agupubs.onlinelibrary.wiley.com Open in urlscan Pro
2606:4700:7::a29f:8157  Public Scan

Form analysis
1 forms found in the DOM

POST #

<form method="post" action="#" role="search">
  <div class="fieldWithButton">
    <input class="search-field" placeholder="Search this document...">
    <button type="submit" class="btn--cta__light small submit">
      <span class="d-none d-md-inline-block">SEARCH</span>
      <span class="d-inline-block d-md-none icon material-icons">search</span>
    </button>
  </div>
</form>

Text Content

PUBLICATION LOADED

error_outline


JAVASCRIPT DISABLED

You have to enable JavaScript in your browser's settings in order to use the
eReader.

Or try downloading the content offline

DOWNLOAD
menu close
info list link
CLOSE

ARTICLE


SATURN'S WEATHER‐DRIVEN AURORAE MODULATE OSCILLATIONS IN THE MAGNETIC FIELD AND
RADIO EMISSIONS

 * navigate_before BACK info Details
   
   
   DETAILS
   
   ARTICLE
   
   SATURN'S WEATHER-DRIVEN AURORAE MODULATE OSCILLATIONS IN THE MAGNETIC FIELD
   AND RADIO EMISSIONS
   
   M. N. Chowdhury, T. S. Stallard, K. H. Baines, G. Provan, H. Melin, G. J.
   Hunt, L. Moore, J. O’Donoghue, E. M. Thomas, R. Wang, S. Miller, S. V. Badman
   format_quote CITE
   © 2021. The Authors.
   https://doi.org/10.1029/2021GL096492
   Published inGeophysical Research Letters
   PublisherJohn Wiley & Sons, Ltd
   ISSN0094-8276
   eISSN1944-8007
   
   
   
   Received7 October 2021
   
   
   
   Accepted20 December 2021
   Volume49
   Issue3
   Pagesn/a - n/a
   
   ABSTRACT
   
   The Cassini spacecraft revealed that Saturn's magnetic field displayed
   oscillations at a period originally thought to match the planetary rotation
   rate but later found not to. One of many proposed theories predicts that a
   polar twin-cell neutral weather system drives this variation, producing
   observable differences in flows within Saturn's ionosphere. Here, using
   spectral observations of auroral emission lines taken by the Keck
   Observatory's Near Infrared Echelle Spectrograph (Keck-NIRSPEC) in 2017, we
   derive ion line-of-sight velocity maps after grouping spectra into rotational
   quadrants matching phases of the planetary magnetic field. We measure 0.5 km
   s−1 wind systems in the ionosphere consistent with predicted neutral
   twin-vortex flow patterns. These findings demonstrate that neutral winds in
   Saturn's polar regions cause the rotational period, as determined via the
   magnetic field, to exhibit differences and time variabilities relative to the
   planet's true period of rotation in a process never before seen within
   planetary atmospheres.

 * navigate_before BACK list Outline
   
   
   OUTLINE
   
    1. Saturn's Weather-Driven Aurorae Modulate Oscillations in the Magnetic
       Field and Radio Emissions
       1. Abstract
       2. Plain Language Summary
       3. 1. Introduction
       4. 2. Data Analysis
       5. 3. Results
       6. 4. Discussion and Conclusions
       7. Data Availability Statement
       8. References

 * navigate_before BACK perm_media Materials
   
   
   MATERIALS
   
      
    * FIGURES navigate_next
    * SUPPLEMENTS navigate_next
   
   Figure 1
   
   Predicted ion wind flows in Saturn's northern thermosphere (as observed from
   Earth) based on a magnetospheric (a) and thermospheric (b) source with
   planetary dawn on the left and dusk to the right. We adapt the predicted ion
   flows for a magnetospheric (G. J. Hunt et al., 2014) and atmospheric (G. Hunt
   et al., 2015) driver into what would be observed in the line‐of‐sight from
   Earth when the central meridian planetary phase (ΨN) is 0°. Colors indicate
   in broad‐scale the expected line‐of‐sight blue‐ and red‐Doppler shifts.
   Magnetic fields are not shown here for the sake of the clarity, but can be
   found in the original works.
   
