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

Gamborino, Diana; Vorburger, Audrey; Würz, Peter

doi:10.5194/angeo-37-455-2019

Cited by 18 publications

(24 citation statements)

References 70 publications

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“…(2015); Gamborino and Wurz (2018); Gamborino et al. (2019). Since they are caused by the solar radiation, TD and PSD are confined to the dayside, while bombardment with micro‐meteoroids yields, on average, a radially symmetric release of sodium particles (Wurz et al., 2010).…”

Section: Exosphere Modelmentioning

confidence: 99%

“…Gamborino et al. (2019) conducted a Monte‐Carlo simulation for the exosphere which included gravitational escape, radiation pressure, and surface adsorption as loss processes. They were able to explain the sodium column density profile derived from MASCS/UVVS measurements on 23 April 2012 (Cassidy et al., 2015).…”

Section: Exosphere Modelmentioning

confidence: 99%

“…The total sodium density of the exosphere will be used as a free parameter, considering surface densities derived from an exosphere model by Gamborino et al. (2019), the results of James et al. (2019) and even larger values that would correspond to Venus/Mars‐like interaction scenarios.…”

Section: Introductionmentioning

confidence: 99%

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Influence of Mercury's Exosphere on the Structure of the Magnetosphere

et al. 2020

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Mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma‐surface interactions. The absolute values of the sodium ion density are still under debate. Observations by MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. However, MESSENGER magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. To investigate how a dense exosphere would affect the current systems within Mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet‐like configurations. We demonstrate how a sufficiently dense exosphere leads to self‐shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of Mercury's magnetosphere, which is decreasingly affected by the dipole offset. Once the sodium ion density is sufficiently high, Region 2 field‐aligned currents emerge close to the planet. The modeled Region 2 currents are located below the orbit of MESSENGER, thereby providing a possible explanation for the absence of these currents in observations. The sodium exosphere also closes a significant fraction of the Region 1 currents through Pedersen and Hall currents before the “guiding” magnetic field lines even reach the planetary surface. The modeled sodium ion and solar wind densities agree well with observations.

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Section: Exosphere Modelmentioning

confidence: 99%

Section: Exosphere Modelmentioning

confidence: 99%

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Influence of Mercury's Exosphere on the Structure of the Magnetosphere

et al. 2020

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“…Because of the intense solar irradiation at Mercury's surface, which is due to the proximity of Mercury to the Sun, thermal and photon stimulated desorption are suggested to have the largest contribution to the formation of Mercury's neutral exosphere (e.g., Gamborino et al, ; Leblanc & Johnson, ; McGrath et al, ; Mura et al, ; Wurz & Lammer, ; Wurz et al, ). However, the particles released by these two processes have relatively low velocities and thus are more gravitationally bound to the planet, compared to the particles released by micrometeorite impact vaporization and solar wind ion sputtering (e.g., Leblanc & Johnson, ; Schmidt et al, , ; Schmidt, ; Wurz et al, ).…”

Section: Introductionmentioning

confidence: 99%

“…Finally, we provide detailed analysis on plasma precipitation to the surface of Mercury under different solar wind plasma dynamic pressures and IMF orientations as well as the cusp dynamics and plasma precipitation through the cusps to explain some of the observed features in Na exosphere. Our global precipitation maps provide a better understanding of the magnetosphere‐exosphere‐surface coupling at Mercury (Milillo et al, ; Orsini et al, ) and can be applied in Monte Carlo simulations of Mercury's exosphere (e.g., Gamborino et al, ; Mura et al, ; Schmidt, ; Wurz & Lammer, ), a tool that is required to better interpret observations by the European Space Agency (ESA)'s BepiColombo mission (Benkhoff et al, ; Milillo et al, ) and future ground‐based telescope observations.…”

Section: Introductionmentioning

confidence: 99%

Hybrid Simulations of Solar Wind Proton Precipitation to the Surface of Mercury

2020

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We examine the effects of the interplanetary magnetic field (IMF) orientation and solar wind dynamic pressure on the solar wind proton precipitation to the surface of Mercury using a hybrid-kinetic model. We use our model to explain observations of Mercury's neutral sodium exosphere and compare our results with MESSENGER observations. For the typical solar wind dynamic pressure at Mercury our model shows a high proton flux precipitates through the magnetospheric cusps to the high latitudes on both hemispheres on the dayside, centered near the noon meridian with ∼11 • latitudinal extent in the north and ∼21 • latitudinal extent in the south, which is consistent with MESSENGER observations. We show that this two-peak pattern is controlled by the radial component (B x ) of the IMF and not the B z . Our model suggests that the southward IMF and its associated magnetic reconnection do not play a major role in controlling plasma precipitation to the surface of Mercury through the cusps. We found that the total precipitation rate through both of the cusps remain constant and independent of the IMF orientation. We also show that the solar wind proton incidence rate to the entire surface of Mercury is higher when the IMF has a northward component and nearly half of the incidence flux impacts the low latitudes on the nightside. During extreme solar events (e.g., coronal mass ejections), our model suggests that over 70 nPa solar wind dynamic pressure is required for the entire surface of Mercury to be exposed to the solar wind plasma.

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Rationale for BepiColombo Studies of Mercury’s Surface and Composition

et al. 2020

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BepiColombo has a larger and in many ways more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of Mercury’s surface than the suite carried by NASA’s MESSENGER spacecraft. Moreover, BepiColombo’s data rate is substantially higher. This equips it to confirm, elaborate upon, and go beyond many of MESSENGER’s remarkable achievements. Furthermore, the geometry of BepiColombo’s orbital science campaign, beginning in 2026, will enable it to make uniformly resolved observations of both northern and southern hemispheres. This will offer more detailed and complete imaging and topographic mapping, element mapping with better sensitivity and improved spatial resolution, and totally new mineralogical mapping. We discuss MESSENGER data in the context of preparing for BepiColombo, and describe the contributions that we expect BepiColombo to make towards increased knowledge and understanding of Mercury’s surface and its composition. Much current work, including analysis of analogue materials, is directed towards better preparing ourselves to understand what BepiColombo might reveal. Some of MESSENGER’s more remarkable observations were obtained under unique or extreme conditions. BepiColombo should be able to confirm the validity of these observations and reveal the extent to which they are representative of the planet as a whole. It will also make new observations to clarify geological processes governing and reflecting crustal origin and evolution. We anticipate that the insights gained into Mercury’s geological history and its current space weathering environment will enable us to better understand the relationships of surface chemistry, morphologies and structures with the composition of crustal types, including the nature and mobility of volatile species. This will enable estimation of the composition of the mantle from which the crust was derived, and lead to tighter constraints on models for Mercury’s origin including the nature and original heliocentric distance of the material from which it formed.

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

Cited by 18 publications

References 70 publications

Influence of Mercury's Exosphere on the Structure of the Magnetosphere

Influence of Mercury's Exosphere on the Structure of the Magnetosphere

Hybrid Simulations of Solar Wind Proton Precipitation to the Surface of Mercury

Rationale for BepiColombo Studies of Mercury’s Surface and Composition

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