Proton Pitch Angle Distributions Research Articles

In‐situ observations of a neutral gas torus at Europa

A persistent pattern in the pitch angle distributions of energetic protons near the orbit of Europa has been observed with the Energetic Particles Detector (EPD) on board the Galileo spacecraft during each of the Europa orbit crossings in the last 7 years. The proton fluxes at energies larger than 220 keV peak at equatorial pitch angles of 90° whereas fluxes of lower energy protons (80–220 keV) at this pitch angle are depleted. We propose that a Jupiter‐surrounding neutral gas torus in the vicinity of the orbit of Europa is responsible for the depletion of energetic particle fluxes by charge exchange collisions. In order to reproduce the observed depletion signature an average neutral number density of 20 to 50 cm−3 is required.

Locations of proton isotropic boundaries as measured by conjugate high-altitude and low-altitude satellites

Polar CAMMICE MICS data of proton pitch angle distributions with energies of 31–80 keV for the period of 1997–1998 were analyzed to determine the locations where anisotropic pitch angle distributions (perpendicular flux dominating) change to isotropic distributions. We study this high-altitude isotropic distribution boundary (IDB) in terms of its location in L-shell and MLT. Statistical results showed that this boundary is located at lower L-shells on the nightside and at higher L-shells on the dayside with most distant location at dawn. With the increase of magnetic activity, the IDB positions shift towards lower L-shells at all MLT. The locations of IDBs were also determined for different storm phases. A superposed epoch analysis revealed that the L-shells of the IDBs move to lower L-shells with decreasing Dst and to higher L-shells during Dst recovery. We made a comparison between Polar measurements of IDB at high altitudes and DMSP measurements of b2i boundary at low altitudes selecting several events during which simultaneous observations in the same local time sector were available. The magnetic field mapping using the Tsyganenko T01 model with the observed solar wind input parameters showed that Polar and DMSP were placed on nearly the same field lines, which leads us to suggest that the Polar IDB and the b2i boundary at DMSP are related.

Observation of the proton aurora with IMAGE FUV imager and simultaneous ion flux in situ measurements

The far ultraviolet cameras on board the IMAGE satellite images the aurora in three different spectral regions. One of the channels of the spectrographic imager SI12 observes the Doppler‐shifted Lyman α emission of precipitating protons. It makes it possible to spectrally discriminate between the proton and electron FUV aurora and to globally map the energetic protons. Its response depends on the auroral Lyman α line shape which reflects the characteristics of the proton pitch angle and energy distributions. We illustrate the dependence of the SI12 count rate on the characteristic energy of the proton precipitation and the viewing geometry. Simultaneous in situ observations of the precipitated protons have been collected during a FAST satellite pass when IMAGE was observing the global north polar region. The premidnight region located at the equatorward boundary of the oval is dominated by proton precipitation with a mean energy Ē = 7 keV which is separated from the electron component. The prenoon crossing exhibits a softer proton energy spectrum with Ē = 0.9 keV. The measured proton energy distribution is used as an input to a Monte Carlo model to calculate the expected SI12 signal along the magnetic footprint of the satellite orbit. If the different spatial resolution of the two types of measurements is accounted for, a good quantitative agreement is found with the IMAGE observations. Similarly, ion flux measurements collected on board the Defense Meteorological Satellite Program Fl5 satellite during an overflight in the postmidnight sector provide good agreement with the SI12 observations at the footprint aurora. The comparisons confirm the reliability of the FUV IMAGE cameras to remotely discriminate between the electron and the proton precipitations. The vertical emission rate profiles of the N2 Lyman‐Birge‐Hopfield and OI(1356Å) emissions are calculated in the proton‐dominated premidnight region. It is shown that the protons and the electrons produce FUV emissions with contributions peaking at different altitudes. Excitation by secondary electrons dominates the production of both emissions.

