STUDIES OF GEOMAGNETIC PULSATIONS USING MAGNETOMETER DATA FROM THE

CHAMP LOW-EARTH-ORBIT SATELLITE AND GROUND-BASED STATIONS

Peter R Sutcliffe

Hermanus Magnetic Observatory (HMO), South Africa

Hermann Lühr

Helmholtz Centre Potsdam – GFZ, Germany

Balazs Heilig

Tihany Geophysical Observatory, Hungary

Pulsation Studies using CHAMP Satellite and Ground-based Magnetometer Data

Geomagnetic pulsations

Observations of Pi2 pulsations at low latitudes

Observations of Pc3 pulsation field line resonances

Objective:In this presentation we show how research on geomagnetic pulsations can be carried out using magnetic field data from a low-Earth-orbit (LEO) satellite and ground-based stations. We also illustrate the value and importance of high quality magnetometer data, i.e. good resolution and accurate timing, for pulsation studies.

Schematic of the Earth’s magnetic field represented by:• (a) Dipole field• (b) The relationship to a box model of the magnetosphere• (c) Perturbations of field and plasma in a shear Alfvén wave• (d) Compressional fast mode wave.

Geomagnetic pulsationsGeomagnetic pulsations are the magnetic signatures of ultra low frequency (ULF) waves in the Earth’s magnetosphere.

These oscillations have short periods (usually of the order of seconds to minutes) and small amplitudes (usually less than one part in 104 of the Earth’s main field).

ULF waves serve as extremely useful and powerful diagnostics of the Earth’s magnetosphere; consequently, they can play an important role in space weather forecasting.

(a): Plots of the perturbation electric and magnetic fields versus distance along the field line from one ionosphere to the other.(b) & (c): Schematic illustrations of the field displacements in a fundamental (top) and second harmonic (bottom) of the poloidal and toroidal field-line resonant modes. Dashed lines are the displaced field lines.

(a) (b) (c)

Poloidal and Toroidal field-line resonant (FLR) modes

Ionospheric shielding and rotation

Schematic model showing the field-aligned and ionospheric currents for a shear Alfvén wave incident upon the ionosphere. The ionosphere affects the pulsation signal observed on the ground:• Shields the magnetic field (due to field aligned currents).• Rotates the signal by 90º.• Attenuates short scale oscillations.

Geomagnetic pulsations may be divided into two broad classes based on their observed characteristics.

The first class, of a regular and mainly continuous character, are known as Pc:

Classes of Geomagnetic Pulsations

The second main class, characterized by their irregular form, are known as Pi:

Classification Period Range (sec)

Pi 1 1 - 40

Pi 2 40 - 150

Classification Period Range (sec.)

Pc 1 0.2 - 5

Pc 2 5 - 10

Pc 3 10 - 45

Pc 4 45 - 150

Pc 5 150 - 600

The CHAMP satellite was launched on 15 July 2000 into a near polar circular orbit at an initial altitude of 454 km. The magnetic field measurements from CHAMP are of unprecedented accuracy and resolution.

The orbit, which lies roughly a similar distance above the E-region ionosphere as ground stations are below, provides a unique opportunity to study geomagnetic pulsations and the effects of the ionosphere on their propagation.

Data sources: CHAMP LEO magnetometer data

The magnetic field observations from CHAMP have a particular advantage over ground-based data for pulsation studies:• It is difficult to discriminate between shear Alfvén and fast mode waves using ground-based data, since both manifest themselves in the H (magnetic north) component, due to the effects of the ionosphere.• An advantage of using LEO satellite data is that the two wave modes manifest themselves in different components above the ionosphere, so that it is easier to discriminate between them.

Data sources: Ground-based magnetometer data

Data from ground-based meridian chains of magnetometers in South Africa and at the approximately conjugate MM100 stations in Europe are used for our investigations.

Geographic coordinates AACGM coordinates* Station

Latitude Longitude Latitude Longitude

THY 46.90° N 17.89° E 42.44° 92.39°

BEL 51.83° N 20.80° E 48.01° 96.15°

TAR 58.26° N 26.46° E 54.80° 103.04°

NUR 60.52° N 24.65° E 57.23° 102.34°

HAN 62.30° N 26.65° E 59.01° 104.78°

KIL 69.02° N 20.79° E 66.10° 104.00°

SUT 32.40º S 20.67º E -41.34º 84.61º

HER 34.43º S 19.22º E -42.57º 82.16º

Pulsation Studies using CHAMP Satellite and Ground-based Magnetometer Data

Geomagnetic pulsations

Observations of Pi2 pulsations at low latitudes

Observations of Pc3 pulsation field line resonances

Illustration of plasma flow within the magnetosphere (convection) driven by magnetic reconnection. The inset shows the positions of the feet of field lines in the northern high-latitude ionosphere and the corresponding plasma flows, an antisunward flow in the polar cap, and a return flow at lower latitudes.

