publications

Theses

2023

1. PhD Thesis

   Diving Deep into Saturn’s Equatorial Ionosphere with Cassini : Insights from the Grand Finale

   Joshua Dreyer

   Uppsala University, Department of Physics and Astronomy; Swedish Institute of Space Physics, Uppsala Division, 2023

   Abs HTML

   In the summer of 2017, the Cassini mission concluded its nearly 13 years orbiting Saturn with a series of daring dives between the rings and the upper reaches of Saturn’s atmosphere. This last phase of the mission, called the Grand Finale, revealed a highly variable equatorial ionosphere dominated by a large influx of ring material from Saturn’s D ring. The papers included in this thesis utilize data gathered during these proximal orbits to gain insights into the nature and effects of the infalling ring material. Initially, we derive upper limits for the effective recombination coefficient in Saturn’s equatorial ionosphere at altitudes below 2500 km, where photochemical equilibrium can be assumed, to constrain the composition of the positive ion species. Our inceptive results indicate that ion species with low recombination coefficients are dominant. We follow up on this by developing a photochemical model, incorporating grain charging, to investigate the effects of the ring influx on the plasma composition. The model results at an altitude of 1700 km yield vastly different abundances of two types of neutral species when compared to those derived from measurements, ultimately representing the difficulty of reconciling the observed H+ and H3+ densities with our and other model results. Exploring the nature of narrow decreases in the ionospheric H2+ densities reveals a time shift in the ion data. After correcting for this, the decreases line up very well with calculated shadows for substructures in Saturn’s C ring. We can further estimate the optical depths of these substructures and investigate at which altitudes photochemical equilibrium for H2+ is applicable. The direct measurement of heavier neutral species during the proximal orbits is complicated by the high spacecraft speed. We devise a method to utilize helium ion chemistry to independently derive the mixing ratios of these heavier neutrals in Saturn’s ionosphere. Our results show considerable variability, which may suggest temporal and/or spatial changes in the ring influx. A comparison with other studies indicates that potentially only the most volatile ring-sourced species significantly ablate to enter the gas phase in this region of Saturn’s ionosphere. Finally, we compare the fixed-bias Langmuir probe electron densities and the light ion densities. They exhibit a strong positive correlation for most parts of the proximal orbits even on short timescales. We find three distinct regions in the proximal orbits, which can provide further insight into the ionospheric composition, connection to the rings, and measurement uncertainties.

2021

1. Licentiate Thesis

   Saturn’s dusty equatorial ionosphere from Cassini’s Grand Finale observations

   Joshua Dreyer

   Uppsala University, Department of Physics and Astronomy; Swedish Institute of Space Physics, Uppsala Division, Nov 2021

   Abs HTML

   The Cassini mission provided unprecedented insights into the Kronian system and revealed many unexpected phenomena relating to dusty plasma environments around Saturn and its rings. Almost 13 years after its arrival, Cassini began a series of 22 orbits which brought it between Saturn’s upper atmosphere and the innermost D ring, culminating in a final plunge into Saturn’s ionosphere. This concluding period of Cassini’s journey is referred to as the Grand Finale and data collected during these final orbits serves as the foundation for the two papers included in this thesis. We constrain the composition of positive ions in Saturn’s equatorial ionosphere by deriving upper limits for the effective recombination coefficient. Our results suggest an ionosphere dominated by species with low recombination coefficients, such as HCO+. The analysis of Grand Finale data further revealed small-scale variations in the H2+ density for two of the orbits. We link these to narrow substructures in Saturn’s C ring with increased optical depths, which attenuate the solar extreme ultraviolet flux and thus reduce photoionisation rates within their respective shadowed regions.

2019

1. MSc Thesis

   A detailed study of auroral fragments

   Joshua Dreyer

   Uppsala UniversityUppsala University, Department of Physics and Astronomy, Swedish Institute of Space Physics, Uppsala Division, Jun 2019

   Abs HTML

   Aurora occurs in various shapes, one of which is the hitherto unreported phenomenon of auroral fragments. For three periods of occurrence of these fragments their properties were studied in detail during this master’s thesis, using mainly ground-based instrumentation located near Longyearbyen on Svalbard, Norway. A base dataset was constructed from 103 all-sky camera images, manually marking 305 fragments for further analysis. This thesis reports and describes the fragment observations during the observed events, including the auroral and geomagnetic context. Fragments generally seem to fall into two categories, the first being singular, apparently randomly distributed fragments, and the second being periodic fragments that occur in groups with a regular spacing close to auroral arcs. A typical fragment has a small horizontal size below 20 km, a short lifetime of less than a minute and shows no field-aligned extent in the emission. The fragments appear mainly west of zenith (73%) during the three observation nights, whereas their north-south distribution is symmetric around the zenith. Almost all of them exhibit westward drift, the estimated speed for one of the fragments passing the field of view of ASK is ∼1 km/s. A spectral signature can be seen in the green auroral wavelength of O at 557.7 nm and red emission line of N2 at 673.0 nm, but no emission enhancement was observed in the blue wavelengths. One fragment passing the EISCAT Svalbard radar’s field of view shows a local ion temperature increase in a small altitude range of ∼15 km, whereas there is no visible increase in electron density. This could be explained by fragment generation due to locally strong horizontal electric fields. A potential mechanism for this might be electric fields of atmospheric waves superposing with the converging electric fields of auroral arcs created by particle precipitation and the corresponding field-aligned currents. The resulting field would be perpendicular to the magnetic field and the auroral arcs, leading to wave-like density variations of excited plasma close to the arcs. Further study is required to verify this hypothesis and improve the understanding of fragment properties determined from the limited dataset used for this thesis.

