earthquake, propagate through the ground and boulder, and reflect back to the ground, involving a variety of their interaction. An elastic dynamic analysis is carried out on the basis of a one-dimensional continuum model consisting of the ground and boulder. It is subjected to the input of the Ricker wave, which is intended to simulate an earthquake-generated wave, emanating from the bottom of the model ground. The upthrow of a boulder is taken to occur when the dynamic response at the bottom of the boulder satisfies certain conditions. It turns out that the possibility of upthrow occurrence is high when the period of the Ricker wave coincides with the fundamental period of the ground vibration. It leads to the conclusion that the upthrow takes place due to resonance in the response of the system of the ground and boulder to the external wave input. The upthrow possibility increases as the input acceleration increases. Trial is made of predicting the maximum acceleration and velocity of an earthquake, based on this consideration of the up throw phenomenon.

Head Accelerations during Particle Repositioning Manoeuvres

Benign paroxysmal positional vertigo (BPPV) due to canalithiasis can be treated with particle repositioning manoeuvres, which aim to evacuate trapped particles from the semicircular canals (SCC). The movement of particles within the SCC is affected by gravity as well as by the accelerations of the head during the manoeuvres. Moreover, as experienced by the particles, gravity is indistinguishable from an upward acceleration of the SCC in free space. We used a set of three orthogonal linear accelerometers to measure the net three-dimensional linear acceleration vector acting on the head during the Hallpike manoeuvre and Epley and Semont particle repositioning manoeuvres (which are used to treat posterior canal BPPV). The projection of the net acceleration vector onto the SCC planes showed that both the Epley and Semont manoeuvres approximated to stepwise, 360°, backward rotations in the plane of the targeted posterior canal. Angular velocity measurements however showed that the rotational component during the central stages of these two manoeuvres is opposite in direction. A simple model of head rotations during particle repositioning manoeuvres was created which showed good agreement to the linear acceleration measurements. Analysis of modelled and measured data identified that speed of movement during the Semont manoeuvre should be critical to its clinical success.

Benign Paroxysmal Positional Vertigo in Mountain Bikers

We evaluated 4 men who had benign paroxysmal positional vertigo (BPPV) that occured several hours after intensive mountain biking but without head trauma. The positional maneuvers in the planes of the posterior and horizontal canals elicited BPPV, as well as transitory nystagmus. This was attributed to both the posterior and horizontal semicircular canals (SCCs) on the left side in 1 patient, in these 2 SCCs on the right side in another patient, and to the right posterior SCC in the other 2 patients. The symptoms disappeared after physiotherapeutic maneuvers in 2 patients and spontaneously in the other 2 patients. Cross-country or downhill mountain biking generates frequent vibratory impacts, which are only partially filtered through the suspension fork and the upper parts of the body. Biomechanically, during a moderate jump, before landing, the head is subjected to an acceleration close to negative 1 g, and during impact it is subjected to an upward acceleration of more than 2g. Repeated acceleration-deceleration events during intensive off-road biking might generate displacement and/or dislocation of otoconia from the otolithic organs, inducing the typical symptoms of BPPV. This new cause of posttraumatic BPPV should be considered as an injury of minor severity attributed to the practice of mountain biking.

Kink‐induced Catastrophe in a Coronal Eruption

We investigate the kinking motion and its role in the eruption of a filament/cavity system that occurred on 2002 October 31. The evolution of the eruptive filament consists of four distinct phases. After an initial slow upward acceleration, the filament experiences a quasi-static phase exhibiting kinking motions of the filament axis. The kinking phase is followed by a sudden jump, coincident with the onset of the unkinking of the filament. The loss of equilibrium initiates a gradual relaxation phase at the end of which the filament reattains a similar unkinked configuration as its initial state. The filament/cavity structure, evident in the white-light observations, interacts with a large-scale coronal helmet streamer to the north, and material is observed to eject outward, aligned with a preexisting, low-density, dark channel that originally separated the northern helmet streamer from the southern streamer, where the dark cavity resides. The bulk of the filament, however, remains confined in the lower corona throughout the eruption along the channel. This suggests a partial eruption of the filament/cavity structure. The observations presented here manifest a catastrophic loss of equilibrium in response to the evolution of kinking motions in the filament activation.

