Previously hidden low-energy ions: a better map of near-Earth space and the terrestrial mass balance

This is a review of the mass balance of planet Earth, intended also for scientists not usually working with space physics or geophysics. The discussion includes both outflow of ions and neutrals from the ionosphere and upper atmosphere, and the inflow of meteoroids and larger objects. The focus is on ions with energies less than tens of eV originating from the ionosphere. Positive low-energy ions are complicated to detect onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of volts. We have invented a technique to observe low-energy ions based on the detection of the wake behind a charged spacecraft in a supersonic ion flow. We find that low-energy ions usually dominate the ion density and the outward flux in large volumes in the magnetosphere. The global outflow is of the order of 1026 ions s–1. This is a significant fraction of the total number outflow of particles from Earth, and changes plasma processes in near-Earth space. We compare order of magnitude estimates of the mass outflow and inflow for planet Earth and find that they are similar, at around 1 kg s−1 (30 000 ton yr−1). We briefly discuss atmospheric and ionospheric outflow from other planets and the connection to evolution of extraterrestrial life.


Introduction
Space is not empty. The sun is emitting not only electromagnetic radiation but also a solar wind. This unsteady wind includes charged particles (a plasma) and a magnetic field. When this supersonic wind hits the magnetic field of the Earth and is deflected, an enormous cavity is created, the magnetosphere. The visible aurora was studied for hundreds of years without realizing that this phenomenon is caused by charged particles accelerated at several thousand kilometers altitude within the magnetosphere, which then hit the atmosphere (Gilmore 1997, Brekke and Broms 2013). Studies of plasma in the solar wind and the magnetosphere are today closely linked to investigations of laboratory, fusion and astrophysical plasmas (Kivelson and Russell 1995, Aschwanden 2004, Koepke 2008, Koskinen 2011.
A consequence of solar radiation and the solar wind is that matter can escape from planet Earth. In the following we consider ions leaving Earth and discuss how these particles change the properties of near-Earth space. We then compare this outflow with the outflow of neutrals and with the inflow of meteoroids and larger objects.
More than a hundred years ago, magnetic field observations on the ground, and reflections of radio waves, showed the existence of currents and charged particles in what would | Royal Swedish Academy of Sciences Physica Scripta Phys. Scr. 90 (2015) 128005 (11pp) doi: 10.1088/0031-8949/90/12/128005 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. become known as the ionosphere. Before the launch of the first satellites, studies of natural electromagnetic so-called whistler waves indicated the presence of the plasmasphere (Gillmore 1997), figure 1. The ionospheric plasma is created by ionization of the upper atmosphere at altitudes of about eighty to several hundred km by solar UV and EUV radiation, and to some extent by precipitating particles, including auroral electrons at energies from one to tens of keV. The peak plasma density is about 10 6 cm −3 near noontime in the socalled F-layer at about 250 km altitude. The thermal plasma energy is of the order of 0.1 eV (about 1000 K), mainly determined by collisions with the denser neutral gas (Kelley 2009). In this simple ionospheric scenario, essentially all ions have velocities well below escape velocity. In comparison with the solar wind, particle energies in the ionosphere are low. The solar wind velocity of at least a few hundred km s −1 corresponds to energies of about a keV for the major ion species, H + . In the following we discuss low-energy ions leaving the ionosphere, some being energized to several keV, but a major fraction staying cold also far away from Earth.
While collisions are important in the dense ionosphere at altitudes below about 500 km most of the magnetosphere has much lower density and is essentially collisionless. The charged particles are still strongly coupled via electromagnetic fields. Upstream from Earth a collionless shock is formed at about 90 000 km altitude (15 R E , Earth radii) when the supersonic solar wind is deflected by the terrestrial magnetic field. At around 10 R E a current sheet, the magnetopause, is separating the shocked solar wind from the regions dominated by the terrestrial magnetic field. Some of the solar wind energy and particles can enter the magnetosphere behind the magnetopause. One major process permitting mass and energy exchange across this plasma boundary is so-called magnetic reconnection (Priest andForbes 2000, Paschmann et al 2013). Plasma can be further energized in the reconnection processes, transferring energy from the magnetic field to particle kinetic energy.
