MAVEN Observations of the Effects of Crustal Magnetic Fields on Electron Density and Temperature in the Martian Dayside Ionosphere
MAVEN对火星白天侧电离层中地壳磁场对电子密度和温度的影响的观测

1 Introduction  1 引言

At Earth, the planet's strong, global-scale, dipolar magnetic field plays a major role in influencing the interaction of the planet with the surrounding space environment (e.g., Cravens, 2004; Gurnett & Bhattacharjee, 2005; Kivelson & Russell, 1995). Moreover, Earth's magnetic field strongly affects ionospheric processes and properties (e.g., Kelley, 1989; Schunk & Nagy, 2009). Is the same true at Mars, which has a very different magnetic environment?
在地球上，地球强大的全球性双极磁场在影响地球与周围空间环境相互作用方面发挥着重要作用（例如，Cravens，2004；Gurnett和Bhattacharjee，2005；Kivelson和Russell，1995）。此外，地球磁场对电离层过程和性质有着强烈影响（例如，Kelley，1989；Schunk & Nagy，2009）。火星的磁场环境截然不同，是否也存在同样的情况？

Mars is unique among the solar system's planets: it lacks a global intrinsic magnetic field but possesses concentrated regions of strong crustal magnetic field. The crustal magnetic fields are primarily radial in direction, have magnitudes up to several hundred nanotesla (nT) at 400 km altitude, and are strongest at southern latitudes and longitudes ~140°E–240°E (Brain et al., 2003; Connerney et al., 2001).
火星在太阳系行星中独一无二：它没有全球性的固有磁场，但拥有强地壳磁场的集中区域。地壳磁场主要呈径向分布，在400公里高空的强度可达数百毫微特斯拉（nT），在纬度为南纬140°至东经240°的地区最强（Brain等人，2003年；Connerney等人，2001年）。

The question of how Mars's unique magnetic field affects the planet's interaction with the space environment is important for the disciplines of planetary science and plasma science. In planetary science, this interaction affects the present-day behavior of the atmosphere-ionosphere-magnetosphere system. It also affects the climate of Mars by its long-term effects on escape processes. In plasma science, Mars offers a unique laboratory for probing conditions not found elsewhere in the solar system, such as variations in magnetic field strength, direction, and connectedness to the solar wind on small length scales.
火星独特的磁场如何影响行星与太空环境的相互作用，这个问题对于行星科学和等离子体科学学科非常重要。在行星科学中，这种相互作用会影响大气层-电离层-磁层系统的当前行为。它还会对逃逸过程产生长期影响，从而影响火星的气候。在等离子体科学领域，火星为研究太阳系其他地方所没有的条件提供了独一无二的实验室，例如磁场强度、方向以及与太阳风在较小尺度上的关联性。

Previous work has established that Mars' crustal magnetic fields, although spatially restricted, do have significant effects on the planet's interaction with the solar wind (e.g., Edberg et al., 2008; Mitchell et al., 2001).
之前的研究已经证实，火星的地壳磁场虽然空间有限，但确实对火星与太阳风的相互作用产生了重大影响（例如，Edberg等人，2008年；Mitchell等人，2001年）。

It is also clear that Mars' crustal magnetic fields have important effects on the structure of the Martian ionosphere (e.g., Gurnett et al., 2008; Withers et al., 2005). The presence of strong crustal magnetic fields can influence the ionospheric electron density on regional scales. The ionosphere of Mars is summarized in Withers (2009). The key aspects for this paper are that dayside conditions are dominated by photoionization and associated chemical processes below ~200 km, while transport processes are significant above ~200 km. Andrews, Andersson, et al. (2015) compared dayside electron densities above 300 km measured by Mars Express to an empirical model of the Martian ionosphere and found that the measured values exceeded the model predictions in regions of strongest crustal fields. The magnetic field orientation and topology can influence the local electron density. Nielsen et al. (2007) reported enhanced peak electron densities in magnetic cusps or regions of strongly vertical magnetic field that are likely connected to the solar wind. However, the large-scale effects of crustal magnetic fields on ionospheric conditions have not been fully characterized by previous work.
同样显而易见的是，火星的地壳磁场对火星电离层的结构有着重要影响（例如，Gurnett等人，2008；Withers等人，2005）。强地壳磁场的存在会影响区域范围内的电离层电子密度。火星电离层在Withers（2009）中进行了总结。本文的关键内容是：近地面的条件主要由光离子化和200公里以下的化学过程决定，而200公里以上的传输过程则非常重要。安德鲁斯、安德森等人（2015）将火星快车号测量的300公里以上日面电子密度与火星电离层的经验模型进行了比较，发现测量值在最强地壳磁场区域超过了模型预测值。磁场的方向和拓扑结构会影响局部电子密度。Nielsen等人（2007年）报告称，在磁尖或垂直磁场强的区域，峰值电子密度增强，这可能与太阳风有关。然而，前人的工作尚未充分揭示地壳磁场对电离层条件的大规模影响。

