237 results found
Archer M, Southwood D, Hartinger M, et al., 2022, How a realistic magnetosphere alters the polarizations of surface, fast magnetosonic, and Alfvén waves, Journal of Geophysical Research: Space Physics, Vol: 127, ISSN: 2169-9380
System-scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here, we show global MHD simulations of resonant waves impulsively excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause, the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods that might be applied to spacecraft observations.
Archer M, Hartinger M, Plaschke F, et al., 2021, Magnetopause ripples going against the flow form azimuthally stationary surface waves, Nature Communications, Vol: 12, Pages: 1-14, ISSN: 2041-1723
Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09–15h) the waves’ Poynting flux exactly balances the flow’s advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems.
Southwood DJ, Cao H, Shebanits O, et al., 2021, Discovery of Alfven waves planetward of Saturn's rings, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-18, ISSN: 2169-9380
Between April and September 2017 in the final stages of the Cassini Saturn Orbiter mission the spacecraft executed 22 orbits passing planetward of the innermost ring, the D-ring. During all periapsis passes oscillations were detected in the azimuthal magnetic field components on typical time scales of a few minutes. We argue that these time-varying magnetic signals detected on the spacecraft are also primarily time-varying in the plasma frame. Furthermore, we show that nearly all signals exhibit a spatial feature, namely a magnetic node near the effective field line equator. We propose that the oscillations are associated with Alfvén waves excited in local field line resonances, most likely driven from global sources.
Southwood D, Kivelson M, 2020, An improbable collaboration, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380
Fifty years of collaboration between the authors are reviewed. Common themes cover magnetospheric magnetohydrodynamic phenomena: MHD waves, wave‐particle interactions, circulation, global modes and field line resonances in the terrestrial context, and magnetosphere‐moon interactions, transport processes, instabilities and global structure in the magnetospheres of giant planets. Over the period reviewed, instrumentation has improved, particularly in particle detectors, and interpretations that seemed radical when first suggested are now supported by measurements and seem commonplace.
Southwood D, Cao H, Hunt G, et al., 2020, Discovery of Alfven waves planetward of the Rings of Saturn, Europlanet Science Congress 2020, Publisher: American Geophysical Union
Between April and September 2017 in the final stages of the Cassini Saturn Orbiter mission the spacecraft executed 22 orbits passing planetward of the innermost ring, the D-ring. During periapsis passes on all these orbits oscillations were detected in the azimuthal magnetic field components on typical time scales from a few minutes to 10 minutes. We argue that the time-varying signals detected on the spacecraft are also primarily time-varying in the plasma frame. Nonetheless, we show that nearly all signals exhibit a distinct spatial effect, namely a magnetic node near the effective field line equator. The oscillations thus have a standing structure along the background magnetic field and it follows that they are field line resonances associated with Alfvén waves. The form of the signals suggests that the local field line resonances are most likely pumped from global sources. This is the first detection in a giant planet magnetosphere of a phenomenon known to be important at Earth.
Martin CJ, Ray LC, Constable DA, et al., 2020, Evaluating the ionospheric mass source for Jupiter's magnetosphere: An ionospheric outflow model for the auroral regions, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380
Ionospheric outflow is the flow of plasma initiated by a loss of equilibrium along a magnetic field line which induces an ambipolar electric field due to the separation of electrons and ions in a gravitational field and other mass dependant sources. We have developed an ionospheric outflow model using the transport equations to determine the number of particles that flow into the outer magnetosphere of Jupiter. The model ranges from 1400 km in altitude above the 1 bar level to 2.5 RJ along the magnetic field line and considers H+ and H3+ as the main ion constituents. Previously, only pressure gradients and gravitational forces were considered in modelling polar wind. However, at Jupiter we need to evaluate the affect of field‐aligned currents present in the auroral regions due to the breakdown of corotation in the magnetosphere, along with the centrifugal force exerted on the particles due to the fast planetary rotation rate. The total number flux from both hemispheres is found to be 1.3‐1.8 x 1028 s‐1 comparable in total number flux to the Io plasma source. The mass flux is lower due to the difference in ion species. This influx of protons from the ionosphere into the inner and middle magnetosphere needs to be included in future assessments of global flux tube dynamics and composition of the magnetosphere system.
