No abstract

We develop and exploit a new catalog of coronal pressure waves modeled in 3D to study the potential role of these waves in accelerating solar energetic particles (SEPs) measured in situ. Our sample comprises modeled shocks and SEP events detected during solar cycle 24 observed over a broad range of longitudes. From the 3D reconstruction of shock waves using coronagraphic observations we derived the 3D velocity along the entire front as a function of time. Combining new reconstruction techniques with global models of the solar corona, we derive the 3D distribution of basic shock parameters such as Mach numbers, compression ratios, and shock geometry. We then model in a time-dependent manner how the shock wave connects magnetically with spacecraft making in situ measurements of SEPs. This allows us to compare modeled shock parameters deduced at the magnetically wellconnected regions, with different key parameters of SEPs such as their maximum intensity. This approach accounts for projection effects associated with remote-sensing observations and constitutes the most extensive study to date of shock waves in the corona and their relation to SEPs. We find a high correlation between the maximum flux of SEPs and the strength of coronal shock waves quantified, for instance, by the Mach number. We discuss the implications of that work for understanding particle acceleration in the corona.

No abstract

The physical mechanisms that produce the slow solar wind are still highly debated. Parker Solar Probe's (PSP 's) second solar encounter provided a new opportunity to relate in situ measurements of the nascent slow solar wind with white-light images of streamer flows. We exploit data taken by the Solar and Heliospheric Observatory (SOHO), the Solar TErrestrial RElations Observatory (STEREO) and the Wide Imager on Solar Probe to reveal for the first time a close link between imaged streamer flows and the high-density plasma measured by the Solar Wind Electrons Alphas and Protons (SWEAP) experiment. We identify different types of slow winds measured by PSP that we relate to the spacecraft's magnetic connectivity (or not) to streamer flows. SWEAP measured highdensity and highly variable plasma when PSP was well connected to streamers but more tenuous wind with much weaker density variations when it exited streamer flows. STEREO imaging of the release and propagation of small transients from the Sun to PSP reveals that the spacecraft was continually impacted by the southern edge of streamer transients. The impact of specific density structures is marked by a higher occurrence of magnetic field reversals measured by the FIELDS magnetometers. Magnetic reversals originating from the streamers are associated with larger density variations compared with reversals originating outside streamers. We tentatively interpret these findings in terms Corresponding author: Alexis P. Rouillard arouillard@irap.omp.eu arXiv:2001.01993v1 [astro-ph.SR] 7 Jan 2020 2 Rouillard et al. of magnetic reconnection between open magnetic fields and coronal loops with different properties, providing support for the formation of a subset of the slow wind by magnetic reconnection.

[1] Land clearing for agricultural purposes in southwest Australia has created a landscape where a 750 km rabbit-proof fence separates 13 million hectares of croplands from the remnant native vegetation to the east. The Bunny Fence Experiment (BuFex) was conducted in the vicinity of the intended vermin-proof boundary in December 2005 and August 2007. The experiment examined the role of land cover change (LCC) on the preferential formation of clouds over the native vegetation that often terminates along the vermin-proof fence as well as the regional rainfall reduction observed in this region. Observations and numerical model analysis show that the formation and development of the west coast trough (WCT), which is a synoptic-scale feature that initiates spring and summertime convection, is impacted by land cover change and that the cloud fields induced by the WCT would extend farther west in the absence of the LCC. The surface convergence patterns associated with the wintertime WCT circulation are substantially altered by LCC, due to changes in both WCT dynamics and surface aerodynamic roughness, leading to a rainfall decrease to the west of the rabbit fence. Although this study focuses on only two events, it further illustrates that LCC has significant regional impacts in southwest Western Australia regardless of large-scale shifts in the climate system.

We study the solar eruptive event on 2017 September 10 that produced long-lasting >100 MeV γ-ray emission and a ground level enhancement (GLE72). The origin of the high-energy ions producing latephase gamma-ray emission (LPGRE) is still an open question, but a possible explanation is proton acceleration at coronal shocks produced by coronal mass ejections. We examine a common shock acceleration origin for both the LPGRE and GLE72. The γ-ray emission observed by the Fermi-Large Area Telescope exhibits a weak impulsive phase, consistent with that observed in hard X-and γ-ray line flare emissions, and what appear to be two distinct stages of LPGRE. From a detailed modeling of the shock wave, we derive the 3D distribution and temporal evolution of the shock parameters, and we examine the shock wave magnetic connection with the visible solar disk. The evolution of shock parameters on field lines returning to the visible disk, mirrors the two stages of LPGRE. We find good agreement between the time history of >100 MeV γ-rays and one produced by a basic shock acceleration model. The time history of shock parameters magnetically mapped to Earth agrees with the rates observed by the Fort Smith neutron monitor during the first hour of the GLE72 if we include a 30% contribution of flare-accelerated protons during the first 10 minutes, having a release time following the time history of nuclear γ-rays. Our analysis provides compelling evidence for a common shock origin for protons producing the LPGRE and most of the particles observed in GLE72.

