Sudden Impulse Research Articles

Probing the relationship between electromagnetic ion cyclotron waves and plasmaspheric plumes near geosynchronous orbit

Plasmaspheric plumes created during disturbed geomagnetic conditions have been suggested as a major cause of increased occurrences of electromagnetic ion cyclotron (EMIC) waves at these times. We have catalogued occurrences of strong Pc1 EMIC waves from 1996 through 2003 at three automated geophysical observatories operated by the British Antarctic Survey at auroral zone latitudes in Antarctica (L = 6.28, 7.68, and 8.07) and have compared them to the occurrence of plasmaspheric plumes in space, using simultaneous data from the Magnetospheric Plasma Analyzer on the Los Alamos National Laboratory 1990–095 spacecraft, in geosynchronous orbit at the same magnetic longitude. A superposed epoch analysis of these data was conducted for several categories of disturbed geomagnetic conditions, including magnetic storms, high‐speed streams, and storm sudden commencements. We found only a weak correspondence between the occurrence of strong Pc1 waves observed on the ground and either plasmaspheric plumes or intervals of extended plasmasphere at geosynchronous orbit before, during, or after the onset of any of these categories. Strong Pc1 activity peaked near or slightly after local noon during all storm phases, consistent with equatorial observations by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer satellite at these L shells. The highest Pc1 occurrence probability was at or 1–2 days before storm onset and during the late recovery phase. Occurrence was lowest during the early recovery phase, consistent with the decrease in solar wind pressure often seen at this time. The peak at onset is consistent with earlier observations of waves in the outer magnetosphere stimulated by sudden impulses and magnetospheric compressions.

ISSLS Prize Winner: How Loading Rate Influences Disc Failure Mechanics

Mechanically induced disruption and subsequent microscopic investigation of lumbar intervertebral discs following a previously published testing protocol, but using a much higher rate of loading. To explore if loading rate affects the internal disruption mechanics of lumbar intervertebral discs. The failure mechanics of some bone-ligament-bone constructs vary with the rate of tensile load application. Like many ligaments, recent reports indicate that the mechanical response of the disc wall varies with strain-rate. It is possible that the internal failure mechanics of the disc wall also varies with strain-rate. Nuclear pressurization was used to deliver sudden pressure impulses directly to the nucleus of ovine lumbar motion segments. Pressure impulses were delivered to 12 neutrally positioned motion segments, and 15 motion segments held at 7° flexion. Aside from loading rate, testing was conducted in the same manner as 2 previously published studies that employed a gradual nuclear pressurization regime. Following testing, the internal damage resulting to each disc was analyzed using micro-CT and serial microscopy in tandem. Radial tears of the medioposterior disc wall were the most frequent cause of disc failure. In most cases, radial tears involved a combination of annular and endplate disruption: Neutrally positioned discs frequently suffered tears within the superior cartilaginous endplate adjacent to the transition zone and/or inner anulus. Flexed discs frequently suffered tears adjacent to the outer anulus at the cartilaginous/vertebral endplate junction, or within the vertebral endplate. Both groups frequently suffered endplate tears adjacent to the mid anulus at the inferior cartilaginous/vertebral endplate junction. The internal morphologies of the disc disruptions created in this study using high strain-rate impulse pressurization differed significantly from those documented previously for both neutrally positioned and flexed discs subjected to gradual low strain-rate pressurization. These morphologic differences show that the internal failure mechanics of lumbar intervertebral discs vary with the rate of internal radial load application.

An empirical relationship between coronal mass ejection initial speed and solar wind dynamic pressure

Interplanetary shocks that precede coronal mass ejections (CMEs) are mainly responsible for sudden impulses, which are characterized by a simple step‐like increase in the horizontal H component. Such a magnetic field change has been explained as a compression of the magnetosphere by the passage of a sudden increase in the solar wind dynamic pressure. Strong compression of the dayside magnetopause could cause geosynchronous satellites to be exposed to solar wind environments where large fluctuations of the interplanetary magnetic field and highly energetic particles are present. In this study, we chose 26 event pairs consisting of a type II burst/CME occurring in conjunction with a sudden commencement/sudden impulse (SC/SI) whose solar wind, and Earth magnetic field data are available. We then investigated relationships among three physical properties (kinetic energy, directional parameter, and speed) of near‐Sun CMEs, solar wind dynamic pressure, and SC/SI amplitude. As a result, we found that (1) the CME speed is more highly correlated with SC/SI amplitude than its kinetic energy and direction parameter; (2) by adopting the empirical relationship between solar wind dynamic pressure and amplitude of symmetric H (a steplike increase in the horizontal H component at low latitude), we could derive an empirical formula for the relationship between solar wind dynamic pressure near the Earth and the CME speed; (3) the CME speed has a linear relationship with the difference of magnetopause locations derived by using the model of Shue et al. (1998) at the subsolar point before and after the shock arrivals; (4) a fast CME greater than 1600 km s−1 could be a driver of the magnetopause crossing of a spacecraft at geosynchronous orbit. Our results show that the CME speed is an important parameter for early prediction of geosynchronous magnetopause crossing.

