Imperial College London

Professor Southwood

Faculty of Natural SciencesDepartment of Physics

Senior Research Investigator
 
 
 
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Contact

 

+44 (0)20 7594 7770d.southwood CV

 
 
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Location

 

711AHuxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

229 results found

Hunt G, Cowley S, Provan G, Cao H, Bunce E, Dougherty M, Southwood Det al., Currents associated with Saturn's intra-D ring azimuthal field perturbations, Journal of Geophysical Research: Space Physics, 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.

Journal article

Khurana KK, Dougherty MK, Provan G, Hunt GJ, Kivelson MG, Cowley SWH, Southwood DJ, Russell CTet 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.

Journal article

Dougherty MK, Cao H, Khurana KK, Hunt GJ, Provan G, Kellock S, Burton ME, Burk TA, Bunce EJ, Cowley SWH, Kivelson MG, Russell CT, Southwood DJet al., 2018, Saturn's magnetic field revealed by the Cassini Grand Finale (vol 362, eaat5434, 2018), SCIENCE, Vol: 362, ISSN: 0036-8075

Journal article

Dougherty MK, Cao H, Khurana KK, Hunt GJ, Provan G, Kellock S, Burton ME, Burk TA, Bunce EJ, Cowley SWH, Kivelson MG, Russell CT, Southwood DJet al., 2018, Saturn's magnetic field revealed by the Cassini Grand Finale, SCIENCE, Vol: 362, Pages: 46-+, ISSN: 0036-8075

Journal article

Hunt GJ, Provan G, Bunce EJ, Cowley SWH, Dougherty MK, Southwood DJet al., Field-aligned currents in Saturn’s magnetosphere: Observations from the F-ring orbits., Journal of Geophysical Research: Space Physics, Vol: 123, 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.

Journal article

Hunt GJ, Provan G, Cowley SWH, Dougherty MK, Southwood DJet al., 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.

Journal article

Southwood D, Brekke P, 2017, Norway's most celebrated scientist, Astronomy and Geophysics, Vol: 58, Pages: 5.28-5.31, ISSN: 1366-8781

Journal article

Radioti A, Grodent D, Gerard JC, Southwood DJ, Chane E, Bonfond B, Pryor Wet al., 2017, Stagnation of Saturn’s auroral emission at noon, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 6078-6087, ISSN: 2169-9402

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.

Journal article

Yates JN, Southwood DJ, Dougherty MK, Sulaiman AH, Masters A, Cowley SWH, Kivelson MG, Chen CHK, Provan G, Mitchell DG, Hospodarsky GB, Achilleos N, Sorba AM, Coates AJet al., 2016, Saturn's quasiperiodic magnetohydrodynamic waves, Geophysical Research Letters, Vol: 43, Pages: 102-111, ISSN: 1944-8007

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. [2009]. 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

Journal article

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.

Journal article

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.

Journal article

Southwood D, 2015, James Wynne Dungey 1923-2015 OBITUARY, Astronomy & Geophysics, Vol: 56, Pages: 8-8, ISSN: 1468-4004

Journal article

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.

Book

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.

Book chapter

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.

Book chapter

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

Journal article

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

Journal article

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

Book chapter

Southwood D, 2014, Space science and policy, ASTRONOMY & GEOPHYSICS, Vol: 55, Pages: 26-32, ISSN: 1366-8781

Journal article

Jackman CM, Slavin JA, Kivelson MG, Southwood DJ, Achilleos N, Thomsen MF, DiBraccio GA, Eastwood JP, Freeman MP, Dougherty MK, Vogt MFet 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

Journal article

Southwood D, 2014, Crowded Orbits: Conflict and Cooperation in Space, NATURE, Vol: 509, Pages: 32-32, ISSN: 0028-0836

Journal article

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

Journal article

Southwood D, 2014, Saturn's mysterious magnetism, ASTRONOMY & GEOPHYSICS, Vol: 55, Pages: 13-18, ISSN: 1366-8781

Journal article

Southwood D, 2012, When international partnerships go wrong, NATURE, Vol: 488, Pages: 451-453, ISSN: 0028-0836

Journal article

Andrews DJ, Cowley SWH, Dougherty MK, Lamy L, Provan G, Southwood DJet al., 2012, Planetary period oscillations in Saturn’s magnetosphere: Evolution of magnetic oscillation properties from southern summer to post-equinox, J. Geophys. Res., Vol: 117

Journal article

Farrugia CJ, Chen L-J, Torbert RB, Southwood DJ, Cowley SWH, Vrublevskis A, Mouikis C, Vaivads A, Andre M, Decreau P, Vaith H, Owen CJ, Sibeck DJ, Lucek E, Smith CWet al., 2011, "Crater" flux transfer events: Highroad to the X line?, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 116, ISSN: 0148-0227

Journal article

Southwood DJ, 2011, Direct evidence of differences in magnetic rotation rate between Saturn’s northern andsouthern polar regions, J. Geophys. Res., Vol: 116

The discovery of a difference in apparent rotation rate between Saturn kilometricradio emission from northern and southern hemispheres leads immediately to thequestion of whether there is a counterpart detectable in the Saturnian magnetic field. Wereport direct detection of a difference in the rotating magnetic signal measured by theCassini spacecraft magnetometer consistent with that seen in the radio signals.Theoretical consequences of the direct detection are discussed.

Journal article

Southwood DJ, Kivelson MG, 2009, The source of Saturn's periodic radio emission, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 114, ISSN: 2169-9380

Journal article

Bunce EJ, Arridge CS, Clarke JT, Coates AJ, Cowley SWH, Dougherty MK, Gerard J-C, Grodent D, Hansen KC, Nichols JD, Southwood DJ, Talboys DLet al., 2008, Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 113, ISSN: 2169-9380

Journal article

Andrews DJ, Bunce EJ, Cowley SWH, Dougherty MK, Provan G, Southwood DJet al., 2008, Planetary period oscillations in Saturn's magnetosphere: Phase relation of equatorial magnetic field oscillations and Saturn kilometric radiation modulation, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 113, ISSN: 2169-9380

Journal article

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