Research Article
Published: 26 Jan, 2018

SF Journal of Aviation and Aeronautical Science
Classification and Communication of Aviation RelatedSpace Weather Radiation Events
Meier MM* and Matthiä D
German Aerospace Center, Institute of Aerospace Medicine, Radiation Biology Department, Linder Höhe, Germany

Abstract
Space Weather impacts on the radiation field at aviation altitudes have been a matter of concern
and discussion for many years. This situation has led to a growing demand for corresponding highquality information in the aviation industry. As a consequence, the Space Weather D-scale wasintroduced to provide aviation users and the interested public with relevant information about
the development of the radiation field at flight altitudes during solar particle events (SPEs). The
corresponding D-index for warnings of elevated radiation levels in the atmosphere can be assessedboth by measurements and model calculations. The classification of the different grades of this space
weather scale for the radiation field at aviation altitudes and operational aspects of the D-index are
discussed in this paper. For example, it is demonstrated that the use of the D-index can avoid false
alarms and facilitate proportionate mitigation measures. Furthermore, guidelines for action levels
using the D-scale are derived from current FAA recommendations.
Keywords: Space weather information for aviation; Solar particle events; Radiation exposure ofaircrew; False alarms; D-Index

Introduction

OPEN ACCESS*

Correspondence:
Meier MM, Radiation Biology, German
Aerospace Center, Institute of
Aerospace Medicine, Radiation Biology
Department, Linder Höhe, 51147 Köln,
Germany.
Tel: +49 2203 601 2789
E-mail: Matthias.Meier@dlr.deReceived Date: 10 Nov 2017
Accepted Date: 15 Jan 2018
Published Date: 26 Jan 2018
Citation: Meier MM, Matthiä D.
Classification and Communicationof Aviation Related Space Weather
Radiation Events. SF J Aviation
Aeronaut Sci. 2018; 1(1): 1002.
Copyright © 2018 Meier MM. This is
an open access article distributed under
the Creative Commons Attribution
License, which permits unrestricted
use, distribution, and reproduction inany medium, provided the original work
is properly cited.

The assessment of space weather events in general and warning situations associated with actionlevels in particular depends on the variation of suitable observable or measurable physical quantities
as well as on their impacts and consequences. The general challenge of the quantification for allpractical purposes consists in finding a relevant space weather index that connects an observable
physical quantity with the degree of the impact in question. Furthermore, it should also facilitate
the understanding of the relative severity of the corresponding impacts and of the consequencesof a particular space weather event which is important for the effective communication to thenon-expert. For this purpose, the National Oceanic and Atmospheric Administration (NOAA)
already introduced their Space Weather Scales, i.e. the R-Index for radio blackouts, the G-Index forgeomagnetic storms, and the S-Index for solar radiation storms in 1999 [1,2]. However, an index
that has proved to give good assessment for a particular field of application might fail in another,even if the applications appear to be similar at first sight. This could be particularly learned from theassessment of the impacts of the well-known Halloween storms on aviation in 2003, when several
airlines reacted to the information of an ongoing severe solar radiation storm, S 4 on NOAA Space
Weather S-scale, on 29th October 2003. As a consequence of this radiation alert, some flights onroutes from the U.S. to Asia and Europe were altered, e.g. flown at lower altitudes, during this solar
radiation storm, since the corresponding airlines had established their radiation storm action levelat the S 3 threshold [3,4]. Although there might have been some positive response to these wellmeant precautionary measures in the media during these events, a detailed retrospective analysis
showed that they were more or less ineffective, at least in terms of mitigating radiation exposure on
the corresponding flights. Nevertheless, these measures resulted in higher cost, fuel consumption
and atmospheric pollution.
The outcome of the analysis of the impact of the Halloween storm events on the radiation field
at aviation altitudes seems to be surprising at first sight, since the NOAA Space Weather S-scale
for solar radiation storms is based on the flux of cosmic particles. The restriction, however, isthat the S-index is based on the integral particle flux level with energies above 10MeV, which aredetected onboard the corresponding GOES spacecraft in a geosynchronous orbit outside the Earth’s
atmosphere. Thus, it provides useful information for the prompt assessment of radiation impacts
on the near Earth space environment, e.g. on the operation of satellites and manned spaceflight,
and facilitates rapid action in order to mitigate consequences. At aviation altitudes, however, the

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Table 1: Values of the Space Weather D-index from D 0 up to D 5 and their classification.
Index DClassification

D0

Quiet

D1

Nominal

Dose rate interval [µSv/h]

