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Recipe for Initial Turning of Bz

A Decision Aid for determining the initial turning (northward or southward) of the IMF Bz at 1 AU following a solar disturbance, based upon the work by Chin-Chun Wu and Murray Dryer, as described below, and in the References page.

Initial turning of Bz - Decision Aid:
  • What was the heliographic latitude of the source of the disturbance with respect to the Earth?

Source was NORTH of Earth's helio-latitude
Source SOUTH of Earth's helio-latitude
Source and Earth at roughly the SAME latitude

  • What sector will the Earth be in at the time of arrival of the disturbance?

Toward (IMF B-vector pointing toward sun)
Away
Not sure

  • Get initial turning of Bz:

 

Description:

Wu and Dryer used a 3-D MHD numerical solar wind model to calculate the temporal evolution of the interplanetary magnetic field (IMF) at 1 AU (astronomical unit). They found that the initial turning of the IMF Bz component after a solar event at 1 AU could be predicted with better than 80% accuracy. (Southward Bz conditions in the IMF relate to increased geomagnetic activity.)

IMF Bz Turning: If the pre-event IMF at 1 AU Bz component is positive (i.e., pointing to the north), and the prediction for initial Bz-turning is "southward", this indicates i) a "less northward" IMF or ii) the IMF will actually turn to the south. Conversely, a prediction of "northward" turning when the pre-event IMF Bz is pointing south indicates the likelihood the IMF will become less southward, or ii) will become northward.

Based on these studies, Wu and Dryer have developed a recipe for predicting the initial turning of Bz, given the following information:

  1. The heliographic latitude of the source of the solar disturbance with respect to the Earth's latitude (North of, South of, or same).
  2. The location of the observer in the IMF (i.e., whether in a "toward" or "away" sector) prior to the geomagnetic storm sudden commencement (SSC). Presumably the SSC is roughly 1-2 days after the initiation of the disturbance.

Remarkably, additional information about the location of the heliospheric current sheet (HCS) is not important for predicting the initial turning, unless the Earth is near the HCS (i.e., near a sector boundary).

Notes:

  • "Please note that this "recipe" is intended to direct attention only to the prediction of the INITIAL turning of the Bz component when the interplanetary shock arrives at Earth. Prediction of Bz(t), thereafter, requires application of the full 3D MHD code that is given in the References." - Murray Dryer
  • This is a research tool and has not yet been validated for operational use. - expi
 

Copyright 2006, Exploration Physics International, Inc.