Imperial College London

Dr Ravindra T. Desai

Faculty of Natural SciencesDepartment of Physics

Research Associate
 
 
 
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Contact

 

ravindra.desai Website

 
 
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Location

 

Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

19 results found

Desai R, Freeman M, Eastwood J, Eggington J, Archer M, Shprits Y, Meredith N, Staples F, Ian R, Hietala H, Mejnertsen L, Chittenden J, Horne Ret al., 2021, Interplanetary shock-induced magnetopause motion: Comparison between theory and global magnetohydrodynamic simulations, Geophysical Research Letters, ISSN: 0094-8276

The magnetopause marks the outer edge of the Earth’s magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this letter, we use global magneto-hydrodynamic simulations to examine the response of the terrestrial magnetopause to fast-forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterised by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance travelled, and large-scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991.

Journal article

Yao Z, Dunn WR, Woodfield EE, Clark G, Mauk BH, Ebert RW, Grodent D, Bonfond B, Pan D, Rae IJ, Ni B, Guo R, Branduardi-Raymont G, Wibisono AD, Rodriguez P, Kotsiaros S, Ness J-U, Allegrini F, Kurth WS, Gladstone GR, Kraft R, Sulaiman AH, Manners H, Desai RT, Bolton SJet al., 2021, Revealing the source of Jupiter's x-ray auroral flares., Sci Adv, Vol: 7

Jupiter's rapidly rotating, strong magnetic field provides a natural laboratory that is key to understanding the dynamics of high-energy plasmas. Spectacular auroral x-ray flares are diagnostic of the most energetic processes governing magnetospheres but seemingly unique to Jupiter. Since their discovery 40 years ago, the processes that produce Jupiter's x-ray flares have remained unknown. Here, we report simultaneous in situ satellite and space-based telescope observations that reveal the processes that produce Jupiter's x-ray flares, showing surprising similarities to terrestrial ion aurora. Planetary-scale electromagnetic waves are observed to modulate electromagnetic ion cyclotron waves, periodically causing heavy ions to precipitate and produce Jupiter's x-ray pulses. Our findings show that ion aurorae share common mechanisms across planetary systems, despite temporal, spatial, and energetic scales varying by orders of magnitude.

Journal article

Zhang Z, Desai R, Miyake Y, Usui H, Shebanits Oet al., 2021, Particle-in-cell simulations of the Cassini spacecraft’s interaction with Saturn’s ionosphere during the Grand Finale, Monthly Notices of the Royal Astronomical Society, Vol: 504, Pages: 964-973, ISSN: 0035-8711

A surprising and unexpected phenomenon observed during Cassini’s Grand Finale was the spacecraft charging to positive potentials in Saturn’s ionosphere. Here, the ionospheric plasma was depleted of free electrons with negatively charged ions and dust accumulating up to over 95 per cent of the negative charge density. To further understand the spacecraft–plasma interaction, we perform a three-dimensional Particle-In-Cell study of a model Cassini spacecraft immersed in plasma representative of Saturn’s ionosphere. The simulations reveal complex interaction features such as electron wings and a highly structured wake containing spacecraft-scale vortices. The results show how a large negative ion concentration combined with a large negative to positive ion mass ratio is able to drive the spacecraft to the observed positive potentials. Despite the high electron depletions, the electron properties are found as a significant controlling factor for the spacecraft potential together with the magnetic field orientation which induces a potential gradient directed across Cassini’s asymmetric body. This study reveals the global spacecraft interaction experienced by Cassini during the Grand Finale and how this is influenced by the unexpected negative ion and dust populations.

Journal article

Desai RT, Zhang Z, Wu X, Lue Cet al., 2021, Photodetachment and Test-Particle Simulation Constraints on Negative Ions in Solar System Plasmas

Negative ions have been detected in abundance in recent years by spacecraftacross the solar system. These detections were, however, made by instrumentsnot designed for this purpose and, as such, significant uncertainties remainregarding the prevalence of these unexpected plasma components. In thisarticle, the phenomenon of photodetachment is examined and experimentally andtheoretically derived cross-sections are used to calculate photodetachmentrates for a range of atomic and molecular negative ions subjected to the solarphoton spectrum. These rates are applied to negative ions outflowing fromEuropa, Enceladus, Titan, Dione and Rhea and their trajectories are traced toconstrain source production rates and the extent to which negative ions areable to pervade the surrounding space environments. Predictions are also madefor further negative ion populations in the outer solar system with Triton usedas an illustrative example. This study demonstrates how, at increasedheliocentric distances, negative ions can form stable ambient plasmapopulations and can be exploited by future missions to the outer solar system.

