Auroral energy flow budget as a function of altitude:
How does energy arrive from the magnetosphere?

Pekka Janhunen, FMI/Space/Helsinki

Annika Olsson, Unaffiliated (was:IRF/Uppsala)

Christopher Russell, UCLA/Los Angeles

Harri Laakso, ESA/ESTEC

Nikolai Tsyganenko, NASA/GSFC

Lars Blomberg, KTH/Stockholm

Acknowledgements: Andris Vaivads

Plan of presentation

Poynting flux as function of altitude

Comprehensive statistical study using 5 years of Polar data (EFI + MFE)

Locus of Alfvenic electron acceleration (“Alfven Resonosphere”)

Fraction of electron precipitation powered by Alfvenic acceleration

Example
Polar
EFI
and
MFE
data:
E-field

Polar
B-field

Poynting
flux
from
Polar
E-field
and
B-field

Basic
statistical
plot
from
Polar
data
MLT 18-06,
ILAT 65-74
Kp>2 red,
Kp 0-2 blue

MLT-decomposed Polar statistics

MLT 02-06

MLT 22-02

MLT 18-22

Auroral energy budget

DC Poynting dominates

AC small, but stepwise transition for Kp>2

Low Kp

High Kp

Same data;
only AC;
3 Kp ranges
(blue: 0-2,
green: 2-4,
red: >4)

Electron precipitation energy budget

AC Poynting sink at 4-5 R_E

Compared to electron precipitation (Hardy)

The “Alfven Resonosphere”

Alfven speed and electron thermal speed in Landau resonance at 4-5 R_E

Natural explanation for “Alfven Resonosphere” and Poynting sink

Independent support from dens. & E-field statistics

Conclusions

30% particle precipitation (mainly electron), 70% Joule heating

Alfven waves (~100 km scale) responsible for 10-20-40% of electron precipitation for low-mid-high Kp, respectively

Alfvenic electron acceleration occurs in the “Alfven Resonosphere” at 4-5 R_E (not in the normal acceleration region)

ARS explanation: Landau resonance

References

Janhunen, P., A. Olsson, N.A. Tsyganenko, C.T. Russell, H. Laakso and L.G. Blomberg, Statistics of parallel Poynting vector in the auroral zone as a function of altitude using Polar EFI and MFE data and Astrid-2 EMMA data, Ann. Geophys., in review, 2005.

Olsson, A., P. Janhunen, C.T. Russell and H. Laakso, Alfvenic electron acceleration in aurora occurs in global Alfven Resonosphere region, Phys. Rev. Lett., submitted, 2005.

Janhunen, P., A. Olsson, J. Hanasz, C.T. Russell, H. Laakso and J.C. Samson, Different Alfven wave acceleration processes of electrons in substorms at 4-5 R_E and 2-3 R_E radial distance, Ann. Geophys., 22, 2213-2227, 2004.

Olsson, A., P. Janhunen, J. Hanasz, M. Mogilevsky, S. Perraut and J.D. Menietti, Observational study of generation mechanism of substorm-associated low-frequency AKR emissions, Ann. Geophys., 22, 3571-3582, 2004.

Olsson, A., P. Janhunen, T. Karlsson, N. Ivchenko and L.G. Blomberg, Statistics of Joule heating in the auroral zone and polar cap using Astrid-2 satellite Poynting flux, Ann. Geophys., 22, 4133-4142, 2004.

Janhunen, P., A. Olsson and H. Laakso, Altitude dependence of plasma density in the auroral zone, Ann. Geophys., 20, 1743-1750, 2002.

Janhunen, P., A. Olsson and H. Laakso, The occurrence frequency of auroral potential structures and electric fields as a function of altitude using Polar/EFI data, Ann. Geophys., 22, 1233-1250, 2004.

These and more papers: http://www.space.fmi.fi/~pjanhune/papers/

Plasma physics and magnetosphere

2005-03-17T13:00:06