Measurement of the radial density gradient of cosmic ray in the heliosphere by the GRAPES-3 experiment

A radial anisotropy in the flux of cosmic rays in heliosphere was theoretically predicted by Parker and others within the framework of the diffusion–convection mechanism. The solar wind is responsible for sweeping out the galactic cosmic rays, creating a radial density gradient within the heliosphere. This gradient coupled with the interplanetary magnetic field induces a flow of charged particles perpendicular to the ecliptic plane which was measured and correctly explained by Swinson, and is hereafter referred as ‘Swinson flow’. The large area GRAPES-3 tracking muon telescope offers a powerful probe to measure the Swinson flow and the underlying radial density gradient of the galactic cosmic rays at a relatively high rigidity of ∼100 GV. The GRAPES-3 data collected over a period of six years (2000–2005) were analyzed and the amplitude of the Swinson flow was estimated to be (0.0644 ± 0.0008)% of cosmic ray flux which was an ∼80σ effect. The phase of the maximum flow was at a sidereal time of (17.70 ± 0.05) h which was 18 min earlier than the expected value of 18 h. This small 18 min phase difference had a significance of ∼6σ indicating the inherent precision of the GRAPES-3 measurement. The radial density gradient of the galactic cosmic rays at a median rigidity of 77 GV was found to be 0.65% AU⁻¹.     

Driven waves in a two-fluid plasma

We study the physics of wave propagation in a weakly ionized plasma, as it applies to the formation of multifluid, magnetohydrodynamics (MHD) shock waves. We model the plasma as separate charged and neutral fluids which are coupled by ion–neutral friction. At times much less than the ion–neutral drag time, the fluids are decoupled and so evolve independently. At later times, the evolution is determined by the large inertial mismatch between the charged and neutral particles. The neutral flow continues to evolve independently; the charged flow is driven by and slaved to the neutral flow by friction. We calculate this driven flow analytically by considering the special but realistic case where the charged fluid obeys linearized equations of motion. We carry out an extensive analysis of linear, driven, MHD waves. The physics of driven MHD waves is embodied in certain Green functions which describe wave propagation on short time-scales, ambipolar diffusion on long time-scales and transitional behaviour at intermediate times. By way of illustration, we give an approximate solution for the formation of a multifluid shock during the collision of two identical interstellar clouds. The collision produces forward and reverse J shocks in the neutral fluid and a transient in the charged fluid. The latter rapidly evolves into a pair of magnetic precursors on the J shocks, wherein the ions undergo force-free motion and the magnetic field grows monotonically with time. The flow appears to be self-similar at the time when linear analysis ceases to be valid.     

Dispersive shock waves in the solar wind

Compressional waves in the solar wind propagating over large distances are likely to steepen into shock waves where the increase in the amplitude is balanced by dissipation. Dispersive effects caused by, e.g. Hall currents perpendicular to the ambient magnetic field can influence the generation and propagation of shock waves. In the present study the dispersion is considered weak but in time its importance can grow. When the effect of dispersion is strong enough, it can balance the nonlinear steepening of waves leading to the formation of solitons. The obtained results show that the weak dispersion will alter the amplitude and propagation speed of the shock wave. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mapping of the cusp plasma precipitation on the surface of Mercury

The presence of a magnetosphere around Mercury plays a fundamental role on the way the solar wind plasma interacts with the planet. Since the observations suggest that Mercury should occupy a large fraction of its magnetosphere and because of lack of an atmosphere, significant differences in solar wind-magnetosphere coupling are expected to exist with respect to the Earth case. On the basis of a modified Tsyganenko T96 model we describe the geometry of the magnetic field that could characterize Mercury, and its response to the variations of the impinging solar wind and of the interplanetary magnetic field. The investigation is focused on the shape and dimension of the open magnetic field regions (cusps) that allow the direct penetration of magnetosheath plasma through the exosphere of Mercury, down to its surface. The precipitating particle flux and energy are evaluated as a function of the open field line position, according to different solar wind conditions. A target of this study is the evaluation of the sputtered particles from the crust of the planet, and their contribution to the exospheric neutral particle populations. Such estimates are valuable in the frame of a neutral particle analyser to be proposed on board of the ESA/BepiColombo mission.     

