Many missions to Mars have helped to show the rich and ever-changing nature of the plasma environment around Mars. Together, the absence of a strong global-scale field and the presence of localized regions of crustal magnetization ensure that the magnetosphere and ionosphere of Mars display unique behavior not seen anywhere else in the solar system. The need to determine how the ionosphere varies on small spatial and temporal scales motivates the three science goals of this proposal.
- How does the ionosphere respond to solar wind dynamics?
- What small-scale structures exist in the lower ionosphere of Mars?
- How is the ionosphere on the night side maintained?
Figure 1: Density and magnetic field profiles of 3 inbound passes by the MAVEN spacecraft close to subsolar point (information presented here is only measured every other orbit). The densities are shown in logarithmic scale e-, O2+, O+ and CO2+ represented by black, red, blue, and green lines. The total magnetic field is shown in linear scale by the pink lines; where the vertical lines represent 50, 100, 150, and 200 nT. Light ion densities (eg H+) are not shown and account for differences between ion and e- densities. In panel b, MAVEN was in the solar wind above 380 km, as indicated by the cutoff of the Ne data.
Discoveries by the recently arrived MAVEN mission [Jakosky et al., 2015a,b] have emphasized the dynamic and complex nature of the Martian ionosphere [Bougher et al., 2015]. Figure 1, which shows vertical profiles of electron and ion densities and magnetic field from three closely-spaced orbits from the MAVEN mission. It is clear that large changes in ionospheric conditions occur on time scales much shorter than the ~5 hour orbital period. Understanding the complex interaction of the Mars space environment with the solar wind and with the crustal field of the planet requires high cadence observations. Given the ever-increasing pace and sophistication of exploration activity at Mars, it is important to establish the extent to which the ionosphere will impact navigation and communications. That requires knowledge of how the ionosphere varies over small spatial and temporal scales that no mission has yet provided. The MarsCAT mission will acquire multipoint measurements from a constellation of CubeSats that will determine this operational risk and answer significant scientific questions about how the unique magnetic fields of Mars affect the planet’s surrounding plasma environment.
How does the ionosphere respond to solar wind dynamics?
On the dayside, the lower ionosphere, which encompasses the main ionosphere peak and contains >80% of the plasma, is well-described by simple photochemical theory [Nair et al., 1994]. It is only above ~180 km that plasma transport becomes a significant process. Here plasma densities are elevated above those expected from photochemical processes alone. Above ~300 km, ionospheric densities are remarkably uniform with altitude (Fig 1a). There is often no ionopause, or sharp upper boundary at which plasma densities abruptly decrease [Vogt et al., 2015]. Instead, the shocked solar wind inside the shock frequently compresses the dayside ionosphere significantly (Fig 1b). In this example, the shocked solar wind penetrated to ~350 km altitude as indicated by the cutoff of the Ne data and the heaviest ions (O2+, CO2+) are eroded away down to altitudes as low as 300 km. Such changes occur rapidly, as the bulk of the upper ionosphere departs and returns (Fig 1c) in less than MAVEN’s five hour orbital period. The single-point measurements of MAVEN preclude robust separation of temporal and spatial effects. MarsCAT will be able to determine whether such dramatic changes in the upper ionosphere occur over the entire ionosphere on short timescales or in localized regions with potentially longer timescales. Given the complex topology of the draped solar wind magnetic field interacting with localized crustal fields [Mitchell et al., 2001], variations on small spatial scales are a viable possibility. MarsCAT will measure local electron density at each spacecraft and also the integrated line-of-sight electron density between each spacecraft. The combination of these measurements will be very useful for identifying spatial variations in the ionosphere.
What small-scale structures exist in the lower ionosphere of Mars?
MAVEN and pre-MAVEN observations have shown that small-scale variations in plasma density are common in the ionosphere of Mars [Nielsen et al., 2007]. At Earth, such fluctuations degrade the effectiveness of radio navigation and communications systems. The corresponding effects at Mars are uncertain due to inability in current observations to distinguish spatial and temporal variations. The MarsCAT constellation of spacecraft will provide the observations needed to assess the impact of Mars plasma fluctuations on radio nav/comm systems.
