Inbound Leg Research Articles

Serendipitous Archival Observations of a New Ultradistant Comet C/2019 E3 (ATLAS)

We identified a new ultradistant comet C/2019 E3 (ATLAS) exhibiting preperihelion cometary activity at heliocentric distances ≳20 au, making it the fourth member of this population after C/2010 U3 (Boattini), C/2014 UN271 (Bernardinelli–Bernstein), and C/2017 K2 (PANSTARRS). From serendipitous archival data, we conducted analyses of the comet, finding that the activity was consistent with steady-state behavior, suggestive of sublimation of supervolatiles; that the cross section of dust increased gradually on the inbound leg of the orbit, varying with heliocentric distances as rH−1.5±0.4 ; and that the dust was produced at a rate of ≳102 kg s−1 within the observed timespan. Our modeling of the largely symmetric morphology of the comet suggests that the dust environment was likely dominated by mm-scale dust grains ejected at speeds ≲0.4 m s−1 from the sunlit hemisphere of the nucleus. Assuming a typical geometric albedo of 0.05 and adopting several simplistic thermophysical models, we estimated the nucleus to be at least ∼3 km across. We also measured the color of the comet to be consistent with other long-period comets, except being slightly bluer in g − r. With our astrometric measurements, we determined an improved orbit of the comet, based upon which we derived that the comet is dynamically new and that its perihelion distance will further shrink due to the Galactic tide. We conclude the paper by comparing the known characteristics of the known ultradistant comets.

NEOWISE Observations of Distant Active Long-period Comets C/2014 B1 (Schwartz), C/2017 K2 (Pan-STARRS), and C/2010 U3 (Boattini)

Hyperactive comet activity typically becomes evident beyond the frost line (∼3–4 au) where it becomes too cold for water-ice to sublimate. If carbon monoxide (CO) and carbon dioxide (CO2) are the species that drive activity at sufficiently large distances, then detailed studies on the production rates of these species are extremely valuable to examine the formation of the solar system because these two species (beyond water) are next culpable for driving cometary activity. The NEOWISE reactivated mission operates at two imaging bandpasses, W1 and W2 at 3.4 μm and 4.6 μm, respectively, with the W2 channel being fully capable of detecting CO and CO2 at 4.67 μm and 4.23 μm in the same bandpass. It is extremely difficult to study CO2 from the ground due to contamination in Earth’s atmosphere. We present our W1 and W2 photometry, dust measurements, and findings for comets C/2014 B1 (Schwartz), C/2017 K2 (Pan-STARRS), and C/2010 U3 (Boattini), hereafter, B1, K2, and U3, respectively. Our results assess CO and CO2 gas production rates observed by NEOWISE. We have determined: (1) comets B1 and K2 have CO2 and CO gas production rates of ∼1027 and ∼1029 molecules s−1, respectively, if one assumes the excess emission is attributed to either all CO or all CO2; (2) B1 and K2 are considered hyperactive in that their measured Af ρ dust production values are on the order of ≳103 cm; and (3) the CO and CO2 production rates do not always follow the expected convention of increasing with decreased heliocentric distance, while B1 and K2 exhibit noticeable dust activity on their inbound leg orbits.

Multispacecraft Energetic Particle Enhancements Associated with a Single Corotating Interaction Region

The radial evolution of particles accelerated at corotating interaction regions (CIRs) is not fully understood, particularly the distance range over which this particle acceleration occurs and how the energy spectra are modulated by transport through the inner heliosphere. Here, we present observations of energetic proton enhancements associated with a CIR observed by Parker Solar Probe on 2021 April 25 during the inbound leg of its orbit near ∼46 R s (∼0.21 au). The CIR is identified at additional spacecraft (Solar Terrestrial Relations Observatory, STEREO-A; Solar Orbiter, SolO; and Advanced Composition Explorer, ACE) using a corotation time delay estimation, and energetic proton spectra from each spacecraft are compared. We find that (1) energetic protons are observed near 46 R s streaming sunward ahead of the CIR; (2) the CIR persists for at least one solar rotation and the corresponding energetic proton enhancements are observed at STEREO-A, SolO, and ACE; and (3) the proton energy spectrum is steeper near the Sun and hardens near 1 au. This observation presents the closest in situ energetic particle observation of a CIR to the Sun ever recorded. Results presented here suggest that particles can be accelerated by CIR structures within 1 au and these particles can penetrate very deep into the inner heliosphere.

