Enhanced Acceleration and Escape of Hydrogen Pickup Ions Upstream from Mars by Interplanetary Shocks

The pickup process of newly ionized hydrogen (H) from the Martian extended exosphere represents an important H+ escape channel on Mars. It is generally believed that interplanetary (IP) shocks can accelerate the pickup ions and, in turn, affect the pickup process. However, the underlying processes inherent to the acceleration are not yet fully understood. Here, we concentrate on the dynamic processes involved in acceleration arising from IP shock compression. We examine two typical IP shock events characterized by a sudden increase in the average energy of upstream H+ pickup ions across the shock. The H+ pickup ions continuously enter the field of view of Supra-Thermal And Thermal Ion Composition or Solar Wind Ion Analyzer on board the Mars Atmosphere and Volatile EvolutioN spacecraft in a narrow angular range. Moreover, they correspond to a similar part of their respective ring distributions in velocity space. By comparing measured and theoretical H+ pickup ion energies, we attribute the more energetic H+ pickup ions to the enhanced convection electric field acceleration caused by the shock compression. An increase in the guiding center drift speed across the shock implies a higher escape rate of the H+ pickup ions. Furthermore, the pitch-angle scattering could facilitate the escape of the higher energy H+ pickup ions from Mars. The results may shed light on the understanding of the energization and escape of planetary pickup ions in the solar system.