Transient dipolar sound wave in a viscous compressible fluid

The dipolar sound wave generated by a sudden impulse in an unbounded viscous compressible fluid is studied on the basis of the linearized Navier–Stokes equations. Due to viscosity the spherical wavefront is diffuse with a width which grows with the square root of time. The wavefront is followed by a spherical shell of static potential flow. The shell itself surrounds an expanding spherical region of viscous flow. At long times the energy in each of the three regions is of comparable magnitude, and decays with a t − 3 / 2 power law.

Interplanetary shock characteristics and associated geosynchronous magnetic field variations estimated from sudden impulses observed on the ground

Interplanetary (IP) shocks disturb the magnetosphere‐ionosphere system resulting in geosynchronous magnetic field changes and sudden impulses observed by ground‐based magnetometers. We extend the implications of a previous statistical study and show that sudden impulses (SIs) can be used to estimate some parameters at the L1 point and geosynchronous orbit, including the change of the square root of solar wind dynamic pressure across the shock and the associated geosynchronous magnetic field changes near the subsolar region. It should be pointed out that the relationship between magnetospheric field change and SIs amplitude and the solar wind dynamic pressure is not a single valued one, but a statistical relationship is useful in cases when interplanetary data are not available. Empirical formulae deduced from observations can be used to estimate certain IP shock characteristics and geosynchronous magnetic field changes from sudden impulse data observed on the ground, with the prediction efficiency as high as 90% and 86%, respectively. These estimates are useful for studying historic, pre‐space era data or if the L1 and geosynchronous data are not available at some future time.

Loss of momentum in a viscous compressible fluid due to no-slip boundary condition at one or two planar walls

The loss of fluid momentum due to friction at one or two planar walls bounding a viscous compressible fluid is studied as a function of time for the situation where the flow is due to a sudden impulse applied at a selected point in initially quiescent fluid. The no-slip condition is assumed to hold at the walls, and the initial impulse is assumed to be sufficiently small, so that the linearized Navier-Stokes equations may be used. When the initial impulse is directed parallel to the walls the time-dependent total fluid momentum is independent of compressibility and volume viscosity. For initial impulse directed perpendicular to the walls an echoing effect, corresponding to sound bouncing between the two walls, is observed.

High‐latitude reconnection effect observed at the dayside dip equator as a precursor of a sudden impulse

Geomagnetic sudden impulses (SI) observed at the dayside dip equator normally show a decrease and then an increase in the H magnetic field component, i.e., a preliminary reverse impulse (PRI) followed by a main impulse. Using global geomagnetic field measurements, we examine an unusual SI event observed at the dayside dip equator, which shows a clear precursor ∼1 min before the PRI onset. The precursor was observed simultaneously at both the dayside dip equator and in the southern polar region but was not observed at all in the northern polar region. The global ground variations after the PRI onset were, however, consistent with a conventional SI model of the magnetospheric response to a sudden enhancement of the solar wind dynamic pressure. Considering that the interplanetary magnetic field Bx, By, and Bz components were positive with the By component dominant for this event, we suggest that the SI precursor was caused by high‐latitude magnetic reconnection that occurred only (or first) in the dawn quadrant of the southern hemisphere, the effect of which has for the first time been clearly identified at the dayside dip equator. This implies that electric field effects occurring in the polar ionosphere due to magnetopause reconnection may be rapidly monitored at the dayside dip equator. In addition, we argue that the quasi‐simultaneous (time difference is less than 10 s) appearance of the disturbance fields both in the polar region and at the dip equator confirmed in this study is extremely important for theoretically explaining the transmission of the polar electric field to the dayside dip equator, because different theories give different travel times for a disturbance field propagating from polar region to the dip equator. Our observations lend strong evidence for validity of the waveguide model.

