Morris B Cohen

Morris B Cohen
Georgia Institute of Technology | GT · School of Electrical & Computer Engineering

PhD

About

107
Publications
20,233
Reads
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1,946
Citations
Citations since 2017
2 Research Items
950 Citations
2017201820192020202120222023050100150
2017201820192020202120222023050100150
2017201820192020202120222023050100150
2017201820192020202120222023050100150
Additional affiliations
October 2013 - present
Georgia Institute of Technology
Position
  • Professor (Assistant)
November 2010 - August 2013
Stanford University

Publications

Publications (107)
Article
Gigantic Jets are electric discharges from thunderstorm cloud tops to the bottom of ionosphere at ~90 km altitude and electrically connect the troposphere and lower ionosphere. Since their first report in 2002, sporadic observations have been reported from ground and space based observations. Here we report first observations of Gigantic Jets in In...
Article
Full-text available
Gigantic Jets are electric discharges from thunderstorm cloud tops to the bottom of ionosphere at ~90 km altitude and electrically connect the troposphere and lower ionosphere. Since their first report in 2002, sporadic observations have been reported from ground and space based observations. Here we report first observations of Gigantic Jets in In...
Article
Full-text available
Early Very Low Frequency (VLF) events are perturbations to subionospherically propagating VLF radio transmitters which sometimes occur when lightning activity is near the transmitter-receiver path. They are often correlated to Transient Luminous Events (TLEs). Recent analysis have focused on a new type of early events whose recovery time persists f...
Article
Approximately 90% of natural lightning occurs over land but recent observations, using Global Lightning Detection (GLD360) geo-location peak current estimates and satellite optical data, suggested that cloud-to-ground flashes are on average stronger over the ocean. We present initial statistics from a novel experiment using a Low Frequency (LF) mag...
Article
Full-text available
Modulated ionospheric heating experiments are performed with the High Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska for the purpose of generating extremely low frequency (ELF) and very low frequency (VLF) waves. Observations are made at three different azimuths from the heating facility and at distances from 37 km to...
Article
We present analysis of more than 2000 lightning-generated whistlers recorded at a low latitude station, located at Allahabad (Geog. lat., 25.40° N; Geog. long. 81.93° E; L=1.081), India, during December 2010 to November 2011. The main focus of this work is on the correlation between observed low latitude whistlers and lightning activity detected by...
Article
Natural lightning is both frequent and variable, and thus a good subject for statistical studies. A typical negative cloud-to-ground (CG) flash consists of multiple individual return strokes. The spatial and temporal distributions of various lightning events throughout the discharge provide a surrogate look inside the CG flash and offer insight int...
Article
During the return stroke in downward negative cloud-to-ground lightning, a current wave propagates upward from the ground along the lightning channel. The current wave causes rapid heating of the channel and induces intense optical radiation. The optical radiation wave propagation speed along the channel has been measured to be between and of the s...
Article
Full-text available
Controlled experiments with dedicated ground-based ELF/VLF (0.3–30 kHz) transmitters are invaluable in investigating nonlinear whistler mode wave-particle interactions in the Earth's magnetosphere. The most productive such experiment operated between 1973 and 1988 near L = 4 at Siple Station, Antarctica. A major effort has been undertaken to digiti...
Article
[1] Lightning strokes are known to cause direct heating and ionization of the D region, some of which are detected via scattering of VLF transmitter signals and are known as Early VLF events. The disturbed ionosphere typically recovers in many tens of seconds. New experimental evidence is presented demonstrating that the scattering pattern and onse...
Article
[1] Extremely low frequency/very low frequency (ELF/VLF) radio waves are difficult to generate with conventional antennas. Ionospheric high frequency (HF) heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural...
Article
Full-text available
Extremely low frequency/very low frequency (ELF/VLF) radio waves are difficult to generate with conventional antennas. Ionospheric high frequency (HF) heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural curr...