Economic assessment of transporting refrigerated cargo between West-Africa and Europe: a chain cost analysis approach

Determining which ports to call at in a maritime loop is considered as an important determinant factor for shipping companies which impacts not only on efficiency, and productivity but also on transportation costs. The port selection process becomes more challenging and sensitive if the market is for perishable goods such as fruits and vegetables. Specifically, this paper aims to develop an efficient maritime supply chain for reefer cargo. A case study of refrigerated cargo trade between West-Africa and West-Europe is investigated for which the main objective is to minimize the trade route's supply chain costs. To do so, the paper analyzes various scenarios based upon the different types of cargo (transshipment and non-transshipment), commodity types (dry and reefer), and a combination of different (un)loading ports in and the Rotterdam–Gibraltar range. Four African ports, namely Tema (Ghana), Douala (Cameroon), Abidjan (Ivory Coast), and Dakar (Senegal), are considered for the outbound leg (Africa–Europe), while several ports in the E.U. and the U.K. are selected as inbound leg. The analysis is initiated by applying a data-gathering strategy to get the relevant container flow from European ports to West-African ports. Then, a chain cost modelling approach is used to determine what the sailing schedule of the vessel should be. By performing the import/export cargo volume and maritime and supply chain costs are calculated, which can be used by the involved stakeholders to decide which ports are the best to call on a specific route.

The yearly variability of the sodium exosphere of Mercury: A toy model

Observations of the sodium exosphere of Mercury show a peculiar yearly variability, with two intensity maxima at aphelion and perihelion. Here we present an analytical model for the total Na exosphere content, and we compare our results with ground-based observations. The model is able to reproduce the observed data, both in magnitude and in the seasonal variability. The combined effect of the planetary rotation with the modulation of sources and losses magnitude along the orbit, is able to produce a source of Na at dawn, which is needed to explain the observed maximum at aphelion. Also, we demonstrate that a process producing a consistent Na supply rate at the nightside, which can either be plasma or micrometeoroid precipitation, is needed as well. With the help of the model, we also propose a possible explanation for the dusk enhancement of Na that was seen in the MESSENGER data during the inbound leg of Mercury's orbit.

Accounting for Heterogeneity in Stop Frequency Models of Work Tours Using Latent Class Poisson Models

Stop frequency models, as one of the elements of activity based models, represent an important part of travel behavior. Unobserved heterogeneity across the travelers should be taken into consideration to prevent biasedness and inconsistency in the estimated parameters in the stop frequency models. Additionally, previous studies on the stop frequency have mostly been done in larger metropolitan areas and less attention has been paid to the areas with less population. This study addresses these gaps by using 2012 travel data from a medium sized U.S. urban area using the work tour for the case study. Stop in the work tour were classified into three groups of outbound leg, work based subtour, and inbound leg of the commutes. Latent Class Poisson Regression Models were used to analyze the data. The results indicate the presence of heterogeneity across the commuters. Using latent class models significantly improves the predictive power of the models compared to regular one class Poisson regression models. In contrast to one class Poisson models, gender becomes insignificant in predicting the number of tours when unobserved heterogeneity is accounted for. The commuters are associated with increased stops on their work based subtour when the employment density of service-related occupations increases in their work zone, but employment density of retail employment does not significantly contribute to the stop making likelihood of the commuters. Additionally, an increase in the number of work tours was associated with fewer stops on the inbound leg of the commute. The results of this study suggest the consideration of unobserved heterogeneity in the stop frequency models and help transportation agencies and policy makers make better inferences from such models.

