Proton Pitch Angle Distributions Research Articles

Dynamic acceleration of energetic protons by an interplanetary collisionless shock

Context. Interplanetary collisionless shocks are known to be capable of accelerating charged particles up to hundreds of MeV. However, the underlying acceleration mechanisms are still under debate. Aims. We present the dynamic behaviors of energetic protons that are accelerated by an interplanetary shock that was observed with unprecedented high-resolution measurements by the Electron-Proton Telescope sensor of the Energetic Particle Detector suite on board the Solar Orbiter spacecraft on 2021 November 3. We constrain the potential acceleration mechanisms and processes. Methods. We first reconstructed the proton pitch-angle distributions (PADs) in the solar wind frame. Then, we examined the temporal flux profile, PAD, and the velocity distribution function of energetic protons close to the shock, and we qualitatively compared the observations with theoretical predictions. Moreover, we applied a velocity dispersion analysis (VDA) to an observed velocity dispersion event and derived the proton path length and release time at the shock. Then, we tested this derivation by comparing it with the shock motion and the magnetic field configuration. Results. We find that ∼1000–4000 keV protons exhibit a rapid-rise, rapid-decay temporal flux profile with a clear velocity dispersion ∼2 min before the shock, similar to impulsive solar energetic particle events. The proton path length based on the VDA of this event is consistent with the length derived from the shock motion and magnetic field configuration. The peak spectrum in this event appears to be steeper than the spectrum at the shock. Furthermore, we find that ∼50–200 keV proton fluxes peak between ∼10 and ∼20 s before the shock, with an inverse velocity dispersion. The velocity dispersion event and the inverse velocity dispersion event are both accompanied by magnetic kinks or switchbacks. In addition, two distinct proton populations appear near the shock. The first population at energies below ∼300 keV is characterized by a power-law spectrum with an index of ∼6–7 and a flux profile that increases before and decreases after the shock. The other population at energies above ∼300 keV shows a long-lasting, anti-sunward-beamed PAD across the shock and a flux profile that remains relatively constant before and increases slightly after the shock. Conclusions. These results suggest that the shock acceleration of energetic protons is highly dynamic due to temporal and/or spatial variations at the shock front. The observation of the velocity dispersion event further suggests that shock acceleration can be impulsive and efficient, which may be due to the interaction between the shock and magnetic kinks or switchbacks. Moreover, these results may support shock-drift acceleration and diffusive shock acceleration as candidate acceleration mechanisms at interplanetary shocks.

Proton Equatorial Pitch Angle Distributions in Jupiter's Inner Magnetosphere

AbstractWe use data from the plasma and energetic particle instruments on the Juno spacecraft, JADE and JEDI, to study the equatorial pitch angle distributions of energetic protons within Jupiter's inner magnetosphere during Juno's prime mission from 2016 to 2021. Averaging over all observations made by Juno within M‐shell bins between M = 6 and M = 30, we find protons at energies between 10 and 50 keV are predominantly field‐aligned at all M‐shells. In contrast, those at energies above 200 keV transition from being predominantly field‐aligned at M = 20 to having pancake‐like distributions at M < 10. We find the observed distributions are consistent either with charge exchange of protons with neutral toroidal clouds, or with adiabatic acceleration of protons originating from Jupiter that are transported inward from the middle magnetosphere.

