Concurrent Magnetic Field Research Articles

A possible mechanism for the formation of an eastward moving auroral spiral

The generation process of auroral spirals is described by different theories varying for their morphology and surrounding conditions. Here, a possible mechanism is proposed for an eastward moving auroral spiral, which was observed in Tromsø, Norway, during the expansion phase of a substorm on 18 September 2013. Measurements from the THEMIS-A and Cluster spacecraft were analyzed, which were located up to ∼10 RE duskward from the spiral generator region in the magnetosphere. Precursory to the spiral observation, concurrent magnetic field dipolarizations, flow bursts and electron injections were measured by the Cluster satellites between 13.6 and 14.2 RE radial distance from Earth. A local Kelvin-Helmholtz-like vortex street in the magnetic field was detected at the same time, which was likely caused by bursty bulk flows. The vortex street was oriented approximately in the X-Y (GSE) plane and presumably propagated towards the source region of the spiral due to a high dawnward velocity component in the flow bursts. The observations suggest that the spiral can have been generated by an associated vortex in the magnetotail and then mapped along the magnetic field lines to the ionosphere. To better understand the role of the ionosphere in auroral spiral generation, in future more mesoscale observations are required.

Transient Flashes in Saturn’s UV Aurora: An Analysis of Hubble Space Telescope 2013–2017 Campaigns and Cassini Magnetic Field Measurements

We examined Hubble Space Telescope images of Saturn’s northern UV aurora in 2013–2017, identified 29 short-lived flashes, and examined simultaneous magnetometer data collected by the Cassini orbiter. When observation cadence permitted, a flash lifetime of 4–17 min (subject to exposure time-related uncertainties), and a 40–70 min recurrence period were found. An occurrence map shows a strong preference in both local time (14–19 LT) and latitude (75–85°). These transient flashes are identified in both the presence and absence of Saturn’s main auroral oval, indicating the lack of dependence on the main emission power. The concurrent magnetic field pulsations generally form a sawtooth shape, and the local field strength experiences a change of 0.2 to 2.0 nT (depending on the distance of Cassini). The quasiperiodic pulsation events were all detected when the spacecraft was in the southern hemisphere with conjugate flashes in northern aurora, suggesting these events occur on closed field lines, and typically showing a sudden transition to a less lagging, more southward magnetic field configuration. We also found the ionospheric footprint of the spacecraft must be close to the region of flashes for magnetic field pulsations to be detected, implying a localised rather than global driving process.

Formation of 30 KeV Proton Isotropic Boundaries During Geomagnetic Storms

AbstractWe study the origin of the 30 keV proton isotropic boundary (IB) in the nightside auroral zone during geomagnetic storms, particularly, to address the recent results that the adiabaticity parameter K (ratio of the magnetic field line curvature radius to the particle gyroradius at the equator) on the IB field line can be much larger comparing to its theoretical estimate K ∼ 8 for the field line curvature (FLC) scattering mechanism. During nine storms in 2011–2013, we investigate ∼2,000 IBs observed by low‐altitude Polar Operational Environmental Satellites (POES) satellites and apply the TS05 magnetospheric model to estimate the K value in the equatorial part of the IB field line. The statistical distribution of the estimated K parameter, while being rather broad, is centered on K = 9–13. For smaller subset of ∼250 IBs, the concurrent magnetic field measurements on board Time History of Events and Macroscale Interaction During Substorms probes in the equatorial magnetotail were used to correct the estimated K‐values accounting for the TS05 deviations from the real magnetic configuration. After correction, the K distribution becomes narrower, being still centered on K = 9–12. Different estimates give percentages of events with K < 13, which can be attributed to IBs formed by FLC scattering, between 60% and 80%. Finally, we have not found any dependence of the K distribution on magnetic local time and IB latitude, except for events with IB located at extremely low latitudes (<59°). These findings imply that the FLC scattering is a dominant mechanism of IB formation operating in a variety of magnetospheric conditions.

Ultrafast Ligand Self-Exchanging Gadolinium Complexes in Ionic Liquids for NMR Field Probes.

