Imperial College London

ProfessorMicheleDougherty

Faculty of Natural SciencesDepartment of Physics

Head of Department of Physics, Professor of Space Physics
 
 
 
//

Contact

 

+44 (0)20 7594 7770m.dougherty Website

 
 
//

Assistant

 

Miss Katherine Nesbitt +44 (0)20 7594 7503

 
//

Location

 

Blackett 900aBlackett LaboratorySouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

370 results found

Bradley TJ, Cowley SWH, Bunce EJ, Melin H, Provan G, Nichols JD, Dougherty MK, Roussos E, Krupp N, Tao C, Lamy L, Pryor WR, Hunt GJet al., 2020, Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380

Journal article

Cao H, Dougherty MK, Hunt GJ, Provan G, Cowley SWH, Bunce EJ, Kellock S, Stevenson DJet al., 2020, The landscape of Saturn's internal magnetic field from the Cassini Grand Finale, ICARUS, Vol: 344, ISSN: 0019-1035

Journal article

Shebanits O, Hadid LZ, Cao H, Morooka MW, Hunt G, Dougherty MK, Wahlund J-E, Waite Jr JH, Mueller-Wodarg Iet al., 2020, Saturn’s near-equatorial ionospheric conductivities from in situ measurements, Scientific Reports, Vol: 10, ISSN: 2045-2322

Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.

Journal article

Hunt GJ, Bunce EJ, Cao H, Cowley SWH, Dougherty MK, Provan G, Southwood DJet 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.

Journal article

Staniland N, Dougherty M, Masters A, Bunce Eet al., 2020, Determining the nominal thickness and variability of the magnetodisc current sheet at saturn, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-15, ISSN: 2169-9380

The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.

Journal article

Agiwal O, Hunt GJ, Dougherty MK, Cowley SWH, Provan Get al., 2020, Modeling the Temporal Variability in Saturn's Magnetotail Current Sheet From the Cassini F-ring Orbits, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380

Journal article

Cao Y, Wellbrock A, Coates AJ, CaroCarretero R, Jones GH, Cui J, Galand M, Dougherty MKet al., 2020, Field‐aligned photoelectron energy peaks at high altitude and on the nightside of titan, Journal of Geophysical Research: Planets, Vol: 125, Pages: 1-13, ISSN: 2169-9097

The ionization of N urn:x-wiley:jgre:media:jgre21272:jgre21272-math-0001 by strong solar He II 30.4‐nm photons produces distinctive spectral peaks near 24.1 eV in Titan's upper atmosphere, which have been observed by the Electron Spectrometer (ELS) as part of the Cassini Plasma Spectrometer. The ELS observations reveal that, in addition to the dayside, photoelectron peaks were also detected on the deep nightside where photoionization is switched off, as well as at sufficiently high altitudes where the ambient neutral density is low. These photoelectron peaks are unlikely to be produced locally but instead must be contributed by transport along the magnetic field lines from their dayside source regions. In this study, we present a statistical survey of all photoelectron peaks identified with an automatic finite impulse response algorithm based on the available ELS data accumulated during 56 Titan flybys. The spatial distribution of photoelectron peaks indicates that most photoelectrons detected at an altitude above 4,000 km and a solar zenith angle above 100° are field aligned, which is consistent with the scenario of photoelectron transport along the magnetic field lines. Our analysis also reveals the presence of a photoelectron gap in the deep nightside ionosphere where almost no photoelectrons were detected. It appears to be very difficult for photoelectrons to travel to this region, and such a feature may not be driven by the changes in the orientation between the solar and corotation wakes.

Journal article

Jackman CM, Thomsen MF, Dougherty MK, 2019, Survey of Saturn's Magnetopause and Bow Shock Positions Over the Entire Cassini Mission: Boundary Statistical Properties and Exploration of Associated Upstream Conditions, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 8865-8883, ISSN: 2169-9380

Journal article

Provan G, Cowley SWH, Bradley TJ, Bunce EJ, Hunt GJ, Cao H, Dougherty MKet al., 2019, Magnetic Field Observations on Cassini's Proximal Periapsis Passes: Planetary Period Oscillations and Mean Residual Fields, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 8814-8864, ISSN: 2169-9380

Journal article

Guo RL, Yao ZH, Sergis N, Wei Y, Xu XJ, Coates AJ, Delamere PA, Roussos E, Arridge CS, Waite JH, Krupp N, Mitche D, Burch J, Dougherty MK, Wan WXet al., 2019, Long-standing Small-scale Reconnection Processes at Saturn Revealed by Cassini, ASTROPHYSICAL JOURNAL LETTERS, Vol: 884, ISSN: 2041-8205

Journal article

Sorba AM, Achilleos NA, Sergis N, Guio P, Arridge CS, Dougherty MKet al., 2019, Local Time Variation in the Large-Scale Structure of Saturn's Magnetosphere, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7425-7441, ISSN: 2169-9380

Journal article

Hunt G, Cowley S, Provan G, Cao H, Bunce E, Dougherty M, Southwood Det 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.