Article
Subionospheric VLF recordings are investigated in relation with intense cloud-to-ground (CG) lightning data. Lightning impacts the lower ionosphere via heating and ionization changes which produce VLF signal perturbations known as early VLF events. Typically, early events recover in about 100 s, but a small subclass does not recover for many minute...
Article
Full-text available
1] Lightning strokes are capable of initiating disturbances in the lower ionosphere, whose recoveries persist for many minutes. These events are remotely sensed via monitoring subionospherically propagating very low frequency (VLF) transmitter signals, which are perturbed as they pass through the region above the lightning stroke. In this paper we...
Article
1] We present the first global distribution of the average estimated peak currents in negative lightning flashes using 1 year of continuous data from the Vaisala global lightning data set GLD360. The data set, composed of 353 million flashes, was compared with the National Lightning Detection Network TM for peak current accuracy, location accuracy,...
Article
1] Experiments at the ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) are performed employing dual HF beams amplitude modulated at ELF/VLF with a phase offset between the two modulation waveforms. The amplitude of the observed ELF/VLF waves is strongly dependent on the imposed ELF/VLF phase offset, the mod...
Article
1] Accurate models of trans-ionospheric propagation are needed to assess the role of Earth-originating very low frequency (VLF) electromagnetic waves in radiation belt dynamics. Recent studies have called the relatively crude early trans-ionospheric models into question, finding that they underestimate the attenuation by 20–100 dB. A full wave mode...
Article
1] Lightning discharges are nature's way of neutralizing the electrical buildup in thunderclouds. Thus, if an individual discharge destroys a substantial fraction of the cloud charge, the probability of a subsequent flash is reduced until the cloud charge separation rebuilds. The temporal pattern of lightning activity in a localized region may thus...
Article
Full-text available
The present study reports the VLF (very low frequency) sub-ionospheric perturbations observed on transmitter JJI (22.1 kHz), Japan, received at the Indian low-latitude station, Allahabad (geographic lat. 25.41 • N, long 81.93 • E), due to Wenchuan earthquake (EQ) that occurred on 12 May 2008 with the magnitude 7.9 and at the depth of 19 km in Sichu...
Article
1] Modulated high-frequency heating of the D region ionosphere near the auroral electrojet can generate extremely low frequency (ELF; 3 Hz–3 kHz) radio waves. The modulated heating process is nonlinear and generates harmonics at integer multiples of the ELF modulation frequency. Quaternary phase shift keying, a digital modulation technique is appli...
Conference Paper
The 3D magnetospheric wave power density between adjoined conjugate regions associated with ground based VLF transmitters is deduced from paired measurements of wave field intensities acquired by the low-altitude DEMETER satellite within respective hemispheres.
Article
Full-text available
1] Ground based Very Low Frequency (VLF, 3–30 kHz) radio transmitters play a role in precipitation of energetic Van Allen electrons. Initial analyses of the contribution of VLF transmitters to radiation belt losses were based on early models of trans-ionospheric propagation known as the Helliwell absorption curves, but some recent studies have foun...
Article
Full-text available
1] It is well established that Extremely Low Frequency (ELF, 0.3–3 kHz) and Very Low Frequency (VLF, 3–30 kHz) radio waves can be generated via modulated High Frequency (HF, 3–10 MHz) heating of the lower ionosphere (60–100 km). The ionospheric absorption of HF power modifies the conductivity of the lower ionosphere, which in the presence of natura...
Article
Full-text available
1] Modulated High Frequency (few MHz) heating of the D-region ionosphere under the auroral electrojet is capable of generating extremely low frequency (ELF) radio waves in the few kilohertz range by affecting the conductivity of the D-region. The HF heating is nonlinear and results in the generation of harmonics at integer multiples of the ELF modu...