Journal article

Desai R, Zhang H, Davies E, Stawarz J, Mico-Gomez J, Iváñez-Ballesteros Pet al., 2020, Three dimensional simulations of solar wind preconditioning and the 23 July 2012 Interplanetary Coronal Mass Ejection, Solar Physics: a journal for solar and solar-stellar research and the study of solar terrestrial physics, Vol: 295, Pages: 1-14, ISSN: 0038-0938

Predicting the large-scale eruptions from the solar corona and theirpropagation through interplanetary space remains an outstanding challenge in solar- and helio-physics research. In this article, we describe three dimensional magnetohydrodynamic simulations of the inner heliosphere leading up to and including the extreme interplanetary coronal mass ejection (ICME) of 23 July 2012, developed using the code PLUTO. The simulations are driven using the output of coronal models for Carrington rotations 2125 and 2126 and, given the uncertainties in the initial conditions, are able to reproduce an event of comparable magnitude to the 23 July ICME, with similar velocity and densityprofi les at 1 au. The launch-time of this event is then varied with regards to an initial 19 July ICME and the effects of solar wind preconditioning are found to be signi ficant for an event of this magnitude and to decrease over a time-window consistent with the ballistic re filling of the depleted heliospheric sector. These results indicate that the 23 July ICME was mostly unaffected by events prior, but would have travelled even faster had it erupted closer in time to the 19 July event where it would have experienced even lower drag forces. We discuss this systematic study of solar wind preconditioning in the context of space weatherforecasting.

Journal article

Mihailescu AT, Desai R, Shebanits O, Haythornthwaite R, Wellbrock A, Coates A, Eastwood J, Waite JHet al., 2020, Spatial variations of low mass negative ions in Titan's upper atmosphere, The Planetary Science Journal, Vol: 1, Pages: 1-8, ISSN: 2632-3338

Observations with Cassini’s Electron Spectrometer discovered negative ions in Titan’s ionosphere,at altitudes between 1400 and 950 km. Within the broad mass distribution extending up to severalt housand amu, two distinct peaks were identified at 25.8-26.0 and 49.0-50.1 amu/q, corresponding to the carbon chain anions CN−and/orC2H−for the first peak and C3N−and/orC4H−for the second peak. In this study we present the spatial distribution of these low mass negative ions from 28 Titanflybys with favourable observations between 26 October 2004 and 22 May 2012. We report a trend of lower densities on the night side and increased densities up to twice as high on the day side at small solar zenith angles. To further understand this trend, we compare the negative ion densities to the total electron density measured by Cassini’s Langmuir Probe. We find the low mass negative ion density and the electron density to be proportional to each other on the dayside, but independent of each other on the night side. This indicates photochemical processes and is in agreement with the primary production route for the low mass negative ions being initiated by dissociative reactions with suprathermal electron populations produced by photoionisation. We also find the ratio ofCN−/C2H−toC3N−/C4H−highly constrained on the day-side, in agreement with this production channel, but notably displays large variations on the nightside.

Journal article

Eggington JWB, Eastwood JP, Mejnertsen L, Desai RT, Chittenden JPet al., 2020, Dipole tilt effect on magnetopause reconnection and the steady‐state magnetosphere‐ionosphere system: global MHD simulation, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-17, ISSN: 2169-9380

The Earth’s dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere‐ionosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying IMF orientation and non‐uniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle‐dependence, we perform MHD simulations of the steady‐state magnetosphere‐ionosphere system under purely southward IMF conditions for tilt angles from 0°‐90°. We identify the location of the magnetic separator in each case, and find that an increasing tilt angle shifts the 3‐D X‐line southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50° tilt angle, characteristic of regular FTE generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90°. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region‐I field‐aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north‐south asymmetry in magnetospheric convection, thus driving more FAC in the northern (sunward‐facing) hemisphere for large tilt angles than in the south independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.