The purpose of this paper is to report on the influence of crustal fields on electron densities and temperatures in the dayside Martian ionosphere as observed by NASA's Mars Atmospheric and Volatile EvolutioN (MAVEN) mission. MAVEN entered Mars orbit in September 2014 and provides the best opportunity to date for studying the effects of crustal magnetic fields on the Martian ionosphere. The spacecraft is equipped with a Langmuir Probes and Waves (LPW) instrument (Andersson et al., 2015) that provides electron density measurements over a larger altitude range than was possible with Mars Global Surveyor or Mars Express radio occultations. LPW also provides the first electron temperature measurements in the Martian ionosphere since the single profile measured by the Viking landers (Hanson & Mantas, 1988).
本文旨在报告美国国家航空航天局（NASA）火星大气与挥发物演化（MAVEN）任务对火星电离层日侧电子密度和温度的观测结果。MAVEN于2014年9月进入火星轨道，为研究地壳磁场对火星电离层的影响提供了迄今为止的最佳机会。该航天器配备了一个Langmuir探针和波（LPW）仪器（Andersson等人，2015），该仪器能够测量比火星全球勘测者或火星快车无线电掩星仪所能达到的更大高度范围内的电子密度。LPW还提供了自Viking登陆器测量单一剖面以来（Hanson和Mantas，1988年）首次对火星电离层进行电子温度测量。

An initial survey of MAVEN data showed that regions of strong crustal fields have increased electron densities compared to surrounding regions (Andrews, Edberg, et al., 2015). Here we expand on that study by adding roughly one Earth year of additional electron density data, considering dependence on altitude, and also examining the effects of crustal fields on the electron temperature.
对MAVEN数据的初步调查显示，与周围地区相比，地壳磁场强的地区的电子密度更高（Andrews、Edberg等，2015）。在此，我们对该研究进行了扩展，增加了大约一个地球年的电子密度数据，考虑了与海拔高度的依存关系，并研究了地壳磁场对电子温度的影响。

This paper is organized as follows. Section 2 presents the MAVEN observations used in this study and an analysis of how the electron density and temperature are affected by strong crustal magnetic fields. Section 3 discusses the implications of our findings and possible mechanisms to explain the influence of crustal fields on the ionospheric density and temperature. Section 4 summarizes the findings of this project.
本文结构如下。第2节介绍了本研究中使用的MAVEN观测数据，并分析了电子密度和温度如何受到强地壳磁场的影响。第3部分讨论了我们的发现所具有的意义，以及解释地壳磁场对电离层密度和温度的影响的可能机制。第4部分总结了本项目的研究结果。

2 Observations  2 观察

MAVEN is located in a 4.5 h elliptical orbit around Mars, with periapsis typically at ~150 km, apoapsis at ~6,200 km, and a 75° inclination (Jakosky et al., 2015). We have surveyed MAVEN LPW measurements of electron density and temperature from October 2014 through the end of April 2016. This interval includes four “deep dips” in which the spacecraft orbital periapsis was lowered to ~120–130 km for about a week. The interval also covers most of a Mars year, starting in northern fall (L_s ~206°) and going through northern late summer (L_s ~145°). We have restricted our analysis to data assessed by the LPW team to be suitable for scientific interpretation and to solar zenith angles less than 90° because the dayside and nightside ionospheres are controlled by different physical processes (e.g., Withers, 2009). We shall examine the effects of crustal magnetic field using maps of how dayside electron density and temperature vary with areographic latitude and longitude (defined positive eastward throughout).
MAVEN位于环绕火星的4.5小时椭圆轨道上，近地点通常为150公里，远地点为6200公里，倾角为75°（Jakosky等人，2015）。我们调查了2014年10月至2016年4月底期间MAVEN LPW对电子密度和温度的测量结果。这一区间包括四次“深度下降”，其中航天器的轨道近地点降低到约120-130公里，持续约一周时间。这一区间还涵盖了火星年中的大部分时间，从北半球秋季（L _s ~206°）开始，到北半球夏末（L _s ~145°）结束。我们仅限于分析LPW团队评估的数据，这些数据适合科学解释，且太阳天顶角小于90°，因为白天和黑夜的电离层受不同的物理过程控制（例如，Withers，2009）。我们将使用日面电子密度和温度随纬度和经度变化（定义为向东为正）的地图来研究地壳磁场的影响。

Figure 1 shows how the median LPW electron density and temperature vary with areographic latitude and longitude at three different altitude ranges: 180–200 km, 240–260 km, and 300–320 km. The conclusions of this work are the same whether considering the median or mean electron density or temperature. In each panel the color of each 10° latitude by 10° longitude bin indicates the median electron density or electron temperature in each bin. Contours of crustal magnetic field as predicted by the Cain et al. (2003) model are overplotted in each panel and show that regions of strongest crustal magnetic fields are largely confined to southern latitudes, particularly at longitudes ~140°–240°. Each 10° latitude by 10° longitude bin in Figure 1 typically contains at least 50 data points at altitudes above 300 km and more than 100 data points per bin at altitudes below 200 km.
图1显示了三个不同高度范围内（180-200公里、240-260公里和300-320公里）LPW电子密度和温度的中值随纬度和经度的变化情况。180-200公里、240-260公里和300-320公里。无论考虑电子密度或温度的中位数还是平均值，这项工作的结论都是相同的。在每个面板中，每个10°纬度乘10°经度的网格的颜色表示每个网格中的电子密度或电子温度的中位数。每个面板上叠加了Cain等人（2003）模型预测的地壳磁场等值线，显示地壳磁场最强的区域主要位于南纬地区，特别是经度约为140°-240°的区域。图1中每个10°纬度乘10°经度的网格通常包含至少50个300公里以上高度的数据点，以及每个网格中超过100个200公里以下高度的数据点。

Inspection of Figure 1 leads to two immediate impressions. First, there are substantial differences between observed conditions in the northern and southern hemispheres. Second, observed conditions at high altitudes, southern latitudes, and longitudes near 180° are noticeably different from observed conditions at the same altitudes and latitudes but different longitudes.
观察图1，我们立刻得出两个印象。首先，南北半球观测到的条件存在显著差异。其次，在高空、南纬和接近180°的经度观测到的条件与在同一高度和纬度但不同经度观测到的条件明显不同。