Acceleration of suprathermal protons near an interplanetary shock

Context.Interplanetary collisionless shocks are known to be sources of energetic charged particles up to hundreds of MeV. However, the underlying acceleration mechanisms are still under debate.Aims.We determine the properties of suprathermal protons accelerated by the interplanetary shock on 2021 November 3 with the unprecedented high-resolution measurements by the SupraThermal Electron Proton sensor of the Energetic Particle Detector on board the Solar Orbiter spacecraft, in order to constrain the potential shock acceleration mechanisms.Methods.We first reconstructed the pitch-angle distributions (PADs) of suprathermal protons in the solar wind frame. Then, we studied the evolution of the PADs, the temporal flux profile, and the velocity distribution function of this proton population close to the shock and compared the observations to theoretical predictions.Results.We find that the suprathermal proton fluxes peak ∼12 to ∼24 s before the shock in the upstream region. The proton fluxes rapidly decrease by ∼50% in a thin layer (∼8000 km) adjacent to the shock in the downstream region and become constant farther downstream. Furthermore, the proton velocity distribution functions in the upstream (downstream) region fit a double power law,f(v)∼v−γ, at ∼1000 − 3600 km s−1, with aγof ∼3.4 ± 0.2 (∼4.3 ± 0.7) at velocities (v) below a break at ∼1800 ± 100 km s−1(∼1600 ± 200 km s−1) and aγof ∼5.8 ± 0.3 (∼5.8 ± 0.2) at velocities higher than this. These indices are all smaller than predicted by first-order Fermi acceleration. In addition, the proton PADs in the covered pitch-angle range show anisotropies in the direction away from the shock in the region close to the upstream region and become nearly isotropic farther upstream, while downstream of the shock, they show a tendency of anisotropies towards 90° PA.Conclusions.These results suggest that the acceleration of suprathermal protons at interplanetary shocks are dynamic on a timescale of ∼10 s, that is, few proton gyroperiods. Furthermore, shock-drift acceleration likely plays an important role in accelerating these suprathermal protons.

Energetic proton acceleration by EMIC waves in Io’s footprint tail

In this study, we present a survey of energetic proton observations associated with Io’s footprint tail (FPT) and compare their signatures with in situ measurements of the plasma waves and lower-energy electron environments. We find further supporting evidence that proton acceleration in Io’s FPT is likely a consequence of wave–particle interactions via electromagnetic ion cyclotron waves that are generated by precipitating electrons into Jupiter’s ionosphere. This idea was originally proposed by Clark et al. (2020) and Sulaiman et al. (2020) based on NASA’s Juno mission likely transiting Io’s Main Alfvén Wing (MAW) during its twelfth orbit (i.e., PJ12). Additionally, the analysis of > 50 keV protons presented here highlights important observational details about the Io–Jupiter interaction as follows: 1) proton acceleration in Io’s FPT is a persistent feature and the energy flux carried by the protons is highest at smaller Io-Alfvén tail distances; 2) energetic protons exhibit positive correlations with both plasma waves and <100 keV/Q electrons; 3) during a small number of Io FPT crossings, the protons display finer spatial/temporal structure reminiscent of the electron observations reported by Szalay et al. (2018); and 4) the proton pitch angle distributions are characterized by two types: conic distributions in or near Io’s MAW and isotropic elsewhere.

Proton pitch angle distributions in the Martian induced magnetosphere: A survey of Tianwen-1 Mars Ion and Neutral Particle Analyzer observations

Proton pitch angle distributions in the Martian induced magnetosphere: A survey of Tianwen-1 Mars Ion and Neutral Particle Analyzer observations

Modeling Inner Proton Belt Variability at Energies 1 to 10 MeV Using BAS‐PRO

AbstractGeomagnetically trapped protons forming Earth's proton radiation belt pose a hazard to orbiting spacecraft. In particular, solar cell degradation is caused by non‐ionising collisions with protons at energies of several megaelectron volts (MeV), which can shorten mission lifespan. Dynamic enhancements in trapped proton flux following solar energetic particle events have been observed to last several months, and there is a strong need for physics‐based modeling to predict the impact on spacecraft. However, modeling proton belt variability at this energy is challenging because radial diffusion coefficients are not well constrained. We address this by using the British Antarctic Survey proton belt model BAS‐PRO to perform 3D simulations of the proton belt in the region 1.15 ≤ L ≤ 2 from 2014 to 2018. The model is driven by measurements from the Radiation Belt Storm Probes Ion Composition Experiment and Magnetic Electron Ion Spectrometer instruments carried by the Van Allen Probe satellites. To investigate sensitivity, simulations are repeated for three different sets of proton radial diffusion coefficients DLL taken from previous literature. Comparing the time evolution of each result, we find that solar cycle variability can drive up to a ∼75% increase in 7.5 MeV flux at L = 1.3 over four years due to the increased importance of collisional loss at low energies. We also show how the anisotropy of proton pitch angle distributions varies with L and energy, depending on DLL. However we find that phase space density can vary by three orders of magnitude at L = 1.4 and μ = 20 MeV/G due to uncertainty in DLL, highlighting the need to better constrain proton DLL at low energy.

