JHU APL

Bryans Road, MD, United States
Bryans Road, MD, United States
SEARCH FILTERS
Time filter
Source Type

Cheng A.F.,JHU APL | Reed C.L.B.,JHU APL
Proceedings of the International Astronautical Congress, IAC | Year: 2016

The Asteroid Impact Deflection Assessment (AIDA) mission will be the first space experiment to demonstrate asteroid impact hazard mitigation by using a kinetic impactor. AIDA is a joint ESA-NASA cooperative project, consisting of the ESA Asteroid Impact Mission (AIM) rendezvous mission and the NASA Double Asteroid Redirection Test (DART) mission which is the kinetic impactor. The AIDA target is the near-Earth binary asteroid 65803 Didymos, which will make an unusually close approach to Earth in October, 2022. The DART spacecraft is designed to impact the Didymos secondary at 7 km/s and demonstrate the ability to modify its trajectory through momentum transfer. DART and AIM are currently Phase A studies supported by NASA and ESA respectively. The primary goals of AIDA are (1) perform a full-scale demonstration of the spacecraft kinetic impact technique for deflection of an asteroid; (2) measure the resulting asteroid deflection; and (3) study hyper-velocity collision effects on an asteroid, validating models for momentum transfer in asteroid impacts based on measured physical properties of the asteroid surface and sub-surface, and including long-term dynamics of impact ejecta. The DART impact on the moon of the Didymos binary system will change the orbital period of the binary. This change can be measured by supporting Earth-based optical and radar observations. The baseline DART mission launches in December, 2020 to impact the Didymos secondary in September, 2022. The AIM spacecraft will be launched in October, 2020 and arrive at Didymos in spring, 2022, several months before the DART impact. AIM will characterize the Didymos binary system by means of remote sensing and in-situ instruments both before and after the DART impact. The asteroid deflection will be measured to higher accuracy, and additional results of the DART impact, like the impact crater, will be studied in detail by the AIM mission. The combined DART and AIM missions will provide the first measurements of momentum transfer efficiency from a kinetic impact at full scale on an asteroid, where the impact conditions of the projectile are known, and physical properties and internal structures of the target asteroid are also characterized. The DART kinetic impact is predicted to make a crater of 6 to 17 meters diameter, depending on target physical properties, but will also release a large volume of particulate ejecta that may be directly observable from Earth or even resolvable as a coma or an ejecta tail by ground-based telescopes.


Lui A.T.Y.,JHU APL
Geoscience Letters | Year: 2015

We examine similarities and differences between the concepts of current disruption (CD) and magnetic reconnection (MR). Both concepts have been invoked to account for explosive phenomena that involve energy transformation from magnetic field to charged particles. Similarities of these two concepts include (1) the occurrence of breakdown in magnetic connectivity, (2) magnetic energy as the primary energy source, and (3) plasma energization as a product of the process. Differences include (1) plasma flow across separatrix surfaces in an X-type magnetic field geometry being essential for MR but not for CD, (2) plasma flow ordered by the magnetic field geometry for MR but not for CD, (3) field line topology change essential for MR but not necessary for CD, and (4) CD exhibiting multifractal and symmetry breaking behavior while no such behavior has been investigated for MR. Overall, CD can be viewed as a form of generalized MR (GMR) in which the requirement for a specific magnetic field geometry and its constraint on the plasma flow pattern are removed. Therefore, the CD concept has a broader scope in applications than the MR concept alone. © 2015, Lui.


Lui A.T.Y.,JHU APL
Geoscience Letters | Year: 2015

Recent emphasis on dipolarization fronts (DFs) has led to the impression that DFs play a significant role in bringing magnetic flux to the inner magnetosphere during substorms. In this work, we investigate the amount of magnetic flux transport associated with DFs by examining the frozen-in field line condition (FIC) for previously reported DF events. A study of 18 DF cases shows that the FIC does not hold for 17 cases when the ratio of |[Ey+(V×B)y]/(V×B)y| exceeds 0.5, i.e., the mismatch of Ey and −(V × B)y exceeds 50 %; this criterion is applied only when the electric field magnitude exceeds 0.5 mV/m to eliminate times of low-level electric fluctuations. Furthermore, the peak magnetic flux transport rate for DFs in which FIC holds is found to be in the range of ~8–42 kWb/s/RE while the accumulated flux transport within the DF intervals to be ~0.1–2.8 MWb/RE. Assuming a dawn-dusk dimension of 3 RE for a DF, the accumulated magnetic flux transport is ~0.3–8 MWb, which amounts to ~0.1–2.2 % of what is needed to account for magnetic flux increase in the near-earth dipolarization during substorms. This result casts doubt on the idea that DFs play a significant role in substorm dipolarization. © 2015, Lui.


