427 research outputs found
Approaches for modeling magnetic nanoparticle dynamics
Magnetic nanoparticles are useful biological probes as well as therapeutic
agents. There have been several approaches used to model nanoparticle
magnetization dynamics for both Brownian as well as N\'eel rotation. The
magnetizations are often of interest and can be compared with experimental
results. Here we summarize these approaches including the Stoner-Wohlfarth
approach, and stochastic approaches including thermal fluctuations.
Non-equilibrium related temperature effects can be described by a distribution
function approach (Fokker-Planck equation) or a stochastic differential
equation (Langevin equation). Approximate models in several regimes can be
derived from these general approaches to simplify implementation
Generalized Scaling and the Master Variable for Brownian Magnetic Nanoparticle Dynamics
Understanding the dynamics of magnetic particles can help to advance several biomedical nanotechnologies. Previously, scaling relationships have been used in magnetic spectroscopy of nanoparticle Brownian motion (MSB) to measure biologically relevant properties (e.g., temperature, viscosity, bound state) surrounding nanoparticles in vivo. Those scaling relationships can be generalized with the introduction of a master variable found from non-dimensionalizing the dynamical Langevin equation. The variable encapsulates the dynamical variables of the surroundings and additionally includes the particles’ size distribution and moment and the applied field’s amplitude and frequency. From an applied perspective, the master variable allows tuning to an optimal MSB biosensing sensitivity range by manipulating both frequency and field amplitude. Calculation of magnetization harmonics in an oscillating applied field is also possible with an approximate closed-form solution in terms of the master variable and a single free parameter
Current Sheet and Reconnection Inflow-Outflow Observations During Solar Eruptions
Magnetic reconnection is widely accepted as a dominant source of energy during solar flares; however, observations of it have been indirect and/or incomplete. Using the suite of instruments available spanning wavelength space, we will provide observations and measurements of both the inputs and outputs predicted from reconnection in the form of inflows preceding outflows (i.e. supra-arcade downflows, supra-arcade downflowing loops, upflows, and disconnection events). We will also present evidence for current sheets through which reconnection is expected to occur and discuss current sheet motion during flare progression
The Closest View of a Fast Coronal Mass Ejection: How Faulty Assumptions near Perihelion Lead to Unrealistic Interpretations of PSP/WISPR Observations
We report on the closest view of a coronal mass ejection observed by the
Parker Solar Probe (PSP)/Wide-field Imager for {Parker} Solar PRobe (WISPR)
instrument on September 05, 2022, when PSP was traversing from a distance of
15.3~to~13.5~R from the Sun. The CME leading edge and an arc-shaped
{\emph{concave-up} structure near the core} was tracked in WISPR~field of view
using the polar coordinate system, for the first time. Using the impact
distance on Thomson surface, we measured average speeds of CME leading edge and
concave-up structure as 2500~~270\,km\,s and
400~~70\,km\,s with a deceleration of
20~m~s for the later. {The use of the plane-of-sky approach
yielded an unrealistic speed of more than three times of this estimate.} We
also used single viewpoint STEREO/COR-2A images to fit the Graduated
Cylindrical Shell (GCS) model to the CME while incorporating the source region
location from EUI of Solar Orbiter and estimated a 3D speed of
2700\,km\,s. We conclude that this CME exhibits the highest
speed during the ascending phase of solar cycle 25. This places it in the
category of extreme speed CMEs, which account for only 0.15\% of all CMEs
listed in the CDAW CME catalog.Comment: 13 Pages, 6 Figures; Accepted in The Astrophysical Journal Letter
Asteroid Redirect Mission Proximity Operations for Reference Target Asteroid 2008 EV5
NASA's Asteroid Redirect Mission (ARM) is composed of two segments, the Asteroid Redirect Robotic Mission (ARRM), and the Asteroid Redirect Crewed Mission (ARCM). In March of 2015, NASA selected the Robotic Boulder Capture Option1 as the baseline for the ARRM. This option will capture a multi-ton boulder, (typically 2-4 meters in size) from the surface of a large (greater than approx.100 m diameter) Near-Earth Asteroid (NEA) and return it to cis-lunar space for subsequent human exploration during the ARCM. Further human and robotic missions to the asteroidal material would also be facilitated by its return to cis-lunar space. In addition, prior to departing the asteroid, the Asteroid Redirect Vehicle (ARV) will perform a demonstration of the Enhanced Gravity Tractor (EGT) planetary defense technique2. This paper will discuss the proximity operations which have been broken into three phases: Approach and Characterization, Boulder Capture, and Planetary Defense Demonstration. Each of these phases has been analyzed for the ARRM reference target, 2008 EV5, and a detailed baseline operations concept has been developed
Production of eta Mesons in Double Pomeron Exchange
We estimate the production cross sections for and mesons
via pomeron-pomeron fusion in peripheral heavy-ion collisions. Total and
elastic PP cross sections are calculated in an equivalent pomeron
approximation.Comment: 9 pages, 3 Postscript figure
Enhanced Gravity Tractor Technique for Planetary Defense
Given sufficient warning time, Earth-impacting asteroids and comets can be deflected with a variety of different "slow push/pull" techniques. The gravity tractor is one technique that uses the gravitational attraction of a rendezvous spacecraft to the impactor and a low-thrust, high-efficiency propulsion system to provide a gradual velocity change and alter its trajectory. An innovation to this technique, known as the Enhanced Gravity Tractor (EGT), uses mass collected in-situ to augment the mass of the spacecraft, thereby greatly increasing the gravitational force between the objects. The collected material can be a single boulder, multiple boulders, regolith or a combination of different sources. The collected mass would likely range from tens to hundreds of metric tons depending on the size of the impactor and warning time available. Depending on the propulsion system's capability and the mass collected, the EGT approach can reduce the deflection times by a factor of 10 to 50 or more, thus reducing the deflection times of several decades to years or less and overcoming the main criticism of the traditional gravity tractor approach. Additionally, multiple spacecraft can orbit the target in formation to provide the necessary velocity change and further reduce the time needed by the EGT technique to divert hazardous asteroids and comets. The robotic segment of NASA's Asteroid Redirect Mission (ARM) will collect a multi-ton boulder from the surface of a large Near-Earth Asteroid (NEA) and will provide the first ever demonstration of the EGT technique and validate one method of collecting in-situ mass on an asteroid of hazardous size
Production in Peripheral Heavy-Ion Collisions
We estimate the impact parameter dependence of the production cross section
for and mesons in peripheral heavy-ion collisions collisions.
Total and elastic cross sections are calculated in an equivalent
photon approximation.Comment: 9 pages, uuencoded postscrip
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