Enhanced Radiation Levels at Aviation Altitudes and Their Relationship to Plasma Waves in the Inner Magnetosphere

It is believed that galactic cosmic rays and solar energetic particles are the two major sources of ionizing radiation. However, the radiation source may also be due to relativistic electrons that are associated with precipitation from the Van Allen radiation belts. In this study, we use Automated Radiation Measurements for Aerospace Safety (ARMAS) measurements to investigate the precipitation mechanism of energetic radiation belt electrons. ARMAS instruments are flown on agency‐sponsored (NASA, National Oceanic and Atmospheric Administration, National Science Foundation, Federal Aviation Administration, DOE) flights, commercial space transportation companies and airliners (>9 km) in automated radiation collection mode. We identified magnetic conjunction events between ARMAS and NASA's Van Allen Probes to study the highly variable, dynamic mesoscale radiation events observed by ARMAS instruments at aviation altitudes and their relationship to various plasma waves in the inner magnetosphere measured by the Van Allen Probes. The results show that there is a strong correlation between dose rates observed by ARMAS and plasmaspheric hiss wave power measured by the Van Allen Probes, but no such relationship with electromagnetic ion cyclotron waves and only a modest correlation with whistler mode chorus waves. These results suggest that the space environment could have a potentially significant effect on passenger safety.


Data and Methodology
The ARMAS instruments are flown in an automated radiation collection mode on agency-sponsored flights such as, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), the National Center for Atmospheric Research (NCAR), the NASA Armstrong Flight Research Center (AFRC), the Federal Aviation Administration (FAA) William J. Hughes Technical Center (WJHTC) Bombardier Global 5000 (BG5), Gulfstream 5 (G-5), and commercial aircrafts (Tobiska et al., 2016(Tobiska et al., , 2018(Tobiska et al., , 2022)).The ARMAS monitoring system consists of two components: (a) a flight instrument that measures the environment absorbed dose in silicon on an aircraft and (b) a real-time data stream from the aircraft to the ground (Tobiska et al., 2016(Tobiska et al., , 2018(Tobiska et al., , 2022)).The data is then processed to Level 4 effective dose rates for location and time.The absorbed dose (Si) is measured within the aircraft, it is relayed to the ground via an Iridium satellite link or aircraft WiFi.The measurements are made using the Teledyne micro dosimeter uDOS001 (uDOS) in combination with a microprocessor, a GPS chip, an Iridium transceiver or a Bluetooth transmitter, and associated electronics.The uDOS chip is sensitive to heavy ions (Fe+), alphas, protons, neutrons, electrons, and γ-rays, especially above 1 MeV based on extensive ground beam line testing (Tobiska et al., 2016).In this study, we use almost 7 years of ARMAS real-time radiation measurements (2013)(2014)(2015)(2016)(2017)(2018)(2019) from ground to 107 km altitude for >700 flights consisting >700,000 10-s measured absorbed dose and derived effective dose rates.The background dose, in this study, are estimated by NASA Langley Research Center (LaRC) Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) model (Mertens et al., 2013), which includes GCR.The background will also include SEPs.
The Van Allen Probes were launched on 30 August 2012 and were operational until 2019.The two identical probes were launched into a 10° inclination orbit with a perigee of approximately 1.1 RE and an apogee of 5.8 RE geocentric (Mauk et al., 2013).Each probe carried an Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) wave instrument to measure wave power spectral density (PSD) between 10 Hz and 12 kHz using a Waveform Receiver (Kletzing et al., 2013;Wygant et al., 2013) and the high-frequency receiver (HFR) that measured electric spectral intensity between 10 and 400 kHz.The background magnetic fields were measured via a triaxial fluxgate magnetometer while the high frequency wave magnetic field fluctuations were measured by a triaxial search coil magnetometer (Kletzing et al., 2013).The EMFISIS waves instruments provided a comprehensive set of electric and magnetic field wave measurements needed to identify various wave types within the inner magnetosphere.We use the comprehensive wave data measurements from the EMFISIS instruments, for the entire Van Allen Probes mission, to identify different types of waves and study the relationship between each wave type with enhanced radiation events measured by ARMAS during conjunction events with each Van Allen Probe.The 3D magnetic field waveforms, which were captured in a continuous waveform burst mode with selected ∼6 s snapshots from search coil magnetometers, are used to calculate the wave intensities and the HFR are used to calculate the high frequency wave intensities.
Figure 1 shows a schematic of the Van Allen Probes observing plasma waves in the inner magnetosphere while in magnetic conjunction with ARMAS instrument at aviation altitudes (top panel).The middle panel shows the background dose rate (black line) and the observed dose rate (red bars) measured by ARMAS instruments.Plasma wave observations by the Van Allen Probes are shown in the bottom panel.In this particular example, the Van Allen Probe B observed plasmaspheric hiss waves while in magnetic conjunction with ARMAS on 29 March 2018.The solid and dotted blue lines represent the difference in L-Shell (ΔL (RE)) and MLT (ΔMLT (h)) respectively between ARMAS and Van Allen Probe B. At the same time ARMAS also observed dose rates much higher than the background dose rate.The two observations coincide well, as the highest dose rate was observed when the most intense plasmaspheric hiss wave power was recorded by Van Allen Probe B between 19:36UT to 19:39UT during the closest conjunction period (between 19:36UT to 19:42UT) with both ΔL and ΔMLT less than 1 (the gray shaded area highlights times when ARMAS and Van Allen Probe B is not in close conjunction i.e., ΔL or/and ΔMLT is greater than 1).