Convection in the Earth’s magnetosphereDynamic processes in the magnetosphere-ionosphere system are largely controlled by the interplanetary magnetic field (IMF). In particular, activity is strongly dependent on the strength and orientation of the transverse y and z components of the field. When the IMF points southward, the system is dominated by Dungey cycle flow and substorm activity. Recently, however, there has been increasing interest in activity associated with northward IMF.

Model of substorm expansion phase onset and Pi2s

Schematic diagram of the modified near-Earth neutral line substorm model [Shiokawa et al., 1998].

The braking of high-speed ion flows in the near-Earth central plasma sheet, at the boundary between regions of dipolar and tail-like field, produce the substorm current wedge and compressional pulses, which lead to Pi2 pulsations at high and low latitudes respectively.

Characteristics of Pi2 PulsationsPi2 pulsations are impulsive, damped geomagnetic field oscillations, which occur at the time of magnetospheric substorm onsets and intensifications.

At high latitudes, Pi2 pulsations are associated with the “switch on” of the substorm current wedge and are observed only close to local midnight.

However, at low latitude ground stations, Pi2 pulsations are observed at all local times at night and often observed during local daytime.

The diagram shows plots of a Pi2 pulsation observed at the:• Night-side high-latitude (Hornsund: 77.0° N; 0232 LT)• Night-side low-latitude (Hermanus: 34.4° S; 0247 LT)• Dayside low-latitude (Kakioka: 36.2° N; 1051 LT).

Substorm associated Pi2 pulsations: At high latitudesAt high latitudes, Pi2s are associated with the “switch on” of the substorm current wedge by the short-circuiting of the cross-tail current to the auroral oval via field-aligned currents.

Diversion of the cross-tail current through the midnight ionosphere to form the substorm current wedge [Ritter and Lühr, 2008].

Ground and space-based observations of a Pi2 pulsation; X-component at Abisko (upper curve) and By component at the TC-1 satellite (lower curve) [Wang et al., 2008].

Data selection and processing to study low latitude Pi2s• Times when CHAMP was located within 30° of longitude of Hermanus and at latitudes lower than 50° were selected.

• The Hermanus induction magnetometer data were scanned for Pi2 pulsations.

• CHAMP fluxgate magnetometer data were then checked for Pi2s.

The satellite vector data were rotated into a field-aligned coordinate system, i.e. compressional (Bcom) component aligned in the ambient magnetic field direction (positive North), toroidal (Btor) component in the azimuthal direction (positive East), and poloidal (Bpol) component in magnetic meridian plane (positive inward).

Crustal anomaly effects on satellite dataWe found it necessary to subtract a lithospheric magnetic field anomaly model from the data (to remove variations in the Pi2 frequency band).

Lithospheric magnetic field anomaly model for the southern Africa region.

Comparison of CHAMP compressional component, before and after removal of the lithospheric field respectively, and the HER H-component.

CHAMP observation of southern hemisphere Pi2

(a)Nightside Pi2 pulsation observed on 2002-05-17 showing the good correlation between the CHAMP compressional and poloidal components and the Hermanus H component.

(b)Complex demodulates of the poloidal and H components.

(a) (b)

H

D

H

(a)Nightside Pi2 pulsation observed on 2002-02-02 showing the good correlation between the CHAMP compressional and poloidal components and the Hermanus H component.

(b)Complex demodulates of the poloidal and H components.

(a) (b)CHAMP observation of northern hemisphere Pi2

H

D

- H

More observations of nighttime Pi2s

(a) (b)

Observations of substorm related Pi2 pulsations on the CHAMP satellite (solid line) and the near-conjugate ground stations HER (dashed line) and THY (dotted line) when (a) CHAMP was in the southern hemisphere and (b) CHAMP was in the northern hemisphere [Sutcliffe and Lühr, 2003].

Magnetic meridian view of the field line configuration of a cavity mode oscillation in a box-model magnetosphere at two epochs half an oscillation period apart [Takahashi et al., 1995].

Model for low-latitude Pi2 pulsations

The CHAMP and ground-based observations of nighttime Pi2s are indicative of a cavity mode resonance and their spatial phase structure is consistent with a compressional cavity mode resonance as proposed by Takahashi et al., [1995].

Pi2 pulsations are often observed during local daytime at low-latitude ground stations [Sutcliffe and Yumoto; 1989, 1991]. However, it is not clear whether or not daytime Pi2 pulsations are observed in space. Consequently, we used CHAMP magnetometer data to search for the occurrence of daytime Pi2 pulsations in the F-region ionosphere.

Events were selected for times when CHAMP and HER were located in the dayside hemisphere and KAK observed Pi2s located on the nightside.

