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A&A
Volume 679, November 2023
Article Number A79
Number of page(s) 8
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202346644
Published online 10 November 2023

© The Authors 2023

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

The weak gravity on Mars allows for the formation of an extended exosphere. Its range extends far beyond the bow shock (BS), up to tens of Martian radii (RM ~ 3397 km). Thus, the exo-sphere interacts directly with the impacting solar wind plasma. The exospheric neutral atoms, predominantly hydrogen (H) and hot oxygen (O), can be ionized by three dominant mechanisms: photoionization by solar extreme ultraviolet (EUV) irradiance, charge exchange with solar wind protons, and impact ionization by solar wind electrons. The newly formed ions are immediately picked up by the solar wind electromagnetic fields and then accelerated by the convection electric field of the solar wind. This process is known as ion pickup, and the ions are called pickup ions. The pickup ions move on cycloidal trajectories perpendicular to the local magnetic field in the Mars rest frame, corresponding to a ring distribution in velocity space.

Heavy pickup ions like O+ generated upstream of the bow shock have gyroradii of the order of 20 000 km; in the magnetosheath, their gyroradii are of the same order of the characteristic scale of the Marian size, usually on a scale of several Mars radii. Thus, the O+ pickup ions often leap over Mars or enter the Martian upper atmosphere in their first gyration. Their spatial distributions around Mars exhibit a pronounced asymmetry with respect to the direction of the solar wind convection electric field, E = −Vsw × B (Fang et al. 2008; Dong et al. 2015b; Johnson 2018, and references therein). In the +E hemisphere, where the convection electric field points away from Mars, the O+ pickup ions escape Mars by forming a plume-like structure, predicted by many numerical models (Cloutier et al. 1974; Luhmann & Schwingenschuh 1990; Brecht & Ledvina 2006; Fang et al. 2008, 2010; Brain et al. 2010; Najib et al. 2011; Dong et al. 2014; Curry et al. 2015; Jarvinen et al. 2016) and unambiguously confirmed by the observations taken from Mars Express (MEX) and Mars Atmosphere and Volatile EvolutioN (MAVEN; Liemohn et al. 2014; Dong et al. 2015b; Johnson 2018). Indeed, there was already some evidence of incident energetic ion beams with similar characteristics to the plume ions in the earlier spacecraft measurements from Phobos-2 and MEX (Kallio et al. 1995, 2006, 2008; Kallio & Koskinen 1999; Carlsson et al. 2006, 2008; Dubinin et al. 2006, 2012; Boesswetter et al. 2007), although the plume structure has not yet been clearly defined due to instrument limitations (Liemohn et al. 2014). The MAVEN observations have demonstrated that the plume O+ ion fluxes are prominent above the magnetic pileup boundary (MPB) and gradually transition to tailward fluxes in the nightside plasma wake region (another main planetary ion escape channel); in addition, the plume fluxes have been shown to be slightly stronger than the tailward ones (Dong et al. 2015b). It should be noted that in addition to the O+ pickup ions of high-altitude exospheric origin mentioned above, the O+ plume structure observed above MPB may possibly contain a minor population of accelerated O+ ions of ionospheric origin (Curry et al. 2013). In the -E hemisphere, where the convection electric field points towards Mars, the O+ pickup ions precipitate into the Martian upper atmosphere (Fang et al. 2013; Hara et al. 2013; Dong et al. 2015b). The precipitating O+ ions are generally weaker in flux and have lower speeds compared to the plume ions (Dong et al. 2015b).