The first impression, hemispheric asymmetry, is not significant for this work. The hemispheric asymmetry that is visible is simply caused by variations in the Mars-Sun distance and solar zenith angle between northern and southern observations. We do not discuss these further in this work.
对于这件作品来说，第一印象——半球不对称——并不重要。可见的半球不对称性只是由于火星与太阳的距离以及南北半球观测时太阳天顶角的变化引起的。我们在此不作进一步讨论。

The second impression, variations in ionospheric conditions with longitude, is the focus of this work. The Mars-Sun distance, season, and the latitude, local time, solar zenith angle of MAVEN's periapsis change slowly and systematically between successive orbits. Longitude, on the other hand, changes appreciably between successive orbits. Differences in ionospheric conditions at two different longitudes and the same latitude cannot be caused by changes in Mars-Sun distance, season, latitude, local time, or solar zenith angle. Only factors fixed with respect to the solid body of Mars can do so—such as crustal magnetic fields.
第二印象，即电离层条件随经度的变化，是这项工作的重点。火星与太阳的距离、季节以及MAVEN卫星近地点的纬度、当地时间和太阳天顶角在连续的轨道之间缓慢而有规律地变化。另一方面，经度在连续的轨道之间变化明显。火星-太阳距离、季节、纬度、当地时间或太阳天顶角的变化不会导致两个不同经度和相同纬度的电离层条件出现差异。只有与火星固体相关的固定因素（如地壳磁场）才能造成这种差异。

Temporal variations in solar forcing, such as the solar ionizing flux or external solar wind conditions, could, in principle, cause variations in ionospheric conditions with longitude. However, any such temporal variations in solar forcing are unlikely to occur with a periodicity commensurate with the periodicity of MAVEN's longitudinal sampling. MAVEN completes approximately five orbits in each Martian day (sol). Occasional extreme solar events, such as solar flares, which occur on timescales of minutes to hours, could perhaps create longitudinal structure in observed ionospheric conditions. Since the MAVEN observations come from a very weak solar cycle with few solar flares, we deem this unlikely ( e.g., Thiemann et al., 2017). We conclude that variations with longitude in this set of ionospheric observations cannot be caused by external factors.
原则上，太阳离子通量或太阳风外部条件等太阳作用力的时间变化可能会导致电离层条件随经度变化。然而，太阳辐射的任何这种时间变化都不太可能与MAVEN的纵向采样周期相吻合。MAVEN在火星的一天（sol）内大约完成五个轨道。偶尔发生的极端太阳活动（如太阳耀斑）发生在几分钟到几小时的时间尺度上，可能会在观测到的电离层条件下形成纵向结构。由于MAVEN的观测来自太阳活动周期非常弱且太阳耀斑很少的时期，我们认为这种情况不太可能发生（例如，Thiemann等人，2017）。我们得出结论，这组电离层观测数据中的经度变化不可能由外部因素引起。

Figure 1 shows that the electron density and temperature are largely unaffected by the presence or absence of strong crustal magnetic fields at low altitudes (180–200 km panel). At higher altitudes (240–260 km and 300–320 km) the strong crustal magnetic field regions feature larger electron densities and smaller electron temperatures than the surrounding regions. This is most noticeable in the largest region of strong crustal fields, located at southern latitudes and longitudes ~140°–240°, but is also noticeable in the more equatorial crustal field regions (e.g., latitudes ±~15°, longitudes ~10–50°, and ~300–360°). At these higher altitudes, the electron density increases by ~25–30% and the electron temperature decreases by ~10–15% in regions of strong crustal fields compared to other regions. For example, we can compare the electron density and temperature at 240–260 km altitude in a region of weak crustal field, southern latitudes between −30° and −60°, and longitudes 0–140°, to the electron density and temperature in the nearby region of strong crustal field, southern latitudes between −30° and −60°, and longitudes 140–240°. In this weak crustal field region the electron density is typically ~3.5–3.6 × 10³ cm⁻³ compared to ~4.4 × 10³ cm⁻³ in the strong crustal field region, while the electron temperature is typically ~1,750 K in the weak crustal field region compared to ~1,950 K in the strong crustal field region.
图1显示，在低海拔（180-200公里面板）地区，电子密度和温度在很大程度上不受强地壳磁场存在与否的影响。在更高的高度（240-260公里和300-320公里），强地壳磁场区域的电子密度比周围区域更大，电子温度更低。这种现象在位于南纬和经度约140°–240°的最大强地壳磁场区域最为明显，但在赤道附近的地壳磁场区域（例如纬度±~15°、经度约10–50°和约300–360°）也很明显。在这些较高的高度上，与其他区域相比，强地壳磁场区域的电子密度增加约25-30%，电子温度降低约10-15%。例如，我们可以将位于地壳磁场较弱区域（南纬30°至60°，东经0°至140°）240至260公里高空的电子密度和温度与附近地壳磁场较强区域（南纬30°至60°，东经140°至240°）的电子密度和温度进行比较。在这个弱地壳磁场区域，电子密度通常为~3.5–3.6 × 10 ³ cm ⁻³ ，而在强地壳磁场区域，电子密度通常为~4.4 × 10 ³ cm ⁻³ ；弱地壳磁场区域的电子温度通常为~1,750 K，而强地壳磁场区域的电子温度通常为~1,950 K。