Lui A.T.Y.,JHU APL | Zong Q.-G.,Beijing Institute of Technology | Zong Q.-G.,University of Massachusetts Lowell | Wang C.,Chinese Academy of Sciences | Dunlop M.W.,Rutherford Appleton Laboratory
Journal of Geophysical Research: Space Physics | Year: 2012

We examine the strength of the electron source associated with dipolarization at the outer boundary of the radiation belts using multisatellite observations from THEMIS. This topic is relevant to the determination on the relative roles of inward radial diffusion versus internal local acceleration for the origin of the relativistic electrons in the outer radiation belt. We focus on the electron phase space density (PSD) as a function of the first adiabatic invariant () for equatorially mirroring population over a broad energy range. It is found that the source strength associated with dipolarization for non-storm periods at the outer boundary of the radiation belts can be well above the observed fluxes of relativistic electrons inside the outer radiation belt. The PSD change due to the magnetic field strength variation dominates over PSD change from the energy flux increase with dipolarization, resulting in a strong anticorrelation between magnetic field strength and PSD values at a given . If observations from closely spaced satellites during the same event can be used to indicate radial transport of electrons with dipolarization, then the observed PSD at these satellites indicates frequent occurrence of non-adiabatic process during their radial transport. © 2012. American Geophysical Union. All Rights Reserved.


Connors M.,Athabasca University | McPherron R.L.,University of California at Los Angeles | Anderson B.J.,JHU APL | Korth H.,JHU APL | And 2 more authors.
Geophysical Research Letters | Year: 2014

The three-dimensional "substorm current wedge" (SCW) was postulated by McPherron et al. (1973) to explain substorm magnetic perturbations. The origin and coherence as a physical system of this important paradigm of modern space physics remained unclear, however, with progress hindered by gross undersampling, and uniqueness problems in data inversion. Complementing AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) space-derived radial electric currents with ground magnetic data allowing us to determine currents from the ionosphere up, we overcome problems of uniqueness identified by Fukushima (1969, 1994). For a substorm on 24 February 2010, we quantify SCW development consistently from ground and space data. Its westward electrojet carries 0.5 MA in the more poleward part of the auroral oval, in Region 1 (R1) sense spanning midnight. The evening sector electrojet also feeds into its upward current. We thus validate the SCW concept and obtain parameters needed for quantitative study of substorms. Key Points Auroral zone electric currents should be determined from both ground and space The substorm current wedge to first order represents substorm currents well Magnetic inversions can give useful quantitative parameters of the current wedge ©2014. American Geophysical Union. All Rights Reserved.


Grant
Agency: Department of Defense | Branch: Missile Defense Agency | Program: STTR | Phase: Phase II | Award Amount: 877.97K | Year: 2014

Corvid Technologies is pleased to offer this STTR Phase II proposal in collaboration with The Johns Hopkins University Applied Physics Laboratory (JHU/APL), Spectral Sciences Inc. and Torch Technologies. Capabilities from each collaborator are being combined toward the ability to perform accurate, fast-running electro-optical and infrared (EO/IR) signature predictions. This collaborative effort will focus on the realization of consolidated modeling capabilities and benchmarking against empirical data. When coupled with existing Corvid radar modeling efforts, the resulting capability will provide a unified description across disparate sensor domains including radar and EO/IR. Approved for Public Release 14-MDA-7739 (18 March 14).