It is now generally accepted that the O+ ion plume represents an important nonthermal ion escape channel for Martian atmospheric erosion, at least at the present epoch. The characteristics of the plume and its contribution to ion escape have received increasing attention over the past decade. Dong et al. (2015b) conducted a statistical study of escaping O+ ion distributions using three months of data obtained by MAVEN and found that ions escaping through the dayside plume contribute about 23% of the total ion losses for O+ ions with energies greater than 25 eV. It is well known that the variations among the solar EUV and solar wind forcing can affect the atmosphere and ionosphere of Mars and the ionization rate of neutral atoms, as well as the plasma boundaries (i.e., bow shock and MPB); consequently, these variations could affect the energization and dynamics of the pickup ions and cause subsequent variations in O+ ion escape rate. Dong et al. (2017) investigated the potential impacts of EUV on O+ ions with energies greater than 6 eV and found that the ratio of the plume escape to the total escape decreases from ~30 to ~20% as the EUV intensity increases. Dubinin et al. (2017) examined the impacts of solar wind on O+ ion escape, including the plume escape, and found that fluxes of O+ ions with energies less than 30 eV decrease with increasing solar wind flux (or dynamic pressure), while those with energies greater than 30 eV exhibit the opposite trend. As a result, the averaged total O+ fluxes show little variability with the solar wind flux (or dynamic pressure). Also, there have been some studies investigating the global ion escape during space weather events, such as the interplanetary coronal mass ejection (ICME) impacting Mars on March 8, 2015, when the solar wind flux and dynamic pressure were significantly enhanced. Their results suggest that the inferred escaping ion fluxes are enhanced by an order of magnitude or more at the initial shock-sheath phase with compressed solar wind plasma and field (Jakosky et al. 2015a; Curry et al. 2015; Dong et al. 2015a; Luhmann et al. 2017), which is likely due to additional ionization via charge exchange and electron impact facilitated by the interplanetary (IP) shock compression. There are seemingly contradictory reports regarding the relative contribution from the plume to the total ion escape. For example, the ratio of O+ escape rates from the plume is increased from ~8.5 to ~14.2% in response to the ICME passage on September 13, 2017 (Romanelli et al. 2018), while that for heavy ions (including O+) is reduced from ~30 to ~10% in response to the ICME passage on March 8, 2015 (Jakosky et al. 2015a). We note that in this case, the heavier the ions, the more significant the escape plume (Dong et al. 2015a). More importantly, the shock-sheath phase can produce a high-energy escaping plume >10 keV, as reported by Curry et al. (2015). This indicates that the plume O+ ions are accelerated immediately after the IP shock arrival. Nevertheless, the acceleration mechanisms responsible for such rapid energization have not yet been fully elucidated.

The underlying acceleration processes of the plume ions may be classified into two broad classes: energization by enhanced electric field due to IP shock compression and energization by the IP shock itself. The latter is typically attributed to shock drift (Pesses et al. 1982) and diffusive shock (Axford et al. 1977; Bell 1978a,b; Blandford & Ostriker 1978) acceleration. Additionally, adiabatic acceleration and wave-particle energization (if ultra-low frequency waves were to be induced by IP shock) may also be involved in the acceleration of the plume ions. The MAVEN spacecraft is outfitted with a comprehensive suite of plasma and magnetic field instruments (Jakosky et al. 2015b), including two ion spectrometers, namely the Supra-Thermal And Thermal Ion Composition (STATIC; McFadden et al. 2015) and the Solar Wind Ion Analyzer (SWIA; Halekas et al. 2015). These tools could potentially help shine some light on the acceleration mechanisms and the detailed processes involved. We are thus motivated to identify plume ion energization events associated with the arrival of IP shocks, using in situ measurements from MAVEN. For this purpose, we first examined the MAVEN data from October 2014 to February 2023 for IP shock encounters, in particular, the available lists of IP shocks detected by MAVEN from October 2014 to November 2018 (see Huang et al. 2021). The ion energization events are required to meet two criteria: (1) the plume O+ ions continuously enter the field of view (FoV) of STATIC at times both before and after the shock passage (i.e., STATIC is favorably located to record the response of the plume O+ ions to the shock arrival) and (2) changes in the magnetic field orientation are negligible across IP shocks, ensuring that the detected plume O+ ions launch from similar source regions. We successfully identified one plume ion energization event, namely, one that is associated with a transmitted IP shock propagating through the Martian magnetosheath on March 3, 2015. We further analyzed the characteristics of the O+ pickup ions over an extended interval around the shock arrival. It is shown that the enhanced convection electric field due to IP shock compression is the primary factor contributing to the enhancement of energization of plume O+ ions. This paper provides a report of this event.