For many years the solar wind was believed to be the obvious source of magnetospheric plasma, which was consistent with observations. In contrast, we have found that the magnetosphere often is dominated by low-energy (eV) plasma of ionospheric origin (figure 1). These ions are hard to detect onboard a sunlit spacecraft in a low-density plasma. The spacecraft emits photoelectrons and in steady state often charges to several tens of volts positive, preventing positive low-energy ions from reaching the onboard detectors. We show how this problem can be solved. Rather than using a particle detector onboard a spacecraft, we observe the electric field caused by a wake behind a spacecraft in a supersonic ion flow. We discuss the contribution of these ions to the overall mass balance of planet Earth. We conclude by considering atmospheric and ionospheric outflow from other planets and the importance for extraterrestrial life.

Ions in near-Earth space
In the beginning of the space age, scientific spacecraft included instruments to detect high-energy (MeV) particles, leading to the discovery of the so-called Van Allen belts (Van Allen andFrank 1959, Foerstner 2007). These particles are of practical importance since they may damage spacecraft but they contribute only a minor fraction of the mass in the magnetosphere. Later spacecraft detectors could cover the energy range from several eV to hundreds of keV. For a long time it was believed that the magnetosphere was dominated by plasma of solar wind origin. Observations of O + ions at kev energies (occurring only at very low densities in the solar wind) made it clear that ions flowing out from the ionosphere can contribute to the near Earth plasma (Shelley et al 1972, Sharp et al 1977. Today the outflow of H + , O + , He + and other ions from the ionosphere is discussed in review articles and textbooks , Moore and Horwitz 2007. However, these ions typically have energies of at least several eV, while ions at lower energies often have not been possible to detect.
Ions originating in the polar cap ionosphere (regions around the magnetic poles) often have low (eV) energies and can form a supersonic 'polar wind' along 'open' magnetic field lines (connected to the solar wind) expanding into the anti-sunward 'magnetotail' of the magnetosphere (Yau et al 2007). Such a supersonic outflow of low-energy ions was predicted by Axford (1968) and Banks and Holzer (1968). Early observations of this outflow were made at altitudes up to a few thousand kilometers by the Explorer 31 (Hoffman 1970) and DE-1 satellites (Chandler et al 1991). Many of the following studies of the high-latitude ion outflow, including the polar wind, used observations up to several R E by the Akebono, DE-1 and Polar spacecraft , Cully et al 2003, Abe et al 2004, Huddleston et al 2005, Peterson et al 2006, Yau et al 2007. The source of the polar wind can also be observed from the ground by radars, up to altitudes of several hundred kilometers (Ogawa et al 2009). H + , O + and other ions originating in the auroral region equatorward of the polar cap can typically reach higher (keV) energies due to various energization processes Yau 1997, Moore andHorwitz 2007). Due to convection, keV ions from the dayside can appear also above the polar cap. Observations of such ions by the Cluster spacecraft cover altitudes up to about 20 R E (Nilsson et al 2012, Liao et al 2012. However, concerning low-energy ions, at altitudes above a few R E observations of such ions onboard a sunlit and charged spacecraft, have been rare. Ions originating from the equatorial ionosphere often end up in the plasmasphere. This is a torus-like region with lowenergy (eV) and dense (compared to the rest of the magnetosphere) plasma of ionospheric origin, on nearly dipolar magnetic field lines. In the plasmasphere, plasma co-rotates with the Earth, while plasma outside its outer boundary (the plasmapause) is controlled by magnetospheric convection. The plasmapause is typically located inside geostationary orbit at 6.6 R E for moderate magnetic activity (Borovsky and Denton 2008, Darrouzet et al 2008). At high geomagnetic activity the outer regions of the plasmasphere (around 50% of the mass) are removed by increased magnetospheric convection, forming plumes. Plumes are usually found in the afternoon and last for days, but weaken in density, width and flux with time (Chandler et al 1999, Borovsky and Denton 2008, Darrouzet et al 2008, Moore et al 2008, Spasojevic and Sandel 2010. Low-energy ions are found just inside the magnetopause, not only in plasmaspheric plumes (Matsui et al 1999, Lemaire 2006, Lee andAngelopoulos 2014), and indications of a cold 'plasmaspheric wind' have been found (Dandouras 2013). However, as for the high latitude case, observations of low-energy ions onboard a sunlit, and charged spacecraft have been rare.

