Isotropic Pitch Angle Distribution Research Articles

Energetic neutral atom images of a narrow flow channel from the plasma sheet: Astrid‐1 observations

We present two energetic neutral atom (ENA) images in the energy range 26–37 keV from the ENA imager on board the Swedish microsatellite Astrid‐1. The images are taken 24 hours apart from below the ring current at 0300 magnetic local time (MLT) around the equator at 1000 km altitude and show a narrow region of ENA emissions extending along the field lines in the postmidnight region. A parameterized model of the ring current is used to extract the ion distribution. The extracted ion distribution is 25° wide in longitude within L = 4 − 6 with an almost isotropic pitch angle distribution (PAD). One ENA image from a polar vantage point shows that the loss cones of the PAD are partially filled. The z component of the interplanetary magnetic field (IMF) was +2 and −3 nT at the times of the two observations. For IMF Bz = −3 nT the ENA flux is stronger than that for the observation when IMF Bz = +2 nT. We use a kinetic ring current model to show that the morphology of the extracted ion distribution can be explained by a narrow flow channel from the plasma sheet at L = 10 and 0200–0300 MLT to L = 5 and 0100–0200 MLT for 26 keV protons. In order to match the absolute flux of the extracted ion distribution with the results from the kinetic model, the plasma sheet density at L = 10 had to be set to 10 cm−3. This value is unreasonably high, and we discuss some scenarios that may justify a more reasonable value. During the previous 5 days the Dst had been between −50 and −15 nT, and the interplanetary magnetic field Bz had been fluctuating between −10 and +10 nT.

Approximate Expression of Ellipse Polarization Degree for Gyrosynchrotron Radiation

We study the elliptical polarization degree (EPD) of gyrosynchrotronradiation for the case of transverse propagation. The EPD decreaseswith increasing ellipticity R (i.e. ratio of the major and minoraxial lengths) and with decreasing electron energy spectral indexδ. The EPD value varies from 0.55 to 0.15 in the whole rangeof 3⩽R⩽10 and 2⩽δ⩽7. An approximateexpression for the EPD is proposed, in which we assume that theemitting electrons have an isotropic pitch angle distribution and apower-law distribution in energy. The expression is designed for theelectron energy spectral index δ from 2 to 7, as well as theharmonic numbers s from 2 to 70 with a statistical error of ±6%. The expression can be applied to the polarization analysis andmagnetic field strength diagnostic for solar limb events.

Ring current dynamics during the 13-18 July 2000 storm period

We study the development of the terrestrial ring current during the time interval of 13–18 July, 2000, which consisted of two small to moderate geomagnetic storms followed by a great storm with indices Dst=−300 nT and Kp=9. This period of intense geomagnetic activity was caused by three interplanetary coronal mass ejecta (ICME) each driving interplanetary shocks, the last shock being very strong and reaching Earth at ∼ 14 UT on 15 July. We note that (a) the sheath region behind the third shock was characterized by B z fluctuations of ∼35 nT peak-to-peak amplitude, and (b) the ICME contained a negative to positive B z variation extending for about 1 day, with a ∼ 6-hour long negative phase and a minimum B z of about −55 nT. Both of these interplanetary sources caused considerable geomagnetic activity (Kp=8 to 9) despite their disparity as interplanetary triggers. We used our global ring current-atmosphere interaction model with initial and boundary conditions inferred from measurements from the hot plasma instruments on the Polar spacecraft and the geosynchronous Los Alamos satellites, and simulated the time evolution of H+, O+, and He+ ring current ion distributions. We found that the O+ content of the ring current increased after each shock and reached maximum values of ∼ 60% near minimum Dst of the great storm. We calculated the growth rate of electromagnetic ion cyclotron waves considering for the first time wave excitation at frequencies below O+ gyrofrequency. We found that the wave gain of O+ band waves is greater and is located at larger L shells than that of the He+ band waves during this storm interval. Isotropic pitch angle distributions indicating strong plasma wave scattering were observed by the imaging proton sensor (IPS) on Polar at the locations of maximum predicted wave gain, in good agreement with model simulations.

