I need to search a directory for all files staring with downtime and add a line into each one. 

Right now I have the following:

```
os.chdir("/misc/downtimerecords")

for file in glob.glob("downtime*"):
    fo=open("","a+")
    fo.write("Network Cause")
    fo.close()

```

How can I change this so that no matter how many files I have in the directory this will work? Currently I will get an error because of the `""` in `fo=open`. I want this to be the name of every file that begins with downtime.