The problem of hidden low-energy ions
In our definition, low-energy ions have thermal energies, and drift energies relative to Earth, of less than tens of eV. These ions are hard to detect onboard a sunlit spacecraft in a lowdensity plasma. The spacecraft emits photoelectrons and in steady state often charges to several tens of volts positive, preventing positive low-energy ions from reaching the onboard detectors. There have still been suggestions for a few decades that low-energy ions of ionospheric origin can be a significant source of magnetospheric plasma (Chappell et al 1980, 1987, Olsen 1982, Olsen et al 1985, Chappell 2015. However, in most of the magnetosphere it has not been possible to get more than glimpses of this low-energy population. For comparison, in the ionosphere the density is often high enough that the electrons in the plasma will cause the spacecraft to charge negatively. Here an ion detector can determine thermal energies of a few tenths of an eV and upflow velocities of a few hundred m s −1 (Burchill et al 2010).

The way to a solution
Combining data analysis and simulations we have invented a new method to detect low-energy (eV) positive ions from the ionosphere on a positively charged (tens of volts) spacecraft. One important part of the new method is the electric field and wave (EFW) instruments on the four Cluster spacecraft. EFW uses two pairs of spherical probes deployed on wire booms in the satellite spin plane to measure the electric field, figure 2 (Gustafsson et al 2001, Eriksson et al 2006. The probe-toprobe separation is 88 m. The instrument can be used to obtain the electric field by taking the potential difference between two probes. We also use measurements of the potential difference between the probes and spacecraft, often refereed to as the spacecraft potential, to estimate the plasma density , Lybekk et al 2012. The Cluster spacecraft were launched in two pairs during the summer of 2000 (Escoubet et al 2001). After launch the EFW instruments performed well and mainly as expected. However, in large regions in the magnetotail lobes (regions with low density plasma magnetically connected to the polar caps) the observations did not make sense. We expected the lobes to be regions of low activity and with small electric fields. However, rather often large regions with electric fields of several mV m −1 were observed. Although this may at first seem to be small fields, particles accelerated in such an electric field over thousands of kilometers (smaller than the typical lengths scales in this region) would gain keV energies. These particles would easily be detected by onboard particle detectors. One hint to what was going on was that no such high energy particles were observed. Another hint was that when the spacecraft potential control instrument ASPOC was on, the EFW fields looked reasonable. ASPOC emits an ion beam to reduce spacecraft charging, which is beneficial for direct particle observations (Torkar et al 2001). In addition, when ASPOC was on, our observations agreed well with the electron drift instrument (EDI) electric field instrument (Paschmann et al 2001). This instrument is detecting the quasi-static electric field based on a completely different technique: the field is inferred by measuring the drift of artificially emitted high-energy (keV) electrons as they gyrate back to the spacecraft under the influence of the geophysical magnetic field. In the lobes, these electrons have gyroradii of several kilometers, much longer than the EFW wire booms (Eriksson et al 2006). Although it at first seems contradictory, it turns out that both electric field instruments give correct values also when ASPOC is off.
What started out as a dedicated attempt to understand the EFW instrument performance turned out to initiate a new way to detect low-energy ions. The key is that EFW and EDI measure the electric field in different regions. EFW locally (order 100 m) around the spacecraft and EDI averaged over much larger distances (few km). Much of the initial work on this method, and on the geophysical importance of lowenergy ions, was performed by Anders Eriksson and his graduate student Erik Engwall , 2006b, Eriksson et al 2006, Engwall et al 2009a, 2009b.