The Radiation Belt Storm Probe (RBSP) spacecraft used the Direct Field Acoustic Testing (DFAT) method to meet the program's acoustic test requirement. Leading up to the flight test, a representative test article (Mockup) was tested by the direct field acoustic method and in a reverberant chamber. The Mockup was designed to be a pathfinder for RBSP spacecraft by mimicking the overall shape and construction of the flight satellites. Microphone placement relative to the Mockup and accelerometer locations remained consistent between the tests allowing for detailed comparisons. Mockup accelerometer responses to a reverberant field are compared to accelerometer responses generated from a direct acoustic field using MIMO control. In addition, microphones 9" from the panel surfaces are compared between reverberant and DFAT tests. Flight RBSP accelerometer responses are presented for selected areas.


Lui A.T.Y.,JHU APL
Geophysical Research Letters | Year: 2016

The physical process responsible for the onset of substorm expansion is still unresolved in spite of decades of research on the topic. Detailed properties of the spatially periodic auroral beads on prebreakup auroral arcs that initiate substorm expansion onset are now available. These auroral bead properties impose severe observational constraints on the onset process. In this work, theoretical predictions of the cross-field current instability are evaluated in terms of these constraints. The growth rates and wavelengths associated with auroral beads in several previously published events are reproduced by the cross-field current instability, implying that the instability can indeed account for the characteristics of auroral beads that eventually lead to substorm onset. The present results differ from the conclusion reached by a previous analysis that the shear flow ballooning instability can account for the growth and spatial scales of auroral beads better than the cross-field current instability. ©2016. American Geophysical Union. All Rights Reserved.


Lui A.T.Y.,JHU APL
Geophysical Research Letters | Year: 2016

The modification of current density on the dawn-dusk cross section of the magnetotail with the earthward approach of a dipolarization front (DF) is examined through the recently published results of a three-dimensional (3-D) particle-in-cell (PIC) simulation. It is found that the current density intensifies by ~37% abruptly within ~1.5 ion gyrotime as the DF approaches and shows localized regions with north-south extrusions. After reaching its peak value, it undergoes a drastic current reduction (DCR) by ~65% within ~2 ion gyrotime. Breakdown of the frozen-in condition occurs in the neutral sheet region in association with DCR, demonstrating the non-MHD behavior of the phenomenon. The evolution of current density from this 3-D PIC simulation bears several similarities to those observed for the current disruption (CD) phenomenon, such as explosive growth and disruption of the current density leading to a breakdown of the frozen-in condition. The evolution is also similar to those from a previous two-dimensional (2-D) PIC simulation specially designed to investigate the nonlinear evolution of the cross-field current instability for CD. One interpretation of these findings is that CD and substorm triggering can be associated with earthward intrusion of a DF into the near-Earth plasma sheet as indicated by previous Cluster and Time History of Events and Macroscale Interactions during Substorms observations. An alternative interpretation is that both DF and CD are consequences of a global evolution from an ion-tearing-like instability of the magnetotail. ©2016. American Geophysical Union. All Rights Reserved.


Lui A.T.Y.,JHU APL
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2014

There is a long history in the study of current-disruptions/dipolarizations (CDDs) in the Earth's magnetotail. A recent trend for this topic is the focus on very transient (~1-2min) positive pulses of the Bz component in the magnetotail coined as dipolarization fronts (DFs). We reviewed several salient features of CDDs reported previously and compare them with those of DFs. We find several major differences between them. First, their temporal profiles differ significantly-DFs have pulse-like form while CDDs have the sustained dipolarization lasting for many minutes. Second, CDDs are typically associated with initial large magnetic fluctuations having the characteristics of turbulence while DFs are not. Third, DFs typically propagate Earthward while CDDs spread tailward. Fourth, DFs are mainly spatial structures acting as discontinuities to separate the regions ahead and behind them with a north-south oriented thin current sheet. On the other hand, CDDs are mainly temporal manifestations of a local dynamic process that reduces the east-west cross-tail current near the neutral sheet. There is indication that CDDs in the near-Earth region (within the downstream distance of ~15RE) occur prior to substorm onset and DFs are typically found in the midtail region after substorm onsets. These differences justify renaming DFs as dipolarization pulses so that the fronts of CDDs can be distinguished from dipolarization pulses without the confusion brought about by the present terminology. © 2013 Elsevier Ltd.

Loading JHU APL collaborators
Loading JHU APL collaborators