Modeling of Anisotropy Dynamics of the Proton Pitch Angle Distribution in the Earth’s Magnetosphere

Last years the attention to research of anisotropy of the charged particle pitch angle distribution has considerably increased. Therefore for research of anisotropy dynamics of the proton pitch angle distribution is used the two-dimensional Phenomenological Model of the Ring Current (PheMRC 2-D), which includes the radial and pitch angle diffusions with consideration of losses due to wave-particle interactions. Experimental data are collected on the Polar/MICS satellite during the magnetic storm on October 21–22, 1999. Solving the non-stationary two-dimensional equation of pitch angle and radial diffusions, numerically was determined the proton pitch angle distribution anisotropy index (or parameter of the proton pitch angle distribution) for the pitch angle of 90 degrees during the magnetic storm, when the geomagnetic activity Kp-index changed from 2 in the beginning of a storm up to 7+ in the end of a storm. Dependence of the perpendicular proton pitch angle distribution anisotropy index with energy E = 90 keV during the different moments of time from the McIlwain parameter L (2.26 < L < 6.6) is received. It is certain at a quantitative level for the magnetic storm on October 21–22, 1999, when and where on the nightside of the Earth’s magnetosphere (MLT = 2300) to increase in the geomagnetic activity Kp-index there is a transition from normal (pancake) proton pitch angle distributions to butterfly proton pitch angle distributions. That has allowed to determine unequivocally and precisely the anisotropy dynamics of the proton pitch angle distribution in the given concrete case. It is shown, that with increase of the geomagnetic activity Kp-index the boundary of isotropic proton pitch angle distribution comes nearer to the Earth, reaching L ≈ 3.6 at Kp = 7+

Simultaneous Observations of Electromagnetic Ion Cyclotron (EMIC) Waves and Pitch Angle Scattering During a Van Allen Probes Conjunction

AbstractOn 22 December 2015, the two Van Allen Probes observed two sets of electromagnetic ion cyclotron (EMIC) wave bursts during a close conjunction when both Probe A and Probe B were separated by 0.57 to 0.68 RE. The EMIC waves occurred during an active period in the recovery phase of a coronal mass ejection‐driven geomagnetic storm. Both spacecraft observed EMIC wave bursts that had similar spatial structure within a 1–2 min time delay. The EMIC waves occurred outside the plasmasphere, within ΔL ≈ 1–2 of the plasmapause and within a few degrees in magnetic latitude of the equatorial plane. The spatial structure of the EMIC wave bursts may have been related to the proton drift paths outside the plasmasphere and influenced by total magnetic field strength variations associated with solar wind pressure enhancements. The EMIC waves were observed in a narrow L shell region from L ≈ 4.55–5.32 between 10 and 11 magnetic local time (MLT) on the outbound halves of the spacecraft orbits and from L ≈ 4.82–5.51 between 13 and 14 MLT on the inbound halves of the spacecraft orbits. However, Pc1 pulsations were observed on the ground over a broad range of local times. The anisotropy of the proton pitch angle distributions was enhanced when the EMIC waves were observed. Although the overall radiation belt response during this storm was dominated by acceleration and transport processes, the EMIC waves produced local pitch angle scattering of 13–15 keV protons and 2.1–2.6 MeV electrons, consistent with calculations of the expected resonant energies.

