Search NASA⌕ Search

Engineering topics

Kivelson, M. G.

Publications and source records attributed to Kivelson, M. G..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Spatial Correlation of Solar-Wind Turbulence from Two-Point Measurements

Interplanetary turbulence, the best studied case of low frequency plasma turbulence, is the only directly quantified instance of astrophysical turbulence. Here, magnetic field correlation analysis, using for the first time only proper two-point, single time measurements, provides a key step in unraveling the space-time structure of interplanetary turbulence. Simultaneous magnetic field data from the Wind, ACE, and Cluster spacecraft are analyzed to determine the correlation (outer) scale, and the Taylor microscale near Earth's orbit.

Matthaeus, W. H.↗
NASA NTRS2005

Dynamic Harris current sheet thickness from Cluster current density and plasma measurements

We use the first accurate measurements of current densities in the plasma sheet to calculate the half-thickness and position of the current sheet as a function of time. Our technique assumes a Harris current sheet model, which is parameterized by lobe magnetic field B(o), current sheet half-thickness h, and current sheet position z(sub o). Cluster measurements of magnetic field, current density, and plasma pressure are used to infer the three parameters as a function of time. We find that most long timescale (6-12 hours) current sheet crossings observed by Cluster cannot be described by a static Harris current sheet with a single set of parameters B(sub o), h, and z(sub o). Noting the presence of high-frequency fluctuations that appear to be superimposed on lower frequency variations, we average over running 6-min intervals and use the smoothed data to infer the parameters h(t) and z(sub o)(t), constrained by the pressure balance lobe magnetic field B(sub o)(t). Whereas this approach has been used in previous studies, the spatial gnuhen& now provided by the Cluster magnetometers were unavailable or not well constrained in earlier studies. We place the calculated hdf&cknessa in a magnetospheric context by examining the change in thickness with substorm phase for three case study events and 21 events in a superposed epoch analysis. We find that the inferred half-thickness in many cases reflects the nominal changes experienced by the plasma sheet during substorms (i.e., thinning during growth phase, thickening following substorm onset). We conclude with an analysis of the relative contribution of (Delta)B(sub z)/(Delta)X to the cross-tail current density during substorms. We find that (Delta)B(sub z)/(Delta)X can contribute a significant portion of the cross-tail c m n t around substorm onset.

Thompson, S. M.↗
NASA NTRS2005

Plasma sheet turbulence observed by Cluster II

Cluster fluxgate magnetometer (FGM) and ion spectrometer (CIS) data are employed to analyze magnetic field fluctuations within the plasma sheet during passages through the magnetotail region in the summers of 2001 and 2002 and, in particular, to look for characteristics of magnetohydrodynamic (MHD) turbulence. Power spectral indices determined from power spectral density functions are on average larger than Kolmogorov's theoretical value for fluid turbulence as well as Kraichnan's theoretical value for MHD plasma turbulence. Probability distribution functions of the magnetic fluctuations show a scaling law over a large range of temporal scales with non-Gaussian distributions at small dissipative scales and inertial scales and more Gaussian distribution at large driving scales. Furthermore, a multifractal analysis of the magnetic field components shows scaling behavior in the inertial range of the fluctuations from about 20 s to 13 min for moments through the fifth order. Both the scaling behavior of the probability distribution functions and the multifractal structure function suggest that intermittent turbulence is present within the plasma sheet. The unique multispacecraft aspect and fortuitous spacecraft spacing allow us to examine the turbulent eddy scale sizes. Dynamic autocorrelation and cross correlation analysis of the magnetic field components allow us to determine that eddy scale sizes fit within the plasma sheet. These results suggest that magnetic field turbulence is occurring within the plasma sheet resulting in turbulent energy dissipation.

Weygand, James M.↗
NASA NTRS2005

The response of the near earth magnetotail to substorm activity

The large scale structure of the current sheet in the terrestrial magnetotail is often represented as the superposition of a constant northward-oriented magnetic field component (B(sub z)) and a component along the Earth-Sun direction (B(sub x)) that varies with distance from the center of the sheet (z(sub o) in GSM) as in a Hams neutral sheet. The latter implies that B(sub x) = B(sub Lx) tanh((z - z(sub o))/h) where B(sub Lx) is the magnitude of the B(sub x) component in the northern lobe. Correspondingly, the cross-tail current should be approximated by J(sub y) = (B(sub Lx)/h) sech(sup 2)((z - z(sub o))/h). Using data from the fluxgate magnetometer (FGM) on the Cluster I1 spacecraft tetrad, we have used measured fields and currents to ask if this model represents the large-scale properties of the system. During very quiet crossings of the plasmasheet, we find that the model gives a reasonable estimate of the trend of the average current and field distributions, but during disturbed intervals, the best fit fails to represent the data. If, however, the parameters z(sub o) and h of the model are taken as variable functions of time, the fits can be reasonably good. The temporal variation of the fit parameter h that characterizes the thickness of the current sheet can be interpreted in terms of thinning during the growth phase of a substorm and thickening following the expansion phase. Ground signatures that give insight into the local time of substorm onset can be used to interpret the response of the plasmasheet to substorm related changes of the global system. During a substorm, the field magnitude in the central plasmasheet fluctuates at the period of Pi2 pulsations.

