Geraint H. Jones

Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics

[1]xa0We study the interplanetary magnetic field (IMF) data obtained by the Cassini spacecraft during a ∼6.5-month interval when the spacecraft was approaching Saturn at heliocentric distances between ∼8.5 and ∼8.9 AU. It is shown that the structure of the IMF is consistent with that expected to be formed by corotating interaction regions (CIRs) during the declining phase of the solar cycle, with two sectors during each solar rotation, and crossings of the heliospheric current sheet generally embedded within few-day higher-field compression regions, separated by several-day lower-field rarefaction regions. This pattern was disrupted in November 2003, however, by an interval of high activity on the Sun. These data have then been employed to estimate the voltage associated with open flux production at Saturns magnetopause using an empirical formula adapted from Earth. The results show that the CIR-related structuring of the IMF leads to corresponding structuring of the interplanetary interaction with Saturns magnetosphere and hence also to intervals of very different dynamical behavior. During few-day compression regions where the IMF strength is ∼0.5–2 nT, the average Dungey cycle voltage is estimated to be ∼100 kV, such that the open flux produced over such intervals is ∼30–40 GWb, similar to the typical total amount present in Saturns magnetosphere. The magnetosphere is thus significantly driven by the solar wind interaction during such intervals. During some rarefaction intervals, on the other hand, the field strength remains ∼0.1 nT or less over several days, implying reconnection voltages of ∼10 kV or less, with negligible production of open flux. The magnetosphere is then expected to enter a quiescent state, dominated by internal processes. Overall, ∼100 GWb of open flux is estimated to be produced during each ∼25-day solar rotation, about 3 times the typical flux contained in the tail, and sufficient to drive three to five substorms. We point out, however, that CIR-related variations in solar wind dynamic pressure will also occur in synchronism with the field variations, which may also play a role in modulating the open flux in the system, thus reinforcing the synchronization of the pattern of growth and decay of open flux to the CIR pattern. Estimates of open flux production associated with the period of strong solar activity indicate that major magnetospheric dynamics were excited by reconnection-mediated solar wind interaction during this interval.

The auroral footprint of Enceladus on Saturn

Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io–Jupiter coupling, for example, create an auroral footprint in Jupiter’s ionosphere. Auroral ultraviolet emission associated with Enceladus–Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io’s footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon’s footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters—and as such is probably indicative of variable plume activity.

Long‐ and short‐term variability of Saturn's ionic radiation belts

Earlier studies of Saturns inner ionic radiation belts revealed that their content was surprisingly constant while their evolution appeared decoupled from dynamics of the Saturnian magnetosphere. Saturns icy moons in combination with the neutral gas and dust that surround the planet seem to effectively restrict radial transport of energetic ions and are responsible for all these unusual characteristics. A possible process through which MeV ions may be populating the regions between the icy moons is cosmic ray albedo neutron decay (CRAND). While some circumstantial evidence suggests that this process actually occurs, the concept of CRAND has only been applied to the proton energy spectrum above similar to 10 MeV; the source of ions below 10 MeV is not yet obvious. Additional hints about the nature of this source are now becoming evident by monitoring Saturns radiation belts about half a solar cycle (from the declining phase of the solar maximum to solar minimum). Using Cassinis magnetosphere imaging instrument and low-energy magnetospheric measurement system (MIMI/LEMMS) data from June 2004 to June 2010, we detect a weak intensification of the trapped proton component that probably originates from CRAND (> 10 MeV). This anticipated enhancement, due to the solar cycle modulation of the galactic cosmic ray influx at Saturn, is closely followed by ions in the 1-10 MeV range. This observation sets constraints on the nature of those ions source: this source should be connected (directly or indirectly) to the access of galactic cosmic rays in the Saturnian system. We also find evidence indicating that the ionic belts experience short-term variability following the occurrence of solar energetic particle events at Saturns distance, probably associated with coronal mass ejections that propagate in the heliosphere. LEMMS data contain clear evidence of Earth-like Forbush decreases following such events. These decreases may explain the lack of an (expected) ionic belt intensification between 2004 and 2006.

COMETARY IONS TRAPPED IN A CORONAL MASS EJECTION

Ion tails of comets are known to extend radially away from the Sun over very large distances. Crossing these tails by spacecraft not specifically targeted to intercept them was believed to be extremely improbable, since that requires precise angular alignment of the spacecraft with a comet. We report here the fortuitous detection of cometary ions at large angular separation far from the comet. To explain this unexpected discovery, we conclude that these ions were ducted laterally along magnetic fields that were randomly distorted by a coronal mass ejection and that such transport increases the probability of an unplanned detection of comets.