Synchrotron Radiation as the Source of Gamma‐Ray Burst Spectra

We investigate synchrotron emission models as the source of gamma-ray burst spectra. We show that including the possibility for synchrotron self-absorption, a ``smooth cutoff'' to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters - in particular, the value of the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be). From this correlation and knowledge of the E_p distribution, we show how to derive the expected distribution of alpha. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of alpha below alpha=-2/3. This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low energy cutoff in the spectrum of accelerated electrons, and allow for a more gradual break. We show that this low energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with -2/3 < alpha <= 0$. The very few bursts with low energy spectral indices that fall above alpha=0 may be due the presence of a the synchrotron self-absorption frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between alpha and E_p during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how the behavior are consistent with the above models.

On the Spatial Distribution of Hard X‐Rays from Solar Flare Loops

The aim of this paper is to investigate the spatial structure of the impulsive phase hard X-ray emission from solar flares. This work is motivated by the YOHKOH and the forthcoming HESSI observations. Summarizing past results, it is shown that the transport effects can account for the observations by inhomogeneous loops where there is a strong field convergence and/or density enhancement at the top of the flaring loop. Scattering by plasma turbulence at the acceleration site or pancake type pitch angle distribution of the accelerated electrons can also give rise to enhanced emission at the loop tops. These could be a natural consequence of acceleration by plasma waves. This paper considers a general case of stochastic scattering and acceleration that leads to an isotropic pitch angle distribution and an enhanced emission from the loop tops or the acceleration site. Following the formalism developed in earlier papers the strength and the spectrum of the radiation expected from the acceleration site and the foot points are evaluated and their dependence on the parameters describing the acceleration process and the flare plasma are determined. The theoretical ratio of these two intensities and relative values of their spectral indices are compared with the YOHKOH observations, demonstrating that the above mentioned parameters can be constrained with such observations. It is shown that future high spatial and spectral resolution observations, for example those expected from HESSI, can begin to distinguish between different models and constrain their parameters.

Deconvolution of Directly Precipitating and Trap‐precipitating Electrons in Solar Flare Hard X‐Rays. III.YohkohHard X‐Ray Telescope Data Analysis

We analyze the footpoint separation d and flux asymmetry A of magnetically conjugate double footpoint sources in hard X-ray images from the Yohkoh Hard X-Ray Telescope (HXT). The data set of 54 solar flares includes all events simultaneously observed with the Compton Gamma Ray Observatory (CGRO) in high time resolution mode. From the CGRO data we deconvolved the direct-precipitation and trap-precipitation components previously (in Paper II). Using the combined measurements from CGRO and HXT, we develop an asymmetric trap model that allows us to quantify the relative fractions of four different electron components, i.e., the ratios of direct-precipitating (qP1, qP2) and trap-precipitating electrons (qT1, qT2) at both magnetically conjugate footpoints. We find mean ratios of qP1=0.14 ± 0.06, qP2=0.26 ± 0.10, and qT=qT1+qT2=0.60 ± 0.13. We assume an isotropic pitch-angle distribution at the acceleration site and double-sided trap precipitation (qT2/qT1=qP2/qP1) to determine the conjugate loss-cone angles (α1=42° ± 11° and α2=52° ± 10°) and magnetic mirror ratiosat both footpoints (R1=1.6,...,4.0 and R2=1.3,...,2.5). From the relative displacement of footpoint sources we also measure altitude differences of hard X-ray emission at different energies, which are found to decrease systematically with higher energies, with a statistical height difference of hLo-hM1=980 ± 250 km and hM1-hM2=310 ± 300 km between the three lower HXT energy channels (Lo, M1, M2).

Monte-Carlo simulations of proton aurora

The spreading of a proton beam in the upper atmosphere is calculated based onMonte-Carlo simulations. The transport of the atoms is modelled in a magnetic field with dipolestrength. Neuralisation, ionisation and excitation mechanisms of the incoming particles areincluded from collision cross-sections of protons and hydrogen with an effective N 2atmosphere. Assuming an isotropic pitch angle distribution for the incoming protons, theirspreading in the upper atmosphere and the return flux of the charged and neutral component ofthe hydrogen beam has been calculated. Depending on energy and the tilt angle of the magneticfield about 10% of the incoming particles return from the atmosphere as ENA (Energetic NeutralAtoms). The ENA returning from the atmosphere show a source region below 500 km for theincoming high energy protons. For low energy protons, the ENA originate mainly from twodifferent regions, one around 700 km and the other at 400 km altitude, reflecting their sensitivityto several charge exchange processes.