In Situ Ion Composition Observations of Ganymede's Outflowing Ionosphere

AbstractOn 7 June 2021 the Juno spacecraft passed through the Ganymede magnetosphere, with a closest approach altitude of 1,046 km. The Jovian Auroral Distributions Experiment‐Ion (JADE‐I) sensor observed ionospheric ions, consisting of O2+, O+, H2+, H+, and H3+. These ions were outflowing, with no bi‐directional flow except possibly near the magnetopause. Relative ion densities with respect to time agree the electron density determined by the Waves instrument, but are ∼2.5 times larger. The light ions appear to be in hydrostatic equilibrium because the altitude profile is generally symmetric between inbound and outbound legs of the flyby. H3+ ions are an exception to this, with the ratio of H3+/H2+ being ∼a factor 4 lower on the outbound than the inbound leg. The heavy ions have higher densities outbound than inbound. The outflowing flux of light ions peak near closest approach, but the heavy ions peak outbound of the flyby.

Photoionization Loss of Mercury's Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope.

We present the first investigation and quantification of the photoionization loss process to Mercury's sodium exosphere from spacecraft and ground‐based observations. We analyze plasma and neutral sodium measurements from NASA's MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na+) content and therefore the significance of photoionization varies with Mercury's orbit around the Sun (i.e., true anomaly angle: TAA). Na+ production is affected by the neutral sodium solar‐radiation acceleration loss process. More Na+ was measured on the inbound leg of Mercury's orbit at 180°–360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼1024 atoms/s from the sodium exosphere (maxima of 4 × 1024 atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration.

Short timeframe changes of environmental impacts on summer phytoplankton in the Chukchi Sea and surrounding areas in a regional scaling

Recent changes observed in the Arctic associated with climate change call for further studies of its habitats and ecosystems, including those of the microbial communities that are the base of the food web. Limited information is available on short timeframe changes of summer phytoplankton and its driving factors. In the present study, space and time resolution in the phytoplankton community (>10 μm) and chlorophyll a (Chl a) were analyzed in Chukchi shelf and northern deep-sea areas (66°N ~ 80°N) during the Fourth Chinese National Arctic Research Expedition in the summer of 2010. Potential environmental drivers were linked to assess effects of Pacific inflow, light intensity and seasonal ice retreat on summer phytoplankton community. Results indicate a strong temporal variability in species composition, abundance and Chl a within a short interval (July 20 to August 30), as well as significant difference in regional distribution (Chukchi shelf, coast Alaska and deep-sea). A total of 159 taxa belonging to 69 genera from 9 phyla were identified by Utermöhl method. The Chukchi shelf in early summer (July 20 to 25) had a depth-integrated Chl a of 97.24 mg/m2 with a depth-averaged abundance of 101.37 × 103 cells/L. The community was dominated by Navicula pelagica, Thalassiosira nordenskiöeldii, Pseudonitzschia seriata, Leptocylindrus danicus, Nitzschia longissimi and Navicula vanhoeffenii. An ongoing ice-edge diatom bloom was observed associated to Anadyr Water. In contrast, from the deep-sea to Chukchi shelf in late summer (August 23 to 30), depth-integrated Chl a and depth-averaged abundance were lower than the previous weeks with only 25.31 mg/m2 plus 2.10 × 103 cells/L in deep-sea and 71.37 mg/m2 plus 27.00 × 103 cells/L in Chukchi shelf, respectively. At this stage, dominant species now include Thalassionema nitzschioides, Chaetoceros spp., Cylindrotheca closterium, Nitzchia longissima in deep-sea and Chaetoceros diadema, Letocylindrus danicus, N. longissima, Navicula pelagica, C. closterium, T. nitzschioides in Chukchi shelf, respectively. Generally, both Chl a and abundance were higher in the Chukchi shelf than those in the deep-sea, but was also higher in July than in August. Deep chlorophyll maxima was exclusively fixed at around 50 m in deep-sea, while a peak in Chl a appeared randomly at each layer in Chukchi shelf. A succession from pennate to centric diatoms was observed in northern Chukchi shelf closely related to ice retreat, while the opposite trend was common in the ice-free southern Chukchi shelf. Revisited stations in Chukchi shelf demonstrated the decrease in light intensity from July to August chiefly resulted in the reduction in Chl a and abundance during the return leg, while nutrients from Pacific inflows played a main role during the inbound leg.

