Keywords

Inner source pickup ions (PUIs) are thought to be produced by the interaction between solar wind ions and interplanetary dust particles (IDPs). The amount of PUIs produced and their velocity distribution depend on the composition, density, porosity, and size of the IDPs. Quinn et al. simulated the production of PUIs from chondritic porous (CP) IDPs. The study showed that the composition of PUIs produced from CP IDPs nearly resembled the solar wind. The purpose of this study is to expand upon Quinn et al. to chondritic smooth (CS) IDPs to get a more complete description of inner source PUI production. We simulate the production and transport of C⁺ and O⁺ PUIs using the Stopping and Range of Ions in Matter and the Energetic Particle Radiation Environment Module. We consider five production mechanisms: solar wind recycling, neutralization, backscattering, sputtering, and sputtering-induced recycling. Comparisons are made to observational studies that used the Charge-Time-Of-Flight instrument on board the SOlar and Heliospheric Observatory. Results indicate that sputtering is the dominant mechanism. This results in an inner source PUI composition that resembles the dust grains, which are rich in species such as C and O and poor in species such as Ne. However, studies by Ulysses show that inner source PUIs produced inside of ∼0.5 au have a composition similar to that of the solar wind. Thus, we conclude that the IDP population close to the Sun is dominated by CP IDPs rather than CS IDPs.

We present determinations of the meteoroid differential mass index, s, using over a decade of meteor observations from the Southern Argentina Agile MEteor Radar (SAAMER). For this, we employ an autonomous statistical technique to determine this parameter from the measured radar echo amplitudes. Unlike previous studies, we examine the role of the system noise in the determination of this parameter and found that if not taken into account appropriately, the results can yield significant over estimations of the mass index. In general we found that a value of s = 2.0 represents SAAMER's results in general agreement with recent studies performed in the northern hemisphere. We explore both the index interannual and seasonal variability and, unlike previous studies, we found them to be constant, except during the presence of the Southern δ Aquariids meteor shower which is so strong that it dominates the meteor counts when present. Our study suggests that using the maximum echo amplitude for these studies is not ideal as it can be biased by many factors which make the inaccuracies larger than the precision estimated by the fitting routine. A method that results in a more direct estimate of the electron line density would be required which takes into account range, gain pattern, system noise, etc.

Photometry from the Helios and STEREO spacecraft revealed regions of enhanced sky surface-brightness suggesting a narrow circumsolar ring of dust associated with Venus's orbit. We model this phenomenon by integrating the orbits of 10,000,000+ dust particles subject to gravitational and non-gravitational forces, considering several different kinds of plausible dust sources. We find that only particles from a hypothetical population of Venus co-orbital asteroids can produce enough signal in a narrow ring to match the observations. Previous works had suggested such objects would be dynamically unstable. However, we re-examined the stability of asteroids in 1:1 resonance with Venus and found that ∼8% should survive for the age of the solar system, enough to supply the observed ring.

To test a technique to be used on the white-light imager onboard the recently launched Parker Solar Probe mission, we performed a numerical differentiation of the brightness profiles along the photometric axis of the F-corona models that are derived from STEREO Ahead Sun Earth Connection Heliospheric Investigation observations recorded with the HI-1 instrument between 2007 December and 2014 March. We found a consistent pattern in the derivatives that can be observed from any S/C longitude between about 18° and 23° elongation with a maximum at about 21°. These findings indicate the presence of a circumsolar dust density enhancement that peaks at about 23° elongation. A straightforward integration of the excess signal in the derivative space indicates that the brightness increase over the background F-corona is on the order of 1.5%–2.5%, which implies an excess dust density of about 3%–5% at the center of the ring. This study has also revealed (1) a large-scale azimuthal modulation of the inner boundary of the pattern, which is in clear association with Mercury's orbit; and (2) a localized modulation of the inner boundary that is attributable to the dust trail of Comet 2P/Encke, which occurs near ecliptic longitudes corresponding to the crossing of Encke's and Mercury's orbital paths. Moreover, evidence of dust near the S/C in two restricted ranges of ecliptic longitudes has also been revealed by this technique, which is attributable to the dust trails of (1) comet 73P/Schwassmann–Wachmann 3, and (2) 169P/NEAT.

