Dust sputtering within the inner heliosphere

The aim of this study is to investigate how sputtering by impacting solar wind particles influences the lifetime of dust particles in the inner heliosphere near the Sun. We consider three typical dust materials: silicate, Fe0.4Mg0.6O and carbon and describe their sputtering yields based on atomic yields given by the Stopping and Range of Ions in Matter (SRIM) package. The influence of the solar wind is characterized by plasma density, solar wind speed and solar wind composition and we assume for these parameters values that are typical 5 for fast solar wind, slow solar wind and CME conditions to calculate the sputtering lifetimes of dust. To compare the sputtering lifetimes to typical sublimation lifetimes we use temperature estimates based on Mie calculations and material vapour pressure derived with the chemical equilibrium code MAGMA. We also compare the sputtering lifetimes to the Poynting-Robertson lifetime and to the collision lifetime. We present a set of sputtering rates and lifetimes that can be used for estimating dust destruction in the fast and slow solar 10 wind and during CME conditions. Our results can be applied to solid particles of a few nm and larger. The sputtering lifetimes increase linearly with the size of particles. We show that sputtering rates increase during CME conditions, primarily because of the high number densities of heavy ions in the CME plasma. The shortest sputtering lifetimes we find are for silicate, followed by Fe0.4Mg0.6O and carbon. In a comparison between sputtering and sublimation lifetimes we concentrate on the nanodust population. The comparison shows that sublimation is the faster destruction process within 0.1 AU for Fe0.4Mg0.6O, within 15 0.05 AU for carbon dust and within 0.07 AU for silicate dust. The destruction by sputtering can play a role in the vicinity of the Sun. We discuss our findings in the context of recent F-corona intensity measurements onboard Parker Solar Probe.

sublimation process for dust approaching the vicinity of the Sun. The comparison of both dust loss processes and its implication 60 for dust near the Sun is described within Section 4. Finally, Section 5 draws the conclusions of this study.

Dust sputtering
Sputtering is the physical process of atom ejection from a solid through the bombardment of energetic ions (Behrisch and Eckstein, 2007). This process usually is performed within a laboratory environment where a cathode is bombarded with noble gas ions and the ejected cathode atom deposit and form high quality surfaces. However, this process is also well known in the 65 context of dust destruction for interplanetary (e.g. Mukai and Schwehm, 1981) and interstellar dust grains (e.g. Barlow, 1978;Draine and Salpeter, 1979).
For the calculation of nanodust sputtering, we divide our study into three different sputtering scenarios. These are the slow solar wind conditions, fast solar wind conditions and CME conditions. In the following, the heliospheric conditions of these scenarios are discussed in detail. Subsequently, we introduce the calculation of the dust's sputtering lifetimes. This is followed 70 by an analysis of dust sputtering at 1 AU and in the inner heliosphere. Figure 1 shows the composition of the three solar wind scenarios considered. The SW/CME composition used here is based on the work of Killen et al. (2012, their Table 6) and contains ten different species including protons (H), Helium ions (He) and the heavier species Carbon (C), Oxygen (O), Nitrogen (N), Iron (Fe), Neon (Ne), Magnesium (Mg), Silicon (Si) and Sulfur (S) 75 ions. Generally, the solar wind is composed of a big fraction of protons, a small fraction of Helium and traces of the heavier species. The composition changes between 95% H / 4.5% He for the slow solar wind and 98% H / 1.5% He for the fast solar wind, with heavier species around 0.5%. The composition of CME's however is much more dominated by heavier species, 66% H 30% He and 4% heavier species. In addition to the plasma composition, also the plasma speed and density is different for each solar wind scenario. Table 1 summarizes the values that have been used for the solar wind / CME conditions within 80 this study. It has to be noted that the composition, speed and plasma density of the solar wind or CMEs is highly variable. The given values represent average conditions. Sputtering is the impact process of an energetic ion or atom on a target and the subsequent removal of target atoms from its surface. The sputtering yield is the main parameter of the sputtering process itself. This yield denotes the number of target atoms sputtered by one incident ion and is a function of the ion's kinetic energy and the targets material properties (e.g. Behrisch 85 and Eckstein, 2007, and references therein package (Ziegler et al., 2008), that derives the sputtering yields and also stopping powers and ranges of ions within compounds.

