Study on the Impact ofCombined Magnetic and Electric Field Analysis

and of Ocean Circulation Effects on Swarm Mission Performance

byS. Vennerstrom, E. Friis-Christensen, H. Lühr

T. Moretto, N. Olsen, C. Manoj, P. Ritter,L. Rastätter, A. Kuvshinov, S. Maus

Study Organization

 

  Ocean circulation study

External field study

Task 1:Forward modeling DSRI and GFZ

DSRI in cooperation with the Community Coordinated Modeling Center (CCMC)

Task 2:Inversion based on the Swarm constellation

GFZ GFZ

Sources to the near Earth magnetic field

• Internal magnetic field ~ 98%• External magnetic field < 2%

– Current systems in the ionosphere and magnetosphere,generated in the interaction with the sun and the solar wind.

Highly time-variable!

External Field Study - Research Objectives

• To what extent can the Swarm constellation be used to determine the external electric currents, and thereby to recover the external magnetic signal ?

(i.e. separate this from the internal contributions).

• Can the combined magnetic and electric field measurements be utilized in this effort?

Presentation overview

• Forward modeling– Presentation of the model used– Development of algorithm for magnetic field computation– Simulation results

• Inversion– Field-aligned currents– Ionospheric currents– Activity indices based on the satellite data.

• Suggestions for further studies

Currents systems generated in the interaction with the solar wind

UCLA Geospace General Circulation Model (GGCM)Global MHD simulation of the magnetospherecombined with an ionospheric model for FAC closure

Developed at UCLA, Raeder et al., 1998

The ionospheric model

Two-dimensional spherical shellat 90 km’s altitude:

Conductivity determined by-solar UV (through F10.7)-electronprecipitation (through magne-tospheric T and N, and FAC)

The 3D-current density distribution

The outer magnetosphere: The inner magnetosphere:

The ionosphere:

Algorithms for magnetic field computation

• Poloidal/toroidal decomposition– Works on any distribution of J, provided that

divJ=0– Very fast, computes B at the whole grid in a few

minutes

• Direct Biot-Savart Integration– Works on any distribution of J, including separat

parts of the distribution– Present implementation very slow, computes B at

the grid at swarm altitudes in 6 hours

Used for data-processing

Used for testing and estimating relativesize of individual contributions

Test of magnetic field computation Comparing results of the two methods

Selection of solar wind input

Three Selected Model Runs

1. IMF Bz changes slowly from 5nT – -5nT• All other parameters kept constant.• IMF By = 0, Dipole tilt angle = 0 (equinox)

2. Identical to 1. except for the tilt angle• Dipole tilt angle = -26° (summer/winter)

3. Real event• IMF Bz and By varies roughly between 5 and

–5 nT, Dipole tilt angle -16 °• Kp varies between 0 and 3-

The main contributionIonospheric and field-aligned currents

in the polar ionosphereCCMC run 1

Estimated magnetic field at swarm altitudesin the polar region, CCMC run 1

Estimated electric and magnetic fields compared

Input solar wind – real event

Real event – Comparison with observationsNorthern hemisphere

Real event – Comparison with observationsSouthern hemisphere

Ionospheric Conductivity

Summary of the forward modelling

• We have developed and implemented an algorithm for fast computation of the magnetic field due to a general distribution of current density on a spherical grid.

• We have performed three runs of a state-of-the-art model of solar wind interaction processes and calculated on this basis the 3-D distributions of current, and magnetic and electric field.

• We have compared the simulation results with observations with good results.

• Due to the highly variable ionospheric conductivity there is no 1-1 correspondance between electric and magnetic field, however quiet intervals can be distinguished by the global pattern (in the polar region) of both fields.

Presentation overview

• Forward modeling– Presentation of the model used– Development of algorithm for magnetic field computation– Simulation results

• Inversion– Field-aligned currents– Ionospheric currents– Activity indices based on the satellite data.

• Suggestions for further studies

Determination of field-aligned current density j along track, using two satellites

Single-satellite approach:

Simulated and recovered field-aligned currents compared

One satellite Two satellites

One satellite Two satellites

One satellite Two satellites

Method for determining ionospheric currents as observed by low-altitude polar orbiting satellites

High precision measurements necessary

F

J

No E-field information E-field ”measurement”included

New auroral region index for improved data selection

1. Local determination of field-aligned current density (correlated with residuals in field intensity F)

2. Along track integration of field-aligned current density, weighted by the cosine to the angle between the track and the electric field. (Proxy for the Pedersen current along track).

Two attemps:

Magnetic perturbation from a field-aligned current filament

assuming homogeneous conductivity

Orbit by orbit correlation between peak values of jpar and F

For quiet conditions…

..and including more activity

Test of the concept against CHAMP-data

Comparison with CHAMP data

Simulated data: Equinox

CHAMP data:Winter conditions

Summary of Task 2, inversion.

• It has been demonstrated that the Swarm constellation provides for the first time an excellent opportunity for deriving field-aligned currents uniquely. FAC are important for the Science Objectives: Magnetospheric and ionospheric current systems and Upper atmospheric dynamics.

• Tools have been developed (but not completed to full satisfaction) to estimate the along track ionospheric currents, including the position and intensity of the auroral electrojets. This information is needed in high-resolution lithospheric field recovery (Maus et al., 2004).

• A polar region activity index has been suggested which could be of importance for the selection of quiet polar passes. This may help to improve the accuracy of magnetic field models.

Suggestions for further studies

1. Global pattern of field-aligned and ionospheric currents in the polar region.

2. Space weather model validation and development.

3. Night-time ionospheric currents at mid- and low latitude.

Global pattern of field-aligned and ionospheric currents in the polar region.

•Is it possible to parametrize the FAC and ionospheric current system based on parameters such as intensity, width and location of the separate parts? (Region 1, Region 2, NBZ)•Can these parameters be retrieved by the Swarm magnetic and electric field components? Which additional data will help?

Space weather model validation and development.

•Which factors are most important for differencies between models and observations? •Can the Swarm data be used systematically to drive space weather prediction models?

Night-time ionospheric currents at mid- and low latitudes.

• Recently observational evidence has been found for the occurence of F-region ionospheric currents in the equatorial region at the night-side. (Lühr et al. 2002)

• The currents are associated with plasma instabilities and their effect in the satellite data is of the order of 5 nT.

• These currents are important in connection with Swarm because night-side data are used in internal field studies in order to minimize the effect of ionospheric currents.

• A comprehensive study of night time mid- and low latitude ionospheric currents, in order to optimize the use of the Swarm data is suggested.