On March 10, 2011 the operational voltage of Segment B of the COS/FUV detector was increased to the value used initially in SMOV and during routine ground testing. The operational voltage of Segment A was not modified. This change was needed to mitigate the effects of gain sag, which had become particularly severe on Segment B, leading to the appearance of absorption-like features, i.e. gain sag "holes", in the regions of the detector illuminated by geocoronal Lyman alpha emission when observing with G130M settings. [Gain sag of the COS/FUV detector has been discussed in the April 2011 and November 2010 STANS].
Increasing the operational voltage of Segment B allows the COS/FUV data to remain "hole" free until later in the Summer and has minimal impact on the data: the flat field characteristics at higher HV are within 3% or better of those at lower HV, while the throughputs are within 1%. The dark current level as well as the wavelength scales remain constant within the uncertainties.
In the coming months the COS team will be considering other strategies to mitigate the effects of gain sag on the FUV detector, such as increasing further the operational voltage of both segments, excluding gain-sag regions when co-adding data obtained at different FP-POS, and moving routine operations to a pristine area of the COS/FUV detector (i.e., new lifetime position).