Heliospheric conditions
For each solar wind scenario, the SRIM program has been initialized by the above discussed plasma composition and speed 90 (energy/nucleii). In order to derive the sputtering yield for the dust species i for a given scenario, we summed up the sputtering yields for each atom j sputtered by solar wind ion k.
The index j denotes the target atoms Mg/Fe/Si/O for astronomical silicate and Fe/Mg/O for the Fe 0.4 Mg 0.6 O composition.
The yields correspond to the atomic ratios of the dust composition and the ion ratio of the solar wind composition. For the 95 monoatomic carbon case we have used the analytic formula from (Eckstein and Preuss, 2003) together with the experimentally fitted sputtering parameters (Behrisch and Eckstein, 2007, and   lations are sputtering yields for silicate, Fe 0.4 Mg 0.6 O and carbon for the three different sputtering scenarios, i.e. slow SW / fast SW / CME. The individually derived yields can be found in the supplemented material. Figure 2 shows the results of the assessment, i.e. the sputtering yield as a function of ion species (H -Sulfur S), for the three 100 different materials. The results indicate a strong ion mass dependence as well as a target material dependence. The sputtering yields for carbon are significantly lower than for the silicate and Fe 0.4 Mg 0.6 O materials. Furthermore, the sputtering yield for Iron ions are two orders of magnitude higher than the sputtering by light protons, that behavior is valid for all target materials.
We derived the sputtering yields for all speeds stated in Tab. 1, with the 300 km/s case resulting in the highest values and 500 km/s being 20% lower and the 800 km/s 40% lower. The effect of ion impact speed on the sputtering yield is rather low 105 compared to the importance of SW composition or impact material.
To show the importance of heavy ion sputtering especially during CME conditions we have calculated the relative sputtering yield Y r k weighed with the plasma composition as follows: c k denotes the composition of the solar wind plasma as shown in Fig. 1. The relative sputtering for the case of carbon as 110 target material is shown in Fig. 3, the other target materials show similar results. It is evident that the slightly higher amount of heavier ions within the CME plasma result in a much higher contribution of the sputtering yield from these ions according to Fig. 2. It can be expected that this effect will enhance the sputtering effectiveness enormously. Usually, when considering   the fast and slow SW it is reasonable to account only for H and He sputtering (e.g. Wurz et al., 2010). However, we have also included the heavier ions in our sputtering calculations for the fast and slow SW scenarios.

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The sputtering yields used in this study have only been derived by considering a normal impact of an ion onto the target particle and a resulting collision cascade governed by atomic forces within the target lattice. However, it has to be noted that the sputtering process is also heavily influenced by a number of additional parameters, which cannot be accounted for in this study. Eventual sputtering yield enhancement can occur due to high target temperatures (Roth and Möller, 1985), non-normal impact angle, ion charge state for sputtering of insulators (Hayderer et al., 2001), and a size dependence of the yield when 120 considering nanometer sized dust (Järvi et al., 2008). All these processes might enhance the sputtering yield substantially. On the other hand microscopic surface roughness can increase but also decrease the sputtering yield under certain circumstances, e.g. slant sputtering (Ruzic, 1990). In addition, we also do not consider composition change due to eventual implementation of solar wind ions into the nanodust or fractional depletion of a certain atom type within the dust.

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For the derivation of nanodust sputtering lifetimes we follow the formalism given by Wurz (2012). The mass loss rate from a surface through sputtering in the solar wind is given by the following: Here, A is the cross section of the dust, f SW the solar wind ion flux, Y tot the total sputtering yield of the target material and m A is the mean mass of the sputtered atoms.