ARMAS Conjunctions With the Van Allen Probes
We identified conjunction events between ARMAS and the Van Allen Probes for enhanced radiation events observed by ARMAS at aviation altitudes.Figure 2 shows the number of conjunctions between ARMAS and the Van Allen Probes as a function of ΔL and ΔMLT.The number of conjunctions increases with increasing ΔL and ΔMLT.The aim is to keep the conjunction window, in terms of time (ΔMLT) and space (ΔL), as small as possible to ensure the observations between ARMAS and the Van Allen Probes are related, whilst ensuring there are reasonable number of data (conjunctions) to study the relationship between enhanced radiation events at aviation altitudes (>9 km) and various plasma waves in the inner magnetosphere.The results, presented in Figure 2, shows that there are more than 1,000 conjunction points for a conjunction window of ΔL < 1RE and ΔMLT < 1 hr, which is a reasonable size, and enough to conduct the statistical study (please note that 1RE is considerably larger than the scale of the waves discussed here; Aryan et al., 2021Aryan et al., , 2022;;Blum et al., 2017;Zhang et al., 2021).

Correlation Between Enhanced Radiation at Aviation Altitude and EMIC Waves
It has been suggested, in the past, that the third source of enhanced radiation observed at aviation altitude might be related to the occurrence of EMIC waves in the inner magnetosphere (Tobiska et al., 2022;Tsurutani et al., 2016).EMIC waves are generally excited by anisotropic protons in the ring current when heavy ions, such as He+ and O+ are present within the background plasma.They are often observed within three distinct bands (the H+ band, He+ band, and O+ band) in the terrestrial magnetosphere and due to the presence of He+, O+, and H+ in the background plasma, new resonances arise at the gyrofrequencies of He+ and O+ (Allen et al., 2015;Min et al., 2012;Saikin et al., 2015;Yu et al., 2015).In this section we study statistically the correlation between the ARMAS observed dose rate and wave power in the EMIC frequency range.Figure 3 shows the relationship between dose rate (ΔD) observed by ARMAS and wave PSD observed by the Van Allen Probes during conjunction events, over the frequency ranges , and f  f  + for H + , He + , and O + bands respectively (Note: ΔD is the difference between the measured dose by ARMAS and the background dose estimated by NAIRAS model.Also each point represents 1 min of measurement and the EMIC wave intensity is the sum of the intensities of the three bands).The results show that there is no correlation between enhanced radiation and wave power in the EMIC frequency range with the correlation coefficient of r = −0.0898.This indicates that the source might be related to a different type of plasma that influences the dynamics of the radiation belts.

Correlation Between Enhanced Radiation at Aviation Altitude and Plasmaspheric Hiss Waves
Plasmaspheric hiss waves are excited naturally in the high plasma density region inside the plasmasphere and dayside plasmaspheric plumes (Chan & Holzer, 1976;Hayakawa et al., 1986;Parrot & Lefeuvre, 1986) and are known to be largely responsible for the continuous scattering of the inner radiation belt electrons into the atmospheric loss cone, the decay of energetic electrons in the outer radiation belt during relatively quiet geomagnetic conditions (Lyons et al., 1972;Summers et al., 2007) due to resonant pitch angle scattering of energetic electrons (Lyons et al., 1972), and are responsible for the formation of the slot region between the inner and outer radiation belts (Abel & Thorne, 1998;Lyons & Thorne, 1973;Lyons et al., 1972).Plasmaspheric hiss waves are right hand polarized electromagnetic whistler mode wave (Bortnik et al., 2008(Bortnik et al., , 2009;;Thorne et al., 1973) observed as a steady, incoherent noise band (Falkowski et al., 2017;Tsurutani et al., 2015Tsurutani et al., , 2018) ) and typically observed in the frequency range 100 Hz < f < 2 kHz with a lower wave power than chorus (Meredith et al., 2004).
Figure 4 shows the relationship between dose rate (ΔD) observed by ARMAS and plasmaspheric hiss wave PSD observed by the Van Allen Probes during conjunction events.The results show that there is a strong correlation between enhanced radiation levels observed at aviation altitudes and plasmaspheric hiss wave power observed along the same magnetic field line in the plasmasphere with a correlation coefficient of r = 0.7309.This indicates that the radiation source at aviation altitudes may also be due to the continuous scattering of radiation belt electrons by plasmaspheric hiss waves.

