One knows since the beginning of spatial era that the relation between solar activity and the extent and intensity of polar aurorae on Earth comes from the solar wind plasma and magnetic field. The interaction between solar wind and the magnetosphere occurs at the border between the closed lines of terrestrial magnetic field and those which are open on the interplanetary medium. This interaction takes the form of magnetic reconnections between the terrestrial and solar fields. From that results a magnetospheric activity - and thus an auroral activity - permanent, of relatively low level, located primarily on the night side. When the passage of an interplanetary shock compresses the magnetosphere brutally, the equilibrium of the magnetospheric plasma is affected, and precipitations of energetical charged particles grow in intensity and extent towards the low latitudes : it is a geomagnetic storm accompanied of very bright and very wide aurorae. One also knows since the Voyager missions the existence of permanent aurorae on the (magnetized) planets Jupiter, Saturn, Uranus and Neptune, and one knows that they are strongly correlated with the fluctuations of speed, density and pressure of the solar wind. But it had never been possible until now to observe directly the equivalent of a geomagnetic storm in response to the interplanetary passage. Thus it was not known whether a shock keeps intact its properties of interaction with the magnetospheres during its propagation through the solar system, and whether the nature of interaction is the same for all magnetized planets.
Since 1992 for Jupiter, and 1996 for Saturn, R. Prangé and L Pallier observe the planetary aurorae with the Hubble Space Telescope (HST). The images taken in far UV with very high space resolution (1 pixel 75 km on the Jupiter disc and 150 km on that of Saturn) allow a very precise study of the morphology of the aurorae and its variability. The Saturn aurorae obtained during several years appeared rather simple, consisting in a bright ring around each pole (as on the Earth) with an asymmetry morning-evening in latitude and intensity. December 7, 2000 on the other hand, this ring, more intense than usual, is accompanied by an active region, very bright at0 high latitudes on the polar cap. The following day, all returned back to order (figure 1). One then suspects that it is the first observation of a magnetic storm on Saturn. But no space probe is then in the immediate vicinity of Saturn there to measure the solar wind and to confirm this assumption.
Helped by an American colleague who develops a magnetohydrodynamic code (MHD) of solar wind propagation from Earth until several AU, the LESIA team benefits from the exceptional planetary conjunction at the time of these observations : the Sun, the Earth, Jupiter and Saturn then were almost aligned, and measurements of the solar wind parameters were realized a little upstream of Jupiter by the Cassini probe (on the way towards Saturn). The same solar wind thus met successively three planets, as shown on the graph at left, and the validity of the MHD model could be tested by comparing its extrapolations with the measurements made by Cassini within 5 AU of the Sun. The conclusions are summarized by figure 2 : a series of solar coronal matter ejections (CME) observed by SOHO generated a series of shocks in solar wind, measured two days later by WIND and ACE close to the Earth. At the Jupiter distance, after 15 days of propagation, these shocks amalgamate in a wider shock (as simulated by the MHD code and measured by Cassini). One month later, this shock reaches Saturn at the time of the violent auroral disturbance, whose magnetic storm nature is thus shown. The left-hand column of figure 2 also shows that magnetic storms were started on each three planet successively at the time of interaction of this shock with their magnetosphere. Finally, one notes resemblances between the auroral answers of the Earth and Saturn (intensification of the aurorae on the night side, activation of open lines of magnetic field), but also differences (no expansion of the auroral oval on Saturn, and asymmetry morning-evening) which carry the signature of coupling processes specific to each planet. We now expect, from the Cassini mission in orbit around Saturn, a harvest of new data which will allow to understand more completely the interactions solar wind/planet magnetospheres at the scale of the solar system.
Reference
- An interplanetary shock traced by planetary auroral storms from the Sun to Saturn Renée Prangé, Laurent Pallier, Kenneth C. Hansen, Russ Howard, Angelos Vourlidas, Régis Courtin & Chris Parkinson Nature 432, 78 (2004) 4 November
Contact
- Renée Prangé (Observatoire de Paris, LESIA)
Dernière modification le 21 décembre 2021