Detection of the high energy component of Jovian electrons in Low Earth Orbit with the PAMELA experiment

The PAMELA experiment is devoted to the study of cosmic rays in Low Earth Orbit with an apparatus optimized to perform a precise determination of the galactic antimatter component of c.r. It is constituted by a number of detectors built around a permanent magnet spectrometer. PAMELA was launched in space on June 15th 2006 on board the Russian Resurs-DK1 satellite for a mission duration of 3 years. The characteristics of the detectors, the long lifetime and the orbit of the satellite, will allow to address several aspects of cosmic-ray physics. In this work we discuss the observational capabilities of PAMELA to detect the electron component above 50 MeV. The magnetic spectrometer allows a detailed measurement of the energy spectrum of electrons of galactic and Jovian origin. Long term measurements and correlations with Earth–Jupiter 13 months synodic period will allow to separate these two contributions and to measure the primary electron Jovian component, dominant in the 50–70 MeV energy range. With this technique it will also be possible to study the contribution to the electron spectrum of Jovian e⁻ reaccelerated up to 2 GeV at the Solar Wind Termination Shock.     

The PAMELA experiment: A space-borne observatory for heliospheric phenomena

PAMELA is a multi-purpose apparatus composed of a series of scintillator counters arranged at the extremities of a permanent magnet spectrometer to provide charge, time-of-flight and rigidity information. Lepton/hadron identification is performed by a silicon–tungsten calorimeter and a Neutron detector placed at the bottom of the device. An Anticounter system is used offline to reject false triggers coming from the satellite. The device was put into orbit on June 15th 2006 in a pressurized container on board the Russian Resurs-DK1 satellite. The satellite is flying along a high inclination (70°), low Earth orbit (350–600 km), allowing to perform measurements in different points and conditions of the geomagnetosphere. PAMELA main goal is a precise measurement of the antimatter ( 80 MeV–190 GeV, e⁺ 50 MeV–270 GeV) and matter (p 80–700 GeV, e⁻ 50 MeV–400 GeV) component of the galactic cosmic rays. In this paper we focus on the capabilites of observations of heliospheric cosmic rays: trapped and semi-trapped particles in the proton and electron belts, solar particle events, Jovian electrons will be studied in the three years of expected mission.     

Effects of solar modulation on the cosmic ray positron fraction

We implemented a 2D Monte Carlo model to simulate the solar modulation of galactic cosmic rays. The model is based on the Parker’s transport equation which contains diffusion, convection, particle drift and energy loss. Following the evolution in time of the solar activity, we are able to modulate a local interstellar spectrum (LIS), that we assumed isotropic beyond the termination shock, down to the Earth position inside the heliosphere. In this work we focused our attention to the cosmic ray positron fraction at energy below ∼10 GeV, showing how the particle drift processes could explain different results for AMS-01 and PAMELA. We compare our modulated spectra with observations at Earth, and then make a prediction of the cosmic ray positron fraction for the AMS-02 experiment.     

On the determination of energy spectra of MeV electrons by the Ulysses COSPIN/KET

The Ulysses mission has provided a wealth of data, particularly regarding the transport of low-energy cosmic ray electrons. These data have been used to derive significant constraints for the anisotropic spatial diffusion of these particles. Detailed model simulations allowed, in addition, to determine the relative contributions of galactic and Jovian electrons to the total flux at a given time and position in the heliosphere. Despite these insights, energy spectra have not been reliably determined as yet. This is a consequence of the uncertainty due to a background connected to proton interactions with the spacecraft. Recently, however, it was demonstrated that this uncertainty can, with some difficulty, be reduced, thus opening the opportunity to understand such spectra in the energy range 3–30 MeV, i.e., the part mostly dominated by Jovian electrons. We present results of a corresponding re-analysis of COSPIN/KET data.     

Modelling and observing Jovian electron propagation times in the inner heliosphere

The propagation of Jovian electrons in interplanetary space was modelled by solving the relevant transport equation numerically through the use of stochastic differential equations. This approach allows us to calculate, for the first time, the propagation time of Jovian electrons from the Jovian magnetosphere to Earth. Using observed quiet-time increases of electron intensities at Earth, we also derive values for this quantity. Comparing the modelled and observed propagation times we can gauge the magnitude of the transport parameters sufficiently to place a limit on the 6 MeV Jovian electron flux reaching Earth. We also investigate how the modelled propagation time, and corresponding Jovian electron flux, varies with the well-known ∼13 month periodicity in the magnetic connectivity of Earth and Jupiter. The results show that the Jovian electron intensity varies by a factor of ∼10 during this cycle of magnetic connectivity.     

