The loan move was announced on January 5, 2013 (per Arsenal.com):
Johan Djourou will join side Hannover 96 on loan for the remainder of the 2012/13 season, subject to the completion of regulatory processes.
The Switzerland international has featured 144 times for Arsène Wenger’s side in all competitions since making his first-team debut in a 2-1 League Cup victory against Manchester City in October 2004.
A fortnight or so removed from life as a Gunner, Johan Djourou was thrown in at the deep end against Schalke—the result was disastrous with Hannover 96 conceding five times.
The Arsenal loanee had now been on teams that conceded a combined 10 goals in his last two official club outings (Reading 5-7, with Arsenal being the other match).
No criticisms should be directed at Hannover manager Mirko Slomka, whose thought process was logical: Ciprian Marica, a run-of-the-mill forward, was projected to start at No. 9 for Schalke 04; Schalke hadn't won their last six league games; Slomka was without Christian Schulz; and Karim Haggui's displays were getting progressively worse .
Not to mention, Djourou was solid in several club friendlies leading into the game versus Schalke (per Dietmar Nolte at Bundesliga.com): "In the warm-up matches, Hannover always kept a clean sheet with you [Johan] in the side."
 Still hasn't recovered from the psychological battering he took when a ruthless Bayern Munich side methodically targeted him in a 5-0 win last November—goalkeeper Ron-Robert Zieler ensured the the scoreline didn't run into double-digits.
Mirko Slomka's approach to football: "lads, the opposing team will score four times, but we'll put seven past them."
Slomka has also built a name as a talent gambler.
Forget about giving Mesut Özil and Benedikt Höwedes their league debuts, Slomka's decision to stick with some 20-year-old novice called Manuel Neuer over Frank Rost  in 2006 demonstrates how accurate Mirko's talent evaluating ability is.
Buying Mohammed Abdellaoue from Vålerenga was a masterstroke with Moa becoming a reliable source for goals.
Szabolcs Huszti is a run-away hit with a combined goals scored and created tally of 18 league goals this season; seven more than Fernando Torres.
Djourou leaves Arsenal for 6-month Hannover loan. H96 have made a habit of plucking unwanted EPL players. Previous success w/ Diouf & Zieler— Clark Whitney (@Mr_Bundesliga) January 5, 2013
Mame Biram Diouf went from Premier League flop to one of the rising forwards in German football.
Ron-Robert Zieler has transitioned from Manchester United academy player to Bundesliga-standard goalie.
Johan Djourou's lack of self-belief is the primary reason for him failing to live up to Arsène Wenger's expectations.
Whenever Johan started (out of position) at right-back, it was like a lamb to the slaughter, hence why his body language as an Arsenal player was so bad.
Manager Mirko Slomka will play a vital part in Johan's renaissance next season by rebuilding the Swiss international's confidence.
Djourou came to Hannover 96 on loan without match fitness, no-form and is still in the process of acclimatising to life in Germany.
He hasn't been Jeffrey Bruma-bad, but I would give the Arsenal loanee a C- grading for his displays at centre-back for Hannover so far.
If Johan did qualify for the kicker's player ratings list, his current rating (3.78)  would rank him as the 52nd best player in the Bundesliga, which can be spun as a positive considering his circumstances.
For those wondering if Djourou has a future with the Gunners: NO!
He's about to begin preliminary discussions with Hannover regarding his future (via Sky Sports' Thomas Zocher).
Johan has the tangibles: at times, he is unbeatable in the air; excellent blocking ability; calm under pressure; and has shown glimpses that he can frequently cut off passes (eight interceptions in two UEFA Europa League games).
Should H96 sign him to a long-term contract , it's not ridiculous to say that Djourou has the potential to become one of the league's top defenders.
Don't forget that Dante's first impressions in Germany were even worse than Johan—look at the Brazilian now.
Cristian Molinaro went from a laughing stock at Juventus to Stuttgart's best defender during the 2009-10 season.
It was the reverse for Andrea Barzagli, who was an overpriced flop at Wolfsburg, but is now producing defensive masterclasses for Juve week in, week out.
 the lower rating, the better.
 Slomka generally always green-lights risky deals.
Carlos Grossmüller at Schalke comes to mind and recently, Franca at Hannover.
There are two possible scenarios where Djourou might not end up being a H96 player come this time next season.
1) Arsène Wenger tries to draw blood from a stone.
2) Johan's agent attempts to inflate his client's salary to which the Germany club responds: "verpiss dich!"