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STEREO Observations of Langmuir Waves and IonEvents in the Terrestrial Foreshock

M. Pulupa, S.D. Bale, R.P. Lin, J. Luhmann, D.E. Larson,R. Ergun, K. Goetz, M. Kaiser, M. Maksimovic

STEREO Science Working Group Meeting20-22 April 2008

Meudon, France

Outline

Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Conclusions andFuture Work

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Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Conclusions and Future Work

The Foreshock

Introduction

The Foreshock

Instruments

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Conclusions andFuture Work

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• The foreshock is the regionupstream of Earth’s bowshock that is magneticallyconnected to the shock.

• Particles in the interplan-etary medium travel alongfield lines and can be mir-rored by the bow shock.

• The reflected beams causebump-on-tail velocity distri-butions and generate Lang-muir wave emission.

(Filbert and Kellogg 1979 andmany others)

Instruments

Introduction

The Foreshock

Instruments

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Conclusions andFuture Work

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STE:

• 2-100keV• Solid state detector• Electrons, ions, neutrals• 4 look directions, down-

stream and upstream (STE-U is not working)

XGSE

YGSE

D0

D1

D2

D3Sun

Nominal Parker spiral

SWEA:

• 8-2000 eV• Electrostatic detector• Nearly 4π angular resolution

Late 2006: Petal Orbit

Introduction

Early STEREO Orbit

Late 2006: PetalOrbitEarly 2007:STEREO/Ahead inthe Foreshock

Electron Foreshock

Ion Foreshock

Conclusions andFuture Work

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Early 2007: STEREO/Ahead in the Foreshock

Introduction

Early STEREO Orbit

Late 2006: PetalOrbitEarly 2007:STEREO/Ahead inthe Foreshock

Electron Foreshock

Ion Foreshock

Conclusions andFuture Work

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Electron Foreshock

Introduction

Early STEREO Orbit

Electron Foreshock

Electron Event

Electron spectra

Distribution ofelectron events

Ion Foreshock

Conclusions andFuture Work

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Electron Foreshock Event

Intense Langmuir waves at fp accom-panied by STE and SWEA electronsindicate foreshock electron beam.

SWEA PADs

STE PADs

Energy spectra for each STE lookdirection. Electron beam is visible inSTE D0 and D3.

Electron spectra

Introduction

Early STEREO Orbit

Electron Foreshock

Electron Event

Electron spectra

Distribution ofelectron events

Ion Foreshock

Conclusions andFuture Work

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• Blue = Parallel, Red = Perpendicular.• Electron beam present in both SWEA and STE.• Pitch angle coverage limited by fixed look directions of STE.

Distribution of electron events

Introduction

Early STEREO Orbit

Electron Foreshock

Electron Event

Electron spectra

Distribution ofelectron events

Ion Foreshock

Conclusions andFuture Work

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Hundreds of events, no imme-diately obvious organization byspacecraft position.

Electrons continue to be ob-served months into 2007. (Theblank space is due to incompletecataloguing, not a data gap ora dearth of events.)

Ion Foreshock

Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Ion Foreshock

Ion spectra

Foreshock eventwith fine structureDistribution of ionevents

Conclusions andFuture Work

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Ions in the ULF Foreshock

Strong ULF fluctuations

Only intermittent Langmuirwave activity

Monoenergetic, mostlyisotropic beam from around10-80 keV suggests diffuseions rather than electrons.Note anticorrelation be-tween peak flux and peakwave activity.

Field aligned ion beam orelectrons near beginning ofevent?

(Mazelle 2002 and others)

Ion spectra

Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Ion Foreshock

Ion spectra

Foreshock eventwith fine structureDistribution of ionevents

Conclusions andFuture Work

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• Blue = Parallel, Red = Perpendicular.

Ion Foreshock Event with FineStructure

Evolution of lower and upper energycutoffs, at times upper limit of ener-gies reaches ≥ 100 keV

Same general structure seen in allSTE look directions, but there issome difference in intensity.

No plasma wave activity

Magnetic field mostly dominatedby Bx with By and Bz fluctuatingaround 0.

No evidence of electrons

Distribution of ion events

Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Ion Foreshock

Ion spectra

Foreshock eventwith fine structureDistribution of ionevents

Conclusions andFuture Work

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Light blue dot is an ion event,dark blue dot is an ion eventwith associated ULF activity.

Ion events continue, but ULFevents die out. Issue of distanceversus depth.

Conclusions and Future Work

Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Conclusions andFuture WorkConclusions andFuture Work

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• STE and SWEA see many examples of electrons and ionsupstream of the foreshock. STEREO/A continues to observeupstream particles well into 2007.

• STE can measure the energy spectrum of foreshock electronbeams with good sensitivity and energy resolution. However, itis limited by incomplete pitch angle coverage.

• STE also sees ion events, including ULF events. The strengthand frequency of ULF events decreases as STEREO/A movesfarther from the bow shock, but it is unclear whether this isdue to distance or depth in the foreshock.

• Put observations in magnetic coordinates (organized by θBN

and depth in foreshock) rather than GSE coordinates. A betterbow shock model is necessary.

• Resolve ambiguous events and use current SWEA data.

Introduction

Early STEREO Orbit

Electron Foreshock

Ion Foreshock

Conclusions andFuture WorkConclusions andFuture Work

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Thank you!