High-Energy Ground Level Events Measured with Neutron Monitors and the Milagro Instrument James M. Ryan University of New Hampshire and the Milagro Collaboration

The Network Northeast US –Mount Washington IGY; P c = 1.68 GV; 2028 m –Durham NM64; P c = 1.88 GV; sea level Southwest US –Climax IGY; P c = 2.96 GV; 3650 m –Milagro TeV  telescope; P c = 3.90 GV; 2630 m  = 10.6˚  = 23.2˚ Don Smart; priv. comm.

Concept Have pairs of detectors with similar cutoffs, similar asymptotic directions but different yield functions. Each pair provides two integral measures of solar particle spectrum, minimizing anisotropy effects.

For the New Hampshire (northeast US) NMs: –2000 m of altitude (180 mb) For the southeast US detectors –1 GV in cutoff –1000 m in altitude –NM and muon detector

Technique For either pair: For well connected events use count-rate differential to estimate shape of proton spectrum. Absolute amplitude of signal provides normalization. Two pairs can be used to identify breaking spectrum.

Milagro is a TeV ground-level  - ray telescope. It detects electromagnetic (or hadronic) showers in a 1-acre pond of water, outfitted with photomultiplier tubes. The differential timing signal provides the incident direction, allowing one to identify TeV sources. Crab Nebula

Milagro Cross-sectional schematic Charged particles enter the pond and emit Cherenkov light, registered by the photomultiplier tubes. Can be either ‘single’ muons or full blown showers produced by galactic or solar cosmic rays. Records rates with differing numbers of ‘hit’ PMTs. Single low-E muons often trigger only 1 PMT, while showers trigger dozens. So-called High Threshold is the lowest threshold, ~5 GV.

Lockwood et al November 6 Ratio indicates p –6.2, agreeing with Duldig & Humble (1999) Ratio Index

The combined Climax and Milagro data necessitate either a steepening of the spectrum or an abrupt cutoff. Falcone et al. 2003

2005 January 20 Solar excess ratio between MW and Durham of 1.7 indicates p –6.5 spectrum. Rise time and initial decay in both Climax and Milagro are identical.

Where was the proton acceleration and release? CME (SXI loops) liftoff time UT* Solar Wind Speed ~560 km-s – 1 Pitch angle cone half angle 20˚ Milagro GLE onset 0652 UT (tenth max) CME speed 2500 km-s – 1  R s (Lin et al. 05) (Galvin, priv. comm.) (Lin et al. 05)

 rise = 110 s  fall = 70 s Extremely fast and intense event

Understanding the Acceleration Process Difficulties: 2.3 R s — shock formation time High CME speed may work 70 s decay — large  in shock sheath ( ~10 2 r g ) Low total fluence consistent with . Picture may be that shock somehow sets up early, accelerates a few protons quickly but loses them just as quickly.

Complicating factors for Milagro 1)Signal is seen in 6-fold PMT coincidences and higher — indicating high energies. Distribution in multiplicity does not conform to an isotropic distribution. 2)Unfolding spectrum and anisotropy remains to be done. Can be addressed in modeling — no spectrum yet.