Table of Contents ECSS Model Page
Background Information Background Information
Jovian Trapped Particle Radiation Models

Table of contents

1 Introduction

2 The Trapped Particle Radiation Environment of Jupiter

2.1 The Magnetosphere of Jupiter

2.2 Jovian radiation belts

2.2.1 The trapped proton population

2.2.2 The trapped electron population

3 Effects of trapped particle radiation on spacecraft and components

4 Implementation in SPENVIS

4.1 The Divene and Garrett model

4.2 The GIRE model

4.3 The ONERA/Salammbo model for Jupiter

References

1 Introduction

The existence of the Jovian magnetosphere has been known since the early 1960s, soon after the realisation that the UHF radio emissions from Jupiter could be explained in terms of trapped energetic electrons [ Drake & Hvatum, 1959]. We now know that Jupiter, like the other giant planets, possess a large magnetosphere. Thus, spacecrafts orbiting in its vicinity can encounter intense trapped particle environments including direct radiation effects and internal charging.

In the following, we present a brief review of the main characteristics of the Jovian trapped particle radiation belts and the engineering models that are used to evaluate mission fluences and doses.

2 The Trapped Particle Radiation Environment of Jupiter

2.1 The Magnetosphere of Jupiter

The magnetosphere of Jupiter is in many aspects somewhat different from the one of Earth [Dessler, 1983]. First, Jupiter shows weak yet genuine pulsar behaviour. Thus the energy required to drive the Jovian magnetosphere is apparently extracted from the rotational energy of the planet rather than from the solar wind as in the case of the equivalent terrestrial system. Second, the Jovian magnetosphere contains a ring and several satellites that absorb energetic particles. At the same time, they behave as a plasma source that can have an extensive effect on the structure and dynamics of the radiation belts of Jupiter. Finally, Jupiter is a strong source of energetic charged particles that can be detected as far as the orbit of Mercury. Nevertheless, there are also similarities and some ideas from the study of Earth's magnetosphere has been successfully applied in the case of Jupiter.

The Jovian magnetosphere is normally described in terms of three major regions [Dessler, 1983]:

  1. The inner magnetosphere extends to the orbit of Io to a distance of approximately 6 Rj, where Rj (~ 71400 km) is the mean equatorial radius of Jupiter. This is the region where the principal magnetic field is created by sources internal to the planet. Outside of this region the effects of an azimuthal current sheet in the equatorial plane produce a significant perturbation, leading to the stretching of the magnetic field lines in the radial direction;
  2. The middle magnetosphere is situated between ~ 6 Rj and 30-50 Rj and is the region where the equatorial currents flow;
  3. The outer magnetosphere extends from ~ 30-50 Rj to the magnetopause. In this region the magnetic field has a large southward component and changes in the solar wind pressure can cause big temporal and/or spatial variations in magnitude and direction.

2.2 Jovian radiation belts

In the Jovian radiation belts, trapped particles are about ten times more energetic than the ones from the equivalent radiation belts of Earth. In addition, they are several orders of magnitude more abundant. Jupiter's magnetosphere contains a major internal source of material. It is now understood that most of the particles come ultimately from the volcanic moon, Io. As a result, the densest part of the magnetosphere is the Io torus situated at 5.5 - 8 Rj [Rogers, 1995].

2.2.1 The trapped proton population

Measurements from the energetic particle detector (EPD) onboard the Galileo spacecraft showed that the number and energy intensities of the protons are higher compared to that of the other energetic ions inside 20-25 Rj [Mauk et al, 2004].

Also, the passage of Ulysses spacecraft through the Jovian magnetosphere revealed stable pancake-shape pitch angle distributions for protons inside ~ 17 Rj. The same mission showed that the equatorial proton omni-directional flux decreases approximately exponentially with magnetic equatorial distance and is nearly longitudinally symmetric [Aglin et al, 1997].