Concurrent magnetic field monitoring in MRI with an array of NMR field probes allows for reducing image imperfections. High 19F concentrations together with short relaxation times are basic probe properties. We present the NMR properties of [Gd(NTf2)4]- dissolved in an ionic liquid consisting of an imidazolium cation and the [NTf2]- anion. These solutions achieve fluorine concentrations as high as 26 M and rapid relaxation times in the sub-ms time range. The second order self-exchange rate of coordinated [NTf2]- with [NTf2]- as determined with [Y(NTf2)4]- is 4.5 × 105·M-1·s-1. X-ray structure analyses confirm the coordination number of eight around Gd3+ and Y3+. These ionic-liquid solutions are thus excellent candidates for dynamically probing the NMR fields in MRI.

Cumulative Residual Photon Power Density of ~10<sup>-12</sup> W•m<sup>-2</sup> During Mild “Distress” in the Same Space: Implications for Temporal Entanglement

The potential for entanglement of photons generated within the space-time continuum to remain as residuals of photon flux density within the same space requires excess correlations between successive temporal increments. Our model predicted that the quantitative relationship with the fundamental quantity of 10 -20 J multiplied by the inverse diffusivity from the wave impedance and magnetic susceptibility of space and the electron orbital frequency would reflect excess correlation. The value would be ~10 -12 W• m -2 . To test this prediction experimentally, different mice were serially exposed within the same container box or each mouse was placed in different container boxes placed in the same space for 3 min per mouse while photons were measured from the dorsal surfaces in hyper-dark settings. Before asymptote was evident around 30 to 35 min of serial exposures the net increase in photon flux densities within that same space was 10 -12 W•m -2 . These results suggest that the same “space” may “store” photon-related information as indicated by previous experiments involving chemiluminescent reactions. We postulate that entanglement between photons emitted from biological systems during distress within the same space and specific concurrent magnetic field patterns may create the conditions for the “retrieval” of these photon patterns at some later date when these fields recur.

Diffusion MRI with concurrent magnetic field monitoring.

Diffusion MRI is compromised by unknown field perturbation during image encoding. The purpose of this study was to address this problem using the recently described approach of concurrent magnetic field monitoring. Magnetic field dynamics were monitored during the echo planar imaging readout of a common diffusion-weighted MRI sequence using an integrated magnetic field camera setup. The image encoding including encoding changes over the duration of entire scans were quantified and analyzed. Field perturbations were corrected by accounting for them in generalized image reconstruction. The impact on image quality along with geometrical congruence among different diffusion-weighted images was assessed both qualitatively and quantitatively. The most significant field perturbations were found to be related to higher-order eddy currents from diffusion-weighting gradients and B0 field drift as well as gradual changes of short-term eddy current behavior and mechanical oscillations during the scan. All artifacts relating to dynamic field perturbations were eliminated by incorporating the measured encoding in image reconstruction. Concurrent field monitoring combined with generalized reconstruction enhances depiction fidelity in diffusion imaging. In addition to artifact reduction, it improves geometric congruence and thus facilitates image combination for quantitative diffusion analysis.

Monitoring, analysis, and correction of magnetic field fluctuations in echo planar imaging time series.

To assess the utility of concurrent magnetic field monitoring for observing and correcting for variations in k-space trajectories and global background fields that occur in single-shot echo planar imaging (EPI) time series as typically used in functional MRI (fMRI). Field monitoring was performed using an array of NMR field probes operated concurrently with series of single-shot EPI acquisitions from a static phantom. The observed fluctuations in field evolution were analyzed in terms of their temporal and spatial behavior at the field level as well as at the level of reconstructed image series. The potential to correct for such fluctuations was assessed by accounting for them upon image reconstruction. An indication of the number and relative magnitude of underlying effects was obtained via principal component analysis. Trajectory and global field variations were found to induce substantial image fluctuations. Global field fluctuations induced standard deviations in image intensity up to 31%. Fluctuations in the trajectory induced ghosting artifacts with standard deviations up to 2%. Concurrent magnetic field monitoring reduced the fluctuations in the EPI time series to a maximum of 1.2%. Concurrent magnetic field monitoring holds the potential to improve the net sensitivity of fMRI by reducing signal fluctuations unrelated to brain activity.