The MAVEN mission has demonstrated that Rayleigh Taylor (RT) instability [Fowler et al., 2016] can grow at Mars. Observations below 200 km suggest this instability mainly occurs at dawn and dusk but potentially also at noon [Nielsen, op cit.]. The dayside density profile of Figure 1a has higher density at 350 km than at 300 km suggesting if this is RT unstable then plasma bubbles can be created here.
How is the ionosphere on the night side maintained?
Without ionization from sunlight, densities in the nightside ionosphere might be expected to decrease rapidly [Verigin et al., 1991; Lillis et al., 2009; Fowler et al., 2015]. Yet significant densities have been observed on the deep nightside. Idealized modeling suggests that constant electron precipitation associated with field-aligned currents (FAC) could maintain the observed nightside densities. However, fluxes of precipitating electrons, which produce aurora, are neither constant nor uniform. It remains unclear whether temporally and spatially varying fluxes of precipitating electrons can maintain the nightside ionosphere [Xu et al., 2016]. FAC are often observed on the outer edges of crustal field regions where the field geometry has a boundary between closed and open regions. Density variations of a factor of 6 have been observed over a distance of 4 km, which indicates there is small-scale structure in the ionosphere. The night ionosphere observations by MarsCAT will allow a better understanding of FAC and the source to the night side ionosphere. MarsCAT will be able to evaluate how important crustal fields are for FAC that can create aurora [Lilensten et al., 2015].
The MarsCAT mission consists of two 6U CubeSats placed in Mars orbit. It is equipped with magnetometers, Faraday cups, double-Langmuir probes, and an interspacecraft total electron content radio occulation experiment. Together these will measure the magnetic fields, particle density, charge, velocity, and temperature across a range of energies and enable us to achieve the following goals:
- Determine the cause of the density anomalies seen in the Martian ionosphere
- Quantify the energy balance of the Mars ionosphere
- Determine the response of the ionospheric plasma temperature to solar activity and the solar cycle
- Quantify the role of residual magnetism upon the structure and dynamics of the Martian ionosphere
- Separate spatial from temporal effects to understand the dynamics of minimagnetospheres in the Martian system
- Quantify the physics of reconnection in the weak magnetic field environment ofminimagnetospheres
- Determine the role of plasma disturbances such as flux ropes and Kelvin-Helmholtz waves in the solar wind-Martian interaction
- Boundary Interactions
Most exploration of the ionosphere of Mars has taken the form of electron density measurements made by either the radio occultation technique [liore et al., 1965, 1973], or vertical profiles made by topside sounding [Nielsen, 2004; Nielsen et al., 2007]. The retarding potential analyzers (RPAs) on the Viking landers have provided the only ion composition, ion temperature and electron temperature data available [Hanson et al., 1977]. Understanding the aeronomy of Mars is a necessary and important part of understanding the history of the Martian climate [Withers et al., 2011]. As the first aeronomy mission to Mars, MAVEN is the first spacecraft to provide simultaneous electron density and temperature, ion density, composition and temperature, and magnetic field data. It may not provide ion flow velocity data. The first MAVEN data will not be published until after this proposal is due. It is clear that a single mission will not be able to provide sufficient data to answer all open questions about Martian aeronomy. In particular, a single spacecraft cannot separate spatial from temporal variations, nor can it study extended structures in two dimensions. Finally, the primary MAVEN mission is one year, and the most optimistic extended mission scenario lasts 9 years. The MarsCAT mission will address two of the four top Martian aeronomy goals identified in the aeronomy white paper submitted to the recent Decadal Survey [Withers et al., 2011]
Determine the cause of the density anomalies seen in the Martian ionosphere
The overall lack of existing data on the Mars ionosphere have precluded development of such basic data/modeling products as maps of the ionosphere. Models predict that the Mars ionosphere should be a simple photochemical Chapman layer [Nair et al., 1994]. Generally, the observed solar zenith angle dependence confirms this view. However, the limited existing data show anomalies. The ionosphere is not as uniform as expected. There is an enhancement at 90° E longitude [Krymskii et al., 2003]. The density data exhibit a much larger variance than the models predict [Mahajan et al., 2007; Nielsen et al., 2007]. Possible explanations include the presence of transport processes, variations in the underlying neutral density and plasma wave processes. More density data are required to enable us to explore the parameter space of possible sources of this variance. The initial phase of the MarsCAT mission will include global mapping observations of electron density made by two spacecraft in nearly circular low Mars orbit. The ability of the MarsCATs to work together to measure the total electron content between them will extend and speed up the mapping process. Use of two spacecraft will enable separation of spatial from temporal features.
