Variation Of Magnetic Field Strength Research Articles

Intercluster conduction in lightly doped La1 − x Ca x MnO3 manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La 1 − xCaxMnO3 polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature TC. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ TC is fitted by the relation logρ ∼ T−1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.

Magnetic signatures and conjugate features of low‐latitude plasma blobs as observed by the CHAMP satellite

We investigated the magnetic signatures of plasma blobs, as observed by the CHAMP satellite. In total, we have identified 52 blobs with clear density enhancements and magnetic signatures. In each blob the magnetic field strength was depressed, and the components perpendicular to the main magnetic field direction showed fluctuations. The variation in magnetic field strength implies that the enhanced plasma pressure is balanced by a magnetic pressure reduction, and the deflection in the perpendicular components indicates the presence of field‐aligned currents. Both characteristics are consistent with bubble magnetic signatures. Concurrent observations of CHAMP (at ∼350 km altitude) with ROCSAT‐1 (∼600 km), STSAT‐1 (∼680 km), and DMSP F15 (∼840 km) strongly suggest that the blobs have a field‐aligned structure spanning several hundred kilometers. We also investigated the seasonal/longitudinal distribution of the detected blob events. From that a rough coincidence with the bubble occurrence distribution is found, although the blob distribution is biased heavily toward the winter hemisphere. Plasma blob encounters at CHAMP altitude are quite frequent, even after local midnight. We believe that our results corroborate the close relationship between equatorial plasma bubbles and blobs.

On the edge of the foreshock: model-data comparisons

Abstract. We present the results of a global hybrid code simulation for the solar wind-interaction with the Earth's magnetosphere during an interval of steady radial IMF. The model predicts a foreshock marked by innumerable localized, correlated, and large amplitude, density and magnetic field strength variations, depressed velocities, and enhanced temperatures. The foreshock is bounded by a broad (~0.8 RE) region of enhanced densities, temperatures, and magnetic field strengths that extends far (~8.6 RE) upstream from the bow shock. Flow perturbations within the boundary are directed perpendicular to the boundary, towards the unperturbed solar wind and away from the foreshock. Cluster observations of the ion foreshock and pristine solar wind confirm the predictions of the model. The observations suggest that foreshock cavities, crater-like density and magnetic field strength structures whose cores are filled with suprathermal particles, can be interpreted in terms of transient encounters with the foreshock boundary.

Dynamics of conical wire array Z-pinch implosions

A modification of the wire array Z pinch, the conical wire array, has applications to the understanding of wire array implosions and potentially to pulse shaping relevant to inertial confinement fusion. Results are presented from imploding conical wire array experiments performed on university scale 1 MA generators—the MAGPIE generator (1 MA, 240 ns) at Imperial College London [I. H. Mitchell et al., Rev. Sci Instrum. 67, 1533 (1996)] and the Nevada Terawatt Facility’s Zebra generator (1 MA, 100 ns) at the University of Nevada, Reno [B. Bauer et al., in Dense Z-Pinches, edited by N. Pereira, J. Davis, and P. Pulsifer (AIP, New York, 1997), Vol. 409, p. 153]. This paper will discuss the implosion dynamics of conical wire arrays. Data indicate that mass ablation from the wires in this complex system can be reproduced with a rocket model with fixed ablation velocity. Modulations in the ablated plasma are present, the wavelength of which is invariant to a threefold variation in magnetic field strength. The axial variation in the array leads to a zippered precursor column formation. An initial implosion of a magnetic bubble near the cathode is followed by the implosion zippering upwards. Spectroscopic data demonstrating a variation of plasma parameters (e.g., electron temperature) along the Z-pinch axis is discussed, and experimental data are compared to magnetohydrodynamic simulations.

