Spectral observations of the extreme ultraviolet background

Area of Science:

• Space Physics
• Astrophysics
• Spectroscopy

Background:

• The diffuse extreme ultraviolet (EUV) background is crucial for understanding the local interstellar medium.
• Previous measurements lacked spectral resolution below 520 angstroms, limiting detailed analysis.

Purpose of the Study:

• To obtain the first spectral measurements of the diffuse EUV background below 520 angstroms.
• To identify emission features and characterize the properties of the emitting gas.

Main Methods:

• A sounding rocket was equipped with a grazing incidence spectrometer for EUV measurements.
• Data were collected on the diffuse background radiation between 80 and 650 angstroms.

Main Results:

• Detected interplanetary He I 584 angstroms and geocoronal He II 304 angstroms emissions.
• Observed features potentially from hot ionized interstellar gas, indicating multiple temperature components.
• Strongest feature consistent with O V emission (630 angstroms) restricted to log T = 5.5-5.7.
• Feature near 99 angstroms fitted by Fe XVIII/XIX lines (log T = 6.6-6.8).
• Feature near 178 angstroms consistent with Fe X/XI emission (log T = 6).

Conclusions:

• Measurements provide new constraints on the temperature and pressure of hot ionized interstellar gas.
• Results support models of saturated evaporation of clouds in a hot medium.
• The observed O V emission could be a significant ionization source.
• Findings align with previous low-resolution soft X-ray observations.