Article
Full-text available
Dispersive atmospherics (tweeks) observed during 2010 simultaneously at two low-latitude stations, Allahabad (geomagnetic latitude, 16.05N) and Nainital (geomagnetic latitude, 20.48N), have been used to estimate the nighttime D region electron density at the ionospheric reflection height under the local nighttime propagation (21:00-02:00 LT or 15:3...
Article
Full-text available
For decades, whistlers observed on the ground at mid and high latitudes have been used for diagnostics of Earth's plasmasphere. Whistlers have also been observed at low latitudes however, the propagation characteristics of low latitude whistlers are poorly understood thus they have not been used effectively as a diagnostic for the low latitude iono...
Article
Full-text available
1] Modulated high frequency (HF, 3–10 MHz) heating of the ionosphere in the presence of the auroral electrojet cur-rents is an effective method for generating extremely low frequency (ELF, 3–3000 Hz) and very low frequency (VLF, 3–30 kHz) radio waves. The amplitudes of ELF/VLF waves generated in this manner depend sensitively on the auroral electro...
Article
Full-text available
1] Broadband ELF/VLF radio recordings (0.2–40 kHz) were made at a site in Onagawa, Japan located 102 km from the epicenter of the M9.0 11 March 2011 earthquake, the fifth most powerful earthquake in recorded history. The receiver operated for about two minutes after the start of the earthquake, after which the receiver lost power. Examination of th...
Conference Paper
We describe two enhancements to Stanford Full Wave Method (StanfordFWM) for calculation of electromagnetic field radiated into a stratified medium by arbitrarily positioned monochromatic sources [1,2]. In the first part, we consider the problem of scattering on strong ionospheric disturbances. We introduce a novel computational technique which comb...
Conference Paper
form only given. The lightning return stroke is one of the most energetic phenomena known to mankind. One of the outstanding puzzles about this phenomena is a relatively slow (c/3-c/3) speed of the return stroke current front. In order to tackle this problem, we model the return stroke channel as a three layer cylindrical waveguide, consisting of (...
Article
Full-text available
1] Observations show that intense +CG lightning dis-charges which trigger both an elve and a sprite are associated with long-lasting conductivity modifications in the upper D-region ionosphere. They are observed as strong pertur-bations in VLF signals propagating through the disturbed region, manifested as LOng Recovery Early VLF events (LORE), whi...
Article
Full-text available
1] Long D-region ionospheric recovery perturbations are a recently discovered and poorly understood subcategory of early VLF events, distinguished by exceptionally long ionospheric recovery times of up to 20 min (compared to more typical $1 min recovery times). Characteristics and occurrence rates of long ionospheric recovery events on the NWC tran...
Article
Full-text available
1] Very Low Frequency (VLF, 3–30 kHz) radio waves emitted from ground sources (transmitters and lightning) strongly impact the radiation belts, driving electron precipitation via whistler-electron gyroresonance, and contributing to the formation of the slot region. However, calculations of the global impacts of VLF waves are based on models of tran...
Article
It is well understood that a return strokes of lightning discharges generates electromagnetic waves in a wide frequency spectrum from few Hz to few tens of MHz, major portion of which propagates in Earth-Ionosphere Waveguide via multiple reflections as sferics. A small portion of the lightning radiated energy penetrates the lower ionosphere and tra...
Article
Full-text available
1] ELF/VLF (0.3–30 kHz) wave generation is achievable via modulated HF (3–30 MHz) heating of the lower ionosphere in the presence of natural currents such as the auroral electrojet. Using the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, AK, we investigate the effect of HF frequency and beam size on the generat...
Article
Full-text available
at Allahabad, a low-latitude station in the Indian sector, has been used to study seasonal occurrence which shows maximum tweek occurrence of about 63% during summer season and about 19% and 18% occurrences during equinox and winter seasons. Maximum occurrence of tweeks during summer season is consistent with the larger number of lightnings detecte...
Article
Every day millions of lightning flashes occur around the globe but the understanding of this natural phenomenon is still lacking. Fundamentally, lightning is nature's way of destroying charge separation in clouds and restoring electric neutrality. Thus, statistical patterns of lightning activity indicate the scope of these electric discharges and o...