Tsallis distributions of magnetic field strength variations in the heliosphere: 5 to 90 AU

The Tsallis (q‐exponential) distribution function, derived from the entropy principle of nonextensive statistical mechanics, describes fluctuations in the magnetic field strength on many scales throughout the heliosphere. This paper shows that a one‐dimensional multifluid magnetohydrodynamic (MHD) model, with Advanced Composition Explorer (ACE) observations at 1 AU as input, predicts Tsallis distributions between 5 and 90 AU on scales from 1 to 128 days. At a scale of 1 day, the radial variation of the entropic indexqdecreases fromq≥ 5/3 atR≤ 50 AU toq≤ 5/3 atR≥ 60 AU, corresponding to a change from a divergent to a convergent second moment of the Tsallis distribution, suggesting the possibility of a “phase transition” and/or a relaxation effect at ≈60 AU. The Tsallis distribution derived from the time series of one‐dimensional MHD model is nearly identical to those observed by Voyager 1 at ∼80 AU over the scales from 1 to 64 days during the year 2000. The Tsallis distribution appears over a wide range of scales and distances despite the complex nonlinear dynamical evolution of the heliospheric magnetic field during 1999/2000.

New scaling relations in cluster radio haloes and the re-acceleration model

In this paper we derive new expected scaling relations for clusters with giant radio haloes in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P R ) versus size of the radio emitting region (R H ), and P R versus total cluster mass (M H ) contained in the emitting region and cluster velocity dispersion (σ H ) in this region. We search for these correlations by analysing the most recent radio and X-ray data available in the literature for a well-known sample of clusters with giant radio haloes. In particular we find a good correlation between P R and R H and a very tight 'geometrical' scaling between M H and R H . From these correlations P R is also expected to scale with M H and σ H and this is confirmed by our analysis. We show that all the observed trends can be well reconciled with expectations in the case of a slight variation of the mean magnetic field strength in the radio halo volume with M H . A byproduct correlation between R H and σ H is also found, and can be further tested by optical studies. In addition, we find that observationally R H scales non-linearly with the virial radius of the host cluster, and this immediately means that the fraction of the cluster volume which is radio emitting increases with cluster mass and thus that the non-thermal component in clusters is not self-similar.

Role of coronal mass ejections in the heliospheric Hale cycle

The 11‐year solar cycle variation in the heliospheric magnetic field strength can be explained by the temporary buildup of closed flux released by coronal mass ejections (CMEs). If this explanation is correct, and the total open magnetic flux is conserved, then the interplanetary‐CME closed flux must eventually open via reconnection with open flux close to the Sun. In this case each CME will move the reconnected open flux by at least the CME footpoint separation distance. Since the polarity of CME footpoints tends to follow a pattern similar to the Hale cycle of sunspot polarity, repeated CME eruption and subsequent reconnection will naturally result in latitudinal transport of open solar flux. We demonstrate how this process can reverse the coronal and heliospheric fields, and we calculate that the amount of flux involved is sufficient to accomplish the reversal within the 11 years of the solar cycle.

Long-term geomagnetic indices and their use in inferring solar wind parameters in the past

We discuss three new geomagnetic indices [the Inter-Hour Variability ( IHV), the Inter-Diurnal Variability ( IDV), Svalgaard, L., Cliver, E.W. The IDV index: its derivation and use in inferring long-term variations of the interplanetary magnetic field strength. J. Geophys. Res. 110, A12103. doi:10.1029/2005JA011203, 2005; and the Polar Cap Potential ( PCP) index, Le Sager, P., Svalgaard, L. No increase of the interplanetary electric field since 1926. J. Geophys. Res. 109 (A7), A07106. doi:10.1029/2004JA010411, 2004], that are derivable from data available for a century or more. Each of these indices responds directly to either the solar wind magnetic field strength ( B) or to different combinations of B and the solar wind speed ( V). This over-determined system permits a reconstruction of these parameters for the past ∼150 years. The variation of yearly averages of B can be described as a constant value (4.6 nT) plus a component varying with the square root of the sunspot number. Because the latter seems to exhibit a ∼100 year Gleissberg cycle, B does as well. Since 1890, annual averages of V range from a low of ∼300 km/s in 1902 to 545 km/s in 2003. The IHV-index fords a way to check the calibration of other long-term geomagnetic indices. We find that the ap-index tracks the variation of IHV, back to 1932 but that the aa-index (extended back to 1844) is systematically too low (3–6 nT) before 1957, relative to modern values.

