Cometary Activity beyond the Planets

Recent observations show activity in long-period comet C/2017 K2 at heliocentric distances beyond the orbit of Uranus. With this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. The model reveals a localized increase in carbon monoxide (CO) sublimation close to heliocentric distance r H = 150 au (local blackbody temperature ∼23 K), followed by a plateau and then a slow increase in activity toward smaller distances. This localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the CO front at larger distances and nearly isothermal CO at smaller distances. As this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system.

Gas-phase Modeling of the Cometary Coma of Interstellar Comet 2I/Borisov

Comet 2I/Borisov is the first interstellar comet observed in the solar system, providing a unique opportunity to understand the physical conditions that prevailed in a distant unknown planetary system. Observations of the comet show that the CO/H2O ratio is higher than that observed in solar system comets at a heliocentric distance r h < 2.5 au. We aim to study the gas-phase coma of comet 2I/Borisov using a multifluid chemical-hydrodynamical model. The gas-phase model includes a host of chemical reactions, with the neutrals, ions, and electrons treated as three separate fluids. Energy exchange between the three fluids due to elastic and inelastic scattering and radiative losses are also considered. Our model results show that in the region of the coma beyond ∼100 km of the nucleus, e−−CO inelastic collisions leading to vibrational excitation of CO causes a loss of energy from the electron fluid. We find a high abundance of CO+ and HCO+ ions, and we show how these two ions affect the creation/destruction rates of other ions such as H2O+, H3O+, N-bearing ions, and large organic ions. We find that the presence of CO leads to a higher abundance of large organic ions and neutrals such as CH 3 OH 2 + , CH 3 OCH 4 + , and CH3OCH3, as compared to a typical H2O-rich solar system comet. We conclude that the presence of a large amount of CO in the coma of comet 2I/Borisov, combined with a low production rate, affects the coma temperature profile and flux of major ionic species significantly.

Search for the metal-weak thick disk from the LAMOST DR5

Based on the data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey (LAMOST DR5) and the Gaia Early Data Release 3 (Gaia EDR3), we construct a sample containing 46,109 giant (log g 3.5 dex) stars with heliocentric distance d 4 kpc, and the sample is further divided into two groups of the inner (RGC < 8.34 kpc) and outer region (RGC > 8.34 kpc). The LZ distributions of our program stars in the panels with different [Fe/H] and [α/Fe] suggest that the thick-disk consists of two distinct components with different chemical compositions and kinematic properties. For the inner region, the metal-weak thick disk (MWTD) contributes significantly when [α/Fe] > +0.2 dex and [Fe/H] < −0.8 dex, while the canonical thick-disk (TD) dominates when [Fe/H]> −0.8 dex. However, MWTD clear appears only when [α/Fe] > +0.2 dex and [Fe/H] < −1.2 dex for the outer region, and its proportion is lower than that of the inner region within the same metallicity. Similar result can be obtained from the Vφ distribution.

Occurrence rate of ultra-low frequency waves in the foreshock of Mercury increases with heliocentric distance

Studies of Mercury’s foreshock have analyzed in detail the properties of ultra-low frequency waves. However, an open question remains in regards to understanding favorable conditions for these planetary foreshocks waves. Here, we report that 0.05–0.41 Hz quasi-monochromatic waves are mostly present under quasi-radial and relatively low intensity Interplanetary Magnetic Field, based on 17 Mercury years of MESSENGER Magnetometer data. These conditions are consistent with larger foreshock size and reflection of solar wind protons, their most likely source. Consequently, we find that the wave occurrence rate increases with Mercury’s heliocentric distance. Detection of these waves throughout Mercury’s highly eccentric orbit suggests the conditions for backstreaming protons are potentially present for all of Mercury’s heliocentric distances, despite the relatively low solar wind Alfvén Mach number regime. These results are relevant for planetary magnetospheres throughout the solar system, and the magnetospheres of exoplanets, and provide knowledge of particle acceleration mechanisms occurring inside foreshocks.