The heliospheric modulation of 3–10 MeV electrons: Modeling of changes in the solar wind speed in relation to perpendicular polar diffusion

The discrepancy between cosmic ray model predictions representing solar minimum conditions in the heliosphere and the 3–10 MeV post-1998 electrons observations by the Kiel Electron Telescope (KET) onboard Ulysses suggests the need for consistent changes in model parameters with increasing solar activity. In order to reduce this discrepancy, an effort is made to model the KET observations realistically during periods of increased solar activity by applying an advanced three-dimensional, steady-state electron modulation model based on Parker’s transport equation including the Jovian electron source. Some elements of the diffusion tensor which were not previously emphasized are revisited. A new relation is also established between the latitudinal dependence of the solar wind speed and the perpendicular polar diffusion. Based on this relation, a transition of an average solar wind speed from solar minimum to solar maximum conditions, as observed on board the Ulysses spacecraft, is modeled on the concept of the time-evolution of large polar coronal holes. These changes are correlated to different scenarios of the enhancement of perpendicular polar diffusion. Effects of these scenarios are illustrated, as a series of steady-state solutions, on the computed 7 MeV Jovian and galactic electrons in comparison with 3–10 MeV electrons observed from the period 1998 to the end of 2003. It is shown that this approach improves compatibility with the KET observations but it also points to the need for a time-dependent electron modulation model to fully describe modulation during moderate to extreme solar maximum conditions.     

The role of drift and convection–diffusion mechanisms in small energy global cosmic ray modulation: Application to the hysteresis phenomenon of proton and alpha particle satellite data

The hysteresis effect for small energies of galactic cosmic rays is due to two effects. The first is the same as for neutron monitor energies – the delay of the interplanetary processes responsible for cosmic ray modulation with respect to the initiating solar processes, according to the effective velocity of solar wind and shock waves propagation. Then, the observed cosmic ray intensity is connected to the solar activity variations during many months before the time of cosmic ray measurement. The second is caused by the time delay of small energy cosmic ray diffusion from the boundary of modulation region to the Earth’s orbit. The model describing the connection between solar activity variation and cosmic ray convection–diffusion global modulation for neutron monitor energies is here developed by taking into account also the time-lag of the small energy particle diffusion in the Heliosphere. We use theoretical results on drifts and analytically approximate the dependences of drifts from tilt angle, and take into account the dependence from the sign of primary particles, and from the sign of polar magnetic field (A > 0 or A < 0). The obtained results are applied on proton and alpha-particle satellite data. We analyze satellite 5-min data of proton fluxes with energies >1 MeV, >2 MeV, >5 MeV, >10 MeV, >30 MeV, >50 MeV, >60 MeV, >100 MeV, and in intervals 10–30 MeV, 30–60 MeV, and 60–100 MeV during January 1986–December 1999. We exclude periods with great cosmic ray increases caused by particle acceleration in solar flare events. Then, we determine monthly averaged fluxes, as well as 5-month and 11-month smoothed data. We analyze also satellite 5-min data on alpha-particle fluxes in the energy intervals 60-160 MeV, 160–260 MeV and 330–500 MeV during January 1986–May 2000. We correct observation data for drifts and then compare with what is expected according to the convection–diffusion mechanism. We assume different dimensions of the modulation region (by the time propagation X₀ of solar wind from the Sun to the boundary of modulation region), for X₀ values from 1 to 60 average months, by one-month steps. For each value of X₀ we determine the correlation coefficient between variations of expected and observed cosmic ray intensities (the estimation of cosmic ray intensities values is given in Section 3 by Eq. (9), and the determination of correlation and regression coefficients in Section 3 by Eq. (8)). The dimension of modulation region is determined by the value of X_0 max, for which the correlation coefficient reaches the maximum value. Then the effective radial diffusion coefficient and residual modulation in small energy region can be estimated.     

Where does the heliospheric modulation of galactic cosmic rays start?

