EUSO-SPB1 Mission and Science

The Extreme Universe Space Observatory on a Super Pressure Balloon 1 (EUSO-SPB1) was launched in 2017 April from Wanaka, New Zealand. The plan of this mission of opportunity on a NASA super pressure balloon test flight was to circle the southern hemisphere. The primary scientific goal was to make the first observations of ultra-high-energy cosmic-ray extensive air showers (EASs) by looking down on the atmosphere with an ultraviolet (UV) fluorescence telescope from suborbital altitude (33~km). After 12~days and 4~hours aloft, the flight was terminated prematurely in the Pacific Ocean. Before the flight, the instrument was tested extensively in the West Desert of Utah, USA, with UV point sources and lasers. The test results indicated that the instrument had sensitivity to EASs of approximately 3 EeV. Simulations of the telescope system, telescope on time, and realized flight trajectory predicted an observation of about 1 event assuming clear sky conditions. The effects of high clouds were estimated to reduce this value by approximately a factor of 2. A manual search and a machine-learning-based search did not find any EAS signals in these data. Here we review the EUSO-SPB1 instrument and flight and the EAS search.


INTRODUCTION
The sources and acceleration mechanisms that produce the highest energy particles ever observed remain unknown.With measured energies that can exceed 100 EeV (Abu-Zayyad et al. 2013;Aab et al. 2020), ultra-high-energy cosmic rays (UHECRs 1 ) occupy a tantalizing position in the multi-messenger view of the cosmos.The 100 EeV scale is six orders of magnitude above the highest energy gamma rays (Cao et al. 2021) and five orders of magnitude above the highest energy neutrinos (Aartsen et al. 2021;Abbasi et al. 2021) observed to date.Because UHECRs are charged, they are the only known high energy multi-messengers that can be accelerated directly by the sources of interest.The low flux of UHECRs requires indirect measurement techniques that use the atmosphere as a giant calorimeter.UHECRs that reach Earth's atmosphere convert their kinetic energy into extensive air showers (EASs), which generate fluorescence light via excitation of atmospheric nitrogen.The atmospheric fluorescence technique has been developed and used successfully by Fly's Eye (Bird et al. 1994), the High-Resolution Fly's Eye (HiRes) (Abbasi et al. 2008), the Pierre Auger Observatory (Aab et al. 2015), and Telescope Array (Abbasi et al. 2016) to measure EASs in 3D from the ground.The atmosphere is, by definition, the largest-volume calorimeter on Earth.
The Extreme Universe Space Observatory on a Super Pressure Balloon 1 (EUSO-SPB1) was the Joint Experiment Missions for Extreme Universe Space Observatory (JEM-EUSO) collaboration's first mission targeting UHECR EASs by looking down on the atmosphere from suborbital space.This mission represents an important step toward establishing a UHECR detector in space (Benson & Linsley 1981;Stecker et al. 2004;Krizmanic et al. 2013;Haungs et al. 2015;Bertaina 2019) that would view a much larger (≈100×) atmospheric footprint, map the entire sky at extreme energies, and discover the sources of UHECRs.The Probe of Extreme Multi-Messenger Astrophysics (POEMMA) Figure 1.EUSO-SPB1 shortly before its 2017 launch from Wanaka, New Zealand.The instrument hangs below the crane on the far left.(Olinto et al. 2021) would also have sensitivity to ultra-high-energy photons, monopoles and super heavy forms of dark matter, and, via interactions in Earth's limb, very high energy neutrinos (Anchordoqui et al. 2020).
EUSO-SPB1 flew as a NASA mission of opportunity.It was suspended below a NASA super pressure balloon that was launched as a test flight on 2017 April 24 23:51 UT from Wanaka, New Zealand (Figure 1).The EUSO-SPB1 science goals included the following: 1. making the first observations of UHECR EASs by looking down from suborbital space with an air fluorescence detector, 2. measuring background UV light at night over ocean and clouds, and 3. searching for fast UV pulse-like signatures from other objects.
2. ULTRA-LONG-DURATION SCIENTIFIC BALLOON FLIGHTS FROM WANAKA, NEW ZEALAND Located at 45 • S latitude, Wanaka lies below a fast stratospheric air circulation that develops twice a year about 33 km (7 mbar) above the Southern Ocean.This circulation flows easterly in the southern autumn and westerly in the southern spring.Super pressure balloons (SPBs) are designed to float at a constant displacement volume and consequently at a constant altitude, even at night.Thus a payload launched from Wanaka under a specially prepared SPB could ride this stratospheric circulation for months, completing multiple suborbital circumnavigations followed by controlled termination over land.Like conventional zero-pressure stratospheric balloons, SPBs also drift with the balloon-altitude wind currents.NASA's first engineering SPB mission launched from Wanaka (Cathey et al. 2017) flew for 32 days in 2015 and landed in Australia.The first scientific payload, the Compton Spectrometer and Imager (COSI) (Lowell et al. 2017), flew the following year for 46 days, circled the Southern Ocean, and landed in Peru.The 2017 flight was the third SPB test flight from Wanaka, with a nominal duration target of 100 days.The paths of these three flights are shown in Figure 2.
Unfortunately, the 2017 balloon developed a helium leak that necessitated an early controlled termination of the mission (Figure 3).The entire flight train was "valved down" about 300 km southeast of Easter Island 12 days after launch.It currently rests on the deep ocean floor.
Despite the setbacks, the EUSO-SPB1 instrument (Figure 4) operated successfully while aloft and returned about 60 GB of data.Here we describe the instrument, preflight testing in the laboratory and desert, the mission, the data, and the search for EASs.Preparations for the EUSO-SPB2 mission (Section 7) are underway.