C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance

&lt;p&gt;We report an identification of long-period comet C/2010 U3 (Boattini) active at a new record inbound heliocentric distance of &lt;em&gt;r&lt;/em&gt;&lt;sub&gt;H&lt;/sub&gt; &amp;#8776; 26 au. Two outburst events around 2009 and 2017 were observed. The dust morphology of the coma and tail cannot be explained unless the Lorentz force, solar gravitation, and solar radiation pressure force are all taken into account. Optically dominant dust grains have radii of ~10 &amp;#956;m and are ejected protractedly at speeds &amp;#8804;50 m s&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; near the subsolar point. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO&lt;sub&gt;2&lt;/sub&gt;) is at play. Similar to other long-period comets, the colour of the cometary dust is redder than the solar colours. We also observed potential colour variations when the comet was at 10 &lt; &lt;em&gt;r&lt;/em&gt;&lt;sub&gt;H&lt;/sub&gt; &lt; 15 au, concurrent with the onset of crystallisation of amorphous water ice, if any. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet, propagated it backward to its previous perihelion, and found that the comet visited the planetary region ~2 Myr ago at perihelion distance &lt;em&gt;q&lt;/em&gt; &amp;#8776; 8 au. Thus, C/2010 U3 (Boattini) is almost certainly a dynamically old comet from the Oort cloud, and the observed activity cannot be caused by retained heat from the previous apparition. The detailed study is presented in Hui et al. (2019, AJ, 157, 162).&lt;/p&gt;

Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2

&lt;p class=&quot;p1&quot;&gt;We discuss the development of activity in the extraordinary, distant long-period comet C/2017 K2 over the heliocentric distance range&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;9 &lt; r&lt;sub&gt;H&lt;/sub&gt; &lt; 16 AU.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; C/2017 K2 is an incoming long-period comet with a period so long (~ 3 Myr) that no heat from the previous perihelion can be retained; we can be sure that the observed mass-loss is driven by the current insolation and not by a thermal lag.&amp;#160; &lt;/span&gt;The comet is characterized by a steady-state coma of sub-millimeter and larger particles ejected at low (4 m/s) velocity, filling a roughly spheroidal coma with a characteristic scale of 80,000 km.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; &lt;/span&gt;In a fixed, co-moving volume around the nucleus we find that the scattering cross-section of the coma, C, is related to the heliocentric distance by a power law, C ~ r&lt;sub&gt;H&lt;/sub&gt;&lt;sup&gt;-s&lt;/sup&gt;, with heliocentric index s = 1.14+/-0.05. This dependence is significantly weaker than the r&lt;sub&gt;H&lt;/sub&gt;&lt;sup&gt;-2&lt;/sup&gt;, variation of the&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;insolation as a result of two effects.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; &lt;/span&gt;These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;particles ejected long before the observations were taken. &lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; &lt;/span&gt;A Monte Carlo&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;model of the photometry shows that dust production beginning at r&lt;sub&gt;H&lt;/sub&gt; ~ 35 AU is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; &lt;/span&gt;Dust mass loss rates at 10 AU are of order dM/dt ~ 10&lt;sup&gt;3 &lt;/sup&gt;a&lt;sub&gt;1&lt;/sub&gt; kg/s, where 0.1 &lt; a&lt;sub&gt;1&lt;/sub&gt; &lt; 1 is the effective particle radius expressed in millimeters.&lt;/p&gt; &lt;p class=&quot;p1&quot;&gt;The expulsion of submillimeter and larger grains, beginning at Kuiper belt distances, is likely the result of the sublimation of near-surface supervolatile ice (probably CO, as suggested by the recent detection of this molecule at 6.7 AU; Yang et al. Ap. J. Letters, in press). Water ice is involatile over the observed distance range and even the energy and gas release triggered by the crystallization of amorphous ice, if present, cannot produce activity at 35 AU.&amp;#160; Comet C/2017 K2 will reach perihelion near Mars' orbit in December 2022.&amp;#160;&amp;#160;&lt;/p&gt; &lt;p class=&quot;p1&quot;&gt;&amp;#160;&lt;/p&gt; &lt;p class=&quot;p1&quot;&gt;This work is described in D. Jewitt, Y. Kim. M. Mutchler, J. Agarwal, J. Li and H. Weaver (2021).&amp;#160; Astronomical Journal, 161:188 (11pp)&amp;#160;&lt;/p&gt;

In-Situ Multi-Spacecraft and Remote Imaging Observations of the First CME Detected by Solar Orbiter and BepiColombo

&lt;p&gt;On April 19th 2020 a CME was detected by Solar Orbiter at a heliocentric distance of 0.8 AU and was also observed in-situ on April 20th by both Wind and BepiColombo. During this time, BepiColombo had just completed a flyby of the Earth and therefore the longitudinal separation between BepiColombo and Wind was just 1.4&amp;#176;. The total longitudinal separation of Solar Orbiter and both spacecraft near the Earth was less than 5&amp;#176;, providing an excellent opportunity for a radial alignment study of the CME. We use the in-situ observations of the magnetic field at Solar Orbiter with those at Wind and BepiColombo to analyse the large-scale properties of the CME and compare results to those predicted using remote observations at STEREO-A, providing a global picture of the CME as it propagated from the Sun to 1 AU.&lt;/p&gt;