The high-energy particle package onboard CSES

The China Seismo-Electromagnetic Satellite (CSES) aims to monitor space electromagnetic fields, ionospheric plasma, high-energy charged particles and other features of the global space environment. The high-energy particle package (HEPP), which can effectively detect the energy spectrum, flux and pitch angle distribution of space electrons and protons, and soft X-ray emission from solar flares, is one of the main payloads of CSES. In this study, we designed, developed and calibrated the high-energy particle package and launched it into orbit with CSES. HEPP consists of the high-energy detector (HEPP-H), the low-energy detector (HEPP-L), and the solar X-ray monitor (HEPP-X). The three sub-detectors mainly use silicon detector and crystal calorimeter detection technology. Before launching, we calibrated the three sub-detectors in detail by using radiation source and accelerator beam. All the three sub-detectors have good energy linearity. After launching into orbit, the space energetic particles and X-ray distribution detected by HEPP are consistent with expectations. The performance indices of the detector reach the advanced level of the same kind of detector in the world. HEPP has a wide energy detection range, good energy resolution and high angular resolution ability for electrons and protons. It will play an important role in the study of space particle response and space physics of seismic activity.

Saturn's Innermost Radiation Belt Throughout and Inward of the D‐Ring

AbstractCassini discovered Saturn's innermost radiation belt during the end of its mission. The belt is populated with relativistic protons, probably up to the trapping limit of ≈20 GeV. It extends from Saturn's dense atmosphere into and throughout the D‐ring. The A–C rings separate this belt entirely from the previously known radiation belts, suggesting that the innermost radiation belt is populated entirely via cosmic ray albedo neutron decay. We find that the proton pitch angle distributions are consistent with being shaped by losses to the D‐ring and the upper atmosphere rather than, for example, wave‐particle interactions. This supports that the main loss process of this new radiation belt is energy loss in neutral material, different from Saturn's other radiation belts. This property constrains the overall scale height of Saturn's exosphere to <700 km and the average D‐ring water molecule column density to being about 1 order of magnitude below the Enceladus gas torus.

On the dynamics of latitudinal profiles of low-energy solar protons in the Earth magnetosphere

Many works have been devoted to studying the boundaries of the penetration of solar protons into the Earth’s magnetosphere. This work first considers the dynamics of not only the boundary, but the latitudinal profiles of penetration in general depending on the energy and local time of measurement according to the data of the low-altitude CORONAS-F satellite. When flying through the polar cap, the isotropic pitchangle distribution of protons leads to the equality of the recorded precipitating flux and the proton flux in the interplanetary space. Beginning at a particular latitude, the proton flux begins to drop and, over time, reaches the level of the background of galactic cosmic rays. The latitudinal profile measured in this manner on the night side reaches the bending point when the Larmor radius of the proton becomes comparable with the radius of the curvature of the line of force; after partial trapping, the flux of precipitating protons successively drops. The protons are transferred to the day side by the magnetic drift and, unlike the night profile, the character of the day profile depends on the configuration of the entire magnetosphere. The character of latitudinal profiles has been studied depending on the local time and energy of the particles, which enabled the features of the magnetosphere deformation to be evaluated at certain times of magnetic storms.

A statistical study of proton pitch angle distributions measured by the Radiation Belt Storm Probes Ion Composition Experiment

