Papers by Konstantinos Dennis Papadopoulos
Sensors, 2019
The Dense Trajectories concept is one of the most successful approaches in action recognition, su... more The Dense Trajectories concept is one of the most successful approaches in action recognition, suitable for scenarios involving a significant amount of motion. However, due to noise and background motion, many generated trajectories are irrelevant to the actual human activity and can potentially lead to performance degradation. In this paper, we propose Localized Trajectories as an improved version of Dense Trajectories where motion trajectories are clustered around human body joints provided by RGB-D cameras and then encoded by local Bag-of-Words. As a result, the Localized Trajectories concept provides an advanced discriminative representation of actions. Moreover, we generalize Localized Trajectories to 3D by using the depth modality. One of the main advantages of 3D Localized Trajectories is that they describe radial displacements that are perpendicular to the image plane. Extensive experiments and analysis were carried out on five different datasets.
A series of Lyon-Fedder-Mobarry (LFM) global MHD code runs were conducted to study the behavior o... more A series of Lyon-Fedder-Mobarry (LFM) global MHD code runs were conducted to study the behavior of the cross polar cap potential (CPCP) under strong solar wind conditions. The runs were developed so as to reveal a dependence of CPCP on the solar wind electric field in a wide range while keeping magnetosonic Mach number Mms higher than 2. The solar wind convective electric field was changed from 4 to 16 mV/m and Mms was in the range from 2.02 to 3.74. All runs were repeated for two values of ionospheric Pedersen conductance of 5 and 10 mhos. The simulation shows that the cross polar cap potential is always reduced compared to the corresponding potential in the solar wind due to the stagnation of the magnetosheath flow and existance of parallel potentials. However, it is the ionospheric conductance that affects the value of the CPCP the most: for the low value of the conductance the above mentioned reduction leads to unrealistically high values of the CPCP. Thus, saturation of CPCP is...
Journal of Electromagnetic Waves and Applications, 2017
Abstract A mobile heater for ionospheric modification studies requires a new megawatt (MW) class ... more Abstract A mobile heater for ionospheric modification studies requires a new megawatt (MW) class radio frequency (RF) source operating with an antenna array 1/20 the area of the High-Frequency Active Auroral Research Program (HAARP). To deliver an effective power density comparable to HAARP, the total source power must be in the range of 16 MW, thus demanding highly efficient sources. While the development of a whole multi-megawatt system for mobile ionospheric heaters is a complex engineering problem, in the present paper we describe only the work of our group on studying main features of a prototype MW-class vacuum electronics RF source for such system. The source design we are currently pursuing assumes class D operation using a modified version of the inductive output tube. The electron beam is a thin annular beam, switched on and off by a mod-anode as opposed to a grid. The beam is then passed through a decelerating gap, and its kinetic energy is extracted using a tunable resonant circuit that presents a constant impedance in the range of 3–10 MHz. With this design the beam is almost completely decelerated at all frequencies, thus achieving high efficiency.
A new concept of generating ionospheric currents in the ULF/ELF range with modulated HF heating u... more A new concept of generating ionospheric currents in the ULF/ELF range with modulated HF heating using ground-based transmitters even in the absence of electrojet currents is presented. The new concept relies on using HF heating of the F-region to modulate the electron temperature and has been given the name Ionospheric Current Drive (ICD). In ICD, the pressure gradient associated with anomalous or collisional F-region electron heating drives a local diamagnetic current that acts as an antenna to inject mainly Magneto-Sonic (MS) waves in the ionospheric plasma. The electric field associated with the MS wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that inject waves in the Earth-Ionosphere Waveguide (EIW) below and shear Alfven waves or EMIC waves upwards towards the conjugate regions. The paper presents: (i) Theoretical results using a cold Hall MHD model to study ICD and the generation of ULF/ELF waves by the m...