2 MAVEN data

There are nine instruments aboard MAVEN. The main data used in this work are obtained by the Supra-Thermal And Thermal Ion Composition (STATIC; McFadden et al. 2015), the Solar Wind Ion Analyzer (SWIA; Halekas et al. 2015), the Solar Wind Electron Analyzer (SWEA; Mitchell et al. 2016), and the Magnetometer (MAG; Connerney et al. 2015). STATIC operates over an energy range of 0.1 eV up to 30 keV, with a time resolution of 4 s. It consists of a toroidal electrostatic analyzer with a 360° × 90° field-of-view, combined with a time-of-flight velocity analyzer with a 22.5° resolution in the detection plane. The “d0” product, consisting of the mass, energy and angle of ions with 16 azimuth bins, 4 elevation bins, and 8 mass bins, detected with a time resolution of 128 s, is used to analyze the ion velocity distribution functions (VDFs). The ions in the mass ranges of 12–20 amu are taken as O+ ions. The “d1” data products of STATIC with a time resolution of 16 s are also used to analyze ion energy spectrograms. SWIA is an energy and angular ion spectrometer with electrostatic deflectors that measures ion fluxes. The ion moments are computed from the fine 3D velocity distributions measured by SWIA, under the assumption that all ions are protons. We utilized the ion velocity and energy spectra computed onboard, with a 4 s time resolution. SWEA provides the energy and angular distributions of 3–4600 eV electrons. MAG provides vector magnetic field measurements with a sampling frequency of 32 Hz with an 0.05% absolute vector accuracy.

thumbnail Fig. 1

MAVEN orbit information (left panel): (a) MAVEN trajectory (green line) for the time period of orbit 822, on March 3, 2015, presented in a cylindrical (CYL) MSO coordinate frame. The blue curve and the black curve are the mean positions of the MPB and BS, respectively. The red point marks the position of MAVEN at the shock arrival time. (b−c) Projected MAVEN trajectory during 18:00–18:30 UT (a segment near the apoapsis of orbit 822) on the MSO XZ and XY planes, respectively. The local E, B, and E × B directions are shown by the orange, blue and purple lines on the MAVEN position at the shock arrival time, respectively. Overview plots of the shock encountered by MAVEN (right panel): (d) the spacecraft altitude and position, (e) ion energy spectra, (f) electron energy spectra, (g) ions mass spectra, (h) proton density, (i) proton temperature, (j) proton velocity components in MSO coordinates and proton bulk speed, (k) magnetic field components in MSO coordinates and magnetic field magnitude, (l) angle between V and B, and (m) convection electric field components in MSO coordinates and electric field magnitude. The red and blue vertical dashed lines indicate a fast shock and a discontinuity, respectively.

3 Event overview

MAVEN recorded an ICME, and an IP shock driven by the ICME, passing through Mars between March 3–4, 2015 (Jakosky et al. 2015a; Thampi et al. 2018). Figure 1 shows MAVEN measurements during the period 18:00–18:30 UT on March 3, 2015, a segment near the apoapsis of orbit 822, encompassing the IP shock. We note that orbit 822 started at the periapsis passage time of 16:08:21 UT, and did not extend out of the BS. The shock arrived at the MAVEN spacecraft at 18:17:52 UT on March 3, when the spacecraft was located inside the Martian magnetosheath of the southern hemisphere in the Mars Solar Orbital (MSO) coordinates, where the local convection electric field points away from Mars (i.e., the +E hemisphere in the Mars Solar Electric field (MSE) coordinates; see Figs. 1b–c). It is clearly distinguished by abrupt increases in the magnetic field magnitude and fluctuation level and a sharp increase in proton bulk velocity, along with a pronounced broadening of ion and electron energy spectra across the shock from the upstream to the downstream. Moreover, the energetic electron fluxes increase abruptly at the shock (see Fig. 1 of Jakosky et al. 2015a). It is a transmitted IP shock propagating through the magnetosheath, as a consequence of the interaction of the IP shock and the Martian BS. The transmitted shock is followed by a newly created discontinuity at 18:18:32 UT, in agreement with the prediction of MHD theory (Samsonov et al. 2006). The discontinuity is characterized by an increase in proton density and a decrease in proton temperature.