2017

1. BSc Thesis

   Identification and Study of M-dwarfs with a Cross-match of the Gaia DR1, AllWISE, ROSAT and XMM-Newton Serendipitous Source Catalogues

   Joshua Dreyer

   Universität Hamburg, May 2017

Conference Articles

2023

1. EGU

   Deriving mixing ratios of heavier neutral species in Saturn’s ionosphere from light ion measurements and helium chemistry

   Joshua Dreyer, Erik Vigren, Fredrik L. Johansson, and J. Hunter Waite

   In EGU General Assembly Conference Abstracts, Feb 2023

2022

1. EPSC

   Deriving mixing ratios of heavier neutral species in Saturn’s ionosphere from light ion measurements

   Joshua Dreyer, and Erik Vigren

   In European Planetary Science Congress, Sep 2022
2. EGU

   Characteristics of Fragmented Aurora-like Emissions (FAEs)

   Joshua Dreyer, Noora Partamies, Daniel Whiter, Pål G. Ellingsen, Lisa Baddeley, and Stephan C. Buchert

   In EGU General Assembly Conference Abstracts, May 2022

2021

1. EGU

   Ionospheric shadowing signatures of ringlets and plateaus in Saturn’s C Ring

   Joshua Dreyer, and Erik Vigren

   In EGU General Assembly Conference Abstracts, Apr 2021
2. AGU

   Identifying Ionospheric Shadowing Signatures of Ringlets and Plateaus in Saturn’s C Ring

   Joshua Dreyer, Erik Vigren, Oleg Shebanits, Michiko Morooka, Jan-Erik Wahlund, and Jack Waite

   In AGU Fall Meeting Abstracts, Dec 2021
3. EPSC

   Identifying ionospheric shadowing signatures of ringlets and plateaus in Saturn’s C Ring

   Joshua Dreyer, Erik Vigren, Oleg Shebanits, Michiko Morooka, Jan-Erik Wahlund, and Jack Hunter Waite

   In European Planetary Science Congress, Sep 2021

2020

1. EPSC

   Using the effective recombination coefficient to constrain the positive ion composition in Saturn’s ionosphere

   Joshua Dreyer, Erik Vigren, Michiko Morooka, Jan-Erik Wahlund, Stephan Buchert, and J. Hunter Waite

   In European Planetary Science Congress, Sep 2020
2. EGU

   On the effective recombination coefficient in Saturn’s ionosphere

   Joshua Dreyer, Erik Vigren, Michiko Morooka, Jan-Erik Wahlund, Stephan Buchert, and J. Hunter Waite

   In EGU General Assembly Conference Abstracts, May 2020

Journal Articles

2023

1. JGR:Space

   Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn’s Equatorial Ionosphere

   Joshua Dreyer, Erik Vigren, Fredrik L. Johansson, and J. Hunter Waite

   Journal of Geophysical Research: Space Physics, Jun 2023

   Abs HTML

   A surprisingly strong influx of organic-rich material into Saturn’s upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn’s ionosphere through a steady-state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn’s equatorial ionosphere is through reactions with species other than H2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini’s proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of ∼2 × 10−4 at closest approaches and order-of-magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn’s atmosphere and are thus unavailable for reactions.

2022

1. PSJ

   Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn’s Ionosphere

   Joshua Dreyer, Erik Vigren, Fredrik L. Johansson, Oleg Shebanits, Michiko Morooka, Jan-Erik Wahlund, Rebecca S. Perryman, and Jack Hunter Waite

   The Planetary Science Journal, Jul 2022

   Abs HTML

   For orbits 288 and 292 of Cassini’s Grand Finale, clear dips (sharp and narrow decreases) are visible in the H2+ densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini’s position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn’s C ring. The calculated shadowed altitudes along Cassini’s trajectory line up well with the dips in the H2+ data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H2+ densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H2+ at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini’s trajectory. The resulting estimate of  0.3 km s-1 at an altitude of  3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

2. MNRAS

   Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

   F. L. Johansson, E. Vigren, J. H. Waite, K. Miller, A. I. Eriksson, N. J. T. Edberg, and J. Dreyer

   Monthly Notices of the Royal Astronomical Society, Jul 2022

   Abs HTML

   We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn’s ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn’s ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

3. PSJ

   Empirical Photochemical Modeling of Saturn’s Ionization Balance Including Grain Charging

   E. Vigren, J. Dreyer, A. I. Eriksson, F. L. Johansson, M. Morooka, and J.-E. Wahlund

   The Planetary Science Journal, Feb 2022

   Abs HTML

   We present a semianalytical photochemical model of Saturn’s near-equatorial ionosphere and adapt it to two regions ( 2200 and  1700 km above the 1 bar level) probed during the inbound portion of Cassini’s orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near  1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 Å.