AbstractA statistical study of ring current‐energy proton pitch angle distributions (PADs) in Earth's inner magnetosphere is reported here. The data are from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on board the Van Allen Probe B spacecraft from 1 January 2013 to 15 April 2015. By fitting the data to the functional form sinnα, where α is the proton pitch angle, we examine proton PADs at the energies 50, 100, 180, 328, and 488 keV in the L shell range from L = 2.5 to L = 6. Three PAD types are classified: trapped (90° peaked), butterfly, and isotropic. The proton PAD dependence on the particle energy, magnetic local time (MLT), L shell, and geomagnetic activity are analyzed in detail. The results show a strong dependence of the proton PADs on MLT. On the nightside, the n values outside the plasmapause are clearly lower than those inside the plasmapause. At higher energies and during intense magnetic activity, nightside butterfly PADs can be observed at L shells down to the vicinity of the plasmapause. The averaged n values on the dayside are larger than on the nightside. A maximum of the averaged n values occurs around L = 4.5 in the postnoon sector (12–16 MLT). The averaged n values show a dawn‐dusk asymmetry with lower values on the dawnside at high L shells, which is consistent with previous studies of butterfly PADs. The MLT dependence of the proton PADs becomes more distinct with increasing particle energy. These features suggest that drift shell splitting coupled with a radial flux gradient play an important role in the formation of PADs, particularly at L > ~ 4.5.

Recent Particle Measurements from Voyagers 1 and 2

We summarize recent measurements made by the LECP instrument on Voyager 1 near its crossing of the heliopause and entry into the interstellar medium on or about day 238 of 2012 at 121.6 AU, and on Voyager 2 mainly during the period 2012-2014.4 characterized by large variations in the intensities and angular distributions of low-energy heliosheath ions and the reappearance of low-energy heliosheath electrons. Results from Voyager 1 not previously published include the energy dependence of ion intensity decreases prior to the heliopause crossing and a quantitative measure of the evolution of low-energy heliosheath proton pitch angle distributions that extend across the heliopause. For Voyager 2 we describe the evolution in time and with ion energy of the tangential streaming of ions directed from the nose toward the tail of the heliosheath, summarize the recovery of low-energy heliosheath ion intensities since their decline to minimum levels during 2013.0-3013.3, and discuss the effects of this intensity minimum and subsequent recovery on the ion partial pressure in the heliosheath and on its magnitude relative to that of the thermal plasma and the magnetic field.

A NEW UNDERSTANDING OF THE EUROPA ATMOSPHERE AND LIMITS ON GEOPHYSICAL ACTIVITY

Deep extreme ultraviolet spectrograph exposures of the plasma sheet at the orbit of Europa, obtained in 2001 using the Cassini Ultraviolet Imaging Spectrograph experiment, have been analyzed to determine the state of the gas. The results are in basic agreement with earlier results, in particular with Voyager encounter measurements of electron density and temperature. Mass loading rates and lack of detectable neutrals in the plasma sheet, however, are in conflict with earlier determinations of atmospheric composition and density at Europa. A substantial fraction of the plasma species at the Europa orbit are long-lived sulfur ions originating at Io, with ~25% derived from Europa. During the outward radial diffusion process to the Europa orbit, heat deposition forces a significant rise in plasma electron temperature and latitudinal size accompanied with conversion to higher order ions, a clear indication that mass loading from Europa is very low. Analysis of far ultraviolet spectra from exposures on Europa leads to the conclusion that earlier reported atmospheric measurements have been misinterpreted. The results in the present work are also in conflict with a report that energetic neutral particles imaged by the Cassini ion and neutral camera experiment originate at the Europa orbit. An interpretation of persistent energetic proton pitch angle distributions near the Europa orbit as an effect of a significant population of neutral gas is also in conflict with the results of the present work. The general conclusion drawn here is that Europa is geophysically far less active than inferred in previous research, with mass loading of the plasma sheet ≤4.5 x 10^(25) atoms s^(-1) two orders of magnitude below earlier published calculations. Temporal variability in the region joining the Io and Europa orbits, based on the accumulated evidence, is forced by the response of the system to geophysical activity at Io. No evidence for the direct injection of H_2O into the Europa atmosphere or from Europa into the magnetosphere system, as has been observed at Enceladus in the Saturn system, is obtained in the present investigation.