Journal article

Sulaiman AH, Farrell WM, Ye S-Y, Kurth WS, Gurnett DA, Hospodarsky GB, Menietti JD, Pisa D, Hunt GJ, Agiwal O, Dougherty MKet al., 2019, A Persistent, Large-Scale, and Ordered Electrodynamic Connection Between Saturn and Its Main Rings, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7166-7172, ISSN: 0094-8276

Journal article

Provan G, Cowley SWH, Bunce EJ, Bradley TJ, Hunt GJ, Cao H, Dougherty MKet al., 2019, Variability of Intra-D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 379-404, ISSN: 2169-9380

Journal article

Brown P, Auster U, Bergman JES, Fredriksson J, Kasaba Y, Mansour M, Pollinger A, Baughen R, Berglund M, Hercik D, Misawa H, Retino A, Bendyk M, Magnes W, Cecconi B, Dougherty MK, Fischer Get al., 2019, MEETING THE MAGNETIC EMC CHALLENGES FOR THE IN-SITU FIELD MEASUREMENTS ON THE JUICE MISSION, ESA Workshop on Aerospace EMC (Aerospace EMC), Publisher: IEEE

Conference paper

Dougherty M, Christensen U, Cao H, Khurana Ket al., 2019, Saturn's Magnetic Field and Dynamo, Saturn in the 21st Century, Editors: Baines, Flasar, Krupp, Stallard, Publisher: Cambridge University Press, Pages: 69-96, ISBN: 978-1-107-10677-2

Book chapter

Guo RL, Yao ZH, Sergis N, Wei Y, Mitchell D, Roussos E, Palmaerts B, Dunn WR, Radioti A, Ray LC, Coates AJ, Grodent D, Arridge CS, Kollmann P, Krupp N, Waite JH, Dougherty MK, Burch JL, Wan WXet al., 2018, Reconnection Acceleration in Saturn's Dayside Magnetodisk: A Multicase Study with Cassini, ASTROPHYSICAL JOURNAL LETTERS, Vol: 868, ISSN: 2041-8205

Journal article

Krupp N, Roussos E, Kollmann P, Mitchell DG, Paranicas CP, Krimigis SM, Hamilton DC, Hedman M, Dougherty MKet al., 2018, Energetic Neutral and Charged Particle Measurements in the Inner Saturnian Magnetosphere During the Grand Finale Orbits of Cassini 2016/2017, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 10847-10854, ISSN: 0094-8276

Journal article

Dougherty M, Buratti BJ, Seidelmann PK, Spencer JRet al., 2018, Enceladus as an active world: History and discovery. In Enceladus and the Icy, Enceladus and the Icy Moons of Saturn, Editors: Schenk, Clark, Howett, Verbiscer, Waite, Publisher: University of Arizona Press, Pages: 3-16, ISBN: 9780816537075

Dougherty M. K., Buratti B. J., Seidelmann P. K., and Spencer J. R. (2018) Enceladus as an active world: History and discovery. In Enceladus and the Icy Moons of Saturn (P. M. Schenk et al., eds.), pp. 3–16. Univ. of Arizona, Tucson, DOI: ...

Book chapter

Sorba AM, Achilleos NA, Guio P, Arridge CS, Sergis N, Dougherty MKet al., 2018, The Periodic Flapping and Breathing of Saturn's Magnetodisk During Equinox, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 8292-8316, ISSN: 2169-9380

Journal article

Yao ZH, Radioti A, Grodent D, Ray LC, Palmaerts B, Sergis N, Dialynas K, Coates AJ, Arridge CS, Roussos E, Badman SV, Ye S-Y, Gerard J-C, Delamere PA, Guo RL, Pu ZY, Waite JH, Krupp N, Mitchell DG, Dougherty MKet al., 2018, Recurrent Magnetic Dipolarization at Saturn: Revealed by Cassini, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 8502-8517, ISSN: 2169-9380

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, SCIENCE, Vol: 362, Pages: 46-+, ISSN: 0036-8075

Journal article

Staniland N, Dougherty M, Masters A, 2018, Quantifying the stress of the Saturnian magnetosphere during the Cassini era, Geophysical Research Letters, Vol: 45, Pages: 8704-8711, ISSN: 0094-8276

We quantify the magnetospheric stress state of Saturn, revealing the nature of the planetary environment and its current systems. The complete magnetic field data set collected by the Cassini spacecraft is used to track the global behavior of the Saturnian magnetosphere during the Cassini era. Variations in the magnetodisc current model parameter μoIo determine when the system is stretched, compressed, or near the ground state. Of the 111 orbits that pass through our chosen region, 69 are well described by the model, indicating a steady state current sheet during this interval. While the stress state displays a dependence on local time, it is also shown to vary temporally. We conclude that the Saturnian magnetosphere remained in a quiet state for a significant period of the Cassini orbital mission at Saturn, with occasional large‐scale deviations observed.