The long outstanding question of where the heliospheric (solar) modulation of galactic cosmic rays actually begins, in terms of spatial position, as well as at what high kinetic energy, can now be answered. Both answers are possible by using the results of an advanced numerical model, together with appropriate observations. Voyager 1 has been exploring the outskirts of the heliosphere and is presently entering what can be called the very local interstellar medium. It has been generally expected, and accepted, that once the heliopause is crossed, the local interstellar spectrum (LIS) should be measured in situ by the Voyager spacecraft. However, we show that this may not be the case and that modulation effects on galactic cosmic rays can persist well beyond the heliopause. For example, proton observations at 100 MeV close to the heliopause can be lower by ∼$\sim$25% to 40% than the LIS, depending on solar modulation conditions. It is also illustrated quantitatively that significant solar modulation diminishes above ∼$\sim$50 GeV at Earth. It is found that cosmic ray observations above this energy contain less that 5%$5\%$ solar modulation effects and should therefore reflect the LIS for galactic cosmic rays. Input spectra, in other words the very LIS, for solar modulation models are now constrained by in situ observations and can therefore not any longer be treated arbitrarily. It is also possible for the first time to determine the lower limit of the very LIS from a few MeV/nuc to very high energies.     

Solar system planets observed with Suzaku

Recent results of solar system planets observed with the Japanese X-ray astronomy satellite Suzaku are reviewed. Thanks to the low instrumental background and good energy resolution, X-ray CCDs onboard Suzaku are one of the best probes to study diffuse X-ray emission. An overview of the Suzaku data of Jupiter and Earth is presented, along with preliminary results of Mars. Firstly, diffuse hard X-ray emission is discovered in 1–5 keV at Jovian radiation belts. Its spectrum is represented by a power-law continuum with a photon index of ∼1.4. This emission could originate from inverse-Compton scattering of solar photons by tens MeV electrons. Secondly, variable diffuse soft X-rays are serendipitously found during observations in the directions of the north ecliptic pole and galactic ridge. Good time correlations with the solar wind and emission lines found in the X-ray spectra are firm evidences of a solar wind charge exchange emission with Earth’s exosphere. Thirdly, diffuse X-ray emission from Martian exosphere via the solar wind charge exchange is investigated for the first time at solar minimum. A stringent upper limit on the density of the Martian exosphere is placed from the Suzaku data.     

Evaluation of solar proton spectra using balloon cosmic ray observations and Monte Carlo simulation results

We have developed a method to evaluate the spectrum of solar energetic protons at the top of the Earth’s atmosphere from the measurements of our balloon cosmic ray experiment. By using the Monte Carlo PLANETOCOSMICS code based on Geant4 we compute the interaction of solar protons [10 MeV–10 GeV] with the Earth’s atmosphere. We obtain the angular and energy distributions of secondary particles (p, e⁻, e⁺, photons, muons) at different atmospheric levels as a function of primary proton spectra. By comparing the calculated depth dependence of the particle flux with the data obtained by our balloon experiment we can deduce the parameters of the solar proton spectrum that best fit the observations. In this paper we discuss our solar proton spectrum estimation method, and present results of its application to selected solar proton events from 2001 to 2005.     

Space qualification tests of the PAMELA instrument

PAMELA is a satellite-borne experiment which will measure the antiparticle component of cosmic rays over an extended energy range and with unprecedented accuracy. The apparatus consists of a permanent magnetic spectrometer equipped with a double-sided silicon microstrip tracking system and surrounded by a scintillator anticoincidence system. A silicon–tungsten imaging calorimeter, complemented by a scintillator shower tail catcher, and a transition radiation detector perform the particle identification task. Fast scintillators are used for Time-of-Flight measurements and to provide the primary trigger. A neutron detector is finally provided to extend the range of particle measurements to the TeV region.PAMELA will fly on-board of the Resurs-DK1 satellite, which will be put into a semi-polar orbit in 2005 by a Soyuz rocket. We give a brief review of the scientific issues of the mission and report about the status of the experiment few months before the launch.     