Kivelson, M. G.↗
NASA NTRS2004

Cluster observations of quasi-periodic impulsive signatures in the dayside northern lobe: High-latitude flux transfer events?

We report on a series of quasi-periodic reversals in GSM B(sub Z) observed by the four Cluster spacecraft in the northern dayside lobe poleward of the cusp on 23 February 2001. During an interval of about 35 min, multiple reversals (negative to positive) in B(sub Z) of approximately 1-min duration with an approximate 8-min recurrence time were observed. The individual structures do not resemble low-latitude flux transfer events (FTE) [Russell and Elphic, 1979] but the 8-min recurrence frequency suggests that intermittent reconnection may be occurring .Measurements (appropriately lagged) of the solar wind at ACE show that the IMF was southward-oriented with a strong B(sub X) and that a modest dynamic pressure increased as the events started. The multi-point observations afforded by the Cluster spacecraft were used to infer the motion (direction and speed) of the observed magnetic field reversals. The associated currents were also calculated and they are consistent with the spatial confinement of the observed magnetic field reversals. We propose that the observed reversals are due to flux tubes reconnecting with closed field lines on the dayside. Ancillary data from the Cluster Ion Spectrometry (CIS) and Plasma Electron And Current Experiment (PEACE) instruments were used to develop a physical picture of the reversals.

Thompson, S. M.↗
NASA NTRS2003

Cluster electric current density measurements within a magnetic flux rope in the plasma sheet

On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, Beta(sub i) approx. 1-2, near the leading edge of a bursty bulk flow. The "time-of-flight" of the flux rope across the 4 spacecraft yielded V(sub x) approx. 700 km/s and a diameter of approx.1 R(sub e). The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping-Burlaga force-free flux rope model. The results indicated that the center of the flux rope passed northward (above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single-s/c models were approx.15-19 nA/sq m. The 4-spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA/m2 near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force-free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single-spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force-free condition.

Slavin, J. A.↗
NASA NTRS2002

The Dusk Flank of Jupiter's Magnetosphere

Limited single-spacecraft observations of Jupiter's magnetopause have been used to infer that the boundary moves inward or outward in response to variations in the dynamic pressure of the solar wind. At Earth, multiple-spacecraft observations have been implemented to understand the physics of how this motion occurs, because they can provide a snapshot of a transient event in progress. Here we present a set of nearly simultaneous two-point measurements of the jovian magnetopause at a time when the jovian magnetopause was in a state of transition from a relatively larger to a relatively smaller size in response to an increase in solar-wind pressure. The response of Jupiter's magnetopause is very similar to that of the Earth, confirming that the understanding built on studies of the Earth's magnetosphere is valid. The data also reveal evidence for a well-developed boundary layer just inside the magnetopause.

Kurth, W. S.↗
NASA NTRS2002

The Permanent and Inductive Magnetic Moments of Ganymede

Data acquired by the Galileo magnetometer on five passes by Ganymede have been used to characterize Ganymede's internal magnetic moments. Three of the five passes were useful for determination of the internal moments through quadrupole order. Models representing the internal field as the sum of dipole and quadrupole terms or as the sum of a permanent dipole field upon which is superimposed an induced magnetic dipole driven by the time varying component of the externally imposed magnetic field of Jupiter's magnetosphere give equally satisfactory fits to the data. The permanent dipole moment has an equatorial field magnitude 719 nT and is tilted by 176 degrees from the spin axis with the pole in the southern hemisphere rotated by 24 degrees from the Jupiter-facing meridian plane towards the trailing hemisphere. The data are consistent with an inductive response of a good electrical conductor of radius approximately 1 Ganymede radius. Although the data do not enable us to establish the presence of an inductive response beyond doubt, we favor the inductive response model because it gives a good fit to the data using only 4 parameters to describe the internal sources of fields, whereas the equally good dipole plus quadrupole fit requires 8 parameters. An inductive response is consistent with a buried conducting shell, probably liquid water with dissolved electrolytes, somewhere in the first few hundred km below Ganymede's surface. The depth at which the ocean is buried beneath the surface is somewhat uncertain, but our favored model suggests a depth of order 150 kilometers. As both temperature and pressure increase with depth and the melting temperature of pure ice decreases to a minimum at approximately 170 kilometer depth, it seems possible that near this location, a layer of water would be sandwiched between layers of ice.

