Scrapy process.crawl() to export data to json

This might be a subquestion of Passing arguments to process.crawl in Scrapy python but the author marked the answer (that doesn't answer the subquestion i'm asking myself) as a satisfying one.

Here's my problem : I cannot use scrapy crawl mySpider -a start_urls(myUrl) -o myData.json
Instead i want/need to use crawlerProcess.crawl(spider) I have already figured out several way to pass the arguments (and anyway it is answered in the question I linked) but i can't grasp how i am supposed to tell it to dump the data into myData.json... the -o myData.json part
Anyone got a suggestion ? Or am I just not understanding how it is supposed to work..?

Here is the code :

crawlerProcess = CrawlerProcess(settings)
crawlerProcess.install()
crawlerProcess.configure()

spider = challenges(start_urls=["http://www.myUrl.html"])
crawlerProcess.crawl(spider)
#For now i am just trying to get that bit of code to work but obviously it will become a loop later.

dispatcher.connect(handleSpiderIdle, signals.spider_idle)

log.start()
print "Starting crawler."
crawlerProcess.start()
print "Crawler stopped."