Introduction
The Martian hydrogen (H) exosphere or H corona, populated by thermal H atoms, was first detected by Mariner-6 and Mariner-7 at the wavelength of Lyα (Anderson & Hord 1971;Anderson 1974).The thermal H atoms originated from the photodissociation of water vapor near the Martian surface (<20 km), implying that neutral H is a crucial indicator of the water loss on Mars (Hunten & McElroy 1970;McElroy & Donahue 1972;Parkinson & Hunten 1972;Bhattacharyya et al. 2017).The generated H atoms diffuse upward to the exobase (∼200 km), where a portion of H atoms in the high energy tail of the Maxwell-Boltzmann distribution overcome the gravitational attraction of Mars, and thus can potentially escape from the planet through Jeans process (Shizgal & Arkos 1996;Jakosky et al. 2018), and help to create the H exosphere.The H exosphere can extend out to large distances, up to ∼10 R M (R M ∼ 3400 km is the Mars radius).Once exospheric H atoms are ionized through photoionization, charge exchange, and electron impact, the newborn H + ions are immediately picked up by the solar wind electromagnetic fields, and then accelerated by the solar wind convection electric field E, precessing along cycloidal trajectories perpendicular to the local magnetic field B with a guiding center drift speed V gc = (E × B)/B 2 in the Mars rest frame.In velocity space, the cycloid translates to a ring.The ring distribution is unstable to plasma waves and instabilities, and subsequently, the ion distribution becomes scattered in pitch angle.The H + pickup ions will eventually be isotropized and move with the solar wind flow.Note that some H + pickup ions created upstream of Mars gyrate downstream back toward Mars, and then they may undergo reflection from the Martian bow shock (BS; Dubinin et al. 2006).These reflected pickup ions will eventually escape into interplanetary (IP) space.It is now widely recognized that the loss of H + pickup ions to the solar wind represents an important H escape channel from Mars (Dubinin et al. 2012;Halekas et al. 2017;Rahmati et al. 2017Rahmati et al. , 2018)).
Past observations have established that intense space weather events, typically associated with the passage of interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs), can significantly affect the state of the Martian atmospheric environment, the ionization rate of neutral atoms, as well as the plasma boundaries (i.e., BS and magnetic pileup boundary), leading to increased production rates of newborn H + ions and accelerated H + pickup ion loss.For example, Romanelli et al. (2018) analyzed the interaction between an ICME and Mars on 2017 September 13 by means of simulation and Mars Atmosphere and Volatile EvolutioN (MAVEN) measurements, and found that the inferred H + loss rate, including upstream H + pickup ions, was increased by a factor of 10, as a result of the ICME passage through the Martian plasma environment.Zhao et al. (2023aZhao et al. ( , 2023b) ) performed a statistical investigation of the occurrence rate of upstream proton cyclotron waves generated by ion/ion instabilities due to the interaction between the solar wind and the pickup H + ions using MAVEN observations.Their results indicate an enhancement of upstream pickup H + production during the passage of ICMEs and SIRs, mainly due to the increased exospheric H density above the Martian BS and the elevated solar wind proton and electron fluxes (i.e., higher ionization rates of exospheric H atoms via charge exchange and electron impact).
In addition to ICMEs and SIRs, IP shocks driven by them can also lead to significant enhancements of ion loss via the pickup process.For instance, Curry et al. (2015) found that the plume O + ions (escaping pickup heavy ions along the solar wind electric field direction are termed plume) were accelerated immediately after the arrival of the IP shock driven by the 2015 March 8 ICME.In theory, there exist two main categories of acceleration mechanisms: acceleration by enhanced electric field due to IP shock compression, and acceleration by the shock itself, such as diffusive shock acceleration (DSA; Axford et al. 1977;Bell 1978aBell , 1978b;;Blandford & Ostriker 1978), shock drift acceleration (SDA; Pesses et al. 1982), and shock reflect or further shock-surfing acceleration (Lee et al. 1996;Zank et al. 1996).SDA and shock reflect (non-Fermi mechanisms) are often dominant at quasi-perpendicular shocks, while DSA dominates at quasi-parallel shocks via the firstorder Fermi mechanism.Certainly, a combination of DSA with other processes, such as magnetic reconnection, might overcome the conceptual difficulties of DSA at quasi-perpendicular shocks.It should be mentioned here that most of (∼92%) the IP shocks propagating close to the Martian environment are quasiperpendicular shocks (Huang et al. 2021).Additionally, stochastic acceleration (mainly via second-order Fermi mechanism), adiabatic acceleration, and wave-particle energization (if plasma waves are induced by shock) might also be involved on some occasions.More recently, by examining distributions of plume ions before and after a quasi-perpendicular IP shock, He et al. (2023) gained valuable insights into acceleration mechanisms of pickup heavy ions.Their results indicate that the acceleration mechanisms related to the shock itself probably did not take part in the acceleration process, because the pickup heavy ions could overcome the electrostatic shock potential and proceed downstream, owing to their large energy.Instead, the enhanced convection electric field E caused by the IP shock compression primarily contributed to the energization of the pickup heavy ions in the magnetosheath.Despite recent progress based both on simulations and observations, some aspects of the dynamics and the acceleration process of pickup ions with different charges and masses associated with the shock arrival have not yet been fully understood.Moreover, one may anticipate that the detailed scenario for light ions like upstream H + pickup ions is somewhat different from the heavy species, and correspondingly, the effectiveness of escape is also different.Nevertheless, a comprehensive survey of the H + pickup ions response to IP shock impact has not been undertaken, partly because of the scattering of solar wind protons within the instrument that obscure the pickup H + ions signature in the data.
We intend to explore the dynamics and the acceleration process of upstream H + pickup ions associated with the arrival of IP shocks using in situ measurements from the MAVEN spacecraft.MAVEN is outfitted with a comprehensive suite of plasma and magnetic field instruments (Jakosky et al. 2015), including two ion spectrometers, namely Supra-Thermal And Thermal Ion Composition (STATIC; McFadden et al. 2015) and Solar Wind Ion Analyzer (SWIA; Halekas et al. 2015), which are able to detect pickup ions over a much wider field of view (FOV), up to 70% of the sky in 64 look directions.More details on the STATIC and SWIA instruments are provided in the next section.Our preliminary efforts have been made toward confident identification of H + pickup ions from the ion data set, by visually surveying the difference in energy per charge between pickup ions and solar wind ions in ion energy spectra (i.e., pickup ion signatures), and by analyzing the characteristic ion velocity distribution functions (VDFs).Then we examine 38 fast forward IP shock events detected by MAVEN from 2014 October to 2018 November (see Huang et al. 2021) for the signatures of acceleration of upstream H + pickup ions associated with the arrival of IP shocks.As expected, we find evidence that both of the mechanisms, i.e., acceleration by the enhanced electric field due to IP shock compression and acceleration by the shock itself, can lead to the energization of upstream H + pickup ions.The former is a relatively simple physical scenario, but essential for understanding the loss of H to space, particularly in early Martian history when IP shocks driven by strong solar storms (e.g., ICMEs) are believed to occur more frequently.In the present paper, we report two representative events of this scenario.In contrast, the microphysics processes involved in the latter scenario are much more complex and will be described in detail in a subsequent paper.