Proton ring current pitch angle distributions: Comparison of simulations with CRRES observations

In this study we compare the proton pitch angle distributions (PADs) in the ring current region (L ∼ 3–4) obtained from Combined Release and Radiation Effects Satellite (CRRES) observations during the large magnetic storm (minimum Dst = −170 nT) on August 19, 1991, with results of phase‐space mapping simulations in which we trace the bounce‐averaged drift of protons during storm‐associated enhancements in a model of the convection electric field. We map the phase‐space density ƒ according to Liouville's theorem except for attenuation by charge exchange, which we compute for both an empirical model [Rairden et al., 1986] and a theory‐based model [Hodges, 1994] of the neutral H density distribution. We compare simulated pitch angle distributions at 48 keV, 81 keV, and 140 keV at L = 3 and L = 4 directly with the CRRES distributions at the same energies and L values before and during the storm. A steady‐state application of our transport model, using the empirical neutral H density model of Rairden et al. [1986], reproduces the absolute intensities well except for E = 140 keV at L = 3 (M = 10 MeV/G) and E = 48 keV at L = 4 (M = 13 MeV/G). The anisotropies (A ∼ 0.2–0.8) of the CRRES and modeled pre‐storm pitch angle distributions agree within factors ≲ 2. Time‐dependent application of our transport model reproduces measured recovery phase anisotropies (t = 10–12 h after storm onset; A ∼ 0.4–1.2) similarly well at the selected energies and L values, but agreement between modeled and measured absolute intensities is energy‐dependent and not consistently good. Our model underpredicts the proton intensities found by CRRES for E > 80 keV at L = 4 in early recovery phase (t = 10–12 h). Perhaps the impulsive stormtime convection electric field was stronger during the main phase than we have assumed here. Comparisons were more difficult in late recovery phase (t = 20 h) because CRRES was too far off the magnetic equator. Proton life‐times inferred from the CRRES data during the recovery phase of this storm are considerably shorter than charge‐exchange lifetimes for either model, but the empirical neutral H density model of Rairden et al. [1986] leads to smaller discrepancies with the CRRES data at all the selected energies and L values than the theory‐based neutral H density model of Hodges [1994] for parameters that most closely represent the seasonal and solar maximum conditions of the August 19, 1991, storm. It appears that charge exchange alone is not enough to explain the observed rapid decay of the ring current proton intensities during the recovery phase of this storm.

Particle propagation channel detected at 4.7 AU inside a corotating interaction region

This study presents an interplanetary particle (electrons and ions) event which was detected at 4.7 AU by the instruments on the Ulysses spacecraft inside a corotating interaction region between a fast and slow solar wind stream. A particle propagation channel is identified where the particles can propagate from the Sun nearly scatter free within a region bounded by tangential discontinuities. This channel is found to be rooted in the vicinity of a solar active region where the flare associated with the interplanetary event was produced. The ions (energy over 600 keV) detected inside this channel are shown to be of solar origin: dispersion occurs in the proton arrival time versus proton energy, there is a strong anisotropy in the proton pitch angle distribution, and a hard energy‐spectrum for the protons. The electrons were already present when the spacecraft crossed the structure. The chemical composition is also indicative of a solar origin, with a decrease in the 4He/H (4He in the energy range 389–1278 keV and H in the range 480–1204 keV) abundance ratio by a factor of 2 inside the channel. Mapped back to the solar surface, the size of the propagation channel is estimated to be of the order of 30,000 km. The channel is found to be magnetically quiet, without a preferred direction for the minimum of variance.

Simulations of ring current proton pitch angle distributions

We employ a three‐dimensional ring current model to trace the bounce‐averaged drift of singly charged ions during storm‐associated enhancements in the convection electric field. Using the simulation results, we map proton phase space density during the main and recovery phases of a storm in accordance with conservation of phase space density ƒ. We map from an initial quiescent phase space distribution that is obtained by solving the steady state transport equation (bounce‐averaged charge exchange balancing bounce‐averaged radial diffusion) with observed plasma sheet proton spectra as outer boundary conditions. We obtain proton pitch angle distributions at L ∼ 3–4.5 by evaluating ƒ at representative ring current energies (∼20–170 keV). We find that the prestorm and stormtime proton pitch angle anisotropy at any given L between 3 and 4.5 increases with particle energy in agreement with observations. The actual anisotropy at specific energies depends strongly on the shape of the plasma sheet source spectrum at ∼0.5–3 keV. Relatively large enhancements in the stormtime phase space density from the quiescent distribution occurs at all pitch angles for low energies (≲80 keV) except where the quiescent distribution lies on open drift shells. These increases result primarily from stormtime access to L ≲ 4.5 along open drift trajectories from the plasma sheet. For higher‐energy (≳150 keV) protons, which are transported via radial diffusion, there is little change in the anisotropy over a 3‐hour storm. At intermediate energies (E ∼ 80–150 keV) the stormtime enhancements in the phase space density can vary quite strongly with equatorial pitch angle. For such particles the stormtime transport is intermediate between directly convective and quasi‐diffusive, which can complicate the analysis of stormtime pitch angle anisotropies at intermediate energies. Decay lifetimes obtained from CRRES data during the recovery phase of a moderate storm are found to be considerably shorter than charge‐exchange lifetimes. It thus appears that charge exchange alone is not enough to explain the observed rapid decay of the proton ring current.