Echoing in a viscous compressible fluid confined between two parallel plane walls

The dynamics of a viscous compressible fluid, confined between two parallel plane walls and excited by a sudden impulse transverse to the walls, is studied on the basis of the linearized Navier–Stokes equations. It is shown that the time-dependent flow depends strongly on the sound velocity and on the shear and volume viscosity. Under favourable conditions an echoing effect can be observed, with a sound pulse bouncing many times between the two plates. The velocity correlation function of a Brownian particle immersed in the fluid is calculated in point approximation. It shows a similar strong dependence on fluid properties.

Plasma sheet changes caused by sudden enhancements of the solar wind pressure

Using Geotail data, we have studied the response of the plasma sheet in the magnetotail to sudden enhancements of the solar wind pressure. We have selected three events in which there were no indications of substorm expansions around the corresponding sudden impulses (SIs) on the ground. It is shown that the ion number density and temperature, as well as the ion pressure, increase significantly due to the plasma sheet compression. The specific entropy does not significantly change, suggesting that the plasma sheet compression is nearly adiabatic. This plasma behavior is in contrast to the nonadiabatic substorm‐associated processes. Hence examining the correlation between the number density and the temperature, and especially the specific entropy is critical for distinguishing between substorm‐associated and SI‐associated changes of the plasma sheet. The northward magnetic field in the plasma sheet also increases, or at least does not decrease, associated with the compression, implying that the lateral magnetotail compression suppresses the triggering of magnetic reconnection and therefore substorm expansion onsets.

A search for dayside geomagnetic Pi2 pulsations in the CHAMP low‐Earth‐orbit data

Pi2 pulsations are typically observed at night and often during local daytime at low‐latitude ground stations. Their daytime occurrence is most common and clearest near the dip equator; however, daytime Pi2s are regularly observed at low‐latitude ground stations away from the dip equator. In contrast, a study utilizing satellite data at low latitudes within the plasmasphere could find no evidence for dayside Pi2s. Consequently, in this investigation we used high‐resolution vector magnetometer data from the CHAMP satellite to carry out a search for daytime Pi2 pulsations in the F region ionosphere. We confirm that nighttime Pi2s are observed simultaneously on CHAMP and at low‐latitude ground stations and that their characteristics are consistent with a cavity mode resonance. However, our comparisons of daytime Pi2 pulsation events identified in ground station data with data from CHAMP clearly demonstrate that Pi2s in the dayside hemisphere differ from their nighttime counterparts. Specifically, we find no convincing evidence of their occurrence in CHAMP data. Our results suggest that the generation mechanism for daytime Pi2s may be similar to the mechanisms that other authors have proposed to explain sudden impulses and DP2 current systems. We propose a model in which the electric field imprinted by the field‐aligned currents connected to the substorm current wedge is distributed globally in the atmospheric waveguide. Toroidal currents flowing on the dayside between ionosphere and ground generate magnetic fields that are confined to the waveguide. As a consequence, no Pi2‐related magnetic signals can be detected in the topside ionosphere.

Transient flow of a viscous compressible fluid in a circular tube after a sudden point impulse transverse to the axis

The flow of a viscous compressible fluid in a circular tube generated by a sudden impulse at a point on the axis and directed transverse to the axis is studied on the basis of the linearized Navier–Stokes equations. A no-slip boundary condition is assumed to hold on the wall of the tube. The flow behaviour differs qualitatively from that for a point impulse in the direction of the axis in that there is no coupling to a diffusive sound mode. As a consequence, the transverse velocity autocorrelation function of a suspended Brownian particle decays at long times faster than t−3/2.

USGS 1-min Dst index

We produce a 1-min time resolution storm-time disturbance index, the USGS Dst, called Dst 8507-4 SM . This index is based on minute resolution horizontal magnetic field intensity from low-latitude observatories in Honolulu, Kakioka, San Juan and Hermanus, for the years 1985–2007. The method used to produce the index uses a combination of time- and frequency-domain techniques, which more clearly identifies and excises solar-quiet variation from the horizontal intensity time series of an individual station than the strictly time-domain method used in the Kyoto Dst index. The USGS 1-min Dst is compared against the Kyoto Dst, Kyoto Sym-H, and the USGS 1-h Dst ( Dst 5807-4 SH ). In a time series comparison, Sym-H is found to produce more extreme values during both sudden impulses and main phase maximum deviation, possibly due to the latitude of its contributing observatories. Both Kyoto indices are shown to have a peak in their distributions below zero, while the USGS indices have a peak near zero. The USGS 1-min Dst is shown to have the higher time resolution benefits of Sym-H, while using the more typical low-latitude observatories of Kyoto Dst.

