bluuebunny/crossref_metadata_2025 huggingface dataset:
Global numerical simulation of the growth phase and the expansion onset for a substorm observed by Viking
Authors: Fedder J. A. , Slinker S. P. , Lyon J. G. , Elphinstone R. D.
Published: February 2004

We report the first global magnetohydrodynamic (MHD) simulation of an actual magnetospheric substorm, which was recorded by the Viking spacecraft on October 19, 1986. The simulation is driven by IMP 8 solar wind parameters measured upstream of the Earth's bow shock. The substorm, which had expansion onset at 1132 UT, was caused by a brief period of southward interplanetary magnetic field (IMF) and two weak solar wind shocks. The simulation model includes a self‐consistent auroral ionospheric conductance depending directly on the MHD magnetospheric plasma parameters and magnetic field. Synthetic auroral emissions, derived from simulation results, are compared to the Viking images, which show considerable dayside activity preceding the substorm. We also compare model‐derived synthetic AU and AL indices to geomagnetic measurements. The simulation results are seen to be in reasonable agreement with the observations throughout the growth phase and expansion onset. Moreover, the results allow us to form conclusions concerning which essential processes were responsible for the substorm occurence. These results are a highly encouraging first step leading toward development of a space weather forecasting methodology based on the directly measured solar input.
Theory and simulation of low‐frequency plasma waves and comparison to Freja satellite observations
Authors: Seyler C. E. , Wahlund J.‐E. , Holback B.
Published: February 2004

One‐dimensional models of obliquely propagating nonlinear plasma waves were formulated and solved both analytically and numerically to interpret recent Freja satellite observations of low‐frequency plasma waves detected in the low‐altitude auroral magnetosphere. Analytic calculations revealed four types of steady state waves solutions. Time dependent initial value numerical calculations were compared to the steady state solutions and to Freja observations. One type of steady state wave solution emerged in the long time limit from the initial sinusoidal waves; however, the initial value simulations agreed best with the observations during the nonlinear steepening phase of the initial waveform at a time well before a steady state was reached. From this result we concluded that many of the low altitude auroral waves Freja has detected were oblique inertial Alfvén waves that had nonlinearly steepened due to propagation into a region of lower Alfvén speed. The nonlinear steepening was found to produce very large parallel currents. The current is sufficiently high to excite parallel electron drift instabilities, which may lead to electron and ion energization and enhanced dissipation of auroral arc energy.
Relationship between large horizontal electric fields and auroral arc elements
Authors: Lanchester B. S. , Kaila K. , McCrea I. W.
Published: February 2004

High time resolution optical measurements in the magnetic zenith are compared with European Incoherent Scatter (EISCAT) field‐aligned measurements of electron density at 0.2‐s resolution and with horizontal electric field measurements made at 278 km with resolution of 9 s. In one event, 20 min after a spectacular auroral breakup, a system of narrow and active arc elements moved southward into the magnetic zenith, where it remained for several minutes. During a 30‐s interval of activity in a narrow arc element very close to the radar beam, the electric field vectors at 3‐s resolution were found to be extremely large (up to 400 mV m−1) and to point toward the bright optical features in the arc, which moved along its length. It is proposed that the large electric fields are short‐lived and are directly associated with the particle precipitation that causes the bright features in auroral arc elements.
Ionospheric plasma response to HF radio waves operating at frequencies close to the third harmonic of the electron gyrofrequency
Authors: Honary F. , Stocker A. J. , Robinson T. R. , Jones T. B. , Stubbe P.
Published: February 2004

Experimental results concerning European incoherent scatter observations of heater‐induced electron temperature enhancements, anomalous absorption of low‐power HF probe waves, and the spectrum of stimulated electromagnetic emission (SEE) in the sidebands of a high‐power HF electromagnetic wave are presented. For the experiments reported in this paper, an O mode pump wave was transmitted vertically into the F region above Tromsø, Norway, while the injected frequency was varied in small steps around the third harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the data obtained from all three diagnostics. Measurements of anomalous absorption, the downshifted maximum (DM) spectral feature, and heater‐induced electron temperature enhancements all exhibited broad minima as the heater frequency approached the third harmonic of the electron gyrofrequency. In addition, the signal strength of the HF probe wave measured during heater off periods is also reduced at these and higher heater frequencies. The experimental findings suggest that at heater frequencies in the vicinity of the third gyroharmonic, small‐scale field‐aligned irregularities are not excited, whereas very small scale irregularities, of the order of a few electron cyclotron radii, which are responsible for the production of fast electrons, may be generated. The observed reduction in the diagnostic signal strength is then attributed to the ionized patches produced by these energetic electrons.
GEOTAIL observation of magnetospheric convection in the distant tail at 200 RE in quiet times
Authors: Nishida A. , Mukai T. , Yamamoto T. , Saito Y. , Kokubun S. , Maezawa K.
Published: February 2004