Millisecond Imaging of Radio Transients with the Pocket Correlator

We demonstrate a signal-processing concept for imaging the sky at millisecond rates with radio interferometers. The Pocket Correlator (PoCo) correlates the signals from multiple elements of a radio interferometer fast enough to image brief, dispersed pulses. By the nature of interferometry, a millisecond correlator functions like a large, single-dish telescope, but with improved survey speed, spatial localization, calibration, and interference rejection. To test the concept, we installed PoCo at the Allen Telescope Array (ATA) to search for dispersed pulses from the Crab pulsar, B0329+54, and M31 using total-power, visibility-based, and image-plane techniques. In 1.7 hr of observing, PoCo detected 191 giant pulses from the Crab pulsar brighter than a typical 5σ sensitivity limit of 60 Jy over pulse widths of 3 ms. Roughly 40% of pulses from pulsar B0329+54 were detected by using novel visibility-based techniques. Observations of M31 constrain the rate of pulses brighter than 190 Jy in a three-degree region surrounding the galaxy to <4.3 hr^(–1). We calculate the computational demand of various visibility-based pulse search algorithms and demonstrate how compute clusters can help meet this demand. Larger implementations of the fast imaging concept will conduct blind searches for millisecond pulses in our Galaxy and beyond, providing a valuable probe of the interstellar/intergalactic media, discovering new kinds of radio transients, and localizing them to constrain models of their origin.

The interaction of comet 153P/Ikeya‐Zhang with interplanetary coronal mass ejections: Identification of fast ICME signatures

[1]xa0The active comet 153P/Ikeya-Zhang possessed a highly-variable plasma tail. Favorable circumstances allowed the identification of the impact of fast ICMEs with the comet. The impact produces a specific morphology including a characteristic scalloped appearance which suggests that the ICME magnetic field drapes around preexisting tail density enhancements. This appears to be the first direct association between fast ICMEs and plasma tail structure and the specific structure should permit the identification of fast ICME locations in the heliosphere.

Possible Distortion of the Interplanetary Magnetic Field by the Dust Trail of Comet 122P/de Vico

Interplanetary field enhancements are unexplained localized increases in the heliospheric magnetic field magnitude. Four such enhancements, detected by the Pioneer Venus Orbiter and Ulysses spacecraft, have been found to be very close to the orbital plane of comet 122P/de Vico. Analysis of the magnetic field during these events reveals that the field perturbations were close to those expected if the interplanetary magnetic field draped at the comets orbit and that the field enhancements represent crossings of a sheet of disturbed solar wind in the comets orbital plane. The results suggest that a dense dust stream may occupy at least part of the orbit of de Vico and that its effects on the solar wind are apparent almost 1.4 AU downstream. One field enhancement suggests the existence of an annulus of dust extending sunward of the dust trail itself.

Corotating Magnetic Reconnection Site in Saturn's Magnetosphere

Using measurements from the Cassini spacecraft in Saturns magnetosphere, we propose a 3D physical picture of a corotating reconnection site, which can only be driven by an internally generated source. Our results demonstrate that the corotating magnetic reconnection can drive an expansion of the current sheet in Saturns magnetosphere and, consequently, can produce Fermi acceleration of electrons. This reconnection site lasted for longer than one of Saturns rotation period. The long-lasting and corotating natures of the magnetic reconnection site at Saturn suggest fundamentally different roles of magnetic reconnection in driving magnetospheric dynamics (e.g., the auroral precipitation) from the Earth. Our corotating reconnection picture could also potentially shed light on the fast rotating magnetized plasma environments in the solar system and beyond.

Cassini Plasma Observations of Saturn's Magnetospheric Cusp

The magnetospheric cusp is a funnel-shaped region where shocked solar wind plasma is able to enter the high latitude magnetosphere via the process of magnetic reconnection. The plasma observations include various cusp signatures such as ion energy dispersions as well as diamagnetic effects. We present an overview analysis of the cusp plasma observations at the Saturnian magnetosphere from the Cassini spacecraft era. A comparison of the observations is made as well as classification into groups due to varying characteristics. The locations of the reconnection site are calculated and shown to vary along the subsolar magnetopause. We show the first in situ evidence for lobe reconnection that occurred at nearly the same time as dayside reconnection for one of the cusp crossings. Evidence for bursty and more continous reconnection signatures are observed in different cusp events. The events are compared to solar wind propagation models and it is shown that magnetic reconnection and plasma injection into the cusp can occur for a variety of upstream conditions. These are important results because they show that Saturns magnetospheric interaction with the solar wind and the resulting cusp signatures are dynamic, and that plasma injection in the cusp occurs due to a variety of solar wind conditions. Furthermore, reconnection can proceed at a variety of locations along the magnetopause.

Sources, sinks and transport of energetic electrons near Saturn's main rings

The inner boundary of Saturns electron radiation belts, near the planets A‐ring (∼2.27 Rs), is studied using Cassinis Proximal orbit measurements. We find that variable convective flows transport energetic electrons to the A‐ring, which absorbs them instantaneously, forming the inner belt boundary. These flows are also responsible for a variable and longitudinally asymmetric boundary configuration. Pre‐noon, the boundary oscillates towards and away from the A‐ring with a two‐week period. Post‐noon, it maps persistently near the F‐ring (∼2.32 Rs) and coexists with localized MeV electron intensity enhancements (microbelts). We propose that the microbelts contain electrons in drift resonance with corotation, trapped in local‐time confined trajectories which result from the aforementioned convective flows. The microbelts collocation with the F‐ring implies either a local, secondary electron production due to Galactic Cosmic Ray collisions with F‐ring dust, or an enhanced resonant electron trapping due to an electrodynamic interaction between the F‐ring and Saturns magnetosphere.