2014 instrument and mission
The 2017 EUSO-SPB1 mission succeeded the 2014 EUSO-Balloon overnight mission sponsored by the French Space Agency and launched from Timmins, Ontario, Canada, by the Canadian Space Agency on a zero-pressure balloon.The fluorescence telescope (FT) camera was triggered externally by an onboard 20 Hz clock.The instrument recorded UV terrestrial emission levels (Abdellaoui et al. 2019)  fluorescence telescope looking down on the atmosphere (Abdellaoui et al. 2018).The entire flight train splashed down in a small lake and the instrument was recovered intact.

2017 instrument
The 2014 instrument was upgraded extensively for the 2017 launch.Specifications of the 2017 instrument (Bacholle 2017) are listed in Table 1.The upgrades featured a trigger to identify EAS candidates, a new set of Fresnel lenses, a new UV camera (Figure 5) with higher quantum efficiency multi-anode photomultiplier tubes (MAPMTs), an integrated high voltage (HV) system, a new flight CPU, interfaces to the NASA Support Integration Package (SIP) and telemetry, new control software, thermal sensors interfaced to the SIP, a solar power system, and a gondola exoskeleton frame on which all equipment was mounted, including an antenna boom.The solar power system was designed to support the mission through the southern winter including a trajectory excursion to about 60 • S.
EUSO-SPB1 was designed to operate at stratospheric altitude during nights with little or no moon to detect UV tracks from EASs.The subsystem architecture is diagrammed in Figure 6.Two 1 m 2 polymethyl methacrylate (PMMA) Fresnel lenses focus light from below onto a UV-sensitive, custom high-speed camera.The focal surface of the EUSO-SPB1 telescope features a photodetector module (PDM) that counts single photoelectrons (SPEs).For assembly purposes, 4 MAPMTs, of 64 channels each, are covered by a square BG-3 UV-transmitting optical filter to   form an elementary cell (EC).Nine ECs in a 3×3 arrangement make up one PDM.Located in the PDM behind the ECs are six circuit boards that count the numbers of photoelectrons.These boards plug into a central control buffer board that hosts the VHDL trigger logic.
The PDM is operated with −1100 V applied to the photocathodes.The nominal gain of the MAPMTs is 10 6 .A Cockcroft-Walton circuit (Plebaniak et al. 2017) generates the HV.This circuit is implemented in a board potted into each EC subassembly.The MAPMT anodes are held at ground and coupled directly to the digitization electronics.This DC coupling permits photometric calibrations using pulsed or DC light sources and also permits direct measurements of background light levels.The digitization electronics identify the small current pulses that are initiated by the emission of single photoelectrons from the photocathodes.The double pulse resolution of the digitization electronics is about 6 ns.The number of SPE counts in each MAPMT channel is tallied in 2.5 µs time bins.One time bin is referred to as a gate time unit (GTU) in this paper.The onboard trigger system (Abdellaoui et al. 2017;Bayer et al. 2017) operates at the MAPMT level and scans buffered pixel count lists for locally persistent signals above background as averaged over a specified time, within 3×3 pixel cells.Persistence settings of 1, 2, and 5 GTUs were used during the flight.A persistence setting of 2 GTUs, for example, corresponds to 5 µs.The local background threshold level is adjusted dynamically to significantly reduce the number of fake triggers caused by slowly moving objects such as airplanes, and electrical storms.On receipt of a trigger, the data processing (DP) system (Scotti et al. 2019) copies 128 consecutive data frames (40 before and 88 after the trigger for a total of 320 µs) from a system buffer to an onboard 1 TB raid array.A single data frame contains a list of the number of photoelectrons recorded in each of the 2304 individual pixels over the same 2.5 µs interval.The DP system is also interfaced to ancillary systems, as diagrammed in Figure 6.The flight computer hosts a comprehensive modular control software package (Fornaro et al. 2019) that can be readily configured for testing and flight.Field tests of the trigger system are described in Section 4.
As a technology test of silicon photomultipliers (SiPMs) in near space, the EUSO-SPB1 focal surface also includes a 256-channel silicon photomultiplier elementary cell array (SiECA) (Painter et al. 2017;Renschler et al. 2018) mounted next to the MAPMTs of the PDM (Figures 5 and 7).This add-on system was flown in a stand-alone sampling mode.
An infrared (IR) camera system was developed and flown to record IR images of the scene below the balloon to identify clouds and estimate the heights of the cloud tops.Measuring clouds is important because high clouds reduce the instantaneous aperture to detect EASs by the EUSO-SPB1 instrument (Adams et al. 2015).The University of Chicago Infrared Camera (UCIRC) (Rezazadeh et al. 2017) featured two identical IR cameras that pointed down toward the same region.The field of view was 24 • ×30 • .Each camera had an IR filter.One filter transmitted IR light between wavelengths of 11.5 and 12.9 µm.The other filter transmitted IR light between wavelengths of 9.6 and 11.6 µm.These values were selected because they fall near the typical blackbody peak for clouds.The method for measuring cloud color temperature from which the cloud top height can be derived is described elsewhere (Anzalone et al. 2019).
The mechanical upgrades to the payload complied with NASA requirements for balloon gondolas, with the added requirement that the overall height, including antennas, be lower than the door of the aircraft hangar from which the mission is staged.The NASA SIP and antennas were mounted on top of the gondola frame, and two ballast hoppers were mounted on opposite sides of the frame to allow the UV fluorescence telescope and IR camera system an unobstructed downward field of view.The fluorescence telescope module could be rolled into the gondola structure and attached in about 30 minutes.A four-sided solar array and a light baffle were connected to the payload outside the hangar.