Kivelson, M. G.↗
NASA NTRS2001

Wave Activity in Europa's Wake: Implications for Ion Pickup

Intense wave power at frequencies near and below the cyclotron frequencies of heavy ions was detected in Europa's wake during the E11 and E15 flybys. The fluctuations are mainly transverse to the background magnetic field. Wave characteristics indicate that they are ion cyclotron waves driven by positively charged pickup ions. In both flybys there is evidence, derived from the wave polarization, for pickup of negatively charged chlorine ions. When the moon is near the center of the Jovian current sheet, the pickup rate inferred for the E15 flyby is larger than that for the E11 flyby, when the moon is outside the Jovian current sheet. The wave power does not provide exact pickup density values because the waves are observed in regions where their growth has not yet fully developed. At the edges of the wake region, low-frequency (< K+ gyrofrequency) magnetohydrodynamic waves are also present. We identify magnetic field signatures that are reminiscent of interchange/ballooning of mass-loaded flux tubes from the wake/pickup region expanding into ambient medium that is less dense.

Volwerk, M.↗
NASA NTRS2001

Structure of the Jovian Magnetodisk Current Sheet: Initial Galileo Observations

The ten-degree tilt of the Jovian magnetic dipole causes the magnetic equator to move back and forth across Jupiter's rotational equator and tile Galileo orbit that lies therein. Beyond about 24 Jovian radii, the equatorial current sheet thins and tile magnetic structure changes from quasi-dipolar into magnetodisk-like with two regions of nearly radial but antiparallel magnetic field separated by a strong current layer. The magnetic field at the center of the current sheet is very weak in this region. Herein we examine tile current sheet at radial distances from 24 55 Jovian radii. We find that the magnetic structure very much resembles tile structure seen at planetary magnetopause and tail current sheet crossings. Tile magnetic field variation is mainly linear with little rotation of the field direction, At times there is almost no small-scale structure present and the normal component of the magnetic field is almost constant through the current sheet. At other times there are strong small-scale structures present in both the southward and northward directions. This small-scale structure appears to grow with radial distance and may provide the seeds for tile explosive reconnection observed at even greater radial distances oil tile nightside. Beyond about 40 Jovian radii, the thin current sheet also appears to be almost constantly in oscillatory motion with periods of about 10 min. The amplitude of these oscillations also appears to grow with radial distance. The source of these fluctuations may be dynamical events in tile more distant magnetodisk.

Russell, C. T.↗
NASA NTRS2001

Sounding of Icy Galilean Satellites by Surface Observatories

Several independent geological and geophysical investigations suggest that Europa and Ganymede contain subsurface oceans. Using Jupiter's rotating magnetic field as a primary signal, the magnetometer experiment onboard Galileo has measured secondary induction signals emanating from Europa, Ganymede, and surprisingly Callisto. The strong electromagnetic induction from these moons suggests that large global electrical conductors are located just below their icy crusts. A detailed analysis reveals that global salty oceans with salinity similar to the Earth's ocean and thicknesses in the range of approx. 6-100 kms can explain the induction observed by the Galileo magnetometer. Additional information is contained in the original extended abstract.

Khurana, K. K.↗
NASA NTRS2000

Implications of Depleted flux Tubes in the Jovian Magnetosphere

A rare but persistent phenomenon in the jovian magnetosphere is the occurrence of apparently depleted flux tubes, whose magnetic pressures are significantly above ambient levels. These flux tubes occur about 0.25% of the observing time in the region of the Io torus in the Galileo high resolution data. The importance of these tubes is that they can return to the inner magnetosphere the magnetic flux that has been convected radially outward with the iogenic plasma to the tail. The paucity of these tubes is consistent with the expected flux return rates if the tubes are moving inward at an average rate of about 5-10 km/s in the torus. Depleted flux tubes have yet to be observed inside of the lo orbit where the plasma beta is lower than in the hot torus. Estimates of the plasma density outside the tube from plasma wave measurements enable the average perpendicular temperature to be obtained from the magnetic field change. Extrapolating this temperature back to lo, we obtain an average ion temperature of approximately 60 eV. These values are generally consistent with earlier Voyager observations but on the low side of their range of uncertainty, and agree quite well with contemporaneous Galileo measurements where these are available.