Correlation Between Enhanced Radiation at Aviation Altitude and Chorus Waves
Chorus waves are excited by the injection of substorm electrons in the vicinity of the geomagnetic equator (Burtis & Helliwell, 1969;Lauben et al., 2002;LeDocq et al., 1998;Meredith et al., 2001;Tsurutani & Smith, 1974) associated with the electron temperature anisotropy plasma instability (Kennel & Petschek, 1966).Chorus waves are very intense right hand polarized electromagnetic whistler mode waves that make significant contributions to the production of the diffuse aurora (Ni et al., 2008;Thorne, 2010) and are an important source of plasmaspheric hiss waves (Agapitov et al., 2018;Bortnik et al., 2008Bortnik et al., , 2009Bortnik et al., , 2016;;Meredith et al., 2013).Chorus emissions are observed in the low density region outside the plasmasphere as short coherent pulses (Burtis & Helliwell, 1969;Tsurutani & Smith, 1974) in the frequency range of 0.1f ce < f < f ce (0.1f ce < f < 0.5f ce is defined as the lower band chorus and 0.5f ce < f < f ce is the upper band chorus), where f ce is the local electron gyrofrequency.The lower band chorus is believed to be the dominant scattering process leading to pulsating auroral precipitation (Nishimura et al., 2010(Nishimura et al., , 2011)).
Figure 5 shows the relationship between the dose rate (ΔD) observed by ARMAS and chorus wave PSD observed by the Van Allen Probes during conjunction events.The results show that there may be a relationship between enhanced radiation at aviation altitude and chorus wave power in the inner magnetosphere with a correlation coefficient of r = 0.4987.Also, there are relatively fewer chorus wave observations during the conjunction events, and therefore the relationship may be better defined with a larger number of chorus wave observations.

Correlation Between Enhanced Radiation at Aviation Altitude and High Frequency Waves
For completeness we also tested the relationship between enhanced radiation at aviation altitude and wave PSD in the high frequency range (i.e., f > f ce ) which includes potential scattering from Electrostatic Electron Cyclotron Harmonic (ECH) waves (Fukizawa et al., 2018;Horne et al., 2003;Kennel et al., 1970;Kurita et al., 2014).Figure 6 shows the relationship between dose rate (ΔD) observed by ARMAS and wave PSD in the high frequency range observed by the Van Allen Probes during conjunction events.The results show that there is no correlation between enhanced radiation and high frequency wave power with the correlation coefficient of r = −0.0252.This indicates that the source might not be related to any plasma waves above f ce , and highlights plasmaspheric hiss as the leading scattering candidate.

Dose Rate Versus Plasmaspheric Hiss Wave Power as a Function of Geomagnetic Activity
In Section 3.3 it was shown that there is a strong correlation between enhanced radiation at aviation altitudes and plasmaspheric hiss wave power in the plasmasphere with a correlation coefficient of r = 0.7309.It has been shown that plasmaspheric hiss wave power is determined by the level of geomagnetic activity as defined by geomagnetic index AE (Aryan et al., 2021).In this section we study how the relationship between dose rate and plasmaspheric hiss wave power, shown in Section 3.3, is affected by the level of geomagnetic activity.Figure 7 shows the relationship between dose rate (ΔD) observed by ARMAS and plasmaspheric hiss wave PSD observed by the Van Allen Probes during conjunction events as a function of low (AE < 100 nT), medium (100 nT < AE < 300 nT), and high (AE > 300 nT) geomagnetic activities.The results show that the correlation coefficient between observed dose rate by ARMAS at aviation altitudes and plasmaspheric hiss wave power observed in the inner magnetosphere decreases as geomagnetic activity increases, which may be related.Also, higher geomagnetic activity leads to more intense plasmaspheric hiss wave power that ultimately leads to more enhanced radiation at aviation altitudes at all activity levels.
Here we show how the relationship between the dose rate observed by ARMAS and the plasmaspheric hiss wave power observed by the Van Allen Probes, as shown in Section 3.3, is affected by plasma density.
Figure 8 shows the relationship between dose rate (ΔD) observed by ARMAS and plasmaspheric hiss wave PSD observed by the Van Allen Probes during conjunction events as a function of low (n e < 100 cm −3 ), medium (100 cm −3 < n e < 300 cm −3 ), and high (n e > 300 cm −3 ) plasma densities.The results show that the correlation coefficient between dose rate and plasmaspheric hiss wave power is generally similar for all differ-  ent densities, although, higher density events are slightly more likely to occur at more intense plasmaspheric hiss wave power.