Magnetic field and plasma observations by GEOTAIL in the distant tail at x = −200 RE are studied for a geomagnetically quiet interval when IMF Bz is predominantly northward and |By| is larger than B on average. In the distant tail during this interval, Bz is northward and Ey is directed dusk‐to‐dawn on average. This combination of Bz and Ey does not seem to represent the tailward convection of the closed field lines, because Ey is much weaker in the lobe than in the plasma sheet so that the field lines would be piled up at the plasma sheet boundary if they were closed. The observations suggest instead that the plasma and field lines are convected parallel to the neutral sheet across the tail at the same time as they flow tailward. The direction of this cross‐tail convection depends on the polarity of IMF By and is antisymmetric with respect to the neutral sheet, which can be twisted by tens of degrees under the influence of IMFBy. A consistent picture is obtained from observations both inside the tail and at the tail magnetopause. This convection profile agrees in topology with the cusp reconnection model, but it occurs mainly in the plasma sheet while only the lobe field lines are expected to be involved according to this model. Observations show that not only the cold‐dense ions which can be linked directly with the entrant solar wind plasma but also the hot‐tenuous ions in the plasma sheet take part in the convection.
Radar measurements of very small aspect angles in the equatorial ionosphere
Authors: Farley D. T. , Hysell D. L.
Published: February 2004

Equatorial spread F (ESF) irregularities are very highly aligned with the geomagnetic field. In order to understand the physics of the irregularities, that is, the unstable plasma waves, it is important to measure this degree of alignment as accurately as possible. Measuring the angles with a radar is no easy task, however, since the angles in question, perhaps a few times 10−2 degrees or less, are much smaller than the radar beam widths available at any equatorial radar, and there are numerous potential sources of error that can make the angles appear larger than they really are. Nevertheless, the measurement can be done. At Jicamarca we have determined an upper limit on the rms deviations from perfect alignment that is often as small as 0.015°. This paper describes the measurement technique and presents a few illustrative examples of the data. The following companion paper [Hysell and Farley, this issue] discusses the theoretical implications of these measurements.
Lorentzian ion exosphere model
Authors: Pierrard V. , Lemaire J.
Published: February 2004

Since observed velocity distributions of particles in the magnetosphere generally have a suprathermal tail instead of an exponential one, we propose to recalculate the density and temperature distributions in a nonrotating ion exosphere with a Lorentzian velocity distribution function (VDF) instead of a Maxwellian. The number density, the flux of particles, parallel and perpendicular pressures, and energy flux of the different classes of particles in the exosphere have been determined for any value of the index κ characterizing the Lorentzian VDF. The barometric density and temperature distributions for a Maxwellian VDF and for a Lorentzian VDF are compared. It is shown that for particles in an attractive potential, the barometric density decreases more slowly with altitude for the Lorentzian VDF. Furthermore, the temperature increases with altitude in this case, while for a Maxwellian VDF, it is independent of altitude. It is suggested that positive gradients of the ion and electron temperatures observed between the topside ionosphere and the outer plasmasphere can be explained by this effect, that is, a non‐Maxwellian VDF with an enhanced suprathermal tail.
First detection of charged dust particles in the Earth's mesosphere
Authors: Havnes O. , Trøim J. , Blix T. , Mortensen W. , Næsheim L. I. , Thrane E. , Tønnesen T.
Published: February 2004

Some theories for the observed anomalous radar backscatter during the summer (polar mesospheric summer echoes, or PMSE) and electron bite outs measured by rockets require the presence of charged dust. To investigate this, two dust probes have been launched in 1994 from Andøya Rocket Range and we here report the results from the dust and an electron probe on the two payloads. The dust probes were designed to block out the electron and ion components at the mesopause but to detect primary currents due to impacts of charged dust and also to detect secondary plasma production during dust impacts. The results indicate that both during PMSE and noctilucent cloud (NLC) conditions, large amounts of dust, with average sizes apparently of about 0.1 μm and less, were present. The number densities Nd can be up to many thousand per cubic centimeter, and the charge density NdZd likewise. Large local gradients in density and charge density of dust are detected. Dust carrying both positive and negative charges can apparently be present on different occasions. In some parts of the NLC/PMSE layers we find that the negative charge density locked in grains is so large that the number of free electrons is significantly reduced there because the dust acts like sinks for electrons, and an electron bite out results. We also find that in one case the presence of positive dust leads to an increase in the local electron density by photoionization. The main uncertainties in the data analysis are the structure of the dust and the secondary plasma production at the comparatively low dust impact velocities (1 km s−1) in the experiment.
On determining polarization characteristics of ion cyclotron wave magnetic field fluctuations
Authors: Anderson B. J. , Denton R. E. , Fuselier S. A.
Published: February 2004