A quantitative calculation of the shock parameters is intrinsically difficult because of the low reliability in deriving magnetosheath ion moments for protons and alpha particles separately. Moreover, the selection of the upstream and downstream time intervals of a shock is somewhat subjective. To make a rough estimate of the shock angle θBn (the angle between the shock normal and the upstream magnetic field), we used the method proposed by Huang et al. (2021). Specifically, the shock angle θBn is calculated by θBn=180πarccos(| Bup⋅n || Bup || n |),${\theta _{{\rm{Bn}}}} = {{180} \over \pi }\arccos \left( {{{\left| {{{\bf{B}}^{{\rm{up}}}} \cdot {\bf{n}}} \right|} \over {\left| {{{\bf{B}}^{{\rm{up}}}}} \right|\left| {\bf{n}} \right|}}} \right),$(1)

where Bup is the upstream magnetic field, and the n is the shock normal. The shock normal, n, is estimated by: n=±(Bdown−Bup)×((Bdown−Bup)×(Vdown−Vup))| (Bdown−Bup)×((Bdown−Bup)×(Vdown−Vup)) |,${\bf{n}} = \pm {{\left( {{{\bf{B}}^{{\rm{down}}}} - {{\bf{B}}^{{\rm{up}}}}} \right) \times \left( {\left( {{{\bf{B}}^{{\rm{down}}}} - {{\bf{B}}^{{\rm{up}}}}} \right) \times \left( {{{\bf{V}}^{{\rm{down}}}} - {{\bf{V}}^{{\rm{up}}}}} \right)} \right)} \over {\left| {\left( {{{\bf{B}}^{{\rm{down}}}} - {{\bf{B}}^{{\rm{up}}}}} \right) \times \left( {\left( {{{\bf{B}}^{{\rm{down}}}} - {{\bf{B}}^{{\rm{up}}}}} \right) \times \left( {{{\bf{V}}^{{\rm{down}}}} - {{\bf{V}}^{{\rm{up}}}}} \right)} \right)} \right|}},$(2)

where Bup and Bdown are the upstream and downstream magnetic field, respectively, and Vuv and Vdown are the upstream and downstream proton velocity, respectively. The estimated shock angle is larger than 45°, indicating a quasi-perpendicular shock. Herein, we focus primarily on the changes of the local plasma parameters across the shock. The proton speed V increases from 280 to 430 km s−1, mainly along the -X direction in MSO coordinates. The magnetic field, B, that is originally dominant in the +Z direction increase in strength from 14 to 27 nT, and then the increase is further enhanced to a maximum value of ~38 nT across the discontinuity, mostly in the +Z direction. It is important to note that the shock compression enhances the magnitude of the magnetic field, but keeps it in roughly the same direction (predominantly aligned in the +Z direction). This feature will aid in analyzing the cycloidal trajectories and sources of pickup ions entering into the FoV of STATIC, at times before and after the shock arrival. As a consequence of these changes in V and B, the convection electric field E (E = −V × B) is enhanced by a factor of about 3, from 3.8 mVm−1 to about 11.7 mVm−1, mostly in the -Y direction, and then begins a surge to higher levels near 18.2 mVm−1 (about five times its original value). In the following analysis, the changes across the discontinuity are attributed to the shock impact based on a simple consideration.