Matched-filter acquisition for BOLD fMRI

We introduce matched-filter fMRI, which improves BOLD (blood oxygen level dependent) sensitivity by variable-density image acquisition tailored to subsequent image smoothing. Image smoothing is an established post-processing technique used in the vast majority of fMRI studies. Here we show that the signal-to-noise ratio of the resulting smoothed data can be substantially increased by acquisition weighting with a weighting function that matches the k-space filter imposed by the smoothing operation. We derive the theoretical SNR advantage of this strategy and propose a practical implementation of 2D echo-planar acquisition matched to common Gaussian smoothing. To reliably perform the involved variable-speed trajectories, concurrent magnetic field monitoring with NMR probes is used. Using this technique, phantom and in vivo measurements confirm reliable SNR improvement in the order of 30% in a “resting-state” condition and prove robust in different regimes of physiological noise. Furthermore, a preliminary task-based visual fMRI experiment equally suggests a consistent BOLD sensitivity increase in terms of statistical sensitivity (average t-value increase of about 35%). In summary, our study suggests that matched-filter acquisition is an effective means of improving BOLD SNR in studies that rely on image smoothing at the post-processing level.

Phase Space Density matching of relativistic electrons using the Van Allen Probes: REPT results

Phase Space Density (PSD) matching can be used to identify the presence of nonadiabatic processes, evaluate accuracy of magnetic field models, or to cross‐calibrate instruments. Calculating PSD in adiabatic invariant coordinates requires a global specification of the magnetic field. For a well specified global magnetic field, nonadiabatic processes or inadequate cross calibration will give a poor PSD match. We have calculated PSD(μ, K) for both Van Allen Probes using a range of models and compare these PSDs at conjunctions in L* (for given μ, K). We quantitatively assess the relative goodness of each model for radiation belt applications. We also quantify the uncertainty in the model magnetic field magnitude and the related uncertainties in PSD, which has applications for modeling and particle data without concurrent magnetic field measurements. Using this technique, we show the error in PSD for an energy spectrum observed by the relativistic electron‐proton telescope (REPT) is a factor of ∼1.2 using the TS04 model.

Structure of the dayside magnetopause for low magnetic shear

We have analyzed 22 AMPTE/IRM satellite passes through the low‐latitude magnetopause region for which the magnetic shear, i.e., the field rotation angle on transit from the magnetosheath to the magnetosphere, was less than 30°. We found that on all passes a key time could be identified where the proton temperature, and usually also in the electron temperature and the temperature anisotropies, show net and often discontinuous changes. A change in plasma flow direction also occurs at this time. The small field rotations that occur have no fixed relationship to the key time. Earthward of the key time, the plasma has the characteristics of the magnetopause boundary layer, i.e., reduced density and bulk velocity, and in particular an electron temperature anisotropy with Te∥ > Te⊥, until further and usually more dramatic changes in plasma thermal properties mark the entry into the magnetosphere proper. Boundary layer durations varied widely, from 4s to 14 min. On the magnetosheath side we observed a layer of plasma density depletion (by a factor of two or more) and concurrent magnetic field pile‐up in less than half the crossings. This depletion layer was 3 min wide on average and often characterized by a drop in Tp∥, as predicted. From the observational evidence we conclude that the changes in plasma thermal and flow properties at the key time mark the crossing of the magnetopause under conditions of low magnetic shear. It is tempting to attribute these changes to the crossing of a topological boundary, e.g., a transition from open interplanetary to closed geomagnetic field lines. There is evidence, however, that in some cases the field lines immediately earthward of the key time are not closed. The consistent presence of a boundary layer inside the low‐shear magnetopause confirms earlier inferences that solar wind plasma can enter the magnetosphere regardless of field orientation. Inspection of the velocity distribution functions indicates that the plasma is heated upon entry. Taking measured plasma velocities along the magnetopause normal direction, we have inferred an average magnetopause speed of 11 km/s, an indication that the low‐shear magnetopause moves more slowly. The duration of the temperature transition translates into a thickness of often less than 50 km, comparable with the typical proton gyroradius. For the plasma depletion layer and the boundary layer we obtained average thicknesses of 0.4 and 0.3 RE, respectively.

Multi-station observation of IPDP micropulsations - Two-dimensional distribution and evolution of the source regions.