Quantify the energy balance of the Mars ionosphere
Determine the response of the ionospheric plasma temperature to solar activity and the solar cycle
Goals 3 and 4 are closely related. The historical data record contains just two single profile temperature measurements of the Martian ionospheric plasma. MAVEN is beginning the task of rectifying this deficiency. MarsCAT will expand this effort, adding many months of data taking to the effort. The mission will map the temperatures globally at altitudes of 180-5000 km for the entire primary mission. The mission will use “formation flying” in which the MarsCATs will be in elliptical orbits with periareion at ~180 km at mid-latitude in the Southern hemisphere and apoareion at or above ~5000 km. The orbital planes will be separated by a few degrees. Using these approaches, we will make both global maps of plasma properties at low altitude and altitude profiles featuring the region of the ionosphere where the most structure is expected.
Ionospheric energy balance and hence temperatures are driven by solar EUV radiation input. Our investigation will include correlation studies of the response of the plasma temperatures to solar activity as well as validation studies of ionospheric energy balance models. MarsCAT data will provide a unique opportunity to examine the ionospheric energy balance by comparing predicted to observed temperatures and by using observed temperature gradients to estimate heat flow. The observation campaign will be conducted in conjunction with a modeling effort that will explore the effect of both solar wind and neutral atmosphere dynamics on the energy balance of the ionosphere
Quantify the role of residual magnetism upon the structure and dynamics of the Martian ionosphere
Mars Global Surveyor (operating 1997-2006) measured and mapped the vector magnetic field locally at its operating altitude of 378 km with two triaxial fluxgate magnetometers [Acuña et al., 2001]. In addition, the MGS used an electron reflectometry technique to infer the total magnetic intensity at ~170 km altitude [Mitchell et al., 2001]. These data show regions of peak magnetic intensity near 180° E longitude in the Southern hemisphere. The MarsCAT mission plan will include intervals with periareion at or below 200 km in this area. The triaxial MarsCAT magnetometers will enable us to make a vector map of the magnetic field in the high intensity regions. These maps will substantially improve our understanding of the effect these regions have on the structure and dynamics of the ionosphere.
Separate spatial from temporal effects to understand the dynamics of minimagnetospheres in the Martian system
The complex crustal fields observed by MGS have high field regions where the field is so strong that closed “minimagnetospheres” can form [Mitchell et al., 2001]. These regions must have some closed field lines. It has been speculated that the outermost field lines can reconnect to the IMF and be opened. Lower altitude (~200km) vector measurements by MarsCAT will be used to investigate the validity of this speculation. The plasma in these minimagnetosphere regions should have different dynamics and thermal balance than the surrounding regions. MarsCAT will study the plasma thermal balance and dynamics in these regions by observing electron and ion temperatures, electron density and ion flow velocity. By taking data at two nearby locations, MarsCAT will permit the separation of spatial structure from temporal variations.
We will investigate the magnetosheath density variations predicted by multifluid models in the presence of minimagnetospheres [Harnett and Winglee, 2003; 2005; 2006]. According to these models, minimagnetospheres enhance flank sheath densities by factor of 2. The exception is when draped magnetosheath magnetic fields lie antiparallel to the minimagnetosphere magnetic fields. At these times, reconnection may occur, and create magnetosheath density voids in the regions downstream from the minimagnetospheres.
The presence of minimagnetospheres and reconnection may also account for observations interpreted as indicating that the altitude of the pile-up boundary rises for eastward IMF orientations but falls for southwest IMF orientations when the subsolar latitude lies in the northern hemisphere [Brain et al., 2005].
