S11-3

Magnetosphere-Ionosphere-Thermosphere Coupling——
Ryoichi Fujii, Satonori Nozawa and Yasunobu Ogawa 
Solar-Terrestrial Environment Laboratory, Nagoya University,Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan !"# $464-8601 &'()*+,

Magnetosphere-ionosphere-thermosphere (M-I-T) coupling is one of the most fundamental physicalprocesses in the solar-terrestrial physics. Unlike previous studies, this paper focuses on the active role ofthe ionosphere on M-I-T coupling. Statistical studies based on data from EISCAT radar measurements
show that sunlit/shade ionospheric conditions significantly regulates electromagnetic energy from themagnetosphere to the ionosphere and that the thermosphere, i.e., neutral wind, plays a crucial role todissipate the electromagnetic energy.

1. Introduction
The magnetosphere and the ionosphere strongly
couple each other with exchanging energy either inthe form of electromagnetic energy flux
accompanied by currents and electric fields, in theform of particle fluxes associated with plasma
precipitation and outflow, or in the form of plasma
waves. It is the ionosphere and thermosphere thatdissipate energy input from the magnetosphere. The
ionosphere is a weakly ionized plasma region, inparticular, the E-region of which, where collisions
with neutrals dominate and most of the energy
dissipation thereby occurs, is of only 0.00001%
ionization rate, while the magnetosphere is a 100%
ionized collisionless plasma region. This actuallymeans that two plasma regions of different states interms of the ionization rate stay in contact with eachother. Understanding the physical processes of theenergy transfer between the magnetosphere,ionosphere and thermosphere (M–I–T) is one of thekey issues not only for the M–I–T coupling but
even also for the solar-terrestrial physics, sincesome planets such as Jupiter and Saturn have moreor less same situation to the Earth and others haverather different situations from it.
It was received wisdom that the ionosphereplayed a somewhat passive role in M-I-T couplingin comparison to the magnetosphere where plasma
motion, electric fields and currents were driven.
Recent observations have clearly shown, however,that the ionosphere plays a much more active roleon the M-I coupling processes than we considered
before. Indeed, observations with the FAST and
Freja satellites have shown that ionospheric thermal
electrons are sometimes greatly accelerated into
field-aligned upward directions by downwarddirected parallel electric fields existing in the lower

magnetosphere, carrying downward field-alignedcurrents (e.g., [1]). These downward field-aligned
currents were indeed thought to be secondary andmerely return currents of the primary upward fieldaligned currents carried by precipitation electrons
from the magnetosphere. Such upward moving
electrons were observed almost exclusively in awinter hemisphere, suggesting that the ionospheremust play an indispensable role in their formation.
On the other hand, Newell et al. [2] indicated thatthe ionosphere played an essential role on theoccurrence/intensity of auroras, that is, energetic
electron precipitation. One of the outstanding
problems is then how the ionosphere actively
regulates the electromagnetic energy input from themagnetosphere to the ionosphere.
Another key issue is to understand how theionosphere and the thermosphere interact, inparticular, how the electromagnetic energy from themagnetosphere is distributed to Joule heating thatheats up both ions/electrons and neutrals and todynamical energy of neutrals, which in turn giveenergy to ions/electrons. The rate of theelectromagnetic energy transfer is expressed by J·E(J is the electric current and E is the electric field),which is, as mentioned above, further converted in
the lower ionosphere both to the Joule heating rate
J·E' and to the mechanical energy transfer rate of
the neutral gas U·(J¥B), where U is the neutral
wind velocity and E'=E+U¥B. Hence, the estimateof the Joule heating rate and the mechanical energytransfer rate is not possible without determining theneutral wind velocity. Measuring the neutral windvelocity, which is highly height dependent, is avery difficult task, however, which can be derived
observationally only from incoherent scatter (IS)energy input from the magnetosphere to theionosphere is distributed to both Joule heating J·E'and the mechanical energy U·(J¥B). J·E' isgenerally larger than U·(J¥B), but the latter one is
generally not negligible and becomes comparable
when the magnetosphere is disturbed. It is also
noted that U·(J¥B) can be negative, meaning thatthe energy is transferred from neutral wind toplasmas.

radar measurements with good altitude resolution,
covering the whole E and lower F region heightrange (e.g., [3]). Quantitative characteristics of J·E,
J·E' and U·(J¥B) based on observations that are notyet determined will certainly contribute to thefurther understandings of the M–I–T coupling
processes.
This paper is going to address the above
mentioned two issues using long-term EuropeanIncoherent Scatter (EISCAT) radar measurements

Tromsø and conjugate: shadedTromsø: shaded, conjugate: sunlit
Tromsø: sunlit, conjugate: shaded
Tromsø: and conjugate: sunlit

100£52-£Kp£

100

100

Occurrence (%)

3-9 MLT

15-21MLT

21-03 MLT

80

80

80

60

60

60

40

40

40

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20

0004812 16 20004812 16 20048

12 16 20

Electromagnetic energy flux (mW/m2). Figure 1. Histograms of J·E in the dawn, dusk andnight regions. The occurrence is normalized, with thelargest occurrence for each region set to 100%.
3. Ionosphere-Thermosphere Interaction
We have studied characteristics of the energy
coupling processes between the lower thermosphere,ionosphere and magnetosphere through an analysis
of J·E, J·E' and U·(J¥B), based on EISCAT radar
data obtained from 1989 to 1991.
The present study shows that the electromagnetic

J·E

32J·E'U·(JxB)

1Energy flux per meter (mW/m 3 )2. Active Role of Ionosphere on M-I Coupling
Based on a statistical analysis of EISCAT KST(Kiruna-Sodankylae-Tromsoe) tri-static radar data
we have quantitatively investigated how actively
the ionospheric conditions relate to the M-Icoupling. The electromagnetic energy J·E from themagnetosphere to the ionosphere and the electricfield relate strongly to the sunlit/shade condition of
the ionosphere not only at the local observation
point above the EISCAT radar but also at its
magnetic conjugate point in the southernhemisphere. Both J·E and the electric field strength
are enhanced more frequently when both of oreither of the local and conjugate ionospheres are in
the shade than when these two regions are sunlit,
suggesting strongly that the ionosphere is not
passive at all but regulates the magnetosphere tosome extent.

0KP > 50-12102 0 < KP < 5+-1210-1

KP < 2+0612180612180Magnetic local time (hours)

61218Figure 2. The magnetic local time (MLT) distribution of
the electromagnetic energy transfer rate J·E (left), theJoule heating rate J·E' (middle) and the mechanicalenergy transfer rate U·(J¥B) (right) under four
geomagnetic disturbance levels: KP > 5 (top), 2 < KP ≤ 5(middle), and KP ≤ 2 (bottom)

References[1] Carlson et al., Geophys. Res. Lett., 25 (1998) 2017.
[2] Newell et al., Nature, 381 (1996) 766.
[3] Fujii et al., J. Geophys. Res., 104 (1999) 2357.

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