AGU allows authors to upload Supporting Information along with a manuscript to one of its journals. The description about it is here. It is to provide a digital archive of information that is not essential to the study but readers might find useful for a deeper understanding of the topic, methodology, underlying data sets, software, or results.
This used to be called an “electronic supplement” but that name is outdated now that the paper itself is electronic. Thus the new name, supporting information. Another change from the recent past is that it now should be uploaded as a single file, if possible. Yes, a single file, with embedded figures, tables, video, audio, or code. You can use multiple files for the different pieces of supporting information, but AGU greatly prefers for it to be embedded/concatenated into a single file. The link above is a short description of Supporting Information; the more detailed site is located here.
There are templates for the supporting information file in Word and LaTeX formats. The file should contain header information to identify the original article, introductory text to give an overview of the content, detail the source of the material, list any known caveats to usage of the information, and explain its potential usefulness to the research community. A caption should be written for each piece of supporting information.
Note that this has to be uploaded as part of the submission process and that it undergoes peer review scrutiny just like the rest of the article. Its existence has to be justified and appropriate. It also has to be deemed nonessential to the main findings of the study. So, like I recently advocated, think carefully about which figures below in the main article. Perhaps some of them could go in the Supporting Information instead.
On the plus side, supporting information does not count towards the Publication Unit count and is therefore “free” content to the author. Or, more appropriately, it is built in to the base publication fee, which is $1000 for JGR Space Physics. Also note that color figures are the same price (free!) as black-and-white images, and, at least at this point, there is no size limit to audio or video.