Keilhauer, B.

Pierre Auger Collaboration

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Rapid monitoring of the atmosphere after the detection of high-energy showers at the Pierre Auger Observatory

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PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009 1Rapid atmospheric monitoring after the detection of high-energyshowers at the Pierre Auger ObservatoryBianca Keilhauer∗, for the Pierre Auger Collaboration†∗Karlsruhe Institute of Technology (KIT),Forschungszentrum Karlsruhe, Institut für Kernphysik, P.O.Box 3640, 76021 Karlsruhe, Germany†Observatorio Pierre Auger, Av. San Martin Norte 304, 5613 Malargüe, ArgentinaAbstract. The atmospheric monitoring program ofthe Pierre Auger Observatory has been upgradedto make measurements of atmospheric conditionspossible after the detection of very high-energy show-ers. Measurements of the optical transmittance dueto aerosols and clouds are time-critical. Therefore,observations of atmospheric regions close to a showertrack of interest are performed within ten minutes ofa shower detection using LIDAR and telescope mon-itors. Measurements of the altitude dependence ofatmospheric state variables such as air temperature,pressure, and humidity are performed within abouttwo hours following the detection of a very high-energy event using meteorological radio soundings.Both programs are triggered using a full onlinereconstruction with analysis-level quality cuts. Wedescribe the implementation of the online trigger, anddiscuss the impact of the monitoring data with highresolution on the analysis of air shower events.Keywords: rapid atmospheric monitoring, PierreAuger Observatory, high-energy air showersI. I NTRODUCTIONAt the Pierre Auger Observatory [1], extensive airshowers (EAS) induced by ultra-high energy cosmic raysare studied. The observatory consists of two detectortypes, a surface detector (SD) for secondary particles ofEAS and fluorescence detector (FD) telescopes for UV-emissions by nitrogen molecules in the atmosphere. Thefluorescence technique provides an almost calorimetricmeasurement of the primary energy of cosmic rays.However, the constantly changing conditions of theatmosphere demand a sophisticated monitoring sys-tem [2]. The reconstruction of air showers from theirUV-emission requires proper characterisation of atmo-spheric state variables such as pressure, temperature, andhumidity, as well as the optical transmittance due toaerosol contamination and the presence of clouds [3].The state variables of the atmosphere above the PierreAuger Observatory are determined using meteorologicalradio soundings, while aerosol and cloud conditions aremeasured by two central lasers, four elastic LIDARs,and four cloud cameras [4].The sounding data have been incorporated intomonthly models, and aerosol and cloud data into anhourly database [4]. However, for events of particularphysical interest, such as very high-energy showers, it isdesirable to measure the properties of the atmosphere asaccurately as possible. To improve the resolution of theatmospheric database for such events, dedicated radiosoundings and LIDAR measurements can be triggeredby an online event reconstruction. We will discuss themotivation for such measurements (Section II), the op-eration of the online trigger (Section III), and the useof dedicated atmospheric measurements in the offlinereconstruction (Section IV).II. M OTIVATION FOR RAPID MONITORINGBetween 2002 and 2005, radio soundings were per-formed at the observatory during dedicated measurementcampaigns. Since mid-2005, the soundings have beenperformed approximately every fifth day. The measure-ments obtained by launching weather balloons providealtitude profiles of the air temperature, pressure, andrelative humidity up to about23 km above sea level.Due to the limited statistics of the measurements, thedata have been incorporated into monthly models ofconditions near Malargüe, Argentina, the site of thesouthern part of the Pierre Auger Observatory [4], [5].Using monthly models instead of actual profilesintroduces an uncertainty of the primary energy of∆E/E= 1.5%−3% for showers with energies between≈ 1017.7 eV and1020 eV, and a corresponding uncer-tainty ∆Xmax= 7.2− 8.4 g cm−2 of the