MAVEN Data
The MAVEN spacecraft has been orbiting Mars since 2014 September.It is in an elliptical orbit with periapsis altitude of ∼150 km and apoapsis altitude of ∼6500 km, and precesses in both local time and longitude (Jakosky et al. 2015).During periods of the mission when apoapsis is located upstream of the BS, it provides in situ measurements in the solar wind.
The primary ion data used in this study are collected from STATIC (McFadden et al. 2015) and SWIA (Halekas et al. 2015), which are able to measure the 3D distribution of the lower energy pickup ions created closer to Mars.STATIC measures ions over an energy range of 0.1 eV up to 30 keV with a 15% energy resolution, and a 4 s base time resolution.It consists of a toroidal top-hat electrostatic analyzer with a 360°× 90°FOV and a time-of-flight velocity analyzer, which enables the instrument to distinguish ion species with a mass resolution of 25%.It can be used to distinguish between pickup ions of different species.The ions in the mass range of 0.75-1.25 amu are taken as H + in this study.Two STATIC data products named "d0" with a time resolution of 32 s and "d1" with a time resolution of 4 s, including 32 energy bins, 16 azimuth bins, 4 elevation bins, and 8 ion mass bins, are used to analyze ion VDFs and ion energy spectra, respectively.SWIA is a toroidal energy analyzer with electrostatic deflectors that measures ions over a broad 360°× 90°FOV.It provides 3D velocity distributions of ions over an energy range of 25 eV-25 keV with a 14.5% energy resolution.SWIA does not discriminate between ion species.The proton moments (the proton velocity, density, and temperature) could be computed from the Fine 3D velocity distributions under the assumption that all ions are protons.The onboard-computed proton moments and ion energy spectra derived from the Fine 3D data with 4 s time resolution are utilized.The coarse data products with 8 s time resolution are also used to analyze ion energy spectra.In the coarse data, each of the two neighboring energy bins are grouped together, resulting in 48 energy bins; the 10 fine anodes are binned in groups of 5, resulting in two 22°.5 bins in the solar wind direction, which in addition to the 14 coarse anodes create a total of 16 azimuth bins of 22°. 5, and there are four elevation bins with a resolution of 22°.5.It is important to point out that although SWIA does not have the capability to discriminate ions based on mass, it can distinguish pickup ions from solar wind protons in some cases when they have clearly different energies per charge.Thus, the FOVs of STATIC and SWIA can complement each other in pickup ions detection.Note that only pickup ions created within a few R M can be detected by STATIC and SWIA, owing to their relatively low energy coverage and geometric factor (Rahmati et al. 2017).
In addition, Magnetometer (MAG) provides vector magnetic field measurements with a sampling frequency of 32 Hz with 0.05% absolute vector accuracy (Connerney et al. 2015), and the Solar Wind Electron Analyzer (SWEA) provides the energy and angular distributions of 3-4600 eV electrons (Mitchell et al. 2016).The MAVEN data are utilized in the Mars Solar Orbital (MSO) coordinate system, in which X points from Mars toward the Sun, Y points in the opposite direction of the Mars orbital velocity component perpendicular to X, and Z completes the right-handed system.