2.2.2 The trapped electron population

The 2-dimensional images of synchrotron emission provided by radio telescope observations show two distinct populations of high energy electrons (1-100 MeV) in the inner belts. One that has pitch angles near 90° and produces radiation concentrated at the magnetic equator, with maximum intensity near R ~ 1.5 Rj. The other one has a wide pitch angle distribution and produces radiation at a large range of magnetic latitudes [Sicard & Bourdarie, 2004]. These observations are in agreement with in situ measurements

The population of energetic electrons is limited mainly by absorption by the satellites and the Jovian ring rather than by radiation losses.

3 Effects of trapped particle radiation on spacecraft and components

Due to their large energy coverage, trapped particles cause a variety of effects in spacecraft, components and biological systems.

Low energy electrons contribute to spacecraft surface charging. High energy electrons can cause dielectric charge buildup that may lead in turn to destructive arcing. Electrons also contribute to ionising doses through direct energy deposition and bremsstrahlung effects.

High energy protons are the main contributors to ionising dose deposition in shielded components. They also dominate Single Event Upset (SEU) rates. Lower energy protons (up to 10 MeV) contribute to Non-Ionising Energy Loss (NIEL) dose that affects Charged-Coupled Devices (CCD) and other detectors.

4 Implementation in SPENVIS

The following trapped particle models for Jupiter are now available in SPENVIS: Table 1 contains the main characteristics of the various models. The range of the various models is expressed in terms of the McIlwain L-parameter measured in units of Rj.

Table 1. Characteristics of the trapped particle radiation models for Jupiter.
Model Energy range
(MeV)
Coordinate range
(Rj)
Magnetic field models Reference
Proton models
D&G83 >  0.6 L <=  12 D4  (Smith et al, 1976) Divine & Garrett, 1983
Salammbo 1 - 1000 L <= 6 Internal: O6 (Connerney, 1993)
External: Khurana (1992,1997)
Bourdarie & Sicard, 2006
Electron models
D&G83 >  0.06 L <= 16 (in)
L > 16 (out)
D4  (Smith et al, 1976) Divine & Garrett, 1983
GIRE 0.5 - 30 8 <= L <= 16 VIP4 (Connerney et al, 1998) Garrett et al, 2003
Salammbo 1 - 600 L <= 9.5 Internal: O6 (Connerney, 1993)
External: Khurana (1992,1997)
Bourdarie & Sicard, 2006
Ion models
JOSE HIC 6-200 MeV/nucl 2.8 to > 30 N/A Jun, 2005
JPL-Heavy Ion 5-40 MeV/nucl 5 ≤ Radius ≤ 25 Rj N/A JPL Publication 11-16, 2011

For energies below model validity range (see Table 1), flux values are extrapolated and subsequently used in other SPENVIS applications (e.g. SHIELDOSE).

4.1 The Divine and Garrett model

The model developed by Divine and Garrett is the only global model available today and is based on data collected by the Pioneer and Voyager flybys of Jupiter combined with earth-based observations [Divine and Garrett, 1983]. For the electrons, it covers the whole range from the surface of the planet to the tail of the magnetosphere while for the protons it applies from the surface to L = 12. The energy spectrum for electrons and protons includes energies higher than 0.06 and 0.6 MeV respectively.

The magnetic field used is based on the D4 model derived from Pioneer Helium vector magnetometer data [Smith et al , 1976]. Note that this model performs fine for radial distances inside Io's orbit but it fails for higher distances due to effects of the plasma torus.

4.2 The GIRE model

Following the success of the Galileo mission, a new empirical model, known as GIRE (Galileo Interim Electron Environment), has been developed at JPL It is derived from electron data collected by the Galileo, Pioneer 10 and 11 missions at L = 8-16 Rj.

Though GIRE covers the equatorial plane of Jupiter, it can be extended by assuming the pitch-angle distribution provided by the Divine and Garrett model. It applies for electrons with energies from 0.5 to 30 MeV [Bourdarie & Sicard , 2006].