Comment on “The IDV index: Its derivation and use in inferring long‐term variations of the interplanetary magnetic field strength” by Leif Svalgaard and Edward W. Cliver

[1] Svalgaard and Cliver [2005] (hereinafter referred to as SC05) use the IDV geomagnetic index to infer the variation of the interplanetary magnetic field strength, B, since 1872. They find that ‘‘B increased by 25% from the 1900s to the 1950s’’ (paragraphs 1 and 24) and that this ‘‘is in contrast to the more than doubling of B during the 20th century obtained from an analysis of the aa index by Lockwood et al. [1999]’’ (paragraph 24). We agree with neither statement. We identify a number of errors and biases in the analysis of SC05, each of which acts in the same direction, namely to reduce the true long-term drift. The key result of Lockwood et al. (hereinafter referred to as LEA99) is a doubling, not in B, but rather in the total coronal source flux which is proportional to the radial IMF component, Br. This is an important distinction which we discus here in section 3. We show (in section 2) that SC05’s own analysis gives a drift in decadal averages in B of 38%, not the ‘‘ 25%’’ that they quote, but we also point out (section 4) that SC05’s method for dealing with data gaps gives rise to a (small) bias and that their regression procedure (which is compared to others in section 5) is not robust (sections 6 and 7), in addition to them not taking into account the distinction between B and Br . We here argue that a simple ordinary linear regression procedure, as used by SC05, is inferior to the method employed by LEA99 (section 9) but does nevertheless show that the IDV index is fully consistent with the doubling in the open solar flux found by LEA99 (section 8).

The IDV index: Its derivation and use in inferring long‐term variations of the interplanetary magnetic field strength

On the basis of a consideration of Bartels' historical u index of geomagnetic activity, we devise an equivalent index that we refer to as the interdiurnal variability (IDV). The IDV index has the interesting and useful property of being highly correlated with the strength of the interplanetary magnetic field (B; R2 = 0.75) and essentially unaffected by the solar wind speed (V; R2 = 0.01) as measured by spacecraft. This enables us to obtain the variation of B from 1872 to the present, providing an independent check on previously reported results for the evolution of this parameter. We find that solar cycle average B increased by ∼25% from the 1900s to the 1950s and has been lower since. If predictions for a small solar cycle 24 bear out, solar cycle average B will return to levels of ∼100 years ago during the coming cycle(s).

Radial dependence of foreshock cavities: a case study

Abstract. We present a case study of Geotail, Interball-1, IMP-8, and Wind observations of density and magnetic field strength cavities excavated by the enhanced pressures associated with bursts of energetic ions in the foreshock. Consistent with theoretical predictions, the pressure of the energetic ions diminishes rapidly with upstream distance due to a decrease in the flux of energetic ions and a transition from near-isotropic to streaming pitch angle distributions. Consequently, the cavities can only be observed immediately upstream from the bow shock. A comparison of conditions upstream from the pre- and post-noon bow shock demonstrates that foreshock cavities introduce perturbations into the oncoming solar wind flow with dimensions smaller than those of the magnetosphere. Dayside geosynchronous magnetic field strength variations observed by GOES-8 do not track the density variations seen by any of the spacecraft upstream from the bow shock in a one-to-one manner, indicating that none of these spacecraft observed the precise sequence of density variations that actually struck the subsolar magnetopause. Key words. Interplanetary physics (energetic particles; planetary bow shocks) – Magnetospheric physics (solar wind-magnetosphere interactions)