Numerical simulation of proton pitch angle distribution in the real magnetosphere of the Earth

A new method of numerical simulation of the proton pitch angle distribution (PAD) based on Liouville's theorem is presented. It yields good results for that region of the Earth's magnetosphere where the transport rate far exceeds the loss processes (charge exchange and Coulomb collisions) rate. Obtained numerical models of the proton PAD are in good agreement with the experimental data and explain all features of the PAD of proton fluxes with energy E > 50 keV experimentally measured in the Earth's magnetosphere at 3 < R/ R E < 7.5 ( R E is Earth's radius) and different MLT (magnetic local time) during geomagnetically quiet periods.

Influence of the charge exchange and coulomb collisions on proton pitch angle distributions form in the Earth's radiation belts

Radial evolution of the proton pitch angle distributions at L<5.25 for 0.03< E<10.0 MeV is considered for various models of the radial diffusion (with various correlation between coefficients of the magnetic and electric diffusion and various dependencies of these coefficients on geomagnetic latitude) and selected spacial distributions of the neutral exosphere atoms and cold plasma of the plamasphere. The results of the numerical simulation are compared with experimental data. It is shown that the best agreement of numerical models with experimental data is achieved using for the numerical simulation radial diffusion coefficients that are independent of geomagnetic latitude, given by D E = 5 × 10 −6 L 10 (L 4 + μ 2) and D M = 5 × 10 −9 L 10R E 2day −1.

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.

Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening MLT sector. Season‐dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely‐directed, field‐aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies &gt;5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near‐Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field‐aligned currents and conductivity gradients. The generation of field‐aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotail's current sheet.

Comparison of picked‐up protons and water group ions upstream of comet Halley's bow shock

Data are presented on the properties of picked‐up cometary protons and water group (WG) ions observed upstream of the bow shock of comet Halley by the ion mass spectrometer and Johnstone plasma analyzer experiments on the Giotto spacecraft. The number of WG ions exceeded the number of cometary protons at cometocentric distances r &lt; 1.3×106 km, while the WG mass density exceeded the proton mass density when r &lt; 6×106 km. The small scale variations of proton and WG densities were well correlated, which argues against the "expanding halo" model which has been used to explain the quasi‐periodicity of energetic particles observed by VEGA and Giotto. Two parameters are used to describe the pitch angle distributions of the picked‐up ions: a 10% width, which is the full angular width at 10% of the maximum of the pitch angle distribution, and a mean width. The cometocentric distance profiles of both parameters exhibited a great deal of scatter, and both parameters showed a steeper average radial gradient for WG ions than for protons. The 10% widths of WG ions were consistent with less than 10∶1 anisotropies for r &lt; 2.5×106 km, but the proton anisotropy did not drop below 10∶1 until the spacecraft entered the “foreshock” region at r = 1.4×106 km. After subtraction of the variation in field direction, the mean width of the proton pitch angle distribution was nearly independent of distance everywhere outside the shock. The WG mean width, on the other hand, increased with increasing WG density (as expected) and with increasing angle α between the interplanetary field and the solar wind velocity vector (an unexpected result). No increase in shell radii, due to either adiabatic compression or first‐order Fermi acceleration, could be discerned for either ion species until the spacecraft was very close to the bow shock. The thickness of the picked‐up proton shell slowly increased as r decreased inside 2.5×106 km, whereas the water group shell thickness remained fairly constant until just outside the shock. While some of the differences between the two ion species can be attributed to the greater ionization scale length of cometary H atoms compared to WG atoms and molecules, the very different dependences of their pitch angle and energy diffusion rates on ion densities and on α are difficult to understand in terms of presently available theories and models.

Propagation of upstream protons in the near-earth solar wind

The propagation of energetic protons (35–1600 keV) from the Earth's magnetosphere to the ISEE-3 spacecraft located about 240 earth radii (R E) upstream in the solar wind is used as a tool to study the interaction between these protons and the solar wind. In this preliminary study we present proton pitch angle distributions seen at different times during the development of upstream events that occur in relatively quiet interplanetary conditions. In general a highly anisotropic sunward flow is seen at the beginning of the events. During the course of the events pitch angle distributions may vary between streaming along the field lines (peaked around 0° pitch angle), a uniform intensity between 0° and 90°, and a peaked distribution around a preferred pitch angle that is often near 90°.