Also, the more recent VIP4 magnetic field model was used [Connerney et al, 1998]. However, it has the same spatial limitations as the D4 model.

4.3 The ONERA/Salammbo model for Jupiter

The model is based on the Earth Salammbo code, developed by ONERA/DESP, that has been adapted for Jupiter. It has been validated for protons and electrons for radial distances from the planet's surface up to the orbit of Europa [Bourdarie & Sicard , 2006].

The spatial range of the model extends from the surface to L = 9.5 for the electrons and L = 6 for the protons. It includes electrons and protons with energies from 1 to 600 MeV and 1 MeV to 1 GeV respectively.

In order to have a more accurate picture of the radiation belts outside Io's orbit, the O6 model [Connerney, 1993] is used to describe the inner magnetic field. In addition, an external field model from Khurana is used, providing more realistic field lines for large distances [Khurana, 1992,1997].

4.4 The JOREM/JOSE model for Jupiter

The Jovian Specification Environment (JOSE) model is a new model for the trapped proton and electron environment near Jupiter, based on all relevant data. The model overcomes inaccuracies and discontinuities identified in previous models such as JOE/JOP and GIRE. Fluxes predicted by JOSE have been compared with in-situ measurements and ground based data (in the case of electron only). These comparisons show a good agreement between JOSE mean model and in-situ data, and allow validation of the model in the energy range from a few hundred keV up to few ten MeV for both electron and proton models and in the spatial range from the inner part to the magnetosphere to 100 RJ. A key benefit from JOSE is that it allows flux predictions to be made as a function of confidence level, and the results have also been validated by comparison with in-situ data. Indeed, flux resulting from JOSE model with confidence level of 0.99 include almost all in-situ data used for this validation phase.

The spatial range of the model extends from the surface to L = 9.5 for the electrons and L = 6 for the protons. It includes electrons and protons with energies from 1 to 600 MeV and 1 MeV to 1 GeV respectively.

The heavy ion model integrated in JOSE is the HIC model developed by Garrett et al. [Jun 2005][Evans, 2008].This version of the HIC model uses data from 31 of the 35 Galileo orbits of Jupiter. The HIC model covers radial distances of 2.5 Rj to well past 30 Rj and defines three heavy ion populations: carbon, oxygen, and sulphur. The HIC model is composed of two cases: the average and the worst case. However, until now, only the average model is publicly available and consequently is the only one implemented in JOSE.

In order to have a more accurate picture of the radiation belts outside Io's orbit, the O6 model [Connerney, 1993] is used to describe the inner magnetic field. In addition, an external field model from Khurana is used, providing more realistic field lines for large distances [Khurana, 1992,1997].

4.5 The JPL Jovian Equatorial Heavy Ion Radiation Environment model

The JPL Jovian equatorial heavy ion radiation environment model has been based on data from the Galileo HIC experiment. The data covered the period from 1995 to 2003 and included orbits C03 through J35 (excluding J5, J13 and A34) and the heavy ion range from 6C to 28Ni. The model defines the fluxes for oxygen (5-40 MeV/nucl), carbon (5-40 MeV/nucl) and sulphur (6.3-40 MeV/nucl) between ~5 and 25 RJ. Average differential flux spectra for these three components are presented in terms of energy for selected radial bins. A simple fit has been developed in terms of energy and radial distance that allows interpolation of the fluxes at intermediate values of the two variables. As the model is based on averages over pitch angle from Galileo, which primarily orbits in the jovian equatorial plane, the model is considered valid for approximately 2-3 RJ above or below that plane between 5-25 RJ. The model defaults to the ambient GCR levels for carbon, oxygen and sulphur values for fluxes below 10-6(cm2 s sr MeV/nuc)-1 for carbon and oxygen and 10-8 (cm2 s sr MeV/nucl)-1 for sulphur.