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Under the assumption of constant composition and size independent sputtering yield, the sputtering lifetime can be integrated from the sputtering mass loss rate of a circular surface exposed to the SW/CME plasma: Here, r 0 is the initial radius of the dust, N A is the Avogadro constant, M and ρ are the molar mass and mass density of the sputtered material. For the solar wind flux f SW = n p · v p the values from Tab. 1 have been used.  One can see that carbon nanodust survives longest among all three studied composition, i.e. 1 nm dust survives 5000 days at 1 AU under CME conditions. Fe 0.4 Mg 0.6 O sputtering lifetimes are by a factor of 20 shorter. The lifetimes of silicate are a factor of 60 shorter than the carbon sputtering lifetimes. These factors vary slightly with all solar wind condition. When 140 comparing the different solar wind conditions, CME sputtering lifetimes are the shortest. Sputtering lifetime in the slow solar wind are 20 fold longer. The lifetimes for fast solar wind conditions are 20 times longer than the lifetimes in CME condition.
This behavior varies a bit from one dust composition to the other. The short lifetimes in the CME scenario occur due to the presence of heavy ions in an overall denser plasma cloud. However, CME's are distinct solar eruptions and these sputtering conditions do not last longer than one or two days and occur only locally in the heliosphere. The given lifetimes of several ten 145 days and more at 1 AU for 1 nm dust make a full destruction due to CME sputtering not possible. However, for the case of fast and slow SW, which is present within the heliosphere at all times, the sputtering life times are close to ten orbital periods in the case of silicate nanodust and thirty years for Fe 0.4 Mg 0.6 O nanodust.
The sputtering lifetime described in Eq. 4 is linear in initial dust particle radius and enables easy calculation of lifetimes for other dust sizes. In Fig. 5 we show the derived lifetimes of dust particles in the size range from 1 nm to 1 µm at the Earth's 150 orbit. Sub-micron dust particles at Earth's orbit have sputtering lifetimes which can reach several hundred thousands of days, Poynting-Robertson lifetime (τP R) are shown in black for comparison (data taken from Grün et al., 1985).
i.e. thousands of orbital periods. For a better comparison, the sputtering lifetimes are plotted together with the collisional and Poynting-Robertson lifetimes given by Grün et al. (1985). As mentioned above, only nanodust in the small size limit can be significantly removed by solar wind sputtering in reasonable timescales. For slow and fast solar wind conditions at 1 AU, we find that the sputtering lifetime of silicate particles smaller than 60 nm is clearly below their Poynting-Robertson 155 and collision lifetime. That is also the case for Fe 0.4 Mg 0.6 O dust below 30 nm and carbon dust below 20 nm. We point out, that for CME conditions at 1 AU, we also find that the sputtering lifetime of silicate and Fe 0.4 Mg 0.6 O particles is well below Poynting-Robertson and collision lifetime of the dust in the whole considered size interval of 1 to 1000 nm. In practice, this has no consequence because of the short time duration of CME. This situation changes when considering sputtering at shorter distances from the Sun, as the SW and CME plasma density increases.

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For this approach we consider a SW plasma density following a power law with exponent minus two: Here, the distance from the Sun d is given in astronomical units. The used exponent lies within the range of published values, e.g. Maksimovic et al. (2005) report a value of -2.2±0.1. This values was found for the fast SW conditions which we are going to apply for the slow SW and CME conditions as well. Figure 6 shows the lifetime of 1 nm dust at distances from the Sun 165 from 0.01 AU to 1 AU, derived for the three different SW scenarios and three dust materials. The high vulnerability of silicate to sputtering is visible here too as their solar wind sputtering lifetimes are in the range of the carbon's lifetimes for CME conditions.  slow SW conditions 9.3 · 10 −2 3.3 · 10 −2 5.4 · 10 −4 CME conditions 0.41 1.6 2.6 · 10 −2 As stated above, carbon is a very resistant material with respect to sputtering. Carbon dust with only 1 nm can survive several ten days at 0.1 AU. Only in the case of sputtering within the CME conditions the carbon sputtering lifetimes is below the typical 170 duration of a CME of 1-2 days within the shortest distances from the Sun.
From the mass loss rate (Eq.3) it is also possible to derive the erosion rate of a dust particle due to sputtering. This erosion rate, i.e. the shrinkage of dust per unit time (dr/dt), is also independent of dust size.
For a distance of 0.1 AU, we derived the erosion rates of the three dust components for the three solar wind conditions in Tab.

2.
As the dust erosion rate (Eq. 6) is independent of initial dust radius, the sputtering of dust larger than 1 micron can also be considered. For example a silicate dust particle with a size of 10 µm has a 10000 fold lifetime of a 1 nm dust grain. When assuming the dust is in at a distance of 0.1 AU the 1 nm dust survives 0.6 days under CME conditions, i.e. it will be destroyed by a single CME. A dust grain of 10 µm size has a lifetime of 6000 days under CME condition. That means this can be hit by 180 3000 strong CMEs at a distance of 0.1 AU until it will be finally destroyed. Within our solar system, CME rates vary during a solar cycle . The rate can peak up to 400 per month during high solar activity and can be as low as 10 CME per month during solar minimum (Lamy et al., 2019). When assuming a mean value of 100 CMEs per month, the duration a 10 µm can survive at 0.1 AU is at least 2.5 years. It has to be noted that this requires that the dust particle is hit by every CME ejected by the Sun.
This seems to be unlikely due to the randomness of the CME propagation and its allocated size within the heliosphere. Another 185 reason why the lifetime of bigger dust particles might be unrealistic is that during this period the dust size and its orbit changes drastically. This leads to a different sputtering environment and the assumption of a constant erosion rate breaks down.
As dust particles approach the vicinity of Sun their temperature increases drastically. To investigate the relevance of the sputtering process, the seemingly low nanodust sputtering lifetimes has to be compared to dust destruction by sublimation into free space.