Charged particle anisotropies in Saturn's magnetosphere

We report observations of anisotropies and pitch angle distributions for 0.5–1.8 MeV protons, 7–17 MeV electrons and ⪞3.4 MeV electrons in Saturn's magnetosphere made with the University of Chicago experiments on Pioneer 11. In the outer magnetosphere (L &gt;6) there is clear evidence for corotation of the proton flux, and the proton pitch angle distribution shows maximum flux perpendicular to the magnetic field (‘pancake’ distribution). Observed changes in the amplitude and shape of the pitch angle distributions suggest the existence of significant temporal variations in the outer magnetosphere. From L=6 to L=4, the proton intensity decreased by more than two orders of magnitude, and the pitch angle distribution shifted to a ‘dumbbell’ form (maximum flux parallel to magnetic field). The shift in pitch angle distribution most likely results from preferential absorption of large pitch angle particles by the tenuous E‐ ring found in the equatorial plane out to at least R=5 Rs. For L &lt;4, the proton intensity increased inwards, implying an inner edge for the E ring at R ≈4 Rs. Except for regions where the flux was reduced by satellite absorption, the pitch angle distributions remained dumbbell. In absorption regions, pancake distributions were found. The observations are consistent with the suggestion by McKibben and Simpson (this issue) that inward diffusion and acceleration at Saturn may proceed primarily via large, infrequent disturbances. Electron anisotropies were measurable only for L ⪝4.5, and the pitch angle distributions were found to be pancake for the entire region L &lt;4.5, suggesting that the absorbing particles in the E ring have radii that lie between the range of ∼1 MeV proton and ∼10 MeV electrons, or of the order of millimeters.

Field line topology in the dayside cusp region inferred from low altitude particle observations

Dayside low altitude satellite observations of the pitch angle and energy distribution of electrons and protons in the energy range 1 eV to 100 eV during quite geomagnetic conditions reveal that at times there is a clear latitudinal separation between the precipitating low energy (keV) electrons and protons, with the protons precipitating poleward of the electrons. The high energy (100 keV) proton precipitation overlaps both the low energy (keV) electron and proton precipitation. These observations are consistent with a model where magnetosheath particles stream in along the cusp field lines and are at the same time convected poleward by an electric field. The electrons with energies of a few keV move fast and give the “ionospheric footprint” of the distant cusp. The protons are partly convected poleward of the cusp and into the polar cap. Here the mirroring protons populate the plasma mantle. Equatorward of the cusp the pitch angle distribution of both electrons and protons with energies above a few keV is pancake shaped indicating closed geomagnetic field lines. The 1 keV electrons, penetrate, however, into this region of closed field line structure maintaining an isotropic pitch angle distribution. The intensity is, however, reduced with respect to what it was in the cusp region. It is suggested that these electrons, the lowest energies measured on the satellite, are associated with the entry layer.

Toward a unified view of diffuse auroral precipitation

A theoretical description of the mechanisms responsible for diffuse aurora is presented. Emphasis is placed on explaining the origin of the quiet time proton strong diffusion precipitation and its approximate coincidence with the zone of more intense electron precipitation. During quiet geomagnetic conditions, resonant scattering by electrostatic waves is considered the most viable mechanism. It is shown that the broadband electrostatic noise recently detected on auroral field lines by Gurnett and Frank can adequately account for the observed precipitation of ions up to 100 keV. Near the equatorial plane these waves can be destabilized by the normal loss cone pitch angle distribution of plasma sheet protons. Closer to the earth the waves are more readily excited by the field‐aligned currents which are a permanent feature of the auroral flux tubes. A numerical simulation of the convective growth rate in these physically distinct regions is presented, and examples are given of the anticipated polarization and spectral characteristics of the ensuing turbulence. An analysis is also made of the resonant and nonresonant diffusion of plasma on the auroral field lines. Of particular importance is the rapid Landau resonant heating of ionospheric electrons by the ion mode turbulence. The ultimate demise of the outflowing ionospheric electrons is of crucial importance, since a small concentration of cold electrons can quench the loss cone instability of ion cyclotron waves near the equator. This is invoked to explain the restriction of intense broadband electrostatic noise and the concomitant plasma sheet proton precipitation to auroral flux tubes which carry sufficient field‐aligned current to excite ion mode waves (and thus heat electrons) in the topside ionosphere.