Peculiarities of entropy and magnetosonic waves in optically thin cosmic plasma

The stability problem for small magnetohydrodynamic (MHD) perturbations in an optically thin, perfectly conducting uniform plasma with a cosmic abundance of elements is solved in the linear approximation. The electron heat conduction along the magnetic field and the proton heat conduction across the field are taken into account. We have shown for the first time that the entropy waves can grow exponentially, while the magnetosonic waves are damped in a wide range of physical conditions closest to the conditions in stellar coronae with the proper allowance for radiative losses. Slow magnetosonic waves are damped particularly rapidly. For the solar corona, the calculated damping decrement of slow magnetosonic waves agrees well with the averaged one in 11 quasi-periodic events observed from the TRACE satellite in extreme ultraviolet radiation. Other possible astrophysical applications of the results obtained are briefly discussed.     

Preservation potential of implanted solar wind volatiles in lunar paleoregolith deposits buried by lava flows

The lunar surface is bathed in a variety of impacting particles originating from the solar wind, solar flares, and galactic cosmic rays. These particles can become embedded in the regolith and/or produce a range of other molecules as they pass through the target material. The Moon therefore contains a record of the variability of the solar and galactic particle fluxes through time. To obtain useful temporal snapshots of these processes, discrete regolith units must be shielded from continued bombardment that would rewrite the record over time. One mechanism for achieving this preservation is the burial of a regolith deposit by a later lava flow. The archival value of such deposits sandwiched between lava layers is enhanced by the fact that both the under- and over-lying lava can be dated by radiometric techniques, thereby precisely defining the age of the regolith layer and the geologic record contained therein. The implanted volatile species would be vulnerable to outgassing by the heat of the over-lying flow, at temperatures exceeding 300-700 °C. However, the insulating properties of the finely particulate regolith would restrict significant heating to shallow depths. We have therefore modeled the heat transfer between lunar mare basalt lavas and the regolith in order to establish the range of depths below which implanted volatiles would be preserved. We find that the full suite of solar wind volatiles, consisting predominantly of H and He, would survive at depths of ∼13-290 cm (for 1-10 m thick lava flows, respectively). A substantial amount of CO, CO₂, N₂ and Xe would be preserved at depths as shallow as 3.7 cm beneath meter-thick flows. Given typical regolith accumulation rates during mare volcanism, the optimal localities for collecting viable solar wind samples would involve stacks of thin mare lava flows emplaced a few tens to a few hundred Ma apart, in order for sufficient regolith to develop between burial events. Obtaining useful archives of Solar System processes would therefore require extraction of regolith deposits buried at quite shallow depths beneath radiometrically-dated mare lava flows. These results provide a basis for possible lunar exploration activities.     

Cascades in the Earth’s magnetosphere initiated by photons with the parameters of the highest energy AGASA events

We investigate the cascading effects of extremely high energy (EHE) photons in the Earth’s magnetosphere assuming that these photons arrive with the parameters of the highest energy AGASA events (energies, arrival directions). For the location of the AGASA Observatory, we determine the directions in the sky from which photons can cascade with a high (low) probability. In the case of the primary photons with the parameters of the events with the energies >10²⁰ eV, we compute the average cascade spectra of secondary photons entering the Earth’s atmosphere, and estimate their fluctuations around these average values by selecting the events with the largest and smallest number of secondary cascade photons. It is shown that most photons with the parameters of the highest energy AGASA events should initiate cascades in the Earth’s magnetosphere with a high probability even though they tend to arrive from directions in the sky for which the perpendicular component of the magnetic field is weaker. On the other hand, if these events are caused by the photons with lower energies, then the fluctuations in their shower development in the magnetosphere and the atmosphere should be higher than in the case of photons with the energies estimated by the AGASA experiment.     