Journal article

Sulaiman AH, Kurth WS, Hospodarsky GB, Averkamp TF, Ye S-Y, Menietti JD, Farrell WM, Gurnett DA, Persoon AM, Dougherty MK, Hunt GJet al., 2018, Enceladus auroral hiss emissions during Cassini's grand finale, Geophysical Research Letters, Vol: 45, Pages: 7347-7353, ISSN: 0094-8276

Cassini's Radio and Plasma Wave Science (RPWS) instrument detected intense auroral hiss emissions during one of its perikrone passes of the Grand Finale orbits. The emissions were detected when Cassini traversed a flux tube connected to Enceladus' orbit (L‐shell = 4) and at a time when both the spacecraft and the icy moon were in similar longitudes. Previous observations of auroral hiss related to Enceladus were made only during close flybys and here we present the first observation of such emissions close to Saturn. Further, ray‐tracing analysis shows the source location at a latitude of 63°, in excellent agreement with earlier UVIS observations of Enceladus' auroral footprint by Pryor et al. (2011, https://doi.org/10.1038/nature09928). The detection has been afforded exclusively by the Grand Finale phase, which enabled sampling of Enceladus' high‐latitude flux tube near Saturn. This result provides new insight into the spatial extent of the electrodynamic interaction between Saturn and Enceladus.

Journal article

Guo RL, Yao ZH, Wei Y, Ray LC, Rae IJ, Arridge CS, Coates AJ, Delamere PA, Sergis N, Kollmann P, Grodent D, Dunn WR, Waite JH, Burch JL, Pu ZY, Palmaerts B, Dougherty MKet al., 2018, Rotationally driven magnetic reconnection in Saturn's dayside, Nature Astronomy, Vol: 2, Pages: 640-645, ISSN: 2397-3366

Magnetic reconnection is a key process that explosively accelerates charged particles, generating phenomena such as nebular flares1, solar flares2 and stunning aurorae3. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisc, where thin-current-sheet conditions are conducive to reconnection4. The dayside magnetodisc is usually considered thicker than the nightside due to the compression of solar wind, and is therefore not an ideal environment for reconnection. In contrast, a recent statistical study of magnetic flux circulation strongly suggests that magnetic reconnection must occur throughout Saturn’s dayside magnetosphere5. Additionally, the source of energetic plasma can be present in the noon sector of giant planetary magnetospheres6. However, so far, dayside magnetic reconnection has only been identified at the magnetopause. Here, we report direct evidence of near-noon reconnection within Saturn’s magnetodisc using measurements from the Cassini spacecraft. The measured energetic electrons and ions (ranging from tens to hundreds of keV) and the estimated energy flux of ~2.6 mW m–2 within the reconnection region are sufficient to power aurorae. We suggest that dayside magnetodisc reconnection can explain bursty phenomena in the dayside magnetospheres of giant planets, which can potentially advance our understanding of quasi-periodic injections of relativistic electrons6 and auroral pulsations7.

Journal article

Sergis N, Achilleos N, Guio P, Arridge CS, Sorba AM, Roussos E, Krimigis SM, Paranicas C, Hamilton DC, Krupp N, Mitchell DG, Dougherty MK, Balasis G, Giannakis Oet al., 2018, Mapping Saturn's nightside plasma sheet using Cassini's proximal orbits, Geophysical Research Letters, Vol: 45, Pages: 6798-6804, ISSN: 0094-8276

Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements.

Journal article

Sulaiman AH, Kurth WS, Hospodarsky GB, Averkamp TF, Persoon AM, Menietti JD, Ye S-Y, Gurnett DA, Pisa D, Farrell WM, Dougherty MKet al., 2018, Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 6782-6789, ISSN: 0094-8276

The Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel‐shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn's environment.

Journal article

Dougherty MK, Spilker LJ, 2018, Review of Saturn's icy moons following the Cassini mission, Reports on Progress in Physics, Vol: 81, ISSN: 0034-4885

We review our knowledge of the icy moons of Saturn prior to the Cassini orbital mission, describe the discoveries made by the instrumentation onboard the Cassini spacecraft.

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00029186&limit=30&person=true