Conclusion
In this study, we used almost 7 years of ARMAS real-time radiation measurements taken at aviation altitudes (>9 km) and plasma wave data observed by NASA's Van Allen Probes in the inner magnetosphere (between 2013 and 2019).We identified conjunction events between ARMAS and Van Allen Probes, focusing on enhanced radiation events, and studied the relationship between dose rate and various magnetospheric waves.The results show that there is a strong correlation between the observed dose rate and plasmaspheric hiss wave power.However, there is no correlation between dose rate and other plasma wave types in the inner magnetosphere, such as, EMIC waves, high-frequency waves, and possibly also whistler mode chorus, although the results for chorus waves were somewhat unclear.As we understand the two major sources of ionizing radiation hazards are the GCRs and SEPs.However, the results presented here shows that an additional radiation source is also due to plasmaspheric hiss waves within the inner magnetosphere.The results also show that geomagnetic activity may also play an important role in defining the relationship between enhanced radiation and plasmaspheric hiss waves in the inner magnetosphere.Overall the results suggest that the space environment could have a potentially significant effect to pilots, aircraft passengers, and commercial space travellers.

Figure 1 .
Figure 1.A schematic of the Van Allen Probes observing plasma waves in the inner magnetosphere while in magnetic conjunction with Automated Radiation Measurements for Aerospace Safety (ARMAS) instrument at aviation altitudes (top panel).The middle panel shows the background dose rate (black line) and the observed dose rate (red bars) measured by ARMAS.Plasma wave observations by the Van Allen Probes are shown in the bottom panel.In this particular example the Van Allen Probe B is observing plasmaspheric hiss waves while in magnetic conjunction with ARMAS on 29 March 2018.The solid and dotted blue lines in the middle panel shows ΔL and ΔMLT respectively between ARMAS and Van Allen Probe B

Figure 2 .
Figure 2. The conjunctions events between Automated Radiation Measurements for Aerospace Safety (ARMAS) and the Van Allen Probes.The number of conjunctions between ARMAS and the Van Allen Probes are shown as a function of ΔL and ΔMLT.

Figure 3 .
Figure 3.The correlation between the dose rate observed by Automated Radiation Measurements for Aerospace Safety at aviation altitudes and wave power in the electromagnetic ion cyclotron frequency range observed by the Van Allen Probes in the inner magnetosphere.The correlation coefficient is given in the top right.

Figure 4 .
Figure 4.The correlation between the dose rate observed by Automated Radiation Measurements for Aerospace Safety at aviation altitudes and plasmaspheric hiss wave power observed by the Van Allen Probes in the inner magnetosphere.The correlation coefficient is given in the top right.

Figure 5 .
Figure 5.The correlation between the dose rate observed by Automated Radiation Measurements for Aerospace Safety at aviation altitudes and chorus wave power observed by the Van Allen Probes in the inner magnetosphere.The correlation coefficient is given in the top right.

Figure 6 .
Figure 6.The correlation between the dose rate observed by Automated Radiation Measurements for Aerospace Safety at aviation altitudes and wave power in the high frequency range observed by the Van Allen Probes in the inner magnetosphere.The correlation coefficient is given in the top right.

Figure 7 .
Figure 7.The relationship between the dose rate (ΔD) observed by Automated Radiation Measurements for Aerospace Safety and plasmaspheric hiss wave power spectral density observed by the Van Allen Probes during conjunction events as a function of geomagnetic activity.The blue, green, and red stars represents low (AE < 100 nT), medium (100 nT < AE < 300 nT), and high (AE > 300 nT) geomagnetic activities respectively.

Figure 8 .
Figure8.The relationship between dose rate (ΔD) observed by Automated Radiation Measurements for Aerospace Safety and plasmaspheric hiss wave power spectral density observed by the Van Allen Probes during conjunction events as a function of plasma density.The blue, green, and red stars represents low (n e < 100 cm −3 ), medium (100 cm −3 < n e < 300 cm −3 ), and high (n e > 300 cm −3 ) plasma densities respectively.