NDS-2018

Neptune's Dark Vortex NDS-2018 ("NDS-2018")

Primary Investigator: Michael H. Wong

Contributing Program PI: Amy A. Simon

HLSP Authors: Michael H. Wong, Patrick M. Fry, Lawrence A. Sromovsky

Released: 2022-08-01

Updated: 2022-08-01

Primary Reference(s): Wong et al. (2022)

DOI: 10.17909/99vs-b998

Citations: See ADS Statistics

Read Me

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Animation showing a projection of Neptune's surface. A dark spot emerges and changes latitude over time. — A sequence of Hubble “difference images” highlights Neptune’s dark vortex from its formation in 2018 through the end of 2020. Mysterious transient secondary dark features seen in 2019 and 2020 have never been seen accompanying any previous Neptune dark spot, nor have similar features been reproduced in numerical simulations. The lower left is a representation of the change of the dark vortex latitude over time.

An animation showing the projected surface of Neptune and a simulated vortex near the center, at lower Eddy pseudo potential velocity. — A numerical simulation of an anticyclonic vortex on Neptune, with an equatorward drift similar to the observations. The model requires a modified zonal wind profile compared to the winds measured from Voyager data. The transient secondary dark spots could not be reproduced in this simulation. The simulation movie was created by Raúl Morales-Juberías; see Wong et al. (2022) for details.

Overview

A Northern Dark Spot discovered in 2018 (NDS-2018) was detected in the annual observations of Neptune from the OPAL (https://archive.stsci.edu/prepds/opal) program. Once 2019 OPAL data showed that the spot was drifting very close to the equator, where numerical simulations find that spots may perhaps cause planetary-scale disturbances as they disrupt themselves (LeBeau and Dowling 1998), additional observing programs were conducted to understand how the dark vortex was changing on shorter time scales, and to capture finer detail by taking a higher number of blue (F467M) images in each HST orbit. Dark spots require high spatial resolution at blue wavelengths to be observed, so no ground-based observatory has yet published a detection of one. Only the Voyager 2 spacecraft and Hubble have made published observations of dark vortices on Neptune.

Wong et al. (2022) reviews the overall collection of HST imaging data from the WFC3/UVIS instrument. These have been processed to correct for geometric distortion, cosmic ray hits, and fringing (at narrowband red wavelengths), then navigated to define latitude, longitude, emission, and incidence angles for each pixel as described in Wong et al. (2020). Frames have been corrected for loss of contrast due to scattered light from the wings of the point spread function as described in Sromovsky and Fry (2002), and limb darkening and large-scale zonal brightness variation were removed by constructing difference images as described in Sromovsky et al. (2001). The team prepared stacks of coadded difference images to maximize PSF sampling and to reduce noise from pixel instability (Borque et al. 2018, ISR WFC3-2018-15; Medina 2021, ISR WFC3-2021-14).

Data Products

Four types of data are provided. In order of the processing stage, they are:

Type	Description
NAV	Navigated individual frame; FITS extensions give latitude, longitude, emission angle, and incidence angle for each pixel. All files in the the NAV bundle are FITS files, combined with tar and compressed with gzip. Science data for NAV files are stored in the first extension, following the convention for standard pipeline drizzled WFC3 images. Units of the science data (in the first extension) are e-/sec, like the drz.fits files, with a PHOTIF header keyword provided to convert the data to I/F reflectivity units.
REG	Cylindrical map projection of a single frame of HST data. FITS extensions give emission angle and incidence angle for each pixel; no limb-darkening correction has been applied. FITS keywords give latitude/longitude bounds of maps. All files in the the REG bundle are FITS files, combined with tar and compressed with gzip. Science data for REG files are stored in the primary data array, with emission angle and incidence angle backplanes provided in extensions. Units of the science data (in the first extension) are e-/sec, like the drz.fits files, with a PHOTIF header keyword provided to convert the data to I/F reflectivity units.
STACKED-DIFF	Coadded stacks of frames, where limb darkening and zonal brightness variations are removed by background-subtracting an included background image. The background image (background-nav.fits inside the tar bundle) is a coadded stack of frames with no discrete dark features present, stored in extension 6 of the background FITS files (the first FITS extension should be ignored for the background files). Stacked difference bundles contain data in both NAV (sky coordinates) and REG (latitude/longitude grid coordinates) formats. All files in the the STACKED-DIFF bundle are FITS files, combined with tar and compressed with gzip. HISTORY lines in the FITS headers list the individual frames included, but the names are listed as local research filenames that use a non-MAST standard: "YYMMDD_FILT_HHMM_<nav/reg>.fits". Units of the science data are I/F reflectivity for "background-nav.fits" files, and I/F difference for the "*stacked-diff-<nav/reg>.fits" files.
GLOBAL-DIFFMAP	Mosaics covering 360 degrees of longitude, constructed using the stacked difference maps. These are cylindrical projections in latitude-longitude space, provided in FITS format (with additional info in the headers) and graphical PNG and PDF formats (with axes and image brightness scale bars).

The files have the following naming convention:

hlsp_nds-2018_hst_wfc3-uvis_neptune-<timestamp>_<filters>_v1_<product>.<ext>

where:

<filters> = The name of the HST filter or filters that went into this product. If more than one, they are joined by a dash (-). Example: "f467m-f547m-f763m-f845m-fq619n".
<timestamp> = A timestamp in either "YYYYMMDD" format for the tar files (example: "20200107") or "YYYYMMDDutHHMMSS" format (example: "20200107ut224424") for the other products.
<product> = The type of data product, one of "nav", "reg", "stacked-diff", "global-diffmap", or "global-diffmap-small".
<ext> = The file extension, one of "tar.gz", "fits", "pdf", "png", or "jpg".

Data file types, in order of processing level:

_nav.tar.gz	Navigated images, in FITS format, for a given timestamp.
_reg.tar.gz	Cylindrical map projections, in FITS format, of single frames of HST data for a given timestamp.
_stacked-diff.tar.gz	Coadded stack of frames of HST data, in FITS format, that have been background subtracted for a given timestamp.
_global-diffmap.{fits,pdf,png}	Mosaics covering 360 degrees of longitude made from the stacked difference maps.
_global-diffmap-small.jpg	Smaller versions of the global-diffmap to use as previews, e.g., on webpages.

Data Access

First Obs. Date (UT)	Global Difference Maps	Bundles of Individual Frames
2018-09-09	PNG PDF FITS	NAV REG STACKED-DIFF
2018-11-05	PNG PDF FITS	NAV REG STACKED-DIFF
2019-09-28	PNG PDF FITS	NAV REG STACKED-DIFF
2020-01-07	PNG PDF FITS	NAV REG STACKED-DIFF
2020-06-23	PNG PDF FITS	NAV REG STACKED-DIFF
2020-08-19	PNG PDF FITS	NAV REG STACKED-DIFF
2020-12-12	PNG PDF FITS	NAV REG STACKED-DIFF