I think you can call about any game in the Big 10 a rivalry game simply because of the importance.
Some are more important than others to fans. For instance, no one could call the Iowa/OSU games a rivalry.
To be a true rivalry, I think the following must be met:
1. A long history of interconference play.
2. Both teams winning, thus the series is about even.
3. When teams knock out their rival for an important bowl spot or league title.
4. Perhaps an incident that precipitates tensions between fans.
5. Fans on both teams "hate" the other team
6. Both teams have good teams regularly and are both are ranked when they meet.
7. Two instate schools with fervent fans. BUT both teams have to regularly win. If one team dominates the series, then only one fanbase probably considers the series a rivalry.
8. Border states often have nice rivalries, especially if they are in the same conference.
9. Both teams go after the same players and "take" them from one another.
There are probably more?
The obvious largest rival for Iowa is Wisconsin...not even close.
Minnesota is close although they have been down the past few years.
Michigan State is a huge rival.
Illinois is a huge rival.
NW is getting there, no doubt.
NE...we haven't played them enough yet to consider the series a rivalry. Iowa played most of the games against NE when Iowa wasn't very good. Once Hayden beat them, they scheduled no more games with Iowa. All of the points made are there for the taking though to create a huge rivalry.
Does a winning streak against the opponent make a series suddenly a rivalry? For instance, if we beat Michigan five times in a row, would they consider Iowa a bigtime rival. I doubt it...Iowa would have to beat Michigan quite a few times and spoil many of their seasons before they would consider Iowa a rival...their rivals-- OSU, Notre Dame, and MSU?
So does PSU consider Iowa a rival? I doubt it, with their history I would imagine most fans would consider a more "name" team to be a main rival. Iowa has definitely gotten under the skin of PSU fans and players though. I would imagine many PSU fans dislike Iowa a lot, simply because Iowa has been able to beat them more than regularly. I also think if PSU were to win two or three in a row, they would consider Iowa's dominance over them this past decade a fluke. Kind of like Iowa does with the clowns. If however, Iowa continues to win or split wins (home-home), the series will turn into a good rivalry. Because of their history, I imagine PSU would like to call a team like Nebraska, Michigan, or OSU their real rivals. Pitt was their natural rival but they refuse to play Pitt and have since they joined the big 10.
Is Northwestern a rival of Iowa? Both fanbases greatly dislike each. One team has dominated the series for the past few years but overall Iowa has dominated the series. Both teams have a long history against each other. Northwestern has hurt Iowa by beating them, games that Iowa should have probably won. Rarely is NW ranked when Iowa plays them. They don't have much of a fanbase. They don't recruit against each other too much.