Applications of adiabatic half passage to NQR

The application of nuclear quadrupole resonance (NQR) to the detection of materials can be hampered by the low sensitivity of the technique. The use of surface coils for remote detection only exacerbates this problem. In this paper we demonstrate the advantages of adiabatic half passage (AHP) for NQR detection ofI=1 spins in powder samples. Under optimal conditions, AHP provides a 15% sensitivity enhancement over traditional optimized, pulsed excitation. AHP excitation is independent of ω1 over more than an order of magnitude variation in radio-frequency (rf) field strength, and can provide up to a factor of two or more sensitivity enhancement over traditional pulsed excitation in inhomogeneous rf magnetic fields. In pulsed spin-locking-type experiments, AHP as a prepulse can provide near constant signal amplitude over a factor of two variation in rf magnetic field strength.

Geoeffectiveness of interplanetary shocks during solar minimum (1995–1996) and solar maximum (2000)

Plasma and magnetic field parameter variations across fast forward interplanetary shocks are analyzed during the last solar cycle minimum (1995–1996, 15 shocks), and maximum year 2000 (50 shocks). It was observed that the solar wind velocity and magnetic field strength variation across the shocks were the parameters better correlated with Dst. Superposed epoch analysis centered on the shock showed that, during solar minimum, B z profiles had a southward, long-duration variation superposed with fluctuations, whereas in solar maximum the B z profile presented 2 peaks. The first peak occurred 4 hr after the shock, and seems to be associated with the magnetic field disturbed by the shock in the sheath region. The second peak occurred 19 hr after the shock, and seems to be associated with the ejecta fields. The difference in shape and peak in solar maximum (Dst peak =−50 nT, moderate activity) and minimum (Dst peak =−30 nT, weak activity) in average Dst profiles after shocks are, probably, a consequence of the energy injection in the magnetosphere being driven by different interplanetary southward magnetic structures. A statistical distribution of geomagnetic activity levels following interplanetary shocks was also obtained. It was observed that during solar maximum, 36% of interplanetary shocks were followed by intense (Dst≤−100 nT) and 28% by moderate (−50≤Dst <−100 nT) geomagnetic activity. During solar minimum, 13% and 33% of the shocks were followed by intense and moderate geomagnetic activity, respectively. Thus, during solar maximum a higher relative number of interplanetary shocks might be followed by intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar cycle a shock has a probability of around 50–60% to be followed by intense/moderate geomagnetic activity.

Thin current sheets and loss of equilibrium: Three‐dimensional theory and simulations

Using quasi‐static equilibrium theory and three‐dimensional MHD simulations, we confirm earlier two‐dimensional results that finite boundary deformations of magnetotail equilibria can lead to large local current density enhancements, that is, the formation of thin current sheets. Equilibrium configurations that satisfy flux, entropy, and topology conservation cease to exist when the boundary deformations exceed critical limits. This provides a strong argument for onset of instability or the loss of equilibrium, regardless of the dissipation mechanism. Catastrophe points can be reached for relatively modest perturbations of the boundary. The most important internal property that influences the critical limit is the variation of the lobe magnetic field strength with distance from the Earth, related to the overall tail flaring. Stronger flaring, corresponding to a more rapid decrease of the lobe field with distance downtail, tends to stabilize the tail. That means such configurations require stronger perturbations to reach the critical limit or show less significant thinning and current density intensification for a given perturbation. Due to the nonlinearity of the tail response to the perturbations, the current intensification tends to be more localized than the perturbation.