Article
This study evaluates the effectiveness of spectrophotometric measurements from space in revealing properties of lightning flashes. The multicolor optical waveform data obtained by FORMOSAT-2/ISUAL were analyzed in relation to the National Lightning Detection Network (NLDN), North Alabama Lightning Mapping Array (LMA). As of July 2011, we found six...
Article
We have examined two negative gigantic jets that occurred sufficiently near a very high-frequency (VHF) lightning mapping network that the associated lightning development is well characterized. Remote sensing of broadband (<1 Hz to 400 kHz) magnetic fields provides extra insights into the charge transfer and detailed sequence of fast discharge eve...
Article
ELF/VLF waves (0.3 - 30 kHz) can be generated via HF (2-10 MHz) heating of the lower ionosphere. Since the D-region ionospheric conductivity is a function of electron temperature, in the presence of natural currents such as the auroral electrojet, a modulated or steered HF beam can turn a patch of the lower ionosphere into a large radiating antenna...
Article
George Jin Maria Spasojevic Morris Cohen Umran Inan Stanford University Modulated HF heating of the D-region ionosphere near the auroral electrojet can generate extremely low frequency (ELF) waves in the kilohertz range. This process is nonlinear and generates harmonics at integer multiples of the ELF modulation frequency. The nonlinear distortion...
Article
Nonlinear amplification of whistler mode waves in the Earth's magnetosphere, due to gyroresonance with energetic electrons, has been investigated extensively with theory, experimental observations and computer simulations. It is generally accepted that non-linear effects are dominated by resonant and near-resonant electrons that are phase-trapped i...
Article
Controlled experiments with ground-based ELF/VLF transmitters have proven invaluable in studying the interactions between whistler waves and energetic electrons in the radiation belts. These gyrosesonant interactions result in wave growth and nonlinear triggered emissions that can be observed both at the geomagnetic conjugate point after reflection...
Article
Full-text available
Terrestrial gamma ray flashes (TGFs) are bremsstrahlung emissions from relativistic electrons accelerated in electric fields associated with thunder storms, with photon energies up to at least 40 MeV, which sets the lowest estimate of the total potential of 40 MV. The electric field that produces TGFs will be reflected by the initial angular distri...
Article
Full-text available
Observations of tweeks with higher harmonics (n > 1) at low latitude stations Allahabad and Nainital, in the Indian sector, during the total solar eclipse on 22 July 2009, are presented. Allahabad and Nainital stations were in 100% and 85% of the totality paths. Observations suggest that about 30-40% obscuration of solar disc can lead to the tweeks...
Conference Paper
Full-text available
The electrical properties of the lightning channel are determined by a complex interplay of dielectric breakdown, plasma heating, expansion, and cooling. These details are observable through their effect on the channel current and electromagnetic wave emissions. This suggests that electromagnetic observations of lightning can be used to study chann...
Conference Paper
Full-text available
We present the first constraints on the TGF emission cone, based on accurately geolocated TGFs. The distribution of the observation angles for 106 TGFs are compared to Monte Carlo simulations. We find that TGF emissions within an isotropic half angle >30° can generate the observations. In addition, 36 events are used for spectral analysis. The resu...
Conference Paper
We report the observations of two gigantic jets, one during daytime over sea and one at nighttime over land, in the context of detailed associated observations of lightning flash and discharge evolution provided by a variety of radio measurements.
Conference Paper
Lightning is nature's way of destroying electrical buildup in thunderclouds. Thus, the pattern of lightning activity is inherently a proxy measure of the timescales for charge separation and the lateral extent of the charge reunification in a flash. Using data from lightning stroke geolocation networks such as NLDN and GLD360, we deduce statistical...
