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Frequently Asked Questions (FAQ)


PLEASE, PLEASE, PLEASE … READ THE VALIDATION PAPER   It answers the vast majority of the most common questions users have.


  1. Why is Br a constant in the files?

    The short answer is that Div B = 0. Remember we are doing a 1-D propagation and are therefore assuming that there are no gradients in the direction perpendicular to the direction of propagation. Think about this in Cartesian coordinates, taking the direction of propagation to be the z direction,

    Div B = dBx/dx + dBy/dy + dBz/dz = dBz/dz = 0

    So, the 1-D approximation results in the requirement that Bz cannot vary spatially and therefore Bz also cannot change in time. In spherical coordinates, this means that Br falls off like 1/r^2 but beyond that there cannot be any spatial or time variation. This is an inherent limitation of 1D MHD.
  2. I care the most about Bz, how accurate is it?

    Unfortunately, Bn (closest coordinate to Bz) is not very accurate, please read the validation paper to learn the details. In the paper we note that Vr is very accurate, followed by the density. Unfortunately, the magnetic field is not modeled with the same level of correlation to the data.
  3. I want to schedule my observations using your predictions! Can you tell me exactly when to schedule the telescope time?

    NO! Keep in mind that while we have tried to validate the model in detail, we don’t have any way to calculate the error in arrival of any specific shock. Our validation gives the statistical likelihood of the arrival time error. The error for any given shock could be higher or lower than our reported average. We encourage people to use our predictive propagations to help with their planning but using mSWiM should not be the sole means for determining when to take data.
  4. What input data do you use?

    Propagations from “Earth” use the low resolution OMNI2 data product available on the OMNIweb site:

    http://omniweb.gsfc.nasa.gov/ow.html

    We specifically use the omni_m data product which provides hour averages in RTN coordinates.

    Propagations from Stereo A & B use the Stereo COHO 1HR average merged plasma and magnetic field data product available from:

    http://cdaweb.gsfc.nasa.gov/
  5. Does the method capture solar wind transients?

    The method pages on this web site describe the fact that the 1D nature of the propagation requires several different rotations in heliographic longitude. These rotations, by their inherent nature, assume that the sun and heliosphere are in steady state during the times of the rotations. The rotations are then the most accurate when there are no transients in the solar wind. In contrast, the fact that we use data measured at the Earth and the Stereo spacecraft as inputs implies that any transients measured by those spacecraft are present in the propagations. However, transients are generally confined to a limited range of longitudes. So, while we do model solar wind transients, these transients are most accurately modeled only near the time of alignment of the two bodies.
  6. What is contained in the full resolution files?

    Full resolution files are the direct output of the solar wind simulations. They contain ASCII data for approximately 1 orbital period centered on the apparent opposition date. The file is not exactly one year due to the rotations to and from the inertial line on which we make the propagations. Typical time steps in these files is several seconds. As with all mSWiM results, the propagations are most accurate near the apparent opposition date.
  7. What is contained in the hourly average files?

    Hourly resolution files are generated from the full resolution files. Each file contains the solar wind propagation averaged over hourly bins as ASCII data for 1 calendar year. This means that multiple full resolution files are combined together to make this product. Files are merged at times corresponding to 180-degree separation in helioecliptic longitude from apparent opposition. Therefore solar wind predictions in the date range ± one or two months of the 180-degree longitudinal separation should not be used for correlative studies. They are, however, suitable for things like statistical studies of solar wind conditions.
  8. What does the term Recurrence Index mean?

    The recurrence index of solar wind speed is simply the cross correlation of the observed solar wind speed during one solar rotation with the solar wind speed in the preceding rotation. A high recurrence index indicates that the 1-D approximation that we make and the required rotations of input and output data are most appropriate and therefore the propagations are most accurate. Low recurrence index indicates the opposite. Thus the highest prediction efficiency is expected during the late declining phase of the solar cycle when long-lived high-speed recurrent streams dominate the corona in the ecliptic plane.
  9. What does the number for Solar Wind Coverage in the hourly average files mean?

    In order for the simulation to run, we must have continuous solar wind conditions to use as input at the inner boundary. When there are gaps in the data, we therefore must interpolate to obtain these input values. Large data gaps in the boundary conditions that can seriously influence the accuracy of the solar wind predictions. A data coverage of 80% or higher is essential to ensure reliable solar wind prediction.

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