Journal article

Nordheim TA, Wellbrock A, Jones GH, Desai RT, Coates AJ, Teolis BD, Jasinski JMet al., 2020, Detection of Negative Pickup Ions at Saturn's Moon Dione, GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276

Journal article

Wellbrock A, Coates AJ, Jones GH, Vuitton V, Lavvas P, Desai RT, Waite JHet al., 2019, Heavy negative ion growth in Titan's polar winter, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 490, Pages: 2254-2261, ISSN: 0035-8711

Journal article

Dubois D, Carrasco N, Bourgalais J, Vettier L, Desai RT, Wellbrock A, Coates AJet al., 2019, Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations, ASTROPHYSICAL JOURNAL LETTERS, Vol: 872, ISSN: 2041-8205

Journal article

Eggington J, Mejnertsen L, Desai R, Eastwood J, Chittenden Jet al., 2018, Forging links in Earth's plasma environment, Astronomy and Geophysics, Vol: 59, Pages: 6.26-6.28, ISSN: 1366-8781

Journal article

Gingell IL, Schwartz SJ, Gershman DJ, Paterson WR, Desai RT, Giles BL, Pollock CJ, Avanov LAet al., 2018, Production of negative hydrogen ions within MMS Fast Plasma Investigation due to solar wind bombardment, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 6161-6170, ISSN: 2169-9380

The particle data delivered by Fast Plasma Investigation (FPI) instrument aboard NASA's Magnetospheric Multiscale (MMS) mission allows for exceptionally high-resolution examination of the electron and ion phase space in the near-Earth plasma environment. It is necessary to identify populations which originate from instrumental effects. Using FPI's Dual Electron Spectrometers (DES) we isolate a high energy (~keV) beam, present while the spacecraft are in the solar wind, which exhibits an azimuthal drift with period associated with the spacecraft spin. We show that this population is consistent with negative hydrogen ions H- generated by a double charge exchange interaction between the incident solar wind H+ ions and the metallic surfaces within the instrument. This interaction is likely to occur at the deflector plates close to the instrument aperture. The H- density is shown to be approximately 0.2-0.4% of the solar wind ion density, and the energy of the negative ion population is shown to be 70% of the incident solar wind energy. These negative ions may introduce errors in electron velocity moments on the order of 0.2-0.4% of the solar wind velocity, and significantly higher errors in the electron temperature.

Journal article

Desai RT, Taylor SA, Regoli LH, Coates AJ, Nordheim TA, Cordiner MA, Teolis BD, Thomsen MF, Johnson RE, Jones GH, Cowee MM, Waite JHet al., 2018, Cassini CAPS identification of pickup ion compositions at Rhea, Geophysical Research Letters, Vol: 45, Pages: 1704-1712, ISSN: 0094-8276

Saturn's largest icy moon, Rhea, hosts a tenuous surface‐sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing urn:x-wiley:grl:media:grl56939:grl56939-math-0001 during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along‐track densities of ∼2 × 10−3 cm−3. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon‐based compounds, such as CN−, urn:x-wiley:grl:media:grl56939:grl56939-math-0002, urn:x-wiley:grl:media:grl56939:grl56939-math-0003, or HCO−, sputtered from carbonaceous material on the moon's surface. The negative ions are calculated to possess along‐track densities of ∼5 × 10−4 cm−3 and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO2 exosphere and surprisingly low O2 sputtering yields. These pickup ions provide important context for understanding the exospheric and surface ice composition of Rhea and of other icy moons which exhibit similar characteristics.

Journal article

Taylor SA, Coates AJ, Jones GH, Wellbrock A, Fazakerley AN, Desai RT, Caro-Carretero R, Michiko MW, Schippers P, Waite JHet al., 2018, Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 287-296, ISSN: 2169-9380

Journal article

Desai RT, Cowee MM, Wei H, Fu X, Gary SP, Volwerk M, Coates AJet al., 2017, Hybrid Simulations of Positively and Negatively Charged Pickup Ions and Cyclotron Wave Generation at Europa, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 10408-10420, ISSN: 2169-9380

