Variable scope in node.js or casper.js

I feel stupid asking this question. But I have an annoying issue with variable scoping aaa below. Shouldn't it output test at the end? But that wasn't the case. What did I do wrong to use aaa within a function?

var casper = require('casper').create();

var aaa = 'test';
casper.start('http://google.com/', function() {
    this.echo("I'm in");
});

casper.run(function(aaa) {
  this.echo(aaa);
  this.exit();
});