Figure 2 shows O+ energy-time spectrograms for all 64 look directions of STATIC (16 azimuth bins of 22.5° and 4 elevation bins of 22.5°) during the interval of 18:00–18:30 UT. The white curves overlaid on each spectrogram are the maximum energy Emax that pickup O+ can achieve, which is given by Emax = 2mV2 sin2 θ, where m is the mass of the pickup ions (in this case oxygen), V is the proton velocity, and θ is the angle between the proton velocity and the magnetic field direction. We note that the calculation of Emax is based on the assumption that the proton velocity and magnetic field are spatially uniform and temporally stable, and the calculated Emax is only used for qualitative diagnostics of ion populations, rather than quantitative analyses. It is not uncommon to observe solar wind ion contamination within the O+ ion data sampled in the magnetosheath. The solar wind ions are mainly seen in four look directions, namely, A12D1, A12D2, A13D1, and A13D2. The O+ pickup ions, in particular those near or at their maximum energy, are easily distinguished from solar wind ions and scattered solar wind ions (ghost counts), due to their higher energies. They are mainly present in the 16 look directions, corresponding to azimuth bins A12-A15 and elevation bins D0-D3. It is worth noting that a part of O+ pickup ions with energies exceeding the upper energy detection limit are not detected by STATIC, mainly at times after the shock arrival (marked by the vertical dashed line). We can see a high-flux O+ ion population, concentrated in a narrow energy band below the maximum energy, enters STATIC in a narrow angular range, corresponding to A13D0 and A14D0. A striking feature of this ion population is the sudden jump of the ion energies at the shock, and thus there is no temporal ambiguity for the IP shock related phenomena. More specifically, the average energy increases from ~10 to ~25 keV, indicating an enhanced ion energization associated with the shock arrival, which is the focus of this paper. Despite possessing much higher energies due to the shock impact, the O+ ion population detected after the shock impact likely originates from similar source regions as that detected before the shock arrival, in view of the fact that: (1) the direction of the magnetic field remains roughly stable and the ion cyclotron radius R(R=mVqB)$R\left( {R = {{mV} \over {qB}}} \right)$ decreases (but slightly, by a factor of 0.29, across the shock) and, thus, the initial trajectories of O+ pickup ions change only slightly as well; and (2) they enter STATIC from the same look directions (A13D0 and A14D0) as those before the shock passage. We argue that this ion population corresponds to the O+ ion plume accelerated toward the convection electric field direction.

4 Analysis and discussion

In order to explore the sources and acceleration mechanisms of the ion population seen in the look directions A13D0 and A14D0, we conducted a analysis of the O+ ion VDFs measured by STATIC. Here, the data product of “d0” with a time resolution of 128 s was used. Figure 3 shows the characteristic O+ ion VDFs projected on the plane perpendicular to the local magnetic field at times before and after the shock arrival. At ~ 18:13 UT (Fig. 3a), about 4 min before the shock arrival, three O+ ion populations can be clearly seen in the VDF. The first one is O+ ions, with energies of 1 ~ 10 keV on the initial phase of the ring distribution for newborn pickup ions (red dashed circle). The ideal ring has a radius V sin θ, where V is the proton velocity and θ is the angle between the proton velocity and the ambient magnetic field direction (Fig. 1l), which corresponds to the perpendicular proton velocity to the local magnetic field. The O+ ions form a narrow plume-like escaping ion population that is moving approximately in the local convection electric field direction. The plume is probably composed of both O+ ions picked up in the magnetosheath and those originated in the high altitude ionosphere (Curry et al. 2013; Liemohn et al. 2013; Johnson 2018). The plume ions at high energies (6 ~ 10 keV), located around the end of the first quadrant of the ideal ring, have large phase space densities (up to ~10−12 s3 m−6) and correspond primarily to the O+ ions in the look directions A13D0 and A14D0 (Fig. 2). Also, we note that some plume ions leak into the neighboring look directions, such as A13D1, A14D1, and A15D1 (Fig. 2). The second one is O+ ions with high energies (~20 keV) and relatively low phase space densities (~10−14 s3 m−6), near the middle phase of the ring distribution (i.e., near the maximum energy of the ring distribution). They are local O+ pickup ions in the magnetosheath, and enter the FoV of STATIC near the peak of their gyro-motion, corresponding primarily to the O+ ions in the look directions A12D0 and A12D1 (Fig. 2). The third one is the energetic (>10 keV) but relatively low phase space densities (~10−14 s3 m−6) quasi-Marsward precipitating O+ ions, coming from the upstream solar wind (Masunaga et al. 2016, Masunaga et al. 2017). At ~18:16 UT (Fig. 3b), just before the shock arrival, we can see two O+ ion populations in the VDF, similar to the first and second populations (displayed in Fig. 3a).