A search for daytime Pi2s in CHAMP data

Observations of oscillations on 4 Dec 2004 when CHAMP (1203 LT) and HER (1422 LT) were located in the dayside hemisphere and KAK (2226 LT) was located on the nightside. (a)Clear Pi2 pulsations are observed at HER (dashed lines) and KAK (dotted lines). The oscillation at CHAMP (solid lines) do not appear to match the oscillations at HER or KAK.(b)& (c) This is confirmed by plots of the spectral characteristics.

A search for daytime Pi2s in CHAMP data

We regularly observed Pi2 pulsations during local daytime at HER and KAK; however, to date, we have not been able to clearly identify Pi2 pulsations above the ionosphere using CHAMP magnetometer data.

Summary and conclusions of Pi2 study

The correlation between satellite and ground Pi2s is improved by subtracting a lithospheric magnetic field anomaly model from the satellite data.

The H-component signal on the ground is correlated with the compressional (Bcom) and poloidal (Bpol) components above the ionosphere. This correlation is indicative of a cavity mode resonance.

In the southern hemisphere Bcom and Bpol oscillate in phase with H, while, in the northern hemisphere Bpol oscillates in anti-phase with Bcom and H. The spatial phase structure is consistent with a compressional cavity-mode resonance [Takahashi et al., 1995].

Although daytime Pi2 pulsations are regularly observed on the ground, we can find no convincing evidence of their occurrence in CHAMP data. Consequently, we conclude that Pi2 pulsations on the dayside differ from their night-time counterparts.

Pulsation Studies using CHAMP Satellite and Ground-based Magnetometer Data

Geomagnetic pulsations

Observations of Pi2 pulsations at low latitudes

Observations of Pc3 pulsation field line resonances

• Frequencies: 25-100 mHz

• Amplitudes: 0.1-1.0 nT

• Sources: UWs and FLRs

Characteristics and sources of Pc3 Pulsations

Pc3 pulsations are the pulsation type most commonly observed during local daytime at low to middle latitudes.

Field line resonances (FLRs) and frequency determination

FLRs are transverse standing Alfvén waves on geomagnetic field lines, that is, equivalent to the concept of a vibrating field line fixed between the ionospheres in opposite hemispheres.

ωFLR = ω (VA, field line length) where VA = B/(μo ρ)½

Eigen frequency determination using amplitude ratio or difference method [Baransky et al., 1985] and phase difference method [Waters et al., 1991]

Example of FLR frequency determination

Example of Pc3 FLR frequency determination using amplitude difference and phase difference methods from SUT and HER induction magnetometer data.

SUT

HER

Accurate timing essential for studies of FLRsExamples where there is a timing error in the data from one of the stations (SUT)

Time correct Time error = -10 sec

Time error = 33 sec Time error = -97 sec

Data selection for Pc3 FLR study

Map of the southern African region showing the CHAMP ground-track as it traversed the region during a Pc3 pulsation on 15 Feb 2003. The locations of the HER and SUT ground stations are also shown.

• Times when CHAMP was located within 20º of longitude of Hermanus and between latitudes 10º and 60º S were selected.

• Ground and satellite data were then scanned for Pc3 pulsations.

Dynamic spectra of the Pc3 pulsation H-component observed on the ground. Top panel: dynamic log power at HER, Middle panel: amplitude difference and bottom panel: phase difference between HER and SUT.

Dynamic FFT spectra for a Pc3 event

Hermanus and CHAMP observations of a Pc3 pulsation

Pc3 pulsation observed on the ground at Hermanus and along the CHAMP trajectory on 15 Feb 2003.

MESA dynamic spectra for magnetic field components observed at Hermanus and CHAMP on 15 Feb 2003. Axes for geocentric latitude and L-value along the satellite track are included,

MESA dynamic spectra

CHAMP (top panel) and ground (bottom panel) wave hodograms for three consecutive 20 second intervals at the time when the satellite was passing over the Hermanus ground station.

Polarisation hodograms

MESA dynamic spectra of L-dependent FLR oscillations

MESA dynamic spectra for magnetic field components observed at Hermanus and CHAMP on 13 Feb 2002.

Conclusions of Pc3 study

An analysis of Pc3 pulsations observed on the ground and on CHAMP shows that:

When a number of discrete frequency components are observed in the fast mode wave:

These drive field line resonances (FLRs) at latitudes where they match the field line resonant frequency. The toroidal mode observed on CHAMP experiences a Doppler frequency shift due to the rapid motion across the resonance region. Polarization hodograms in the resonance region clearly show the expected 90º rotation of the field line resonant magnetic field components.

When a broadband compressional source spectrum is observed in the fast mode wave:

Toroidal mode resonant oscillations with continuous L-dependent frequency are observed on CHAMP.

The combination of LEO satellite and ground-based magnetometer data provides an extremely powerful

research tool for the study of geomagnetic pulsations.

Consequently, we look forward to the launch of Swarm.