Additional radiation exposure is comparable withE sol < 5
 < 10
5≤E

Variation of the natural background at
cruising altitudes
Natural background at high latitudes up to
FL400
Natural background at high latitudes
between about FL400 and FL600
Average dose rate inside the International
Space Station (ISS)Average dose rate during an extra-vehicular
activity (EVA) on the ISS
Dose of a North Atlantic return flight per
hour

sol

 < 2010 ≤ Esol < 4020 ≤ E

D2

Minor

D3

Moderate

D4Strong < 8040 ≤ E
sol

D5

Severe < 160
80 ≤ Esol

sol

situation is quite different from the near Earth space environment
due to the shielding influence of the Earth's magnetosphere andatmosphere. Solar particles that are energetic enough to penetrate
the magnetosphere reach the atmosphere and interact with theirconstituents.

solar contribution to the radiation exposure in the atmosphere andderived from the corresponding effective dose rate E sol, which canbe assessed both by measurements of the ambient dose equivalent
rate and model calculations. The space weather D-index permits tocover a wide range of solar contributions to the radiation exposure
at aviation altitudes with comparatively small natural numbers using
a multiple of 2 of the corresponding dose rates [8]. Although thetotal dose rate at flight altitudes is an interesting quantity as well, thereduction to the solar contribution offers a more favorable base fora specific space weather index in terms of dynamics. This importantcharacteristic is easy to understand: when the solar contribution issmaller or comparable to the omnipresent galactic background,the D-index is not dominated by this background and can provide
information about an ongoing space weather event more specifically.
During intense space weather events, however, the radiation exposure
at flight altitudes is strongly dominated by the solar contribution
which renders the galactic background temporarily negligible. The
total dose of an affected flight can be calculated as the sum of thedose due to the regular galactic cosmic radiation background, whichis already assessed in many countries using correspondingly qualified
programs, and the additional solar contribution E sol.

While the flux of the primary particles decreases with increasing
atmospheric shielding, a secondary radiation field is generated. It
could be shown using model calculations that the threshold for
impinging cosmic protons to overcome the atmospheric shielding
and to contribute to the radiation field at commercial aviation
altitudes is about 600MeV [5]. However, most of the particles
impinging on Earth during the Halloween storms, i.e. far more than
99%, had energies below this threshold and did not contribute to theradiation field. As a result, the radiation intensity in the atmosphere
was temporarily only slightly increased during the Halloween storms,
e.g. direct measurements at aviation altitudes showed an increase indose rates of only about 30% [6].
Furthermore, the neutron monitor of the University of Oulu in
Finland, an instrument to observe effects of cosmic radiation on theatmosphere, proved a variation in count rates of about 5% at sea level
during the course of the associated ground level enhancement GLE
65. This demonstrated the shielding capability of the magnetosphereand the atmosphere which reduced an increase in the flux of theimpinging cosmic particles > 10MeV of about 5 orders of magnitude
observed onboard the GOES satellite to only moderate increases
in dose rates at aviation altitudes and an increase of only about 5%
measured using a neutron monitor in Oulu. The subsequent shielding
from a part of the galactic cosmic radiation component by the socalled forbush effect even brought about a decrease in the radiation
intensity in the atmosphere which lasted for several days. A detailed
discussion of this event and its impact on the radiation field at aviation
altitudes can be found e.g. in Meier & Matthiä [8].

The practical meaning of the different grades of the D-scale canbe described similar to the grades of the NOAA space weather scales.
The D-indices from D 0 up to D 5, their classification, corresponding
ranges of dose rates, and comparison with other natural radiation
sources are listed in Table 1. An index D 0 represents a quiet space
weather situation in terms of radiation exposure at flight altitudes
within the variation of the natural galactic cosmic ray background.
An event at D 1 level indicates a slight increase in radiation exposurein the atmosphere. Although the corresponding solar contribution
is only of the order of the natural background, it reflects a nominal
space weather radiation event. The next grade D 2 describes minor
events when solar contributions begin to dominate the radiation
field. A moderate space weather radiation event is characterized bya D-index of 3. Although even an exposure of 10 hours at D 3 level
would hardly contribute to exceeding the monthly limit for pregnant
women recommended by the Federal Aviation Administration (fordetails see FAA Advisory Circular No: 120-61B, https://www.faa.
gov/documentLibrary/media/Advisory_Circular/AC_120-61B.pdf),
it might be considered as an action level for moderate mitigation
measures, in particular for comparatively long-lasting space weather
radiation events and long haul flights. The following grades represent
strong (D 4) and severe (D 5) radiation events. Since the recommended
monthly limit for pregnant women could be already exceeded after
three hours when exposed at D 5 level, mitigation measures should
be considered to avoid this scenario, in particular for long haul flights
at high latitudes.