LISA-PF radiation monitor performance during the evolution of SEP events for the monitoring of test-mass charging

Cosmic rays of solar and galactic origin at energies >100 MeV/n charge and induce spurious forces on free-floating test masses on board interferometers devoted to gravitational wave detection in space. LISA Pathfinder (LISA-PF), the technology testing mission for eLISA/NGO, will carry radiation monitors for on board test-mass charging monitoring. We present here the results of a simulation of radiation monitor performance during the evolution of solar energetic particle (SEP) events of different intensity. This simulation was carried out with the Fluka Monte Carlo package by taking into account for the first time both energy and spatial distributions of solar protons for the SEP events of 23 February 1956, 15 November 1960 and 7 May 1978. Input data for the Monte Carlo simulations was inferred from neutron monitor measurements. Conversely, for the SEP event of 13 December 2006 observed by the PAMELA experiment in space, we used the proton pitch angle distribution (PAD) computed from the Particle Acceleration and Transport in the Heliosphere (PATH) code. We plan to adopt this approach at the time of LISA-PF data analysis in order to optimize the correlation between radiation monitor observations and test-mass charging. The results of this work can be extended to the future space interferometers and other space missions carrying instruments for SEP detection.

Comparison of TWINS and THEMIS observations of proton pitch angle distributions in the ring current during the 29 May 2010 geomagnetic storm

AbstractThe first comparison between energy‐dependent ion pitch angle distributions in the ring current deconvolved from Two Wide‐angle Imagining Neutral‐atom Spectrometers (TWINS) energetic neutral atom (ENA) images and measured by Time History of Events and Macroscale Interactions during Substorms (THEMIS) D and E during a magnetic storm on 29 May 2010 is shown. The ion intensities and energy spectra along the THEMIS path are also compared with those deconvolved from the TWINS ENA images. The global plots of the ion intensities show an asymmetric ring current in the early recovery phase consistent with the ASY/H index. The comparison between the in situ pitch angle distributions observed by THEMIS and those obtained here from TWINS ENA images lends credence to the global plots of pitch angle anisotropy provided by the TWINS data. The spatial dependence of the pitch angle anisotropy provides information relevant to ion precipitation and lifetimes of trapped ring current ions not available from in situ measurements.

Average magnetotail electron and proton pitch angle distributions from Cluster PEACE and CIS observations

We present results from the first systematic survey of proton and electron pitch angle distributions in the magnetotail, based on Cluster CIS and PEACE data binned by proton plasma beta (beta(p)). The proton distributions conform to the canonical picture of magnetotail ions - a made up of Earthward streaming and bidirectional field-aligned particles, consistent with recent observations of time-varying beamlets, which gives way to a broadly isotropic central plasma sheet when beta(p) similar to 3. The electron distributions are significantly different from the canonical picture. A boundary layer made up of bidirectional field-aligned electrons is observed to values of beta(p) as high as 17. This quickly gives way to perpendicular-dominated electrons close to the neutral sheet. Hence, our results suggest that, on average, there is no extended, isotropic electron plasma sheet and that the proton plasma sheet is not routinely encountered until higher beta(p) than commonly assumed. Citation: Walsh, A. P., C. J. Owen, A. N. Fazakerley, C. Forsyth, and I. Dandouras (2011), Average magnetotail electron and proton pitch angle distributions from Cluster PEACE and CIS observations, Geophys. Res. Lett., 38, L06103, doi:10.1029/2011GL046770.

