AN ENGINEERING MODEL FOR SOLAR ENERGETIC PARTICLES IN INTERPLANETARY SPACE


B. Sanahuja*, A. Aran* and D. Lario**

*Departament d'Astronomia i Meteorologia. Universitat de Barcelona. Spain

and  Institut d'Estudis Espacials de Catalunya. Spain.

**Applied Physics Laboratory. The John Hopkins University. USA

Sponsored by ESA/ESTEC, the DGCYT (MCyT) and NASA
 

 

1. Introduction

2. The model

3. Application to particle flux calculation

 

1. Introduction

The response of the space environment to the constantly changing Sun is known as "Space Weather". We are concerned with one of the elements of Space Weather that is important for many systems operating in space, either near the Earth, or in interplanetary space: solar energetic particles.

Most of the low energy particles (E<100 MeV) observed in the interplanetary medium are found in the form of solar energetic particle (SEP) events, they are time limited increases of particle fluxes, mostly protons, lasting from a few hours to some days. They were first detected at the Earth high latitude regions as polar cap absorption envents in radio noise. The SEPs are sporadic in nature, but are more frequent at the time of high solar activity. Energetic particles generally result from the acceleration of the ambient plasma as magnetohydrodynamic shocks propagate through the solar corona and the solar wind, as reflected in their composition. Most of the shocks are driven by solar coronal mass ejections (CMEs). Some of these CMEs are accompanied by a solar flare (often visible as impulsive events in H-alpha light and X-rays). In such cases the composition may reflect the large heating or ionization of the local impulsive acceleration in the solar corona.

Since SEP events present one of the most severe hazards in space environment it is important to be able to predict when and how important an event may be on the basis of routine observation of the solar corona and the near-sun solar wind. Besides the observational data we need a model that descrives how the acceleration of energetic particles occurs at these shocks as they propagate within the heliosphere and how the particles accelerated near the shock leak into the solar wind and propagate throughout interplanetary space. A summary of the state-of-the-art and historical development of the modeling of SEP events associated with CME-driven shocks can be found in this link: History

 

2. The model

The used model is a development based on the interplanetary shock- plus- particles of the propagation model described by Heras et al. [1992, 1995]. Further improvements and changes can be found in Lario et al., 1998, and references quoted there. We use the concept of cobpoint (Connecting-with-the-OBserver POINT), the point on the intersection between the front of the interplanetary shock and the interplanetary magnetic field (IMF) line that passes through the observer's position. Particles accelerated at this point of the MHD shock will propagate through the magnetic flux tube defined by this magnetic line. As the shock expands and propagates through the interplanetary medium, the cobpoint moves along the front of the shock; that means that the conditions for particle on acceleration at this point, where shock-accelerated particles are injected, change as function of time (figure 1)

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to get full size image

Fig.1: Snapshots of the simulation of the shock propagation for the CM-event. The density contours (log cm-3), and some IMF lines are represented at four different times. The shocks are explicitly located within the steep density gradients. The locations of several spacecraft are indicated.

The cobpoint describes different paths along the shock front, depending on the heliolongitude of the parent solar activity that generates the shock, and on the position of the observer with respect to the shock. For a given solar event, which triggers a shock, the shock propagation model provides the values of the magnetohydrodinamic variables at the cobpoint. The effects of the propagation of these particles through the interplanetary medium, along the IMF, are estimated by means of a focused-diffusion transport equation. By combining both models we are able to evaluate the number of particles to be injected into the IMF line rooted at the cobpoint. The result is flux and anistropy profiles which can be compared with observations, or used as fiducial profiles.

Presently the code has been used to derive the injection rate and its evolution for different events, and to test its reliability. First applications to synthesise flux profiles have been presented in Lario et al., 1998. The code has been applied to several particle events detected by ISEE-3 spacecraft, and by Helios-2, for energies between 56 keV and 50/100 MeV (depending on the event). Now it is going to be used to interpret SEP events observed by ACE spacecraft and in the future to relate them to observations from the Ulysses spacecraft (figure 2).

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to get full size image

Fig.2: Observed (thin lines) and fitted (thick-dashed lines) flux and first order anisotropy for an WS-event. The thick arrow indicates the time of the solar activity, the dotted-dashed vertical line indicates the passage of the shock, and the short solid vertical line is t_c. The extra flux profiles, at 5-10 MeV and at 10-20 MeV, are plotted with dashed lines.

