Radiative properties of a plasma moving across a magnetic field. I: Theoretical analysis

The early‐time evolution of plasmas moving across a background magnetic field is addressed with a two‐dimensional model in which a plasma cloud is assumed to have formed instantaneously with a velocity across a uniform background magnetic field and with a Gaussian density profile in the two dimensions perpendicular to the direction of motion. This model treats both the dynamics associated with the formation of a polarization field and the generation and propagation of electromagnetic waves. In general, the results indicate that, to zeroth order, the plasma cloud behaves like a large dipole antenna oriented in the direction of the polarization field which oscillates at frequencies defined by the normal mode of the system. The magnitude of the radiation field and the amount of plasma momentum and energy carried away by and stored instantaneously in the fields are discussed only qualitatively in this paper, quantitative results for specific cloud parameters and scaling laws for the magnitude of the fields and the slowing down of the plasma cloud are presented in a companion manuscript.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70196/2/PFBPEI-5-4-1289-1.pd

Radiative properties of a plasma moving across a magnetic field. II: Numerical results

A theoretical analysis developed in a companion paper to treat the early‐time evolution of plasmas moving across a background magnetic field is applied to the modeling of low‐beta, barium chemical releases in the magnetosphere. The results indicate that radiation damping plays an important role in defining the plasma cloud evolution, causing a rapid decay of the polarization field and a loss of plasma kinetic energy and momentum on time scales comparable to several ion gyroperiods. The radiation spectrum consists of a burst of chirped, high‐frequency (in the range of the cloud plasma frequencies) waves, followed by a pulse of whistler waves, and subsequently by ion cyclotron emission. Scaling laws are derived for the plasma momentum and energy loss rates and predictions for the braking time, the amplitude and spectrum of the radiation field, and the total radiated power are presented for conditions relevant to the recent Combined Release and Radiation Effects Satellite (CRRES) experiments [Phys. Fluids B 4, 2249 (1992)].Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70589/2/PFBPEI-5-4-1306-1.pd

FORTE satellite constraints on ultra-high energy cosmic particle fluxes

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultra-high energy (UHE) neutrino fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE neutrinos. We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE neutrino shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.Comment: added a table, updated references and Figure 8, this version is submitted to Phys. Rev.

Relativistic electron beams above thunderclouds

Non-luminous relativistic electron beams above thunderclouds have been detected by the radio signals of low frequency ~ 40–400 kHz which they radiate. The electron beams occur ~ 2–9 ms after positive cloud-to-ground lightning discharges at heights between ~22–72 km above thunderclouds. Intense positive lightning discharges can also cause sprites which occur either above or prior to the electron beam. One electron beam was detected without any luminous sprite which suggests that electron beams may also occur independently of sprites. Numerical simulations show that beams of electrons partially discharge the lightning electric field above thunderclouds and thereby gain a mean energy of ~ 7MeV to transport a total charge of ~ −10mC upwards. The impulsive current ~3×10−3 Am−2 associated with relativistic electron beams above thunderclouds is directed downwards and needs to be considered as a novel element of the global atmospheric electric circuit.Peer Reviewe