Observation and Analysis
It is well known that a sudden jump of the ion energies at shock in ion energy spectra is a clear manifestation of ion acceleration by shock compression.In order to identify such a signature in the ion energy spectra, we first pick out representative events that meet the following criteria: (1) the H + pickup ions continuously enter the FOV of either or both ion instruments at times both before and after the shock passage; (2) the H + pickup ions have energies quite different from those of solar wind ions and ghost counts (scattered solar wind ions), allowing for a clear distinction of H + pickup ions; (3) the recorded H + pickup ions correspond to a similar part of their respective ring distributions in the velocity space.As we will see later, these criteria enable a quantitative comparison between measurements and theoretical calculations, and thus aid in revealing pickup ion acceleration due to shock compression.
Among the 38 fast forward IP shock events, 26 events are found to satisfy the above criteria.In this section, we present two typical events of upstream H + pickup ion acceleration by shock compression.One occurred on 2016 July 19 and the H + pickup ions were recorded by STATIC (Event 1).The other occurred on 2016 August 14, and the H + pickup ions were recorded by SWIA (Event 2).

Shock Event on 2016 July 19
Figure 1 shows the MAVEN measurements during the period 19:30-20:30 UT on 2016 July 19, encompassing a fast forward IP shock.The shock arrived at the MAVEN spacecraft at 20:05:26 UT, when the spacecraft was located in the upstream solar wind (Figure 1(a)).The shock is clearly distinguished by sharp increases in proton speed, density, and temperature, along with abrupt increases in magnetic field magnitude and disturbance level.Moreover, the ion and electron spectra are significantly broadened and strengthened across the shock.It is a quasi-perpendicular shock with shock angle θ Bn = 83°.4(the angle between the shock normal and the upstream magnetic field) and driven by an SIR, which passed the MAVEN spacecraft during 2016 July 19-21 (Huang et al. 2019(Huang et al. , 2021)).In order to investigate the potential impact of this shock on the pickup ion dynamics, we first analyze the changes in the plasma and magnetic field parameters across the shock.The proton speed V p increases from ∼347 to ∼400 km s −1 , predominantly along the −X direction in the MSO coordinates.The magnitude of magnetic field B increases from ∼2 to ∼6 nT, whereas the direction of B remains roughly the same, predominantly aligned in the +Y direction.As a consequence of the changes in V p and B, the angle θ between V p and B remains almost unchanged.The convection electric field E (E = −V p × B) is enhanced by a factor of about 2.4, from ∼0.7 to ∼1.7 mV m −1 , mostly in the +Z direction.The H + pickup ions with energies near or at their maximum energy E max that they can achieve continuously enter the FOV of STATIC during the time interval shown in Figure 1.The maximum energy is given by 2 , where m is the mass of H + ions, V p is the proton velocity, and θ is the angle between the proton velocity and the magnetic field direction.As an example, the recorded H + pickup ions in the look direction A14D1 are illustrated in Figure 1(e).A notable characteristic of the H + pickup ion population is the rapid increase in the ion energies at the shock, from ∼2.1 to ∼2.8 keV, and thus there is no temporal ambiguity for the IP-shock-related phenomena.
Figure 2 shows H + energy-time spectrograms for all 64 look directions of STATIC, with 4 elevation bins separated horizontally and 16 anodes (azimuth bins) separated vertically, during the interval 19:30-20:30 UT.The white curve overlaid on each spectrogram represents the E max that H + pickup ions can achieve.The solar wind ions are mainly present in four look directions, namely, A12D1, A12D2, A13D1, and A13D2.The scattered solar wind ions in the instrument are seen as ghost counts that uniformly spread among all look directions.The H + pickup ions with energies near their maximum energy are barely contaminated by solar wind ions and ghost counts, and thus can be clearly distinguished.During the time interval of ±5 minutes around the shock arrival, they continuously enter the FOV of STATIC in a narrow angular range, corresponding to A14D0, A14D1, and A14D2.The H + pickup ion energies jump across the shock.As noted earlier, the direction of the magnetic field remains roughly stable across the shock (pickup ions gyrate about the magnetic field in cycloid motion), and we can infer that the H + pickup ions detected after the shock impact likely originate from similar upstream source regions as those detected before the shock arrival.In any case, however, this is not a necessary condition for the present diagnostics of upstream H + pickup ion acceleration by shock compression.