Russell, C. T.↗
NASA NTRS2000

Substorms At Jupiter: Galileo Observations of Transient Reconnection in The Near Tail

The magnetic flux content of the Jovian magnetosphere is set by the internal dynamo, but those magnetic field lines are constantly being loaded by heavy ions at the orbit of lo and dragged inexorably outward by the centrifugal force. Vasyliunas has proposed a steady state reconnecting magnetospheric model that sheds plasma islands of zero net magnetic flux and returns nearly empty flux tubes to the inner magnetosphere. The Galileo observations indicate that beyond 40 Rj the current sheet begins to tear and beyond 50 Rj on the nightside explosively reconnects as the tearing site reaches the low density lobe region above and below the current sheet. Small events occur irregularly but on average about every 4 hours and large events about once a day. The magnetic flux reconnected in such events amounts up to about 70,000 Webers/sec and is sufficient to return the outwardly convected magnetic flux to the inner magnetosphere. Since this process releases plasmoids into the jovian tail, as do terrestrial substorms; since this process involves explosive reconnection across the current sheet on the nightside of the planet, as do terrestrial substorms; and since the process is a key in closing the circulation pattern of the magnetic and plasma flux, as it is in terrestrial substorms; we refer to these events as jovian substorms.

Russell, C. T.↗
NASA NTRS1999

Probing Ganymede's Magnetosphere with Field Line Resonances

We report on the spectrum of field line resonances identified in data acquired by the Galileo spacecraft within Ganymede's magnetosphere on a relatively low latitude pass. We infer properties of the plasma distribution and its transport from the observed spectrum. The harmonic structure in the spectrum of the magnetometer data agrees very well with the frequencies predicted for resonances of a dipole field. The spectrum implies a density of 2 amu per cubic centimeter near the equator on closed field lines of Ganymede's magnetic field inside of 2 RG (Ganymede radii). This density is significantly reduced relative to the local density of the Jovian plasma sheet near Ganymede (less than or equal to 8 electrons per cubic centimeter, or approximately equal to 100 amu per cubic centimeter for an average ion mass per charge of 20 amu and average charge of 1.5 electron charges). A shadowing effect of Ganymede for the flow of particles injected at a reconnection layer on the side of the moon downstream relative to the direction of torus plasma flow accounts for the marked density depletion. Implications for conducting paths near Ganymede's surface are considered.

Volwerk, M.↗
NASA NTRS1995

Solar wind interaction with small bodies. 1: Whistler wing signatures near to Gaspra and Ida

Data from Galileo's two asteroid flybys reveal magnetic fluctuations that we interpret as perturbations of the solar wind magnetic field caused by an interaction with the nearby asteroid. The scale sizes of the bodies (approximately 14 km for Gaspra and approximately 30 km for Ida) are intermediate between the ion and electron gyroradii, which implies that the asteroid-imposed perturbations propagate in the whistler mode. Special properties of the whistler mode include phase phase velocities that can exceed the solar wind speed and confinement of the disturbance to directions nearly aligned with the magnetic field. These features of the interaction impose a structure on the solar wind disturbance that differs greatly from the forms familiar for either magnetized or unmagnetized bodies of magnetohydrodynamic spatial scales. We examine both data and computer simulations of the interaction with special attention to what can be inferred about the interaction itself from analysis of the data.

Wang, Z.↗
NASA NTRS1995

Solar wind interaction with small bodies. 2: What can Galileo's detection of magnetic rotations tell us about Gaspra and Ida

As the Galileo spacecraft passed the asteroids Gaspra in 1990 and Ida in 1993, the magnetometer recorded changes in the solar wind magnetic field that we associate with the presence of the nearby body. This paper focuses on the types of interactions that can produce perturbations in the solar wind. We have suggested that the interaction at Gaspra is consistent with expectations of flow diversion by a magnetic dipole moment and an associated 'magnetosphere' whose scale size is much larger than the diameter of the solid body. The conditions for the Ida flyby leave more room for ambiguity. The observations could plausibly be related to either interaction with a magnetized body or with a conducting body. We will report on details of the observations that may enable us to distinguish between the different types of interaction and to provide quantitative estimates of the physical properties of the asteroids themselves.

Kivelson, M. G.↗
NASA NTRS1995

Galileo flybys of Earth: The nature of the distant shock

We report on observations of the Earth's bow shock at unprecedentedly large downtail distances, some as remote as 360 R(sub E). Suprisingly, we find that even at these large distances, the bow shock signature remains clear. The cases we report are among the weakest shocks ever clearly identified. These shocks reveal patterns of field changes remarkably similar to those observed for stronger shocks. Indeed, several of the shocks could serve as textbook examples because they occur in unusually quiet and steady solar wind conditions. The quasi- perpendicular shocks are, in some cases preceded by whistler wave trains. Several of the shocks which have normal vectors in the transitional region between quasiperpendicular and quasiparallel are associated with large amplitude wave disturbances in the downstream plasma. Although low Mach number, these shocks are the source of large amplitude turbulence. We show a range of fits to the locations of the shock crossings, and argue that, for steady solar wind conditions, there is no sign of tail flapping. The displacement of the shock from the tail axis appears to be governed by the interplanetary magnetic field orientation as previously reported for Venus.

Kivelson, M. G.↗