It is well known that modulated heating of the ionospheric plasma by high-frequency (HF) radio wa... more It is well known that modulated heating of the ionospheric plasma by high-frequency (HF) radio waves creates electron temperature perturbations that can act as sources of magnetohydrodynamic (MHD) waves at the modulation frequency. Despite several important works on the subject, a self-consistent picture of ionospheric heating and the subsequent generation and propagation of MHD waves has yet to be considered. In this work we present a model of underdense HF heating of the auroral ionosphere and the subsequent generation and propagation of MHD waves. The model presented here is composed of two stages. Initially an HF heating equation with collisional and transport losses is numerically solved to yield a temporal electron temperature profile caused by the underdense modulated heating. This electron temperature profile is then fed as a source into a 2D MHD model, which computes the propagation of the waves, their penetration to the ground, and their injection to the magnetosphere. The...
Delcourt and Sauvaud [1999] trace energetic particles in a statistical magnetospheric model [Mead... more Delcourt and Sauvaud [1999] trace energetic particles in a statistical magnetospheric model [Mead and Fairfield, 1975] and show that the existence of a magnetic field minimum in the outer cusp region causes transport of energetic (hundreds of keV) equatorial particles toward high latitudes at the dayside magnetopause. It is shown in Delcourt and Sauvaud [1999] that this process may be significant for popoulating the cusp region by energetic particles during substorms. Recently, we developed a 3D particle tracing code which traces particles in the electric and magnetic field obtained from the Lyon-Fedder-Mobbary (LFM) [Fedder and Lyon, 1987] global MHD simulation. We observed the large-scale transport of the equatorial particles to the cusp region and the pseudo-trapping at the cusp region. Two global MHD simulations are carried out. In the first case, the solar wind interplanetary magnetic field (IMF) turns from northward to southward and an idealized substorm is simulated. In the s...
By analyzing CRRES and GOES observations on Aug. 27 1991, Tan et al. [2004] reported evidence of ... more By analyzing CRRES and GOES observations on Aug. 27 1991, Tan et al. [2004] reported evidence of magnetospheric relativistic electron acceleration by resonant interactions with PC5 ULF waves. The event showed strong ULF wave activities after a storm sudden commencement (SSC) and energetic electron fluxes were enhanced in 2 hours. The electron flux peak observed in energy channels (0.6 - 1.1 MeV) were modulated by local electric field observed by CRRES. In this study, we set up a drift-resonant interaction model between ULF wave and magnetospheric relativistic electrons to model the observed electron flux in the event. In this model, the poloidal mode wave is concentrated in the dayside and the toroidal mode wave is concentrated in two flanks. The toroidal mode waves in the dawn and dusk flanks are in anti-phase. We found that electron can be accelerated jointly by the poloidal wave in the dayside and toroidal wave in flanks. The dayside poloidal wave serves as the dominant source of...
ABSTRACT Current modulation of D/E region ionospheric currents at ULF frequencies results in gene... more ABSTRACT Current modulation of D/E region ionospheric currents at ULF frequencies results in generation of Shear Alfven Waves injected upwards and guided by the magnetic field lines towards the conjugate ionosphere. Under particular ionospheric conditions frequencies in the PC1 range (.2-6Hz) are reflected by the gradient in the Alfven velocity above the F-region resulting in the well-known Ionospheric Alfven Resonator (IAR) structure. Ground detection of ULF waves due to current modulation on the ground is thus limited to the vicinity of the heated spot since at these frequencies the coupling to the earth ionosphere waveguide is evanescent. Propagation of ULF waves at significant lateral distances requires generation of magnetosonic waves since they are the only mode that propagates isotropically and can thus couple efficiently in the Alfvenic duct. In this paper we present a completely new mechanism to generate magnetosonic waves by modulated ionospheric heating that does not require the presence of electrojet currents. The process relies in anomalous electron heating near the F-region peak by preferably using O-mode upper hybrid heating modulated at ULF Pc-1 frequencies. The modulation in the electron pressure drives a Bxgrad(p) oscillatory current. The resultant field