Polarization characteristics of magnetospheric proton cyclotron waves should provide definitive tests of mechanisms for wave propagation and growth. Previous studies used Fourier spectral analysis to determine the ellipticity ε and the minimum variance direction êmin, which gives θmin, the angle between êmin and the background field B0. Comparison with theoretical models depends critically on accurate determination of ε and θmin. However, observed fluctuations might not be sets of phase‐coherent sine waves, as implicitly assumed in Fourier analysis, but may consist of series of packets whose phase and azimuthal orientation vary randomly. By constructing synthetic nonstationary signals, we find that spectral analysis of data intervals containing several wave packets systematically underestimates θmin, often by 45° or more, and overestimates |ε|. The problem is caused by fluctuations in the polarization ellipse azimuth orientation. We present a minimum variance analysis technique, called wave‐step analysis, which requires only a few wave cycles of data. Tests of the wave‐step procedure show that it is valid for signals with bandwidths up to ∼30% full width at half maximum and is therefore applicable to the majority of proton cyclotron wave events. Comparison of the wave‐step and Fourier analyses for cyclotron wave events confirms that cyclotron wave fluctuations display features characteristic of nonstationary signals. Relative to the wave‐step results, the Fourier results underestimate θmin, overestimate |ε|, and display the predicted variations of these parameters with each other and with azimuth angle fluctuations. The opposite relationship between Fourier and wave‐step θmin should result if the signals were too broadbanded for the wave‐step algorithm. Thus the θmin results provide an unambiguous indication of nonstationarity. Time windows of 30 s proved to be too long for analysis of ∼0.5 Hz signals, indicating that analysis needs to be carried out on timescales shorter than tens of wave periods. Previous analyses reported θmin ≤ 30°, but the wavestep results for one linearly polarized event analyzed here show that θmin can be larger than 70°.
High‐latitude Hall and Pedersen conductances during substorm activity in the SUNDIAL‐ATLAS campaign
Authors: Lester M. , Davies J. A. , Virdi T. S.
Published: February 2004

Simultaneous high time resolution observations of the Hall and Pedersen conductances, ΣH and ΣP, respectively, the ionospheric electric field and the ground magnetic field during a magnetospheric substorm are reported. The measurements discussed here were taken during the SUNDIAL/ATLAS 1 campaign of March 24 to April 2, 1992. The European Incoherent Scatter (EISCAT) UHF special programme SP‐UK‐ATLAS, which operated on March 27, 1992, provided continuous measurements of the electron density and the ion vector velocity from which ΣH and ΣP and the ionospheric electric field were calculated. During the substorm growth phase, ΣP and ΣH were less than 10 S and the ratio, R = ΣH/ΣP, was less than 1. Although both ΣH and ΣP increased at the onset of the expansion phase, R remained close to 1. This ratio provides information on the mean energy of the precipitating particles responsible for the enhanced conductances. A ratio of 1 implies a mean energy of the particles of 2.56 keV. Two distinct expansion phases were identified, the second of which included a number of intensifications. Both ΣH and ΣP increased toward the end of the second expansion phase with peak values of 71 S and 34 S, respectively. The ratio R also increased to values exceeding 2, equivalent to mean energies of more than 5.78 keV. The largest value of R was 3.25, which occurred during the substorm recovery phase and is equivalent to mean energies of more than 10 keV. The increase in mean energy as the substorm progresses may be interpreted in terms of changes in the acceleration processes in the magnetosphere. The maximum zonal (east‐west) current during this interval was 2.20 A m−1 and occurred toward the end of the second substorm expansion phase. At a number of intensifications, reversals or enhancements in the zonal current were observed, for which the electric field was responsible in most cases. While the peak value of the Pedersen conductance of 40 S is similar to recent published results, the Hall conductance peak of 75 S is less than recent published measurements. Furthermore, the westward current during the initial part of the expansion phase is dominated by the electric field, in contrast to a previously published model which suggested that this current would be conductivity‐dominated. It is suggested that further spatial structure of the current is necessary to explain these observations.
A statistical survey of 5‐MeV proton events at transient interplanetary shocks
Authors: Kallenrode May‐Britt
Published: February 2004

Between 1974 and 1985 the two Helios spacecraft observed 351 transient interplanetary shocks. For 5‐MeV protons the particle events associated with these shocks can be divided into three groups: (1) events without intensity increase above quiet time or increased background (47%), (2) solar and interplanetary particle (SIP) events consisting of particles accelerated on or close to the Sun (solar or near‐Sun component) as well as at the interplanetary shock (24%), and (3) pure interplanetary particle (PIP) events (29%) which consist of particles accelerated at the shock in interplanetary space but do not show evidence for significant or even excess particle acceleration on the Sun. This classification shows that (1) only about half of the shocks accelerate MeV protons in interplanetary space and (2) MeV protons accelerated oh the Sun are neither a necessary nor a sufficient condition for the acceleration of MeV protons in interplanetary space. Shock parameters such as speed or shock strength alone do not give an indication for the class of the associated particle event, because in the parameter range which covers most of the shocks, all three classes are distributed rather evenly. However, the shocks strongest in these parameters tend to accelerate particles. The intensity at the time of shock passage, which can be used as a crude measure for the local acceleration efficiency, is correlated with the local shock speed and the magnetic compression. The correlation coefficients are small but statistically significant, indicating that (1) the correlations are real and (2) the intensity is influenced by additional parameters, which are not necessarily shock inherent. As an example I will show that the local acceleration at the shock decreases roughly symmetrically with increasing distance from the nose of the shock with a median e‐folding angle of 10°. Occasionally, larger e‐folding angles are observed close to the nose of the shock. The question of how the shock accelerates protons in the MeV range could not be answered here, but I will suggest future studies that could shed a new light on this problem.
Role of current sheets in the modulation of solar energetic particle events
Authors: Kahler S. W. , Kunches J. M. , Smith D. F.
Published: February 2004