Optimization of analytical performance of a graphite furnace atomic absorption spectrometer with Zeeman-effect background correction using variable magnetic field strength

This paper describes a graphite furnace atomic absorption spectrometer of the third generation with Zeeman-effect background correction and variable magnetic field strength. Compared to earlier generations it is focused on magnetic field strength control and technical and analytical performance parameters. The Zeeman ratios for 10 elements at different magnetic field strengths were determined. The capability of increasing the magnetic field strength up to a maximum of 1.0 T allows the analytical sensitivity for Cu to be improved by up to 38%, resulting in an enhanced Zeeman ratio, and two-folds increase of the linear working range. Alternatively, the introduction of the 3-field mode–measurement at zero, maximum and a medium magnetic field strength – allows the sensitivity to be adapted for higher analyte concentrations. With this mode, Cu, Fe and Zn were determined in serum, with analyte contents in a concentration range common for flame atomic absorption spectrometry. The results were in good agreement with the expected values. The dynamic mode combines the sensitive measurement mode of graphite furnace atomic absorption spectrometry with the reduced-sensitivity 3-field mode. The present work demonstrates how this mode enables to extend calibration over 3 orders of magnitude for Pb.

RXTE,ROSAT, andASCAObservations of G347.3−0.5 (RX J1713.7−3946): Probing Cosmic‐Ray Acceleration by a Galactic Shell‐Type Supernova Remnant

We present an analysis of the X-ray spectrum of the Galactic shell-type supernova remnant (SNR) G347.3-0.5 (RX J1713.7-3946). This SNR is a member of a growing class of dynamically young, shell-type SNRs that emit nonthermal X-rays from specific regions on their outer shells. By performing a joint spectral analysis of data from observations made of G347.3-0.5 using the ROSAT PSPC, the ASCA GIS, and the RXTE PCA, we have fitted the spectra of particular regions of this SNR (including the bright northwestern and southwestern rims, the northeast rim, and the interior diffuse emission) over the approximate energy range of 0.5-30 keV. We find that fits to the spectra of this SNR over this energy range using the SRCUT model were superior to a simple power-law model or the SRESC model. We also find that the inclusion of a thermal model with the SRCUT model helps to improve the fit to the observed X-ray spectrum: this represents the first detection of thermal X-ray emission from G347.3-0.5. Thermal emission appears to be more clearly associated with the diffuse emission in the interior of the SNR than with the bright X-ray-emitting rims. A weak emission feature seen near 6.4 keV in the RXTE PCA spectrum most likely originates from diffuse X-ray emission from the surrounding Galactic ridge rather than from G347.3-0.5 itself. We have analyzed our RXTE PCA data to search for pulsations from a recently discovered radio pulsar (PSR J1713-3949) which may be associated with G347.3-0.5, and we do not detect any X-ray pulsations at the measured radio period of 392 ms. Using the best-fit parameters obtained from the SRCUT model, we estimate the maximum energy of cosmic-ray electrons accelerated by the rims of G347.3-0.5 to be 36-48 TeV (assuming a magnetic field strength of B = 10 μG). We present a broadband (radio to γ-ray) photon energy-flux spectrum for the northwestern rim of G347.3-0.5, where we have fitted the spectrum using a more sophisticated synchrotron-inverse Compton model with a variable magnetic field strength. Our fit derived from this model yields a maximum energy of only 8.8 TeV for the accelerated cosmic-ray electrons and a much greater magnetic field strength of 150 μG; however, our derived ratio of volumes for TeV emission and X-ray emission based on this fit, VTeV/VX-ray ≈ 1000, is too large to be physically acceptable. We argue that neither nonthermal bremsstrahlung nor neutral pion particle decay can adequately explain the TeV emission from this rim, and therefore the physical process responsible for this emission at this site is currently uncertain. Finally, we compare the gross properties of G347.3-0.5 with other SNRs known to possess X-ray spectra dominated by nonthermal emission.