In order to understand the dynamics of the recorded H + pickup ion population, we perform a detailed analysis of the H + ion VDFs measured by STATIC.The "d0" data product with a time resolution of 32 s is used.Figure 3 shows the characteristic H + ion VDFs perpendicular to the local magnetic field at times before and after shock.The red dashed circle with a radius q V sin p , where V p is the proton velocity and θ is the angle between V p and B (Figure 1(j)), represents the ideal ring distribution of newborn H + pickup ions.Considering that H + ion VDFs have a denser solar wind proton core component, the component with a radius 0.6 times V p (proton velocity perpendicular to the local magnetic field) in the velocity space is removed (Lin et al. 2022 and references therein).
At times 1-3 minutes before the shock arrival (Figures 3(a) and (b)), we can see two distinct H + ion populations in the VDFs.One is H + ions with energies of ∼2.1 keV and relatively low phase space density of ∼10 −15 sec 3 m −6 , near the middle phase of the ring distribution (i.e., near the maximum energy of the ring distribution).They are local H + pickup ions in the upstream region, primarily corresponding to the H + pickup ions seen in the directions A14D0, A14D1, and A14D2 (Figure 2).They might originate from several R M upstream of Mars (Rahmati et al. 2017), gyrate back toward Mars, and enter the FOV of STATIC near the peak of their cycloidal trajectories.The other population consists of H + ions with higher phase space density of ∼10 −13 sec 3 m −6 and lower energies.It is probably composed of both ghost counts and solar wind protons that have not been completely removed.This ion population occurs persistently, and will not be described again later.
Around the shock arrival time (20:05:09-20:05:41 UT; Figure 3(c)), we also see the H + pickup ions near the middle phase of ring distribution, but with higher energies of From top to bottom: (b) the spacecraft altitude and position in units of the radius of Mars R M , (c) ion energy spectra, (d) electron energy spectra, (e) H + energy spectra for the look direction of STATIC A14D1, overlaid with the calculated maximum energy E max curve for H + pickup ions (white curve), (f) proton density, (g) proton temperature, (h) proton velocity components in MSO coordinates and proton bulk speed, (i) magnetic field components in MSO coordinates and magnetic field magnitude, (j) the angle between the proton velocity and the magnetic field direction, (k) convection electric field components in MSO coordinates and electric field magnitude, and (l) guiding center drift speed of pickup ions.
∼2.81 keV.Correspondingly, the ring has a larger radius, mainly due to the increase of the proton speed V p (note that the angle θ between V p and B does not show pronounced changes across the shock, see Figure 1(j)).This indicates that H + pickup ions created upstream of MAVEN gain more energy from the enhanced E, as they reach the peak of their cycloidal trajectories.At times 1-5 minutes after the shock arrival (Figures 3(d)-(f)), the H + pickup ions exhibit similar characteristics to those in Figure 3(c).
As stated above, the H + pickup ions correspond to a similar part (i.e., near the middle phase) of their respective ring distribution at times before and after the shock passage.This offers us an opportunity to qualitatively understand the pickup ions energization associated with the shock.We take the look direction A14D1 as an example, and conduct a comparison analysis between the calculated H + pickup ion energies gained from E, based on the pickup ion motion equation (where the pickup ion transport is dictated by the Lorentz force and convection electric field), and the observed values.The calculation assumes that the newborn ions are created with approximately zero velocity in the planetary rest frame, and they enter the FOV of STATIC at time   with STATIC measurements are shown in Figure 4(b).The slope of the linear regression between observed and calculated energies is 1.02, with a correlation coefficient of 0.96.This result implies that the enhanced E is the primary factor contributing to the enhanced ion energization.It is worth noting, however, that the H + pickup ions have traveled a shorter distance along E to reach the peak of their cycloidal trajectories after the shock passage, compared to the scenario before the shock passage.

