1. Early Alert of Solar Radiation Hazard
Paul Evenson T. Kuwabara, J. W. Bieber, J. Clem, R. Pyle
Bartol Research Institute and Department of Physics and Astronomy,　University of Delaware
Abstract We describe our automated Ground Level Enhancement (GLE) Alarm that is now operational in beta testing. Using our neutron monitor stations at Fort Smith, Inuvik, McMurdo, Nain, Newark, Peawanuck, South Pole, and Thule, automated alarms can be sent to subscribers within minutes of large GLE onsets. In brief, the system generates a WATCH when one station observes an increase of 4% or more in a three minute moving average, a WARNING when two stations observe such an increase, and an ALERT when three or more stations record such an increase. (A higher threshold for percent increase may be imposed for some stations with lower count rates.) A scientific discussion of the alarm system can be found in the article "Development of a Ground Level Enhancement Alarm System Based upon Neutron Monitors", T. Kuwabara, J. W. Bieber, J. Clem, P. Evenson, and R. Pyle, Space Weather, 4, S10001, doi: /2006SW000223, A web page specifically devoted to the GLE alarm, including graphical displays is available at:
Algorithm for issuing an alarm
We define three levels of alarm (Watch, Warning, and Alert) based on the number of stations that record a significant intensity increase.
A threshold level Ith is set for the cosmic ray intensity increase, and an alarm is generated when the number of stations that exceed the threshold level is
1 - Watch, 2 - Warning, 3 –Alert.
Baseline to calculate the intensity increase is defined by two parameters tb and t0. Then, by using a trailing moving average value for the current count rate, the intensity at time t = t is calculated each minute from the observed count rate N(t) averaged over the preceding tc minutes, shown as left equation.
To reduce fluctuations and accurately detect the GLE, all parameters (threshold and baseline) were optimized as by backtesting against past neutron monitor data as
tc=3 min, tb =75min, t0=10min, Ith = 4%.
At this condition, the false alarm rate for Watch, Warning, and Alert was ~40/yr, less than 1/yr, and 0/yr respectively, during the 4.4-year period of our backtesting study,
Condition for issuing three levels of alarm. Intensity increase recorded at three stations during a schematic GLE are illustrated.
tc : average time
tb : duration of the
base-line
t0 : time interval between the
baseline and current time
Ith : threshold level
GLE event on January 20, 2005
Alarm issued for January 20, 2005 event
On Jan 20, 2005 event, alarm times for Warning and Alert are 6:49 and 6:50 respectively; a separate Watch alarm was not generated. The SEC alert was issued at 7:02 from >10 MeV data, and 7:04 from >100 MeV data.
Proton flux observed in the >10 MeV channel exceeds 100 pfu at 6:55. This flux level is defined as a “Moderate Storm” (S2 on NOAA Space Weather Scale for Solar Radiation Storms) that has the possibility to cause single-event upsets aboard satellites, and our system can produce alarm before this.
Space Weather Message Code: ALTPX2
Serial Number: 27
Issue Time: 2005 Jan UTC
ALERT: Proton Event 10MeV Integral Flux exceeded 100pfu
Begin Time: 2005 Jan UTC
NOAA Scale: S2 - Moderate
GOES proton flux and cosmic ray intensity increase. In upper panel, black arrows indicate the onset time of SEP event(>100 MeV), and the start time of the Moderate storm. Vertical lines drawn in lower panels also show the onset time of GLE. Colored arrows indicate the time when each alarm is issued (proton monitor) or generated (neutron monitor).
Space Weather Message Code: ALTPC0
Serial Number: 17
Issue Time: 2005 Jan UTC
ALERT: Proton Event 100MeV Integral Flux exceeded 1pfu
Begin Time: 2005 Jan UTC　
Comparison between the alert times from our system
and alarm issue times from proton data at SEC/NOAA for nine GLE events.
GLE event on January 20, Upper panel shows the low energy proton integral flux recorded by the GOES-11 satellite (solid line >10MeV, dotted line >100MeV). Lower panel shows neutron rates detected in several neutron monitors. These data are normalized to average counting rate at Inuvik (05:30-06:30).
The GLE (Ground Level Enhancement) of January 20, 2005 was the largest in half a century. The onset of the intensity increase in neutron monitors is earlier than that of the low energy proton flux, and time to reach maximum intensity is shorter for neutron monitors.
The GLE particles have large mean free paths and travel almost at the speed of light, and can be detected with high accuracy by detectors with large volume such as ground-based neutron monitors. A GLE alarm can provide a very useful early warning of an impending solar radiation storm. We developed a system that watches (in real time) for count rate increases in our data, and gives an alarm when a GLE is detected. We compare the alarm time produced in our system with that of the system operated by SEC/NOAA by using the GOES proton data.
Number of Minutes by which each alert precedes Moderate Storm (>100pfu in >10 MeV channel)
Number of Minutes by which GLE Alert precedes earliest SEC Proton Alert
Summary
Eight neutron monitors are used in this work. The station name, detector type, average cosmic ray background count rate (in 2005),
geographical latitude, longitude, and altitude are listed.
We have developed a real-time GLE detection system using eight high-latitude neutron monitors
GLE alarms are produced at three levels (Watch, Warning, and Alert) corresponding to the number of stations that exceed the 4% intensity threshold
Intensity increase are calculated from a 3-minute moving average counting rate relative to a 75-minute baseline extending from 85 minutes to 10 minutes before the current time
During the 4.4-year period of our backtesting study, the false alarm rate for Watch, Warning, and Alert was ~40/yr, less than 1/yr, and 0/yr respectively
Alert times decided from this algorithm in the past 9 GLE events were compared with the earliest alert issued by SEC/NOAA based upon GOES (100 MeV or 10 MeV protons) data
Alert times produced by our system are ~10-30 minutes earlier than alert issue times from SEC/NOAA
Alert times are also substantially earlier (around 60 minutes) than the time when dangerous amounts of low energy particles reach the satellite (S2 storm level)
These results suggest that our system can provide valuable added minutes of advance warning for radiation events of concern for satellites, astronauts, and air crews.
Station
Type
Count/hour
Lat., Long., Altitude
Inuvik, Canada
18NM64
6.6×105
68.4 N, W, 21 m
Fort Smith, Canada
7.4×105
60.0 N, W, 203 m
Peawanuck, Canada
7.3×105
55.0 N, 85.4 W, 52 m
Nain, Canada
56.5 N, 61.7 W, 46 m
Thule, Greenland
8.0×105
76.5 N, 68.7 W, 44 m
McMurdo, Antarctica
9.4×105
77.9 S, E, 48 m
South Pole, Antarctica
3NM64
10.3×105
90.0 S, 0.0 E, 2820 m
South Pole Bares*
6NM64
3.2×105
*Bare : Neutron counters without the usual lead shielding that respond to a slightly lower energy primary cosmic ray than the standard monitor.