DESERT FIELD TESTS
The EUSO-SPB1 fluorescence telescope system was tested in the laboratory and in the West Desert of Utah, USA (Adams et al. 2021), at the Telescope Array site.Measurements of a 365 nm calibrated point source on a mast yielded an estimate of end-to-end absolute photometric calibration as 0.10±0.01SPE counts per incident photon.This value is comparable to a laboratory piecewise calibration.A pulsed UV laser system (Hunt et al. 2016) having a 10 ns pulse width was placed 24 km from EUSO-SPB1 and used to measure the trigger efficiency to speed-of-light tracks in the atmosphere.Like an EAS, the pulsed laser produces a moving spot of UV light traveling at the speed of light.Laser measurements with the beam tilted 45 • away from the telescope position were recorded (Figure 8) to approximate geometrically the expected distance from EUSO-SPB1 at float altitude to an EAS of a typical inclination traversing the telescope field of view below.A comparison of these configurations is diagrammed in Figure 9.The geometrical equivalence means that the rate of travel of the light spot crossing the camera is equivalent in the two orientations, as is the 1/r effect for a line source where r is the distance between the telescope and the laser pulse.The trigger efficiency was found to be 50% for a laser energy of 0.94±0.02mJ and approached 100% efficiency at 1.5 mJ (Figure 10).The data were collected in two energy sweeps over about 3 hours.A change in the atmospheric clarity during this period across the 24 km separating the laser and detector is the most likely reason that the points in the threshold range are separated beyond the error bars which are statistical.The data points to the left and right of the fitted curve in the region below 50% trigger efficiency are separated by about two hours in time.
The inclined 0.8 mJ laser track, as viewed from the side in the desert field test, was estimated to appear about as bright as an inclined EAS of 3 EeV would appear when the fluorescence telescope was looking down from the balloon float altitude.This estimate assumed a nominal aerosol optical depth for the desert laser measurement.
Noise trigger rates were typically at the level of 1 Hz over the full camera during the field tests.These trigger rates increased slightly when bright stars or meteors crossed the field of view, or when there was lightning outside the field of view.