Dark matter search with the CALET detector on-board ISS

The CALorimetric Electron Telescope, CALET, mission is proposed for the observation of high-energy electrons and gamma-rays at the Exposed Facility of the Japanese Experiment Module on the International Space Station. The CALET has a capability to observe the electrons (without separation between e⁺ and e⁻) in 1 GeV–10 TeV and the gamma-rays in 20 MeV–several TeV with a high-energy resolution of 2% at 100 GeV, a good angular resolution of 0.06 degree at 100 GeV, and a high proton-rejection power of nearly 10⁶. The CALET has a geometrical factor of 1 m²sr, and the observation period is expected for more than three years. The very precise measurement of electrons enables us to detect a distinctive feature in the energy spectrum caused from WIMP dark matter in the Galactic halo. The excellent energy resolution of CALET, which is much better than GLAST or air Cherenkov telescopes over 10 GeV, enables us to detect gamma-ray lines in the sub-TeV region from WIMP dark matter annihilations. The CALET has, therefore, a unique capability to search for WIMP dark matter by the hybrid observations of electrons and gamma-rays.     

Neutron monitor asymptotic directions of viewing during the event of 13 December 2006

During the recent ground level enhancement of 13 December 2006, also known as GLE70, solar cosmic ray particles of energy bigger that ∼500 MeV/nucleon propagated inside the Earth’s magnetosphere and finally accessed low-altitude satellites and ground level neutron monitors. The magnitude and the characteristics of this event registered at different neutron monitor stations of the worldwide network can be interpreted adequately on the basis of an estimation of the solar particle trajectories in the near Earth interplanetary space. In this work, an extended representation of the Earth’s magnetic field was realized applying the Tsyganenko 1989 model. Using a numerical back-tracing technique the solar proton trajectories inside the magnetospheric field of the Earth were calculated for a variety of particles, initializing their travel at different locations, covering a wide range of energies. In this way, the asymptotic directions of viewing were calculated for a significant number of neutron monitor stations, providing crucial information on the Earth’s “magnetospheric optics” for primary solar cosmic rays, on the top of the atmosphere, during the big solar event of December 2006. The neutron monitor network has been treated, therefore, as a multidimensional tool that gives insights into the arrival directions of solar cosmic ray particles as well as their spatial and energy distributions during extreme solar events.     

Study of correlations between waves and particle fluxes measured on board the DEMETER satellite

The topic of relativistic electron dynamics in the outer radiation belt has received considerable attention for many years. Nevertheless, the problem of understanding the physical phenomenon involved is far from being resolved. In this paper, we use DEMETER observations to examine the variations of the energetic electron fluxes and ELF/VLF wave intensities in the inner magnetosphere during the intense 8 November 2004 magnetic storm. Electron flux spectra and associated wave intensity spectra are analysed throughout the magnetic storm and common characteristics or differences to other storm events are retained. The overall objective of this study is to identify and derive parameters that are relevant for particle flux modelling; the time constant characterizing the persistent decay after particle enhancement was found to be one of these important model parameters.The analysis of the 8 November 2004 event reveals that for L-shell parameter higher than 4, an electron flux dropout is observed during the storm’s main phase for electrons in the energy range 0.1–1 MeV, as has been reported from other measurements. Characteristic wave spectra accompanying this phase are analysed. They show a typical enhancement in the frequency range 0.3–10 kHz at onset for all L-shell values under consideration (2 < L < 5). During the first stage of the recovery phase, the electron fluxes are increased to a level higher than the pre-storm level, whereas the level of wave intensity in the frequency range observed below 300 Hz is at its highest. In the second stage, the particle flux decrease goes hand in hand with a global wave activity decline, the relaxation time of the latter being smaller than the former’s one. In some other cases, long-lasting electron enhancement associated with constant wave activity has been observed during this latter stage. For the above mentioned storm, while at low L values the decay time constants are higher for low energy electrons than for high energy electrons, this order is reversed at high L values. At about L = 3.6 the time constant is independent of electron energy.     