Journal article

Desai RT, Coates AJ, Wellbrock A, Vuitton V, Crary FJ, Gonzalez-Caniulef D, Shebanits O, Jones GH, Lewis GR, Waite JH, Cordiner M, Taylor SA, Kataria DO, Wahlund J-E, Edberg NJT, Sittler ECet al., 2017, Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere, ASTROPHYSICAL JOURNAL LETTERS, Vol: 844, ISSN: 2041-8205

Journal article

Shebanits O, Wahlund J-E, Edberg NJT, Crary FJ, Wellbrock A, Andrews DJ, Vigren E, Desai RT, Coates AJ, Mandt KE, Waite JHet al., 2016, Ion and aerosol precursor densities in Titan's ionosphere: A multi-instrument case study, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 121, Pages: 10075-10090, ISSN: 2169-9380

Journal article

Daykin-Iliopoulos A, Desai R, 2014, Performance evaluation of micropropulsion systems with the application of Active Debris Removal

Recent micropropulsion developments have increased the operational capabilities of microsatellites (< 100 kg), to the point where they are being considered for demanding missions such as Active Debris Removal (ADR). This study examines the state-of-the-art micropropulsion options that can be applied to a microsatellite performing a range of orbital manoeuvres such as ADR. To this end a generic system architecture is used for this example scenario with ranging delta-v requirements. A variety of propulsion systems are sized accordingly, and the trade-offs between the differing propulsion technologies evaluated with respect to the power requirements, System-Specific Impulse , and technology readiness level (TRL), as well as a ‘newly defined’ volume driven System-Specific Impulse . It was shown that the high specific impulse (> 2000s) and consequent mass and volume savings of miniaturised Electric Propulsion (EP) systems, such as the miniature ion and colloid thrusters, results in optimal ISSP’s and ISSP’s for the ADR scenario examined. Moreover an in-depth combined quantitative and qualitative analysis identifies delta-v regions most suited to both EP and Chemical Propulsion (CP) systems, as well as highlighting the inherent differences in the varying technologies with respect to volume as well as mass.

Conference paper

Rymer A, Mandt K, Hurley D, Lisse C, Izenberg N, Smith HT, Westlake J, Bunce E, Arridge C, Masters A, Hofstadter M, Simon A, Brandt P, Clark G, Cohen I, Allen R, Vine S, Hansen K, Hospodarsky G, Kurth W, Romani P, Lamy L, Zarka P, Cao H, Paty C, Hedman M, Roussos E, Cruikshank D, Farrell W, Fieseler P, Coates A, Yelle R, Parkinson C, Militzer B, Grodent D, Kollmann P, McNutt R, André N, Strange N, Barnes J, Dones L, Denk T, Rathbun J, Lunine J, Desai R, Cochrane C, Sayanagi KM, Postberg F, Ebert R, Hill T, Mueller-Wodarg I, Regoli L, Pontius D, Stanley S, Greathouse T, Saur J, Marouf E, Bergman J, Higgins C, Johnson R, Thomsen M, Soderlund K, Jia X, Wilson R, Englander J, Burch J, Nordheim T, Grava C, Baines K, Quick L, Russell C, Cravens T, Cecconi B, Aslam S, Bray V, Garcia-Sage K, Richardson J, Clark J, Hsu S, Achterberg R, Sergis N, Paganelli F, Kempf S, Orton G, Portyankina G, Jones G, Economou T, Livengood T, Krimigi S, Szalay J, Jackman C, Valek P, Lecacheux A, Colwell J, Jasinski J, Tosi F, Sulaiman A, Galand M, Kotova A, Khurana K, Kivelson M, Strobel D, Radiota A, Estrada P, Livi S, Azari A, Yates J, Allegrini F, Vogt M, Felici M, Luhmann J, Filacchione G, Moore Let al., Solar System Ice Giants: Exoplanets in our Backyard

Future remote sensing of exoplanets will be enhanced by a thoroughinvestigation of our solar system Ice Giants (Neptune-size planets). What canthe configuration of the magnetic field tell us (remotely) about the interior,and what implications does that field have for the structure of themagnetosphere; energy input into the atmosphere, and surface geophysics (forexample surface weathering of satellites that might harbour sub-surfaceoceans). How can monitoring of auroral emission help inform future remoteobservations of emission from exoplanets? Our Solar System provides the onlylaboratory in which we can perform in-situ experiments to understand exoplanetformation, dynamos, systems and magnetospheres.

Working paper

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