How to copy some ElasticSearch data to a new index

Since ElasticSearch 2.3 you can now use the built in _reindex API

for example:

POST /_reindex
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter"
  }
}

Or only a specific part by adding a filter/query

POST /_reindex
{
  "source": {
    "index": "twitter",
    "query": {
      "term": {
        "user": "kimchy"
      }
    }
  },
  "dest": {
    "index": "new_twitter"
  }
}

Read more: https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html