Dust sublimation
The processes of sublimation, evaporation and condensation is usually described by Langmuir's equation of evaporation: A.
In case of sublimation, it describes the sublimated mass per time unit as a function of vapour pressure p v and temperature T of the sublimating material. A is the whole surface of the dust and R is the gas constant. In the context of free space, atoms leave 195 the materials surface into the vacuum, while the adsorption of atoms onto surfaces can also occur under certain conditions, e.g. the resupply of Saturn E rings by the adsorption of Enceladus water vapour (e.g. Hansen et al., 2006). The sublimation of dust particles has been studied within different astrophysical context, e.g. protoplanetary systems (e.g. Duschl et al., 1996) and interstellar dust (e.g. Draine and Salpeter, 1979). For a self-consistent study, the sublimation of the same dust materials as in the sputtering part will be considered. In order to quantify the dust sublimation two parameters are needed, i.e. dust temperature equilibrium of absorbed solar radiation and re-emitted thermal radiation of the dust particle (Myrvang, 2018). The effect of dust cooling due to evaporation has been quantified to be only 10% of the re-emitted power (e.g. Schwehm, 1980), which we neglect in this study. Figure 7 shows the temperature of 1 nm dust particles made of carbon, silicate and Fe 0.4 Mg 0.6 O, for comparison the temperature of a black body is also shown. All nanodust is significantly hotter than a blackbody, except for 205 silicate near 1 AU which has similar equilibrium temperatures. Near the Sun, the dust temperatures of all materials exceed the black body. At 0.01 AU the Fe 0.4 Mg 0.6 O 1 nm dust is ≈ 700 K hotter than a black body, carbon 500 K and silicate 400 K. All three materials with a dust size of 1 nm are hotter than 3000 K near the Sun. The temperature change from 1 nm to 100 nm is below 100 K for each dust material(not shown). These temperatures have been derived using Mie theory and the refractive The second quantity for the description of sublimation is the vapour pressure. For the derivation of the vapour pressure for the oxides Fe 0.4 Mg 0.6 O and astronomical silicate we used the MAGMA code (Fegley Jr and Cameron, 1987;Schaefer and Fegley, 2004). The program is very flexible with regard to material composition and the derived vapour pressures have been checked with a vast number of experimental data. The MAGMA code has been used mainly for the change of planets 215 and planetesimals due to geological activity but also for the evaporation of meteoroids within the Earth's atmosphere (e.g. Schult et al., 2015). The MAGMA model is a multicomponent gas-melt chemical equilibrium code and is able to derive vapour as well. The vapour pressure for carbon was used from the literature (Leider et al., 1973;Lide, 2003). To derive the sublimation lifetime of nanometer sized dust particles, Eq. 7 is integrated using spherical geometry (Lamy, 1974): Where p v is the vapour pressure of the dust material, T dust is the temperature of the nanodust as a function of distance from the Sun d, and R is the universal gas constant. In Fig. 8  As the vapour pressure is a very steep function of temperature, according to Eq. 8 the relationship is inversely translated to 235 the sublimation lifetime. At temperatures below 1000 K the lifetime of all different kinds of 1 nm dust are greater than 10 5 d.
Nanodust with temperatures above 2500 K have already sublimation lifetimes below 10 −5 d, these lifetimes are so short that the dust can be regarded as non-existing.
The next step will be the direct comparison of sublimation and sputtering lifetime for nanodust within the near Sun environment.