The white-light Fraunhofer-corona (F-corona) arises from light scattered by the circumsolar dust. Using weekly minimum background models of ST-A/HI-1 observations, we characterized the flattening of the F-corona between 5° and 24° elongation by measuring the radii of constant-intensity contours along, and at a 25° angle to, the photometric axis. The ratio of these quantities (the pseudo-flattening index $\tilde{f}={R}_{\mathrm{eq}}/R(25^\circ )-1$) is analogous to the definition of the flattening index ($f={R}_{\mathrm{eq}}/{R}_{\mathrm{pol}}-1$). Measurements of the pseudo-flattening in the north and south hemispheres reveal a periodic asymmetry in the appearance of the F-corona (attributable to changes in polar brightness due to the elevation of the spacecraft from the dust symmetry surface), as well as a north/south asymmetry possibly introduced by the warped dust symmetry surface. The north/south averaged pseudo-flattening was used to infer the flattening for each weekly model. We found that the inferred flattening index (1) varies periodically with spacecraft position, reaching a maximum in the range 262° ≲ λ ≲ 290° due to a brightening along the photometric axis when the spacecraft passes through the surface of maximum dust density, and a minimum at λ ≈ 200°, and (2) slightly varies as a function of time (at least partially due to the displacement of the circumsolar dust with respect to the Sun). Comparison of the flattening index with previous works suggests a cubic dependence of the flattening index with log elongation for $| \epsilon | \lesssim 24^\circ$.

We reanalyze the Imaging Photopolarimeter data from Pioneer 10 to study the zodiacal light in the B and R bands beyond Earth orbit, applying an improved method to subtract integrated star light (ISL) and diffuse Galactic light (DGL). We found that there exists a significant instrumental offset, making it difficult to examine the absolute sky brightness. Instead, we analyzed the differential brightness, i.e., the difference in sky brightness from the average at high ecliptic latitude, and compared with that expected from the model zodiacal light. At a heliocentric distance of r < 2 au, we found a fairly good correlation between the J-band model zodiacal light and the residual sky brightness after subtracting the ISL and DGL. The reflectances of the interplanetary dust derived from the correlation study are marginally consistent with previous works. The zodiacal light is not significantly detectable at r > 3 au, as previously reported. However, a clear discrepancy from the model is found at r = 2.94 au which indicates the existence of a local dust cloud produced by the collision of asteroids or dust trail from active asteroids (or main-belt comets). Our result confirms that the main component of the zodiacal light (smooth cloud) is consistent with the model even beyond the earth orbit, which justifies the detection of the extragalactic background light after subtracting the zodiacal light based on the model.

To characterize the meteoroid environment around Mercury and its contribution to the planet's exosphere, we combined four distinctive sources of meteoroids in the solar system: main-belt asteroids, Jupiter-family comets, Halley-type comets, and Oort Cloud comets. All meteoroid populations are described by currently available dynamical models. We used a recent calibration of the meteoroid influx onto Earth as a constraint for the combined population model on Mercury. We predict vastly different distributions of orbital elements, impact velocities, and directions of arrival for all four meteoroid populations at Mercury. We demonstrate that the most likely model of Mercury's meteoroid environment—in the sense of agreement with Earth—provides good agreement with previously reported observations of Mercury's exosphere by the MESSENGER spacecraft and is not highly sensitive to variations of uncertain parameters such as the ratio of these populations at Earth, the size–frequency distribution, and the collisional lifetime of meteoroids. Finally, we provide a fully calibrated model consisting of high-resolution maps of mass influx and surface vaporization rates for different values of Mercury's true anomaly angle.