Deconvolution of Directly Precipitating and Trap‐Precipitating Electrons in Solar Flare Hard X‐Rays. II.Compton Gamma Ray ObservatoryData Analysis

Based on the deconvolution method developed in the first paper of this series, we present here the data analysis of 20-200 keV hard X-ray (HXR) data from the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) recorded during 103 solar flares in 1991-1995. These are all of the flares simultaneously observed by CGRO with high time resolution (64 ms) and by Yohkoh in flare mode. The deconvolution method takes the measured HXR count rates as function of energy and time, I(ε, t), and computes the following self-consistently: the electron injection function n(E, t), the directly precipitating electron flux nprec(E, t), the trapped-precipitating flux ntrap(E, t), the fraction of directly precipitating electrons (qprec), the electron time-of-flight distance (lTOF), and the electron density at the loss cone site of the trap (ne). We find that the electron time-of-flight distances (lTOF = 20.0 ± 7.3 Mm) inferred with the deconvolution method are fully consistent with those obtained earlier using a Fourier filter method. The trap electron densities (ne = 1010.96±0.57 cm-3) obtained from deconvolving the e-folding decay times of HXR pulses (according to the trap model of Melrose & Brown) are found to be statistically a factor of 1.5 lower than those inferred from cross-correlation delays. The fraction qprec of directly precipitating electrons, measured for the first time here, is found to have a mean (and standard deviation) of qprec = 0.42 ± 0.16. Based on this precipitation fraction, we infer loss cone angles of α0 ≈ 20°-70° and magnetic mirror ratios of R = Bloss/Binj ≈ 1.2 - 3 (with a median value of Rmedian = 1.6) between the loss cone site and injection/acceleration site, assuming an isotropic pitch angle distribution at the injection site. The TOF distances and mirror ratios constrain magnetic scale heights in flare loops to λB = 10-150 Mm. The fact that this two-component model (with free-streaming and trapped electrons) satisfactorily fits the energy-dependent time delays in virtually all flares provides strong evidence that electron time-of-flight differences and collisional scattering times dominate the observed HXR timing, while the injection of electrons appears to be synchronized (independent of energy) and does not reveal the timing of the acceleration process.

Deconvolution of Directly Precipitating and Trap‐Precipitating Electrons in Solar Flare Hard X‐Rays. I. Method and Tests

We develop and test a numerical code that provides a self-consistent deconvolution of energy-dependent hard X-ray (HXR) time profiles I(e, t) into two HXR-producing electron components, i.e., directly precipitating and trap-precipitating electrons. These two HXR components can be physically distinguished because their energy-dependent time delays have an opposite sign. The deconvolution is based on the following model assumptions: (1) nonthermal electrons are injected from the acceleration site into coronal flare loops by an injection function f(E, α, t) that consists of synchronized pulses in energy E and pitch angle α, (2) electrons with initially small pitch angles (α ≤ α0) precipitate directly to the HXR emission site, (3) electrons with initially large pitch angles (α ≥ α0) are temporarily trapped and precipitate after the collisional deflection time, and (4) nonthermal electrons lose their energy by Coulomb collisions and emit thick-target HXR bremsstrahlung in a high-density (fully collisional) site (near or inside the chromosphere). The numerical deconvolution provides a self-consistent determination of three physical parameters: (1) the electron time-of-flight distance lTOF between the acceleration/injection site and the HXR emission site, (2) the electron density ne in the trap region, and (3) the fraction of HXR-emitting electrons that precipitate directly, qprec, which relates to the loss cone angle by qprec(α0) = (1 - cos α0) for isotropic pitch angle distributions. This yields the magnetic mirror ratio R = Bloss/Binj = 1/sin2 (α0) between the injection and loss cone site. With this method, we measure for the first time magnetic field ratios in coronal loops by means of HXR data. Based on this ratio, together with the knowledge of the photospheric field at the footpoint, a direct measurement of the magnetic field in the coronal acceleration region can be obtained. We simulate energy-dependent HXR data I(e, t) with typical solar flare parameters (lTOF = 15,000 km, ne = 1011 cm-3, qprec = 0.5) and test the accuracy of the inversion code. We perform the inversion in 30 different simulations over the entire physically plausible parameter space and demonstrate that a satisfactory inversion of all three physical parameters lTOF, ne, and qprec is achieved in a density range of ne = 1010-1012 cm-3 for precipitation ratios of qprec = 0.1-0.9 and for signal-to-noise ratios of 100 (requiring HXR count rates of 104 counts s-1). Applications of this inversion method to solar flare observations in hard X-rays (CGRO/BATSE, Yohkoh/Hard X-Ray Telescope) and microwaves (Nobeyama) will be presented in subsequent papers.

