In Situ Measurements of Auroral Acceleration Regions Wu Tong

Auroral Acceleration Regions The direct cause of the aurora is the precipitation of energetic (keV) electrons and ions into the atmosphere leading to excitation of the ambient atmospheric gases which then emit light characteristic of their chemical composition. The auroral process is far more complex than the simple guiding of energetic charged particles from the sun to the atmosphere by the Earth's magnetic field lines. Precipitating electrons are accelerated in certain areas of the polar region.

History of discovery of AAR and field aligned E field The E field can not be measurements directly by instruments. We introduce parallel E field to explain the acceleration of electrons in upward current region. 1958: Balloon measurements of X-rays with energies up to 70 keV in association with active aurora implied that electrons were accelerated to large energies. 1960, 1974: Sounding rockets measurements to electron spectra. 1970: Optical observe of discrete arc indicated that there must be large (>1V/m) convergent E ⊥ at high altitudes. But such fields are not observed in the lower ionosphere. 1976: Upgoing ion beams observed by S3-3 satellites. 1976: Acceleration of an artificial Ba ion cloud observed. 1977: Direct Measurements of large (>1V/m) convergent E ⊥ at high altitudes by S3-3 satellites.

Processes in AAR is complex Electric field formation Particle acceleration & dissipation Different kinds of waves Wave-particle interactions I-M coupling ……

Roughly divide AAR into 3 regions by electric current structures Region 1,2 current system: km Upward current region: km Downward current region: 10-30km

Satellites that have provided important auroral in situ measurements

A FAST observation of AARs Green: downward current. Blue: upward current. Red: Alfvenic current.

About FAST Fast Auroral Snapshot (FAST) A Recursive acronym (e.g. Linux is not UNIX) Launched: by NASA SMEX (small explorer). 4175*350km polar orbit. T=133min. 4 passes though aurora zone per cycle. Detection of E & B field, plasma waves, high energy electrons and protons, ions mass spectra, density and temperature of hot plasma, etc. in auroral precipitation region.

Upward Current Region

Defining in in-situ measurements An enhanced flux of electrons with several keV energy. The current carried predominantly by these downward accelerated electrons. Phenomena in the upward current region can be summarized as inverted-V arc system. Often corresponds to bright, discrete visible aurora. Arc / Diffuse area in in-situ measurements: Arc: electron precipitation with signatures of acceleration by E parallel. Diffuse: just precipitate, no acceleration. Arc area in ground-based measurements: Arc: localized brightening in visible light.

An upward current arc event Inverted-V arc (a spatial, quasistatic distribution) Changes in the electron E flux are usually due to changes in the characteristic E rather than in N flux. Decrease in B is steady Relatively flat N flux Electron characteristic Energy correlates best with Energy flux

An upward current arc event Note: An inverted-V arc can be associated with several visible aurora arcs. An inverted-V arc may contain 2 or more peak.

Electron spectra in inverted V arc Monoenergeti c Plateau unstable distribution observed in A & B Wave observed in C. Generating field aligned acceleration. A. UnderB. Within C. Wave- accelerated

Electron spectra in inverted V arc Low energy Electrons in A (90deg direction): Ionospheric plasma Backscattered electrons Atmospheric secondaries Low energy Electrons in B: Interaction between solar UV & SC surface A.UnderB.Within C.Wave- accelerated

Information got from the electron spectra Temperature of the source population Width of the electron loss cone

Ion beams and density cavity Ions and electrons tell us the information of different directions. Ions: Info below. Electrons: Info above. Observation

Ion beams and density cavity Ion beams region: E field exists under observation point. Also called ΔΦ Region Observation

Ion beams and density cavity 2 ion beams density cavity

Ion beams and density cavity Monoenergetic Highly field aligned 180deg pitch angel T ⊥ >T ∥， transversely heated. May be heated by EMIC waves. Go upward by mirror force

Ion beams and density cavity density cavity Hot ion beams prevent cold plasma from moving into their regions. Only ion beams and precipitation electrons exist.

Other Phenomena observed Electrostatic shocks Shear flow

Other Phenomena observed Non-uniform altitude profile of the potential drop

Downward Current Region

Defining in in-situ measurements Regions where intense upgoing field-aligned accelerated cold ionospheric electrons are observed. Also called return current (versus primary current corresponding to bright aurora arc) Phenomena in the downward current region can be summarized as reverse arc system. Many clear parallels with upward current region, but also some interesting differences.

A downward current arc event Left: upward current region. Inverted-V structure, ion beams, etc. Right: downward current region

Electron spectra Extreme field-alignment Difference between the upgoing and downgoing population T ⊥ is no more than few eV (cold origin) Peak of V ∥ is -1.5×10 4 km/s (600eV) Hot upgoing tail extends to over 1keV (boarder distribution than upgoing ion beams)

Ion conics and pressure cooker Plasma sheet ions Ion conics Pressure cooker Broadband ELF wave Heats local ions Raising H of the ionosphere

Ion conics and pressure cooker Cold Ions Ion conics Pressure cooker Downward Current Region Upward Current Region Alfven E Field Region Pressure Cooker Region

Downward acceleration of ions Low energy (not from plasma sheet) Only seen in Downward Current Region Not related to current densities Acceleration: Ambipolar E field caused by the large difference in speeds of energetic electrons and ions in plasma sheet Source: assumed to be the ionized atmospheric neutrals

Divergent E field observations

Exceptions of quasistatic arc modes 2 main questions: wave-particle interactions. Feedback loops. (Couplings) Acceleration by Alfven waves.(aurora during substorms) Wave acceleration of low energy electrons below the potential drop regions. Where potential contours close?

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