A solar or stellar coronal mass ejection (CME) exerts thermal, ram and magnetic pressure on the magnetospheres of orbiting (exo-)planets. A new dynamical equilibrium is then achieved between planetary magnetospheres and the enhanced stellar wind, this time with a stand-off, magnetopause distance that is shorter than its unperturbed value. If this distance falls below a certain threshold, magnetospheric compression causes heating and significant ionization in a planet’s upper atmosphere, prompting atmospheric escape. Persistent atmospheric erosion results in the ultimate lack of planetary atmospheres, if these are not replenished fast enough by independently acting planetary processes.
In an attempt to investigate how far we can reach with a first-principles description of planetary forcing by Sun or magnetically active stars, we (a) focus on magnetic pressure, as a lower limit of forcing by stellar transients, (b) adopt the solar analog of planeto-effective eruptions, and (c) apply explicitly the fundamental principle of magnetic helicity conservation in the solar corona. This enables nominal estimates of the near-Sun and interplanetary CME axial magnetic field, under the magnetic flux rope hypothesis. It further allows estimates of magnetospheric compression based on known planetary equatorial magnetic fields. In view of the general lack of stellar CME observations, we connect observed, bolometric stellar flare energies with expected magnetic helicity contents of the corresponding ejecta.
We find that (i) Earth, with its robust magnetosphere, is not in danger of significant atmospheric loss even for much stronger eruptions than those expected by Sun, (ii) this is not the case for Mars, and (iii) some famous exoplanet cases that intrigued imagination for the existence of alien life also seem unable to pass this test. Atmospheric survival is further hampered by the expected tidal locking of many exoplanets orbiting dwarf stars with orbital radii that are fractions of an astronomical unit. We discuss the repercussions and a possible outlook in this line of research, again with fundamental stellar magnetohydrodynamics in mind.