Short period geomagnetic pulsations called IPDP (intervals of pulsations of diminishing periods) are investigated, using ground data obtained at 35 campaign stations in the northern polar region. Comparison of the frequency-time (f-t) spectrograms of data from the stations shows that the internal structures of f-t pattern of IPDP pulsations vary with a small separation between stations. The variability of the internal structures suggests that “an IPDP event” consists of many discrete wave packets each of which comes from individual sources. Spatial-temporal developments of IPDP wave power are also examined. The two-dimensional distribution of IPDP power is both highly structured in space, suggesting presence of discrete source regions, and also highly variable with time, indicating characteristic dynamic features of the source regions. The source regions are located in the dusk half of the magnetosphere in a range of L value from 4 to 10. Three types of dynamics of the source regions are found; 1) fast westward motion consistent with magnetic-drift velocity of 60-70keV protons in the magnetosphere, 2) quasi-stationary sources, which are almost stationary in the magnetospheric frame for periods longer than 10 minutes, 3) evolution of the IPDP region toward the night and earthward with increase in center frequency. When the westward moving sources arrive at the location of the stationary sources, they merge with them and do not go further westward beyond the stationary sources. These time developments of the IPDP source regions comprehensively explain most of the IPDP characteristics so far obtained from observations in a limited area. Not only well-defined IPDP's but also IPDP-like events with small and slower frequency rise were observed in high latitudes associated with concurrent magnetic field depressions at synchronous orbit and with positive magnetic bays. For both, the magnetic field depression is probably due to hot plasma injection into the evening sector, although clear substorm onsets were only identified for well-defined IPDP's. The occurrence rate of IPDP events monitored within the semi-global coverage of the network was once every six days or 20 percent in hourly occurrence in the dusk sector, more than twice the previously reported rate. The occurrence rate of IPDP is still much smaller than that of substorms. The small occurrence shows that IPDP events are not caused by every “plasma injection” associated with substorms.

Impulsive particle precipitation and concurrent magnetic field changes observed in conjugate areas near L = 4

Using sensitive riometers (30 MHz) and magnetometers at Siple Station, Antarctica, and at four locations in the conjugate area in the northeastern United States and Southern Quebec, we have studied the temporal and spatial extent of the impulsive losses of electrons from the radiation belts and the concurrent accompanying magnetic field variations produced by the changes in ionospheric current systems. The impulsive variations have a short (∼2–4 s) onset time and a ∼5–10 s decay time. The observed decay times in the riometer and magnetometer signals are consistent with the recombination times in the ionosphere at ∼90–100 km. Often the magnetic field fluctuations are observed to occur primarily in the east‐west direction and are out of phase between the southern and northern hemispheres. These observations suggest that predominantly north‐south ionospheric currents are modified and/or produced by such impulsive precipitation. Time delays in the occurrence of the events between the northern and southern hemispheres, as well as time and spatial differences observed between the northern hemisphere stations, suggest that irregularly shaped, gradient B drifting patches of precipitating electrons could be the source of these impulsive events.

An observed characteristic of concurrent magnetic field and electron precipitation pulsations

In each case for a collection of seven large-amplitude events, spectral peaks in the 1 to 10 mHz range for electron precipitation pulsations were observed to correspond to spectral peaks for concurrent magnetic field pulsations. The converse is not true, i.e. a magnetic field pulsation spectral peak need not correspond to a precipitation spectral peak, and in fact there may not even be a concurrent precipitation pulsation event. The data are consistent with occurrences of interactions between a portion of the hydromagnetic wave spectrum and trapped electrons in the equatorial region of the field line of the site.

A study of concurrent magnetic field and particle precipitation pulsations, 0.005 to 0.5 Hz, recorded near College, Alaska

Precipitation pulsations, as detected by the riometer, were studied together with induction magnetometer and earth current recordings of magnetic field pulsations. Type Pi (broadband) magnetic field pulsations were found to be well correlated with riometer pulsations in the same frequency range and abrupt onsets were simultaneous to within a few seconds. The results imply, at least for the evening-midnight impulsive events (Type Pi B), that the first-arriving Pi waves originate in the ionosphere and are a consequence of the precipitation pulsations. Three Pc 1–2 events were found in the riometer pulsation records and the Pc 1–2 riometer pulsations lagged the magnetic field pulsations by 5–8 sec.

A study of concurrent magnetic field and particle precipitation pulsations, 0.005 to 0.5 Hz, recorded near College, Alaska

Precipitation pulsations, as detected by the riometer, were studied together with induction magnetometer and earth current recordings of magnetic field pulsations. Type Pi (broadband) magnetic field pulsations were found to be well correlated with riometer pulsations in the same frequency range and abrupt onsets were simultaneous to within a few seconds. The results imply, at least for the evening-midnight impulsive events (Type Pi B), that the first-arriving Pi waves originate in the ionosphere and are a consequence of the precipitation pulsations. Three Pc 1–2 events were found in the riometer pulsation records and the Pc 1–2 riometer pulsations lagged the magnetic field pulsations by 5–8 sec.