Margaret Ann Shea

A comparison of the tsyganenko model predicted and measured geomagnetic cutoff latitudes

Abstract We have computed world grids of geomagnetic vertical cutoff rigidities employing the Tsyganenko magnetospheric model for all integer magnetic activity levels specified by the Kp magnetic index. In order to test the accuracy of these results, we present comparisons with the measured cutoff latitudes observed by the SAMPEX spacecraft. In general we find that during magnetically active times the measured cutoffs are about one degree lower (equatorward) in latitude than the cutoffs determined from the trajectory-tracing method using the Tsyganenko magnetospheric model. We also find that the 3-hour averaged Kp index is not responsive to dynamic magnetospheric activity, and a more frequent temporal description of the magnetic activity is desirable.

Comment on the use of GOES solar proton data and spectra in solar proton dose calculations

There is a need to understand the calibration and response of the GOES solar particle detectors since the GOES data are being used to evaluate high energy solar particle events. We share some of our experience in utilizing these data in the analysis of solar particle ground-level events (GLEs). For the 29 September 1989 event, we have evaluated the solar proton and alpha particle spectral characteristics throughout the event. The results show that the solar cosmic ray spectrum is extremely hard at low energies with the magnitude of the slope increasing with increasing energy and with time.

World-wide radiation dosage calculations for air crew members

A greatly improved version of the computer program to calculate radiation dosage to air crew members is now available. Designated CARI-6, this program incorporates an updated geomagnetic cutoff rigidity model and a revision of the primary cosmic ray spectrum based on recent work by Gaisser and Stanev (1998). We believe CARI-6 provides the most accurate available method for calculating the radiation dosage to air crew members. The program is now utilized by airline companies around the world and provides unification for subsequent world-wide studies on the effects of natural radiation on aircrew members.

A review of solar proton events during the 22nd solar cycle.

Solar cycle 22 had significant, large fluence, energetic particle events on a scale reminiscent of the 19th solar cycle. Examination of the characteristics of these large events suggests that some of the old concepts of spectral form, intensity-time envelope and energy extrapolations, used to estimate the dose from large events that occurred during previous solar cycles should be re-evaluated. There has also been a dramatic change in perspective regarding the source of solar protons observed in interplanetary space. Very large fluence events are associated with powerful fast interplanetary shocks. The elemental composition and charge state of these events is suggestive of a dominate source in the solar corona and not from a very hot plasma. Furthermore, there is a strong suggestion that the intensity-time profile observed in space is dominated by the connection of the observer to an interplanetary shock source rather than to a unique location near the surface of the sun. These concepts will be examined from the perspective of energetic particles contributing to the dose experienced by an astronaut on an interplanetary space mission.

Identification of major proton fluence events from nitrates in polar ice cores

Large transient concentrations of nitrates in polar ice cores have been identified as the signature of some major solar proton fluence events between 1940 and 1991. We review this solar proton proxy identification technique using nitrate concentration measurements in ice cores from the Arctic and Antarctic. Using this identification technique we go back in time in an attempt to identify major solar proton events during the past several centuries. There is a very large nitrate increase corresponding to the Carrington flare of 1859 evident in the Arctic ice core. Other significant nitrate increases may indicate that major solar proton events occurred toward the end of the last century. The problems associated with this new technique of using nitrates as proxies to identify solar proton events are discussed.

Evaluation of the Dynamic Cutoff Rigidity Model Using Dosimetry Data From the STS-28 Flight

We have a developed a dynamic cutoff rigidity model based on computed world grids of vertical cutoff rigidities derived from employing the Tsyganenko magnetospheric model. The dynamic range of this model covers all magnetic activity levels specified by integer values of the Kp magnetic index. We present comparisons of the measured dose observed on the space shuttle during the August 1989 solar proton event with the dose computed from solar particles predicted to be allowed through the magnetosphere to the space shuttle position. We find a one-to-one correspondence between the portion of the orbit predicted to be subjected to solar protons and the portion of the orbit where solar particle dose measurements were obtained.

Solar proton event patterns: the rising portion of five solar cycles

Using solar proton data from the past four solar cycles we have found that while there is no consistent pattern in the distribution of solar proton events throughout each cycle, there is a surprising consistency in the total number of events over each cycle. In comparing the distribution of solar proton events during the rising portion of cycles 19–23, we find a similarity between the 20th and the 23rd solar cycles. During solar cycle 20, only 17% of the proton events for the entire cycle had occurred during the first three years of the cycle and 28% had occurred during the first four years. If cycles 20 and 23 are similar, we would expect the majority of solar proton events to occur during years 5–8 of the 23rd cycle (i.e. from 2001–2004).

Global real‐time dose measurements using the Automated Radiation Measurements for Aerospace Safety (ARMAS) system

The Automated Radiation Measurements for Aerospace Safety (ARMAS) program has successfully deployed a fleet of six instruments measuring the ambient radiation environment at commercial aircraft altitudes. ARMAS transmits real-time data to the ground and provides quality, tissue-relevant ambient dose equivalent rates with 5-minute latency for dose rates on 213 flights up to 17.3 km (56,700 ft.). We show five cases from different aircraft; the source particles are dominated by Galactic Cosmic Rays but include particle fluxes for minor radiation periods and geomagnetically disturbed conditions. The measurements from 2013–2016 do not cover a period of time to quantify Galactic Cosmic Rays’ (GCRs) dependence on solar cycle variation and their effect on aviation radiation. However, we report on small radiation “clouds” in specific magnetic latitude regions and note that active geomagnetic, variable space weather conditions may sufficiently modify the magnetospheric magnetic field that can enhance the radiation environment, particularly at high altitudes and mid- to high-latitudes. When there is no significant space weather, high latitude flights produce a dose rate analogous to a chest X-ray every 12.5 hours, every 25 hours for mid-latitudes, and every 100 hours for equatorial latitudes at typical commercial flight altitudes of 37,000 ft. (~11 km). The dose rate doubles every 2 km altitude increase, suggesting a radiation event management strategy for pilots or air traffic control, i.e., where event-driven radiation regions can be identified, they can be treated like volcanic ash clouds to achieve radiation safety goals with slightly lower flight altitudes or more equatorial flight paths.

The limitations of using vertical cutoff rigidities determined from the IGRF magnetic field models for computing aircraft radiation dose

Vertical cutoff rigidities derived from the International Geomagnetic Reference Fields (IGRF) are normally used to compute the radiation dose at a specific location and to organize the radiation dose measurements acquired at aircraft altitudes. This paper presents some of the usually ignored limits on the accuracy of the vertical cutoff rigidity models and describes some of the computational artifacts present in these models. It is noted that recent aircraft surveys of the radiation dose experienced along specific flight paths is sufficiently precise that the secular variation of the geomagnetic field is observable.

NASA Marshall Engineering Thermosphere model — 1999 version (MET-99) and implications for satellite lifetime predictions

Abstract The use of thermospheric density models in the prediction of atmospheric drag (the major perturbation for orbiting spacecraft) is of great importance. Issues associated with these predictions include lifetime estimates, orbit determination and tracking, attitude dynamics, and re-entry prediction. Logistics planning through attitude control requirements and re-boost planning are also influenced by future orbital altitude density estimates. The MET-99 model and its predecessors were developed to provide the inputs needed to address these issues. The sensitivity of the future estimation of solar activity, and thus thermospheric density and the prediction of a satellites lifetime, to the selection of Solar Cycle 23 minimum other than the conventionally identified mathematical minimum is shown. This can produce about 25 percent error in predicted satellite lifetime for a typical low Earth orbit example.