The Heliospheric Current Sheet in the Inner Heliosphere Observed by the Parker Solar Probe

Abstract The Parker Solar Probe (PSP) completed its first solar encounter in 2018 November, bringing it closer to the Sun than any previous mission. This allowed in situ investigation of the heliospheric current sheet (HCS) inside the orbit of Venus. The Parker observations reveal a well defined magnetic sector structure placing the spacecraft in a negative polarity region for most of the encounter. The observed current sheet crossings are compared to the predictions of both potential field source surface and magnetohydrodynamic models. All the model predictions are in good qualitative agreement with the observed crossings of the HCS. The models also generally agree that the HCS was nearly parallel with the solar equator during the inbound leg of the encounter and more significantly inclined during the outbound portion. The current sheet crossings at PSP are also compared to similar measurements made by the Wind spacecraft near Earth at 1 au. After allowing for orbital geometry and propagation effects, a remarkable agreement has been found between the observations of these two spacecraft underlying the large-scale stability of the HCS. Finally, the detailed magnetic field and plasma structure of each crossing is analyzed. Marked differences were observed between PSP and Wind measurements in the type of structures found near the HCS. This suggests that significant evolution of these small solar wind structures takes place before they reach 1 au.

Pre-discovery Activity of New Interstellar Comet 2I/Borisov beyond 5 au

Abstract Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in 2019 August at ∼3 au from the Sun on its inbound leg. No pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet’s proximity to the Sun as seen from the Earth. Here we present a search for pre-discovery detections of comet Borisov using images taken by the Catalina Sky Survey, Pan-STARRS, and the Zwicky Transient Facility (ZTF), with a further comprehensive follow-up campaign being presented in Bolin et al. We identified comet Borisov in ZTF images taken in 2019 May and use these data to update its orbit. This allowed us to identify the comet in images acquired as far back as 2018 December, when it was 7.8 au from the Sun. The comet was not detected in 2018 November when it was 8.6 au from the Sun, possibly implying an onset of activity around this time. This suggests that the activity of the comet is either driven by a more volatile species other than H2O, such as CO or CO2, or by exothermic crystallization of amorphous ice. We derive the radius of the nucleus to be <7 km using the non-detection in 2018 November, and estimate an area of ∼0.5– has been active between 2018 December and 2019 September, though this number is model-dependent and is highly uncertain. The behavior of comet Borisov during its inbound leg is observationally consistent with dynamically new comets observed in our solar system, suggesting some similarities between the two.

First observations of magnetic holes deep within the coma of a comet

The Rosetta spacecraft of the European Space Agency made ground-breaking observations of comet 67P/Churyumov-Gerasimenko and of its cometary environment. We search for magnetic holes in that environment, i.e., significant depressions in the magnetic field strength, measured by the Rosetta fluxgate Magnetometer (MAG) in April and May 2015. In that time frame of two months, we identified 23 magnetic holes. The cometary activity was intermediate and increasing because Rosetta was on the inbound leg toward the Sun. While in April solar wind protons were still observed by Rosetta near the comet, in May these protons were already mostly replaced by heavy cometary ions. Magnetic holes have frequently been observed in the solar wind. We find, for the first time, that magnetic holes exist in the cometary environment even when solar wind protons are almost absent. Some of the properties of the magnetic holes are comparable to those of solar wind holes; they are associated with density enhancements, sometimes associated with co-located current sheets and fast solar wind streams, and are of similar scales. However, particularly in May, the magnetic holes near the comet appear to be more processed, featuring shifted density enhancements and, sometimes, bipolar signatures in magnetic field strength rather than simple depressions. The magnetic holes are of global size with respect to the coma. However, at the comet, they are compressed owing to magnetic field pile-up and draping so that they change in shape. There, the magnetic holes become of comparable size to heavy cometary ion gyroradii, potentially enabling kinetic interactions.

