elasticsearchのあるindexをcloseしようと、curlを実行したところ 「curl: (52) Empty reply from server」と表示

$ curl --cacert config/certs/http_ca.crt -u elastic
  -XPOST http://localhost:9200/test_index/_close

curl: (52) Empty reply from server

また、 elasticsearch_server.json には以下のようなログ

{"@timestamp":"2024-10-09T22:49:47.010Z", "log.level": "WARN",
"message":"received plaintext http traffic on an https channel,
           closing connection Netty4HttpChannel{
          localAddress=/127.0.0.1:9200, remoteAddress=/127.0.0.1:42808}",
 "ecs.version": "1.2.0", "service.name":"ES_ECS","event.dataset":"elasticsearch.server",
 "process.thread.name":"elasticsearch[a64][transport_worker][T#20]",
 "log.logger":"org.elasticsearch.http.netty4.Netty4HttpServerTransport",
 "elasticsearch.cluster.uuid":"_1Zg8GmwSdS8pk94jvNoPQ",
 "elasticsearch.node.id":"F5B59GreTvaM_Jz4P2wJFQ",
 "elasticsearch.node.name":"a64",
 "elasticsearch.cluster.name":"elasticsearch"}

elasticsearch.yml を以下のように変更することで解消しましたが、 elasticsearch.yml や xpack.security.enabled の意味は理解できていません

old) xpack.security.enabled: true
new) xpack.security.enabled: false