Magnetoinertial Oscillations of Jupiter’s Inner Radiation Belt

Abstract

Quasi-periodic 40 minute (QP-40) bursts of both relativistic electrons (energy E D 10 MeV) and radio emissions (frequency kHz) from the south Jovian pole were detected by the instrument onboard l [ 700 the Ulysses spacecraft at high jovigraphic latitudes (D30È40 S¡). We show here that JupiterÏs inner radiation belt (IRB) may oscillate globally with a quasi-period of D40 minutes or so and propose that, as results of asymmetries and small-scale circumpolar magnetic irregularities/anomalies, a fraction of relativistic electrons normally trapped within the IRB may go astray from a roughly circular zone surrounding the Jovian magnetic pole during certain phases of such IRB magnetoinertial oscillations. The escaped relativistic electrons rapidly convert their transverse motions to (anti)parallel motions along the magnetic Ðeld lines by the mirror force. The observed QP-40 radio bursts are closely associated with such QP-40 leaks of relativistic electrons in near-antiparallel motions. Our prediction for QP-40 magnetoinertial oscillations of the IRB can be tested by directly observing variations of IRB decimetric brightness using ground-based radio telescopes, especially upon the arrival of high-speed solar winds at Jupiter. Several relevant aspects of this scenario are also discussed.

Stellar magnetic cycles

“…The shorter period may be a consequence of the faster stellar rotation in the framework of the model by Lou (2000) that interprets Rieger cycles as a manifestation of Rossby-type waves trapped in the outer layers of the convection zone. An alternative explanation calls into play a possible influence of the close-in hot Jupiter because the cycle period is very close to ten synodic periods of the planet with respect to the mean stellar rotation period.…”

Quasi-periodic magnetospheric activities of Jupiter and Saturn and magneto{inertial oscillations of their inner radiation belts (extended abstract).

“…It is also important to directly probe in-situ such QP variations in relativistic electron number density and flux, magnetic field, polarized radio emissions, auroral diagnostics as well as electron energy spectrum inside the IRB. In general, a rotating magnetic dipole of Jupiter's IRB can support magneto-inertial oscillations with longer periods of ∼ 40 − 50 min and shorter periods for higher harmonics with nonzero integers m and n [Lou, 1987;2001]. Similar types of magneto-inertial oscillation modes can also appear in rotating magnetized solar and stellar atmospheric layers including those for magnetospheres of exoplanets and for a thin dense magnetized "ocean" over spinning neutron stars.…”

“…Such QP-40 radio bursts feature right-hand circular polarization, and their occurrence strongly correlates with recurrent arrivals of fast-speed solar winds [MacDowall et al 1993]. Based on model analysis, empirical evidence and physical considerations, Lou [2001] advanced the scenario that these QP-40 relativistic electron bursts originate around circumpolar zones from Jupiter's inner radiation belt (IRB) occupying ∼ 1.5 − 3 R J (Jovian radii) wherein the intense synchrotron radiation reveals trapped relativistic electrons (E ≥ 50 MeV with Lorentz factor γ up to ∼ 200) and predicted then that such QP-40 polar burst activities should be global involving both Jovian poles, as indeed confirmed by Ulysses observations towards the north pole direction of Jupiter 12 years later. As Jupiter's dipole field points inward/outward at its south/north pole, outstreaming extremely relativistic electrons gyrate very rapidly around south/north circumpolar magnetic field lines (antiparallel/parallel) with very small pitch angles and emit low-frequency (ν ≤ 0.7 MHz) beamed radio bursts with partially right-hand/left-hand circular polarizations.…”

“…Thus inside the Jovian magnetosphere, one should also detect QP-40 bursts of relativistic electrons together with QP-40 radio bursts from the north pole as from the south pole. Such QP-40 activities are linked to QP-40 magneto-inertial global IRB oscillations [Lou, 2001] that are excited by intermittent magnetized high-speed solar winds [Lou, 1996]. At a certain phase of IRB oscillations, magnetic irregularities along the circumpolar zones adjacent to the IRB would leak out relativistic electrons drifting outwards across field lines from the IRB.…”

“…The low-frequency receiver of JUNO is capable of detecting QP-40 radio bursts from both Jovian poles but without polarization information. QP-40 bursts of relativistic electrons with energy E ≥ 10 MeV may penetrate the shield for protecting the instruments onboard.In reference to the model scenario and physical interpretations for Jupiter's QP-40 phenomena [Lou 2001;Lou et al, 2012] and especially by striking similarities in several major aspects Palmaerts et al, 2016], we advance here an analogous scenario of global magneto-inertial oscillations of Kronian inner radiation belt (KIRB) for QP-60 phenomena of Saturn as widely detected by various diagnostics of Cassini in-situ observations since early July 2004 [Badman et al, 2016;Mitchell et al, 2016; Palmaerts et al, 2016 and references therein]. With a weaker field, KIRB traps relativistic electrons (up to E ∼ 12 MeV and likely higher) and energetic protons within ∼ 5 R S (Saturn radii) [Kollmann et al, 2011], and the KIRB electron number density is ∼ 50−100 cm −3 [Persoon et al, 2005].…”