Gabrielle Provan

Bio: Gabrielle Provan is an academic researcher from University of Leicester. The author has contributed to research in topics: Magnetosphere & Magnetosphere of Saturn. The author has an hindex of 26, co-authored 41 publications receiving 2204 citations.

Magnetic flux transport in the Dungey cycle: A survey of dayside and nightside reconnection rates

Abstract: [1] Changes in the open flux content of the ionospheric polar cap, estimated from auroral, radar, and low-Earth orbit particle measurements, are used to determine dayside and nightside reconnection rates during 73 hours of observation spread over nine intervals. We identify 25 episodes of nightside reconnection and examine statistically the rates and durations of reconnection, as well as possible triggers for the onset of reconnection, such as changes in solar wind ram pressure or orientation of the interplanetary magnetic field. Approximately half of the events can possibly be identified with a trigger, the other half appearing spontaneous. On average 0.3 GWb of open flux are closed in each event, with average durations and reconnection rates being 70 min and 85 kV. We find no evidence for a low background rate of nightside reconnection between these events and conclude that substorms and other large reconnection bursts provide the major or only source of flux closure on the nightside.

CUTLASS Finland radar observations of the ionospheric signatures of flux transfer events and the resulting plasma flows

Abstract: . The CUTLASS Finland radar has been run in a two-beam special scan mode, which offered excellent temporal and spatial information on the flows in the high-latitude ionosphere. A detailed study of one day of this data revealed a convection reversal boundary (CRB) in the CUTLASS field of view (f.o.v) on the dayside, the direction of plasma flow either side of the boundary being typical of a dawn-cell convection pattern. Poleward of the CRB a number of pulsed transients are observed, seemingly moving away from the radar. These transients are identified here as the ionospheric signature of flux transfer events (FTEs). Equatorward of the CRB continuous backscatter was observed, believed to be due to the return flow on closed field lines. The two-beam scan offered a new and innovative opportunity to determine the size and velocity of the ionospheric signatures associated with flux transfer events and the related plasma flow pattern. The transient signature was found to have an azimuthal extent of 1900 ± 900 km and an poleward extent of ∼250 km. The motion of the transient features was in a predominantly westward azimuthal direction, at a velocity of 7.5 ± 3 km. Key words. Magnetospheric physics (auroral phenomena; magnetopause · cusp and boundary layers; magnetosphere - ionosphere interaction)

Saturn's magnetic field revealed by the Cassini Grand Finale.

Abstract: During 2017, the Cassini fluxgate magnetometer made in situ measurements of Saturn's magnetic field at distances ~2550 ± 1290 kilometers above the 1-bar surface during 22 highly inclined Grand Finale orbits. These observations refine the extreme axisymmetry of Saturn's internal magnetic field and show displacement of the magnetic equator northward from the planet's physical equator. Persistent small-scale magnetic structures, corresponding to high-degree (>3) axisymmetric magnetic moments, were observed. This suggests secondary shallow dynamo action in the semiconducting region of Saturn's interior. Some high-degree magnetic moments could arise from strong high-latitude concentrations of magnetic flux within the planet's deep dynamo. A strong field-aligned current (FAC) system is located between Saturn and the inner edge of its D-ring, with strength comparable to the high-latitude auroral FACs.

Magnetospheric period oscillations at Saturn: Comparison of equatorial and high-latitude magnetic field periods with north and south Saturn kilometric radiation periods

Abstract: [1] It has recently been shown using Cassini radio data that Saturn kilometric radiation (SKR) emissions from the Northern and Southern hemispheres of Saturn are modulated at distinctly different periods, ∼10.6 h in the north and ∼10.8 h in the south, during the southern summer conditions that prevailed during the interval from 2004 to near‐equinox in mid‐2009. Here we examine Cassini magnetospheric magnetic field data over the same interval and show that two corresponding systems of magnetic field oscillations that have the same overall periods, as the corresponding SKR modulations, to within ∼0.01% are also present. Specifically, we show that the rotating quasi‐dipolar field perturbations on southern open field lines and the rotating quasi‐uniform field in the inner region of closed field lines have the same period as the southern SKR modulations, although with some intervals of slow long‐term phase drift of unknown origin, while the rotating quasi‐dipolar field perturbations on northern open field lines have the same period as the northern SKR modulations. We also show that while the equatorial quasi‐uniform field and effective southern transverse dipole are directed down tail and toward dawn at southern SKR maxima, as found in previous studies, the corresponding northern transverse dipole is directed approximately opposite, pointing sunward and also slightly toward dawn at northern SKR maxima. We discuss these findings in terms of the presence of two independent high‐latitude field‐aligned current systems that rotate with different periods in the two hemispheres.

Polarization and phase of planetary-period magnetic field oscillations on high-latitude field lines in Saturn's magnetosphere

Abstract: [1] We examine magnetic field data obtained by the Cassini spacecraft on a sequence of high-latitude orbits in Saturn's magnetosphere spanning October 2006 to May 2007 to determine whether planetary-period oscillations are present on polar open field lines, such as have been found previously in near-equatorial magnetic field data. Such oscillations are found generally to be present with amplitudes ∼0.5–1 nT, somewhat smaller than the few nT amplitudes typical of the quasi-dipolar equatorial region. The polarization characteristics in the northern and southern polar regions are determined and found to differ significantly from those in the equatorial region. The phases of the oscillations in the northern and southern hemispheres are also determined relative to the equatorial oscillations, and hence relative to each other, requiring extension of the equatorial oscillation phase model to the end of 2007, spanning the interval of high-latitude orbits. The results show that the overall pattern of field oscillations is not consistent with a rotating external current system that mimics a rotating transverse dipole in the outer regions. Rather, we suggest that the overall field perturbations are associated with a rotating partial ring current and its field-aligned closure currents, the latter favoring the southern ionosphere during the southern summer conditions examined. A physical picture is presented that links together observed planetary-period modulations in the middle and outer magnetospheric field, plasma, and radio emissions that may be subject to further test and makes predictions as to how these phenomena will evolve during future Saturn equinox and northern summer conditions.

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

Abstract: The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.

Variations in the polar cap area during two substorm cycles

Abstract: This study employs observations from several sources to determine the location of the polar cap bound- ary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral im- ages from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the mag- netosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, al- lowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and de- creases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to de- crease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap bound- ary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times sup- ports this finding.

Substorm Current Wedge Revisited

Abstract: Almost 40 years ago the concept of the substorm current wedge was developed to explain the magnetic signatures observed on the ground and in geosynchronous orbit during substorm expansion. In the ensuing decades new observations, including radar and low- altitude spacecraft, MHD simulations, and theoretical considerations have tremendously ad- vanced our understanding of this system. The AMPTE/IRM, THEMIS and Cluster missions have added considerable observational knowledge, especially on the important role of fast flows in producing the stresses that generate the substorm current wedge. Recent detailed, multi-spacecraft, multi-instrument observations both in the magnetosphere and in the iono- sphere have brought a wealth of new information about the details of the temporal evolution and structure of the current system. While the large-scale picture remains valid, the new

Discovery of a north-south asymmetry in Saturn's radio rotation period

Abstract: [1] For many years it has been known that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies by up to one percent on a time scale of years. Recent radio observations from the Cassini spacecraft have revealed the appearance of a second component, with a rotation period of about 10.6 hours, significantly less than the period of the previously known component, which is currently about 10.8 hours. In this paper we show that the first component originates from the southern auroral region, and that the second component originates from the northern auroral region. This north-south asymmetry in the rotation period has potentially important implications on how angular momentum is transferred from the interior to the magnetosphere.