Shock Event on 2016 August 14
Figure 5 shows the in situ measurements of a fast forward IP shock that arrived at the MAVEN spacecraft at 16:38:51 UT on 2016 August 14, when the spacecraft was located outside of the BS (Figure 5(a)).This shock is characterized by increases in proton density, proton temperature, proton velocity, and magnetic field magnitude, together with broadened electron spectra and ion spectra.It is a quasi-perpendicular shock with shock angle θ Bn = 51°and driven by an ICME, which was encountered by MAVEN from 21:47 UT on August 14 to 19:42 UT on August 15 (Huang et al. 2021;Zhao et al. 2021).In much the same way as Event 1, we analyze the changes in the plasma and magnetic field parameters across the shock.The proton speed V p increases slightly from 330 to 357 km s −1 , predominantly along the −X direction.The magnitude of magnetic field B is enhanced by a factor of about 1.5, from ∼2.2 to ∼3.4 nT, and the enhancement occurs in both +Y and +Z directions first and then mostly in the +Y direction.As a consequence of these changes in V p and B, the convection electric field E increases from 0.6 to ∼1.2 mV m −1 , in both +Y and +Z directions first and then mostly in the +Z direction.The angle θ between V p and B changes from ∼45°to ∼90°.As we shall see later, the H + pickup ions with energies lower than their maximum energy E max continuously enter the FOV of SWIA at times both before and after the shock passage.As an example, the recorded H + pickup ions in the look direction A15D3 are illustrated in Figure 5(e).In response to the shock arrival, the average energies of the H + pickup ion population increase rapidly, from ∼1.2 to ∼2.0 keV.continuously enter the FOV of SWIA in a narrow angular range, specifically A0D3, A15D3, A14D3, and A13D3.It is interesting to note that their average energies appear to vary linearly with E max , approximately given by 0.8E max .This implies that the recorded H + pickup ions correspond to a similar part of their respective ring distributions in the velocity space.This feature enables a quantitative comparison between measurements and theoretical calculations, as described below.
We take the look direction A15D3 as an example and conduct a comparison analysis between the calculated H + pickup ion energies gained from E, based on the pickup ion motion equation, and the observed values.The calculation also assumes that the newborn ions are created with approximately zero velocity in the planetary rest frame, and they enter the FOV of SWIA at time t = (n + 0.35)T (n is a positive integer or zero; T is the gyroperiod of H + pickup ions), when the H + pickup ions obtain energies of 0.8E max .The intervals between the pairs of yellow dashed lines, before and after the shock, indicate the shock upstream (16:35:27 UT-16:38:27 UT) and downstream (16:39:15 UT-16:42:15 UT), over which our analysis takes samples from (Figure 7(a)).Comparisons of H + pickup ion energies from the calculations and SWIA measurements are shown in Figure 7(b).The slope of the linear regression between observed and calculated energies is 1.02, with a correlation coefficient of 0.95.This result clearly demonstrates that the enhanced ion energization is mainly attributed to the enhanced E. The H + pickup ions have actually traveled a shorter distance along E at the time they reach the sampling point when compared to the scenario before the shock passage.The H + pickup ions have enough space to reach the peak of their gyromotion and reach the maximum pickup energy by means of convection electric field acceleration, although not recorded by SWIA.The enhancements of the maximum pickup energy across the shock are dictated by the increase in the proton speed and the changes in the direction of the magnetic field.