The white-light F-corona arises from light scattered by circumsolar dust. Using weekly models of the eastern side of the F-corona between 5° and 24° elongation, we analyzed the elongation and time dependence of the brightness of its photometric axis. The models were constructed from STEREO-A SECCHI/HI-1 images taken between 2007 December and 2014 March. We found that the brightness profiles can be approximated by power laws, with the coefficients of the models depending upon the observer's ecliptic longitude. Their variation is not symmetric with respect to the orbital nodes of the dust plane, nor is the behavior similar in the two halves of the spacecraft orbit delimited by the line of nodes. The exponents range between −2.31 and −2.35, the former occurring when the observer is at the nodes. The asymmetry observed in the behavior of the proportionality constant is indicative of the projected center of the dust cloud being offset from the Sun's center by ∼0.4 R_⊙. The coefficients exhibit a secular variation correlated with the location of the barycenter of the solar system. We also used the HI-1 frames obtained during STEREO-A calibration rolls to model the 360° F-corona. We found that (1) its flattening index ($f={R}_{\mathrm{eq}}/{R}_{\mathrm{pol}}-1$) decreases from ∼0.66 to ∼0.46 with decreasing elongation and (2) the isophotes' shape can be approximated by a series of superellipses, with the superellipse index n increasing (nonlinearly) with brightness ($\sim 1.54\lt n\lt \sim 1.65$). Cubic extrapolation of the results below 5° elongation points to a circular F-corona below 1° elongation.

Inner source pickup ions (PUIs) are believed to be created by the interaction between the solar wind and interplanetary dust grains. The production mechanism, however, is not well understood. We use the Stopping Range of Ions in Matter and Energetic Particle Radiation Environment Module to simulate the production and transport of inner source C⁺ and O⁺ produced by five mechanisms: solar wind recycling, neutralization, backscattering, sputtering, and sputtering-induced recycling. This is the first study to consider backscattering and sputtering-induced recycling. We compare the velocity distribution function (VDF) and C⁺/O⁺ abundance ratio to observations from the charge-time-of-flight instrument on board the SOlar and Heliospheric Observatory. Observations reveal a new constraint: a broad VDF at 1 au with a possible cutoff near twice the solar wind speed—suggesting that inner source PUIs are injected into the solar wind at near-zero speeds. In light of this constraint and our model-data comparison, backscattering and sputtering-induced recycling satisfy the most production constraints. However, based on intensity, sputtering and sputtering-induced recycling are the dominant mechanisms.

The Hunt for Observable Signatures of Terrestrial Systems survey searches for dust near the habitable zones (HZs) around nearby, bright main-sequence stars. We use nulling interferometry in the N band to suppress the bright stellar light and to probe for low levels of HZ dust around the 30 stars observed so far. Our overall detection rate is 18%, including four new detections, among which are the first three around Sun-like stars and the first two around stars without any previously known circumstellar dust. The inferred occurrence rates are comparable for early-type and Sun-like stars, but decrease from ${60}_{-21}^{+16}$% for stars with previously detected cold dust to ${8}_{-3}^{+10}$% for stars without such excess, confirming earlier results at higher sensitivity. For completed observations on individual stars, our sensitivity is five to ten times better than previous results. Assuming a lognormal excess luminosity function, we put upper limits on the median HZ dust level of 13 zodis (95% confidence) for a sample of stars without cold dust and of 26 zodis when focusing on Sun-like stars without cold dust. However, our data suggest that a more complex luminosity function may be more appropriate. For stars without detectable Large Binocular Telescope Interferometer (LBTI) excess, our upper limits are almost reduced by a factor of two, demonstrating the strength of LBTI target vetting for future exo-Earth imaging missions. Our statistics are limited so far, and extending the survey is critical to informing the design of future exo-Earth imaging surveys.