The evaluation of the interplanetary diffusion coefficient for energetic particles employing real magnetic field data

A numerical simulation of energetic particle motion in the interplanetary medium is carried out using HEOS-2 magnetometer data in order to computeD(μ), the pitch angle diffusion coefficient, where μ is cosine of pitch angle defined with respect to the local field. WhileD(μ) exceeds that given by quasi-linear theory near 90° pitch angle, it is significantly less at higher values of μ, leading to a parallel transport coefficient in good accord with that given by experimental studies of solar proton propagation. In particular, α∥=0.031 AU at a particle magnetic rigidity of 455 MV, while experimental results range from 0.05 to 0.07 AU (+100%, −50%) in this rigidity region. Furthermore, observed approximately μ-dependent solar proton pitch angle distributions are consistent with the computed findingD(μ)/(1 −μ2)2 ~ constant.

Proton observations supporting the ion cyclotron wave heating theory of SAR arc formation

Low altitude satellite observations of precipitated and locally mirroring protons during periods of ground-based SAR arc observations are presented. The SAR arcs are found to be located in a region with significantly enhanced proton pitch angle scattering and enhanced electron temperature, but inside the plasmapause where the proton pitch angle distribution is anisotropic. The increase in the pitch angle scattering takes place in a localized region having a width of a few tenths of a L-value. The observations can favourably be accounted for by the Cornwall et al. (1971) theory for the SAR arc formation. Using observed proton fluxes and typical energy spectra, the expected Hβ intensity in the SAR arc region is estimated to be a few Rayleighs, and the energy flux from precipitated protons above a few keV to be 10 −2−10 −1erg/cm 2s. These estimates are in reasonable agreement with previously published theoretical and experimental values. Simultaneous groundbased observations of Hα emissions were found in the region of intense, isotropic proton precipitation located outside the plasmapause.

On the energy dependence of the ring current proton precipitation

Low altitude satellite measurements of protons in the 1–100 keV range indicate two energy dependent proton precipitation boundaries. At low invariant latitudes mostly below 60° there is a region of moderately weak proton precipitation. The poleward boundary of this region tends to be at higher latitudes for the high energy protons than for the low energy protons. At high invariant latitudes there is a region where both the low and high energy protons precipitate with an isotropic pitch-angle distribution. The equatorward boundary of this region tends to be at lower latitudes for protons with energy more than 100 keV than for those in the 1–6 keV range. This region with isotropic pitch-angle distribution is located well outside the plasmapause both for the 1–6 and 100-keV protons. Between these two precipitation zones there is a region where the proton pitch-angle distribution is highly anisotropic with almost no protons in the loss cone. This region tends to be wider and more pronounced in the 1–6 than in the 100-keV protons. These findings lend further support to the mechanism of ion-cyclotron instability as the cause of proton pitch-angle diffusion in the low and intermediate regions. The process responsible for the strong diffusion at auroral latitudes has not yet been identified.

The solar proton event of April 16, 1970, 2. Transformation of proton pitch angle distributions from the solar wind into the magnetosheath

During the event of April 16, 1970, energetic (∼0.7 MeV) protons were observed by Vela 6B in the solar wind and by Vela 5B in the magnetosheath. During one 4-hour period the omnidirectional flux measured by 5B was a factor of 3 larger than that measured by 6B. The pitch angle distribution observed at this time by 6B was characterized by a strong peak at 0° pitch angle. Transformation of this distribution according to the Liouville equation accounts for the enhanced omnidirectional flux and the shape of the pitch angle distributions observed at 5B. At a later time in the event, distorted pitch angle distributions, which were observed in the magnetosheath, are accounted for by a Liouville transformation of an assumed unidirectional distribution in interplanetary space. We conclude that particle entry into the magnetosheath was approximately adiabatic.

The effect of transient localized sources on equatorial pitch angle distributions in the magnetosphere

The maxima and minima that have been observed in the storm time equatorial pitch angle distributions of radiation belt electrons and protons can be attributed to the combination of a spatially localized time-dependent source and dispersion by azimuthal drift. A numerical analysis illustrates the variety in the evolution of the pitch angle distributions of trapped particles that can be produced simply by invoking different time and space profiles for the injection event.