The solution
Plasma flowing past a spacecraft will create a wake downstream of this obstacle, as has been studied from the beginning of the space age (e.g., Kraus andWatson 1958, Rand 1960). However, most earlier wake studies are not directly applicable to the case of a positively charged spacecraft in a low-density supersonic polar wind.
Low-energy positive ions in a supersonic flow can create an enhanced wake behind a positively charged spacecraft. Here the ions are diverted not by the mechanical structure but rather by the surrounding potential structure , Miyake et al 2013. This enhanced wake can be much larger than the direct wake caused by particles hitting the spacecraft. The conditions for the enhanced wake formation sketched in figure 3 (top) are that the ion flow energy mv 2 /2 not only exceeds the thermal energy KT but also is lower than the equivalent energy of the spacecraft potential eV SC The ion wake will be filled with electrons, whose thermal energy is higher than the ram kinetic energy, in contrast to that of the ions. Due to the large geometric dimensions of the enhanced wake, the negative space charge density can then create a substantial local wake electric field close to the spacecraft. For typical parameters in the polar wind, the local wake has a scale length of 100 m. The wake electric field can be observed with the EFW wire boom instrument, while the electrons emitted by EDI have a much larger gyroradius and are not significantly affected.
For a given velocity, lighter ions such as H + will be more affected by the spacecraft and hence create a larger wake. Figure 3 (bottom) shows an example of a wake electric field observation. The non-sinusoidal repetitive pattern is due to the wake, and the 4 s spin of the spacecraft, and indicates the presence of low-energy ions , 2006b, 2009a, 2009b, André et al 2010.
The electric field observed by the EFW wire boom instrument is a combination of the geophysical and the wake electric fields. The wake electric field is obtained as the difference between the local electric field at the spacecraft (observed by EFW) and the geophysical quasi-static electric field (EDI). The perpendicular E×B ion drift is given by the geophysical electric field and the ambient magnetic field observed by the fluxgate magnetometer (FGM) (Balogh et al 2001). Assuming that the ions are unmagnetized on the wake length scale, the direction of the wake electric field gives the direction of the flow. Since the perpendicular velocity component of the flow is known, the parallel component can be inferred. This technique has been verified in the magnetotail (Engwall et al 2006b), studied with simulations  , and is further discussed by Engwall et al (2009b).
The plasma density can be estimated by calibrating observations of the spacecraft potential obtained by the Cluster EFW instrument , Svenes et al 2008, Lybekk et al 2012, Haaland et al 2012b.
In summary, the presence of a supersonic flow of lowenergy ions can be inferred by detecting a wake electric field, obtained as a large enough difference between the quasi-static electric fields observed by the EFW (total electric field) and EDI (geophysical electric field) instruments. To estimate the drift velocity, observations of the perpendicular E×B drift velocity from the geophysical quasi-static electric field (EDI) and the geophysical magnetic field (FGM) are needed, together with the direction of the wake electric field. The ion flux can then be estimated from the drift velocity and the density. Details concerning the data analysis are given by Engwall et al (2009b) and André et al (2015).

Low-energy ions and a better map of near-Earth space
The new 'wake' method to detect low-energy ions has been used to map these ions both in the magnetotail lobes (Engwall et al 2009a, 2009b and in the equatorial dayside region (André and Cully 2012). The improved map of near-Earth space is shown in figure 1.
To study the outflow of ions from the polar caps into the magnetotail lobes, André et al (2015) used two Cluster spacecraft and ten years of data, 2001-2010, i. e. from the peak of solar cycle 23 to beyond the first minimum of cycle 24. The study includes altitudes from 5 R E geocentric (about 25 000 km altitude), above the denser plasma where other methods are useful, up to 20 R E , the apogee of the Cluster mission. This is a large data set and even after applying rather strict selection rules on the data to minimize errors, the resulting statistics are good.