Recent results have clarified the way in which solar energetic (E > 10 MeV) particles (SEPs) are produced in the solar corona and interplanetary medium. The principal result is that most SEPs in large events are accelerated on open field lines by large‐scale shocks driven by coronal mass ejections (CMEs). However, the large range of timescales for different SEP events associated with solar source regions at comparable longitudes suggests that the shocks are somehow strongly modulated near the Sun. We investigate the possibility that the coronal streamer belt and current sheet provides a barrier for shock propagation and associated SEP acceleration, as suggested in several recent studies. We use solar flares as proxies for CMEs and plot on the Stanford source surface maps the positions of flares associated with large (F > 10 p/cm2sr s at E > 10 MeV) SEP events observed at the Earth. The basic question is whether SEPs are preferentially observed when the Earth occupies the same magnetic sector as the associated solar flare. We find that onset times, risetimes, and peak fluxes are essentially the same for SEP events with flares in the same sector as for those in the opposite polarity sectors. This suggests that the streamer structure has no detectible effect on the development of the shock and CME driver. We discuss several alternative possibilities to explain the large variation in SEP event timescales.
The role of coronal mass ejections and interplanetary shocks in interplanetary magnetic field statistics and solar magnetic flux ejection
Authors: Smith Charles W. , Phillips John L.
Published: February 2004

We examine the role of coronal mass ejections (CMEs) and interplanetary shocks in modifying the large‐scale winding of the interplanetary magnetic field (IMF) by extracting CME and shock observations from the ISEE 3 data set and analyzing periods of the disturbed and undisturbed solar wind separately. We use the full ISEE 3 data set representing the entire L1 mission (1978 – 1982). We conclude that CMEs, the shocks upstream of CMEs, and other interplanetary shocks are responsible for the apparent overwinding of the IMF spiral relative to the Parker prediction. The IMF winding angle asymmetry is preserved following the removal of the interplanetary disturbances. We also examine the IMF components, the IMF magnitude and the solar wind speed, and the dependence of those averages and asymmetries on CMEs and shock disturbances. An estimate is obtained for the anomalous azimuthal field contained within CMEs which apparently results from the closed‐field topology. We provide new evidence for a nonzero field component crossing the heliospheric current sheet. Last, we examine the role of CMEs and shocks in the measurement of solar magnetic flux ejection. We provide estimates for the average amount of flux transported by CMEs and the error in flux transport analyses that include shock data, and we examine the possible north‐south asymmetry of the flux.
Rotational relaxation of high‐N states of OH (X2Π, v=1–3) by O2
Authors: Holtzclaw K. W. , Upschulte B. L. , Caledonia G. E. , Cronin J. F. , Green B. D. , Lipson S. J. , Blumberg W. A. M. , Dodd J. A.
Published: February 2004

We have studied the relaxation of OH(X2Π, v, N) produced by the reaction O(1D) + H2 → OH* + H. Infrared emission measurements of the fundamental vibration‐rotation band of OH were acquired at a temperature of 100 K in a large cryogenic chamber. Trace amounts of ozone were added to mixtures of H2, O2, and Ar at low (≤38 mT) pressures. The ozone was photolyzed to produce O(1D). Rapid reaction with H2 produced OH*. Spectrally and temporally resolved emissions from levels up to the exothermic limit were observed with sufficient spectral resolution to permit kinetic analysis of individual level populations. The production rate was observed to scale as the rotational quantum number except at the highest levels populated. At early times we observed inverted rotational state distributions that subsequently relaxed to form thermal distributions in each vibrational level. Under these conditions, rotational relaxation was rapid in comparison with vibrational relaxation. Rotational relaxation within a given vibrational state could be represented by single quantum collisional exchange at near‐gas‐kinetic rates. The rotational level dependence of the deduced relaxation was determined.
Stress wave attenuation in shock‐damaged rock
Authors: Liu Cangli , Ahrens Thomas J.
Published: February 2004