Hunt GJ, Bunce EJ, Cao H, et al., 2020, Saturn's auroral field-aligned currents: observations from the Northern Hemisphere dawn sector during cassini's proximal orbits, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380
We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LT). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept‐forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO‐independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass‐by‐pass we find that the main upward current at dawn is stronger compared to near‐noon. This results in the current density been ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO‐independent upward current electron power ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.
Hunt G, Cowley S, Provan G, et al., 2019, Currents associated with Saturn's intra-D ring azimuthal field perturbations, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5675-5691, ISSN: 2169-9380
During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn’s D ring and the denser layers of the planet’s atmosphere, revealing the presence of an unanticipated low-latitude current system via the associated azimuthal perturbation field peaking typically at ~10-30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field-aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field-aligned current densities above the ionosphere are typically ~5-10 nA m-2 , more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field-aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north-south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller-scale structures, possibly due to rapid temporal or local time variations.
Khurana KK, Dougherty MK, Provan G, et al., 2018, Discovery of atmospheric-wind-driven electric currents in Saturn's magnetosphere in the gap between Saturn and its rings, Geophysical Research Letters, Vol: 45, Pages: 10068-10074, ISSN: 0094-8276
Magnetic field observations obtained by the Cassini spacecraft as it traversed regions inside of Saturn's D ring packed a genuine surprise. The azimuthal component of the magnetic field recorded a consistent positive perturbation with a strength of 15–25 nT near closest approach. The closest approaches were near the equatorial plane of Saturn and were distributed narrowly around local noon and brought the spacecraft to within 2,550 km of Saturn's cloud tops. Modeling of this perturbation shows that it is not of internal origin but is produced by external currents that couple the low‐latitude northern ionosphere to the low‐latitude southern ionosphere. The azimuthal perturbations diminish at higher latitudes on field lines that connect to Saturn's icy rings. The sense of the current system suggests that the southern feet of the field lines in the ionosphere leads their northern counterparts. We show that the observed field perturbations are consistent with a field‐aligned current whose strength is ~1 MA/radian, that is, comparable in strength to the planetary‐period‐oscillation‐related current systems observed in the auroral zone. We show that the Lorentz force in the ionosphere extracts momentum from the faster moving low‐latitude zonal belt and delivers it to the northern ionosphere. We further show that the electric current is generated when the two ends of a field line are embedded in zonal flows with differing wind speeds in the low‐latitude thermosphere. The wind‐generated currents dissipate 2 × 1011W of thermal power, similar to the input from the solar extreme ultraviolet flux in this region.
Dougherty MK, Cao H, Khurana KK, et al., 2018, Erratum for the Research Article “Saturn’s magnetic field revealed by the Cassini Grand Finale” by M. K. Dougherty, H. Cao, K. K. Khurana, G. J. Hunt, G. Provan, S. Kellock, M. E. Burton, T. A. Burk, E. J. Bunce, S. W. H. Cowley, M. G. Kivelson, C. T. Russell, D. J. Southwood, Science, Vol: 362, ISSN: 0036-8075
Dougherty MK, Cao H, Khurana KK, et al., 2018, Saturn's magnetic field revealed by the Cassini Grand Finale, SCIENCE, Vol: 362, Pages: 46-+, ISSN: 0036-8075
Hunt GJ, Provan G, Bunce EJ, et al., 2018, Field-aligned currents in Saturn’s magnetosphere: Observations from the F-ring orbits, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3806-3821, ISSN: 2169-9402
We investigate the azimuthal magnetic field signatures associated with high‐latitude field‐aligned currents observed during Cassini's F‐ring orbits (October 2016–April 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the field‐aligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the F‐ring data cover the post‐dawn and dusk sectors in the northern and southern hemispheres, respectively. The F‐ring field‐aligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the field‐aligned currents in a hemisphere are modulated by that hemisphere's “planetary period oscillation” (PPO) systems. We separate the PPO‐independent and PPO‐related currents in both hemispheres using their opposite symmetry. The average PPO‐independent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPO‐related currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPO‐related currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete field‐aligned currents that act to “turn off” the PPOs.