   Figure
   
   get_app
   
   
   
   
   Figure 2
   
   Auroral emission intensity maps and ion wind velocity difference maps for
   Ψ0°–180° and Ψ90°–270° with planetary dawn to the left and dusk to the right.
   We make use of the individual emission intensity and ion line‐of‐sight
   velocity maps for each of the four quadrant groupings of rotational phase
   (shown in Figure S1 in the Supporting Information) to produce average
   emission intensity maps for Ψ0° and Ψ180° (Ψ0°+180°), shown in panel (a), and
   for Ψ90° and Ψ270° (Ψ90°+270°), shown in panel (d). We also subtract the
   observed ion velocity at Ψ180° from Ψ0° and the velocity at Ψ270° from Ψ90°
   to create ion wind difference maps for Ψ0°–180° and Ψ90°–270° displayed in
   panels (b) and (e), respectively. Overall emission structures shown in (a)
   and (d) reveal a bright spot on the dusk of Ψ0°+180° and broadly symmetric
   emission in Ψ90°+270°. The line‐of‐sight velocity difference maps, seen in
   (b) and (e), show clear structures with Ψ0°–180° dominated by a blue‐shift
   over the pole and red‐shift on the flanks, and Ψ90°–270° highlighting more
   complexity. The average emission intensities and ion winds for each (taken
   from between the dotted horizontal lines in the maps) are illustrated in (c)
   and (f). These show intensities of ∼3.5 W m−2 sr−1 across the auroral region
   and ion wind difference flows up to ∼±0.5 km s−1, well above the errors.
   
   Figure
   
   get_app
   
   
   
   
   Figure 3
   
   Our combined observed ion flows as seen from Earth with Saturnian dawn to the
   left and dusk to the right. Velocities are again shown at a noon magnetic
   phase of Ψ0°, as in Figure 1. The vectors represent flows from Figures 2b
   and 2e, with blue‐shifted Ψ0°–180° flowing from midnight to midday (top to
   bottom), and Ψ90°–270° flowing from dusk to dawn (right to left). The
   magnitude of the arrows is scaled with velocity up to ∼2.4 km s−1, and the
   blue‐to‐red color represents the magnitude in the Ψ0°–180° direction. Please
   note that the magnitudes of the arrows shown here correspond exactly with
   velocities presented in Figures 2b and 2e. A clear blue‐shift across the pole
   and red‐shift along the lower latitude flanks matches well with Figure 1b,
   making it consistent with a thermospheric origin for the ion winds. It is as
   yet unclear why the dawn (left) side vortex is more intense than the much
   weaker corresponding dusk (right) side vortex. One possibility may be that
   the current emerging from the dawn side vortex (Ψ90°) is higher than the one
   which emerges from the dusk side vortex (Ψ270°). Furthermore, it is not
   unreasonable to surmise that the dawn side vortex will weaken as it rotates
   into noon (Ψ0°) and through to the dusk side while the dusk side vortex will
   strengthen as it rotates into midnight (Ψ180°) through to the dawn side.
   
   Figure
   
   get_app
   perm_media
   Click here to view supplementary materials of this publication.
 * navigate_before BACK image Figures
   
 * navigate_before BACK perm_media Supplements
   
 * navigate_before BACK link Links
   
   
   LINKS
   
      
    * REFERENCES navigate_next
    * CITED BY navigate_next
    * RECOMMENDED navigate_next
   
   
 * navigate_before BACK link References
   
 * navigate_before BACK format_quote Cited by
   
 * navigate_before BACK more_horiz Recommended
   

open_in_new View in Publisher's site
Page1/7
 * skip_next Next
 * skip_previous Previous
 * Go to start
 * Go to the end
 * go to page
   

format_size
Alignment
format_align_left format_align_justify
Font Size
remove add
Hyphenation
close Return to default styles
Reading Flow
swap_vert swap_horiz
fullscreen fullscreen_exit remove_circle_outline add_circle_outline

