Solar wind interaction with comet 67P: impacts of corotating interaction regions

International audienceWe present observations from the Rosetta Plasma Consortium of the effects of stormy solar wind on comet 67P/Churyumov-Gerasimenko. Four corotating interaction regions (CIRs), where the first event has possibly merged with a coronal mass ejection, are traced from Earth via Mars (using Mars Express and Mars Atmosphere and Volatile EvolutioN mission) to comet 67P from October to December 2014. When the comet is 3.1–2.7 AU from the Sun and the neutral outgassing rate ∼1025–1026 s−1, the CIRs significantly influence the cometary plasma environment at altitudes down to 10–30 km. The ionospheric low-energy (∼5 eV) plasma density increases significantly in all events, by a factor of >2 in events 1 and 2 but less in events 3 and 4. The spacecraft potential drops below −20 V upon impact when the flux of electrons increases. The increased density is likely caused by compression of the plasma environment, increased particle impact ionization, and possibly charge exchange processes and acceleration of mass-loaded plasma back to the comet ionosphere. During all events, the fluxes of suprathermal (∼10–100 eV) electrons increase significantly, suggesting that the heating mechanism of these electrons is coupled to the solar wind energy input. At impact the magnetic field strength in the coma increases by a factor of 2–5 as more interplanetary magnetic field piles up around the comet. During two CIR impact events, we observe possible plasma boundaries forming, or moving past Rosetta, as the strong solar wind compresses the cometary plasma environment. We also discuss the possibility of seeing some signatures of the ionospheric response to tail disconnection events

Spatial distribution of low-energy plasma around 2 comet 67P/CG from Rosetta measurements

International audienceWe use measurements from the Rosetta plasma consortium (RPC) Langmuir probe (LAP) and mutual impedance probe (MIP) to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e. the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1-2·10 −6 , at a cometocentric distance of 10 km and at 3.1 AU from the sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisonless plasma within 260 km from the nucleus falls of with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet

Laboratory and tentative interstellar detection of trans-methyl formate using the publicly available Green Bank Telescope PRIMOS survey

The rotational spectrum of the higher-energy trans conformational isomer of methyl formate has been assigned for the first time using several pulsed-jet Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This species has also been sought toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. We detect seven absorption features in the survey that coincide with laboratory transitions of trans-methyl formate, from which we derive a column density of 3.1 (+2.6, -1.2) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 1.5 K. This excitation temperature is significantly lower than that of the more stable cis conformer in the same source but is consistent with that of other complex molecular species recently detected in Sgr B2(N). The difference in the rotational temperatures of the two conformers suggests that they have different spatial distributions in this source. As the abundance of trans-methyl formate is far higher than would be expected if the cis and trans conformers are in thermodynamic equilibrium, processes that could preferentially form trans-methyl formate in this region are discussed. We also discuss measurements that could be performed to make this detection more certain. This manuscript demonstrates how publicly available broadband radio astronomical surveys of chemically rich molecular clouds can be used in conjunction with laboratory rotational spectroscopy to search for new molecules in the interstellar medium.Comment: 40 pages, 7 figures, 4 tables; accepted for publication in Ap

Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA

Rosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses

Effective ion speeds at ~200–250 km from comet 67P/Churyumov–Gerasimenko near perihelion

In 2015 August, comet 67P/Churyumov–Gerasimenko, the target comet of the ESA Rosetta mission, reached its perihelion at ~1.24 au. Here, we estimate for a three-day period near perihelion, effective ion speeds at distances ~200–250 km from the nucleus. We utilize two different methods combining measurements from the Rosetta Plasma Consortium (RPC)/Mutual Impedance Probe with measurements either from the RPC/Langmuir Probe or from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Comet Pressure Sensor (COPS) (the latter method can only be applied to estimate the effective ion drift speed). The obtained ion speeds, typically in the range 2–8 km s⁻¹, are markedly higher than the expected neutral outflow velocity of ~1 km s⁻¹. This indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. For the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions’ kinetic energy associated with radial drift motion is decreasing

The Morphology of the Topside Martian Ionosphere: Implications on Bulk Ion Flow

Prior to the Mars Atmosphere and Volatile Evolution mission, the only information on the composition of the Martian ionosphere came from the Viking Retarding Potential Analyzer data, revealing the presence of substantial ion outflow on the dayside of Mars. Extensive measurements made by the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer allow us to examine the morphology of the Martian ionosphere not only in unprecedented detail but also on both the dayside and the nightside of the planet. Above 300km, various ionospheric species present a roughly constant density scale height around 100km on the dayside and 180km on the nightside. An evaluation of the ion force balance, appropriate for regions with near-horizontal magnetic field lines, suggests the presence of supersonic ion outflow predominantly driven by the ambient magnetic pressure, with characteristic dayside and nightside flow velocities of 4 and 20km/s, respectively, both referred to an altitude of 500km. The corresponding total ion outflow rates are estimated to be 5x10(25)s(-1) on the dayside and 1x10(25)s(-1) on the nightside. The data also indicate a prominent variation with magnetic field orientation in that the ion distribution over regions with near-vertical field lines tends to be more extended on the dayside but more concentrated on the nightside, as compared to regions with near-horizontal field lines. These observations should have important implications on the pattern of ion dynamics in the vicinity of Mars. Plain Language Summary Prior to the Mars Atmosphere and Volatile Evolution mission, the only information on the composition of the Martian ionosphere came from the Viking Retarding Potential Analyzer data acquired on the dayside of Mars. Recently, extensive measurements made by the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer allow us to examine the Martian ionosphere not only in unprecedented detail but also on both the dayside and the nightside of the planet. By analyzing these data, we find that on each side, many of the detected ion species share a common density structure at altitudes above 300km. Meanwhile, such a structure is clearly influenced by the ambient magnetic fields, which are well known to be inhomogeneous on Mars and cluster over the Southern Hemisphere. Near strong magnetic fields, the Martian ionosphere tends to be more extended on the dayside but more concentrated on the nightside. These findings reveal the presence of supersonic ion outflow on Mars. Such an ion outflow makes a significant contribution to plasma escape, which influences the long-term evolution of the planet.National Natural Science Foundation of China [41525015, 41774186, 41525016]; Science and Technology Development Fund of Macau SAR [039/2013/A2, 119/2017/A3]; National Aeronautics and Space Administration (NASA); Swedish National Space Agency [135/13, 166/14]; Swedish Research Council (VR grant) [621-2013-4191]6 month embargo; published online: 13 February 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]