At ~18:20 UT (Fig. 3c), about 3 min after the shock arrival, we also see two O+ ion populations. One is similar to the first population seen in the VDFs before the shock arrival, namely, plume O+ ions with relatively low phase space densities (~10−14 s3 m−6) in the initial phase of the ring distribution, but with a larger ring radius that is mainly due to the increase of the proton speed (note that the angle θ between the proton velocity and the magnetic field direction does not show remarkable changes across the shock). This implies that the plume ions gain more energy from the enhanced convection electric field, as they travel to the same altitude as those detected before the shock arrival. They correspond primarily to the O+ ions in the look directions A13D0 and A14D0 (Fig. 2). This result lends strong support to our earlier preliminary assertion that the O+ ions in the look directions A13D0 and A14D0 detected after the shock impact originate from similar source regions as those detected before the shock arrival. The other is quasi-Marsward precipitating O+ ions with low phase space densities (~10−15 s3 m−6), corresponding primarily to the O+ ions in the look directions A09D0, A09D1, and A09D3 (Fig. 2). At ~ 18:22 UT (Fig. 3d), about 5 min after the shock arrival, we can see two ion populations that are quite similar to those shown in Fig. 3c.

As stated above, thanks to the fact that the initial trajectories of O+ pickup ions change only slightly, the O+ ions originated from approximately the same source region enter STATIC in the look directions A13D0 and A14D0 at times both before and after the shock passage, which can be seen more clearly in the zoomed-in energy spectra in Fig. 4. This offers us a unique opportunity to investigate the enhanced ion energization associated with the shock. We can turn our attention to comparing the calculated pickup ion energies gained from the convection electric field based on the pickup ion motion equation (where the pickup ion transport is dictated by the Lorentz force and convection electric field) with the observed values. In the calculation, we assume that the newly formed ions are created with approximately zero velocity in the planetary rest frame and they enter the FoV of STATIC at time t=T4$t = {T \over 4}$ (T is the gyro-period of O+ pickup ions given by T=2πmq| B |$\left. {T = {{2\pi m} \over {q\left| B \right|}}} $), when the pickup ions reach the end of the first quarter of their cycloidal trajectories, as shown in Fig. 3. The intervals between two yellow dashed lines before and after the shock indicates the shock upstream (18:10:20– 18:17:20 UT) and downstream (18:18:32–18:25:32 UT), over which our analyses are sampled (Figs. 4a–b). The sampled ions with much higher energies (>0.75Emax) or lower energies (<0.3Emax) are discarded, considering that they could not have entered the FoV at T4${T \over 4}$. Comparisons of O+ ion energies from the calculations and STATIC observations are shown in Figs. 4c and d. The slope of the linear regression between observed and calculated energies is 0.94 (0.91), with a correlation coefficient of 0.95 (0.94), for the look direction A13D0 (A14D0). It suggests that the enhanced convection electric field would will be the primary factor contributing to the enhanced ion energization. On the other hand, during the selected upstream and downstream intervals, MAVEN probed at the altitudes of ~6159–6212 km and ~6217–6227 km, respectively. The increase of the altitude could also lead to an increase of the energy of O+ ions obtained from the local convection electric field, with values of only up to 1.2 keV in this case, which is much lower compared to the observed jump of ~15 keV at the shock. Specifically, the increase in the altitude makes only a small contribution to the enhanced ion energization.