Coronal holes and the solar polar field reversal

The relationship between the solar polar magnetic field reversal and coronal hole evolution is investigated for the time interval from 1996 to 2001. The distribution of coronal holes shows some evolutionary changes in relation to the polarity reversals, and these changes appear to be coordinated with changes in the global magnetic field structure, suggesting that the polar reversal originates from global processes. There are periods when the character of the coronal hole distribution on the solar disk changes significantly, indicating that the magnetic field structure changes. These can be interpreted as periods of rearrangement of the multipole magnetic field structure. It is found that the evolution of the geometric structure of the photospheric magnetic field during these periods is not characterized by a continuous transition from one dominant structure to another, but by relatively sudden rearrangement of the dominant geometrical structure of the magnetic field. These coronal holes and photospheric magnetic field rearrangements coincide with the polar photospheric magnetic field strength variations. The minimum phase of the solar cycle is dominated by the dipole component of the global solar magnetic field. The zonal magnetic field structure and zonal non-polar coronal hole distribution existed at that time. The sectorial magnetic field structure appears when the polar field strength reaches 0.7 of its maximum value in the corresponding hemisphere. This structure was established at dierent times in each hemisphere. In the north hemisphere it has existed since November 1998 and in the south hemisphere it has existed since October 1997. The rearrangement from one configuration to another occurs during a short time period, about 1 2 solar rotations. The coronal hole number and area evolution indicates a redistribution of positive-polarity and negative-polarity photospheric magnetic fields inside these longitudinal sectors, which reflects the solar polar field reversal.

Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23

The Voyager 1 (V1) observations of the heliospheric magnetic field strength B agree with Parker's model of the global heliospheric magnetic field from 1 to 81.0 AU and from 1978 to 2001.34 when one considers the solar cycle variations in the source magnetic field strength and the latitude/time variation in the solar wind speed. In particular, Parker's model, without adjustable parameters, describes the general tendency for B to decrease with increasing distance R from the Sun, the three broad increases of B around 1980, 1990, and 2000, and the minima of B in 1987 and 1997. During 1987 and 1997, B appears to be lower than Parker's model predicts, but that can be attributed to the presence of a heliospheric vortex street at these times and/or uncertainty in the observations. There is no evidence for a significant flux deficit increasing monotonically from 1 to 81.0 AU. By extrapolating these results and considering the limitations of the observations, V1 should continue to make useful measurements during the next few years at least. The magnetic field polarity in the distant heliosphere at V1 and Voyager 2 (V2) changed during the ascending phase of solar cycle 23. In the Northern Hemisphere, V1 observed a decrease in the percentage of positive polarities from ≈100% during 1997 to ≈50% during 2000. In the Southern Hemisphere, V2 observed the opposite behavior, an increase in the percentage of positive polarities from ≈0% during 1997 to ≈50% during 2000. The variation of magnetic polarity observed by V1 and V2 was caused by the increasing latitudinal width of the sector zone with increasing solar activity, which in turn was related to the increasing maximum latitudinal extent and the decreasing minimum latitudinal extent of the footprints of the heliospheric current sheet (HCS). There was a tendency for the speed and proton temperature to decrease and the density to increase at V2 from 1997 (when it observed flows from polar coronal holes) to 2001 (when it observed more complex and dynamic flows).

Time profile of solar energetic particles fit using a mean free path considering the radial dependence of both magnetic field strength and fluctuations

The radially dependent mean free path of solar energetic particles was calculated by considering the radially dependent magnetic field fluctuation, its correlation length and the variation of magnetic field strength along an Archimedean interplanetary magnetic field. A longer mean free path can be deduced as radial distance approaches the sun. That is, the mean free path at 0.1 AU is increased by 10 times for some typical values of radial dependence of magnetic field fluctuation (e.g., power index of radial dependence = −2), compared to the value at 1 AU. The focused pitch angle transport equation without adiabatic deceleration was solved for the obtained mean free path. The calculated time profiles are compared with observations of solar energetic particles. In some cases, very good matches to observed data for longer periods are obtained with shorter mean free paths than the original authors mentioned.

The dynamical nature of the coronal streamer belt

SOHO/LASCO observations of the white-light corona have revealed a number of remarkable small-scale phenomena, including plasma blobs that are ejected continually from the cusps of streamers, fine raylike structures that pervade the streamer belt, and ubiquitous inflows during times of high solar activity. We discuss the implications of these observations for the origin of the heliospheric plasma sheet, the sources of the slow solar wind, and the variation of the interplanetary magnetic field strength.