Summary and Discussion of Implications
In order to understand the energization processes of upstream H + pickup ions associated with IP shock compression, we have examined two shock events, witnessing a sudden increase in the average energy of upstream H + pickup ions across the shock.Another notable feature in the two events is that the H + pickup ions continuously enter the FOV of STATIC or SWIA on board the MAVEN spacecraft in a narrow angular range during the time interval of ±5 minutes around the shock arrival, thanks to their favorable FOV configurations.Moreover, they correspond to a similar part of their respective ring distributions in velocity space.By comparing H + pickup ion energies from theoretical calculations based on the pickup ion motion equation and measurements, we have found that the more energetic H + pickup ions are consistent with being accelerated by enhanced convection electric field caused by shock compression.
This finding carries important implications for H + escape from Mars, first via the direct relationship between the pickup ion drift speed and the escape rate.The guiding center drift speed V gc is given by q , where V p is the proton speed, and θ is the angle between V p and B. It often experiences a remarkable increase across the shock owing to the increase in V p there, which is closely related to the shock strength, except that θ decreases significantly.Referring back to Figures 1(l) and 5(l), we note that the drift speed V gc increases abruptly from ∼320 to ∼370 km s −1 across the shock in Event 1, and from ∼260 to ∼320 km s −1 across the shock in Event 2. As a consequence, the escape rate of the H + pickup ions could have been substantially elevated following the passage of the two shocks.
On the other hand, the pitch-angle scattering of the more energetic H + pickup ions can also lead to a higher escape rate.Specifically, the ion pickup process produces ring distributions in velocity space, which are unstable to ambient and selfgenerated plasma waves.The wave-particle interactions cause pitch-angle scattering of the H + pickup ions.Theoretically, the initial unstable velocity space ring distribution of the H + pickup ions evolves toward bispherical shell distributions  (Coates 2004).Furthermore, diffusion in energy may take place but on a longer timescale.Note that observationally the pitch angle and energy diffusion characteristics are often ambiguous at Mars, mainly owing to the scattering of solar wind protons within the instrument, to our best knowledge.The pitch-angle scattering can cause the higher energy H + pickup ions to escape from Mars more quickly, particularly for those moving along the mean magnetic field direction, which results in an enhanced escape rate of the H + pickup ions.Additionally, a higher escape rate of the H + pickup ions might occur via alternative pathways, such as neutralization of the pickup ions as proposed by Russell et al. (2006).The H + pickup ions are accelerated by the enhanced convection electric field, and subsequently neutralized by charge exchange, producing fast neutrals.The fast neutrals are not subjected to external electric and magnetic fields, and thus they escape from Mars more quickly.Together, these combined results support an increased escape rate of H + in response to IP shock compression.

Figure 1 .
Figure 1.(a) MAVEN orbit #3516 (green line) on 2016 July 19, presented in the cylindrical (CYL) MSO coordinate frame.The two black dashed curves represent the average locations of the bow shock (BS) and magnetic pileup boundary (MPB).The red point indicates the position of MAVEN at the shock arrival time.(b)-(l) In situ observations from MAVEN during the period 19:30-20:30 UT of orbit #3516, encompassing a SIR-driven shock (marked by the red vertical dashed line).From top to bottom: (b) the spacecraft altitude and position in units of the radius of Mars R M , (c) ion energy spectra, (d) electron energy spectra, (e) H + energy spectra for the look direction of STATIC A14D1, overlaid with the calculated maximum energy E max curve for H + pickup ions (white curve), (f) proton density, (g) proton temperature, (h) proton velocity components in MSO coordinates and proton bulk speed, (i) magnetic field components in MSO coordinates and magnetic field magnitude, (j) the angle between the proton velocity and the magnetic field direction, (k) convection electric field components in MSO coordinates and electric field magnitude, and (l) guiding center drift speed of pickup ions.

