Both discharges due to negative particles (blue) and positive particles (red) occurs in the column. Interval \(a\) and interval \(b\) are similar, where negative particle coalesce into neutral then positive particles. Interval \(a\) is longer than \(b\) because \(b\) occurs in the relatively more positive region closer to the anode and the discharge region at the cathode creates more negative charges than all subsequent blue regions.
The background red in the positive column is due to positive particles repelled by the anode and is returning to the cathode. Beyond \(v_{boom}\) speed the positive particles do not cause the low pressure gas in the tube to discharge. On collision with the cathode, the cathode glows.
Some negative particles that reach the anode causes it to glow with a blue tint.
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Note: A red glow is consistent with a larger sized positive particles that have lower energy. A blue glow is consistent with a smaller sized negative particles that have higher energy.
