Abstract. Combined Release and Radiation Effects

[1] A new empirical model of the plasmapause location has been developed using density data from the plasma wave receiver onboard the CRRES spacecraft for nearly 1000 orbits. The ''plasmapause'' is identified here as the innermost sharp gradient in density (change of a factor of 5 in <0.5 L). Such a sharp gradient was observed on 73% of the CRRES inbound and outbound orbits that returned data. The plasmapause location is expressed as a linear function of Kp (previous 12 hour maximum) and local time. The model gives the linear best fit location of the plasmapause as well as the standard deviations of the model parameters. We found a slight noon-midnight asymmetry with the plasmapause located on average an L shell farther from the Earth at midnight than in the noon sector. This is in the opposite sense to the noon-midnight asymmetry found previously. Significant variability (with standard deviations up to +/À 1 L shell) in the plasmapause location is seen and suggests that though the mean plasmapause is roughly circular, the instantaneous plasmapause has significant time variable localized structure at all local times but most especially in the duskside sector.

[1] X-ray and electric field measurements were made during five nearby negative natural lightning strikes in north central Florida during the summer of 2004. The observed X-ray emission typically was detected $1 ms before the first return stroke, during the stepped-leader phase, and had energies extending up to a few hundred keV. The X rays were produced in discrete, intense bursts emitted in coincidence with the formation of the leader steps, demonstrating unambiguously that the source of lightning X rays is closely related to the stepping process. The X-ray emission from lightning stepped leaders is found to be remarkably similar to that from lightning dart leaders, suggesting that these different types of leaders share a common mechanism. The reported observations have important implications for understanding how runaway breakdown occurs and how lightning leaders propagate. Citation: Dwyer, J. R., et al. (2005), X-ray bursts associated with leader steps in cloud-to-ground lightning,

This paper presents a model calculation of solar energetic particle propagation in a three-dimensional interplanetary magnetic field. The model includes essentially all the particle transport mechanisms: streaming along magnetic field lines, convection with the solar wind, pitch-angle diffusion, focusing by the inhomogeneous interplanetary magnetic field, perpendicular diffusion, and pitch-angle dependent adiabatic cooling by the expanding solar wind. We solve the Fokker-Planck transport equation with simulation of backward stochastic processes in a fixed reference frame in which any spacecraft is roughly stationary. As an example we model the propagation of those high-energy (E 10 MeV) solar energetic particles in gradual events that are accelerated by large coronal mass ejection shocks in the corona and released near the Sun into interplanetary space of a Parker spiral magnetic field. Modeled with different scenarios, the source of solar energetic particles can have a full or various limited coverages of latitude and longitude on the solar surface. We compute the long-term time profiles of particle flux and anisotropy at various locations in the heliosphere up to 3 AU, from the ecliptic to high latitudes. Features from particle perpendicular diffusion are revealed. Our simulation reproduces the observed reservoir phenomenon of solar energetic particles with constraints on either solar particle source or the magnitude of perpendicular diffusion.

[1] The Thunderstorm Energetic Radiation Array (TERA) is located at the University of Florida, Florida Tech International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. The array includes forty-five 7.6-cm-diameter NaI/photomultiplier tube detectors enclosed in 24 separate aluminum boxes that shield the detectors from light, moisture, and RF noise. The array covers the $1 km 2 ICLRT facility, centered on the rocket launch tower, used to trigger lightning. From 2005 to 2007, TERA recorded seven rocket-triggered lightning flashes. In this paper we present an analysis of the X-ray emission of three of these flashes. The X-ray emission is observed to occur during the dart leader phase of each stroke, just prior to the time of the return stroke. Significant X-rays are observed on all the detectors to a distance of 500 m from the lightning channel for times up to 200 ms prior to the start of the return stroke. Using Monte Carlo simulations to model the X-ray propagation, we find that the energetic electrons that emit the X-rays have a characteristic energy of about 1 MeV for one particular dart-stepped leader event. The X-ray emission for all three events has a radial fall off proportional to [exp (Àr/120)]/r and is most consistent with the energetic source electrons being emitted isotropically from the leader. It is also found that the X-ray and energetic electron luminosities of the leader channel decreases with increasing height above the ground. These results help shed light onto the mechanism for producing energetic radiation from lightning. For instance, a characteristic energy of 1 MeV is not consistent with the relativistic runaway electron avalanche mechanism, suggesting that so-called cold runaway electrons, produced by very strong electric fields, dominate the production of the X-rays.

We report the observation of an intense gamma‐ray burst observed on the ground at sea level, produced in association with the initial‐stage of rocket‐triggered lightning at the International Center for Lightning Research and Testing at Camp Blanding, FL. The burst was observed simultaneously on three NaI(Tl)/photomultiplier tube detectors that were located 650 m from the triggered lightning channel with gamma‐ray energies extending up to more than 10 MeV. The burst consisted of 227 individual gamma‐rays that arrived over a 300 μs time period in coincidence with an 11 kA current pulse. The burst of gamma‐rays had very different characteristics from the x‐ray emission frequently seen in association with the dart leader/return stroke sequences of triggered lightning and may represent a new kind of event, likely originating from cloud processes thousands of meters overhead.

Using a NaI(Tl) scintillation detector designed to operate in electrically noisy environments, we observed intense bursts of energetic radiation (>> 10 kiloelectron volts) during the dart leader phase of rocket-triggered lightning, just before and possibly at the very start of 31 out of the 37 return strokes measured. The bursts had typical durations of less than 100 microseconds and deposited many tens of megaelectron volts into the detector. These results provide strong evidence that the production of runaway electrons is an important process during lightning.

[1] We report measurements of the x-ray emission from rocket-triggered lightning, made during the summer of 2003, using four instruments placed between 15 and 40 m from the lightning channels. X-rays were measured 0 -80 ms just prior to and at the beginning of 73% of the 26 return strokes observed. The emission was composed of multiple, very brief bursts of x-rays in the 30-250 keV range, with each burst typically lasting less than 1 ms. The x-rays were primarily observed to be spatially and temporally associated with the dart leaders with a possible contribution from the beginning of the return strokes, with the most intense x-ray bursts coming from the part of the lightning channel within $50 m of the ground. Because triggered lightning strokes are similar to subsequent strokes in natural lightning, it is likely that x-ray emission is a common property of natural lightning.