How Alfvén waves energize the solar wind: heat versus work

A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfvén-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ( $P_\textrm {AWb}$ ) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$ , which we denote by $\chi _{H}(r)$ , and the fraction that is transferred via work, which we denote by $\chi _{W}(r)$ . We find that $\chi _{W}(r_{A})$ ranges from 0.15 to 0.3, where $r_{A}$ is the Alfvén critical point. This value is small compared with one because the Alfvén speed $v_{A}$ exceeds the outflow velocity $U$ at $r < r_{A}$ , so the AWs race through the plasma without doing much work. At $r>r_{A}$ , where $v_{A} < U$ , the AWs are in an approximate sense ‘stuck to the plasma’, which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $r=r_{A}$ , so the total rate at which AWs do work on the plasma at $r>r_{A}$ is a modest fraction of $P_\textrm {AWb}$ . We find that heating is more effective than work at $r < r_{A}$ , with $\chi _{H}(r_{A})$ ranging from 0.5 to 0.7. The reason that $\chi _{H} \geq 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfvén-speed scale height in RDAWT, and there are a few Alfvén-speed scale heights between the coronal base and $r_{A}$ . A given amount of heating produces more magnetic moment in regions of weaker magnetic field. Thus, paradoxically, the average proton magnetic moment increases robustly with increasing $r$ at $r>r_{A}$ , even though the total rate at which AW energy is transferred to particles at $r>r_{A}$ is a small fraction of $P_\textrm {AWb}$ .

Whistler waves observed by Solar Orbiter during its first orbit

&lt;p&gt;Plasma waves can play an important role in the evolution of the solar wind and the&amp;#160;particle&amp;#160;velocity distribution functions in particular. We analyzed the electromagnetic waves&amp;#160;observed above a few Hz by the Radio Plasma Waves (RPW) instrument suite onboard Solar Orbiter, during its first orbit,&amp;#160;which covered a distance&amp;#160;from the Sun between 1 AU and 0.5&amp;#160;AU. &amp;#160;We identified the majority of the detected waves as whistler waves with frequency around &amp;#160;0.1 f_ce and right handed&amp;#160;circular&amp;#160;polarisation. We found these waves to be mostly&amp;#160;aligned or anti aligned with the&amp;#160;ambient&amp;#160;magnetic field, and rarely oblique. We also present and discuss their direction of propagation and the variation of the waves' properties with&amp;#160;heliocentric distance.&lt;/p&gt;

Solar Energetic Electron Events Associated with Hard X-ray Flares

&lt;p&gt;&lt;span&gt;We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all 16 cases, the presence of an SEE power-law spectrum extending down to &lt;/span&gt;&lt;span&gt;6&lt;/span&gt;&lt;span&gt;5 keV at 1 AU implies that the SEE source would be high in the corona, at a heliocentric distance of &lt;/span&gt;&lt;span&gt;&gt;&lt;/span&gt;&lt;span&gt;1.3 &lt;/span&gt;&lt;span&gt;solar radii&lt;/span&gt;&lt;span&gt;, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by thick target bremsstrahlung processes very low in the corona. &lt;/span&gt;&lt;span&gt;We find that in 8 cases (the other 8 cases), the power-law spectral index of HXR-producing electrons, estimated under the relativistic thick-target bremsstrahlung model, is significantly larger than (similar to) the observed high-energy spectral index of SEEs, with a positive correlation. In addition, the estimated number of SEEs is only &lt;/span&gt;&lt;span&gt;&amp;#8764;&lt;/span&gt;&lt;span&gt;10&lt;/span&gt;&lt;span&gt;-&lt;/span&gt;&lt;span&gt;4 &lt;/span&gt;&lt;span&gt;- &lt;/span&gt;&lt;span&gt;10&lt;/span&gt;&lt;span&gt;-&lt;/span&gt;&lt;span&gt;2 &lt;/span&gt;&lt;span&gt;of the estimated number of HXRproducing electrons at energies above 30 keV, but also with a positive correlation. &lt;/span&gt;&lt;span&gt;These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated &lt;/span&gt;&lt;span&gt;3&lt;/span&gt;&lt;span&gt;He&lt;/span&gt;&lt;span&gt;=&lt;/span&gt;&lt;span&gt;4&lt;/span&gt;&lt;span&gt;He ratio is positively correlated with &lt;/span&gt;&lt;span&gt;the observed high-energy spectral index of SEEs&lt;/span&gt;&lt;span&gt;, indicating a possible relation of the &lt;/span&gt;&lt;span&gt;3&lt;/span&gt;&lt;span&gt;He ion acceleration with high-energy SEEs&lt;/span&gt;&lt;/p&gt;