search
SEARCH search
close

group_add
Copy Access
more_horiz
 * printPrint document
 * Offline reading
 * stop offline_pin
   Save for offline reading Saving for offline reading
    •  556.6 KB
   Saved for offline reading Remove
   Removing from offline documents
 * library_books View Offline documents
 * Login / Register

get_app

1.IntroductionAn abiding mystery following Cassini's extended tour of Saturn is
also one of the first questions the spacecraft raised about the planet:
measurements showed that Saturn's day appeared to be 6 min longer at ∼10h 45 m
(Gur-nett et al., 2005) than that measured by the Voyager 1 and 2 spacecraft
(Kaiser et al., 1980). Since it is improb-able that the interior of Saturn
changed its rotation period over the course of only two decades, it was clear
that somehow the magnetic fields above the planet were slipping relative to
those generated deep within the interior (Stevenson, 2006). This mystery has
remained unresolved despite nearly two decades of Cassini observations at
Saturn.First detected in the radio Saturn kilometric radiation and subsequently
throughout the Cassini mission, evidence for a varying planetary rotation rate
has been found in numerous parameters including: magnetospheric variations in
the plasma (Gurnett et al., 2007), energetic neutrals (Paranicas et al., 2005)
and the axisymmetric magnetic field (Giampieri et al., 2006), as well as
ultraviolet (Nichols et al., 2008) and infrared (Badman et al., 2012) auroral
emissions. The rotation rate also measurably drifts with time (Kurth et al.,
2008), with two independent rates in the northern and southern hemispheres
(Gurnett et al., 2009), linked to the changing season (Gurnett et al., 2010).
These variations are propagated throughout the Saturnian environment by two
large-scale planetary period current systems flowing between the ionosphere and
magnetosphere (G. J. Hunt et al., 2014).AbstractThe Cassini spacecraft revealed
that Saturn's magnetic field displayed oscillations at a period originally
thought to match the planetary rotation rate but later found not to. One of many
proposed theories predicts that a polar twin-cell neutral weather system drives
this variation, producing observable differences in flows within Saturn's
ionosphere. Here, using spectral observations of auroral 𝐴𝐴H+3 emission lines
taken by the Keck Observatory's Near Infrared Echelle Spectrograph
(Keck-NIRSPEC) in 2017, we derive ion line-of-sight velocity maps after grouping
spectra into rotational quadrants matching phases of the planetary magnetic
field. We measure 0.5 km s−1 wind systems in the ionosphere consistent with
predicted neutral twin-vortex flow patterns. These findings demonstrate that
neutral winds in Saturn's polar regions cause the rotational period, as
determined via the magnetic field, to exhibit differences and time variabilities
relative to the planet's true period of rotation in a process never before seen
within planetary atmospheres.Plain Language SummaryWe observed Saturn's northern
aurorae in the infrared using the Keck Observatory in Mauna Kea, Hawaii over the
course of June, July and August of 2017. Using this data we investigate the
motion of an ion, 𝐴𝐴H+3, in the planet's upper atmosphere. This is done after
first placing the data into four groups corresponding to the rotational phase of
the planet's magnetic field. By doing so we are able to detect twin-vortex flows
in the upper atmosphere of Saturn, consistent with theories that predict the
presence of such a polar feature, thus providing direct evidence that Saturn's
measured variable rotation rate is driven by these flows. These twin-vortex
flows are ultimately responsible for the time differences relative to the
planet's true rotation period observed throughout Saturn's planetary
environment.CHOWDHURY ET AL.© 2021. The Authors.This is an open access article
under the terms of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original work is
properly cited.Saturn's Weather-Driven Aurorae Modulate Oscillations in the
Magnetic Field and Radio EmissionsM. N. Chowdhury1, T. S. Stallard1, K. H.
Baines2,3, G. Provan1, H. Melin1, G. J. Hunt4, L. Moore5, J. O’Donoghue6, E. M.
Thomas1, R. Wang1, S. Miller7, and S. V. Badman81School of Physics and
Astronomy, University of Leicester, Leicester, UK, 2Jet Propulsion Laboratory,
California Institute of Technology, Pasadena, CA, USA, 3Space Science and
Engineering Center, University of Wisconsin-Madison, Madison, WI, USA, 4Blackett
Laboratory, Imperial College London, London, UK, 5Center for Space Physics,
Boston University, Boston, MA, USA, 6Institute for Space and Astronautical
Science, JAXA, Kanagawa, Japan, 7Department of Physics and Astronomy, University
College London, London, UK, 8Department of Physics, Lancaster University,
Lancaster, UKKey Points:•Keck-NIRSPEC observations of Saturn's northern H3+
infrared auroral emission from 2017 are analyzed•First clear picture of how the
ionosphere moves in relation to planetary period currents is provided•Saturn's
measured variable rotation rate is driven by twin-vortex flows in the upper
atmosphereSupporting Information:Supporting Information may be found in the
online version of this article.Correspondence to:M. N.
Chowdhury,nahidc14@gmail.comCitation:Chowdhury, M. N., Stallard, T. S., Baines,
K. H., Provan, G., Melin, H., Hunt, G. J., et al. (2022). Saturn's
weather-driven aurorae modulate oscillations in the magnetic field and radio
emissions. Geophysical Research Letters, 49, e2021GL096492.
https://doi.org/10.1029/2021GL096492Received 7 OCT 2021Accepted 20 DEC
2021Author Contributions:Conceptualization: T. S. Stallard, H. Melin, G. J.
Hunt, L. Moore, J. O’Donoghue, E. M. Thomas, R. Wang, S. Miller, S. V.
BadmanData curation: M. N. Chowdhury, T. S. Stallard, G. Provan, H. MelinFormal
analysis: M. N. Chowdhury, T. S. StallardFunding acquisition: T. S. Stallard, K.
H. BainesInvestigation: T. S. StallardMethodology: M. N. Chowdhury, T. S.
Stallard, G. Provan, H. Melin, G. J. Hunt, S. MillerProject Administration: T.
S. Stallard, K. H. Baines10.1029/2021GL096492RESEARCH LETTER1 of 7