Since the Halloween Storms, airlines and aircraft operators havebeen interested in timely and accurate information about actuallysignificant increases in the radiation intensity in the atmosphere. This
information can help reduce the radiation exposure of aircrew andpassengers to a level which is as low as is reasonably achievable, e.g.
by temporarily reducing flight altitudes [7].

The Space Weather D-Index
The misconception of the radiation exposure situation at aviation
altitudes during the Halloween storms, which was based on the
S-index, raised the awareness of the demand for a relevant space
weather index in the aviation industry. This led to the development of
the concept of the space weather D-Scale with an index for warningsof elevated radiation levels that is directly based on the additional

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A comparatively quiet space weather situation is characterized by
a D 0-, D 1- or D 2-level. The D 3-level, i.e. an additional dose rateof more than 20µSv/h, indicates elevated radiation intensity that isused by the FAA to trigger a corresponding radiation alert using its
Solar Radiation Alert System (ESRAS) if it is exceeded at any altitude
between 30,000 and 70,000 ft. for each of three consecutive 5-min
periods [9]. Thus, the concept of the D-index can also be used within
the framework of already existing warning systems.
An important feature of the D-index is its application for theassessment of the exposure situation at a particular position in
the atmosphere. Thus, it can be used to gain and communicate a
differentiated picture about the radiation field in different geographic
regions and at different altitudes similar to the communicationof fundamental quantities in the field of terrestrial weather, e.g.
temperature, air pressure, etc. The analysis of increased radiation
exposure in a particular region can also be generalized for civil aviation
and a local warning index DL can be derived from the maximum
dose rate in that region at a flight level of 41,000ft. (FL410), whichcharacterizes the upper airspace as worst case scenario. Individual
flights would usually be exposed at lower radiation levels dependingon the actual cruising altitude of a particular flight. Furthermore, aglobal D-index DG might be used as the maximum local DL-index,
anywhere in the world, as worst case scenario for general information
about an ongoing space weather event. However, such a global index
is not specific anymore and would not help towards proportionate
mitigation measures in other regions.

Figure 1: GOES 5-minute averaged integral proton flux (protons  cm-2  s-1 sr-1) as measured by the SWPC primary GOES satellite for energy thresholdsof ≥ 10 (red line), ≥ 50 (blue line), and ≥ 100MeV (green line) in comparison
with the relative variation of the count rate of the Oulu Neutron Monitor
between 22th and 25th January 2012.

Operational Aspects of the D-Index
The concept of the space weather D-index supports not only theprovision of appropriate information about space weather radiation
events for aircrew and the public but also operational aspects forairlines, air traffic controllers (ATC), etc. The D-index has alreadybeen used to provide several European airlines with space weather
information since 2014. In future applications, this information will
be used to avoid false warnings, detect real warning situations andfacilitate proportionate mitigation measures.

for an assessment of the radiation field at aviation altitudes and theadvantage of the application of the D-index can be demonstrated
with the example of GLE 70, which took place on 13th December
2006. The comparison of measurements of the cosmic ray intensity
by the ground based neutron monitor network and GOES reveals
how an analysis in terms of radiation exposure at aviation altitudes
based on GOES data alone, i.e. the S-scale, can be misleading. The
solar radiation storm associated with GLE 70 triggered only an S 2alert (Figure 2), i.e. below the chosen action level, although it brought
about a significant increase in dose rates at cruising altitudes up to
D 5 level at maximum exposure which was reached at 03:10 UTC,
about 15 minutes after the onset of the event. The initial hot spots
had leveled out according to the geomagnetic latitude at 03:35 UTC,
about 40 minutes after the onset of the event, and the derived global
DG-index declined from D 5 to D 3 (Figure 3).

Avoidance of false warnings
The use of the S-scale to generate warnings of increased radiation
intensity at aviation altitudes has caused an unacceptably highnumber of false alarms, based on S 3 level or higher, for many years.
Reactions to false alarms result usually in additional costs without any
benefit and should be avoided. During the solar maximum of cycle 24
in 2012, for instance, false positive alarm situations in terms of theradiation field at aviation altitudes were inferred from S 3 alerts issued
on 23rd January 2012 and 7th March 2012 (no GLEs). The situation
on 23rd January 2012 is another good example that an S 3 alert canbe compatible with a quiet radiation situation at cruising altitudes
as demonstrated in Figure 1, which shows the 5-minute averaged
integral proton flux ≥ 10MeV (protons  cm-2  s-1  sr1) as measured
by the SWPC primary GOES satellite in comparison with the increasein the count rate of the Oulu Neutron Monitor between 22th and 25th
January 2012.