position of theshower maximum. While it is not practical to performa radio sounding every night, the reconstruction can beimproved for a subset of the EAS data by concentratingthe soundings in periods when high-quality events areobserved. This subset of EAS events is particularlyimportant because they contribute to the energy scaledetermination of the entire observatory [6].For aerosol measurements, the LIDAR stations con-duct automated hourly sweeps of the atmosphere abovethe observatory to estimate the vertical aerosol opticaldepth, cloud height, and cloud coverage [7]. The hourlysweeps are sufficient to characterise changing aerosolconditions, but a more rapid response is necessary toidentify moving clouds between shower tracks and theFD telescopes observing the event. To accomplish this,he LIDARs are capable of interrupting their hourlysweeps to scan interesting shower tracks for atmosphericnon-uniformities [7], [8].2 B. KEILHAUER et al. RAPID ATMOSPHERIC MONITORINGIII. O NLINE TRIGGERTo select events for monitoring with radio soundingsand/or LIDAR scans, an online reconstruction is usedto trigger balloon launches and the LIDAR hardware.As data are acquired from the FD telescopes and SD,they are merged by an event builder into hybrid eventfiles, and passed to the reconstruction software. Thesoftware is the same as that used forOff line eventreconstruction [9], including the latest versions of thedetector calibration databases. In this way, the LIDARand balloon triggers can be constructed with the samequality as the offline physics analysis.The reconstruction loop runs every 60–90 seconds,and reconstructs events between 2 and 10 minutes aftertheir detection1. Events with reconstructibledE/dXlongitudinal profiles are used to trigger LIDAR andsounding measurements following the application ofbasic quality cuts. The LIDARs trigger on showers withE ≥ 1019 eV in combination with given quality cutson the reconstruction of the shape of the longitudinalprofile. These events are typically of high quality andthe rapid monitoring is to ensure that no atmosphericimpurity has altered the reconstruction result. To allowthe investigation of shower observations affected byclouds and other non-uniformities in the atmosphere forpossible longitudinal profile corrections in the future,few events of lower quality withE & 1018.78 eV canalso pass the trigger conditions. This yields up to onescan per night. A balloon launch is triggered for eventswith E ≥ 1019.3 eV and a profile fitχ2/NDF < 2.5.All trigger conditions have in common that the positionof shower maximum has to be well in the field of viewand that the observed track has an expedient length.The quality of the online reconstruction has beenchecked by comparing with results from theOff linereconstruction. Even though some minor differences inthe reconstruction chains are present, the reconstructionquality is excellent. Only some events are missed bythe online reconstruction below1018 eV, which is wellbelow the required energy threshold for both rapidmonitoring programmes. At primary energies of interest,the energy of the primary cosmic ray and the positionof the shower maximum are reconstructed very well bycomparison with theOff line reconstruction: only below1% difference for the energy and 2 g cm−2 in Xmax areexpected. The reconstruction cuts for triggering radiosoundings yield a trigger rate of 3 to 13 radio soundingsper shift2 depending on season, see Fig 1. In practice,only one launch is performed within 5 hours resultingin about 2 to 6 launches per FD shift.Triggers for the LIDAR systems are handled auto-matically by these stations: the hourly scans are haltedand the LIDARs sweep into the field of view of theFD telescopes to probe the shower track [7]. To avoid1The delay is caused by buffering of station data from the SD.2To infer these numbers, the EAS data sample from 2008 wasanalysed.Shift Number1 2 3 4 5 6 7 8 9 10 11 12 13          Triggers per Shift02468101214Fig. 1. All triggers for each FD shift in 2008 of events which wouldhave passed the sounding trigger conditions. A seasonal effect due tolonger nights in winter can be seen.triggering the telescopes with stray light, the FD dataacquisition is vetoed for four minutes, the maximumduration of a