From Halopedia, the Halo wiki
The plasma charge is a Covenant air-to-ground plasma weapon utilized by Type-31 Seraph fighters for ground bombardment. They are initially focused pillars of superheated gas, but quickly lose integrity and billow out into destructive conflagrations that can easily overwhelm most UNSC infantry armor, including the SPARTAN-IIIs' Semi-Powered Infiltration armor. This was seen during Operation: TORPEDO, where a sustained usage of plasma charges resulted in the slaughter dozens of the commandos of Beta Company in the open. The only way for the Spartan-IIIs to survive these attacks was for them to find cover.
These seem to be the Covenant equivalent of unguided bombs or rockets, used in a similar manner and functioning in a similar capacity.
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