To study the occurrence of drifting low-energy ions, a total of 1 680 000 data points (4 s spacecraft spins) can be used to search for a wake. We find wakes indicating lowenergy ions in 64% of the data, consistent with earlier studies using smaller data sets (Engwall et al 2009a(Engwall et al , 2009b. Lowenergy ions are common during all parts of the solar cycle . To quantitatively study the outward flux of low-energy ions, even stricter data selection rules are applied by André et al (2015). Using a total of 320 000 data points, they obtain density, outward velocity along the magnetic field, outward ion flux and flux mapped to 1000 km altitude, figure 4. (The flux is mapped downwards to a stronger magnetic field, taking into account the converging magnetic field.) The density increases as a function of increasing solar EUV flux during the solar cycle (as indicated by the F10.7 index, André et al 2015) while the parallel velocity does not vary much. The resulting mapped flux increases about a factor of 2 with increased solar EUV.
Our wake method is more sensitive to lighter ion species since for a given velocity their lower energy will make them more affected by the spacecraft potential and thus create a larger wake. Our method can not tell the difference between ion populations with different masses. However, protons are the dominant ion species in the low-energy high-latitude ion outflow (Su et al 1998), but the ion composition can vary substantially with geomagnetic and solar activity (e.g. Cully et al 2003). As a first approximation, when calculating the ion outflow in the magnetotail, we lower the total density by a factor of 0.8 (Su et al 1998, Engwall et al 2009b. To estimate the total outflow, we assume a total polar cap area of 3-6×10 17 cm 2 (low to high geomagnetic activity), (Li et al 2012, Milan et al 2009. The mapped outward ion flux is about 2 to about 4×10 8 cm −2 s −1 (low to high solar EUV, figure 4). This gives a total outflow, from both hemispheres, of about 0.6-2.4×10 26 ions s −1 , increasing with increasing polar cap size (due to increased geomagnetic activity) and increased solar EUV .
Comparing this low-energy ion outflow observed at high altitude by Cluster with the low-energy ion outflow observed at low altitude (where spacecraft charging is less of a problem) shows good agreement (Peterson et al 2008). At high altitudes, the low-energy ion outflow is higher than the hot ion outflow, see tables of Peterson et al (2006) and Engwall et al (2009a) for a comparison of spacecraft, energies and altitudes.
To detect low-energy ions at the dayside magnetopause, André and Cully (2012) used three complete seasons of Cluster dayside coverage (November 2006 through July 2009). This allows studies of both high-latitude and lowlatitude locations on the dayside magnetopause. Two hours of data (about 1 R E for a stationary magnetopause) just inside a total of 370 magnetopause crossings was searched for lowenergy ions. Three methods were used. The first was based on the detection of a spacecraft wake to find low-energy ions. The second method was based on the fact that occasionally strong electric fields occur close to the magnetopause, accelerating cold ion populations into onboard particle detectors. Such observations can be used to put rough lower bounds on the occurrence of low-energy ions. The third method was to compare partial density moments from onboard particle detectors with density estimates from observations of waves at the plasma frequency. (The plasma frequency is a resonant frequency, proportional to the square root of the density.) The findings are summarized in figure 1.
On the dayside, the outflow of low-energy ions is very variable. When plasmaspheric plumes are not present, the outflow is typically a few times 10 25 ions s −1 integrated over the dayside. In plumes (occurring about 20% of the time) the outflow can be up to 10 27 ions s −1 . Our new results show that low-energy ions can dominate 50%-70% of the time just inside the magnetopause, even when there are no plasmaspheric plumes.
The results in figure 1 give an updated map of the magnetosphere, showing that low-energy ions dominate the density and number flux for large volumes in the magnetosphere, a large fraction of the time. The change in density gives a change in the Alfvén velocity (inversely proportional to the square root of the density) and hence in the velocity of much energy transfer in the plasma. This velocity change also affects the rate of energy conversion in magnetic reconnection. Low-energy ions also change how the microphysics of magnetic reconnection works since the gyro-radius introduces a new length-scale between the gyro-radii of electrons and hot ions (André et al 2010, Toledo-Redondo et al 2015.

