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Home , Magnetosphere, Voyager 2

WHITE PAPER : SOLAR AND PHYSICS DECADAL SURVEY 2013-2022 EXPLORATION OF THE

Primary Author: Sébastien Hess LASP – University of Colorado at Boulder

Co-Authors/Endorsers:

Mackenzie Lystrup, LASP – University of Colorado Fran Bagenal, LASP – University of Colorado Licia C. Ray, LASP – University of Colorado Abigail Rymer, APL – Johns Hopkins University William S. Kurth, University of Iowa Chris Arridge, MSSL – University College London, UK Philippe Zarka, LESIA - Observatoire de Paris, France

1) Overview

Our understanding of the interaction between the and 's magnetosphere has been deepened by the exploration of the Jovian and Saturnian by the and Cassini spacecrafts. In the last decade we have discovered that the magnetospheres of and are much more different from Earth’s than previously thought, involving many processes which have no direct terrestrial analogs. In particular, the magnetospheres of these gas giants revealed themselves to be full of , created through the volcanic activity of inner satellites (mostly and ). These internal plasma sources dictate a large part of the dynamics of the Jovian and Saturnian magnetospheres. The study of these different magnetosphere gave us a point of comparison for the study of the processes occurring in the Earth magnetosphere.

The better understanding of the Jupiter and Saturn magnetospheres these missions gave us also allowed the community to better define the unanswered questions and design better adapted to answer them. Two such examples are the mission, which will study auroral phenomena and the Jovian magnetic and gravitational from a orbit, and the EJSM mission, which will, in part, study the interactions between the magnetosphere and its satellites.

However, if we are to fully understand what it means to live with a , we must gain a full understanding of how every magnetosphere in the interacts with our star, including those magnetospheres in the outer solar system that exhibit extreme dynamics.

Despite the gains we have made understanding planetary magnetospheres and their interactions with the sun, there are still major gaps in our understanding of the magnetospheres of Uranus and . These systems are characteristically very different from the other magnetospheres in the solar system. These ice giants were briefly visited by Voyager 2, which showed and magnetospheres whose characteristics are very different from the terrestrial and gas giant systems. From a magnetospheric physics point of view, the Uranian system is the most fascinating as not only is Uranus’s rotation axis nearly aligned with its orbital plane (i.e. tilt ~ 98°), but also its magnetic dipole axis is tilted ~59° from the rotation axis and very off-centered [Q3 magnetic model, Connerney, et al. 1987]. This atypical magnetic configuration introduces intriguing questions. We propose here a Middle size mission to explore the Uranus magnetosphere.

The next Flagship mission will explore the Jovian or the Saturnian system. Although Uranus and Neptune are not slated for a flagship mission, the Neptunian system would be a logical choice for a following Flagship mission, particularly due to the presence of cryovolcanism on its satellite . Hence, no large mission to Uranus might be sent before a few decades. Nevertheless, Uranus is closer and presents a more intriguing magnetosphere, urging for an exploration mission. The smaller middle size mission we propose is designed to answer to the most compelling questions about the Uranus interior and magnetosphere, and can be launched in a shorter term.

This mission will provide data that can lead to a better understanding of the Uranus magnetosphere and how it interacts with the solar . As we show in the following, the Uranus magnetosphere is a perfect laboratory to test our knowledge of the magnetosphere interaction with the under unusual conditions.

Figure1. Sketch of the tilts of the rotational and magnetic axis of the magnetized planets of our solar system. The sketch shows Uranus rotational axis and dipole tilt at the solstice, in the configuration observed by Voyager 2.

2) Scientific motivations

We expose hereafter a summary of the most compelling questions about the Uranus magnetosphere, and the way its study will benefit to our understanding of the magnetosphere physics in general. A more detailed description of the scientific motivations can be found in a companion white paper [Rymer et al., 2010]

How does the Uranus dynamo generates the 's unusually tilted ? The most compelling scientific goals of a mission to Uranus are the mapping of Uranus’s gravitational and magnetic fields, the latter being of the greatest importance for magnetosphere physics. Polar orbits are the most successful for obtaining this mapping, as they permit the measurement of the fields at all latitudes and longitudes, as well as passing through the auroral regions. However, such orbits are generally difficult and fuel consuming to achieve. Only Earth, and soon Jupiter (with the Juno mission) have had polar orbiters. The large inclination of Uranus is ideal for a polar orbiter as any trajectory confined to the ecliptic plane is a nearly polar one, allowing for a complete mapping of Uranus gravitational and magnetic fields.