A question that naturally arises here concerns whether or not the plume O+ ions are accelerated at the shock via quasi-perpendicular shock-associated acceleration mechanisms, such as shock-drift acceleration (Pesses et al. 1982) and shock-surfing acceleration (Sagdeev 1966; Ohsawa 1986; Lee et al. 1996). We can imagine that the plume O+ ions created from neutral O atoms within a short time period prior to the shock arrival will encounter the shock during their journey through the magnetosheath of Mars. A part of the plume O+ ions could overcome the electrostatic shock potential and proceed downstream, owing to their high energy, as proposed in Masunaga et al. (2016). Another part with small normal velocity to the shock surface cannot overcome the shock potential and will be reflected. The reflected ions would probably precipitate into the upper atmosphere of Mars, rather than be trapped upstream of the shock through the combined action of the electrostatic potential and the Lorentz force, because of the shorter transit time that the shock take to pass through the dayside magnetosheath (note: the propagation velocities of the fast shocks driven by ICMEs are generally faster than 200 km s−1, Huang et al. 2021), compared to the gyro-period of the O+ ions. This scenario is quite different from that for interstellar pickup ion acceleration at IP shocks (e.g., Zirnstein et al. 2018). In the present case, the plume O+ ions sampled in the downstream consist of primarily of newly formed pickup ions created after the shock passage and, thus, they do not have the chance to encounter the shock at all. It should be emphasized, however, that they are subject to the electromagnetic fields that have been greatly amplified by the shock.

thumbnail Fig. 2

O+ energy-time spectrograms for all of the look directions of STATIC with its four deflection channels (D0-D3) and 16 anodes (A00-A15) during 18:00–18:30 UT on March 3, 2015. The vertical dashed lines indicate the shock at ~18:17:52 UT. The white curves overlaid on each spectrogram are the calculated maximum energy for O+ pickup ions.
thumbnail Fig. 3

O+ velocity distribution functions (VDFs) projected on the plane perpendicular to the local magnetic field observed by STATIC (“d0” products). Gray circles correspond to contours of 100 eV, 1 keV and 10 keV, respectively. The red dashed circle corresponds to an ideal ring distribution calculated by local proton velocity perpendicular to the local magnetic field.
thumbnail Fig. 4

O+ energy-time spectrogram for the look directions of STATIC A13D0 (a) and A14D0 (b), shown at the top. The maximum energy curves for O+ pickup ions are overlaid on each spectrogram. The red vertical dashed line indicates the shock at ~18:17:52 UT. The intervals between two yellow dashed lines before and after the shock correspond to the shock upstream and downstream, respectively. Calculated energy gained from local convection electric field E vs. observed energy of O+ pickup ions in the look directions of STATIC A13D0 (c) and A14D0 (d) for the selected upstream (gray symbols) and downstream (brown symbols) intervals are shown at the bottom. The vertical error bars are measurement errors of ion energy. The horizontal error bars represent the standard deviation around the mean. The linear regression fitting slope and correlation coefficient are also shown.

5 Summary

The objective of this paper is to understand the mechanisms responsible for the energization of plume O+ ions associated with the interplanetary (IP) shock arrival. Using in situ measurements by MAVEN, we have been able to identity a plume O+ energization event associated with a transmitted IP shock propagating through the Martian magnetosheath on March 3, 2015. The event is unique in that during the apoapsis segment of spacecraft orbit 822, encompassing the IP shock passage, the plume O+ ions were continuously captured by MAVEN STATIC in a narrow angular range (i.e., look directions A13D0 and A14D0), thanks to the favorable FoV configurations of STATIC. Moreover, because the direction of the magnetic field remained roughly stable across the shock, the plume O+ ions detected before and after the shock arrival should launch from approximately the same source region. The average energy of the plume O+ ions suddenly increased from ~10 to ~25 keV across the shock. Comparisons of O+ ion energies from theoretical calculations and STATIC observations suggest that the enhanced ion energization is mainly due to the enhanced convection electric field caused by shock compression. This result provides a crucial clue towards the understanding of how IP shocks facilitate ion escape through the plume and, in addition, provides direction and constraints that will be useful in future modeling efforts.