Outflow of ions
The average global outflow of H + is of the order of 10 26 ions s −1 . Concerning the outflow of He + , O + and other heavier ions, these ions are less often hidden due to spacecraft charging, since for a given velocity their higher energy will make them less affected by the spacecraft potential. On average, O + dominates the global outflow of heavier ions. Estimates range from less than 10 25 to more than 10 26 ions s −1 , higher at higher geomagnetic activity and for more intense solar EUV radiation (Yau et al 1988, Cully et al 2003, Peterson et al 2006, Parks et al 2015. An order of magnitude estimate of the average over a solar cycle is a few times 10 25 ions s −1 . This would mean that for ions, H + dominates the number outflow, while O + dominates the mass outflow, at about 1 kg s −1 (30 000 ton yr −1 ).
Multiple mechanisms contribute to energization of outflowing ions (Wahlund et al 1992, André et al 1998, Strangeway et al 2005, Moore and Horwitz 2007. As examples, energy can be transferred to the ionosphere from above, via Poynting flux (energy flux of electromagnetic waves) and via downgoing electrons accelerated in the magnetosphere. This can increase the electron temperature and hence the electron scale height, causing an ambipolar electric field starting an outflow of ions. For this process to continue dynamic effects are needed, such as horizontal convection of new ionospheric plasma into the region where such heating occurs. When the ions have reached collisionless altitudes other mechanisms can add energy, and the ions can obtain keV energies. Waves can heat the ions in the direction perpendicular to the magnetic field and the magnetic mirror force will then cause outflow in the diverging geomagnetic field. Ions can also be accelerated upward by the parallel electric fields causing downward energization of auroral electrons.
Above we have discussed only oxygen as the heavier particles (larger mass than hydrogen) originating from the atmosphere and ionosphere. In the thermosphere, above around 100 km, photodissociation of O 2 is fast, while recombination is slow. In contrast, the N 2 bond is more difficult to break directly, and should this happen the molecule is rapidly regenerated through reactions with NO molecules (Strobel 2002). Although N 2 is more abundant than O 2 in the lower atmosphere, processes in the upper atmosphere make O and O + more common than the heavier O 2 , + O , 2 N 2 and + N 2 at a few 100 km (Ghosh 2002). The exobase is the altitude where collisions are no longer important, for Earth about 500 km. There are some observations of N + ions in the magnetosphere  and some mass spectrometers on spacecraft do not have the resolution to distinguish between oxygen and nitrogen. While other species contribute, O + ions seem to dominate the outflow of heavier species. These ions are common around the exobase and nonthermal electromagnetic mechanisms can energize these ions.

Outflow of neutrals
To put the outflow of ions into perspective, we discuss the total mass outflow from planet Earth. Here we regard the solid Earth together with the collisional atmosphere and ionosphere as one system. Hence we disregard processes such as volcanism and combustion of fossil fuels and condensation into the hydro-and litospheres. The estimates for both outflow and inflow are still just order of magnitude calculations, see also Engwall (2009).
For the escape of neutrals, an obvious process is the loss of particles from the high-energy tail of the velocity distribution at the exobase. This thermal process is sometimes called Jeans escape (Jeans 1925). Estimating this process is complicated by the fact that particle distributions at the exobase are not Maxwellian at high energies due to the constant loss of particles. Using typical conditions at the exobase gives a global outflow of the order of 10 26 particles s −1 for H, much smaller for He, and yet again much smaller for heavier species such as O (Hunten and Strobel 1974, Hunten 2002, Strobel 2002. The major non-thermal process leading to the escape of neutrals is charge exchange (Shizgal andArkos 1996, Lie-Svendsen et al 1992). In typical reactions plasmaspheric protons (energy of the order of 1 eV) hit H or O atoms with much lower energies in the upper atmosphere. The resulting H atoms often have velocities above escape velocity, while the resulting oxygen atoms usually do not. (The escape velocity at the exobase is 11 km s −1 , which for H corresponds to an energy of 0.6 eV, and for the heavier O to nearly 10 eV). The estimated global order of magnitude outflow is similar to that of the neutral thermal outflow, 10 26 particles s −1 , and again hydrogen dominates. The hydrogen originates mainly from the oceans, releasing oxygen into the atmosphere (Hanslmeier 2007, Engwall 2009).