aligned magnetic moment generates predominantly magnetosonic waves that are injected laterally into the Alfvenic duct and can also detected above the F-peak by over-flying satellites over distances larger than spanned by the field lines connecting to heated area. In addition to the concept and analytic results the paper will present simulations results using the ZEUS-MP MHD. Non-uniform grids are used to adapt to non-uniform ionospheric plasma density and thin layer of heating source. The effective heating region is placed at about 200-300 km in altitude (F-layer ionosphere). The modulated heating source is modeled as a source with perturbed density, temperature and magnetic field and it transmits modulated-HF electromagnetic waves into a stable ionosphere. Ratios of perturbed magnetic field and density to their background values are extracted from simulations. Different radiation patterns from different polarization component of magnetic field perturbation are investigated. Effects of different profiles of non-uniform ionospheric plasma density on ULF wave propagation are also studied through simulation. Preliminary experimental evidence of the process will also be presented. This work was sponsored by ONR MURI Grant 5-28828
ABSTRACT The stably trapped electrons in the inner radiation belt have lifetimes of years and ene... more ABSTRACT The stably trapped electrons in the inner radiation belt have lifetimes of years and energies higher than a few hundred keV. These energetic electrons can have serious effects on the way spacecrafts and satellites operate and cause significant hazards to low Earth orbit (LEO) satellites. For mitigating these hazards it is necessary to investigate ways for reducing the electron life times, for example, through pitch angle scattering by waves. For these waves, the gyro-resoance condition yields the minimum wavelength requirement for given particle energy and local magnetic field. For example, at the magnetic equator at L = 2 the waves resonant with 1 MeV electrons should have wavelengths less than 10 km. Low frequency Electromagnetic Ion-Cyclotron (EMIC) waves occur in three bands with frequencies below the hydrogen, helium, and oxygen ion gyro-frequencies, respectively. At frequencies close to the ion gyro-frequencies, the EMIC waves can have wavelength short enough to gyro-resonate with energetic electrons, which can lead to significant changes in the lifetimes of electrons in the inner ration belt. However at these altitudes the amplitudes of naturally excited EMIC waves do not yield significant scattering of the energetic electrons and artificial sources are needed. In order to define the characteristics of such sources we investigated the lifetime of inner belt energetic electrons subject to pitch angle scattering with EMIC waves. The resonant wave characteristics are obtained using the global core plasma model (GCPM). The lifetimes of the electrons in the presence of these waves are computed using the pitch angle diffusion coefficient for broadband waves. For several hundred Watts of broadband EMIC waves in the shell volume enclosed by magnetic field lines at L = 2.0 with width dL = 0.1, the lifetime of 1 MeV electrons can be reduced to a few months. This is a considerable reduction compared to the average life times of about years and have important consequences, including remediation of artificially enhanced energetic electron fluxes.
The VLF waves excited by powerful ground-based transmitter propagate in the Earth-ionosphere wave... more The VLF waves excited by powerful ground-based transmitter propagate in the Earth-ionosphere waveguide and leaks through the ionosphere to the magnetosphere. Recent studies [Starks et al. 2008] using combined Earthionosphere waveguide model and ray-tracing model found that the model systematically overestimates the VLF wave field strength in the plasmasphere owing to VLF transmitter by ∼20 dB at night and ∼10dB during the day. We present a numerical model to simulate linear mode conversion between whistler wave and lower hybrid wave due to the interaction with short scale density striations such as field-aligned irregularities in the Earth's ionosphere. It is found that at the altitudes between 90 to 120 km in the ionosphere, the energy of whistler wave energy can be converted to the lower hybrid wave and the lower hybrid wave can be subsequently damped by ion-neutral collisions. The mode conversion of whistler wave to lower hybrid wave in the E region ionosphere may play an important role in the whistler wave damping. This work is supported by ONR MURI grant.
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Papers by Konstantinos Dennis Papadopoulos