Mitigation measures should be proportionate depending on
the D-index at the corresponding altitude in a particular region. A
reduction to moderate D levels might be taken into consideration
if possible, i.e. other critical factors such as weather taken into
account. The effectiveness of mitigation measures and their economic
impacts in terms of delay and fuel consumption for a transatlantic
flight during GLE 70 were investigated by Matthiä et al. (2015) [7].
If the increase in radiation intensity, expressed by the D-index, werecommunicated to the cockpit through the Aircraft Communications
Addressing and Reporting System (ACARS) in time, appropriate
measures could be taken. The investigated reaction to the increased
Space Weather D-index was lowering the flight altitude after theincrease of the dose rates, adapting the flight speed and returning to

Mitigation measures
The acceptance of an action level triggered by an S 3 alert has
not only produced a lot of false positive alarms but also false negativealarms, i.e. mitigation measures were not taken although theradiation intensity in the atmosphere was strongly increased. Thisadditional unwanted shortcoming of the use of the NOAA S-scale
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SF Journal of Aviation and Aeronautical Scienceassurance of the assessment of the D-index by measurements inregional airspace, which might also foster the competition amongmodels and contribute to improving their quality.
Although a global forecast of the occurrence and developmentof SPEs which cause an increase in radiation intensity at aviation
altitudes is still not - and perhaps will never be - possible, thegeographic anisotropy of the increase in dose rates during the onsetof e.g. GLE 70 is an interesting feature that deserves closer attention
in the future. A nowcast of a radiation event by an RWC in an initiallyaffected region, represented by a corresponding D-index, might offer
a possibility for issuing regional warnings by other RWCs for areas
which are not yet affected during the initial phase.

Conclusion
The intensity of the radiation field due to cosmic radiation ataviation altitudes during quiet space weather conditions is, in terms ofdose rate, more than one order of magnitude higher than the averageradiation environment from all natural sources on ground in mostcountries. As a consequence, aircrew and frequent flyers are exposed
to higher levels of ionizing radiation than the average population.
Space weather radiation events might bring about an additional,
albeit short-term increase in radiation exposure of several orders ofmagnitude. Although the actual total exposure at cruising altitudes
during such an event would be quite moderate in comparison tothe ordinary annual radiation exposure from other natural andartificial sources in most cases, the situation might leave many people
insecure about the way their health might have been affected. This
unsettledness is only natural since human beings do not have asense to perceive ionizing radiation. Thus, the psychological effectsof being exposed to additional contributions of radiation during aspace weather event must not be underestimated, in particular whenpotentially exaggerated by the media. Therefore, it is necessary toprovide appropriate information about the actual variation of theradiation field during an event to stakeholders and the public, which
can easily be communicated using a corresponding index.

Figure 2: GOES 5-minute averaged integral proton flux (protons  cm-2  s-1 sr-1) as measured by the SWPC primary GOES satellite for energy thresholdsof ≥ 10 (red line), ≥ 50 (blue line), and ≥ 100MeV (green line) in comparison
with the relative variation of the count rate of the Oulu Neutron Monitor
between 12th and 15th December 2006.

nominal flight altitudes after the additional dose rates had dropped
below a threshold. It was demonstrated that in the case of a prompt
response to the radiation event caused by this GLE, the total effective
dose on the flight could have been reduced by up to 42% by lowering
the flight altitude and using the contingency fuel of the aircraft, i.e. a
5% additional fuel consumption. For the investigated flight scenario,
the D-index would have remained at D 1 rather than at D 3 in caseof no response.

The usability of a space weather index for the assessment of
the radiation field at aviation altitudes during space weather events
requires a close connection between an observable physical quantity
and the degree of the impact. This requirement led to the developmentof the D-index, a bespoke space weather index for this scenario thatis directly based on the dose rate, which is the fundamental physicalquantity used for the characterization of radiation fields in radiation
protection. Thus, the D-index is an application-related tool to provide
the capability of giving timely and useful space weather information
to the users and the public in order to avoid false alarms and facilitate
mitigating measures. In contrast to a global space weather index, the
D-index permits more specific assessment of the effects on particulargeographic regions. Furthermore, the application of the D-index
is independent of the radiation model used for its assessment. In
this respect, it also supports the concept of RWCs which has beensuccessfully used in the field of terrestrial weather for many years,
i.e. the responsible authorities for different countries or regions canchoose a model in due consideration of their particular needs.