Physical activity of the selected nearly isotropic comets with perihelia at large heliocentric distance

Context.The systematic investigation of comets in a wide range of heliocentric distances can contribute to a better understanding of the physical mechanisms that trigger activity at large distances from the Sun and reveals possible differences in the composition of outer solar system bodies belonging to various dynamical groups.Aims.We seek to analyze the dust environment of the selected nearly isotropic comets with a perihelion distance between 4.5 and 9.1 au, where sublimation of water ice is considered to be negligible.Methods.We present results of multicolor broadband photometric observations for 14 distant active objects conducted between 2008 and 2015 with various telescopes. Images obtained with broadband filters were used to investigate optical colors of the cometary comae and to quantify physical activity of the comet nuclei.Results.The activity level was estimated withAfρparameters ranging between 95 ± 10 cm and 9600 ± 300 cm. Three returning comets were less active than the dynamically new comets. Dust production rates of the comet nuclei were estimated between 1 and 100 kg s−1based on some assumptions about the physical properties of dust particles populating comae. The measured colors point out reddening of the continuum for all the comets. The mean values of a normalized reflectivity gradient within the group of the comets amount to 14 ± 2% per 1000 Å and 3 ± 2% per 1000 Å in the BV and VR spectral domains, respectively. The comae of the dynamically new comets, which were observed on their inbound legs, may be slightly redder in the blue spectral interval than comae of the comets observed after the perihelion passages. The dynamically new comets observed both pre- and post-perihelion, seem to have higher production rates post-perihelion than pre-perihelion for similar heliocentric distances.

Evolution of H2O production in comet C/2012 S1 (ISON) as inferred from forbidden oxygen and OH emission

We present H2O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H2O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case < 1000 km). We find that ISON had an active area of around 10 km2 for heliocentric distances Rh > 1.2 AU, which then decreased to about half this value from Rh = 1.2–0.9 AU. This was followed by a rapid increase in active area at about Rh = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 km2 active area observed outside of Rh = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H2O production rates are consistent with values derived using other methods, we conclude that the contribution of O2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H2O for ISON. This is consistent with the expected contribution of O2 photodissociation if O2/H2O ∼ 4%, meaning [O I] emission can still be utilized as a reliable proxy for H2O production in comets as long as O2/H2O ≲ 4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.

The global surface composition of 67P/Churyumov-Gerasimenko nucleus by Rosetta/VIRTIS. II) Diurnal and seasonal variability

VIRTIS-M observations of the nucleus of comet 67P/Churyumov–Gerasimenko acquired from 2014 August to 2015 May have been analysed to investigate surface temporal variability at both seasonal and diurnal scales. The measured reflectance spectra are studied by means of comet spectral indicators (CSI) such as slopes in the visible and infrared ranges, and 3.2 μm band area and band centre. CSI maps derived from data acquired at different heliocentric distances (from 3.62 to 1.72 au) along the inbound leg of the comet's orbit are used to infer surface water ice abundance. We measure a global scale enrichment of water ice from 2014 August to 2015 May across the body of the comet, along with variability at small spatial scale, possibly related with the local insolation conditions. Analysis of water ice diurnal variability is performed on 2014 August observations. Water ice appears at the border of receding shadows in the neck of the comet (Hapi), sublimating in less than 1 h, after exposure to sunlight. As similar variability is not observed in other regions of the comet, we interpreted this as the expression of a diurnal cycle of sublimation and re-condensation of water ice, triggered by sudden shadowing produced on the neck by the body and the head of the nucleus.

Seasonal variations of Mercury's magnesium dayside exosphere from MESSENGER observations

The Ultraviolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer instrument aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft made near-daily observations of solar-scattered resonant emission from magnesium in Mercury's exosphere during the mission's orbital phase (March 2011–April 2015, ∼17 Mercury years). In this paper, a subset of these data (March 2013–April 2015) is described and analyzed to illustrate Mg's spatial and temporal variations. Dayside altitude profiles of emission are used to make estimates of the Mg density and temperature. The main characteristics of the Mg exosphere are (a) a predominant enhancement of emission in the morning (6am–10am) near perihelion, (b) a bulk temperature of ∼6000K, consistent with impact vaporization as the predominant ejection process, (c) a near-surface density that varies from 5cm−3 to 50cm−3 and (d) a production rate that is strongest in the morning on the inbound leg of Mercury's orbit with rates ranging from 1 × 105cm−2s−1 to 8 × 105cm−2s−1.