Next, we focus on the southern hemisphere only and consider how the electron density and temperature change with longitude. This approach allows us to continue to examine the effects of crustal fields on the ionosphere, since the regions of strongest crustal fields are located in the southern hemisphere and the crustal fields not only are most longitudinally constrained in the southern hemisphere but also remove the observational bias and other factors that contribute to the north-south hemispheric asymmetry in both the electron density and electron temperature.
接下来，我们只关注南半球，并考虑电子密度和温度如何随经度变化。这种方法使我们能够继续研究地壳磁场对电离层的影响，因为地壳磁场最强的区域位于南半球，而且地壳磁场不仅在南半球纵向变化最大，而且消除了观测偏差和其他因素，这些因素导致南北半球在电子密度和电子温度方面不对称。

Figures 2a and 2b show how the electron density and temperature vary with altitude and longitude for latitudes poleward of 30°S. This latitude restriction highlights latitudes where the crustal magnetic fields are the most longitudinally restricted, though we note that expanding the analysis to data at all southern latitudes shows similar trends to those seen in Figure 2. Below ~200 km altitude both the electron density and temperature are uniform with longitude, while above ~200 km altitude, both quantities show a dependence on longitude. At a given altitude above ~200 km, the electron densities are increased and electron temperatures are decreased at longitudes ~110–250° compared to values for noncrustal field regions, as seen in Figure 1.
图2a和2b显示了电子密度和温度在南纬30°极地纬度上如何随海拔和经度变化。这一纬度限制突出了地壳磁场纵向变化最明显的纬度，但我们注意到，将分析扩展到所有南纬数据后，发现与图2中显示的趋势类似。在海拔200公里以下，电子密度和温度均与经度一致，而在海拔200公里以上，这两个量均与经度相关。在200公里以上的特定高度，电子密度增加，电子温度降低，与非地壳磁场区域的数值相比，如图1所示，电子密度和电子温度在经度110-250°处变化。

Figures 2c and 2d show how the median electron density and electron temperature in four different 20 km altitude bins vary with longitude poleward of 30°S. At 160–180 km altitude (black line) the electron density and temperature are independent of longitude. As in Figures 2a and 2b, a longitudinal dependence is evident for both the electron density and temperature at higher altitudes. For example, in Figure 2c, at altitudes 320–340 km (red line), the median electron density increases from values of ~1.7 × 10³ cm⁻³ in noncrustal field regions to ~2.0 × 10³ cm⁻³ in the strong crustal field regions at longitudes 110–250°. In Figure 2d, at altitudes 320–340 km (red line), the median electron temperature decreases from values of ~2,400 K in noncrustal field regions to ~2,100 K in the strong crustal field regions at longitudes 110–250°.
图2c和2d显示了四个不同海拔20公里网格的电子密度和电子温度中值如何随南纬30°经度变化。在160-180公里高度（黑线），电子密度和温度与经度无关。如图2a和2b所示，在较高海拔处，电子密度和温度均与纬度有关。例如，在图2c中，在海拔320-340公里处（红线），电子密度中值从非地壳磁场区域约1.7×10 ³ cm ⁻³ 增加到强地壳磁场区域约2.0×10 ³ cm ⁻³ ，经度为110-250°。在图2d中，在纬度110-250°的非地壳磁场区域，电子温度的中值从约2400K下降到地壳强磁场区域的约2100K（红线）。

Upper and lower quartiles are shown for data from 160–180 km and 220–240 km. The median value for all longitudes is generally within those quartiles, so the statistical significance of the putative variations with longitude is not immediately clear. Further inspection, however, of the data at 220–240 km shows that the 24 of the 26 median values of electron density in 5° longitude bins between 120° and 250° are greater than the zonal average. Such a high proportion of observed values exceeding the median is extremely unlikely to have occurred by chance. By contrast, 10 of the 26 median values at 160–180 km exceed the zonal average, close to the half expected when longitude has no effect on electron density. Similarly, 1 of 26 electron temperature values exceeds the zonal average at 220–240 km.
上四分位数和下四分位数分别对应160-180公里和220-240公里的数据。所有经度的中值通常都在这些四分位数内，因此经度变化在统计学上的意义并不明显。然而，进一步检查220-240公里处的数据后发现，在120°至250°之间，5°经度间隔的电子密度中值有26个，其中24个大于区域平均值。如此高的观测值超出中位数的情况极不可能是偶然发生的。相比之下，在160-180公里处，26个中值中有10个超过了区域平均值，接近于当经度对电子密度没有影响时所预期的值的一半。同样，在26个电子温度值中，有1个在220-240公里处超过了区域平均值。

Based on Figure 2, we conclude that the longitudinal dependence of dayside electron density and electron temperature at high altitudes and southern latitudes is due to the presence of strong crustal magnetic fields at longitudes ~110–250°.
根据图2，我们得出结论：在高空和南纬地区，日面电子密度和电子温度的纵向变化与经度~110–250°处存在强地壳磁场有关。

In Figure 3 we present median altitude profiles of the electron density and temperature in the southern hemisphere for five different longitude regions. Electron density and temperature measurements are typically presented as altitude profiles, so it is interesting to compare median profiles in strong crustal field regions to profiles from regions without strong crustal fields. The two green lines represent longitudes with strong crustal magnetic fields (160–180° in dark green, 180–200° in light green), while the other colored lines represent longitudes without crustal fields. Each point in this figure below (above) 380 km is the median of over 200 (over 100) measurements.
图3展示了南半球五个不同经度区域电子密度和温度的中值高度分布。电子密度和温度测量通常以高度剖面形式呈现，因此将强地壳磁场区域的中值剖面与无强地壳磁场区域的中值剖面进行比较很有意义。两条绿色线代表存在强地壳磁场的经度（深绿色为160-180°，浅绿色为180-200°），而其他彩色线代表不存在地壳磁场的经度。下图（上图）中每一点距离380公里，是200（100）多次测量结果的平均值。

