Enabling Search Slow Log in ElasticSearch

How can I enable the search slow log in elastic search.

I am using ES version 5.2

I have tried running the below command but this doesn't seem to be working. Nothing appears to be written to file.

PUT /articles-dev-19-06-2017-15-20-48/_settings
{
"index.search.slowlog.threshold.query.warn": "10s",
"index.search.slowlog.threshold.query.info": "5s",
"index.search.slowlog.threshold.query.debug": "2s",
"index.search.slowlog.threshold.query.trace": "500ms",
"index.search.slowlog.threshold.fetch.warn": "1s",
"index.search.slowlog.threshold.fetch.info": "800ms",
"index.search.slowlog.threshold.fetch.debug": "500ms",
"index.search.slowlog.threshold.fetch.trace": "200ms",
"index.indexing.slowlog.threshold.index.warn": "10s",
"index.indexing.slowlog.threshold.index.info": "5s",
"index.indexing.slowlog.threshold.index.debug": "2s",
"index.indexing.slowlog.threshold.index.trace": "500ms",
"index.indexing.slowlog.level": "trace",
"index.indexing.slowlog.source": "100"
}

Here are the settings on the index

{
  "articles-dev-19-06-2017-15-20-48": {
    "settings": {
      "index": {
        "search": {
          "slowlog": {
            "threshold": {
              "fetch": {
                "warn": "1s",
                "trace": "200ms",
                "debug": "500ms",
                "info": "800ms"
              },
              "query": {
                "warn": "10s",
                "trace": "500ms",
                "debug": "2s",
                "info": "5s"
              }
            }
          }
        },
        "indexing": {
          "slowlog": {
            "level": "trace",
            "threshold": {
              "index": {
                "warn": "10s",
                "trace": "500ms",
                "debug": "2s",
                "info": "5s"
              }
            },
            "source": "100"
          }
        },
        "number_of_shards": "2",
        "provided_name": "advice-articles-dev-19-06-2017-15-20-48",
        "creation_date": "1497885649676",