EUSO-SPB1 CAMPAIGN AND INSTRUMENT MONITORING
After the EUSO-SPB1 components reached New Zealand in late January 2017, the payload was reassembled.To obtain a final flat-field calibration, the payload was suspended from a crane above an illuminated Tyvek screen at night and rotated in azimuth while the FT collected a data set of full-camera triggers.The instrument was declared flight-ready on 2017 March 25.Following seven aborted attempts, the payload was launched successfully on April 24 23:30 UT, 1 day before the new moon.On reaching the New Zealand coast, the balloon drifted northward, passing about 30 km east of Christchurch before heading out over the Pacific Ocean.Instrument monitoring and operations were handled through centers in Europe, Japan, and the USA.To facilitate downloading, telescope data files were limited to a duration of 2 minutes.A shorter (30 s) file was recorded at the start of each hour and downloaded with highest priority to provide telescope monitoring information.
Examples of thermal monitoring measurements recorded during the flight are compared with predictions of the instrument preflight thermal model in Figure 11.This model was developed to predict the hottest and coldest cases for a long-duration flight at 33 km altitude.The hot case assumed a 45 • S latitude flight and a March 1 launch, whereas the cold case assumed a 65 • S latitude with a June 22 launch.The largest excursions of the data below the warm case prediction started on April 30, when the balloon did not maintain a super-pressure state at night and the payload descended at night to 18 km, reaching the colder air of the tropopause.The excursions of the front lens temperature above the warm case may be due the model underestimating the heat transfer effect of direct sunlight on the telescope walls and/or from indirect sunlight reflecting from the ocean and clouds onto the front lens.
To monitor the PDM camera and readout function, a UV "health" LED was fired twice every 16 s during flight (Figure 12).The sample of the LED measurements downloaded over the flight demonstrates the stability of the camera system response to the LED (Figure 13).Most of the data points fall within ±5% of the mean, despite nighttime temperature swings of 30 • C outside the telescope.
The functionality of the IR camera and the telescope point-spread function were tested (through serendipity) as the balloon drifted over the eastern coast of the South Island of New Zealand about 8 hr after launch.An im-  age from the IR camera shows the Pacific coastline and the edge of Lake Ellesmere in detail.The fluorescence telescope happened to be triggered by a light source on the ground (Figure 14) and recorded a pixel pattern consistent with the expected point-spread function of the optics.To analyze the variability of the light source, we created a Lomb-Scargle Lomb (1976); Scargle (1982) periodogram consisting of 3000 consecutive measurements of  the brightest pixel (after this time, the source moved to other pixels).The most prominent peak was at the period of 0.01 s.The corresponding 100 Hz frequency is consistent with the zero-volt crossing flicker of a fluorescent light bulb with an older magnetic ballast driven at 50 Hz, which is the frequency of the New Zealand electrical grid.https://www.overleaf.com/project/6487340c1579dcd88bac5c80Using 100 Hz we were able to create a smooth phased light curve of the source.This analysis provided an in situ sanity check of the camera system's internal timing.
Information about the presence of clouds in the PDM field of view can also be obtained from the PDM data.Variations in the average background rate can show clouds passing under the balloon, as demonstrated in Figure 15.Clouds tend to be more reflective than the ground or ocean.At night they scatter light from airglow, stars and other sources, and appear in the FT as regions of higher average background rate.
The telemetry bandwidth for data downloads was reduced when one of the two Iridium Pilot data links failed.Thirty of the 40 hr of data recorded on board were downloaded.The downloaded data included 175,000 recorded triggers.To optimize bandwidth prioritization, most of the data from the last 3 nights were not downloaded because the instrument was over high-cloud weather systems with poor viewing conditions.The flight was extended by controlled ballast releases.Unfortunately, the combination of the balloon leak and the emptied ballast hoppers led to an early controlled termination into the Pacific Ocean on May 6. Preparations had been underway to fly a Cessna 421C aircraft instrumented with UV LEDs and a UV laser under the balloon after one circumnavigation (Mastafa et al. 2017), but this didn't happen.