Initial FAST observations of acceleration processes in the cusp

FAST satellite observations during two encounters with the Earth's cusps near 4000 km are presented. In addition to precipitating magnetosheath particles with isotropic pitch angle distributions, the energetic particle data reveal upgoing ion beams, ion conics, downgoing “inverted‐V” electron distributions, and very narrow, intense upgoing electron beams. A rich assortment of DC‐coupled electric fields and plasma waves accompany these acceleration processes. In one example, step‐like injections of magnetosheath ions are correlated with bursts of upgoing ion beams and downgoing electrons, suggesting that the currents associated with these injections set up local electric potential structures that subsequently drive the acceleration processes. The observations underscore the fact that the cusp is not merely a conduit for the passage of magnetosheath particles and fields, but contains a dynamic internal electrical structure that accelerates particles locally and may alter the precipitating magnetosheath particle populations themselves.

Strong fluctuations of energetic electrons at low altitudes

Strong fluctuations (SF) of energetic electrons within the local loss cone (LC) from high resolution data on Active, a low altitude, high inclination satellite, are reviewed. The pitch angle diffusion rate of electrons at L > 4 is rapidly changing by one order up to the isotropic pitch angle distribution, corresponding to strong diffusion. The duration of the spikes to nearly strong diffusion limit is increasing with latitude. The statistical study shows the maximum occurrence of fluctuations within LC is in the noon sector and at latitudes 65 – 70°. Strong geomagnetic activity increases the probability of SF occurrence. One of the possible mechanisms consistent with the observations is the nonlinear precipitation oscillator (Davidson 1986; Davidson and Chiu, 1991).

Effects of spatial diffusion in the inner plasma sheet and the structure of the diffusion tensor

Abstract. A standard pair of equations is used to describe the behaviour of a single monoenergetic particle (proton or electron) population on a geomagnetic flux tube drifting in the magnetosphere. When particle losses from the drifting flux tube into the ionosphere are neglected, this behaviour is adiabatic in a thermodynamic sense. For a population of particles with an isotropic pitch-angle distribution, the generalization of that system of equations is obtained by adding radial and azimuthal spatial diffusion terms. The magnetic field is taken to be dipolar in the inner magnetosphere. The potential electric field is assumed to consist of magnetospheric convection and corotation components. Experimental data are used to estimate the radial equatorial profiles of the plasma sheet pressure. Assuming that the local time and L-shell variations are separable and supposing steady-state conditions, the expressions for the diffusion tensor components are evaluated. The influence of spatial diffusion on the radial and azimuthal profiles of the plasma pressure in the inner plasma sheet is also discussed.

Spatial relationships between field‐aligned currents and suprathermal electron beams observed at the poleward boundary of the nightside auroral oval

Magnetic field and particle data obtained from the Akebono satellite in the period from December 1989 to February 1990 are used for examining the relationships between electron populations and field‐aligned currents in the poleward boundary region of the nightside auroral oval. It is found that suprathermal electron beams frequently observed at the poleward boundary of the auroral oval associated with a latitudinally narrow (∼ 1°) field‐aligned current system located at the poleward edge, which has been designated as “the boundary current system” by Fukunishi et al. (1993, p. 11,250). In addition, an unstructured band of plasma‐sheet‐like electrons with isotropic pitch angle distributions except for loss cone appears through the auroral oval including the boundary current region. As a result, the electron energy spectra at the poleward boundary are characterized by a superposition of two Maxwellian functions: the isotropic high‐temperature component and the field‐aligned low‐temperature component. From the Maxwellian fitting procedure, the temperature and the density of the high‐temperature component are estimated to be 0.3 ‐ 1.7 keV and ≤ 1.0 cm−3, respectively, and those of the low‐temperature component are estimated to be 10 ‐ 80 eV and 2 ‐ 16 cm−3, respectively. An important finding is that the temperature and the density of the low‐temperature component and the density of the high‐temperature component are significantly enhanced in the upward current region occupying the equatorward portion of the boundary current system, while the temperature of the high‐temperature component is nearly constant throughout this region. These characteristics strongly suggest that the high‐temperature component originates from plasma sheet electrons, while the low‐temperature component originates from ionospheric electrons. It is likely that ionospheric thermal electrons flow away from the polewardmost downward current region into the magnetosphere and are accelerated and heated by some wave‐particle interaction process during their inward motion due to E × B drift.