Hunt GJ, Provan G, Cowley SWH, et al., 2018, Saturn's planetary period oscillations during the closest approach of Cassini's ring grazing orbits, Geophysical Research Letters, Vol: 45, Pages: 4692-4700, ISSN: 0094-8276
Saturn's planetary period oscillations (PPOs) are ubiquitous throughout its magnetosphere. We investigate the PPO's azimuthal magnetic field amplitude interior to the field‐aligned currents, during the closest approaches of Cassini's ring‐grazing orbits (October 2016 to April 2017), with periapses at ~2.5 RS. The amplitudes of the northern and southern PPO systems are shown to vary as a function of latitude. The amplitude ratio between the two PPO systems shows that the northern system is dominant by a factor of ~1.3 in the equatorial plane, and it is dominant to ~ −15° latitude in the southern hemisphere. The dayside amplitudes are approximately half of the 2008 nightside amplitudes, which agree with previous local time‐related amplitude observations. Overall, there is clear evidence that the PPOs are present on field lines that map to the outer edge of Saturn's rings, closer to Saturn than previously confirmed.
Southwood D, Brekke P, 2017, Norway's most celebrated scientist, Astronomy and Geophysics, Vol: 58, Pages: 5.28-5.31, ISSN: 1366-8781
Auroral emissions serve as a powerful tool to investigate themagnetospheric processes at Saturn. Solar wind and internally driven processes largely control Saturn’s auroral morphology. The main auroral emission at Saturn is suggested to be connected with the magnetosphere - solar wind interaction, through the flow shear related to rotational dynamics. Dawn auroral enhancements are associated with intense field-aligned currents generated by hot tenuous plasma carried towards the planet in fast moving flux tubes as they return from tail reconnection site to the dayside. In this work we demonstrate, based on Cassini auroral observations, that the main auroral emission at Saturn, as it rotates from midnight to dusk via noon, occasionally stagnates near noon over a couple of hours. In half of the sequences examined, the auroral emission is blocked close to noon, while in three out of four cases, the blockage of the auroral emission is accompanied with signatures of dayside reconnection. We discuss some possible interpretations of the auroral ’blockage’ near noon. According to the first one it could be related to local time variations of the flow shear close to noon. Auroral local time variations are also suggested to be initiated by radial transport process. Alternatively, the auroral blockage at noon could be associated with a plasma circulation theory, according to which tenuously populated closed flux tubes as they return from the nightside to the morning sector experience a blockage in the equatorial plane and they cannot rotate beyond noon.
Quasi-periodic ∼1-hour fluctuations have been recently reported by numerous instruments on-board the Cassini spacecraft. The interpretation of the sources of these fluctuations has remained elusive to date. Here we provide an explanation for the origin of these fluctuations using magnetometer observations. We find that magnetic field fluctuations at high northern latitudes are Alfvénic, with small amplitudes (∼0.4 nT), and are concentrated in wave-packets similar to those observed in Kleindienst et al. . The wave-packets recur periodically at the northern magnetic oscillation period. We use a magnetospheric box model to provide an interpretation of the wave periods. Our model results suggest that the observed magnetic fluctuations are second harmonic Alfvén waves standing between the northern and southern ionospheres in Saturn’s outer magnetosphere
Southwood DJ, Chane E, 2016, High latitude circulation in giant planet magnetospheres, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 5394-5403, ISSN: 2169-9402
We follow-up the proposal by Cowley et al. (2004) that the plasma circulation in the magnetospheres of the giant planets is a combination of two cycles or circulation systems. The Vasyliunas cycle transports heavy material ionized deep within the magnetosphere eventually to loss in the magnetotail. The second cycle is driven by magnetic reconnection between the planetary and the solar wind magnetic fields (the Dungey cycle) and is found on flux tubes poleward of those of the Vasyliunas cycle. We examine features of the Dungey system, particularly what occurs out of the equatorial plane. The Dungey cycle requires reconnection on the dayside, and we suggest that at the giant planets the dayside reconnection occurs preferentially in the morning sector. Second, we suggest that most of the solar wind material that enters through reconnection on to open flux tubes on the dayside never gets trapped on closed field lines but makes less than one circuit of the planet and exits down tail. In its passage to the nightside, the streaming ex-solar wind material is accelerated centrifugally by the planetary rotation primarily along the field; thus, in the tail it will appear very like a planetary wind. The escaping wind will be found on the edges of the tail plasma sheet, and reports of light ion streams in the tail are likely due to this source. The paper concludes with a discussion of high-latitude circulation in the absence of reconnection between the solar wind and planetary field.
