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

<p>We report an identification of long-period comet C/2010 U3 (Boattini) active at a new record inbound heliocentric distance of <em>r</em><sub>H</sub> ≈ 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 μm and are ejected protractedly at speeds ≤50 m s<sup>−1</sup> near the subsolar point. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO<sub>2</sub>) 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 < <em>r</em><sub>H</sub> < 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 <em>q</em> ≈ 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).</p>

Ripples going against the flow: How energy propagation determines the global structure of magnetopause surface waves

<p>Impulsive solar wind transients, such as pressure pulses and shocks, excite surface waves on the magnetopause. While much of this surface wave energy is advected downtail by the magnetosheath flow, recently it has been shown that some of these waves can be trapped locally forming a standing wave between the northern and southern ionospheres. It appears that this process can occur across most of the dayside magnetopause, however, it is not clear how these surface waves can resist the advective effect of the tailward flow. Through multispacecraft observations, global MHD simulations, and analytic MHD theory we show that azimuthally standing magnetopause surface waves are possible between 9-15h MLT. In this region, surface waves with Poynting vectors directed towards the subsolar point can exactly balance the advective effect of the magnetosheath flow, leading to no overall energy flow. Further downtail, however, the wave’s propagation cannot overcome advection and the usual tailward energy flow occurs. This trapping of magnetopause surface wave energy following the drivers of intense space weather may in turn have important implications on radiation belt, ionospheric, and auroral dynamics.</p>

Structure and Coalescence of Magnetopause Flux Ropes and Their Dependence on IMF Clock Angle: Three-Dimensional Global Hybrid Simulations

<p>Flux ropes are ubiquitous at Earth’s magnetopause and play important roles in energy transport between the solar wind and Earth’s magnetosphere. In this paper, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three-dimensional global hybrid simulations. As the IMF clock angle decreases from 180°, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward towards cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move towards cusps. However, when the IMF clock angle is small (≤ 135° ), we do not find coalescence between flux ropes.</p>

Inactivation times from 290 to 315 nm UVB in sunlight for SARS coronaviruses CoV and CoV-2 using OMI satellite data for the sunlit Earth

UVB in sunlight, 290–315 nm, can inactivate SARS CoV and SARS CoV-2 viruses on surfaces and in the air. Laboratory exposure to ultraviolet irradiance in the UVC range inactivates many viruses and bacteria in times less than 30 min. Estimated UVB inactivation doses from sunlight in J/m2 are obtained from UVC measurements and radiative transfer calculations, weighted by a virus inactivation action spectrum, using OMI satellite atmospheric data for ozone, clouds, and aerosols. For SARS CoV, using an assumed UVC dose near the mid-range of measured values, D90 = 40 J/m2, 90% inactivation times T90 are estimated for exposure to midday 10:00–14:00 direct plus diffuse sunlight and for nearby locations in the shade (diffuse UVB only). For the assumed D90 = 40 J/m2 model applicable to SARS CoV viruses, calculated estimates show that near noon 11:00–13:00 clear-sky direct sunlight gives values of T90 < 90 min for mid-latitude sites between March and September and less than 60 min for many equatorial sites for 12 months of the year. Recent direct measurements of UVB sunlight inactivation of the SARS CoV-2 virus that causes COVID-19 show shorter T90 inactivation times less than 10 min depending on latitude, season, and hour. The equivalent UVC 254 nm D90 dose for SARS CoV-2 is estimated as 3.2 ± 0.7 J/m2 for viruses on a steel mesh surface and 6.5 ± 1.4 J/m2 for viruses in a growth medium. For SARS CoV-2 clear-sky T90 on a surface ranges from 4 min in the equatorial zone to less than 30 min in a geographic area forming a near circle with solar zenith angle < 60O centered on the subsolar point for local solar times from 09:00 to 15:00 h.

Wave Propagation and Global Implications of Magnetopause Surface Eigenmodes

&lt;p&gt;Using global magnetohydrodynamic simulations we investigate the recently discovered eigenmode of the magnetopause surface &amp;#8211; the natural response of the boundary to impulsive solar wind transients. We show that following the directly driven motion of the magnetopause by a pressure pulse, decaying oscillations of the boundary follow in agreement with theoretical predications and previous simulations of the magnetopause surface eigenmode. Across the equatorial magnetosphere these oscillations originate at the subsolar point and maintain a near-constant frequency through all local times, though into the flanks a secondary higher-frequency signal emerges consistent with the expectations of Kelvin-Helmholtz generated surface waves. Focusing only on the eigenmode shows its amplitude grows with local time away from the subsolar point, with the waves showing no azimuthal propagation in the region 9-15h MLT &amp;#8211; surprising given the convecting effect downtail of the magnetosheath flow.&amp;#160; In the noon-midnight meridian the eigenmode is confined to the dayside magnetosphere. Comparing these results to MHD theory, we propose how the structure of the magnetopause surface eigenmode is determined by the properties of the magnetospheric system and how it may influence global dynamics during impulsive events.&lt;/p&gt;

Comments on: Confirming geomagnetic Sfe by means of a solar flare detector based on GNSS

We report two comments affecting the paper “Curto JJ, Juan JM & Timoté CC, 2019. Confirming geomagnetic Sfe by means of a solar flare detector based on GNSS. J Space Weather Space Clim 9: A42. https://doi.org/10.1051/swsc/2019040”: The first comment is the reporting of two mistakes which distorts the central model used for the measurement and detection of solar flares with GNSS, that might affect as well the most part of results and discussions contained in the paper. And the second comment is the clarification about the authors’ claim of presenting the first work of using the electron content enhancement estimation at the subsolar point for characterizing solar flares with GNSS data, which is not accurate due to the existence of such previous definition and usage.

Mercury's subsolar sodium exosphere: an ab initio calculation to interpret MASCS/UVVS observations from MESSENGER

The optical spectroscopy measurements of sodium in Mercury's exosphere near the subsolar point by MESSENGER Mercury Atmospheric and Surface Composition Spectrometer Ultraviolet and Visible Spectrometer (MASCS/UVVS) have been interpreted before with a model employing two exospheric components of different temperatures. Here we use an updated version of the Monte Carlo (MC) exosphere model developed by Wurz and Lammer (2003) to calculate the Na content of the exosphere for the observation conditions ab initio. In addition, we compare our results to the ones according to Chamberlain theory. Studying several release mechanisms, we find that close to the surface, thermal desorption dominates driven by a surface temperature of 594 K, whereas at higher altitudes micro-meteorite impact vaporization prevails with a characteristic energy of 0.34 eV. From the surface up to 500 km the MC model results agree with the Chamberlain model, and both agree well with the observations. At higher altitudes, the MC model using micro-meteorite impact vaporization explains the observation well. We find that the combination of thermal desorption and micro-meteorite impact vaporization reproduces the observation of the selected day quantitatively over the entire observed altitude range, with the calculations performed based on the prevailing environment and orbit parameters. These findings help in improving our understanding of the physical conditions at Mercury's exosphere as well as in better interpreting mass-spectrometry data obtained to date and in future missions such as BepiColombo.

Parameters of current systems in the magnetosphere as derived from observations of cosmic rays during the June 2015 magnetic storm

Basing on measurements of cosmic rays at the worldwide network of stations, we calculate variations in the planetary system of geomagnetic cutoff rigidity for the 2015 June moderate geomagnetic storm. Using the axisymmetric model of the limited magnetosphere taking into account magnetopause currents and the ring current, we determine the distance to the subsolar point and the ring current radius, as well as the contribution of the ring current to variations in the geomagnetic cutoff rigidity and to the Dst index.