Effects of spatial diffusion in the inner plasma sheet and the structure of the diffusion tensor

<strong class="journal-contentHeaderColor">Abstract.</strong> A standard pair of equations is used to describe the behaviour of a single monoenergetic particle (proton or electron) population on a geomagnetic flux tube drifting in the magnetosphere. When particle losses from the drifting flux tube into the ionosphere are neglected, this behaviour is adiabatic in a thermodynamic sense. For a population of particles with an isotropic pitch-angle distribution, the generalization of that system of equations is obtained by adding radial and azimuthal spatial diffusion terms. The magnetic field is taken to be dipolar in the inner magnetosphere. The potential electric field is assumed to consist of magnetospheric convection and corotation components. Experimental data are used to estimate the radial equatorial profiles of the plasma sheet pressure. Assuming that the local time and <i>L</i>-shell variations are separable and supposing steady-state conditions, the expressions for the diffusion tensor components are evaluated. The influence of spatial diffusion on the radial and azimuthal profiles of the plasma pressure in the inner plasma sheet is also discussed.

Low‐frequency instabilities and the resulting velocity distributions of pickup ions at comet Halley

The interaction between the solar wind and newborn cometary ions is studied using a new analytical theory as well as one‐ and two‐dimensional hybrid simulations. Using the observed parameters upstream of the comet Halley, a detailed study of wave excitation and the resulting particle distributions is presented. Linear theory as well as simulations show that a variety of modes such as the fast magnetosonic mode, high frequency whistlers and obliquely propagating Alfvén ion cyclotron waves can be excited. However, parallel propagating waves are found to be dominant in the wave spectrum and to control the scattering of the pickup ions. Several features of the observed distributions of pickup protons are explained. In particular, it is shown that the observed asymmetric pitch angle distribution for the pickup protons is due to the small saturation amplitude of the waves for the given parameters. Water group associated waves can lead to energy diffusion and further pitch angle scattering of protons. This effect is most likely to be important in the vicinity of the bow shock of comet Halley where the density of water group ions becomes comparable to that of protons. It is shown that the observed increase in the radius of the proton velocity shell just outside the bow shock can be due to water group waves. The nearly isotropic proton pitch angle distribution observed by Neugebauer et al. [1989] just outside the bow shock may, however, be related to the presence of a rotational discontinuity which has been identified in the magnetic field data. Just outside the bow shock, simulations show that parallel propagating water group waves can steepen with attached whistler wave packets. The steepening process at parallel propagation is a transient effect, in an important contrast to the case of steepening at oblique angles. The smaller beam densities at comet Halley appears to be the main reason not only why waves at comet Halley have smaller amplitudes but also why oblique, steepening magnetosonic waves have not been detected at comet Halley, whereas they have been seen at comet Giacobini‐Zinner.

Velocity space diffusion and nongyrotropy of pickup water group ions at comet Grigg‐Skjellerup

The diffusion of water group cometary ions in velocity space at comet Grigg‐Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three‐dimensional distributions are examined. Transformation of these parameters into a field‐aligned solar wind frame allows us to test the gyrotropy of the distributions. The observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach.

Particle drift in the Earth's plasma sheet

We generalize the derivation of the average gradient/curvature‐drift for a flux tube filled with an isotropic distribution of particles at specified kinetic energy. The present treatment is restricted to a two‐dimensional magnetic field with zero electric field, but it includes all chaotic and Speiser orbits, which do not correspond to the simple picture of gradient/curvature drift. We assume that particles are evenly distributed throughout the regions of phase space allowed by their energy and canonical momentum. This assumption is closely related but not exactly equivalent to the assumption of isotropic pitch‐angle distribution. Our derivation assumes that the maximum Larmor radius is small compared to the scale length for equatorial variations in the flux tube volume, but it does not involve any restrictions on the curvature of the field line. The resulting expression for the drift rate is valid for situations where the particle drift velocity is comparable to the thermal speed in some regions. The apparent implication of this generalized treatment is that the existence of very complex non‐adiabatic particle trajectories in the plasma sheet may not invalidate previous estimates of the average rate of particle drift out the sides of the tail, estimates that were made under the assumption of simple guiding‐center drifts.