Evidence of surface wave on the dusk flank of Saturn's magnetopause possibly caused by the Kelvin-Helmholtz instability

Boundary waves on Saturn's dusk side magnetopause were identified from magnetopause crossings made by the Cassini spacecraft. These crossings occurred over a 48 h period from 14 July to 15 July 2007 when Cassini was on the inbound leg of revolution (Rev.) 48, and were located at around 17:00 SLT at low latitudes above the equatorial plane. The boundary normal for each magnetopause crossing was determined from minimum variance analysis (MVA) on the magnetic field within the current sheet interval. MVA was then performed on the normals themselves to identify a maximum variance direction, in an attempt to identify a preferred plane in which the normals varied. The first few crossings of the group exhibited no wave activity. The following 17 crossings showed clear oscillation of consecutive boundary normals about a mean direction in a well defined plane, with a clear periodicity of the order 50 min, suggesting the presence of surface waves with similar characteristics to those observed by Masters et al. (2009). Subsequent normals also exhibited clear consecutive oscillation in a preferred plane, but with no apparent periodicity. The direction along which maximum variance of the normals occurred was assumed to be the direction of wave propagation, and was found to be almost perpendicular to the average magnetospheric field for the duration of the crossings. Both the wave periodicity and propagation direction support the Kelvin-Helmholtz instability as a strong candidate mechanism for generating these waves.

Encounters of the dust trails of comet 45P/Honda-Mrkos-Pajdusakova with Venus in 2006

Aims. We aim to investigate the dynamical fate of meteoroids ejected during past perihelion passages of comet 45P/Honda-Mrkos-Pajdusakova that intersect the orbit of Venus. Of particular interest is the possibility, borne of previous work, that a significant flux of these particles will reach the planet during early June and late August 2006, when the Venus Express spacecraft will be operating in orbit around Venus. Methods. We have simulated the generation of meteoroid trails ejected by the comet at some past perihelion passages, and numericaly integrated them forward in time until they approach Venus in 2006. Results. On the inbound leg of the comet's path towards perihelion, we find a trail composed of dust particles ejected between 1943 and 1980 that encounters Venus on 9 June 2006 at a distance of 0.009 AU. On the outbound leg, we observe a dense trail of particles ejected between 1985 and 2001 that measures 3.4 × 10−4 AU (5 × 104 km) in half-width, and passes under the planet at a distance of 1.72 × 10−3 AU (2.6 × 105 km) on 30 August. Based on these results, we conclude that, on both occasions, the incident flux at Venus will likely be too low to allow for the detection of a shower by optical means. We discuss the circumstances under which Venus Express may encounter a significant flux of small particles detectable through impact ionization or disturbances in the interplanetary magnetic field.

The global plasma environment of Titan as observed by Cassini Plasma Spectrometer during the first two close encounters with Titan

The Cassini spacecraft flew by Titan on October 26, 2004 and December 13, 2004. In both cases it entered the ionosphere of Titan, allowing exploration of its plasma environment. Using observations from the Cassini Plasma Spectrometer (CAPS) and the Cassini magnetometer along the inbound legs of both flybys, we examine Titan's global plasma environment. On both occasions CAPS detected plasma populations distinct from those of the Kronian magnetosphere at about 1–1.5 Saturn radii from the moon. Closer to Titan CAPS observed drifting ion ring distributions originating from Titan and, in addition, a corotating flow that was significantly decelerated around the moon due to mass loading. Near the moon, but above the ionosphere, very cold plasma was dominant. We also compare the CAPS data to those of Voyager 1.

Ion cyclotron waves in the Saturnian magnetosphere associated with Cassini's engine exhaust

Five hours after the orbit insertion maneuver that placed Cassini into orbit about Saturn, a long (90 minute) burst of ion cyclotron waves were seen, very different than any waves on the inbound leg of the orbit or on succeeding orbits. The ion cyclotron waves were left‐hand elliptically polarized and propagating at a moderately large angle to the local magnetic field. The waves had a noticeable compressional component consistent with their off angle propagation. The frequency band of the signals was moderately broad and consistent with the singly ionized components of the engine exhaust gases: CO2, N2, CO, and H2O. While H2O waves are seen elsewhere, and associated with the E ring, these waves were stronger and propagated at a much larger angle to the magnetic field. It appears that the engine exhaust products became ionized by solar EUV and electron impact and then accelerated by the magnetospheric electric field associated with co‐rotation. This acceleration produced a ring‐beam in velocity space much like that produced by Io pickup ions in the jovian magnetosphere.