Regional warning centers
The radiation field at flight altitudes is very complex in termsof its composition of different sorts of particles and their energy
distribution, which depend on the incoming direction of the primaryparticles, local geomagnetic shielding and altitude. The corresponding
dose rate characterizes this radiation field for operational purposesand shows a comparatively strong variation as well (Figure 3). In
contrast to a global space weather index, the D-index permits more
specific assessment of these effects on particular geographic regions.
Furthermore, the application of the D-index is independent of
the radiation model used for its assessment. In this respect, it also
supports the concept of Regional Warning Centers (RWCs) orregional elements of a global system, which has been successfully usedin the field of terrestrial weather for many years, i.e. the responsible
authorities for different countries or regions can choose a model indue consideration of their particular needs. In this context, it is worth
mentioning that the regional assessment of the D-index can be verified
with qualified measuring instruments, similar to other weather
parameters, e.g. temperature [10,11]. Thus, RWCs are provided withthe capability of establishing corresponding procedures for quality
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The feasibility of mitigating measures during a space weather
event was discussed and demonstrated in a first study using theexample of GLE 70 by Matthiä et al. [7]. It could be shown thattimely space weather information based on the D-index might havebeen used to reduce the radiation exposure of crew and passengers

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Figure 3: World map showing areas of increased dose rates due to solar protons represented by the corresponding D- indices at an altitude of 41,000ft.

during this event from D 3 to D 1 for the investigated scenario. The
geographic anisotropy of the increase in dose rates during the onset ofa space weather radiation event, e.g. an initial hot spot in a particularlocation of the globe represented by the D-index, might offer apossibility for issuing regional warnings for areas which are not yet
affected during the initial phase. This could provide a lead time for
mitigating measures, e.g. delay of flights, in the order of about half
an hour.

2. Poppe B, Jorden K. Sentinels of the Sun, Johnson Books. ISBN 1-55566379-6, 2006.
3. Lieber R. Solar Storm Rekindles Concern Over Whether Radiation Hurts
Fliers. The Wall Street Journal. 2003.
4. U.S. Department of Commerce, Service Assessment – Intense Space
Weather Storms. 2003, 2004: 17-18.
5. Matthiä D, Meier MM and Reitz G. Numerical calculation of theradiation exposure from galactic cosmic rays at aviation altitudes with the
PANDOCA core model. Space Weather. 2014; 12: 161-171.

The capabilities and the use of the D-Index for characterizing
space weather radiation events for aviation have been intensively
discussed since 2014, e.g. at the Space Weather Workshop, theEuropean Space Weather Week, etc. It was recommended as effectmetric by the Radiation and Plasma Effects Working Group at the first
Community Coordinated Modeling Center’s (CCMC) International
CCMC-LWS Working Meeting held in 2017.

6. Beck P, Latocha M, Rollet S and Stehno G. TEPC reference measurements
at aircraft altitudes during a solar storm, Advances in Space Research.
2005; 36: 1627–1633.
7. Matthiä D, Schaefer M, and Meier MM. Economic impact and effectivenessof radiation protection measures in aviation during a ground level
enhancement. J. Space Weather Space Clim. 2015; 5: A17.

Acknowledgements

8. Meier MM and Matthiä D. A space weather index for the radiation field ataviation altitudes. J. Space Weather Space Clim. 2014; 4: A13.

We would like to thank the Space Weather Prediction Center of
the National Oceanic and Atmospheric Administration for providing
the GOES proton flux data (http://www.swpc.noaa.gov/) and the
Sodankylä Geophysical Observatory for providing the Oulu neutron
monitor data (http://cosmicrays.oulu.fi).

9. Copeland K, Sauer HH and Friedberg W. Solar Radiation Alert System.
Office of Aerospace Medicine, Washington, DC. 2009.
10. Meier MM, Hubiak M, Matthiä D, Wirtz M and Reitz G. Dosimetry ataviation altitudes. Radiat. Prot. Dosim. 2009; 136; 251–255.
11. Meier MM, Trompier F, Ambrozova I, Kubancak J, Matthiä D, Ploc O,
et al. CONCORD: Comparison of Cosmic Radiation Detectors in the
Radiation Field at Aviation Altitudes. J. Space Weather Space Clim. 2016;
6: A24.

References
1. Poppe B. New Scales Help Public, Technicians Understand Space Weather,
Eos, Transactions. American Geophysical Union. 2000; 81: 322–328.

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