2021

1. PSJ

   Constraining the Positive Ion Composition in Saturn’s Lower Ionosphere with the Effective Recombination Coefficient

   Joshua Dreyer, Erik Vigren, Michiko Morooka, Jan-Erik Wahlund, Stephan C. Buchert, Fredrik L. Johansson, and Jack Hunter Waite

   The Planetary Science Journal, Feb 2021

   Abs HTML

   The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini’s last four orbits into Saturn’s lower ionosphere to constrain the effective recombination coefficient α300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α300 of ≲ 3 × 10-7 cm3 s-1 for altitudes below 2000 km. This suggests that Saturn’s ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10-7 cm3 s-1 at an electron temperature of 300 K.

2. AnnGeo

   Characteristics of fragmented aurora-like emissions (FAEs) observed on Svalbard

   J. Dreyer, N. Partamies, D. Whiter, P. G. Ellingsen, L. Baddeley, and S. C. Buchert

   Annales Geophysicae, Mar 2021

   Abs HTML

   This study analyses the observations of a new type of small-scale aurora-like feature, which is further referred to as fragmented aurora-like emission(s) (FAEs). An all-sky camera captured these FAEs on three separate occasions in 2015 and 2017 at the Kjell Henriksen Observatory near the arctic town of Longyearbyen, Svalbard, Norway. A total of 305 FAE candidates were identified. They seem to appear in two categories - randomly occurring individual FAEs and wave-like structures with regular spacing between FAEs alongside auroral arcs. FAEs show horizontal sizes typically below 20 km, a lack of field-aligned emission extent, and short lifetimes of less than a minute. Emissions were observed at the 557.7 nm line of atomic oxygen and at 673.0 nm (N2; first positive band system) but not at the 427.8 nm emission of N2+ or the 777.4 nm line of atomic oxygen. This suggests an upper limit to the energy that can be produced by the generating mechanism. Their lack of field-aligned extent indicates a different generation mechanism than for aurorae, which are caused by particle precipitation. Instead, these FAEs could be the result of excitation by thermal ionospheric electrons. FAE observations are seemingly accompanied by elevated electron temperatures between 110-120 km and increased ion temperatures at F-region altitudes. One possible explanation for this is Farley-Buneman instabilities of strong local currents. In the present study, we provide an overview of the observations and discuss their characteristics and potential generation mechanisms.

3. AnnGeo

   Fine-scale dynamics of fragmented aurora-like emissions

   Daniel K. Whiter, Hanna Sundberg, Betty S. Lanchester, Joshua Dreyer, Noora Partamies, Nickolay Ivchenko, Marco Zaccaria Di Fraia, Rosie Oliver, and 3 more authors

   Annales Geophysicae, Nov 2021

   Abs HTML

   Fragmented aurora-like emissions (FAEs) are small (few kilometres) optical structures which have been observed close to the poleward boundary of the aurora from the high-latitude location of Svalbard (magnetic latitude 75.3 °N). The FAEs are only visible in certain emissions, and their shape has no magnetic-field-aligned component, suggesting that they are not caused by energetic particle precipitation and are, therefore, not aurora in the normal sense of the word. The FAEs sometimes form wave-like structures parallel to an auroral arc, with regular spacing between each FAE. They drift at a constant speed and exhibit internal dynamics moving at a faster speed than the envelope structure. The formation mechanism of FAEs is currently unknown. We present an analysis of high-resolution optical observations of FAEs made during two separate events. Based on their appearance and dynamics, we make the assumption that the FAEs are a signature of a dispersive wave in the lower E-region ionosphere, co-located with enhanced electron and ion temperatures detected by incoherent scatter radar. Their drift speed (group speed) is found to be 580-700 m s-1, and the speed of their internal dynamics (phase speed) is found to be 2200-2500 m s-1, both for an assumed altitude of 100 km. The speeds are similar for both events which are observed during different auroral conditions. We consider two possible waves which could produce the FAEs, i.e. electrostatic ion cyclotron waves (EIC) and Farley-Buneman waves, and find that the observations could be consistent with either wave under certain assumptions. In the case of EIC waves, the FAEs must be located at an altitude above about 140 km, and our measured speeds scaled accordingly. In the case of Farley-Buneman waves a very strong electric field of about 365 mV m-1 is required to produce the observed speeds of the FAEs; such a strong electric field may be a requirement for FAEs to occur.