The PAMELA Storage and Control Unit

The PAMELA experiment aims to measure with great precision the antimatter present in our Galaxy in the form of high energy particles; in the same time it will measure the galactic, solar and trapped components of cosmic rays. The experiment will be housed on board a Russian Resurs-DK1 satellite and launched in the year 2005 to fly a 350–600 km orbit with an inclination of 70.4°. All operations of the instrument – including data storage – are handled by the PAMELA Storage and Control Unit (PSCU), which is divided in a Central Processing Unit (CPU) and a Mass Memory (MM). The CPU of the experiment is based on a ERC-32 architecture (a SPARC v7 implementation) running a real time operating system (RTEMS). The main purpose of the CPU is to handle slow control, acquire and store data on a 2 GB MM. Communications between PAMELA and the satellite are performed via a 1553B bus. Data acquisition from the sub-detectors (Time-of-Flight counter, Magnetic Spectrometer, Electromagnetic Calorimeter, Anticoincidence shield, Neutron Detector, and Bottom scintillator S4) is performed via a 2 MB/s interface. Download from the PAMELA MM towards the satellite main storage unit is handled by a 16 MB/s bus. The daily amount of data transmitted to ground has been evaluated in not more 20 GB. In this work, we describe the CPU of the experiment and the general software scheme.     

Electric currents from thunderstorms to the ionosphere during a solar cycle: Quasi-static modeling of the coupling mechanism

Conduction and displacement currents, and their sum the Maxwell current, generated over a thunderstorm (TS) with recurrent lightning discharges are investigated theoretically. The aim is to study better the influence of different factors on these currents, which form the link between thunderstorms and the ionosphere in the global atmospheric electrical circuit. The factors studied concern the thunderstorm characteristics (the charge separation current, and the lightning discharge parameters), as well as the atmospheric and cloud conductivity. Some of these factors may show long-term changes with the 11-year solar cycle, possibly realized through an inverse dependence of the cosmic ray flux on solar activity. Earlier investigations have suggested that the lightning-related charge redistribution and subsequent relaxation, rather than the high intensity current, is mainly the source of the energy coupled to the ionosphere. With respect to this, a quasi-electrostatic analytical model is proposed, based on Maxwell’s equations. The currents are generated by a TS modeled as a positive vertical dipole with charges which are first accumulated and then destroyed by lightning. Our computations show that the mean and peak values of the conduction and total Maxwell currents to the ionosphere depend significantly on the charge moment change. The mean currents are also sensitive to the reduction of the conductivity in thunderclouds. Small variations of the stratospheric conductivity (20% at geomagnetic latitude 40° and 40–50% at 55°) with the solar activity do not influence the currents to the ionosphere very much.     

Ionization effect of solar protons in the Earth atmosphere – Case study of the 20 January 2005 SEP event

The cosmic ray ground level enhancement on January 20, 2005 is among the largest recorded events in the history of cosmic ray measurements. The solar protons of MeV energies cause an excess of ionization in the atmosphere, specifically over polar caps following major solar disturbances. The ionization effect in the Earth atmosphere is obtained for various latitudes on the basis of solar proton energy spectra, reconstructed from GOES 11 measurements and subsequent full Monte Carlo simulation of cosmic ray induced atmospheric cascade. The estimation of ionization rates is based on a numerical model for cosmic ray induced ionization. The evolution of atmospheric cascade is performed with the CORSIKA 6.52 code using FLUKA 2006b and QGSJET II hadron interaction models. The atmospheric ion rate ionization is explicitly obtained for 40°N, 60°N and 80°N latitudes. The time evolution of obtained ion rates is presented. It is demonstrated that ionization effect is negative for 40°N and small for 60°N, because of accompanying Forbush decrease. The ionization effect is significant only in sub-polar and polar atmosphere during the major ground level enhancement of 20 January 2005.     

Launch and commissioning of the PAMELA experiment on board the Resurs-DK1 satellite

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature in a wide energy range (protons: 80 MeV–700 GeV, electrons 50 MeV–400 GeV). Main objective is the study of the antimatter component: antiprotons (80 MeV–190 GeV), positrons (50 MeV–270 GeV) and search for antimatter (with a precision of the order of 10⁻⁸). The experiment, housed on board the Russian Resurs-DK1 satellite, was launched on June, 15th 2006 in a 350 × 600 km orbit with an inclination of 70°. The detector consists of a permanent magnet spectrometer core to provide rigidity and charge sign information, a Time-of-Flight system for velocity and charge information, a silicon–tungsten calorimeter and a neutron detector for lepton/hadron identification. An anticounter system is used off-line to reject false triggers coming from the satellite. In self-trigger mode the calorimeter, the neutron detector and a shower tail catcher are capable of an independent measure of the lepton (e⁺ + e⁻) component up to 2 TeV. In this work we focus on the first months of operations of the experiment during the commissioning phase.