2435 hi l2 updated systematic error calculation

[subagonsouth]

Change Summary

Overview

This fixes differences seen in Hi L2 Uncertainties during validation

Updated Files

• imap_processing/hi/hi_l2.py
  • Fix equation for computing map systematic error
  • Only use "improved statistical variance" to combine calibration products
  • Don't remove negative values in total count rates for uncertainty
• imap_processing/tests/hi/test_hi_l2.py
  • Update equation in test to match change in hi_l2.py

Closes: #2435

[Copilot]

Pull request overview

This pull request fixes differences in Hi L2 Uncertainties identified during validation by updating the systematic error calculation and the method for combining calibration products.

Key Changes:

• Updated systematic error calculation to derive from Poisson statistics of background rates
• Modified calibration product combination to use only improved statistical variance (excluding systematic error from weighting)
• Removed negative value clamping in uncertainty calculations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
imap_processing/hi/hi_l2.py	Updated systematic error formula to compute from background Poisson statistics; changed flux weighting to use only improved statistical variance; removed negative value clamping
imap_processing/tests/hi/test_hi_l2.py	Updated test expectations to match new flux weighting calculation that excludes systematic error

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