The velocity and attenuation of ultrasonic stress waves in gabbroic rock samples (San Marcos, California) subjected to shock loading in the 2 GPa range were studied. Prom P wave velocity measurements we determined the damage parameter Dp and crack density ε of the samples and related these to the attenuation coefficient (quality factor) under dynamic strains of 2×10−7 and at a frequency of 2 MHz using the ultrasonic pulse‐echo method. A fit to the data yields the P wave spatial attenuation coefficient at a frequency of 2 MHz, αp(Dp) = 1.1 + 28.2DP (decibels per centimeter). From the relation between the attenuation coefficient and quality factor, the quality factor Q is given by Q−1 = 0.011(1 + 25.6Dp)(1 − Dp)½. Using O'Connell‐Budiansky theory relating crack density to velocity, the parameter in Walsh's theory was determined based on experimental data. An approximate method is also proposed to estimate the average half‐length of cracks based on the attenuation measurements.
Alfvén wave far field from steady‐current tethers
Authors: Sanmartín Juan R. , Estes Robert D.
Published: February 2004

We analyze the Alfvén wave signature left behind in the ionosphere by orbiting insulated tethers operating as thrusters or generators with steady currents. Using a recent description of tether radiation to determine the far field, we show that, at their leading edges, the Alfvén wings have an Airy function crosswise structure. The field amplitude falls off weakly as the inverse cube root of the distance along the wave front. The front itself, which carries a negligible fraction of the power radiated as Alfven waves, broadens proportionally to the cube root of that distance. We show that collisional decay becomes important at about 103 km along the front. For tethers longer than about 2 km, the top and bottom structures (“wings”) are fully disjoint.
Statistical nature of geomagnetic storms
Authors: Yokoyama N. , Kamide Y.
Published: February 2004

On the basis of geomagnetic activity indices and solar wind parameters, a superposed epoch analysis has been conducted for more than 300 geomagnetic storms. The intensity of magnetic storms is found to depend on the duration of the main phase; larger storms have longer timescales. For intense storms, however, not only the duration of energy injection into the ring current but also the strength of injection is important in determining their size. It is confirmed that the southward component of the interplanetary magnetic field plays a crucial role both in triggering the storm main phase and in determining the magnitude of magnetic storms. It is also found that the time profile of the energy injection rate during the main phase tends to have two peaks. This is particularly the case for intense magnetic storms, where the second peak is more intense than the first. Implications of our findings are discussed in terms of the existing concept of geomagnetic storms and recent observations of ring current particles and of interplanetary disturbances.
Models and DE II observations of inertial‐regime irregularities in equatorial spread F
Authors: McDaniel R. D. , Hysell D. L.
Published: February 2004

We investigate theoretically and experimentally the spectral characteristics of plasma density irregularities in the postsunset equatorial F region ionosphere during spread F conditions. We focus on irregularities occuring in the so‐called inertial regime of the ionosphere, where polarization currents dominate Pedersen currents in the maintenance of charge neutrality on the boundaries of field‐aligned plasma depletions. The equations of motion for the inertial regime flow are similar to the Navier Stokes equation for two‐dimensional, neutral fluid turbulence. A model is presented for determining the location of the inertial regime, which may be found in a narrow window of local times and magnetic apex heights for a given longitude, season, and solar cycle. Large‐amplitude depletions are shown to be necessary for inertial regime flow. We examined the most auspicious DE II plasma density fluctuation data for signatures of Navier Stokes turbulence or other processes that may be unique to the inertial regime. Approximately 50 orbital elements have been analyzed. Just 11 of these are predicted by our model to fall within the inertial regime. The density power spectra computed from data from all 11 exhibit κ−5/3 power law scaling, suggesting that iriertial‐range turbulence operates at intermediate and large scales in the topside ionosphere. Meanwhile, spectra computed from plasma density data determined to lie outside the inertial regime exhibit a broad range of spectral indices.
Phase bunching during substorm dipolarization
Authors: Delcourt D. C. , Sauvaud J.‐A. , Moore T. E.
Published: February 2004

We investigate the dynamics of near‐Earth plasma sheet ions during storm time dipolarization of the magnetospheric field lines. We more specifically examine the behavior of O+ ions that are trapped in the equatorial vicinity. We show that during field line dipolarization, these particles may be transported in a nonadiabatic manner and experience large magnetic moment enhancement together with prominent bunching in gyration phase. An analytical estimate is obtained that is in good agreement with numerical trajectory calculations and allows detailed analysis of the adiabatic‐nonadiabatic transition. We focus on the gyrophase bunching effect and demonstrate that it occurs below some threshold energy at the dipolarization onset, this threshold energy being controlled by the amplitude of the magnetic transition and by the injection depth in the magnetotail. In the near‐Earth tail, the phase‐bunched particles possibly experience intense (up to the hundred of keV range) nonadiabatic energization and are radially distributed in a well‐structured manner. As a result of this, systematic trajectory computations reveal that phase bunching during storm time dipolarization can lead to significant though localized variations of the cross‐tail current and may thus be responsible for large fluctuations of the magnetic field.
Artificial auroral effects from a bare conducting tether
Authors: Martínez‐Sanchez M. , Sanmartín J. R.
Published: February 2004

