[1] We analyze dust impacts recorded by the S/WAVES radio instrument onboard the two STEREO spacecraft near 1 A.U. during the period 2007-2010. The impact of a dust particle on a spacecraft produces a plasma cloud whose associated electric field can be detected by on-board electric antennas. For this study we use the electric potential time series recorded by the waveform sampler of the instrument. The high time resolution and long sampling times of this measurement enable us to deduce considerably more information than in previous studies based on the dynamic power spectra provided by the same instrument or by radio instruments onboard other spacecraft. The large detection area compared to conventional dust detectors provides flux data with a better statistics. We show that the dust-generated signals are of two kinds, corresponding to impacts of dust from distinctly different mass ranges. We propose calibration formulas for these signals and show that we are able to use S/WAVES as a dust detector with convincing results both in the nanometer and micrometer size ranges. In the latter, the orbital motion of the spacecraft enables us to distinguish between interstellar and interplanetary dust components. Our measurements cover the mass intervals $10 À22 -10 À20 kg and $10 À17 À 5 Â 10 À16 kg. The flux of the larger dust agrees with measurements of other instruments on different spacecraft.

The STEREO wave instrument (S/WAVES) has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.

We study the dynamics of nano dust grains in the region inward from 1 AU. Assuming that the grains are created with the velocities close to Keplerian, we find that, despite the strong coupling to magnetic field, there is a population of trapped nano grains within about 0.2 AU from the Sun. The nano dust grains produced outside of the trapped region are accelerated to high velocities, of the order of 300 km s −1 , provided that the charge to mass ratio is not much less than 10 −5 e/m p . These values correspond to dust sizes equal or smaller than approximately 10 nm.

Interstellar neutral gas atoms penetrate the heliopause and reach 1 au, where they are detected by IBEX. The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in creation of the secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of interstellar He + population at (8.98 ± 0.12) × 10 −3 cm −3 . With this, we obtain the absolute density of interstellar H + 5.4 × 10 −2 cm −3 and electrons 6.3 × 10 −2 cm −3 , and ionization degrees of H 0.26 and He 0.37. The results agree with estimates of the Very Local Interstellar Matter parameters obtained from fitting the observed spectra of diffuse interstellar EUV and soft X-Ray background.

The need for replenishment of the stable gas observed in the Pictoris system raises a question about the origin of the gas. Correlations between the gas and the dust distribution suggest that the source is related to the dust. Spectroscopic observations imply that the gas is rotating at Keplerian velocity: this includes also the species that, in absence of braking, would be accelerated away from the star by the radiation pressure. We examine the possibility that the gas originates from collisional vaporization of the dust in the disk and the consequences for the gas velocity distribution and the line profiles of spectral features generated by the gas. A simple model of dust distribution and a model of individual dust-dust collision are used to calculate the gas production rate in the disk. Comparing with the gas column densities derived from observations, the escape times of the atoms from the disk are estimated. For the dust distribution and collision model considered, the vaporization of dust leads to the gas production rates of the order between 0:5 ; 10 12 and 2 ; 10 13 g s À1 for the grains with the collisional properties close to those of silicate and ice, respectively. We point out the uncertainties in the collision models. We also found that, for the lines of sight bypassing the star, velocity distributions of gas particles released from orbiting bodies can show a peak at Keplerian velocity even in the absence of braking, despite large acceleration by radiation pressure.

We simulated the signal due to neutral He atoms, observed by Interstellar Boundary Explorer (IBEX), assuming that charge exchange collisions between neutral He atoms and He + ions operate everywhere between the heliopause and a distant source region in the local interstellar cloud, where the neutral and charged components are in thermal equilibrium. We simulated several test cases of the plasma flow within the outer heliosheath and investigated the signal generation for plasma flows both in the absence and in the presence of the interstellar magnetic field. We found that a signal in the portion of IBEX data identified as due to the Warm Breeze does not arise when a homogeneous plasma flow in front of the heliopause is assumed, but it appears immediately when any reasonable disturbance in its flow due to the presence of the heliosphere is assumed. We obtained a good qualitative agreement between the data selected for comparison and the simulations for a model flow with the velocity vector of the unperturbed gas and the direction and intensity of magnetic field adopted from recent determinations. We conclude that direct-sampling observations of neutral He atoms at 1 AU from the Sun are a sensitive tool for investigating the flow of interstellar matter in the outer heliosheath, that the Warm Breeze is indeed the secondary population of interstellar helium, as it was hypothesized earlier, and that the WB signal is consistent with the heliosphere distorted from axial symmetry by the interstellar magnetic field.

The brightest and most surprising feature in the first all-sky maps of Energetic Neutral Atoms (ENA) emissions (0.26 keV) produced by the Interstellar Boundary Explorer (IBEX) is an almost circular ribbon of a ~140° opening angle, centered at (l,b) = (33°, 55°), covering the part of the celestial sphere with the lowest column densities of the Local Interstellar Cloud (LIC). We propose a novel interpretation of the IBEX results based on the idea of ENA produced by charge-exchange between the neutral H atoms at the nearby edge of the LIC and the hot protons of the Local Bubble (LB). These ENAs can reach the Sun's vicinity because of very low column density of the intervening LIC material. We show that a plane-parallel or slightly curved interface layer of contact between the LIC H atoms (n H = 0.2 cm -3 , T = 60007000 K) and the LB protons (n p = 0.005 cm -3 , T ~ 10 6 K), together with indirect contribution coming from multiply-scattered ENAs from the LB, may be able to explain both the shape of the ribbon and the observed intensities provided that the edge is < (5002000) AU away, the LIC proton density is (correspondingly) < (0.040.01) cm -3 , and the LB contains ~1% of non-thermal protons over the IBEX energy range. If this model is correct, then IBEX, for the first time, has imaged in ENAs a celestial object from beyond the confines of the heliosphere and can directly diagnose the plasma conditions in the LB.

We suggest that dust-dust collisions feed ions into the interplanetary medium and produce a significant amount of the heavy inner-source pickup ions detected by the Solar Wind Ion Composition Spectrometer aboard the Ulysses spacecraft. Organic refractory material contained in the cometary dust could explain the existence of carbon among the measured pickup ions. Impact-produced ions may locally influence the plasma parameters of the solar wind. It will be possible to study in situ these dust-plasma interactions and the central region of meteoroid activity through measurements in the inner solar system proposed for the currently discussed Solar Probe and Solar Orbiter missions.