Implications for nanodust near the Sun
In the earlier sections, it has been shown that sputtering and sublimation can be significant sinks for nanodust. The loss of nanodust due to solar wind sputtering increases with ion number density and ion mass (see 2). The effect of sublimation however, is a steep function of dust temperature (see Sect.3). For the comparison of sputtering and sublimation of nanodust we have chosen the CME scenario. We find the shortest sputtering lifetimes for CME conditions, but the short duration of single 245 CMEs has to be taken into account.
The comparison of the lifetimes is done in the small size limit of the dust population, i.e. the sizes 0.2 nm, 1 nm, and 5 nm.
There is no experimental prove for the existence of sub nanometer dust. However, it will be hypothesized that these clusters of molecules exist. This assumption will help to better assess the importance of nanodust sputtering in this study.
Here, we compare the sputtering and sublimation lifetimes of the three different nanodust compositions, namely carbon,

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Fe 0.4 Mg 0.6 O and silicate nanodust. Figure 9 a) shows the sputtering and sublimation lifetimes of carbon dust. All lifetimes are compared to a duration of 2 days, which is used as the upper limit for the duration of a CME. In the case of carbon, which is a rather sturdy material, the nanodust can survive in the near proximity of the Sun. The sublimation of carbon nanodust within 2 days occurs at a distance of 0.03 AU from the Sun, that is because of carbon's comparably high evaporation temperature of 2600K at low pressures (Whittaker, 1978). However, the sputtering lifetime of carbon is longer than sublimation counterpart.
considering the duration of a CME the sputtering and sublimation of only the smallest nanodust are comparably. In the case of carbon nanodust we state that during a typical CME sputtering is not a relevant destruction process within the inner heliosphere.
The lifetimes of Fe 0.4 Mg 0.6 O dust for destruction by sublimation and sputtering are much shorter, see Fig. 9 Fig. 2) as carbon, a single CME cannot destroy nanodust by a single hit. A 1 nm Fe 0.4 Mg 0.6 O dust grain would be completely sputtered by a CME if it reached 0.1 AU but will sublimate earlier due to its high temperature.

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Regarding the sputtering and sublimation lifetime of silicate nanodust we find a different situation compared to the aforementioned compositions. The actual lifetimes of silicate nanodust are shown in Fig. 9 c). The sublimation lifetime of Silicate nanodust equals the two day period at distances from the Sun of around 0.15 AU. The complete sputtering of the silicate nanodust during a CME impact occurs at solar distances of 0.2 AU, 0.07 AU and 0.03 AU for the respective grain sizes 1 nm, 5 nm and 20 nm. We can conclude here that a region void of silicate nanodust forms after the passage of a single CME. This region 270 lies between 0.1 and 0.15 AU for the 1-3 nm dust, larger dust rather sublimates than being fully sputtered by a single CME. The existence of this sputtering region is due to the comparably low temperatures of silicate dust that leads to lower sublimation rates for the same distances as compared to the Fe 0.4 Mg 0.6 O dust that is destroyed by sublimation.

Discussion
The results shown in Fig. 9 (a-c) indicate a diverse influence of sublimation and sputtering on the nanodust environment near 275 the Sun. The following remarks shall put the results into a context for current and upcoming dust measurements near the Sun.
When dust particles approach the Sun, they heat up quickly and along with that the sublimation becomes the governing destruction process. One finding is that sublimation for nanodust is much less size dependent compared to the sputtering process. The derived sublimation lifetimes show that the governing parameters are the distance from the Sun, the resulting equilibrium temperature and their composition.

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The sputtering process on the other hand is much more size dependent but also show distinct dependencies for dust composition and the increasingly harsh plasma environment near the Sun. Also the type of plasma environment, i.e. slow or fast solar wind or CME impacts, present in the heliosphere play an important role for the sputtering of nanodust. The importance of sputtering for the destruction of nanodust even at 1 AU can be seen in Fig. 5 where the dust sputtering lifetimes are well below the collisional and Poynting-Robertson lifetimes given by Grün et al. (1985). The change of the nanodust population through 285 sputtering can result in different dust fluxes at 1 AU as expected so far. Additional measurements and dust flux modelling are needed to verify this finding.
As the sputtering and sublimation of nanodust is an important destruction process near the Sun, it can also be expected that the nanodust population changes its composition when approaching the Sun. While nanodust particles have probably a diverse composition at 1 AU and further away, only the most durable nanodust can survive near the Sun. Closer to the Sun, the nanodust population becomes even more variable under the influence of CME impacts. The sputtering lifetimes of nanodust under CME conditions are several orders of magnitude lower than for the solar wind conditions (Fig. 6).
Void zones for silicate nanodust in the small size limit are identified after the passage of a mature CME impact. This finding would impact the nanodust population locally and during certain times, especially at solar maximum conditions when CMEs are frequent (up to 400 per month (Lamy et al., 2019)). This variability of the nanodust population might be quantified by 295 impact measurements onboard of Parker Solar Probe and Solar Orbiter taking sputtering and also sublimation into account.
Together with the onboard plasma and optical instruments further constraints on the near Sun nanodust population are possibly deducted.
The F-corona brightness at mid infrared to visible wavelength can be attributed to thermal emission from micron sized dust particles (Kimura and Mann, 1998). Recent WISPR observations on PSP (Howard et al., 2019) show that F-corona intensity 300 leaves its linearity around 17 solar radii (0.08 AU). These observations would support the existence of the predicted dust free zones within the F-corona (Lamy, 1974;Mann, 1992). In section 2.2 we have identified sputtering by CME impacts as a possible destruction process also for µm-dust. From Fig. 6, we find that a ten µm dust particle can be fully destroyed within three years at a distance of 0.1 AU from the Sun when struck by multiple CMEs (assuming around 100 CMEs per month) and under constant exposure to the solar wind.