Southwood DJ, 2016, Space in 150 years: From fantasy through fiction to fact and function, Aeronautical Journal, Vol: 120, Pages: 201-208, ISSN: 0001-9240
In the last century and half, space has moved from the realm of fantasy to everyday reality.In parallel the way space has been regarded by the person in the street and the ideas of whataccess to space might be used for have evolved extraordinarily.
Southwood D, 2015, James Wynne Dungey 1923-2015 OBITUARY, Astronomy & Geophysics, Vol: 56, Pages: 8-8, ISSN: 1468-4004
Southwood DJ, 2015, Introduction: Jim Dungey and Magnetospheric Plasma Physics, Magnetospheric Plasma Physics: The Impact of Jim Dungey’s Research, Editors: Southwood, Cowley, Mitton, Publisher: Springer, ISBN: 9783319183596
This book makes good background reading for much of modern magnetospheric physics.
Southwood DJ, 2015, From the Carrington Storm to the Dungey Magnetosphere, Magnetospheric Plasma Physics: The Impact of Jim Dungey’s Research, Editors: Southwood, Cowley, Mitton, Publisher: Springer, Pages: 253-271, ISBN: 9783319183596
This book makes good background reading for much of modern magnetospheric physics.
Southwood D, FRS SWHC, Mitton S, 2015, Magnetospheric Plasma Physics: The Impact of Jim Dungey’s Research, Publisher: Springer, ISBN: 9783319183596
This book makes good background reading for much of modern magnetospheric physics.
Yates JN, Southwood DJ, Dougherty MK, 2015, Reply to the comment by Cowley et al. on “Magneticphase structure of Saturn’s 10.7h oscillations”, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 5691-5693, ISSN: 2169-9402
Yates JN, Southwood DJ, Dougherty MK, 2015, Magnetic phase structure of Saturn's 10.7h oscillations, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 120, Pages: 2631-2648, ISSN: 2169-9380
Southwood DJ, 2015, Formation of Magnetotails: Fast and Slow Rotators Compared, Magnetotails in the Solar System, Editors: Keiling, Jackman, Publisher: John Wiley & Sons, Pages: 199-215, ISBN: 9781118842348
Southwood D, 2014, Space science and policy, ASTRONOMY & GEOPHYSICS, Vol: 55, Pages: 26-32, ISSN: 1366-8781
Jackman CM, Slavin JA, Kivelson MG, et al., 2014, Saturn's dynamic magnetotail: A comprehensive magnetic field and plasma survey of plasmoids and traveling compression regions and their role in global magnetospheric dynamics, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 119, Pages: 5465-5494, ISSN: 2169-9380
Southwood D, 2014, Crowded Orbits: Conflict and Cooperation in Space, NATURE, Vol: 509, Pages: 32-32, ISSN: 0028-0836
Southwood DJ, Cowley SWH, 2014, The origin of Saturn's magnetic periodicities: Northern and southern current systems, Journal of Geophysical Research: Space Physics, Vol: 119, Pages: 1563-1571, ISSN: 2169-9380
The recent survey by Andrews et al. (2012) of the separate northern and southern ~10.7 h periodic magnetic signals in Saturn's magnetosphere limits very much their governing current systems. The existence of signals with pure or close to pure northern or southern periods in respective polar caps taken with the relatively narrow bandwidth of the signals indicates that the actual periodicities are imposed independently from northern and southern polar regions, i.e., the open field line regions. Field‐aligned currents must flow on the boundaries of these regions to exclude signals from the other hemisphere. Equatorward of the polar cap, on closed magnetic shells, there are distinct north and south “cam” source currents, the distinction being made clear by a difference in polarization. We outline the consequences for the governing current systems and the implications for sustaining the energy and power dissipation in the system.
Southwood D, 2014, Saturn's mysterious magnetism, ASTRONOMY & GEOPHYSICS, Vol: 55, Pages: 13-18, ISSN: 1366-8781
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