Geophysical Research LettersCHOWDHURY ET AL.10.1029/2021GL0964922 of 7A wide
range of models have been developed in order to explain the source of these
planetary period cur-rents with limited observational constraints to
substantiate many of them. Some models place the source with-in the
magnetosphere, caused by the Enceladus torus (Burch et al., 2008; Goldreich &
Farmer, 2007; Gurnett et al., 2007), interactions with Titan (Winglee et al.,
2013), or periodic latitudinal oscillations in the plasma sheet (Carbary et al.,
2007). It is worth noting that magnetosphere-driven models are generally unable
to robustly explain the distinction between the two independent periodicities
observed in each polar region. Other models suggest that the source originates
within Saturn's atmosphere, the result of changing ionospheric conductivity
(Gurnett et al., 2007), flows in Saturn's stratosphere driving Hall drift
(Smith, 2014), or the westward drift of ionospheric Rossby waves (Smith, 2018).
Conversely, an alternative theory of how these currents are driven is that ions
are forced to move through polar magnetic fields by collisions with rotating
twin-vortex flows within Saturn's neutral polar thermosphere (Jia et al., 2012;
Smith, 2006, 2011; Southwood & Cowley, 2014)—effective-ly a form of
weather-driven aurora that is itself the result of neutral and ion winds flowing
in the planet's upper atmospheric layers.Tests of these theories using
magnetospheric observations are difficult since both the magnetospheric currents
and generated aurora are nearly identical whether they originate in the
magnetosphere or the atmosphere. The only observational evidence for an
atmospheric source comes from an apparent 1° modulation in the location of the
planetary period auroral current layer, with maximum and minimum modulations
occurring at southern magnetic phases of 270° and 90°, respectively, where local
magnetic phases are analogous to magnetic longitude (G. J. Hunt et al., 2014).
However, since the model predicting that these currents are driven by a
thermospheric twin-cell vortex explicitly requires that the ionosphere flows in
the opposite direction from other models, the motion of ions within the
ionosphere provides a unique diagnostic of the source of planetary period
currents (Smith, 2014).Figure 1 shows these expected ion flows as would be seen
by an observer at Earth with planetary dawn to the left and dusk to the right.
In the case of a magnetospheric driver, at a noon northern magnetic phase of 0°
(Ψ0°) as shown in Figure 1a, ion-neutral collisions in the ionosphere lead to an
anti-sunward flow (red-shifted) away from Earth-based observers over the central
polar region with a return flow (blue-shifted) toward observers at lower
latitudes. For an atmospheric driver at Ψ0°, seen in Figure 1b, the neutral wind
drives parallel plasma flow in the ionosphere, resulting in a sunward flow
(blue-shifted) toward the observer over the polar region and an anti-sunward
return flow (red-shifted) away from the observer at lower latitudes (G. J. Hunt