Quantify the physics of reconnection in the weak magnetic field environment of minimagnetospheres
The fact that Mars may have “minimagnetosphere” regions with associated reconnection raises an interesting possibility. One might look for parallel currents, plasma heating and out flows, particle acceleration and even substorms. This question lies in the realm of fascinating speculation.
Determine the role of plasma disturbances such as flux ropes and Kelvin-Helmholtz waves in the solar wind-martian interaction
The plasma wave spectrum within the Mars ionosphere is largely unknown. A preliminary survey is being performed by MAVEN as the proposal is being written. It is clear that a single mission survey will barely begin to catalog and understand either the large scale disturbances or the plasma wave spectrum of the Mars ionosphere. Transient events are common in the vicinity of the ionopause. Their significance in removing the planetary atmosphere [Brace et al., 1982; Russell et al., 1982] depends upon their dimensions, motion, and occurrence rates as a function of solar wind conditions, aspects more readily determined from two-point measurements than from the single point measurements of past missions.
Plasma blobs [Brace et al., 1980; 1982] and magnetic flux ropes [Russell and Elphic, 1979] observed in the vicinity of the ionopause have been interpreted in terms of the Kelvin- Helmholtz instability [Wolff et al., 1980], curvature forces pulling field lines into the ionosphere [Russell et al., 1987], and magnetic reconnection, whether between sheared magnetosheath magnetic fields or between sheared magnetosheath [Dreher et al., 1995] and minimagnetosphere magnetic fields [Ma et al., 2002; Harnett, 2009; Brain et al., GRL, 2010]. Two point measurements will enable to simultaneously monitor the input or boundary conditions for the instabilities that generate these structures and the resulting outflow, e.g., to test for the predicted effects of density gradients [Duru et al., 2009] and magnetosheath magnetic field orientations. They will allow us to gauge event dimensions and determine event velocities by timing methods. Minimum variance algorithms applied to the magnetic field measurements will permit determination of boundary orientations [Bertucci et al., 2005]. Particle observations within the structures will aid in the determination of magnetic topologies.
In particular, we will use MarsCAT observations to test the criteria governing the Kelvin- Helmholtz instability when wavelike structures are observed, and the likelihood of reconnection when flux ropes are observed. We will seek evidence for the giant plasma flow vortices reported by Pope et al. . Finally, we will test the prediction that the occurrence of the Kelvin-Helmholtz instability is favored in the hemisphere with the outward pointing magnetosheath convection electric field [Terada et al., 2002]. MarsCAT will make wave observations with the magnetometers, and with the plasma instruments. When the plasma instruments are sweeping, the sweep rate will set the upper frequency limit of the observations. Higher frequency modes will be observed by setting the Langmuir probe and Faraday cup to fixed bias and measuring current collection.
Previous observations [e.g., Mahajan et al., 2007] suggest that the structure of Mars ionosphere cannot be explained by a photochemically formed layer. The interactions between the ionosphere and lower neutral atmosphere probably play important roles in shaping the Mars ionosphere. In addition, solar variations influence Mars ionosphere by modifying the ionospheric energy budget. The proposed global observations of Mars ionosphere by MarsCAT will help us further characterize Mars ionosphere. The modeling effort will run three- dimensional (3-D) simulations that will help us understand the physics behind the 3-D structure of Mars ionosphere. In addition, newly-developed 3-D simulations of Mars ionosphere that involve the lower neutral atmosphere can investigate the influence of solar activity and atmospheric dynamics (e.g., interaction with the natural atmosphere waves, diffusion, transport) on Mars ionosphere.
Therefore, we propose to include comparative studies between measurements and numerical simulations to further advance understanding of Mars ionosphere. These comparative studies will benefit both the observation and model communities. First, the proposed global measurements will help validate and improved numerical models. Second, the full physics and dynamics in the numerical models will help us explain the observations. To be consistent with the proposed global mapping observations, we will use time-dependent 3-D models to simulate Mars ionosphere and its spatio-temporal structures as described in the implementation section.
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