Inflow
A large fraction of the mass inflow to planet Earth is from meteoroids and larger objects, hitting the atmosphere. Much of the number flux of material hitting the atmosphere comes in the form of small particles, less than about a mg, or one mm in radius (Love andBrownlee 1993, Ceplecha et al 1998). Different techniques are used to detect incoming particles of different sizes, and the inflow varies with location, day of the year and time during the day (Szasz et al 2004, Mann et al 2011. For observations in the atmosphere, meteor radars and optical camera networks are used (Ceplecha et al 1998, Flynn and Sutton 2006, Campbell-Brown et al 2012. Smaller particles can be measured by impact detectors on spacecraft (Love and Brownlee 1993). Also, studies of marine isotope records can be used to estimate the long term average inflow (Peucker-Ehrenbrink 1996).
Some contribution comes from much larger objects, such as the Chelyabinsk airburst 2013 and the Tunguska event 1908, caused by objects tens of meters across (Popova et al 2013).
To include larger and much less frequent impacts, lunar craters and observations of near-Earth asteroids can be taken into account (Ceplecha et al 1998). Over geological timescales large objects can dominate the average mass inflow and also carry enough energy to change the global conditions for life (Toon et al 1997). For a typical year, many estimates are consistent with a total inflow of at least a few tons d −1 (100 g s −1 ) to hundreds of tons d −1 (a few kg s −1 ) (Love and Brownlee 1993, Engwall 2009, Mann et al 2011, Plane 2012. This is similar to our estimates of the mass outflow from Earth. Also charged particles can reach the collisional ionosphere. Electrons accelerated to keV energies cause auroras when they hit the upper atmosphere, but carry only a small mass flux. Some ions can also precipitate and cause so-called proton auroras (e.g. Frey et al 2003). Also, studies related to the investigations of outflowing low-energy ions show that many of these ions do not directly leave the magnetospheric system. Rather, they reach the plasma sheet (the current sheet in the magnetotail), are probably energized, and circulate in the magnetosphere (Haaland et al 2012a, Li et al 2012. However, statistics from low altitude spacecraft indicate that only a small fraction of these ions return to the ionosphere (Newell et al 2009). One reason that only a few ions reach the collisional ionosphere is that they have to travel downward nearly along the magnetic field, in the so-called loss-cone, in order not to be reflected by the magnetic mirror force.

Other planets, moons and exo-planets
The upper atmospheres of other solar system bodies are ionized in a way similar to what happens at Earth, creating ionospheres. In the case of Mars, with a well developed ionosphere but no global magnetic field, interaction with the solar wind leads to the formation of an induced magnetosphere. Observations during high solar activity with a Langmuir probe on Phobos-2 indicated regions of low-energy plasma and the total outflow was estimated to a few times 10 25 ions s −1 (Nairn et al 1991) consistent with recent analysis of particle detector data from the same spacecraft (Ramstad et al 2013). Some properties of low-energy ion outflows from the ionosphere can be detected with particle instruments, e.g., on Mars Express (Lundin et al 2009) and careful statistical treatment including low energy particle data indicates an outflow of heavy ions during low solar activity of 2×10 24 ions s −1 (Nilsson et al 2011) varying with solar EUV flux (Lundin et al 2013). However, results from the sounding radar MARSIS onboard Mars Express show high plasma densities in regions around Mars where almost no plasma is detected by particle instruments (Dubinin et al 2008). This suggests a situation similar to what is being revealed around Earth: low-energy plasmas are more important than previously anticipated. On Venus, the situation may be similar. Observations with Langmuir probes on Pioneer Venus Orbiter indicated regions of low-energy plasma (Brace et al 1987). Particle detectors give a global outflow of the order of 10 25 ions s −1 using Venus Express and Pioneer Venus Orbiter data (Barabash et al 2007, Fedorov et al 2011, Nordström et al 2013 but ions at a few eV cannot always be reliably detected with previous or present instruments. The planet sized moon Titan orbits most of the time inside the magnetosphere of Saturn. Langmuir probe data from multiple Titan flybys of the Cassini spacecraft indicate a plume of lowenergy ionospheric plasma (Edberg et al 2011, Coates et al 2012. The associated outflow is estimated to around 10 25 ions s −1 , showing the importance of low-energy plasma around yet another celestial body.
