It is not unusual to have transient problems with downloads. Try again and if you still have a problem, you should post more details. Have you successfully downloaded similar files in the past? Have you tried downloading from a different OS/browser/system? Do you have enough free space on your (virtual) disk? Do you have direct internet access or a proxy? Are you downloading using a browser or a command-line tool (e.g., wget or curl in a linux terminal)?
In some cases, a failed download may leave you a small file with some html code. Since you have Ubuntu 16.04, you can use the
filecommand in a terminal as follows:
$ file Downloads/A20173052017334.L3b_MO_CHL.nc. NetCDF4 files are actually HDF5 files (but using a restricted set of features), so this the expected result. If you get "ASCII text", you can open the file in an editor and look for messages. If you get or "HTML document, ASCII text", you may be able to view the file in a browser using the URL "file://<path to file>" to look for messages.
Downloads/A20173052017334.L3b_MO_CHL.nc: Hierarchical Data Format (version 5) data
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Assuming that you have ASCII data records containing latitude, longitude,
and geophysical value (output from l3bindump, for example), then you could
follow this basic outline.
(1) Use your favorite map projection (e.g. Mercator, stereographic, etc.)
to convert your (lon,lat,value) records to Cartesian (x,y,value) records.
(2) Use a gridding algorithm (e.g. nearest neighbor, spline surface, triangulation,
etc.) to get interpolated values at every point in a uniform grid. This
grid becomes the basis for your image raster.
(3) Scale and color the gridded values to taste.
(4) Output the raster in your favorite image format.