A Description of a D-3He Fusion Reactor Based on a Dipole Magnetic Field

This paper discusses an ideal magnetic container for a D-{sup 3}He fusion reactor which must ensure both the stability of the confined plasma and the ability to control the confinement of fusion products. A dipole magnetic field may be suitable for D-{sup 3}He fusion since it is predicted to be able to confine high-beta plasmas while allowing extraction of the high-energy charged fusion products for direct conversion as well as removal of fusion ash using resonant and/or nonresonant magnetic perturbations. New calculations of fusion product control and plasma stability with isotropic pitch-angle distributions are described. In addition, the parameters of a new, higher field dipole reactor design are discussed.

Center-to-limb variations of characteristics of solar flare hard X-ray and gamma-ray emission

view Abstract Citations (37) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Center-to-Limb Variations of Characteristics of Solar Flare Hard X-Ray and Gamma-Ray Emission McTiernan, James M. ; Petrosian, Vahe Abstract An analysis is presented of the observed distribution of fluxes and spectral indices of flares at X-ray and gamma-ray range, with particular emphasis on the variation with heliocentric longitude of these distributions and their moments. It is shown how these distributions can be used to set constraints on the distribution and average values of model parameters describing the characteristics of the accelerated electrons and flare plasma. A comparison of X-ray and gamma-ray spectral indices show that the spectrum of the accelerated electrons must flatten at higher energies. The Gaussian width of the pitch angle distribution of electrons beamed along the field lines and the convergence rate of the magnetic field are delimited. It is concluded that isotropic pitch angle distributions or distributions peaking perpendicular to the field lines, and field convergence rate with more than a fivefold increase in the magnetic field from corona to the photosphere can be ruled out. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170513 Bibcode: 1991ApJ...379..381M Keywords: Gamma Ray Bursts; Solar Electrons; Solar Flares; Solar Limb; Solar X-Rays; Electron Acceleration; Electron Distribution; Flux Density; Power Spectra; Solar Physics; GAMMA RAYS: BURSTS; PARTICLE ACCELERATION; RADIATION MECHANISMS; SUN: FLARES; SUN: X-RAYS full text sources ADS |

Rocket measurements of relativistic electrons: New features in fluxes, spectra and pitch angle distributions

We report new features of precipitating relativistic electron fluxes measured on a spinning sounding rocket payload at midday between altitudes of 70 and 130 km in the auroral region (Poker Flat, Alaska, 65.1°N, 147.5°W, and L=5.5). The sounding rocket (NASA 33.059) was launched at 21:29 UT on May 13, 1990 during a relativistic electron enhancement event of modest intensity. Electron fluxes were measured for a total of about 210 seconds at energies from 0.1 to 3.8 MeV, while pitch angle was sampled from 0° to 90° every spin cycle. Flux levels during the initial 90 seconds were about 5 to 8 times higher than in the next 120 seconds, revealing a time scale of more than 100 seconds for large amplitude intensity variations. A shorter time scale appeared for downward electron bursts lasting 10 to 20 seconds. Electrons with energies below about 0.2 MeV showed isotropic pitch angle distributions during most of the first 90 seconds of data, while at higher energies the electrons had highest fluxes near the mirroring angle (90°); when they occurred, the noted downward bursts were seen at all energies. Data obtained during the second half of the flight showed little variation in the shape of the pitch angle distribution for energies greater than 0.5 MeV; the flux at 90° was about 100 times the flux at 0°. We have compared the low altitude fluxes with those measured at geostationary orbit (L=6.6), and find that the low altitude fluxes are much higher than expected from a simple mapping of a pancake distribution at high altitudes (at the equator). Energy deposition of this modest event is estimated to increase rapidly above 45 km, already exceeding the cosmic ray background at 45 km.