The model is constructed from two components: a tabular radial function, F0,j(R), and a double power law energy fit, as in the equation below.

where
Fj differential flux in units of (#/cm²/s/sr/(MeV/nucl))
Fo,j(R) flux constant at a given radius, fit from the data.
E Energy in MeV/nucl
R radial position (Jovian Radii)
Aj,Bj power law constants fit to the data
E0,j energy constant fit to the data (MeV/nucl)
j subscript indicating the ion species: carbon, oxygen or sulphur

The Spenvis implementation makes no attempts to propagate the equatorial fluxes to higher latitudes, nor to follow field lines to determine the corresponding equatorial radius that the field line crosses.  Instead, the radial distance of the orbital location is used to access the JPL model and the corresponding equatorial fluxes for that radius are returned. It is left as an exercise for the user to assess the validity of higher latitude results.

To provide the integral spectrum, the the Spenvis implementation employs a numerical integration of the differential spectrum and adds the corresponding ISO 15390 GCR integral flux spectrum as the integration constant, or 9.02E10-4, 8.36E10-4 and 2.75E10-4 (cm2 s sr)-1 for carbon, oxygen and sulphur, respectively.

References

Anglin J. D. et al, Trapped energetic ions in Jupiter's inner magnetosphere, J. Geophys. Res., 102, 1-36, 1997.

Bourdarie S. and Sicard A., Jupiter environmental modelling, ONERA technical note 120 issue 1.2, ESA contract 19735/NL/HB, FR 1/11189 DESP, October 2006

Connerney, J. E. P., Magnetic fields of the outer planets, J. Geophys. Res., 98, 18659-18679, 1993.

Connerney, J. E. P. et al, New models of Jupiter's magnetic field constrained by the Io flux tube footprint, J. Geophys. Res., 103, 11929-11939, 1998.

Dessler, A. J. (Ed.), Physics of the Jovian magnetosphere, Cambridge University Press, New York, 1983

Divine N. and Garrett H. B., Charged particle distribution in Jupiter's magnetosphere, J. Geophys. Res., 88, 6889-6903, 1983

Drake F. D. and Hvatum H., Non-thermal microwave radiation from Jupiter, Astron. J., 64, 329, 1959

Garrett H. B. et al, Galileo Interim Radiation Electron Model, Jet Propulsion Laboratory, California Inst. of Technol., JPL 03-006, Pasadena, CA, 2003

Khurana K., A generalized hinged-magnetodisc model of Jupiter's nightside current sheet, J. Geophys. Res., 97, 6269-6276, 1992

Khurana K., Euler potential models of Jupiter's magnetospheric field, J. Geophys. Res., 102, 11295-11306, 1997

Mauk B. H., Energetic ion characterestics and neutral gas interactions in Jupiter's magnetosphere, 109(A9), A09S12, doi:10.1029/2003JA010270, 2004

McIlwain, C. E., Coordinates for mapping the distribution of magnetically trapped particles, J. Geophys. Res., 66, 3681-3691, 1961

Rogers J. H.,The giant planet Jupiter, Cambridge University Press, Cambridge 1995

Sicard A. and Bourdarie S., Physical electron belts model from Jupiter's surface to the orbit of Europa, J. Geophys. Res., 109, 2004

Smith E., Davis L. and Jones D., Jupiter's magnetic field and magnetosphere, in Jupiter (Gehrels T. ed.), University of Arizona press, Tucson, 788-829, 1976

Garret H.B., Kokorowski M., Kang S., Evans R.W., Cohen C.M.S., The Jovian Equatorial Heavy Ion Radiation Environment, JPL Publication 11-16, November 2011.

Jun I., Henry B. Garrett and Robin W. Evans, "High-Energy trapped Particle Environments at Jupiter: an update", IEEE Transactions on Nuclear Science, vol. 52, No. 6, 2005.

Evans R. W., H.B. Garrett, I. Jun, C.M.S. Cohen, E.C. Stone,and S.J. Drouilhet, "Galileo Heavy Ion Radiation Model: Update to the HIC model", AGU poster, 2008.


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