dedicated scan. In contrast to the LIDAR,the balloon launches require human intervention. There-fore, a sounding trigger initiates a SMS text message toa technician in Malargüe. The technician then drives tothe balloon launching facility and performs the soundingtypically within two hours of the detection of the event.This measurement has no interference with any otherdata acquisition of the Pierre Auger Observatory.IV. A NALYSISDuring the March – April 2009 FD shift, the rapidmonitoring with radio soundings was activated for thefirst time. We had two nights with successful triggers forthe radio soundings. In the second night, it was a stereoevent. Both radio soundings could be performed within1.5 hours after the high-energy air shower. The first trig-ger was sent at the end of March and the second one atthe beginning of April. In Fig. 2, the difference betweenthe actual measured atmospheric profiles from the radiosoundings and the monthly models for the area of theAuger Observatory valid for that month are displayed forthe temperature, atmospheric depth, and vapour pressure.For the event in March, the differences between themeasured temperature and atmospheric depth profilesand the monthly average model are small. However theconsiderable amount of water vapour in the lower atmo-sphere indicates possible distortions of the longitudinalshower profile compared with a reconstruction using theadequate monthly model. A reconstruction of the firstevent with the actual atmospheric profiles compared withthat using monthly models yields a∆E/E of +0.9% anda∆Xmax of +6 g cm−2. For the event in April, the watervapour content is nearly the same as in the correspondingmonthly model. However, the higher temperature closeto ground resulting in lower atmospheric depth valueswill change the reconstructed air shower event. The sametwo versions of reconstruction as for the first event yielda ∆E/E of -0.5% and -1.0% for the two different FDPROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009 3altitude (km a.s.l.)5 10 15 20 25T_launch - T_model (K)-10-8-6-4-20246810event in Marchevent in Aprilaltitude (km a.s.l.)5 10 15 20 25)-2X_launch - X_model (g cm-3-2-1012event in Marchevent in Aprilaltitude (km a.s.l.)2 3 4 5 6 7 8 9 10e_launch - e_model (hPa)-2-10123456event in Marchevent in AprilFig. 2. Difference between two actual measured atmosphericprofiles in March and April 2009 from the radio soundings and the correspondingmonthly models for the area of the Auger Observatory. Left: Temperature. Middle: Atmospheric Depth. Right: Vapour Pressure.stations which observed this stereo event and a∆Xmaxof +4 g cm−2 and +3 g cm−2.In the second shift running this programme, we had 10triggers in 6 nights. The first one was again a stereo eventand in the fourth night, there were 3 triggers within 2.5hours. The fifth night also provided two triggers in 2.5hours, and in the last night there were 2 triggers within1 hour. In total, we had 5 radio soundings initiated byhigh-energy air shower events, because the SMS duringthe last night were lost.All events have been reconstructed using two differentconfigurations. The first one represents the status of cur-rently best knowledge, so using the actual atmosphericprofiles from the radio soundings in combination withdescriptions of fluorescence emission [10] and trans-mission taking into account all temperature, pressure,density, and humidity effects. The second reconstructionrelies on the same descriptions but uses the monthlymodels for the site of the Pierre Auger Observatorywhich provide also profiles of water vapour. In Fig. 3,the resulting differences of the reconstruction proceduresare shown for all events during March and April 2009.The stereo events have been reconstructed independentlyfor the two FD stations which observed the extensiveair shower. The primary energies of these events varyfrom the threshold energy up to almost1019.7 eV andfor the position of shower maximum, values between654 and 924 g cm−2 slant depth are observed. Thegiven differences are between reconstruction with actualatmospheric profiles and that with monthly models. Forthe primary energy, we expect an uncertainty of±2.5% atE0 = 1019.3 eV while using monthly models.The differences between reconstructions using