Conference Paper
The Russian 'Alpha' transmitters broadcast alternating pulses between 11-15 kHz for navigation. A fraction of the VLF energy escapes into the magnetosphere, is guided by ducts, amplified by interaction with radiation belt particles, and observed at the geomagnetic conjugate point. We analyze VLF data from Adelaide, Australia, conjugate to Komsomols...
Article
Full-text available
Experiments for generating extremely low frequency (ELF) radio waves using modulated HF heating of the auroral ionosphere have been conducted and refined at the High Frequency Active Auroral Research Program (HAARP) facility at Gakona, Alaska. Because this technique is dependent on strength of the naturally generated electrojet current system, the...
Article
Full-text available
1] We report observations of two negative polarity gigantic jets sufficiently near very high‐frequency (VHF) lightning mapping networks that the associated lightning character-istics and charge transfer could be investigated. In both cases the gigantic jet‐producing flash began as ordinary intracloud lightning with upper level channels attempting t...
Article
Full-text available
1] ELF/VLF waves have been generated via steerable HF heating of the lower ionosphere. The temperature‐dependent conductivity of the lower ionospheric plasma enables HF heating (and subsequent recovery) to modulate natural current systems such as the auroral electrojet, thus generating an antenna embedded in the ionospheric plasma. We apply a reali...
Article
Full-text available
The ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) has been used extensively in the last 3 years for injection of ELF/VLF waves into the magnetosphere via modulated heating of the overhead auroral electrojet currents. Of particular interest are waves that are observed to be nonlinearly amplified after int...
Article
Full-text available
Variations in the global atmospheric electric circuit are investigated using a wide range of globally spaced instruments observing VLF (∼10 kHz) waves, ELF (∼300 Hz) waves, Schumann resonances (4–60 Hz), and the atmospheric fair weather electric field. For the ELF/VLF observations, propagation effects are accounted for in a novel approach using est...
Conference Paper
Strong electric fields associated with lightning generate brief (~1 ms) but intense Terrestrial Gamma-ray Flashes (TGFs). A few events are thought to be the signature of a relativistic electron beam escaping the atmosphere. Such an event is distinguishable from a TGF since the lightning discharge is along the geomagnetic field line from the spacecr...
Conference Paper
Electromagnetic simulations of lightning are useful for prediction of lightning properties and exploration of the underlying physical behavior. Fractal lightning models predict the spatial structure of the discharge, but thus far do not provide much information about discharge behavior in time and therefore cannot predict electromagnetic wave emiss...
Article
Techniques for using modulated HF heating of the D-region ionosphere near the auroral electrojet to generate extremely low frequency (ELF) waves in the kilohertz range have been refined over many decades. Even with the high power of the newest ionospheric heater at the High-Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alask...
Article
The High Frequency Active Auroral Research Program (HAARP) transmitter array (3.6 MW, 2.8 - 10 MHz) is used to generate Extremely Low Frequency (ELF, 300 - 3000 Hz) waves through periodic heating of the ionospheric D-layer and subsequent modulation of the conductivity of the auroral electrojet. The generated ELF waves can be used in various manners...
Article
The HF heating facility of the High Frequency Active Auroral Research Program (HAARP) has been used to generate ELF/VLF waves via modulated heating of the lower ionosphere for several years. Here we investigate the effective altitude of the modulated electrojet currents as a function of HF frequency and harmonic number of the ELF/VLF modulation. In...
Conference Paper
Measurement of thunderstorm electric fields typically require balloon-borne measurements in the region of interest. Such measurements are cumbersome and provide limited information at a single point. Remote sensing of electric fields by Kerr-effect induced optical polarization changes of background skylight circumvents many of these difficulties an...
Conference Paper
Cosmic gamma-ray bursts ionize the upper atmosphere, affecting sub-ionospheric propagation of very-low-frequency (VLF) radio waves. Perturbations of VLF radio signals can therefore be used to study gamma-ray burst effects on the ionosphere. We present observations of VLF radio signal perturbations coincident with bursts observed by the Fermi satell...