Giant Planet Magnetospheres

Bagenal, F.

doi:10.1146/annurev.ea.20.050192.001445

Cited by 112 publications

(45 citation statements)

References 123 publications

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“…The ratio of the planetary magnetospheric radius, Rm, to the planetary radius, Rp, is derived from the equilibrium between the pressure from the wind and the outward force of (14) ,where Bp is the strength of the planetary magnetic field at the pole. Since there have been no measurements of the magnetic field strength of hot Jupiters to date, we have assumed a planetary magnetic field strength similar to that of Jupiter, which is a maximum of ∼ 14 G (Bagenal 1992). The values of Rm/Rp evaluated over the eight epochs are shown in Table 5.…”

Section: Planetary Magnetospheric Behaviourmentioning

confidence: 99%

Temporal variability of the wind from the star τ Boötis

Nicholson

Vidotto

Mengel

et al. 2016

Mon. Not. R. Astron. Soc.

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We present new wind models for τ Boötis (τ Boo), a hot-Jupiter-host-star whose observable magnetic cycles makes it a uniquely useful target for our goal of monitoring the temporal variability of stellar winds and their exoplanetary impacts. Using spectropolarimetric observations from May 2009 to January 2015, the most extensive information of this type yet available, to reconstruct the stellar magnetic field, we produce multiple 3D magnetohydrodynamic stellar wind models. Our results show that characteristic changes in the large-scale magnetic field as the star undergoes magnetic cycles produce changes in the wind properties, both globally and locally at the position of the orbiting planet. Whilst the mass loss rate of the star varies by only a minimal amount (∼ 4 percent), the rates of angular momentum loss and associated spin-down timescales are seen to vary widely (up to ∼ 140 percent), findings consistent with and extending previous research. In addition, we find that temporal variation in the global wind is governed mainly by changes in total magnetic flux rather than changes in wind plasma properties. The magnetic pressure varies with time and location and dominates the stellar wind pressure at the planetary orbit. By assuming a Jovian planetary magnetic field for τ Boo b, we nevertheless conclude that the planetary magnetosphere can remain stable in size for all observed stellar cycle epochs, despite significant changes in the stellar field and the resulting local space weather environment.

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Section: Planetary Magnetospheric Behaviourmentioning

confidence: 99%

Temporal variability of the wind from the star τ Boötis

Nicholson

Vidotto

Mengel

et al. 2016

Mon. Not. R. Astron. Soc.

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“…On the planetary side, pressures resulting from thermal motions, mass loss (atmospheric escape), and magnetic effects can all contribute to the pressure equilibrium. The Earth magnetosphere is located at r M 10−15 R ⊕ (Bagenal 1992), where planetary thermal pressure can safely be neglected. The Earth also does not have a significant atmospheric escape ( a few kg/s, Fahr & Szizgal 1983).…”

Section: Interaction Between the Planet And The Corona Of Its Host Starmentioning

confidence: 99%

Effects of M dwarf magnetic fields on potentially habitable planets

Vidotto

Jardine

Morin

et al. 2013

A&A

148

161

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We investigate the effect of the magnetic fields of M dwarf (dM) stars on potentially habitable Earth-like planets. These fields can reduce the size of planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. We used a sample of 15 active dM stars, for which surface magnetic-field maps were reconstructed, to determine the magnetic pressure at the planet orbit and hence the largest size of its magnetosphere, which would only be decreased by considering the stellar wind. Our method provides a fast means to assess which planets are most affected by the stellar magnetic field, which can be used as a first study to be followed by more sophisticated models. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (∼1 G) orbiting at the inner (outer) edge of the habitable zone of these stars would present magnetospheres that extend at most up to 6 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G to up to a few thousand G. By assuming a magnetospheric size that is more appropriate for the young-Earth (3.4 Gyr ago), the required planetary magnetic fields are one order of magnitude weaker. However, in this case, the polar-cap area of the planet, which is unprotected from transport of particles to/from interplanetary space, is twice as large. At present, we do not know how small the smallest area of the planetary surface is that could be exposed and would still not affect the potential for formation and development of life in a planet. As the star becomes older and, therefore, its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets probably expands. Using an empirically derived rotation-activity/magnetism relation, we provide an analytical expression for estimating the shortest stellar rotation period for which an Earth-analogue in the habitable zone could sustain an Earth-sized magnetosphere. We find that the required rotation rate of the early-and mid-dM stars (with periods 37-202 days) is slower than the solar one, and even slower for the late-dM stars ( 63-263 days). Planets orbiting in the habitable zone of dM stars that rotate faster than this have smaller magnetospheric sizes than that of the Earth magnetosphere. Because many late-dM stars are fast rotators, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early-and mid-dM stars.

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“…The periapsis distance of Voyager 2 is indicated by a white arrow inside of the orbit of Puck. The typical distance to the subsolar magnetopause (Bagenal, 1992) is indicated by the vertical red line. Images of the major regular satellites are from Voyager 2 (credit: NASA/JPL) and have not been photometrically corrected.…”

Section: An Ice Giant Planetary System: Rings and Natural Satellitesmentioning

confidence: 99%