SEARCHING FOR EXTENSIVE AIR SHOWERS
The average background rates and trigger rates recorded during the mission are displayed in Figure 16.The higher trigger rates during the first 3 nights occurred while the PDM was operating with a 1 GTU persistence trigger setting.When the persistence setting was changed to 2 GTU, the typical trigger rate dropped below 2 Hz for the first 2 nights after the change, and then fell below 1 Hz for the rest of the mission.The instantaneous aperture was estimated as a function of air shower energy for four balloon altitudes (Figure 17) that represent the periods when data were collected and analyzed.These altitudes ranged from the nominal 33 km float altitude, when the balloon envelope was in a super-pressure state, to 17 km, when the leaking balloon was descending over a cold storm system.The effect of the lower altitude was twofold.It reduced the highest energy aperture by about a factor of 3. It also lowered the energy threshold by about a factor of 2 because the telescope was closer to the troposphere, where nearly all EAS light production occurs.
A simulated energy distribution, based on the duration and nighttime altitudes of the SPB trajectory and the cosmic ray spectrum from (Fenu 2017), is shown in Figure 18.The distribution yields expected an event rate of 0.76±0.03event/25.1 hr when scaled to the duration of the mission flown, and assuming a clear atmosphere and low background conditions.(A total of 25.1 hr of FT data were downloaded and analyzed.)The uncertainty in the event rate is statistical, driven by the number of events in the simulation.For part of the mission, the balloon flew over high clouds that obscured the field of view.This effect (Bruno et al. 2019;Monte et al. 2019;Shinozaki et al. 2019) was estimated to reduce the event rate by about a factor of 2 for an expected rate of 0.4 event/25 hr.Two independent searches for EAS events were performed on this data set.One search (Díaz Damian 2019) involved scanning the data for triggers of potential interest, which were then classified into seven types.Of these triggers, 4128 were identified as having a duration between 3 and 50 GTUs and were visually scanned in detail.None showed a signature of a small or elongated cluster of pixels moving in a nearly straight line in the PDM at a speed consistent with that of an EAS, which moves through the atmosphere infinitesimally close to the speed of light.The second search for EAS events involved using a feature extraction method to form a simpler representation of an event, after which data were classified as EAS or noise using established machine learning techniques (Vrábel et al. 2019).A training data set combined simulated EAS samples and noise samples from EUSO-SPB1 data.The efficiency of the method was tested on laser tracks from the field campaign as a function of laser energy and on simulated EAS events as a function of primary particle energy.This search also did not yield any obvious EAS candidates.
Both searches did identify background track-like triggers that appear to have been caused by very low energy cosmic particles, most likely muons, striking the PDM directly.An example of one of these events is shown in the sequence of track-like images in Figure 19.A muon passing across the front face of the camera within in the optical filter and/or MaPMT windows could generate cherenkov light Because these tracks cross the PDM within a single GTU time frame, they are readily distinguishable from an EAS track.An EAS track would require many GTUs in order to cross the PDM field of view because the fluorescence light is produced far below the telescope.In addition, an EAS track would not exhibit the persistence observed in this event (center and left panels of 19), The reason for this persistence is not well-understood.It may be due to a lingering ionization effect along the particle's path after it skimmed the front of the camera, for example.This persistence effect was not observed in the distant laser tracks recorded during field tests.Preparations are underway for a EUSO-SPB2 mission (Eser et al. 2021) to expand the science goals beyond those of EUSO-SPB1 in support of a future space-based observatory for UHECRs and neutrinos, such as POEMMA.EUSO-SPB2 will fly two astroparticle optical telescopes that use reflective optics (Kungel et al. 2021) to improve the optical efficiency relative to EUSO-SPB1.A fluorescence telescope (Osteria et al. 2021) with a field of view three times larger than that of EUSO-SPB1 and a 1 µs GTU will point down to observe EAS tracks (Filippatos et al. 2021a,b) and search for dark matter candidates (Paul et al. 2021).A Cherenkov telescope (Bagheri et al. 2021), featuring a SiPM camera with 10 ns time bins, will point near Earth's limb to observe direct Cherenkov light from lowerenergy cosmic rays above the limb (Cummings et al. 2021a) and to search for neutrino signatures from tau neutrino interaction a few degrees below Earth's limb (Cummings et al. 2021b).The optical backgrounds for such events are currently unexplored for suborbital altitudes.The balloon flight train will include an azimuth rotator.It will slew the gondola (and by extension the Cherenkov telescope) in azimuth to enable multi-messenger target-of-opportunity neutrino searches (Venters et al. 2020) in follow-up of selected international alerts from gravitational wave events, tidal disruption events, and gamma-ray bursts, for example.A 2023 launch from Wanaka is planned.

Figure 2 .Figure 3 .
Figure 2. Trajectories of the NASA super pressure balloon test flights launched from Wanaka, New Zealand through 2017.The green portion of a trajectory denotes a completed circumnavigation

Figure 4 .
Figure 4. EUSO-SPB1 instrument in flight-ready configuration.See text for description of subsystems.