An electrically floating metallic bare tether in a low Earth orbit would be highly negative with respect to the ambient plasma over most of its length, and would be bombarded by ambient ions. This would liberate secondary electrons, which, after acceleration through the same voltage, would form a magnetically guided two‐sided planar e beam, and result in auroral effects (ionization and light emission) upon impact on the atmospheric E layer, at about 120–140 km altitude. This paper examines in a preliminary way the feasibility of using this effect as an upper atmospheric probe. Ionization rates can reach up to 105 cm−3 s−1 if a tape, instead of a wire, is used as tether. Contrary to standard e beams, the beam from the tether is free of spacecraft charging and plasma interaction problems, and its energy flux varies across the cross section, which is quite large; this would make possible continuous observation from the satellite, with high resolution, both spectral and vertical, of the induced optical emissions. Ground observation might be possible at latitudes around 40°, for night, magnetically quiet conditions.
A comparison of a model for the theta aurora with observations from Polar, Wind, and SuperDARN
Authors: Chang S.‐W. , Scudder J. D. , Sigwarth J. B. , Frank L. A. , Maynard N. C. , Burke W. J. , Peterson W. K. , Shelley E. G. , Friedel R. , Blake J. B. , Greenwald R. A. , Lepping R. P. , Sofko G. J. , Villain J.‐P. , Lester M.
Published: February 2004

A model is presented according to which theta auroral arcs form after southward turnings of interplanetary magnetic field (IMF) and/or large variations in IMF By, following prolonged periods of northward IMF or very small Bz, with |By| ≳ |Bz|. The arcs start on the dawnside (duskside) of the auroral oval and drift duskward (dawnward) across the polar cap for positive (negative) By in the northern hemisphere and conversely in the southern hemisphere. After the theta aurora has formed, changes in IMF By or Bz readjust the merging configuration and continue the auroral pattern. The transpolar arcs are on closed magnetic field lines that bifurcate two open sections of the polar cap and map to the outer plasma sheet. Four theta auroral events were studied using data from the ISTP/GGS Polar and Wind spacecraft and the ground‐based SuperDARN radars. Observations that are correctly predicted by our model include the following: (1) The formation and evolution of theta auroras observed by the visible imaging system are closely related to the IMF patterns measured by the Wind magnetic field investigation. (2) Both electrons and ions in the transpolar arc and poleward part of the night side auroral oval exhibit similar spectral characteristics, identified from the data acquired with Hydra and the comprehensive energetic particle and pitch angle distribution experiment. The low‐energy electrons show counterstreaming distributions, consistent with their being on closed field lines that magnetically connect to the boundary plasma sheet in the magnetotail. (3) Ion composition measurements obtained from the toroidal imaging mass‐angle spectrograph show cold plasma outflows from the ionosphere and hot, Isotropic magnetospheric ions in the two regions, also indicating transpolar arcs are on closed field lines. (4) Large scale polar cap convection inferred by SuperDARN observations is well correlated with IMF patterns. (5) Plasma convection in the transpolar arcs, inferred from the electric field instrument and the magnetic field investigation measurements, is sunward.
Oblique turbulence driven by field‐aligned electron fluxes in the auroral ionosphere
Authors: Muschietti L. , Roth I. , Delory G.
Published: February 2004

This paper investigates how the various waves of the whistler resonance cone compete for the free energy available in field‐aligned fluxes of precipitating electrons in the auroral ionosphere. A two‐dimensional numerical simulation code, based on the coupled pair of quasi‐linear equations, is used for the investigation. The description provided by these equations allows us to follow the nonlinear development of the instability and analyze the interplay between the modes as mediated by the changing distribution function. We present results for a parameter regime corresponding to altitudes probed by sounding rockets, around 103 km and Ωe ≳ ωp. The presence of a halo of energetic electrons significantly affects the evolution of the turbulence. The spectrum is shown to shift with time toward increasingly oblique propagation angles including a substantial share of quasi perpendicular, short‐wavelength modes close to the lower hybrid frequency.
Disturbances in Mercury's magnetosphere: Are the Mariner 10 “substorms” simply driven?
Authors: Luhmann J. G. , Russell C. T. , Tsyganenko N. A.
Published: February 2004

In addition to providing the first in situ evidence of a magnetosphere at Mercury, the first flyby by Mariner 10 inspired reports of Earth‐like substorms. While the small scales at Mercury should make the substorm timescale there much shorter than it is at the Earth, these early interpretations may have too readily assumed that the substorm requirement of an energy storage and release phase occurs. Instead, its size should make Mercury's magnetosphere especially prone to disturbances that are purely “driven” by the changing external boundary conditions on the magnetosphere imposed by the solar wind. These result simply from the magnetosphere's relatively unhindered reconfiguration in response to the varying interplanetary parameters, including the IMF southward component. In this paper we demonstrate that the “disturbed” structure observed outbound from closest approach during the first Mariner 10 flyby can alternately be explained as a consequence of a typical period of rotating IMF. We use an appropriately modified IMF‐dependent terrestrial magnetosphere model scaled for Mercury, together with an assumed, reasonable IMF time series, to reproduce the magnetic field signature during the disturbed outbound pass segment. This result suggests that rapid restructuring of the small magnetosphere in response to changing local interplanetary conditions may dominate the magnetospheric dynamics at Mercury. Future Mercury magnetosphere missions should be instrumented to distinguish between this driven magnetospheric dynamism and any internal Earth‐like substorm processes. These results also remind us that driven reconfigurations must always be considered in studies of transients in the terrestrial magnetosphere.
Dynamic simulations of slip on a smooth fault in an elastic solid
Authors: Ben‐Zion Yehuda , Rice James R.
Published: February 2004