On the effect of the martian crustal magnetic field on atmospheric erosion

Without the shielding of a strong intrinsic magnetic field, the martian atmosphere directly interacts with the impacting solar wind. The neutral constituents of the atmospheric corona can be ionized, and then picked up and accelerated by the magnetic field and convection electric field in the solar wind. A significant fraction of pickup ions escape Mars’ gravitational pull and are lost to space. This non-thermal escape process of heavy species is an important mechanism responsible for atmospheric erosion. While there is a perception that the martian magnetic anomalies are significant for the ionospheric density distribution and the bow shock standoff location, little is known about the quantitative influence of the martian crustal magnetic field on the global distribution of escaping pickup ions. In this paper, we apply a newly developed Monte Carlo ion transport model to resolve the crustal field effect on the pickup oxygen ion distribution around Mars. The background magnetic and electric fields, in which test particles are followed, are calculated using an independent three-dimensional multispecies MHD model. The effects of the crustal magnetic field on particle escape are quantified by varying the crustal field orientation in the model setup and comparing the corresponding test particle simulation results. The comparison is made by turning on or off the crustal field or changing the local time of the strongest field from the dayside to the dawnside. It is found that without the protection of the crustal magnetic field, the total amount of atmospheric escape through the tail region would be enhanced by more than a factor of two. It is shown that the crustal magnetic field not only regionally deflects the solar wind around the martian atmosphere, but also has an important global effect on atmospheric erosion and thus on long-term atmospheric evolution.     

From cosmic ray physics to cosmic ray astronomy: Bruno Rossi and the opening of new windows on the universe

Bruno Rossi is considered one of the fathers of modern physics, being also a pioneer in virtually every aspect of what is today called high-energy astrophysics. At the beginning of 1930s he was the pioneer of cosmic ray research in Italy, and, as one of the leading actors in the study of the nature and behavior of the cosmic radiation, he witnessed the birth of particle physics and was one of the main investigators in this fields for many years. While cosmic ray physics moved more and more towards astrophysics, Rossi continued to be one of the inspirers of this line of research. When outer space became a reality, he did not hesitate to leap into this new scientific dimension. Rossi’s intuition on the importance of exploiting new technological windows to look at the universe with new eyes, is a fundamental key to understand the profound unity which guided his scientific research path up to its culminating moments at the beginning of 1960s, when his group at MIT performed the first in situ measurements of the density, speed and direction of the solar wind at the boundary of Earth’s magnetosphere, and when he promoted the search for extra-solar sources of X rays. A visionary idea which eventually led to the breakthrough experiment which discovered Scorpius X-1 in 1962, and inaugurated X-ray astronomy.     

Effect of solar heliospheric parameters on different components of daily variation in cosmic rays

In the present work the data of three different neutron monitoring stations, Deep River, Tokyo and Inuvik located at different geomagnetic cutoff rigidities and altitudes has been harmonically analysed for the period 1980–1993, 1980–1990 and 1981–1993 respectively to investigate for a comparative study of diurnal, semi-diurnal and tri-diurnal anisotropies in cosmic ray (CR) intensity in connection with the change in IMF Bz component and solar wind velocity on 60 quietest days. It is observed that the amplitude of first harmonic is highly anti-correlated to the solar wind velocity during the period of high-speed solar wind stream (HSSWS) epoch on quiet days for three neutron monitor stations at different geomagnetic rigidity thresholds. During quiet days the amplitude of all the three harmonics significantly deviates on the onset of HSSWS epoch, whereas the direction of the anisotropy of all the three harmonics remains time invariant at three different cut off rigidity stations. The amplitude as well as the direction of anisotropy of all the three harmonics does not have time variation characteristics associated with Bz component of IMF on geo-magnetically most quiet days.     