Magnetic coupling of the ring current and the radiation belt

The magnetic influence of the storm time ring current on high‐energy particles is demonstrated by using a simulation of the ring current incorporating self‐consistent magnetic and electric fields. Observations by the Polar satellite show that the magnetic field is occasionally depressed by 50% or more near the equatorial plane at <6 RE. We call them equatorially magnetic depression events (EMDEs) and focus on the most intense EMDE observed during an intense storm on 22 October 1999. The simulation predicts that under a strong convection electric field, the magnetic field strength is highly depressed around L = 5 by newly injected ions of energy 80 keV or less. The depressed magnetic field causes a significant adiabatic decrease in the high‐energy ion flux at pitch angles near 90° to conserve the first adiabatic invariant. A more tail‐like (shortened) magnetic field line causes an enhancement of the flux at pitch angles near 0° and 180° to conserve the second adiabatic invariant. Consequently, a butterfly‐like pitch angle distribution (PAD) is formed, which agrees with the Polar observation. We propose that the adiabatic process could have acted not only on the high‐energy component of the protons but also on relativistic electrons in the outer radiation belt. This notion is supported by simultaneous Polar observation of relativistic electron fluxes that show a decrease at pitch angles near 90° and a slight increase at pitch angles near 0° and 180°. PADs of protons and electrons can be used to distinguish nonadiabatic processes acting selectively on electrons from adiabatic ones.

Loss of relativistic electrons: Evidence for pitch angle scattering by electromagnetic ion cyclotron waves excited by unstable ring current protons

During geomagnetic storms the flux of radiation belt electrons can increase, decrease, or stay constant, depending on the competition between acceleration and loss mechanisms. We focus on loss of relativistic electrons. We use low‐altitude polar‐orbiting spacecraft and analyze fluxes of tens to hundreds of keV protons and relativistic (>1.5 MeV) electrons during a moderate geomagnetic storm, with a long‐lasting recovery phase (4–5 d). Using data from four local times, we find that the loss of relativistic electrons is confined within the anisotropic proton zone and that a spatially limited loss of relativistic electrons is spatially collocated with increased loss of protons. The proton pitch angle distributions within these peaks are consistent with moderate to strong pitch angle scattering due to electromagnetic ion cyclotron (EMIC) waves. The loss of relativistic electrons collocated with protons is found at all four local times considered (0300, 0700, 1400, 1700 MLT). Since anisotropic proton distributions can under certain conditions generate EMIC waves, we find strong indications that the observed relativistic electrons are scattered into the atmospheric loss cone by EMIC waves. EMIC wave scattering is less efficient at high equatorial pitch angles but very efficient near the loss cone, thereby controlling the loss rate of relativistic electrons to the atmosphere. Our observations in and near the loss cone support theoretical work suggesting that EMIC waves can cause scattering loss to the atmosphere of relativistic electrons over the course of a geomagnetic storm.

Proton isotropy boundaries as measured on mid- and low-altitude satellites

Abstract. Polar CAMMICE MICS proton pitch angle distributions with energies of 31-80 keV were analyzed to determine the locations where anisotropic pitch angle distributions (perpendicular flux dominating) change to isotropic distributions. We compared the positions of these mid-altitude isotropic distribution boundaries (IDB) for different activity conditions with low-altitude isotropic boundaries (IB) observed by NOAA 12. Although the obtained statistical properties of IDBs were quite similar to those of IBs, a small difference in latitudes, most pronounced on the nightside and dayside, was found. We selected several events during which simultaneous observations in the same local time sector were available from Polar at mid-altitudes, and NOAA or DMSP at low-altitudes. Magnetic field mapping using the Tsyganenko T01 model with the observed solar wind input parameters showed that the low- and mid-altitude isotropization boundaries were closely located, which leads us to suggest that the Polar IDB and low-altitude IBs are related. Furthermore, we introduced a procedure to control the difference between the observed and model magnetic field to reduce the large scatter in the mapping. We showed that the isotropic distribution boundary (IDB) lies in the region where Rc/ρ~6, that is at the boundary of the region where the non-adiabatic pitch angle scattering is strong enough. We therefore conclude that the scattering in the large field line curvature regions in the nightside current sheet is the main mechanism producing isotropization for the main portion of proton population in the tail current sheet. This mechanism controls the observed positions of both IB and IDB boundaries. Thus, this tail region can be probed, in its turn, with observations of these isotropy boundaries. Keywords. Magnetospheric physics (Energetic particles, Precipitating; Magnetospheric configuration and dynamics; Magnetotail)