We report on numerical simulations of slip evolution along a two dimensional (slip varies only with depth) vertical strike‐slip fault in an elastic half‐space, using a framework incorporating full inertial elastodynamics. The model is a follow‐up on earlier quasi‐static and quasi‐dynamic simulations of deformations along smooth fault systems in elastic continua. The fault is driven below a crustal depth of 24 km by a constant plate velocity of 35‐mm/yr. Deformation at each fault location in the crustal zone is the sum of slip rate contributions from rate‐ and state‐dependent friction and power law creep, where both processes have temperature‐dependent (and hence depth‐varying) coefficients and both take place locally under the same stress. The simulations employ two versions of rate‐ and state‐dependent friction: a “slip” law, which requires nonzero velocity for state evolution, and an ”ageing/slowness“ law, which incorporates state evolution and restrengthening in stationary contact. The assumed constitutive laws and distribution of frictional parameters are compatible with laboratory experiments. The elastodynamic calculations are based on spectral representations of variables and a new algorithm providing a unified computational framework for calculations of long deformational histories containing short periods of rapid instabilities. The simulations show dynamic rupture propagation and wave phenomena not accounted for in the previous quasi‐static and quasi‐dynamic works. However, the results are qualitatively similar to those obtained by corresponding quasi‐static and quasi‐dynamic calculations. Slip histories along a smooth fault, simulated here with full elastodynamics for various constitutive laws and model parameters, consist mostly of quasi‐periodic large events. This finding indicates that inertial dynamics does not provide a generic mechanism for generating spatio‐temporal complexities of slip. On the other hand, calculations done for cases representing, approximately, strongly disordered systems do show rich slip histories with a range of event sizes. This result is compatible with our previous conclusions that the origin of observed broad distributions of earthquake sizes is strong fault zone heterogeneities. The fully dynamic calculations illustrate the evolution of nucleation phases of instabilities associated with accelerating and expanding creep. Final slip values of model earthquakes in full elastodynamic calculations are larger than those of corresponding quasi‐static and quasi‐dynamic events. The dynamic overshoot in simulations with the slip version of friction is larger than in those employing the ageing/slowness law.
Stability of a frictional, cohesive layer on a viscous substratum: Validity of asymptotic solution and influence of material properties
Authors: Triantafyllidis N. , Leroy Y. M.
Published: February 2004

This study deals with the stability of a stratified structure composed of a cohesive and frictional overburden, a viscous substratum, and a rigid basement. That structure should be seen as a prototype for various salt tectonics and lithospheric plates stability analyses. The destabilizing factors are the density contrast, the tectonic compressive stress, and the possible erosion and deposition at the top surface. The overburden stiffness, a nonlinear function of in situ stress, has a stabilizing role. Two solutions are extracted from the variational formulation of the stability problem previously proposed [Leroy and Triantafyllidis, 1996]: the first is analytical and is obtained by disregarding gravity, and the second is numerical and is based on the finite element method. The latter is used to assess the validity of the previously presented asymptotic solution. It is shown that the asymptotic solution is accurate even for values of the small parameter, defined as the perturbation wavenumber times the overburden thickness, as large as 0.4. Furthermore, the possibility for the cohesive material in the overburden to accommodate part of the deformation by slip along a population of small pervasive faults is accounted for by the introduction of a deformation theory of plasticity. Stability predictions based on this theory indicate that structural modes, such as folding, and localized faulting modes are triggered for similar stress magnitudes. The parametric study presented includes the previously undetected influences of the stress gradient with depth and of the work hardening properties of the competent overburden. The role of erosion and deposition in destabilizing shallow overburdens, regardless of the magnitude of the tectonic stress, is also established. The stability predictions are then applied to a folded section through the Campos basin, offshore Brazil, revealing that the deformation theory of plasticity is necessary to explain the buckling that occurred during the Albian.
Tropospheric water vapor derived from solar spectrometer, radiometer, and GPS measurements
Authors: Sierk Bernd , Bürki Beat , Becker‐Ross Helmut , Florek Stefan , Neubert Reinhart , Kruse Lars Peter , Kahle Hans‐Gert
Published: February 2004