Abstract. This study examines dynamical and microphysical features of convective clouds that affect mercury (Hg) wet scavenging and concentrations in rainfall. Using idealized numerical model simulations in the Regional Atmospheric Modeling System (RAMS), we diagnose vertical transport and scavenging of soluble Hg species -gaseous oxidized mercury (GOM) and particle-bound mercury (HgP), collectively Hg(II) -in thunderstorms under typical environmental conditions found in the Northeast and Southeast United States (US). Mercury scavenging efficiencies from various initial altitudes are diagnosed for a case study of a typical strong convective storm in the Southeast US. Assuming that soluble mercury concentrations are initially vertically uniform, the model results suggest that 60 % of mercury deposited to the surface in rainwater originates from above the boundary layer (> 2 km). The free troposphere could supply a larger fraction of mercury wet deposition if GOM and HgP concentrations increase with altitude. We use radiosonde observations in the Northeast and Southeast to characterize three important environmental characteristics that influence thunderstorm morphology: convective available potential energy (CAPE), vertical shear (0-6 km) of horizontal wind (SHEAR) and precipitable water (PW). The Southeast US generally has lower SHEAR and higher CAPE and PW. We then use RAMS to test how PW and SHEAR impact mercury scavenging and deposition, while keeping the initial Hg(II) concentrations fixed in all experiments. We found that the mercury concentration in rainfall is sensitive to SHEAR with the nature of sensitivity differing depending upon the PW. Since CAPE and PW cannot be perturbed independently, we test their combined influence using an ensemble of thunderstorm simulations initialized with environmental conditions for the Northeast and Southeast US. These simulations, which begin with identical Hg(II) concentrations, predict higher mercury concentrations in rainfall from thunderstorms forming in the environmental conditions over the Southeast US compared to the Northeast US. A final simulation of a stratiform rain event produces lower mercury concentrations than in thunderstorms forming in environments typical of the Southeast US. The stratiform cloud scavenges mercury from the lowest ∼ 4 km of the atmosphere, while thunderstorms scavenge up to ∼ 10 km.

Context. The Solar Orbiter spacecraft will be equipped with a wide range of remote-sensing (RS) and in-situ (IS) instruments to record novel and unprecedented measurements of the solar atmosphere and the inner heliosphere. To take full advantage of these new datasets, tools and techniques must be developed to ease multi-instrument and multi-spacecraft studies. In particular the currently inaccessible low solar corona below two solar radii can only be observed remotely. Furthermore techniques must be used to retrieve coronal plasma properties in time and in three dimensional (3D) space. Solar Orbiter will run complex observation campaigns that provide interesting opportunities to maximise the likelihood of linking IS data to their source region near the Sun. Several RS instruments can be directed to specific targets situated on the solar disk just days before data acquisition. To compare IS and RS, data we must improve our understanding of how heliospheric probes magnetically connect to the solar disk. Aims. The aim of the present paper is to briefly review how the current modelling of the Sun and its atmosphere can support Solar Orbiter science. We describe the results of a community-led effort by European Space Agency (ESA)'s Modelling and Data Analysis Working Group (MADAWG) to develop different models, tools, and techniques deemed necessary to test different theories for the physical processes that may occur in the solar plasma. The focus here is on the large scales and little is described with regards to kinetic processes. To exploit future IS and RS data fully, many techniques have been adapted to model the evolving 3D solar magneto-plasma from the solar interior to the solar wind. A particular focus in the paper is placed on techniques that can estimate how Solar Orbiter will connect magnetically through the complex coronal magnetic fields to various photospheric and coronal features in support of spacecraft operations and future scientific studies. Methods. Recent missions such as STEREO, provided great opportunities for RS, IS, and multi-spacecraft studies. We summarise the achievements and highlight the challenges faced during these investigations, many of which motivated the Solar Orbiter mission. We present the new tools and techniques developed by the MADAWG to support the science operations and the analysis of the data from the many instruments on Solar Orbiter. Results. This article reviews current modelling and tool developments that ease the comparison of model results with RS and IS data made available by current and upcoming missions. It also describes the modelling strategy to support the science operations and subsequent exploitation of Solar Orbiter data in order to maximise the scientific output of the mission. Conclusions. The on-going community effort presented in this paper has provided new models and tools necessary to support mission operations as well as the science exploitation of the Solar Orbiter data. The tools and techniques will no doubt evolve significantly as we refine our proc...