Transient flow of a viscous compressible fluid in a circular tube after a sudden point impulse

The flow of a viscous compressible fluid in a circular tube generated by a sudden impulse at a point on the axis is studied on the basis of the linearized Navier–Stokes equations. A no-slip boundary condition is assumed to hold on the wall of the tube. Owing to the finite velocity of sound the flow behaviour differs qualitatively from that of an incompressible fluid. The flow velocity and the pressure disturbance at any fixed point different from the source point vanish at short time and decay at long times with a t−3/2 power law.

Sudden impulses at geosynchronous orbit and at ground

Sudden impulses of the magnetospheric and geomagnetic field are caused by sudden increases in the dynamic pressure of the solar wind, generally associated with the Earth’s arrival of interplanetary shock waves or discontinuities. We compared the observed field jumps, at geosynchronous orbit and at ground, with those predicted by a theoretical model (T04, Tsyganenko and Sitnov, 2005) for transitions between two steady states representations of the magnetosphere under different solar wind conditions. The geosynchronous response (mostly along the northward component, B z) in the dayside hemisphere is basically consistent with the magnetic field jump expected for changes of the magnetopause current alone. A similar conclusion holds for the positive changes observed in the nightside hemisphere; by contrast, the competing contributions of several current systems (from the magnetopause, cross-tail current, ring current, field aligned currents) determine in this time sector a large variety of small amplitude and negative responses that cannot be univocally interpreted. A different situation emerges in ground observations, at low latitudes. Here the observed responses along the northward component, H, in the dawn sector are consistent with changes of the magnetopause current; by contrast, in the entire postnoon hemisphere, they are greater than expected. The H variations are also typically accompanied by a significant change of the eastward component, D. These aspects reveal additional ionospheric contributions which determine a negative change of the D component through the entire day and a remarkable enhancement (≈50%) of the H component in the postnoon sector. The comparison between observations and model representations in several cases allows to discriminate between the roles of the current systems which compete in determining the field variations. Occasionally, SI manifestations are accompanied by trains of ULF fluctuations (f≈1–100 mHz); we present a case in which the experimental observations suggest evidence for a cavity/waveguide oscillation at f≈3.2 mHz.

Propagation of inclined interplanetary shock through the magnetosheath

Normals of most interplanetary shocks are nearly aligned with the Sun–Earth line. But some shocks, especially those connected with corotating interaction regions, are sufficiently diverted from the typical orientation near 1 AU. We obtain that shocks with normal lying in the XY plane and inclined at an angle about 40° or more from the Sun–Earth line can result in sudden impulse variations of different magnitudes in the dawn and dusk magnetosphere. Using the Rankine–Hugoniot equations, we calculate the downstream velocity in dependence on the interplanetary shock orientation. We find for given upstream parameters that the downstream velocity V y ≃ 0.2 V x , when n y ≃ n x and the upstream velocity is directed exactly along the Sun–Earth line. For more inclined shocks, the ratio V y / V x may exceed 30 percent. Numerical three-dimensional (3-D) MHD simulations predict a set of MHD discontinuities propagating through the magnetosheath after interaction between an inclined shock and the bow shock. It is shown a clear difference between variations in the dusk magnetosheath downstream of the quasi-perpendicular bow shock (the region passed first by the inclined interplanetary shock) and in the dawn magnetosheath downstream of the quasi-parallel bow shock. In the dusk flank, the predicted variations are mainly similar to those obtained previously for a radially propagating shock at the Sun–Earth line. In the dawn flank, the forward fast shock with a small variation of the magnetic field magnitude is followed by another compound discontinuity bringing an increase of the density and magnetic field, but a decrease of the velocity and temperature. We suppose that this discontinuity consists of several basic MHD discontinuities moving with close velocities, therefore its composition cannot be determined exactly in 3-D simulations. Using an estimation of the Alfvén velocity in the magnetosphere, we find the transit time of the fast shock from the first impact at the bow shock to the ionosphere. This transit time is obtained to be 0.5–1 min longer for the inclined shock than for a radially propagating shock with a similar amplitude.

