Presentation is loading. Please wait.

Presentation is loading. Please wait.

DEFINITION, CALCULATION, AND PROPERTIES OF THE Dst INDEX R.L. McPherron Institute of Geophysics and Planetary Physics University of California Los Angeles.

Similar presentations


Presentation on theme: "DEFINITION, CALCULATION, AND PROPERTIES OF THE Dst INDEX R.L. McPherron Institute of Geophysics and Planetary Physics University of California Los Angeles."— Presentation transcript:

1 DEFINITION, CALCULATION, AND PROPERTIES OF THE Dst INDEX R.L. McPherron Institute of Geophysics and Planetary Physics University of California Los Angeles Presentation at GEM 1998 Workshop Snowmass, Colorado June 15-19, 1998

2 MINOR MAGNETIC STORM RECORDED AT SAN JUAN - 11/24/96

3 INTERPLANETARY MAGNETIC FIELD, AE AND Dst INDICES DURING STORM n Coronal mass ejection produce intervals of strong southward Bz at the earth n Magnetic reconnection drives magnetospheric convection n Convection drives currents along field lines and through ionosphere n Ground magnetometers record effects of ionospheric currents in H and other components n H traces are used to construct the AE and Dst index

4 GEOGRAPHIC COORDINATES USED IN MAGNETIC MEASUREMENTS n Dipole is tilted and inverted relative to rotation axis n Dipole field lines are nearly vertical above 60  latitude n Cartesian geographic coordinates are defined in a plane tangent to earth at observer’s location n X component is towards geographic north pole n Y component is east along a circle of latitude n Z component is radially inward or down

5 LOCAL VIEW OF VARIOUS COORDINATE SYSTEMS USED IN GEOMAGNETISM n Origin is located at observer n X points north, Y points east, Z points down in the local tangent plane n F is the total vector field n H is the horizontal projection of the vector F n D is the east declination of H from geographic north in tangent plane n I is the inclination of F below the tangent plane n X, Y, Z are the geographic Cartesian components of F

6 SCHEMATIC ILLUSTRATION OF EFFECTS OF RING CURRENT IN H COMPONENT Projection of a uniform axial field onto Earth’s surface Magnetic effects of a symmetric equatorial ring current

7 MAGNETIC EFFECT OF A RING CURRENT AT EARTH’S CENTER n Axial field from a circular ring current n Field at center of ring n Convenient units X Westward Ring Current LRReLRRe Z

8 LONGITUDINAL PROFILE OF  B j FROM MAGNETOSPHERIC CURRENTS n Symmetric ring should create nearly constant longitudinal profile in H component n Local time average of  H at equator approximates  B at center of Earth n But other magnetospheric currents create local time dependent deviations from symmetry n Assume asymmetric component has zero mean when averaged over local time n Define the disturbance storm time index Dst as local time average of observed  H profile

9 DISTRIBUTION OF RING CURRENT AND ITS PERTURBATION IN A MERIDIAN n Most of the current is concentrated close to the equator n Eastward current inside and westward outside n Perturbations curl around the volume of current n The perturbation over the earth is nearly uniform and axial

10 THE SOLENOIDAL EFFECT OF THE RADIATION BELT CURRENTS n A more realistic model of the ring current n Shows the magnetic perturbations n Shows the distortion of dipole current contours n Perturbation field from ring current

11 DESSLER-PARKER-SCKOPKE DERIVATION

12 THE DESSLER-PARKER-SCKOPKE RELATION

13 CONTRIBUTIONS TO THE VARIATION IN THE H COMPONENT

14 CURRENTS CONTRIBUTING TO MIDLATITUDE MAGNETIC PERTURBATIONS n View is from behind and aabove earth looking toward Sun n Current systems illustrated –Symmetric ring current –Dayside magnetopause current –Partial ring current –Tail current –Substorm current wedge –Region 1 current –Region 2 current n Current systems not shown –Solar quiet day ionospheric current –Secular variation within earth –Main field of Earth

15 EFFECTS OF MAGNETOPAUSE ON THE Dst INDEX n Balance magnetic pressure against dynamic pressure X (Re) Z (Re) Solar Wind Neutral Point