Figure 5 .
Figure 5. Left: The photodetector module (PDM) of the EUSO-SPB1 instrument.Right: as installed at the focal plane.The SiECA prototype is positioned to the right of the PDM.

Figure 7 .
Figure 7. Left: SiECA 256-channel SiPM array of four 64-channel Hamamatsu S13361-3050AS-08 sensors was flown as a test on the EUSO-SPB1 mission.Right: laboratory data recorded from an uncollimated DC light spot source.

Figure 8 .
Figure 8. Example of a 1.3 mJ 355 nm laser track recorded during desert field tests of EUSO-SPB1.The laser steering optics were aimed at an elevation angle 45 • away from the telescope position.The top of the track corresponds to a height of 9.3 km and viewing distance of 35 km.The pixels displayed are a sum over ten 2.5 µs frames of the trigger signal.A single 2.5 µs frame that captures the moving light spot from the laser is shown in the inset.

Figure 9 .Figure 10 .
Figure 9. Arrangement the EUSO-SPB1 fluorescence telescope and a laser during field tests is shown on the left.The diagram on the right shows this arrangement after rotation by 90 • , so that the telescope optical axis is pointing down, as it did under the balloon.The laser and EAS axes are in the same geometrical position relative to the telescope optical axis.Both diagrams are side views.

Figure 11 .
Figure 11.Examples of temperature data (top panel: outside the Fresnel lens at the telescope optics aperture; bottom panel: batteries) recorded during the flight compared with predictions of the preflight thermal model (see text).

Figure 12 .
Figure 12.Left: A double light pulse generated by the onboard health LED system as recorded by EUSO-SPB1.The response shown here gives the SPE counts/pixel averaged over all pixels as a function of (2.5 µs) GTU number.The error bars represent the corresponding standard deviation about each average divided by the square root of the number of pixels in the camera.Right: The health LED was mounted in the middle of the Fresnel lens in front of the UV camera.

Figure 13 .
Figure 13.Response of the central cell of the EUSO-SPB1 PDM to flashes from the health LED (dots).The dashed lines indicate ±5% about the mean.The lower curves show the temperature measured at the outside of the Fresnel lens at the telescope optics aperture.Dates and times are in UT.Some gaps in the data occurred because the LED was turned off to avoid RF pickup from an onboard radio beacon that was enabled when the balloon descended below 21.3 km (70,000 ft).

Figure 14 .
Figure 14.At 8:32 UT, April 25, the balloon drifted over the New Zealand coast at an altitude of 33 km.Left: the X in the center of the satellite reference image Google-Maps (Retrived Nov 2017) denotes the coordinates of the balloon: 43.86 • S, 172.35 • E. Center: the shape of the coast is reproduced in the overlaid image recorded by the IR camera.Its projected field of view on the ground as shown here is 14×17 km.Right: An unidentified ground light that the fluorescence telescope happened to observe (inset) provided additional in situ checks (see text).

Figure 15 .
Figure 15.Sequence of time-averaged FT camera images that show clouds drifting across the field of view of the instrument.The four images in the figure are each separated by 30 s and each represents the average of about 1000 2.5 µs images.Darker colors represent more SPE counts (light) recorded.The dark regions are clouds, and the observed average SPE counts from these regions are about 2 photoelectrons/pixel/2.5 µs.The average count rates in the light-colored regions are about half this amount.These data were recorded on 2017 April 28.

Figure 16 .
Figure 16.Top panel: The trigger rate measured during the mission.The trigger persistence setting is listed above.Bottom panel: The nighttime background light levels recorded during the mission.The units are SPE counts/pixel/2.5 µs. 17

Figure 17 .
Figure17.Estimated aperture of the EUSO-SPB1 fluorescence telescope as a function of primary particle energy (proton) for four altitudes that are representative of those encountered at night during data collection.The margin of statistical uncertainty of these estimates is ≤ 35%. 17

Figure 18 .
Figure18.Expected event rate distribution shown with the contributions for the four representative altitudes.This simulation assumed a clear atmosphere and low background conditions for an expected event rate of 0.76±0.03event over the 25.1 hr of data collected.

Figure 19 .. 7 .
Figure 19.A direct cosmic ray traveling through the 48 × 48 pixel PDM camera.The three panels correspond to three consecutive 2.5 µs time bins.The track structure in the first frame persists in two following time bins for this event.See text for discussion. .7. EUSO-SPB2
SPIFigure6.EUSO-SPB1 data readout system and its connections to the various subsystems.