soundingdata and the monthly models fit these expectations(Fig. 3 left). For the position of shower maximum, theexpected uncertainty atE0 = 1019.3 eV is ± 8 g cm−2.The reconstruction with monthly models nearly matchesthese expectation but is biased to one direction for thisseason (Fig. 3 right).The rapid monitoring with LIDARs started in Febru-ary 2009 and through the beginning of May 2009, thefour LIDAR stations at the Pierre Auger Observatorywere triggered 29 times. The intention is to investigateatmospheric conditions for those high-energy showersthat fail strict analysis cuts due to distortions caused byclouds and aerosols.For high-energy showers of high reconstruction qual-ity, the LIDAR scans can be used to verify the qualityof the atmosphere. In this manner, the scans allow forthe investigation of atmospheric selection effects on thehighest energy showers. Of the 29 showers probed bydedicated LIDAR scans, 17 passed the strict quality cutsused in the analysis of FD data. The energies of theseshowers ranges from1019 to 1019.52 eV. The observedshower maxima are between 678 and 808 g cm−2.In nearly all cases, the profile fit is of high quality,and the LIDAR data do not indicate the presence oflarge amounts of aerosols or heavy cloud coverage. Oneexception is shown in Fig. 4, in which the light from theupper segment of a shower track is blocked by a thickcloud layer. The backscattered light from the LIDARscan shows a strong echo near8 km above groundlevel, or 650 g cm−2 slant depth along the shower track,confirming the presence of a cloud.At present, the rapid monitoring with LIDARs ismainly used as a check of the quality of the atmosphereafter the observation of high-energy showers. This isquite important for analyses that rely on unusual featuresi shower tracks, such as exotic particle searches. TheLIDAR shots can also be used to remove obscured ordistorted sections of a shower track from the analysis.Once sufficient statistics have been collected, it shouldbe possible to use the LIDAR data to correct observedshower tracks for inhomogeneities in the atmosphere.4 B. KEILHAUER et al. RAPID ATMOSPHERIC MONITORING E/E [%]∆-3 -2 -1 0 1 2 3Entries00.511.522.5]-2 [g cmmax X∆0 2 4 6 8 10Entries00.511.522.5Fig. 3. Comparison of two different versions of reconstruction for air shower events observed in March and April 2009. The first reconstructionuses actual atmospheric profiles from radio soundings performed shortly after the detection of the EAS. The second one uses monthly modelsdeveloped for the site of the Pierre Auger Observatory.]-2slant depth [g cm400 600 800 1000 1200 1400)]-2dE/dX [PeV / (g cm01020304050/Ndf= 147.96/1672χ                  distance from lidar [km]0 5 10 15 20          height above lidar [km]051015               ]                          −2                 slant depth [g cm200400600800100012001400Fig. 4. Left: A 1019.48 eV shower profile obscured below650 g cm−2 by a cloud. The backscattered light from a LIDAR scan of theshower-detector plane (right) confirms the presence of a cloud ayer (the dark horizontal band) in the telescope field of view.REFERENCES[1] J. Abrahamet al. [Pierre Auger Collab.], Nucl. Instr. Meth.A523(2004) 50[2] S. BenZvi for the Pierre Auger Collab., Proc. 31st Int. Cos. RayConf. Lodz, Poland, (2009)[3] L. Valore for the Pierre Auger Collab., Proc. 31st Int. Cos. RayConf. Lodz, Poland, (2009)[4] J. Abrahamet al. [Pierre Auger Collab.],A Study of the Effect ofMolecular and Aerosol Conditions on Air Fluorescence Measure-ments at the Pierre Auger Observatory, submitted to Astropart.Phys. (2009)[5] J. Blümeret al. for the Pierre Auger Collab., Proc. 29th Int. Cos.Ray Conf. Pune, India,7 (2005) 123[6] J. Abrahamet al. [Pierre Auger Collab.], PRL101 (2008) 061101[7] S.Y. BenZvi et al., Nucl. Instr. Meth.A574 (2007) 171[8] J. Boyer et al. for the HiRes Collab., Proc. 28th Int. Cos.RayConf. Tsukuba, Japan, 1 (2003) 481[9] S. Argiro et al., Nucl. Instr. Meth.A580 (2007) 1485[10] B. Keilhauer, M. Unger, Proc. 31st Int. Cos. Ray Conf. Lodz,Poland, (2009)

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Rapid monitoring of the atmosphere after the detection of high-energy showers at the Pierre Auger Observatory

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