Tropospheric water vapor is of central interest in a large variety of geoscientific fields, such as geodesy, geodynamics, climate research, and meteorology. A new instrumental approach to ground‐based mapping of tropospheric water vapor has been developed. It utilizes high‐resolution absorption measurements in the near‐infrared region by means of a solar spectrometer (SSM). The processing algorithm for retrieval of the precipitable water vapor (PW) is based on a line‐by‐line calculation of the observed solar spectrum in a narrow wavelength interval (1 nm) using a simple absorption model of the troposphere. To prove the feasibility and accuracy potential of the new technique, we carried out a 30‐day field experiment. Simultaneous measurements of colocated SSM, water vapor radiometers (WVR) and Global Positioning System (GPS) receivers were performed, exploiting absorption, emission and refraction properties of water vapor, respectively. A comparison of the three different techniques demonstrated the potential of solar spectrometry for precise and absolute determinaton of PW without meteorological a priori information. Apart from apparent systematic errors of the GPS measurements, a good agreement between the SSM and WVR results within their individual accuracy limits was observed. The PW standard deviations of the techniques were determined to 0.37 mm for the WVR, 0.75 mm for the SSM, and 1.40 mm for the GPS retrievals. The independence of SSM from external calibration by radiosondes and the high potential for further development may qualify this new technique to contribute to developing an error budget for other techniques, such as GPS meteorology.
A new reference spectrum for the EUV irradiance of the quiet Sun: 1. Emission measure formulation
Authors: Warren H. P. , Mariska J. T. , Lean J.
Published: February 2004

We present a quiet Sun irradiance spectrum from 50–1200 Å based primarily on intensities computed from a newly constructed quiet Sun emission measure distribution. We derive the emission measure from a spectrum of a portion of the quiet solar disk measured with the Harvard instrument on Skylab and recent compilations of atomic data. For some specific emission lines and continua which are not optically thin and cannot be computed using an emission measure, we either use intensities from the Harvard spectrum directly or infer them from other observations. Application of a simple center‐to‐limb variation converts the intensities to equivalent full‐disk quiet Sun fluxes.
Evolution of the Carlsberg Ridge between 60 and 45 Ma: Ridge propagation, spreading asymmetry, and the Deccan‐Reunion hotspot
Authors: Dyment J.
Published: February 2004

Despite the availability of numerous geophysical data, no convincing magnetic anomaly identification and structural map of a major part of the Arabian and eastern Somali Basins have been published to date. I propose a new interpretation of the available magnetic data based on the identification of ”tiny wiggles,” i.e., singular second‐order patterns of the magnetic signal. Only one fracture zone and a set of propagating rifts existed between the Owen Fracture Zone and the Chagos‐Laccadive Ridge between anomalies 26 and 20. At anomalies 26–25, at least two eastward propagating rifts are identified, while at anomalies 24–20, seven westward propagating rifts are confidently recognized. The major consequence of this systematic ridge propagation is a tremendous regional asymmetry: between anomalies 26 and 25, about 65% of the crust formed at the Carlsberg Ridge was accreted to the African Plate, while at anomalies 24–20, more than 75% benefited to the Indian Plate. I suggest that these asymmetries result from the relative position of the Carlsberg Ridge and the nearby Deccan‐Reunion hotspot, the ridge tending to remain near the hotspot.
Peroxisome proliferator‐activated receptor‐γ: too much of a good thing causes harm
Authors: Cock Terrie‐Anne , Houten Sander M , Auwerx Johan
Published: January 2004

The nuclear receptor peroxisome proliferator‐activated receptor‐γ (PPARγ) helps to translate ‘what you eat’ into ‘what you are’ because it allows dietary fatty acids (PPARγ ligands) to modulate gene transcription. Treatments for diabetes include PPARγ activators, as they sensitize the body to insulin. Our understanding of PPARγ function has recently been enhanced by a flurry of human and mouse genetic studies, and the characterization of new PPARγ ligands. This insight has led us to propose that modulating PPARγ activity, rather than activating it, might be the most effective strategy for treating metabolic disorders, as this will improve glucose homeostasis while preventing adipogenesis.
Biological activities of HAP46/BAG‐1
Authors: Gehring Ulrich
Published: January 2004

HAP46/BAG‐1M and its isoforms affect the protein‐folding activities of mammalian HSP70 chaperones. They interact with the ATP‐binding domain of HSP70 or HSC70, leaving the substrate‐binding site available for further interactions. Trimeric complexes can therefore form with, for example, transcription factors. Moreover, HAP46/BAG‐1M and the larger isoform HAP50/BAG‐1L bind to DNA non‐specifically and enhance transcription in vitro and upon overexpression in intact cells. These factors are linked to positive effects on cell proliferation and survival. This review focuses on DNA‐binding activity and transcriptional stimulation by HAP46/BAG‐1M, and presents a molecular model for the underlying mechanism. It is proposed that transcription factors are recruited into complexes with HAP46/BAG‐1M or HAP50/BAG‐1L through HSP70/HSC70 and that response‐element‐bound complexes that contain HAP46/BAG‐1M and/or HAP50/BAG‐1L along with HSP70s target and affect the basal transcription machinery.