For both the electron density and temperature the green crustal field-influenced profiles diverge from the other three profiles at altitudes ~200 km. The electron density in the green crustal field-influenced profiles is ~30% larger than in the noncrustal field profiles at ~200–250 km altitude, ~40% larger at ~250–350 km altitude, and ~50–60% larger at ~350–500 km altitude. The electron temperature in the green crustal field-influenced profiles is ~95% of the electron temperature in the noncrustal field profiles at ~200–250 km altitude, ~90% at ~250–320 km altitude, and ~85% at ~320–500 km altitude. This represents a temperature difference of ~250 K at ~250–320 km altitude and ~450 K above 320 km altitude.
在电子密度和温度方面，受地壳磁场影响的绿色剖面在海拔约200公里处与其他三个剖面不同。在200-250公里高度，受地壳磁场影响的绿色剖面中的电子密度比非地壳磁场剖面中的电子密度高约30%，在250-350公里高度高约40%，在350-500公里高度高约50-60%。在200-250公里高度，受地壳磁场影响的绿色剖面中的电子温度约为非地壳磁场剖面中电子温度的95%，在250-320公里高度约为90%，在320-500公里高度约为85%。这意味着在250-320公里高度，温差约为250 K，而在320公里以上，温差约为450 K。

Upper and lower quartiles are shown for data at 60°–80° (weak crustal fields) and 160°–180° (strong crustal fields). Median electron densities in the strong crustal field region are not generally larger than the upper quartile densities in the weak crustal field region, so the statistical significance of the putative variations with longitude is not immediately clear. Further inspection, however, of the data shows that each of the 60 median electron density values reported at 5 km intervals between 200 km and 500 km is greater for the strong crustal field longitudes than for the weak crustal field longitudes. That is extremely unlikely to have occurred by chance. Similar trends are seen for electron temperature.
上四分位数和下四分位数分别对应60°–80°（弱地壳磁场）和160°–180°（强地壳磁场）的数据。强地壳磁场区域的中位电子密度一般不会大于弱地壳磁场区域的上四分位数密度，因此，推测的经度变化在统计学上的意义并不明显。然而，进一步检查数据后发现，在200公里至500公里之间，以5公里为间隔报告的60个电子密度中值中，强地壳磁场经度的每个数值都大于弱地壳磁场经度的每个数值。这种情况极不可能是偶然发生的。电子温度也呈现类似的趋势。

Figure 3c shows electron temperature as a function of electron density for the five sets of results shown as vertical profiles in Figures 3a and 3b. Strikingly, the five profiles are very similar. The two profiles from strong crustal field regions do not appear different from the other three profiles in this representation.
图3c显示了电子温度与电子密度的关系，图3a和3b中的五组结果以垂直剖面形式显示。值得注意的是，这五个剖面非常相似。图中来自强地壳磁场区域的两个剖面与其他三个剖面看起来并无不同。

Overall, Figures 1-3 show that above 200 km altitude in the dayside Martian ionosphere, regions of strong crustal magnetic fields feature larger electron densities and colder electron temperatures than regions without crustal magnetic fields. The influence of the crustal fields on both density and temperature increases with altitude, but between ~200 and ~400 km altitude, the presence of strong crustal fields typically increases the electron density by ~30% from values in noncrustal field regions and decreases the electron temperature by ~10–15% from values in noncrustal field regions, or about ~250–450 K.
总体而言，图1-3显示，在火星电离层日侧海拔200公里以上的地方，强地壳磁场的区域电子密度和电子温度比没有地壳磁场的区域更大、更低。地壳磁场对密度和温度的影响随高度增加而增强，但在200至400公里高度之间，强地壳磁场的存在通常会使电子密度比非地壳磁场区域高出约30%，而电子温度则比非地壳磁场区域低约10-15%，即约250-450开尔文。

3 Discussion  3 讨论

Our survey of dayside MAVEN LPW data shows how the electron density and temperature changes in regions of strong crustal fields, which provides useful information for determining the processes by which crustal fields influence the structure of the dayside Martian ionosphere. The key finding is that electron densities are larger but electron temperatures are smaller, in regions of strong crustal field than in regions of weak crustal field.
我们对日面MAVEN LPW数据的调查显示了强地壳磁场区域电子密度和温度的变化，这为确定地壳磁场影响火星电离层结构的过程提供了有用的信息。主要发现是，在强地壳磁场区域，电子密度更大，但电子温度更低。

Our findings for electron density are consistent with the results of previous work. For example, using Mars Express MARSIS AIS measurements, Andrews, Andersson, et al. (2015) reported that the dayside electron density at altitudes 350–400 km was ~40% larger in regions of strong crustal magnetic fields compared to average values. Similarly, we found that the green crustal field-influenced median profiles in Figure 3 are ~50% larger than noncrustal field profiles at altitudes 350–400 km.
我们的电子密度测量结果与之前的工作结果一致。例如，根据火星快车MARSIS AIS测量结果，安德鲁斯、安德森等人（2015）报告称，在强地壳磁场区域，350-400公里高度的白天电子密度比平均值高出约40%。同样，我们发现，在350-400公里高度上，图3中受地壳磁场影响的绿色中间剖面比非地壳磁场剖面大50%左右。