Diffusion in momentum space as a picture of second-order Fermi acceleration

Energetic particles in a turbulent medium can be subject to second-order Fermi acceleration due to scattering on moving plasma waves. This mechanism leads to growing particle momentum dispersion and, at the same time, increases the mean particle energy. In the most frequently met situations both processes can be represented by a single momentum diffusion term in the particle kinetic equation. In the present paper we discuss the conditions allowing the additional term for regular acceleration to arise. For forward-backward asymmetric scattering centres, besides the diffusive term one should explicitly consider the regular acceleration term in momentum space, which can consist of the first-order (∝ V), as well as the second-order (∝ V²) part in the wave velocity V. We derive the condition for the scattering probability in the wave rest frame requied for vanishing the regular acceleration term and provide a simple mechanical example illustrating the theoretical concepts. Finally, we address its possible role in cosmic ray acceleration processes.     

Lunar helium-3 in marine sediments: Implications for a late Eocene asteroid shower

A late Eocene asteroid shower to the Earth-Moon system resulted in an increased flux of impact ejected ³He-rich lunar matter to Earth, which is recorded by a 2 Ma enduring ³He-anomaly in marine sediments.     

Oblique propagation and dissipation of Alfvén waves in coronal holes

We investigate the effect of viscosity and magnetic diffusivity on the oblique propagation and dissipation of Alfvén waves with respect to the normal outward direction, making use of MHD equations, density, temperature and magnetic field structure in coronal holes and underlying magnetic funnels. We find reduction in the damping length scale, group velocity and energy flux density as the propagation angle of Alfvén waves increases inside the coronal holes. For any propagation angle, the energy flux density and damping length scale also show a decrement in the source region of the solar wind (< 1.05 R_⊙) where these may be one of the primary energy sources, which can convert the inflow of the solar wind into the outflow. In the outer region (> 1.21 R_⊙), for any propagation angle, the energy flux density peaks match with the peaks of MgX 609.78 Å and 624.78 Å linewidths observed from the Coronal Diagnostic Spectrometer (CDS) on SOHO and the non-thermal velocity derived from these observations, justify the observed spectroscopic signature of the Alfvén wave dissipation.     

Nonlinear interaction between Alfvén and acoustic-gravity waves in the solar atmosphere

Based on a plane-parallel isothermal model solar atmosphere permeated by a uniform magnetic field directed against the action of gravity, we considered the nonlinear interaction between vertically propagating Alfvén and acoustic-gravity waves. We established that Alfvén waves are efficiently generated at the difference and sum frequencies. We ascertained that no acoustic-gravity waves are formed at the corresponding combination frequencies. A horizontal magnetohydrodynamic wind whose direction changes with height was found to be formed in the solar atmosphere at zero difference frequency.     

AR6659光球色球的磁场速度场的演化特征及其与大耀斑的关系

ＡＲ６６５９是２２周以来最重要的一个活动区，它爆发了２２周最强大的高能事件。本文用云南天文台的光球、色球精细结构照片和北京天文台怀柔站的磁场速度场资料，分析了该活动区磁场速度场的二维位形和大耀斑期间的演化特征。本文分析的４个大耀斑均爆发在中性线附近的Ｎ极区磁场梯度大的地方及色球速度场的红移区。偏带观测也显示耀斑物质是向红端移动的。耀斑波沿横场传播在离本黑子群几万至十几万公里的地方激起感生耀斑，在原生耀斑与感生耀斑之间往往有耀斑环相连。此外，本文还从演化特征出发分析了耀斑爆发前活动区等离子体的宏观不稳定性。     

A new numerical method for simulating the solar wind Alfvén waves: Development of the Vlasov-MHD model

A novel scheme of plasma simulation particularly suited for computing the one-dimensional nonlinear evolution of parallel propagating solar wind Alfvén waves is presented. The scheme is based on the Vlasov and the MHD models, for solving the longitudinal and the transverse components, respectively. As long as the nonlinearity is not very large (so that the longitudinal and transverse components are well separated), our Vlasov-MHD model can correctly describe evolution of finite amplitude parallel Alfvén waves, which are typical in the solar wind, both in the linear and nonlinear stages. The present model can be applied to discussions of phenomena where the parallel Alfvén waves play major roles, for example, the solar coronal heating and solar wind acceleration by the Alfvén waves propagating from the photosphere.