Contribution of solar cosmic rays to the formation of the Earth’s proton radiation belt

The significance of the contribution of solar protons to fluxes of trapped radiation in the Earth’s outer radiation belt (L > 2) is estimated for various phases of solar activity. In periods of high solar activity, proton fluxes with the energy 1–5 MeV at L = 2–3 for the bulk of time have SCR as a source, during a minimum of solar activity, trapped proton fluxes are determined by the conventional diffusive mechanism under the action of sudden IMF impulses.

Transient flow of a viscous incompressible fluid in a circular tube after a sudden point impulse

The flow of a viscous incompressible fluid in a circular tube generated by a sudden impulse on the axis is studied on the basis of the linearized Navier–Stokes equations. A no-slip boundary condition is assumed to hold on the wall of the tube. At short time the flow is irrotational and may be described by a potential which varies with the square root of time. At later times there is a sequence of moving and decaying vortex rings. At long times the flow velocity decays with an algebraic long-time tail. The impulse generates a time-dependent pressure difference between the ends of the tube.

Numerical simulation of magnetospheric ULF waves excited by positive and negative impulses of solar wind dynamic pressure

The sources of ultra low frequency (ULF) waves in the magnetosphere are generally believed to be either the external solar wind perturbations or the internal plasma instabilities. When a sudden impulse of the solar wind dynamic pressure impinges on the magnetopause, ULF waves might be excited and thus the solar wind energy is transported into the earth’s magnetosphere. In this paper, we study the ULF waves excited by different kinds of sudden solar wind pressure impulses through an MHD simulation. We primarily focus on the responses of the earth’s magnetosphere to positive/negative impulses of solar wind dynamic pressure, and positive-negative impulse pairs. The simulation results show that the ULF waves excited by positive and negative impulse have the same amplitude and frequency, with 180° difference in phase, if the amplitude and durations of the input impulses are the same. In addition, it is found that field line resonances (FLRs) occur at certain L-shell regions of the earth’s magnetosphere after the impact of different positive-negative impulse pairs, which appear to be related to the duration of the impulses and the time interval between the sequential impulses. Another result is that the energy from the solar wind could be transported deeper into the inner magnetosphere by an impulse pair than by a single pulse impact. The results presented in this paper could help us to better understand how energy is transported from solar wind to the earth’s magnetosphere via ULF waves. Also, these results provide some new clues to understanding of how energetic particles in the inner magnetosphere response to different kinds of solar wind pressure impulse impacts including interplanetary shocks.

Responses of the magnetotail plasma sheet to two interplanetary shocks: TC-1 observations

Two interplanetary shocks are examined to determine the responses of the magnetic field and plasma in the plasma sheet upon the shock impacts by using TC-1 observational data. The two shocks are observed by WIND on November 7, 2004. Prior to and after the shock, the IMF is either weakly southward or northward. The responses of the plasma sheet to the two shocks are intense and much similar. When the shock interacts with the magnetosphere, the magnetic field impulsively increases 1–2 min after the geomagnetic field sudden impulse (SI) judged from the Sym-H index change, and the magnetic field line is stretched. On the other hand, all of the ion density, the ion temperature, and the velocity of ion flow in the plasma sheet increase. Interestingly, quasi-periodical oscillations of the ion flow are suddenly enhanced, and the plasma flow is basically perpendicular to the local magnetic field. The responses of the magnetic field and the plasma are nearly simultaneous. The responses in the plasma sheet are probably caused by the lateral compression due to the dynamic pressure enhancement downstream the shock when the shock propagates antisunward in the magnetosheath.

Flow of a viscous incompressible fluid after a sudden point impulse near a wall

The flow of a viscous incompressible fluid generated by a sudden impulse near a wall with no-slip boundary condition is studied on the basis of the linearized Navier–Stokes equations. It turns out that the flow differs significantly from that for the perfect slip boundary condition, except far from the wall and at short times. At short time the flow is irrotational and can be described by a potential which varies with the square root of time. Correspondingly the pressure disturbance is quite large at short times. It shows an oscillation at later times if the impulse is directed parallel to the wall and decays monotonically for impulse perpendicular to the wall.