et al., 2014; G. Hunt et al., 2015; Jia et al., 2012; Southwood & Cowley, 2014).
In addition, these patterns rotate with phase at the measured period of the
magnetic field oscillations.2.Data AnalysisTo test the thermospheric twin-cell
vortex hypothesis, we used the Keck Observatory's Near Infrared Echelle
Spectrograph (Keck-NIRSPEC) (McLean et al., 1998) to scan the auroral region in
a manner similar to a study at Jupiter (Johnson et al., 2017), measuring
emission from 𝐴𝐴H+3—a dominant molecular ion species in Saturn's ionosphere.
The peak emission altitude of 𝐴𝐴H+3 is at around 1,155 km above the
atmospheric 1 bar level (T. S. Stallard et al., 2012) and is more generally
found to exist between 1,000 and 2,000 km of the same altitudinal level (T.
Stallard et al., 2010) which is, as demonstrated by Moore et al. (2010) and
Shebanits et al. (2020), in the electrical dynamo region of the ionosphere. In a
previous publication by T. S. Stallard et al. (2019), techniques were
established to measure the line-of-sight velocity of 𝐴𝐴H+3 emission lines on
individual nights, providing a view of the varying ionospheric velocity in the
Earth's reference frame. Similar work by O’Donoghue et al. (2016) and Chowdhury
et al. (2019) have also provided insights into the 𝐴𝐴H+3 aurorae at Saturn in
recent years.Here, we measure the ion velocities after mapping data into four
rotational bins matching the phase (ΨN) of the magnetic field oscillation
resulting from the rotating current system (Provan et al., 2018): Ψ0°
(315°–45°), Ψ90°(45°–135°), Ψ180° (135°–225°) and Ψ270° (225°–315°), in order to
reveal the underlying ion winds that are associ-ated with the rotating current
system. Tables S1 and S2 in the Supporting Information outline the corresponding
night-by-night and quadrant-by-quadrant breakdown of spectra into the four
aforementioned groupings of north-ern planetary magnetic phase.Saturn's auroral
currents and thus our line-of-sight velocity maps consist of subcorotational
flows associated with the outer magnetosphere. Both the co-rotational flows and
the outer magnetosphere are thought to be broadly Resources: T. S. Stallard, G.
Provan, H. MelinSoftware: M. N. Chowdhury, T. S. Stallard, G. Provan, H. Melin,
E. M. Thomas, R. WangSupervision: T. S. StallardValidation: T. S. Stallard, S.
Miller, S. V. BadmanVisualization: M. N. Chowdhury, T. S. Stallard, J.
O’Donoghue, S. V. BadmanWriting – original draft: M. N. Chowdhury, T. S.
StallardWriting – review & editing: M. N. Chowdhury, T. S. Stallard, K. H.
Baines, G. Provan, H. Melin, G. J. Hunt, L. Moore, J. O’Donoghue, S. Miller, S.
V. Badman








sort
play_circle_filled play_circle_filled
arrow_back BACK


CHAPTER TITLE HERE


FIGURE TITLE HERE

get_app
All Materials (0)
SHOW All materials  0 Figures  0 Tables  0 Videos  0 Audio files  0 Code
snippets  0 Other files  0
fullscreen fullscreen_exit remove_circle_outline add_circle_outline
play_circle_filled play_circle_filled
arrow_back BACK

CLOSE


FIGURE TITLE HERE

get_app




ALL MATERIALS

PUBLICATION TITLE HERE