[Paper Review] Modeling the Optical Cherenkov Signals by Cosmic Ray Extensive Air Showers Directly Observed from Sub-Orbital and Orbital Altitudes

Austin Cummings, Roberto Aloisio+2 more|arXiv (Cornell University)|May 7, 2021

Astrophysics and Cosmic Phenomena47 references28 citations

TL;DR

This paper models optical Cherenkov signals from high-energy cosmic ray extensive air showers (EAS) observed above Earth's limb from sub-orbital and orbital platforms, using a customized simulation framework. It predicts event rates exceeding hundreds per hour for EUSO-SPB2 and POEMMA, demonstrating the feasibility of using cosmic ray backgrounds as in-flight calibration sources for Cherenkov detection instruments.

ABSTRACT

Future experiments based on the observation of Earth's atmosphere from sub-orbital and orbital altitudes plan to include optical Cherenkov cameras to observe extensive air showers produced by high-energy cosmic radiation via its interaction with both the Earth and its atmosphere. As discussed elsewhere, particularly relevant is the case of upward-moving showers initiated by astrophysical neutrinos skimming and interacting in the Earth. The Cherenkov cameras, by looking above Earth's limb, can also detect cosmic rays with energies starting from less than a PeV up to the highest energies (tens of EeV). Using a customized computation scheme to determine the expected optical Cherenkov signal from these high-energy cosmic rays, we estimate the sensitivity and event rate for balloon-borne and satellite-based instruments, focusing our analysis on the Extreme Universe Space Observatory aboard a Super Pressure Balloon 2 (EUSO-SPB2) and the Probe of Extreme Multi-Messenger Astrophysics (POEMMA) experiments. We find the expected event rates to be larger than hundreds of events per hour of experimental live time, enabling a promising overall test of the Cherenkov detection technique from sub-orbital and orbital altitudes as well as a guaranteed signal that can be used for understanding the response of the instrument.

Motivation & Objective

• To model the optical Cherenkov emission from high-energy cosmic ray extensive air showers (EAS) observed from above Earth's limb using sub-orbital and orbital platforms.
• To assess the sensitivity and expected event rates for future instruments like EUSO-SPB2 and POEMMA, focusing on the Cherenkov detection technique from high altitudes.
• To evaluate the potential of above-the-limb cosmic ray events as a high-rate, in-flight calibration source for Cherenkov telescopes.
• To quantify the background from refracted Cherenkov light due to atmospheric refraction, which could mimic neutrino-induced events.
• To explore the feasibility of using these signals for instrument response characterization and primary particle energy reconstruction.

Proposed method

• Adapted a customized computation scheme from prior work on neutrino-induced EAS to model Cherenkov emission from cosmic ray-induced EAS with above-the-limb trajectories.
• Simulated the development of particle cascades in the rarefied upper atmosphere, accounting for the geometry of high-altitude observation above Earth's limb.
• Calculated Cherenkov photon yield and propagation, including atmospheric refraction effects that can misreconstruct the shower origin.
• Used detailed atmospheric models for refractive index and extinction to compute signal attenuation and angular distribution.
• Applied the simulation to two mission concepts: EUSO-SPB2 (balloon-borne at 33 km) and POEMMA (orbital at 525 km), with distinct optical systems and fields of view.
• Integrated time-resolved signal modeling with fast imaging capabilities (10–20 ns integration) to assess detectability and trigger efficiency.

Experimental results

Research questions

• RQ1 What is the expected event rate for optical Cherenkov signals from above-the-limb cosmic ray EAS in EUSO-SPB2 and POEMMA missions?
• RQ2 How does atmospheric refraction affect the reconstruction of above-the-limb cosmic ray showers, and what is its impact as a background for neutrino detection?
• RQ3 To what extent do the Cherenkov signal characteristics (spatial profile, time structure, photon spectrum) of cosmic ray EAS resemble those of neutrino-induced EAS?
• RQ4 Can above-the-limb cosmic ray events serve as a reliable in-flight calibration source for Cherenkov telescopes on sub-orbital and orbital platforms?
• RQ5 What is the potential for using these signals to validate the energy reconstruction and instrument response of high-altitude Cherenkov detectors?

Key findings

• The expected event rate for above-the-limb cosmic ray EAS exceeds hundreds of events per hour for both EUSO-SPB2 and POEMMA, providing a strong, continuous calibration signal.
• The Cherenkov signals from above-the-limb cosmic ray EAS have nearly identical spatial and temporal profiles, as well as similar photon spectra, to those from below-the-limb neutrino-induced EAS.
• Atmospheric refraction can cause above-the-limb cosmic ray signals to be reconstructed as originating below the limb, creating a significant background for neutrino detection.
• The high flux of cosmic ray events (4 orders of magnitude higher than neutrino flux at similar energies) makes them ideal for in-flight instrument response testing.
• The signals are detectable with existing Cherenkov telescope designs (e.g., EUSO-SPB2 and POEMMA), confirming the feasibility of the detection technique from high altitudes.
• The results support the use of cosmic ray events as a benchmark for validating the full chain of Cherenkov signal reconstruction, from optics to trigger systems, in real flight conditions.