How does the solar wind interact with a magnetic dipole oriented toward the sun? All of the magnetospheres explored up to now have magnetic dipole axes oriented almost perpendicularly to the ecliptic plane. Therefore, the nature of the interaction between the solar wind and a magnetic dipole oriented toward the sun is largely unknown. The exploration of such a system will open for magnetosphere physics. Due to the 59o tilt of the Uranian magnetic dipole, the magnetosphere will be in such a configuration about seven years before equinox, in 2042. In 2028, for the solstice, the dipole orientation will be almost constant at 59o from the sun direction, constituting the less exotic configuration. Still, this angle is smaller than at Earth (~67o), Jupiter (~80o) or Saturn (~90o). No spacecraft can reach Uranus before 2030, when the minimum angle between the sun direction and the dipole will be about 50o and decreasing. This small angle presents an interesting laboratory in which to test theories of the interaction between a magnetosphere and the solar wind.

Figure 2. (upper panel) Sketch of the magnetic field lines deformations due to the interactions of the magnetosphere with the interplanetary magnetic field and with the equatorial current sheet at Uranus, at the time of the Voyager 2 (solstice). (Lower panel) Simulation of the magnetic field line toloplogy for a solstice configuration. The large angle between the magnetic and rotational axis lead to a twisted current sheet. These sketches, adapted from Bagenal [2009], an simulation were realized from a single flyby, a the detail of the magnetic field topology and of the current sheet behavior are not precisely known.

How does the magnetosphere reacts when its configuration relative to the solar wind changes rapidly? The interaction between a magnetosphere and the solar wind depends strongly on the relative directions of their magnetic fields at the interface. The non-alignment of Uranus rotation and magnetic dipole axes leads to a complete change of the magnetic configuration in a few hours. When the rotation axis is oriented toward the sun, the magnetosphere and solar wind magnetic fields change from parallel to anti-parallel in less than 8 hours. Since large-scale , which is the main driver of the Earth-solar wind interaction, occurs only for anti- parallel fields, this quickly changing magnetic field orientation at the boundary must strongly affect the driving mechanisms of the Uranian magnetosphere (Figure 2). This makes Uranus the perfect laboratory to test the models against a quickly and strongly varying environment.

How are current sheets influenced by the tilt of the magnetic dipole relative to the planet's rotation axis and by the magnetosphere configuration relative to the solar wind flow? In the terrestrial magnetosphere, and even more in the gaseous giant magnetospheres, the equatorial current sheet strongly influences and drives magnetospheric dynamics. The equatorial current sheet consists of plasma confined in the centrifugal plane, which is located between the rotational and magnetic equatorial planes. The currents circulating through this plasma are responsible for a large part of the magnetosphere dynamics, and result in auroral phenomena. It is unclear where this current sheet exists in a system where the rotational and magnetic equatorial planes are almost orthogonal and, if a Uranian current sheet does exist, what role it plays in magnetospheric dynamics and in the auroral phenomena. Understanding the Uranian current sheet is essential in determining the origin of the auroral emissions seen by Voyager 2 and from the ground [Herbert, 1994].

3) Observations to perform, Instrumentation required

Magnetosphere dynamics is a consequence of plasma-magnetic field interactions. The primary goal of the mission is thus to measure the characteristics of both the plasma and magnetic fields present in the Uranian system. This requires a complete mapping of the magnetic field, which is easier at Uranus than at any other planets as polar orbits are easily achieved. To perform these measurements the spacecraft will have to carry similar to those on-board Juno. It also requires to measure the density, temperature, composition and flow direction and velocity of the magnetospheric plasma. A Langmuir probe permits to determine the plasma density, and ions and electrons spectrometers to determine the other parameters. For these instruments to perform best, a spinning spacecraft is preferred to allow for complete 3D coverage.

Finally, measurements of the electric currents and electromagnetic fields are essential for understanding the interactions. These can be performed by an electric field sensor and magnetic search coil for the local measurements, whereas magnetospheric currents and acceleration processes, characterized by electrons accelerated to energies ranging from a few keV to a few tens of keV, can be remotely characterized through the auroral emissions they generate.

