Investigation of Field-aligned Currents Onboard of Interkosmos Bulgaria-1300 Satellite

Dimitar Danov

Solar-Terrestrial Influences Laboratory,

Bulgarian academy of sciences

Introduction

BULGARIA-1300 SATELLITE

Launch dateLaunch date: : 07-08-1981 07-08-1981 Termination dateTermination date::    17-02-1983 17-02-1983

Perigee (km)Perigee (km):: 825  825  Apogee (km)Apogee (km):: 906 906

Inclination (°)Inclination (°)::    81.2  81.2  EccentricityEccentricity:: 0.005 0.005

Spin rate (rpm)Spin rate (rpm):: 00

Introduction

BULGARIA-1300 SATELLITEBULGARIA-1300 SATELLITE

Triaxial Fluxgate Magnetometer

MagneticMagnetic field component field componentss BBxx, B, Byy, B, Bzz

Dynamic range Dynamic range ±64000 nT ±64000 nT

Sensitivity Sensitivity 5 nT5 nT

Time sampling Time sampling 8080 or or 320 ms 320 ms

Number of Fine scales Number of Fine scales 256256

Fine scale (Dynamic range) Fine scale (Dynamic range) ~~500 nT500 nT

Introduction

Reason for …Reason for …

The statistical picture of large-scale field-aligned current (FAC) distribution has been obtained by Iijima and Potemra [1976] and by many satellite observations including latest Iridium results (Waters et al. [2001]).

At the same time it is well known that large-scale field aligned current sheets are frequently stratified on multiple sheets (see Kamide and Rostoker [1977]). Antonova et al. [1998] discussed the mechanisms of such stratification.

Investigations of the stratifications of large-scale field-aligned Investigations of the stratifications of large-scale field-aligned current bands on multiple field-aligned current sheets were current bands on multiple field-aligned current sheets were limited by the assumption of the existence of infinite limited by the assumption of the existence of infinite homogeneous current sheetshomogeneous current sheets. However, this assumption is valid . However, this assumption is valid when the width of the current sheet in longitudinal direction when the width of the current sheet in longitudinal direction exceeds significantly its size along the meridian.exceeds significantly its size along the meridian.

This allowed reducing the three-dimensional problem to one-This allowed reducing the three-dimensional problem to one-dimensional one and deriving FAC’s structure from one-satellite dimensional one and deriving FAC’s structure from one-satellite

measurementsmeasurements.

IntroductionReason for …Reason for …

We try to evaluate the corrections considering a finite size of We try to evaluate the corrections considering a finite size of the sheet in the longitudinal direction and show the efficiency of our the sheet in the longitudinal direction and show the efficiency of our methodology using Interkosmos–Bulgaria-1300 magnetic field methodology using Interkosmos–Bulgaria-1300 magnetic field

measurements.measurements.

We identify current sheets in magnetic field data “manually” We identify current sheets in magnetic field data “manually” by the following algorithm:by the following algorithm:

1. Process the telemetry and obtain three magnetic components in satellite coordinate system

2. Subtract the model IGRF geomagnetic field from the

measured field

3. Transform the magnetic vector to three components in spherical Solar Magnetic (SM) coordinate system

4. Analyse the graphs of these components to identify the sheets

5. The value (and sign) of the current density is evaluated by a simple program

In the past we have done all these steps automatically, but we obtained many false currents.

Whenever the current density is estimated from the curl(B) the following approximation is very popular

curl(B) ~ By/x ~ By/Vxt

Sources of errors ??Sources of errors ??

MODELS SELECTEDMODELS SELECTED

We use two models of current sheets, to understand possible

errors :

1.1. Model of a finite plane current sheetModel of a finite plane current sheet

current sheet in X,Y directions with a rectangular form and constant current density inside the rectangular:

• j=const if a1 x a2, b1 y b2 and

• j=0 outside this interval.

2.2. Conical model (Conical model (used byused by Tsyganenko 2001) Tsyganenko 2001)

current sheet is defined inside the spherical cone :

• j=const if a1 a2 and

• j=0 outside this cone.

Model of a finite current sheetModel of a finite current sheet

The normal (meridional) component of the magnetic field disturbance for such configuration is symmetric with respect to the tangential surface of symmetry and antysymmetric with respect to the normal one.

The tangential (zonal) component is antysymmetrical with respect to the tangential surface of symmetry and symmetrical with respect to the normal one.

Model of a finite current sheet Model of a finite current sheet One sheet, s/c trajectories are normal to it

Changes in the tangential and normal components of the magnetic field corresponding to the field-aligned current sheet shown at the top, the satellite orbit is perpendicular to the sheet.