Our most significant result concerns the influence of crustal fields on the electron temperature, as MAVEN's electron temperature measurements are the first since Viking's single vertical profile. This work shows that Mars's crustal magnetic fields affect its ionospheric electron temperature. We found that regions of strong crustal magnetic field feature electron temperatures that are ~10–15%, or ~250–400 K, cooler than regions without strong crustal fields at altitudes between ~200 and ~400 km.
我们最重要的成果是地壳磁场对电子温度的影响，因为MAVEN的电子温度测量是自Viking的单一垂直剖面以来的首次测量。这项工作表明，火星的地壳磁场会影响其电离层电子温度。我们发现，在高度约为200至400公里的区域，强地壳磁场区域的电子温度比无强地壳磁场的区域低约10-15%，即约250-400 K。

One hypothesis for the observations presented in this work is that ionospheric properties, including density and temperature, track fixed pressure levels. This hypothesis is suggested by Figure 3c, which shows that electron temperature is fixed with respect to electron density regardless of crustal magnetic field strength. This hypothesis is reasonable at low altitudes, where the ionosphere is photochemically controlled (Bougher et al., 2001), but questionable at high altitudes, where transport plays a significant role. Nevertheless, if that supposition were true, then the longitudinal variations in ionospheric properties could simply be a consequence of longitudinal variations in the neutral atmosphere associated with crustal magnetic fields. To test this hypothesis, we examined MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS, Mahaffy et al., 2015) data from a period that overlaps the period from which LPW data were selected. Figure 4 shows the number density of neutral argon in a format equivalent to the display of electron density and temperature in Figure 1. In order to reach the highest altitudes possible, the inert species argon is selected as it is the species whose density is measured most reliably by NGIMS. As in Figure 1, the variations with latitude that are visible in Figure 4 are caused by variations in periapsis latitude, the Mars-Sun distance, and solar zenith angle over the observing period. Our interest is in whether the neutral measurements show longitudinal variations similar to those displayed by the plasma measurements. They do not, so the hypothesis is not supported. Electron temperature is fixed with respect to electron density in these observations, but these ionospheric properties are not fixed with respect to neutral density.
本研究中提出的观测结果的一个假设是，电离层的特性（包括密度和温度）与固定的压力水平有关。图3c表明，电子温度与电子密度有关，而与地壳磁场强度无关，从而提出了这一假设。这个假设在低纬度地区是合理的，因为那里的电离层受光化学控制（Bougher等人，2001年），但在高纬度地区却值得商榷，因为那里的传输起着重要作用。然而，如果假设成立，那么电离层属性的纵向变化可能只是与地壳磁场相关的中性大气纵向变化的结果。为了验证这一假设，我们检查了MAVEN中性气体和离子质谱仪（NGIMS，Mahaffy等人，2015）的数据，该时间段与LPW数据所选时间段重叠。图4显示的是中性氩的数密度，其格式与图1中电子密度和温度的显示格式相同。为了尽可能达到最高海拔，我们选择了惰性气体氩，因为它是NGIMS测量密度最可靠的物质。如图1所示，图4中随纬度变化而产生的变化是由观测期间近地点纬度、火星-太阳距离和太阳天顶角的变化引起的。我们感兴趣的是中性测量是否显示出与等离子测量相似的纵向变化。两者并不相同，因此该假设并不成立。在这些观测中，电子温度与电子密度是固定的，但电离层的这些特性与中性密度并非固定不变。

To interpret the finding that regions of strong crustal field feature larger electron densities and smaller electron temperatures than regions of weak crustal field, we consider two idealized end-member scenarios.
为了解释地壳磁场强的区域电子密度更大、电子温度更低的发现，我们考虑了两种理想化的极端情况。

First, in photochemically controlled regions like the lower ionosphere, if the electron temperature is isothermal, electron densities decrease exponentially with altitude with a plasma scale height that is twice the neutral scale height ( e.g., Withers, 2009, and references therein). This behavior is modified if the electron temperature is not isothermal, because the electron temperature controls the main plasma loss process, the dissociative recombination of the dominant O₂⁺ ion species. The dissociative recombination coefficient for O₂⁺ is proportional to T_e^−0.7 (Schunk & Nagy, 2009) so that the electron density is proportional to T_e^0.35, where T_e is the electron temperature (e.g., Withers, 2009, and references therein). Higher electron temperatures should be associated with higher electron densities.
首先，在光化学控制区域（如低电离层），如果电子温度是等温的，电子密度会随着高度呈指数下降，等离子体尺度高度是中性尺度高度的两倍（例如，Withers，2009年，及其中的参考文献）。如果电子温度不是等温的，这种行为就会改变，因为电子温度控制着主要的等离子体损耗过程，即主要O ₂ ⁺ 离子物种的解离重组。O ₂ ⁺ 的解离复合系数与T _e ^−0.7 成正比（Schunk和Nagy，2009），因此电子密度与T _e ^0.35 成正比，其中T _e 是电子温度（例如，Withers，2009，及其中的参考文献）。电子温度越高，电子密度应该越高。

Second, in transport-controlled regions like the high-altitude ionosphere, electron temperature may also affect electron densities. Hotter electron temperatures create larger pressure gradients, which lead to larger electron densities at high altitude as plasma flows upward more strongly. In the idealized case of diffusive equilibrium, the plasma scale height H_p is proportional to the sum of the ion and electron temperatures. Since the electron temperature is much greater than the ion temperature (Schunk & Nagy, 2009), the plasma scale height is effectively proportional to the electron temperature. Again, higher electron temperatures should be associated with higher electron densities.
其次，在运输控制区域，如高空电离层，电子温度也会影响电子密度。电子温度越高，压力梯度越大，导致高空电子密度增加，因为等离子体向上流动得越强烈。在理想的扩散平衡情况下，等离子体尺度高度H _p 与离子和电子温度之和成正比。由于电子温度远高于离子温度（Schunk & Nagy，2009），等离子体尺度高度实际上与电子温度成正比。同样，电子温度越高，电子密度也越高。