A full set of auroral measurement instruments generally consists of a Low-Frequency Radio (LFR) receiver and both Infrared (IR) and (UV) spectro-imagers. Each of these wavelengths gives different, complementary information about the auroral current system(s): ● UV observations are used to derive the power precipitated on the planet and the energy of the precipitating electrons. These two parameters characterize the auroral current systems by providing into the auroral acceleration processes. ● IR observations also permit one to obtain the power precipitated, as well as yielding information about the ionospheric response to the current system. ● LFR observations provide a large set of parameters, perhaps most importantly the rotation rate of the planet, which is a key parameter in nearly all physical aspects of a planetary system. LFR observations also provide the power precipitated, the energy of the electrons involved and eventually the nature of the auroral acceleration processes, and images the auroral emissions at different altitudes above the planet [Cecconi et al., 2009]. It can also detect the presence, or not, of lightnings in the , which is important to understand the climate on Uranus. Considering that the mass and power constraints for a Uranus mission may not allow for all remote sensing instruments, a radio receiver should have first priority.

In conclusion, a magnetosphere-oriented mission to Uranus must at least carry: ● Magnetometers to determine the magnetic field structure (similar to Juno's FGM+SHM instruments). ● A radio and plasma wave package, including direction finding capabilities, to accurately measure the planet rotation rate, provide auroral observations and measure currents (similar to CASSINI's RPWS package, or Juno's WAVES+direction finding). ● An ion mass spectrometer and an electron spectrometer to determine the plasma composition, density and temperature (Similar to Juno's JADE) and an energetic particle detector (similar to New Horizons' PEPPSI). ● Dual-frequency radio transmitters, which have almost no power or mass costs and provide important information about the planet interior. They also are part of the minimum recommendations of the Uranus mission white paper submitted to the decadal survey [Hofstadter, 2010].

Such a set of instruments answers the most compelling questions about Uranus’s magnetosphere and about its interactions with the solar wind. Based on the experience of the previous New Frontiers missions, such a minimal payload can be optimized to run with less than 50 W of electrical power (not accounting for the radio transmitters).

In case of sufficient mass and power allocation, a payload similar to JUNO's would provide a set of instruments sufficient to answer most of the fundamental physics questions for a planetary magnetosphere, atmosphere and interior.

4) Mission feasibility

Mission : launch windows and mass limitation

A recent JPL study investigated the logistics of sending an orbiter to Uranus. This study showed that, in term of mass, a middle size mission carrying about 100 kg of science payload could be put on a polar orbit. This is sufficient for the minimum payload we propose, and therefore the mass of the spacecraft not the primary limitation to a Uranus mission. The best is in 2018 for a chemical propulsion only scenario, as it allows for a Jupiter gravitational assistance. A much broader window is possible if electric propulsion is used. Flight times to Uranus orbit insertion are typically 8 to 12 years, meaning an orbit insertion in 2030 for a 2018 launch.

The JPL study envisioned a solar panel powered spacecraft, with solar panels producing 100 W at Uranus. This power is not sufficient, and therefore a larger surface area of solar panels is required at the cost of several hundreds of additional kilograms. However, if the solar panels were to be replaced by RTGs more mass could be allocated for the science payload, and a science payload equivalent to Juno's (a bit more than 170 kg) would be possible. Another possibility envisioned in the JPL study is an electric propulsion system, such as the one used for NASA's and missions. This option would greatly benefit to any solar-powered mission. As the spacecraft need a power of 100 to 200W to run at Uranus, a much larger power would be generated by the solar panels along the way to the planet, which could be used for the spacecraft propulsion.

Electrical power supply: Solar Panels versus Radioisotope Thermal Generator (RTG) The biggest problem with a mission to Uranus comes from the electrical power supply. The New Horizons spacecraft – which has strict power limitations – needs 150 W to run, plus 30 W for the science payload. A spinning spacecraft requires a few tens of Watts less, since it does not require power-consuming components. Nevertheless, the New Horizons spacecraft electrical production expected at – a bit less than 200 W – may constitute a reasonable lower limit, given the minimal set of instruments we defined.

The solar constant at Uranus is ~3.7 W/m2, implying that even the most efficient solar panels would not generate much more than 1 W/m2. Therefore, even taking into account improvements in solar panel technology, a solar-powered spacecraft at Uranus needs much more than 100m2 of solar panels, which has a high mass cost. Hence, an RTG-powered spacecraft is the best option, as it weighs much less and solves power supply difficulties, which allows for a larger science payload. However, new supplies of Plutonium 238 are necessary to build new RTGs. NASA funding has been approved, so that preliminary studies can be performed while waiting for the Department of Energy funding approval. Another possibility is a joint mission with the , which is planning to build Americium 241 based RTGs before then end of the decade. Nevertheless US-built RTGs are preferable, since 241Am has a four times lower power-per-mass efficiency than 238Pu.