Acknowledgements

This work is supported by the Strategic Priority Research Program of Chinese Academy of Sciences, Grant No. XDB 41000000, the National Natural Science Foundation of China (NSFC) (42074212, 42374214, 42030202, 12073005, 12021003), the Key Research Program of the Institute of Geology and Geophysics, CAS, Grant IGGCAS-201904, the pre-research Project on Civil Aerospace Technologies No. D020104 funded by Chinese National Space Administration. The MAVEN project is supported by NASA through the Mars Exploration Program. We have used the MAVEN plasma and magnetic field data throughout. The PDS Planetary Plasma Interactions Node (https://pds-ppi.igpp.ucla.edu/) makes the MAVEN data publically available. We appreciate the data being made available by the PIs of MAG (J.E.P. Connerney), SWIA (J.S. Halekas), and SWEA (D.L. Mitchell) on board MAVEN.

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All Figures

thumbnail Fig. 1

MAVEN orbit information (left panel): (a) MAVEN trajectory (green line) for the time period of orbit 822, on March 3, 2015, presented in a cylindrical (CYL) MSO coordinate frame. The blue curve and the black curve are the mean positions of the MPB and BS, respectively. The red point marks the position of MAVEN at the shock arrival time. (b−c) Projected MAVEN trajectory during 18:00–18:30 UT (a segment near the apoapsis of orbit 822) on the MSO XZ and XY planes, respectively. The local E, B, and E × B directions are shown by the orange, blue and purple lines on the MAVEN position at the shock arrival time, respectively. Overview plots of the shock encountered by MAVEN (right panel): (d) the spacecraft altitude and position, (e) ion energy spectra, (f) electron energy spectra, (g) ions mass spectra, (h) proton density, (i) proton temperature, (j) proton velocity components in MSO coordinates and proton bulk speed, (k) magnetic field components in MSO coordinates and magnetic field magnitude, (l) angle between V and B, and (m) convection electric field components in MSO coordinates and electric field magnitude. The red and blue vertical dashed lines indicate a fast shock and a discontinuity, respectively.
In the text
thumbnail Fig. 2

O+ energy-time spectrograms for all of the look directions of STATIC with its four deflection channels (D0-D3) and 16 anodes (A00-A15) during 18:00–18:30 UT on March 3, 2015. The vertical dashed lines indicate the shock at ~18:17:52 UT. The white curves overlaid on each spectrogram are the calculated maximum energy for O+ pickup ions.
In the text
thumbnail Fig. 3

O+ velocity distribution functions (VDFs) projected on the plane perpendicular to the local magnetic field observed by STATIC (“d0” products). Gray circles correspond to contours of 100 eV, 1 keV and 10 keV, respectively. The red dashed circle corresponds to an ideal ring distribution calculated by local proton velocity perpendicular to the local magnetic field.
In the text
thumbnail Fig. 4

O+ energy-time spectrogram for the look directions of STATIC A13D0 (a) and A14D0 (b), shown at the top. The maximum energy curves for O+ pickup ions are overlaid on each spectrogram. The red vertical dashed line indicates the shock at ~18:17:52 UT. The intervals between two yellow dashed lines before and after the shock correspond to the shock upstream and downstream, respectively. Calculated energy gained from local convection electric field E vs. observed energy of O+ pickup ions in the look directions of STATIC A13D0 (c) and A14D0 (d) for the selected upstream (gray symbols) and downstream (brown symbols) intervals are shown at the bottom. The vertical error bars are measurement errors of ion energy. The horizontal error bars represent the standard deviation around the mean. The linear regression fitting slope and correlation coefficient are also shown.
In the text

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