Thus, changes in electron temperature may significantly and directly affect electron density. So if the crustal magnetic fields somehow change electron temperatures, then they will also change electron densities. However, in both these idealized end-member scenarios, higher electron temperatures are associated with higher electron densities. In the observations, higher electron temperatures are associated with low electron densities. It appears that processes associated with the crustal magnetic fields are directly affecting both the densities and temperatures of ionospheric electrons.
因此，电子温度的变化可能会显著且直接地影响电子密度。因此，如果地壳磁场以某种方式改变了电子温度，那么它们也会改变电子密度。然而，在这两种理想化的极端情况下，电子温度越高，电子密度也越高。在观测中，较高的电子温度与较低的电子密度相关。与地壳磁场相关的过程似乎直接影响了电离层电子的密度和温度。

What processes could explain the observed changes in the electron density and electron temperature in regions of strong crustal field? Models and data suggest that the transition between the low-altitude photochemically controlled region and the high-altitude transport controlled region in the Martian ionosphere occurs at ~200 km altitude (Barth et al., 1992; Nagy & Cravens, 2002; Withers et al., 2012). The observation that ionospheric effects of strong crustal fields are noticeable only above ~200 km altitude on the dayside, along with the fact that the changes in electron density and temperature are inconsistent with photochemical equilibrium theory, suggests that the processes by which crustal magnetic fields influence the Martian ionosphere over spatially extended regions have minimal effect on photochemical production and loss processes in the dayside ionosphere. This conclusion does not necessarily apply to localized regions of cusp-like magnetic field.
什么过程可以解释在强地壳磁场区域观察到的电子密度和电子温度的变化？模型和数据表明，火星电离层中低纬度光化学控制区域和高纬度传输控制区域之间的过渡发生在约200公里高度处（Barth等人，1992；Nagy和Cravens，2002；Withers等人，2012）。根据观察，只有在日面200公里以上的高空，才能明显感受到地壳强磁场对电离层的影响，同时电子密度和温度的变化与光化学平衡理论不符，这表明地壳磁场影响火星电离层空间扩展区域的过程对日面电离层的光化学产生和损失过程影响甚微。这一结论并不一定适用于局部区域的尖峰状磁场。

In previous studies that reported enhanced electron densities at the ionospheric peak in cusp-like magnetic fields, Nielsen et al. (2007) and Gurnett et al. (2008) suggested that the electron density enhancement was due to plasma heating by either the two-stream plasma instability driven by the solar wind induced electric field or by solar wind electrons that can access the lower ionosphere along open magnetic field lines. This mechanism cannot explain the observations reported here, which concern higher altitudes and are not focused solely on cusp-like regions.
在此前的研究中，尼尔森等人（2007）和古内特等人（2008）报告了在类似尖峰的磁场中电离层峰值处的电子密度增强，他们认为电子密度增强是由于太阳风引起的电场驱动的两流等离子体不稳定或太阳风电子沿开放的磁场线进入低电离层，从而引起等离子体加热。这种机制无法解释本文所报告的观测结果，这些观测结果涉及更高的高度，且不仅仅关注类似尖端的区域。

Andrews, Andersson, et al. (2015) suggested that the electron density increase observed in regions of strong crustal fields at altitudes 350–400 km is caused by changes to the magnetic field inclination, or the angle that a field line makes with respect to the planetary surface. The field is, on average, less horizontal in strong crustal field regions than elsewhere, which affects the vertical and horizontal transport of plasma. It is unclear whether this can explain the observed behavior of electron temperatures.
安德鲁斯、安德森等人（2015）提出，在高度为350-400公里的强地壳磁场区域观测到的电子密度增加是由磁场倾斜度变化或磁场线与行星表面夹角的变化引起的。在强地壳磁场区域，磁场平均水平度低于其他区域，这会影响等离子体的垂直和水平传输。目前尚不清楚这是否能够解释所观测到的电子温度行为。

The ionospheric magnetic field lines in regions of strong crustal field may also have a different field topology, indicating whether one, neither, or both ends of a field line are rooted in the planet, compared to the magnetic field lines in the surrounding areas. Recent analysis by Xu et al. (2017) suggests that the magnetic fields at dayside altitudes where we have observed enhanced electron densities are most likely to be closed, or have both ends rooted in the planet, rather than having one or both ends open to the solar wind. They studied electron energy and pitch angle distributions measured by the MAVEN Solar Wind Electron Analyzer instrument from December 2014 through the beginning of May 2016, a similar interval to the one we have used here, and created 3-dimensional maps of how the magnetic topology varies with altitude and Mars latitude and longitude. They found that at altitudes between 200 and 400 km, the most common field line topology was closed (occurrence rate greater than 50%) but also that, at these altitudes, field lines were even more likely to be closed and less likely to be open in regions of strong crustal fields than in regions of weak crustal fields.
与周围区域的磁场线相比，地壳磁场强的区域中的电离层磁场线也可能具有不同的磁场拓扑结构，表明磁场线的两端或一端是否植根于地球。徐等人（2017）的最新分析表明，在电离层中，我们观察到电子密度增强的日侧高度处的磁场最可能是闭合的，或者两端都植根于行星，而不是一端或两端都暴露在太阳风中。他们研究了2014年12月至2016年5月初期间（与我们这里使用的间隔时间相似）由MAVEN太阳风电子分析仪仪器测量的电子能量和俯仰角分布，并创建了磁拓扑随高度和火星经纬度变化的三维地图。他们发现，在200至400公里的高度，最常见的磁场线拓扑结构是闭合的（发生率超过50%），但同样在这些高度，与弱地壳磁场区域相比，强地壳磁场区域更容易出现闭合磁场线，而出现开放磁场线的可能性更小。