Cost estimate

The JPL study showed that a middle size mission, launched by a 521 rocket, can be put into orbit around Uranus after a ten year travel, and the science payload is comparable to that of the previous New Frontiers mission, New Horizons and JUNO. Thus, this mission can fit in the New Frontiers cost constraints. The biggest issue, which could lead to higher costs, is the electric power supply. Depending on the availability of RPGs, or on the progress in solar panel technologies, the cost may be very different. An international collaboration would permit to share the cost of the development of new power supplies.

5) Summary

If we are to fully understand what it means to live with a star, we must gain a full understanding of how every magnetosphere in the solar system interacts with our star. After the exploration of the inner rocky planets, which revealed very different interactions with the solar wind, depending on the presence or not of a strong planetary magnetic field, and after the exploration of the giant gaseous planets, with the huge magnetospheres whose dynamics is dominated by the internal plasma sources, the next step of the solar system exploration is the study of the magnetospheres of the ice giants, Uranus and Neptune.

We propose here a middle size mission to Uranus. This mission will reveal many of the Uranus magnetospheres mysteries with a limited instrument package and a limited cost. It will also provide important information for a following mission to Neptune, which may be more suitable for a next Flagship mission, particularly because of its one-of-a-kind satellite Triton.

Hence, we propose a smaller Middle size orbiter to Uranus, with three main goals:

● Determine the planet's magnetic field and explore its exotic magnetosphere

● Test the models which have been developed for more steady magnetospheres, to improve our general understanding of the magnetosphere dynamics, and of the interaction of the magnetosphere with the solar wind under different conditions.

● Investigate the source of Uranus unusual magnetic field, by measuring the planet gravitational and magnetic fields and precise its rotation period.

Such a mission requires a small number of instruments, which are part of the basic payload of most spacecraft, and in particular of all of the New Frontiers missions. JPL's preliminary studies showed that such a mission could be done for a cost consistent with a middle size mission.

It will be the first orbiter sent to the only class of planets which have never had any, the ice giants. This first small mission would provide new knowledge about our solar system, about the magnetosphere physics and about the planetary science in general. This may have important consequences for the study of the Neptune-Mass extra-solar planets which are detected, thus improving our knowledge of how extra-solar planet live with their own .

Finally, being the first mission sent to the ice giants, it would provide very important constraints on the design of a following larger mission to Uranus or Neptune.

There are two possible options for the mission:

● A RTG-powered mission, with only tested technologies. This is the safest option, which also allows for the largest science payload. Nevertheless, new supplies of Plutonium 238 have to be found, or a collaboration with ESA has to be set to obtain RTGs.

● A technology development mission, powered by large solar panels, which have to be developed, and propelled by plasma thrusters. This is more risky, and may be more costly. Moreover, due to the larger weight of the solar panels, and the reduced power available, this option does not allow for a large science payload. Nevertheless, as more missions to the outer solar system will be planned, such a mission would help to develop new technologies which may reveal themselves less expensive over time.

In both cases, international collaborations would be beneficial to reduce the costs and solve technological difficulties.

References

Bagenal F., Comparative Planetary Environments, in : Plasma Physics of the Local Cosmos, C.J. Schrijver, G.L. Siscoe (eds), Cambridge University Press, pp 360-398, 2009 Cecconi, B.; Lamy, L.; Zarka, P.; Prangé, R.; Kurth, W. S.; Louarn, P., Goniopolarimetric study of the revolution 29 perikrone using the Cassini Radio and Plasma Wave Science instrument high- frequency radio receiver, J. Geophys. Res., vol. 114, A03215, 2009 Connerney, J. E. P., M. H. Acuña, and N. F. Ness, The magnetic field of Uranus, J. Geophys. Res., 92, 15329–15336, 1987. Herbert, F.; Sandel, B. R., The Uranian and its relationship to the magnetosphere, J. Geophys. Res. , vol. 99, A3, p. 4143-4160, 1994 Hofstadter, M., and co-authors, The case for an Uranus orbiter, Planetary Science Decadal Survey, 2010. Rymer, A. and co-authors, The Case for Exploring Uranus’ Magnetosphere, Solar and Heliophysics Decadal Survey, 2010