U.S. Dollar Index
The U.S. dollar represents a gauge measuring the markets' move toward high return/higher risk ($US falling) or risk aversion ($US rising). A failing global economy tends to prompt the risk-off trade and a rise in the $US. The fact that oil price can fall as well, is an outcome of falling global demand due to failing global economic conditions rather than a rising $US. The dollar index rallied as the risk aversion trade was on and a great deal of uncertainty shrouded the European debt crises of Greece, Ireland, Portugal and others. QE initiatives and low interest rates tend to prompt risk on behaviours and a falling $US. Concern around a global contraction may be causing a flight back to the $US. Clear evidence of risk off.
The U.S. Dollar Index is a weighted gauge against the Euro, Yen, Pound, and three other currencies.