Figure 2 .
Figure 2. H + ion energy spectra from all of the look directions of STATIC, with its 4 deflection channels (D0-D3) and 16 anodes (A0-A15), during the period 19:30-20:30 UT on 2016 July 19.The vertical dashed lines indicate the shock at ∼20:05:26 UT.The white curves overlaid on each spectrogram represent the maximum energy that H + pickup ions can achieve.

Figure 3 .
Figure 3. H + ion velocity distribution functions (VDFs) projected on the plane perpendicular to the local magnetic field.The gray solid circle corresponds to the contour of 100 eV.The gray dashed circle corresponds to the calculated maximum energy of H + pickup ions (the values are shown).The red dashed circle corresponds to an ideal ring distribution of newborn H + pickup ions, calculated according to local proton velocity perpendicular to the local magnetic field.

Figure 4 .
Figure 4. (a) H + ion energy spectra during 19:30-20:30 UT on 2016 July 19 for the look direction of STATIC A14D1, overlaid with the calculated maximum energy E max curve for H + pickup ions.The red vertical dashed line indicates the shock at ∼20:05:26 UT.The intervals between pairs of yellow dashed lines before and after shock represent the upstream and downstream periods we sampled.(b) Calculated energy gained from local convection electric field vs. observed energy of H + pickup ions in the look direction of STATIC A14D1 for the selected upstream (gray symbols) and downstream (brown symbols) intervals.The vertical error bars are measurement errors of ion energy, and the horizontal error bars represent the standard deviation around the mean.The linear regression fitting slope and correlation coefficient (CC) are shown.

Figure 5 .
Figure 5. (a) MAVEN orbit #3658 (green line) on 2016 August 14, presented in the cylindrical (CYL) MSO coordinate frame.The two black dashed curves represent the average locations of the bow shock (BS) and magnetic pileup boundary (MPB).The red point indicates the position of MAVEN at the shock arrival time.(b)-(l) In situ observations from MAVEN during the period 16:00-17:00 UT of orbit #3658, encompassing an ICME-driven shock (marked by the red vertical dashed line).The format is the same as Figure1, except that panel (e) shows H + ion energy spectra for the look direction of SWIA A15D3.

Figure 6
Figure 6 shows ion energy-time spectrograms for all 64 look directions of SWIA, with 4 elevation bins (i.e., D0-D3) separated horizontally and 16 azimuth bins (i.e., A7-A6) separated vertically, during the interval 16:00-17:00 UT.The white curve overlaid on each spectrogram represents E max that H + pickup ions can achieve.The solar wind ions are mainly seen in four look directions, namely, A14D1, A14D2, A15D1, and A15D2.The ghost counts spread among all look directions.The rest of what is seen mainly corresponds to O + pickup ions and H + pickup ions.During the time interval of ±5 minutes around the shock arrival, the H + pickup ions

Figure 6 .
Figure 6.Ion energy spectra from all of the look directions of SWIA, with its 4 elevation angles (D0-D3) and 16 azimuth angles (the vertical direction labeled by A7 → A6 corresponds to the azimuth angle continuously from 0°to 360°), during the period 16:00-17:00 UT on 2016 August 14.The vertical dashed lines indicate the shock at ∼16:38:51 UT.The white curves overlaid on each spectrogram are the maximum energy that H + pickup ions can achieve.

Figure 7 .
Figure 7. (a) Ion energy spectra during 16:00-17:00 UT on 2016 August 14 for the look direction of SWIA A15D3, overlaid with the calculated maximum energy E max curve for H + pickup ions.The red vertical dashed line indicates the shock at ∼16:38:51 UT.The intervals between pairs of yellow dashed lines before and after shock represent the upstream and downstream periods we sampled.(b) Calculated energy gained from local convection electric field vs. observed energy of H + pickup ions in the look directions of SWIA A15D3 for the selected upstream (gray symbols) and downstream (brown symbols) intervals.The vertical error bars are measurement errors of ion energy, and the horizontal error bars represent the standard deviation around the mean.The linear regression fitting slope and correlation coefficient (CC) are shown.