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In the end, it has to be noted that the given lifetimes are only valid for dust on near circular orbits. Dust affected by sublimation or sputtering is subject to a constant reduction of its size, which will result in alteration of its present orbit.
The given results only represent a general description of these destruction processes. However, conclusions on the impact of sputtering and sublimation on individual dust grains along their orbits cannot be drawn and are not subject of this work.

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Interplanetary dust enters a harsh environment when approaching the proximity of our central star. Especially the fragile nanodust is prone to destruction through sputtering by the solar wind or sublimation near the Sun. Studies on dust destructions mechanisms near the Sun already showed that there are distinct regions dominated by sublimation and sputtering in the heliosphere (e.g. Mukai and Schwehm, 1981). This study has investigated dust sputtering during more extreme conditions of Coronal Mass ejection (CME) events. CME plasma in addition to its high number density contains a large fraction of heavy 315 ions. We find that dust is sputtered most effectively in the CME case followed by sputtering within the slow solar wind. The weakest sputtering we find in the low-density plasma of the fast solar wind. However, the sputtering process is also very composition dependent. Carbon has been found to be more stable against sputtering than the silicate and Fe 0.4 Mg 0.6 O composition.
The case of nanodust has been studied in more detail for sputtering and sublimation during a the passage of a single CME.
Nanodust free zones can occur after two day CMEs for silicate (0.1 to 0.15 AU) but not for Fe 0.4 Mg 0.6 O and carbon.

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The dust component near the Sun is in the process of being probed in unprecedented detail. While Parker Solar Probe is closing in to a proximity of the Sun as close as nine solar radii, Solar Orbiter will reach 0.3 AU but also observe the Sun outside the ecliptic plane. Both missions carry instruments to measure the local electric field (Bale et al., 2016;Maksimovic et al., 2007) which also enable the detection of dust impacts on the spacecraft. Taking into account sublimation and sputtering will be crucial to the modelling of the measured dust fluxes. This present work gives the needed insights and want to encourage 325 to mind these processes when interpreting the satellite measurements.
The implementation of sputtering and sublimation as destruction mechanisms needs to be included into dust flux models especially for the case of dust in the small size limit. Taking these processes into account is definitely important when considering the dust population near the Sun or other central stars. But also when considering dust trajectory modelling, the rough environment near stars lead to a shrinking of dust particles due to sublimation and sputtering. That leads to an increase of the 330 often used charge to mass ratio of dust in these trajectory models for the small dust component. We expect that integrating the change of the dust size together with its full equation of motion will lead to new insights of the nanodust population near central stars. A recent study by Shestakova and Demchenko (2018) derived the orbital evolution of µm dust within the sublimation zone and included the dust size reduction due to sublimation. They find either elongated dust trajectories after partial sublimation or trajectories leading to complete sublimation after spiralling further into the evaporation zone of the Sun. A future study 335 which also takes the sputtering of dust into account will find deeper insight into the fate of nanodust near the Sun and during the passage of a CME.
Variations in the F-corona intensity has usually been explained by the destruction of dust through sublimation or orbital changes (Lamy, 1974;Mann, 1992). The results of our work have shown that sputtering of micron sized dust during the passage of multiple CME can play a role in the explanation of dust free zones in the F-corona.

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Furthermore, we also expect that standard solar wind conditions can lead to significant sputtering in timescales which are shorter than the dynamical removal times of dust within intermediate distances from the Sun, i.e. 1 AU and greater.
Nevertheless, further laboratory as well as theoretical research is necessary to pin point sputtering yields for small dust grains of various composition. At the moment, experimental, theoretical and modelling results of sputtering yields show a diverse picture where scientific consensus is missing.