The enhanced electron densities in regions of strong crustal fields could then be explained by the fact that electrons in these regions are trapped on closed field lines, with relatively little loss to the solar wind compared to on open or draped field lines. Low electron temperatures in strong crustal field regions could perhaps be caused by the longer lifetime of plasma trapped on closed field lines. Electrons, when first generated, are hotter than neutrals and ions and therefore cool over time (e.g., Schunk & Nagy, 2009). This hypothesis predicts that ion temperatures are also relatively cool in regions of strong crustal field, which can be tested using data from the MAVEN SupraThermal and Thermal Ion Composition instrument (McFadden et al., 2015). Further work is necessary to test whether this hypothesis can explain the fixed density/temperature relationship illustrated by Figure 3c.
强地壳磁场区域电子密度的增强可以用以下事实来解释：这些区域中的电子被困在闭合的磁场线上，与开放或垂悬的磁场线相比，它们对太阳风的损耗相对较小。强地壳磁场区域电子温度较低，可能是因为被困在闭合磁场线上的等离子体寿命较长。电子在最初产生时比中子和离子更热，因此会随着时间的推移而冷却（例如，Schunk和Nagy，2009）。该假设预测，在强地壳磁场区域，离子温度也相对较低，这可以通过MAVEN超热和热离子成分仪器的数据来检验（McFadden等人，2015）。需要进一步研究才能验证该假设能否解释图3c所示的固定密度/温度关系。

4 Conclusions  4 结论

We have surveyed MAVEN LPW measurements of the electron density and temperature in the dayside Martian ionosphere and studied how these quantities change in response to the presence of strong crustal magnetic fields. Specifically, we have studied how the electron density and temperature vary with planetary latitude and longitude, which are excellent proxies for the spatially restricted crustal fields. We found that the presence of strong crustal fields has no effect on the electron density or temperature below 200 km altitude. Above 200 km altitude, areas with strong crustal fields feature enhanced electron density and cooler electron temperature than in surrounding regions. The effect of crustal fields on the electron density and temperature increases with increasing altitude, but it typically represents about a ~30% increase in the electron density and a ~10–15% decrease in the electron temperature, corresponding to ~250–400 K colder temperatures.
我们调查了MAVEN LPW对火星电离层白天一侧电子密度和温度的测量结果，并研究了这些量如何随强地壳磁场的存在而变化。具体来说，我们研究了电子密度和温度如何随行星纬度和经度变化，这些纬度和经度是空间受限的地壳磁场的极佳替代指标。我们发现，强地壳磁场对200公里高度以下的电子密度或温度没有影响。在200公里以上的高空，强地壳磁场区域的电子密度和电子温度比周围区域更高和更低。地壳磁场对电子密度和温度的影响会随着海拔的升高而增强，但通常表现为电子密度增加约30%，电子温度降低约10-15%，相当于温度降低约250-400 K。

Our finding with respect to the electron density is consistent with previous studies that reported enhanced electron densities in magnetic cusps, as well as larger surveys that showed enhanced electron densities in regions of strong crustal fields at high altitudes. However, our finding that electron temperatures are coolest in regions of strong crustal fields is both new and surprising, since MAVEN provides the first electron temperature measurements in the Martian ionosphere since the single profile measured by the Viking lander and previous studies have attributed the increase in electron density near crustal fields to plasma heating.
我们关于电子密度的发现与之前的研究一致，即磁尖峰处的电子密度增强，而大型调查则显示，高纬度强地壳磁场区域的电子密度增强。然而，我们发现强地壳磁场区域的电子温度最低，这既是一个新发现，也令人惊讶，因为自维京号着陆器测量单一剖面以来，MAVEN首次提供了火星电离层的电子温度测量数据，而此前的研究将地壳磁场附近电子密度的增加归因于等离子体加热。

The observed anticorrelation of electron densities and temperatures is not consistent with explanations based on idealized photochemical equilibrium, which is anyway unlikely to apply at the relevant altitudes, or idealized plasma diffusion. However, processes associated with plasma transport are favored by the fact that these effects of crustal fields are observed only above 200 km, which is the altitude at which plasma transport becomes significant. One possible explanation for the observations is that closed field lines on which electrons can be trapped are more prevalent in strong crustal field regions than elsewhere. Trapped on closed field lines, electrons are protected against loss processes involving the solar wind. This would lead to longer plasma lifetimes, higher densities, and lower temperatures.
所观测到的电子密度和温度的反相关性不符合基于理想化光化学平衡的解释，因为这种平衡无论如何都不可能适用于相关高度，也不符合理想化等离子体扩散的解释。然而，与等离子体传输相关的过程却得到了证实，因为地壳磁场的影响仅在200公里以上的高度才能观察到，而200公里正是等离子体传输开始显著的高度。对观测结果的一种可能的解释是，在强地壳磁场区域，电子被困的闭合磁场线比其他地方更普遍。被困在闭合的磁场线上的电子可以免受太阳风带来的损耗。这将延长等离子体的寿命，提高密度，降低温度。