Trace - 1near the ‘center’ of current sheet

Trace - 3near the ‘termination’ of current sheet

As seen from graphs on this slide, By/x approximation and the possible limited size of the current in Y-direction can be a cause for the appearance of ‘false’ currents.

Model of a finite current sheet Model of a finite current sheet One sheet, s/c trajectories are normal to it

Trace - 1near the ‘center’ of current sheet

Trace - 3near the ‘termination’ of current sheet

the current sheet

False current sheet with

By/x >0

Another false current sheet

By/x >0

Model of a finite current sheet Model of a finite current sheet Two sheets, trajectories are normal to it

Trace - 1 Trace - 2

Trace - 3

The value of the current density is equal in both sheets, but their directions are opposite.

In case of TRACE 1 & 2 one can estimate equivalent (and opposite) current densities.

NO false currents

In case of TRACE 3 one can estimate different (and opposite) current densities.

AS WELL false currents

Model of a finite current sheet Model of a finite current sheet Very complicated case

It is good that B-1300 satellite has such It is good that B-1300 satellite has such Inclination,Inclination,

and and cuts across FAC sheets near it’s normal.cuts across FAC sheets near it’s normal.

Cross with fair approach to normal Cross with fair

approach to tangent

Cross with big angle

Obtaining of ‘false’ currents sheets can be demonstrated also with the conikal model.

Conical model (Conical model (used byused by Tsyganenko Tsyganenko in his work fromin his work from 2001 2001))

This model is defined in spherical coordinate system. It is complicated but self consistent.

outside the interval a1 a2

and

2

1

)(

m

mr jj

)cos(1)sin(1)cos()sin(

)sin().sin(.

1cos

)sin()(sin21

1

02

)(

mB

rj

m

m

mm

where

in the interval a1 a2

),0,0( rjj

0,0,0j

Conical model (Conical model (used byused by Tsiganenko 2001 Tsiganenko 2001))

Formulas for meridional and zonal components of the magnetic field are to long, for that reason we will show only graphs.

Trajectory provisionally parallel to 5 MLT

zonal

meridional

the current sheet False current sheet with

By/x <0

False current sheet with

By/x <0

The X-axis represents the distance (in percents) from satellite trajectory to symmetry surface (100%)

1. The calculated current density is always less than

the real one; the By/x approximation works well only in case when s/c crosses the sheet near its middle;

2. There are cases when the satellite does not cross the sheet, but the change in the tangential component is significant – one can obtain false current.

In the model of finite current sheet we evaluate the difference between the value of the modeled tangential component of the magnetic field at both edges of the sheet.

Magnetic data & Field Aligned CurrentsMagnetic data & Field Aligned Currents

The procedure we use to avoid obtaining false currents:

We calculate the currents only for those measurements which:

1. The graph of the longitudinallongitudinal component Bis linear;

B between both edges of the linear interval is greater than 50nT;

max(B) – min(B) < 30nT (for the meridional component);

4. The correlation coefficient between B and is greater 0.9

5. j = 0 -1 Bcoefficient of the linear regression B to

Magnetic data & Field Aligned CurrentsMagnetic data & Field Aligned Currents

Seance 217. Midnight satellite trajectory. Series of FACs are present.

Tsyganenko-2001 model field is plotted with symbols. Tsy-2001

c/ sheet

Tsy-2001 c/ sheet

Magnetic data & Field Aligned CurrentsMagnetic data & Field Aligned Currents

Seance 183. Midnight satellite trajectory. FAC is directed into ionosphere.

Tsyganenko-2001 model field is plotted with symbols.

Tsy-2001 c/ sheet

Tsy-2001

c/ sheet

Statistics of current sheets

FAC density thickness distribution measured aboard ICB-1300 satellite

It is possible to see that more than 60% of observed current sheets

have thickness less than 1.50.

Conclusions and discussionConclusions and discussion• The conducted analysis shows that both components (meridional and

zonal) of the magnetic field must be taken into account when estimating the FAC density and position, to eliminate the false groups of sheets.

• Estimated from measurements FAC density is always smaller than the real

one, up to 25% at the end of the sheet. • The FAC density is greater and thickness is smaller than usually accepted.

We compared the results of our analysis with Tsyganenko [2002] and

Papitashvili et al. [2001] models and results of Lukianova et al. [2001]. It is possible to mention that Intercosmos-Bulgaria-1300 observations show that FAC density is greater and thickness is usually smaller, than in these works.

Unfortunately we have no sufficient data to investigate the dependence of FAC's density or thickness on other parameters like Kp, Dst or Solar Wind parameters

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