16 A SHEET CURRENT MODEL OF EFFECT OF TAIL CURRENT ON Dst Xgsm (Re) Bz (nT) Normal Tail Inner Edge Total Earth n Tail Current Model n Magnetic Effects BzBz xxx xxx xxx RiRi RoRo

17 MAGNETIC EFFECTS OF A SUBSTORM CURRENT WEDGE n Transverse currents in the magnetosphere are diverted along field lines to the ionosphere n Viewed from above north pole the projection of the current system has a wedge shape n Midlatitude stations are primarily affected by field-aligned currents and the equatorial closure (an equivalent eastward current) n The local time profile of H component is symmetric with respect to the central meridian of wedge n The D component is asymmetric with respect to center of wedge

18 STEPS IN THE CALCULATION OF Dst INDEX n Define the reference level for H component on a monthly basis n Fit a polynomial to reference H values (secular variation) n Adjust H observed on a given day by subtracting secular variation n Identify quiet days from same season and phase of solar cycle n Remove storm effects in quiet values and offset traces so that there is zero magnetic perturbation at station midnight n Flag all values recorded during disturbed times and interpolate from adjacent quiet intervals n Create some type of smoothed ensemble average of all quiet days n Subtract average quiet day from adjusted daily variation to obtain disturbance daily variation for station n Repeat for a number of stations distributed around the world at midlatitudes n Project the local H variations to obtain axial field from ring current and average over all stations

19 ESTIMATION OF THE SECULAR TREND IN H COMPONENT AT SAN JUAN x 10 4 Year H (nT) Fourth Order Trend Daily Average 80% Point

20 REMOVAL OF SECULAR TREND FROM HOURLY VALUES OF H AT GUAM DURING STORM x 10 4 Observed H (nT) COMPARISON OF GUAM H WITH SECULAR TREND IN Day in 1986 Transient H (nT) DEVIATION OF GUAM H FROM SECULAR TREND IN 1986 Secular Trend

21 REMOVAL OF STORM EFFECTS IN QUIET DAY (Sq) ESTIMATION Day in Disturbance (nT) COMPARISON OF DETRENDED GUAM H TO MIDNIGHT SPLINE Residual H (nT) DETRENDED AND STORM CORRECTED GUAM H IN 1986 Midnight Spline H Comp

22 QUIET VALUES DURING STORM USED IN QUIET DAY (Sq) ESTIMATION Day in 1986 Transient H (nT) Flagged Point Quiet Value

23 Sq FOR H AT SAN JUAN IN 1978 AS FUNCTION OF DAY OF YEAR AND UT UT Hour Day of Year Diurnal Variation (nT)

24 QUIET GUAM H TRACE AT EQUINOX AND SOLSTICE 1986

25 COMPARISON OF SEVERAL OBSERVED AND PREDICTED QUIET DAYS AT GUAM IN Day in 1986 Disturbance (nT) Observed Quiet Residual

26 CORRECTED H AT GUAM DURING RECOVERY FROM A MAGNETIC STORM Day in 1986 Disturbance (nT) Quiet H Observed H Corrected H

27 DELTA H AT MIDLATITUDES DURING MAGNETIC STORM

28 MAJOR SUBSTORMS DURING MAGNETIC STORM OF APRIL 3-5, 1979

29 CONCLUSIONS n The Dst index is defined to be linearly proportional to the total energy of particles drifting in the radiation belts (symmetric ring current) n Dst must be estimated from surface measurements of the horizontal component of the magnetic field n Surface field measurements include effects of many electrical currents other than the symmetric ring current n These effects must be estimated or eliminated by the algorithm that calculates the Dst index n Extraneous currents include: secular variation, Sq, magnetopause, tail, Region 1&2, partial ring current, substorm current wedge, magnetic induction n There are numerous assumptions and errors involved in Dst calculations and the index contains systematic and random errors as a consequence Be aware of these problems and take them into account in interpreting Dst!


Download ppt "DEFINITION, CALCULATION, AND PROPERTIES OF THE Dst INDEX R.L. McPherron Institute of Geophysics and Planetary Physics University of California Los Angeles."

Similar presentations


Ads by Google