It appears that the log 'returns' of the VIX index have a (negative) correlation to the log 'returns' of e.g. the S&P 500 index. The r-squared is on the order of 0.7. I thought VIX was supposed to be a measure of volatility, both up and down. However, it appears to spike up when the market spikes down, and has a fairly strong negative correlation to the market (in fact, the correlation is much stronger than e.g. for your garden variety tech stock).
Can anyone explain why? The mean 'returns' of both indices are accounted for in this correlation, so this is not a result of the expectation of the market to increase at ~7% p.a.
Is there a more pure volatility index or instrument?