It is possible to detect the outflow of particles from planets from a distance, also for exo-planets orbiting distant stars. HD 209458b is a Jupiter-type gas giant planet orbiting its host star HD 209458, which is a solar-like G-type star with an age of about 4 Gyr. Absorption in the stellar Lyman-a line observed during the transit of HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet (Vidal-Majar et al 2003) and the escape mechanisms are compared to mechanism in our solar system (Holmström et al 2008, Linsky et al 2010. Also studies of more Earth-like exoplanets in the habitable zone with temperatures suitable for liquid water are in progress (Kislyakova et al 2013). The evolution of atmospheres and ionospheres is an important factor when considering the conditions for biological life on other planets and moons (Yamauchi and Wahlund 2007, Lammer et al 2008. Parameters describing distant stars and planets vary, but the basic physical mechanisms remain the same. Understanding the outflow of planetary atmospheres in our solar system is one important piece in understanding the possibilities for life to evolve on exo-planets in the rest of the Galaxy.

Summary
The ion density and number flux in most of the volume of the Earth´s magnetosphere is dominated by low-energy (eV) positive ions from the ionsophere. Previously these ions could often not be detected on sunlit, positively charged, spacecraft. We use a technique based on the detection of the wake behind a charged spacecraft in a supersonic ion flow to detect these low-energy ions. Including this low-energy ion component gives a better map of the plasma abundance in near-Earth space. This updated map improves our understanding of plasma processes such as the speed of Alfvén waves and associated energy transport. This update also improves our knowledge concerning the rate and micro-physics of magnetic reconnection.
The order of magnitude global outflow of particles originating as low-energy ions, mainly H + , is 10 26 ions s −1 , increasing with geomagnetic activity and solar EUV radiation. This is similar to the estimates of thermal escape, and escape due to charge exchange, of mainly neutral hydrogen from the upper atmosphere. On average this gives an order of magnitude estimate of the global outflow of ionized and neutral hydrogen of a few times 10 26 particles s −1 . The estimated number outflow of O + ions is more variable and lower, with an average around a few times 10 25 particles s −1 . Due to their larger mass these particles might still dominate the average mass outflow, at around 1 kg s −1 . A large fraction of the mass outflow seems to be due to charged particles. One reason is that electromagnetic energy originating from the solar wind can give escape velocity to a small fraction of the upper ionosphere also when there is not at all enough energy to heat the bulk of the atmosphere.
The particle number inflow on a typical year is dominated by bodies smaller than about 1 mm in radius, with an estimated order of magnitude total mass inflow of about 1 kg s −1 . Hence, averaged over a solar cycle of about 11 yr, present reasonable order of magnitude estimates of the global mass outflow and inflow to Earth are similar, at 1 kg s −1 (30 000 ton yr −1 ).
The mass of Earth´s atmosphere is about 5×10 18 kg. Even without any inflow or supply from the litho-and hydrospheres, the present outflow on geological timescales of a Gyr corresponds to less than one percent of the total mass. On such long time scales, impacts of large (km) size asteroids and comets can be a major fraction of the mass inflow but can also remove significant parts of the atmosphere.
Several processes determine the stability of an atmosphere of a planet or large moon. Biological life as we know it benefits from a stable atmosphere. Understanding atmospheric and ionospheric outflow from Earth and other planets in our solar system is one important step to understand the possibilities for life to evolve on exo-planets around distant stars.