The long term objective is to develop an engineering code for characterising solar energetic particle population at user-specified locations in space from outside the solar corona up to the orbit of Mars. The model will estimate time-dependent particle fluxes and fluences as a function of the energy over the range 50 keV to 100 MeV, with a familiar user interface for running the engineering tool.

The inputs will be those that are now available in real time from NOAA. Those include:
      - Coronal Mass Ejection and/or flare location;
      - time of the optical/H-alpha detection;
      - onset of GOES X-ray flux;
      - solar wind velocity, from L1 spacecraft if available (default: 400 km/s); and
      - spacecraft location, at L1, earth, or any point relative to fixed Sun-Earth axis.

The outputs will be in a form that is readily available to the mission designer or operational user. For example, as a graphical time series that includes the solar event of interest and will be continuously updated in real time, or as a table of values for different scenarios. The plots will display the following information:
       - the expected flux (particles/cm2-s-sr-MeV), which will be continuously compared on the same plot, in designated energy channels,
       - the expected duration of the event, which will be continuously updated;
      - the expected flux, which will continuously be compared on the same plot with actual observations, if available in real time, plots would also be       configured for post-event comparisons.
       - integrated fluences in the designated energy channels.

The plots will indicate the initial arrival time of energetic particle events, the expected time of the flux maximum, and the expected time of shock arrival. The operator will see also tabular data.

Figures 3 and 4 give an idea of the variety of parameters that we need to have into account to predict the event that will be seen at the observer's location. Figure 3 shows typical events produced at different heliolongitudes relative to the observer. Figure 4 compares two events produced at similar relative heliolongitudes, but where the shock properties or/and the particle propagation conditions are different. In both figures we can see how the time profile of the fluxes at different energies depends on different conditions.

The application that follows is based on the creation of a grid of fiducial particle flux profiles and an interpolation tool. The large number of free parameters is a consequence of the limited information and theoretical development. Our model is in development and the observations are yet far from providing essential data, for instance, at present there is not a reliable method to measure the shock speed near the Sun.

 

3. Application to particle flux calculation

A first version of a code that calculates proton flux and cumulative fluence profiles at 0.5 MeV and 2.0 MeV, and for observers located at 1.0 AU and 0.4 AU is available upon request to aaran@am.ub.es. The code is written in IDL language to be run under version IDL 5.4. The main programme reads the data base and asks the user to choose several characteristics of the event. A number of non-standard IDL routines are needed to run the programme. The software and data files occupy 155 Mbytes.

The application assumes that the observer is located in the ecliptic plane. The following inputs need to be provided:

    1) Heliocentric distace of the observer in the range 0.4 through 1.4 AU

    2) Heliologitud of the parent event (flare or CME)

    3) Shock transit time, in hours (i.e., the time interval the shock spends traveling from the Sun to the Earth)

    4) Shock Width, in the range 40 through 140 degrees.

    5) Mean Free Path, in AU (0.2/0.8). The transport conditions of energetic particles (specified by the mean free path of 0.5 MeV protons). With the present form of the model you can choose between 0.2 and 0.8 AU

    6) Turbulence Foreshock Region (yes/no). In some events the arrival of the shock at the observer is characterized by an energetic particle flux enhancement (also called ESP event). Our model reproduces these ESP events by assuming a foreshock turbulent region.

The profile of proton fluxes and anisotropy at the observer location are given in graphical form for 6 energy levels between 0.25 and 8 MeV.

Details on the selection choices and interpolation procedures to obtain flux profiles between the given set of parameters can be found in Aran et al. [2001]. The code has not yet been validated.

Here you may see, as an example, the aplication of the model for a limited set of choices. Just fill in or reply to the following questions:

Heliocentric Distance AU
Heliolongitude deg (EXX,WXX)
Transit Time hr
Shock Width deg
Mean Free Path AU
Turbulence Foreshock Region

Note: For longitude and transit time, the closest values to the specified parameters will be selected from our dataset.

   

REFERENCES

Aran, A., B. Sanahuja, D. Lario, in "Solar Encounter: The First Solar Orbiter Workshop", ESA SP-493, 157, 2001. [PDF]

Heras, A.M., et al., ApJ, 391, 359, 1992. [PDF]

Heras, A.M., et al., ApJ, 445, 497, 1995. [PDF]

Lario, D., B. Sanahuja, A.M. Heras, ApJ, 509, 414, 1998. [PDF]

 

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