Counter Movement Jump Characteristics Pre and Post Fatiguing Strength Exercise in Elderly Individuals

Failure to perform functional motor task in elderly individuals may be linked to muscle fatigue. Leg muscle peak power generation obtained during counter movement jump (CMJ) test has previously demonstrated close correlations to various functional motor tasks in old individuals. It is not known, however, which impact fatiguing leg strength exercise has on the various descriptive parameters of the CMJ-test. PURPOSE: To examine changes in CMJ parameters pre and post fatiguing leg strength exercise in elderly women and to correlate changes in jump height and mechanical CMJ output, respectively. METHODS: Eighteen elderly women (age 69.7 ±3.4 years, mean ± SD) were randomly assigned CMJ tests with or without prior fatiguing leg muscle exercise. The fatiguing protocol consisted of 5 sets of 10 RM incline leg press. Maximal CMJs were performed on a force plate (Kistler 9281 B). Peak upward acceleration (apeak) was measured in the eccentric phase, while maximal jump height (JH), peak power (Ppeak) and mean power (Pmean) were calculated for concentric phase. Ppeak was further decomposed into its velocity (Vppeak) and force (Fppeak) components. Furthermore, plantar flexor phase mean power (PF Pmean) was denned in the final part of the concentric phase where the position of the center of mass is lifted vertically above the starting position. A separate familiarization session was performed 1–4 days prior to the test session. Students paired t-test was used to evaluate differences in mean values pre versus post fatiguing leg exercise and linear correlation of JH-changes and descriptive parameters were evaluated by the Pearson product-moment method. RESULTS: A significant decrease in JH (−9.3%) was followed by decreases in Pmean (−3.8%), Vppeak (−2.9%), and apeak (−20.4%). When correlating the decrease in JH with the decreases in mechanical CMJ output the Pmean and PF Pmean explained a large proportion of the variation in JH-decrease (77.6% and 84.6%, respectively). Moderate correlations were observed for Ppeak, Vppeak, and apeak (range: 24.8% and 30.2%). CONCLUSIONS: Changes in PF Pmean and Pmean largely explained the decrease in JH. Fatigue related decrements in plantar flexor function and functional consequences of muscle fatigue should be evaluated further in elderly subjects.

Reply to Comments on “A turning point in auroral physics”

Haerendel's conviction (An Apparent Controversy in Auroral Physics, Eos, this issue) that a potential difference is responsible for the downward acceleration of auroral electrons is based on there being an upward acceleration of positive ions, caused apparently by a corresponding electric field. However, even if one accepts on face value this indication of an upward directed quasi‐static electric field, the inevitable presence of a downward directed field farther along the line, where the equipotentials must close and the ions are slowed down again, prevents the negative potential well having any net effect on the energy of either ions or electrons traversing it. The conviction is therefore unfounded.Stern (Origin of Auroral Arcs, Eos, this issue) is swayed to the same conclusion by the fact that in order to deliver the necessary power, electrons need to be forced through the magnetic mirror field. Again, though, even setting aside the fact that static fields cannot produce net acceleration, the conclusion overlooks the fact that any process leading to downward acceleration will accomplish the same feat. Similarly, his claim that a voltage drop has been shown to exist is false. The measurements in question were of electrons, not electric fields. They demonstrate that downward acceleration occurs but do not reveal the process responsible.

A two‐dimensional particle code simulation of inertial Alfvén waves and auroral electron acceleration

A two‐dimensional particle code used to simulate the propagation of inertial Alfvén waves down auroral field lines. The simulation domain is 20,000 km parallel to the magnetic field and some tens of kilometers perpendicular to the field. Electrons are tied to the magnetic field. Effects of a variable magnetic field and magnetic mirror force are included. A spatially narrow Alfvén pulse is launched by application of a variable potential with peak amplitude of 100 V at the top of the simulation domain. If the thickness of the pulse is less than the electron inertial length, the pulse tends to broaden as it propagates down the field line. The result is a parallel electric field considerably smaller than might be expected, assuming a wave whose perpendicular wavelength maps along magnetic field lines. In this model the strongest interaction between the electrons and the Alfvén wave occurs in the 3000 to 7000 km altitude range with an energy gain of the order 100 eV. The downward acceleration was strongest when the applied excitation consisted of a moving positive pulse followed by a negative pulse. Strongest upward acceleration was seen when the polarity was reversed. When the magnitude of the driving potential was doubled, there appeared to be some acceleration in the 9000 to 18,000 km altitude range and acceleration of some particles to 1.5 keV.

