How do I keep the application logged in? (Firebase and Ionic)

I need some help I'm using firebase to register and login to my app, but every time I close the app, it asks me to sign in again when I open ... How do I log in until the user clicks the exit button?

I saw that the firebase has a method for this

firebase.auth.Auth.Persistence.LOCAL

but I do not know how to use

My login method is this

signInUser(){

this.fire.auth.signInWithEmailAndPassword(this.user.value, this.password.value)
.then(data => {
  console.log('got some data ', this.fire.auth.currentUser);

  this.alert('Parabéns! Você está na sua conta!')
  this.navCtrl.setRoot("PrincipalPage")
})
.catch(error => {
  console.log('got an error ', error);

  this.alert('Username ou senha inválido!')

});
console.log(this.user.value, this.password.value)

}

and in the app.component it looks like this:

export class MyApp {
 rootPage:string = "HomePage";

   constructor(platform: Platform, statusBar: StatusBar, splashScreen: SplashScreen) {
      platform.ready().then(() => {
      statusBar.styleDefault();
      splashScreen.hide();
   });
 }
}