Wave impact loads: The role of the flip-through

The impact of waves upon a vertical, rigid wall during sloshing is analyzed with specific focus on the modes that lead to the generation of a flip-through [M. J. Cooker and D. H. Peregrine, “A model for breaking wave impact pressures,” in Proceedings of the 22nd International Conference on Coastal Engineering (ASCE, Delft, 1990), Vol. 2, pp. 1473–1486]. Experimental data, based on a time-resolved particle image velocimetry technique and on a novel free-surface tracking method [M. Miozzi, “Particle image velocimetry using feature tracking and Delaunay tessellation,” in Proceedings of the 12th International Symposium on Applications of Laser Techniques to Fluid Mechanics (2004)], are used to characterize the details of the flip-through dynamics while wave loads are computed by integrating the experimental pressure distributions. Three different flip-through modes are observed and studied in dependence on the amount and modes of air trapping. No air entrapment characterizes a “mode (a) flip-through,” engulfment of a single, well-formed air bubble is typical of a “mode (b)” event, while the generation of a fine-scale air-water mixing occurs for a “mode (c)” event. Upward accelerations of the flip-through jet exceeding 1500g have been measured and the generation/collapse process of a small air cavity is described in conjunction with the available pressure time histories. Predictions of the vertical pressure distributions made with the pressure-impulse model of Cooker and Peregrine [M. J. Cooker and D. H. Peregrine, “Pressure-impulse theory for liquid impact problems,” J. Fluid Mech. 297, 193 (1995)] show good agreement with the experimental data.

Circulating currents in magnetic flux tubes

There are several phenomena which require the application of a force in the axial direction of a magnetic flux tube, for example: solar plasma loops, where support of the plasma mass against gravity is required within those sections of the flux tube that have vertical components, the observed upward acceleration of the plasma in flux tubes forming the legs of quiescent prominences, and the acceleration of the fast solar wind. In this paper, we show that the origin of such axial forces is the magnetic pressure of an azimuthal magnetic field Bθ generated by currents flowing in circuits passing inside and outside the flux tube, and that the source of the EMF driving this circulating current is the difference between the temperature gradients inside and outside the flux tube, which generates a thermoelectric potential.

Tornadoes in Environments with Small Helicity and/or High LCL Heights

Abstract Recent studies have suggested that supercell tornado environments are usually associated with large 0–1-km storm-relative helicity (SRH) and relatively low lifting condensation levels (LCL heights). However, occasional tornadoes of significance occur in environments having characteristics that appear less supportive of supercell tornadoes, including small SRH values and/or relatively high LCL heights. Such tornadoes, whether associated with supercell or nonsupercell processes (more precisely termed mesocyclone and nonmesocyclone processes), present a challenge for forecasters. This empirical study uses a database of soundings derived from the Rapid Update Cycle model to examine thermodynamic characteristics of F1 and greater intensity tornado events associated with small SRH and/or high LCL heights. Results strongly suggest that many such tornado events are associated with steep lapse rates in the lowest few kilometers above ground. The low level of free convection heights, small convective inhibition, and sizable convective available potential energy below 3 km were also found to be of possible importance. These thermodynamic characteristics combined would likely reduce resistance to upward accelerations, potentially enhancing ascent for low-level parcels entering thunderstorm updrafts and, hence, low-level stretching. From prior research, if preexisting boundaries were available to provide surface vertical vorticity for stretching, such thermodynamic characteristics could be an important component of tornado events that involve nonmesocyclone processes. These same thermodynamic characteristics may also offer clues for the investigation of mesocyclone tornado events that do not fit well with accepted tornado forecasting parameters from prior studies.

Gas slug ascent through changes in conduit diameter: Laboratory insights into a volcano‐seismic source process in low‐viscosity magmas

Seismic signals generated during the flow and degassing of low‐viscosity magmas include long‐period (LP) and very‐long‐period (VLP) events, whose sources are often attributed to dynamic fluid processes within the conduit. We present the results of laboratory experiments designed to investigate whether the passage of a gas slug through regions of changing conduit diameter could act as a suitable source mechanism. A vertical, liquid‐filled glass tube featuring a concentric diameter change was used to provide canonical insights into potentially deep or shallow seismic sources. As gas slugs ascend the tube, we observe systematic pressure changes varying with slug size, liquid depth, tube diameter, and liquid viscosity. Gas slugs undergoing an abrupt flow pattern change upon entering a section of significantly increased tube diameter induce a transient pressure decrease in and above the flare and an associated pressure increase below it, which stimulates acoustic and inertial resonant oscillations. When the liquid flow is not dominantly controlled by viscosity, net vertical forces on the apparatus are also detected. The net force is a function of the magnitude of the pressure transients generated and the tube geometry, which dictates where, and hence when, the traveling pressure pulses can couple into the tube. In contrast to interpretations of related volcano‐seismic data, where a single downward force is assumed to result from an upward acceleration of the center of mass in the conduit, our experiments suggest that significant downward forces can result from the rapid deceleration of relatively small volumes of downward‐moving liquid.